The role of extracellular polymeric substances (EPS) in chemical-degradation of persistent organic pollutants in soil: A review.

Persistent organic pollutants (POPs) in soil show high environmental risk due to their high toxicity and low biodegradability. Studies have demonstrated the degradation function of microbial extracellular polymeric substances (EPS) on POPs in various matrices. However, the degradation mechanisms and the factors that influence the process in soil have not been clearly illustrated. In this review, the characteristics of EPS were introduced and the possible mechanisms of EPS on degradation of organic pollutants (e.g., external electron transfer, photodegradation, and enzyme catalysis) were comprehensively discussed. In addition, the environmental conditions (e.g., UV, nutrients, and redox potential) that could influence the production and degradation-related active components of EPS were addressed. Moreover, the current approaches on the application of EPS in biotechnology were summarized. Further, the future perspectives of enhancement on degradation of POPs by regulating EPS were discussed. Overall, this review could provide a new thought on remediation of POPs by widely-existing EPS in soil with low-cost and minimized eco-disturbance.

Biochar-based materials as remediation strategy in petroleum hydrocarbon-contaminated soil and water: Performances, mechanisms, and environmental impact.

Petroleum contamination is considered as a major risk to the health of humans and environment. Biochars as low-cost and eco-friendly carbon materials, have been widely used for the removal of petroleum hydrocarbon in the environment. The purpose of this paper is to review the performance, mechanisms, and potential environmental toxicity of biochar, modified biochar and its integration use with other materials in petroleum contaminated soil and water. Specifically, the use of biochar in oil-contaminated water and soil as well as the factors that could influence the removal ability of biochar were systematically evaluated. In addition, the modification and integrated use of biochar for improving the removal efficiency were summarized from the aspects of sorption, biodegradation, chemical degradation, and reusability. Moreover, the functional impacts and associated ecotoxicity of pristine and modified biochars in various environments were demonstrated. Finally, some shortcoming of current approaches, and future research needs were provided for the future direction and challenges of modified biochar research. Overall, this paper gain insight into biochar application in petroleum remediation from the perspectives of performance enhancement and environmental sustainability.

Tea-based biochar-mediated changes in cation diffusion homeostasis in rice grown in heavy metal (loid) contaminated mining soil.

Foreseeable future scenarios highlight the urgency of applying eco-safe avoidance methods or tolerance to heavy metal(loid) (HM) stress in agricultural production areas of contamination. The analyses show that the Ni, Mn, As, and Cr concentrations detected in the soils of the paddy fields in the Black Sea region vary between 123.60 and 263.30; 687-1271; 8.90-14.50; 162.00-340.00 mg kg-1 proving high accumulation of Ni, Mn, As, Cr in rice. Overconsumption of rice farmed extensively on these soils might also lead to human HM-related health problems. Therefore, in the current study, the approach of using tea-based biochar (BC) proven to have one of the most significant potentials as a soil amendment to reduce HM transmission to in-vitro-grown rice plants was investigated in the soil medium naturally contaminated with HMs. The tea-BC was produced from readily available local black tea waste of a conventional fermentation process and applied in the in-vitro experiments. Among the tested doses examined, 1% tea-BC showed a more positive effect on rice plant growth and development characterized by a better relative growth rate (59.7 and 84 mg g-1 d-1 for root and shoot tissues), photosynthetic pigment intactness (62.48 µg mL-1), cellular membrane integrity (93%), and relative water (96%) than the other rates (0% BC, 3%BC, 5%BC). The mRNA expression data highlights the probability of a cation diffusion facilitator (CDF) (OsMTP11) in concert with catalase isozyme (CATa) and dehydration-responsive element binding protein (DREB1a) linking the HM detoxification, oxidative defense, and dehydration pathways with the help of tea-BC. At the optimum concentration (1%BC), this approach might reduce HM accumulation levels of crops planted in HM-contaminated farmlands.

Solid digestate biochar amendment on pig manure composting: Nitrogen cycle and balance.

