Stabilization of nanoscale zero-valent iron (nZVI) with modified biochar for Cr(VI) removal from aqueous solution

Efficient remediation of different concentrations of Cr-contaminated soils by nano zero-valent iron modified with carboxymethyl cellulose and biochar

Nano zero-valent iron (nZVI) is widely used in soil remediation due to its high reactivity. However, the easy agglomeration, poor antioxidant ability and passivation layer of Fe-Cr coprecipitates of nZVI have limited its application scale in Cr-contaminated soil remediation, especially in high concentration of Cr-contaminated soil. Herein, we found that the carboxymethyl cellulose on nZVI particles could increase the zeta potential value of soil and change the phase of nZVI. Along with the presence of biochar, 97.0% and 96.6% Cr immobilization efficiency through CMC-nZVI/BC were respectively achieved in high and low concentrations of Cr-contaminated soils after 90-days remediation. In addition, the immobilization efficiency of Cr(VI) only decreased by 5.1% through CMC-nZVI/BC treatment after 10 weeks aging in air, attributing to the strong antioxidation ability. As for the surrounding Cr-contaminated groundwater, the Cr(VI) removal capacity of CMC-nZVI/BC was evaluated under different reaction conditions through column experiments and COMSOL Multiphysics. CMC-nZVI/BC could efficiently remove 85% of Cr(VI) in about 400 hr when the initial Cr(VI) concentration was 40 mg/L and the flow rate was 0.5 mL/min. This study demonstrates that uniformly dispersed CMC-nZVI/BC has an excellent remediation effect on different concentrations of Cr-contaminated soils.

Effects of nZVI on the migration and availability of Cr(VI) in soils under simulated acid rain leaching conditions

Hexavalent chromium, Cr(VI), is a ubiquitous toxic metal that can be reduced to Cr(III) by nano-zero-valent iron (nZVI). Finding out effects of continuous rainfall leaching on the Cr(VI) release and availability remains a problem, needing to be addressed. Whether the Cr(VI) reduction by nZVI and continuous rainfall leaching lead to localized heterogeneity in soil is unclear. Therefore, two in situ high-resolution (HR) techniques of the diffusive gradients in thin-films (DGT) and planar optode were combined with ex situ sampling experiments here. Results demonstrate that nZVI decreased Cr(VI) leaching by 5.60-8.50 % compared to control soils. DGT-measured concentrations of Cr(VI), CDGT-Cr(VI), ranged from 7.31 to 19.4 μg L-1 in the control soils, increasing with depth while CDGT-Cr(VI) in nZVI-treated soils (2.41-6.18 μg L-1) decreased or remained stable with depth. However, simulated acid-rain leaching increases CDGT-Cr(VI) by 1.61-fold in nZVI-treated soils, negatively affecting the remediation. DGT measurements in bulk soils using disc devices are better at capturing the change of Cr(VI) availability at different conditions, whereas 2D-HR DGT mappings did not characterize significant mobilization of Cr(VI) at the micro-scale. These findings emphasize the importance of monitoring Cr(VI) release and availability in remediated soil under acid-rain leaching conditions for effective environment management.

Novel Fe/Ca oxide co-embedded coconut shell biochar for phosphorus recovery from agricultural return flows

Efficient elimination and recovery of phosphorus from agricultural return flows are crucial for effective eutrophication management and phosphorus reuse. In this study, a neutral Fe/Ca oxide co-embedded biochar (FCBC) was synthesized using calcium peroxide and ferrous chloride as precursors for phosphate recovery from agricultural return flows. FCBC possesses a highly intricate pore structure and an abundance of surface-active groups. Fe/Ca oxides were loaded onto the biochar in the form of Ca2Fe2O5, Fe2O3, and CaCO3. FCBC demonstrated a broad pH tolerance range (pH = 6-12) in the aquatic environment. The maximum saturation adsorption capacity was 53.31 mg g-1. Phosphorus removal is influenced by Ca3(PO4)2 generation, intra-particle diffusion, and electrostatic attraction. The produced FCBC showed exceptional phosphorus removal efficiency in the presence of various anions, except for wastewater with high concentrations of SO4 2-, CO3 2-, HCO3 -, and F- (>500 mg L-1). FCBC can effectively remove phosphorus from agricultural return flows and reduce the risk of the water environment. Returning it to the field can also mitigate the depletion of phosphorus resources, effectively reduce carbon emissions from farmland, improve soil fertility, and realize multiple benefits.

Insights into the pH-dependent mechanism of peracetic acid activation by biochar-supported zero-valent iron/cobalt bimetallic nanoparticles: The shift of reactive sites and the dual role of hydrogen peroxide

The peracetic acid (PAA)-based water purification process is often controlled by the solution pH. Herein, we explored the usage of biochar (BC) supported zero-valent iron/cobalt nanoparticles (Fe/Co@BC) for triggering PAA oxidation of sulfamethazine (SMT), and discovered the PAA activation mechanisms at different pHs. Fe/Co@BC exhibited extraordinary PAA activation efficiency over the pH range of 3.0-8.2, effectively broadening the working pH of the zero-valent iron nanoparticles (NZVI)-PAA process. Specifically, the SMT removal efficiency increased by 8.3 times in Fe/Co@BC-PAA system compared to the NZVI-PAA system at pH 8.2. Besides, the leaching and recycling experiments indicated the improved stability and reusability of the materials. For the mechanism study, the main reactive species was •OH under acidic conditions and R-O•/Fe(IV) under neutral/alkaline conditions. More interestingly, the reactive sites on Fe/Co@BC shifted from Fe species to Co species as pH increased, and the role of H2O2 in this reaction system also shifted from a radical precursor to a radical scavenger with increasing pH. This study highlights the distinct mechanism of PAA activation by bimetallic composites under different pH conditions and provides a new efficient approach for PAA activation to degrade organic contaminants.

Persulfate activation with biochar supported nanoscale zero- valent iron: Engineering application for effective degradation of NCB in soil

Nitrochlorobenzene (NCB) is very common in pesticide and chemical industries, which has become a major problem in soil environment. However, the remediation of NCB contaminated soil is received finite concern. Using biochar as a substrate for nanoscale-zero valent iron (nZVI/p-BC) to activate peroxodisulfate (PDS), a novel heterogeneous oxidative system had been applied in the current study to remediate NCB contaminants in soil. The degradation efficiencies and kinetics of m-NCB, p-NCB, and o-NCB by various systems were contrasted in soil slurry. Key factors including the dosage of nZVI/p-BC, the molar ratio of nZVI/PDS, initial pH and temperature on degradation of NCB were further examined. The results confirmed that the nZVI/p-BC/PDS displayed the remarkable performance for removing NCB compared with other systems. Higher temperature with nZVI/PDS molar ratio of 2:1 under the acidic condition favored the reduction of NCB. The treatment for NCB with optimal conditions were evaluated for the engineering application. The mechanism of nZVI/p-BC/PDS indicated that electron transfer between p-BC and nZVI was responsible for activation of PDS, generating active species (SO4•-, •OH and 1O2) via both the free and non-free radical pathways. Experimental results revealed prominent availability of nZVI/p-BC/PDS system in remediation of actual contaminated field by NCB.

Efficient reduction of Cr(VI) by guava (Psidium guajava) leaf extract and its mitigation effect on Cr toxicity in rice seedlings

Hexavalent chromium (Cr(VI)) is a toxic element that has negative impacts on crop growth and yield. Using plant extracts to convert toxic Cr(VI) into less toxic Cr(III) may be a more favorable option compared to chemical reducing agents. In this study, the potential effects and mechanisms of using an aqueous extract of Psidium guajava L. leaves (AEP) in reducing Cr(VI) toxicity in rice were comprehensively studied. Firstly, the reducing power of AEP for Cr(VI) was confirmed by the cyclic voltammetry combined with X-ray photoelectron spectroscopy (XPS) assays. The highest Cr(VI) reduction efficiency reached approximately 78% under 1.5 mg gallic acid equivalent (GAE)/mL of AEP and 10 mg/L Cr(VI) condition. Additionally, Cr(VI) stress had a significant inhibitory effect on rice growth. However, the exogenous application of AEP alleviated the growth inhibition and oxidative damage of rice under Cr(VI) stress by increasing the activity and level of enzymatic and non-enzymatic antioxidants. Furthermore, the addition of AEP restored the ultrastructure of root cells, promoted Cr adsorption onto root cell walls, and limited the translocation Cr to shoots. In shoots, AEP application also triggered the expression of specific genes involved in Cr defense and detoxification response, including photosynthesis pathways, antioxidant systems, flavonoids biosynthesis, and plant hormone signal transduction. These results suggest that AEP is an efficient reduction agent for Cr(VI), and exogenous application of AEP may be a promising strategy to mitigate the harm of Cr(VI) on rice, ultimately contributing to improved crop yield in Cr-contaminated environments.

