Preparation of highly-conductive pyrogenic carbon-supported zero-valent iron for enhanced Cr(Ⅵ) reduction

Valorization of agricultural waste and red mud through CO2-mediated pyrolysis system

This study presents a strategic method to improve carbon utilization in agricultural waste, specifically spent mushroom substrate (SMS), while valorizing metal-rich waste, red mud, through pyrolysis platform. To offer sustainability, CO2 was utilized as reaction medium. The utilization of CO2 in SMS pyrolysis promotes syngas production, particularly CO, through homogeneous reactions (HRs) with volatile compounds (VCs). Indeed, the amount of syngas in N2 and CO2 were 2.42 and 4.04 mmol gsms-1, respectively. The inclusion of red mud in CO2-mediated pyrolysis of SMS accelerated the reaction kinetics of HRs lined to CO2, resulting in enhanced syngas production. The iron-functionalized biochars (Fe-FBs) produced at 700 °C in N2 and CO2 were evaluated for Cr(VI) removal. Faster kinetics for Cr(VI) removal was observed in Fe-FB in N2 compared with that in CO2. This is likely due to the higher capability of elemental iron (Fe0) in reduction of Cr(VI). However, Fe-FBs in CO2 exhibited superior adsorption capacities relative to that in N2, indicating that Fe3O4 is a primary contributor to Cr(VI) removal through adsorption. Notably, removal of Fe-FBs in N2 and CO2 via adsorption reached up to 17.2 and 82.8 %, respectively. Considering toxic nature of Cr species (Cr(VI) and Cr(III)), their immobilization through adsorption on Fe-FBs in CO2 may offer a more environmentally benign strategy. These findings suggest that the pyrolysis of SMS with red mud using CO2 as a reactant provides two benefits: enhanced syngas production and the fabrication of Fe-FBs that can serve as environmentally benign materials for Cr(VI) removal.

Comigration Behavior of Cr(VI) and Microplastics and Remediation of Microplastics-Facilitated Cr(VI) Transportation in Saturated Porous Media

The study of the co-transport of Cr(VI) and microplastics (MPs) in porous media is important for predicting migration behavior and for achieving pollution removal in natural soils and groundwater. In this work, the effect of MPs on Cr(VI) migration in saturated porous media was investigated at different ionic strengths (ISs) and pHs. The results showed that pH 7 and low IS (5 mM), respectively, promoted the movement of Cr(VI), which was further promoted by the presence of MPs. The Derjaguin-Landau-Verwey-Overbeek (DLVO) results showed that the repulsive energy barrier between MPs and quartz sand decreased with increasing IS and decreasing pH, respectively, which promoted the retention of MPs in quartz sand and constrained the competition of Cr(VI) for adsorption sites on the surface of the quartz sand, thus facilitating the enhanced migration of Cr(VI), while Cr(VI) behaved conversely. Sodium alginate/nano zero-valent iron-reduced graphene oxide (SA/NZVI-rGO) gel beads could achieve the removal of MPs through a π-π interaction, hydrogen bonding, and electrostatic attraction, but the MPs removal would be reduced by 40% due to the competitive adsorption of Cr(VI). Notably, 97% Cr(VI) removal could still be achieved by the gel beads in the presence of MPs. Therefore, the gel beads can be used as a permeation reaction barrier to inhibit the MP-induced high migration of Cr(VI). The Cr(VI) breakthrough curves in reactive migration were well-fitted with the two-site chemical nonequilibrium model. Overall, the findings of this work contribute to the understanding of the migration behavior of Cr(VI) and MPs in saturated porous media and provide a theoretical basis for the remediation of soils and groundwater contaminated with Cr(VI) and MPs.

Enhanced Cr(VI) removal and stabilization from bioleached wastewater by zero-valent iron coupled with hetero and autotrophic bacteria

Zero-valent iron (Fe0) usually suffers from organic acid complexation and ferrochrome layer passivation in Cr(VI) removal from bioleached wastewater of Cr slag. In this work, a synergetic system combined Fe0 and mixed hetero/autotrophic bacteria was established to reduce and stabilize Cr(VI) from bioleached wastewater. Due to bacterial consumption of organic acid and hydrogen, severe iron corrosion and structured-Fe(II) mineral generation (e.g., magnetite and green rust) occurred on biotic Fe0 surface in terms of solid-phase characterization, which was crucial for Cr(VI) adsorption and reduction. Therefore, compared with the abiotic Fe0 system, this integrated system exhibited a 6.1-fold increase in Cr(VI) removal, with heterotrophic reduction contributing 3.4-fold and abiotic part promoted by hydrogen-autotrophic bacteria enhancing 2.7-fold. After reaction, the Cr valence distribution and X-ray photoelectron spectroscopy indicated that most Cr(VI) was converted into immobilized products such as FexCr1-x(OH)3, Cr2O3, and FeCr2O4 by biotic Fe0. Reoxidation experiment revealed that these products exhibited superior stability to the immobilized products generated by Fe0 or bacteria. Additionally, organic acid concentration and Fe0 dosage showed significantly positive correlation with Cr(VI) removal within the range of biological adaptation, which emphasized that heterotrophic and autotrophic bacteria acted essential roles in Cr(VI) removal. This work highlighted the enhanced effect of heterotrophic and autotrophic activities on Cr(VI) reduction and stabilization by Fe0 and offered a promising approach for bioleached wastewater treatment.

Tea saponin co-ball milled commercial micro zero-valent iron for boosting Cr(VI) removal

Tea saponins (TS), a natural biosurfactant extracted from tea trees, were co-ball milled with commercial micro zero-valent iron (mZVI) to produce TS modified mZVI (TS-BZVI) for efficient hexavalent chromium (Cr(VI)) removal. The findings demonstrated that TS-BZVI could nearly remove 100% of Cr(VI) within 2 h, which was 1.43 times higher than that by ball milled mZVI (BZVI) (70%). Kinetics analysis demonstrated a high degree of compatibility with the pseudo-second-order (PSO), revealing that TS-BZVI exhibited a 2.83 times faster Cr(VI) removal rate involved primarily chemisorption. Further, X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES) measurements indicated that the TS co-ball milling process improved the exposure of Fe(II) and Fe(0) on mZVI, which further promoted the Cr(VI) reduction process. Impressively, the introduction of TS increased the hydrophobicity of ZVI, effectively inhibiting the H2 evolution by 95%, thus improved electron selectivity for efficient Cr(VI) removal. Ultimately, after operating for 10 days, a simulated permeable reactive barrier (PRB) column experiment revealed that TS-BZVI had a higher Cr(VI) elimination efficiency than BZVI, indicating that TS-BZVI was promising for practical environment remediation.

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.

