A process-based model for describing redox kinetics of Cr(VI) in natural sediments containing variable reactive Fe(II) species
Graphical abstract
Introduction
Natural sediments contain a variety of Fe-bearing minerals, including Fe-(oxyhydr)oxides and clay minerals (Stucki et al., 2012). Fe occurs in two redox states, Fe(II) and Fe(III). Redox transformation between Fe(II) and Fe(III) has a significant impact on other redox-sensitive elements (Kappler et al., 2021). For example, Fe(II) is a natural reductant that plays an important role in the transport and fate of groundwater metals and contaminants such as Cr(VI), As(V), Tc(VII), and U(VI) (Brookshaw et al., 2015; Whitaker et al., 2018). Since natural sediments contain various Fe(II) species, including aqueous Fe(II), sorbed Fe(II) and other solid forms of Fe(II) (Hofstetter et al., 2003), they exhibit diverse reactive properties based on Fe(II) speciation and availability (Brookshaw et al., 2016; Fredrickson et al., 2004; Whitaker et al., 2018; Zhao et al., 2015). Consequently, redox cycling of Fe(II) and Fe(III) and the heterogeneous reactivity of Fe(II) pose a significant challenge for determining and scaling the geochemical reaction parameters of sediments from laboratory to field scale applications, especially under temporally and spatially varying redox conditions (Ginn et al., 2017; Joe-Wong et al., 2017; Qafoku et al., 2017; Xu et al., 2018).
The reactivity of Fe(II)-containing sediment toward the reduction of Cr(VI), a common groundwater contaminant found naturally and from industrial wastes (Gorny et al., 2016; Yan et al., 2022), has been intensively studied because Cr(VI) reductive immobilization to Cr(III) is an effective groundwater remediation method (Bishop et al., 2019; Joe-Wong et al., 2017). Owing to the complexity of Fe(II) speciation in natural sediments, the reported Cr(VI) reduction rates in natural sediments varied from ∼0.05 to ∼5 mg−1 g−1 h−1 (Cr(VI) reduction rates were normalized by Fe content in the sediments) (Jung et al., 2020; Xu et al., 2018, 2017), implying large uncertainty in sediment-associated Fe(II) reactivity. In our previous studies, a multirate model was proposed to describe Fe(II) reactivity toward Cr(VI) reduction in natural sediments (Liu et al., 2017; Xu et al., 2018). The multirate model assumes that the reaction in a sediment can be described by linearly adding the reactions of individual rate fractions of Fe(II). Since it is extremely difficult to characterize all the reactive Fe(II) species in natural sediments, a three-step sequential extraction method was developed to categorize the reactive Fe(II) into three fractions based on Fe(II) extractabilities in sodium acetate and HCl solutions (Xu et al., 2018). The model well described Cr(VI) reduction in different natural sediments collected from the Columbia River hyporheic zone at the Hanford site in Washington, USA.
In natural sediments, the sequential nature of electron acceptor utilization by microorganisms results in variation in redox conditions, thus influencing redox cycling of Fe(II) and Fe(III) (Ferris et al., 2021). Under anoxic conditions, Fe transformation in natural sediments is tightly coupled with other elements (such as C, N and Mn) through complex biogeochemical processes (Kappler et al., 2021; Tian et al., 2020). Fe(II) can be enzymatically oxidized coupled with nitrate reduction by nitrate-reducing Fe(II)-oxidizing bacteria (Klueglein et al., 2013) and chemically oxidized by Mn-(oxyhydr)oxides (Madison et al., 2013). Nitrate and Mn-(oxyhydr)oxides have higher redox potentials (EH) than Fe(III) and would be preferentially utilized as terminal electron acceptors by microorganisms from a thermodynamic point of view (Bhanja et al., 2019). In either way, nitrate and Mn-(oxyhydr)oxides might inhibit or compete with Fe(III) reduction and have an indirect effect on the redox reactivity of the sediments. For example, the longevity of the in situ redox manipulation groundwater barrier, which was installed at the Hanford site in Washington, USA, to prevent migration of groundwater Cr(VI) via its reduction by Fe(II) generated in the barrier, was reduced by 50% due to the presence of nitrate in the groundwater (Szecsody et al., 2005). Therefore, extensive experiments have been conducted to investigate the coupled biogeochemical processes of C, N, Fe, and Mn (Chen et al., 2020; Lin and Taillefert, 2014; Madison et al., 2013), and a few studies have developed numerical models to describe their coupled reaction kinetics under different scenarios (Bhanja et al., 2019; Liu et al., 2017; van Bodegom and Scholten, 2001; Wang and Choi, 2013). Although the influence of coupled biogeochemical processes on Fe transformation has attracted much attention, little is known about how the reactive Fe(II) fractions proposed in the multirate model change with Fe transformation, limiting the field applications of the model.
