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[Display omitted] • Reduction of Cr(VI) occurs mainly on solid surfaces of the multiple interfaces from ternary composites. • Micron zero-valent iron powder activated by low-cost natural pyrite and biochar provide a long-lasting reduction for Cr(VI). • Biochar acts as a conductor, adsorber, and disperser in the reaction system. • The electron selectivity and utilization rate of the composite toward Cr(VI) were 14.20% and 48.08%, respectively. • 70.07% of Cr(VI) removal was due to reduction/precipitation and 29.93% was ascribed to surface sorption. Current studies have confirmed that sulfidation is an effective method to enhance the reduction capacity of zero-valent iron (ZVI), however, the cost of sulfidation and the durability of the material are still the challenges of material reducibility. In this work, ternary composites of natural pyrite (FeS 2), zero-valent iron powder, and pine wood biochar (BC) were synthesized by simple ball milling technology (BM) and applied for the removal of hexavalent chromium (Cr(VI)) in aqueous solution. Batch sorption experiments of each component and composites at various mass rates conformed that the adsorption capacity of BMFeS 2 /ZVI/BC at 1:1:1 (81.5 mg·g−1) exceeded two to three-fold of Cr (VI) than their individuals. Comparative experiments on two groups with various raw material ratios indicated the balance between biochar-related adsorption capacity and reduction-related S/Fe was imperative. The analysis and calculation of the products yielded 70.07 % of Cr(VI) removal due to reduction/precipitation and 29.93 % was ascribed to surface sorption at the equilibrium Cr(VI) concentration of 33.76 mg·L−1. The electron selectivity (ES) and utilization rate (UR) of 1:1:1 toward Cr(VI) were determined to be 14.20 % and 48.08 %, respectively. A negligible metal ion leakage was observed upon pH > 5.5. Shielding experiments of Fe2+, TEM, and XPS analysis proved the reduction of Cr(VI) occurred mainly on solid surfaces. The role of biochar in the reaction system was identified as conductor, adsorber, and disperser. The multiple interfaces caused by ball milling provide different reduction pathways. Nevertheless, reduction, adsorption, and surface complexation were the dominant mechanisms for Cr(VI) removal. This work demonstrates the potential of natural pyrite for the preparation of BMFeS 2 /ZVI/BC composite to remove Cr(VI) from water and wastewater. [ABSTRACT FROM AUTHOR]

A novel biochar supported nanoscale iron sulfide (FeS) composite (CMC-FeS@biochar) combining the advantages of biochar, carboxymethyl cellulose (CMC), and FeS was synthesized and tested for Cr(VI) removal efficiency and mechanisms. FeS particles were effectively soldered onto the surface of biochar through OH, C C, O C O, C O, and Si O functional groups. The composite at a mass ratio of FeS:CMC: biochar = 1:1:1 displayed an enhanced Cr(VI) adsorption capacity of 130.5 mg/g at pH 5.5 compared to 38.6 mg/g for FeS and 25.4 mg/g for biochar. Surface sorption and reduction were the dominant removal mechanisms. At the equilibrium Cr(VI) concentration of 13.4 mg/L, 57% of Cr(VI) removal was attributed to reduction and 43% was ascribed to surface sorption. The adsorption kinetic data were adequately simulated with pseudo second-order kinetic model and mass transfer model, suggesting that sorption kinetics were the combination of chemisorption and external mass transfer. The Redlich-Peterson model fitted better than the Langmuir and Freundlich models in simulating the adsorption isotherm data, again suggesting a hybrid chemical reaction-sorption process. The Dubinin–Radushkevich isotherm model resulted in an adsorption energy of 10.0 kJ/mol, implying a chemisorption between Cr(VI) and CMC-FeS@biochar. The present study demonstrated the promise of CMC-FeS@biochar composite as a low-cost, “green”, and effective sorbent for removal of Cr(VI) in the environment. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fe 1 -N 1 -BC 1 composites were synthesized by carbothermal reduction at 300 – 900 °C. • Low- and high-temperature composites showed superior N and Fe, respectively. • Fe 1 -N 1 -BC 1 -300 removed Cr(VI), and Fe 1 -N 1 -BC 1 -700 removed NB completely from solution. • Fe 1 -N 1 -BC 1 composites showed better efficiency in simultaneous removal of Cr(VI) and NB. • Fe 1 -N 1 -BC 1 composites exhibited restored removal capacity for more than four months. Synthesizing new composites with better efficiency and stability from abundant sources offers an appealing prospect for removal of heavy metals and organic pollutants to address water pollution concerns. In this study, iron (Fe) and nitrogen (N) doped biochar (BC) composites were investigated for hexavalent chromium (Cr(VI)) and nitrobenzene (NB) removal. Specifically, temperature-dependent Fe 1 -N 1 -BC 1 composites were synthesized with different mass ratios of FeCl 3 as Fe, melamine as N, and pinewood sawdust (PWS) as BC precursors via carbothermal reduction at 300, 500, 700, and 900 °C. Detailed characterizations indicating low-temperature incorporated superior N and high-temperature incorporated more Fe species in Fe 1 -N 1 -BC 1 composites. The Cr(VI) and NB were removed with Fe 1 -N 1 -BC 1 , and experimental results attested with standard isotherm and kinetic models. The N species (pyridinic-N, pyrrolic-N, graphitic-N, quaternary-N, and oxidized-N) and Fe species (Fe 3 C, Fe0, Fe 2p 3/2 , and Fe 2p 1/2) in temperature-dependent Fe 1 -N 1 -BC 1 composites participated in pollutants removal. Mass ratio of 1:1:1 of Fe, N, and BC was effective, and under optimum conditions, low-temperature (Fe 1 -N 1 -BC 1 -300) and high-temperature (Fe 1 -N 1 -BC 1 -700) composites completely removed Cr(VI) and NB, respectively. In comparison to pinewood sawdust BC (PWBC) and composite of FeCl 3 with PWS (Fe-BC), melamine with PWS (N-BC), and FeCl 3 with melamine (Fe-N), the Fe 1 -N 1 -BC 1 was more efficient due to synergistic effect of Fe, N, and BC for better pollutant interaction and removal. Effect of different factors on pollutants removal with Fe 1 -N 1 -BC 1 was studied to reveal removal mechanism insights. Additionally, Fe 1 -N 1 -BC 1 showed better efficiency in simultaneous removal of Cr(VI) and NB, stability for > 4 months, reusability, and applicability for multiple pollutants. This study offers a benign approach for synthesizing and applying Fe 1 -N 1 -BC 1 for pollutants removal and exploring reaction mechanisms. [ABSTRACT FROM AUTHOR]

