Degradation of chlorinated solvents with reactive iron minerals in subsurface sediments from redox transition zones

Highlights

• Reactive Fe mineral coatings play an important role in contaminant dehalogenation.

• Dechlorination for 1,4-DCB was observed with reactive Fe minerals coatings.

• 90% degradation of 1,4-DCB, TCE, and PCE with RTZ sediments less than 39 days.

• In degradation, the following trend was found: Fe(II) sulfides>magnetite>siderite.

Abstract

Reactive iron (Fe) mineral coatings found in subsurface reduction-oxidation transition zones (RTZs) contribute to the attenuation of contaminants. An 18.3-m anoxic core was collected from the site, where constituents of concern (COCs) in groundwater included chlorinated solvents. Reactive Fe mineral coatings were found to be abundant in the RTZs. This research focused on evaluating reaction kinetics with anoxic sediments bearing ferrous mineral nano-coatings spiked with either tetrachloroethylene (PCE), trichloroethylene (TCE), or 1,4-dichlorobenzene (1,4-DCB). Reaction kinetics with RTZ sediments followed pseudo-first-order reactions for the three contaminants with 90% degradation achieved in less than 39 days. The second-order rate constants for the three COCs ranged from 6.20 × 10^-4 to 1.73 × 10^-3 Lg^-1h^-1 with pyrite (FeS₂), 4.97 × 10^-5 to 1.24 × 10^-3 Lg^-1h^-1with mackinawite (FeS), 1.25 × 10^-4 to 1.89 × 10^-4 Lg^-1h^-1 with siderite (FeCO₃), and 1.79 × 10^-4 to 1.10 × 10^-3 Lg^-1h^-1 with magnetite (Fe₃O₄). For these three chlorinated solvents, the trend for the rate constants followed: Fe(II) sulfide minerals > magnetite > siderite. The high reactivity of Fe mineral coatings is hypothesized to be due to the large surface areas of the nano-mineral coatings. As a result, these surfaces are expected to play an important role in the attenuation of chlorinated solvents in contaminated subsurface environments.

Introduction

Contamination of subsurface systems with chlorinated solvents, such as 1,4-dichlorobenzene (1,4-DCB), tetrachloroethylene (PCE), and trichloroethylene (TCE), is a pressing problem given their past usage, mobility in the environment, and legacy at sites worldwide (Jordan et al., 2021). The U.S. produced approximately 2 billion pounds of chlorinated solvents each year between 1940 and 1980 (Moran et al., 2007). Because of their low solubility, relatively slow attenuation rates, and past disposal practices, releases into the subsurface and groundwater have been problematic.

The chlorinated benzene 1,4-DCB is chemically stable (Table S1) and is the byproduct of producing long-chain aromatic compounds, such as alpha-hexachlorobenzene (γ-HCH). Liu et al. (2003) reported that 1,4-DCB was a dominant byproduct of γ-HCH transformation via reductive elimination and dehydrochlorination by mackinawite (FeS). Reduction of γ-HCH was also observed in a bimetallic system where Fe was catalyzed by less active metals such as Ag (Xu and Zhang, 2000). However, products from further reduction of 1,4-DCB in these systems have not been reported. Abiotic degradation of 1,4-DCB has been reported with hydroxyl radicals, activated persulfate (Garcia-Cervilla et al., 2022), and ozone (Gushchin et al., 2020). On the other hand, reductive biodegradation of 1,4-DCB has been studied to a greater extent under anaerobic conditions (Fung et al., 2009, Lawrence, 2006, Qiao et al., 2018). For example, degradation of 1,4-DCB by microcosms in the sediment collected from a contaminated industrial site was observed within 11 days (Fung et al., 2009). In other studies (De Vera et al., 2022, Qiao et al., 2018), Dehalobacter has been found to degrade 1,4-DCB; byproducts included chlorobenzene (CB) and benzene (BZ) via the hydrogenolysis pathway. Although many studies focused on 1,4-DCB biodegradation, reductive dechlorination of 1,4-DCB has not received the same attention. With a stable aromatic ring, degradation of 1,4-DCB through reductive abiotic pathways is challenging.

