Passivation and Remediation of Pb and Cr in Contaminated Soil by Sewage Sludge Biochar Tubule

Passivation and Remediation of Pb and Cr in Contaminated Soil by Sewage Sludge Biochar Tubule | 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 Passivation and Remediation of Pb and Cr in Contaminated Soil by Sewage Sludge Biochar Tubule Lin Chen, Qi Ni, Yan Wu, Chuan Fu, Wei Ping, Hongyu Bai, Mengnan Li, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-173906/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 3 You are reading this latest preprint version Abstract Currently, numerous studies have carried out to research the effect of biochars remediation soil heavy metals (HMs) contaminated, but there have been fewer explorations of the effect of biochars tubule on soil HMs remediation. This work aimed was to study the effect of passivation and remediation of lead (Pb) and chromium (Cr) contaminated soil after insert sewage sludge biochar (SSB) tubule. The results showed that the high risky fractions of Pb and Cr could be transformed into more stable fractions, also, Pb and Cr total contents are significantly decreased by SSB tubule. The mechanisms including adsorption, ion exchange, complexation and precipitation which are concluded from the characteristic analysis. Detailly, the passivation of Pb and Cr are better when the moisture at 25% and 35%, respectively, [Pb: exchangeable (F1), carbonate bound (F2) decreased by 25.1%, 16.8%, Fe-Mn oxides bound (F3) increased by 18.5%; Cr: F1 decreased by 73.0%, F2, F3, Organic matter bound (F4) increased by 13.2%, 23.9%, 30.8%), respectively]. The remediation of Pb and Cr is better when the moisture at 25% and 35%, respectively, (Pb: decreased by 23.3%; Cr: decreased by 38.4%, respectively). The findings showed that the SSB tubule is effective when used for soil HMs contaminated. Environmental Engineering Environmental Policy Biochar tubule Sewage sludge Soil heavy metals contaminated Passivation and remediation Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction The soil environment has been a global concern due to its complexity and significance. However, with the rapid urbanization and industrialization various contaminants [such as HMs, radioactive elements and organics, etc.] are being introduced into the soil system through directly discharged to contain more than the allowable standard contaminant concentrations (Wang et al. 2020 ; Rybak et al. 2018 ; Feng et al. 2018 ). HMs have ranked the first among all soil contaminant types due to their high toxicity, bioaccumulation, persistence and mobility in soils (He et al. 2020 ; Yang et al. 2020 ). These HMs (Pb, Cr, Cd, Hg, As, etc.) can lead to potential health risks to predators and humans through the accumulation of food chains. For instance, Pb is a commonly recognized carcinogen and led to neurotoxicity, stomach and lung lesions; Cr(VI) is a strong oxidant and also act as carcinogenic and teratogenic characteristics, and both of them have been listed as priority monitoring and control pollutants (Hu et al. 2020 ). Excess of Pb and Cr in soil is mainly derived from intensifying anthropogenic activities including mining, sewage irrigation, and pesticide abuse (He et al. 2020 ), which could lead to a deterioration in various functions and stability of soil systems (Duan et al. 2018 ). In the last several decades, various remediation technologies, including phytoremediation, excavation, landfilling, electrokinetic remediation, soil washing and their blending, have been used to Pb and Cr contaminated soils for removing or reducing high toxic element amounts (Liu et al. 2018 ; Sarwar et al. 2017 ; Trellu et al. 2016 ). However, most of these technologies due to their long remediation cycle, high energy consumption, low efficiency and generating significant secondary environmental impacts (Gerhardt et al. 2009 ), which are unsuitable for the most area to decelerate their development. Accordingly, there is an urgent need for inexpensive, efficient and stable amendment materials. To date, various materials [resin (Chen et al. 2020 ); activated carbon (Dong et al. 2016 ); peat (Lee et al. 2015 )] are used to adsorb HMs. Unfortunately, the resin has a potential risk with secondary pollution, activated carbon is expensive and peat has a characteristically low surface. Biochar is a material with carbon-rich, porous and high aromaticity which is obtained from the pyrolysis of biomass at relatively low temperature (< 700℃) under the presence of limited oxygen (Yu et al. 2020 ; Chen et al. 2020 ). Biochar as an environmental sorbent has become one of the most attractive research hotspots due to having abundant raw materials, easy preparation and stable performance, etc. (Hung et al. 2020 ; Zhang et al. 2020 ; Xiao et al.2019). In addition, biochar can increase crop yield through improve soil fertility and remediate soil by immobilizing HMs (Azeem et al. 2020; Xi et al. 2020 ). Numerous researches have showed that biochar plays important role in decreasing the HMs (such as Cr, Cd, Pb, Cu and Zn, etc.) total and unstable concentration (Puga et al. 2015 ; Gao et al. 2020 ; Liu et al. 2018 ). Thus, Biochar has great advantages as a green environmental sorbent in remediating HMs contaminated soil. Biochar can be prepared from a wide variety of raw materials such as municipal sludge, livestock manure, crop straw (Al-Wabel et al. 2018 ). In recent years, municipal sewage sludge has increased sharply with the increasing improvement of sewage treatment facilities (Zhou et al. 2020a ), it is a by-product of the sewage treatment municipal and contains a variety of harmful substances (pathogens, refractory organics and toxic HMs, etc.), which is easy to cause secondary pollution (Zhou et al. 2020b ). Pyrolysis of sewage sludge is promising since it enables to decrease the harmful substances and volume. Meanwhile, the solid carbonaceous residues after pyrolysis (SSB) may be used as the amendment of HMs in soil (Zhang et al. 2018). At present, Some researches have showed that the excellent effect of sludge biochar on the remediation of HMs contaminated soil, For example, Penido et al. ( 2019 ) observed a significant reduction of Cd, Pb, and Zn bioavailability in HMs contaminated soil from a Zn-mining area by applying SSB. Fang et al. ( 2016 ) used the SSB to remediate soils that had been contaminated with cationic Pb(II), etc. and anionic Cr(VI), etc. respectively. However, there are still concerns regarding potential soil secondary contamination with toxic HMs due to SSB were thoroughly mixed with soil and require further investigations to the assessment of long-term risks (Fang et al. 2016 ; Figueiredo et al. 2019 ). Thereby, it is requisite to obtain a piece of equipment which is able to separate biochar from the soil when remediation ended. In this study, biochar derived from sewage sludge and was placed in polymethyl methacrylate (PMMA) tubule and as an amendment inserted HMs contaminated soils. Herein, Pb and Cr were selected as representative of toxic HMs. The objectives of this study are to 1) investigate the effect of SSB on the fractions of Pb and Cr in soil, to 2) determine the changes of the total content of Pb and Cr in soil with the remediation distance from the SSB tubule and time. The research results will provide some theoretical references for HMs-contaminated soil remediation and passivation by biochar in future research and application. Material And Method Biochar preparation In this study, sewage sludge (SS) was collected from the dewatering workshop of the Xintian Wastewater Treatment Plant (XWTP) in Wanzhou District, Chongqing City, China, where an Orbal oxidation ditch wastewater treatment system is operated. The basic properties of sewage sludge were given in Table 1 . The raw sewage sludge was dewatered through a vacuum pump (SHZ-DIII, Yuezong, China) and then was placed in an oven (DHG-9420A, Honghua, China) at a temperature of 103 ± 2 ℃ to constant weight. The dried samples were ground and passed through 20-mesh nylon sieves, The samples were thoroughly mixed and stored in plastic bags for further study. Table 1 Properties of sewage sludge MLSS mg·L − 1 MLVSS / mg·L − 1 SVI / mL·g − 1 Moisture / % 2425.5 1659.4 128.5 99.1 The oxygen-limited pyrolysis method was used to prepare the biochar. In brief, approximate 50.0g of sewage sludge samples loaded in the crucible and covered with fitting lids and alumina foils, which were loaded into the muffle furnace (SX-10-12, Yiheng, China), the pyrolysis process was performed by raising the target temperature to 500°C with a heating rate of 10°C min − 1 and residence time of 2 h. The obtained sewage sludge biochar (SSB) was grounded to pass through 100-mesh nylon sieves, and dip in hydrochloric acid for 24 h to eliminate effects of surface and internal impurities, and improve the porous structure of SSB (Liu et al. 2018 ), then the SSB was cleaned with deionized water until reaching a neutral pH. Finally, SSB was dried and store in plastic bags. Characterization of biochar The yield was calculated from the ratio between the mass of SSB and the raw materials. Ash was measured as the residual remaining after heating to 800°C and maintaining for 4 h. The pH values were measured with a pH meter (FE28, Mettler Toledo, China) and the method from wang et al. (2020). Levels of Pb and Cr were determined by inductively coupled plasma optical emission spectrometry (ICP-OES; Optima7000DV, Perkin Elmer, USA) after acid digestion according to Figueiredo et al. (2020). The surface structure of SSB was analyzed using a scanning electron