Removal of Pb pollution using alginate-coupled magnetic sludge biochar: Solidification and stabilization behavior and electron promotion mechanisms

Environmental and human health problems caused by Pb pollution have attracted much attention, and solidification and stabilization are effective means for its remediation. Improving the ability of biochar to remediate heavy metals through modification is the focus of current biochar research. This study used calcium-alginate gel (GB) and Fe3+ (magnetic) to encapsulate and improve sludge biochar (SB), and explored the adsorption behavior and passivation mechanism of Pb2+ on it from outside to inside. The magnetic-biochar (MB) in magnetic-biochar-gel microspheres (MBGB) showed a homogeneous dispersion and part of the Fe ion was detached from the MB into the three-dimensional pores of the gel. The results of kinetic, isothermal and pH adsorption experiments showed that the MBGB has 108.4 % and 200 % higher Pb2+ adsorption capacity and rate than SB and can be applied to pH 3-9. The adsorption of Pb2+ by MBGB is a multilayer adsorption with both physical and chemical mechanisms. Mineralogical and electrochemical results demonstrate that the cross-linking of the gel with magnetic-biochar (MB) can provide a directional diffusion channel for Pb2+ from the outside to the inside. The electron transfer rate of MBGB was significantly higher than that of SB (222.2 %) after the reaction. The dissolved cations and electrons on the MB guide Pb2+ from the MBGB surface to the internal MB quickly via accelerating the electron transfer and migration rate between Pb2+ and MB. Subsequently, the abundance of PO43- on the MB ensures stable mineral precipitation (Pyromorphite). Moreover, four-step extraction analysis confirmed that most of Pb2+ in MBGB was stable (36.2 % acid-soluble and 47.6 % non-bioavailable). Meanwhile, the Pb adsorption efficiency of MBGB was still >93.0 % after three cycles of adsorption-desorption. Excellent reuse performance and stability guarantee the environmental security of MBGB. The results of the study provide theoretical support for the efficient treatment of Pb2+ polluted water assisted by gel materials.

Green synthesis of nZVI-modified biochar significantly enhanced the removal of Cr(VI) from aqueous solution

Water contamination by hexavalent chromium (Cr(VI)) seriously jeopardizes human health, which is a pressing environmental concern. Biochar-loaded green-synthesized nZVI, as a green and environmentally friendly material, can efficiently reduce Cr(VI) to Cr(III) while removing Cr(VI) from water. Therefore, in this study, an efficient green-modified biochar material (TP-nZVI/BC) was successfully prepared using tea polyphenol (TP) and sludge biochar (BC) using a low-cost and environmentally friendly green synthesis method. The preparation conditions of TP-nZVI/BC were optimized using response surface methodology (RSM), revealing that the dosage of tea polyphenols plays a crucial role in the removal performance (R2 = 1271.09), followed by reaction time and temperature. The quadratic regression model proved accurate. The optimal preparation conditions are as follows: tea polyphenols (TP) dosage at 48 g/L, reaction temperature at 75 ℃, and a reaction time of 3 h. TP-nZVI/BC removed Cr(VI) from water at a rate 7.6 times greater than BC. The pseudo-second-order kinetic model (R2 = 0.987) accurately describes the adsorption process, suggesting that chemical adsorption predominantly controls the removal process. The adsorption of Cr(VI) by TP-nZVI/BC can be well described by the Langmuir model, and the maximum adsorption capacity reached 105.65 mg/g. FTIR and XPS analyses before and after adsorption demonstrate that nZVI plays a crucial role in the reduction process of Cr(VI), and the synergistic effects of surface adsorption, reduction, and co-precipitation enhance Cr(VI) removal. In summary, using green-modified biochar for Cr(VI) removal is a feasible and promising method with significant potential.

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.

Efficient Cr(VI) removal by pyrite/porous biochar: Critical role of potassium salt and sulphur

The excessive accumulation of hexavalent chromium (Cr(VI)) in the environment poses a risk to environment and human health. In the present study, a potassium bicarbonate-modified pyrite/porous biochar composite (PKBC) was prepared in a one-step process and applied for the efficient removal of Cr(VI) in wastewater. The results showed that PKBC can significantly remove Cr(VI) within 4 h over a wide range of pH (2-11). Meanwhile, the PKBC demonstrated remarkable resistance towards interference from complex ions. The addition of potassium bicarbonate increased the pore structure of the material and promoted the release of Fe2+. The reduction of Cr(VI) in aqueous solution was primarily attributed to the Fe(II)/Fe(III) redox cycle. The sulphur species achieved Fe(II)/Fe(III) cycle through electron transfer with iron, thus ensuring the continuous reduction capacity of PKBC. Besides, the removal rate was also maintained at more than 85% in the actual water samples treatment process. This work provides a new way to remove hexavalent chromium from wastewater and demonstrates the potential critical role of potassium bicarbonate and sulphur.

Enhanced remediation of Cr(VI)-contaminated soil by modified zero-valent iron with oxalic acid on biochar

Hexavalent chromium (Cr(VI)) is carcinogenic and widely presented in soil. In this study, modified zero-valent iron (ZVI) with oxalic acid on biochar (OA-ZVI/BC) was prepared using wet ball milling method for the remediation of Cr(VI)-contaminated soil. Microscopic characterizations showed that ZVI were distributed on the biochar uniformly and confirmed the enhanced interface interaction between biochar and ZVI by wet ball milling. Electrochemical analysis indicated the strong electron transfer ability and enhanced corrosion behavior of OA-ZVI/BC. Moreover, inhibitory efficiencies of Cr(VI) removal with the addition of 1,10-phenanthroline suggested abundant Fe2+ generation in OA-ZVI/BC, which might facilitate the reduction of Cr(VI) to Cr(III). Theory calculation further demonstrated the ZVI modified by oxalic acid was more susceptible to solid-solid interfacial reactions with Cr(VI), and more electrons were transferred to Cr(VI). When applied to Cr(VI)-contaminated soil, OA-ZVI/BC could passivate 96.7 % total Cr(VI) and maintained for 90 days. The toxicity characteristic leaching procedure (TCLP) and simple based extraction test (SBET) were used to evaluate the leaching toxicity and bioaccessibility of Cr(VI), respectively. The TCLP-Cr(VI) decreased to 0.11 mg·L-1 after OA-ZVI/BC treatment, much lower than that of soils with ZVI/BC and OA-ZVI remediation (1.5 mg·L-1 and 4.1 mg·L-1). The bioaccessibility of Cr(VI) reduced by 93.5 % after 3-month remediation. Sequential extraction showed that Cr fractions in the soil after OA-ZVI/BC remediation was converted from acetic acid-extractable (HOAc-extractable) to more stable forms (e.g., residual and oxidizable forms). Benefiting from the synergies of oxalic acid, biochar and wet ball milling, OA-ZVI/BC exhibited an excellent performance on the remediation of Cr(VI)-contaminated soil, whose mechanisms involved adsorption, reduction (Fe0/Fe2+, Fe2+/Fe3+) and co-precipitation. This study herein develops a promising ZVI technology in the remediation of heavy metal-contaminated soil.

