Dual modification strategy improves the adsorption efficiency of corn straw hydrochar phosphate

Dual modification strategy improves the adsorption efficiency of corn straw hydrochar phosphate | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 18 March 2026 V1 Latest version Share on Dual modification strategy improves the adsorption efficiency of corn straw hydrochar phosphate Authors : sinan Wang 0000-0002-4910-0454 , Xin Li , Chengrao Li , Ru Ye , Ci Wang , Youquan Xu , Lei Yang , … Show All … , Hongshuo Zhao , Jiahao Li , Enping Shi , Wenyue Sima , Jiying Liu , Sen Dou , Jihua Liu , Jingmin Yang , and zhongqing zhang [email protected] Show Fewer Authors Info & Affiliations https://doi.org/10.22541/au.177383744.42570452/v1 117 views 55 downloads Contents Abstract Supplementary Material Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Search for environment-friendly phosphate adsorption materials in polluted water to realize the high-value utilization of agricultural wastes corn straw under mild hydrothermal conditions, the effects of KOH and FeCl3 modification on the corn straw hydrochar were investigated with corn straw as raw material. And Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), XRD, and XPS were used to characterize the surface functional groups, structure, element content, and morphology of the modified hydrochar. The adsorption mechanism of phosphate in water was explored through adsorption kinetics and adsorption thermodynamics tests. The results showed that the adsorption kinetics of phosphate on the modified hydrochar conformed to the quasi-second order kinetic equation (R2>0.95, P ≤ 0.05), and the adsorption thermodynamics conformed to the Langmuir equation (R2 ≥ 0.94, P ≤ 0.05). The adsorption of phosphate was a spontaneous endothermic reaction (ΔGθ＜0, ΔHθ＞0) and monolayer adsorption and controlled by rapid reaction. Both FeCl3 and KOH modified hydrochar can improve the ability to adsorb phosphate, and the adsorption mechanism was different. The main reason of FeCl3 modified hydrochar can adsorb phosphate was that it has good electrostatic attraction. After KOH modification, phosphate adsorption mainly depended on large specific surface area and ion exchange. The corn straw hydrochar modified with FeCl3 had a large adsorption capacity for phosphate, and the maximum adsorption capacity at 45 ℃ was 2.25 mg/g, which can be used as a potential adsorption material for phosphate in polluted water. Dual modification strategy improves the adsorption efficiency of corn straw hydrochar phosphate Abstract: Search for environment-friendly phosphate adsorption materials in polluted water to realize the high-value utilization of agricultural wastes corn straw under mild hydrothermal conditions, the effects of KOH and FeCl 3 modification on the corn straw hydrochar were investigated with corn straw as raw material. And Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), XRD, and XPS were used to characterize the surface functional groups, structure, element content, and morphology of the modified hydrochar. The adsorption mechanism of phosphate in water was explored through adsorption kinetics and adsorption thermodynamics tests. The results showed that the adsorption kinetics of phosphate on the modified hydrochar conformed to the quasi-second order kinetic equation (R 2 >0.95, P ≤ 0.05), and the adsorption thermodynamics conformed to the Langmuir equation (R 2 ≥ 0.94, P ≤ 0.05). The adsorption of phosphate was a spontaneous endothermic reaction (ΔG θ ＜0, ΔH θ ＞0) and monolayer adsorption and controlled by rapid reaction. Both FeCl 3 and KOH modified hydrochar can improve the ability to adsorb phosphate, and the adsorption mechanism was different. The main reason of FeCl 3 modified hydrochar can adsorb phosphate was that it has good electrostatic attraction. After KOH modification, phosphate adsorption mainly depended on large specific surface area and ion exchange. The corn straw hydrochar modified with FeCl 3 had a large adsorption capacity for phosphate, and the maximum adsorption capacity at 45 ℃ was 2.25 mg/g, which can be used as a potential adsorption material for phosphate in polluted water. Keyword: Corn straw; Hydrochar; Phosphate; Adsorption