UV modification of biochar for enhanced hexavalent chromium removal from aqueous solution

A complete review on the oxygen-containing functional groups of biochar: Formation mechanisms, detection methods, engineering, and applications

Biochar is a porous carbon material generated by the thermal treatment of biomass under anaerobic or anoxic conditions with wealthy Oxygen-containing functional groups (OCFGs). To date, OCFGs of biochar have been extensively studied for their significant utility in pollutant removal, catalysis, capacitive applications, etc. This review adopted a whole system philosophy and systematically summarizes up-to-date knowledge of formation, detection methods, engineering, and application for OCFGs. The formation mechanisms and detection methods of OCFGs, as well as the relationships between OCFGs and pyrolysis conditions (such as feedstocks, temperature, atmosphere, and heating rate), were discussed in detail. The review also summarized strategies and mechanisms for the oxidation of biochar to afford OCFGs, with the performances and mechanisms of OCFGs in the various application fields (environmental remediation, catalytic biorefinery, and electrode material) being highlighted. In the end, the future research direction of biochar OCFGs was put forward.

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.

KHCO3-activated high surface area biochar derived from brown algae: A case study for efficient adsorption of Cr(VI) in aqueous solution

The current study aimed to assess the effectiveness of biochar formed from algae in the removal of Cr(VI) through the process of impregnating brown algae Sargassum hemiphyllum with KHCO3. The synthesis of KHCO3-activated biochar (KBAB-3), demonstrating remarkable adsorption capabilities for Cr(VI), was accomplished utilizing a mixture of brown algae and KHCO3 in a mass ratio of 1:3, followed by calcination at a temperature of 700 °C. Based on the empirical evidence, it can be observed that KBAB-3 shown a significant ability to adsorb Cr(VI) within a range of 60-160 mg g-1 across different environmental conditions. In addition, the KBAB-3 material demonstrated the advantageous characteristic of easy separation, allowing for the continued maintenance of a high efficiency in removing Cr(VI) even after undergoing numerous cycles of reuse. In conclusion, the application of KBAB-3, a novel adsorbent, exhibits considerable prospects for effective removal of Cr(VI) from diverse water sources in the near future.

Amino-Functionalized Lotus Stem Hydrochar for Rapid Adsorption and In Situ Detoxification of Cr(VI) from Water

The development of low-cost, efficient, and environmentally friendly adsorbents is the key to highly toxic hexavalent chromium [Cr(VI)] removal by adsorption. In this paper, amino-functionalized lotus stem hydrochar (ALSHC) was prepared from an agricultural waste lotus stem (LS) for the adsorption removal of Cr(VI) from water. The effects of the initial Cr(VI) concentration, contact time, temperature, coexisting anions, and reusability of ALSHC on Cr(VI) removal were examined in detail. The adsorption mechanism was further discussed by investigating the impact of the solution's initial pH, the relation between the pH change in solution and Cr(VI) removal during the process, the changes of chromium (Cr) species in solution and on ALSHC during adsorption, and the XPS characterization. The results demonstrated that ALSHC effectively removed Cr(VI) from water with rapid adsorption (the removal rate reached 80.90% in only 10 min) and in situ detoxification. Most importantly, ALSHC still had better adsorption performance (adsorption capacity of 30.95 mg g-1) than commercially activated carbon, even at pH = 9.00. The adsorption of Cr(VI) by ALSHC accorded with the pseudo-second-order kinetic model and Langmuir isotherm model, indicating a monolayer chemisorption process. The adsorption process was shown to be spontaneous and endothermic based on the thermodynamic characteristics (ΔG0 < 0, ΔH0 > 0, and ΔS0 > 0). The mechanism of Cr(VI) removal was mainly composed of three parts in sequence: Firstly, Cr(VI) in solution was quickly adsorbed onto ALSHC with protonated -NH2 through electrostatic attraction; subsequently, the adsorbed Cr(VI) on ALSHC was mostly detoxicated by in situ reduction; and finally, the reduced Cr(III) and the remaining Cr(VI) were fixed on the ALSHC surface by complexation. The prepared ALSHC displayed a certain superiority in Cr(VI) adsorption and had the prospect of further development.

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.

Taguchi method for optimization of Cr(VI) removal, isotherm, kinetic and thermodynamic studies

In this study, Taguchi optimization method was applied to determine the optimum operating conditions for batch adsorption of Cr(VI) from aqueous solution. Initial pH of solution, adsorbent dose, initial hexavalent chromium concentration, contact time and adsorbent type were selected as the variables, and the removal efficiency of Cr(VI) was chosen for the designated response. L18(35) orthogonal array, signal-to-noise (S/N) ratio and analysis of variance statistical procedures were applied to identify the effect of each operating parameter on the removal of Cr(VI) from aqueous solution. The signal-to-noise (S/N) ratio results showed that the optimal combination for Cr(VI) removal was at pH 1.0, adsorbent dose of 3.6 g.L-1, Cr(VI) concentration of 30 mg.L-1, contact time of 95 min and olive leaves as adsorbent type. A removal of 95.09% was obtained at these optimum conditions. The analysis of variance of the data revealed that initial pH of solution was the most dominant parameter affecting Cr(VI) removal efficiency, followed by adsorbent type, adsorbent dose, contact time and initial metal concentration. Under optimal conditions, adsorption kinetic of Cr(VI) was studied and modeled using the pseudo first-order, pseudo-second-order and intraparticle diffusion models. It was found that the pseudo-second-order model fitted the adsorption data most with the highest determination coefficient (R2 = 0.996). Freundlich isotherm model, with regression coefficient R2 of 0.953, fit well with the equilibrium isotherm data. The Langmuir maximum adsorption capacity was found to be 62.5 mg.g-1. The experimental values of ΔH°, ΔG° and ΔS° revealed that the adsorption process was spontaneous and endothermic.

Simultaneous decontamination of phosphorus and bisphenol A from livestock wastewater with boehmite-modified carbon composite

In this work, a novel boehmite-modified carbon adsorbent (BMCC) derived from moldy corn was used for simultaneous removal of P and bisphenol A (BPA) from livestock wastewater. The results showed that BMCC had a high specific surface area (308.82 m2/g) with boehmite nanoparticles anchored on its surface. BMCC showed high P and BPA decontamination capabilities (40.98 mg/g for P and 54.65 mg/g for BPA by Langmuir model). The adsorbed amount of P declined as pH increased from 4 to 10, while the adsorbed amount of BPA remained steady until pH increased to 10. After 6 cycles of BMCC use, the P and BPA adsorption efficiencies reduced by 21.75 % and 19.41 %, respectively. The adsorption of P was dominated by electrostatic attraction and complexation, while the adsorption of BPA was controlled by hydrogen bonding, electrostatic interaction, and π-π association. In conclusion, BMCC is an effective treatment for decontaminating P- and BPA-contaminated livestock wastewater.