Highly Stable, Mechanically Enhanced, and Easy-to-Collect Sodium Alginate/NZVI-rGO Gel Beads for Efficient Removal of Cr(VI)

Nanoscale zero-valent iron (NZVI) is a material that is extensively applied for water pollution treatment, but its poor dispersibility, easy oxidation, and inconvenient collection limit its application. To overcome these drawbacks and limit secondary contamination of nanomaterials, we confine NZVI supported by reduced graphene oxide (rGO) in the scaffold of sodium alginate (SA) gel beads (SA/NZVI-rGO). Scanning electron microscopy showed that the NZVI was uniformly dispersed in the gel beads. Fourier transform infrared spectroscopy demonstrated that the hydrogen bonding and conjugation between SA and rGO allowed the NZVI-rGO to be successfully embedded in SA. Furthermore, the mechanical strength, swelling resistance, and Cr(VI) removal capacity of SA/NZVI-rGO were enhanced by optimizing the ratio of NZVI and rGO. Interestingly, cation exchange may drive Cr(VI) removal above 82% over a wide pH range. In the complex environment of actual Cr(VI) wastewater, Cr(VI) removal efficiency still reached 70.25%. Pseudo-first-order kinetics and Langmuir adsorption isotherm are preferred to explain the removal process. The mechanism of Cr(VI) removal by SA/NZVI-rGO is dominated by reduction and adsorption. The sustainable removal of Cr(VI) by packed columns could be well fitted by the Thomas, Adams-Bohart, and Yoon-Nelson models, and importantly, the gel beads maintained integrity during the prolonged removal. These results will contribute significant insights into the practical application of SA/NZVI-rGO beads for the Cr(VI) removal in aqueous environments.

Application potential of zero-valent aluminum in nitrophenols wastewater decontamination: Enhanced reactivity, electron selectivity and anti-passivation capability

Nitrophenols (NPs) are highly toxic and easy to accumulate to high concentrations (> 500 mg/L) in real wastewater. The nitro group contained in NPs is an electron-absorbing group that is easy to reduce and difficult to oxidize, so there is an urgent need to develop reduction removal technology. Zero-valent aluminum (ZVAl) is an excellent electron donor that can reductively transform various refractory pollutants. However, ZVAl is prone to rapid deactivation due to non-selective reactions with water, ions, etc. To overcome this critical limitation, we prepared a new type of carbon nanotubes (CNTs) modified microscale ZVAl, CNTs@mZVAl, through a facile mechanochemical ball milling method. CNTs@mZVAl had outstanding high reactivity in degrading p-nitrophenol even 1000 mg/L and showed up to 95.50% electron utilization efficiency. Moreover, CNTs@mZVAl was highly resistant to the passivation by dissolved oxygen, ions and natural organic matters coexisting in water matrix, and remained highly reactive after aging in the air for 10 days. Furthermore, CNTs@mZVAl could effectively remove dinitrodiazophenol from real explosive wastewater. The excellent performance of CNTs@mZVAl is due to the combination of selective adsorption of NPs and CNTs-mediated e-transfer. CNTs@mZVAl looks promising for the efficient and selective degradation of NPs, with broader prospects for real wastewater treatment.

Activation of sulfite by micron-scale iron-carbon composite for metronidazole degradation: Theoretical and experimental studies

In recent years, sulfite (S(Ⅳ)), as an alternative to persulfates, has played a crucial role in eliminating antibiotics in wastewater, so there is an urgent need to develop a cheap, environmentally friendly, and effective catalyst. Zero-valent iron (ZVI) has great potential for activated S(Ⅳ) removal of organic pollutants, but its reactivity in water is reduced due to passivation. In this study, a micron-scale iron-carbon composite(mZVI@C-800) prepared via high-temperature calcination was coupled with S(Ⅳ) to degrade metronidazole (MNZ). Under the optimized reaction conditions of mZVI@C-800 dosage of 0.2 g/L and S(Ⅳ) concentration of 0.1 g/L, the MNZ removal rate was up to 81.5 % in acidic and neutral environments. The surface chemical properties of the catalysts were characterized by different analytical techniques, and the corresponding catalytic mechanism was analyzed based on these analytical results. As a result, Fe²⁺ is the main active site, and ^·OH and SO₄^·- were the dominant active species. The increase in efficiency was attributed to the introduction of carbon to enhance the corrosion of mZVI further releasing more Fe²⁺. Additionally proposed were the potential response mechanism, the degradation path, and the toxicity change rule. These results demonstrate that the catalytic breakdown of antibiotics in wastewater treatment can be accelerated by the use of the outstanding catalytic material mZVI@C-800.

Supporting nanoscale zero-valent iron onto shrimp shell-derived N-doped biochar to boost its reactivity and electron utilization for selenite sequestration

Nanoscale zero-valent iron (nZVI) has been widely used in the reductive removal of contaminants from water, yet it still fights against the inherent passive cover and the raise of medium pH. In this study, nZVI was supported onto a nitrogen-doped biochar (NBC) that was prepared by pyrolyzing shrimp shell for efficiently sequestrating aqueous selenite (Se(IV)). The resultant composite (NBC-nZVI) revealed a higher reactivity and electron utilization efficiency (EUE) than the bare nZVI in Se(IV) sequestration because of the positive charge, the buffering effect and the good conductivity of NBC. The kinetic rate and EUE of NBC-nZVI were increased by 143.4% and 15.3% compared to the bare nZVI, respectively, at initial pH of 3.0. The high removal capacity of 605.4 mg g^-1 for NBC-nZVI was obtained at Se(IV) concentration of 1000 mg L^-1, initial pH of 3.0, NBC-nZVI dosage of 1.0 g L^-1 and contact time of 12 h. Moreover, NBC-nZVI exhibited a strong tolerance to solution pHs and coexisting compounds (e.g., humic acid) and could reduce the Se(IV) concentration from 5.0 mg L^-1 to below the limit of drinking water (50 μg L^-1) in real-world samples. This work exemplified a utilization of shrimp shell-derived NBC to simultaneously enhance the reactivity and EUE of nZVI for reductively removing contaminants.

Goethite and riboflavin synergistically enhance Cr(VI) reduction by Shewanella oneidensis MR-1

Bioreduction of Cr(VI) is cost-effective and environmentally friendly, however, the slow bioreduction rate limits its application. In this study, the potential synergistic enhancement of Cr(VI) bioreduction by shewanella oneidensis MR-1 (S. oneidensis) with goethite and riboflavin (RF) was investigated. The results showed that the S. oneidensis reaction system reduce 29.2% of 20 mg/L Cr(VI) after 42 h reaction, while the S. oneidensis/goethite/RF reaction system increased the Cr(VI) reduction rate to 87.74%. RF as an efficient electron shuttle and Fe(II) from goethite bioreduction were identified as the crucial components in Cr(VI) reduction. XPS analysis showed that the final precipitates of Cr(VI) reduction were Cr(CH₃C(O)CHC(O)CH₃)₃ and Cr₂O₃ and adhered to the bacterial cell surface. In this process, the microbial surface functional groups such as hydroxyl and carboxyl groups participated in the adsorption and reduction of Cr(VI). Meanwhile, an increase in cytochrome c led to an increase in electron transfer system activity (ETSA), causing a significant enhancement in extracellular electron transfer efficiency. This study provides insight into the mechanism of Cr(VI) reduction in a complex environment where microorganisms, iron minerals and RF coexist, and the synergistic treatment method of Fe(III) minerals and RF has great potential application for Cr(VI) detoxification in aqueous environment.

