Effect of straw biochar magnesium lanthanum hydrotalcite on phosphorus recovery from acidified biogas slurry

Effect of straw biochar magnesium lanthanum hydrotalcite on phosphorus recovery from acidified biogas slurry | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Effect of straw biochar magnesium lanthanum hydrotalcite on phosphorus recovery from acidified biogas slurry Yanru Ma, Yujun Shen, Hongsheng Cheng, Haibin Zhou, Jingtao Ding, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6321421/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 21 Jul, 2025 Read the published version in Waste and Biomass Valorization → Version 1 posted 5 You are reading this latest preprint version Abstract Biogas slurry is rich in phosphorus and serves as an important source for phosphorus recovery. To improve the efficiency of phosphorus recovery from biogas slurry and strengthen the recovery process, H 2 SO 4 is used for the acidification pretreatment of biogas slurry. However, the primary mechanism underlying phosphorus recovery from acidified biogas slurry remains unclear. In this study, the effects of different acidification conditions on the transformation of phosphorus forms and physicochemical properties of biogas slurry as well as the phosphorus recovery effect of a straw charcoal magnesium–lanthanum hydrotalcite composite material (6YBC-LDO) were investigated. The results showed that soluble orthophosphate (Ortho-P) was the main form of phosphorus recovered from the slurry by the 6YBC-LDO adsorbent. When H 2 SO 4 was added to adjust the pH of the biogas slurry to 6.0, 6.5, and 7.0, the mass concentration of Ortho-P increased by a factor of 20, 9, and 4, respectively, compared with the original biogas slurry. The total phosphorus recovery rates of 6YBC-LDO from acidified biogas slurry at pH 6.0, 6.5, and 7.0 were 150.99, 144.73, and 40.32 mg/g, respectively, with phosphorus recovery rates of 64%, 85%, and 99%, respectively. The results showed that the pH of biogas slurry acidification was the main factor affecting the phosphorus recovery efficiency of biogas slurry. Acidification with pH 7.0 significantly affected phosphorus recovery. The phosphorus recovery products from biogas slurry treated with pH 7 were mainly phosphate complexes with higher purity. High purity phosphorus recovery materials could be used as for efficient phosphorus fertilizers production in the future, which has great market application potential. biogas slurry acidification phosphorus species adsorption recovery Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 1 Introduction With the continuous improvement in livestock and poultry breeding, the number of intensive biogas projects in China is increasing [ 1 ]. According to statistics, more than 120000 large-scale biogas projects are conducted in China, with an annual output of 25 billion m 3 of biogas and an annual production of ~ 30 million tons of biogas slurry. The production is increasing at a rate of ~ 14% per year [ 2 ]. The processing of large amounts of biogas slurry is a problem. Biogas slurry is rich in phosphorus and its direct discharge wastes resources and pollutes the environment [ 3 ], the improper discharge of biogas slurry could lead to the eutrophication problem of water environment. In recent years, technologies for recovering phosphorus nutrients from biogas slurry have received increasing attention [ 4 , 5 ], the main technologies include struvite crystallization, membrane concentration for fertilizer production, and nutrient adsorption and utilization [ 6 ]. Struvite crystallization technology is relatively mature for phosphorus recovery, but the purity of the recovered struvite is generally low, and struvite loss is significant [ 7 ]. Membrane concentration technology exhibits excellent filtration efficiency for nitrogen and phosphorus in biogas slurry and is often used in advanced treatment processes. However, due to severe membrane fouling and high operational energy consumption, its application in nutrient recovery from biogas slurry is limited. In contrast, the adsorption method is one of the primary technologies for nutrient recovery, and adsorption processes using cost-effective and environmentally friendly materials have garnered increasing attention [ 8 ]. The use of adsorption methods to recover phosphorus from biogas slurry reduces the loss of phosphorus resources and environmental eutrophication caused by improper discharge, decreases the cost of subsequent treatment of biogas slurry, and improves economic benefits [ 9 , 10 ]. Relevant data show that in the world's 71 billion tons of phosphorus rock reserves, China only accounts for 5.2% of the world's total, the current global phosphorus reserves may be exhausted in 50 ~ 100 years. Phosphorus resources are increasingly scarce, and phosphorus recovery is a problem that cannot be ignored. As a recyclable agricultural waste resource, biogas slurry is rich in nitrogen, phosphorus and potassium elements, which is considered be the second largest phosphate mineral[ 11 , 12 ]. The full recovery of nutrients such as nitrogen and phosphorus from biogas slurry has become a research hotspot. In recent years, more and more attention has been paid to biogas slurry phosphorus recovery technology [ 4 – 5 , 13 ]. Some studies have concluded that replacing existing wastewater treatment systems with resource recovery technologies can increase the global nutrient and energy recovery potential by about 50%~79%. Therefore, the recovery of phosphorus from biogas slurry can realize the management and utilization of nutrients [ 14 ], avoid the waste of resources caused by direct discharge of biogas slurry, and help to solve the problem of rational utilization of phosphorus in biogas slurry. Realizing the recycling of phosphorus resources is a new way to achieve green and sustainable development of circular agriculture. The magnesium–lanthanum hydrotalcite composite material exhibited excellent adsorption performance for phosphorus, especially orthophosphate(Ortho-P) [ 15 ]. Compared with other phosphorus adsorbents, this material has high selectivity and efficient phosphorus adsorption in biogas slurry [ 16 , 17 ], making it more suitable for phosphorus recovery from biogas slurry [ 18 , 19 ]. However, the Ortho-P content in the digestate is less than 30% of the total phosphorus (TP) content. Because of the high concentration of suspended solids and viscosity, most of the phosphorus in the digestate is present in the form of solid-phase phosphorus (TP S ) in suspended solids, which cannot be directly recovered [ 20 ]. Therefore, a single adsorption technology cannot ensure the efficient recovery of phosphorus from biogas slurry and it is crucial to convert the solid phosphorus in biogas slurry into a dissolved state. The results of previous studies showed that acidification is a feasible method for promoting the conversion of particulate to soluble phosphorus in biogas slurry and for increasing the proportion of dissolved TP [ 21 , 22 ]. Acidification of biogas slurry is considered to be an effective method for improving the utilisation efficiency of nitrogen and phosphorus nutrients [ 23 ], which can promote the dissolution of nutrients, such as calcium (Ca) and magnesium (Mg) [ 24 ], increase the nutrient availability, and reduce the total suspended solid (TSS) concentration and turbidity. This technology has been applied in several countries, including Denmark and the United Kingdom [ 25 ]. Regueiro et al. demonstrated that sulfuric acid acidification of biogas slurry promotes phosphorus solubilisation, resulting in a TP leaching rate of 90% [ 26 ]. Zeng et al. reported that sulfuric acid has a significant effect on the dissolution of phosphorus in particulate matter and demonstrated that sulfuric acid provides more stable pH conditions for biogas slurry [ 27 ], reduces the cost of biogas slurry treatment.. In summary, acidification technology is suitable for biogas slurry treatment. However, research on the recovery of phosphorus from acidified biogas slurry using adsorbent materials is limited. The aim of this study was to improve the efficiency of phosphorus recovery from biogas slurry and enhance the phosphorus recovery process. A sulfuric acid acidifier was used for the pretreatment of biogas slurry to promote the recovery of phosphorus from biogas slurry using a 6YBC-LDO. By exploring the effects of different acidification conditions on the transformation of phosphorus forms and the physicochemical properties in the biogas slurry, the dissolution characteristics of phosphorus in the acidified biogas slurry were determined. By analysing the recovery characteristics of the 6YBC-LDO adsorbent material for phosphorus from acidified biogas slurry, the main mechanism and efficiency of phosphorus recovery from acidified biogas slurry were explored. Correlations between the phosphorus recovery efficiency and characteristics of the acidified biogas slurry were analysed to clarify the factors influencing the phosphorus recovery from acidified biogas slurry. To provide an effective pathway for regulating phosphorus recovery, the benefits and application potential of the acidification slurry phosphorus recovery process were evaluated in this study. 2 Materials and methods 2.1 Materials Biogas slurry was obtained from a pig farm in the Shunyi District, Beijing, China, stored for 60 d after anaerobic fermentation, and then discharged from the secondary sedimentation tank. The biogas slurry was collected and sealed for sedimentation for 1 week and then subjected to primary filtration using a 0.15-mm filter to remove large suspended particles and excess impurities before subsequent use. The acidification slurry container consisted of a 50-L polyethylene acid-resistant bucket. 2.2 Preparation of 6YBC-LDO composite material The mixed Mg(NO 3 ) 2 ·6H 2 O and La(NO 3 ) 3 ·6H 2 O was prepared at a volume ratio (n (Mg 2+ )/n (La 3+ )) of 10/1 at 25°C, with 3.5 mol/L NaOH solution and 0.94 mol/L Na 2 CO 3 solutions, and the straw was weighed at 50% of the total mass of Mg/La salt. The straw (0.2 g) was dispersed in deionised water (1 mL) for 30 min. The deionised water with straw was then poured into the mixed solution of Mg(NO 3 ) 2 ·6H 2 O and La(NO 3 ) 3 ·6H 2 O, was heated to 90°C and rapidly stirred for 10 min at 200 rpm, then stirred at 25°C for 2 h. The straw-Mg/La 0.1 -LDH complex was then prepared by adding the solution of Na 2 CO 3 and NaOH to the straw Mg/La solution drop by drop, heating to 65 ± 5°C, and stirring for more than 16 h. The obtained solution was centrifuged at 3500 rpm for 10 min repeatedly, washed, dried, ground to a uniform granular molecular size, and sieved through a 0.15 mm molecular sieve (W.S. Tyler, USA). The straw-Mg/La 0.1 -LDH complex was then pyrolysed and carbonised in a tube furnace at 600°C for 2 h under an N 2 atmosphere. The final product was named 6YBC-LDO and was stored in airtight glass bottles for further analysis. 2.3 Analysis of the effect of phosphorus conformation in biogas slurry Under natural storage conditions, the biogas slurry was placed in a 50-L polyethylene acid-resistant bucket. A sulfuric acid acidifier was added to the slurry, and the pH values were adjusted to 6.0, 6.5, and 7.0, respectively, labelled as pH 6.0, 6.5, and 7.0, respectively. The original slurry was used as the control (CK) under the same conditions. The experimental period was 28 d. Samples were collected on days 0, 7, 14, 21, and 28 of storage. The mass concentrations of TP, liquid TP (TP L ), total dissolved phosphorus (TDP), and dissolved Ortho-P in the biogas slurry during the acidification storage process as well as the pH, conductivity, electrical conductivity (EC), turbidity, chemical oxygen demand (COD), TSS, NH 4 + -N, and total organic carbon (TOC) contents of the biogas slurry before and after acidification were measured. The effects of acidification on the physicochemical properties and phosphorus form conversion of the biogas slurry were analysed. 2.4 Phosphorus recovery experiment The 6YBC-LDO adsorbent material was used to adsorb biogas slurry acidified to pH 6.0, 6.5, and 7.0 as well as non-acidified biogas slurry (CK). The experiment was conducted in a 50-mL polyethylene centrifuge tube. Adsorbent (0.08 g) was dispersed in 40 mL of biogas slurry and oscillated at a speed of 180 r/min for 24 h at constant temperature (25 ± 1°C). After the adsorption equilibrium was reached, the biogas slurry was left to stand for 15 min, subsequently centrifuged at a speed of 4000 r/min for 20 min, and finally filtered through a 0.45- µ m membrane to obtain the supernatant for measurement. The phosphorus recovery equation is as follows: where Q is the amount of phosphorus recovered by the adsorbent at adsorption equilibrium (mg/g), C 0 is the pre-adsorption acidification biogas slurry (mg/L), C e is the biogas slurry phosphorus concentration at adsorption equilibrium (mg/L), V is the adsorption volume of the biogas slurry, and m is the amount of adsorbent used (g). 