microscopy (SEM; Supra55, Zeiss Germany). The surface functional groups of SSB were determined through Fourier transform infrared (FTIR) spectroscopy (Nicolet iS 10, Thermo Fisher Scientific, USA), the infrared spectra were obtained over the 4000 ~ 500 cm − 1 . The crystal structure of the SSB was characterized by X-ray diffraction (XRD; D8 ADVANCE, Bruker, Germany). Test soil collection and preparation Surface soil (0–20 cm) was collected from Three Gorges Reservoir area located in Wanzhou District, Chongqing City, China (30 ° 42 ′ 53 " N, 108 ° 25 ′ 55 " E). The soil sample was thoroughly mixed, air-dried at room temperature, excess roots and gravels were removed, ground to pass through 20-mesh nylon sieves before initiating the experiments. The physiochemical properties of the soil were determined according to Chinese standard methods (Liu et al. 1996 ) and were given in Table 2 . To obtain a more significant remediation effect of SSB on Pb and Cr and reduce influences of other HMs in the soil, we replenish external Pb (Pb(NO 3 ) 2 ) or Cr (K 2 Cr 2 O 7 ) to the air-dried soil and to reach the First or Second type of Risk Intervention Values for Soil Contamination of Development Land (RIVSCDL) of China, respectively. Meanwhile, the soil moisture content was controlled at 25% or 35% in each test group. Finally, all test soils were allowed to incubate for 7 days before the experiments. The type of soils required and marking was given in Table 3 . Table 2 The properties of the raw soil. Volume-weight / (g·cm − 3 ) Moisture / % pH Total Pb / (mg·kg − 1 ) Total Cr / (mg·kg − 1 ) 1.3 ± 0.1 29.4 ± 2.1 7.3 ± 0.4 26.4 ± 1.3 35.6 ± 1.5 Table 3 The types of soils required Adscititious HMs Value / (mg·kg − 1 ) expectantly / actually Moisture / % Amendments Marking Pb 800 / 707.3 25 and 35 Raw soil CN Pb 800 / 707.3 25 and 35 SSB SSB-F Pb 2500 / 1998.2 25 and 35 SSB SSB-S Cr 30 / 31.8 25 and 35 Raw soil CN Cr 30 / 31.8 25 and 35 SSB SSB-F Cr 80 / 76.5 25 and 35 SSB SSB-S Notes: CN: Control. HMs contaminated soil (concentration were controlled at the First type of RIVSCDL) which were remediated by raw soil. SSB-F and SSB-S: Treatment groups. HMs contaminated soil (concentration was controlled at the First and Second type of RIVSCDL) which were remediated by SSB. Experimental design The tested soils and SSB were placed in PMMA column [H (height) = 10, D (diameter) = 30 cm] and PMMA tubule (H = 10, d = 1.5 cm, many pores is full of the tubule wall), respectively, and the tubule was located center of the column (Fig. 1 ). To minimize the migration of Pb and Cr through the column sidewalls, PMMA columns were coated with a thin layer of paraffin wax and spread a layer of 1.5 cm gravel. Then, the soil was filled with layer upon layer and packed. The filling height up to 8.0 cm both soil and biochar and at a ratio of 1:600 (dry mass, biochar/soils). Finally, they were incubated in a lightproof, sealed, and room temperature sustain 35 days, soil samples were collected every 5 days and 8 times in total. Approximate 0.5 g of soil samples were collected at equal depth and distance to measure the total contents of Pb or Cr, and the needless soil was backfilled and compacted. After the last collection, SSB tubules were recycled, soil in the PMMA column was thoroughly mixed and measure the Pb and Cr concentration of fractions. All of the soil samples were dried, grounded to pass through 100-mesh nylon sieves and stores. Each experiment was implemented in duplicate. The schematic diagram of the experimental design was showed in Fig. 1 . Analyses of Pb and Cr Total contents of Pb and Cr In this study, total Pb and Cr were determined using a microwave digester (Speedwave Xpert, Berghof, Germany). Particularly, samples (0.2 g) were digested with a dup-acid mixture of HF (1 mL) and HNO 3 (8 mL) according to Khadhar et al. ( 2020 ) and the residual solution through membrane filters (0.22 µm) and diluted to 50 mL. Chemical fractions of Pb and Cr Pb and Cr potentially toxic were measured according to Tessier sequential extraction method (Tessier et al. 1979 ), this method divides HMs into five fractions due to their are successively less bioavailability and mobility effect (Chen et al. 2020 ). In brief, extraction steps were as follows: (F1) exchangeable (1mol/L MgCl 2 ); (F2)carbonate bound (1 mol/L NaAc); (F3) Fe-Mn oxides bound (0.04 mol/L NH 2 OH·HCI in 25% HAc); (F4) organic matter bound [(0.02 mol/L HNO 3 ), (30% H 2 O 2 ), (3.2 mol/L NH 4 Ac in 20% HNO 3 )]; (F5) residual (digested with HF-HNO 3 acids). After each step, the sample was separated at 4000 rpm for 20min, supernatants were used for detecting the concentration of F1- F5 through membrane filters (0.22 µm). The Pb and Cr concentrations were measured by ICP-OES. Results And Discussion Passivation effect of SSB on Pb and Cr HMs exist in the soil with diverse chemical fractions and the bioavailability and potentially toxic mainly depend on their specific fraction. F1 presents the most unstable and high risky and F2 slightly, F5 was identified as a stable fraction. bioavailability and toxic of those five fractions from high to low was F1 > F2 > F3 > F4 > F5 (Chen et al. 2020 ). Fraction distribution of Pb and Cr obtained from Tessier sequential extraction method and were showed in Fig. 2 which reveal the fraction distribution difference of initially and finally. In brief, the results showed that CN had a varied slightly of fraction distribution and SSB had was significant. Compare SSB-F with SSB-S groups. The F1 and F2 decreased by 25.1% and 16.8%, F3 increased by 18.5%; the F1 and F2 decreased by 28.8% and 2.9%, F3 increased by 9.8% (Fig. 4 a). The F2 and F3 decreased by 8.8% and 4.8%, F4 increased by 75.1%; the F1 decreased by 42.1%, F3 and F4 increased by 4.5% and 12.9% (Fig. 4 b). The F1 decreased by 33.6%, F3 and F4 increased by 10.1% and 22.1%; the F1 decreased by 25.3%, F3 and F4 increased by 13.8% and 14.8% (Fig. 4 c). The F1 decreased by 73.0%, F2, F3 and F4 increased by 13.2%, 23.9% and 30.8%; the F1 decreased by 37.2%, F3 and F4 increased by 25.4% and 33.5% (Fig. 4 d). Munir et al. ( 2020 ) found that Cr and Pb be immobilized and removed by -OH and -COOH groups in biochar. Results showed that SSB could decrease Pb and Cr bioavailability and mobility through transforming the F1 and F2 to more stable forms and that is better when the moisture at 25% and 35%, respectively, (Pb: F1, F2 decreased by 25.1%, 16.8%. Cr: F1 decreased by 73.0%, respectively). These results were compared with others. Liu et al. ( 2018 ) added 5% (w/w) modified coconut shell biochar to multi-metals contaminated soil in Mianzhu, Sichuan, China, which decreased the acid-soluble Cd, Ni and Zn concentration by 30.1%, 57.2% and 12.7%, respectively. Munir et al. ( 2020 ) added 2% (w/w) bamboo-biochar to Pb and Cr contaminated soil in Huainan, Anhui, China, which decreased their F1 concentration by 8.5% and 29%, respectively. Therefore, SSB tubules have a practical effect on soil passivation. The remediation effect of SSB on Pb and Cr The variation of Pb and Cr total contents in the soil with the incubation period and distance were showed in Fig. 3 (A line represents the variation of the Pb or Cr total content in this sampling point with time). Insert SSB tubules in contaminated soils resulted showed that Pb and Cr total contents were significantly decreased. Presented diverse trends under different experiment conditions which phenomenons indicate these conditions (moisture and pollution intensity) have a definite influence on the remediation results. At end of the remediation, results revealed that CN had a varied slightly of Pb and Cr concentration compares with initially (concentration decreased by -2.1%~8.0% and − 10.3%~4.0%). On the contrary, the effect were significantly of SSB-F and SSB-S [Pb concentration decreased by 8.5%~21.8% and 14.6%~23.3% (Fig. 3 a), 13.0%~17.3% and 8.2%~16.2% (Fig. 3 b); Cr concentration decreased by 9.5%~22.2% and 5.0%~15.5% (Fig. 3 c), 25.1%~38.4% and 24.8%~36.1% (Fig. 3 d), respectively]. Maximum of Pb is located at the farthest and the closest point when moisture was 25% and 35%, respectively, and maximum of Cr located were showed in Fig. 3 c-d. Results indicated that a better remediation effect of Pb and Cr when the soil moisture content was 25% and 35% (concentration of Pb and Cr decreased by 23.3% and 38.4%, maximally). This remediation effect was compared with others. Wang et al. ( 2020 ) added 10% (w/w) kitchen waste-based biochar to Ba contaminated soil in landfill areas, Tibet, China, which decreased the concentration by 10.1%. Li et al. ( 2016 ) added biochar to Cd contaminated soil, found decreased the total content by 46.4%. These studies indicate that added biochars could remove HMs in soil. In addition, for Pb contaminated soil, these points (≤ 7.5cm) presented initial decrease, subsequent increased and final decreased with the remediation time, and others presented initial increased, finally decreased. These phenomena could be attributed to SSB had a rich porous structure (Fig. 4 b). Lin et al. ( 2020 ) indicated biochars adsorb HMs are divided into the surface monolayer sorption initially and the intraparticle diffusion sorption later. Initially, SSB adsorbs Pb ion at closer points, with the adsorption persistent, Pb ion could gradually migrate to SSB tubule from a broader area, which leads to Pb ion short-dated increase and final continuous decrease, Mitzia et al. ( 2020 ) consider that cause this phenomenon may attribute multiple interactions (such as soil matrix and water content), which can provoke an Eh-pH