Nanoscale zero-valent iron for metal/metalloid removal from model hydraulic fracturing wastewater

Insights into micro-and nano-zero valent iron materials: synthesis methods and multifaceted applications

The growing threat of environmental pollution to global environmental health necessitates a focus on the search for sustainable wastewater remediation materials coupled with innovative remediation strategies. Nano and micro zero-valent iron materials have attracted substantial researchers' attention due to their distinct physiochemical properties. This review article delves into novel micro- and nano-zero valent iron (ZVI) materials, analysing their synthesis methods, and exploring their multifaceted potential as a powerful tool for environmental remediation. This analysis contributes to the ongoing search of effective solutions for environmental remediation. Synthesis techniques are analysed based on their efficacy, scalability, and environmental impact, providing insights into existing methodologies, current challenges, and future directions for optimisation. Factors influencing ZVI materials' physicochemical properties and multifunctional engineering applications, including their role in wastewater and soil remediation, are highlighted. Environmental concerns, pros and cons, and the potential industrial applications of these materials are also discussed, accenting the importance of understanding the synthesis methods, materials' applications and their impacts on humans and the environment. The review is designed to provide insights into nano-and micro-ZVI materials, and their potential engineering applications, as well as guide researchers in the choice of ZVI materials' synthesis methods from a variety of nanoparticle synthesis strategies fostering nexus between these methods and industrial applications.

Applying Red Mud in Cadmium Contamination Remediation: A Scoping Review

Red mud is an industrial solid waste rarely utilized and often disposed of in landfills, resulting in resource waste and environmental pollution. However, due to its high pH and abundance of iron and aluminum oxides and hydroxides, red mud has excellent adsorption properties which can effectively remove heavy metals through ion exchange, adsorption, and precipitation. Therefore, red mud is a valuable resource rather than a waste byproduct. In recent years, red mud has been increasingly studied for its potential in wastewater treatment and soil improvement. Red mud can effectively reduce the migration and impact of heavy metals in soils and water bodies. This paper reviews the research results from using red mud to mitigate cadmium pollution in water bodies and soils, discusses the environmental risks of red mud, and proposes key research directions for the future management of red mud in cadmium-contaminated environments.

Stereoselective analysis of chiral pesticides: a review

Chiral organic pollutants, including pesticides, herbicides, medicines, flame retardants, and polycyclic musk, represent a significant threat to both the environment and human health. The presence of asymmetric centers in the structure of chiral pesticides introduces stereoisomers with distinct distributions, fates, biomagnification capacities, and cytotoxicities. In aquatic environments, pesticides, as persistent/pseudo-persistent compounds, have been detected in substantial quantities, posing severe risks to non-target species and, ultimately, public health through water supply and food exposures. In response to this environmental challenge, stereoselective analytical methods have gained prominence for the identification of pesticide/drug enantiomers in recent years. This review examines the environmental impact of chiral pesticides, emphasizing the distinct biological activities and distribution patterns of their stereoisomers. By highlighting the advancements in liquid chromatography for enantiomeric analysis, the review aims to underscore the urgent need for a comprehensive understanding of these pollutants to facilitate informed remediation strategies and ensure the safer dispersal of chiral organic pollutants in the environment, thereby addressing the potential risks they pose to ecosystems and human health. Future research should focus on developing sustainable and efficient methodologies for the precise analysis of stereoisomers in complex matrices, particularly in sewage water, emphasizing the importance of sewage processing plants in ensuring water quality.

Influence of CoOx surface passivation and Sn/Zr-co-doping on the photocatalytic activity of Fe2O3 nanorod photocatalysts for bacterial inactivation and photo-Fenton degradation

Hydrothermal and wet impregnation methods are presented in this study for synthesizing CoO_x(1 wt%)/Sn-Zr codoped-Fe₂O₃ nanorod photocatalysts for the degradation of organic pollutants and deactivation of bacteria. A hydrothermal route was used to synthesize self-assembled rod-like hierarchical structures of Sn(0-6%) doped Zr-Fe₂O₃ NRs. Additionally, a wet impregnation method was used to load CoO_x onto the surface of photocatalysts (Sn(0-6%)-doped Zr-Fe₂O₃ NRs). A series of 1 wt% CoO_x modified Sn(0-6%)-doped Zr-Fe₂O₃ NRs were synthesized, characterized, and utilized for the photocatalytic decomposition of organic contaminants, along with the killing of E. coli and S. aureus. In comparison with 0, 2, and 6% Sn co-doped Zr-Fe₂O₃ NRs, the CoO_x(1 wt%)/4%Sn/Zr-Fe₂O₃ NRs photocatalyst exhibited an E. coli and S. aureus inactivation efficiencies (90 and 98%). A bio-TEM study of treated and untreated bacterial cells revealed that the CoO_x(1 wt%)/4%Sn/Zr-Fe₂O₃ NRs photocatalyst led to considerable changes in the bacterial cell membranes' morphology. The optimal CoO_x(1 wt%)/Sn(4%) co-doped Zr-Fe₂O₃ NRs photocatalyst achieved degradation efficiencies of 98.5% and 94.6% for BPA and orange II dye. As a result, this work will provide a facile and effective method for developing visible light-active photocatalysts for bacterial inactivation and organic pollutants degradation.

Mechanistic insights into sulfate and phosphate-mediated hexavalent chromium removal by tea polyphenols wrapped nano-zero-valent iron

Nano zero-valent iron via green synthesis (g-nZVI) has great potential in removing toxic hexavalent Cr(VI) from industrial wastewater. Sulfate and phosphate in wastewater can influence Cr(VI) removal by g-nZVI. In this study, the Cr(VI) removal kinetics by different g-nZVI materials were investigated with the existence of sulfate and/or phosphate, and the corresponding mechanisms were first revealed using multiple characterizations, including X-ray absorption near-edge spectra (XANES) and X-ray photoelectron spectroscopy (XPS). The results showed that Cr(OH)₃ was the dominant species initially formed on the surface of g-nZVI particles before transforming to Cr₂O₃ during the reaction of g-nZVI with Cr(VI). Sulfate in wastewater can promote the reduction from Cr(VI) to Cr(OH)₃ by g-nZVI, because sulfate triggers the release of Fe(II) and tea polyphenols (from tea extracts) from the g-nZVI surface due to the corrosion of Fe⁰ core, which is in line with an obvious increase in pseudo-second-order rate constant (k₂) and subtle change in Cr(VI) removal capacity (q_e). However, phosphate impedes the g-nZVI corrosion and inhibits q_e because of the inner-sphere complexation of phosphate onto g-nZVI decreasing the released Fe(II) for Cr₂O₃ production. When sulfate and phosphate coexisted in contaminated water, the inhibition effect of phosphate in Cr(VI) removal by g-nZVI was stronger than the promotion of sulfate. Accordingly, q_e value of g-nZVI declined from 93.4 mg g^-1 to 77.5 mg g^-1, while k₂ remained constant as the molar ratio of phosphate/sulfate increased from 0.1 to 10 in water. This study provides new insights into applying g-nZVI in efficient Cr(VI) removal from contaminated water with enrichment of sulphates and phosphates.

