Enhanced trichloroethylene dechlorination by carbon-modified zero-valent iron: Revisiting the role of carbon additives

Nonmetallic modified zero-valent iron for remediating halogenated organic compounds and heavy metals: A comprehensive review

Zero Valent Iron (ZVI), an ideal reductant treating persistent pollutants, is hampered by issues like corrosion, passivation, and suboptimal utilization. Recent advancements in nonmetallic modified ZVI (NM-ZVI) show promising potential in circumventing these challenges by modifying ZVI's surface and internal physicochemical properties. Despite its promise, a thorough synthesis of research advancements in this domain remains elusive. Here we review the innovative methodologies, regulatory principles, and reduction-centric mechanisms underpinning NM-ZVI's effectiveness against two prevalent persistent pollutants: halogenated organic compounds and heavy metals. We start by evaluating different nonmetallic modification techniques, such as liquid-phase reduction, mechanical ball milling, and pyrolysis, and their respective advantages. The discussion progresses towards a critical analysis of current strategies and mechanisms used for NM-ZVI to enhance its reactivity, electron selectivity, and electron utilization efficiency. This is achieved by optimizing the elemental compositions, content ratios, lattice constants, hydrophobicity, and conductivity. Furthermore, we propose novel approaches for augmenting NM-ZVI's capability to address complex pollution challenges. This review highlights NM-ZVI's potential as an alternative to remediate water environments contaminated with halogenated organic compounds or heavy metals, contributing to the broader discourse on green remediation technologies.

Achieving sustainable trichloroethylene removal from nitrate-containing groundwater: Effects of particle size and dosage of microscale zero-valent iron on its synergistic action with anaerobic bacteria

The coupling of microscale zero-valent iron (mZVI) and anaerobic bacteria (AB) has gained increasing attention due to its ability to enhance dechlorination efficiency by combining the advantages of chemical and microbial reduction. However, the implementation of these coupling technologies at the field scale is challenging in terms of sustainability goals due to the coexistence of various natural electron acceptors in groundwater, which leads to limited electron selectivity and increased secondary risk. Therefore, this study used trichloroethylene (TCE) as a probe contaminant and nitrate (NO3-) as a typical co-occurring natural electron acceptor to optimize the overall sustainable remediation performance of an mZVI/AB coupled system by adjusting the mZVI particle size and dosage. Results revealed that mZVI particles of different sizes exhibit different microorganism activation capabilities. In contrast to its 2 μm and 7 μm counterparts, the 30 μm mZVI/AB system demonstrated a strong dosage-dependency in TCE removal and its product selectivity. Finally, multi-criteria analysis (MCA) methods were established to comprehensively rank the alternatives, and 30 μm mZVI (15 g/L dosage) was determined to be the best remediation strategy with the highest total sustainability score under all studied hydro-chemical conditions when equal weights were applied to technical, environmental, and economic indicators. Our work provides a paradigm for comprehensively assessing the sustainable remediation performance of chlorinated aliphatic hydrocarbons polluted groundwater in practical applications.

Micro-nano H2 bubbles enhanced hydrodehalogenation of 3-chloro-4-fluoroaniline: Mass transfer and action mechanism

3-chloro-4-fluoraniline (FCA) is an important intermediate for the synthesis of antibiotics, herbicides and insecticides, and has significant environmental health hazards. Catalytic hydrogenation technology is widely used in pretreatment of halogenated organics due to its simple process and excellent performance. However, compared with the research of high activity hydrogenation catalyst, the research of efficient utilization of hydrogen source under mild conditions is not sufficient. In this work, micro-nano H2 bubbles are produced in situ by electrolytic water and active metal replacement, and their apparent properties are studied. The result show that the H2 bubbles have a size distribution in the range of 150-900 nm, which can rapidly reduce the REDOX potential of the water and maintain it in a hydrogen-rich state for a long time. Under the action of Pd/C catalyst, atomic hydrogen (H•) produced by dissociative adsorption can sequentially hydrogenate FCA to aniline. The H• utilization ratios of the above two hydrogen supply pathways reach 6.20% and 4.94% respectively, and H2 consumption is reduced by tens of times (≥50 → ≈1.0 mL/min). The research provides technical support for the efficient removal of halogenated refractory pollutants in water and the development of hydrogen economy.