Enhanced Cr (VI) reduction and removal by Fe/Mn oxide biochar composites under acidic simulated wastewater

Chromium (Cr (VI)) can cause severe damage to the ecosystem and humans because of its toxicity. In this paper, the adsorbed Fe/Mn ions Bacillus cereus ZNT-03, lotus seeds, and graphene oxide were co-cultured as the raw materials. Fe/Mn oxide biochar composite (FMBC) was prepared to treat Cr (VI) by one-step pyrolysis. FMBC has high-density micropores, and the average pore size is about 0.82 nm. Fe (II), Mn (II), and N-containing functional groups could serve as electron donors for Cr (VI) reduction. The removal of Cr (VI) is monolayer chemisorption and pH-dependent. The maximum adsorption capacity of FMBC is 21.25 mg g^-1. Cr (VI) is reduced and adsorbed on FMBC by physical adsorption, reduction, complexation, electrostatic attraction, and coprecipitation. The contribution ratio of the reduction mechanism to Cr (VI) is 72.25%. The packed column and regeneration experiments indicated that FMBC had excellent adsorption stability even after soaking in acidic simulated wastewater after 180 days (pH 1.5). These results indicate that FMBC can provide rapid reduction and efficient adsorption for Cr (VI), making it possible to apply in water treatment.

Catalytic activation of peroxydisulfate by secondary mineral derived self-modified iron-based composite for florfenicol degradation: Performance and mechanism

The advanced oxidation processes (AOPs) driven by iron-based materials are the highly efficient technology for refractory organic pollutants treatment. In this work, self-modified iron-based catalysts were prepared using secondary mineral as the precursor by one-step pyrolysis process without additional dopants. The prepared catalysts exhibited excellent performance in catalytic degradation of florfenicol (FF), especially C-AJ, which was derived from ammoniojarosite [(NH₄, H₃O)Fe₃(OH)₆(SO₄)₂], activated PDS to degrade 93% FF with initial concentration of 50 mg/L. Quenching tests and electron paramagnetic resonance (ESR) studies showed that SO₄^•-, ^•OH, and ^•O₂^- were the main reactive species for FF degradation and their contribution degree was SO₄^•- > ^•OH > •O₂^-. The Fe⁰ and the cycle of Fe(II)/Fe(III) both contributed to the PDS activation, and the reduction of Fe(III) to Fe(II) was accelerated by S^2- on the catalyst surface. In addition, Fe₃O₄ on the C-AJ indirectly catalyzes PDS by promoting electron transfer. The effects of catalyst dosage, PDS concentration, pH, inorganic anions, and real aqueous matrices on FF degradation, TOC analysis, and cycling test were investigated. The results showed that iron-based catalysts have superior environmental durability due to their excellent catalytic properties in the real aqueous matrices with common inorganic anions and pH 3-9 and its steady catalytic capacity with multiple cycles. Overall, this study sheds new light on the rational design of self-modified iron-based composite and develops low-cost technology toward remediation of FF-contaminated wastewater.

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.

In situ formation of Ca(OH)2 coating shell to extend the longevity of zero-valent iron biochar composite derived from Fe-rich sludge for aqueous phosphorus removal

Despite being an effective and attractive functional strategy for aqueous phosphorus (P) removal, the use of zero valent iron (ZVI) biochar composites has been severely impeded by rapid self-erosion. We describe a new approach for extending the lifespan of Fe-rich sludge-derived ZVI biochar composites via CaCl2 modification. Preliminary results showed that composites obtained at 900 °C without modification (MBC900) and at 900 °C with 100 g Cl/kg addition (MBC900100) had the highest P removal efficiency. In subsequent batch experiments, MBC900100 exhibited more stable P adsorption capacities than MBC900 over a wide pH range (4-10) and at various dosages, which was enhanced by the presence of HCO3-. The theoretical maximum P adsorption capacities of MBC900 and MBC900100 were 227.14 and 224.15 mg g-1, respectively. Kinetic analysis indicated that chemisorption dominated the removal process. Continuous experimental data using the Yoon-Nelson model indicated that MBC900100 had a considerably longer half-penetration time. The primary mechanism of P removal by MBC900 was Fe/C micro-electrolysis. As the embedded CaO formed a dissolvable Ca(OH)2 shell in situ on the surface of MBC900100, the phosphate formed a precipitate with free Ca2+ before being removed via micro-electrolysis. Overall, CaCl2 modification successfully enhanced the longevity of the ZVI biochar composites.

A novel slag composite for the adsorption of heavy metals: Preparation, characterization and mechanisms

The utilization of solid waste for resource recovery and production of value-added products is the theme of green chemistry. Currently, how to using solid wastes to prepare environmentally-functional materials with high performance and strength is one of the hot topics. In this research, electrolytic manganese residue (EMR) was thermally activated with calcite to prepare a silicon-based functionalized adsorbent (C-EMR) for the removal of cadmium (Cd²⁺, 467.14 mg/g) and lead (Pb²⁺, 972 mg/g). The thermodynamic results indicated that the removal process of Cd²⁺ and Pb²⁺ by C-EMR were endothermic and spontaneous. HNO₃ can effectively strip the two adsorbed metals from C-EMR with the stripping efficiency of nearly 80% for Cd²⁺ and 99.92% for Pb²⁺, indicating that adsorption and ion exchange may be the main reason for the removal of the metals on C-EMR. Besides, surface precipitation was also responsible for removing some Pb²⁺ from the aquatic environment according to the X-ray photoelectron spectrometry (XPS) analysis. Results indicate that -SiO₃^- has stronger affinity with Pb²⁺ and Cd²⁺ than other groups ((-MnO₂), -OH) by theoretical calculation (VASP, GGA-PBE). This study shows that this novel adsorbent (C-EMR) can be adopted as an environmentally-friendly, inexpensive and efficient adsorbent for removal of Cd²⁺ and Pb²⁺ from aquatic solution. This technique not only provides potential adsorbent for the elimination of heavy metals but also proposes an alternative route for the treatment and utilization of waste solid.

An iron-biochar composite from co-pyrolysis of incinerated sewage sludge ash and peanut shell for arsenic removal: Role of silica

Modification of biochar by low-cost iron sources has gained increasing attention to improve pollutants removal performance and reduce production costs compared to conventional chemical modifications. While such iron sources generally have complex compositions, their effects on properties of the iron-biochar composite are not well investigated. This study produced an iron-biochar (RBC) composite from co-pyrolysis of incinerated sewage sludge ash (ISSA) and peanut shell, and examined the role of silica with widespread existence in ISSA and other low-cost iron sources on properties of the iron-biochar composite relevant to As(III)/As(V) removal. Silica was found to react with iron during the pyrolysis process at 850 °C and formed iron silicon at the expense of producing zero valent iron and Fe₃O₄ which resulted in a poorer removal efficacy for As(III) and As(V) compared to the iron-biochar (FBC) made from pure Fe₂O₃ and peanut shell. Moreover, a high leaching of reactive silica from RBC was observed which affected the formation of corrosion products of ZVI and competed with arsenic for active adsorption sites. Despite this, RBC still exhibited a maximum adsorption capacity of 17.44 and 57.56 mg/g towards As(III) and As(V) respectively at pH 3.0. Overall, this study provides an interesting insight into upcycling ISSA into useful media for sorptive removal of arsenic from aqueous solutions.