fluctuation. For Cr contaminated soil, presented a similar trend for all lines which are rapid initial decrease, subsequent increase (starting on the 5th day), and final decrease (starting on the 20th day) and presented an indistinctive effect at remediation distance. Characteristics of SSB and its removal mechanisms of Pb and Cr Characteristics of SSB The basic properties of SSB were given in Table 4 . Results showed that yield and ash of SSB were higher compare with other biochar (biochars derived from rice straw, sawdust and phragmites etc.) which was mainly due to the SSB have high contents of inorganic constituents (Pellera et al. 2020) Furthermore, SSB was alkaline which was due to the surface of SSB including multitudinous alkaline aromatization groups and them could immobilize HMs through increase the pH of the soil (Mitzia et al. 2020 ). Pellera et al. (2020) found that biochar leads to higher soil pH and ECE and causes metal precipitation, finally. Table 4 Basic properties of SSB Yield / % Ash / % pH Total Pb / mg·kg − 1 Total Cr / mg·kg − 1 47.8 ± 0.2 30.6 ± 0.3 8.2 ± 0.1 18.8 ± 2.4 53.5 ± 4.0 SEM analysis The SEM was carried out to characterize the microstructure of SSB and different magnifications are showed in Fig. 4 . Results showed that the SSB had uneven size distribution and visible loose fold (Fig. 4 a), and had a rich porous structure (Fig. 4 b), which could be due to the breakdown of the volatile compounds at higher temperatures that leadto the energy (gas) to escape from sewage sludge (Shakya et al. 2019). These forming mesopores and micropores to obtain greater surface area and porous volume, these porous structures could provide enough adsorption area for metal binding (Wang et al. 2018 ). Furthermore, visible inorganic ash particles were random distributed around of porous structure, Yuan et al. ( 2020 ) found that these particles would via ion exchange, complexation and precipitation reactions to reduce metal ions. FTIR spectra analysis Figure 5 a shows the infrared spectrum of SSB. Five main absorption peaks centered at 3734, 2360, 1540, 1015 and 775 cm − 1 were recorded. The peaks observed near 3734 cm − 1 was attributed to the stretching and bending vibration of –OH (Lin et al. 2017 ), the peaks at 2360 cm − 1 were corresponded to the stretching vibration of CO 2 (Lin et al. 2017 ) and the peaks at 1540 cm − 1 was ascribed to the stretching vibration of C = O, C = C aromatic rings (Shin et al. 2020 ), the peaks at 1015 cm − 1 and 775 cm − 1 were attributed to the stretching vibration of -OH and C-H of aromatic rings (Liu et al. 2020 ), respectively. Indicating that SSB contained several kinds of functional groups, These groups play a heavy role to reduce Pb and Cr in soil have been widely reported, for example, Zhao et al. ( 2021 ) found that O-containing groups positive participation Cr(VI) removal, Wang et al. ( 2018 ) reported that -OH was involved in the interaction with Pb(II) ion by the ion exchange reaction. XRD analysis Figure 5 b shows the mineralogical composition of SSB. Illustrating highly crystalline structures due to revealing numerous sharp peaks (Liu et al. 2020 ). Results revealed C, SiO 2 , SiS 2 and AlPO 4 as the predominant minerals in SSB. The characteristic peaks of carbon were detected corresponding to the presence of a number of aromatic carbon sheets and this phenomenon was confirmed from the FTIR results. Also, the components of SiO 2 , SiS 2 and AlPO 4 were responsible to immobilize Pb and Cr in soil. Li et al. ( 2018 ) found that SiO 2 could induce complexation and coordination with Cr(VI), the SiS 2 could induce reduction what was Cr(Ⅵ) reduce to Cr(Ⅲ) and the AlPO 4 were converted to Pb 5 (PO 4 ) 3 OH precipitations (Zhao et al. 2018 ). Based on the previous discussions, the potential mechanism of removal and passivation of Pb and Cr are surmised. Generally, SSB could decrease Pb and Cr toxicity and mobility due to form precipitations under alkaline conditions (Khan et al. 2020 ), on the other hand, abundant pore structures of SSB provide more adsorption sites for metal complexing and precipitating (Wang et al. 2018 ). Furthermore, the FTIR spectrum revealed that functional groups (such as –OH, C = O, C = C, et al.) of SSB could bind Pb and Cr via ion exchange, complexation and coordination (Yuan et al. 2020 ). XRD analysis indicated that SiO 2 and AlPO 4 could immobilize Pb and Cr via complexation and precipitation (Shakya et al. 2019; Zhao et al. 2018 ). To sum up, these characters of SSB could reduce the bioavailability and total contents of Pb and Cr in soil. Cost analysis of SSB In this study, SSB was prepared in a muffle furnace with an oxygen-limited pyrolysis method. Specific costs of raw materials, reagents and disposal were estimated, which provides a basis for large-scale production in the further. To date, more than 60 million tonnes of municipal sewage sludge were produced in China (Zhang et al. 2020 ). The raw materials were gathered with costless. Furthermore, the cost of electricity during the production procedures is 0.51 USD/kg (including pyrolysis and dry of biochar). Finally, the cost of chemicals (hydrochloric acid) during removing SSB impurities procedures is 0.71USD/kg. The cost of SSB has a low level compares with anterior reports and the price will lower when large-scale production (Cai et al. 2020). Therefore. the SSB has a great potential in HMs contaminated soil remediation, meanwhile, there are signs to investigate further. Conclusion The fraction of unstable and high risky and total content of Pb and Cr could be significantly decreased when inserting a SSB tubule in the contaminated soil, which was achieved via adsorption, ion exchange, complexation and precipitation, etc. This study revealed that have a better passivation effect of Pb and Cr when the soil moisture at 25% and 35%, respectively, [Pb: (F1) decreased by 4.9%, (F2) decreased by 14.8%, (F3) increased by 22.5%; Cr: (F1) decreased by 60.2%, (F3) increased by 21.5%), maximally]. The remediation effect is better of Pb and Cr when the moisture content at 25% and 35% (the total content decreased by 23.3% and 38.4%, maximally) which is located at outermost and innermost, respectively. Furthermore, SSB exhibited higher efficacy in remediating and passivating Cr contamination than Pb. Declarations Authors’ contributions LC and CF: Investigation, visualization and writing the original manuscript. QN and YW: Analysis and writing (review and editing). WP and HL: Supervision, resources and writing (review and editing). HB, CH and ML conducting experiments. All authors read and approved the final manuscript. Funding This study was supported by the National Natural Science Foundation of China (grant numbers 31670467, 51808089), Science and Technology Research Program of Chongqing Municipal Education Commission of China (grant number KJZDK201801202). Data and materials availability All data and materials generated or analyzed during this study are included in this article. Compliance with ethical standards Conflict of interest The authors declare that they have no competing interests. Ethics approval Not applicable. Consent to participate Not applicable. Consent for publication Not applicable. References Azeem M, Hassan TU, Tahir MI, Ali A, Jeyasundar PGSA, Hussain Q, Bashir S, Mehmood S, Zhang Z (2021) Tea leaves biochar as a carrier of Bacillus cereus improves the soil function and crop productivity. 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Environ Sci Pollut Res 27: 6312-6325. https://doi.org/10.1007/s11356-019-07353-7 Yuan S, Hong M, Li H, Ye Z, Gong H, Zhang J, Huang Q, Tan Z (2020) Contributions and mechanisms of components in modified biochar to adsorb cadmium in aqueous solution. Sci Total Environ 733: 139320. https://doi.org/10.1016/j.scitotenv.2020.139320 Zhang J, Hu H, Wang M, Li Y, Wu S, Cao Y, Lian P, Zhang J, Naidu R, Liu Y, Man YB, Wong MH, Zhang C, Shan S (2021) Land application of sewage sludge biochar: Assessments of soil-plant-human health risks from potentially toxic metals. Sci Total Environ 756: 114137. https://doi.org/10.1016/j.scitotenv.2020.144137 Zhang J, Jin J, Wang M, Naidu R, Liu Y, Man YB, Liang X, Wong MH, Christie P, Zhang Y, Song C, Shan S (2020) Co-pyrolysis of sewage sludge and rice husk/ bamboo sawdust for biochar with high aromaticity and low metal mobility. Environ Res 191: 110034. https://doi.org/10.10.1016/j.envres.2020.110034 Zhang W, Du W, Wang F, Xu H, Zhao T, Zhang H, Ding Y, Zhu W (2020) Comparative study on Pb 2+ removal from aqueous solutions using biochars derived from cow manure and its vermicompost. Sci Total Environ 716: 137108. https://doi.org/10.1016/j.scitotenv.2020.137108 Zhao N, Zhao C, Tsang DCW, Liu K, Zhu L, Zhang W, Zhang J, Tang Y, Qiu R (2021) Microscopic mechanism about the selective adsorption of Cr(VI) from salt solution on O-rich and N-rich biochars. J Hazard Mater 404: 124162. https://doi.org/10.1016/j.jhazmat.2020.124162 Zhao T, Yao Y, Li D, Wu F, Zhang C, Gao B (2018) Facile low-temperature one-step synthesis of pomelo peel biochar under air atmosphere and its adsorption behaviors for Ag(I) and Pb(II). Sci Total Environ 640-641: 73-79. https://doi.org/10.1016/j.scitotenv.2018.05.251 Zhou G, Gu Y, Yuan H, Gong Y, Wu Y (2020a) Selecting sustainable technologies for disposal of municipal sewage sludge using a multi-criterion decision-making method: A case study from China. Resour Conserv Recycl 161: 104881. https://doi.org/10.1016/j.resconrec.2020.104881 Zhou W, Yang F, Zhu R, Dai G, Wang W, Wang W, Guo X, Jiang J, Wang Z (2020b) Mechanism analysis of pore structure