2.5 Characterisation and analysis methods for recycled products The surface structure of the composite material before and after phosphorus adsorption was observed using field-emission scanning electron microscopy (FE-SEM, SU8010, HITACHI, Japan). Energy-dispersive spectroscopy (EDS, EDAX Octane Plus, AMETEK, USA) and transmission electron microscopy (TEM, H7650, HITACHI, Japan) were used to determine the surface elemental distribution of the composite materials. Data were acquired using an X-ray diffractometer (XRD, Bruker D8 Venture, Bruker, Japan). The crystal structure of the material was analysed using MDI jade6.0 software. A Fourier transform infrared spectrometer (FTIR, Nicolet i50, Thermo Fisher Scientific, USA) was employed to characterise the functional groups on the material surfaces. An inductively coupled plasma emission spectrometer (ICPE-9000; Shimadzu, Japan) was used to determine the concentrations of phosphorus (P) and magnesium ions (Mg 2+ ) in the biogas slurry before and after adsorption. 2.6 Analysis of physicochemical properties of biogas slurry The pH of biogas slurry was measured using a pH meter (Thunderbolt DDS-307A; Mettler). EC was analysed using a conductivity meter (Thunderbolt DDS-307A, Mettler). The TP and ammonia nitrogen concentrations were determined using a UV–Visible spectrophotometer (UV8100, LabTech, USA). The turbidity was analysed using a turbidity meter (Thunderbolt WZB-175, Mettler). The concentrations of Cu 2+ , Zn 2+ , and Cr 3+ were determined using inductively coupled plasma mass spectrometry (ICP-MS; Agilent, USA). The COD was determined using the dichromate method. Each sample was analysed in triplicate and the average of the test results was reported. 2.7 Data analysis Experimental data were statistically analysed using Microsoft Excel 2021. A factor analysis of variance and significance test (LSD method, P < 0.05) were performed using SPSS (version 20.0; SPSS Inc., Chicago, USA). The experimental results were plotted using Origin 2021 (Origin 2021, Northampton, MA, USA). The ‘Correlation Plot’ package of Origin 2021 was utilised for correlation analysis. Redundancy Analysis (RDA) was performed using Canoco 5.0 software. Penetration curve data were fitted using a logistic model from Origin 2021. 3. Results and discussion 3.1 Characterisation of 6YBC‑LDO composite material and biogas slurry The N 2 adsorption-desorption isotherm of 6YBC-LDO is shown in Fig. 1 a, which is a Type IV adsorption isotherm with an H3 hysteresis loop. The pores of this material are slit-shaped, formed by the stacking of plate-like particles. The 6YBC-LDO adsorption material exhibits a high specific surface area of 123.65 m²/g, a pore volume of 0.149 cm³/g, and an average pore diameter of 7.70 nm, indicating that the pore size falls within the mesoporous range. The surface morphology and elemental distribution of the 6YBC-LDO composite material were characterized using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). As observed in Figs. 1 b and 1 c, Mg/La 0.1 -LDO is uniformly distributed on the straw carbon, and the layered structure of Mg/La 0.1 -LDO is well preserved. EDS mapping revealed that the main elemental components of the composite material are C, O, Mg, La, Na, Ca, and Si. In the 6YBC-LDO composite material, the mass percentages of the elements are 45.06% for C, 8% for Mg, and 4.12% for La. The physicochemical properties of biogas slurry are listed in Table 1 . Table 1 Physicochemical properties of biogas slurry Index pH EC(mS/cm) Turbidity(NTU) TSS(mg/L) COD(mg/L) NH 4 + -N(mg/L) TP(mg/L) Content 8.3 4.6 528 277 2210 1455.69 472.91 Table 2 Comparison of economic benefits of phosphorus recovery technology in biogas slurry Types of fertilizers Phosphorus recovery from biogas slurry(kg/m 3 ) Purity of phosphate fertilizer(%) Selling price(RMB/t) Reference Struvite(MgNH 4 PO 4 ·6H 2 O) 1.87 12.62 2010 ~ 2589 [ 45 ] Granular phosphate fertilizer(Ca(H₂PO₄)₂) / 12 ~ 20 3450 Selling price Liquid phosphate fertilizer(KH 2 PO 4 ) / 22.75 3000 ~ 10000 Selling price Straw biochar hydrotalcite powder composite phosphate fertilizer 1.92 23.81 ~ 30.35 3000 This study 3.2 Effect of acidification on the phosphorus dissolution characteristics of biogas slurry 3.2.1 Effect of acidizing pH on the distribution of phosphorus conformation in solid and liquid phases The morphological changes of solid and liquid phosphorus in the biogas slurry after different acidification treatments are shown in Fig. 2 . The TP L content gradually increased with decreasing pH, whereas the TP content decreased with decreasing pH. The main reason for this was that acidification dissolved inorganic phosphorus, leading to an increase in the TP L concentration [ 28 , 29 ]. Compared with other treatments, the TP content dissolved at pH 6.0 was higher and 80% of the phosphorus was released, which is consistent with the results of previous studies [ 30 , 31 ]. Improving the degree of acidification of the biogas slurry is beneficial for the gradual conversion of TP S to TP L [ 32 ]. The storage time of acidified biogas slurry also affects the distribution of TP S and TP L . Based on the comparison of the distribution of TP S and TP L in the early and late stages of acidified storage, TP L accounted for 94.93%, 54.54%, 53.64%, and 32.78% of TP in biogas slurry treated with pH 6.0, 6.5, 7.0, and CK, respectively, in the early stage of storage. In the later stages of storage, TP L accounted for 97.27%, 84.02%, 34.32%, and 10.02% of TP, respectively. As the storage time increased, the TP L content of the acidified slurry increased, whereas the TP L concentration of the untreated slurry gradually decreased. This indicates that, without acidification treatment, the prolonged storage period of the biogas slurry led to the conversion of TP L to TP S , which is not conducive to phosphorus recovery. 3.2.2 Effect of pH on the distribution of liquid phosphorus forms The transformation of liquid phosphorus in biogas slurry after different acidification treatments is shown in Fig. 3 . After H 2 SO 4 acidification of biogas slurry, the TP L content significantly increased and the phosphorus solubility and pH value of the acidified biogas slurry were significantly negatively correlated. The lower the pH is, the greater is the amount of phosphorus dissolved in biogas slurry. For example, when the initial pH value of acidified biogas slurry was 6.0, the TP L content varied between 340.01 and 460.02 mg/L within 28 d, which was 14–48 times higher than that of the non-acidified biogas slurry (CK). The main form of TP L , that is, TDP, included Ortho-P and dissolved reduced phosphorus (RDP). After treatment at pH 6.0, 6.5, and 7.0, the Ortho-P content was 75.27–93.71%, 52.78–79.12%, and 48.29–33.91% of TP, respectively. The degree of H 2 SO 4 acidification significantly affected the concentration of Ortho-P in the biogas slurry. The concentration range of Ortho-P in biogas slurry treated with pH 6.0, 6.5, and 7.0 was 283.60–416.27, 150.42–309.10, and 69.41–86.8 mg/L, respectively. The storage time also affected the transformation of phosphorus forms. The changes in liquid-phase phosphorus forms with storage time are shown in Table S1 . The phosphorus concentration of non-acidified biogas slurry (CK) gradually decreased with the extension of the storage period. The mass concentration of TP L in the early stage of acidification ranged from 8.36 to 22.68 mg/L. In the late stage of acidification (28 d), the mass concentration of Ortho-P in the original biogas slurry decreased by 55.42% compared with the initial value. This may be due to the gradual conversion of organic to inorganic phosphorus in the biogas slurry. The degradation of organic matter causes the conversion of dissolved into particulate phosphorus [ 33 ]. After H 2 SO 4 acidification treatment, the TP L and Ortho-P contents of the digestate treated with pH 6.0 and 6.5 significantly increased with the extension of the storage period. The content of Ortho-P dissolved after storage at pH 6.0, 6.5, and 7.0 is 29, 20, and 4 times that of the original digestate, respectively, indicating that storage strengthened H 2 SO 4 acidification and dissolution of phosphorus and that acidification improved the conversion of recoverable phosphorus in digestate. Acidification reduced the loss of dissolved phosphorus during the storage of biogas slurry. 3.2.3 Effect of acidification on the physicochemical properties of biogas slurry Acidification pH is a key factor affecting the changes in the characteristics of biogas slurry. The changes in various indicators of the acidified biogas slurry (pH 6.0, 6.5, and 7.0) and non-acidified biogas slurry (CK) are shown in Fig. 4 . The changes in the pH and EC of the digestate are shown in Fig. 4 a. After different acidification treatments, the pH of the digestate increased significantly ( P < 0.05), indicating alkalinity. This was due to the change in the microbial activity of the digestate after acidification [ 34 ]. The degradation of organic matter produced NH 4 + -N, resulting in an increase in the pH value [ 35 ]. In contrast, acidification led to a significant decrease in the EC value of the biogas slurry ( P < 0.05), but the EC value of the pH 6.0 acidification treatment remained high. Changes in the TSS content and turbidity of the biogas slurry are shown in Fig. 4 b. The TSS content and turbidity of acidified biogas slurry were significantly reduced compared with those of the original digestate. Both showed the same trend, indicating that sulfuric acid dissolved a large amount of suspended solids in the digestate, effectively reducing the dry matter content. Changes in the COD and TOC content of the biogas slurry are shown in Fig. 4 d. The COD and TOC content were significantly reduced after H 2 SO 4 acidification ( P < 0.05), which was directly related to the degradation of organic matter. Carbon in organic matter can be lost as CH 4 and CO 2 [ 36 ], indicating that acidification promotes organic matter degradation. Changes in the NH 4 + -N and TP L concentrations in the biogas slurry are shown in Fig. 4 c. The NH 4 + -N content of acidified biogas slurry decreased compared with that of original biogas slurry, which was due to the loss of NH 4 + -N in the form of ammonia gas. Popovic and Jensen also reported the phenomenon of NH 4 + -N loss after acidification [ 37 ]. The TP L content in the biogas slurry increased with the decrease in the acidification pH, which was due to the dissolution of solid phosphorus in the sediment of the biogas slurry and a large amount of particulate phosphorus (PP) in suspended solids by acidification, causing phosphorus to diffuse into the liquid phase in a dissolved form. This indicates that H 2 SO 4 promotes the release of soluble phosphorus. Therefore, it can be inferred that the physicochemical properties of the biogas slurry are influenced by the acidification pH. 3.3 Effects of composite materials on phosphorus recovery from acidified biogas slurry In this study, 6YBC-LDO was used to adsorb and recover H 2 SO 4 -acidified biogas slurry after storage. The phosphorus recovery effect of the 6YBC-LDO composite material on the acidified biogas slurry is shown in Fig. 5 . Significant differences in the phosphorus recovery can be observed among the treatments ( P < 0.05). As the initial pH value of acidification decreased, phosphorus recovery gradually increased. The phosphorus recovery capacities of the pH 7.0, 6.5, and 6.0, treatments were 40.32, 144.73, and 150.99 mg/g, respectively. The phosphorus recovery of the original biogas slurry (CK) was only 2.71 mg/g, indicating that H 2 SO 4 acidification significantly improved the phosphorus recovery efficiency of the slurry. The adsorption effect of 6YBC-LDO on phosphorus in acidified biogas slurry is shown in Fig. 6 . The adsorption rates of acidified biogas slurry were 99.17%, 85.03%, and 64.10% at pH 7.0, 6.5, and 6.0, respectively. Treatment at pH 6.0 with the highest recovery capacity resulted in a lower adsorption rate, indicating that the recoverable phosphorus dissolved by acidification cannot be completely adsorbed and 6YBC-LDO has reached adsorption saturation. However, no significant difference ( P > 0.05) was observed in the phosphorus recovery efficiency of 6YBC-LDO in biogas slurry treated with pH 6.5 compared with that treated with pH 6.0. However, the adsorption rate of the pH 6.5 treatment exceeded 85%, indicating that pH 6.5 treatment has advantages. In contrast, the phosphorus adsorption rate of the CK treatment was only 52.45% and the phosphorus recovery is also relatively low. This indicates that phosphorus in the original biogas slurry existed very rarely in the form of Ortho-P and mostly in the form of PP, which affected the phosphorus recovery efficiency. The effect before and after adsorption on chromaticity and turbidity of acidified biogas slurry is shown in Fig. 6 . The recovery of acidified biogas slurry by 6YBC-LDO had a certain effect on removing the chromaticity and turbidity of the biogas slurry. In summary, an initial pH range of the biogas slurry of 6.5–7.0 after H2SO4 acidification was optimal for the recovery of phosphorus from biogas slurry by 6YBC-LDO. Compared with the original slurry, the recovery efficiency of phosphate could be improved by a factor ranging from 14 to 52. 