Effect of solid digestate biochar (DB) on nitrogen cycle and balance was evaluated during composting by adding DB into mixtures of pig manure and Lycium chinensis branch filings. Results indicated that DB addition improved composting microenvironment and increased the total N content of the final product. Furthermore, N balance calculation indicated that the NH3 and N2O emissions accounted for 72.14%-81.39% and 0.49%-2.37% of the total N loss without DB addition, respectively. After using DB, the N reductions in the form of NH3 and N2O reduced from 10.78% to < 5.73% and from 0.34% to < 0.041% of total N, respectively. Addition of DB affected N fixation with 92.32%-93.67% of total N fixed in the compost than that of the T1 treatment (85.63%). DB amendment enhanced the aerobic bacterial communities and hindered anaerobic bacterial growth, thus benefiting the NH3 and N2O emission mitigation and N conservation.

Effects of biochar and N-stabilizers on greenhouse gas emissions from a subtropical pasture field applied with organic and inorganic nitrogen fertilizers.

Pasturelands contribute significantly to the global CO2, CH4 and N2O emissions. These gas emissions are influenced by the amount and type of N-fertilizers applied and local climate. Recent studies showed potential of biochar and N-stabilizer compounds in minimizing CO2, CH4 and N2O emissions by regulating N-release from N-fertilizers. The present study was aimed at determining and comparing the effects of biochar and N-(n-butyl) thiophosphoric triamide + dicyandiamide (N-stabilizer) on CO2, N2O and CH4 emissions from a pasture fertilized with cattle manure or urea. The study was conducted during 2015 and 2016 in an established bermudagrass (Cynodon dactylon L. Pers.). Treatments consisted of combination of N-sources (manure, and urea) and two mitigation technologies [pine hardwood biochar (BC) and N-stabilizer] along with control. Emissions of GHGs were measured from each plot using static chamber systems. Both BC and N-stabilizer applications with manure applied to the hay field significantly decreased N2O emissions by 42% and 45%, respectively, in the year-2, and emission factors compared to manure only treatment. Addition of N-stabilizer to urea had significantly decreased N2O emissions compared to urea alone, while BC had statistically insignificant effect although numerically lowered N2O emissions in both the years. Application of manure to the soil resulted in significantly higher CO2 emissions in both years and CH4 emissions in 2016 compared to unfertilized soil. Urea application had significant effect on CO2 emissions in 2016, while no effect on CH4 emissions compared to control. Application of either biochar or N-stabilizer did not significantly affect CO2 and CH4 emissions.

Transcriptional insights into Cu related tolerance strategies in maize linked to a novel tea-biochar.

One-third of maize cultivation in Turkey has been performed in nutrient-rich soils of the coastal agricultural lands of the Black Sea Region, which is among the country's granaries. However, the yield of this chief crop is affected by Cu toxicity due to a decades-long abandoned opencast Cu-mine. As part of the modern agenda, against this problem, we valorized one of the region's signature plant waste by synthesizing a tea-derived biochar (BC) and evaluated for remediation effect on maize Cu tolerance. Among other rates (0%, 0.4%, 0.8%, 1.6%), maximum Cu absorption (168.27 mg kg-1) was found in the 5%BC in in-vitro spiking experiments where natural Cu contamination levels were mimicked. Obvious increasing trends in both root and shoot tissues of maize plantlets growing in Cu-spiked soil (260.26 ± 5.19 mg Cu kg-1) were recorded with proportionally increasing BC application rates. The black tea waste-BC (5%) amendment remarkably reduced the Cu uptake from Cu spiked-soil and showed no phenotypic retardation in maize. Accordingly, it boosted the metabolic and transcriptomic profile owing to up-regulation in the aquaporin and defense genes (PIP1;5 and POD1) by 1.31 and 1.6 fold. The tea-BC application also improved the soil-plant water relations by minimizing cytosolic volume changes between 85 and 90%, increasing chlorophyll intactness (65%) and membrane stability up to 41%. The tea-BC could be a strong agent with potential agronomic benefits in the remediation of the cationic Cu toxicity that occurred in the mining-contaminated agricultural soils.

Integrated application effects of biochar and plant residue on ammonia loss, heavy metal immobilization, and estrogen dissipation during the composting of poultry manure.