Insights into the contrasting effects of sulfidation on dechlorination of chlorinated aliphatic hydrocarbons by zero-valent iron

Contrasting effects of sulfidation on contaminants reduction by zero-valent iron (ZVI) has been reported in literature but the underlying mechanisms remain unclear. Here, under well-controlled conditions, we compared the performance of ZVI and sulfidated ZVI (S-ZVI) toward a series of chlorinated compounds. Results revealed that, although S-ZVI was more reactive than ZVI toward hexachloroethane, pentachloroethane, tetrachloroethylene, and trichloroethene, sulfidation hindered the dechlorination of the other ten tested chlorinated aliphatics by a factor of 1.5-125. Moreover, S-ZVI may lead to an accumulation of toxic partially-dechlorinated products. Analogous to its effects on ZVI reactivity, sulfidation also exerted positive, negligible, or negative effects on the electron efficiency of ZVI. Solvent kinetic isotope effect analysis suggested that direct electron transfer rather than reaction with atomic hydrogen was the dominant reduction mechanism in S-ZVI system. Hence, the sulfidation enhancing effects could be expected only when direct electron transfer is the preferred reduction route for target contaminants. Furthermore, linear free energy relationships analysis indicated one-electron reduction potential could be used to predict the transformation of chlorinated ethanes by S-ZVI, whereas for chlorinated ethenes, their adsorption properties on S-ZVI determined the dechlorination process. All these findings may offer guidance for the decision-making regarding the application of S-ZVI.

Regulating the FeSx assembly pattern of sulfidated zero-valent iron: All-in-one interface modulation with activated carbon

Specifically designing the heterogeneous interface in sulfidated zero-valent iron (S-ZVI) has been an effective, yet usually overlooked method to improve the decontamination ability. However, the mechanism behind FeSx assembly remains elusive and the lack of modulating strategies that can essentially tune the applicability of S-ZVI further imposes difficulties in creating better-performing S-ZVI with heterogeneous interface. In this study, by introducing powdered activated carbon (PAC) during S-ZVI preparation, S-ZVI/PAC microparticles were prepared to modulate the assembly pattern of FeSx for the applicability and reactivity of the material. S-ZVI/PAC showed robust performance in Cr(VI) sequestration, with 11.16 and 1.78 fold increase in Cr(VI) reactivity compared to ZVI and S-ZVI, respectively. This was attributed to the fact that the introduced PAC could acquire FeSx to enhance the electron transfer capacity matching its adsorption threshold, thus helping to accommodate the transfer of the reduction center to PAC in S-ZVI/PAC. In optimizing the FeSx allocation between ZVI and PAC, the chemical assembly of FeSx on S-ZVI was superior to physical adsorption. Critically, we found that isolated FeSx in the prepared solution was physically adsorbed by the PAC, allowing chemically assembled FeSx on the S-ZVI. This was achieved by controlling the addition sequence of Na2S and PAC, as it effectively controlled the release rate and content of Fe(II) in the preparation solution. S-ZVI/PAC was demonstrated to be extremely effective in simulated wastewater and electrokinetics-permeable reactive barrier (EK-PRB) treatments. Introducing PAC enriches the diversity of sulfidation mechanisms and may realize the universality of the S-ZVI/PAC application scenarios. This study provides a new interface optimization strategy for S-ZVI targeted design towards environmental applications.

Tailoring Fe0 Nanoparticles via Lattice Engineering for Environmental Remediation

Lattice engineering of nanomaterials holds promise in simultaneously regulating their geometric and electronic effects to promote their performance. However, local microenvironment engineering of Fe0 nanoparticles (nFe0) for efficient and selective environmental remediation is still in its infancy and lacks deep understanding. Here, we present the design principles and characterization techniques of lattice-doped nFe0 from the point of view of microenvironment chemistry at both atomic and elemental levels, revealing their crystalline structure, electronic effects, and physicochemical properties. We summarize the current knowledge about the impacts of doping nonmetal p-block elements, transition-metal d-block elements, and hybrid elements into nFe0 crystals on their local coordination environment, which largely determines their structure-property-activity relationships. The materials' reactivity-selectivity trade-off can be altered via facile and feasible approaches, e.g., controlling doping elements' amounts, types, and speciation. We also discuss the remaining challenges and future outlooks of using lattice-doped nFe0 materials in real applications. This perspective provides an intuitive interpretation for the rational design of lattice-doped nFe0, which is conducive to real practice for efficient and selective environmental remediation.

Enhanced removal of cis-1,2-dichloroethene and vinyl chloride in groundwater using ball-milled sulfur- and biochar-modified zero-valent iron: From the laboratory to the field

Sulfidated zero-valent iron (ZVI) and biochar-supported ZVI have received increasing attention for their potential to dechlorinate trichloroethylene. However, minimal data are available regarding the combined effect of sulfur and biochar ZVI on trichloroethylene byproducts. The primary aim of the current study is to determine whether sulfur- and biochar-modified ZVI (ZVI-BC-S) enhances the removal of cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC) from groundwater. Results show that biochar and sulfur facilitated the milling of ZVI-BC-S into micro- and nanoscale particles and increased FeS formation. Moreover, the rates of cDCE and VC removal by ZVI-S increased by 30.1% and 30.2%, respectively, compared to those obtained with ZVI, owing to enhanced dechlorination via β-elimination by sulfur. Meanwhile, treatment with ZVI-BC-S harnessed the benefits of biochar and sulfur to enhance the cDCE and VC removal rates by 62.0% and 67.7%, respectively. Mechanistically, biochar enhanced the corrosion of ZVI-S to increase FeS production and enhance the electron transfer, β-elimination, and hydrogenolysis involved in cDCE and VC dechlorination. The effectiveness of ZVI-BC-S was confirmed in a field demonstration, during which cDCE and VC concentrations significantly decreased within 10 days following injection. The findings of this study can help inform the rational design of ZVI for in-situ remediation of chlorinated hydrocarbons in groundwater.