characteristics 1 Introduction China is a major agricultural country with a wide variety of crop straw types and high yields every year [1] , with corn straw accounting for 41.92% of the total crop straw production [2] . The traditional methods for treating corn straw mainly include incineration [3] , straw returning to the field [4] , and silage [5] . In recent years, pyrolysis carbonization [6] and hydrolysis carbonization [7] have also gradually attracted attention from researchers in related fields. Biochar prepared by hydrothermal carbonization method has received widespread attention as an economical and efficient adsorption material in recent years [8] . The price of modified hydrothermal biochar is closed to half of that of activated carbon, and its adsorption capacity is equivalent to that of activated carbon [9] . Furthermore, the adsorption capacity was significantly higher than that of traditional adsorbents [10] . Therefore, the preparation of hydrothermal biochar provided an effective meaning for the comprehensive utilization of corn straw. Water eutrophication seriously endangered aquatic ecosystems and poses various risks to humans, animals, and plants. Phosphate enrichment was one of the main reasons for water eutrophicating [11] and it lead to the proliferation of algae and the production of microcystins in surface water [12] . At present, the main methods and technologies for reducing phosphate in water bodies included biological method [13] , chemical precipitation method [12] , electrolysis [14] , membrane technology [15] , and adsorption method [16-17] . Among them, the adsorption method was widely used due to its simplicity and ease of operation. Biochar have the characteristics of large specific surface area, strong adsorption capacity, and wide sources [18-19] , and was gradually being applied to reduce water eutrophication. [20] used biochar prepared from corn straw at different carbonization temperatures to adsorb nitrogen and phosphorus in aqueous solutions. The results indicate that the saturated adsorption capacity of phosphorus was B450>B350>B250. The biochar prepared at 250 ℃ had a strong phosphorus release effect on the adsorption process of phosphorus, and showed significant negative adsorption of phosphorus. The surface of biochar was negatively charged and usually has weak adsorption capacity for phosphate anions. Surface modification of biochar effectively improve its phosphorus adsorption performance [21] . Sun et al [22] studied the use of FeCl 3 modified hydrothermal biochar for phosphorus removal in biological retention ponds. The results indicate that the modified hydrothermal biochar effectively adsorb soluble phosphate in water, with a saturated adsorption capacity of 1.025 mg/g. Wang et al [23] prepared iron modified biochar using agricultural waste Ma Gen as a precursor to adsorb phosphate in aquaculture wastewater. The results indicate that iron modified Ma Gen biochar had a good adsorption effect on phosphate in water, with a maximum adsorption capacity of 6.9 mg/g. Xu et al [24] prepared iron modified biochar using water hyacinth as raw material to adsorb phosphate in the water environment. The results indicate that the removal efficiency of phosphate was 85%, and the treatment effect of synthetic phosphorus containing wastewater was improved by 5%. Wang et al [25] prepared iron modified biochar using peanut shells as raw material to adsorb phosphate. The result indicate that the maximum adsorption capacity of modified peanut shell biochar for phosphate was 1.11mg/g. Shen et al [26] prepared lanthanum modified biochar from hawthorn kernels and studied its adsorption behavior for phosphate. The result indicate that the adsorption capacity for phosphate can reach 45.38mg/g. At present, there are many studies on using pyrolysis method to prepare corn straw biochar as an adsorbent, but there are few literature reports on using the hydrothermal method to prepare corn straw biochar as an adsorbent. This article use corn straw as raw material and uses KOH and FeCl 3 modification methods to prepare corn straw hydrothermal biochar. The adsorption of phosphate in an biochar-aqueous solution is studied, and the structural characteristics of modified corn straw hydrothermal carbon before and after adsorption are compared and analyzed. At the same time, the adsorption mechanism of corn straw modified hydrothermal biochar on phosphate is explored. The study will provided the theoretical support for eutrophic water treatment. 