Inadequate handling of poultry manure can cause significant releases of NH3, heavy metals, and estrogen, thereby impairing environmental quality. This study was a composting experiment involving the combination of poultry manure with plant residues (corn stalks, mushroom residues, and vegetable straw), as well as with either wheat stalk biochar (WB) or rice husk biochar (RB). The integrated effects of plant residues and biochar on NH3 loss, heavy metal (Cu, Zn, As, and Cd) stabilization, and 17ß-estradiol (E2) dissipation were investigated during composting. The poultry manure co-composted with corn straw and mushroom residue showed the highest potential for E2 degradation. Biochar enhanced E2 dissipation and decreased estrogenic activity in all treatments, with RB showing a stronger effect than WB. Both biochars decreased microbial diversity and increased bacterial groups related to E2 and organic matter degradation·NH3 emission was reduced by 50-82% with 15% WB and 86-97% with 15% RB. The extractable fraction of As was reduced by 0-53% with WB and 50-84% with RB, while that of Cd was reduced by 5-28% for WB and 25-41% for RB in poultry manure compost. However, biochar showed little effect on Cu and Zn. Biochar appears to have a promotional effect and enhances the microbial degradation of E2. Specifically, the integration of corn stalks, mushroom residues, and RB in poultry manure compost had a positive effect by preventing nitrogen loss while reducing the bioavailability of heavy metals and hormones.

Removal of potentially toxic metal by biochar derived from rendered solid residue with high content of protein and bone tissue.

The purpose of this study was to produce rendering animal carcass residue char (RACR-C) by pyrolyzing the solid residues of low-recyclable rendered pig carcasses and to evaluate their cadmium (Cd) adsorption characteristics and mechanisms. As the pyrolysis temperature increased, the inorganic content of RACR-C increased, while the carbon content decreased. In particular, the surface structure and chemistry of RACR-Cs prepared at different pyrolysis temperatures were well described by SEM-EDS, XRD, XRF, TGA, and FTIR. The Cd adsorption characteristics of RACR-C were in good agreement with the Langmuir isotherm and pseudo-second-order models, and the Cd adsorption capacities of RACR-Cs prepared at various pyrolysis temperatures were in the order of RACR-C500 (73.5 mg/g)> RACR-C600 (53.8 mg/g)> RACR-C400 (41.5 mg/g) " RACR-C250 (15.9 mg/g). The intraparticle diffusion model suggested that the adsorption of Cd by RACR-C is greatly influenced by internal diffusion as well as external boundary. Since the Cd adsorption capacity of RACR-C is greatly influenced by the initial dosage, pH, and co-existing metals, it is necessary to manage these influencing factors when treating wastewater containing heavy metals. Our results suggest that Cd adsorption by RACR-C is a complex adsorption phenomenon by various mechanisms such as adsorption by functional group (CË­C and C-O), precipitation of Cd-P and ion exchange reaction by exchangeable cation occurring rather than by a single specific mechanism.

Application of biochar in estrogen hormone-contaminated and manure-affected soils: Impact on soil respiration, microbial community and enzyme activity.

Biochar as a soil amendment has been proposed for enhancing carbon sequestration and manure-borne hormone contaminant remediation. However, little is known about the ecological risk of biochar application in the soil with hormone contamination. This study investigated the influence of biochar in three manure-impacted soils contaminated with estrogen hormones, natural estrogen 17ß-estradiol and synthesized estrogen 17α-ethinylestradiol in a microcosm experiment. Specifically, microbial respiration was periodically determined during microcosm incubation while microbial community phospholipid fatty acids and activities of nutrient (C, N, P, S) cycling related enzymes (ß-glucosidase, urease, phosphodiesterase, arylsulfatase) were characterized after the incubation. Results showed that the manure-impacted soils with high SOC generally had greater total microbial biomass, ratios of fungi/bacteria and Gram-positive bacteria/Gram-negative bacteria, and phosphodiesterase activity, but lower urease activity. Additionally, hormones stimulated microbial respiration and biomass, while had little impact on activity of the enzymes. On the other hand, biochar showed negative priming effect by significantly decreasing total microbial biomass by 8.7%-26.4%, CO2 production by 16.6%-33.5%, and glucosidase activity by 27.1%-41.0% in the three soils. Biochar significantly increased the activity of phosphodiesterase, showed no impact on arylsulfatase, while decreased the activity of urease. Overall, the study suggests that when used in hormone remediation in manure-impact soils, biochar could improve phosphodiesterase activity, but may decrease soil microbial activity and the activity of soil glucosidase and urease.