Rust triggers rapid reduction of Cr6+ by red phosphorus: The importance of electronic transfer medium of Fe3

Red phosphorus (P) is one of the metalloid materials, with five external electrons, it should be an excellent electron donor. However, the activity of red P to reduce Cr⁶⁺ is limited. Due to electrostatic repulsion, it is difficult for the electrons on the red P to transfer to the chromate anion (Cr⁶⁺). Interestingly, we found that Fe³⁺ derived from rust, waste iron or Fe³⁺ reagents can be used as the electron transport medium to solve the electron transport obstacles between red P and Cr⁶⁺. As a result, the reduction of Cr⁶⁺ by red P/rust system takes only 20 min, which is far lower than the 140 min of red P. The reduction rate of Cr⁶⁺ in the red P/rust system is about 12.3 times that of red P. The reaction mechanism is that red P is not easy to access chromate anions but can easily adsorb Fe³⁺. The adsorbed Fe³⁺ will be reduced to Fe²⁺ by red P, and the regenerated Fe²⁺ will diffuse into the solution to rapidly reduce Cr⁶⁺. Therefore, this work provides an alternative waste iron reuse pathway and also sheds light on the important role of electron medium in reduction reaction.

Biochar-supported nZVI for the removal of Cr(VI) from soil and water: Advances in experimental research and engineering applications

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.

Boron doping positively enhances the catalytic activity of carbon materials for the removal of bisphenol A

Boron-doped carbon materials (BCs), low-cost and environmentally friendly carbocatalysts, were prepared for the activation of persulfate (PS) for the removal of bisphenol A (BPA). Compared with B-free carbon materials (Cs), the adsorption and catalytic activity were significantly enhanced by the boron modification. Fast and efficient removal of BPA was achieved using the BCs/PS system. The BPA removal rate constant increased linearly with the adsorption capacity of BCs. Electron paramagnetic resonance (EPR) spectroscopy and radical quenching experiments indicated that the degradation mechanisms in the BCs/PS system were different from conventional radical-based oxidation pathways. On the contrary, nonradical pathways were demonstrated to dominate the oxidation processes in the removal of BPA using the BCs/PS system. Herein, a mechanism is proposed where PS is activated by the carbon material to form a reactive electron-deficient carbocatalyst ([BCs]*) complex with a high redox potential, driving a nonradical oxidation pathway to achieve BPA removal. Through experimental investigation and the use of electrochemical techniques (cyclic voltammetry, Tafel corrosion analysis and open circuit voltages), B-doped carbon materials for the activation of PS elevate the potential of the derived nonradical [BCs]* complexes, and then accelerate the BPA removal efficiency via an electron transfer process. Utilizing adsorption and nonradical oxidation processes, the BCs/PS system possesses great potential for the removal of BPA in practical applications such as wastewater treatment.

Boron-doped carbon materials, based on coffee grounds, sodium bicarbonate and boric acid, were synthesized via a simple hydrothermal process. The ability of a boron-doped carbon material/persulfate system to remove bisphenol A was systematically studied.

Integration of biochar into Ag3PO4/α-Fe2O3 heterojunction for enhanced reactive oxygen species generation towards organic pollutants removal

A biochar (BC) harbored Ag₃PO₄/α-Fe₂O₃ type-Ⅰ heterojunction (Ag-Fe-BC) was prepared by a hydrothermal-impregnation method to transfer active center of heterojunctions. The electrochemical and spectroscopic tests demonstrated that BC enhanced the catalytic performance of the heterojunction by enhancing photocurrent, reducing fluorescence intensity, and facilitating separation of electron-hole pairs. The photocatalytic activity showed the Ag-Fe-BC (5:1:3) could degrade Rhodamine B (20 mg/L) by up to 92.7%, which was 3.35 times higher than Ag₃PO₄/α-Fe₂O₃. Tetracycline and ciprofloxacin (20 mg/L) were degraded efficiently by 58.3% and 79.4% within 2 h, respectively. Electron paramagnetic resonance and scavenging experiments confirmed the major reactive oxygen species (ROS) consisted of singlet oxygen (¹O₂) and superoxide (·O₂^-). Excellent RhB adsorption and electrons capturing capacity of BC facilitated electron-hole pairs separation and ROS transferring to target organics followed by elevated degradation. Thus, a facile method was proposed to synthesize a highly efficient visible-light responsive photocatalyst for degradation of various organics in water.

Pyrolysis temperature and feedstock affected Cr(VI) removal capacity of sulfidated zerovalent iron: Importance of surface area and electrical conductivity

Herein, feedstock (pinewood, rice straw, and dairy manure) and pyrolysis temperature (300, 500, and 700 °C) were selected as the influencing factors of properties of biochar (BC) to identify the contribution of biochar's matrix on Cr(VI) removal by BC-supported sulfidated zero-valent iron (S-ZVI/BC). Results showed that higher temperature was more conducible to improve the electrochemical properties and specific surface areas of composites. Raman spectra of S-ZVI supported by pinewood-derived BC (S-ZVI/PBC) showed the I_D/I_G ratio increased from 0.639 to 0.975 for the composites prepared at 300-700 °C, indicating the increased structural defects and resulting in the greatest Cr(VI) removal (35.81 mg g^-1) and reduction (30.21 mg g^-1) amounts of S-ZVI/PBC700. Besides, S-ZVI/PBC exhibited greater electrochemical reactivity and surface area than S-ZVI harbored by BC from dairy manure and rice straw. Additionally, Pearson correlation analysis revealed that Cr(VI) removal was significantly positively correlated to surface area (R² = 0.90) and negatively correlated to Tafel corrosive potential (R² = 0.88). Both desorption experiment and XPS spectra of spent sorbents showed that reduction predominated the detoxifying mechanism of Cr(VI) followed by adsorption (due to corrosively-generated iron oxides and BC) and precipitation (Cr₂S₃). This suggested that biochar with greater specific surface area and electrical conductivity is more favorable to immobilize S-ZVI with respect to Cr(VI) removal.

New insights into iron/nickel-carbon ternary micro-electrolysis toward 4-nitrochlorobenzene removal: Enhancing reduction and unveiling removal mechanisms

The ternary micro-electrolysis material iron/nickel-carbon (Fe/Ni-AC) with enhanced reducibility was constructed by introducing the trace transition metal Ni based on the iron/carbon (Fe/AC) system and used for the removal of 4-nitrochlorobenzene (4-NCB) in solution. The composition and structures of the Fe/Ni-AC were analyzed by various characterizations to estimate its feasibility as reductants for pollutants. The removal efficiency of 4-NCB by Fe/Ni-AC was considerably greater than that of Fe/AC and iron/nickel (Fe/Ni) binary systems. This was mainly due to the enhanced reducibility of 4-NCB by the synergism between anode and double-cathode in the ternary micro-electrolysis system (MES). In the Fe/Ni-AC ternary MES, zero-iron (Fe⁰) served as anode involved in the formation of galvanic couples with activated carbon (AC) and zero-nickel (Ni⁰), respectively, where AC and Ni⁰ functioned as double-cathode, thereby promoting the electron transfer and the corrosion of Fe⁰. The cathodic and catalytic effects of Ni⁰ that existed simultaneously could not only facilitate the corrosion of Fe⁰ but also catalyze H₂ to form active hydrogen (H*), which was responsible for 4-NCB transformation. Besides, AC acted as a supporter which could offer the reaction interface for in-situ reduction, and at the same time provide interconnection space for electrons and H₂ to transfer from Fe⁰ to the surface of Ni⁰. The results suggest that a double-cathode of Ni⁰ and AC could drive much more electrons, Fe²⁺ and H*, thus serving as effective reductants for 4-NCB reduction.

