A comparative study on biochar properties and Cd adsorption behavior under effects of ageing processes of leaching, acidification and oxidation

A systematic review of biochar aging and the potential eco-environmental risk in heavy metal contaminated soil

Biochar is widely accepted as a green and effective amendment for remediating heavy metals (HMs) contaminated soil, but its long-term efficiency and safety changes with biochar aging in fields. Currently, some reviews have qualitatively summarized biochar aging methods and mechanisms, aging-induced changes in biochar properties, and often ignored the potential eco-environmental risk during biochar aging process. Therefore, this review systematically summarizes the study methods of biochar aging, quantitatively compares the effects of different biochar aging process on its properties, and discusses the potential eco-environmental risk due to biochar aging in HMs contaminated soil. At present, various artificial aging methods (physical aging, chemical aging and biological aging) rather than natural field aging have been applied to study the changes of biochar's properties. Generally, biochar aging increases specific surface area (SSA), pore volume (PV), surface oxygen-containing functional group (OFGs) and O content, while decreases pH, ash, H, C and N content. Chemical aging method has a greater effect on the properties of biochar than other aging methods. In addition, biochar aging may lead to HMs remobilization and produce new types of pollutants, such as polycyclic aromatic hydrocarbons (PAHs), environmentally persistent free radicals (EPFRs) and colloidal/nano biochar particles, which consequently bring secondary eco-environmental risk. Finally, future research directions are suggested to establish a more accurate assessment method and model on biochar aging behavior and evaluate the environmental safety of aged biochar, in order to promote its wider application for remediating HMs contaminated soil.

Physicochemical properties and performance of non-woody derived biochars for the sustainable removal of aquatic pollutants: A systematic review

Biochar is a carbon-rich material produced from the partial combustion of different biomass residues. It can be used as a promising material for adsorbing pollutants from soil and water and promoting environmental sustainability. Extensive research has been conducted on biochars prepared from different feedstocks used for pollutant removal. However, a comprehensive review of biochar derived from non-woody feedstocks (NWF) and its physiochemical attributes, adsorption capacities, and performance in removing heavy metals, antibiotics, and organic pollutants from water systems needs to be included. This review revealed that the biochars derived from NWF and their adsorption efficiency varied greatly according to pyrolysis temperatures. However, biochars (NWF) pyrolyzed at higher temperatures (400-800 °C) manifested excellent physiochemical and structural attributes as well as significant removal effectiveness against antibiotics, heavy metals, and organic compounds from contaminated water. This review further highlighted why biochars prepared from NWF are most valuable/beneficial for water treatment. What preparatory conditions (pyrolysis temperature, residence time, heating rate, and gas flow rate) are necessary to design a desirable biochar containing superior physiochemical and structural properties, and adsorption efficiency for aquatic pollutants? The findings of this review will provide new research directions in the field of water decontamination through the application of NWF-derived adsorbents.

A critical review of biochar as an environmental functional material in soil ecosystems for migration and transformation mechanisms and ecological risk assessment

At present, biochar has a large application potential in soil amelioration, pollution remediation, carbon sequestration and emission reduction, and research on the effect of biochar on soil ecology and environment has made positive progress. However, under natural and anthropogenic perturbations, biochar may undergo a series of environmental behaviors such as migratory transformation, mineralization and decomposition, and synergistic transport, thus posing certain potential risks. This paper outlines the multi-interfacial migration pathway of biochar in "air-soil-plant-animal-water", and analyzes the migration process and mechanism at different interfaces during the preparation, transportation and application of biochar. The two stages of the biochar mineralization process (mineralization of easily degradable aliphatic carbon components in the early stage and mineralization of relatively stable aromatic carbon components in the later stage) were described, the self-influencing factors and external environmental factors of biochar mineralization were analyzed, and the mineral stabilization mechanism and positive/negative excitation effects of biochar into the soil were elucidated. The proximity between field natural and artificially simulated aging of biochar were analyzed, and the change of its properties showed a trend of biological aging > chemical aging > physical aging > natural aging, and in order to improve the simulation and prediction, the artificially simulated aging party needs to be changed from a qualitative method to a quantitative method. The technical advantages, application scope and potential drawbacks of different biochar modification methods were compared, and biological modification can create new materials with enhanced environmental application. The stability performance of modified biochar was compared, indicating that raw materials, pyrolysis temperature and modification method were the key factors affecting the stability of biochar. The potential risks to the soil environment from different pollutants carried by biochar were summarized, the levels of pollutants released from biochar in the soil environment were highlighted, and a comprehensive selection of ecological risk assessment methods was suggested in terms of evaluation requirements, data acquisition and operation difficulty. Dynamic tracing of migration decomposition behavior, long-term assessment of pollution remediation effects, and directional design of modified composite biochar materials were proposed as scientific issues worthy of focused attention. The results can provide a certain reference basis for the theoretical research and technological development of biochar.

Respective evolution of soil and biochar on competitive adsorption mechanisms for Cd(II), Ni(II), and Cu(II) after 2-year natural ageing

To reveal the respective evolution of soil and biochar on competitive heavy metal adsorption mechanisms after natural ageing, three soils and two biochars were tested in this study. The soil-biochar interlayer samples were buried in the field for 0.5, 1, and 2 years, for which competitive adsorption characteristics and mechanisms of soils and biochars in four systems (Cd, Cd+Ni, Cd+Cu, and Cd+Ni+Cu) were investigated. Results showed that physicochemical properties, adsorption capacity and mechanisms of soils and biochars all changed the most in the first 0.5 years. The properties and adsorption capacity of biochars gradually weakened with the ageing time, meanwhile, those of soils gradually enhanced. After co-ageing with acidic soil for 0.5 years, the Cd(II) adsorption capacity of modified biochar decreased by 86.59% in the ternary system; meanwhile, that of acidic soil increased by 65.52%. The contributions of mineral mechanisms decreased significantly, while non-mineral mechanisms were slightly affected by ageing. This study highlighted that when using biochar to remediate heavy metal-contaminated soils, biochar should be applied at least half a year in advance before planting crops so that biochar can fully contact and react with the soil.

The effect of the ageing process on the desorption of nonylphenol in black carbon-sediment systems: a kineto-mechanistic and modeling investigation

Black carbon (BC) exhibits promising potential as a sediment amendment owing to its commendable adsorption capacity for hydrophobic organic contaminants (HOCs), thereby resulting in HOC-laden sediments. Desorption kinetic studies play a crucial role in comprehending the release potential of HOCs from BC-sediment systems. Although the adsorption capacity of BC for HOCs has been found to decrease with aging, there is limited research on its impact on HOC desorption kinetics. In this study, BCs derived from agricultural waste (rice straw carbon, RC) and industrial waste (fly ash carbon, FC), respectively, were used to investigate the desorption kinetics of nonylphenol (NP). Additionally, a predictive model was established using the fitting parameters obtained from the modified two-domain model. The results showed that desorption of NP was divided into three fractions: rapid fraction (Frap), slow fraction (Fslow) and resistant fraction (Fr). BCs significantly decreased, while ageing increased the desorption amount and rate of NP. The performance of RC in controlling NP release was superior to that of FC. The predicted values calculated by the established model exhibit significant positive correlations with the measured values (p < 0.01). Additionally, the correlation analysis between sorption sites and desorption fractions revealed that the concentration of NP in the desorbing fraction was nearly equivalent to that of NP in partition sites within aged sediment/FC-sediment systems. However, the aged RC-sediment systems do not conform well to this rule. In other words, the estimation of NP release risk from sediments with a strong adsorbent would be overestimated, if Frap + Fsolw is considered equivalent to the desorbing fraction.

Catalytic oxidation of lignite by Pt/TiO2 can enhance cadmium adsorption capacity

Addressing global warming necessitates innovative strategies in fossil fuel management. This study evaluates lignite, a low-rank coal with limited calorific value, exploring applications beyond its use as fuel. Utilizing Pt/TiO2 catalytic oxidation, the research aims to enhance the cadmium adsorption capacity of lignite in wastewater. Lignite, treated with 0.5% Pt/TiO2 at 125 °C for 2 h, demonstrated a threefold increase in cadmium adsorption capacity. Characterization using TGA-DSC confirmed the modification process as exothermic and self-sustainable. Spectroscopic analysis and Boehm titration revealed significant alterations in pore structure, surface area, and oxygen-containing functional groups, emphasizing the effectiveness of catalytic oxidation. Adsorption mechanisms such as complexation, cation exchange, and cation-π interactions were identified, enhancing Cd adsorption. Techniques, including the d-band model, H2-TPR, and O2-TPD, indicated that dissociative adsorption of molecular O2 and the subsequent generation of reactive oxygen species introduced additional oxygen-containing functional groups on the lignite surface. These findings provide essential strategies for the alternative use of lignite in environmental remediation, promoting sustainable resource utilization and enhancing cost-effectiveness in remediation processes. ENVIRONMENTAL IMPLICATION: This study innovates in using lignite to reduce cadmium (Cd) contamination in wastewater. Employing Pt/TiO2 catalytic oxidation, lignite is transformed, enhancing its cadmium adsorption capacity. This process, being exothermic, contributes to decreased energy consumption. The approach not only mitigates the hazardous impacts of cadmium but also aligns with sustainability by reducing greenhouse gas emissions and energy use, showcasing a multifaceted environmental advancement.