Removal of nitrate and pesticides from groundwater by nano zero-valent iron injection pulses under biostimulation and bioaugmentation scenarios in continuous-flow packed soil columns

This study evaluates the NO₃^- removal from groundwater through Heterotrophic Denitrification (HDN) (promoted by the addition of acetate and/or an inoculum rich in denitrifiers) and Abiotic Chemical Nitrate Reduction (ACNR) (promoted by pulse injection of zerovalent iron nanoparticles (nZVI)). HDN and ACNR were applied, separately or combined, in packed soil column experiments to complement the scarce research on pulse-injected nZVI in continuous-flow systems mimicking a Well-based Denitrification Barrier. Together with NO₃^-, the removal of two common pesticides (dieldrin and lindane) was evaluated. Results showed that total NO₃^- removal (>97%) could be achieved by either bioestimulation with acetate (converting NO₃^- to N₂(g) via HDN) or by injecting nZVI (removing NO₃^- via ACNR). In the presence of nZVI, NO₃^- was partially converted to N₂(g) and to a lower extent NO₂^-, with unreacted NO₃^- being likely adsorbed onto Fe-(oxy)hydroxides. Combination of both HDN and ACNR resulted in even a higher NO₃^- removal (>99%). Interestingly, nZVI did not seem to pose any toxic effect on denitrifiers. These results showed that both processes can be alterned or combined to take advantage of the benefits of each individual process while overcoming their disadvantages if applied alone. With regard to the target pesticides, the removal was high for dieldrin (>93%) and moderate for lindane (38%), and it was not due to biodegradation but to adsorption onto soil. When nZVI was applied, the removal increased (generally >91%) due to chemical degradation by nZVI and/or adsorption onto formed Fe-(oxy)hydroxides.

Effects of Red Mud on Cadmium Uptake and Accumulation by Rice and Chemical Changes in Rhizospheres by Rhizobox Method

Red mud (RM), a byproduct of aluminum production, is used as amendments to increase the pH and reduce the available Cd in soil, but the effects of RM treatments on rice and rhizosphere chemistry changes at different radial-oxygen-loss (ROL) rates and developmental stages remain unclear. To address this concern, a rhizobox trial was conducted to investigate the effect of 0%, 0.5%, and 1.0% RM, on Cd accumulation by rice cultivars differing in ROL rate (‘Zheyou12’ (ZY12), ‘Qianyou1’ (QY1), and ‘Chunjiangnuo2’ (CJN2)) at two growth stages (tillering and bolting). The results showed that mobility factors of Cd in the soil were decreased significantly at both stages. The Cd mobility factor (MF) of CJN2 was decreased by 33.01% under 1% RM treatment at bolting stage. The pH value was increased by 0.39–0.53 units at two stages. RM contains large amounts of metals, which can increase soil iron (Fe) and manganese (Mn) concentrations, reduce redox potential, and transform the available Cd into Fe/Mn oxide-bound Cd. In addition, the Fe plaque further increased to inhibit the transformation of Cd. These changes reduced the available Cd in the soil and further decreased Cd absorption by rice. With the increase in RM concentration, the shoot and root biomass increased, and Cd accumulation in the plant significantly decreased. Compared with that under 0% RM treatment, the shoot Cd concentrations of ZY12, QY1, and CJN2 under 1% RM treatment at the bolting stage decreased by 27.59%, 36.00%, and 46.03%, respectively. The relative Cd accumulation ability of the three rice cultivars was CJN2 < QY1 < ZY12. The ROL promotes Fe plaque formation on the root surface. The Fe plaque is an obstacle or buffer between Cd and rice, which can immobilize Cd in Fe plaque and further reduce Cd absorption by rice. The addition of RM, in combination with a high-ROL rice cultivar, is a potential strategy for the safe production of rice on Cd-contaminated soils.

Rhizobacteria communities reshaped by red mud based passivators is vital for reducing soil Cd accumulation in edible amaranth

Red mud (RM) was constantly reported to immobilize soil cadmium (Cd) and reduce Cd uptake by crops, but few studies investigated whether and how RM influenced rhizobacteria communities, which was a vital factor determining Cd bioavailability and plant growth. To address this concern, high-throughput sequencing and bioinformatics were used to analyze microbiological mechanisms underlying RM application reducing Cd accumulation in edible amaranth. Based on multiple statistical models (Detrended correspondence analysis, Bray-Curtis, weighted UniFrac, and Phylogenetic tree), this study found that RM reduced Cd content in plants not only through increasing rhizosphere soil pH, but by reshaping rhizobacteria communities. Special taxa (Alphaproteobacteria, Gammaproteobacteria, Actinobacteriota, and Gemmatimonadota) associated with growth promotion, anti-disease ability, and Cd resistance of plants preferentially colonized in the rhizosphere. Moreover, RM distinctly facilitated soil microbes' proliferation and microbial biofilm formation by up-regulating intracellular organic metabolism pathways and down-regulating cell motility metabolic pathways, and these microbial metabolites/microbial biofilm (e.g., organic acid, carbohydrates, proteins, S^2-, and PO₄^3-) and microbial cells immobilized rhizosphere soil Cd via the biosorption and chemical chelation. This study revealed an important role of reshaped rhizobacteria communities acting in reducing Cd content in plants after RM application.

Detection and treatment of organic matters in hydraulic fracturing wastewater from shale gas extraction: A critical review

Although shale gas has shown promising potential to alleviate energy crisis as a clean energy resource, more attention has been paid to the harmful environmental impacts during exploitation. It is a critical issue for the management of shale gas wastewater (SGW), especially the organic compounds. This review focuses on analytical methods and corresponding treatment technologies targeting organic matters in SGW. Firstly, detailed information about specific shale-derived organics and related organic compounds in SGW were overviewed. Secondly, the state-of-the art analytical methods for detecting organics in SGW were summarized. The gas chromatography paired with mass spectrometry was the most commonly used technique. Thirdly, relevant treatment technologies for SGW organic matters were systematically explored. Forward osmosis and membrane distillation ranked the top two most frequently used treatment processes. Moreover, quantitative analyses on the removal of general and single organic compounds by treatment technologies were conducted. Finally, challenges for the analytical methods and treatment technologies of organic matters in SGW were addressed. The lack of effective trace organic detection techniques and high cost of treatment technologies are the urgent problems to be solved. Advances in the extraction, detection, identification and disposal of trace organic matters are critical to address the issues.

Portable SA/CMC entrapped bimetallic magnetic fly ash zeolite spheres for heavy metals contaminated industrial effluents treatment via batch and column studies

Heavy metals are perceived as a significant environmental concern because of their toxic effect, bioaccumulation, and persistence. In this work, a novel sodium alginate (SA) and carboxymethylcellulose (CMC) entrapped with fly ash derived zeolite stabilized nano zero-valent iron and nickel (ZFN) (SA/CMC-ZFN), followed by crosslinking with CaCl2, is synthesized and applied for remediation of Cu(II) and Cr(VI) from industrial effluent. The characterization of the adsorbent and its surface mechanism for removing metals were investigated using advanced instrumental techniques, including XRD, FT-IR, SEM-EDX, BET, and XPS. The outcomes from the batch experiments indicated that monolayer adsorption on homogeneous surfaces (Langmuir isotherm model) was the rate-limiting step in both heavy metals sorption processes. The maximum adsorption capacity of as-prepared SA/CMC-ZFN was 63.29 and 10.15 mg/g for Cu(II) and Cr(VI), respectively. Owing to the fact that the wastewater released from industries are large and continuous, a continuous column is installed for simultaneous removal of heavy metal ions from real industrial wastewater. The outcomes revealed the potential of SA/CMC-ZFN as an efficient adsorbent. The experimental breakthrough curves fitted well with the theoretical values of Thomas and Yoon-Nelson models. Overall, the results indicated that SA/CMC-ZFN is a viable, efficient, and cost-effective water treatment both interms of batch and column processes.