Biotoxicity attenuation and the underlying physicochemical mechanism of biochar aged under simulated natural environmental conditions

Biochar (BC), with the benefits of enhancing soil fertility, absorbing heavy metals, carbon sequestration, and mitigating the greenhouse effect, has been extensively used for soil remediation. However, the long-term changes in the biotoxicity of BC under complex environmental conditions, which are the key factors influencing the sustainable application of BC in soil, are still unclear. Herein, the biotoxicity of BC aged with various processes, including dry‒wet cycle (DW) aging, freeze‒thaw cycle (FT) aging, ultraviolet irradiation (UV) aging, and low molecular weight organic acid (OA) aging, was systematically investigated by Escherichia coli (E. coli) culture experiments. The toxicity attenuation rate (%·week-1) was proposed to more concisely and clearly compare the influence of different aging methods on BC toxicity. The results indicated that after 5 weeks of aging, the toxicity attenuation rate during the four aging modes followed the order OA aging > FT aging > UV aging > DW aging. BC was nontoxic after 1 week of OA aging, 4 weeks of FT aging, 7 weeks of UV aging, and 14 weeks of DW aging. Spectroscopic characterizations revealed that humic acids in the dissolved organic matter of BC were the main reason for the biotoxicity. In addition, the attenuation of environmentally persistent free radicals on BC during aging was also an important factor for reducing environmental toxicity. This work provides insight into the detoxification mechanism of the BC aging process under ordinary environmental conditions and guidance for the safe application of BC in soil.

Biochar preparation and evaluation of its effect in composting mechanism: A review

This article provides an overview of biochar application for organic waste co-composting and its biochemical transformation mechanism. As a composting amendment, biochar work in the adsorption of nutrients, the retention of oxygen and water, and the promotion of electron transfer. These functions serve the micro-organisms (physical support of niche) and determine changes in community structure beyond the succession of composing primary microorganisms. Biochar mediates resistance genes, mobile gene elements, and biochemical metabolic activities of organic matter degrading. The participation of biochar enriched the α-diversity of microbial communities at all stages of composting, and ultimately reflects the high γ-diversity. Finally, easy and convincing biochar preparation methods and characteristic need to be explored, in turn, the mechanism of biochar on composting microbes at the microscopic level can be studied in depth.

Enhanced adsorptive removal of hexavalent chromium in aqueous media using chitosan-modified biochar: Synthesis, sorption mechanism, and reusability

Hexavalent chromium (Cr(VI)) is deemed a priority contaminant owing to its carcinogenicity, teratogenicity, and mutagenicity towards flora and fauna. A novel Chitosan-modified Mimosa pigra biochar (CMPBC) was fabricated and efficiency of Cr(VI) oxyanion removal in aqueous systems was compared with the pristine biochar. The gross composition of pyrolyzed biomass was determined through the proximate analysis. The instrumental characterization of X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FT-IR) confirmed the amino modification of MPBC when treated with chitosan. Characteristic features of the Cr(VI) sorptive process by CMPBC and MPBC were examined by performing batch sorption studies. Experimental data suggested that sorption is heavily dependent on pH, with the highest adsorption capacity (14.4 ± 0.9 mg g^-1) occurring at pH 3. It was further noted that the removal efficiency of CMPBC (92%) was considerably greater than that of MPBC (75%) when the biochar dose and initial concentration of Cr(VI) are 1 g L^-1 and 5 mg L^-1 respectively. The kinetic data were best interpreted by the power function model (R² = 0.97) suggesting a homogenous chemisorption process. The isotherm data of removal of Cr(VI) by CMPBC was inferred well by Redlich Peterson and Temkin isotherms. Results of sorption-desorption regeneration cycles indicated that the Cr(VI) uptake by CMPBC is not fully reversible. The electrostatic attractions between cationic surface functionalities and Cr(VI) oxyanions, partial reductive transformation of Cr(VI) species to Cr(III), as well as complexation of Cr(III) onto CMPBC were the possible mechanisms of mitigation of Cr(VI) by CMPBC. The results and outcomes of this research suggest the possibility of utilizing the chitosan-modified Mimosa pigra biochar as an easily available, environmentally sustainable, and inexpensive sorbent to decontaminate Cr(VI) pollution from aqueous media.

Effect of different aging treatments on the transport of nano-biochar in saturated porous media

Widely used for soil amendment, carbon sequestration, and remediation of contaminated soils, biochars (BCs) inevitably produce a large number of nanoparticles with relatively high mobility. Geochemical aging alters chemical structure of these nanoparticles and thus affect their colloidal aggregation and transport behavior. In this study, the transport of ramie derived nano-BCs (after ball-milling) was investigated by different aging treatments (i.e., photo (PBC) and chemical aging (NBC)) as well as the managing BC under different physicochemical factors (i.e., flow rates, ionic strengths (IS), pH, and coexisting cations). Consequences of the column experiments indicated aging promoted the mobility of the nano-BCs. Compared to the nonaging BC, consequences of spectroscopic analysis demonstrated the aging BCs exhibited a number of tiny corrosion pores. Both of these aging treatments contribute to a more negative zeta potential and a higher dispersion stability of the nano-BCs, which is caused by the abundance of O-functional groups. Also the specific surface area and mesoporous volume of both aging BCs increased significantly, with the increase being more pronounced for NBC. The breakthrough curves (BTCs) obtained for the three nano-BCs were modelled by the advection-dispersion equation (ADE), which included first-order deposition and release terms. The ADE revealed high mobility of aging BCs, which meant their retention in saturated porous media was reduced. This work contributes to a comprehensive understanding of the transport of aging nano-BCs in the environment.

Applicability of new sustainable and efficient green metal-based nanoparticles for removal of Cr(VI): Adsorption anti-microbial, and DFT studies

Artemisia absinthium leaves were utilized as a reducing agent for green synthesis of Zinc oxide nanoparticles (particle size 17 nm). Synthesized green-ZnO (g-ZnO) were characterized by SEM/EDX, FTIR, XRD, UV, and BET analyses and then further used as an adsorbent to remove Cr(VI) ions from simulated wastewater. Optimal pH, temperature and adsorbent dosage were determined through batch mode studies. High removal efficiency and adsorption capacity were observed at pH 4, 0.25 g L^-1 dosage, and 25 mg L^-1 concentration of Cr(VI). Experimental data were modelled with different adsorption kinetics (Elovich model, PFO, PSO, IDP model) and isotherms (Langmuir, Freundlich, and Temkin), and it was found the adsorption process was well fitted to Langmuir with an R² value greater than>0.99. Computational calculation showed that the g-ZnO nanoparticles became ∼14 times more dynamic with delocalized surface states making them a relevant platform to adsorb Cr with greater work function compatibility supporting the experimental findings. The Q_max adsorption capacity of g-ZnO was 315.46 mg g^-1 from Langmuir calculations. Thermodynamic calculations reveal that the Cr (VI) adsorption process was spontaneous and endothermic, with a positive ΔS value representing the disorder at the solid-solution interface during the adsorption. In addition, the present study has demonstrated that these g-ZnO nanoparticles show strong antibacterial activities against P. aeruginosa (MTCC 1688) and E. coli (MTCC 1687). Also, the novel g-ZnO adsorbent capacity to remove Cr(VI) from simulated water revealed that it could be reused at least six times with higher removal rates during regeneration experiments. The results obtained from adsorption and antimicrobial activities suggest that g-ZnO nanoparticles could be used effectively in real-time wastewater and agricultural safety applications.