Nitrogen amended graphene catalyses fast reduction of vinyl chloride by nano zerovalent iron

Vinyl chloride (VC) is a dominant carcinogenic residual in many aged chlorinated solvent plumes, and it remains a huge challenge to clean it up. Zerovalent iron (ZVI) is an effective reductant for many chlorinated compounds but shows low VC removal efficiency at field scale. Amendment of ZVI with a carbonaceous material may be used to both preconcentrate VC and facilitate redox reactions. In this study, nitrogen-doped graphene (NG) produced by a simple co-pyrolysis method using urea as nitrogen (N) source, was tested as a catalyst for VC reduction by nanoscale ZVI (nZVI). The extent of VC reduction to ethylene in the presence of 2 g/L of nZVI was less than 1% after 3 days, and barely improved with the addition of 4 g/L of graphene. In contrast, with amendment of nZVI with NG produced at pyrolysis temperature (PT) of 950 °C, the VC reduction extent increased more than 10-fold to 69%. The reactivity increased with NG PT increasing from 400 °C to an optimum at 950 °C, and it increased linearly with NG loadings. Interestingly, N dosage had little effect on reactivity if NG was produced at PT of 950 °C, while a positive correlation was observed for NG produced at PT of 600 °C. XPS and Raman analyses revealed that for NG produced at lower PT (<800 °C) mainly the content of pyridine-N-oxide (PNO) groups correlates with reactivity, while for NG produced at higher PT up to 950 °C, reactivity correlates mainly with N induced structural defects in graphene. The results of quenching and hydrogen yield experiments indicated that NG promote reduction of VC by storage of atomic hydrogen, thus increasing its availability for VC reduction, while likely also enabling electron transfer from nZVI to VC. Overall, these findings demonstrate effective chemical reduction of VC by a nZVI-NG composite, and they give insights into the effects of N doping on redox reactivity and hydrogen storage potential of carbonaceous materials.

Tetrabromobisphenol A transformation by biochar supported post-sulfidated nanoscale zero-valent iron: Mechanistic insights from shell control and solvent kinetic isotope effects

Post-sulfidated nanoscale zero-valent iron with a controlled FeS_X shell thickness deposited on biochar (S-nZVI/BC) was synthesized to degrade tetrabromobisphenol A (TBBPA). Detailed characterizations revealed that the increasing sulfidation degree altered shell thickness/morphology, S content/speciation/distribution, hydrophobicity, and electron transfer capacity. Meanwhile, the BC improved electron transfer capacity and hydrophobicity and inhibited the surface oxidation of S-nZVI. These properties endowed S-nZVI/BC with highly reactive (∼8.9-13.2 times) and selective (∼58.4-228.9 times) over nZVI/BC in TBBPA transformation. BC modification improved the reactivity and selectivity of S-nZVI by 1.77 and 1.96 times, respectively. The difference of S-nZVI/BC in reactivity was related to hydrophobicity and electron transfer, particularly FeS_X shell thickness and morphology. Optimal shell thickness of ∼32 nm allowed the maximum association between Fe⁰ core and exterior FeS_X, resulting in superior reactivity. A thicker shell with abundant networks increased the roughness but decreased the surface area and electron transfer. The higher [S/Fe]_surface and [S/Fe]_particle were conducive to the selectivity, and [S/Fe]_particle was more influential than [S/Fe]_surface on selectivity upon similar hydrophobicity. The solvent kinetic isotope effects (SKIEs) exhibited that increasing [S/Fe]_dose tuned the relative contributions of atomic H and electron in TBBPA debromination but failed to alter the dominant debromination pathway (i.e., direct electron transfer) in (S)-nZVI/BC systems. Mechanism of electron transfer rather than atomic H contributed to higher selectivity. This work demonstrated that S-nZVI/BC was a prospective material for the remediation of TBBPA-contaminated groundwater.

