Long-term performance evaluation of zero-valent iron amended permeable reactive barriers for groundwater remediation - A mechanistic approach

Long-term field study of nitrate and ammonium remediation using a permeable reactive barrier at a livestock feeding operation

Protecting groundwater is a global challenge in modern agriculture. Nutrients from livestock detritus and manure have caused adverse effects on ecosystems and posed health risks associated with use of contaminated groundwater. Such occurred at a concentrated animal feeding operation (CAFO) where failed manure containment and buried carcasses led to toxic concentrations of ammonium and nitrate in groundwater. Here we evaluate a two-step approach to remediate ammonium using pump-and-treat technology, and nitrate using a permeable reactive barrier (PRB) composed of locally sourced hay as a carbon source to drive denitrification. Long-term monitoring (10-year dataset) revealed that effective mitigation of total nitrogen was accomplished through several mechanisms. Ammonium initially developed in the PRB from mineralization of protein in the hay; however, groundwater conditions permitted the possibility of dissimilatory nitrate reduction. Total nitrogen mitigation is attributed to anaerobic ammonium oxidation and denitrification. The reducing environment induced by the PRB caused reduction of iron oxyhydroxides as evidenced by increased dissolved iron and manganese in groundwater. Increased total phosphorus and arsenic mobilization was also locally observed. Some monitoring wells contained high levels of ammonium released from buried detritus. While the PRB effectively removed nitrate, elevated total phosphorus in stream water exceeded recommended limits and placed surface water at continued risk for eutrophication even ten years after installation. Locally sourced carbon sources deployed in a PRB can effectively mitigate nitrogen contamination in groundwater; however, future applications of organic carbon systems should consider the possible mobilization of secondary contaminants including phosphorus, arsenic, iron, and manganese.

Prediction of heavy metal contamination in soil-groundwater systems at contaminated sites

The diffusion of heavy metal pollutants in polluted industrial areas can cause severe environmental pollution in surrounding areas. However, the migration of pollutants into groundwater is a complex process that requires consideration of local geological and hydrological conditions, solute transport, and geochemistry factors to better predict the flow paths and plume dispersion of pollutants. This study is based on numerical models of Darcy's law and the Richards equation. A numerical model is used to predict the pollution risk of a certain abandoned metallurgical site. The results indicate that the risk of heavy metal leaching is extremely high under natural conditions, potentially affecting downstream reservoirs after 1500 days. The use of permeable reactive barriers (PRBs) can effectively prevent the migration of heavy metals. However, even with PRBs, 28%-30% of pollutants may still continue to spread outward through lateral flow pathways. The use of impermeable Funnel-and-gate PRB design can effectively reduce lateral pollutant migration, reducing lateral leakage by up to 27%. Based on these results, the rational design of PRBs can effectively reduce remediation costs and time, enhance groundwater remediation effectiveness, and provide strong support for environmental protection and ecological health.

Chemical aging of biochar-zero-valent iron composites in groundwater: Impact on Cd(II) and Cr(VI) co-removal

Biochar (BC), zero-valent iron (ZVI), and their composites are promising materials for use in permeable reactive barriers, although further research is needed to understand how their properties change during long-term aging in groundwater. In this study, BC, ZVI and their composites (4BC-1ZVI) were subjected to the chemical aging tests in five media (deionized water, NaCl, NaHCO3, CaCl2 and a mixture of CaCl2 and NaHCO3 solutions) for 20 days. After treatment, the microscopic analysis and performance tests for the co-removal of Cd(II) and Cr(VI) were carried out. The results indicated that the removal of Cd(II) by aged 4BC-1ZVI followed a pseudo-second-order model, whereas the removal of Cr(VI) was better fitted with a pseudo-first order model. The aging mechanism of 4BC-1ZVI was primarily governed by iron corrosion/passivation, the reduction of soluble components, and the formation of carbonate minerals. Less Fe3O4/ γ-Fe2O3 was formed during aging in deionized water, NaCl and CaCl2 solutions. The corrosion products, Fe3O4/ γ-Fe2O3, FeCO3 and α/γ-FeOOH, were observed after aging in NaHCO3 and a mixture of NaHCO3 and CaCl2 solutions. The decrease in the soluble components of biochar led to a decrease in cation exchange, while carbonate minerals contributed to Cd(II) precipitation. This work provides insights into the aging processes of BC-ZVI composites for long-term groundwater remediation applications.

New insights on zero-valent iron permeable reactive barrier for Cr(VI) removal: The function of FeS reaction zone downstream in-situ generated by sulfate-reducing bacteria

The biogeochemical behavior downstream of the zero-valent iron permeable reactive barrier (ZVI-PRB) plays an enormous positive role in the remediation of contaminated-groundwater, but has been completely neglected for a long time. Therefore, this study conducted a 240-day SRB-enhanced ZVI-PRB column experiment, focusing on what exactly happens downstream of ZVI-PRB. Results show that biosulfidation of SRB inside ZVI-PRB prolonged the complete Cr(VI) removal longevity of ZVI-PRB from 38 days to at least 240 days. More importantly, unlike previous studies that focused on improving the performance of ZVI-PRB itself, this study found an in-situ generated FeS reduction reaction zone downstream of the ZVI-PRB. When the ZVI-PRB fails, the downstream reaction zone can continue to play a role in Cr(VI) removal. The maximum Cr(VI) removal capacity of the aquifer media from the reaction zone reached 155.1 mg/kg, which was 39.7 % of commercial ZVI capacity. The reduction zone was further confirmed to be predominantly FeS rather than FeS2. Biogeochemistry occurring within and downstream of ZVI-PRB leads to the formation of FeS. Gene sequencing revealed significantly higher SRB abundance downstream of ZVI-PRB than within the ZVI-PRB. The understanding of the downstream FeS reaction zone provides new insights for more effective remediation using ZVI-PRB.