In field studies (Weatherill et al., 2018), degradation of chlorinated ethenes primarily followed biotic pathways such as halorespiration and cometabolism under anaerobic conditions. With a better understanding of iron (Fe) minerals in natural systems, abiotic dehalogenation has been recognized in the degradation of chlorinated ethenes in monitored natural attenuation (MNA) (He et al., 2015). Most studies of PCE and TCE degradation have been reported based on laboratory experiments with synthesized zero-valent iron (ZVI) (Garcia et al., 2021), pyrite (FeS₂) (Butler and Hayes, 1999), mackinawite (FeS) (Jeong et al., 2007), vivianite (Fe²⁺₃(PO₄)₂•8 H₂O) (Bae and Lee, 2012), magnetite (Fe₃O₄) (Culpepper et al., 2018), and green rust (Fe²⁺_4.5Mg_1.5Fe³⁺₂(OH)₁₈·4(H₂O)) (Ai et al., 2021). In the abiotic studies, reductive elimination is the dominant pathway for chlorinated ethene, with acetylene as the major byproduct.(He et al., 2015) While hydrogenolysis has been reported to some extent (Jeong et al., 2007, Liang et al., 2009), it is the primary pathway for biotic transformation with byproducts 1,1-dichloroethylene (1,1-DCE), cis-1,2-dichloroethylene (cis-DCE), vinyl chloride (VC), and ethene. The byproduct VC is more toxic than PCE and TCE (Lin et al., 2022), thus abiotic degradation may be a more attractive strategy. In field studies, abiotic degradation of PCE and TCE was observed with Fe(II) minerals in low-permeability source zones (Berns et al., 2019), dense non-aqueous phase liquids (DNAPLs) (Puigserver et al., 2022), and clay soils (Entwistle et al., 2019, Schaefer et al., 2017). However, contributions of nano-size reactive Fe mineral coatings in the degradation process have not yet been reported.

Reactive Fe minerals play an important role at the mineral-water interface. Under sulfate- and iron-reducing conditions, abiotic reduction of chlorinated solvents driven by synthesized reactive Fe minerals has been widely reported in laboratory-scale studies (He et al., 2009). The target compounds have included chlorinated ethenes (e.g., PCE (Butler and Hayes, 1999, Jeong and Hayes, 2007, Nunez Garcia et al., 2020) and TCE (Audí-Miró et al., 2015; Velimirovic et al., 2013)), chlorinated methanes (e.g., carbon tetrachloride (CCl₄) (Rodríguez-Fernández et al., 2018; Zhang et al., 2021b)), and chlorinated alkanes (e.g., 1,1,1-trichloroethane (Ji et al., 2019)). Based on studies with pure minerals, the general trend of mineral activity for chlorinated solvent degradation was summarized by He et al. (2015) as follows: disordered mackinawite > mackinawite > ZVI > pyrite > sorbed Fe²⁺ > green rust = magnetite > biotite > vermiculite. On the other hand, the reactivity of iron minerals in natural systems has been more difficult to resolve. Multiple studies demonstrated that Fe(II)-bearing clay (e.g., illite, chlorite, and riebeckite) and pyrite present in rock matrices participated in PCE and TCE dechlorination (Entwistle et al., 2019, Schaefer et al., 2017, Yu et al., 2018). However, several minerals in the matrix contribute to contaminant attenuation. With rates that are environmentally relevant, further work is needed in resolving degradation kinetics from Fe(II) mineral contributions (Berns et al., 2019, Schaefer et al., 2013).

In this study, an 18.3-m anoxic core (Landis et al., 2021) was collected and preserved from a site with historical contamination. In earlier work, the core was characterized as a function of depth (Yin et al., 2021) from which five RTZs were identified (Fig. 1). Analyses of mineralogy and morphology (Hua et al., 2020) revealed reactive Fe(II) mineral nano-coatings in RTZs that included mackinawite, pyrite, greigite (Fe₃S₄), siderite, and magnetite. Furthermore, a six-step sequential extraction analysis was applied to quantify Fe mineral coatings in sediment samples (Yin et al., 2022) (Fig. 1). To help resolve contributions of the reactive Fe mineral coatings in abiotic attenuation, studies were designed to evaluate reaction kinetics for constituents of concern (COCs): 1,4-DCB, PCE, and TCE. The study focuses on applying well-characterized sediments with Fe mineral coatings from the RTZs to address reaction rate expressions and rate constants in the abiotic degradation process for select COCs. To the best of our knowledge, it is the first study to resolve the abiotic dechlorination processes of chlorinated ethene and benzene in the presence of natural Fe(II) mineral nano-coatings. Importantly, this study relied on investigating nano-scale coatings that include metastable minerals, and therefore potential changes or impacts to Fe minerology and redox conditions must be obviated (e.g., O₂ exposure (Benning et al., 2000; Zhang et al., 2021a)). Methods used to prevent biological processes include physical sterilization with high level gamma-radiation or autoclaving (Bank et al., 2008, Berns et al., 2008, Tanaka et al., 2003), and chemical sterilization with, for example, sodium azide and methyl bromide (Otte et al., 2018, Tanaka et al., 2003). Because of the purpose of this study, these methods could not be employed. This study helps support MNA with reactive Fe mineral coatings found in the subsurface and in RTZs.