• The conductivity and conjugated O-containing functional groups of BMBCs(ball mill biochar) at higher pyrolysis temperature (≥ 700 °C) were responsible for synergistic effect with MR-1. • Low electronegative charges of BMBC700 and BMBC800 were more beneficial to the synergistic interaction with MR-1. • Cr(VI) removal reaction of MR-1 with BMBCs could be classified into two steps-fast reaction and slow reaction. Biochar, carbonaceous material produced from biomass pyrolysis, has been demonstrated to have electron transfer property (associated with redox active groups and multi condensed aromatic moiety), and to be also involved in biogeochemical redox reactions. In this study, the enhanced removal of Cr(VI) by Shewanella oneidensis MR-1(MR-1) in the presence of biochars with different pyrolysis temperatures (300 to 800 °C) was investigated to understand how biochar interacts with Cr(VI) reducing bacteria under anaerobic condition. The promotion effects of biochar (as high as 1.07~1.47 fold) were discovered in this process, of which the synergistic effect of BMBC700(ball milled biochar) and BMBC800 with MR-1 was noticeable, in contrast, the synergistic effect of BMBCs (300–600 °C) with MR-1 was not recognized. The more enhanced removal effect was observed with the increase of BMBC dosage for BMBC700+MR-1 group. The conductivity and conjugated O-containing functional groups of BMBC700 particles themselves has been proposed to become a dominant factor for the synergistic action with this strain. And, the smallest negative Zeta potential of BMBC700 and BMBC800 is thought to favor decreasing the distance from microbe than other BMBCs. The results are expected to provide some technical considerations and scientific insight for the optimization of bioreduction by useful microbes combining with biochar composites to be newly developed. [Display omitted] [ABSTRACT FROM AUTHOR]