Carbothermal synthesis of sulfurized nano zero-valent iron from sulfate-reducing bacteria biomass for mercury removal: The first application of biomass sulfur source

The development of low-cost, highly efficient adsorbent materials is of significant importance for environmental remediation. In this study, a novel material, sulfurized nano zero-valent iron loaded biomass carbon (S-nZVI/BC), was successfully synthesized by a simple manufacturing process. The preparation of S-nZVI/BC does not require the use of expensive and hazardous chemicals. Instead, residual sludge, a solid waste product, is used as feedstock. The sludge is rich in Sulfate-Reducing Bacteria (SRB), which can provide carbon and sulfur sources for the synthesis of S-nZVI/BC. It was observed that S-nZVI particles formed in situ were dispersed within BC and covered by it. Additionally, S-nZVI/BC inherited the large specific surface area and porosity of BC. The adsorption capacity of S-nZVI/BC can reach 857.55 mg g-1 Hg (II) during the remediation of mercury-polluted water. This research offers new perspectives for developing composites in terms of the low cost and harmlessness of raw materials.

Processes and mechanisms in remediation of aqueous chromium contamination by sulfidated nano-scale zerovalent iron (S-nZVI): Experimental and computational investigations

Sulfidated nano-scale zerovalent iron (S-nZVI) has emerged as an advanced functional nanomaterial for efficiently remediating Cr(VI) contamination in aqueous environments. However, there is an insufficient understanding of its coherent process, removal pathway, and hydrochemical reactive mechanisms, presenting potential challenges for its future environmental applications. To address this gap, this study successfully synthesized S-nZVI through a chemical precipitation method and effectively applied it for the removal of Cr(VI). Additional characterization revealed that the removal of Cr(VI) followed a sequence of rapid chemisorption and intraparticle diffusion processes, concomitant with an increase in pH and a decrease in oxidation-reduction potential. The remediation mechanism encompassed a synergistic reduction of Cr(VI) to Cr(III) and simultaneous immobilization via Cr2FeO4 coprecipitation. The highest Cr(VI) removal capacity of 75 mg/g was attained during dynamic removal experiments in the sand column packed with S-nZVI. Further computational analysis, employing density functional theory calculations based on the experimental data, revealed the involvement of multiple molecular orbitals of Cr(VI) in the removal process. It also elucidated a step-by-step reduction pathway for Cr(VI) characterized by decreasing free energy. These findings provide evidence-based insights into Cr(VI) remediation using S-nZVI and can serve as valuable technical support for future environmental management of heavy metals.

Improvement strategy of citrate and biochar assisted nano-palladium/iron composite for effective dechlorination of 2,4-dichlorophenol

Rapid passivation and aggregation of nanoscale zero-valent iron (nZVI) seriously limit its performance in the remediation of different contaminants from wastewater. To overcome such issues, in the present study, nano-palladium/iron (nPd/Fe) was simultaneously improved by biochar (BC) prepared from discarded peanut shells and green complexing agent sodium citrate (SC). For this purpose, a composite (SC-nPd/Fe@BC) was successfully synthesized to remove 2,4-dichlorophenol (2,4-DCP) from wastewater. In the SC-nPd/Fe@BC, BC acts as a carrier with dispersed nPd/Fe particles to effectively prevent its agglomeration, and increased the specific surface area of the composite, thereby improving the reactivity and stability of nPd/Fe. Characterization results demonstrated that the SC-nPd/Fe@BC composites were well dispersed, and the agglomeration was weakened. The formation of the passivation layer on the surface of the particles was inhibited, and the mechanism of SC and BC improving the reactivity of nPd/Fe was clarified. Different factors were found to influence the reductive dichlorination of 2,4-DCP, including Pd loading, Fe:C, SC addition, temperature, initial pH, and initial pollutant concentration. The dechlorination results revealed that the synergistic effect of the BC and SC made the removal efficiency and dechlorination rate of 2,4-DCP by SC-nPd/Fe@BC reached to 96.0 and 95.6%, respectively, which was better than that of nPd/Fe (removal: 46.2%, dechlorination: 45.3%). Kinetic studies explained that the dechlorination reaction of 2,4-DCP and the data were better represented by the pseudo-first-order kinetic model. The reaction rate constants followed the order of SC-nPd/Fe@BC (0.0264 min-1) > nPd/Fe@BC (0.0089 min-1) > SC-nPd/Fe (0.0081 min-1) > nPd/Fe (0.0043 min-1). Thus, SC-nPd/Fe@BC was capable of efficiently reducing 2,4-DCP and the dechlorination efficiency of BC and SC synergistically assisted composite on 2,4-DCP was much better than that of SC-nPd/Fe, nPd/Fe@BC and nPd/Fe. Findings suggested that SC-nPd/Fe@BC can be promising for efficient treatment of chlorinated pollutants.

In situ carbothermal synthesis of carbonized bacterial cellulose embedded with nano zero-valent iron for removal of Cr(VI)

Carbonized bacterial cellulose embedded with highly dispersed nano zero-valent iron (nZVI), denoted as nZVI@CBC, was prepared through one-step in situ carbothermal treatment of bacterial cellulose adsorbing iron(III) nitrate. The structure characteristics of nZVI@CBC and its performance in removing hexavalent chromium Cr(VI) were investigated. Results showed the formation of nZVI@CBC with a surface area of 409.61 m2/g at 800 °C, with nZVI particles of mean size 28.2 nm well distributed within the fibrous network of CBC. The stability of nZVI was enhanced by its carbon coating, despite some inevitable oxidation of exposed nZVI. Batch experiments demonstrated that nZVI@CBC exhibited superior removal efficiency compared to bare nZVI and CBC. Under optimal conditions, nZVI@CBC exhibited a high Cr(VI) adsorption capacity of up to 372.42 mg/g. Therefore, nZVI@CBC shows promise as an effective adsorbent for remediating Cr(VI) pollution in water.

Sustainable remediation of Cr(VI)-contaminated soil by soil washing and subsequent recovery of washing agents using biochar supported nanoscale zero-valent iron

Soil contamination by Cr(VI) has attracted widespread attention globally in recent years, but it remains a significant challenge in developing an environmentally friendly and eco-sustainable technique for the disposal of Cr(VI)-contaminated soil. Herein, a sustainable cyclic soil washing system for Cr(VI)-polluted soil remediation and the recovery of washing agents using biochar supported nanoscale zero-valent iron (nZVI-BC) was established. Citric acid (CA) was initially screened to desorb Cr(VI) from contaminated soil, mobilizing Cr from the highly bioaccessible fractions. The nZVI-BC exhibited superior properties for Cr(VI) and Cr(total) removal from spent effluent, allowing effective recovery of the washing agents. The elimination mechanism of Cr(total) by nZVI-BC involved the coordinated actions of electrostatic adsorption, reduction, and co-precipitation. The contributions to Cr(VI) reduction by Fe0, surface-bound Fe(II), and soluble Fe(II) were 0.6 %, 39.8 %, and 59.6 %, respectively. Meanwhile, CA favored the activity of surface-bound Fe(II) and Fe0 in nZVI-BC, enhancing the production of soluble Fe(II) to strengthen Cr(VI) removal. Finally, the recovered washing agent was proven to be reused three times. This study showcases that the combined soil washing using biodegradable chelant CA and effluent treatment by nZVI-BC could be a sustainable and promising strategy for Cr(VI)-contaminated soil remediation.

Partially oxidized mackinawite/biochar for photo-Fenton organic contaminant removal: Synergistically improve interfacial electron transfer and H2O2 activation

Immobilizing Fe-based nanoparticles on electron-rich biochar has becoming an attractive heterogeneous Fenton-like catalysts (Fe/BC) for wastewater decontamination. However, the insufficient graphitization of biochar causing low electron transfer and by slow H2O2 activation limited its application. Herein, we firstly constructed FeS/biochar composite through all-solid molten salt method (Fe/MSBCs), which can provide strong polarization force and liquid reaction environment to improve carbonization. As expected, the obtained Fe/MSBCs exhibits high surface area and fast interfacial electron transfer between FeS and biochar. More importantly, the partially oxidized FeS (001) facet facilitate H2O2 adsorption and thermodynamically easily decomposition into •OH. Such a synergistic effect endowed them excellent photo-Fenton degradation performance for methyl orange (MO) with large kinetic rate constants (0.079 min-1) and high H2O2 utilization efficiency (95.9%). This study first demonstrated the critical regulatory role of molten salt method in iron-based biochar composites, which provide an alternative for H2O2 activator in water pollutant control.