Remediation of crude oil-contaminated coastal marsh soil: Integrated effect of biochar, rhamnolipid biosurfactant and nitrogen application.

Decontamination of oil spills from coastal wetland soils requires a delicate approach. A microcosm study was carried out to investigate the impact of integrated application of biochar, rhamnolipid (RL) biosurfactant and nitrogen (N) on petroleum hydrocarbon remediation in a Louisiana coastal saline marsh and their impact on soil microbial community. The soil was artificially contaminated with crude oil and subjected to treatments of different combinations of sugarcane residue biochar, RL, and coated urea. Total petroleum hydrocarbons (TPH) in the contaminated soil were analyzed periodically using gas chromatograph and associated soil bacterial community was studied using 16 s rRNA sequencing technologies. Results showed that integrated application of biochar + RL, biochar + N, and biochar + N+RL reduced 32.3%, 73.2%, 80.9% of TPH, respectively, and exhibited synergic interaction with higher efficiency than application individually. Combined treatments showed distinct functions that biochar increased the sorption of aromatic compounds, while RL and N enhanced the degradation of heavy and light aliphatic compounds. All remediation treatments caused reduction of soil bacterial diversity while RL and N shifted the microbial community to higher abundances of Proteobacteria and Bacteroidetes, respectively. Overall, the findings of this study demonstrate the positivity of applying integrated biochar, biosurfactant, and N treatment in oil remediation in wetland soils.

Potential use of biochar and rhamnolipid biosurfactant for remediation of crude oil-contaminated coastal wetland soil: Ecotoxicity assessment.

Remediation of wetland soils contaminated with petroleum hydrocarbons is a challenging task. Biosurfactant and biochar have been used in oil remediation. However, little is known about the ecotoxicity of these materials when applied in wetland ecosystems. In this study, the ecotoxicity of biochar and rhamnolipid (RL) biosurfactant as crude oil remediation strategies in a Louisiana wetland soil was investigated. A pot experiment was set up with wetland soil treated with/without crude oil followed by subjecting to application of 1% biochar and various levels of RL ranging from 0.1% to 1.4%. The ecotoxicity was evaluated regarding to high plant (S. Alterniflora), algae, and soil microbes. Specifically, after a 30-day growth in a controlled chamber, plant biomass change as well as shoot/root ratio was measured. Algae growth was estimated by quantifying chlorophyll by spectrometry following separation, and soil microbial community was characterized by phospholipid fatty acids analysis. Results showed that plant can tolerate RL level up to 0.8%, while algae growth was strongly inhibited at RL > 0.1%. Algal biomass was significantly increased by biochar, which offset the negative impact of oil and RL. Additionally, soil microbial community shift caused by crude oil and RL was alleviated by biochar with promoting Gram-positive bacteria, actinomycetes, and arbuscular mycorrhizal fungi. Overall, this study shows that integrated treatment of biochar and RL has the lowest ecotoxicity to plant and algae when used in oil remediation of contaminated wetland soils.

Exploring anaerobic CO2 production response to elevated nitrate levels in Gulf of Mexico coastal wetlands: Phenomena and relationships.

