Sewage Sludge Biochar Research Articles

Characterization, fractionation and untapped potential of phosphate-amended sewage sludge biochar in soil-plant systems

Sewage sludge, a byproduct of wastewater treatment, poses serious environmental and health risks due to its content of organic contaminants, heavy metals, and pathogenic microorganisms. With the growing global production of municipal wastewater, finding effective methods for managing and disposing of sewage sludge has become increasingly urgent. Traditional methods such as land disposal, dumping, and incineration have limitations and environmental drawbacks. However, recent advancements have shown promise in the valorization of sewage sludge, particularly through pyrolysis, which converts it into biochar for use in soil amendment and pollutant mitigation. This study aims to characterize and fractionate phosphate-amended sewage sludge biochar produced at 300 °C, 400 °C, and 500 °C, and to evaluate its potential use in soil-plant systems. It examines nutrient bioavailability in soil after the addition of this biochar and its effects on plant growth. The pyrolysis process resulted in biochar with high alkalinity (7.2–11.1), ash content ranging from 56.9% to 87.3%, and significant phosphorus retention, with phosphorus concentrations increasing with pyrolysis temperature (5.35%–9.38%). Phosphorus fractionation showed a shift toward more stable fractions particularly at 500 °C. Soil incubation experiments indicated increased phosphorus availability with HCl-extractable P showing a high extraction efficiency of up to 94.95%. In plant growth experiments, the amended biochar significantly enhanced growth, with corn showing an increase of up to 28.8% and wheat showing an increase of up to 86% compared to the control in the first four weeks after emergence. These findings indicate that phosphate-amended sewage sludge biochar enhances nutrient availability and supports plant growth, providing a sustainable solution for sewage sludge management, contributing to soil improvement and carbon sequestration, thereby addressing global environmental challenges.

Effects of Sewage Sludge Biochar and a Seaweed Extract-Based Biostimulant on Soil Properties, Nutritional Status and Antioxidant Capacity of Lettuce Plants in a Saline Soil with the Risk of Alkalinization

Effects of Sewage Sludge Biochar and a Seaweed Extract-Based Biostimulant on Soil Properties, Nutritional Status and Antioxidant Capacity of Lettuce Plants in a Saline Soil with the Risk of Alkalinization

Effect of Sewage Sludge, Sewage Sludge Compost and Sewage Sludge Biochar on Heavy Metal and It Fractions

Effect of Sewage Sludge, Sewage Sludge Compost and Sewage Sludge Biochar on Heavy Metal and It Fractions

Effects of Sewage Sludge Biochar (Sewchar) Application on Nutrient Uptake and Yield of African Marigold

Effects of Sewage Sludge Biochar (Sewchar) Application on Nutrient Uptake and Yield of African Marigold

Effect of metal-modified sewage sludge biochar tubule on immobilization of chromium in unsaturated soil: Groundwater table fluctuations induced by rainfall

Many studies have studied biochar immobilizing chromium (Cr) in soil. However, few studies were conducted to reduce the environmental risks due to biochar aging in soil. In this study, we adopt FeCl3, MgCl2, and AlCl3 to activate sewage sludge to form modified biochar and produce biochar tubules. Then, the column experiments were carried out to study the effect of fluctuating groundwater table on Cr leaching behavior, total Cr, and fractions distribution with the insertion of biochar tubule. Results showed that the Cr immobilization performance was improved by metal-modification biochar, the biochar tubules can significantly decrease the Cr leaching amounts, retard the Cr downward migration in the soil, and there was a better effect with slightly Cr-contaminated soil. In addition, the immobilization effect is also impacted by the biochar's application mode and the hydrodynamic conditions. Detailedly, the Cr leaching amounts maximally decreased by 33.39%, the residual amounts maximally increased by 10.05% in the soil column, and the exchangeable (EX) and carbonates-bound (CB) fractions were maximally increased by 85.18%, 151.78% at the equal depth of soil column, respectively. BET, SEM-EDS, XRD, and FTIR analyses revealed that biochars' immobilization mechanisms on Cr involved reduction(predominately), physisorption, precipitation, and complexation. Risk assessment demonstrated that the sewage sludge biochar has very low environmental risk. This study indicates that the biochar tubule applied to immobilize Cr in soil has potential and provides new insights into reducing environmental risks due to biochar aging.