Activity and recyclability enhancement of pH-dependent Fe0@BC-mediated heterogeneous sodium percarbonate (SPC)-reducing agents (RA) system

Dyes pose great threats to the aquatic environment and human health. Fe0-based Fenton-like systems have been widely employed for the degradation of organic dyes. However, the regulation of degradability and recyclability was still unclear. In this study, Rhodamine B (RhB) was served as the model pollutant, hydroxylamine hydrochloride was selected as the RA, the natural photocatalysis system demonstrated stable operation. RA, as performance enhancement agent, was firstly reported in micro/nano-Zero-Valent Iron@Biochar (m/nZVI@BC) based SPC-RA system. Carrier size-fractionated m/nZVI@BC was fabricated by one-step carbothermal method. As a result, RA synergistically interacted with SPC, and the reaction time reduced from 15 min to 4 min. In the 0.010 g m/nZVI@BC-mediated SPC-RA system, over 95% of RhB (100 mg·L-1, 1041.667 mg·g-1) was successfully degraded. The maximum degradation ability could still exceed 1g·g-1 via 5 times repeated applications. Meanwhile, the loss of degradability, caused by halving SPC concentration could be compensated by RA dosage measurement. The entire degradation process was predominantly dominated by free radicals (•OH> 1O2> •O2-> •CO3-). Reactive oxidizing species (ROSs) were primarily excited by α-Fe0, Fe3C and N sites of biochar (BC). Light and BC carrier dedicated slight influence. These discoveries shed a light on the activity and recyclability regulation of catalytic material, aligning with the principles of green chemistry and cleaner production. This study demonstrates a novel approach to efficient management of solid waste disposal, reuse of waste biomass, advanced treatment of dye-containing wastewater, pollution control in aquatic environments.

Efficient reduction and adsorption of Cr(VI) using FeCl3-modified biochar: Synergistic roles of persistent free radicals and Fe(II)

Transition metal iron and persistent free radicals (PFRs) both affect the redox properties of biochar, but the electron transfer relationship between them and the coupling reduction mechanism of Cr(VI) requires further investigation. To untangle the interplay between iron and PFRs in biochar and the influences on redox properties, FeCl3-modified rice husk biochar (FBCs) was prepared and its reduction mechanism for Cr(VI) without light was evaluated. The FBCs had higher surface positive charges, oxygen-containing functional groups, and PFRs compared with pristine rice husk biochar (BC). Phenoxyl PFRs with high electron-donating capability formed in biochar. The pronounced electron paramagnetic resonance signals showed that the PFRs preferred to form at lower Fe(III) concentrations. While a high concentration of Fe(III) would be reduced to Fe(II) and consumed the formed PFRs. Adsorption kinetics and X-ray photoelectron spectroscopy analysis indicated that the FBCs effectively enhanced the Cr(VI) removal efficiency by 1.54-8.20 fold and the Cr(VI) reduction efficiency by 1.88-9.29 fold compared to those of BC. PFRs quenching and competitive reductant addition experiments revealed that the higher Cr(VI) reduction performance of FBCs was mainly attributed to the formed PFRs, which could contribute to ∼74.0% of Cr(VI) reduction by direct or indirect electron transfer. The PFRs on FBCs surfaces could promote the Fe(III)/Fe(II) cycle through single electron transfer and synergistically accelerate ∼52.3% of Cr(VI) reduction. This study provides an improved understanding of the reduction mechanism of iron-modified biochar PFRs on Cr(VI) in environments.

Efficient removal and transformation of Cr(VI) from alkaline wastewater to form a ferrochromium spinel multiphase via a modified ferrite process

Chromium-containing wastewater causes serious environmental pollution due to the harmfulness of Cr(VI). The ferrite process is typically used to treat chromium-containing wastewater and recycle the valuable chromium metal. However, the current ferrite process is unable to fully transform Cr(VI) into chromium ferrite under mild reaction conditions. This paper proposes a novel ferrite process to treat chromium-containing wastewater and recover valuable chromium metal. The process combines FeSO4 reduction and hydrothermal treatment to remove Cr(VI) and form chromium ferrite composites. The Cr(VI) concentration in the wastewater was reduced from 1040 mg L-1 to 0.035 mg L-1, and the Cr(VI) leaching toxicity of the precipitate was 0.21 mg L-1 under optimal hydrothermal conditions. The precipitate consisted of micron-sized ferrochromium spinel multiphase with polyhedral structure. The mechanism of Cr(VI) removal involved three steps: 1) partial oxidation of FeSO4 to Fe(III) hydroxide and oxy-hydroxide; 2) reduction of Cr(VI) by FeSO4 to Cr(III) and Fe(III) precipitates; 3) transformation and growth of the precipitates into chromium ferrite composites. This process meets the release standards of industrial wastewater and hazardous waste and can improve the efficiency of the ferrite process for toxic heavy metal removal.

A comprehensive review on the preparation of biochar from digestate sources and its application in environmental pollution remediation

The preparation of biochar from digestate is one of the effective ways to achieve the safe disposal and resource utilization of digestate. Nevertheless, up to now, a comprehensive review encompassing the factors influencing anaerobic digestate-derived biochar production and its applications is scarce in the literature. Therefore, to fill this gap, the present work first outlined the research hotspots of digestate in the last decade using bibliometric statistical analysis with the help of VOSviewer. Then, the characteristics of the different sources of digestate were summarized. Furthermore, the influencing factors of biochar preparation from digestate and the modification methods of digestate-derived biochar and associated mechanisms were analyzed. Notably, a comprehensive synthesis of anaerobic digestate-derived biochar applications is provided, encompassing enhanced anaerobic digestion, heavy metal remediation, aerobic composting, antibiotic/antibiotic resistance gene removal, and phosphorus recovery from digestate liquor. The economic and environmental impacts of digestate-derived biochar were also analyzed. Finally, the development prospect and challenges of using biochar from digestate to combat environmental pollution are foreseen. The aim is to not only address digestate management challenges at the source but also offer a novel path for the resourceful utilization of digestate.

Biochar supported nanoscale zerovalent iron-calcium alginate composite for simultaneous removal of Mn(II) and Cr(VI) from wastewater: Sorption performance and mechanisms

Heavy metal pollution in water caused by industrial activities has become a global environmental issue. Among them, manganese mining and smelting activities have caused the combined pollution of Cr(VI) and Mn(II) in water, posing a serious ecotoxicological risk to ecological environments and human health. To efficiently remove Cr(VI) and Mn(II) from wastewater, a novel biochar supported nanoscale zerovalent iron-calcium alginate composite (CA/nZVI/RSBC) was synthesized by liquid-phase reduction and calcium alginate embedding methods. The adsorption performance and mechanisms of Cr(VI) and Mn(II) by CA/nZVI/RSBC were investigated. The maximum adsorption capacities of Cr(VI) and Mn(II) onto CA/nZVI/RSBC fitted by the Langmuir model were 5.38 and 39.78 mg/g, respectively, which were much higher than the pristine biochar. The iron release from CA/nZVI/RSBC was comparatively lower than that of nZVI/RSBC. Mn(II) presence enhanced the reduction of Cr(VI) by CA/nZVI/RSBC. The results of XRD, XPS, and site energy distribution analysis indicated that redox was the predominant mechanism of Cr(VI) adsorption, while electrostatic attraction dominated Mn(II) adsorption. This study provides a novel alternative way for the simultaneous removal of Cr(VI) and Mn(II) in wastewater.

Reductive sequestration of Cr (VI) by phosphorylated nanoscale zerovalent iron

The cracked phosphorylated nanoscale zerovalent iron (p-nZVI) has a strong electron selectivity towards the reductive removal of many heavy metal ions in waters. However, the unintended environmental risk after interactions with impurities or wastewater are not involved. Therefore, in this study, the phosphate group was successfully adsorbed into p-nZVI, and the cracked p-nZVI was successfully prepared with an optimal P/Fe ratio of 0.5%. The dosages of p-nZVI and temperatures were positively correlated with the removal rates. The removal process of Cr(VI) was more suitable by the Langmuir isothermal model（R2 > 0.99). The process of Cr (VI) (10, 20 and 40 mg/L) removal more fitted the pseudo first-order reaction model, while the process of Cr (VI) (60, 80 mg/L) removal more fitted the pseudo second-order reaction model. The Cr (VI) removal rates gradually decreased when the pH was increased. Dissolved oxygen slowed nanoiron reaction rates. The order of inhibition on the reactivity towards Cr(VI) was SiO32- > SO42- > PO43- > NO3- > HCO3-.The facilitation followed the order of Cd2+>Cu2+>Mg2+>Mn2+>Ca2+. Ca2+ showed an inhibitory effect, but all other cations showed different degrees of facilitation. The promotion effect is relatively similar in presence of Mn2+ or Mg2+. HA had a significant inhibitory effect. Environmental friendly p-nZVI had a good effect in simulated groundwater, seawater, river water and secondary effluent of the urban sewage treatment plant. The main pathway to remove Cr (VI) was in situ reduction by p-nZVI. The improved adsorption and reduction effect of p-nZVI on heavy metal ions in water was due to the structural change and the phosphate group.

