Abiotic dechlorination in the presence of ferrous minerals

Sorption-desorption processes contributing to the natural attenuation of cis-1,2-dichloroethene in porous media

Monitored natural attenuation (MNA) is widely used to manage groundwater plumes with persistent chlorinated solvents exceeding regulatory standards. In heterogeneous aquifers, accumulation and release of these contaminants can impact MNA's effectiveness. Research often focuses on tetrachloroethene (PCE) and trichloroethene (TCE), but incomplete reductive dichlorination can lead to cis-1,2-dichloroethene (DCE) accumulation. This study investigates rate-limited sorption-desorption processes governing DCE release from lower-permeability media. Batch reactor studies with two soils established equilibrium linear distribution coefficients (KD) of 0.15 mL/g and 0.25 mL/g. Column transport studies were then completed using the same soils at two flow rates with flow interruptions to assess rate-limited desorption. A numerical simulator with a "two-site" sorption model was used to fit the effluent concentration data, yielding parameters for the fraction of sorption sites at instantaneous equilibrium (f) and the rate of sorption for time-dependent sites (k) ranging from 0.2 to 0.6 and 0.4-2 1/day, respectively. Soils with small f and k exhibit prolonged DCE release, which can benefit MNA at sites with an active DCE-to-ethene dechlorinating microbial community. These persistent, low concentrations of DCE can support microbial reductive dichlorination by providing sufficient residence time for the complete biodegradation to non-toxic ethene. This work emphasizes the importance of developing conceptual site models that capture sorption-desorption processes contributing to natural attenuation of chlorinated solvents in heterogeneous aquifers.

Predicting trichloroethene attenuation in aquifers with reduced iron minerals under oxygen perturbation: From kinetic model to reactive transport model

Pollutant attenuation in aquifers due to the reactivity of in situ reduced iron minerals (RIM) under dynamic redox conditions has gained popularity because of its value in designing green and sustainable strategies for soil and groundwater remediation. In this study, a novel approach that integrates RIM-based kinetic modeling in reactive transport modeling was initiated to predict trichloroethene (TCE) attenuation in RIM-containing aquifers. The kinetic model was optimized and simplified based on previous efforts and verified with data from batch experiments (R2 > 0.85). Using the refined kinetic model as a reaction module, a multi-scale and multi-process reactive transport model (RTM) was developed, which comprehensively linked geochemical reactions, dispersion, and advection of TCE in the aquifers with RIM and accurately simulated the variations of TCE concentrations in experiments (R2 >0.95). Notably, the importance of RIM oxygenation in TCE attenuation during O2 perturbation was demonstrated by the model; the ratio of contributions from RIM-based anaerobic dichlorination, hydrodynamics, and RIM-based aerobic degradation to TCE removal in low-permeability zones after 10 d was approximately 1:7:21. Besides, the RTM helped to elucidate the effects of variations in key elements (i.e., Fe(II) species, O2) on the TCE attenuation system. This study can facilitate the evaluation and prediction of the TCE fate in aquifers with RIM under O2 perturbation, so that remediation strategies can be formulated. Meanwhile, the limitations of the study motivate future work to determine the effects of more elements (e.g., organic matters, microbes), environmental changes, and scale differences to develop more sophisticated models.

Abiotic natural attenuation of 1,2,3-trichloropropane by natural magnetite under O2 perturbation

1,2,3-Trichloropropane (TCP) is an emerging groundwater pollutant, but there is a lack of reported studies on the abiotic natural attenuation of TCP by iron minerals. Furthermore, perturbation by O2 is common in the shallow subsurface by both natural and artificial processes. In this study, natural magnetite was selected as the reactive iron mineral to investigate its role in the degradation of TCP under O2 perturbation. The results indicated that the mineral structural Fe(II) on magnetite reacted with dissolved oxygen to generate O2-· and HO·. Both O2-· and HO· contributed to TCP degradation, with O2-· playing a more important role. After 56 days of reaction, 66.7% of TCP was completely dechlorinated. This study revealed that higher magnetite concentrations, smaller magnetite particle sizes, and lower initial TCP concentrations favored TCP degradation. The presence of <10 mg/L natural organic matter (NOM) did not affect TCP degradation. These findings significantly advance our understanding of the abiotic natural attenuation mechanisms facilitated by iron minerals under O2 perturbation, providing crucial insights for the study of natural attenuation.