Effects of different aging methods on the ability of biochar to adsorb heavy metal cadmium and its physical and chemical properties

The aging process can affect the physical and chemical properties as well as adsorption capacity of biochar. This study focuses on the heavy metal cadmium (Cd) as the research object, and artificially ages biochar prepared from rice straw and corn straw through accelerated freeze-thaw cycles, alternating dry wet cycles, and ultraviolet light treatment, in order to evaluate the effects of different aging conditions on the physical and chemical properties of the two different types of biochar and on their adsorption capacities for Cd. After aging, the pH of rice and corn biochar decreased to varying degrees, respectively. The surface structure was ruptured, the average pore diameter was decreased, and the specific surface area was increased by 27.3%, 21.9%, and 9.8% (rice) and 95.4%, 27.7%, and 13.4% (corn). Ultraviolet light aging has the most significant impact on the elemental content of biochar, and the C content was decreased by 12.4% (rice) and 9.3% (corn). The O content was increased by 11.2% (rice) and 44.1% (corn), and the numbers of O/C, H/C, (O + N)/C, and oxygen-containing functional groups were increased. These results demonstrate that the aging process reduces the degree of aromatization of biochar, while enhancing its polarity and Cd adsorption capacity. Rice straw biochar (RSB) has a greater ability to adsorb Cd than corn straw biochar (CSB). In addition, ultraviolet light aging is particularly effective in increasing heavy metal adsorption.

Competitive adsorption behaviors and mechanisms of Cd, Ni, and Cu by biochar when coexisting with microplastics under single, binary, and ternary systems

In this study, the effects of coexistence with microplastics and co-ageing with the soil on adsorption behaviors and mechanisms of biochar for heavy metals were investigated. Adsorption experiments of Cd, Ni, and Cu by microplastics, biochar, and their combination were conducted in single, binary, and ternary systems. The results indicated that the heavy metal adsorption by microplastics was ranked as Ni > Cd > Cu, which increased with decreasing particle size, and the adsorption capacity of microplastics was enhanced after dry-wet and freeze-thaw ageing. Biochar preferentially adsorbed Cd in the single system, while the maximum adsorption of Cu was observed in the binary and ternary systems due to the minimizing impact of competition on the Cu adsorption by biochar. The heavy metal adsorption by the combination of microplastics and biochar was less than that by single biochar, and the smaller the particle size of microplastics, the greater the negative effects on heavy metal adsorption. Coexistence with microplastics reduced Cd adsorption of biochar by 0.72 %-50.35 %, Ni adsorption by 1.17 %-30.43 %, and Cu adsorption by 5.78 %-47.88 %, respectively. Moreover, coexistence with microplastics exacerbated the adverse impacts of competition on biochar adsorption for heavy metals. The contribution percentages of biochar mineral mechanisms for heavy metal adsorption were ranked as Cu > Cd > Ni. When coexisting with microplastics or after ageing, the mineral mechanisms of heavy metal adsorption by biochar significantly decreased. This study investigated the competitive adsorption behaviors and mechanisms of heavy metals by biochar when coexisting with microplastics, which highlighted that the application of biochar for the remediation of heavy metal pollution should be concerned with the impacts of microplastics.

Conversion of carbon black recovered from waste tires into activated carbon via chemical/microwave methods for efficient removal of heavy metal ions from wastewater

In this research study, recovered carbon black (rCB) was obtained via pyrolysis of waste tires. The obtained rCB was then converted into activated carbon species through both chemical treatment and microwave coupled with chemical treatment as a two-step activation process. The activated carbon obtained from chemical activation was denoted as C-AC, while that obtained from exposure to microwave followed by chemical activation was labeled as MC-AC. These two structures were consequently introduced as sorbents for the removal of cadmium ions from an aqueous solution. The structural characteristics of the introduced adsorbents were confirmed using various techniques, namely X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, and energy-dispersive X-ray (EDX) spectroscopy. Additionally, textual features of these adsorbents were acquired via both scanning electron microscopy (SEM) and N2 adsorption-desorption BET surface area analyses. These two structures were then introduced for Cd ion adsorption under different operating conditions. Particularly, the effect of pH, contact time, adsorbent dose, and metal ion concentration on the efficiency of adsorption was investigated. The 1maximum adsorption capacity was detected at a pH value of 5.0, a contact time of 30 min, a sorbent dose of 0.4 g L-1, and an initial metal concentration of 50 mg L-1 using MC-AC, which exhibited nearly double the sorption capacity detected for C-AC. Kinetic studies indicated that the process of Cd(ii) adsorption is perfectly described and fitted by the pseudo-second-order model. However, adsorption isotherms for the two adsorbents were found to match the Langmuir model, referring to the occurrence of uniform monolayer adsorption for the metal ions. Thermodynamic analysis demonstrated that the adsorption process was spontaneous and endothermic.

Effects of BC on metal uptake by crops (availability) and the vertical migration behavior in soil: A 3-year field experiments of crop rotation

Biochar (BC) has been substantiated to effectively reduce the available content of heavy metals (HMs) in soil-plant system; however, the risk of biochar (BC)derived dissolved organic matter (DOM) induced metal vertical migration has not been well documented, especially in the long-term field conditions. Therefore, this study investigated HM vertical migration ecological risks and the long-term effectiveness of the amendment of biochar in the three successive years of field trials during the rotation system. The results revealed that biochar application could increase soil pH and DOM with a decrease in soil CaCl2 extractable pool for Pb, Cu, and Cd. Furthermore, the results indicated a significant decrease in acid phosphatase activities and an increase in urease and catalase activities in the soil. Cucumber was shown to be safe during a three-year rotation system in the field. These results suggest that BC has the potential to enhance soil environment and crop yields. BC derived DOM-specific substances were identified using parallel factor analysis of excitation-emission matrix in deep soil (0-60 cm). The study incorporated HM concentration fluctuations in deep soils, providing an additional interpretation of DOM and co-migration of HMs.The environmental risk associated with the increase in DOM hydrophobicity should not be ignored by employing BC for soil HM remediation applications. The study enhances understanding of biochar-derived DOM's migration and stabilization mechanisms on heavy metals, providing guidelines for its use as a soil amendment.

Stability and ecological risk assessment of nickel (Ni) in phytoremediation-derived biochar

Improper treatment of heavy metal-enriched biomass generated after phytoremediation might cause secondary pollution in soil and water. At present, the pyrolysis process is an effective method for the treatment of phytoremediation residue. In this study, Ni-enriched biomass was prepared using hydroponics method and further pyrolyzed at different temperatures (300-700 °C). At low pyrolysis temperatures (below 500 °C), carbonate precipitation was the main reason of Ni stabilization in biochar. Nevertheless, the formed phosphate and aluminosilicate were important factors for immobilizing Ni in biochar at high pyrolysis temperatures (above 500 °C). Moreover, the oxidizable (F3) and residual (F4) components of Ni in biochar increased with increasing pyrolysis temperature, which indicated that higher pyrolysis temperature could effectively reduce the bioavailability of Ni in biochar. The results of deionized water, acidification, oxidation, and toxic characteristic leaching procedure (TCLP) experiments showed that pyrolysis temperature was the dominant factor for Ni stabilization in biochar. The ecological risk assessments further proved that pyrolyzed Ni-enriched biochar could reduce the environmental toxicity and potential ecological risks of Ni. In the soil simulated experiment, the soil microenvironment gradually promoted the transformation of Ni in BCNiX from bioavailable fraction to stable fraction. Overall, this study would expose more reasonable reference for the long-term storage of phytoremediation residues.

Effects of chemical aging on carbonaceous materials: Stability of water-dispersible colloids and their influence on the aggregation of natural-soil colloid

Although hydrochar and biochar have been used as soil conditioners, there is not a clear understanding of how their properties changes due to aging impacts their colloidal particles behavior on the soil system. From this premise, we produced hydrochar and biochar from the same feedstock (cashew bagasse) and aged with different chemical methods: (i) using hydrogen peroxide, (ii) a mixture of nitric and sulfuric acids, and (iii) hot water. It was analyzed the effects of aging on the stability of the carbonaceous materials (CMs) colloids in aqueous medium with different ionic strength (single systems), as well as the stability of the natural-soil colloid when interacting with biochar and hydrochar colloids (binary systems). A chemical composition (C, H, N, and O content) change in CMs due to the chemically induced aging was observed along with minor structural modifications. Chemical aging could increase the amount of oxygen functional groups for both biochar and hydrochar, though in a different level depending on the methodology applied. In this sense, hydrochar was more susceptive to chemical oxidation than biochar. The effectiveness of chemical aging treatments for biochar increased in the order of water < acid < hydrogen peroxide, whereas for hydrochar the order was water < hydrogen peroxide < acid. While the increase in surface oxidation improved the biochar colloidal stability in water medium at different ionic strengths (single systems), the stability and critical coagulation concentration (CCC) slightly changed for hydrochar. Natural-soil clay (NSC) interactions with oxidized carbonaceous material colloids (binary systems) enhanced NSC stability, which is less likely to aggregate. Therefore, the aging of carbonaceous materials modifies the interaction and dynamics of soil small particles, requiring far more attention to the environmental risks due to their application over time.