Cations facilitate sulfidation of zero-valent iron by elemental sulfur: Mechanism and dechlorination application

The solid-solid reaction of microscale zero-valent iron (mZVI) with elemental sulfur (S0) in water can form sulfidated mZVI (S-mZVI) with high reactivity and selectivity. However, the inherent passivation layer of mZVI hinders the sulfidation. In this study, we demonstrate that ionic solutions of Me-chloride (Me: Mg2+, Ca2+, K+, Na+ and Fe2+) can accelerate the sulfidation of mZVI by S0. The S0 with S/Fe molar ratio of 0.1 was fully reacted with mZVI in all solutions to form unevenly distributed FeS species on S-mZVIs as confirmed by SEM-EDX and XANES characterization. The cations depassivated the mZVI surface by driving the proton release from the surface site (FeOH) and resulting in localized acidification. The probe reaction test (tetrachloride dechlorination) and open circuit potential (EOCP) measurement demonstrated that Mg2+ was most efficient in depassivating the mZVI and therefore promoting sulfidation. The decrease of surface proton for hydrogenolysis on the S-mZVI synthesized in MgCl2 solution also inhibited the formation of cis-1,2-dichloroethylene by 14-79% compared to other S-mZVIs during trichloroethylene dechlorination. In addition, the synthesized S-mZVIs exhibited the highest reduction capacity reported so far. These findings provide a theoretical basis for the facile on-site sulfidation of mZVI by S0 with cation-rich natural waters for sustainable remediation of contaminated sites.

Zero-valent iron based materials selection for permeable reactive barrier using machine learning

The zero-valent iron (ZVI) based reactive materials are potential remediation reagents in permeable reactive barriers (PRB). Considering that reactive materials is the essential to determining the long-term stability of PRB and the emergence of a large number of new iron-based materials. Here, we present a new approach using machine learning to screen PRB reactive materials, which proposes to improve the efficiency and practicality of selection of ZVI-based materials. To compensate for the insufficient amount of existing machine learning source data and the real-world implementation, machine learning combines evaluation index (EI) and reactive material experimental evaluations. XGboost model is applied to estimate the kinetic data and SHAP is used to improve the accuracy of model. Batch and column tests were conducted to investigate the geochemical characteristics of groundwater. The study find that specific surface area is a fundamental factor correlated with the kinetic constants of ZVI-based materials, according to SHAP analysis. Reclassifying the data with specific surface area significantly improved prediction accuracy (reducing RMSE from 1.84 to 0.6). Experimental evaluation results showed that ZVI had 3.2 times higher anaerobic corrosion reaction kinetic constants and 3.8 times lower selectivity than AC-ZVI. Mechanistic studies revealed the transformation pathways and endpoint products of iron compounds. Overall, this study is a successful initial attempt to use machine learning for selecting reactive materials.

Physicochemical Effects of Sulfur Precursors on Sulfidated Amorphous Zero-Valent Iron and Its Enhanced Mechanisms for Cr(VI) Removal

Amorphous zerovalent iron (AZVI) has gained considerable attention due to its remarkable reactivity, but there is limited research on sulfidated amorphous zerovalent iron (SAZVI) and the influence of different sulfur precursors on its reactivity remains unclear. In this study, SAZVI materials with an amorphous structure were synthesized using various sulfur precursors, resulting in significantly increased specific surface area and hydrophobicity compared to AZVI. The Cr(VI) removal efficiency of SAZVI-Na₂S, which exhibited the most negative free corrosion potential (-0.82 V) and strongest electron transfer ability, was up to 8.5 times higher than that of AZVI. Correlation analysis revealed that the water contact angle (r = 0.87), free corrosion potential (r = -0.92), and surface Fe(II) proportion (r = 0.98) of the SAZVI samples played crucial roles in Cr(VI) removal. Furthermore, the enhanced elimination ability of SAZVI-Na₂S was analyzed, primarily attributed to the adsorption of Cr(VI) by the FeS_x shell, followed by the rapid release of internal electrons to reduce Cr(VI) to Cr(III). This process ultimately led to the precipitation of FeCr₂O₄ and Cr₂S₃ on the surface of SAZVI-Na₂S, resulting in their removal from the water. This study provides insights into the influence of sulfur precursors on the reactivity of SAZVI and offers a new strategy for designing highly active AZVI for efficient Cr(VI) removal.

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.

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.