Recent studies have shown the effect of nitrate (NO3-) on carbon gas emissions from wetland soils that contradict thermodynamic predictions. In this study, CO2 production in three Mississippi River deltaic plain wetland soils (forest swamp, freshwater and saline marshes) with the presence of different NO3- levels (0.2, 2.0, and 3.2 mM) was evaluated in an anaerobic microcosm. Molecular composition of dissolved organic matter (DOM) of these soils was investigated using pyrolysis-GC/MS, and soil microbial community was characterized based on phosphorus lipid fatty acid (PLFA) method to elucidate the underlying mechanisms. Addition of NO3- promoted CO2 production in swamp forest soil, but inhibited CO2 emission from marsh soils. Pyrolysis-GC/MS analysis showed that swamp soil contained more polysaccharides, whereas both marsh soils had high abundance of phenolic compounds. Total PLFAs of forest swamp soil were 34% and 66% higher than freshwater and saline marsh soils, respectively. The PLFA profiles indicated different microbial distribution along a salinity gradient with the forest swamp having a higher proportion of fungi and NO3- reducers but lower sulfate (SO42-) reducers than marsh soils. Overall, the study indicated that the inherent differences in soil DOM and microbial community led to the contrasting response in soil CO2 respiration between forest swamp and marsh ecosystems to NO3- loading. These differences should be considered in determining the fate of nitrate entering Louisiana coastal wetlands from river diversions and other sources and their management.

Removing tetracycline and Hg(II) with ball-milled magnetic nanobiochar and its potential on polluted irrigation water reclamation.

The feasibility of ball-milled magnetic nanobiochars (BMBCs) derived from wheat straw for adsorptive removal of tetracycline (TC) and Hg(II) from aqueous solution was assessed against that of pristine magnetic biochars (PMBCs). Ball milling conversion of PMBCs into BMBCs greatly improved TC and Hg(II) removal, and ≥ 99% TC and Hg(II) were adsorbed by BMBC prepared at 700 °C (BMBC700) within 12 h. The maximum adsorptive removal capacities of BMBC700 for TC and Hg(II) were 268.3 and 127.4 mg/g, respectively. The amounts of TC and Hg(II) removed by BMBC700 decreased gradually as the ionic strength of the solution increased, but increased as the solution temperature increased from 25 to 45 °C. The further FTIR and XPS analysis confirmed removal of TC was predominately regulated by the combination of electrostatic interactions, hydrogen bonds, and Cπ-Cπ interaction, while, the adsorption of Hg(II) was mainly governed by several mechanisms, including electrostatic attractions, Hg-Cπ bond formation, and surface complexation. Overall, BMBC700 presented great potential for TC and Hg(II) removal from polluted irrigation water and exhibited acceptable recyclability performance as well as magnetic separation advantage in use.

Sorption and Desorption Characteristics of Tylosin in Three Louisiana Soils.

The macrolide antibiotic tylosin is widely used in animal production, but its environmental fate is not fully understood. Objectives of this study were to determine the effect of pH on tylosin A sorption and desorption in three sandy loam soils from Louisiana, USA, that had long histories of poultry waste application, to model sorption and desorption, and to estimate the effect of high soil organic matter on sorption. Twenty-four-hour sorption isotherms (5 to 200 mg L in 0.01 M CaCl) at pH 4.5, 6.0, and pH 7.5 were described by the Freundlich model. Desorption from the 200-mg L set at constant pH by 10 24-h extractions with 0.01 M CaCl recovered 43 to 98% of the added tylosin A, and further desorption with methanol increased recovery from 66 to 100%. Single-point distribution coefficient, as a function of pH from 4 to 9 in 0.01 M CaCl exhibited maxima from pH 6 to 7, reflecting increasing sorption of the positively charged form with increasing pH up to about the pKa. The data were well described, modeling pH-dependent negative charge according to Henderson-Hasselbalch along with tylosin speciation. Using soil from which organic matter had been removed by HO gave three to six times greater K at pH 6 to 7, but without or with reduced maxima. The data could be approximately described without invoking pH-dependent surface charge. Further study is needed to confirm whether an increase in soil organic matter with poultry waste application decreases tylosin sorption, thus increasing environmental risk.

Cadmium adsorption characteristics of biochars derived using various pine tree residues and pyrolysis temperatures.