Nano-biochar as a potential amendment for metal(loid) remediation: Implications for soil quality improvement and stress alleviation

Metal(loid) contamination of agricultural soils has become an alarming issue due to its detrimental impacts on soil health and global agricultural production. Therefore, environmentally sustainable and cost-effective solutions are urgently required for soil remediation. Biochar, particularly nano-biochar, exhibits superior and high-performance capabilities in the remediation of metal(loid)-contaminated soil, owing to its unique structure and large surface area. Current researches on nano-biochar mainly focus on safety design and property improvement, with limited information available regarding the impact of nano-biochar on soil ecosystems and crop defense mechanisms in metal(loid)-contaminated soils. In this review, we systematically summarized recent progress in the application of nano-biochar for remediation of metal(loid)-contaminated soil, with a focus on possible factors influencing metal(loid) uptake and translocation in soil-crop systems. Additionally, we conducted the potential/related mechanisms by which nano-biochar can mitigate the toxic impacts of metal(loid) on crop production and security. Furthermore, the application of nano-biochar in field trials and existing challenges were also outlined. Future studies should integrate agricultural sustainability and ecosystem health targets into biochar design/selection. This review highlighted the potential of nano-biochar as a promising soil amendment for enhancing the remediation of metal(loid)-contaminated agricultural soils, thereby promoting the synthesis and development of highly efficient nano-biochar towards achieving environmental sustainability.

Application and development of zero-valent iron (ZVI)-based materials for environmental remediation: A scientometric and visualization analysis

Zero-valent iron (ZVI)-based materials are among the most widely used engineered particles in the field of environmental remediation. To provide a comprehensive overview of the status and trend of the research on them, this study conducted a quantitative and visual analysis of 6296 relevant publications obtained from Web of Science between 1994 and 2022 using CiteSpace software. By using the bibliometric method, this work systematically analyzed the knowledge structure, research hotspots and trends of ZVI-based materials in this field. The results show that the research on ZVI-based materials in this field developed rapidly over the past 28 years. China is the greatest contributor with the most published articles and collaborations. Still, the USA has the most academic influence with the highest average citations per article. Chinese Academy of Sciences and Tongji University are the primary establishments that produced the greatest number of publications and had the highest h-index. Keyword cluster analysis indicates that the primary research topics are related to reductive dechlorination, sulfate radical, arsenic removal, graphene oxide, porous media, peroxymonosulfate, groundwater remediation, and permeable reactive barrier. Meanwhile, keyword burst analysis reveals that the primary research hotspots and frontiers of ZVI focus on its modification, the refractory and emerging contaminants treatment, persulfate activation, and electron transfer. However, no keywords or topics related to the environmental impact and toxicity of ZVI-based materials are available in the keyword clustering and burst analysis results, indicating this direction deserves more attention in future research. Through a comprehensive and in-depth bibliometric analysis, this paper provides new insight into the research hotspots and development trends of the research on ZVI-based materials in environmental remediation.

nFeS Embedded into Cryogels for High-Efficiency Removal of Cr(VI): From Mechanism to for Treatment of Industrial Wastewater

Most studies have focused on complex strategies for materials preparation instead of industrial wastewater treatment due to emergency treatment requirements for metal pollution. This study evaluated sodium polyacrylate (PSA) as a carbon skeleton and FeS as a functional material to synthesize PSA-nFeS material. The characteristics and interactions of PSA-nFeS composites treated with hexavalent chromium were analyzed by means of various techniques, such as scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectrometry (FTIR), and atomic absorption spectroscopy (AAS). Adsorption-coupled reduction was observed to be the predominant mechanism of Cr(VI) removal. The feasibility of PSA-nFeS composites in reducing toxicity and removing of Cr(VI) from real effluents was investigated through column studies and material properties evaluation. The continuous column studies were conducted using tannery effluents to optimize feed flow rates, initial feed Cr(VI) concentration, and column bed height. The results revealed that PSA-nFeS composites are ideal for filling materials in portable filtration devices due to their lightweight and compact size.

Preparation of chitosan-iron oxide modified sludge-based biochar for effective removal of tetracycline from water: performance and mechanism

The release of antibiotics has attracted wide attention due to their abuse and discharge. How to remove these emerging contaminants is an urgent need to be solved. In the present study, sludge-based biochar combining chitosan and iron oxide was prepared via municipal sewage sludge. The novel biochar modified with chitosan and iron oxide exhibited satisfying performance in eliminating antibiotics from water. The application of modified biochar combined with activated persulfate (PS) showed a remarkable removal efficiency of 96.98% for tetracycline (TC). Analysis of the surface characteristics of the modified biochar showed the presence of structural defects, dispersed iron oxides, abundant functional groups, a porous structure, and a relatively stable crystal structure. These characteristics attributed significant importance to facilitating the degradation of TC. A series of experimental conditions including preparation temperature (600-900 ℃), reaction temperature (15-45 ℃), contaminant concentration (30-180 mg/L), adsorbent usage (0.1-1 g/L), pH (2-10), and persulfate addition concentration (1-5 mmol) were conducted. The results revealed that the highest removal efficiency was achieved at 96.98% under the conditions of TC concentration at 30 mg/L, reaction temperature at 35 ℃, pH of 4, adsorbent addition amount of 0.6 g/L, and PS concentration of 2 mmol, respectively. Three degradation pathways and seven intermediate products of TC were proposed. Therefore, our study provides a promising approach for developing effective removal of antibiotic pollutants.

Removal of toxic metals using iron sulfide particles: A brief overview of modifications and mechanisms

Growing mechanization has released higher concentrations of toxic metals in water and sediment, which is a critical concern for the environment and human health. Recent studies show that naturally occurring and synthetic iron sulfide particles are efficient at removing these hazardous pollutants. This review seeks to provide a concise summary of the evolution in the production of iron sulfide particles, specifically nanoparticles, through the years. This review presents an outline of the synthesis process for the most dominant forms of iron sulfide: mackinawite (FeS), pyrite (FeS2), pyrrhotite (Fe1-x S), and greigite (Fe3S4). The review confirms that both natural forms of iron sulfide and modified forms of iron sulfide are highly effective at removing different heavy metals and metalloids from water. Concurrently, this review reveals the interaction mechanism between toxic metals and iron sulfide, along with the impact of conditions for remedy and rectification. None the less, modifications and future investigations into the synthesis of novel iron sulfides, their use to adsorb diverse environmental pollutants, and their fate after injection into polluted aquifers, remain crucial to maximizing pollution control.

Persulfate activation using leonardite char-supported nano zero-valent iron composites for styrene-contaminated soil and water remediation

Effective in-situ technology to treat carcinogenic compounds in contaminated areas poses a major challenge. Our objective was to load nano-zero-valent iron (nZVI) onto leonardite char (LNDC), an alternative carbon source from industrial waste, for use as a persulfate (PS) activator for styrene treatment in soil and water. By adding a surfactant during synthesis, cetyltrimethylammonium bromide (CTAB) promotes a flower-like morphology and the nZVI formation in smaller sizes. Results showed that nZVI plays a crucial role in PS activation in both homogeneous and heterogeneous reactions to generate reactive oxygen species (ROS), which can remove 98% of styrene within 20 min. Quenching experiments indicated that singlet oxygen (1O2), superoxide radicals (O2•-), and sulfate radicals (SO4•-) were the main species working together to degrade styrene. XPS analysis also revealed a role of surface oxygen-containing groups (i.e., CO, C-OH) in activating PS for SO4•- and 1O2 generation. The possible reaction mechanism of PS activation by LNDC-CTAB-nZVI composite and factors affecting treatment efficiency (i.e., PS concentration, catalyst dosage, pH, and humic acid) were illustrated. The molarity/molality ratio of PS to nZVI should be set greater than 1 for effective styrene removal. GC-MS analysis showed that styrene was degraded to a less toxic benzaldehyde intermediate. However, the excessive use of PS and catalysts can harm plant growth, requiring a combining approach to achieve safer use for real applications. Overall results supported the use of the LNDC-CTAB-nZVI/PS system as an efficient in-situ treatment technology for soil and water remediation.