Sulfidated microscale zero-valent iron/reduced graphene oxide composite (S-mZVI/rGO) for enhanced degradation of trichloroethylene: The role of hydrogen spillover

Atomic hydrogen (H*) has long been thought to play an important role in the dechlorination of trichloroethylene (TCE) by carbon-supported zero-valent iron (ZVI), which offers an alternative pathway for TCE dechlorination. Herein, we demonstrate that the reductive dechlorination of TCE by sulfidated microscale ZVI (S-mZVI) can be further enhanced by promoting the formation of H* through the introduction of reduced graphene oxide (rGO). The completely degradation of 10 mg/L TCE can be achieved by S-mZVI/rGO within 24 h, which was 3.3 times faster than that of S-mZVI. The change in the distribution of TCE degradation products over time suggests that the introduction of rGO leads to a change in the dechlorination pathway. The percentage of ethane in the final products of TCE degradation by S-mZVI/rGO was 34.3 %, while that of S-mZVI was only 21.9 %. The electrochemical tests confirmed the occurrence of hydrogen spillover in the S-mZVI/rGO composite, which promoted the reductive dechlorination of TCE by H*. Although the S-mZVI/rGO composite had stronger hydrogen evolution propensity than S-mZVI, the S-mZVI/rGO composite still exhibited higher electron utilization efficiency than S-mZVI thanks to the increased utilization of hydrogen.

Removal of pharmaceutically active compounds (PhACs) by zero-valent iron: quantification of removal mechanisms consisting of degradation, adsorption and co-precipitation

The removal mechanisms of carbamazepine (CBZ), which is one of pharmaceutically active compounds, using zero-valent iron (ZVI) were quantified by defining three fractions, namely "degradation", "adsorption", and "co-precipitation". The maximum total organic carbon (TOC) removal was obtained at pH 4. The results demonstrate that the adsorption on the ZVI surface is dominant in the TOC removal of CBZ for 4 ≤ pH ≤ 6 while the degradation by oxidative and reductive reactions is efficient exclusively for pH ≤ 3. TOC removal was not obtained for pH ≥ 8. The most dominant mechanism in the removal of CBZ by ZVI is the adsorption onto the iron oxides/hydroxides layer formed on ZVI surface rather than the degradation by oxidative and reductive reactions including Fenton and Fenton-like reactions for pH ≥ 4. A novel kinetic model for removal of CBZ by ZVI was developed to simulate the dynamic concentration profiles of CBZ, TOC, total Fe ions, and dissolved oxygen linked closely with each other and the contributions of degradation, adsorption, and co-precipitation in TOC removal of CBZ. Reasonable agreement between experimental data and model predictions suggests the applicability of the proposed kinetic model to quantitatively analyze the mechanisms of CBZ removal by ZVI.

Fe-C-based materials: synthesis modulation for the remediation of environmental pollutants-a review

Presently, the rapid pace in the discovery of emerging aquatic pollutants is increasing the demand for the remediation and treatment of our natural resources. Regarding this, nanotechnology is being considered the potential solution for contaminated water remediation with techniques such as filtration, adsorption, catalysis, and desalination. For this purpose, zerovalent iron (ZVI) is being widely used in the remediation of environmental pollutants due to its large specific surface area and high reactivity. However, ZVI is easy to agglomerate and oxidize, limiting its application in the real environment. Therefore, the present study was designed to discuss the preparation and characterization methods of ZVI composite materials, factors affecting adsorption, the removal effect, and adsorption mechanism of different pollutants by Fe-C materials because the optimization and modification of nano-zero-valent iron is a hot research topic nowadays in this field. Moreover, this paper does also analyze the possibility of the practical application prospects of the team's technology for preparing iron-carbon materials. Thus, this information will be helpful for the development and application of Fe-C-based technologies for water and soil remediation and the prediction of the future research direction of Fe-C composite materials.

Mechanochemical synthesis of catalysts and reagents for water decontamination: Recent advances and perspective

This paper aims to provide insights on mechanochemistry as a green and versatile tool to synthesize advanced materials for water remediation. In particular, mechanochemical methodologies for preparation of reagents and catalysts for the removal of organic pollutants are reviewed and discussed, focusing on those materials that, directly or indirectly, induce redox reactions in the contaminants (i.e., photo-, persulfate-, ozone-, and Fenton-catalysts, as well as redox reagents). Methods reported in the literature include surface reactivity enhancement for single-component materials, as well as multi-component material design to obtain synergistic effects in catalytic efficiency and/or reactivity. It was also amply demonstrated that mechanochemical surface activation or the incorporation of catalytic/reactive components boost the generation of reactive species in water by accelerating charge transfer, increasing superficial active sites, and developing pollutant absorption. Finally, indications for potential future developments in this field are debated.

Pyrolyzed biomass-derived nanoparticles: a review of surface chemistry, contaminant mobility, and future research avenues to fill the gaps

Nanoparticles are abundant in the subsurface, soil, streams, and water bodies, and are often a critical control on elemental speciation, transport and cycling in the natural environment. This review provides an overview of pyrolyzed biomass-derived nanoparticles (PBNPs), their surface properties and reactivity towards aqueous species. We focus specifically on biochar-derived nanoparticles and activated carbon-derived nanoparticles which fall under our classification of PBNPs. Activated carbon-iron (nano)composites are included in some instances where there are significant gaps in literature because of their environmental relevance. Increased use of activated carbon, along with a resurgence in the manufacture and application of biochar for water treatment and soil amendment, has generated significant concerns about the mobility and toxicity of PBNPs derived from the bulk material in environmental applications. Recent examples are discussed to highlight current progress in understanding the influence of PBNPs on contaminant transport, followed by a critical discussion of gaps and future research directions.