3.4 Effect of acidification on the composition of phosphorus recovered from biogas slurry 3.4.1 Morphology of phosphorus recovery products The morphology of phosphorus recovered by 6YBC-LDO after acidification of biogas slurry is shown in Fig. 7 . Three different morphologies could be observed as pH changes affected the complex ion equilibrium system in the biogas slurry, which in turn affected the composition and phosphorus forms of recovered products. Figures 7 a and 7 c show that the recovered particles treated with pH 6.0 were regular quadrilateral crystals and that a small amount of regular quadrilateral crystals remained in the recovered particles treated with pH 7.0. However, the amount of amorphous substances in the recovered particles increased. According to the XRD results shown in Fig. 10 , this substance may be Ca 3 Mg 3 (PO 4 ) 4 . The surface structure of the recovered material treated at pH 6.5 is shown in Fig. 7 b. Many sheet-like cluster structures can be observed on the recovered material, which are likely LaPO 4 formed after the intercalation of PO 4 3− by magnesium/lanthanum hydrotalcite (Mg/La LDH). The surface structure of the recovered material treated with CK is shown in Fig. 7 d. A large number of long-pointed tubular tree branches appeared on the surface of the recovered material. This substance may be struvite crystals, as determined by XRD analysis. Hu et al. and Karbakhshravari demonstrated that struvite crystals have a double-branched and tubular structure [ 38 , 39 ]. The results of this study were similar to those of other studies in which researchers reported that phosphorus recovery generates both adsorption and precipitation products. Xu et al. stated that adsorption and precipitation reactions occur simultaneously when phosphorus is recovered from Mg-supported biochar materials [ 40 ]. Based on the above-mentioned analysis, the composition of the recovered material significantly differed when H 2 SO 4 was used to acidify biogas slurry to different pH values, indicating the existence of multiple synergistic mechanisms. 3.4.2 Distribution of elements in recycled materials Energy spectrum point measurements and EDS were performed on phosphorus-recovered materials after various acidification treatments to determine the main components of the recovered materials. The EDX spectra of the recovered materials after acidification treatment are shown in Fig. 8 . The EDX analysis showed that C, O, and Mg were the main elements in the recovered material and that N, Ca, K, Si, and La were also present. After adsorption saturation, treatment with pH 6.0, 6.5, 7.0, and CK resulted in a uniform distribution of P on the entire surface of the material, indicating that the adsorbent surface had abundant adsorption sites that could bind and effectively capture phosphate on the material surface. On the surface of the recycled material, the distribution sites of Mg and O strongly overlapped with the distribution sites of P, indicating that Mg and O in the adsorbent participated in the phosphorus adsorption reaction. Kong et al. used La@MgAl and observed a similar phenomenon in the recovered material when using nanocomposites to remove phosphate from water [ 41 ]. In addition, La and P in recovered materials treated with pH 6.5 and CK also strongly overlapped. However, they only partially overlapped in recovered materials treated with pH 6.0 and 6.5. Based on the above-mentioned analysis, La ions participated more in phosphorus adsorption reactions at an acidification pH value of 6.5 and 7.0, whereas an acidification pH value of 6.0 promoted the reaction between Mg and O ions and phosphate. The elemental distribution of phosphorus recovery material after treatment with 6YBC-LDO at pH 6.0, 6.5, 7.0, and CK in biogas slurry is shown in Fig. 9 . The Mg/P mass ratios of 6YBC-LDO of recovered materials of biogas slurry treated with pH 6.0, 6.5, 7.0, and CK were 0.59, 1.28, 0.57, and 4.67, respectively. The similar distribution ratio of Mg/P elements at pH 6.0 and 7.0 indicates that generated magnesium phosphorus compounds had the same structure. XRD detection results showed that both produced Ca 3 Mg 3 (PO 4 ) 4 . According to the analysis of the mass proportion of phosphorus in the recovered material, the phosphorus purities of the recovered products of 6YBC-LDO for pH 6.0, 6.5, 7.0, and CK biogas slurry were 22.33%, 7.02%, 23.81%, and 1.14%, respectively. The results showed that the phosphorus recovery product obtained from pH 7.0-acidified biogas slurry had a higher purity than other condition of pH values. 3.4.3 Analysis of structural components of phosphorus-recycled materials The crystal phase of the recovered material was identified using XRD and the crystal structure of the phosphorus recovered from the acidified biogas slurry was determined. The results are shown in Fig. 10 . Strong peaks were observed at 43.04°, 62.03°, and 36.58° prior to 6YBC-LDO adsorption. Novais et al. showed that these peaks correspond to MgO crystal peaks [ 42 ]. La 2 O 3 crystal peaks were observed at 29.59° and 28.70°. The crystal phase structures of the recovered material treated with CK and pH 6.5 after adsorption were similar. Peaks appearing at 30.48° and 33.91° matched well with those observed for bird guano stone (PDF: 77-2303), indicating the presence of bird guano stone crystals in the recovered material. However, the purities of the two differed from the distributions of the elements. Researchers showed that MgO can participate in the crystallisation reaction of bird guano stone when the pH ranges between 5.5 and 6.5 and the presence of MgO promotes the crystallisation reaction of bird guano stone [ 43 ]. In addition, LaPO 4 crystallisation peaks were observed at 40.90° in phosphorus recovery materials treated with CK and pH 6.5. Phosphorus recovery products treated with pH 6.0 and 7.0 showed Ca 3 Mg 3 (PO 4 ) 4 crystallisation peaks at ~ 31.61°. Based on the above-mentioned analysis, sludge treated with CK and pH 6.5 formed a LaPO 4 complex after adsorption by 6YBC-LDO. The phosphorus in the digestate treated with pH 6.0 and pH 7.0 was effectively recovered through the complexation reaction of Ca 3 (PO 4 ) 2 and Mg 3 (PO 4 ). 3.4.4 Analysis of structure and composition of phosphorus-recycled materials The Mg 2+ mass concentration and pH of acidified biogas slurry at the adsorption equilibrium are shown in Fig. 11 . The Mg 2+ content in the acidified digestate treated with pH 6.0 and 6.5 was lower after adsorption, indicating that MgO participated in the phosphorus adsorption reaction when 6YBC-LDO adsorbed the acidified digestate at pH 6.0 and 6.5. The high amount of dissolved phosphorus in the digestate resulted in significant consumption of MgO. In contrast, the Mg 2+ content in the digestate treated with pH 7.0 and CK after adsorption exceeded that before adsorption, indicating that the dissolved phosphorus at pH 7.0 and non-acidified digestate were completely adsorbed by 6YBC-LDO. In summary, when the initial acidification pH of the biogas slurry was 6.5, the precipitation of bird guano and the complexation reaction synergistically promoted the recovery of phosphorus by 6YBC-LDO. The purity of the phosphorus recovery product was 7.02% and recovered substances included bird guano and phosphate complexes. When the initial acidification pH of the biogas slurry was 6.0 or 7.0, phosphorus was mainly recovered through complexation reactions. The purities of the phosphorus recovery products from the biogas slurry at pH 6.0 and 7.0 were 22.33% and 23.81%, respectively. The purity of the phosphorus recovery product from biogas slurry at pH 7.0 was relatively high. Therefore, pH 7.0 acidification treatment is preferred for phosphorus recovery from biogas slurry. 3.5 Correlation analysis between phosphorus recovery and acidification of biogas slurry 3.5.1 Correlation analysis To further investigate the impact mechanism of the acidification process on phosphorus recovery from biogas slurry, a correlation analysis was conducted between the amount of phosphorus recovered and characteristic indicators of the acidified biogas slurry. The results are shown in Fig. 12 .A significant positive correlation was observed among phosphorus recovery and the contents of TP ( P < 0.001), TP L ( P < 0.001), TDP ( P < 0.001), Ortho-P ( P < 0.001), RDP ( P < 0.01), and PP ( P < 0.01) in the acidified biogas slurry, whereas a significant negative correlation with TPs (P < 0.05) was evident. Significant positive correlations were observed among TP, TP L , TDP, Ortho-P, RDP in the biogas slurry and EC, whereas the pH value, TSS concentration, and heavy metal chromium (Cr) content of the acidified biogas slurry were significantly negatively correlated. In addition, the content of TPs in the acidified biogas slurry was significantly positively correlated with the pH value ( P < 0.01) and significantly negatively correlated with the EC and Cu and Zn contents. The turbidity of the acidified biogas slurry was significantly negatively correlated with the TP content. 3.5.2 RDA analysis RDA was used to further determine the correlations between phosphorus recovery from biogas slurry and environmental factors. The results are shown in Fig. 13 . A strong constraint relation was observed between the amount of phosphorus recovered and the acidification characteristics of the biogas slurry. Figure 13 shows that 98.5% of the change in the phosphorus form can be explained by characteristic indicators of the acidified biogas slurry. The pH value, TSS, EC, and Cr content were significantly correlated with phosphorus forms of acidified biogas slurry ( P < 0.01). The pH value and TSS concentration of the biogas slurry explained 68.8% and 15.1% of the variance, respectively, reaching a significant level. In addition, a positive correlation was observed among the phosphorus recovery amount, phosphorus recoverable state (Ortho-P), and EC and the TPS content, pH, and TSS concentration were negatively correlated. In summary, the recoverable state of phosphorus (Ortho-P) and the pH value, TSS concentration, and EC value of the acidified biogas slurry were strongly correlated. The higher the degree of H 2 SO 4 acidification is, the lower were the pH value and TSS concentration of the biogas slurry and the higher was the content of phosphorus in the recoverable state (Ortho-P), which improved the phosphorus recovery efficiency. 3.6 Benefit of phosphorus recovery from biogas slurry The results of previous studies suggested that the main source of resource recycling revenue is phosphorus. In this study, the income recovered from biogas slurry resources was calculated based on the percentage of phosphorus in the recovered products. The economic benefits of bird droppings and commercially available phosphate fertilisers were analysed, as shown in Table 3. The crystallisation of bird guano stone is another effective method for phosphorus nutrient recovery. Bird guano stone (MgNH 4 PO 4 ·6H 2 O) can be used as a substitute for phosphate fertiliser [ 44 ]. Compared with the economic benefits of bird guano stone crystallisation technology for recovering phosphorus nutrients from biogas slurry, researchers showed that processing 1 m 3 of biogas slurry can yield ~ 14.80 kg of bird guano stone crystals [ 45 ]. Calculated as a mass percentage, the amount of phosphorus recovered from biogas slurry was determined to be ~ 1.87 kg/m 3 . At present, the market price of bird guano stones is ~ 2010–2589 yuan/ton. The price of granular phosphate fertiliser sold on the market ranges between 2000 and 3000 yuan/ton, whereas the price of liquid phosphate fertiliser is relatively high, generally exceeding 3000 yuan/ton. Based on the results of this study, calculated at an average market price of 3000 RMB/t, 1.92 kg of composite phosphate fertiliser can be obtained by treating 1 m 3 of biogas slurry, with a construction cost of 2900 RMB/m 3 and biogas slurry treatment cost of 1.03 RMB/m 3 , representing a profit of 4.44 RMB/m 3 . Overall, the economic benefits of the 6YBC-LDO technology for recovering biogas slurry phosphorus mainly lies in the profit of phosphate fertiliser, representing market application potential. 