Efficient degradation of tetrabromobisphenol A using peroxymonosulfate oxidation activated by a novel nano-CuFe2O4@coconut shell biochar catalyst

In this study, a novel bimetallic complexation-curing nucleation-anaerobic calcination method was developed to synthesize a nano-CuFe2O4@coconut shell biochar (CuFe2O4@CSBC) catalyst to activate peroxymonosulfate for degradation of tetrabromobisphenol A (TBBPA). The reaction processes of the TBBPA on CuFe2O4@CSBC have been investigated using in situ characterization and metal leaching. The effects of initial reaction conditions and degradation mechanism were investigated. Greater than 99% degradation of TBBPA at 10 mg L-1 was achieved in 30 min under the condition of pH 11, a total organic carbon removal rate of up to 70.67% was achieved and the degradation efficiency was 90% after 5 cycles of CuFe2O4@CSBC use. The degradation was in a second-order reaction at a constant of 0.797 M-1 min-1 (R2 = 0.993). The degradation was attributed to the main active species (SO4·-≈·OH < 1O2), and the surface active site of CuFe2O4@CSBC was the key role. The degradation process involved three main degradation pathways. Path A: ·OH attacked the C-Br bonds (TBBPA→TriBBPA→DBBPA→MBBPA→BPA); Path B: Hydroxylation and decarboxylation; Path C: Dehydrocoupling of TBBPA. What's more, the practical application of the system was very positive, achieved >77% degradation in sewage and industrial wastewater.

Regulation of tourmaline-mediated Fenton-like system by biochar: Free radical pathway to non-free radical pathway

The heterogeneous Fenton-like systems induced by Fe-containing minerals have been largely applied for the degradation of organic pollutants. However, few studies have been conducted on biochar (BC) as an additive to Fenton-like systems mediated by iron-containing minerals. In this study, the addition of BC prepared at different temperatures was found to significantly enhance the degradation of contaminants in the tourmaline-mediated Fenton-like system (TM/H2O2) using Rhodamine B (RhB) as the target contaminant. Furthermore, the hydrochloric acid-modified BC prepared at 700 °C (BC700(HCl)) could achieve complete degradation of high concentrations of RhB in the BC700(HCl)/TM/H2O2 system. Free radical quenching experiments showed that TM/H2O2 system removed contaminants mainly mediated by the free radical pathway. After adding BC, the removal of contaminants is mainly mediated by the non-free radical pathway in BC700(HCl)/TM/H2O2 system which was confirmed by the Electron paramagnetic resonance (EPR) experiments and electrochemical impedance spectroscopy (EIS). In addition, BC700(HCl) had broad feasibility in the degradation of other organic pollutants (Methylene Blue (MB) 100%, Methyl Orange (MO) 100%, and tetracycline (TC) 91.47%) in the tourmaline-mediated Fenton-like system. Possible pathways for the degradation of RhB by the BC700(HCl)/TM/H2O2 system were also proposed.

Facilitating New Chromium Reducing Microbes to Enhance Hexavalent Chromium Reduction by In Situ Sonoporation-Mediated Gene Transfer in Soils

Chromium (Cr) is a heavy metal with a high toxicity and pathogenicity. Microbial reduction is an effective strategy to remove Cr(VI) at contaminated sites but suffers from the low populations and activities of Cr-reducing microorganisms in soils. This study proposed an in situ sonoporation-mediated gene transfer approach, which improved soil Cr(VI) reduction performance by delivering exogenous Cr-transporter chrA genes and Cr-reducing yieF genes into soil microorganisms with the aid of ultrasound. Besides the increasing populations of Cr-resistant bacteria and elevated copy numbers of chrA and yieF genes after sonoporation-mediated gene transfer, three new Cr-reducing strains were isolated, among which Comamonas aquatica was confirmed to obtain Cr-resistant capability. In addition, sonoporation-mediated gene transfer was the main driving force significantly shaping soil microbial communities owing to the predominance of Cr-resistant microbes. This study pioneered and evidenced that in situ soil sonoporation-mediated gene transfer could effectively deliver functional genes into soil indigenous microbes to facilitate microbial functions for enhanced bioremediation, e.g., Cr-reduction in this study, showing its feasibility as a chemically green and sustainable remediation strategy for heavy metal contaminated sites.

Recent advancements and challenges in emerging applications of biochar-based catalysts

The sustainable utilization of biochar produced from biomass waste could substantially promote the development of carbon neutrality and a circular economy. Due to their cost-effectiveness, multiple functionalities, tailorable porous structure, and thermal stability, biochar-based catalysts play a vital role in sustainable biorefineries and environmental protection, contributing to a positive, planet-level impact. This review provides an overview of emerging synthesis routes for multifunctional biochar-based catalysts. It discusses recent advances in biorefinery and pollutant degradation in air, soil, and water, providing deeper and more comprehensive information of the catalysts, such as physicochemical properties and surface chemistry. The catalytic performance and deactivation mechanisms under different catalytic systems were critically reviewed, providing new insights into developing efficient and practical biochar-based catalysts for large-scale use in various applications. Machine learning (ML)-based predictions and inverse design have addressed the innovation of biochar-based catalysts with high-performance applications, as ML efficiently predicts the properties and performance of biochar, interprets the underlying mechanisms and complicated relationships, and guides biochar synthesis. Finally, environmental benefit and economic feasibility assessments are proposed for science-based guidelines for industries and policymakers. With concerted effort, upgrading biomass waste into high-performance catalysts for biorefinery and environmental protection could reduce environmental pollution, increase energy safety, and achieve sustainable biomass management, all of which are beneficial for attaining several of the United Nations Sustainable Development Goals (UN SDGs) and Environmental, Social and Governance (ESG).

Nano zero-valent iron enhances the absorption and transport of chromium in rice (Oryza sativa L.): Implication for Cr risks management in paddy fields

Chromium (Cr) accumulating in soil caused serious pollution to cultivated land. At present, nano zero-valent iron (nZVI) is considered to be a promising remediation material for Cr-contaminated soil. However, the nZVI impact on the behavior of Cr in the soil-rice system under high natural geological background value remains unknown. We studied the effects of nZVI on the migration and transformation of Cr in paddy soil-rice by pot experiment. Three different doses of nZVI (0, 0.001 % and 0.1 % (w/w)) treatments and one dose of 0.1 % (w/w) nZVI treatment without plant rice were set up. Under continuous flooding conditions, nZVI significantly increased rice biomass compared with the control. At the same time, nZVI significantly promoted the reduction of Fe in the soil, increased the concentration of oxalate Fe and bioavailable Cr, then facilitated the absorption of Cr in rice roots and the transportation to the aboveground part. In addition, the enrichment of Fe(III)-reducing bacteria and sulfate-reducing bacteria in soil provided electron donors for Cr oxidation, which helps to form bioavailable Cr that is easily absorbed by plants. The results of this study can provide scientific basis and technical support for the remediation of Cr -polluted paddy soil with high geological background.

Effective degradation of tetracycline via persulfate activation using silica-supported zero-valent iron: process optimization, mechanism, degradation pathways and water matrices

Pure zero-valent iron (ZVI) was supported on silica and starch to enhance the activation of persulfate (PS) for tetracycline degradation. The synthesized catalysts were characterized by microscopic and spectroscopic methods to assess their physical and chemical properties. High tetracycline removal (67.55%) occurred using silica modified ZVI (ZVI-Si)/PS system due to the improved hydrophilicity and colloidal stability of ZVI-Si. Incorporating light into the ZVI-Si/PS system improved the degradation performance by 9.45%. Efficient degradation efficiencies were recorded at pH 3-7. The optimum operating parameters determined by the response surface methodology were PS concentration of 0.22 mM, initial tetracycline concentration of 10 mg/L, and ZVI-Si dose of 0.46 g/L, respectively. The rate of tetracycline degradation declined with increasing tetracycline concentration. The degradation efficiencies of tetracycline were 77%, 76.4%, 75.7%, 74.5%, and 73.75% in five repetitive runs at pH 7, 20 mg/L tetracycline concentration, 0.5 g/L ZVI-Si dose, and 0.1 mM PS concentration. The degradation mechanism was explained, and sulfate radicals were the principal reactive oxygen species. The degradation pathway was proposed based on liquid chromatography-mass spectroscopy. Tetracycline degradation was favorable in distilled and tap water. The ubiquitous presence of inorganic ions and dissolved organic matter in the lake, drain, and seawater matrices interfered with the tetracycline degradation. The high reactivity, degradation performance, stability, and reusability of ZVI-Si substantiate the potential practical application of this material for the degradation of real industrial effluents.