Adsorptive removal of diclofenac on nanoporous anoxic sewage sludge biochar: Investigating the influence of carbonization temperature, residence time and specific Fe(III) infusion

This study explores the removal of the emerging contaminant diclofenac using pristine biochar and iron-impregnated biochar derived from anoxic sewage sludge. A biochar ensemble was prepared at varying pyrolysis temperatures (350 °C to 950 °C) and residence times (30 to 240 min). The iron-modified material was synthesized by incorporating FeCl3 with the precursor at 950 °C for 30 min. A comprehensive characterization based on the morphology, crystallinity, and functional groups confirmed the structural and chemical modifications in the pristine and modified biochar of interest. Adsorption experiments demonstrated that iron-modified biochar exhibited superior adsorption capacity for diclofenac (18.958±0.417 mg/g) compared to pristine biochar (0.814±0.001 mg/g), attributable to its larger surface area and more active sites. The adsorption mechanisms were identified as a combination of electrostatic interactions, hydrogen bonding, and complexation, with significant Fe3+ to Fe2+ reduction indicating electron transfer processes. Adsorption kinetics followed the Elovich model, and equilibrium studies fitted best to the Redlich-Peterson model for the iron-amended biochar. The study also assessed the reusability of the adsorbents, with pristine and iron-impregnated biochar maintaining around 70 % removal efficiency up to 3 and 5 cycles, respectively. Consequently, both the biochar serve as potentially efficient adsorbents for diclofenac removal from aqueous solutions.

Enhanced Cu2+ and Cd2+ removal by a novel co-pyrolysis biochar derived from sewage sludge and phosphorus tailings: adsorption performance and mechanisms.

The reutilization of municipal wastes has always been one of the hottest subjects of sustainable development study. In this study, a novel biochar co-pyrolyzed from municipal sewage sludge and phosphorus tailings was produced to enhance the adsorption performance of the composite on Cu2+ and Cd2+. The maximum Cu2+ and Cd2+ adsorption capacity of SSB-PT were 44.34 and 45.91mg/g, respectively, which were much higher than that of sewage sludge biochar (5.21 and 4.58mg/g). Chemisorption dominated the whole adsorption process while multilayer adsorption and indirect interaction were also involved. According to the result of X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectrum (XPS), the load of CO32-, Mg2+, and Ca2+ on the surface of SSB-PT enhanced the precipitation and ion exchange effect. Posnjakite and CdCO3 were formed after the adsorption of Cu2+ and Cd2+, respectively. Besides, complexation, and metal-π interaction were also involved during the adsorption process. Therefore, this study offered a promising method to reuse sewage sludge and phosphorus tailings as an effective adsorbent.

Heavy Metals in Pyrolysis of Contaminated Wastes: Phase Distribution and Leaching Behaviour