A comparative study on the physicochemical properties, reactivity and long-term performance of sulfidized nanoscale zerovalent iron synthesized with different kinds of sulfur precursors and procedures in simulated groundwater

There are plentiful ways to synthesize sulfidized nanoscale zerovalent iron (S-nZVI), and this study investigated the influence of sulfur reagents (Na₂S, Na₂S₂O₃, Na₂S₂O₄) and sulfidation sequence (co-sulfidation and post-sulfidation method) on the physicochemical properties, reactivity, and long-term performance of S-nZVI in simulated groundwater. The results suggested that the co-sulfidized nZVI (S-nZVI_co) has higher reactivity (∼2-fold) than S-nZVI_post due to the stronger electron transfer capacity, deriving from the higher content of Fe⁰ and reductive sulfur species. However, during aging, the reactivity of S-nZVI_co would be lost more rapidly than S-nZVI_post, due to the faster corrosion of Fe⁰ and more oxidation of reductive sulfur species. S-nZVI_post has the superior long-term performance with the degradation rate of trichloroethylene (TCE) remained at 30%∼60% even after 90 d of aging. Sulfur precursors can control the selectivity of S-nZVI by affecting the sulfur speciation on the particle surface. The proportion of reductive sulfur species on S-nZVI_post synthesized by Na₂S was higher than S-nZVI_post synthesized by Na₂S₂O₃ or Na₂S₂O₄, resulting in a higher selectivity of the former S-nZVI_post than the latter S-nZVI_post. In addition, sulfidation procedures and sulfur precursors did not affect the degradation pathway of TCE. Nevertheless, the degradation product distribution can be affected by the different physicochemical transformation of various types of S-nZVI with the aging time. These results indicated that sulfur reagents and sulfidation procedures have crucial effects on the reactivity and long-term performance of S-nZVI, which can be designed for the specific application scenarios.

Evaluation of self-oxidation and selectivity of iron-based reductant in anaerobic pentachlorophenol contaminated soil

Soil contamination due to chlorinated organics prompts an important environmental problem; however, the iron-based reduction materials and complicated ground environment are the main barriers to implementation and promotion of in situ soil remediation. Therefore, this study aims to evaluate the reductants zero-valent iron (ZVI) and its activated carbon composite (AC-ZVI) in terms of their self-oxidation and selectivity in soil experiments. The results indicated that saturated moisture conditions were beneficial for degradation due to the dispersal of the pollutants from soil particles. Particularly, increasing the water/soil ratio to the over-saturated state would decrease the selectivity of ZVI and AC-ZVI. Meanwhile, increasing the reductant loading decreased the selectivity of ZVI and AC-ZVI, whereas the high initial concentration increased the selectivity of AC-ZVI. In addition, the self-oxidation of ZVI (3.0 ×10^-3 h^-1) is 4.2 times higher than that of AC-ZVI (0.7 ×10^-3 h^-1), and the selectivity of AC-ZVI (48%) is 6.9 times higher than that of ZVI (7%), which confirmed that AC-ZVI is a superior iron-based amendment in saturated moisture conditions. Therefore, this study provides a reliable and feasible evaluation method for in situ remediation process, and deepens the understanding of the effects of moisture contents.

Increasing the electron selectivity of nanoscale zero-valent iron in environmental remediation: A review

Nanoscale zero-valent iron nanoparticles (nZVI) have been used for groundwater remediation and wastewater treatment due to their high reactivity, high adsorption capacity and nontoxicity. However, side reactions generally occur in tandem with the target contaminants removal process, resulting in poor electron selectivity (ES) of nZVI, and subsequently restricting its commercial application. Major efforts to increase ES of nZVI have been made in recent years. This review's objective is to provide a progress report on the significant developments in nZVI's ES during the past decade. Firstly, the definition of ES and its quantification approaches were documented, and the intrinsic (i.e. particle size, crystallinity, and surface area) and extrinsic factors (i.e. solutions pH, target contaminant concentration, and presence of co-contaminants) affecting the ES of nZVI were reported. The latest techniques for increasing ES were summarized in detail, with reference made to sulfidation, magnetization, carbon loading and other features. Then the mechanisms of those strategies for ES enhancement were described. Finally, some constructive suggestions on future research directions concerning nZVI's ES in the future were proposed.