Confining polyacrylic acid on the surface of nanoscale zero-valent iron by aluminum hydroxide for in-situ anti-passivation

Aggregation and surface passivation of nanoscale zero-valent iron (NZVI) particles have limited their reactivity and application for environmental remediation. In this study, an aluminum hydroxide/polyacrylic acid (Al(OH)₃/PAA) hybrid shell was homogeneously coated on the NZVI surface to overcome the limitations. PAA molecules were confined onto the NZVI surface by hydration of Al(III) cations. The Al(OH)₃/PAA coating shell provided more electrostatic repulsion forces between NZVI particles to hinder the particle aggregation and preserve the NZVI reactivity. On the other hand, the surface-anchored PAA provided a thickened reactive layer for Cr(VI) reduction. Besides, XPS and TEM results showed that the surface carboxylic groups bound produced Cr(III) and Fe(III) cations and inhibited the precipitation of hydroxides on the NZVI surface. The reduced passivation layer increased the longevity of NZVI for surface reactions. As a result, the 24-h Cr(VI) reduction capacity of NZVI particles was improved from 49.4 to 92.6 mg/g with a 2 wt% (Al/Fe) Al(OH)₃/PAA coating shell. Overall, this study presented a promising strategy to effectively tune the surface properties of nanoparticles and improve the feasibility of NZVI for environmental remediation.

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.

Performance of a novel iron infused biochar developed from Raphanus sativus and Artocarpus heterophyllus refuse for trivalent and pentavalent arsenic adsorption from an aqueous solution: mechanism, isotherm and kinetics study

Fabrication of magnetic biochar was done by pyrolysis of waste leaves of Raphanus sativus (MRB) and Artocarpus heterophyllus (MJB) peel pretreated with FeCl₃ was examined for As(III and V) adsorption from an aqueous solution. The synthesized bioadsorbents were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), particle size analysis (PSA), scanning electron microscope (SEM), energy dispersive x-ray (EDX), zeta potential, Vibrating sample magnetometer (VSM) and point of zero charge (pH_ZPC). MRB-800 exhibits greater efficiency toward the removal of both As species with q_max value 2.08 mg/g for As(III) and 2.03 mg/g for As(V). Whereas, the q_max value was 1.13 mg/g for As (III) and 1.26 mg g^-1 for As (V) adsorption using MJB-800. Temkin and Freundlich isotherm were best fitted to the adsorption of As(III) and As(V) by MRB-800, respectively. Langmuir isotherm was best followed to the adsorption of As (III and V) by MJB-800. Pseudo-second-order kinetics was well simulated by the experimental data of As adsorption using both the bioadsorbents. Surface complexation and electrostatic attraction was dominant mechanism for As (III) and As (V) adsorption. Thermodynamic study shows that removal of As (III) was exothermic while the As (V) adsorption was endothermic for MRB-800 and MJB-800.

Highly efficient nano-Fe/Cu bimetal-loaded mesoporous silica Fe/Cu-MCM-41 for the removal of Cr(VI): Kinetics, mechanism and performance

Zero valent iron (Fe⁰) can reduce Cr(VI) in water, where Fe⁰ and Fe(Ⅱ) are possible electron donors, but passivation and aggregation easily occur to Fe⁰. To improve the performance of Fe⁰, a new hybridization strategy of Fe/Cu bimetal and silica-based mesoporous molecular sieve MCM-41 for the removal of Cr(VI) from water has been proposed. The results show that the two-dimensional mesoporous structure of MCM-41 can provide skeleton support for Fe⁰, improve the mass transfer rate, and overcome the aggregation bottleneck of Fe⁰. The Cr(VI) removal rate reached 98.98% (pH = 2) after 40 min. The analytical results revealed Cr(VI) removal process: Cr(VI) adsorbed onto Fe/Cu-MCM-41 by electrostatic attraction and other molecular inter-atomic forces. The second metal, Cu, can inhibit the passivation of Fe⁰ and promote Fe(Ⅱ)through the formation of Fe/Cu battery, thereby promoting the electron transfer. The resulting Cr(Ⅲ) is precipitated as FeCr₂O₄ and Cr_xFe_1-x(OH)₃.

Applications of red mud as an environmental remediation material: A review

Red mud is an alkaline by-product produced by alumina plants. The accumulation of red mud is becoming an increasingly serious problem with the growth of the aluminum industry. Various waste treatment methods utilizing red mud as an environmental remediation material have been developed. Red mud environmental remediation materials (RM-ERMs) are environmental remediation materials prepared by activating red mud, synergistically using red mud and other ingredients, or by extracting effective components from red mud. There are three general categories of use for RM-ERMs: for waste water purification, waste gas purification and soil remediation. As well as providing an opportunity to improve the environment through purification technologies, the highly alkaline red mud is consumed in the production of RM-ERMs. The use of RM-ERMs has been shown to be a promising strategy for the simultaneous treatment of various wastes. In this paper, the developregeneration characteristics of various red mud granularent status of RM-ERMs is described, the physical and chemical properties of red mud are introduced, and the active mechanism of RM-ERMs on target pollutants in waste water, waste gas and soil is summarized. Moreover, a discussion on the current existing problems of RM-ERMs provides important tips and suggestions for future research on RM-ERMs.

Mechanisms of enhanced hexavalent chromium removal from groundwater by sodium carboxymethyl cellulose stabilized zerovalent iron nanoparticles

Chromium (Cr) contamination poses serious threats to the environment and human health. Thus, batch and column experiments were performed to investigate hexavalent chromium [Cr (VI)] removal from solution and porous media using nanoscale zerovalent iron nanoparticles (NZVI) stabilized by sodium carboxymethyl cellulose (CMC). Batch experiments indicated that the mass ratio of Fe/CMC = 1, the presence of 150-200 mg L^-1 CMC and lower ionic strength led to optimum Cr (VI) removal in aqueous solution. Column experiments demonstrated that Cr (VI) removal was enhanced with decreasing solution pH and increasing CMC-NZVI concentration. The presence of CMC can increase Cr (VI) removal by NZVI in both aqueous solution and porous media by complexation precipitation of Cr (VI) compounds and better dispersion of NZVI. X-ray photoelectron spectroscopy (XPS) analysis revealed that an appropriate amount of CMC supported the redox reaction of Cr (VI) and NZVI. The removal of Cr (VI) through columns was 20.8% and 88.5% under no additional CMC and optimized CMC content, respectively. However, Cr (VI) removal decreased to 64.6% under excessive CMC content. The CMC modified NZVI nanoparticles were characterized by XRD, XPS and TEM techniques. These findings imply that CMC can be used as an effective stabilizer on NZVI which can in turn be applied for the efficient removal of Cr (VI) from industrial wastewater and groundwater.

Practical application potential of microbial-phosphorus minerals-alginate immobilized particles on chromium(VI)-bioreduction

Due to the widespread use of chromium (Cr) across various industrial processes, the process of in-situ remediation of Cr-contaminated groundwater has received extensive attention. Previous studies of solid-phase phosphorus sources and microbial immobilization co-strengthening materials have demonstrated that their performance in continuous flow reactions is of great significance towards practical application of these technologies. It was suggested that Microbial-Phosphorus minerals-Alginate (MPA) immobilized particles showed superior performance (high Cr removal efficiency, low phosphorus surplus, and high environmental resistance) in comparisons of non-immobilization systems and different immobilization methods under continuous flow conditions. Microbial community analysis revealed significant differences between different systems as well as between variations in environmental factors, providing further support for the above conclusions. Synthetic wastewater (synthesized by actual groundwater) was also introduced to further verify the practical application potential of MPA immobilized particles. The results of this study provide a new insight and relevant bench scale data to support the enhancement of in-situ Cr(VI) bioremediation.