Pyrolysis is a recognized alternative for the sustainable management of contaminated organic waste, as it yields energy-rich gas, oil, and a carbon-rich biochar product. Low-volatility compounds, however, such as heavy metals (HMs; As, Cd, Cu, Cr, Ni, Pb, and Zn) typically accumulate in biochars, limiting their application potential, especially for soil improvement. The distribution of HMs in pyrolysis products is influenced by treatment temperature and the properties of both the HMs and the feedstock. There is a significant knowledge gap in our understanding of the mass balances of HMs in full-scale industrial pyrolysis systems. Therefore, the fate of HMs during full-scale relevant pyrolysis (500–800 °C) of seven contaminated feedstocks and a clean wood feedstock were investigated for the first time. Most of the HMs accumulated in the biochar (fixation rates (FR) >70%), but As, Cd, Pb, and Zn partly partitioned into the flue gas at temperatures ≥ 600 °C, as demonstrated by FRs of <30% for some of the feedstocks. Emission factors (EFs, mg per tonne biochar produced) for particle-bound HMs (<0.45 µm) were 0.04–7.7 for As, 0.002–0.41 for Cd, 0.01–208 for Pb, and 0.09–342 for Zn. Only minor fractions of the HMs were found in the condensate (0–11.5%). To investigate the mobility of HMs accumulated in the biochars, a novel leaching test for sustained pH drop (at pH 4, 5.5 and 7) was developed. It was revealed that increasing pyrolysis temperature led to stronger incorporation of HMs in the sludge-based biochar matrix: after pyrolysis at 800 °C, at pH 4, <1% of total Cr, Cu, Ni, and Pb and < 10% of total As and Zn contents in the biochars were leached. Most interestingly, the high HM mobility observed in wood-based biochars compared to sewage-sludge-based biochars indicates the need to develop specific environmental-management thresholds for soil application of sewage-sludge biochars. Accordingly, more research is needed to better understand what governs the mobility of HMs in sewage-sludge biochars to provide a sound basis for future policy-making.

Growth, nutrient uptake, blood metabolites and bone properties in broilers consuming feed with mineral-enriched whole black soldier fly larvae

Abstract The recycling of minerals is crucial for the future circular agriculture. Black soldier fly larvae (BSFL) can accumulate minerals in their body. This study investigated the effects of adding mineral-enriched BSFL, grown on substrates containing sewage sludge recyclates (SSR), to broiler feed to reintroduce minerals from waste streams currently subject to regulatory restrictions back into the nutrient cycle. Feed, nutrient, mineral and heavy metal intake, growth, blood metabolites and immunoglobulins, bone characteristics and mineral status of broilers were studied in response to different mineral-enriched BSFL supplements. Eighty newly-hatched mixed-sex Ross 308 chicks were divided into four groups, with six replicate pens per group. BSFL used in the broiler experiment were grown either on a modified Gainesville fly diet (FD) (L-FD) or on FD supplemented with 4% of sewage sludge biochar (L-BCH), or on the FD supplemented with Single Superphosphate (SSP) SSR (L-SSP). All broilers were fed age-specific diets and either had no access to BSFL (CON) or received 15% of CON birds’ feed intake as defrosted BSFL from three different sources. Inclusion of 15% of mineral-enriched whole BSFL in broiler rations had no adverse effects on growth performance parameters, nutrient intakes, nutrient conversion efficiency, plasma metabolites and immunoglobulins (). Birds in BSFL supply groups had higher serum Ca concentrations than CON birds (). L-BCH supplied birds had a lower serum P than CON birds (). Tibial characteristics and mineral status of birds were not affected by larvae supply (). Heavy metal intake (manganese, iron, zinc, copper, arsenic, cadmium, lead, and mercury) of the birds was not affected by dietary treatments (). In conclusion, 15% of mineral-enriched-BSFL reared on SSP can be included in broiler diets for 42 experimental days without adverse effects on nutrient intakes, growth performance parameters and bone condition.

Enhancing in-situ tetracycline degradation in urine environment using boron-modified sewage sludge biochar: Mechanism of carbonate radical promotion

In this work, we reveal the influencing factors and mechanisms of tetracycline removal by peroxydisulfate activated with boron-modified sewage sludge biochar in urine. Advanced characterization techniques showed that the introduced BCO2 and BC2O were the main activation sites for peroxydisulfate, which maintained a high removal rate of tetracycline in synthetic and actual urine (86.48% and 77.35%). HCO3−/CO32− and some organic matter in the urine contributed to the degradation of the tetracycline. Theoretical calculations showed that CO32− in urine could be efficiently adsorbed by the introduced B atoms and converted to CO3·− by peroxydisulfate to assist the degradation of tetracycline, which was attributed to the special electronic structure of the B atoms as well as the strong electron-enrichment ability of the neighboring O atoms. The potential toxicity of tetracycline and its degradation intermediates was further explored, revealing a significant reduction in toxicity during boron-modified sewage sludge biochar/ peroxydisulfate treatment.