Enhanced removal of aqueous Cr(VI) by the in situ iron loaded activated carbon through a facile impregnation with Fe(II) and Fe(VI) two step method: Mechanism study

In this study, a novel in situ iron-loaded activated carbon (AFPAC) was prepared by a FeSO₄/K₂FeO₄ impregnation and oxidation combination two-step supported on activated carbon for enhanced removal of Cr(VI) from aqueous solutions. Cr(VI) removal efficiency greatly increased by AFPAC more than 70% than that of fresh activated carbon (AC), which is due to rich iron oxides formed in situ and the synergistic effect between iron oxides and activated carbon. Cr(VI) adsorption behaviors on AFPAC under different water quality parameters were investigated. The maximum monolayer adsorption capacities for Cr(VI) by AFPAC are as high as 26.24 mg/g, 28.65 mg/g, and 32.05 mg/g at 25 °C, 35 °C and 45 °C at pH 4, respectively. Density functional theory (DFT) results showed that the adsorption energy of K₂Cr₂O₇ on the surface of FeOOH was - 2.52 eV, which was greater than that on the surface of bare AC, and more charge transfer occurred during the adsorption of K₂Cr₂O₇ on the surface of FeOOH, greatly promoting the formation of Cr = O-Fe. Cr(VI) removal by AFPAC included electrostatic attraction, redox reaction, coordinate complexation, and co-precipitation. Cr(VI) adsorption process on AFPAC consisted of the three reaction steps: (1) AFPAC was fast protonation and Cr₂O₇^2- would electrostatically attract to the positively charged AFPAC surface. (2) Cr₂O₇^2- was reduced into Cr₂O₃ by the carbons bond to the oxygen functionalities on activated carbon and the redox reaction process of FeSO₄ and K₂FeO₄. (3) The inner-sphere complexes were formed, and adsorbed on AFPAC by iron oxides and then co-precipitation.

Synthesis and applications of bismuth-impregnated biochars originated from spent coffee grounds for efficient adsorption of radioactive iodine: A mechanism study

The adsorption of radioactive iodine, which is capable of presenting high mobility in aquatic ecosystems and generating undesirable health effects in humans (e.g., thyroid gland dysfunction), was comprehensively examined using pristine spent coffee ground biochar (SCGB) and bismuth-impregnated spent coffee ground biochar (Bi@SCGB) to provide valuable insights into the variations in the adsorption capacity and mechanisms after pretreatment with Bi(NO₃)₃. The greater adsorption of radioactive iodine toward Bi@SCGB (adsorption capacity (Q_e) = 253.71 μg/g) compared to that for SCGB (Q_e = 23.32 μg/g) and its reduced adsorption capability at higher pH values provide evidence that the adsorption of radioactive iodine with SCGB and Bi@SCGB is strongly influenced by the presence of bismuth materials and the electrostatic repulsion between their negatively charged surfaces and negatively charged radioactive iodine (IO₃^-). The calculated R² values for the adsorption kinetics and isotherms support that chemisorption plays a crucial role in the adsorption of radioactive iodine by SCGB and Bi@SCGB in aqueous phases. The adsorption of radioactive iodine onto SCGB was linearly correlated with the contact time (h^1/2), and the diffusion of intra-particle predominantly determined the adsorption rate of radioactive iodine onto Bi@SCGB (C_{stage II} (129.20) > C_{stage I} (42.33)). Thermodynamic studies revealed that the adsorption of radioactive iodine toward SCGB (ΔG° = -8.47 to -7.83 kJ/mol; ΔH° = -13.93 kJ/mol) occurred exothermically and that for Bi@SCGB (ΔG° = -15.90 to -13.89 kJ/mol; ΔH° = 5.88 kJ/mol) proceeded endothermically and spontaneously. The X-ray photoelectron spectroscopy (XPS) analysis of SCGB and Bi@SCGB before and after the adsorption of radioactive iodine suggest the conclusion that the change in the primary adsorption mechanism from electrostatic attraction to surface precipitation upon the impregnation of bismuth materials on the surfaces of spent coffee ground biochars is beneficial for the adsorption of radioactive iodine in aqueous phases.

Modified and pristine biochars for remediation of chromium contamination in soil and aquatic systems

Chromium (Cr) contamination in soil and water poses high toxicity risks to organisms and threatens food and water security worldwide. Biochar has emerged as a promising material for cleaning up Cr contamination owing to biochar's strong capacity to immobilize Cr. This paper synthesizes information on biochar modification for the efficient remediation of Cr contamination in soil and water, and critically reviews mechanisms of Cr adsorption on pristine and modified biochars. Biochar modification methods include physical activation via ball milling or ultraviolet irradiation, chemical activation via magnetization, alkali/acid treatment, nano-fabrication or loading of reductive agents, and biological activation via integrating biochars with microorganisms and their metabolites. Modified biochars often have multi-fold enhancement in Cr adsorption/reduction capacity than pristine biochars. Iron (Fe)-supported magnetic biochars have the most promising Cr removal abilities with high reusability of the biochars. Pre-pyrolysis modification with Fe could load Fe₃O₄ micro-/nanoparticles on biochars, and increase the surface area and electrostatic attraction between chromate anions and biochar surfaces, and reduce Cr(VI) to Cr(III). Post-pyrolysis modification could enrich oxygen-containing functional groups such as CO and -OH on biochar surfaces and promote Cr reduction and adsorption. Future research directions for Cr mitigation using advanced biochar products are discussed in this review.

Removal of hexavalent chromium via biochar-based adsorbents: State-of-the-art, challenges, and future perspectives

Chromium originates from geogenic and extensive anthropogenic activities and significantly impacts natural ecosystems and human health. Various methods have been applied to remove hexavalent chromium (Cr(VI)) from aquatic environmental matrices, including adsorption via different adsorbents, which is considered to be the most common and low-cost approach. Biochar materials have been recognized as renewable carbon sorbents, pyrolyzed from various biomass at different temperatures under limited/no oxygen conditions for heavy metals remediation. This review summarizes the sources, chemical speciation & toxicity of Cr(VI) ions, and raw and modified biochar applications for Cr(VI) remediation from various contaminated matrices. Mechanistic understanding of Cr(VI) adsorption using different biochar-based materials through batch and saturated column adsorption experiments is documented. Electrostatic interaction and ion exchange dominate the Cr(VI) adsorption onto the biochar materials in acidic pH media. Cr(VI) ions tend to break down as HCrO₄^-, CrO₄^2-, and Cr₂O₇^2- ions in aqueous solutions. At low pH (∼1-4), the availability of HCrO₄^- ions attributes the electrostatic forces of attraction due to the available functional groups such as -NH₄⁺, -COOH, and -OH₂⁺, which encourages higher adsorption of Cr(VI). Equilibrium isotherm, kinetic, and thermodynamic models help to understand Cr(VI)-biochar interactions and their adsorption mechanism. The adsorption studies of Cr(VI) are summarized through the fixed-bed saturated column experiments and Cr-contaminated real groundwater analysis using biochar-based sorbents for practical applicability. This review highlights the significant challenges in biochar-based material applications as green, renewable, and cost-effective adsorbents for the remediation of Cr(VI). Further recommendations and future scope for the implications of advanced novel biochar materials for Cr(VI) removal and other heavy metals are elegantly discussed.

