Potentially toxic metals have become a viable threat to the ecosystem due to their carcinogenic nature. Biochar has gained substantial interest due to its redox-mediated processes and redox-active metals. Biochar has the capacity to directly adsorb the pollutants from contaminated environments through several mechanisms such as coprecipitation, complexation, ion exchange, and electrostatic interaction. Biochar's electron-mediating potential may be influenced by the cyclic transition of surface moieties and conjugated carbon structures. Thus, pyrolysis configuration, biomass material, retention time, oxygen flow, and heating time also affect biochar's redox properties. Generally, reactive oxygen species (ROS) exist as free radicals (FRs) in radical and non-radical forms, i.e., hydroxyl radical, superoxide, nitric oxide, hydrogen peroxide, and singlet oxygen. Heavy metals are involved in the production of FRs during redox-mediated reactions, which may contribute to ROS formation. This review aims to critically evaluate the redox-mediated characteristics of biochar produced from various biomass feedstocks under different pyrolysis conditions. In addition, we assessed the impact of biochar-assisted FRs redox-mediated processes on heavy metal immobilization and mobility. We also revealed new insights into the function of FRs in biochar and its potential uses for environment-friendly remediation and reducing the dependency on fossil-based materials, utilizing local residual biomass as a raw material in terms of sustainability.

Microbial biosurfactants are surface-active molecules that are naturally produced by a range of microorganisms. They have certain advantages over chemical surfactants, such as lower toxicity, higher biodegradability, anti-tumor, and anti-microbial properties. Sophorolipids (SLs) in particular are one of the most promising biosurfactants, as they hold the largest share of the biosurfactant market. Currently, researchers are developing novel approaches for SL production that utilize renewable feedstocks and advanced separation technologies. However, challenges still exist regarding consumption of materials, enzymes, and electricity, that are primarily fossil based. Researchers lack a clear understanding of the associated environmental impacts. It is imperative to quantify and optimize the environmental impacts associated with this emerging technology very early in its design phase to guide a sustainable scale-up. It is necessary to take a collaborative perspective, wherein life cycle assessment (LCA) experts work with experimentalists, to quantify environmental impacts and provide recommendations for improvements in the novel waste-derived SL production pathways. Studies that have analyzed the environmental sustainability of microbial biosurfactant production are very scarce in literature. Hence, in this work, we explore the possibility of applying LCA to evaluate the environmental sustainability of SL production. A dynamic LCA (dLCA) framework that quantifies the environmental impacts of a process in an iterative manner, is proposed and applied to evaluate SL production. The first traversal of the dLCA was associated with the selection of an optimal feedstock, and results identified food waste as a promising feedstock. The second traversal compared fermentation coupled with alternative separation techniques, and highlighted that the fed-batch fermentation of food waste integrated with the in-situ separation technique resulted in less environmental impacts. These results will guide experimentalists to further optimize those processes, and improve the environmental sustainability of SL production. Resultant datasets can be iteratively used in subsequent traversals to account for technological changes and mitigate the corresponding impacts before scaling up.

Forest fires significantly affect the wildlife, vegetation, composition and structure of the forests. This study explores the potential of partially burnt wood recovered in the aftermath of a recent Canadian forest fire incident as a feedstock for generating hydrogen-rich syngas through hydrothermal gasification. Partially burnt wood was gasified in hydrothermal conditions to study the influence of process temperature (300–500 °C), residence time (15–45 min), feed concentration (10–20 wt%) and biomass particle size (0.13 mm and 0.8 mm) using the statistical Taguchi method. Maximum hydrogen yield and total gas yield of 5.26 mmol/g and 11.88 mmol/g, respectively were obtained under optimized process conditions at 500 °C in 45 min with 10 wt% feed concentration using biomass particle size of 0.13 mm. The results from the mean of hydrogen yield show that the contribution of each experimental factors was in the order of temperature > feed concentration > residence time > biomass particle size. Other gaseous products obtained at optimum conditions include CO₂ (3.43 mmol/g), CH₄ (3.13 mmol/g) and C₂–C₄ hydrocarbons (0.06 mmol/g).