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.

Bioremediation of trichloroethylene-contaminated groundwater using green carbon-releasing substrate with pH control capability

In this research, a sustainable substrate, termed green and long-lasting substrate (GLS), featuring a blend of emulsified substrate (ES) and modified rice husk ash (m-RHA) was devised. The primary objective was to facilitate the bioremediation of groundwater contaminated with trichloroethylene (TCE) using innovative GLS for slow carbon release and pH control. The GLS was concocted by homogenizing a mixture of soybean oil, surfactants (Simple Green™ and soya lecithin), and m-RHA, ensuring a gradual release of carbon sources. The hydrothermal synthesis was applied for the production of m-RHA production. The analyses demonstrate that m-RHA were uniform sphere-shape granules with diameters in micro-scale ranges. Results from the microcosm study show that approximately 83% of TCE could be removed (initial TCE concentration = 7.6 mg/L) with GLS supplement after 60 days of operation. Compared to other substrates without RHA addition, higher TCE removal efficiency was obtained, and higher Dehalococcoides sp. (DHC) population and hydA gene (hydrogen-producing gene) copy number were also detected in microcosms with GLS addition. Higher hydrogen concentrations enhanced the DHC growth, which corresponded to the increased DHC populations. The addition of the GLS could provide alkalinity at the initial stage to neutralize the acidified groundwater caused by the produced organic acids after substrate biodegradation, which was advantageous to DHC growth and TCE dechlorination. The addition of m-RHA reached an increased TCE removal efficiency, which was due to the fact that the m-RHA had the zeolite-like structure with a higher surface area and lower granular diameter, and thus, it resulted in a more effective initial adsorption effect. Therefore, a significant amount of TCE could be adsorbed onto the surface of m-RHA, which caused a rapid TCE removal through adsorption. The carbon substrates released from m-RHA could then enhance the subsequent dechlorination. The developed GLS is an environmentally-friendly and green substrate.

Enhanced Biogenic Sulfidation of Zero-Valent Iron in Columns: Implications for Promoting Dechlorination in Permeable Reactive Barriers

Biogenic sulfidation of zero-valent iron (ZVI) using sulfate reducing bacteria (SRB) has shown enhanced dechlorination rates comparable to those produced by chemical sulfidation. However, controlling and sustaining biogenic sulfidation to enhance in situ dechlorination are poorly understood. Detailed interactions between SRB and ZVI were examined for 4 months in column experiments under enhanced biogenic sulfidation conditions. SRB proliferation and changes in ZVI surface properties were characterized along the flow paths. The results show that ZVI can stimulate SRB activity by removing excessive free sulfide (S2-), in addition to lowering reduction potential. ZVI also hinders downgradient movement of SRB via electrostatic repulsion, restricting SRB presence near the upgradient interface. Dissolved organic carbon (e.g., >2.2 mM) was essential for intense biogenic sulfidation in ZVI columns. The presence of SRB in the upgradient zone appeared to promote the formation of iron polysulfides. Biogenic FeSx deposition increased the S content on ZVI surfaces ∼3-fold, corresponding to 3-fold and 2-fold improvements in the trichloroethylene degradation rate and electron efficiency in batch tests. Elucidation of SRB and ZVI interactions enhances sustained sulfidation in ZVI permeable reactive barrier.

A comprehensive review on permeable reactive barrier for the remediation of groundwater contamination

Groundwater quality is deteriorating due to contamination from both natural and anthropogenic sources. Traditional "Pump and Treat" techniques of treating the groundwater suffer from the disadvantages of a small-scale and energy-intensive approach. Permeable reactive barriers (PRBs), owing to their passive operation, offer a more sustainable strategy for remediation. This promising technique focuses on eliminating heavy metal pollutants and hazardous aromatic compounds by physisorption, chemisorption, precipitation, denitrification, and/or biodegradation. Researchers have utilized ZVI, activated carbon, natural and manufactured zeolites, and other by-products as reactive media barriers. Environmental parameters, i.e., pH, initial pollutant concentration, organic substance, dissolved oxygen, and reactive media by-products, all influence a PRB's performance. Although their long-term impact and performance are uncertain, PRBs are still evolving as viable alternatives to pump-and-treat techniques. The use of PRBs to remove anionic contaminants (e.g., Fluoride, Nitrate, etc.) has received less attention since precipitates can clog the reactive barrier and hinder groundwater flow. In this paper, we present an insight into this approach and the tremendous implications for future scientific study that integrates this strategy using sustainability and explores the viability of PRBs for anionic pollutants.