Amino-functionalized biochars for the detoxification and removal of hexavalent chromium in aqueous media

The objectives of the study were to evaluate and compare the efficacy of hexavalent chromium (Cr(VI)) removal by amino-modified (HDA-MPBC) and pristine biochar (MPBC) derived from an invasive plant Mimosa pigra. Prepared biochars were characterized and batch experiments were conducted to check the performance and the mechanisms of Cr(VI) removal. FTIR spectra revealed that the surface of HDA-MPBC is abundant with amino functional groups which was further confirmed by XPS analysis. The highest Cr(VI) removal for both HDA-MPBC (76%) and MPBC (62%) was observed at pH 3.0. The batch sorption data were well fitted to the Freundlich isotherm model and pseudo-second-order kinetic model, suggesting the involvement of both physisorption and chemisorption mechanisms for Cr(VI) removal. X-ray photoelectron spectroscopy studies showed that both Cr(VI) and Cr(III) were presented at the modified biochar surface after adsorption. These results indicated that the electrostatic attraction of Cr(VI) coupled with reduction of Cr(VI) to Cr(III) and complexation of Cr(III) ions with functional groups on HDA-MPBC as the most plausible mechanism for removal of Cr(VI) by modified biochar. Regeneration experiment concluded that adsorbed Cr(VI) onto the surface of HDA-MPBC had the least tendency of being desorbed in basic conditions. HDA-MPBC showed a high performance in adsorptive removal of Cr(VI) compared to pristine biochar signifying the amino modification to enhance adsorption performance of biochar in Cr(VI) removal from wastewater.

Biochar for agronomy, animal farming, anaerobic digestion, composting, water treatment, soil remediation, construction, energy storage, and carbon sequestration: a review

In the context of climate change and the circular economy, biochar has recently found many applications in various sectors as a versatile and recycled material. Here, we review application of biochar-based for carbon sink, covering agronomy, animal farming, anaerobic digestion, composting, environmental remediation, construction, and energy storage. The ultimate storage reservoirs for biochar are soils, civil infrastructure, and landfills. Biochar-based fertilisers, which combine traditional fertilisers with biochar as a nutrient carrier, are promising in agronomy. The use of biochar as a feed additive for animals shows benefits in terms of animal growth, gut microbiota, reduced enteric methane production, egg yield, and endo-toxicant mitigation. Biochar enhances anaerobic digestion operations, primarily for biogas generation and upgrading, performance and sustainability, and the mitigation of inhibitory impurities. In composts, biochar controls the release of greenhouse gases and enhances microbial activity. Co-composted biochar improves soil properties and enhances crop productivity. Pristine and engineered biochar can also be employed for water and soil remediation to remove pollutants. In construction, biochar can be added to cement or asphalt, thus conferring structural and functional advantages. Incorporating biochar in biocomposites improves insulation, electromagnetic radiation protection and moisture control. Finally, synthesising biochar-based materials for energy storage applications requires additional functionalisation.

Application of porous styrene resin loaded carboxymethyl cellulose-stabilized nano-zero-valent iron for highly efficient hexavalent chromium removal

The results of this study provide a new idea for the design of efficient Cr(vi) removal materials based on nZVI.

Bioengineered biochar as smart candidate for resource recovery toward circular bio-economy: a review

Biochar's ability to mediate and facilitate microbial contamination degradation, as well as its carbon-sequestration potential, has sparked interest in recent years. The scope, possible advantages (economic and environmental), and future views are all evaluated in this review. We go over the many designed processes that are taking place and show why it is critical to look into biochar production for resource recovery and the role of bioengineered biochar in waste recycling. We concentrate on current breakthroughs in the fields of engineered biochar application techniques to systematically and sustainable technology. As a result, this paper describes the use of biomass for biochar production using various methods, as well as its use as an effective inclusion material to increase performance. The impact of biochar amendments on microbial colonisation, direct interspecies electron transfer, organic load minimization, and buffering maintenance is explored in detail. The majority of organic and inorganic (heavy metals) contaminants in the environment today are caused by human activities, such as mining and the use of chemical fertilizers and pesticides, which can be treated sustainably by using engineered biochar to promote the establishment of a sustainable engineered process by inducing the circular bioeconomy.

Enhanced sequestration of Cr(VI) onto plant extract anchored on carbon-coated aluminium oxide composite

Aluminium oxide (ALU) and carbon-coated aluminium oxide modified with Kigelia africana leaf extract (KECA) were employed for the removal of toxic hexavalent chromium (Cr(VI)) from the aqueous phase. The adsorbents (ALU and KECA) were characterized by TGA, BET, FESEM, FTIR, Raman and XRD spectroscopic techniques. The potential of KECA and ALU to remove Cr(VI) from simulated wastewater was optimum at pH 2, sorbent dose of 0.025 g and a contact time of 200 min. Meanwhile, the uptake capacity of KECA and ALU was enhanced with an increase in sorbent dose, contact time and initial Cr(VI) concentration. The uptake of Cr(VI) onto the adsorbents ALU and KECA was kinetically best described by the pseudo-second-order and Elovich models, respectively. Besides, the equilibrium data acquired for ALU and KECA obeyed Freundlich and Langmuir isotherm models, respectively. ALU and KECA were observed to have optimum adsorption capacity of 56.45 mg g^-1 and 258.2 mg g^-1, respectively. The adsorption of Cr(VI) onto the adsorbents was thermodynamically feasible, endothermic in nature and entropy-driven. A decrease in efficiency was observed on regeneration of the absorbents, thus limiting their reusability. However, the presence of functional groups with reducing property in the extract of Kigelia africana leaves was noticed to enhance the capacity of the adsorbent to abstract Cr(VI) from the solution. Hence, this study demonstrates the potential of KECA to sequestrate Cr(VI) from an aqueous solution and provides a reference for its application to the treatment of Cr(VI)-laden industrial wastewater.