Freshly biogenic Fe(II) interacts with sediments through sorption of Fe(II), electron transfer and Fe atom exchange between sorbed Fe(II) and structural Fe(III), and mineral phase transformation (Latta et al., 2012). As such, redistribution of freshly generated Fe(II) should influence the reactive Fe(II) speciation in the sediments. Previous studies have suggested that Fe(II) reactivity (represented as reaction rate or rate constant) in aqueous Fe(II)/Fe-(oxyhrdr)oxide suspensions correlated with the density of the bounded Fe(II) and the Fe-(oxyhrdr)oxide species (Amonette et al., 2000). Consequently, the redistribution of biogenic Fe(II) could change the reactivity of sediment-associated Fe(II). A hypothesis to explain this phenomenon is that the values of log(k) of the bounded Fe(II) are linearly correlated with the free energy of the electron transfer reaction (ΔrG), which is correlated with the EH of the bounded Fe(II) if electron transfer occurs preceding the rate-limiting step to reach equilibrium (Stewart et al., 2018). Moreover, EH is approximately linearly correlated with log() if thermodynamic equilibria, such as sorptive equilibrium and redox equilibrium, are achieved (Gorski et al., 2016). To date, such good correlations have been reported only in simple systems, such as redox reactions between Fe(II)/Fe-(oxyhydr)oxide and soluble electron transfer meditators. The two assumptions may not be valid for Cr(VI) reduction in natural sediments. Thus, it is necessary to investigate how anoxic Fe transformation influences sediment-associated reactive Fe(II) speciation in sediments and whether the redistribution of biogenic Fe(II) changes the reactivity of Fe(II) toward Cr(VI).
In this study, sediments collected from a river corridor along the Zhenlong River in Hengxian, Guangxi, China, were used as an example to investigate the kinetics of Fe(II) reactivity toward Cr(VI) in natural sediments during the coupled biogeochemical transformation of N, Mn, Fe, and C under anaerobic conditions. The study site is located in an intensive agricultural area, where nitrate is a common natural water pollutant and some soils and sediments are contaminated with Cr (Shen et al., 2019; Yang et al., 2021). Anaerobic incubation experiments were conducted to investigate the dominant biogeochemical processes in the sediments and their kinetics. During incubation, a three-step sequential extraction method was used to determine the variation in Fe(II) fractions. Then, Cr(VI) reduction experiments were conducted to characterize the variation in reactivities of the three Fe(II) fractions. Eventually, a process-based model was developed to simulate the kinetics of the three Fe(II) fractions and the variation in their reactivity toward Cr(VI) reduction under varying redox conditions. The results were compared with those in sediments from the Hanford site to provide new insights into the dynamic reduction reactivity of Fe(II) toward Cr(VI) in natural sediments, and the model developed in this study is applicable to describe and predict Cr(VI) reduction in natural sediments under varying redox conditions.
Section snippets
Study site, sediment sampling, and sample characterization
The study site is located in an intensive agricultural area in Hengxian County, Guangxi Province, China (Fig. 1). Details of the geological and chemical features of the study area are provided in Text S1. In 2017, uncontaminated sediment samples were collected upstream of the Zhenlong River near Dengxu town to investigate the biogeochemical reaction kinetics that influence the fate of Cr. Sediment samples were collected from 11 sampling points (Fig. 1), at which point three subsamples (∼250 g
Sediment properties and Cr(VI) immobilization pathways in the sediment
The sediment properties, including pH, mineralogy and grain size, are provided in Text S6. The TOC content was high (2.88±0.25%) due to agricultural activities. The total Mn content was 2.99±1.95 μmol/g. The reducible Fe content was 40.44±25.49 μmol/g. The concentrations of the total sediment-associated reactive Fe(II) ranged from 2.30 to 6.48 μmol/g due to the strong heterogeneity of the sediments. The concentration of AVS was near the detection limit (<5 μM), implying the absence of AVS.
An
Conclusions
A process-based model was developed to describe the influence of coupled biogeochemical processes on the variation in reactivity of sediment-associated Fe(II) toward Cr(VI) reduction. The model was used to investigate Cr(VI) reduction rates in natural sediments from Dengxu, China and Hanford, USA.
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The average reduction rate constants (<k>) for Cr(VI) reduction by reactive Fe(II) in different sediments varied by more than 4 orders of magnitude.
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The large variation in Fe(II) reactivity in the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research is funded by the National Key Research and Development Plan of China (Grant No. 2020YFC1807004), National Natural Science Foundation of China (Grant No. 41773111 and 41977169), Jiangsu ‘Double Innovation Plan’, and Ministry of Land and Resources of China [2017-2676].
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