The effectiveness of biochar as a sorptive material to remove contaminants, particularly heavy metals, from water is dependent on biomass type and pyrolysis condition. Biochars were produced from pulp mill sludge (PMS) and rice straw (RS) with nitrogen (N₂) or carbon dioxide (CO₂) as the purging gas. The sorptive capacity of the biochars for cadmium(II), copper(II), nickel(II) and lead(II) was studied. The heavy metal adsorption capacity was mainly affected by biomass type, with biochars adsorption capacities higher for lead(II) (109.9–256.4 mg g⁻¹) than for nickel(II) (40.2–64.1 mg g⁻¹), cadmium(II) (29.5–42.7 mg g⁻¹) and copper(II) (18.5–39.4 mg g⁻¹) based on the Langmuir adsorption model. The highest lead(II) adsorption capacities for PMS and RS biochars were 256.4 and 133.3 mg g⁻¹, respectively, when generated using N₂ as the purging gas. The corresponding lead(II) adsorption capacities were 250.0 and 109.9 mg g⁻¹, respectively, when generated using CO₂ as the purging gas. According to the intraparticle diffusion model, 30–62% of heavy metal adsorption was achieved in 1 h; film diffusion was the rate-dominating step, whereas pore diffusion was a rate-limiting step. Ion exchange and complexation between heavy metals and biochar surface functional groups such as carbonyl and hydroxyl groups were effective mechanisms for heavy metal sorption from the aqueous solution. We conclude that proper selection of both the feedstock type and the purging gas is important in designing biochars for the effective removal of potentially toxic metals from wastewater.

Biochar is a promising immobilizing agent of trace elements (TEs) in contaminated soils. However, several contradictory results have been reported regarding the potential of biochar to immobilize arsenic (As), chromium (Cr), and nickel (Ni) in contaminated soils. We conducted a meta-analysis on the published papers since 2006 until 2019 to examine the effects of biochar on the chemical (im)mobilization of As, Cr, and Ni in contaminated soils and to elucidate the major factors that control their interactions with biochar in soil. We synthesized 48 individual papers comprised of a total of 9351 pairwise comparisons and used the statistical tool of Cohen's d as an appropriate effect size for the comparison between means. We found that the application of biochar often increased the As mobilization in soils. Important variables that modulated the biochar effects on As mobilization in soil were pyrolysis temperature and time (ranging between 8 and 16 times when T > 450 °C and t > 1hr), organic matter (7–16 times when SOM<3%) and further site conditions. In contrast to As, biochar efficiently immobilized Cr and Ni in contaminated soils. The extent of the Cr and Ni immobilization was determined by the feedstock (Cr: 7–18 times for agricultural residue-derived biochar; Ni: 13–32 times for woody biomass-derived biochar). Our meta-analysis provides a compilation on the potential of different types of biochar to reduce/increase the mobilization of As, Cr, and Ni in various soils and under different experimental conditions. This study provides important insights on factors that affect biochar's efficiency for the (im)mobilization of As, Cr, and Ni in contaminated soils. While biochar effectively immobilizes Cr and Ni, a proper management of As-polluted soils with pristine biochar is still challenging. This limitation might be overcome by modification of biochar surfaces to exhibit higher surface area and functionality and active sites for surface complexation with TEs.

Recently, the adsorption-based environmental remediation techniques have gained a considerable attention, due to their economic viability and simplicity over other methods. Hence, detailed presentation and analysis were herein focused on describing the role of biochar in oil spill removal. Oil removal by utilizing biochar is assumed as a green-oriented concept. Biochar is a carbon-rich low-cost material with high porosity and specific surface chemistry, with a tremendous potentiality for oil removal from aqueous solutions. Oil sorption properties of biochar mainly depend on the biochar production/synthesis method, and the biomass feedstock type. In order to preserve the stability of functional groups in the structure, biochar needs to be produced/activated at low temperatures (<700 ᵒC). In general, biochar derived from biomass containing high lignin content via slow pyrolysis is more favorable for oil removal. Exceptional characteristics of biochar which intensify the oil removal capability such as hydrophobicity, oleophilicity or/and specific contaminant-surface interaction of biochar can be enhanced and be tuned by chemical and physical activation methods. Considering all the presented results, future perspectives such as the examination of biochar efficacy on oil removal efficiency in multi-element contaminated aqueous solutions to identify the best biomass feedstocks, the production protocols and large-scale field trials, are also discussed.

Biochar has recently been fascinating for research in many environment areas due to its potential applications. In this research, graphene, and nano zero-valent iron (nZVI) were integrated with biochar and used for copper immobilization in the soil. Initially, the biomass feedstock was pyrolyzed under N₂ atmosphere from 150 to 650 °C and immersed in an aqueous solution containing graphene, and then impregnated with nZVI. Laboratory characterization with different instruments (eg. SEM, TEM, XRD, UV–Vis, VSM, and XPS) showed that graphene sheets and reactive nZVI were loaded on the biochar surface during the development process. The 450 °C was considered as optimum pyrolysis temperature based on the effective surface properties of the obtain biochar material. Boehm titration and functional group analysis confirmed the presence of carboxylic groups, phenolic groups in the corn stack biochar supported graphene oxide/nZVI (CTBC-GO/nZVI). Thermogravimetric analysis showed that nZVI incorporation to biochar surface could improve thermal stability as compared to graphene oxide incorporated biochar and pristine biochar. The material was utilized for copper (Cu) immobilization in the soil and a comparative evaluation was established on the basis of efficiency. The soil experiment showed that the CTBC-GO/nZVI has a superior immobilization efficiency of copper than pristine biochar and GO@BC. The available Cu content decreased by > 65% in CTBC-GO/nZVI amended soil after 14 days. Sequential extraction procedure (SEP) results suggested that CTBC-GO/nZVI promoted the conversion of more accessible Cu into the less accessible and bioavailable forms to reduce the toxicity of Cu. Therefore, CTBC-GO/nZVI composite is a promising and effective amendment for immobilizing Cu in contaminated soils and improving soil properties.This work can put forward a strategy to develop magnetic biochar composites and an application towards toxic heavy metals immobilization in soil.