4. Conclusion To enhance the efficiency of phosphorus recovery from biogas slurry and strengthen the phosphorus recovery process, sulfuric acid was used for the acidification pretreatment of biogas slurry to promote the transformation of phosphorus forms and increase the content of orthophosphate in biogas slurry. Results of previous research show that sulfuric acid acidification promotes the transformation of phosphorus in biogas slurry. TP S gradually transform into TP L , which improves the recovery efficiency of 6YBC-LDO. The lower the pH of sulfuric acid used for the acidification of the biogas slurry is, the greater is the amount of TP S . After acidification, the TP L content of the biogas slurry increases by a factor of 14–48 compared with that in the original slurry. The acidification of the biogas slurry significantly improved the phosphorus recovery efficiency. The new adsorbent (6YBC-LDO) achieved phosphorus recovery rates of 150.99, 144.73, and 40.32 mg/g for biogas slurry acidified to pH 6.0, 6.5, and 7.0, respectively, with phosphorus recovery rates of 64.10%, 85.03%, and 99.17%, respectively. In contrast, the recovery rate of the original biogas slurry was only 2.71 mg/g. The phosphorus recovery efficiency of acidified biogas slurry was 14–52 times higher than that of the original biogas slurry. The phosphorus recovery products of 6YBC-LDO biogas slurry treated with pH 7 were mainly phosphate complexes and the purity of the phosphorus recovery material was high. Declarations Acknowledgements This project was financially supported by the Academy of Agricultural Planning and Engineering Independent research and development projects of China (QX202417). Declaration of Interest Statement: The authors (Yanru Ma，Yujun Shen，Hongcheng Cheng，Haibin Zhou，Jingtao Ding，Pengyue Zhang, Juan Wang, Ying Zhang, Ran Li) declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. Funding This project was financially supported by the Academy of Agricultural Planning and Engineering Independent research and development projects of China (QX202417). References Zeng, W.S., Wang, D.H., Luo, Z.F., Yang, J., Wu, Z.Y.: Phosphorus recovery from pig farm biogas slurry by the catalytic ozonation process with MgO as the catalyst and magnesium source. J. Clean. 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6","display":"","copyAsset":false,"role":"figure","size":230301,"visible":true,"origin":"","legend":"\u003cp\u003eSee image above for figure legend\u003c/p\u003e","description":"","filename":"Figures176.png","url":"https://assets-eu.researchsquare.com/files/rs-6321421/v1/2fe27da9b837e3ac4b6c2ad1.png"},{"id":81404783,"identity":"6dcdea6e-96c5-49b6-bca9-e19a190cab47","added_by":"auto","created_at":"2025-04-25 17:25:24","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":377430,"visible":true,"origin":"","legend":"\u003cp\u003eSee image above for figure legend\u003c/p\u003e","description":"","filename":"Figures177.png","url":"https://assets-eu.researchsquare.com/files/rs-6321421/v1/76c6a39f78018222c5f6473b.png"},{"id":81404789,"identity":"a6d8d6d5-961c-4496-b394-4c10b17b117d","added_by":"auto","created_at":"2025-04-25 17:25:24","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":1792731,"visible":true,"origin":"","legend":"\u003cp\u003eSee image above for figure legend\u003c/p\u003e","description":"","filename":"Figures178.png","url":"https://assets-eu.researchsquare.com/files/rs-6321421/v1/c18ce8076bce7a85cc433c15.png"},{"id":81404790,"identity":"7aa32cca-28ba-449f-b803-60cff38286cb","added_by":"auto","created_at":"2025-04-25 17:25:24","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":22050,"visible":true,"origin":"","legend":"\u003cp\u003eSee image above for figure legend\u003c/p\u003e","description":"","filename":"Figures179.png","url":"https://assets-eu.researchsquare.com/files/rs-6321421/v1/68c4ab7ede9395ba12b2ef6e.png"},{"id":81404958,"identity":"d7908eb3-5bb6-4b51-91a2-a5ba3b3bfec2","added_by":"auto","created_at":"2025-04-25 17:33:24","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":16074,"visible":true,"origin":"","legend":"\u003cp\u003eSee image above for figure legend\u003c/p\u003e","description":"","filename":"Figures1710.png","url":"https://assets-eu.researchsquare.com/files/rs-6321421/v1/fd8d42c1e7ef26cd950614cb.png"},{"id":81404964,"identity":"7082b245-150f-4d11-845e-d30c683adb17","added_by":"auto","created_at":"2025-04-25 17:33:24","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":18581,"visible":true,"origin":"","legend":"\u003cp\u003eSee image above for figure legend\u003c/p\u003e","description":"","filename":"Figures1711.png","url":"https://assets-eu.researchsquare.com/files/rs-6321421/v1/48fd70384f9720bc5700b76e.png"},{"id":81404785,"identity":"f9bfd0f4-f8fc-4f63-b2f0-a457bea1c948","added_by":"auto","created_at":"2025-04-25 17:25:24","extension":"png","order_by":12,"title":"Figure 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16:11:11","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4844663,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6321421/v1/798b5c8f-4e8b-402e-b6fd-95cb99f5c198.pdf"},{"id":81405501,"identity":"5620f263-2881-4c4e-b28b-f5fb1d5e195b","added_by":"auto","created_at":"2025-04-25 17:49:24","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1017067,"visible":true,"origin":"","legend":"","description":"","filename":"GraphicalAbstract.docx","url":"https://assets-eu.researchsquare.com/files/rs-6321421/v1/2bf2e860260dda826f151fc8.docx"},{"id":81404777,"identity":"3acb6499-93d3-4f46-ada2-6dd8e6782740","added_by":"auto","created_at":"2025-04-25 17:25:24","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":51866,"visible":true,"origin":"","legend":"","description":"","filename":"Supportingdata.docx","url":"https://assets-eu.researchsquare.com/files/rs-6321421/v1/6c44f627003ce8d748344648.docx"},{"id":81404776,"identity":"53b276ab-adf3-486b-a263-1ee4484782cb","added_by":"auto","created_at":"2025-04-25 17:25:24","extension":"docx","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":18417,"visible":true,"origin":"","legend":"","description":"","filename":"Highlights.docx","url":"https://assets-eu.researchsquare.com/files/rs-6321421/v1/f99edf30ab3f50a2d3821f32.docx"}],"financialInterests":"","formattedTitle":"Effect of straw biochar magnesium lanthanum hydrotalcite on phosphorus recovery from acidified biogas slurry","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eWith the continuous improvement in livestock and poultry breeding, the number of intensive biogas projects in China is increasing [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. According to statistics, more than 120000 large-scale biogas projects are conducted in China, with an annual output of 25\u0026nbsp;billion m\u003csup\u003e3\u003c/sup\u003e of biogas and an annual production of ~\u0026thinsp;30\u0026nbsp;million tons of biogas slurry. The production is increasing at a rate of ~\u0026thinsp;14% per year [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. The processing of large amounts of biogas slurry is a problem. Biogas slurry is rich in phosphorus and its direct discharge wastes resources and pollutes the environment [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e], the improper discharge of biogas slurry could lead to the eutrophication problem of water environment. In recent years, technologies for recovering phosphorus nutrients from biogas slurry have received increasing attention [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e], the main technologies include struvite crystallization, membrane concentration for fertilizer production, and nutrient adsorption and utilization [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Struvite crystallization technology is relatively mature for phosphorus recovery, but the purity of the recovered struvite is generally low, and struvite loss is significant [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Membrane concentration technology exhibits excellent filtration efficiency for nitrogen and phosphorus in biogas slurry and is often used in advanced treatment processes. However, due to severe membrane fouling and high operational energy consumption, its application in nutrient recovery from biogas slurry is limited. In contrast, the adsorption method is one of the primary technologies for nutrient recovery, and adsorption processes using cost-effective and environmentally friendly materials have garnered increasing attention [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. The use of adsorption methods to recover phosphorus from biogas slurry reduces the loss of phosphorus resources and environmental eutrophication caused by improper discharge, decreases the cost of subsequent treatment of biogas slurry, and improves economic benefits [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eRelevant data show that in the world's 71\u0026nbsp;billion tons of phosphorus rock reserves, China only accounts for 5.2% of the world's total, the current global phosphorus reserves may be exhausted in 50\u0026thinsp;~\u0026thinsp;100 years. Phosphorus resources are increasingly scarce, and phosphorus recovery is a problem that cannot be ignored. As a recyclable agricultural waste resource, biogas slurry is rich in nitrogen, phosphorus and potassium elements, which is considered be the second largest phosphate mineral[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. The full recovery of nutrients such as nitrogen and phosphorus from biogas slurry has become a research hotspot. In recent years, more and more attention has been paid to biogas slurry phosphorus recovery technology [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Some studies have concluded that replacing existing wastewater treatment systems with resource recovery technologies can increase the global nutrient and energy recovery potential by about 50%~79%. Therefore, the recovery of phosphorus from biogas slurry can realize the management and utilization of nutrients [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e], avoid the waste of resources caused by direct discharge of biogas slurry, and help to solve the problem of rational utilization of phosphorus in biogas slurry. Realizing the recycling of phosphorus resources is a new way to achieve green and sustainable development of circular agriculture.\u003c/p\u003e \u003cp\u003eThe magnesium\u0026ndash;lanthanum hydrotalcite composite material exhibited excellent adsorption performance for phosphorus, especially orthophosphate(Ortho-P) [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Compared with other phosphorus adsorbents, this material has high selectivity and efficient phosphorus adsorption in biogas slurry [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e], making it more suitable for phosphorus recovery from biogas slurry [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. However, the Ortho-P content in the digestate is less than 30% of the total phosphorus (TP) content. Because of the high concentration of suspended solids and viscosity, most of the phosphorus in the digestate is present in the form of solid-phase phosphorus (TP\u003csub\u003eS\u003c/sub\u003e) in suspended solids, which cannot be directly recovered [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Therefore, a single adsorption technology cannot ensure the efficient recovery of phosphorus from biogas slurry and it is crucial to convert the solid phosphorus in biogas slurry into a dissolved state.\u003c/p\u003e \u003cp\u003eThe results of previous studies showed that acidification is a feasible method for promoting the conversion of particulate to soluble phosphorus in biogas slurry and for increasing the proportion of dissolved TP [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Acidification of biogas slurry is considered to be an effective method for improving the utilisation efficiency of nitrogen and phosphorus nutrients [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e], which can promote the dissolution of nutrients, such as calcium (Ca) and magnesium (Mg) [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e], increase the nutrient availability, and reduce the total suspended solid (TSS) concentration and turbidity. This technology has been applied in several countries, including Denmark and the United Kingdom [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. Regueiro et al. demonstrated that sulfuric acid acidification of biogas slurry promotes phosphorus solubilisation, resulting in a TP leaching rate of 90% [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. Zeng et al. reported that sulfuric acid has a significant effect on the dissolution of phosphorus in particulate matter and demonstrated that sulfuric acid provides more stable pH conditions for biogas slurry [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e], reduces the cost of biogas slurry treatment..