Stimulation of pyrolytic carbon materials as electron shuttles on the anaerobic transformation of recalcitrant organic pollutants: A review

Pyrolytic carbon materials (PCMs) with various surface functionalities are widely used as environmentally friendly and cost-efficient adsorbents for the removal of organic and inorganic pollutants. Recent studies have illustrated that PCMs as electron shuttles (ESs) could also show excellent performances in promoting the anaerobic transformation of recalcitrant organic pollutants (ROPs). Numerous studies have demonstrated the excellent electron-shuttle capability (ESC) of PCMs to stimulate the anaerobic reductive transformation of ROPs. However, there is a lack of consistent understanding of the mechanism of ESC formation in PCMs and the stimulation mechanism for ROPs anaerobic transformation. To gain a more comprehensive understanding of the latest developments in the study of PCMs as ESs for ROPs anaerobic transformation, this review summarizes the formation mechanism, influencing factors, and stimulation mechanisms of ESC. ESC benefits from redox functional groups (quinone and phenol groups), persistent free radicals (PFRs), redox-active metal ions, conductive graphene phase, and porous nature of their surface. The factors influencing ESC include the highest treatment temperature (HTT), feedstocks, modification methods, and environmental conditions, of which, the HTT is the key factor. PCMs promote the reductive transformation of ROPs under anaerobic conditions via abiotic and biotic pathways. Eventually, the prospects for the ROPs anaerobic transformation enhanced by PCMs are proposed.

Coincorporation of N and S into Zero-Valent Iron to Enhance TCE Dechlorination: Kinetics, Electron Efficiency, and Dechlorination Capacity

Sulfidated zero-valent iron (S-ZVI) enhances the degradation of chlorinated hydrocarbon (CHC) in contaminated groundwater. Despite numerous studies of S-ZVI, a versatile strategy to improve its dechlorination kinetics, electron efficiency (ε_e), and dechlorination capacity is still needed. Here, we used heteroatom incorporation of N(C) and S by ball-milling of microscale ZVI with melamine and sulfur via nitridation and sulfidation to synthesize S-N(C)-mZVI^bm particles that contain reactive Fe-N_X(C) and FeS species. Sulfidation and nitridation synergistically increased the trichloroethene (TCE) dechlorination rate, with reaction constants k_SA of 2.98 × 10^-2 L·h^-1·m^-2 by S-N(C)-mZVI^bm, compared to 1.77 × 10^-3 and 8.15 × 10^-5 L·h^-1·m^-2 by S-mZVI^bm and N(C)-mZVI^bm, respectively. Data show that sulfidation suppressed the reductive dissociation of N(C) from S-N(C)-mZVI^bm, which stabilized the reactive Fe-N_X(C) and reserved electrons for TCE dechlorination. In addition to lowering H₂ production, S-N(C)-mZVI^bm dechlorinated TCE to less reduced products (e.g., acetylene), contributing to the material's higher ε_e and dechlorination capacity. This synergistic effect on TCE degradation can be extended to other recalcitrant CHCs (e.g., chloroform) in both deionized and groundwater. This multiheteroatom incorporation approach to optimize ZVI for groundwater remediation provides a basis for further advances in reactive material synthesis.

Exploring the role of Fe species from biochar-iron composites in the removal and long-term immobilization of SeO42- against competing oxyanions

Carbothermal reduction is a convenient and cost-effective method to produce biochar (BC) supported iron-based nano-particles (INP) for oxyanion contaminants removal. However, considering the possible desorption of the target oxyanion during change of the surrounding environment, the detailed removal mechanisms remain unclear and the long-term efficiency of different INPs cannot be predicted. In this study, different BC/Fe composites were synthesized by controlling the pyrolysis temperatures (500-800 °C). BC/Fe₃O₄ composite synthesized at 500 °C (BC/Fe₅₀₀) possessed the strongest surface acidity thus with the best SeO₄^2- removal performance, and BC/Fe⁰/Fe₃O₄ composite synthesized at 650 °C (BC/Fe₆₅₀) possessed the best reducing ability toward SeO₄^2-. Through the co-removal experiments (SeO₄^2- and common competing oxyanions co-existed) and the investigation of Se stability loaded on BC/Fe composites, the removal of SeO₄^2- by BC/Fe₅₀₀ through highly reversible adsorption could not achieve long-term immobilization of Se, making it an appropriate adsorbent for pre-treatment only, while the efficient reduction of SeO₄^2- to Se⁰ by BC/Fe₆₅₀ could largely improve its long-term stability. This study supplies a possible strategy for Se immobilization against common competing oxyanions.