and crystalline phase of thermal insulation bricks with high municipal sewage sludge content. Constr. Build. Mater 263:120021. https://doi.org/10.10.1016/j.conbuildmat.2020.120021 Supplementary Files 1.Passivation.xlsx 2.Remediation.xlsx Cite Share Download PDF Status: Under Review Version 1 posted Reviews received at journal 14 Feb, 2021 Reviewers invited by journal 08 Feb, 2021 First submitted to journal 27 Jan, 2021 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-173906","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":11099942,"identity":"23de252d-e89b-40d0-a158-08a0ada1e990","order_by":0,"name":"Lin Chen","email":"","orcid":"","institution":"Chongqing Three Gorges University","correspondingAuthor":false,"prefix":"","firstName":"Lin","middleName":"","lastName":"Chen","suffix":""},{"id":11099943,"identity":"82f6b6f3-ffc9-441f-9baa-c2541b7d9a33","order_by":1,"name":"Qi Ni","email":"","orcid":"","institution":"Chongqing Three Gorges University","correspondingAuthor":false,"prefix":"","firstName":"Qi","middleName":"","lastName":"Ni","suffix":""},{"id":11099944,"identity":"0df6b6ce-60ef-43c7-82f1-885754f34626","order_by":2,"name":"Yan Wu","email":"","orcid":"","institution":"Chongqing Three Gorges University","correspondingAuthor":false,"prefix":"","firstName":"Yan","middleName":"","lastName":"Wu","suffix":""},{"id":11099945,"identity":"3ac5ff3a-a4d2-4951-a48c-2c37ea34447d","order_by":3,"name":"Chuan Fu","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA00lEQVRIiWNgGAWjYNCCAhseNvnDBw58+EG0FoM0OX4JtsSDM3uI13LYWHIGj/FhDjYiFPO3H38m8cOAOXHD7Z4Phxl4GOT5xQ7g1yJxJsdMsseALXHDnbMbDhdYMBjOnJ1AwEkMOWwSPAY8iRsO5G44PIOHIcHgNiEt/M+fSf4xkABqyXlwmIeNGC0SCWbSPAYGQO/nMBCnReLGG2NrGYMEOX6eYwbAQJYg7Bf+/vSHN99U/OdhY29+/OHDDxt5fmkCWjBsJU35KBgFo2AUjALsAAD20ETPnNuq4QAAAABJRU5ErkJggg==","orcid":"","institution":"Chongqing Three Gorges University","correspondingAuthor":true,"prefix":"","firstName":"Chuan","middleName":"","lastName":"Fu","suffix":""},{"id":11099946,"identity":"b74b1e72-7423-4819-82d9-408ae294c5e2","order_by":4,"name":"Wei Ping","email":"","orcid":"","institution":"Chongqing Three Gorges University","correspondingAuthor":false,"prefix":"","firstName":"Wei","middleName":"","lastName":"Ping","suffix":""},{"id":11099947,"identity":"80b12d48-8e78-4d89-b11f-872fed66d123","order_by":5,"name":"Hongyu Bai","email":"","orcid":"","institution":"Chongqing Three Gorges University","correspondingAuthor":false,"prefix":"","firstName":"Hongyu","middleName":"","lastName":"Bai","suffix":""},{"id":11099948,"identity":"65b41ff5-44ba-4f75-b9cf-9a22b77a0be6","order_by":6,"name":"Mengnan Li","email":"","orcid":"","institution":"Chongqing Three Gorges University","correspondingAuthor":false,"prefix":"","firstName":"Mengnan","middleName":"","lastName":"Li","suffix":""},{"id":11099949,"identity":"0615bdcb-b2d2-49ed-9b0a-bde59128a268","order_by":7,"name":"HongCheng Huang","email":"","orcid":"","institution":"Chongqing Three Gorges University","correspondingAuthor":false,"prefix":"","firstName":"HongCheng","middleName":"","lastName":"Huang","suffix":""},{"id":11099950,"identity":"15c02652-d15e-4859-a238-ec63044260d1","order_by":8,"name":"Hanshuang Liu","email":"","orcid":"","institution":"Chongqing Three Gorges University","correspondingAuthor":false,"prefix":"","firstName":"Hanshuang","middleName":"","lastName":"Liu","suffix":""}],"badges":[],"createdAt":"2021-01-28 11:46:18","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-173906/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-173906/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":5901638,"identity":"d2ea04f4-b3f9-426e-92a3-2c72f7196f9c","added_by":"auto","created_at":"2021-02-12 15:57:37","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":53995,"visible":true,"origin":"","legend":"A brief diagram of the experimental design","description":"","filename":"Fig1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-173906/v1/d2aea18267dd187a5a170f59.jpg"},{"id":5901517,"identity":"0c757e71-c8c0-4bc4-9cc4-79b36b336a93","added_by":"auto","created_at":"2021-02-12 15:54:37","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":109110,"visible":true,"origin":"","legend":"Fractions distribution of Pb (a-b)and Cr (c-d) in soil\nNotes: F1: Exchangeable; F2: Carbonate bound; F3: Fe-Mn oxides bound; F4: Organic matter bound; F5: Residual. \n(a) and (b): contaminated soil (Pb), at moisture was 25% and 35%; \n(c) and (d): contaminated soil (Cr), at moisture was 25% and 35%; \na and b: Fractions distribution of Pb or Cr from initially and finally; \nCN, SSB-F and SSB-S were showed in Table 3.","description":"","filename":"Fig2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-173906/v1/256807ad5010431d051c5d7d.jpg"},{"id":5901639,"identity":"d32bff14-de7f-44ed-ac17-610f036c5d92","added_by":"auto","created_at":"2021-02-12 15:57:37","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":534233,"visible":true,"origin":"","legend":"Concentration of total Pb (a-b) and Cr (c-d) during remediating","description":"","filename":"Fig3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-173906/v1/d64c181a926ec2d56dc18941.jpg"},{"id":5901752,"identity":"c7f91692-ed41-4049-ad1a-cd2daf46c9ac","added_by":"auto","created_at":"2021-02-12 16:00:37","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":99004,"visible":true,"origin":"","legend":"SEM images of SSB at different magnifications ","description":"","filename":"Fig4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-173906/v1/33ca33d8e2adeb0a42a8f474.jpg"},{"id":5901518,"identity":"82a3640f-a47c-4a98-88af-9ee72964ffbc","added_by":"auto","created_at":"2021-02-12 15:54:37","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":46973,"visible":true,"origin":"","legend":"FTIR spectra (a) and XRD pattern (b) of SSB","description":"","filename":"Fig5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-173906/v1/938be47de09d242c7daac2cf.jpg"},{"id":13660678,"identity":"ab25f36e-778f-447b-84f1-6dce89070c22","added_by":"auto","created_at":"2021-09-17 10:25:52","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":836864,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-173906/v1/a7bb6cbf-62a2-4f8d-a5f3-5da80265f099.pdf"},{"id":5901640,"identity":"9250fb3e-1e12-4b3b-8fc9-1d7edc0388c2","added_by":"auto","created_at":"2021-02-12 15:57:37","extension":"xlsx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":11948,"visible":true,"origin":"","legend":"","description":"","filename":"1.Passivation.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-173906/v1/c4cc12764ed64b71fd3b2eac.xlsx"},{"id":5901520,"identity":"460f267a-c7a8-4a3c-ab19-b7da4a6fd1fe","added_by":"auto","created_at":"2021-02-12 15:54:37","extension":"xlsx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":20452,"visible":true,"origin":"","legend":"","description":"","filename":"2.Remediation.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-173906/v1/2f170f710b321e43c9bb1641.xlsx"}],"financialInterests":"","formattedTitle":"\u003cp\u003ePassivation and Remediation of Pb and Cr in Contaminated Soil by Sewage Sludge Biochar Tubule\u003c/p\u003e","fulltext":[{"header":"Introduction","content":" \u003cp\u003eThe soil environment has been a global concern due to its complexity and significance. However, with the rapid urbanization and industrialization various contaminants [such as HMs, radioactive elements and organics, etc.] are being introduced into the soil system through directly discharged to contain more than the allowable standard contaminant concentrations (Wang et al. \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Rybak et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Feng et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). HMs have ranked the first among all soil contaminant types due to their high toxicity, bioaccumulation, persistence and mobility in soils (He et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Yang et al. \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). These HMs (Pb, Cr, Cd, Hg, As, etc.) can lead to potential health risks to predators and humans through the accumulation of food chains. For instance, Pb is a commonly recognized carcinogen and led to neurotoxicity, stomach and lung lesions; Cr(VI) is a strong oxidant and also act as carcinogenic and teratogenic characteristics, and both of them have been listed as priority monitoring and control pollutants (Hu et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Excess of Pb and Cr in soil is mainly derived from intensifying anthropogenic activities including mining, sewage irrigation, and pesticide abuse (He et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), which could lead to a deterioration in various functions and stability of soil systems (Duan et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn the last several decades, various remediation technologies, including phytoremediation, excavation, landfilling, electrokinetic remediation, soil washing and their blending, have been used to Pb and Cr contaminated soils for removing or reducing high toxic element amounts (Liu et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Sarwar et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Trellu et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). However, most of these technologies due to their long remediation cycle, high energy consumption, low efficiency and generating significant secondary environmental impacts (Gerhardt et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2009\u003c/span\u003e), which are unsuitable for the most area to decelerate their development. Accordingly, there is an urgent need for inexpensive, efficient and stable amendment materials.