Strategy and mechanisms of sulfamethoxazole removal from aqueous systems by single and combined Shewanella oneidensis MR-1 and nanoscale zero-valent iron-enriched biochar

Sulfamethoxazole (SMX, a sulfonamide antibiotic) is ubiquitously present in various aqueous systems, which can accelerate the spread of antibiotic resistance genes, induce genetic mutations, and even disrupt the ecological equilibrium. Considering the potential eco-environmental risk of SMX, this study explored an effective technology using Shewanella oneidensis MR-1 (MR-1) and nanoscale zero-valent iron-enriched biochar (nZVI-HBC) to remove SMX from aqueous systems with different pollution levels (1-30 mg·L^-1). SMX removal by nZVI-HBC and nZVI-HBC + MR-1 (55-100 %) under optimal conditions (iron/HBC ratio of 1:5, 4 g·L^-1 nZVI-HBC, and 10 % v/v MR-1) was more effective than its removal by MR-1 and biochar (HBC) (8-35 %). This was due to the catalytic degradation of SMX in the nZVI-HBC and nZVI-HBC + MR-1 reaction systems because of accelerated electron transfer during oxidation of nZVI and reduction of Fe(III) to Fe(II). When SMX concentration was lower than 10 mg·L^-1, nZVI-HBC + MR-1 effectively removed SMX (removal rate of approximately 100 %) when compared to nZVI-HBC (removal rate of 56-79 %). In addition to oxidation degradation of SMX by nZVI in the nZVI-HBC + MR-1 reaction system, MR-1-driven dissimilatory iron reduction accelerated electron transfer to SMX, thereby enhancing reductive degradation of SMX. However, a considerable decline in SMX removal from the nZVI-HBC + MR-1 system (42 %) was observed when SMX concentrations ranged 15-30 mg·L^-1, which was due to the toxicity of accumulated degradation products of SMX. A high interaction probability between SMX and nZVI-HBC promoted the catalytic degradation of SMX in the nZVI-HBC reaction system. The results of this study provide promising strategies and insights for enhancing antibiotic removal from aqueous systems with different pollution levels.

Fe-N-Doped Conjugated Organic Polymer Efficiently Enhanced the Removal Rate of Cr(VI) from Water

A Fe-N conjugated organic polymer (SMP-Fr-Py) was prepared from ferrocene and pyrrole using a Scholl coupling reaction, which significantly improved the performance of Cr(VI) removal compared to the polymer (HCP-Fr-Py) prepared by adding the cross-linker formaldehyde dimethyl acetal (FDA). The results showed that at a pH of 2 and at 25 °C, the removal of Cr(VI) reached 90% for SMP-Fr-Py and only 58% for HCP-Fr-Py after 20 min of reaction. Subsequently, 99% and 78% were achieved after 120 min of reaction, respectively. The test results showed that the removal reaction followed a pseudo-second-order kinetic model. The removal efficiency decreased with increasing solution pH and initial Cr(VI) concentration, but increased with increasing SMP-Fr-Py dosage, reaching three cycles. The characterization of the reaction complexes and measurements of Cr species conversion revealed the near absence of Cr(VI) species in the solution. Approximately 38% of Cr(VI) was found to be adsorbed on the material surface, with another fraction present in solution (24%) and on the material surface (38%) in the form of Cr(III). The overall study showed that the direct connection of ferrocene and pyrrole in SMP-Fr-Py through C-C bonding increased the conjugated structure of the polymer backbone, which facilitated electron transfer and transport. Furthermore, the Fe-N elements worked synergistically with each other more easily, which improved the removal performance of Cr(VI) and provided a reference for the subsequent work.

Heterogeneous photo-Fenton degradation of acid orange 7 activated by red mud biochar under visible light irradiation

The heterogeneous photo-Fenton process is an effective technology for degrading organic contaminants in wastewater, and Fe-based catalysts are recently preferred due to their low biotoxicity and geological abundance. Herein, we synthesized a Fe-containing red mud biochar (RMBC) via one-step co-pyrolysis of red mud and shaddock peel as a photo-Fenton catalyst to activate H₂O₂ and degrade an azo dye (acid orange 7, AO7). RMBC showed excellent AO7 removal capability with a decolorization efficiency of nearly 100% and a mineralization efficiency of 87% in the heterogeneous photo-Fenton process with visible light irradiation, which were kept stable in five successive reuses. RMBC provided Fe²⁺ for H₂O₂ activation, and the light irradiation facilitated the redox cycle of Fe²⁺/Fe³⁺ in the system to produce more reactive oxygen species (ROS, i.e., •OH) for AO7 degradation. Further investigation revealed that •OH was the predominant ROS responsible for AO7 degradation in the light-free condition, while more ROS were produced in the system with light irradiation, and ¹O₂ was the primary ROS in the photo-Fenton process for AO7 removal, followed by •OH and O₂^•-. This study provides insight into the interfacial mechanisms of RMBC as a photo-Fenton catalyst for treating non-degradable organic contaminants in water through advanced oxidation processes under visible light irradiation.

Enhanced organic pollutant removal in saline wastewater by a tripolyphosphate-Fe0/H2O2 system: Key role of tripolyphosphate and reactive oxygen species generation

The effects of tripolyphosphate (TPP) on organic pollutant degradation in saline wastewater using Fe⁰/H₂O₂ were systematically investigated to elucidate its mechanism and the main reactive oxygen species (ROS). Organic pollutant degradation was dependent on the Fe⁰ and H₂O₂ concentration, Fe⁰/TPP molar ratio, and pH value. The apparent rate constant (k_obs) of TPP-Fe⁰/H₂O₂ was 5.35 times higher than that of Fe⁰/H₂O₂ when orange II (OGII) and NaCl were used as the target pollutant and model salt, respectively. The electron paramagnetic resonance (EPR) and quenching test results showed that ^•OH, O₂^•-, and ¹O₂ participated in OGII removal, and the dominant ROS were influenced by the Fe⁰/TPP molar ratio. The presence of TPP accelerates Fe³⁺/Fe²⁺ recycling and forms Fe-TPP complexes, which ensures sufficient soluble Fe for H₂O₂ activation, prevents excessive Fe⁰ corrosion, and thereby inhibits Fe sludge formation. Additionally, TPP-Fe⁰/H₂O₂/NaCl maintained a performance similar to those of other saline systems and effectively removed various organic pollutants. The OGII degradation intermediates were identified using high-performance liquid chromatography-mass spectrometry (HPLC-MS) and density functional theory (DFT), and possible degradation pathways for OGII were proposed. These findings provide a facile and cost-effective Fe-based AOP method for removing organic pollutants from saline wastewater.

Research on the performance of modified blue coke in adsorbing hexavalent chromium

To solve the issue of hexavalent chromium (Cr(VI)) contamination in water bodies, blue coke powder (LC) was chemically changed using potassium hydroxide to create the modified material (GLC), which was then used to treat a Cr(VI)-containing wastewater solution. The differences between the modified and unmodified blue coke's adsorption characteristics for Cr(VI) were studied, and the impact of pH, starting solution concentration, and adsorption period on the GLC's adsorption performance was investigated. The adsorption behavior of the GLC was analyzed using isothermal adsorption models, kinetic models, and adsorption thermodynamic analysis. The mechanism of Cr(VI) adsorption by the GLC was investigated using characterization techniques such as Fourier Transform Infrared Spectroscopy (FTIR), Field Emission Scanning Electron Microscope (FE-SEM), X-Ray Diffraction (XRD), and X-Ray Photoelectron Spectroscopy (XPS). With the biggest difference in removal rate at pH = 2, which was 2.42 times that of LC, batch adsorption experiments revealed that, under the same adsorption conditions, the GLC always performed better than LC. With a specific surface area that was three times that of LC and an average pore diameter that was 0.67 times that of LC, GLC had a more porous structure than LC. The alteration significantly increased the number of hydroxyls on the surface of GLC by altering the structural makeup of LC. The ideal pH for removing Cr(VI) was 2, and the ideal GLC adsorbent dosage was 2.0 g/L. Pseudo-second-order kinetic (PSO) model and Redlich-Peterson (RP) model can effectively describe the adsorption behavior of GLC for Cr(VI). Physical and chemical adsorption work together to remove Cr(VI) by GLC in a spontaneous, exothermic, and entropy-increasing process, with oxidation-reduction processes playing a key role. GLC is a potent adsorbent that can be used to remove Cr(VI) from aqueous solutions.

Comparative efficacy of Parthenium hysterophorus (L.) derived biochar and iron doped zinc oxide nanoparticle on heavy metals (HMs) mobility and its uptake by Triticum aestivum (L.) in chromite mining contaminated soils

In this study we investigated the efficacy of a novel material parthenium weed (Parthenium hysterophorus L.) biochar (PBC), iron doped zinc oxide nanoparticles (nFe-ZnO), and biochar modified with nFe-ZnO (Fe-ZnO@BC) to adsorb heavy metals (HMs) and reduce their uptake by wheat (Triticum aestivum L.) in a highly chromite mining contaminated soil. The co-application of the applied soil conditioners exhibited a positive effect on the immobilization and restricted the HMs uptake below their threshold levels in shoot content of wheat. The maximum adsorption capacity was because of large surface area, cation exchange capacity, surface precipitation, and complexation of the soil conditioners. The scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) showed porous smooth structure of parthenium weed derived biochar that helped in HMs adsorption, increase the efficiency of soil fertilizers and nutrients retention which help in the enhancement soil condition. Under different application rates the highest translocation factor (TFHMs) was obtained at 2 g nFe-ZnO rate followed the descending order: Mn > Cr > Cu > Ni > Pb. The overall TFHMs was found <1.0 indicating that low content of HMs accumulation in roots from soil slight transferred to shoot, thus satisfying the remediation requirements.