Biochar as a carbonaceous material to enhance soil quality in drylands ecosystems: A review

Drylands are fragile environments that should be carefully managed to improve their quality and functions to achieve sustainable development. Their major problems involve low availability of nutrients and soil organic carbon content. Biochar effect on soil is a joint response of micro to nano sized biochar and soil characteristics. In this review, we attempt to carry out a critical analysis of biochar application to enhance dryland soil quality. Correlating the effects identified from its soil application, we explored the subjects that remains open in the literature. The relation of composition-structure-properties of biochar vary among pyrolysis parameters and biomass sources. Limitations in soil physical quality in drylands, such as low water-holding capacity, can be alleviated by applying biochar at a rate of 10 Mg ha-1 also resulting in beneficial effects on soil aggregation, improved soil porosity, and reduced bulk density. Biochar addition can contribute to the rehabilitation of saline soils, by releasing cations able to displaces sodium in the exchange complex. However, the recovery process of salt-affected soils might be accelerated by the association of biochar with another soil conditioners. This is a promising strategy especially considering the biochar alkalinity and variability in nutrients bioavailability to improve soil fertilization. Further, while higher biochar application rate (>20 Mg ha-1) might change soil C dynamics, a combination of biochar and nitrogen fertilizer can increase microbial biomass carbon in dryland systems. Other aspect of biochar soil application is the economic viability of scale-up production, which is mainly associate to pyrolysis process being biochar production the costliest stage. Nevertheless, the supplying of feedstock might also represent a great input on biochar final costs. Therefore, biochar-based technology is a big opportunity to improve fragile environments such as drylands, integrating sustainable technologies with regional development. Considering the specificity of application area, it might be a model of sustainable agricultural practices protecting the environment in a bioeconomic perspective.

Qualitative and quantitative analysis for Cd2+ removal mechanisms by biochar composites from co-pyrolysis of corn straw and fly ash

Removal of heavy metals (e.g., Cd) from contaminated water using waste-converted adsorbents is promising, but the efficiency still needs to be improved. Here, we prepared a functional biochar composite as novel Cd adsorbents by co-pyrolysis of two typical solid wastes, i.e., agricultural corn straw and industrial fly ash. The adsorption behavior and mechanism were investigated using batch and column adsorption experiments and modern characterization techniques. Results showed that alkali-modified fly ash (AMFA) was loaded onto the surface of the corn straw biochar as some fine particle forms, with quartz (SiO₂) and silicate being the main mineral phases on the surface. The maximum sorption capacity fitted by Langmuir model for functionalized biochar composite (FBC700) was up to 137.1 mg g^-1, which was 7.7 times higher than that of the original corn straw biochar (BC700). Spectroscopic analysis revealed that adsorption mechanisms of Cd onto the FBC700 included mainly precipitation and ion exchange, with complexation and Cd-π interaction also contributing. The AMFA could effectively improve the mineral precipitation with Cd. The adsorption columns filled with FBC700 exhibited a longer breakthrough time than that filled with BC700. The adsorption capacity calculated by Thomas model for FBC700 was also approximately 6.0 times higher than that for BC700, showing that FBC700 was more suited to practical applications. This study provided a novel perspective for recycling solid wastes and treating Cd-contaminated water.

The role of phosphorus speciation of biochar in reducing available Cd and phytoavailability in mining area soil: Effect and mechanism

The effect of phosphorus (P) speciation in biochar on soil available Cd and its mechanism to alleviate plant Cd stress remain largely unknown. Here, ammonium polyphosphate (PABC)-, phosphoric acid (PHBC)-, potassium dihydrogen phosphate (PKBC)-, and ammonium dihydrogen phosphate (PNBC)-modified biochar were used to investigate P speciation. The Cd immobilization mechanism of biochar was analyzed by XPS and ³¹P NMR, and the soil quality and the mechanism for the biochar to alleviate Cd stress were also determined. The results demonstrated that PBC (pristine biochar), PABC, PHBC, PKBC, and PNBC reduced the content of soil DTPA-Cd by 14.96 % - 32.19 %, 40.44 % - 47.26 %, 17.52 % - 41.78 %, and 21.90 % - 36.64 %, respectively. The XPS and ³¹P NMR results demonstrated that the orthophosphate on the surface of PABC, PHBC, PKBC, and PNBC accounted for 82.06 %, 62.77 %, 33.1 %, and 54.46 %, respectively, indicating that PABC has the highest passivation efficiency on soil Cd, which was ascribed to the highest orthophosphate content on the biochar surface. Pot experiments revealed that PABC could reduce the Cd content by 4.18, 4.41, 4.43, 2.94, and 2.57 folds in roots, stems, leaves, pods, and grains, respectively, and at the same time increase the dry and fresh weight of soybean and decrease Cd toxicity to soybean by improving the antioxidant system. In addition, application of the P-modified biochars improved the enzyme activity and physicochemical properties of the soil. This study provides a new perspective for studying the effect of P-modified biochars on soil Cd immobilization.

Low temperature pyrolytic biochar is a preferred choice for sulfonamide-Cu(II) contaminated soil remediation in tropical climate region

Biochar is getting increasing consideration for eco-friendly soil amendment and environmental remediation. Once added to the soil, biochar would undergo the natural ageing process, affecting its physicochemical properties and, as a result, the adsorption and immobilization of pollutants in the water and soil. To evaluate the high/low temperature pyrolyzed biochar performance on complex contaminants and the effect of climate ageing, the batch experiments were conducted on the adsorption of the pollutants of antibiotics sulfapyridine (SPY) and a typical coexisting heavy metal Cu²⁺ as one or binary system on low/high pyrolytic temperature biochars before and after the simulated tropical climate and frigid climate region ageing treatment. The results showed that high-temperature ageing could enhance the SPY adsorption in biochar-amended soil. The SPY sorption mechanism was fully elucidated, and the result confirmed that H-bonding was the dominant role in biochar-amended soil, and π-π electron-donor-acceptor (EDA) interaction and micro-pore filling was another factor for SPY adsorption. This study could lead to the conclusion that low-temperature pyrolytic biochar is a better option for sulfonamide-Cu(II) contaminated soil remediation in tropical regions.

The characteristic difference between non-drilosphere and drilosphere-aged biochar: Revealing that earthworms accelerate the aging of biochar

Numerous researches have been conducted on the effects of biotic and abiotic-induced aging on the physicochemical characteristics and functions of biochar; however, the impacts of earthworm-induced aging on biochar have not been reported. Hence, we conducted a microscopic experiment simulating a 'drilosphere' to explore the influence of earthworm activity on the natural aging of rice husk biochar (RHBC) through the difference in biochar characteristics after aging in drilosphere and non-drilosphere. The earthworm activity increases the available nitrogen (AN) and dissolved organic matter (DOM) contents of aged RHBC and changes its composition. The increase of DOM and AN content may recruit more microorganisms to colonize biochar and accelerate the biological oxidation of biochar. Furthermore, earthworm activity significantly increased the contents of oxygen (O) and O-containing functional groups in the aged RHBC and decreased the stability (aromaticity) of the aged RHBC, suggesting that the earthworm activity accelerates the natural aging of biochar. Earthworm feeding promotes physical damage to biochar. Besides, the earthworm activity decreased the pH, hydrophilicity and specific surface area (SSA) of aged RHBC but enhanced the adsorption capacity of aged RHBC for heavy metals. The higher content of O-containing functional groups on the surface of drilosphere-aged RHBC was the main reason for its higher adsorption performance. Earthworm feeding promotes physical damage to biochar. These results indicate that earthworm activity can accelerate the natural aging of biochar and alter its physicochemical characteristics and functions. This study illustrates how biochar characteristics change in earthworm-soil systems, which will help scientifically evaluate the long-term effectiveness of biochar.