Sewage sludge biochar as a sustainable and water-safe substrate additive for extensive green roofs

Article describes the application of nutrient-rich sewage sludge (SS) biochar as an innovative additive to extensive green roof substrate at application rates of 10 (SB10) and 20% (v/v) (SB20) and its long-term impact on discharge quality. In addition to a routine analysis of pH, EC, TSS, and COD in the discharge, concentrations of nutrients (total nitrogen and total phosphorus) and metals (Cd, Cu, Fe, Mn, Pb, and Zn) were used to monitor leaching from the experimental green plots. SS biochar improves the water-holding capacity and air-filled porosity of the developed substrates SB10 and SB20, where the chemical and physical parameters meet the minimum requirements for multi-layer extensive roof substrate. The addition of biochar didn't substantially increase the concentrations of nutrients in discharge compared to the conventional extensive substrate, even after 30 months. Results suggested that biochar tends to release nutrients gradually when leached out by rain and may therefore serve as a source of P and other micronutrients. Although an increase in the concentrations of metals (Cu, Pb, and Zn) in the SB10 and SB20 substrates was observed, no significant differences in metal leaching were found. Metal concentrations in the aboveground parts of Sedum plants indicated that the pyrolysis of SS transforms water-extractable forms of metals into stable forms, thereby significantly reducing their bioavailability to Sedum species and any direct ecotoxicological risk. Based on our results, we assert that SS biochar could be used as a sustainable, valuable and water-safe component of extensive substrates for green roofs.

The Removal of Phosphorus from Wastewater Using a Sewage Sludge Biochar: A Column Study

This scientific study investigated the adsorption capabilities of biochar samples derived from municipal sewage sludge pellets. Sewage sludge was pyrolyzed at various temperatures (400, 500, and 600 °C), and the biochar’s properties, including specific surface area, pore volume, and pore size distribution, were assessed. The results indicate that the sewage sludge biochar samples are mesoporous materials with significant potential for good adsorption performance. Despite showing a decrease in specific surface area compared to that achieved with pyrolysis at 400 °C, samples pyrolyzed at 600 °C demonstrated an increase in mesopore surface area, enhancing their adsorption potential. Two filtration experiments, conducted at a flow rate of 8 mL/min, revealed that the column containing sewage sludge pyrolyzed at 600 °C retained phosphorus the most effectively during the first and second experiments (with retention efficiencies of 87% and 78%, respectively). The study concludes that municipal sewage sludge biochar could be a promising material for the removal of phosphorus from wastewater and represents a viable solution for sustainable environmental development.

An integrated constructed wetland-Microbial fuel cell system with sewage sludge-biochar to enhance treatment and energy recovery efficiencies

Constructed wetlands and microbial fuel cells (CW-MFCs) are a promising green and sustainable wastewater treatment technology. However, the power production performance of CW-MFCs has long been limited by low-performance cathodes. In this study, we designed two CW-MFC systems using cheap and readily available sewage sludge biochar (SSB) as the substrate for the experimental units. The integration of SSB significantly facilitated the removal of chemical oxygen demand, ammonium nitrogen, total nitrogen, and total phosphorus. Moreover, SSB enhanced the electrochemical performance of the air cathode, resulting in higher power density (44.64 mW/m2) and lower internal resistance (900.00 Ω) compared to gravel-integrated CW-MFC (0.88 mW/m2, 2057.14 Ω). In addition, a typical electroactive bacterium---Geobacter was enriched in the SSB-integrated CW-MFC system. The iron oxides and graphic nitrogen of the SSB favor the growth of Geobacter, and the redox-active groups on the surface of SSB accelerate the extracellular electron transfer process, contributing to the enhancement of treatment performance. Meanwhile, the metal and nitrogen doping improves the oxygen reduction reaction activity and conductivity of SSB, enhancing the cathode area of CW-MFC and improving the energy recovery. Overall, these results demonstrated that SSB-integrated CW-MFC can be a competitive technology for effective wastewater treatment and energy recovery.