\u003c/p\u003e \u003cp\u003eTo date, various materials [resin (Chen et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2020\u003c/span\u003e); activated carbon (Dong et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2016\u003c/span\u003e); peat (Lee et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2015\u003c/span\u003e)] are used to adsorb HMs. Unfortunately, the resin has a potential risk with secondary pollution, activated carbon is expensive and peat has a characteristically low surface. Biochar is a material with carbon-rich, porous and high aromaticity which is obtained from the pyrolysis of biomass at relatively low temperature (\u0026lt;\u0026thinsp;700℃) under the presence of limited oxygen (Yu et al. \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Chen et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Biochar as an environmental sorbent has become one of the most attractive research hotspots due to having abundant raw materials, easy preparation and stable performance, etc. (Hung et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Zhang et al. \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Xiao et al.2019). In addition, biochar can increase crop yield through improve soil fertility and remediate soil by immobilizing HMs (Azeem et al. 2020; Xi et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Numerous researches have showed that biochar plays important role in decreasing the HMs (such as Cr, Cd, Pb, Cu and Zn, etc.) total and unstable concentration (Puga et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Gao et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Liu et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Thus, Biochar has great advantages as a green environmental sorbent in remediating HMs contaminated soil.\u003c/p\u003e \u003cp\u003eBiochar can be prepared from a wide variety of raw materials such as municipal sludge, livestock manure, crop straw (Al-Wabel et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). In recent years, municipal sewage sludge has increased sharply with the increasing improvement of sewage treatment facilities (Zhou et al. \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2020a\u003c/span\u003e), it is a by-product of the sewage treatment municipal and contains a variety of harmful substances (pathogens, refractory organics and toxic HMs, etc.), which is easy to cause secondary pollution (Zhou et al. \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2020b\u003c/span\u003e). Pyrolysis of sewage sludge is promising since it enables to decrease the harmful substances and volume. Meanwhile, the solid carbonaceous residues after pyrolysis (SSB) may be used as the amendment of HMs in soil (Zhang et al. 2018). At present, Some researches have showed that the excellent effect of sludge biochar on the remediation of HMs contaminated soil, For example, Penido et al. (\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) observed a significant reduction of Cd, Pb, and Zn bioavailability in HMs contaminated soil from a Zn-mining area by applying SSB. Fang et al. (\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) used the SSB to remediate soils that had been contaminated with cationic Pb(II), etc. and anionic Cr(VI), etc. respectively. However, there are still concerns regarding potential soil secondary contamination with toxic HMs due to SSB were thoroughly mixed with soil and require further investigations to the assessment of long-term risks (Fang et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Figueiredo et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Thereby, it is requisite to obtain a piece of equipment which is able to separate biochar from the soil when remediation ended.\u003c/p\u003e \u003cp\u003eIn this study, biochar derived from sewage sludge and was placed in polymethyl methacrylate (PMMA) tubule and as an amendment inserted HMs contaminated soils. Herein, Pb and Cr were selected as representative of toxic HMs. The objectives of this study are to 1) investigate the effect of SSB on the fractions of Pb and Cr in soil, to 2) determine the changes of the total content of Pb and Cr in soil with the remediation distance from the SSB tubule and time. The research results will provide some theoretical references for HMs-contaminated soil remediation and passivation by biochar in future research and application.\u003c/p\u003e "},{"header":"Material And Method","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n\u003ch2\u003eBiochar preparation\u003c/h2\u003e\n\u003cp\u003eIn this study, sewage sludge (SS) was collected from the dewatering workshop of the Xintian Wastewater Treatment Plant (XWTP) in Wanzhou District, Chongqing City, China, where an Orbal oxidation ditch wastewater treatment system is operated. The basic properties of sewage sludge were given in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e. The raw sewage sludge was dewatered through a vacuum pump (SHZ-DIII, Yuezong, China) and then was placed in an oven (DHG-9420A, Honghua, China) at a temperature of 103\u0026thinsp;\u0026plusmn;\u0026thinsp;2 ℃ to constant weight. The dried samples were ground and passed through 20-mesh nylon sieves, The samples were thoroughly mixed and stored in plastic bags for further study.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003ctable id=\"Tab1\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003eProperties of sewage sludge\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eMLSS mg\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eMLVSS / mg\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eSVI / mL\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eMoisture / %\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e2425.5\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e1659.4\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e128.5\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e99.1\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n\u003cp\u003eThe oxygen-limited pyrolysis method was used to prepare the biochar. In brief, approximate 50.0g of sewage sludge samples loaded in the crucible and covered with fitting lids and alumina foils, which were loaded into the muffle furnace (SX-10-12, Yiheng, China), the pyrolysis process was performed by raising the target temperature to 500\u0026deg;C with a heating rate of 10\u0026deg;C min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u0026nbsp;and residence time of 2 h. The obtained sewage sludge biochar (SSB) was grounded to pass through 100-mesh nylon sieves, and dip in hydrochloric acid for 24 h to eliminate effects of surface and internal impurities, and improve the porous structure of SSB (Liu et al.\u0026nbsp;\u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e), then the SSB was cleaned with deionized water until reaching a neutral pH. Finally, SSB was dried and store in plastic bags.\u003c/p\u003e\n\u003ch2\u003eCharacterization of biochar\u003c/h2\u003e\n\u003cp\u003eThe yield was calculated from the ratio between the mass of SSB and the raw materials. Ash was measured as the residual remaining after heating to 800\u0026deg;C and maintaining for 4 h. The pH values were measured with a pH meter (FE28, Mettler Toledo, China) and the method from wang et al. (2020). Levels of Pb and Cr were determined by inductively coupled plasma optical emission spectrometry (ICP-OES; Optima7000DV, Perkin Elmer, USA) after acid digestion according to Figueiredo et al. (2020). The surface structure of SSB was analyzed using a scanning electron microscopy (SEM; Supra55, Zeiss Germany). The surface functional groups of SSB were determined through Fourier transform infrared (FTIR) spectroscopy (Nicolet iS 10, Thermo Fisher Scientific, USA), the infrared spectra were obtained over the 4000\u0026thinsp;~\u0026thinsp;500 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The crystal structure of the SSB was characterized by X-ray diffraction (XRD; D8 ADVANCE, Bruker, Germany).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n\u003ch2\u003eTest soil collection and preparation\u003c/h2\u003e\n\u003cp\u003eSurface soil (0\u0026ndash;20 cm) was collected from Three Gorges Reservoir area located in Wanzhou District, Chongqing City, China (30\u003csup\u003e\u0026deg;\u003c/sup\u003e42\u003csup\u003e\u0026prime;\u003c/sup\u003e 53\u003csup\u003e\"\u003c/sup\u003e N, 108\u003csup\u003e\u0026deg;\u003c/sup\u003e25\u003csup\u003e\u0026prime;\u003c/sup\u003e 55\u003csup\u003e\"\u003c/sup\u003e E). The soil sample was thoroughly mixed, air-dried at room temperature, excess roots and gravels were removed, ground to pass through 20-mesh nylon sieves before initiating the experiments. The physiochemical properties of the soil were determined according to Chinese standard methods (Liu et al. \u003cspan class=\"CitationRef\"\u003e1996\u003c/span\u003e) and were given in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e\n\u003cp\u003eTo obtain a more significant remediation effect of SSB on Pb and Cr and reduce influences of other HMs in the soil, we replenish external Pb (Pb(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e) or Cr (K\u003csub\u003e2\u003c/sub\u003eCr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e) to the air-dried soil and to reach the First or Second type of Risk Intervention Values for Soil Contamination of Development Land (RIVSCDL) of China, respectively. Meanwhile, the soil moisture content was controlled at 25% or 35% in each test group. Finally, all test soils were allowed to incubate for 7 days before the experiments. The type of soils required and marking was given in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003ctable id=\"Tab2\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003eThe properties of the raw soil.