2 Materials and Methods 2.1 Chemicals and Equipments Chemicals are all analytical pure and purchased from China National Pharmaceutical Group. Equipment included scanning electron microscope (JSM-7900F, Japan), infrared spectrum scanner (FTIR-IRAffinity-1s, Japan), constant temperature oscillator (SHA-2, Jintan Ruihua Instrument Co., Ltd.), electrothermal constant temperature drying oven (202-3AB, Tianjin test Instrument Co., Ltd.), magnetic stirrer (DJ-1, Jiangsu Scientific Analysis Instrument Co., Ltd.) X-ray diffraction and XPS. 2.2 Preparation method of hydrochar Three types of hydrochar were prepared using corn stalks as raw materials. The specific method was as follows: unmodified hydrochar (C1), accurately weigh 8.00g of cornstalk sieved through a 70 mesh sieve (0.180 mm), add 100 mL of deionized water, stir at room temperature for 30 minutes, transfer to a reactor, place in a 240 ℃ electric constant temperature drying oven, and maintain for 24 hours, cool to room temperature and remove. Rinsing with deionized water until the solution is neutral, then dry to constant weight in a 105 ℃ electric constant temperature drying oven, which was the unmodified corn straw source hydrochar. FeCl 3 (analytical pure) modified hydrochar (C2), weighing 8.00 g of corn straw and 2.00 g of FeCl 3 , mixing them thoroughly, and add them to 100 ml of deionized water. The subsequent preparation method was the same as (C1). Modified hydrochar (C3) was mixed with 3 mol/L KOH (analytically pure) at a solid-liquid ratio of 1:10, then shaken at 25 ℃ for 3 hours (200 r/min), washed with deionized water until the solution was neutral, and finally dried in an oven at 105 ℃ to constant weight. 2.3 Adsorption Test (1) Adsorption Thermodynamics Test The isothermal adsorption test adopted a batch equilibrium method, weighing 0.0500 g of three types of hydrochar and placing them in a 50 mL centrifuge tube. Adding 25.0 mL of phosphate solution at 0, 1, 3, 5, 7, 10, and 20 mg/L, respectively. Shaking at 25 ℃, 35 ℃, and 45 ℃ for 24 hours and filter. Measuring the phosphate concentration in the filtrate using molybdenum antimony resistance spectrophotometry, measuring the phosphate concentration in the filtrate, and calculating the adsorption amount of phosphate on the hydrochar, drawing the isothermal adsorption curve of phosphate, fit the isothermal adsorption equation, and calculate thermodynamic parameters. (2) Adsorption Kinetics Test The adsorption kinetics experiment was conducted using batch equilibrium method. 0.0500 g of three types of hydrochar were weighed and placed in a 50 mL centrifuge tube, 25.0 mL of 20 mg/L phosphate solution was added and shaken at 25 ℃, 35 ℃, and 45 ℃. Take out and filter at 5, 10, 30, 60, 120, 360, and 720 minutes after shaking, measuring the phosphate concentration in the filtrate, calculating the adsorption capacity of hydrochar for phosphate, drawing the phosphate adsorption kinetics curve, fit the adsorption kinetics equation, and calculating the kinetic parameters. (3) Physicochemical characterization of hydrochar Measuring the specific surface area of three types of hydrochar before and after phosphate adsorption using a static nitrogen adsorption instrument (Model 3H-2000PS1, Bester Instrument Technology (Beijing) Co., Ltd.), and selecting the BET model for specific surface area measurement. Using an electron scanning microscope (JSM-7900F, Japan), the apparent morphology of three types of hydrochar before and after phosphate adsorption was determined and analyzed. Using an infrared spectroscopy scanner (FTIR IRAffinity 1s, Japan), the infrared spectral characteristics of three types of hydrochar before and after phosphate adsorption were measured and analyzed. The crystal structures and phase compositions of three types of hydrochar before and after phosphate adsorption were measured and analyzed using X-ray diffraction (Bruker D8 Advance). The chemical elements and functional groups of the three types of hydrochar before and after phosphate adsorption were determined and analyzed using XPS (XPS, ESCALAB 250Xi, USA). 