Efficiency assessment of ZVI-based media as fillers in permeable reactive barrier for multiple heavy metal-contaminated groundwater remediation

Enhancing and sustaining arsenic removal in a zerovalent iron-based magnetic flow-through water treatment system

In areas affected by arsenicosis, zerovalent iron (ZVI)/sand filters are extensively used by households to treat groundwater, but ZVI surface passivation and filter clogging limit their arsenic (As) removal performance. Here we present a magnetic confinement-enabled column reactor coupled with periodic ultrasonic depassivation (MCCR-PUD), which efficiently and sustainably removes As by reaction with continuously generated iron (oxyhydr)oxides from ZVI oxidative corrosion. In the MCCR, ZVI microparticles self-assemble into stable millimeter-scale wires in forest-like arrays in a parallel magnetic field (0.42-0.48 T, produced by two parallel permanent magnets), forming a highly porous structure (87 % porosity) with twice the accessible reactive surface area of a ZVI/sand mixture. For a feed concentration of 100 μg/L As(III), the MCCR-PUD, with a short empty bed contact time (1.6 min), treated ca. 7340 empty bed volume (EBV) of water at breakthrough (10 μg/L), 9.4 folds higher than that of a ZVI/sand filter. Due to the large interspace between ZVI wires, the MCCR-PUD effectively prevented column clogging that occurred in the ZVI/sand filter. The high water treatment capacity was attributed to the much enhanced ZVI reactivity in the magnetic field, sustained through rejuvenation by PUD. Furthermore, most of As was structurally incorporated into the produced iron (oxyhydr)oxides (mostly ferrihydrite) in the MCCR-PUD, as revealed by Mössbauer spectroscopy, X-ray absorption spectroscopy, and sequential extraction experiments. This finding evinced a different mechanism from the surface adsorption in the ZVI/sand filter. The structural incorporation of As also resulted in much less As remobilization from the produced corrosion products during aging in water, in total ∼1 % in 28 days. Furthermore, the MCCR-PUD exihibted robust performance when treating complex synthetic groundwater containing natural organic matter and common ions (∼3700 EBV at breakthrough). Taken together, our study demonstrates the potential of the magnetic confinement-enabled ZVI reactor as a promising decentralized As treatment platform.

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

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

Superb green cycling strategies for microbe-Fe0 neural network-type interaction: Harnessing eight key genes encoding enzymes and mineral transformations to efficiently treat PFOA

To address time-consuming and efficiency-limited challenges in conventional zero-valent iron (ZVI, Fe0) reduction or biotransformation for perfluorooctanoic acid (PFOA) treatment, two calcium alginate-embedded amendments (biochar-immobilized PFOA-degrading bacteria (CB) and ZVI (CZ)) were developed to construct microbe-Fe0 high-rate interaction systems. Interaction mechanisms and key metabolic pathways were systematically explored using metagenomics and a multi-process coupling model for PFOA under microbe-Fe0 interaction. Compared to Fe0 (0.0076 day-1) or microbe (0.0172 day-1) systems, the PFOA removal rate (0.0426 day-1) increased by 1.5 to 4.6 folds in the batch microbe-Fe0 interaction system. Moreover, Pseudomonas accelerated the transformation of Fe0 into Fe3+, which profoundly impacted PFOA transport and fate. Model results demonstrated microbe-Fe0 interaction improved retardation effect for PFOA in columns, with decreased dispersivity a (0.48 to 0.20 cm), increased reaction rate λ (0.15 to 0.22 h-1), distribution coefficient Kd (0.22 to 0.46 cm3∙g-1), and fraction f´(52 % to 60 %) of first-order kinetic sorption of PFOA in microbe-Fe0 interaction column system. Moreover, intermediates analysis showed that microbe-Fe0 interaction diversified PFOA reaction pathways. Three key metabolic pathways (ko00362, ko00626, ko00361), eight functional genes, and corresponding enzymes for PFOA degradation were identified. These findings provide insights into microbe-Fe0 "neural network-type" interaction by unveiling biotransformation and mineral transformation mechanisms for efficient PFOA treatment.