NaHCO3 activated sludge-derived biochar by KMnO4 modification for Cd(II) removal from aqueous solutions

The surface flat pristine biochar provides limited adsorption sites for Cd(II) adsorption. To address this issue, a novel sludge-derived biochar (MNBC) was prepared by NaHCO₃ activation and KMnO₄ modification. The batch adsorption experiments illustrated that the maximum adsorption capacity of MNBC was twice that of pristine biochar and reached equilibrium more quickly. The pseudo-second order and Langmuir model were more suitable for analyzing the Cd(II) adsorption process on MNBC. Na⁺, K⁺, Mg²⁺, Ca²⁺, Cl^- and NO^-₃ had no effect on the Cd(II) removal. Cu²⁺ and Pb²⁺ inhibited the Cd(II) removal, while PO^3-₄ and humic acid (HA) promoted it. After 5 repeated experiments, the Cd(II) removal efficiency on MNBC was 90.24%. The Cd(II) removal efficiency of MNBC in different actual water bodies was over 98%. Furthermore, MNBC owned excellent Cd(II) adsorption performance in fixed bed experiments, and the effective treatment capacity was 450 BV. The co-precipitation, complexation, ion exchange and Cd(II)-π interaction were involved in Cd(II) removal mechanism. XPS analysis showed that NaHCO₃ activation and KMnO₄ modification enhanced the complexation ability of MNBC to Cd(II). The results suggested that MNBC can be used as an effective adsorbent for treating of Cd-contaminated wastewater.

Integrated microbiological and metabolomics analyses to understand the mechanism that allows modified biochar to affect the alkalinity of saline soil and winter wheat growth

In order to understand the mechanism that allows modified biochar (BC) to enhance the salt tolerance and growth of crops in saline-alkali soil, we tested the effects of ordinary BC, nanoparticle-size BC, acidified BC (HBC), and acidified nanoparticle-size BC on winter wheat growth and the soil properties by combining microbiological and metabolomics analyses. The results showed that compared with the control with no BC, the plant height increased by 17.33 % under HBC and the proportion of large soil aggregates increased by 1.25-2.83 times. HBC increased the relative abundances of some dominant genera of bacteria (e.g., Streptococcus) and fungi (e.g., Mycothermus), as well as functions such as bacterial metabolic genetic information processing and cellular processes, and reduced the abundance of pathotrophic fungi. Metabolomics analysis showed that HBC upregulated various metabolites (including amino acids and their derivatives, lipids, flavonoids, and organic acids) and five main metabolic pathways. Among the KEGG pathways, the pyrimidine metabolism pathway was significantly upregulated, as well as crop leaf metabolism, β-alanine metabolism, and valine, leucine, and isoleucine metabolism, and the antioxidant levels and resistance to salt-alkali stress were enhanced in winter wheat leaves. Partial least squares-path modeling suggested that HBC affected the growth of winter wheat by significantly changing the soil physicochemical properties and microbial structure (path coefficients of 0.566 and 0.512, respectively).

Artificial aging induced changes in biochar,s properties and Cd2+ adsorption behaviors

Fresh biochar has been widely applied to the remediation of heavy metals in soil by its property of adsorption, but the changes in its physicochemical properties and in situ adsorption performance over time cannot be ignored. In this study, the sorption of Cd²⁺ by corn straw biochars (CB) and municipal sludge biochars (SB) produced at 350 °C and 650 °C before and after H₂O₂ oxidation, and dry-wet and freeze-thaw aging were investigated using batch sorption experiments. The changes of physicochemical properties of biochar before and after aging were analyzed by various characterization methods. Based on these results, the impact of aging on the Cd²⁺ adsorption behavior could be clarified, which showed that CB650 was able to display the highest adsorption capacity in fresh biochars. Aging treatments reduced the ash content and pH value of CB, and significantly diminished the adsorption performance of Cd²⁺. These changes indicated that precipitation was a critical factor in the adsorption of Cd²⁺ on CB. The adsorption capacity of SB was enhanced after H₂O₂ oxidation, but weakened after dry-wet and freeze-thaw aging. This was closely related to the increase or decrease in the content of oxygen-containing functional groups, which in turn enhanced or inhibited its ability to compound with heavy metals. These results are of great significance for evaluating its long-term application prospects in the natural environment.

Effects and mechanism on cadmium adsorption removal by CaCl2-modified biochar from selenium-rich straw

As every-one knows, cadmium contamination poses a significant and permanent threat to people and aquatic life. Therefore, research on how to remove cadmium from wastewater is essential to protect the natural environment. In this study, agricultural and forestry waste straw sprayed with selenium-enriched foliar fertilizer was prepared as biochar, which was altered by calcium chloride (CaCl₂) to remove Cd²⁺ from water. The outcomes demonstrated that biochar generated by pyrolysis at 700 °C (BC700) had the best adsorption effect. Secondly, pseudo-second-order kinetics and Langmuir adsorption models were used to predict the Cd²⁺ adsorption. Finally, electrostatic adsorption, ion exchange, and complexation of oxygen functional groups (OFGs) were demonstratedto be the main adsorption mechanisms. These conclusions indicate that selenium-rich straw biochar is a novel adsorbent for agroforestry waste recovery. Meanwhile, this work will offer a promising strategy for the overall utilization of rice straw.

Biochar for the Removal of Emerging Pollutants from Aquatic Systems: A Review

Water contaminated with emerging pollutants has become a serious environmental issue globally. Biochar is a porous and carbon-rich material produced from biomass pyrolysis and has the potential to be used as an integrated adsorptive material. Many studies have shown that biochar is capable to adsorb emerging pollutants from aquatic systems and could be used to solve the water pollution problem. Here, we provided a dual perspective on removing emerging pollutants from aquatic systems using biochar and analyzed the emerging pollutant removal efficiency from the aspects of biochar types, pollutant types and coexistence with heavy metals, as well as the associated mechanisms. The potential risks and future research directions of biochar utilization are also presented. This review aims to assist researchers interested in using biochar for emerging pollutants remediation in aquatic systems and facilitate research on emerging pollutants removal, thereby reducing their environmental risk.

The older, the better: Ageing improves the efficiency of biochar-compost mixture to alleviate drought stress in plant and soil

Due to increased drought frequency following climate change, practices improving water use efficiency and reducing water-stress are needed. The efficiency of organic amendments to improve plant growth conditions under drought is poorly known. Our aim was to investigate if organic amendments can attenuate plant water-stress due to their effect on the plant-soil system and if this effect may increase upon ageing. To this end we determined plant and soil responses to water shortage and organic amendments added to soil. We compared fresh biochar/compost mixtures to similar amendments after ageing in soil. Results indicated that amendment application induced few plant physiological responses under water-stress. The reduction of leaf gas exchange under watershortage was alleviated when plants were grown with biochar and compost amendments: stomatal conductance was least reduced with aged mixture aged mixture (-79 % compared to -87 % in control), similarly to transpiration (-69 % in control and not affected with aged mixture). Belowground biomass production (0.25 times) and nodules formation (6.5 times) were enhanced under water-stress by amendment addition. This effect was improved when grown on soil containing the aged as compared to fresh amendments. Plants grown with aged mixtures also showed reduced leaf proline concentrations (two to five times) compared to fresh mixtures indicating stress reduction. Soil enzyme activities were less affected by water-stress in soil with aged amendments. We conclude that the application of biochar-compost mixtures may be a solution to reduce the effect of water-stress to plants. Our findings revealed that this beneficial effect is expected to increase with aged mixtures, leading to a better water-stress resistance over time. However, while being beneficial for plant growth under water-stress, the use of amendments may not be suited to increase water use efficiency.

Effects of modification and co-aging with soils on Cd(II) adsorption behaviors and quantitative mechanisms by biochar

In this study, original and two KMnO₄-modified rice straw biochars (pre- and postmodification) were prepared, which were all pyrolysed at 400 °C. Premodified biochar had the largest Cd adsorption capacity, strongest acid and solute buffering capacity, which benefited from the increase of carbonate content, specific surface area, and the emergence of Mn(II) and MnO_x through modification. Original and premodified biochars were then conducted four types of aging process by an improved three-layer mesh method, namely, aging without soil and co-aging with acid (pH = 5.00), neutral (pH = 7.00), and alkaline (pH = 8.30) soils. The adsorption capacities of modified biochar were always larger than those of original biochar after aging processes. After four aging processes, Cd(II) adsorption capacities were basically in the order of aged biochar without soil > biochar co-aged with alkaline soil > biochar co-aged with neutral soil > biochar co-aged with acid soil, and KMnO₄-modified biochar was always better than original biochar after co-aging with soils. The dominant adsorption mechanism of original and premodified biochars (fresh and aged) for Cd(II) was all the precipitation and adsorption with minerals (accounted for 58.55 ~ 85.55%). In this study, we highlighted that biochar remediation for Cd should be evaluated by co-aging with soil instead of aging without soil participation.