2.4 Data Processing (1) Calculation of adsorption capacity Q=(C 0 －C)/M×V (1) Where Q is the adsorption amount (mg/g). C 0 and C are the initial and equilibrium concentration (mg/L), respectively. V is the volume of adsorption liquid (L) and M is the mass of the adsorbent (g). (2) Adsorption kinetic model quasi-first-order-model：ln (q e -q t ) =ln q e1 -k 1 t (2) quasi-second-order-model：t/q t =1/k 2 q e 2 +t/q e (3) Where qt is the adsorption amount at t (mg/kg). T is the reaction time (s). K is the model parameter, and qe is the maximum adsorption capacity (mg/kg). (3) Adsorption thermodynamics model Where Ce refers to the phosphate concentration (mg/L) at which the adsorption reaction reaches equilibrium. Qe refers to the amount of phosphate adsorbed during adsorption equilibrium (mg/g). Qm refers to the maximum total amount of phosphate adsorbed by the adsorbent (mg/g), and KL is the adsorption constant related to the Langmuir equation, which is influenced by the adsorbent and adsorbate. In the Freundlich equation, KF is an empirical coefficient reflecting the adsorption strength, and n represents the energy variation characteristics of the adsorption sites on the surface of the adsorbent. (4) Van’t Hoff ΔG θ =-RT lnK L (6) ΔS θ =(ΔH θ -ΔG θ )/T (7) Where ∆G θ is the Gibbs free energy. R (8.314 J/K/mol) is the gas constant. T is the absolute temperature. K L is the adsorption equilibrium constant of the Langmuir model. ∆H θ is the change in enthalpy. ∆S θ is the change in entropy. The values of ∆H θ and ∆S θ were calculated according to the slope and intercept of the plot of lnK L vs. 1/t. 3 Results and Discussion 3.1 Thermodynamic characteristics of phosphate adsorption onto hydrochar Figure 1 shows the adsorption thermodynamic curves of three types of hydrochar for phosphate at temperatures of 25 ℃, 35 ℃, and 45 ℃. As the concentration of phosphate in the solution increases, the adsorption capacity of these three types of hydrochar for phosphate also gradually increases. As the temperature increases, the adsorption capacity of phosphate on hydrochar increases, indicating that both the increase in temperature and phosphate concentration promote the adsorption capacity of hydrochar on phosphate. Fig.1 Adsorption thermodynamics of phosphate onto hydrochar Table 1 shows the fitting results of thermodynamic equations for phosphate adsorption on three types of hydrochar. Langmuir model and Freundlich model were used to describe the thermodynamic process of phosphate adsorption on hydrochar, and Langmuir equation can well fit the adsorption isotherms of phosphate on these three types of hydrochar (R 2 ≥0.940). Calculating the slope and intercept of the straight line based on the Van’t Hoff equation ΔH θ and ΔS θ , further seeking ΔG θ value is shown in Table 2, ΔG θ ＜0, ΔH θ ＞0 and ΔS θ ＞0. Table 1 Adsorption isotherm parameters of phosphate onto hydrochar q m /(mg·g -1 ) K L /(L·mg -1 ) R 2 n K F /( L·mg -1 ) R 2 C1 25 0.86 0.0371 0.9478 1.34 0.0400 0.9353 35 0.71 0.1279 0.9799 1.98 0.1179 0.9409 45 0.72 0.1600 0.9851 2.17 0.1450 0.9436 C2 25 1.44 0.0399 0.9912 1.37 0.0731 0.9814 35 1.20 0.1040 0.9963 1.85 0.1669 0.981 45 1.42 0.1168 0.9752 1.92 0.2167 0.9413 C3 25 1.04 0.0498 0.9647 1.43 0.0656 0.9444 35 0.89 0.1248 0.9898 1.94 0.1419 0.9531 45 1.04 0.1548 0.9928 2.13 0.2020 0.9558 Table 2 Thermodynamic parameters for the adsorption of phosphate onto hydrochar C1 25 -8.95 0.2235 57.24 35 -12.42 45 -13.42 C2 25 -9.14 0.1725 41.92 35 -11.89 45 -12.59 C3 25 -9.68 0.1825 44.42 35 -12.36 45 -13.33 3.2 The adsorption kinetics of phosphate onto hydrochar Figure 2 shows the adsorption kinetics curve of modified hydrochar for phosphate, with a fast adsorption stage from 0 to 150 minutes and a slow adsorption stage from 150 to 350 minutes. The adsorption curve tends to stabilize and the adsorption rate is slow. After 350 minutes, the adsorption amount reaches equilibrium, and the adsorption of phosphate by hydrochar reaches its maximum saturation adsorption. However, with the increase of time, the adsorption amount does change insignificantly. The adsorption of phosphate