A framework for estimating soil water characteristic curve and hydraulic conductivity function of permeable reactive media

The unsaturated behavior of permeable reactive barriers (PRB) is a critical component in predicting the removal efficiency through the adsorption of contaminants. This study investigates the framework to estimate the soil water characteristic curve (SWCC) and hydraulic conductivity function (HCF) for iron oxide-coated sand (IOCS) and zeolite, which are common materials used in PRBs. A multistep outflow (MSO) experiment was performed and the results of the MSO experiment were used to optimize associated parameters in Kosugi's SWCC and HCF. In addition, three scenarios of optimization analysis were investigated to evaluate the best-fitting model for estimating SWCC and HCF. The low root mean square error (RMSE) of fitted parameters indicates the Kosugi model well described the observed suction profiles in MSO experiments. In addition, the lowest RMSE and coefficient of variation suggested the inclusion of the additional parameter β provided the best estimation of the three materials (clean sand, IOCS, and zeolite). The physically reasonable estimation of SWCC and HCF of the three materials from the optimized parameters suggests the proposed framework is a reasonable model for the unsaturated behavior of PRBs.

Double-edged effect of frequent freeze-thaw on the stability of zero-valent iron after heavy metal remediation

Freeze-thaw cycles (FTCs) cause dynamic microscale changes in ions and solvents. During freezing, heavy metals adsorbed on zero-valent iron (M-ZVI) and protons are excluded by ice crystals and concentrated in the liquid-like grain boundary region. The high proton concentration in this region leads to the dissolution of the passivation layer of ZVI. To assess the environmental risks of M-ZVI during FTCs, this study evaluated the stability of M-ZVI in this scenario from both microscale and macroscale perspectives. The results showed that the dissolution of the passivation layer had a dual effect on the stability of M-ZVI, which depends on the by-products of M-ZVI. The dissolution of the passivation layer was accompanied by the leaching of heavy metals, such as Ni-ZVI, but it also enhanced the reactivity of ZVI, causing it to re-react with desorbed heavy metals. The stability of Cr-ZVI and Cd-ZVI was improved due to frequent FTCs. Furthermore, changes in the surrounding environment (water dipole moment, ion concentration, etc.) of ZVI affected the crystallization of Fe oxides, increasing the content of amorphous Fe oxide. As low-crystallinity Fe oxides could facilitate ion doping, Ni2+ was doped into Fe3O4 lattice during FTCs, which reduced the mobility of heavy metals. Contrary to traditional views that freezing temperatures slow chemical reactions, this study provides new insights into the application of iron-based materials in cold environments.

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.

Longevity evaluation of non-pumping reactive wells for control of groundwater contamination: Application of upscaling methods

Non-pumping reactive wells (NPRWs) are subsurface structures used for the passive treatment of contaminated groundwater using wells containing reactive media. In the vicinity of NPRWs, a combination of hydrogeological and chemical processes makes it difficult to predict their longevity. In this study, we evaluated the longevity of NPRWs using the upscaling methods. A horizontal two-dimensional sandbox was constructed to mimic the hydrogeological and chemical processes in a single unit of NPRW (unit NPRW). The groundwater flow and solute transport were simulated numerically to validate the processes of contaminant spreading prevention in the sandbox. Dye tracing and arsenic transport tests showed different performance of NPRW due to induced flow and uneven consumption of reactivity, which is dependent on the pathway length and residence time of the coal waste. Through numerical modeling of the experiments, the fate-related processes of contamination around NPRW were described in detail in both spatial and temporal terms. The stepwise approach of the upscaling methods was used to predict the contamination-blocking performance of the entire facility based on the reactivity of the materials and the contamination removal of the unit NPRW.

Health Risk Assessments and Microbial Community Analyses of Groundwater from a Heavy Metal-Contaminated Site in Hezhou City, Southwest China

In industrial site groundwater, heavy metal pollution is relatively common, causing great harm to the surrounding environment and human health. To explore the relationships between the heavy metal concentration, health risks and microbial community distribution, the groundwater from a polluted site at an abandoned processing plant in Hezhou City, China, is taken as the research object. A health risk assessment model recommended by the United States Environmental Protection Agency (US EPA) is used for the evaluation, and high-throughput sequencing technology is used to analyze the characteristics of the microbial community in the groundwater. The results show that the heavy metal pollution levels of five monitoring wells are different. The monitoring well labelled HLJ2 is polluted by Cu, Mn, Ni and Cd, and the other four monitoring wells are polluted by As and Cd to varying degrees. The carcinogenic risk values of heavy metals in the groundwater environments of the five monitoring wells are all greater than the acceptable range, and only the noncarcinogenic risk value of the HLJ2 monitoring well exceeds 1, which greatly impacts health. The risks posed by the contaminants in the site groundwater through the ingestion route of drinking water are greater than those caused by the ingestion route of skin contact. The groundwater environments of the five monitoring wells contain Proteobacteria and Patescibacteria, indicating that these two bacteria have certain tolerances to heavy metal pollution. The microbial community composition varies between the monitoring wells, suggesting that different concentrations and types of heavy metal contamination promote different types of bacterial growth. Studies have shown that Proteobacteria have many heavy metal resistance genes, improving their tolerance in heavy metal-polluted environments; additionally, Proteobacteria can transport heavy metals, which is conducive to the restoration of polluted sites.