\u003c/p\u003e \u003cp\u003eIn summary, acidification technology is suitable for biogas slurry treatment. However, research on the recovery of phosphorus from acidified biogas slurry using adsorbent materials is limited. The aim of this study was to improve the efficiency of phosphorus recovery from biogas slurry and enhance the phosphorus recovery process. A sulfuric acid acidifier was used for the pretreatment of biogas slurry to promote the recovery of phosphorus from biogas slurry using a 6YBC-LDO. By exploring the effects of different acidification conditions on the transformation of phosphorus forms and the physicochemical properties in the biogas slurry, the dissolution characteristics of phosphorus in the acidified biogas slurry were determined. By analysing the recovery characteristics of the 6YBC-LDO adsorbent material for phosphorus from acidified biogas slurry, the main mechanism and efficiency of phosphorus recovery from acidified biogas slurry were explored. Correlations between the phosphorus recovery efficiency and characteristics of the acidified biogas slurry were analysed to clarify the factors influencing the phosphorus recovery from acidified biogas slurry. To provide an effective pathway for regulating phosphorus recovery, the benefits and application potential of the acidification slurry phosphorus recovery process were evaluated in this study.\u003c/p\u003e"},{"header":"2 Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Materials\u003c/h2\u003e \u003cp\u003eBiogas slurry was obtained from a pig farm in the Shunyi District, Beijing, China, stored for 60 d after anaerobic fermentation, and then discharged from the secondary sedimentation tank. The biogas slurry was collected and sealed for sedimentation for 1 week and then subjected to primary filtration using a 0.15-mm filter to remove large suspended particles and excess impurities before subsequent use. The acidification slurry container consisted of a 50-L polyethylene acid-resistant bucket.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Preparation of 6YBC-LDO composite material\u003c/h2\u003e \u003cp\u003eThe mixed Mg(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO and La(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO was prepared at a volume ratio (n (Mg\u003csup\u003e2+\u003c/sup\u003e)/n (La\u003csup\u003e3+\u003c/sup\u003e)) of 10/1 at 25\u0026deg;C, with 3.5 mol/L NaOH solution and 0.94 mol/L Na\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e solutions, and the straw was weighed at 50% of the total mass of Mg/La salt. The straw (0.2 g) was dispersed in deionised water (1 mL) for 30 min. The deionised water with straw was then poured into the mixed solution of Mg(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO and La(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO, was heated to 90\u0026deg;C and rapidly stirred for 10 min at 200 rpm, then stirred at 25\u0026deg;C for 2 h. The straw-Mg/La\u003csub\u003e0.1\u003c/sub\u003e-LDH complex was then prepared by adding the solution of Na\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e and NaOH to the straw Mg/La solution drop by drop, heating to 65\u0026thinsp;\u0026plusmn;\u0026thinsp;5\u0026deg;C, and stirring for more than 16 h. The obtained solution was centrifuged at 3500 rpm for 10 min repeatedly, washed, dried, ground to a uniform granular molecular size, and sieved through a 0.15 mm molecular sieve (W.S. Tyler, USA). The straw-Mg/La\u003csub\u003e0.1\u003c/sub\u003e-LDH complex was then pyrolysed and carbonised in a tube furnace at 600\u0026deg;C for 2 h under an N\u003csub\u003e2\u003c/sub\u003e atmosphere. The final product was named 6YBC-LDO and was stored in airtight glass bottles for further analysis.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Analysis of the effect of phosphorus conformation in biogas slurry\u003c/h2\u003e \u003cp\u003eUnder natural storage conditions, the biogas slurry was placed in a 50-L polyethylene acid-resistant bucket. A sulfuric acid acidifier was added to the slurry, and the pH values were adjusted to 6.0, 6.5, and 7.0, respectively, labelled as pH 6.0, 6.5, and 7.0, respectively. The original slurry was used as the control (CK) under the same conditions. The experimental period was 28 d. Samples were collected on days 0, 7, 14, 21, and 28 of storage. The mass concentrations of TP, liquid TP (TP\u003csub\u003eL\u003c/sub\u003e), total dissolved phosphorus (TDP), and dissolved Ortho-P in the biogas slurry during the acidification storage process as well as the pH, conductivity, electrical conductivity (EC), turbidity, chemical oxygen demand (COD), TSS, NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N, and total organic carbon (TOC) contents of the biogas slurry before and after acidification were measured. The effects of acidification on the physicochemical properties and phosphorus form conversion of the biogas slurry were analysed.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Phosphorus recovery experiment\u003c/h2\u003e \u003cp\u003eThe 6YBC-LDO adsorbent material was used to adsorb biogas slurry acidified to pH 6.0, 6.5, and 7.0 as well as non-acidified biogas slurry (CK). The experiment was conducted in a 50-mL polyethylene centrifuge tube. Adsorbent (0.08 g) was dispersed in 40 mL of biogas slurry and oscillated at a speed of 180 r/min for 24 h at constant temperature (25\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u0026deg;C). After the adsorption equilibrium was reached, the biogas slurry was left to stand for 15 min, subsequently centrifuged at a speed of 4000 r/min for 20 min, and finally filtered through a 0.45-\u003cem\u003e\u0026micro;\u003c/em\u003em membrane to obtain the supernatant for measurement.\u003c/p\u003e \u003cp\u003eThe phosphorus recovery equation is as follows:\u003cp\u003e\u003cimg src=\"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAHwAAAAsCAYAAACnkNxJAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsMAAA7DAcdvqGQAAAWPSURBVHhe7Zw5LHxfFMePf5SWYhSWhhE0IpZQCYJGIjoTFEgYCY21p2aIQiSIQmHvrJ29E0tEIoilsRUKSymZ//te9435jTczbxu/5zf3k7zcuffNzLn3nbucc+6diXBLkCBs+I+ngjBBKDzMEAoPM4TCwwyh8DBDKDzMEAq3EO/v71ReXk4RERGea2Vlhd0bHBz0lOG1buCHC6xFV1eX2263u+/v73mJ2/329uYuKytzz87O8hJ9iBFuQRITE6mwsJASEhJ4CdHW1hbV1NSwywhC4Rbk5OSEv/rk6OiIDg4OqLm5mZfoRyjcYmAdv7u7o6ysLE9+ZmaGWlpaWN4oQuEWQ1qr6ebmhjIyMlh+bGyM6urq/pjejSAUbjEeHx8pMjKSkpKSmIUeGxtLOTk5/K5xhMItBqbzj48P9np7e9uwkeaL5RV+eXlJCwsLPPe7mZycpIeHB55T5vz8nGw2G1u3JfeMoqKi+B1zsLTC8XDQcIfDwUt+N01NTTQ+Ph5Q6bDQ9/f3qbi42LR12xvdCsda093dTXFxcSz6g3Rubo7fNQc8nEDWKSxY7zoUFBTQ2toaW/vkCFUoMCIX7UG7AuFyuaiyspLnzEWXwtGozMxMiomJodvbW0TraGRkhGpra0170PA9X19f/fZydK7k5GT2+vT0lNVhY2OD5ufnqbGxkRk9ocCoXLQH7UL7lJiammKdKWQg3KaF5eVlHIlSDPHV19ez8B/CgEbBd0GWEoHqgHCk0+k0pQ6+mCUX34P2/Q00jXCsPe3t7SRV1q/1CB8SvqQRIOfs7ExxtGBkYCQFqgM+Z7axY6ZcvA/tC2bAhQJNCocB9fLyQh0dHbwkNMA+APHx8Sz1BtMn8FcHTJm9vb08Zx5mypXbJbfzJ1GtcBgq6+vrlJ2dTWlpabz0C/TWvb29b0F/0NDQwIwbfxfuewNf9Pn5mee+CFaHUBEKuWgf2vnTqFa4HPKrqKhQnLYQ3L++vqbq6mpe8gUMEWn58HvhvhrkOpg1ZavtiGbL/ZuoVjimH0znSmAEDA8Pk2SwUUlJCS+1PmZ1RDNQ6nDBLj2oVjjWHcR1fbfuAAL8x8fHNDAwoDgCtE7p/sBSgSUDS4eSwYOOh2iWjHxKxNAJEQmtcvWg1OGCXbqQPqgal8vlttlsHrcELghOZ6DMnwulh8PDQ3d6ejpLfUEZ5OXn57svLi546Wd5T0+Pxy1CHnUDSJW+Swtq5aoBLhy+x2id9KBJ4bKC0U9w4QEgL/V6/g5zgByHw+H3geCBw+f1rYf3Q0fnlDshUuSNokYumJiYYPf8yUS7oHDvI0w/hSaFAzRuZ2fnWyPNJlDgRQ146HKHQYr8TwBZ6AR4Rv5k/prAC3auSktLqaioiKXe4UHEks3c1YK/u7m5yXPagYEpuz1I/RmcZgPjdWhoiIVHsQGiBNoVilhGW1sbs1n6+/t5yXc0WekAAYjV1VW2hZebm8sEYPMAmLmrhU1/xOr9xZyDgQ4pdxikyP8U0ghmW5tKBh7K0C4zDzXIjI6OBjfmPge6NcEaJ40WntMOpk000Yz1Wy2QFUhmX19fyNbu1tZWJvvq6oqXfMfyvw/HiMBygb3k3w5cNyyHPxkl9EX8IUCYocloE4QWzGapqanMLsK5AtgvsI9Qhnu+eT0IhVsIRPQQzZN8dHp6emK/NkEU0263M2UvLS3R9PQ0y+tFKNxiYBMK4FRMZ2cncylh1eNwI7ZfkWIVjo6OZu/TilC4xVhcXGQKdjqdnjxcYO88fHi9u3ZC4RYCmzAY0bJCsU5jiq+qqvLkJZeO8vLy+Ce0IxRuIRDJxGiWt5gxvaekpPyRxy9Ld3d3hdH2L4AoJqxwebr2nb6xfuMsHPx431NFahF+eJghRniYIRQeVhD9DxSoePkcO4rfAAAAAElFTkSuQmCC\"\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003eQ\u003c/em\u003e is the amount of phosphorus recovered by the adsorbent at adsorption equilibrium (mg/g), \u003cem\u003eC\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e is the pre-adsorption acidification biogas slurry (mg/L), \u003cem\u003eC\u003c/em\u003e\u003csub\u003ee\u003c/sub\u003e is the biogas slurry phosphorus concentration at adsorption equilibrium (mg/L), \u003cem\u003eV\u003c/em\u003e is the adsorption volume of the biogas slurry, and \u003cem\u003em\u003c/em\u003e is the amount of adsorbent used (g).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5 Characterisation and analysis methods for recycled products\u003c/h2\u003e \u003cp\u003eThe surface structure of the composite material before and after phosphorus adsorption was observed using field-emission scanning electron microscopy (FE-SEM, SU8010, HITACHI, Japan). Energy-dispersive spectroscopy (EDS, EDAX Octane Plus, AMETEK, USA) and transmission electron microscopy (TEM, H7650, HITACHI, Japan) were used to determine the surface elemental distribution of the composite materials. Data were acquired using an X-ray diffractometer (XRD, Bruker D8 Venture, Bruker, Japan). The crystal structure of the material was analysed using MDI jade6.0 software. A Fourier transform infrared spectrometer (FTIR, Nicolet i50, Thermo Fisher Scientific, USA) was employed to characterise the functional groups on the material surfaces. An inductively coupled plasma emission spectrometer (ICPE-9000; Shimadzu, Japan) was used to determine the concentrations of phosphorus (P) and magnesium ions (Mg\u003csup\u003e2+\u003c/sup\u003e) in the biogas slurry before and after adsorption.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e2.6 Analysis of physicochemical properties of biogas slurry\u003c/h2\u003e \u003cp\u003eThe pH of biogas slurry was measured using a pH meter (Thunderbolt DDS-307A; Mettler). EC was analysed using a conductivity meter (Thunderbolt DDS-307A, Mettler). The TP and ammonia nitrogen concentrations were determined using a UV\u0026ndash;Visible spectrophotometer (UV8100, LabTech, USA). The turbidity was analysed using a turbidity meter (Thunderbolt WZB-175, Mettler). The concentrations of Cu\u003csup\u003e2+\u003c/sup\u003e, Zn\u003csup\u003e2+\u003c/sup\u003e, and Cr\u003csup\u003e3+\u003c/sup\u003ewere determined using inductively coupled plasma mass spectrometry (ICP-MS; Agilent, USA). The COD was determined using the dichromate method. Each sample was analysed in triplicate and the average of the test results was reported.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e2.7 Data analysis\u003c/h2\u003e \u003cp\u003eExperimental data were statistically analysed using Microsoft Excel 2021. A factor analysis of variance and significance test (LSD method, \u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05) were performed using SPSS (version 20.0; SPSS Inc., Chicago, USA). The experimental results were plotted using Origin 2021 (Origin 2021, Northampton, MA, USA). The \u0026lsquo;Correlation Plot\u0026rsquo; package of Origin 2021 was utilised for correlation analysis. Redundancy Analysis (RDA) was performed using Canoco 5.0 software. Penetration curve data were fitted using a logistic model from Origin 2021.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Characterisation of 6YBC‑LDO composite material and biogas slurry\u003c/h2\u003e \u003cp\u003eThe N\u003csub\u003e2\u003c/sub\u003e adsorption-desorption isotherm of 6YBC-LDO is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e1\u003c/span\u003ea, which is a Type IV adsorption isotherm with an H3 hysteresis loop. The pores of this material are slit-shaped, formed by the stacking of plate-like particles. The 6YBC-LDO adsorption material exhibits a high specific surface area of 123.65 m\u0026sup2;/g, a pore volume of 0.149 cm\u0026sup3;/g, and an average pore diameter of 7.70 nm, indicating that the pore size falls within the mesoporous range.