Sorption and post-sorption performances of Cd, Pb and Zn onto peat, compost and biochar

The development of waste-derived sorbents to immobilize potentially toxic elements (PTEs) is a promising strategy, contributing to the achievement of sustainable development goals (SDGs). Therefore, this study aimed to assess the sorption performance of cadmium (Cd), lead (Pb) and zinc (Zn), comparing sorbents derived from organic fraction of municipal solid waste (composts and biochars) with peat. The physicochemical characterization, equilibrium of sorption, post-sorption analyzes and bioaccessibility were investigated. Results showed that the sorbents have distinct characteristics; however, each material have their particularities favorable to sorption. For instance, peat and composts have the highest cation exchange capacity (800-1100 mmol_c kg^-1), while biochar produced at 700 °C has the highest specific surface area (91.21 m² g^-1). The sorption equilibrium data revealed the actual sorption capacity and was well explained by the Freundlich and Langmuir isotherms and, in some cases, by the Dubinin-Radushkevich model. Post-sorption analyzes indicated the occurrence of several sorption mechanisms, driven by the physicochemical properties. Electrostatic interaction stood out for peat and compost. The FTIR spectrum for peat proved the complexation with oxygenated functional groups. The composts showed variations in the released cations (e.g. Ca²⁺ and K⁺), indicating cation exchange. Differently, for biochars, the XRD patterns showed that precipitation or coprecipitation seems to be one of the main mechanisms, especially for Cd and Pb. Regarding human bioaccessibility, the results of the gastric phase simulation (pH∼1.20) revealed lower percentages of Pb (33-81%) than Cd (91-99%) or Zn (82-99%), especially for the highest concentrations. Nevertheless, in numerical terms, all bioaccessible concentrations inspire care. In conclusion, among the sorbents, composts and biochars presented the best sorption performances and, therefore, have great potential for environmental applications. Furthermore, the bioaccessibility findings indicate that these assays, still little used in experiments with sorbents, are an important tool that should be better explored in the assessment of the environmental risk associated with contamination.

Adsorption characteristics and mechanisms of Cd2+ from aqueous solution by biochar derived from corn stover

Corn stover could be pyrolysed to prepare biochar for removing pollutants in water and realizing the resource utilization of biomass. The aims of the present study were to investigate the optimal preparation and adsorption conditions of biochar and to reveal the adsorption characteristics and mechanisms of Cd²⁺ in water by biochar. For this purpose, with Cd²⁺ as the target pollutant, the pyrolysis conditions involved in the pyrolysis temperature, retention time, and heating rate were evaluated and optimized. Additionally, the characteristics, mechanisms and optimal adsorption conditions of Cd²⁺ by biochar were determined. A series of characterization techniques was employed, including scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and specific surface area analysis (S_BET). The optimum pyrolysis parameters were a pyrolysis temperature of 700 °C, a retention time of 2.5 h, and a heating rate of 5 °C/min. Acid/base modification did not improve the adsorption capacity of biochar. The Langmuir and the Elovich model were the most suitable isotherm and kinetic models for equilibrium data, respectively. The maximum adsorption capacity fitted by Langmuir model was 13.4 mg/g. Furthermore, mineral precipitation and π electron interactions were shown to be the main adsorption mechanisms of Cd²⁺. The optimum adsorption conditions for Cd²⁺ in water were a CaCl₂ electrolyte solution of 0.01 mol/L, a pH level of 6.7, and a biochar dosage of 0.4 g. Our results indicated that corn stover biochar was an appropriate approach for improving the status of water with Cd²⁺ contamination in the short term and for promoting a new perspective for the rational utilization of corn stover and the low-cost pollution control of heavy metals in water.

A comparative study on adsorption of cadmium and lead by hydrochars and biochars derived from rice husk and Zizania latifolia straw

In this study, hydrochars and biochars were prepared from rice husk (RH) and Zizania latifolia straw (ZL) at various pyrolysis temperatures as absorbents, for removing toxic ions from single and competitive solutions of cadmium (Cd) and/or lead (Pb). The adsorption efficiencies of Cd and Pb in both hydrochars and biochars were lower in the competitive solution than in the single solution, and the absorbents had a stronger affinity for Pb than for Cd. Compared to hydrochars, biochars showed more favorable Cd and Pb adsorption capacities in the single or competitive solutions, and the ZL biochars had the maximum adsorption capacity among them. The SEM and FTIR analyses suggest that the predominant adsorption mechanisms of biochars and hydrochars are surfaces monolayer adsorption, precipitation, complexation, and coordination with π electrons. However, hydrochars derived from ZL exhibited an optimal additional Pb adsorption capacity in the high-level (5 ~ 10 mg L^-1 Cd and Pb) competitive solution. This extra Pb adsorption of hydrochars was likely attributed to the Si-O-Si groups and more bumpy structure. Zizania latifolia straw biochar had a huge potential removal of Cd or/and Pb, and applying hydrochars as absorbents was beneficial to the removal of Cd and Pb in polluted solutions.

Enhanced chlortetracycline removal by iron oxide modified spent coffee grounds biochar and persulfate system

Chlortetracycline (CTC) is a tetracycline derivative antibiotic that has been widely used in the livestock industry for prophylactic and therapeutic purposes. Effective measures should be taken to decrease the environmental risks associated with CTC-rich waste. Biochar produced by biomass waste showed great potential for organic contaminants removal by adsorption and catalytic degradation. This study prepared iron oxide-modified coffee grounds biochar (CGF) at different temperatures for enhanced CTC removal by adsorption and degradation. The main mechanism for CTC removal was found to be electrostatic interaction. In addition, pore diffusion, hydrogen bonds, and π-π bonds also contributed to CTC adsorption. Maximum CTC adsorption capacity was 223.63 mg/g for CGF800 (CGF prepared at 800 °C pyrolysis). The free radical content of CGF600 (CFG prepared at 600 °C pyrolysis) was higher than CGF800, and there were no significant advantages in using biochar prepared at a higher temperature for persulfate activation. The ion mass-to-charge ratio (M/z) is used to describe the ratio of mass to charge of an ion or peak, which can infer compound structure. The structure of CTC degradation products was analyzed by UPLC-MS, and the M/z values were determined as 444, 273, and 154. Thus, pyrolysis of coffee grounds at higher temperatures increased CTC adsorption capacity, and CGF can indirectly assist in CTC degradation by persulfate activation.

Effect of oxidative aging of biochar on relative distribution of competitive adsorption mechanism of Cd2+ and Pb2

In this study, aged biochar (CCB350 and CCB650) were obtained from pyrolysis of corn stalk biochar (CB350 and CB650) at the degree of 350 °C and 650 °C by artificial oxidation with hydrogen peroxide (H₂O₂). Also, the mechanism of Pb²⁺ and Cd²⁺ on fresh and aged biochars was analyzed qualitatively and quantitatively by batch adsorption experiments combined with characterization. The adsorption isotherm results showed that aging treatment decreased the adsorption capacity of Pb²⁺ and Cd²⁺ and inhibited the competitive adsorption behavior of heavy metals. In the single-metal system, precipitation and cation exchange were considered as the main adsorption mechanisms for CB350 and CB650, with a ratio of 40.07–48.23% and 38.04–57.19%, respectively. Competition between Pb²⁺ and Cd²⁺ increased the relative contribution of mineral precipitation, but decreased the contribution of cation exchange mechanism. Aging resulted in the rise of the contribution of surface complexation to the adsorption of Pb²⁺ and Cd²⁺ on biochars, especially in low-temperature biochars, but weakened the contribution of mineral precipitation to the adsorption. Further, the contribution of other adsorption mechanisms was significantly enhanced for high-temperature aged biochars. These results are important to evaluate its long-term application prospects in the natural environment.

Biochar produced by combining lignocellulosic feedstock and mushroom reduces its heterogeneity

To reduce the feedstock-sourced heterogeneity of biochar, mushrooms, cultivated from lignocellulosic feedstocks (LFs), were used as precursors for biochar preparation. The coefficient of variation (CV) was adopted to show the homogeneity changes. In contrast to LFs, mushrooms produced relatively lower CVs in terms of elemental and proximate analysis. Furthermore, the CV of H/C (9.20%) and O/C (13.32%) of mushroom-based biochars (MRBCs) was lower than that of LF-based biochars (LFBCs), suggesting more homogeneous aromaticity and hydrophilicity. The relatively lower CV of the volatile matter (0.87%), fixed carbon (0.45%), and ash (2.44%) of MRBCs suggested an improvement in the homogeneity of chemical components. The homogenized physical structure was reflected in the lack of a difference in pore characteristics of MRBCs. The lower CVs (1.89-14.82%) for the pollutant adsorption of MRBCs, implied more stable performance. In conclusion, converting LFs to mushrooms reduced the precursor's heterogeneity, consequently homogenizing the biochar's properties and performance.