Bioremediation system consisted with Leclercia adecarboxylata and nZVI@Carbon/Phosphate for lead immobilization: The passivation mechanisms of chemical reaction and biological metabolism in soil

Bioremediation is one of the most promising strategies for heavy metal immobilization. A new remediation system was demonstrated in this research, which combined phosphate solubilizing bacteria (PSB) with nZVI@Carbon/Phosphate (nZVI@C/P) composite to remediate lead contaminated soil. Experimental results indicated that the new system (nZVI@C/P + PSB) could effectively convert the labile Pb into the stable fraction after 30 days of incubation, which increased the maximum residual fraction percentage of Pb by 70.58%. The characterization results showed that lead may exist in the forms of Pb₅(PO₄)₃Cl, PbSO₄ and 3PbCO₃·2Pb(OH)₂·H₂O in the soil treated with nZVI@C/P + PSB. Meanwhile, soil enzyme activities and Leclercia abundance were enhanced in the treated soil compared with CK during the incubation time. In addition, the specialized functions (e.g. ABC transporters, siderophore metabolism, sulfur metabolism and phosphorus metabolism) in PSB and nZVI@C/P + PSB group were also enhanced. These phenomena proved that the key soil metabolic functions may be maintained and enhanced through the synergistic effect of incubated PSB and nZVI@C/P. The study demonstrated that this new bioremediation system provided feasible way to improve the efficacy for lead contaminated soil remediation.

Polycaprolactone-Modified Biochar Supported Nanoscale Zero-Valent Iron Coupling with Shewanella putrefaciens CN32 for 1,1,1-Trichloroethane Removal from Simulated Groundwater: Synthesis, Optimization, and Mechanism

1,1,1-Trichloroethane (1,1,1-TCA) is a typical organochloride solvent in groundwater that poses threats to human health and the environment due to its carcinogenesis and bioaccumulation. In this study, a novel composite with nanoscale zero-valent iron (nZVI) supported by polycaprolac-tone (PCL)-modified biochar (nZVI@PBC) was synthesized via solution intercalation and liquid-phase reduction to address the 1,1,1-TCA pollution problem in groundwater. The synergy effect and improvement mechanism of 1,1,1-TCA removal from simulated groundwater in the presence of nZVI@PBC coupling with Shewanella putrefaciens CN32 were investigated. The results were as follows: (1) The composite surface was rough and porous, and PCL and nZVI were loaded uniformly onto the biochar surface as micro-particles and nanoparticles, respectively; (2) the optimal mass ratio of PCL, biochar, and nZVI was 1:7:2, and the optimal composite dosage was 1.0% (w/v); (3) under the optimal conditions, nZVI@PBC + CN32 exhibited excellent removal performance for 1,1,1-TCA, with a removal rate of 82.98% within 360 h, while the maximum removal rate was only 41.44% in the nZVI + CN32 treatment; (4) the abundance of CN32 and the concentration of adsorbed Fe(II) in the nZVI@PBC + CN32 treatment were significantly higher than that in control treatments, while the total organic carbon (TOC) concentration first increased and then decreased during the culture process; (5) the major improvement mechanisms include the nZVI-mediated chemical reductive dechlorination and the CN32-mediated microbial dissimilatory iron reduction. In conclusion, the nZVI@PBC composite coupling with CN32 can be a potential technique to apply for 1,1,1-TCA removal in groundwater.

Highly efficient removal of hexavalent chromium by magnetic Fe-C composite from reed straw and electric furnace dust waste

Reed straw and electric furnace dust (EFD) waste were used to prepare magnetic Fe-C composite (EFD&C) by co-precipitation and high-temperature activation method to remove Cr(VI) from water. The magnetic EFD&C owned a large specific surface (536.61 m²/g) and a porous structure (micropores and mesopores), and had an efficient removal capacity for Cr(VI). Under conditions of pH (2), the addition amount of EFD&C (1 g/L), the adsorption time (760 min), and the temperature (45 °C), the maximum adsorption capacity reached 111.94 mg/g. The adsorption mechanism mainly attributed to chemical adsorption (redox), Cr(VI) reduced to Cr(III) by Fe(II) and Fe(0) (from Fe₃O₄ and Fe components in EFD) and surface functional groups of -OH, C = C, C-C and O-C = O (from biochar), and secondary attributed to physical adsorption, Cr(VI) and Cr(III) (from reduced Cr(VI)) adsorbed into the porous structure of EFD&C. This study provided a feasible solution for the preparation of adsorbents for adsorbing heavy metals from iron-containing metallurgical solid waste and biomass waste, which contributed to reducing the environmental pollution and lowering the cost of adsorbent preparation.

Simultaneous stabilization of Pb, Cd, and As in soil by rhamnolipid coated sulfidated nano zero-valent iron: Effects and mechanisms

Sulfidation effectively improves the electron transfer efficiency of nanoscale zero-valent iron (nZVI), but decreases the specific surface area of nZVI. In this study, sulfidated nZVI (S-nZVI) coated with rhamnolipid (RL-S-nZVI) was synthesized and used to stabilize Pb, Cd, and As in combined polluted soil. The stabilization efficiency of 0.3% (wt) RL-S-nZVI to water soluble Pb, Cd, and As in soil reached 88.76%, 72%, and 63%, respectively. Rhamnolipid coating inhibited the reduction of specific surface area and successfully encapsulated nZVI, thus reducing the oxidation of Fe⁰. The types of iron oxides in RL-S-nZVI were reduced compared to S-nZVI, but the content and strength of Fe⁰ iron were obviously enhanced. Furthermore, rhamnolipid functional groups (-COOH and -COO^-) were also involved in the stabilization process. In addition, the stabilization efficiency of RL-S-nZVI to the bioavailable Pb, Cd, and As in soil increased by 41%, 41%, and 50%, respectively, compared with nZVI. The presence of organic acids, especially citric acid, improved the stabilization efficiency of RL-S-nZVI to the three metals. The result of BCR sequential extraction indicated that RL-S-nZVI increased the residual state of Pb, Cd, and As and reduced the acid-soluble and reducible state after 28 days of soil incubation. XRD and XPS analyses showed that the stabilization mechanisms of RL-S-nZVI on heavy metals involved in ion exchange, surface complexation, adsorption, co-precipitation, chemisorption, and redox.

Synergistic Cr(VI) reduction and adsorption of Cu(II), Co(II) and Ni(II) by zerovalent iron-loaded hydroxyapatite

Multi-metal contaminated soil, such as Cr(VI), Cu(II), and Co(II), still challenge the environmental remediation. In this work, zerovalent iron-loaded hydroxyapatite (ZVI/HAP) was first applied to simultaneously adsorb multi-metal in contaminated soil. During the remediation, the co-existing Cu(II), Ni(II), and Co(II) were adsorbed and precipitated onto ZVI/HAP. This "spontaneous deposition" simultaneously achieved the adsorption of the cationic metals and improved the isoelectric point of ZVI/HAP to 4.83 from 1.59, thus significantly alleviating the electronegativity to enhance the capture and reduction efficiency of Cr(VI). The application of ZVI/HAP resulted in the reduction of more than 99% of total Cr(VI) in contaminated soil, and the almost complete adsorption of water-soluble and DTPA-extractable Cu, Ni and Co within 20 d. Based on the sequential extraction and risk reduction assessment, soil Cr, Cu, Ni, and Co speciation was transformed from an unstable state (exchangeable and carbonate-bound fractions) to a relatively stable state, reducing the risk of heavy metals in contaminated soil significantly. This study developed an efficient strategy for the remediation of multi-metal contaminated soil.