Carbon matrix of biochar from biomass modeling components facilitates electron transfer from zero-valent iron to Cr(VI)

Biochar-harbored zero-valent iron (ZVI/BC) has been extensively used to detoxify hexavalent chromium (Cr(VI)). However, the role played by biochar in promoting electron transfer of ZVI and Cr(VI) reduction was not fully uncovered. Herein, three biomass modeling components (cellulose, hemicellulose, and lignin) and their blends were utilized to synthesize ZVI/BC via co-pyrolysis with hematite. X-ray diffraction analysis showed that hematite was successfully reduced to ZVI in nitrogen ambience. Batch sorption experiment showed that mass ratio (hematite to lignocellulosic component) of 1:20 is most optimal for reduction of Cr(VI) by ZVI/BCs. ZVI supported by BC derived from cellulose, hemicellulose, and their binary mixture demonstrated better Cr(VI) removal capacity (23.8-38.3 mg g-1) owing to higher ordered and graphitic carbon structure as revealed by Raman spectrum. In addition, lower Tafel corrosion potentials and smaller electrochemical impedance arc radiuses were observed based on electrochemical analysis, suggesting their higher electrical conductivity and faster electron transfer, whereas the BCs derived from lignin and lignin-containing hybrids were not conducive to electron transfer of ZVI due to lower degree of graphitization, thus compromising Cr(VI) removal by ZVI/BC (7.7-17.7 mg g-1). As per X-ray photoelectron spectroscopy analysis, reduction, complexation, and co-precipitation were the main mechanisms for Cr(VI) removal. The present study provided a scientific evidence for screening plant-derived biomass feedstock with high contents of cellulose and hemicellulose and low lignin content to fabricate ZVI/BC to achieve high Cr(VI) removal.

Remediation of Chromium (VI) from Groundwater by Metal-Based Biochar under Anaerobic Conditions

Iron salt-modified biochar has been widely used to remove Cr(VI) pollution due to the combination of the generated iron oxides and biochar, which can bring positive charge and rich redox activity. However, there are few comprehensive studies on the methods of modifying biochar with different iron salts. In this study, two iron salt (FeCl3 and Fe(NO3)3) modification methods were used to prepare two Fe-modified biochar materials for removing Cr(VI) in simulated groundwater environment. It was revealed by systematic characterization that FeCl3@BC prepared via the FeCl3 modification method, has larger pore size, higher zeta potential and iron oxide content, and has higher Cr(VI) adsorption-reduction performance efficiency as compared to Fe(NO3)3@BC prepared via Fe(NO3)3 modification method. Combined with XRD and XPS analyses, Fe3O4 is the key active component for the reduction of Cr(VI) to Cr(III). The experimental results have shown that acidic conditions promoted Cr(VI) removal, while competing ions (SO42− and PO43−) inhibited Cr(VI) removal by FeCl3@BC. The Elovich model and intra-particle diffusion model of FeCl3@BC can describe the adsorption behavior of Cr(VI) well, indicating that both the high activation energy adsorption process and intra-particle diffusion control the removal process of Cr(VI). The Freundlich model (R2 > 0.999) indicated that there were unevenly distributed chemisorptions centers on the FeCl3@BC surface. Stability experiments exposed that FeCl3@BC was stable under neutral, acidic, and alkaline conditions. Furthermore, the main mechanisms of FeCl3@BC removal of Cr(VI) include electrostatic adsorption, chemical reduction, ion exchange, and co-precipitation. In conclusion, our findings provide a new insight for the selection of iron salt-modified biochar methods, and will also be beneficial for the preparation of more efficient Fe-modified biochars in the future.

Rapid and efficient reduction of chromate by novel Pd/Fe@biomass derived from Enterococcus faecalis

Efficient reduction of chromate is highly desirable for its detoxification and remediation of the contaminated environment. This study described a fusion of the concepts of precious metal biorecovery and fabrication of Pd/Fe@biomass derived from simulated wastewater. The effectiveness of Pd/Fe@biomass during reduction process of Cr(VI) was evaluated by comparing with pure nZVI, E. faecalis and Pd@biomass. Results showed that Pd(II) could be recovered by E. faecalis with Fe(II) as the electron donor, and precipitation could yield nZVI anchored onto Pd-loaded E. faecalis. The nano particles (NPs) on Pd/Fe@biomass were well-dispersed, which provided 2.70 folds specific surface area comparing with nZVI. Efficient Cr(VI) reduction could be achieved at a higher catalyst dosage, the most appropriated Pd/Fe molar ratio of 2% and a wide pH range. Typically, 0.5 mM Cr(VI) could be completely reduced in 5 min driven by Pd/Fe@biomass under the conditions of dosage of 1.0 g/L and pH 3. Moreover, the mechanisms of Cr(VI) reduction by Pd/Fe@biomass were proposed, which intimately related to nZVI electron donating capacities, Pd catalysis for hydrogenation and galvanic cell effects between Fe and Pd. Therefore, Pd/Fe@biomass could be an alternative for rapid and complete reduction of Cr(VI).

Mutual-activation between Zero-Valent iron and graphitic carbon for Cr(VI) Removal: Mechanism and inhibition of inherent Side-reaction

The low reactivity of zero-valent iron (ZVI) usually limits its application for pollutant remediation. Therefore, a microscopic galvanic cell (mGC) with short-circuited cathode and anode was synthesized to intensify its galvanic corrosion. The prepared mGC exhibited 7.14 times higher Fe(II) release performance than ordinary nanoscale-ZVI (nZVI), rendering efficient Cr(VI) removal performance. Density functional theory (DFT) revealed mutual-activation of the cathode and anode due to close proximity, dramatically enhancing the galvanic corrosion of Fe(0) in mGC. The corrosion potential of mGC was measured as -0.77 V, which was 100 mV more negative than nZVI. The released electrons and surface-bond Fe(II) from anode in mGC was proved to be the dominant reductive species. More importantly, Cr(VI) reduction was slightly inhibited by hydroxyl radicals generated by a series of inherent side-reactions in the system, which could be well eliminated by low concentrations of 4-acetamido phenol. This study provides a promising strategy for ZVI activation, and sheds light on its environmental applications.

Active biochar-supported iron oxides for Cr(VI) removal from groundwater: Kinetics, stability and the key role of FeO in electron-transfer mechanism

Chromium (Cr), especially in forms of hexavalent chromium (Cr(VI)) remains a serious threat to public health and environmental safety for its high toxicity. Herein, two types of iron-modification methods adopting co-pyrolysis and surface-deposition respectively were carried out to prepare active Fe-biochar composites (FeBC) for Cr(VI) removal in the simulated groundwater environment. The systematic characterization demonstrated that larger BET surface area and diversified iron oxides of FeBC-1 obtained from the co-pyrolysis method contributed to higher adsorption and reduction activity towards Cr(VI) degradation in comparison with FeBC-2 produced from surface-deposition method. Further, FeO was evidenced to be a main active component for transforming Cr(VI) to lower-toxicity Cr(III) uniting XRD and XPS analysis. Also, the designed batch experiments aiming at deeper clarifying FeBC-1 revealed that the pseudo-second-order kinetic and intra-particle diffusion model could well describe the Cr(VI) sorption behaviors, suggesting that a single-layer, chemical adsorption process as well as internal particle diffusion both controlled the removal process of Cr(VI) using FeBC-1. Finally, the stability experiments stated that FeBC-1 was basically stable at acidic and neutral conditions. Thus, it was found that co-pyrolysis of FeBC-1 is a potential technology for Cr(VI) remediation.