Urea formaldehyde modified alginate beads with improved stability and enhanced removal of Pb2+, Cd2+, and Cu2

Urea formaldehyde (UF) was grafted onto the backbone of alginate to prepare microbeads as an adsorbent for the removal of heavy metal ions from aqueous solutions. The expensive alginate was crosslinked with cheaper UF at different ratios (1: 2.5∼1: 12.5) to produce sturdy alginate-UF beads at lower cost. Characterization results showed that UF modification enhanced the pore network and structural stability of the beads, which can be attributed to the reduced intermolecular forces and plentiful of nitrogen and oxygen donor atoms of the beads. The swelling of air-dried alginate-UF beads in different solutions was much lower than that of the unmodified alginate beads, confirming the improved stability. The replacement of alginate with UF at different ratios either did not affect or increased the adsorption of heavy metal ions (Pb²⁺, Cd²⁺, and Cu²⁺) on the beads. For example, the adsorption capacities of Pb²⁺, Cd²⁺, and Cu²⁺ on air-dried alginate-UF (1: 2.5) beads were 1.66, 0.61, and 0.80 mmol/g, which were 39.88%, 9.29%, and 9.52% higher than those of the corresponding unmodified alginate beads, respectively. The adsorption of heavy metals on the alginate-UF beads was mainly controlled by ion exchange, complexation, and electrostatic interaction mechanisms.

Nano-Fe mediated treatment of real hydraulic fracturing flowback and its practical implication on membrane fouling in tandem anaerobic-oxic membrane bioreactor

The rising water-use intensity, and lack of cost-effective treatment strategy and reuse of hydraulic fracturing flowback (HFF) has become an increasing cause of concern. The present work evaluates the integration of parallel sets of tandem anaerobic-oxic membrane bioreactor (AMBR) with and without nano-Fe for treatment and reuse of real HFF obtained from Ordos Basin, China. Treatment efficiencies in terms of organic conversions, micro-pollutants degradation, resource recovery, and effects of nano-Fe release on membrane fouling were evaluated. Nano-Fe mediated AMBR (FAMBR) system effectively reduce target micro-pollutants (such as Acenaphthylene) at 94.4 % compared to the parallel AMBR system (17.1 % without nano-Fe). Moreover, recovery of potential economic chemicals like Al and P (1.0 and 0.6 mg/g spent nano-Fe) availed using FAMBR system. However, colonization of FAMBR membrane surface by Fe-protein/peptide hydroxocomplexes initiated by Fe-catalyzed microbial extrusions present a huge fouling challenge relative to the AMBR system. Additional evidences from microscopic/spectroscopic analysis of the FAMBR membrane system revealed that despite having a promising outlook, mediation of nano-Fe with AMBR system might result in a major fouling event during HFF treatment. Engineered design of nano-Fe to reduced leached nano-Fe ions in pre-treatment step prior to AMBR treatment system may be of potential research consideration.

Co-sorption of Sr2+ and SeO42- as the surrogate of radionuclide by alginate-encapsulated graphene oxide-layered double hydroxide beads

Graphene oxides (GO) and layered double hydroxides (LDHs) were applied to produce alginate beads for the remove of ⁹⁰Sr²⁺ and ⁷⁹SeO₄^2-. The Freundlich isotherm indicated that the Sr²⁺ sorptions were based on the energetically heterogeneous multilayer surfaces. In contrast, the sorption behavior of SeO₄^2- fitted to the Langmuir adsorption isotherm models, indicating that the removal of SeO₄^2- was caused by the ion-exchange of LDHs. The synthesized LDH/GO alginates beads were also applied for setting up small-bore adsorption columns with loading synthetic SeO₄^2- and Sr²⁺ contaminated wastewater. Based on the water chemistry, the adsorbed amount of Sr²⁺ significantly increased after using alginates beads, which was attributed to the functional groups of either GO or alginic acid. The incorporated SeO₄^2- was highly depended on the contents of fabricated LDHs in alginate beads. Specifically, the adsorption capacity of Sr²⁺ (0.85-0.91 mmol/g) on GO slightly increased after alginates fabrication. Therefore, it was deduced that this layered material was partially exfoliated during the manufacture and thus increased the sorption sites. Applications of LDH/GO alginates beads in the removal of both Sr²⁺ and SeO₄^2- in water and soil treatment have a significant impact on the environmental remediation.

Sustainable Synthesis of Nanoscale Zerovalent Iron Particles for Environmental Remediation

Nanoscale zerovalent iron (nZVI) particles represent an important material for diverse environmental applications because of their exceptional electron-donating properties, which can be exploited for applications such as reduction, catalysis, adsorption, and degradation of a broad range of pollutants. The synthesis and assembly of nZVI by using biological and natural sustainable resources is an attractive option for alleviating environmental contamination worldwide. In this Review, various green synthesis pathways for generating nZVI particles are summarized and compared with conventional chemical and physical methods. In addition to describing the latest environmentally benign methods for the synthesis of nZVI, their properties and interactions with diverse biomolecules are discussed, especially in the context of environmental remediation and catalysis. Future prospects in the field are also considered.

Engineered biochar with anisotropic layered double hydroxide nanosheets to simultaneously and efficiently capture Pb2+ and CrO42- from electroplating wastewater

Cationic and anionic heavy metal contaminants generally co-exist in practical industrial effluent, and simultaneously removal of these species is a bottleneck for most of the bio-adsorbents because of their contrary charge. In this work, pinewood sawdust derived engineered biochar (BC) was fabricated with MgAl layered double hydroxide (MgAl-LDH) nanosheets, which could efficiently and simultaneously capture heavy metal cations and oxyanions from wastewater. The synergetic effect between loaded MgAl-LDH and BC substantially improves its adsorption performance towards both cationic and anionic contaminants, i.e., Pb2+ and CrO42-. The adsorption capacity of MgAl-LDH/BC for Pb2+ reached 591.2 mg/g, which is 263% higher than that of BC, and in the case of CrO42-, the adsorption capacity is 330.8 mg/g, 416% higher than that of BC. The elimination of Pb2+ was mainly attributed to forming complexations with surface functional groups. While for oxyanions removal, CrO42- can be reduced to Cr3+ by functional groups, and then generated Cr3+ could replace Al3+ via morphic substitution, consequently formed an MgCr-LDH structure. Further, in the continuous fixed-bed column study, 225 bed volume of simulating electroplating wastewater co-existed with Pb2+ and CrO42- can be efficiently treated. Hence, this study sheds light on the engineered biochar design to efficiently and simultaneously capture heavy metal cations and oxyanions and its feasibility on real wastewater purification.