The effect of Fe-Zn mole ratio (2:1) bimetallic nanoparticles supported by hydroxyethyl cellulose/graphene oxide for high-efficiency removal of doxycycline

In this research, Hydroxyethyl cellulose - graphene oxide HEC-GO and HEC-GO/Fe-Zn mole ratio (2:1) nanocomposite as adsorbents were fabricated by crosslinking ethylene glycol dimethacrylate (EGDMA) to study the thermodynamic, kinetic and isotherm of doxycycline antibiotic adsorption. The morphology and structure of the adsorbents were analyzed by Fourier transform infrared spectroscopy (FT-IR), Field Emission Scanning Electron Microscopy with Energy Dispersive X-Ray Spectroscopy (FE-SEM- EDX), and Transmission electron microscopy (TEM). The adsorption behavior of doxycycline (DOX) was studied with different parameters including doxycycline concentration, pH, the dose of adsorbent (HEC-GO and HEC-GO/Fe-Zn, mole ratio (2:1)), contact time, and temperature. The optimal conditions for the removal of DOX are pH = 3.0, contact time 100 min, and 20 min for HEC-GO and HEC-GO/Fe-Zn mole ratio (2:1). The removal percentage for HEC-GO and HEC-GO/Fe-Zn mole ratio (2:1) was 97% and 95.5%, respectively. Equilibrium adsorption isotherms such as the Langmuir, Freundlich, and Temkin models were analyzed according to the experimental data. Also, four adsorption kinetics were investigated for removing DOX. The Langmuir isotherm and pseudo-second-order kinetic models provided the best fit for experimental data for HEC-GO and HEC-GO/Fe-Zn mole ratio (2:1). Thermodynamic data showed that negative values of Gibbs free energy (ΔG°) and the negative value of enthalpy (ΔH°) of the adsorption process for adsorbents. It means that DOX removal was a spontaneous and exothermic reaction.

Enhanced elimination of Cr(VI) from aqueous media by polyethyleneimine modified corn straw biochar supported sulfide nanoscale zero valent iron: Performance and mechanism

A novel polyethyleneimine modified corn straw biochar supported sulfide nanoscale zero-valent iron (S-nZVI@PBC) was developed to enhance Cr(VI) removal from aqueous media. The characteristics of morphology, chemical composition, and functional groups of S-nZVI@PBC, as well as its kinetics and mechanism for Cr(VI) removal were explored. Characterization verified S-nZVI was successfully loaded onto PEI modified biochar. The adsorption process was well represented pseudo-second-order model (R² = 0.990) and Langmuir isotherm model (R² = 0.962), indicating it was a monolayer chemical adsorption process. The Cr(VI) removal was affected by pH and achieved the maximum when pH = 3.0, which may be ascribed to the better corrosion of nZVI and release of Fe(II) from the S-nZVI@PBC in acidic condition. The primary mechanisms were adsorption, reduction, and co-precipitation. S-nZVI@PBC exhibited higher stability and reusability than nZVI, which makes it more promising in environmental application. Overall, S-nZVI@PBC is of great potential for treating Cr(VI)-containing wastewater.

Removal of Hexavalent Chromium in Aqueous Solution by Cellulose Filter Paper Loaded with Nano-Zero-Valent Iron: Performance Investigation and Numerical Modeling

Cr(VI) pollution in water bodies is very harmful to human health and the environment. Therefore, it is necessary to remove Cr(VI) from water. In this study, the composite (FP-nZVI) was prepared by loading nano-zero-valent iron (nZVI) onto cellulose filter paper (FP) using a liquid-phase reduction method to improve the dispersibility and oxidation resistance of nZVI. In batch experiments, the effects of iron loading of FP-nZVI, initial concentration of Cr(VI), temperature, and pH on Cr(VI) removal were particularly investigated. The maximum removal rate of 98.6% was achieved at 25 °C, pH = 5, initial concentration of Cr(VI) of 20 mg/L, and FeCl3·6H2O solution concentration of 0.8 mol/L. The removal of Cr(VI) by FP-nZVI conformed to a pseudo-second-order kinetic model and Langmuir isotherm model. The mechanism of Cr(VI) removal was a multi-step removal mechanism, involving adsorption, reduction, and coprecipitation. Column experiments investigated the effect of flow rate (1 mL/min, 3 mL/min, and 5 mL/min) on Cr(VI) removal. We found that increasing flow rate slightly decreased the removal rate of Cr(VI). The transport of Cr(VI) in composite porous media was simulated using HYDRUS-1D, and the results show that the two-site model can well simulate the reactive transport of Cr(VI). This study may provide a useful reference for the remediation of groundwater contaminated with Cr(VI) or other similar heavy metals using FP-nZVI.

Sophorolipid modification enables high reactivity and electron selectivity of nanoscale zerovalent iron toward hexavalent chromium

Nanoscale zero-valent iron is considered to be a promising nanostructure for environmental remediation, while increasing the electron selectivity of nanoscale zerovalent iron (nZVI) during target contaminant removal is still a challenge (electron selectivity, defined as the percentage of electrons transferred to the target contaminants over the number of electrons donated by nZVI). In this study, the strategy for increasing the reactivity and electron selectivity of nZVI via sophorolipid (SL-nZVI) modification was proposed. The results showed that the removal efficiency and electron selectivity of SL-nZVI toward Cr(VI) was 99.99% and 56.30%, which was higher than that of nZVI (61.16%, 25.91%). Meanwhile, the particles were well characterized and the mechanism for enhanced reactivity and electron selectivity was investigated. Specially, both the morphology and BET specific surface area characterization suggested that stability against aggregation was higher in SL-nZVI nanoparticles than in nZVI. Besides, X-ray photoelectron spectroscopy (XPS), Tafel polarization curves, and Electrochemical impedance spectroscopy also indicated that the introduction of sophorolipid successfully prevent the nanoparticles from oxidation and benefit the electron transferring. In addition, a water contact angle test revealed that SL-nZVI nanoparticles were less hydrophilic (contact angle = 34.8°) than nZVI (contact angle = 23.9°). Therefore, in terms of reactivity, sophorolipid modification inhibited the aggregation of the nanoparticles and enhanced the electrical conductivity. For electron selectivity, the introduction of sophorolipid not only benefited Cr(VI) adsorption and the electron transfer from Fe⁰ to the surface-adsorbed Cr(VI) that followed but also reduced the possibility of side reactions between Fe⁰ and H₂O. This study demonstrates that the introduction of sophorolipid is an effective strategy for developing a highly efficient nZVI-based nanocomposite system and highlights the potential role of sophorolipid in improving the electron selectivity of nZVI.

Investigate the multipath erasure of nitrobenzene over nanoscale zero-valent-iron/N-doped biochar hybrid with extraordinary reduction performance

In this study, the facile carbothermal reduction method was enforced using urea as dopant to modify the structure and chemical composition of nanoscale zero-valent-iron/biochar hybrid thereby boosting its reduction performance. Through fine-tuning the N-doped amount, the optimal nZVI/N-doped BC was obtained, which exhibited more active sites (nZVI, persistent free radicals (PFRs), pyrrolic-N) and superior electrochemical conductivity. With these blessings, the electrons originating from galvanic cell reaction could zip along the highway within the hybrid. Taking nitrobenzene (NB) as the target pollutant, the quantitative analysis revealed that the NB reduction and adsorption removal efficiency were dramatically improved by 2.42 and 2.78 times, respectively. What's more, combining the in-situ experimental detection and theoretical calculations, unexpected NB reductive multipath with respect to PFRs and pyrrolic-N accelerating the Fe³⁺/Fe²⁺ cycle within the nZVI/N-doped BC system was decoded. The enhancement of Fe³⁺/Fe²⁺ cycle improved the electron utilization efficiency and maintained the reduction reactivity of the hybrid. This work raised awareness of the mechanisms regarding the reduction performance of nZVI/N-doped BC elevated by N-doped and the pollutant reductive pathway within the system, uncovered the dusty roles of PFRs and N-species during the reduction process.

Adsorption-reduction coupling mechanism and reductive species during efficient florfenicol removal by modified biochar supported sulfidized nanoscale zerovalent iron

Sulfidized nanoscale zerovalent iron (S-nZVI) was a promising material for degrading halogenated contaminants, but the easy aggregation limits its application for in-situ groundwater remediation. Hence, S-nZVI was decorated onto modified biochar (mBC) to obtain better dispersity and reactivity with florfenicol (FF), a widely used antibiotic. Uniform dispersion of S-nZVI particles were achieved on the mBC with plentiful oxygen-containing functional groups and negative surface charge. Thus, the removal rate of FF by S-nZVI@mBC was 2.5 and 3.1 times higher than that by S-nZVI and S-nZVI@BC, respectively. Adsorption and dechlorination of FF showed synergistic effect under appropriate mBC addition (e.g., C/Fe mass ratio = 1:3, 1:1), probably due to the enrichment of FF facilitates its reduction. In contrast, the contact between FF and S-nZVI could be hindered under more mBC addition, significantly decrease the reduction rate of FF and the reduction capacity of per unit Fe⁰. In addition, sulfur dose altered the surface species of surface Fe and S, and removal rates of FF correlated well with surface reductive species, i.e., FeS (r = 0.90, p < 0.05) and Fe⁰ (r = 0.98, p < 0.01). These mechanistic insights indicate the importance of rational design for biochar supported S-nZVI, which can lead to more efficient FF degradation.