by hydrochar is a process that starts fast and then slows down, ultimately reaching adsorption equilibrium. At 25℃, the adsorption capacity of hydrochar C2>C3>C1, 1.11 mg/g, 1.00 mg/g, and 0.89 mg/g, respectively. At 35℃, the adsorption capacity of hydrochar C2=C3>C1, 2.21 mg/g, 2.21 mg/g, and 1.89 mg/g, respectively. At 45℃, the adsorption capacity C2>C3>C1，2.31 mg/g, 2.25 mg/g, and 1.88 mg/g, respectively. As the adsorption temperature increases, the adsorption capacity of hydrochar increases, and the maximum adsorption capacity was 2.31 mg/g for C2 at 45 ℃. In order to explore the kinetic mechanism of phosphate adsorption on modified corn straw hydrochar, the adsorption data were fitted and analyzed using quasi first order and quasi second order kinetic equations. The results are listed in Table 3. The fitting parameters of the quasi-first-order kinetic model are R 2 =0.90-0.95, and the fitting parameters of the quasi second order kinetic model are R 2 =0.95-0.98. Fig.2 Adsorption kinetics of phosphate onto hydrochar Table 3 Adsorption kinetic parameters of phosphate onto hydrochar Q 1 (mg·g -1 ) K 1 R 2 Q 2 (mg·g -1 ) K 2 R 2 25 C1 0.89 0.79 0.095 0.93 0.85 0.1468 0.983 C2 1.11 0.96 0.1086 0.9332 1.04 0.1341 0.9764 C3 1.00 0.89 0.0618 0.92 0.96 0.0907 0.9777 35 C1 1.89 1.68 0.0159 0.9075 1.84 0.0122 0.9522 C2 2.21 2.06 0.0217 0.9559 2.25 0.0138 0.9824 C3 2.21 2.09 0.0195 0.9543 2.28 0.0122 0.9809 45 C1 1.88 1.64 0.0620 0.9254 1.79 0.0475 0.9813 C2 2.31 2.06 0.0448 0.9194 2.23 0.0290 0.9777 C3 2.25 2.02 0.0403 0.9171 2.18 0.0281 0.9728 3.3 Specific surface area analysis before and after adsorption of phosphate onto hydrochar Both the methods of BET and BJH adsorption/desorption were used to determine the specific surface area of three types of corn straw hydrochar microspheres. As shown in Table 4, the specific surface area of C2 and C3 significantly increased by 14.81% and 42.79% compared to unmodified hydrochar. The specific surface area of the adsorbed hydrochar C1, C2, and C3 decreased by 34%, 46%, and 70%. The modification of FeCl 3 and KOH improved the pore characteristics of hydrochar, with small pores combining to form large pores, resulting in an increase in specific surface area, which is consistent with many research results [27-30] . Table 4 Specific surface area of hydrochar Before adsorption 15.12 17.36 21.59 After adsorption 10.03 9.30 6.44 3.4 Electron Microscopic Structure Characterization before and after adsorption of phosphate onto hydrochar Three types of hydrochar microstructures were characterized using electron scanning microscopy (Figure 3). C1 (A) shows a smaller carbon ball structure of corn straw hydrochar, but some cores were covered and blocked by impurities. C2 (B) shows a larger and less biochar microsphere structure, and the surface of hydrochar becomes rough. In C3 (C) after KOH modification and activation, some impurities that block the pore structure are decomposed or volatilized [31] , resulting in more obvious pore structure and microsphere structure, and the appearance of monodisperse biochar microspheres. After adsorption, a small amount of biochar microsphere structure is attached to the surface of hydrochar (a) and (b), while the surface of (c) becomes regular, smooth, and dense. Fig.3 Electron microscope scanning of modified nanometer hydrochar microspheres of hydrochar. A(C1) B(C2) C(C3) a(after adsorption C1) b(after adsorption C2) c(after adsorption C3) 3.5 Infrared spectroscopy analysis before and after adsorption of phosphate onto hydrochar From Figure 4, it can be seen that all three types of hydrochar exhibit absorption peaks in the range of 3100-3500 cm -1 and 1610-1667 cm -1 . This absorption peak is mainly caused by the stretching vibration of -OH associated with intermolecular hydrogen bonds [32] and the stretching vibration of aromatic ring carbonyls (-C=O) [33] ; After adsorption, the absorption peak is almost absent, indicating that the hydroxyl (-OH) and aromatic carbonyl (-C=O) groups on the surface of the hydrochar participate in the adsorption of phosphate. The modified C2 and C3 exhibit stretching vibrations with absorption peaks in alcohol or phenol (-C-O) at wave numbers of around 1368-1467 cm -1 [34] . After