Critical Functions of Soil Components for In Situ Persulfate Oxidation of Sulfamethoxazole: Inherent Fe(II) Minerals-Coordinated Nonradical Pathway

Naturally occurring iron (Fe) minerals have been proved to activate persulfate (PS) to generate reactive species, but the role of soil-inherent Fe minerals in activating PS as well as the underlying mechanisms remains poorly understood. Here, we investigated sulfamethoxazole (SMX) degradation by PS in two Fe-rich soils and one Fe-poor soil. Unlike with the radical-dominant oxidation processes in Fe-poor soil, PS was effectively activated through nonradical pathways (i.e., surface electron-transfer) in Fe-rich soils, accounting for 68.4%-85.5% of SMX degradation. The nonradical mechanism was evidenced by multiple methods, including electrochemical, in situ Raman, and competition kinetics tests. Inherent Fe-based minerals, especially those containing Fe(II) were the crucial activators of PS in Fe-rich soils. Compared to Fe(III) minerals, Fe(II) minerals (e.g., ilmenite) were more liable to form Fe(II) mineral-PS* complexes to initiate the nonradical pathways, oxidizing adjacent SMX via electron transfer. Furthermore, mineral structural Fe(II) was the dominant component to coordinate such a direct oxidation process. After PS oxidation, low-crystalline Fe minerals in soils were transformed into high-crystalline Fe phases. Collectively, our study shows that soil-inherent Fe minerals can effectively activate PS in Fe-rich soils, so the addition of exogenous iron might not be required for PS-based in situ chemical oxidation. Outcomes also provide new insights into the activation mechanisms when persulfate is used for the remediation of contaminated soils.

Reciprocal Interactions of Abiotic and Biotic Dechlorination of Chloroethenes in Soil

Chloroethenes (CEs) as common organic pollutants in soil could be attenuated via abiotic and biotic dechlorination. Nonetheless, information on the key catalyzing matter and their reciprocal interactions remains scarce. In this study, FeS was identified as a major catalyzing matter in soil for the abiotic dechlorination of CEs, and acetylene could be employed as an indicator of the FeS-mediated abiotic CE-dechlorination. Organohalide-respiring bacteria (OHRB)-mediated dechlorination enhanced abiotic CEs-to-acetylene potential by providing dichloroethenes (DCEs) and trichloroethene (TCE) since chlorination extent determined CEs-to-acetylene potential with an order of trans-DCE > cis-DCE > TCE > tetrachloroethene/PCE. In contrast, FeS was shown to inhibit OHRB-mediated dechlorination, inhibition of which could be alleviated by the addition of soil humic substances. Moreover, sulfate-reducing bacteria and fermenting microorganisms affected FeS-mediated abiotic dechlorination by re-generation of FeS and providing short chain fatty acids, respectively. A new scenario was proposed to elucidate major abiotic and biotic processes and their reciprocal interactions in determining the fate of CEs in soil. Our results may guide the sustainable management of CE-contaminated sites by providing insights into interactions of the abiotic and biotic dechlorination in soil.

Kinetics of Hydroxyl Radical Production from Oxygenation of Reduced Iron Minerals and Their Reactivity with Trichloroethene: Effects of Iron Amounts, Iron Species, and Sulfate Reducing Bacteria

Reactive oxygen species generated during the oxygenation of different ferrous species have been documented at groundwater field sites, but their effect on pollutant destruction remains an open question. To address this knowledge gap, a kinetic model was developed to probe mechanisms of •OH production and reactivity with trichloroethene (TCE) and competing species in the presence of reduced iron minerals (RIM) and oxygen in batch experiments. RIM slurries were formed by combining different amounts of Fe(II) and sulfide (with Fe(II):S ratios from 1:1 to 50:1) or Fe(II) and sulfate with sulfate reducing bacteria (SRB) added. Extents of TCE oxidation and •OH production were both greater with RIM prepared under more reducing conditions (more added Fe(II)) and then amended with O₂. Kinetic rate constants from modeling indicate that •OH production from free Fe(II) dominates •OH production from solid Fe(II) and that TCE competes for •OH with Fe(II) and organic matter (OM). Competition with OM only occurs in experiments with SRB, which include cells and their exudates. Experimental results indicate that cells and/or exudates also provide electron equivalents to reform Fe(II) from oxidized RIM. Our work provides new insights into mechanisms and environmental significance of TCE oxidation by •OH produced from oxygenation of RIM. However, further work is necessary to confirm the relative importance of reaction pathways identified here and to probe potentially unaccounted for mechanisms that affect abiotic TCE oxidation in natural systems.