\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eVolume-weight / (g\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eMoisture / %\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003epH\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eTotal Pb / (mg\u0026middot;kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eTotal Cr / (mg\u0026middot;kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e1.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e29.4\u0026thinsp;\u0026plusmn;\u0026thinsp;2.1\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e7.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e26.4\u0026thinsp;\u0026plusmn;\u0026thinsp;1.3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e35.6\u0026thinsp;\u0026plusmn;\u0026thinsp;1.5\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003ctable id=\"Tab3\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003eThe types of soils required\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eAdscititious HMs\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eValue / (mg\u0026middot;kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\n\u003cp\u003eexpectantly / actually\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eMoisture / %\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eAmendments\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eMarking\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003ePb\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e800 / 707.3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e25 and 35\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eRaw soil\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eCN\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003ePb\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e800 / 707.3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e25 and 35\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eSSB\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eSSB-F\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003ePb\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e2500 / 1998.2\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e25 and 35\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eSSB\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eSSB-S\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eCr\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e30 / 31.8\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e25 and 35\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eRaw soil\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eCN\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eCr\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e30 / 31.8\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e25 and 35\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eSSB\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eSSB-F\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eCr\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e80 / 76.5\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e25 and 35\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eSSB\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eSSB-S\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003ctfoot\u003e\n\u003ctr\u003e\n\u003ctd colspan=\"5\"\u003eNotes: CN: Control. HMs contaminated soil (concentration were controlled at the First type of RIVSCDL) which were remediated by raw soil.\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd colspan=\"5\"\u003eSSB-F and SSB-S: Treatment groups. HMs contaminated soil (concentration was controlled at the First and Second type of RIVSCDL) which were remediated by SSB.\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tfoot\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n\u003ch2\u003eExperimental design\u003c/h2\u003e\n\u003cp\u003eThe tested soils and SSB were placed in PMMA column [H (height)\u0026thinsp;=\u0026thinsp;10, D (diameter)\u0026thinsp;=\u0026thinsp;30 cm] and PMMA tubule (H\u0026thinsp;=\u0026thinsp;10, d\u0026thinsp;=\u0026thinsp;1.5 cm, many pores is full of the tubule wall), respectively, and the tubule was located center of the column (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e). To minimize the migration of Pb and Cr through the column sidewalls, PMMA columns were coated with a thin layer of paraffin wax and spread a layer of 1.5 cm gravel. Then, the soil was filled with layer upon layer and packed. The filling height up to 8.0 cm both soil and biochar and at a ratio of 1:600 (dry mass, biochar/soils). Finally, they were incubated in a lightproof, sealed, and room temperature sustain 35 days, soil samples were collected every 5 days and 8 times in total. Approximate 0.5 g of soil samples were collected at equal depth and distance to measure the total contents of Pb or Cr, and the needless soil was backfilled and compacted. After the last collection, SSB tubules were recycled, soil in the PMMA column was thoroughly mixed and measure the Pb and Cr concentration of fractions. All of the soil samples were dried, grounded to pass through 100-mesh nylon sieves and stores. Each experiment was implemented in duplicate. The schematic diagram of the experimental design was showed in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\n\u003ch2\u003eAnalyses of Pb and Cr\u003c/h2\u003e\n\u003cdiv id=\"Sec8\" class=\"Section3\"\u003e\n\u003ch2\u003eTotal contents of Pb and Cr\u003c/h2\u003e\n\u003cp\u003eIn this study, total Pb and Cr were determined using a microwave digester (Speedwave Xpert, Berghof, Germany). Particularly, samples (0.2 g) were digested with a dup-acid mixture of HF (1 mL) and HNO\u003csub\u003e3\u003c/sub\u003e (8 mL) according to Khadhar et al. (\u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e) and the residual solution through membrane filters (0.22 \u0026micro;m) and diluted to 50 mL.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec9\" class=\"Section3\"\u003e\n\u003ch2\u003eChemical fractions of Pb and Cr\u003c/h2\u003e\n\u003cp\u003ePb and Cr potentially toxic were measured according to Tessier sequential extraction method (Tessier et al. \u003cspan class=\"CitationRef\"\u003e1979\u003c/span\u003e), this method divides HMs into five fractions due to their are successively less bioavailability and mobility effect (Chen et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e). In brief, extraction steps were as follows: (F1) exchangeable (1mol/L MgCl\u003csub\u003e2\u003c/sub\u003e); (F2)carbonate bound (1 mol/L NaAc); (F3) Fe-Mn oxides bound (0.04 mol/L NH\u003csub\u003e2\u003c/sub\u003eOH\u0026middot;HCI in 25% HAc); (F4) organic matter bound [(0.02 mol/L HNO\u003csub\u003e3\u003c/sub\u003e), (30% H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e), (3.2 mol/L NH\u003csub\u003e4\u003c/sub\u003eAc in 20% HNO\u003csub\u003e3\u003c/sub\u003e)]; (F5) residual (digested with HF-HNO\u003csub\u003e3\u003c/sub\u003e acids). After each step, the sample was separated at 4000 rpm for 20min, supernatants were used for detecting the concentration of F1- F5 through membrane filters (0.22 \u0026micro;m). The Pb and Cr concentrations were measured by ICP-OES.\u003c/p\u003e\n\u003c/div\u003e\n\u003c/div\u003e"},{"header":"Results And Discussion","content":"\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\n\u003ch2\u003ePassivation effect of SSB on Pb and Cr\u003c/h2\u003e\n\u003cp\u003eHMs exist in the soil with diverse chemical fractions and the bioavailability and potentially toxic mainly depend on their specific fraction. F1 presents the most unstable and high risky and F2 slightly, F5 was identified as a stable fraction. bioavailability and toxic of those five fractions from high to low was F1\u0026thinsp;\u0026gt;\u0026thinsp;F2\u0026thinsp;\u0026gt;\u0026thinsp;F3\u0026thinsp;\u0026gt;\u0026thinsp;F4\u0026thinsp;\u0026gt;\u0026thinsp;F5 (Chen et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eFraction distribution of Pb and Cr obtained from Tessier sequential extraction method and were showed in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e which reveal the fraction distribution difference of initially and finally. In brief, the results showed that CN had a varied slightly of fraction distribution and SSB had was significant. Compare SSB-F with SSB-S groups. The F1 and F2 decreased by 25.1% and 16.8%, F3 increased by 18.5%; the F1 and F2 decreased by 28.8% and 2.9%, F3 increased by 9.8% (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea). The F2 and F3 decreased by 8.8% and 4.8%, F4 increased by 75.1%; the F1 decreased by 42.1%, F3 and F4 increased by 4.5% and 12.9% (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eb). The F1 decreased by 33.6%, F3 and F4 increased by 10.1% and 22.1%; the F1 decreased by 25.3%, F3 and F4 increased by 13.8% and 14.8% (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ec). The F1 decreased by 73.0%, F2, F3 and F4 increased by 13.2%, 23.9% and 30.8%; the F1 decreased by 37.2%, F3 and F4 increased by 25.4% and 33.5% (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ed). Munir et al. (\u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e) found that Cr and Pb be immobilized and removed by -OH and -COOH groups in biochar.