As a natural reduction mineral, pyrite (FeS 2), was combined with biochar by simply ball milling technology to synthesize FeS 2 @biochar composite (BM-FeS 2 @BC) and applied for the removal of hexavalent chromium (Cr(VI)) in aqueous solution. SEM, XRD, FTIR, and XPS characterization results showed that the FeS 2 and biochar were successfully combined and biochar suppressed the agglomeration of FeS 2. Batch sorption experiments showed that the BM-FeS 2 @BC700 composite (mass ratio of FeS 2 -to-biochar = 3:1) had enhanced Cr(VI) removal capacity of 134 mg·g−1, which were 3–25 times higher than those of the corresponding pristine and ball-milled biochar and FeS 2. The removal of Cr(VI) by BM-FeS 2 @BC700 was dosage and pH dependent. The addition of oxalic acid (OA) exhibited a promotion effect on the removal of Cr(VI) by increasing the removal rate of Cr(VI) from 56% to 100%. Reduction, adsorption, and surface complexation were the dominate mechanisms for Cr(VI) removal by BM-FeS 2 @BC700. At the equilibrium Cr(VI) concentration of 15.7 mg·L−1, 92.25% of Cr(VI) was removed through reduction/precipitation and 8.75% was removed by adsorption/surface complexation. The fitting results of the Langmuir model proved that the removal of Cr(VI) by BM-FeS 2 @BC700 composite was chemical surface monolayer adsorption. This work demonstrates the potential of ball milling for the preparation of FeS 2 @BC composite to remove Cr(VI) from water and wastewater. [Display omitted] • Preparation of BM-FeS 2 @BC700 achieved the reuse of waste. • Ball milling inhibited the oxidation of the pyrite surface. • BM-FeS 2 @BC dramatically increased the uptake of Cr(VI). • Adsorption and reduction were the main mechanisms to remove Cr(VI). • Acidic pH was more beneficial to the reduction of Cr(VI). [ABSTRACT FROM AUTHOR]

• Impacts of corrosion and passivation on long-term reactivity of S-ZVevaluated. • A rational method determining selectivity was proposed concernin oxidization. • The corrosion behaviors of S-ZVI aged in different environment were reported. • Chloride facilitates iron consumption with corrosion rate rising by 17.329.4 %. • Minor passivation promotes the long-term reactivity of S-ZVI by 33.9 % 131 %. The corrosion and surface passivation of sulfidized zero-valent iron (S-ZVI) by common groundwater ions and contaminants are considered to be the most challenging aspects in the application of S-ZVI for remediation of chlorinated contaminants. This study investigated the impacts of corrosive chloride (Cl−) and passivation of hexavalent chromium (Cr(VI)) on the long-term reactivity, selectivity, corrosion behavior, and physicochemical properties during the 60-day aging process of S-ZVI. Although the co-existing of Cl− promoted the initial reactivity of S-ZVI, the rapid consumption of Fe° content shortened the reactive lifetime owing to the insufficient electron capacity. Severe passivation by Cr(VI) (30 mg L−1) preserved the Fe° content but significantly interfered with the reductive sulfur species, resulting in an increase in electron transfer resistance. In comparison, minor passivated S-ZVI (5.0 mg L−1 Cr(VI)) inhibited the hydrogen evolution while concurrently mitigating the further oxidation of the reductive iron and sulfur species, which significantly enhanced the long-term reactivity and selectivity of S-ZVI. Furthermore, the enhancement effect of minor passivation could be detected in the aging processes of one-step, two-step, and mechanochemically synthesized S-ZVI particles with different S/Fe ratios and precursors, which further verified the advantages of minor passivation. This observation is inspirable for the development of innovative strategies for environmental remediation by S-ZVI-based materials. [Display omitted] [ABSTRACT FROM AUTHOR]

Over the past decade, biochar-supported nZVI composites (nZVI/biochar) have been developed and applied to treat various pollutants due to their excellent physical and chemical properties, especially in the field of chromium (VI) removal. This paper reviewed the factors influencing the preparation and experiments of nZVI/biochar composites, optimization methods, column experimental studies and the mechanism of Cr(VI) removal. The results showed that the difference in raw materials and preparation temperature led to the difference in functional groups and electron transfer capabilities of nZVI/biochar materials. In the experimental process, pH and test temperature can affect the surface chemical properties of materials and involve the electron transfer efficiency. Elemental doping and microbial coupling can effectively improve the performance of nZVI/biochar composites. In conclusion, biochar can stabilize nZVI and enhance electron transfer in nZVI/biochar materials, enabling the composite materials to remove Cr(VI) efficiently. The study of column experiments provides a theoretical basis for applying nZVI/biochar composites in engineering. Finally, the future work prospects of nZVI/biochar composites for heavy metal removal are introduced, and the main challenges and further research directions are proposed. [Display omitted] • NZVI/biochar is a highly efficient Cr(VI) removal composite. • Pyrolysis temperature and feedstock affect functional groups and electron transfer ability. • More efficient and stable effects can be achieved by adding stabilizer and element doping. • Flow rate is the key factor affecting removal efficiency in column experiment. • Multiple removal mechanisms coexist in nZVI/biochar Cr(VI) removal. [ABSTRACT FROM AUTHOR]