Toxicity of biochar influenced by aging time and environmental factors

The extensive application of biochar has drawn more attentions on its potential risk to aquatic organisms. However, the influence of environmental factors (i.e. pH, HA, SDBS and aging time) after they discharged into environment on their toxicity have not been clarified. Here, we synthesized biochar with local pine needles via pyrolysis, and then aged in different media. Followed, the toxicity of pristine and aged biochar was checked with Scenedesmus obliquus (S. obliquus). Our investigation showed that the toxicity of biochar was mitigated when aged in different pH levels or SDBS, while it was opposite in the presence of HA. The increment of pH decreased the toxicity of both the pristine and the aged biochar, while the presence of HA did same impact on the pristine biochar. The presence of SDBS decreased the toxicity of pristine biochar but increased that of aged biochar. Meanwhile, we showed these environmental factors (pH, HA, SDBS and aging time) influenced the biochar toxicity may be due to the adjustment of the aggregation and adhesion of biochar on cell surfaces or the intracellular oxidative stress. Further, the PFRs contained in biochar did influence the toxicity, along with the physicochemical properties of biochar (i.e. carbon structure, functional group or surface charge). Our results aimed to reflect the toxicity profile of biochar in the natural aquatic environment, without misunderstanding of potential ecological risk of biochar in the future application.

Cadmium long-term immobilization by biochar and potential risks in soils with different pH under combined aging

Cd long-term immobilization by biochar and potential risk in soils with different pH were quantified under a combined artificial aging, which simulated five years of aging in the field based on local climate. Two biochars (original and KMnO₄-modified) and five soils with different pH were tested, and an improved three-layer mesh method was employed in this study. Five aging cycles were carried out (Cycle 1-Cycle 5), and each aging cycle quantitatively simulated 1 year of natural aging. As the aging time increased, Cd leaching loss in all soils gradually increased from Cycle 1 to Cycle 5; for relatively stable Cd fraction, it decreased firstly and then stabilized in acidic and neutral soils (S1-S4), while it decreased firstly and then increased in alkaline soil (S5). Biochars significantly promoted Cd immobilization in strongly acidic soil (S1) by increasing relatively stable fractions and decreasing leaching loss. For weakly acidic and neutral soils (S2-S4), although biochars still had positive effects, the immobilization effects were weakened to certain extents compared with S1. The percentage of Cd leaching loss decreased by 19.12% in strongly acidic soil (S1) and by 1.12-11.35% in weakly acidic and neutral soils (S2-S4) after modified biochar treatment. For alkaline soil (S5), the application of biochars had negative effects on Cd immobilization by decreasing relatively stable fractions and increasing leaching loss, and posed risks to the environment. For strongly acidic soil (S1) and weakly acidic and neutral soils (S2-S4), the percentages of relatively stable fractions increased from 6.09-19.93% to 24.98-36.70% after modified biochar treatment. However, for alkaline soil, the percentage of relatively stable fractions decreased from 55.27% to 53.93% after biochar treatment. The more acidic the soil, the more effective the Cd immobilization by biochar. Biochars with high pH level are not suitable for the remediation of alkaline Cd contaminated soil.

Nanobiochar-rhizosphere interactions: Implications for the remediation of heavy-metal contaminated soils

Soil heavy metal contamination has increasingly become a serious environmental issue globally, nearing crisis proportions. There is an urgent need to find environmentally friendly materials to remediate heavy-metal contaminated soils. With the continuing maturation of research on using biochar (BC) for the remediation of contaminated soil, nano-biochar (nano-BC), which is an important fraction of BC, has gradually attracted increasing attention. Compared with BC, nano-BC has unique and useful properties for soil remediation, including a high specific surface area and hydrodynamic dispersivity. The efficacy of nano-BC for immobilization of non-degradable heavy-metal contaminants in soil systems, however, is strongly affected by plant rhizosphere processes, and there is very little known about the role that nano-BC play in these processes. The rhizosphere represents a dynamically complex soil environment, which, although having a small thickness, drives potentially large materials fluxes into and out of plants, notably agricultural foodstuffs, via large diffusive gradients. This article provides a critical review of over 140 peer-reviewed papers regarding nano-BC-rhizosphere interactions and the implications for the remediation of heavy-metal contaminated soils. We conclude that, when using nano-BC to remediate heavy metal-contaminated soil, the relationship between nano-BC and rhizosphere needs to be considered. Moreover, the challenges to extending our knowledge regarding the environmental risk of using nano-BC for remediation, as well as further research needs, are identified.

Effect of ageing on biochar properties and pollutant management

Biochar is a carbon-rich pyrogenic material for multifunctional environmental applications such as carbon sequestration, soil amendment, and pollutant management, etc. Owing to long-term existing in the soil, biochar would inevitably suffer from natural geochemical weathering. Such ageing process could pose nonnegligible impacts on the physicochemical property and functionality of biochar. For an object-oriented design of biochar under different application ageing conditions, the latest research progress on ageing methods, biochar properties, and pollutant sorption performance needs to be fully understood. Specifically, the effect of soil components on biochar ageing is critically reviewed, which is of importance but not fully explored so far. The decrease of ash in aged biochar can inhibit the adsorption of heavy metals. The loss of aromatic components and the formation of three-dimensional water clusters during the ageing process have a negative impact on high-temperature biochar (>500° C) for organic pollutants adsorption. For long-term soil remediation, these results remind us to carefully use high-ash biochar for heavy metals and high-temperature biochar for organic pollutants. The interaction between soil minerals and biochar can form organometallic complexes and change functional groups to enhance the oxidation resistance of biochar. In the present review, the current research on biochar ageing are critical reviewed, and the further researches are prospected including developing advanced artificial ageing methods, exploring the impact of soil components on biochar ageing, and clarifying the long-term environmental behavior of modified biochar.

Effects of competitive adsorption with Ni(II) and Cu(II) on the adsorption of Cd(II) by modified biochar co-aged with acidic soil

To investigate the effects of competitive adsorption with Ni(II) and Cu(II) on the adsorption of Cd(II) by modified biochar co-aged with acidic soil, four biochars were employed in this study, namely original biochar, KMnO₄-modified biochar and two aged biochars which co-aged with an acidic soil using above biochars under freeze-thaw cycling and dry-wet cycling for 54 days simulating 6 years of natural aging. The results showed that biochar adsorption capacities of three heavy metal ions were in the order of Cd(II) > Cu(II) > Ni(II) in the single system while Cu(II) > Cd(II) > Ni(II) in binary and ternary systems. Modification improved biochar adsorption capacity of Cd(II), but competitive adsorption with Ni(II) and Cu(II) weakened the improvement of modification on adsorption performance of modified biochar in binary and ternary systems. The Q_MBC/Q_BC of Cd(II) (Q_MBC and Q_BC are the adsorption capacities of heavy metals by modified and original biochars) decreased from 231.57% (single system) to 216.67%∼219.41% (binary system) and further decreased to 207.74% (ternary system). Co-aging with soil weakened the adsorption capacities of biochars for Cd(II), even worse, competition aggravated this negative effect of co-aging. The Q_AMBC/Q_MBC of Cd(II) (Q_AMBC is the adsorption capacities of heavy metals by aged modified biochar) decreased from 65.41% (single system) to 14.43%∼19.46% (binary and ternary systems). Therefore, the impact of competition should be fully considered when evaluating Cd long-term remediation effects of modified biochar in Cd polluted soils accompanied with other heavy metals.

Sorption of arsenic by composts and biochars derived from the organic fraction of municipal solid wastes: Kinetic, isotherm and oral bioaccessibility study

The historic contamination of water and soils by arsenic (As) is an extremely alarming environmental and public health issue worldwide. This study investigated the relationship between As sorption and physicochemical properties of composts and biochars derived from the organic fraction of municipal solid wastes (OFMSW) towards the development of promising sorbents with value-added solid wastes management solutions. The sorbents were characterized and their effectiveness on the As sorption was tested. Several isothermal and kinetic sorption models were used for the prediction of sorption. Composts did not show promising sorption capacities, and in some cases, the As immobilization was practically null. In contrast, biochars achieved higher sorption performance, and the experimental data fitted well on Dubinin-Rabushkevich and Langmuir models, with higher R² values. The maximum sorption capacities of BC700 estimated by such models were 6.495 and 170.252 mg g^-1, respectively, whereas those of BC500 estimated by D-R and Langmuir models were only 0.066 and 0.070 mg g^-1, respectively. In sorption kinetics, As was retained onto biochars at a faster first stage, reaching equilibrium after approximately 1 h and 2 h for initial concentrations of 10 and 100 mg L^-1. The pseudo-second-order, Ritchie's second-order, Ritchie's, and Elovich models more adequately described the sorption kinetics of As onto biochars with high R² values. Overall, the complexation and precipitation were predominant mechanisms for As sorption by OFMSW-derived biochars. Furthermore, the mathematical models indicated contributions arise from physisorption and external and internal diffusion mechanisms. Although BC700 can immobilize large As amounts, the gastric phase of the oral bioaccessibility test revealed more than 80% of the sorbed As could be released under conditions similar to a human stomach (pH~1.2). Such conclusions have given important insights about the refining of effective and eco-friendly remediation technologies for the management and rehabilitation of As-contaminated soil and water, particularly in developing countries.