\u003c/p\u003e \u003cp\u003eThe surface morphology and elemental distribution of the 6YBC-LDO composite material were characterized using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). As observed in Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e1\u003c/span\u003eb and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e1\u003c/span\u003ec, Mg/La\u003csub\u003e0.1\u003c/sub\u003e-LDO is uniformly distributed on the straw carbon, and the layered structure of Mg/La\u003csub\u003e0.1\u003c/sub\u003e-LDO is well preserved. EDS mapping revealed that the main elemental components of the composite material are C, O, Mg, La, Na, Ca, and Si. In the 6YBC-LDO composite material, the mass percentages of the elements are 45.06% for C, 8% for Mg, and 4.12% for La. The physicochemical properties of biogas slurry are listed in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003ePhysicochemical properties of biogas slurry\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIndex\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003epH\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eEC(mS/cm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eTurbidity(NTU)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eTSS(mg/L)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eCOD(mg/L)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eNH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N(mg/L)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eTP(mg/L)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eContent\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e8.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e528\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e277\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2210\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1455.69\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e472.91\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eComparison of economic benefits of phosphorus recovery technology in biogas slurry\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTypes of fertilizers\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePhosphorus recovery from biogas slurry(kg/m\u003csup\u003e3\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003ePurity of phosphate fertilizer(%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSelling price(RMB/t)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eReference\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eStruvite(MgNH\u003csub\u003e4\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.87\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e12.62\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2010\u0026thinsp;~\u0026thinsp;2589\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e[\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGranular phosphate fertilizer(Ca(H₂PO₄)₂)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e/\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e12\u0026thinsp;~\u0026thinsp;20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3450\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eSelling price\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLiquid phosphate fertilizer(KH\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e/\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e22.75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3000\u0026thinsp;~\u0026thinsp;10000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eSelling price\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eStraw biochar hydrotalcite powder composite phosphate fertilizer\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.92\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e23.81\u0026thinsp;~\u0026thinsp;30.35\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eThis study\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Effect of acidification on the phosphorus dissolution characteristics of biogas slurry\u003c/h2\u003e \u003cdiv id=\"Sec13\" class=\"Section3\"\u003e \u003ch2\u003e3.2.1 Effect of acidizing pH on the distribution of phosphorus conformation in solid and liquid phases\u003c/h2\u003e \u003cp\u003eThe morphological changes of solid and liquid phosphorus in the biogas slurry after different acidification treatments are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The TP\u003csub\u003eL\u003c/sub\u003e content gradually increased with decreasing pH, whereas the TP content decreased with decreasing pH. The main reason for this was that acidification dissolved inorganic phosphorus, leading to an increase in the TP\u003csub\u003eL\u003c/sub\u003e concentration [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Compared with other treatments, the TP content dissolved at pH 6.0 was higher and 80% of the phosphorus was released, which is consistent with the results of previous studies [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. Improving the degree of acidification of the biogas slurry is beneficial for the gradual conversion of TP\u003csub\u003eS\u003c/sub\u003e to TP\u003csub\u003eL\u003c/sub\u003e [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. The storage time of acidified biogas slurry also affects the distribution of TP\u003csub\u003eS\u003c/sub\u003e and TP\u003csub\u003eL\u003c/sub\u003e. Based on the comparison of the distribution of TP\u003csub\u003eS\u003c/sub\u003e and TP\u003csub\u003eL\u003c/sub\u003e in the early and late stages of acidified storage, TP\u003csub\u003eL\u003c/sub\u003e accounted for 94.93%, 54.54%, 53.64%, and 32.78% of TP in biogas slurry treated with pH 6.0, 6.5, 7.0, and CK, respectively, in the early stage of storage. In the later stages of storage, TP\u003csub\u003eL\u003c/sub\u003e accounted for 97.27%, 84.02%, 34.32%, and 10.02% of TP, respectively. As the storage time increased, the TP\u003csub\u003eL\u003c/sub\u003e content of the acidified slurry increased, whereas the TP\u003csub\u003eL\u003c/sub\u003e concentration of the untreated slurry gradually decreased. This indicates that, without acidification treatment, the prolonged storage period of the biogas slurry led to the conversion of TP\u003csub\u003eL\u003c/sub\u003e to TP\u003csub\u003eS\u003c/sub\u003e, which is not conducive to phosphorus recovery.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section3\"\u003e \u003ch2\u003e3.2.2 Effect of pH on the distribution of liquid phosphorus forms\u003c/h2\u003e \u003cp\u003eThe transformation of liquid phosphorus in biogas slurry after different acidification treatments is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e3\u003c/span\u003e. After H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e acidification of biogas slurry, the TP\u003csub\u003eL\u003c/sub\u003e content significantly increased and the phosphorus solubility and pH value of the acidified biogas slurry were significantly negatively correlated. The lower the pH is, the greater is the amount of phosphorus dissolved in biogas slurry. For example, when the initial pH value of acidified biogas slurry was 6.0, the TP\u003csub\u003eL\u003c/sub\u003e content varied between 340.01 and 460.02 mg/L within 28 d, which was 14\u0026ndash;48 times higher than that of the non-acidified biogas slurry (CK). The main form of TP\u003csub\u003eL\u003c/sub\u003e, that is, TDP, included Ortho-P and dissolved reduced phosphorus (RDP). After treatment at pH 6.0, 6.5, and 7.0, the Ortho-P content was 75.27\u0026ndash;93.71%, 52.78\u0026ndash;79.12%, and 48.29\u0026ndash;33.91% of TP, respectively. The degree of H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e acidification significantly affected the concentration of Ortho-P in the biogas slurry. The concentration range of Ortho-P in biogas slurry treated with pH 6.0, 6.5, and 7.0 was 283.60\u0026ndash;416.27, 150.42\u0026ndash;309.10, and 69.41\u0026ndash;86.8 mg/L, respectively.\u003c/p\u003e \u003cp\u003eThe storage time also affected the transformation of phosphorus forms. The changes in liquid-phase phosphorus forms with storage time are shown in Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e. The phosphorus concentration of non-acidified biogas slurry (CK) gradually decreased with the extension of the storage period. The mass concentration of TP\u003csub\u003eL\u003c/sub\u003e in the early stage of acidification ranged from 8.36 to 22.68 mg/L. In the late stage of acidification (28 d), the mass concentration of Ortho-P in the original biogas slurry decreased by 55.42% compared with the initial value. This may be due to the gradual conversion of organic to inorganic phosphorus in the biogas slurry. The degradation of organic matter causes the conversion of dissolved into particulate phosphorus [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. After H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e acidification treatment, the TP\u003csub\u003eL\u003c/sub\u003e and Ortho-P contents of the digestate treated with pH 6.0 and 6.5 significantly increased with the extension of the storage period. The content of Ortho-P dissolved after storage at pH 6.0, 6.5, and 7.0 is 29, 20, and 4 times that of the original digestate, respectively, indicating that storage strengthened H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e acidification and dissolution of phosphorus and that acidification improved the conversion of recoverable phosphorus in digestate. Acidification reduced the loss of dissolved phosphorus during the storage of biogas slurry.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section3\"\u003e \u003ch2\u003e3.2.3 Effect of acidification on the physicochemical properties of biogas slurry\u003c/h2\u003e \u003cp\u003eAcidification pH is a key factor affecting the changes in the characteristics of biogas slurry. The changes in various indicators of the acidified biogas slurry (pH 6.0, 6.5, and 7.0) and non-acidified biogas slurry (CK) are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e4\u003c/span\u003e. The changes in the pH and EC of the digestate are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e4\u003c/span\u003ea. After different acidification treatments, the pH of the digestate increased significantly (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05), indicating alkalinity. This was due to the change in the microbial activity of the digestate after acidification [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. The degradation of organic matter produced NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N, resulting in an increase in the pH value [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. In contrast, acidification led to a significant decrease in the EC value of the biogas slurry (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05), but the EC value of the pH 6.0 acidification treatment remained high.\u003c/p\u003e \u003cp\u003eChanges in the TSS content and turbidity of the biogas slurry are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e4\u003c/span\u003eb. The TSS content and turbidity of acidified biogas slurry were significantly reduced compared with those of the original digestate. Both showed the same trend, indicating that sulfuric acid dissolved a large amount of suspended solids in the digestate, effectively reducing the dry matter content.\u003c/p\u003e \u003cp\u003eChanges in the COD and TOC content of the biogas slurry are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e4\u003c/span\u003ed. The COD and TOC content were significantly reduced after H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e acidification (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05), which was directly related to the degradation of organic matter. Carbon in organic matter can be lost as CH\u003csub\u003e4\u003c/sub\u003e and CO\u003csub\u003e2\u003c/sub\u003e [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e], indicating that acidification promotes organic matter degradation.\u003c/p\u003e \u003cp\u003eChanges in the NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N and TP\u003csub\u003eL\u003c/sub\u003e concentrations in the biogas slurry are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e4\u003c/span\u003ec. The NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N content of acidified biogas slurry decreased compared with that of original biogas slurry, which was due to the loss of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N in the form of ammonia gas. Popovic and Jensen also reported the phenomenon of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N loss after acidification [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. The TP\u003csub\u003eL\u003c/sub\u003e content in the biogas slurry increased with the decrease in the acidification pH, which was due to the dissolution of solid phosphorus in the sediment of the biogas slurry and a large amount of particulate phosphorus (PP) in suspended solids by acidification, causing phosphorus to diffuse into the liquid phase in a dissolved form. This indicates that H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e promotes the release of soluble phosphorus. Therefore, it can be inferred that the physicochemical properties of the biogas slurry are influenced by the acidification pH.