\u003c/p\u003e\n\u003cp\u003eResults showed that SSB could decrease Pb and Cr bioavailability and mobility through transforming the F1 and F2 to more stable forms and that is better when the moisture at 25% and 35%, respectively, (Pb: F1, F2 decreased by 25.1%, 16.8%. Cr: F1 decreased by 73.0%, respectively). These results were compared with others. Liu et al. (\u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e) added 5% (w/w) modified coconut shell biochar to multi-metals contaminated soil in Mianzhu, Sichuan, China, which decreased the acid-soluble Cd, Ni and Zn concentration by 30.1%, 57.2% and 12.7%, respectively. Munir et al. (\u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e) added 2% (w/w) bamboo-biochar to Pb and Cr contaminated soil in Huainan, Anhui, China, which decreased their F1 concentration by 8.5% and 29%, respectively. Therefore, SSB tubules have a practical effect on soil passivation.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\n\u003ch2\u003eThe remediation effect of SSB on Pb and Cr\u003c/h2\u003e\n\u003cp\u003eThe variation of Pb and Cr total contents in the soil with the incubation period and distance were showed in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e (A line represents the variation of the Pb or Cr total content in this sampling point with time). Insert SSB tubules in contaminated soils resulted showed that Pb and Cr total contents were significantly decreased. Presented diverse trends under different experiment conditions which phenomenons indicate these conditions (moisture and pollution intensity) have a definite influence on the remediation results.\u003c/p\u003e\n\u003cp\u003eAt end of the remediation, results revealed that CN had a varied slightly of Pb and Cr concentration compares with initially (concentration decreased by -2.1%~8.0% and \u0026minus;\u0026thinsp;10.3%~4.0%). On the contrary, the effect were significantly of SSB-F and SSB-S [Pb concentration decreased by 8.5%~21.8% and 14.6%~23.3% (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea), 13.0%~17.3% and 8.2%~16.2% (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb); Cr concentration decreased by 9.5%~22.2% and 5.0%~15.5% (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec), 25.1%~38.4% and 24.8%~36.1% (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ed), respectively]. Maximum of Pb is located at the farthest and the closest point when moisture was 25% and 35%, respectively, and maximum of Cr located were showed in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec-d. Results indicated that a better remediation effect of Pb and Cr when the soil moisture content was 25% and 35% (concentration of Pb and Cr decreased by 23.3% and 38.4%, maximally). This remediation effect was compared with others. Wang et al. (\u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e) added 10% (w/w) kitchen waste-based biochar to Ba contaminated soil in landfill areas, Tibet, China, which decreased the concentration by 10.1%. Li et al. (\u003cspan class=\"CitationRef\"\u003e2016\u003c/span\u003e) added biochar to Cd contaminated soil, found decreased the total content by 46.4%. These studies indicate that added biochars could remove HMs in soil.\u003c/p\u003e\n\u003cp\u003eIn addition, for Pb contaminated soil, these points (\u0026le;\u0026thinsp;7.5cm) presented initial decrease, subsequent increased and final decreased with the remediation time, and others presented initial increased, finally decreased. These phenomena could be attributed to SSB had a rich porous structure (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eb). Lin et al. (\u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e) indicated biochars adsorb HMs are divided into the surface monolayer sorption initially and the intraparticle diffusion sorption later. Initially, SSB adsorbs Pb ion at closer points, with the adsorption persistent, Pb ion could gradually migrate to SSB tubule from a broader area, which leads to Pb ion short-dated increase and final continuous decrease, Mitzia et al. (\u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e) consider that cause this phenomenon may attribute multiple interactions (such as soil matrix and water content), which can provoke an Eh-pH fluctuation. For Cr contaminated soil, presented a similar trend for all lines which are rapid initial decrease, subsequent increase (starting on the 5th day), and final decrease (starting on the 20th day) and presented an indistinctive effect at remediation distance.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\n\u003ch2\u003eCharacteristics of SSB and its removal mechanisms of Pb and Cr\u003c/h2\u003e\n\u003cdiv id=\"Sec14\" class=\"Section3\"\u003e\n\u003ch2\u003eCharacteristics of SSB\u003c/h2\u003e\n\u003cp\u003eThe basic properties of SSB were given in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e. Results showed that yield and ash of SSB were higher compare with other biochar (biochars derived from rice straw, sawdust and phragmites etc.) which was mainly due to the SSB have high contents of inorganic constituents (Pellera et al. 2020) Furthermore, SSB was alkaline which was due to the surface of SSB including multitudinous alkaline aromatization groups and them could immobilize HMs through increase the pH of the soil (Mitzia et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e). Pellera et al. (2020) found that biochar leads to higher soil pH and ECE and causes metal precipitation, finally.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003ctable id=\"Tab4\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003eBasic properties of SSB\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eYield / %\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eAsh / %\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003epH\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eTotal Pb / mg\u0026middot;kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eTotal Cr / mg\u0026middot;kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e47.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e30.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e8.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e18.8\u0026thinsp;\u0026plusmn;\u0026thinsp;2.4\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e53.5\u0026thinsp;\u0026plusmn;\u0026thinsp;4.0\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec15\" class=\"Section3\"\u003e\n\u003ch2\u003eSEM analysis\u003c/h2\u003e\n\u003cp\u003eThe SEM was carried out to characterize the microstructure of SSB and different magnifications are showed in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e. Results showed that the SSB had uneven size distribution and visible loose fold (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea), and had a rich porous structure (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eb), which could be due to the breakdown of the volatile compounds at higher temperatures that leadto the energy (gas) to escape from sewage sludge (Shakya et al. 2019). These forming mesopores and micropores to obtain greater surface area and porous volume, these porous structures could provide enough adsorption area for metal binding (Wang et al. \u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e). Furthermore, visible inorganic ash particles were random distributed around of porous structure, Yuan et al. (\u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e) found that these particles would via ion exchange, complexation and precipitation reactions to reduce metal ions.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec16\" class=\"Section3\"\u003e\n\u003ch2\u003eFTIR spectra analysis\u003c/h2\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ea shows the infrared spectrum of SSB. Five main absorption peaks centered at 3734, 2360, 1540, 1015 and 775 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e were recorded. The peaks observed near 3734 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was attributed to the stretching and bending vibration of \u0026ndash;OH (Lin et al. \u003cspan class=\"CitationRef\"\u003e2017\u003c/span\u003e), the peaks at 2360 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e were corresponded to the stretching vibration of CO\u003csub\u003e2\u003c/sub\u003e (Lin et al. \u003cspan class=\"CitationRef\"\u003e2017\u003c/span\u003e) and the peaks at 1540 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was ascribed to the stretching vibration of C\u0026thinsp;=\u0026thinsp;O, C\u0026thinsp;=\u0026thinsp;C aromatic rings (Shin et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e), the peaks at 1015 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 775 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e were attributed to the stretching vibration of -OH and C-H of aromatic rings (Liu et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e), respectively. Indicating that SSB contained several kinds of functional groups, These groups play a heavy role to reduce Pb and Cr in soil have been widely reported, for example, Zhao et al. (\u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e) found that O-containing groups positive participation Cr(VI) removal, Wang et al. (\u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e) reported that -OH was involved in the interaction with Pb(II) ion by the ion exchange reaction.