Long-term immobilization of soil metalloids under simulated aging: Experimental and modeling approach

Aging is an inevitable natural process, leading to faded performances of soil amendments. Understanding long-term aging features is crucial for the risk management of contaminated soil. In this study, a novel quantitative aging method, namely, the "soil coin" method, was developed, which can simulate the effects of natural aging on metal(loid) immobilization performances. To better depict the aging features, two models on the basis of conditional probability-induced failure were developed. To effectively immobilize soil arsenic (As) and antimony (Sb), magnesium (Mg) and iron (Fe) oxides were simultaneously introduced to either fresh or pre-oxidized biochar via a facile method. Although post-application aging is harmful, pre-aging (i.e., pre-oxidation using H₂O₂) in turn served as an effective means to introduce more metal oxides, thereby rendering better short-term and long-term effectiveness for metalloid immobilization. Experimental and modeling approaches suggested that precipitation accounted for long-term immobilization, while a constant aging rate is the key feature for a promising soil amendment. It is suggested that to further calibrate this method and better understand the immobilization performances in the long run, more evidence from the field is needed.

Mechanism of aging of biochars obtained at different temperatures from sewage sludges with different composition and character

The aim of this study was to determine the effect of abiotic aging of sewage sludge (SSL)-derived biochars on their physicochemical properties and in consequence on their stability. Biochars produced at 500 or 700 °C from SSLs with a different composition and properties were incubated at different temperatures (-20, 4, 20, 60, and 90 °C) for 6 and 12 months. Pristine and aged biochars were characterized in terms of their composition and properties using a range of complementary methods. The results showed that SSL-derived biochars will not be as stable as previously thought in the long term. The stability of the SSL-derived biochars was closely related to the content and character of C. The biochars that had more C in their composition and, apart from aromatic C, also aliphatic matter/carbon substances deposited in surface pores (i.e. those produced from SSL with a lower initial ash content and a lower degree of aromaticity) were less stable than the biochars with a lower C content and a typically aromatic character of C (i.e. those derived from SSL with a higher initial ash content and a higher degree of aromaticity). Their oxidation led to partial mineralization of aliphatic chains or organic surface film and manifested itself in a greater changes in their properties. The low-temperature biochars (BC-500) with lower aromaticity were found to be more susceptible to oxidation than the high-temperature ones (BC-700) with higher aromaticity. The more aromatic structure of C limited access of O₂ molecules to biochar interior, due to which the processes occurring during aging were concentrated in their surface layer and their properties were less change. It can therefore be concluded that pyrolysis of SSL with higher aromaticity and a lower organic content and higher pyrolysis temperatures will lead to obtaining more stable SSL-derived biochars.

Novel insights into the adsorption of organic contaminants by biochar: A review

With rising concerns in the practical application of biochar for the remediation of environment influenced by various organic contaminants, a critical review to facilitate insights the crucial role that biochar has played in wastewater and polluted soil decontamination is urgently needed. This research therefore aimed to describe different intriguing dimensions of biochar interactions with organic contaminants, which including: (i) an introduction of biochar preparation and the related physicochemical properties, (ii) an overview of mechanisms and factors controlling the adsorption of organic contaminants onto biochar, and (iii) a summary of the challenges and an outlook of the further research needs in this issue. In the light of the survey consequences, the appearance of biochar indicates the potential in substituting the existing costly adsorbents, and it has been proved that biochar is one promising adsorbent for organic pollutants adsorption removal from water and soil. However, some research gaps, such as dynamic adsorption, potential environmental risks, interactions between biochar and soil microbes, novel modification techniques, need to be further investigated to facilitate its practical application. This research will be conductive to better understanding the adsorption removal of organic contaminants by biochar.

Insights into effects of ageing processes on Cd-adsorbed biochar stability and subsequent sorption performance

The conversion of agricultural biomass wastes into biochar has enormous potential to improve soil quality. Particularly, biochar particles introduced into the natural environment readily bind environmental pollutants. The interaction of biochar and adsorbed pollutants will, however, be impacted by long term ageing. The mechanism of biochar adsorption performance that could be affected by such early-adsorbed pollutant is not understood. Herein, we study the effects of different ageing processes on Cd-adsorbed biochar stability by K₂CrO₇-H₂SO₄ oxidation method for carbon loss evaluation, and sorption capacity towards diethyl phthalate (DEP). We adopted artificially accelerated ageing methods to simulate various processes: HNO₃/H₂SO₄, H₂O₂ oxidative, leaching, high temperature, freeze-thaw cycles, and dry-wet cycles. The results showed that all the Cd-adsorbed aged biochars had more C-C/C-H functional groups and exhibited higher carbon stability than pristine aged biochars. Specially, the carbon loss (20.2-25.2%) of Cd-adsorbed biochar showed almost two times lower than pristine biochar (34.5-35.4%) after high temperature and leaching ageing due to enhancing Cd-π bonding, which could resist strong K₂Cr₂O₇/H₂SO₄ oxidation. Such interaction synergistically promoted the sequestration of adsorbed Cd on biochars. The subsequent sorption performance for DEP on Cd-adsorbed biochars was slightly higher than pristine biochar after H₂O₂ oxidative and leaching ageing. Whereas, the dominate mechanism of DEP removal on Cd-adsorbed biochars changed to hydrophobic partitioning after high temperature, freeze-thaw cycles and dry-wet cycles ageing, leading to a decreased DEP sorption capacity. Hence, the biochar application must be comprehensively considered the susceptibilities due to reciprocal effects of early adsorbed pollutant and natural ageing environment, which helps us both assess and take measures to minimize potential risks in the environment. The present study suggests the continuous irrigation and tropical or temperature climate regions would be suitable for long-term biochar application in soil remediation and carbon sequestration.

Nitric acid-modified hydrochar enhance Cd2+ sorption capacity and reduce the Cd2+ accumulation in rice

Remediating the agricultural soil polluted by cadmium (Cd) is a serious issue in China. Hydrochar showed its potential to purify Cd-contaminated water and improve Cd-contaminated soil due to its vast amounts of macro- and microporous structures. In this study, three concentration gradients of nitric acid (HNO₃, mass fraction: 5%, 10%, 15%) were implemented to age pristine wheat straw hydrochar (N0-HC) aiming to improve surface physiochemical properties. Four HNO₃-aging hydrochars named N0-HC, N5-HC, N10-HC, N15-HC were used to both remove Cd²⁺ from aqueous solution and improve soil properties. Results showed that HNO₃-aging significantly improved the Cd²⁺ adsorption capacity by 1.9-9.9 folds compared to crude hydrochar due to the increased specific surface area (by 1.5-6.5 folds) and oxygen-containing functional group abundance (by 4.5-22.1%). Besides, initial solution pH of 8 or environmental temperature of 318.15 K performed the best Cd²⁺ adsorption capacity. Furthermore, the process of Cd²⁺ adsorption was fitted best to pseudo-second-order (R² = 0.95) and Langmuir models (R² = 0.98), respectively. Nanjing 46 (Oryza sativa L) and HNO₃-aging hydrochars were furtherly applied into Cd-contaminated paddy soil to investigate the mitigation of Cd translation from soil to rice. N15-HC-1% (w/w) performed the best effect on reducing cadmium accumulation in various parts of rice plants. Overall, this research provided an approach to improve hydrochar capacity to remove Cd²⁺ from aqueous solution and mitigate Cd translation from soil to rice.

Effects of simulated diagenesis and mineral amendment on the structure, stability and imidacloprid sorption properties of biochars produced at varied temperatures

In order to successfully apply biochar to soil remediation such as immobilizing contaminants and sequestering carbon, the prerequisite is to maintain its strong sorption capacity for contaminants and stability after long-term aging. Simulated diagenesis was performed here to mimic real geochemical aging process of biochars with different mineral content. The results showed that biochars exhibited increasing aromaticity, thermal and chemical oxidation resistance, microporosity, but declined surface and total polarity after direct diagenesis. The involvement of minerals during diagenesis further increased the surface Fe, Al, and Si content, as well as surface polarity and thermal stability compared to direct diagenesis. Other property changes induced by mineral involvement were dependent on original biochar structure and mineral type. Diagenetic treatment slightly improved the sorption capacity of 450 and 600 °C biochars, regardless of mineral amendment. Only marginal effects were observed relating to their sorption capacity, while the structure of 250 °C biochars was most profoundly affected during diagenesis. Consequently, the simulated geochemical aging had positive effects on biochars long-term stability and ability to immobilize pollutants.