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Effects of composite materials on phosphorus recovery from acidified biogas slurry\u003c/h2\u003e \u003cp\u003eIn this study, 6YBC-LDO was used to adsorb and recover H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e-acidified biogas slurry after storage. The phosphorus recovery effect of the 6YBC-LDO composite material on the acidified biogas slurry is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e5\u003c/span\u003e. Significant differences in the phosphorus recovery can be observed among the treatments (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05). As the initial pH value of acidification decreased, phosphorus recovery gradually increased. The phosphorus recovery capacities of the pH 7.0, 6.5, and 6.0, treatments were 40.32, 144.73, and 150.99 mg/g, respectively. The phosphorus recovery of the original biogas slurry (CK) was only 2.71 mg/g, indicating that H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e acidification significantly improved the phosphorus recovery efficiency of the slurry. The adsorption effect of 6YBC-LDO on phosphorus in acidified biogas slurry is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e6\u003c/span\u003e. The adsorption rates of acidified biogas slurry were 99.17%, 85.03%, and 64.10% at pH 7.0, 6.5, and 6.0, respectively. Treatment at pH 6.0 with the highest recovery capacity resulted in a lower adsorption rate, indicating that the recoverable phosphorus dissolved by acidification cannot be completely adsorbed and 6YBC-LDO has reached adsorption saturation. However, no significant difference (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.05) was observed in the phosphorus recovery efficiency of 6YBC-LDO in biogas slurry treated with pH 6.5 compared with that treated with pH 6.0. However, the adsorption rate of the pH 6.5 treatment exceeded 85%, indicating that pH 6.5 treatment has advantages. In contrast, the phosphorus adsorption rate of the CK treatment was only 52.45% and the phosphorus recovery is also relatively low. This indicates that phosphorus in the original biogas slurry existed very rarely in the form of Ortho-P and mostly in the form of PP, which affected the phosphorus recovery efficiency.\u003c/p\u003e \u003cp\u003eThe effect before and after adsorption on chromaticity and turbidity of acidified biogas slurry is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e6\u003c/span\u003e. The recovery of acidified biogas slurry by 6YBC-LDO had a certain effect on removing the chromaticity and turbidity of the biogas slurry.\u003c/p\u003e \u003cp\u003eIn summary, an initial pH range of the biogas slurry of 6.5\u0026ndash;7.0 after H2SO4 acidification was optimal for the recovery of phosphorus from biogas slurry by 6YBC-LDO. Compared with the original slurry, the recovery efficiency of phosphate could be improved by a factor ranging from 14 to 52.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec17\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Effect of acidification on the composition of phosphorus recovered from biogas slurry\u003c/h2\u003e \u003cdiv id=\"Sec18\" class=\"Section3\"\u003e \u003ch2\u003e3.4.1 Morphology of phosphorus recovery products\u003c/h2\u003e \u003cp\u003eThe morphology of phosphorus recovered by 6YBC-LDO after acidification of biogas slurry is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e7\u003c/span\u003e. Three different morphologies could be observed as pH changes affected the complex ion equilibrium system in the biogas slurry, which in turn affected the composition and phosphorus forms of recovered products. Figures\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e7\u003c/span\u003ea and \u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e7\u003c/span\u003ec show that the recovered particles treated with pH 6.0 were regular quadrilateral crystals and that a small amount of regular quadrilateral crystals remained in the recovered particles treated with pH 7.0. However, the amount of amorphous substances in the recovered particles increased. According to the XRD results shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e10\u003c/span\u003e, this substance may be Ca\u003csub\u003e3\u003c/sub\u003eMg\u003csub\u003e3\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e4\u003c/sub\u003e. The surface structure of the recovered material treated at pH 6.5 is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e7\u003c/span\u003eb. Many sheet-like cluster structures can be observed on the recovered material, which are likely LaPO\u003csub\u003e4\u003c/sub\u003e formed after the intercalation of PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e by magnesium/lanthanum hydrotalcite (Mg/La LDH). The surface structure of the recovered material treated with CK is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e7\u003c/span\u003ed. A large number of long-pointed tubular tree branches appeared on the surface of the recovered material. This substance may be struvite crystals, as determined by XRD analysis. Hu et al. and Karbakhshravari demonstrated that struvite crystals have a double-branched and tubular structure [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. The results of this study were similar to those of other studies in which researchers reported that phosphorus recovery generates both adsorption and precipitation products. Xu et al. stated that adsorption and precipitation reactions occur simultaneously when phosphorus is recovered from Mg-supported biochar materials [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eBased on the above-mentioned analysis, the composition of the recovered material significantly differed when H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e was used to acidify biogas slurry to different pH values, indicating the existence of multiple synergistic mechanisms.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec19\" class=\"Section3\"\u003e \u003ch2\u003e3.4.2 Distribution of elements in recycled materials\u003c/h2\u003e \u003cp\u003eEnergy spectrum point measurements and EDS were performed on phosphorus-recovered materials after various acidification treatments to determine the main components of the recovered materials. The EDX spectra of the recovered materials after acidification treatment are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e8\u003c/span\u003e. The EDX analysis showed that C, O, and Mg were the main elements in the recovered material and that N, Ca, K, Si, and La were also present. After adsorption saturation, treatment with pH 6.0, 6.5, 7.0, and CK resulted in a uniform distribution of P on the entire surface of the material, indicating that the adsorbent surface had abundant adsorption sites that could bind and effectively capture phosphate on the material surface. On the surface of the recycled material, the distribution sites of Mg and O strongly overlapped with the distribution sites of P, indicating that Mg and O in the adsorbent participated in the phosphorus adsorption reaction. Kong et al. used La@MgAl and observed a similar phenomenon in the recovered material when using nanocomposites to remove phosphate from water [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. In addition, La and P in recovered materials treated with pH 6.5 and CK also strongly overlapped. However, they only partially overlapped in recovered materials treated with pH 6.0 and 6.5.\u003c/p\u003e \u003cp\u003eBased on the above-mentioned analysis, La ions participated more in phosphorus adsorption reactions at an acidification pH value of 6.5 and 7.0, whereas an acidification pH value of 6.0 promoted the reaction between Mg and O ions and phosphate.\u003c/p\u003e \u003cp\u003eThe elemental distribution of phosphorus recovery material after treatment with 6YBC-LDO at pH 6.0, 6.5, 7.0, and CK in biogas slurry is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e9\u003c/span\u003e. The Mg/P mass ratios of 6YBC-LDO of recovered materials of biogas slurry treated with pH 6.0, 6.5, 7.0, and CK were 0.59, 1.28, 0.57, and 4.67, respectively. The similar distribution ratio of Mg/P elements at pH 6.0 and 7.0 indicates that generated magnesium phosphorus compounds had the same structure. XRD detection results showed that both produced Ca\u003csub\u003e3\u003c/sub\u003eMg\u003csub\u003e3\u003c/sub\u003e (PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e4\u003c/sub\u003e. According to the analysis of the mass proportion of phosphorus in the recovered material, the phosphorus purities of the recovered products of 6YBC-LDO for pH 6.0, 6.5, 7.0, and CK biogas slurry were 22.33%, 7.02%, 23.81%, and 1.14%, respectively. The results showed that the phosphorus recovery product obtained from pH 7.0-acidified biogas slurry had a higher purity than other condition of pH values.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec20\" class=\"Section3\"\u003e \u003ch2\u003e3.4.3 Analysis of structural components of phosphorus-recycled materials\u003c/h2\u003e \u003cp\u003eThe crystal phase of the recovered material was identified using XRD and the crystal structure of the phosphorus recovered from the acidified biogas slurry was determined. The results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e10\u003c/span\u003e. Strong peaks were observed at 43.04\u0026deg;, 62.03\u0026deg;, and 36.58\u0026deg; prior to 6YBC-LDO adsorption. Novais et al. showed that these peaks correspond to MgO crystal peaks [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e crystal peaks were observed at 29.59\u0026deg; and 28.70\u0026deg;. The crystal phase structures of the recovered material treated with CK and pH 6.5 after adsorption were similar. Peaks appearing at 30.48\u0026deg; and 33.91\u0026deg; matched well with those observed for bird guano stone (PDF: 77-2303), indicating the presence of bird guano stone crystals in the recovered material. However, the purities of the two differed from the distributions of the elements. Researchers showed that MgO can participate in the crystallisation reaction of bird guano stone when the pH ranges between 5.5 and 6.5 and the presence of MgO promotes the crystallisation reaction of bird guano stone [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. In addition, LaPO\u003csub\u003e4\u003c/sub\u003e crystallisation peaks were observed at 40.90\u0026deg; in phosphorus recovery materials treated with CK and pH 6.5. Phosphorus recovery products treated with pH 6.0 and 7.0 showed Ca\u003csub\u003e3\u003c/sub\u003eMg\u003csub\u003e3\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e4\u003c/sub\u003e crystallisation peaks at ~\u0026thinsp;31.61\u0026deg;.\u003c/p\u003e \u003cp\u003eBased on the above-mentioned analysis, sludge treated with CK and pH 6.5 formed a LaPO\u003csub\u003e4\u003c/sub\u003e complex after adsorption by 6YBC-LDO. The phosphorus in the digestate treated with pH 6.0 and pH 7.0 was effectively recovered through the complexation reaction of Ca\u003csub\u003e3\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e and Mg\u003csub\u003e3\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec21\" class=\"Section3\"\u003e \u003ch2\u003e3.4.4 Analysis of structure and composition of phosphorus-recycled materials\u003c/h2\u003e \u003cp\u003eThe Mg\u003csup\u003e2+\u003c/sup\u003e mass concentration and pH of acidified biogas slurry at the adsorption equilibrium are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e11\u003c/span\u003e. The Mg\u003csup\u003e2+\u003c/sup\u003e content in the acidified digestate treated with pH 6.0 and 6.5 was lower after adsorption, indicating that MgO participated in the phosphorus adsorption reaction when 6YBC-LDO adsorbed the acidified digestate at pH 6.0 and 6.5. The high amount of dissolved phosphorus in the digestate resulted in significant consumption of MgO. In contrast, the Mg\u003csup\u003e2+\u003c/sup\u003e content in the digestate treated with pH 7.0 and CK after adsorption exceeded that before adsorption, indicating that the dissolved phosphorus at pH 7.0 and non-acidified digestate were completely adsorbed by 6YBC-LDO.\u003c/p\u003e \u003cp\u003eIn summary, when the initial acidification pH of the biogas slurry was 6.5, the precipitation of bird guano and the complexation reaction synergistically promoted the recovery of phosphorus by 6YBC-LDO. The purity of the phosphorus recovery product was 7.02% and recovered substances included bird guano and phosphate complexes. When the initial acidification pH of the biogas slurry was 6.0 or 7.0, phosphorus was mainly recovered through complexation reactions. The purities of the phosphorus recovery products from the biogas slurry at pH 6.0 and 7.0 were 22.33% and 23.81%, respectively. The purity of the phosphorus recovery product from biogas slurry at pH 7.0 was relatively high. Therefore, pH 7.0 acidification treatment is preferred for phosphorus recovery from biogas slurry.