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec17\" class=\"Section3\"\u003e\n\u003ch2\u003eXRD analysis\u003c/h2\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003eb shows the mineralogical composition of SSB. Illustrating highly crystalline structures due to revealing numerous sharp peaks (Liu et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e). Results revealed C, SiO\u003csub\u003e2\u003c/sub\u003e, SiS\u003csub\u003e2\u003c/sub\u003e and AlPO\u003csub\u003e4\u003c/sub\u003e as the predominant minerals in SSB. The characteristic peaks of carbon were detected corresponding to the presence of a number of aromatic carbon sheets and this phenomenon was confirmed from the FTIR results. Also, the components of SiO\u003csub\u003e2\u003c/sub\u003e, SiS\u003csub\u003e2\u003c/sub\u003e and AlPO\u003csub\u003e4\u003c/sub\u003e were responsible to immobilize Pb and Cr in soil. Li et al. (\u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e) found that SiO\u003csub\u003e2\u003c/sub\u003e could induce complexation and coordination with Cr(VI), the SiS\u003csub\u003e2\u003c/sub\u003e could induce reduction what was Cr(Ⅵ) reduce to Cr(Ⅲ) and the AlPO\u003csub\u003e4\u003c/sub\u003e were converted to Pb\u003csub\u003e5\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003eOH precipitations (Zhao et al. \u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eBased on the previous discussions, the potential mechanism of removal and passivation of Pb and Cr are surmised. Generally, SSB could decrease Pb and Cr toxicity and mobility due to form precipitations under alkaline conditions (Khan et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e), on the other hand, abundant pore structures of SSB provide more adsorption sites for metal complexing and precipitating (Wang et al. \u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e). Furthermore, the FTIR spectrum revealed that functional groups (such as \u0026ndash;OH, C\u0026thinsp;=\u0026thinsp;O, C\u0026thinsp;=\u0026thinsp;C, et al.) of SSB could bind Pb and Cr via ion exchange, complexation and coordination (Yuan et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e). XRD analysis indicated that SiO\u003csub\u003e2\u003c/sub\u003e and AlPO\u003csub\u003e4\u003c/sub\u003e could immobilize Pb and Cr via complexation and precipitation (Shakya et al. 2019; Zhao et al. \u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e). To sum up, these characters of SSB could reduce the bioavailability and total contents of Pb and Cr in soil.\u003c/p\u003e\n\u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec18\" class=\"Section2\"\u003e\n\u003ch2\u003eCost analysis of SSB\u003c/h2\u003e\n\u003cp\u003eIn this study, SSB was prepared in a muffle furnace with an oxygen-limited pyrolysis method. Specific costs of raw materials, reagents and disposal were estimated, which provides a basis for large-scale production in the further. To date, more than 60\u0026nbsp;million tonnes of municipal sewage sludge were produced in China (Zhang et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e). The raw materials were gathered with costless. Furthermore, the cost of electricity during the production procedures is 0.51 USD/kg (including pyrolysis and dry of biochar). Finally, the cost of chemicals (hydrochloric acid) during removing SSB impurities procedures is 0.71USD/kg. The cost of SSB has a low level compares with anterior reports and the price will lower when large-scale production (Cai et al. 2020). Therefore. the SSB has a great potential in HMs contaminated soil remediation, meanwhile, there are signs to investigate further.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"Conclusion","content":" \u003cp\u003eThe fraction of unstable and high risky and total content of Pb and Cr could be significantly decreased when inserting a SSB tubule in the contaminated soil, which was achieved via adsorption, ion exchange, complexation and precipitation, etc. This study revealed that have a better passivation effect of Pb and Cr when the soil moisture at 25% and 35%, respectively, [Pb: (F1) decreased by 4.9%, (F2) decreased by 14.8%, (F3) increased by 22.5%; Cr: (F1) decreased by 60.2%, (F3) increased by 21.5%), maximally]. The remediation effect is better of Pb and Cr when the moisture content at 25% and 35% (the total content decreased by 23.3% and 38.4%, maximally) which is located at outermost and innermost, respectively. Furthermore, SSB exhibited higher efficacy in remediating and passivating Cr contamination than Pb.\u003c/p\u003e "},{"header":"Declarations","content":"\u003cdiv\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u0026rsquo; contributions \u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eLC and CF: Investigation, visualization and writing the original manuscript. QN and YW: Analysis and writing (review and editing). WP and HL: Supervision, resources and writing (review and editing). HB, CH and ML conducting experiments. All authors read and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding \u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study was supported by the National Natural Science Foundation of China (grant numbers 31670467, 51808089), Science and Technology Research Program of Chongqing Municipal Education Commission of China (grant number KJZDK201801202).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData and materials availability \u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data and materials generated or analyzed during this study are included in this article.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompliance with ethical standards \u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of interest \u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics approval \u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication \u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAzeem M, Hassan TU, Tahir MI, Ali A, Jeyasundar PGSA, Hussain Q, Bashir S, Mehmood S, Zhang Z (2021) Tea leaves biochar as a carrier of Bacillus cereus improves the soil function and crop productivity. 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Mater 263:120021. \u003ca href=\"https://doi.org/10.10.1016/j.conbuildmat.2020.120021\"\u003ehttps://doi.org/10.10.1016/j.conbuildmat.2020.120021\u003c/a\u003e\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"environmental-science-and-pollution-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"espr","sideBox":"Learn more about [Environmental Science and Pollution Research](https://www.springer.com/journal/11356)","snPcode":"11356","submissionUrl":"https://submission.nature.com/new-submission/11356/3","title":"Environmental Science and Pollution Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Biochar tubule, Sewage sludge, Soil heavy metals contaminated, Passivation and remediation","lastPublishedDoi":"10.21203/rs.3.rs-173906/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-173906/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eCurrently, numerous studies have carried out to research the effect of biochars remediation\u0026nbsp;soil heavy metals (HMs) contaminated, but there have been fewer explorations of the effect of biochars tubule on soil HMs remediation. This work aimed was to study the effect of passivation and remediation of lead (Pb) and chromium (Cr) contaminated soil after insert sewage sludge biochar (SSB) tubule. The results showed that the high risky fractions of Pb and Cr could be transformed into more stable fractions, also, Pb and Cr total contents are significantly decreased by SSB tubule. The mechanisms including adsorption, ion exchange, complexation and precipitation which are concluded from the characteristic analysis. Detailly, the passivation of Pb and Cr are better when the moisture at 25% and 35%, respectively, [Pb: exchangeable (F1), carbonate bound (F2) decreased by 25.1%, 16.8%, Fe-Mn oxides bound (F3) increased by 18.5%; Cr: F1 decreased by 73.0%, F2, F3, Organic matter bound (F4) increased by 13.2%, 23.9%, 30.8%), respectively]. The remediation of Pb and Cr is better when the moisture at 25% and 35%, respectively, (Pb: decreased by 23.3%; Cr: decreased by 38.4%, respectively). The findings showed that the SSB tubule is effective when used for soil HMs contaminated.\u003c/p\u003e","manuscriptTitle":"Passivation and Remediation of Pb and Cr in Contaminated Soil by Sewage Sludge Biochar Tubule","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2021-02-12 15:54:35","doi":"10.21203/rs.3.rs-173906/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"editorInvitedReview","content":"","date":"2021-02-15T00:00:00+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2021-02-09T00:00:00+00:00","index":"","fulltext":""},{"type":"submitted","content":"Environmental Science and Pollution Research","date":"2021-01-28T03:48:52+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"environmental-science-and-pollution-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"espr","sideBox":"Learn more about [Environmental Science and Pollution Research](https://www.springer.com/journal/11356)","snPcode":"11356","submissionUrl":"https://submission.nature.com/new-submission/11356/3","title":"Environmental Science and Pollution Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"a3692d71-e210-4583-a242-34200e5a4ad5","owner":[],"postedDate":"February 12th, 2021","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":2379918,"name":"Environmental Engineering"},{"id":2379919,"name":"Environmental Policy"}],"tags":[],"updatedAt":"2021-04-01T14:03:23+00:00","versionOfRecord":[],"versionCreatedAt":"2021-02-12 15:54:35","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-173906","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-173906","identity":"rs-173906","version":["v1"]},"buildId":"_2-kVJe1T_tPrBINL-cwx","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}