Removal performance and mechanisms of toxic hexavalent chromium (Cr(VI)) with ZnCl2 enhanced acidic vinegar residue biochar

Acidic vinegar residue (VR) and toxic hexavalent chromium (Cr(VI)) are unfavorable substances due to their toxicity against the environment. In this study, modified biochar was prepared to investigate the removal mechanisms of Cr(VI). The results showed that ZnCl₂ could yield highly aromatic products with improved pore structures. The adsorption capacity of modified biochar reached the highest efficiency (236.81 mg g^-1) when the mass ratio of ZnCl₂/VR was 1, which is higher than the control (9.96 mg g^-1). In addition, Cr(VI) adsorption coexisted with physical and chemical adsorption. The mechanisms of modified biochar to Cr(VI) removal included electrostatic attraction, pore filing, reduction and surface complexation. Notably, as a fermented product, VR biochar was a nitrogen-rich product; the formation of the amino group could provide a direct solid site for Cr(VI) adsorption. Subsequently, amorphous silica could be converted into silanol to provide additional adsorption sites. This work establishes the theoretical basis for efficient Cr(VI) removal and VR reuse.

Evaluation of heavy metal pollutants in salt and seawater under the influence of the Lyari River and potential health risk assessment

Heavy metals intoxication through edible salt poses serious health hazards. The conducted research assessed the levels of heavy metals, health risks of salt samples, and the pollution index of seawater obtained from saltpans located at Sandpit, Karachi, Pakistan, which receive untreated effluents through the Lyari River. Seawater (n = 27) and salt samples (n = 27) were prepared for the detection of Al, Cd, Pb, Cr, Fe, Cu, Hg, Ni, As and Zn through atomic absorption spectroscopy, mean concentrations ± S.D. (mg/mL) were compared with the National Environmental Quality Standards, Pakistan. The levels of Cr (40.06 ± 0.21) were the highest followed by Fe (39.77 ± 0.08) > As (25.12 ± 0.21) > Ni > Pb > Al > Hg > Zn > Cd > Cu. In sea salt (Mean ± S·D mg/Kg), the Cr (47.79 ± 0.19), Fe (47.5 ± 0.15), As (30.62 ± 0.22), and Ni were redundant elements followed by Al > Pb > Hg > Zn > Cu > Cd. The water comprehensive pollution index was 1000 times greater than the critical values. The single factor pollution index was highest for Hg (1321), followed by Cr (40), Ni (36), Pb (32), Al (31.4), Cd (31.3), and As (25). Health risk assessment indices (from salt samples), including target hazard quotient (THQ) for As, Pb, Al, Hg, and Cr were two to six times higher than the noncarcinogenic THQ reference range. Similarly, the hazardous index indicated that salt was 20 times hazardous (HI = 20.29), and the carcinogenic rate index for the heavy metals i.e., Cd, As, Cr, and Ni was above the reference CR value i.e., 1 × 10^-4. In conclusion, the experimental and theoretical approaches recommend that the use of contaminated salt may impart health hazards.

Effects of UV-modified biochar derived from phytoremediation residue on Cd bioavailability and uptake in Coriandrum sativum L. in a Cd-contaminated soil

Biochar has been applied widely as an amendment in the remediation of contaminated soil to immobilize the heavy metals. However, the role of ultraviolet (UV) irradiation modified biochar derived from the residues of phytoremediation plants in the contaminated soil not investigated yet. In this study, the UV-modified biochars were obtained from Brassica napus L. and Lolium perenne L. by pyrolysis at 600 °C. They were applied in a pot experiment to investigate their effect on Cd bioavailability and uptake in Coriandrum sativum L. in a Cd-contaminated soil at four addition rate (0%, 0.2%, 0.4%, and 0.6%). The results showed that the Cd was effectively stabilized in the biochar with environmentally acceptable leaching toxicity. The specific surface area and carboxyl functional group of biochar were greatly increased after UV modification. The application of biochar progressively increased the soil pH and electrical conductivity (EC). Furthermore, the CaCl₂-extractable Cd was significantly reduced by 18.4-51.4% with biochar amendments. The concentration of Cd in shoots and roots was significantly reduced by biochars. In conclusion, the UV-modified biochar obtained from phytoremediation residue could effectively deal with hazardous waste and repair Cd-contaminated soil.

Hydrochar and pyrochar for sorption of pollutants in wastewater and exhaust gas: A critical review

Pollutants in wastewater and exhaust gas bring out serious concerns to public health and the environment. Biochar can be developed as a sustainable adsorbent originating from abundant bio-wastes, such as agricultural waste, forestry residue, food waste and human waste. Here we highlight the state-of-the-art research progress on pyrochar and hydrochar for the sorption of pollutants (heavy metal, organics, gas, etc) in wastewater and exhaust gases. The adsorption performance of pyrochar and hydrochar are compared and discussed in-depth, including preparation procedures (carbonization and activation), sorption possible mechanisms, and physiochemical properties. Challenges and perspective for designing efficient and environmental benign biochar-based adsorbents are finally addressed.

Removal of aqueous Cr(VI) by magnetic biochar derived from bagasse

We prepared a novel adsorbent functionalized by bagasse magnetic biochar (BMBC). To study the removal behaviors and mechanisms of Cr(VI) by BMBC, batch adsorption experiments were conducted by modifying variables, such as pH, adsorption time, BMBC dosages, initial Cr concentration, co-existing ions, and ionic strength, and characterizing BMBC before and after Cr(VI) adsorption. BMBC was primarily composed of Fe₂O₃ and Fe₃O₄ on bagasse boichar with an amorphous structure. The specific surface area of BMBC was 81.94 m² g⁻¹, and the pH_pzc of BMBC was 6.2. The fabricated BMBC showed high adsorption performance of Cr(VI) in aqueous solution. The maximum Cr(VI) adsorption capacity of BMBC was 29.08 mg g⁻¹ at 25 ºC, which was much higher than that of conventional biochar sorbents. The adsorption process followed pseudo-second-order kinetics and could be explained by the involvement of the Langmuir isotherm in monolayer adsorption. The crystalline structure of Fe₃O₄ in the BMBC changed slightly during the adsorption process; Fe₃O₄ improved the adsorption of Cr(VI) on BMB. The desorption capacity of Cr(VI) was 8.21 mg g⁻¹ when 0.2 mol L⁻¹ NaOH was used as the desorption solution. After being reused three times, the removal efficiency is still as high as 80.36%.

Characteristics and applications of biochar for remediating Cr(VI)-contaminated soils and wastewater

Chromium (Cr) is a common environmental contaminant due to industrial processes and anthropogenic activities such as mining of chrome ore, electroplating, timber treatment, leather tanning, fertilizer and pesticide, etc. Cr exists mainly in both hexavalent [Cr(VI)] and trivalent [Cr(III)] form, being Cr(VI) with non-degradability and potential to be hidden, thereby affecting surrounding environment and being toxic to human health. Therefore, researches on remediation of Cr pollution in the environment have received much attention. Biochar is a low-cost adsorbent, which has been identified as a suitable material for Cr(VI) immobilization and removal from soil and wastewater. This review incorporates existing literature to provide a detailed examination into the (1) Cr chemistry, the source and current status of Cr pollution, and Cr toxicity and health; (2) feedstock and characterization of biochar; (3) processes and mechanisms of immobilization and removal of Cr by biochar, including oxidation-reduction, electrostatic interactions, complexation, ion exchange, and precipitation; (4) applications of biochar for Cr(VI) remediation and the modification of biochar to improve its performance; (5) factors affecting removal efficiency of Cr(VI) with respect to its physico-chemical conditions, including pH, temperature, initial concentration, reaction time, biochar characteristics, and coexisting contaminants. Finally, we identify current issues, challenges, and put forward recommendations as well as proposed directions for future research. This review provides a thorough understanding of using biochar as an emerging biomaterial adsorbent in Cr(VI)-contaminated soils and wastewater.