Possibility of New Active Substrates (ASs) to Be Used to Prevent the Migration of Heavy Metals to the Soil and Water Environments

This paper aims to propose an alternative to the known permeable reactive barriers (PRBs). PRB is one of the methods, which is a reactive barrier placed below the ground, to clean up contaminated groundwater. New polymer active substrates (ASs) were used to prevent soil contamination by toxic heavy metals. The active substrates consisted of a mixture of poly(vinyl chloride), Aliquat 336, and bis(2-ethylhexyl)adipate, which was applied to the skeleton material (fiberglass or textile). Aliquat 336 was used as a binding agent for metal ions (Cr(VI), Ni(II), Cu(II), Zn(II), Cd(II), and Pb(II)). In contrast with the PRBs, the ASs (from AS-1 to AS-5) were obtained in a simple way using the pouring method. The obtained ASs could be recycled and reused. The active substrates were used for the binding of various metal ions from aqueous solutions and the examined soil. It was found that the active substrate AS-1 decreased the concentrations of nickel, cadmium, and lead by more than 50% and that of chromium by more than 90% in the aqueous solution. High sorption efficiency for chromium and zinc metals (81% and 66%) with the use of AS-2 was also found, owing to which the migration of metals from soil to water can be limited. In the soil environment, active substrate AS-5 with the addition of a plasticizer showed the greatest effectiveness. This solution resulted in a reduction in each tested metal ion of at least 50%, and reductions in cadmium, lead, and copper of over 70%.

Health Risk Assessment during In Situ Remediation of Cr(VI)-Contaminated Groundwater by Permeable Reactive Barriers: A Field-Scale Study

The presence of residual Cr(VI) in soils causes groundwater contamination in aquifers, affecting the health of exposed populations. Initially, permeable reactive barriers(PRB) effectively removed Cr(VI) from groundwater. However, as PRB clogging increased and Cr(VI) was released from upstream soils, the contamination plume continued to spread downstream. By 2020, the level of contamination in the downstream was nearly identical to that in the upstream. The study results show that during normal operation, the PRB can successfully remove Cr(VI) from contaminated groundwater and reduce the carcinogenic and non-carcinogenic risks to humans from the downstream side of groundwater. However, the remediated groundwater still poses an unacceptable risk to human health. The sensitivity analysis revealed that the concentration of the pollutant was the most sensitive parameter and interacted significantly with other factors. Ultimately, it was determined that the residual Cr(VI) in the soil of the study region continues to contaminate the groundwater and constitutes a serious health danger to residents in the vicinity. As remediated groundwater still poses a severe threat to human health, PRB may not be as effective as people believe.

Versatile strategy of sulfanilamide antibiotics removal via microalgal biochar: Role of oxygen-enriched functional groups

Biochar (BC) adsorption has been widely acknowledged as an efficient approach for the removal of antibiotics. Despite the importance of oxygen-containing functional groups for the antibiotics removal, most of these may be obtained in BC only relying on the addition of oxidants. Herein, an environmentally friendly and oxygen-enriched functional groups adsorbent, namely Chlamydomonas BC (CBC), was fabricated via simple pyrolysis process. Then, the H-bonding, electron donor-acceptor and electrostatic attraction were identified as the main mechanisms regarding sulfathiazole (STZ) adsorption (506.38 mg/g). The carbon-oxygen functional groups on the surface of CBC (61%), especially -COOH and -OH, acted as a pivotal component. Additionally, further theoretical calculation led to the observation that STZ exhibited the highest chemical reactivity (η = 0.04), strong electron exchange capacity (μ = -0.16), remarkable electron accepting capacity (ω = 0.28) and excellent electron transfer efficiency (E_HOMO-E_LUMO gap = 0.29) under the influence of thiazolyl. The electrophilic sulfonamide group and the nucleophilic thiazole were identified as the main active sites of STZ. In summary, the results of this research provide a guiding role for the preparation of adsorbents driven by the structural characteristics of pollutants.