One-step fabrication of dual functional Tb3+ coordinated polymeric micro/nano-structures for Cr(VI) adsorption and detection

Hexavalent chromium Cr(VI) has been considered as one of the most hazardous heavy metals because of its strong and persistent toxicity to the ecosystem and human beings. Herein, we have synthesized a double hydrophilic block co-polyarylene ether nitriles (abbreviated as dhPEN) bearing aromatic backbone as well as pendent carboxyl and sulfonate groups. Afterward, the synthesized dhPEN has been co-assembled with the lanthanide Tb³⁺ via a one-step solvent exchange protocol, leading to generation of Tb³⁺ coordinated dhPEN (Tb-dhPEN) micro/nano-structures that exhibit good adsorption capacity and detection sensitivity towards Cr(VI). More specifically, the direct self-assembly of dhPEN and Tb³⁺ in mixed H₂O/DMF solvents resulted to Tb-dhPEN microparticles with lamellar structures, which exhibited a high Cr(VI) adsorption capacity approaching to 402 mg/g. The detailed characterization confirm that Cr(VI) is adsorbed and partially reduced to Cr(III) by the Tb-dhPEN microparticles via chemical interaction. Furthermore, the self-assembly of dhPEN with Tb³⁺ in the H₂O/DMF mixed solvents containing NaOH contributed to the generation of spherical nanoparticles showing green emission at 545 nm, which can be selectively quenched by the Cr(VI), leading to the specific detection of trace concentration of Cr(VI) down to 0.12 nM as well as reliable determination of Cr(VI) presented in real environmental samples.

The pH-sensitive sorption governed reduction of Cr(VI) by sludge derived biochar and the accelerating effect of organic acids

Reduction coupling immobilization is one of the most commonly adopted strategies for the remediation of Cr(VI) contamination. Biochar is a carbon-rich material with abundant active functional groups for sorption and reduction reactions. In previous reports, phytomass derived biochars and organic functional groups have been emphasized, while the performance of sludge derived biochar (SBC) has often been understated. In the present study, a 30 d kinetic study proved that the removal route involved the sorption of Cr(VI), reduction to Cr(III) and immobilization of Cr(III), and that the sorption process was the primary and rate determining step. As a result of the SBC alkalinity, the solution pH increased, and sorption was largely inhibited, which then governed the overall removal ratio. The FTIR spectra suggested the involvement of hydroxyls in these processes. Low molecular weight organic acids accelerated the removal process in the early phase and improved the reduction process.

Volatilization of Zn and Pb and preparation of integrated micro-electrolysis filter from copper slag and its application for removing Cr(VI) from aqueous solution

In this study, copper slag was treated by carbothermal reduction technology for preparing an integrated micro-electrolysis filter (IMEF) and recovery of Zn and Pb. The influence of roasting conditions on the volatilization of Zn and Pb, and on the performance of IMEF in removing Cr(VI) from water were studied. The results showed that increasing the roasting temperature, time, and dosage of coal facilitated the generation of zero-valent iron (ZVI) and volatilization of Zn and Pb. The IMEF, roasted at 1150 °C for 40 min with 25% anthracite, had the best reduction effect on Cr(VI), and the volatilization efficiencies of Zn and Pb were 97.38% and 96.77%, respectively. The prepared IMEF had a porous structure with a porosity of 75.20%. A great number of nano/micro-sized ZVI particles were generated on the surface of silicate pore, and had super reactivity. The removal of Cr(VI) was promoted by increasing IMEF dosage and solution temperature, and decreasing the pH of the Cr(VI) solution. The IMEF presented good mechanical strength and excellent long-term performance in removing Cr(VI). Cr(VI) was reduced into Cr(III) and then mineralized to FeCr₂O₄ during reaction.

Preparation, characterization, and applications of Fe-based catalysts in advanced oxidation processes for organics removal: A review

Fe-based catalysts as low-cost, high-efficiency, and non-toxic materials display superior catalytic performances in activating hydrogen peroxide, persulfate (PS), peracetic acid (PAA), percarbonate (PC), and ozone to degrade organic contaminants in aqueous solutions. They mainly include ferrous salts, zero-valent iron, iron-metal composites, iron sulfides, iron oxyhydroxides, iron oxides, and supported iron-based catalysts, which have been widely applied in advanced oxidation processes (AOPs). However, there is lack of a comprehensive review systematically reporting their synthesis, characterization, and applications. It is imperative to evaluate the catalytic performances of various Fe-based catalysts in diverse AOPs systems and reveal the activation mechanisms of different oxidants by Fe-based catalysts. This work detailedly summarizes the synthesis methods and characterization technologies of Fe-based catalysts. This paper critically evaluates the catalytic performances of Fe-based catalysts in diverse AOPs systems. The effects of solution pH, reaction temperature, coexisting ions, oxidant concentration, catalyst dosage, and external energy on the degradation of organic contaminants in the Fe-based catalyst/oxidant systems and the stability of Fe-based catalysts are also discussed. The activation mechanisms of various oxidants and the degradation pathways of organic contaminants in the Fe-based catalyst/oxidant systems are revealed by a series of novel detection methods and characterization technologies. Future research prospects on the potential preparation means of Fe-based catalysts, practical applications, assistive technologies, and impact in AOPs are proposed.

The significant role of electron donating capacity and carbon structure of biochar to electron transfer of zerovalent iron

Herein, the major biochar properties were correlated with electron transfer of zerovalent iron (ZVI) and contribution of biomass constituents to biochar property was ascertained to optimize electron transfer of ZVI. To this end, five respective stalk-type and wood-type lignocellulosic biomasses were pyrolzed at 600 °C to prepare biochars to harbor ZVI (ZVI/BC). Thermogravimetric analysis demonstrated woody biomasses decomposed more intensively at higher temperature relative to stalky biomass. ZVI/BC were characterized with Raman, X-ray diffraction, and electrochemical analyses including electron donating capacity (EDC) and electron accepting capacity (EAC). Pearson correlation and partial least-squares (PLS) analyses confirmed that Cr(VI) reduction capacity was negatively related to Tafel corrosion potential and intensity ratio of I_D/I_G, but significantly positively-related to EDC of BC, in which EDC was a predominant attribute to contribute to reductive capacity toward Cr(VI) reduction. That is, greater EDC and higher graphitic carbon structure of biochar due to cellulose and hemicellulose components favor electron transfer of ZVI toward Cr(VI) reduction.