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec22\" class=\"Section2\"\u003e \u003ch2\u003e3.5 Correlation analysis between phosphorus recovery and acidification of biogas slurry\u003c/h2\u003e \u003cdiv id=\"Sec23\" class=\"Section3\"\u003e \u003ch2\u003e3.5.1 Correlation analysis\u003c/h2\u003e \u003cp\u003eTo further investigate the impact mechanism of the acidification process on phosphorus recovery from biogas slurry, a correlation analysis was conducted between the amount of phosphorus recovered and characteristic indicators of the acidified biogas slurry. The results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e12\u003c/span\u003e.A significant positive correlation was observed among phosphorus recovery and the contents of TP (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001), TP\u003csub\u003eL\u003c/sub\u003e (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001), TDP (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001), Ortho-P (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001), RDP (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.01), and PP (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.01) in the acidified biogas slurry, whereas a significant negative correlation with TPs (P\u0026thinsp;\u0026lt;\u0026thinsp;0.05) was evident. Significant positive correlations were observed among TP, TP\u003csub\u003eL\u003c/sub\u003e, TDP, Ortho-P, RDP in the biogas slurry and EC, whereas the pH value, TSS concentration, and heavy metal chromium (Cr) content of the acidified biogas slurry were significantly negatively correlated. In addition, the content of TPs in the acidified biogas slurry was significantly positively correlated with the pH value (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.01) and significantly negatively correlated with the EC and Cu and Zn contents. The turbidity of the acidified biogas slurry was significantly negatively correlated with the TP content.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec24\" class=\"Section3\"\u003e \u003ch2\u003e3.5.2 RDA analysis\u003c/h2\u003e \u003cp\u003eRDA was used to further determine the correlations between phosphorus recovery from biogas slurry and environmental factors. The results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e13\u003c/span\u003e. A strong constraint relation was observed between the amount of phosphorus recovered and the acidification characteristics of the biogas slurry. Figure\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e13\u003c/span\u003e shows that 98.5% of the change in the phosphorus form can be explained by characteristic indicators of the acidified biogas slurry. The pH value, TSS, EC, and Cr content were significantly correlated with phosphorus forms of acidified biogas slurry (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.01). The pH value and TSS concentration of the biogas slurry explained 68.8% and 15.1% of the variance, respectively, reaching a significant level. In addition, a positive correlation was observed among the phosphorus recovery amount, phosphorus recoverable state (Ortho-P), and EC and the TPS content, pH, and TSS concentration were negatively correlated.\u003c/p\u003e \u003cp\u003eIn summary, the recoverable state of phosphorus (Ortho-P) and the pH value, TSS concentration, and EC value of the acidified biogas slurry were strongly correlated. The higher the degree of H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e acidification is, the lower were the pH value and TSS concentration of the biogas slurry and the higher was the content of phosphorus in the recoverable state (Ortho-P), which improved the phosphorus recovery efficiency.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec25\" class=\"Section2\"\u003e \u003ch2\u003e3.6 Benefit of phosphorus recovery from biogas slurry\u003c/h2\u003e \u003cp\u003eThe results of previous studies suggested that the main source of resource recycling revenue is phosphorus. In this study, the income recovered from biogas slurry resources was calculated based on the percentage of phosphorus in the recovered products. The economic benefits of bird droppings and commercially available phosphate fertilisers were analysed, as shown in Table\u0026nbsp;3.\u003c/p\u003e \u003cp\u003eThe crystallisation of bird guano stone is another effective method for phosphorus nutrient recovery. Bird guano stone (MgNH\u003csub\u003e4\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO) can be used as a substitute for phosphate fertiliser [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]. Compared with the economic benefits of bird guano stone crystallisation technology for recovering phosphorus nutrients from biogas slurry, researchers showed that processing 1 m\u003csup\u003e3\u003c/sup\u003e of biogas slurry can yield\u0026thinsp;~\u0026thinsp;14.80 kg of bird guano stone crystals [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]. Calculated as a mass percentage, the amount of phosphorus recovered from biogas slurry was determined to be ~\u0026thinsp;1.87 kg/m\u003csup\u003e3\u003c/sup\u003e. At present, the market price of bird guano stones is ~\u0026thinsp;2010\u0026ndash;2589 yuan/ton. The price of granular phosphate fertiliser sold on the market ranges between 2000 and 3000 yuan/ton, whereas the price of liquid phosphate fertiliser is relatively high, generally exceeding 3000 yuan/ton.\u003c/p\u003e \u003cp\u003eBased on the results of this study, calculated at an average market price of 3000 RMB/t, 1.92 kg of composite phosphate fertiliser can be obtained by treating 1 m\u003csup\u003e3\u003c/sup\u003e of biogas slurry, with a construction cost of 2900 RMB/m\u003csup\u003e3\u003c/sup\u003e and biogas slurry treatment cost of 1.03 RMB/m\u003csup\u003e3\u003c/sup\u003e, representing a profit of 4.44 RMB/m\u003csup\u003e3\u003c/sup\u003e. Overall, the economic benefits of the 6YBC-LDO technology for recovering biogas slurry phosphorus mainly lies in the profit of phosphate fertiliser, representing market application potential.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eTo enhance the efficiency of phosphorus recovery from biogas slurry and strengthen the phosphorus recovery process, sulfuric acid was used for the acidification pretreatment of biogas slurry to promote the transformation of phosphorus forms and increase the content of orthophosphate in biogas slurry. Results of previous research show that sulfuric acid acidification promotes the transformation of phosphorus in biogas slurry. TP\u003csub\u003eS\u003c/sub\u003e gradually transform into TP\u003csub\u003eL\u003c/sub\u003e, which improves the recovery efficiency of 6YBC-LDO. The lower the pH of sulfuric acid used for the acidification of the biogas slurry is, the greater is the amount of TP\u003csub\u003eS\u003c/sub\u003e. After acidification, the TP\u003csub\u003eL\u003c/sub\u003e content of the biogas slurry increases by a factor of 14\u0026ndash;48 compared with that in the original slurry. The acidification of the biogas slurry significantly improved the phosphorus recovery efficiency. The new adsorbent (6YBC-LDO) achieved phosphorus recovery rates of 150.99, 144.73, and 40.32 mg/g for biogas slurry acidified to pH 6.0, 6.5, and 7.0, respectively, with phosphorus recovery rates of 64.10%, 85.03%, and 99.17%, respectively. In contrast, the recovery rate of the original biogas slurry was only 2.71 mg/g. The phosphorus recovery efficiency of acidified biogas slurry was 14\u0026ndash;52 times higher than that of the original biogas slurry. The phosphorus recovery products of 6YBC-LDO biogas slurry treated with pH 7 were mainly phosphate complexes and the purity of the phosphorus recovery material was high.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis project was financially supported by the Academy of Agricultural Planning and Engineering Independent research and development projects of China (QX202417).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of Interest Statement:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors (Yanru Ma，Yujun Shen，Hongcheng Cheng，Haibin Zhou，Jingtao Ding，Pengyue Zhang, Juan Wang, Ying Zhang, Ran Li) declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis project was financially supported by the Academy of Agricultural Planning and Engineering Independent research and development projects of China (QX202417).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eZeng, W.S., Wang, D.H., Luo, Z.F., Yang, J., Wu, Z.Y.: Phosphorus recovery from pig farm biogas slurry by the catalytic ozonation process with MgO as the catalyst and magnesium source. J. Clean. 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Thesis (2021)\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"waste-and-biomass-valorization","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"wave","sideBox":"Learn more about [Waste and Biomass Valorization](http://link.springer.com/journal/12649)","snPcode":"12649","submissionUrl":"https://submission.nature.com/new-submission/12649/3","title":"Waste and Biomass Valorization","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"biogas slurry, acidification, phosphorus species, adsorption, recovery","lastPublishedDoi":"10.21203/rs.3.rs-6321421/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6321421/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eBiogas slurry is rich in phosphorus and serves as an important source for phosphorus recovery. To improve the efficiency of phosphorus recovery from biogas slurry and strengthen the recovery process, H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e is used for the acidification pretreatment of biogas slurry. However, the primary mechanism underlying phosphorus recovery from acidified biogas slurry remains unclear. In this study, the effects of different acidification conditions on the transformation of phosphorus forms and physicochemical properties of biogas slurry as well as the phosphorus recovery effect of a straw charcoal magnesium\u0026ndash;lanthanum hydrotalcite composite material (6YBC-LDO) were investigated. The results showed that soluble orthophosphate (Ortho-P) was the main form of phosphorus recovered from the slurry by the 6YBC-LDO adsorbent. When H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e was added to adjust the pH of the biogas slurry to 6.0, 6.5, and 7.0, the mass concentration of Ortho-P increased by a factor of 20, 9, and 4, respectively, compared with the original biogas slurry. The total phosphorus recovery rates of 6YBC-LDO from acidified biogas slurry at pH 6.0, 6.5, and 7.0 were 150.99, 144.73, and 40.32 mg/g, respectively, with phosphorus recovery rates of 64%, 85%, and 99%, respectively. The results showed that the pH of biogas slurry acidification was the main factor affecting the phosphorus recovery efficiency of biogas slurry. Acidification with pH 7.0 significantly affected phosphorus recovery. The phosphorus recovery products from biogas slurry treated with pH 7 were mainly phosphate complexes with higher purity. High purity phosphorus recovery materials could be used as for efficient phosphorus fertilizers production in the future, which has great market application potential.\u003c/p\u003e","manuscriptTitle":"Effect of straw biochar magnesium lanthanum hydrotalcite on phosphorus recovery from acidified biogas slurry","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-04-25 17:25:19","doi":"10.21203/rs.3.rs-6321421/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2025-04-17T04:26:32+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-04-17T02:00:05+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Waste and Biomass Valorization","date":"2025-04-12T15:15:25+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-03-27T17:01:12+00:00","index":"","fulltext":""},{"type":"submitted","content":"Waste and Biomass Valorization","date":"2025-03-27T10:28:41+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"waste-and-biomass-valorization","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"wave","sideBox":"Learn more about [Waste and Biomass Valorization](http://link.springer.com/journal/12649)","snPcode":"12649","submissionUrl":"https://submission.nature.com/new-submission/12649/3","title":"Waste and Biomass Valorization","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"8273ce36-2366-41f6-b360-94bc754e44b5","owner":[],"postedDate":"April 25th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-07-28T16:10:16+00:00","versionOfRecord":{"articleIdentity":"rs-6321421","link":"https://doi.org/10.1007/s12649-025-03207-1","journal":{"identity":"waste-and-biomass-valorization","isVorOnly":false,"title":"Waste and Biomass Valorization"},"publishedOn":"2025-07-21 15:58:23","publishedOnDateReadable":"July 21st, 2025"},"versionCreatedAt":"2025-04-25 17:25:19","video":"","vorDoi":"10.1007/s12649-025-03207-1","vorDoiUrl":"https://doi.org/10.1007/s12649-025-03207-1","workflowStages":[]},"version":"v1","identity":"rs-6321421","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6321421","identity":"rs-6321421","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}