Nonprecious bimetallic (Mo, Fe)-N/C nanostructures loaded on PVDF membrane for toxic CrVI reduction from water

Nonprecious bimetallic molybdenum and iron embedded into N-doped carbon (MoFe-NC) hybrids were designed and fabricated by pyrolysis of mixed precursors and then immobilized on poly (vinylidene fluoride) (PVDF) films via a phase inversion process to obtain novel catalytic membranes (MoFe-NC@PVDF) for toxic Cr^VI reduction. The catalytic membranes are highly active for aqueous Cr^VI reduction using formic acid (FA) as a sacrificial electron donor under mild conditions. The results demonstrated that the parameters of synthesis process can efficiently adjust the morphology and textural properties of the as-synthesized MoFe-NC@PVDF membrane, and thus have a significant impact on the catalytic behavior. Cr^VI reduction rates significantly increased with increasing FA concentrations (0.234-0.936 M) and reaction temperature (5-35℃), but declined with the increase of Cr^VI concentrations (5-40 mg/L) and pH values of solution (1.87-4.62). Mo-N_x, Fe-N_x, and C-N_x are the active sites, boosting the dissociation of FA molecules into active H* species for effective catalytic reduction of Cr^VI. The catalytic PVDF membrane exhibited distinct porous structure and numerous interaction sites, which not only stabilized metallic nanoparticles, but also promoted mass transfer across the membrane. This cost-effective catalytic membrane provides a new approach toward the treatment of Cr^VI-containing water.

A novel stabilized carbon-coated nZVI as heterogeneous persulfate catalyst for enhanced degradation of 4-chlorophenol

Nano zero-valent iron (nZVI) and its composite materials have been extensively studied in the field of environmental remediation. However, the oxidation and agglomeration of nZVI limits the large-scale application of nZVI in environmental remediation. This study developed a two-step method to prepare stable carbon-coated nZVI (Fe⁰@C) which combined hydrothermal carbonization and carbothermal reduction methods and used glucose and iron oxide (Fe₃O₄) as precursors. When the carbothermal reduction temperature was 700 °C and the elemental molar ratio of carbon to iron was 22:1, stable Fe⁰@C can be generated. The nZVI particles are encapsulated by mesoporous carbon and embedded in the carbon spheres. The unique structure of carbon coating not only inhibits the agglomeration of nZVI, but also makes nZVI stable in air for more than 120 days. Not only that, the as-synthesized Fe⁰@C exhibited high catalytic activity toward the degradation of 4-chlorophenol (4-CP) by activating persulfate. Different from conventional nZVI catalysts in generation of sulfate radicals, Fe⁰@C selectively induced hydroxyl radicals for 4-CP degradation. Moreover, Fe⁰@C has been shown to efficiently degrade 4-CP by using the dissolved oxygen in water to form hydroxyl radicals. This study not only provides a simple, green method for the preparation of stabilized nZVI, but also provides the possibility of large-scale application of nZVI in the field of environmental remediation.

A review of functional sorbents for adsorptive removal of arsenic ions in aqueous systems

The presence of arsenic in the water system has been a universal problem over the past several decades. Inorganic arsenic ions mainly occur in two oxidation states, As(V) and As(III), in the natural environment. These two oxidation states of arsenic ions are ubiquitous in natural waters and pose significant health hazards to humans when present at or above the allowable limits. Therefore, treatment of arsenic ions has become more stringent based on various techniques (e.g., membrane filtration, adsorption, and ion exchange). This paper aims to review the current knowledge on various functional adsorbents through comparison of removal potential for As on the basis of key performance metrics, especially the partition coefficient (PC). As a whole, novel materials exhibited far better removal performance for As(V) and As(III) than conventional materials. Of the materials reviewed, the advanced sorbent like ZrO(OH)₂/CNTs showcased superior performances such as partition coefficient values of 584.6 (As(V) and 143.8 mol kg^-1 M^-1 (As(III) with excellent regenerability (>90 % of desorption efficiency after three sorption cycles). The results of this review are expected to help researchers to establish a powerful strategy for abatement of arsenic ions in wastewater.

Simulated photocatalytic aging of biochar in soil ecosystem: Insight into organic carbon release, surface physicochemical properties and cadmium sorption

Photochemical/photocatalytic reaction, one of the aging pathway of biochar in soil, not only changed the physicochemical properties of biochar, but also affected the migration and transformation of pollutants. Wheat straw biochar was photocatalytic aged in a Fenton-like system using organic acid as buffer solution under light sources, the organic carbon release and surface chemical changes of biochar were investigated to illustrate the adsorption behaviors. With Fe(III) or α-Fe₂O₃ added, the total organic carbon (TOC) of aged biochar solution was influenced more by buffer system than light sources, with the highest of 420.59 mg L^-1 in citric acid system. The production of the hydroxyl radical (OH·) at citric/Fe(III) system was higher than the oxalic/Fe(III) system under the Hg lamp and showed an increasing trend with time. With light exposure, the porous structure of the biochar altered and surface area increased from 7.613 to 29.74 m² g^-1. Meanwhile, the adsorption of cadmium ion by biochar aged in citric/Fe(III) system also showed an increased adsorption capacity with a maximum of 73.54 mg g^-1. So, a well understanding of biochar physicochemical properties changes under natural ecosystem was undoubtedly useful for scientific assessment the long-term feasibility of biochar as soil remediation.

Treatment of As-rich mine effluents and produced residues stability: Current knowledge and research priorities for gold mining

Refractory ores, in which gold is often embedded within As-bearing and acid-generating sulfide minerals, are becoming the main gold source worldwide. These ores require an oxidizing pre-treatment, prior to cyanidation, to efficiently breakdown the sulfides and enhance gold liberation. As a result, large volumes of As-rich effluents (> 500 mg/L) are produced through the pre-oxidation of refractory gold ores and/or the exposure of As-bearing tailings upon exposure to air and water. Limited information is available on performant treatment of these effluents, especially of pre-oxidation effluents characterized by a complex chemistry, extremely acidic or alkaline pH and high concentrations of arsenic. The treatment of As-rich effluents is mainly based on precipitation (using Al or Fe salts and/or Ca-based compounds) and (electro)-chemical or biological oxidation processes. A performant treatment process must maximize As removal from contaminated mine water and allow for the production of residues that are geochemically stable over the long term. An extensive literature review showed that Fe(III)-As(V) precipitates, especially bioscorodite and (nano)scorodite, appear to be the most appropriate forms to immobilize As due to their low solubility and high stability, especially when encapsulated within an inert material such as hydroxyl gels. Research is still required to assess the long-term stability of these As-bearing residues under mine-site conditions for the sustainable exploitation of refractory gold deposits.

Bioremediation of water containing pesticides by microalgae: Mechanisms, methods, and prospects for future research

The application of pesticides reduces the loss of crops while simultaneously increasing crop productivity. However, the frequent use of pesticides can cause serious environmental problems due to their high accumulative and persistent nature. Recently, microalgae technology has received considerable success in the efficient treatment of pesticides pollution. In this review, the metabolic mechanisms responsible for the removal of pesticides are summarized based on previous studies. Different methods used to enhance the ability of microalgae to remove pesticides are critically evaluated. The recycling of microalgae biomass after wastewater treatment for biochar preparation and biodiesel production using the biorefinery approach is also introduced. Furthermore, we present potential future research directions to highlight the prospects of microalgae research in the removal of pesticides along with the production of value-added products.