A novel ball-milled aluminum-carbon composite for enhanced adsorption and degradation of hexabromocyclododecane

Hexabromocyclododecane (HBCD) is one of the priority persistent organic pollutants (POPs), yet a cost-effective technology has been lacking for the removal and degradation of HBCD. Zero-valent aluminum (ZVAl) is an excellent electron donor. However, the inert and hydrophilic surface oxide layer impedes the release of the electrons from the core metallic Al, resulting in poor reactivity towards HBCD. In this research, a new type of modified mZVAl particles (AC@mZVAl^bm/NaCl) were prepared through ball milling mZVAl in the presence of activated carbon (AC) and NaCl, and tested for adsorption and reductive degradation of HBCD in water. AC@mZVAl^bm/NaCl was characterized with a metallic Al core with newly created reactive surface coated with a thin layer of crushed carbon nanoparticles. AC@mZVAl^bm/NaCl was able to rapidly (within 1 h) adsorb HBCD (C₀ = 2 mg L^-1) and thus effectively enriched HBCD on the carbon surface of AC@mZVAl^bm/NaCl. The pre-enriched HBCD was subsequently degraded by the electrons from the core Al, and ∼63.44% of the pre-sorbed HBCD was completely debrominated after 62 h of the contact. A notable time lag (∼12 h) from the onset of the adsorption to the debromination was observed, signifying the importance of the solid-phase mass transfer from the initially adsorbed AC particles to the reactive Al-AC interface. Overall, AC@mZVAl^bm/NaCl synergizes the adsorptive properties of AC and the high reactivity of metallic Al, and enables a novel two-step adsorption and reductive degradation process for treating HBCD or likely other POPs.

Citrate ligand-enhanced microscale zero-valent aluminum corrosion for carbon tetrachloride degradation with high electron utilization efficiency

Carbon tetrachloride (CT) is highly toxic and recalcitrant in groundwater. In recent years, zero-valent aluminum (ZVAl) is highly reductive but limited by its surface passivation film. One of the effective ways to overcome this bottleneck is to add ligands. In this paper, compared with several other ligands, sodium citrate (SC), a natural organic ligand, was introduced to enhance microscale ZVAl (mZVAl) reactivity for the reductive degradation of CT. The results showed that the SC system could effectively reduce but not completely dechlorinate CT and electron utilization efficiency was as high as 94%. However, without ligands, mZVAl is chemically inert for CT degradation. Through SEM-EDS, BET, XRD, and XPS characterizations and H₂ evolution experiments, enhanced mZVAl surface corrosion at the solid-liquid interface of mZVAl/SC system was verified. SC participated in the complexation corrosion reaction with surface inert film to form Al[Cit] complex, which made internal Al⁰ active sites exposed and then promoted mZVAl corrosion. In the five consecutive reuse experiments of mZVAl, CT can be completely degraded, which indicates that mZVAl, with the help of SC, has excellent sustainable utilization efficiency.

New insights into co-treatment of mature landfill leachate with municipal sewage via integrated partial nitrification, Anammox and denitratation

As a low consumption and high efficiency process, Partial Nitrification-Anammox/denitratation (PNAD) was applied to co-treat mature landfill leachate with municipal sewage for 300 days. Specifically, ammonia (670.2 ± 63.7 mg N/L) contained in mature landfill leachate was firstly oxidized to nitrite (611.5 ± 28.1 mg N/L) in sequence batch reactor (SBR_PN); meanwhile, organic matter in municipal sewage was partially removed in another reactor (SBR_OMR); finally, nitrite produced (611.5 ± 28.1 mg N/L) in SBR_PN and ammonia (53.1 ± 6.4 mg N/L) residing in pretreated municipal sewage were simultaneously degraded through combined Anammox-denitratation process in an up-flow anaerobic sludge bed (UASB_AD). A satisfactory effluent quality of 10.3 mg/L TN was obtained after long-term operation, with Anammox and denitrification contributing to 86.2% and 5.8% nitrogen removal efficiency, respectively. Mass balance confirmed 67.2% nitrate generated from Anammox could be reduced to nitrite and in-situ reused. Anammox bacteria genes and nitrate reductase/nitrite reductase ratio were highly detected, accelerating combined Anammox-denitratation. Further, Ca. Brocadia triumph among various Anammox bacteria groups, increasing from 1.2% (day 120) to 3.6% (day 280).

Recent Advances in Magnetic Nanoparticles and Nanocomposites for the Remediation of Water Resources

Water resources are of extreme importance for both human society and the environment. However, human activity has increasingly resulted in the contamination of these resources with a wide range of materials that can prevent their use. Nanomaterials provide a possible means to reduce this contamination, but their removal from water after use may be difficult. The addition of a magnetic character to nanomaterials makes their retrieval after use much easier. The following review comprises a short survey of the most recent reports in this field. It comprises five sections, an introduction into the theme, reports on single magnetic nanoparticles, magnetic nanocomposites containing two of more nanomaterials, magnetic nanocomposites containing material of a biologic origin and finally, observations about the reported research with a view to future developments. This review should provide a snapshot of developments in what is a vibrant and fast-moving area of research.