Effect of biochar modified with magnetite nanoparticles and HNO3 for efficient removal of Cr(VI) from contaminated water: A batch and column scale study

Chromium (Cr) poses serious consequences on human and animal health due to its potential carcinogenicity. The present study aims at preparing a novel biochar derived from Chenopodium quinoa crop residues (QBC), its activation with magnetite nanoparticles (QBC/MNPs) and strong acid HNO₃ (QBC/Acid) to evaluate their batch and column scale potential to remove Cr (VI) from polluted water. The QBC, QBC/MNPs and QBC/Acid were characterized with SEM, FTIR, EDX, XRD as well as point of zero charge (PZC) to get an insight into their adsorption mechanism. The impact of different process parameters including dose of the adsorbent (1-4 g/L), contact time (0-180 min), initial concentration of Cr (25-200 mg/L) as well as solution pH (2-8) was evaluated on the Cr (VI) removal from contaminated water. The results revealed that QBC/MNPs proved more effective (73.35-93.62-%) for the Cr (VI) removal with 77.35 mg/g adsorption capacity as compared with QBC/Acid (55.85-79.8%) and QBC (48.85-75.28-%) when Cr concentration was changed from 200 to 25 mg/L. The isothermal experimental results follow the Freundlich adsorption model rather than Langmuir, Temkin and Dubinin-Radushkevich adsorption isotherm models. While kinetic adsorption results were well demonstrated by pseudo second order kinetic model. Column scale experiments conducted at steady state exhibited excellent retention of Cr (VI) by QBC, QBC/MNPs and QBC/Acid at 50 and 100 mg Cr/L. The results showed that this novel biochar (QBC) and its modified forms (QBC/Acid and QBC/MNPs) are applicable with excellent reusability and stability under acidic conditions for the practical treatment of Cr (VI) contaminated water.

Biochar/iron (BC/Fe) composites for soil and groundwater remediation: Synthesis, applications, and mechanisms

Biochar/iron (BC/Fe) composites, such as nano zero-valent iron (nZVI)/BC, iron sulfide/BC, and iron oxide/BC, have been developed and applied to deal with various contaminants owing to their excellent physicochemical properties. This work summarizes the progress in the preparation of BC/Fe composites, the properties and applications of BC/Fe, and the mechanism of the synergistic effect between Fe and BC in the composites. Various methods, including pyrolysis, hydrothermal carbonization, fractional precipitation, and ball milling, have been used to synthesize BC/Fe composites. In addition, the introduction of stabilizers, such as carboxymethyl cellulose (CMC), in the fractional precipitation process further prevents the agglomeration of Fe particles, which enhances the stability and fluidity of the resultant composites to facilitate the application of the composites in soil and water remediation. The application of BC/Fe composites in water and soil remediation is discussed in three aspects based on the interaction mechanisms, namely adsorption, reduction, and oxidation. Overall, the composites showed the synergistic effect of BC and Fe owing to the combination of the specific properties of Fe, such as reduction, catalysis, and magnetism, which can enhance the properties of BC with a larger surface area, abundant functional groups, and increased electron transfer efficiency. This review systemically summarizes the recent developments in BC/Fe composites to maximize the efficiency of BC/Fe application in soil and groundwater remediation. Key challenges and further research needs are also suggested.

Preparation of a novel iron-based biochar composite for removal of hexavalent chromium in water

The chitosan-stabilized ferrous sulfide nanoparticles were loaded on biochar to prepare a composite material FeS-CS-BC for effective removal of hexavalent chromium in water. BC and FeS-CS-BC were characterized by Brunauer-Emmett-Teller (BET), scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) analyses. Batch experiments were employed to evaluate the Cr(VI) removal performance. The experimental results showed that the removal rate of Cr(VI) by FeS-CS-BC(FeS:CS:BC = 2:2:1) reached 98.34%, which was significantly higher than that of BC (44.58%) and FeS (79.91%). In the pH range of 2-10, the removal of Cr(VI) by FeS-CS-BC was almost independent of pH. The limitation of coexisting anions (Cl^-、SO₄^2-、NO₃^-) on Cr(VI) removal was not too obvious. The removal of Cr(VI) by FeS-CS-BC was fitted with the pseudo-second-order dynamics, which was a hybrid chemical-adsorption reaction. The X-ray photoelectron spectroscopy (XPS) analysis result showed that Cr(VI) was reduced, and the reduced Cr(VI) was fixed on the surface of the material in the form of Cr(VI)-Fe(III). Graphical abstract Removal of hexavalent chromium from wastewater by FeS-CS-BC composite synthesized by impregnation.

Thiol-modified biochar synthesized by a facile ball-milling method for enhanced sorption of inorganic Hg2+ and organic CH3Hg. JOURNAL OF HAZARDOUS MATERIALS 2020;384:121357. [PMID: 31630859 DOI: 10.1016/j.jhazmat.2019.121357]

Modification of thiol on biochar often demands complex synthetic procedures and chemicals. In this work, a simple and environment friendly thiol-modified biochar (BMS-biochar) was successfully synthesized by ball milling pristine biochar with 3-mercaptopropyltrimethoxysilane (3-MPTS). The resultant BMS-biochar was characterized and tested for aqueous inorganic Hg²⁺ and organic CH₃Hg⁺ removal. Characterization results showed that 3-MPTS was loaded on the surface of biochar through oxygen-containing functional groups (i.e., OH and CO) and π-π bond. Ball milling method improved the properties of BMS-biochar, namely, more efficient SH load, a larger surface area, more functional groups, more negatively charged surface, which resulted in higher removal efficiency of Hg²⁺ and CH₃Hg⁺ (320.1 mg/g for Hg²⁺ and 104.9 mg/g for CH₃Hg⁺) compared to the pristine biochar (105.7 mg/g for Hg²⁺ and 8.21 mg/g for CH₃Hg⁺) and thiol-modified biochar through chemical impregnation (CIS-biochar) (175.6 mg/g for Hg²⁺ and 58.0 mg/g for CH₃Hg⁺). Ball milling increased the sorption capacities of Hg²⁺ and CH₃Hg⁺ through surface adsorption, electrostatic attraction, ligand exchange, and surface complexation. Modeling results suggested that the surface diffusion was the rate-limiting adsorption step for BMS-biochar. This work gave prominence to the potential of ball milling for the preparation of thiol-modified biochar to remove mercury especially organic CH₃Hg⁺ by adsorption.