This study investigated the characteristics of biochars derived using various pine tree residues and pyrolysis temperatures and evaluated their Cd adsorption behaviors. The characteristics of pine tree residue biochars (PRBs) were dominantly affected by the pyrolysis temperature, and the optimum pyrolysis temperature for Cd adsorption was 600 °C. The adsorption of Cd by PRBs was divided into two stages: rapid adsorption on the initial boundary layer and slow adsorption by intraparticle diffusion. The Cd adsorption characteristics of all the PRBs were well described by pseudo-second-order and Langmuir isotherm models, and the maximum adsorption capacity was the highest in pine bark biochar (85.8 mg/g). The amounts of the cations released from the mixed pine tree residue biochars (M-PRBs) during Cd adsorption were increased, while the amount of phosphate released was decreased, indicating that exchangeable cations and phosphate on the biochar affected the Cd adsorption. In particular, the amount of Cd removed by the exchangeable cations corresponds to 23.6% of the total adsorption amount. Spectroscopic analyses using FTIR showed that the Cd adsorption on M-PRB was associated with functional groups such as CC, COH and COOH. Overall, the use of biochars derived from pine tree residue as an adsorbent is considered to be effective for both the treatment of wastewater containing heavy metals and the recycling of forest residues.

Effect of biochar amendment on sorption-desorption and dissipation of 17α­ethinylestradiol in sandy loam and clay soils.

Animal manure application in agricultural land has caused the release of steroid estrogens in the soil environment and further movement to aquatic systems. The objective of this study was to investigate the effects of biochar addition on sorption-desorption and dissipation behaviors of 17α­ethinylestradiol (EE2) in two different textured soils. A Commerce sandy loam and a Shakey clay were selected and subjected to sterilization. Soil samples with and without sterilization were reacted with a series of EE2 solutions of different concentrations for sorption followed by desorption and quantification using HPLC-MS/MS. Long-term dissipation of EE2 in the same soils was also evaluated over a 30-d incubation. Biochar amendment increased the maximum EE2 sorption capacity but decreased its water desorption in both sandy loam and clay soils. On other hand, biochar addition increased the Koc in the clay soil which had low EE2 sorption efficiency but decreased Koc in the sandy loam which had high EE2 sorption efficiency. Biochar did significantly increase both desorbable and non-extractable fractions of EE2, while it reduced the bioavailability of EE2 to microbial degradation. The dissipation of EE2 in non-sterilized soils fit to the first-order kinetic model, whereas it was better described by zero-order kinetic for sterilized soil. Biochar increased the half-life of EE2 dissipation in non-sterilized Commerce sandy loam soil by 48% (from 3.63 to 5.37 d) and in non-sterilized Sharkey clay soil by 67% (from 2.28 to 3.81 d). Overall, this study demonstrated positive impacts of biochar on the retention of estrogen hormones in soils.

Lead sorption characteristics of various chicken bone part-derived chars.

Recycling food waste for beneficial use is becoming increasingly important in resource-limited economy. In this study, waste chicken bones of different parts from restaurant industry were pyrolyzed at 600 °C and evaluated for char physicochemical properties and Pb sorption characteristics. Lead adsorption isotherms by different chicken bone chars were carried out with initial Pb concentration range of 1-1000 mg L-1 at pH 5. The Pb adsorption data were better described by the Langmuir model (R2 = 0.9289-0.9937; ARE = 22.7-29.3%) than the Freundlich model (R2 = 0.8684-0.9544; ARE = 35.4-72.0%). Among the chars derived from different chicken bone parts, the tibia bone char exhibited the highest maximum Pb adsorption capacity of 263 mg g-1 followed by the pelvis (222 mg g-1), ribs (208 mg g-1), clavicle (179 mg g-1), vertebrae (159 mg g-1), and humerus (135 mg g-1). The Pb adsorption capacities were significantly and positively correlated with the surface area, phosphate release amount, and total phosphorus content of chicken bone chars (r ≥ 0.9711). On the other hand, approximately 75-88% of the adsorbed Pb on the chicken bone chars was desorbable with 0.1 M HCl, indicating their recyclability for reuse. Results demonstrated that chicken bone char could be used as an effective adsorbent for Pb removal in wastewater.

High-efficiency removal of Pb(II) and humate by a CeO2-MoS2 hybrid magnetic biochar.