adsorption, the absorption peak intensity weakens and the wave peak narrows, indicating that this group also participates in the adsorption of phosphate. At 1116 cm -1 , there were bending vibrations with absorption peaks of alcohol hydroxyl groups (-OH) and stretching vibrations of ethers (-O-) [35] . After adsorption, the absorption peak intensity weakens and the wave peak narrows, indicating that this group also participates in the adsorption of phosphate. Fig.4 FT-IR analysis before and after adsorption of phosphate onto hydrochar 3.6 XRD analysis before and after adsorption of phosphate onto hydrochar In order to investigate the adsorption mechanism of phosphorus, the XRD diffraction patterns of hydrochar prepared under different modification conditions before and after adsorption were analyzed using X-ray powder diffraction. The diffraction results are shown in Figure 5. In the XRD pattern, a strong diffraction peak appears at the position where the diffraction angle (2θ)≈33.15. By comparing with the standard card (JCPDS: 99-0057)matching, it can be seen that this peak represents the phase composition of Fe 2 O 3 compound in the hydrochar material. Indicating that some iron containing compounds are loaded onto hydrochar [36] . A clear diffraction peak appeared at a diffraction angle of approximately 26.5°, which was attributed to the diffraction of the graphite (002) crystal plane by matching with the standard card (JCPDS: 26-1097). In hydrochar, the appearance of this peak may indicate that during the carbonization process, some carbon atoms were rearranged to form a graphite like structure, indicating the presence of a certain degree of graphitized carbon structure in the hydrochar material prepared this time, and reflecting the content or degree of graphitized carbon in the material [37] .By comparing the adsorption of phosphorus by hydrochar before and after, it can be seen that after the adsorption behavior of the three types of hydrochar, the diffraction peak intensity located on the graphite (002) crystal plane showed a significant weakening or even disappearance phenomenon. This phenomenon indicates that the adsorbed substance phosphorus enters the graphitized carbon structure of hydrochar, leading to an increase in lattice spacing and a decrease in diffraction peak intensity. This change may be due to adsorbate molecules or ions occupying some of the space between the originally graphitized carbon layers, affecting the stacking and orderliness between the hydrochar layers [38] . Fig.5 XRD analysis of phosphate adsorption by hydrothermal biochar before and after adsorption 3.7 XPS analysis before and after adsorption of phosphate onto hydrothermal biochar In order to better reveal the adsorption mechanism of three types of hydrochar on phosphate, XPS characterization analysis was conducted on the three types of hydrochar before and after phosphate adsorption, as shown in Figure 6. Through peak analysis of C1s, it can be concluded that before modification, the oxygen groups on the surface of carbon spheres are very abundant. The C1s XPS spectra of C1, C2, and C3 are decomposed into three peaks, corresponding to C=C/C-C, C-O/C=O, and O-C=O, respectively. The peaks of O1s correspond to two peaks, C-O and C=O [39] . These functional groups are formed during hydrothermal processes such as aldol condensation and esterification. As shown in Figure 6 (a, b), it can be seen from the total spectrum that (a is the total spectrum before adsorption of three types of hydrochar, b is the total spectrum after adsorption of three types of hydrochar) after adsorption of phosphate by the three types of hydrochar, a new peak P2p appears, and the peak at 134.6 eV corresponds to P2p. The characteristic peak of P2p shifts after adsorption of phosphate, indicating successful adsorption of phosphate onto hydrochar. The peak appearing at 724.2 eV corresponds to Fe2p, indicating that the modification process successfully loaded FeCl 3 on the surface of the adsorbent. The iron content decreases before and after adsorption. During the process of adsorbing phosphate ions, the characteristic peak of Fe2p shifts, indicating solid-liquid interface adsorption. Fe may react with phosphate ions to form FePO 4 