Stabilization of PFAS-contaminated soil with sewage sludge- and wood-based biochar sorbents

Sustainable and effective remediation technologies for the treatment of soil contaminated with per- and polyfluoroalkyl substances (PFAS) are greatly needed. This study investigated the effects of waste-based biochars on the leaching of PFAS from a sandy soil with a low total organic carbon content (TOC) of 0.57 ± 0.04 % impacted by PFAS from aqueous film forming foam (AFFF) dispersed at a former fire-fighting facility. Six different biochars (pyrolyzed at 700–900 °C) were tested, made from clean wood chips (CWC), waste timber (WT), activated waste timber (aWT), two digested sewage sludges (DSS-1 and DSS-2) and de-watered raw sewage sludge (DWSS). Up-flow column percolation tests (15 days and 16 pore volume replacements) with 1 % biochar indicated that the dominant congener in the soil, perfluorooctane sulphonic acid (PFOS) was retained best by the aWT biochar with a 99.9 % reduction in the leachate concentration, followed by sludge-based DWSS (98.9 %) and DSS-2 and DSS-1 (97.8 % and 91.6 %, respectively). The non-activated wood-based biochars (CWC and WT) on the other hand, reduced leaching by <42.4 %. Extrapolating this to field conditions, 90 % leaching of PFOS would occur after 15 y for unamended soil, and after 1200 y and 12,000 y, respectively, for soil amended with 1 % DWSS-amended and aWT biochar. The high effectiveness of aWT and the three sludge-based biochars in reducing PFAS leaching from the soil was attributed largely to high porosity in a pore size range (>1.5 nm) that can accommodate the large PFAS molecules (>1.02–2.20 nm) combined with a high affinity to the biochar matrix. Other factors like anionic exchange capacity could play a contributing role. Sorbent effectiveness was better for long-chain than for short-chain PFAS, due to weaker, apolar interactions between the biochar and the latter's shorter hydrophobic CF2-tails. The findings were the first to demonstrate that locally sourced activated wood-waste biochars and non-activated sewage sludge biochars could be suitable sorbents for the ex situ stabilization and in situ remediation of PFAS-contaminated soil, bringing this technology one step closer to full-scale field testing.

Effect of carbon dioxide on pyrolytic products characteristics and DOM binding heavy metals copper in sewage sludge biochar

In this study, a new method of calculation and comparison on pyrolysis products is introduced in a sealed tube furnace under different pyrolysis conditions, to explore the effects of carbon dioxide (CO2) as carrier gas on biochar characteristics. In general, the pyrolysis temperature exhibited a more significant impact on biochar characteristics than residence time. The dissolved organic carbon of biochar derived in CO2 reduced by 53.5 % and 81.6 % for 30 min pyrolysis at 500 °C and 700 °C, respectively. In addition, CO2 could reduce the total heavy metals content in biochar compared with nitrogen atmosphere under the same treatment condition. Meanwhile, the soluble microbial by-products and the humic acid-like substances were more susceptible in pyrolysis with CO2, whereas the protein-like substance containing tryptophan, tryptophan related to biological and humic-like substance involved in binding copper. Actually, it was the Fact Sage thermodynamic simulation combined with main syngas composition that identified the CO2 mainly affected the carbonaceous distribution via the dehydrogenation, methane production and Boudouard reaction, made the proportion of carbon monoxide in syngas up to 18.5 % at 700 °C. These results can provide an important path for further applying CO2 as a raw material and enhance the feasibility of sewage sludge biochar application in farmland soil.

Synthesis of sewage sludge biochar in molten salt environment for advanced wastewater treatment: Performance enhancement, carbon footprint and environmental impact reduction

Sewage sludge (SS) pyrolysis to produce biochar is a vital approach for treating and utilizing SS, while reducing the carbon footprint of SS disposal. However, the high inorganic content in SS results in low carbon content and underdeveloped pore structure of biochar prepared under inert atmospheres. There is a significant risk of secondary pollutant emissions, including CO2, SO2, and NOx. In this study, we propose an innovative approach that utilizes excess molten salts, specifically a Li-Na-K molten carbonate (MC) and a Li-Na-K molten chloride (MCH), to create a medium-temperature liquid phase reaction environment (500 °C) for SS pyrolysis. This environment promotes the functional enhancement of biochar (SSB-MC and SSB-MCH) and in-situ absorption of secondary pollutants. The pore structure of SSB-MC and SSB-MCH are greatly optimized. Thanks to the dissolution of calcium-silicon-aluminum-based minerals by molten salt, the carbon content is also significantly increased. The increased specific surface area and surface-enriched functional groups (O, N, P, etc.) of SSB-MC result in greatly enhanced adsorption performance for Rhodamine B (27.9 to 89.1 mg g−1). SSB-MCH, due to the increased iron and phosphorus doping, also exhibits enhanced Fenton oxidation capability. Life cycle assessments demonstrate that the molten salt processes effectively reduce the carbon footprint, energy consumption, and environmental impact.

A combination of conventional extraction and advanced analytical techniques afford a comprehensive understanding of phosphorus distribution and transformation in sewage sludge biochars

Pyrolysis of sewage sludge (SS) to SS biochars (SBCs) is a promising method for maximizing the utilization of phosphorus (P) in SS. A comprehensive understanding of P distribution and transformation in SS and SBCs is fundamental for the recycling of P from these materials as a soil amendment. Most studies have focused on P itself in SS and SBCs, without examining the changes of closely related metals (e.g., Al, Ca, Fe and Mg) that are typically co-extracted with P. Thus, in this study, a series of investigations were performed in which an improved Hedley sequential extraction method of P was used together with advanced analytical techniques to determine the distribution and transformation of P in SS and SBCs. Increasing the pyrolysis temperature increased concentrations of total P and inorganic P in SS and SBCs. In addition, P and Al species in SS and SBCs were predominantly sodium hydroxide-soluble species—denoted NaOH-Al and NaOH-P—and increasing the pyrolysis temperature increased their concentrations in SBCs, especially for NaOH-Al, from 28.35 ± 2.56 mg/g to 38.18 ± 3.29 mg/g. Ca, Mg and Fe were predominantly extracted by HCl solution, showing that HCl-extracted P comprised not only Ca-P but also Fe-P, Mg-P and Al-P. Spectroscopic analyses revealed that the concentration of hydroxyapatite (HAp) in SS and SBCs increased with the pyrolysis temperature, and Al-P was the dominant species in both SS and SBCs. Thus the pyrolysis of SS affords SBCs that are a relatively stable pool of P for plants, enabling SBCs to serve as a slow-release P fertilizer in soil. These findings further contribute to fundamental knowledge on the recycling of P during integrated waste management.