Mechanism of Cr(VI) removal by magnetic greigite/biochar composites

This study synthesized magnetic greigite/biochar composites (MGBs) by a solvothermal method and tested their ability to remove Cr(VI) from heavy metal-polluted wastewater. X-ray diffraction (XRD), Fourier transformed infrared spectrometry (FT-IR) and scanning electron microscopy (SEM) revealed that magnetic greigite (Fe₃S₄) flakes were aggregated and anchored to the biochar surface, resulting in more active sites than pristine biochar. Maximum Cr removal efficiency and capacity of MGB-30 (greigite/biochar = 30%) at an initial Cr(VI) concentration of 20 mg/L were 93% and 23.25 mg/g, respectively. A pseudo-first-order kinetic model was determined for the Cr(VI) removal process and the Cr(VI) removal rate constants were highly dependent on the mass ratios of Fe₃S₄ loaded on biochar, initial MGB and Cr(VI) concentrations and solution pH. X-ray photoelectron spectroscopy (XPS) and flame atomic absorption spectrometric (FAAS) analysis demonstrated that Cr(VI) was preferentially adsorbed on MGBs and subsequently reduced to Cr(III) by MGBs. Electron paramagnetic resonance (EPR) spectroscopy and iron redox transformations revealed that the Cr(VI) removal enhancement was attributed to efficient surface Fe(III)/Fe(II) cycling via electron transfer with the persistent free radicals (PFRs) of biochar. These novel findings provide new insights into the Fe(III)/Fe(II) cycle induced by biochar and the prospects of using magnetic greigite/biochar composites for remediation of Cr(VI)-rich wastewaters.

Removal and enrichment of Cr(VI) from aqueous solutions by lotus seed pods

Chromium (Cr(VI)) is highly toxic and carcinogenic. Cr(VI) water pollution has become more and more serious. This article reports on a study in which lotus seed pods (LSP), an agricultural waste product, was used to efficiently remove Cr(VI) from an aqueous solution, and the carbonization product of LSP after the removal of Cr(VI) (CPLSP) can be regarded as a resource containing Cr. Cr(VI) removed by LSP fits a pseudo-second-order model. pH levels greatly influence the amount of Cr removed. The maximum removal of Cr(VI) by LSP in aqueous solution was 153.85 mg/g. The possible removal mechanism is absorption, redox, and reabsorption based upon SEM/EDS, FT-IR, and XPS spectra results. The Cr content of CPLSP was 42.95% by ammonium persulfate oxidation titrimetric method. These results suggest that LSP can be an excellent, low cost, biomaterial for removing and enriching Cr(VI) from an aqueous solution. PRACTITIONER POINTS: Lotus seed pods are an efficient adsorbent for Cr(VI) from aqueous solutions. The Cr removal by lotus seed pods occurs via absorption, redox, and reabsorption. Cr can be captured after the pods are carbonized. Lotus seed pods can be applied to the removal and enrichment of Cr(VI) from waste water.

Consecutive reduction of Cr(VI) by Fe(II) formed through photo-reaction of iron-dissolved organic matter originated from biochar

Employing biochar for environmental remediation has been widely practiced. Nonetheless, the reduction mechanisms of hexavalent chromium (Cr(VI)) in the presence of biochar have not been fully elucidated (i.e., direct or indirect reduction of Cr(VI) by biochar). In particular, the effect of light on Cr(VI) reduction by biochar was rarely reported. Thus, to clarify the reduction mechanisms of Cr(VI) by biochar at the fundamental level, this study laid great emphasis on the photo-induced reduction of Cr(VI) in the application of biochar. Biochar releases dissolved organic matter (DOM), the DOM can extract Fe(III) from soil by complexation, and the complexes can be photo-reacted under the light. In these respects, Fe(II) formed by the photo-induced reaction of DOM-Fe(III) was particularly evaluated in this study. To evaluate that, three biomass samples (rice straw, granular sludge from an up-flow anaerobic sludge blanket, and spent coffee ground) were torrefied to biochar. To circumvent the adsorption of Cr(VI) onto biochar, biochar extractives (served as a source for DOM) and Fe(III) solution were tested with/without UV light to prove Fe(II) formation. This study experimentally proved that the more Fe(II) under the UV radiation was formed in the co-existence with biochar extractives and Fe(III). All experimental data from three biochar samples were indeed very similar. Cr(VI) reduction by Fe(II) from GB, RB, and CB reached up to 96, 79, and 100%, respectively. The different reduction efficiency signified that the low molecular weight of organic acids, such as oxalate, were more sensitive to the UV light, thereby resulting in the enhanced Fe(II) formation. Such Fe(II) formation subsequently led to the high reduction efficiency of Cr(VI).

A comparative analysis of ball-milled biochar, graphene oxide, and multi-walled carbon nanotubes with respect to toxicity induction in Streptomyces

Ball-milled biochar has recently attracted a lot of attention due to the simplicity of its preparation and low cost. However, it is unknown if the biochar is environmentally safe. Here, the toxic effect of ball-milled biochar on Streptomyces was compared to that of pristine biochar and two other carbon nanomaterials of different shapes-graphene oxide and multi-walled carbon nanotubes. The effect of these different materials on antibiotic production was characterized. The results showed that even at concentrations of up to 10 mg/L, pristine biochar had a negligible effect on toxicity and antibiotic production in Streptomyces. However, after ball milling, the physical and chemical properties of biochar changed dramatically. Cells were severely damaged, and there was a significant increase in antibiotic production after the addition of ball-milled biochar. Exposure to 10 mg/L of ball-milled biochar caused massive cell disruption; the survival rate of Streptomyces coelicolor M145 cells was only 68.2% as compared to 90% after treatment with 10 mg/L graphene oxide and multi-walled carbon nanotubes. The secretion of the antibiotics- the red intracellular pigment undecylprodigiosin (RED) and blue diffusible pigment actinorhodin (ACT) was enhanced with the highest level in treatment with ball milled biochar, as compared to that with the other two carbon nanomaterials. This effect can be attributed to increased expression of pathway-specific regulatory genes redD, redZ and actⅡ-ORF4. Ball-milled biochar can be developed as an effective additive to increase antibiotic yield. However, we should restrict the large-scale use of ball-milled biochar before fully understanding its impact on the environment and human health.