precipitate, which adheres to the surface of the adsorbent [40-42] . After KOH modification of hydrochar, the C content on the surface of hydrochar decreased while the O content relatively increased. During the modification reaction, KOH can react with active oxygen-containing substances in biomass, resulting in complete conversion of KOH to K 2 CO 3 , forming a large amount of gaseous products and phenols, which are converted into gases such as CO and CO 2 and released, leading to a decrease in C content [43,44] . At the same time, a large number of vacancies were generated in the hydrochar during the process, and OH - from KOH quickly entered these vacancies, forming a large number of new O-containing groups (C-O, - OH, - CO, O-C=O, etc.), which led to an increase in oxygen content in the hydrochar [45] . Due to the alkaline modification process of hydrochar, more functional groups on the biomass surface are exposed, especially the increase of oxygen-containing functional groups, which is beneficial for the material’s adsorption of phosphorus, which can further improve the adsorption performance of hydrochar. After adsorption, the type and content of oxygen groups on the surface of hydrochar significantly decrease, indicating that adsorption leads to the consumption of oxygen groups on the material surface. Fig.6 XPS analysis of phosphate adsorption by hydrochar before and after adsorption （a is the three types of carbon before adsorption, and b is the three types of carbon after adsorption） 3.8 The adsorption mechanism of phosphate onto hydrochar The adsorption mechanism is shown in Figure 7. The physical and chemical interactions between phosphate and hydrochar determining the removal of phosphate from aqueous solution. It is important to note that electrostatic interactions exist in both chemical and physical adsorption processes. Compared with other adsorption mechanisms, electrostatic interaction was considered a simple and reversible adsorption step [46] . In addition to electrostatic interactions, other mechanisms such as ion exchange were also involved in the phosphate adsorption process [47] , where one phosphate ion replaces another ion in the hydrochar structure. Ion-exchange is usually related to external sphere complexation. This adsorption was considered reversible, and phosphate ions can be recovered from the surface of biochar. The hydrochar modified with iron chloride had good electrostatic attraction, precipitation ability, and anion exchangeability, thereby improving its adsorption capacity [48-51] . It was generally believed that the modification of hydrochar by KOH improved the pore structure of hydrochar to increase the specific surface area [52] , which was an important factor in increasing the phosphate adsorption ability of KOH modified hydrochar. The oxygen-containing functional groups [53] and ion exchange interaction of KOH-activated hydrochar were also factors in the phosphate adsorption process. Fig.7 Adsorption mechanism of phosphate onto hydrothermal biochar 4 Discussion and conclusion Langmuir equation can better fit the thermodynamic process of phosphate adsorption by these three biochar (R 2 ≥ 0.94). As the temperature increases, the reaction rate constants (K L and K F ) increases, indicating that increasing the temperature can accelerate the adsorption rate of phosphate on hydrochar. The maximum adsorption capacities of C1, C2, and C3 for phosphate at 45 ℃ are 0.523 mg/g, 0.944 mg/g, and 0.762 mg/g, respectively. The overall adsorption capacity of the three types of hydrochar for phosphate is C2>C3>C1. The quasi second-order kinetic equation can describe the kinetic adsorption process of phosphate on hydrochar. According to the quasi second-order kinetic parameter K 2 value [54] , the adsorption of phosphate by the three modified hydrochar is mainly controlled by rapid reactions. The maximum adsorption capacities of C1, C2, and C3 for phosphate at 45 ℃ are 1.875 mg/g, 2.307 mg/g, and 2.25 mg/g, respectively. The adsorption mechanism by three hydrochar is a spontaneous endothermic reaction, in which phosphate spontaneously moves from the solution to the surface of corn straw hydrochar microspheres during the adsorption reaction based on ΔG θ value. The value of ΔG θ decreases with the increase of temperature, therefore, the increased temperature increases the spontaneity of the reaction. It is generally believed that ΔＨ θ > 40 kJ/mol, the adsorption process is mainly chemical adsorption [55] . ΔS θ value is positive, so the reaction is related to an increase in entropy, and the disorder at the solid-liquid interface increases during the adsorption process [56] . The adsorption capacity of the three types of hydrochar increased with the increase of adsorption temperature. The maximum adsorption capacity of C2 for phosphate was significantly greater than that of C1 and C3, and the equilibrium adsorption capacity of phosphate increased by 23%. The experimental results indicate that the adsorption capacity of FeCl 3 modified hydrochar is greater than that of the other two types of hydrochar. The specific surface area of the adsorbed hydrochar C1, C2, and C3 decreased by 34%, 46%, and 70%. The significant reduction in specific surface area of KOH modified hydrochar proves that the adsorption of phosphate relies on its large specific surface area. The absorption peaks at 3100-3500 and 1610-1667 cm -1 after phosphate adsorption changed, indicating that the hydroxyl (-OH) and aromatic cyclic carbonyl (-C=O) groups on the surfaces of the three hydrochar participated in the adsorption of phosphate. The surface distribution of alcohols or phenols (-C-O) and ethers (-O-) on C2 and C3 also participate in the adsorption of phosphate in the solution. The adsorption mechanism is different, and the main reason for FeCl 3 modified phosphate adsorption is its good electrostatic attraction ability. The adsorption of phosphate after KOH modification mainly relies on a large specific surface area and ion exchange interaction. The adsorption capacity of FeCl 3 modified hydrochar is greater than that of the other two types of hydrochar. The maximum adsorption capacity at 45 ℃ is 2.25 mg/g. Ethics approval The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Consent to participate “Informed consent was obtained from all individual participants included in the study.” Consent to publish The authors affirm that this research consent to publish has been received from all participants which appear in the manuscript. Authors Contributions [Redacted for peer-review] Funding [Redacted for peer-review] Competing Interests “The authors have no relevant financial or non-financial interests to disclose.” Availability of data and materials All authors declare that the data supporting the findings of this study are available within the paper and its Supplementary Information files. Should any raw data files be needed in another format they are available from the corresponding author upon reasonable request. Source data are provided with this paper. Reference Shi, Z., Li, X., Wang, J., Wang, F., Sun, R., and Song, C., “Spatial distribution characteristics and utilization mode of straw resources in China,” Resources and environment. 28(7): 202-205 (2018). [2] Zhang, X., “Study on the high-resolution emission characteristics of air pollutants from straw burning in China,” Nanjing University. (2019). 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Keywords adsorption characteristics corn straw hydrochar phosphate Authors Affiliations sinan Wang 0000-0002-4910-0454 View all articles by this author Xin Li View all articles by this author Chengrao Li View all articles by this author Ru Ye View all articles by this author Ci Wang View all articles by this author Youquan Xu View all articles by this author Lei Yang View all articles by this author Hongshuo Zhao View all articles by this author Jiahao Li View all articles by this author Enping Shi View all articles by this author Wenyue Sima View all articles by this author Jiying Liu View all articles by this author Sen Dou View all articles by this author Jihua Liu View all articles by this author Jingmin Yang View all articles by this author zhongqing zhang [email protected] View all articles by this author Metrics & Citations Metrics Article Usage 117 views 55 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation sinan Wang, Xin Li, Chengrao Li, et al. 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