Comparative assessment of phosphate adsorption properties and mechanisms on Mg/Al-engineered sewage sludge biochar in aqueous solution

This study reported the development of Mg/Al-engineered sewage sludge biochars (SSBCs) with adjustable surface properties and enhanced phosphate PO43− (ad)sorption capacity. These biochars successfully addressed the traditional constraints of electric repulsion interactions with PO43− ions at various pH levels, leading to highly efficient and recyclable PO43− enrichment from aqueous solution. It was determined that a ratio of 1:2 of Mg/Al is the optimal concentration of Mg/Al SSBCs for maximum PO43− (ad)sorption capability. In this study, the pseudo-second-order kinetics and Redlich Peterson isotherm worked well for Mg-SSBC, however, the pseudo-first-order kinetics and Langmuir isotherm explained the Al-SSBC interaction with PO43− ions properly. Both Mg-SSBC and Al-SSBC showed strong adsorption specificity to PO43− ions, with all coexisting anions producing negative effects on the PO43− adsorption. The data demonstrate that the variation in adsorption mechanism viz., including electrostatic attraction, complex formation, cationic bridging, and chemical co-precipitation was observed in between Mg-SSBC and Al-SSBC. Comparison between Mg-SSBC and Al-SSBC showed that Mg-SSBC has a more extensive pH range for PO43− removal and greater stability with compound elimination power, i.e., Mg-SSBC is more effective for the elimination of phosphates from aqueous solution.

Dynamics of potassium released from sewage sludge biochar fertilizers in soil

The solid product of sewage sludge (SS) pyrolysis, called SS biochar (SSB), is rich in carbon and nutrients, such as phosphorus (P), nitrogen (N), calcium (Ca), and zinc (Zn). However, SSB has a low potassium (K) concentration because it is released with water during the final stage of sewage treatment. The enrichment of SSB with mineral sources of K can solve the low supply of K in SSB and produce an organomineral fertilizer with a slow release of K. However, the dynamics of K release from these enriched fertilizers in different soil types remain unclear. This study investigated the dynamics of K release from biochar-based fertilizer (BBF) in the form of pellets and granules in two soil types (clayey and sandy) and natural silica. An incubation experiment was conducted for 60 days, and replicates were evaluated at prescribed time intervals. After the incubation period, the levels of K available in the solid fraction were determined, and the dynamics of K release were evaluated using four nonlinear regression models. BBFs achieved a slower release of K than the mineral KCl. The dynamics of K release were affected by the physical form of BBF, such that the pelleted BBF exhibited the slowest K release. Furthermore, regarding the concentration detected in the solid phase, the total released was highest in clayey soil, followed by sandy soil and natural silica. The enriched BBFs reduced K release throughout the experimental period, behaving as slow-release fertilizers with the potential to optimize K uptake by plants throughout the growth cycle. Further studies are required to evaluate K leaching and retention in the soil profile when biochar-based fertilizers are applied.

Effects of ferrous sulfate modification on the fate of phosphorous in sewage sludge biochar and its releasing mechanisms in heavy metal contaminated soils.

Modifications of sludge biochar with metal-based materials can enhance its fertilizing efficiency and improve safety. To elucidate the effects of ferrous sulfate modification on the fate of phosphorus in sludge biochar and its effect on phosphorus fractionation in soil, we investigated the changes in fractionation and bioavailability of phosphorus in modified sludge biochar and studied the changes in soil characteristics, microbial diversity and response, bioavailability, plant uptake of phosphorus, and heavy metals in contaminated soils after treatment with ferrous sulfate modified sludge biochar. The results demonstrated that ferrous sulfate modifications were conducive to the formation of moderately labile phosphorus in sludge biochar, and the concentrations increased by a factor of 2.7 compared to control. The application of ferrous sulfate-modified sludge biochar to alkaline heavy metal-contaminated soils enhanced the bioavailable, labile, and moderately labile phosphorus contents by a factor of 2.9, 3.0, and 1.6, respectively, whereas it obviously reduced the leachability and bioavailability of heavy metals in soils, exhibited great potentials in the fertilization and remediation of actual heavy metal-contaminated soils in mining areas. The biochar-induced reduction in soil pH, enhancement of organic matter, surface oxygen-containing functional groups, the abundance of Gammaproteobacteria, and its phosphonate degradation activity were primarily responsible for the solubilization of phosphorus from modified biochar in heavy metal-contaminated soils.