Ball-milled biochar for galaxolide removal: Sorption performance and governing mechanisms

The environmental risk of galaxolide (HHCB) spurs the need to develop efficient and economical removal technology. Although sorption is one of the best removal approaches, studies on sorption of HHCB by biochar were limited. With the purpose of combining the advantages of ball-milling and sorption technologies, six ball-milled biochars (BM-biochars) varied with biomasses and pyrolysis temperature were produced, characterized, and tested for HHCB removal from aqueous solution. At an initial HHCB concentration of 2 mg L^-1, the unmilled and BM-biochars adsorbed 330-746 and 609-2098 mg kg^-1 of HHCB, respectively. The increase in sorption capacities (about 3-fold increase) was mainly ascribed to the increase in BM-biochar's external and internal surface area, pore volume and pore size, and the exposure of the graphitic structure. The removal of HHCB by the BM-biochars increased with increasing pyrolysis temperature. For lower temperature biochar (300 °C wheat straw biochar, WS300), hydrophobic partitioning played a major role in HHCB sorption onto unmilled biochar (log K_oc/log K_ow value of WS300 was 0.772 at a C_e of 1 mg L^-1). Ball milling reduced the hydrophobicity of 300 °C biochar, which diminished the HHCB sorption. However, increased surface area, pore volume, pore size, and graphitic structure provided additional sorption sites, resulting in enhanced HHCB uptake (log K_oc/log K_ow value of BMWS300 was 1.23 at a C_e of 1 mg L^-1). For higher temperature biochars (500 and 700 °C), ball milling mainly enhanced HHCB sorption onto high temperature biochars via surface adsorption, π-π interaction, and pore filling. For WS500, 77.9% of HHCB removal was due to surface adsorption. Ball milling increased this percentage to 96.7% for BMWS500. This work highlighted the potential of ball milling as an excellent engineering method to improve biochar's sorption properties.

Preparation of various thiol-functionalized carbon-based materials for enhanced removal of mercury from aqueous solution

In this work, biochar (BC), activated carbon (AC), and graphene oxide (GO) were thiol-functionalized using 3-mercaptopropyltrimethoxysilane (3-MPTS) (named as BCS, ACS, and GOS, respectively). BCS, ACS, and GOS were synthesized mainly via the interaction between hydrolyzed 3-MPTS and surface oxygen-containing functional groups (e.g., -OH, O-C=O, and C=O) and π-π interaction. The materials before and after modification were characterized and tested for mercury removal, including sorption kinetics and isotherms, the effects of adsorbent dosage, initial pH, and ionic strength. Pseudo-second-order sorption kinetic model (R² = 0.992~1.000) and Langmuir sorption isotherm model (R² = 0.964~0.998) fitted well with the sorption data of mercury. GOS had the most -SH groups with the largest adsorption capacity for Hg²⁺ and CH₃Hg⁺ (449.6 and 127.5 mg/g), followed by ACS (235.7 and 86.7 mg/g) and BCS (175.6 and 30.3 mg/g), which were much larger than GO (96.7 and 4.9 mg/g), AC (81.1 and 24.6 mg/g), and BC (95.6 and 9.4 mg/g). GOS and ACS showed stable mercury adsorption properties at a wide pH range (2~9) and ionic strength (0.01~0.1 mol/L). Mercury maybe removed by ligand exchange, surface complexation, and electrostatic attraction.

A novel biochar supported CMC stabilized nano zero-valent iron composite for hexavalent chromium removal from water

In this study, novel biochar supported nano-scale zero-valent iron (biochar-CMC-nZVI) stabilized by carboxymethyl cellulose (CMC) was developed and used for the removal of hexavalent chromium (Cr(VI) from aqueous solution. With the stabilization of CMC, nZVI particles (about 80 nm) were effectively dispersed onto the surface of biochar, which inhibited the aggregation of nZVI and resulted in the smaller particle size of nZVI on the surface of biochar. The results showed that the specific surface area of the composite was 11.1 m²/g, lower than that of pristine biochar. The basic element composition was C, O, and Fe with a large number of oxygen-containing functional groups (-COOH, OH, and OCO) observed on the surface. Cr(VI) was reduced to Cr(III) by the composite material, mainly due to the reduction of nZVI on the biochar surface. Upon reaction with Cr(VI), Cr_xFe_1-x(OH)₃ and Fe_xCr_yO₄ were deposited on the surface of biochar-CMC-nZVI composite. Electrostatic attraction, reduction, and surface complexation were the dominant removal mechanisms. The results showed that the 100 mg/L Cr(VI) could be removed completely by biochar-CMC-nZVI within 18 h, at a dosage of 1.25 g/L and an initial pH of 5.6. Cr(VI) removal by biochar-CMC-nZVI was favored by lower pH. The pseudo-second-order kinetic model and the Langmuir isothermal adsorption model fitted well with the sorption kinetic and isotherm data, indicated Cr(VI) adsorption mechanism was a chemisorption based multi-layer adsorption. The present study demonstrated the promise of biochar-CMC-nZVI composite as a low-cost, "green", and effective sorbent for removal of Cr(VI) in the environment.

Unravelled keratin-derived biopolymers as novel biosorbents for the simultaneous removal of multiple trace metals from industrial wastewater

Biopolymers derived from modified poultry feathers (KB) were developed to target a broad range of potentially toxic trace elements for their removal from synthetic wastewater and industrial process affected water. The chemical modifications increased surface functionality of KBs for enhanced metal adsorption. Unmodified KB (SM-03) added to synthetic wastewater spiked with nine transition and redox sensitive elements (30-50 μg L^-1 each) removed >82% of Pb, Ni, Co and Zn, whereas modified KBs (SM-01 and SM-06) removed 68-100% of Se^IV, V^V and Cr^VI. Similar results were observed when spiked process water was used. Experimental observation suggested chemical reduction of redox sensitive elements on the modified KB surfaces to their non-toxic/non-mobile redox states. Biopolymer SM-06 showed a maximum adsorption capacity of 17 mg g^-1 for V^V and 15 mg g^-1 for Cr^VI at ~20 °C. Due to the abundance of raw material and simplicity of the modifications presented here, modified KBs may serve as a useful option for large-scale water treatment.

Removal of hexavalent chromium from aqueous solution by different surface-modified biochars: Acid washing, nanoscale zero-valent iron and ferric iron loading

Willow residue biochar (BC) and modified biochars (hydrochloric acid washing (HBC), HBC loaded with nanoscale zero-valent iron (nZVI-HBC), and HBC loaded with ferric iron (Fe³⁺-HBC)) after aging were used for aqueous Cr(VI) removal. HBC (>98.67%), nZVI-HBC (>98.86%), and Fe³⁺-HBC (>99.64%) kept high Cr(VI) removal rates under the acidic conditions within a wide pH range (<7.0), indicating their good adaptability to pH change because of aging. Cr(VI) reduction to Cr(III) was the dominant removal mechanism. The formation of COOH on BC, HBC, and nZVI-HBC indicates the oxidation of surface functional groups by Cr(VI) and simultaneous Cr(VI) reduction. The disappearance of nZVI peaks indicates the reduction of Cr(VI) to Cr(III) by nZVI. The color reaction result demonstrated that the converted Fe²⁺ in Fe³⁺-HBC contributed to Cr(VI) reduction. Taking into account the removal efficiency, recyclability, cost, preparation process, and stability of adsorbents, Fe³⁺-HBC was recommended for Cr(VI) removal.