Functional group-rich hyperbranched magnetic material for simultaneous efficient removal of heavy metal ions from aqueous solution

In order to achieve the purpose of simultaneous removal of coexisting heavy metal ions, in this work, functionalized magnetic mesoprous nanomaterials (Fe₃O₄-HBPA-ASA) with high density and multiple adsorption sites were designed and prepared. The obtained Fe₃O₄-HBPA-ASA was characterized by SEM, FTIR, VSM, TGA and zeta potential. Cu(II), Pb(II) and Cd(II) were chosen as the model heavy metal ions, the adsorption experiments showed that Fe₃O₄-HBPA-ASA showed hightheoretical adsorption capacitiesin individual system, and the maximum adsorption capacity was 136.66 mg/g, 88.36 mg/g and 165.46 mg/g, respectively. In the binary and ternary systems, the competitive adsorption leads to a decrease in the adsorption capacity of Cu(II), Pb(II) and Cd(II). However, in the ternary system with a concentration lower than 15 mg/L, the simultaneous removal rate was still higher than 90%. The adsorption isotherms and kineticswere well fitted by Langmuir and pseudo-second-order models, respectively. The XPS and density functional theory (DFT) analysis have confirmed that the adsorption of metal ions was related to various types of functional groups on the surface of Fe₃O₄-HBPA-ASA, while the adsorption mechanisms of Cu(II), Cd(II) and Pb(II) were different.

Synergistic utilization of inherent halides and alcohols in hydraulic fracturing wastewater for radical-based treatment: A case study of di-(2-ethylhexyl) phthalate removal

The degradation of di-(2-ethylhexyl) phthalate (DEHP) was examined as an example to capitalize on the potential interactions of peroxydisulfate (PS) and ferrous iron (Fe²⁺) in the model Day-1/Day-90 and on-site hydraulic fracturing wastewater (FWW). The primary oxidative radicals in the Fe²⁺/PS system (i.e., SO₄^- and OH) were less effective for the degradation of DEHP (6.45%) in ultrapure water. Both chloride (Cl^-) and bromide (Br^-) at equivalent molar ratio with PS enhanced DEHP degradation (15.6% and 45.5%, respectively) via the generation of Cl and Br radicals, whereas the degradation rate was inhibited by the excessive amount of Cl^- or Br^- in the Day-1/Day-90 FWW. However, the co-presence of ethylene glycol (C₂H₄(OH)₂, 0.043% v/v in the FWW) and halide ions (Cl^- or Br^-, 0.05 mM) resulted in the highest removal efficiency of 82.6 - 88.5% within 10 min by Fe²⁺/PS. Further investigation revealed that the formation of reductive alcohol radicals (C₂H₃(OH)₂) slowed down or replenished the Fe²⁺ exhaustion. This study demonstrated that the Fe²⁺/PS-based advanced oxidation may show a synergistic interplay with Cl^-/Br^- and C₂H₄(OH)₂ in the FWW, which depends on their relative concentrations. Thus, the inherent constituents in the fracturing wastewater can be utilized for the catalytic degradation of co-existing organic contaminants.

Enhanced passivation of lead with immobilized phosphate solubilizing bacteria beads loaded with biochar/ nanoscale zero valent iron composite

Phosphate solubilizing bacteria (PSBs) can effectively enhance the stability of lead via the formation of insoluble Pb-phosphate compounds. This research presents a bio-beads, which was implemented with the help of a self-designed porous spheres carrier, by immobilized PSBs strains Leclercia adecarboxylata (hereafter referred as L1-5). In addition, the passivation efficiency of lead via bio-beads under different conditions and its mechanism were also investigated in this study. The results indicated that phosphate solubilized by bio-beads could reach 30 mg/L in Ca₃(PO₄)₂ medium containing 1 mM Pb²⁺, and the highest removal rate of Pb²⁺ in beef peptone liquid medium could reach 93%, which is better than that of free bacteria. Furthermore, it was also concluded that the lead could be transformed into stable crystal texture, such as Pb₅(PO₄)₃Cl and Pb₅(PO₄)₃OH. Both hydrophobic and hydrophilic groups in the bio-beads could capture Pb²⁺, which indicated that electrostatic attraction and ion-exchange were also the mechanism of Pb²⁺ adsorption. All the experimental findings demonstrated that this bio-bead could be consider as an efficient way for the lead immobilization in contaminated soil in the future.

Facile inverse micelle fabrication of magnetic ordered mesoporous iron cerium bimetal oxides with excellent performance for arsenic removal from water

In this study, magnetic ordered mesoporous Fe/Ce bimetal oxides (OMICs) were successfully synthesized via the modified sol-gel-based inverse micelle method. The textural/structure properties, surface chemistry and adsorption behavior of OMICs could be easily adjusted by using the calcination temperature. The sintering of samples would decrease the surface area, while expand the pore and crystallite size, which resulted in the formation of highly ordered inner-connected structure. Compared with pure mesoporous iron oxides (MI) and mesoporous cerium oxides (MC), this ordered mesoporous iron-cerium bimetal oxides (OMIC-3, 450 °C) exhibited remarkable arsenic adsorption performance. The maximum adsorption capacities of As(III) and As(V) for OMIC-3 were 281.34 and 216.72 mg/g, respectively, and both As(III)/As(V) adsorption kinetics were well described by the pseudo-second order. The ionic strength and coexisting ions (except SiO₃^2- and PO₄^3-) did not affect arsenic removal, while humic acid (HA) significantly influenced on the arsenic removal even at a lower concentration. The adsorption mechanism study revealed that both the surface charge and surface M-OH groups of OMIC-3 were played the key roles in arsenic removal. The reusable property suggested that this magnetic OMIC-3 was a promising excellent adsorbent for decontamination of arsenic-polluted (especially As(III)-polluted) wastewater.

Waste-derived compost and biochar amendments for stormwater treatment in bioretention column: Co-transport of metals and colloids

Bioretention systems, as one of the most practical management operations for low impact development of water recovery, utilize different soil amendments to remove contaminants from stormwater. For the sake of urban sustainability, the utilization of amendments derived from waste materials has a potential to reduce waste disposal at landfill while improving the quality of stormwater discharge. This study investigated the efficiency of food waste compost and wood waste biochar for metal removal from synthetic stormwater runoff under intermittent flow and co-presence of colloids. Throughout intermittent infiltration of 84 pore volumes of stormwater, columns amended with compost and biochar removed more than 50-70% of influent metals, whereas iron-oxide coated sand was much less effective. Only a small portion of metals adsorbed on the compost (< 0.74%) was reactivated during the drainage of urban pipelines that do not flow frequently, owing to abundant oxygen-containing functional groups in compost. In comparison, co-existing kaolinite enhanced metal removal by biochar owing to the abundance of active sites, whereas co-existing humic acid facilitated mobilization via metal-humate complexation. The results suggest that both waste-derived compost and biochar show promising potential for stormwater harvesting, while biochar is expected to be more recalcitrant and desirable in field-scale bioretention systems.

Biochar-supported nanoscale zero-valent iron as an efficient catalyst for organic degradation in groundwater

High-efficiency and cost-effective catalysts are critical to completely mineralization of organic contaminants for in-situ groundwater remediation via advanced oxidation processes (AOPs). The engineered biochar is a promising method for waste biomass utilization and sustainable remediation. This study engineers maize stalk (S)- and maize cob (C)-derived biochars (i.e., SB300, SB600, CB300, and CB600, respectively) with oxygen-containing functional groups as a carbon-based support for nanoscale zero-valent iron (nZVI). Morphological and physiochemical characterization showed that nZVI could be impregnated within the framework of the synthesized Fe-CB600 composite, which exhibited the largest surface area, pore volume, iron loading capacity, and Fe⁰ proportion. Superior degradation efficiency (100% removal in 20 min) of trichloroethylene (TCE, 0.1 mM) and fast pseudo-first-order kinetics (k_obs =22.0 h^-1) were achieved via peroxymonosulfate (PMS, 5 mM) activation by the Fe-CB600 (1 g L^-1) under groundwater condition (bicarbonate buffer solution at pH = 8.2). Superoxide radical and singlet oxygen mediated by Fe⁰ and oxygen-containing group (i.e., CO) were demonstrated as the major reactive oxygen species (ROSs) responsible for TCE dechlorination. The effectiveness and mechanism of the Fe/C composites for rectifying organic-contaminated groundwater were depicted in this study.