Novel recycling of phosphorus-recovered incinerated sewage sludge ash residues by co-pyrolysis with lignin for reductive/sorptive removal of hexavalent chromium from aqueous solutions

Incinerated sewage sludge ash (ISSA), a by-product generated from the combustion of dewatered sewage sludge, has been extensively studied as a secondary resource for phosphorus recovery by acid extraction methods. Recycling of the P-recovered ISSA residues is crucial to complete and sustain the whole process. In this study, the ISSA residue rich in iron was reused and co-pyrolyzed with lignin at 650, 850 and 1050 °C under N₂ atmosphere for the synthesis of a composite material to remove hexavalent chromium (Cr(VI)) from aqueous solutions. Characterization analysis including XRD, XPS, and FTIR showed that iron oxides in the residue were reduced to zero valent iron at 1050 °C that exhibits the optimal Cr(VI) removal performance. The Cr(VI) removal process was rapid and reached a plateau at around 30 min. The maximum removal rate was obtained at pH 2.0, which was conducive for the treatment of a synthetic Cr(VI)-containing wastewater in fix-bed column experiments, whereby Cr(VI) as well as total Cr were continuously removed. Overall, this study proposed a new routine for the recycling of ISSA residue after phosphorus recovery by the acid extraction method and provided a value-added product for Cr(VI) removal from wastewaters.

Preparation of biochar-interpenetrated iron-alginate hydrogel as a pH-independent sorbent for removal of Cr(VI) and Pb(II)

Herein, a pH-independent interpenetrating polymeric networks (Fe-SA-C) were fabricated from graphitic biochar (BC) and iron-alginate hydrogel (Fe-SA) for removal of Cr(VI) and Pb(II) in aqueous solution. Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and scanning electron microscope (SEM) results demonstrated that graphitic BC interpenetration increased surface porosity and distorted surfaces of Fe-SA, which boosted availability of hydroxyl (-OH) group. Fe³⁺ as a cross-linking agent of the alginate endowed Fe-SA-C with positive surfaces (positive zeta potential) and excellent pH buffering capacity, while excessive Fe³⁺ was soldered on Fe-SA-C matrix as FeO(OH) and Fe₂O₃. Cr(VI) removal at pH of 3 by Fe-SA-C (20.3 mg g^-1) were 30.3% and 410.6% greater than that by Fe-SA and BC, respectively. Fe-SA-C exhibited minor pH dependence over pH range of 2-7 towards Cr(VI) retention. Greater zeta potential of Fe-SA-C over Fe-SA conferred a better electrostatic attraction with Cr(VI). FTIR and XPS of spent sorbents confirmed the reduction accounted for 98.5% for Cr(VI) removal mainly due to participation of -OH. Cr(VI) reduction was further favored by conductive carbon matrix in Fe-SA-C, as evidenced by more negative Tafel corrosion potential. Reductively formed Cr(III) was subsequently complexed with carboxylic groups originating from oxidation of -OH. Thus, Cr(VI) removal invoked electrostatic attraction, reduction, and surface complexation mechanisms. Pb(II) removal with excellent pH independence was mainly ascribed to surface complexation and possible precipitation. Thus, the functionalized, conductive, and positively-charged Fe-SA-C extended its applicability for Cr(VI) and Pb(II) removal from aqueous solutions in a wide pH range. This research could expand the application of hydrogel materials for removal of both cationic and anionic heavy metals in solutions over an extended pH range.

Increased structural defects of graphene oxide compromised reductive capacity of ZVI towards hexavalent chromium

Graphene oxide (GO) was treated with irradiation beams to understand the defective degree of carbon structure of GO in relation to electron transfer property of impregnated zerovalent iron (ZVI). The GO-supported ZVI (ZVI/GO) was synthesized and then characterized by an X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The results showed that the oxygen-bearing functional groups, oxygen content and structural disorder were increased as a function of irradiation beam intensity. ZVI was dominant in the composites, but proportion of iron oxide increased with greater oxygen content. Batch sorption revealed that Cr(VI) removal decreased from 20.11 g kg^-1 to 2.30 g kg^-1 as solution pH rose from 3 to 9. Cr(VI) removal capacity was 26.39 g kg^-1, 23.12 g kg^-1 and 12.35 g kg^-1 for ZVI/GO₀, ZVI/GO_12.3 and ZVI/GO_36.9, respectively. The reduction capacity of sorbents followed similar trends as Cr(VI) sorption as per desorption experiment, which accounted for a major Cr(VI) detoxification mechanism by ZVI/GO composites. The electrochemical tests demonstrated that unfavorable electron transfer rate of ZVI/GO composites was aggravated by greater structural disorder of GO. Thus, higher dose of irradiations could create more disorder in graphitic carbon and promote oxidation of ZVI, which hindered Cr(VI) reduction.

ZVI impregnation altered arsenic sorption by ordered mesoporous carbon in presence of Cr(Ⅵ): A mechanistic investigation

It is challenging to efficiently remove arsenate (As(Ⅴ)) and chromate (Cr(Ⅵ)) simultaneously. Herein, ordered mesoporous carbon (OMC) was fabricated with averaged pore diameter of 6.5 nm and surface area of 997 m² g^-1. Zerovalent iron (ZVI) impregnation reduced surface area of ZVI/OMC (432 m² g^-1) and increased I_D/I_G ratio by 13%. Maximal Cr(Ⅵ) and As(Ⅴ) sorption capacities at pH 3 were 0.66 and 0.019 mmol g^-1 by OMC, and 0.71 and 0.39 mmol g^-1 by ZVI/OMC, respectively. Reduction accounted for over 55% for Cr(Ⅵ) and As(Ⅴ) removal followed by complexation and precipitation. Better ZVI/OMC performance was ascribed to higher electron transfer rate and lower electrical resistance than OMC as per electrochemical analysis. Upon Cr(Ⅵ) introduction, As(Ⅴ) removal increased to 0.28 mmol g^-1 by OMC, but decreased to 0.16 mmol g^-1 by ZVI/OMC. OMC could preferably reduce CrO₄^2- to Cr³⁺ by hydroxyl group, which enhanced its zeta potential facilitating As(Ⅴ) sorption. Regarding ZVI/OMC, Fe⁰ and Fe oxide in ZVI/OMC exhibited better affinity to As(Ⅴ), but the competition for the similar active sites resulted in compromised As(Ⅴ) and Cr(Ⅵ) removal. Thus, the novel OMC is advantageous for removal of binary As(Ⅴ) and Cr(Ⅵ), but ZVI/OMC is robust to detoxify single heavy metal.

Ball milling biochar iron oxide composites for the removal of chromium (Cr(VI)) from water: Performance and mechanisms

As the first of its kind, a novel biochar/iron oxide composite (BM-Fe-HC) was successfully prepared by simply ball milling iron-laden biochar (Fe-HC). The performance and mechanisms of Cr(VI) removal by BM-Fe-HC were investigated. Ball milling effectively reduced particle size, increased specific surface area, more importantly, enhanced the distribution and increased the exposure of iron oxides on biochar surface. As a result, Cr(VI) removal by BM-Fe-HC showed fast kinetics and large adsorption capacity with the Langmuir maximum capacity of 48.1 mg/g, higher than that of other biochar/iron composites reported in the literature. Acidic pH promoted Cr(VI) removal while competition ions (Cl^-, SO₄^2- and PO₄^3-) inhibited Cr(VI) removal by BM-Fe-HC. Comparison of pre- and post-adsorption samples revealed that iron oxides of the BM-Fe-HC played the dominant role in the adsorption and reduction of Cr(VI) during the removal. After adsorption, part of adsorbed Cr(VI) was reduced by Fe(II) and then stabilized by Fe(III) in the form of amorphous Cr_xFe_1-x(OH)₃ on the composite surface. All the results demonstrate that novel ball-milled biochar/iron oxide composites can be used as an effective adsorbent to remove Cr(VI) from water.