Effects of biochar on the migration and transformation of metal species in a highly acid soil contaminated with multiple metals and leached with solutions of different pH

A number of recent studies have been conducted on soil metal immobilization by biochars but there is little information on the migration and transformation of metal species in soils contaminated with multiple metals as affected by biochar and acid rain. Here, a column study investigated the effects of biochar derived from maize straw pyrolyzed at 600 °C on metal (Cu, Pb, Zn and Cd) mobility in a highly acid soil during leaching with simulated acid rain. All four metals examined were released at early stages of the leaching process and the percentages of the metals leached followed the sequence Zn > Cd > Cu > Pb. Acid rain with high acidity resulted in larger amounts of metals leached, particularly at the later stages of leaching. This enhancement of leaching by highly acidic leaching solutions was eliminated by amendment with biochar. However, the effects of biochar on metal mobility depended on metal species, with significant immobilization of soil Cu, Zn and Pb (>90%, 26% and 72%, respectively) but with no effect on soil Cd. Overall, simulated acid rain enhanced soil metal mobility and biochar reduced soil metal mobility and also alleviated the effects of acid rain. More emphasis is needed on metal speciation in the use of biochars for soil metal immobilization in areas with acid rain. The use of biochars in phytoremediation may decrease the toxicity of soil metals to the hyperaccumulator plant.

High-temperature and freeze-thaw aged biochar impacts on sulfonamide sorption and mobility in soil

Biomass-derived biochar is a carbon-rich product for soil amendment and sulfapyridine (SPY) is a typical sulfonamide of antibiotics in the soil. Amendment with biochar for soil could control SPY sorption or mobility. However, the pristine biochar inevitably goes through the long-term ageing in the environment and the information on such ageing impact on SPY sorption is not fully recognized. The simulated ageing process methods were employed for high-temperature and freeze-thraw climate to treat the biochar for two months in the present study. The batch adsorption of SPY and leaching column experiments were conducted for comparison of the fresh/aged biochar-soil system. The results showed that biochar addition could increase soil pH and saturated moisture, aged biochars own more O-containing functional groups and exhibit higher hydrophilicity and polarity. The sorption mechanism of unamended soil with SPY primarily resulted from the weak hydrophobic distribution. All fresh and aged biochar amended soil increased SPY sorption due to improvement of H-bonding interaction between SPY and biochar surface functional groups, indicating such initiative adsorption was stronger than passive partitioning. It is of importance for us to reconsider that aged biochar-amended soil, especially two-month high-temperature aged biochar-amended soil showed the highest adsorption performance and the lowest desorption capacity towards SPY. Both SPY leaching column experiments and the acid rain leaching tests suggested that the application of biochar in tropical or high-temperature climate regions for organics polluted soil remediation is favorable, but we should be aware of the uncertainty of soil amendment with biochar in cold regions.

Rapid Separation and Efficient Removal of Cd Based on Enhancing Surface Precipitation by Carbonate-Modified Biochar

The high buoyancy of biochar makes its application difficult in Cd removal. In this paper, the content of minerals was increased by modifying carbonate on the biochar surface using the vacuum impregnation method. Enhancing surface precipitation between minerals and Cd introduced a correspondingly great number of Cd precipitates on the biochar surface, leading to the rapid precipitation and separation of buoyant biochar. The physical and chemical properties of carbonate-modified biochar and the adsorption mechanism of Cd were comprehensively studied by jar tests, scanning electron microscopy-energy-dispersive X-ray spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The results showed that the adsorption of Cd by carbonate-modified biochar was controlled by multiple mechanisms, including surface precipitation, surface complexation, and Cd-π interaction. Surface precipitation dominated the removal of Cd. The contributions of Cd removal mechanisms indicated that the contribution proportions of minerals increased from 89.73 to 97.9% when the pyrolysis temperature increased from 300 to 600 °C, while the contribution proportions of Cd-π binding decreased from 9.99 to 2.08%. Meanwhile, oxygen functional groups have only a marginal effect on Cd adsorption. Besides, the results revealed that the higher surface hydrophobicity and the lower polarity were conducive to biochar separation from water. The Cd removal method can provide efficient adsorption and rapid separation, making it possible to use biochar in water treatment.

Enhancing biochar sorption properties through self-templating strategy and ultrasonic fore-modified pre-treatment: Characteristic, kinetic and mechanism studies

The traditional anaerobic sewage treatment facilities are unsuitable for the widely uncontrolled spread of antibiotic residues in hospital or on livestock farm, which have raised the risk levels of high concentrations of antibiotic residues leakages and seriously threatened the aquatic ecology safeties. Thus, to develop an effective adsorbent with safe, low cost, and high firmly adsorptive capacity are imminent required. In this investigated, a self-templating hydrothermal alkali fore-modified & ultrasonic treatment was developed to achieve the highly adsorptive capacity and low desorption rate of biochar. As expected, the prepared biochar adsorbents present plenty of surface functional groups and micro pores. The BET value is raised up 1452 cm²·g^-1 for biochar treated by the associated alkali fore-modified and ultrasonic treatment (UFB), whereas it is only 415.8 cm²·g^-1 for the biochar treated by traditional carbonization (AC) and 1205 cm²·g^-1 for the biochar by further hydrothermal alkali fore-modification (FB). Congruously, UFB exhibits the removal abilities of 397.70 mg·g^-1 of levofloxacin (LEV) and 320.99 mg·g^-1 of chlorotetracycline (CTC), 3.5-6.3 times absorbability towards familiar antibiotics than traditional biochar. Moreover, the corresponding the lowest desorption of 1.30 mg·g^-1 (LEV) and 0.43 mg·g^-1 (CTC) mg·g^-1 by UFB have been confirmed. Meanwhile, Furthermore, both the adsorption and desorption mechanisms have been addressed by kinetic studies, pore width distributions, XPS and FTIR surveys. It is proposed the fore-modified treatment is more helpful for carbon functionalization while the ultrasonic treatment dedicates to the largely microporous structures. Consequently, the adsorption's capacity and stability of UFB adsorbents is large promoted due to its more micro- and meso-porous structure through a jointly hydrothermal alkali fore-modified and ultrasonic treatment. The present investigation will provide a novel alternative preparation strategy of the highly efficient adsorbent for emergency medical wastewater treatment.

Biochar application modified growth and physiological parameters of Ocimum ciliatum L. and reduced human risk assessment under cadmium stress

Biochar (BC) is prepared from waste organic material that can improve soil health in the contaminated area. Soil pollution with cadmium (Cd) is one of the worldwide problems. The present study aimed to evaluate the BC influence on some morphophysiological and biochemical characteristics, also Cd concentration of Ocimum ciliatum L. leaves under Cd stress as well as human risk assessment. Therefore, a pot factorial arrangement based on a completely randomized design was done which included three levels of BC (non-BC, 1%, and 2% of the pot soil) and three Cd levels (0, 20, and 40 mg/kg soil) with three replications. The results of the present study indicated that BC application improved morphological traits, photosynthetic pigments, relative water content (RWC), and catalase (CAT) activity of O. ciliatum under Cd stress and reduced total soluble sugars, total phenol, antioxidant activity, proline content, electrolyte leakage (EL), soluble protein content, ascorbate peroxidase (APX), and guaiacol peroxidase (GPX) activities, and Cd concentration as well as target hazard quotient (THQ). In conclusion, based on the findings of this study, BC could be applied as an environmental friendly amendment in Cd-polluted soil to ameliorate the negative influences of Cd stress on O. ciliatum and reduces Cd levels and THQ in the plants due to the absorption properties of BC. This means that BC usage in contaminated soil helps to reduce pollutions and decreases the human risk assessment.

A meta-analysis of heavy metal bioavailability response to biochar aging: Importance of soil and biochar properties

Biochar has been widely applied to remediate the heavy metal-polluted soils, whereas biochar aging can induce the changes of the biochar physic-chemical properties. Afterwards, the bioavailability of heavy metals (BHM) will vary in soils which likely increase the unstable fractions of heavy metals and the following environmental risks. To explore the biochar aging effects on the BHM changes in responses to the variation of experimental conditions and biochar properties, a meta-analysis for the literatures published before May 2020 was conducted. A sum of 257 independent observations from 22 published papers was obtained. The results from the analysis of boosted regression tree showed that the soil pH was the most important factor influencing the BHM changes in biochar amended soil, followed by soil texture, aging time and biochar pyrolysis temperature. The results of this review showed that the BHM was decreased by 16.9%, 28.7% and 6.4% in weakly acid soil (pH 6.00-6.99), coarse- and medium-textured soils, respectively, but increased by 149% and 121% in the alkaline (pH > 8.00) and fine-textured soils. The BHM declined in the soils amended with biochar pyrolyzed at relative high temperature (> 500 °C), and increased during aging in soils amended with biochar pyrolyzed at relatively low temperature (401-500 °C). In terms of diverse immobilized heavy metals, only bioavailable Zn in soil decreased after aging. However, there was no significant changes in Cd, Cu and Pb's bioavalability. Besides, the BHM was decreased by 18.6% within the short-term (less than one year) biochar aging, while showed inverse trend during the longer aging processes. Besides, the application of lignin-enriched biochar may counteract the positive effects of the biochar aging on BHM. Our works may promote the interpretation of the interference factors on the BHM changes and filled the research gaps on biochar aging process in soils.