This work prepares a novel CeO2-MoS2 hybrid magnetic biochar (CMMB) for the adsorptive removal of Pb(II) and humate from aqueous solution. The CMMB was evaluated against magnetic biochar (MB). The results showed that CMMB exhibited strong magnetic separation ability. Hybridization of CMMB greatly improved Pb(II) and humate removal compared to MB, with >99% Pb(II) and humate removed within 6 h. Pb(II) and humate removal capacities of CMMB were 263.6 mg/g and 218.0 mg/g, respectively, with negligible influence of ion strength in the range of 0-0.1 mol/L NaNO3. Pb(II) removal mechanism involved predominately with electrostatic attraction, Cπ-Pb(II) bond interaction, and surface adsorption and complexation combined processes; while pore-filling, partition effect and π-π interaction contributed to the adsorption of humate. Overall, the introduction of graphene-like MoS2 materials into biochar benefits both of the biomass resources recovery and environmental protection.

Removing mercury from aqueous solution using sulfurized biochar and associated mechanisms.

Biochar has been used to remove heavy metals from aqueous solutions. In this study, a sulfurized wood biochar (SWB) by direct impregnation with elemental sulfur was produced and evaluated along with pristine wood biochar (WB) for adsorption characteristics and mechanism of mercury. Mercury adsorption by WB and SWB was well described by Langmuir model and pseudo second order model and the maximum adsorption capacities of WB and SWB were 57.8 and 107.5 mg g-1, respectively. Intraparticle diffusion model showed that mercury adsorption was fast due to boundary layer and slow adsorption due to diffusion into biochar pores. Although, mercury adsorption by both WB and SWB was predominantly influenced by the pH, temperature, salt concentration, and biochar dosage, the SWB showed a relatively stable mercury adsorption compared to WB under different conditions, suggesting the strong affinity of SWB for mercury. The XPS analysis showed different adsorption mechanisms of mercury between WB and SWB. In particular, mercury adsorption in WB was due to Hg-Cπ bond formation and interaction with carboxyl and hydroxyl groups, whereas in SWB it is primarily due to mercury interaction with C-SOx-C and thiophenic groups in addition to Hg-Cπ bond formation and interaction with carboxyl groups. The SEM-EDS mapping also demonstrated that mercury in SWB was related to carbon, oxygen and sulfur. Overall, the sulfurized biochar was effective for removing mercury from aqueous solution, and its direct production through pyrolysis with elemental sulfur impregnation of wood chips could make it an economic option as absorbent for treating mercury-rich wastewater.

Enhanced sorption of hexavalent chromium [Cr(VI)] from aqueous solutions by diluted sulfuric acid-assisted MgO-coated biochar composite.

Metal oxide-Carbon composites have aroused great interesting towards specific anionic contaminants removal from the polluted environment. In this study, aiming at removing toxic chromate ion [Cr(VI)] from aqueous solutions, a novel approach was developed to produce surface-enhanced MgO-coated biochar adsorbent from sugarcane harvest residue (SHR). It was found that sulfuric acid hydrolysis and MgO-coating both facilitated the removal of Cr(VI) by biochars, and the maximum sorption capacities for the pristine biochar (SHR550), MgO-coated biochar (MgSHR550), and acid-assisted MgO-coated biochar (MgASHR550) that derived from the Langmuir isotherm model were 20.79, 54.64, and 62.89 mg g-1, respectively. Additionally, the Cr(VI) removal was a pseudo-second-order kinetic model controlled process with equilibrium reached within 24 h. The mechanism investigation revealed that Cr(VI) ions was directly sorbed by the MgO-coated biochars via the chemical interaction between MgO and Cr(VI), whereas the sorption-coupled reduction of Cr(VI) to Cr(III) governed the sorption of Cr(VI) on the SHR550. Although the increases of solution pH (>2.0) and KNO3 concentration (>0.05 mol L-1) reduced the Cr(VI) removal by biochars, while there were lower secondary pollution risks in MgO-coated biochar treatments due to the suppressed release of Cr(III) in solutions. This work could provide guidance for the production of efficient biochar for the removal of Cr(VI) from wastewater.