Aminocyclopyrachlor is a pyrimidine carboxylic acid herbicide used to control broadleaf weeds and brush. Amending soil with activated charcoal is recommended to prevent off-site transport of aminocyclopyrachlor and non-target plant damage. We used the batch-equilibrium method to determine the concentration of aminocyclopyrachlor in a pseudo-steady state with biochar, soil, and biochar-soil systems (<10% biochar by weight). We observed that aminocyclopyrachlor is mobile in soils. Soil incorporation of activated charcoal removed nearly all of the aqueous aminocyclopyrachlor thereby limiting its bioavailability to non-target flora. On the other hand, biochars were less effective than activated charcoal. Biochar produced from olive mill waste feedstock was the most effective biochar that we assessed for reducing the aqueous herbicide concentration. Although these biochars reduced the aminocyclopyrachlor concentration, they would not be practical remediation media due to the extraordinarily high application rates required to reduce the concentration by 50% (2.13 × 105 kg ha−1–7.27 × 105 kg ha−1).

Polybrominated dibenzo-p-dioxins/furans (PBDD/Fs) and polychlorinated dibenzo-p-dioxins/furans (PCDD/Fs) share similar toxicities and thermal origins, e.g., municipal solid waste incinerator (MSWI). Recently, PBDD/Fs from MSWI attracted rising concern because their important precursors, i.e., brominated flame retardants (BFRs), were frequently found in various wastes for landfill or MSWI feedstock. So far, however, little is known about PBDD/Fs and their associated risks in the vicinal environments of MSWI. Here we analyzed PBDD/Fs and PCDD/Fs in 29 soil samples collected around a multiyear large-scale MSWI, and compared their spatial distributions, sources and risks. PBDD/Fs demonstrated comparable concentrations and toxic equivalent quantities (TEQs) to PCDD/Fs in these samples. Spatially, both the concentrations of PBDD/Fs and PCDD/Fs decreased outwards from the MSWI, and exhibited significant linear correlations with the distances from the MSWI in the southeast downwind soil, suggesting the influence of the MSWI on its vicinal soil environment. However, the existence of other dioxin sources concealed its influence beyond 6 km. PBDD/Fs in the soils were characterized by highly-brominated PBDFs, especially Octa-BDF, and their sources were diagnosed as the MSWI and diesel exhaust; PCDD/Fs, however, were dominated by highly-chlorinated PCDDs, particularly Octa-CDD, and were contributed individually or jointly by the MSWI, automobile exhaust and pentachlorophenol (PCP)/Na-PCP. The non-carcinogenic risks of dioxins in all the soil samples were acceptable, but their carcinogenic risks in 17% of the samples were unacceptable. These samples were all located close to the MSWI and highways, therefore, the land use of these two high-risk zones should be cautiously planed.

Soil fumigation continues to play an important role in soil disinfection, but tools to significantly reduce emissions while providing environmental benefits (e.g., biochar) are lacking. The objective of this study was to determine the effects of biochar products on fumigant 1,3-dichloropropene (1,3-D) and chloropicrin (CP) emissions, their distribution and persistence in soil, nematode control, and potential toxicity to plants in a field trial. Treatments included three biochar products [two derived from almond shells (ASB) at either 550 or 900 °C pyrolysis temperature and one from coconut shells (CSB) at 550 °C] at 30 and 60 t ha⁻¹, a surface covering with a low permeability film (TIF), and no surface covering (control). A mixture of 1,3-D (∼65%) and CP (∼35%) was injected to ∼60 cm soil depth at a combined rate of 640 kg ha⁻¹. All biochar treatments significantly reduced emissions by 38–100% compared to the control. The ASB (900 °C) at both rates reduced emissions as effectively as the TIF (by 99–100%). Both fumigant emission reduction and residue in surface soil were positively correlated with biochar's adsorption capacity while cucumber germination rate and dry biomass were negatively correlated with residual fumigant concentrations in surface soil. This research demonstrated the potential and benefits of using biochar produced from local orchard feedstocks to control fumigant emissions. Additional research is needed to maximize the benefits of biochar on fumigant emission reductions without impacting plant growth.