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

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

Biological treatment of high salinity and low pH produced water in oilfield with immobilized cells of P. aeruginosa NY3 in a pilot-scale

Produced water (PW) in oilfield, as the largest waste streams in the oil and gas production, has posed a huge threat to the ecosystem. In this work, an environmentally friendly and recyclable biofilms have been developed for treating PW. We discovered that the cells of P. aeruginosa NY3 could be easily immobilized on the surface of polyurethane foam (PUF). Removal efficiency of oil and suspended solids (SS) by immobilized P. aeruginosa NY3 was keeping above 80% and 76% both in a laboratory scale and a pilot scale under suitable pH. Low pH and high value of SS had negative effect on the degradation of oil and SS by P. aeruginosa NY3. Recovery test showed that, the activity of biofilms P. aeruginosa NY3 after running in a pilot scale could be recovered in 5 days. Removal ability of oil in the real PW by the recovered biofilms of P. aeruginosa NY3 was even higher than that of the freshly prepared biofilms. These results indicated that, with a simple pH adjustment, immobilized P. aeruginosa NY3 could be recycled for removing oil and SS in the raw PW resulted from oil production.

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

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

Mechanistic insights into red mud, blast furnace slag, or metakaolin-assisted stabilization/solidification of arsenic-contaminated sediment

Elevated level of arsenic (As) in marine sediment via deposition and accumulation presents long-term ecological risks. This study proposed a sustainable stabilization/solidification (S/S) of As-contaminated sediment via novel valorization of red mud waste, blast furnace slag and calcined clay mineral, which were selected to mitigate the increased leaching of As under alkaline environment of S/S treatment. Quantitative X-ray diffraction and thermogravimetric analyses illustrated that stable Ca-As complexes (e.g., Ca₅(AsO₄)₃OH) could be formed at the expense of Ca(OH)₂ consumption, which inevitably hindered the hydration process and S/S efficiency. The ²⁹Si nuclear magnetic resonance analysis revealed that incorporation of metakaolin for As immobilization resulted in a low degree of hydration and polymerization, whereas addition of red mud promoted Fe-As complexation and demonstrated excellent compatibility with As. Transmission electron microscopy and elemental mapping further confirmed the precipitation of crystalline Ca-As and amorphous Fe-As compounds. Therefore, red mud-incorporated S/S binder achieved the highest efficiency of As immobilization (99.9%), which proved to be applicable for both in-situ and ex-situ S/S of As-contaminated sediment. These results advance our mechanistic understanding for the design of green and sustainable remediation approach for effective As immobilization.

Rapid and efficient recovery of silver with nanoscale zerovalent iron supported on high performance activated carbon derived from straw biomass

High performance activated carbon (HPAC) supported nanoscale zerovalent iron (nZVI) was prepared and used for recovery of silver. This composite material was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The removal amount of Ag⁺ increased with pH values and temperature. The removal process achieved equilibrium within 40 min and the maximum removal capacity was 986.5 mg/g at 298 K. The composite material showed fast adsorption rate and high adsorption capacity because the presence of high surface area activated carbon could effectively inhibit aggregation of nanoscale zerovalent iron, thus enhancing its reactivity. The Ag⁺ removal followed pseudo-second-order kinetic model and Langmuir isotherm model. XPS and XRD characterizations were performed to elucidate removal mechanism. It could be concluded that both coordination adsorption and reductive precipitation contributed to removal of Ag⁺ on the nZVI/HPAC.

An advanced sol-gel strategy for enhancing interfacial reactivity of iron oxide nanoparticles on rosin biochar substrate to remove Cr(VI)

The application of iron oxide nanoparticles (IONs) is often limited by agglomeration and low loading. Here, we presented a facile phase change material (PCM) -based sol-gel strategy for the fabrication of α-Fe₂O₃ nanoparticles. Rosin was used as the PCM in the sol-gel process and the carbon-based substrate of α-Fe₂O₃ nanoparticles in the thermal process. The α-Fe₂O₃ nanoparticle embedded rosin-derived biochar(α-Fe₂O₃@HrBc)were highly dispersed. The dispersity of α-Fe₂O₃ nanoparticle could be regulated by the weight ratios of rosin to FeCl₃·6H₂O during the preparation, as evidenced by the scanning electron microscope (SEM) spectrum and the sorption capacity results. Among a series of α-Fe₂O₃@HrBc nanocomposites, the one with the weight ratios of 1/1.5 rosin/FeCl₃·6H₂O had the highest capacity for hexavalent chromium (Cr(VI)) sorption. This phenomenon can be ascribed to a remarkably enhanced interfacial reactivity due to an increase in the dispersity of α-Fe₂O₃ nanoparticle. In addition, SEM showed that the majority of α-Fe₂O₃ nanoparticles was dispersed on and inside the biochar substrate. Batch adsorption experiments revealed that the α-Fe₂O₃@HrBc adsorbed 90% Cr(VI) within one minute, and the maximum capacity was up to 166 mg·g^-1 based on the Langmuir model. The FTIR and XPS spectra revealed that the adsorbed Cr(VI) species were partially reduced to less toxic Cr(III). Considering that α-Fe₂O₃ nanoparticles provided important sorption sites, the newly formed Cr(III) and the remaining Cr(VI) ions could be adsorbed on α-Fe₂O₃@HrBc via the formation of FeCr coprecipitation.

Oxidation resistance of nanoscale zero-valent iron supported on exhausted coffee grounds

In this study, nanoscale zero-valent iron (NZVI) was supported by exhausted coffee grounds. Exhausted coffee grounds are a crucial waste generated in enormous amounts. Since supported nanoscale particles have a lower free energy than bare particles, oxidation resistance of supported NZVI on coffee grounds (NZVI-Coffee ground) is postulated. The main aim of this study was to ascertain the enhanced oxidation resistance of NZVI-Coffee ground. Synthesized materials were dried and stored in the air at temperatures of 4, 20, and 35 °C. Changes in the surface characteristics and cadmium removal efficiency of the supported NZVI were investigated. Fourier transformation infrared spectroscopy and X-ray photoelectron spectroscopy showed that supported NZVI underwent less oxidation compared to bare NZVI. Cadmium removal efficiencies of supported NZVI did not deteriorate with age, while those of bare NZVI decreased by 9.5 ± 0.1, 13.0 ± 0.1, and 18.3 ± 0.2% compared to their initial removal efficiencies when stored 8 weeks at 4, 20, and 35 °C, respectively. This is because the surface free energy of the NZVI decreased via strong interaction with the functional groups of the coffee grounds. According to the results, exhausted coffee grounds are an effective supporting material for NZVI to enhance its storage stability.

Adsorption of hexavalent chromium onto alkali-modified biochar derived from Lepironia articulata: A kinetic, equilibrium, and thermodynamic study

Hydrochar obtained after hydrothermal carbonization (HTC) of Lepironia articulata (LA) was modified into biochar by reacting with a specific amount of KOH in a tubular reactor under slow pyrolysis. The physical and chemical properties of the hydrochar and modified biochar were characterized. The performance of modified biochar (LABC) was investigated through batch sorption experiments. Removal (%) and the maximum adsorption capacity (q_max ) of Cr(VI) onto LABC increased up to 98.9% and 28.75 mg/g relative to 63.44% and 21.90 mg/g in unmodified hydrochar (LAHC) at pH 2.0, 313 K, and 200 mg/L, respectively. The sorption kinetics uptake data were best interpreted with pseudo-second-order model, and sorption isotherm was simulated with the Langmuir adsorption model. The thermodynamic parameters confirm the adsorption process to be an endothermic, spontaneous, and increased disorder. The overall results revealed that LABC can be utilized as an environmentally friendly, inexpensive, and effective adsorbent in Cr(VI) removal. PRACTITIONER POINTS: Hydrochar and modified biochar prepared from a tropical biomass (Lepironia articulata) were successfully used for the removal of Cr(VI) from aqueous solution. Increased specific surface was obtained by applying chemical modification with alkali treatment, contributing to effectiveness as adsorbent. Dimensionless K_c was estimated from the Langmuir fits and then used to estimate thermodynamics of adsorption. The signs of ∆H°, ∆G°, and ∆S° indicate that the adsorption of Cr(VI) onto LABC was an endothermic, spontaneous, and increased disorder.

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

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

Insights into the oxidation of organic contaminants by iron nanoparticles encapsulated within boron and nitrogen co-doped carbon nanoshell: Catalyzed Fenton-like reaction at natural pH

Iron nanoparticles encapsulated within boron and nitrogen co-doped carbon nanoshell (B/N-C@Fe) were synthesized through a novel and green pyrolysis process using melamine, boric acid, and ferric nitrate as the precursors. The surface morphology, structure, and composition of the B/N-C@Fe materials were thoroughly investigated. The materials were employed as novel catalysts for the activation of potassium monopersulfate triple salt (PMS) for the degradation of levofloxacin (LFX). Linear sweep voltammograms and quenching experiments were used to identify the mechanisms of PMS activation and LFX oxidation by B/N-C@Fe, where SO₄^- as well as HO were proved to be the main radicals for the reaction processes. This study also discussed how the fluvic acid and inorganic anions in the aqueous solutions affected the degradation of LFX and use this method to simulate the degradation in the real wastewater. The synthesized materials showed a high efficiency (85.5% of LFX was degraded), outstanding stability, and excellent reusability (77.7% of LFX was degraded in the 5th run) in the Fenton-like reaction of LFX. In view of these advantages, B/N-C@Fe have great potentials as novel strategic materials for environmental catalysis.

A critical review of risks, characteristics, and treatment strategies for potentially toxic elements in wastewater from shale gas extraction

Shale gas extraction via horizontal drilling and hydraulic fracturing (HF) has enhanced gas production worldwide, which has altered global energy markets and reduced the prices of natural gas and oil. Water management has become the most challenging issue of HF, as it demands vast amounts of freshwater and generates high volumes of complex liquid wastes contaminated by diverse potentially toxic elements at variable rates. This critical review focuses on characterizing HF wastewater and establishing strategies to mitigate environmental impacts. High prioritization was given to the constituents with mean concentrations over 10 times greater than the maximum contamination level (MCL) guidelines for drinking water. A number of potentially harmful organic compounds in HF wastewaters were identified via the risk quotient approach to predict the associated toxicity for freshwater organisms in recipient surface waters. Currently, two options for HF wastewater treatment are preferred, i.e., disposal by deep well injection or on-site re-use as a fracturing fluid. Supplementary treatment will be enforced by increasingly rigorous regulations. Partial treatment and reuse remain the preferred method for managing HF wastewater where feasible. Otherwise, advanced technologies such as membrane separation/distillation, forward osmosis, mechanical vapor compression, electrocoagulation, advanced oxidation, and adsorption-biological treatment will be required to satisfy the sustainable requirements for reuse or surface discharge.

Concurrent adsorption and micro-electrolysis of Cr(VI) by nanoscale zerovalent iron/biochar/Ca-alginate composite

This study introduced a new approach for simultaneously enhancing Cr(VI) removal performance and mitigating release of dissolved Fe during nanoscale zero-valent iron (nZVI)-mediated reactions. After entrapping nZVI-impregnated biochar (BC) in the matrix of calcium-alginate (CA) bead, the physicochemical characterization of nZVI/BC/CA composites revealed that nZVI/BC particles were embedded inside CA having a spherical shape and several cracks on its outer layer. The multi-functionality of nZVI/BC/CA composites consisting of reductant (nZVI), porous adsorbent (BC), and external screening layer (CA) enhanced the removal of Cr(VI) with the maximum adsorption capacity of 86.4 mg/g (based on the Langmuir isotherm) and little release of dissolved Fe. With the XPS analysis and fitting results of kinetics (pseudo second order) and isotherms (Redlich-Peterson model), plausible removal mechanisms of Cr(VI) were simultaneous adsorption and micro-electrolysis reactions by nZVI/BC/CA composites. The practical applicability of nZVI/BC/CA composites was further demonstrated through the fixed-bed column experiments. These results provide new insights into the design of high-performance engineered biochar for wastewater treatment.

Multifunctional iron-biochar composites for the removal of potentially toxic elements, inherent cations, and hetero-chloride from hydraulic fracturing wastewater

This paper evaluates a novel sorbent for the removal of potentially toxic elements, inherent cations, and hetero-chloride from hydraulic fracturing wastewater (FWW). A series of iron-biochar (Fe-BC) composites with different Fe/BC impregnation mass ratios (0.5:1, 1:1, and 2:1) were prepared by mixing forestry wood waste-derived BC powder with an aqueous FeCl₃ solution and subsequently pyrolyzing them at 1000 °C in a N₂-purged tubular furnace. The porosity, surface morphology, crystalline structure, and interfacial chemical behavior of the Fe-BC composites were characterized, revealing that Fe chelated with CO bonds as COFe moieties on the BC surface, which were subsequently reduced to a CC bond and nanoscale zerovalent Fe (nZVI) during pyrolysis. The performance of the Fe-BC composites was evaluated for simultaneous removal of potentially toxic elements (Cu(II), Cr(VI), Zn(II), and As(V)), inherent cations (K, Na, Ca, Mg, Ba, and Sr), hetero-chloride (1,1,2-trichlorethane (1,1,2-TCA)), and total organic carbon (TOC) from high-salinity (233 g L^-1 total dissolved solids (TDS)) model FWW. By elucidating the removal mechanisms of different contaminants, we demonstrated that Fe-BC (1:1) had an optimal reducing/charge-transfer reactivity owing to the homogenous distribution of nZVI with the highest Fe⁰/Fe²⁺ ratio. A lower Fe content in Fe-BC (0.5:1) resulted in a rapid exhaustion of Fe⁰, while a higher Fe content in Fe-BC (2:1) caused severe aggregation and oxidization of Fe⁰, contributing to its complexation/(co-)precipitation with Fe²⁺/Fe³⁺. All of the synthesized Fe-BC composites exhibited a high removal capacity for inherent cations (3.2-7.2 g g^-1) in FWW through bridging with the CO bonds and cation-π interactions. Overall, this study illustrated the potential efficacy and mechanistic roles of Fe-BC composites for (pre-)treatment of high-salinity and complex FWW.