Reduction of iron (hydr)oxide-bound arsenate: Evidence from high depth resolution sampling of a reducing aquifer in Yinchuan Plain, China

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Highlights

  • Less reducing, shallower sediments contain abundant oxidized Fe (hydr)oxides and As.

  • Sulfate reducing, deeper sediments contain As-sulfides and less oxidized Fe and As.

  • As(V)+As(III) to Fe(III) (hydr)oxides ratio is 0.64 mmol/mol in Yinchuan sediments.

  • Concurrent reduction of sediment oxidized Fe and As releases Fe(II) and As(III).

  • Highly enriched sediment As (~100 mg kg–1) is due to past groundwater discharge.

Abstract

Sediment in fluvial-deltaic plains with high-As groundwater is heterogenous but its characterization of As and Fe oxidation states lacks resolution, and is rarely attempted for aqueous and solid phases simultaneously. Here, we pair high-resolution (> 1 sample/meter) Fe extended fine-structure spectroscopy (EXAFS, n = 40) and As X-ray absorption near-edge spectroscopy (XANES, n = 49) with groundwater composition and metagenomics measurements for two sediment cores and their associated wells (n = 8) from the Yinchuan Plain in northwest China. At shallower depths, nitrate and Mn/Fe reducing sediment zones are fine textured, contain 9.6 ± 5.6 mg kg−1 of As(V) and 2.3 ± 2.7 mg kg−1 of As(III) with 9.1 ± 8.1 g kg−1 of Fe(III) (hydr)oxides, with bacterial genera capable of As and Fe reduction identified. In four deeper 10-m sections, sulfate-reducing sediments are coarser and contain 2.6 ± 1.3 mg kg−1 of As(V) and 1.1 ± 1.0 mg kg−1 of As(III) with 3.2 ± 2.6 g kg−1 of Fe(III) (hydr)oxides, even though groundwater As concentrations can exceed 200 μg/L, mostly as As(III). Super-enrichment of sediment As (42–133 mg kg−1, n = 7) at shallower depth is due to redox trapping during past groundwater discharge. Active As and Fe reduction is supported by the contrast between the As(III)-dominated groundwater and the As(V)-dominated sediment, and by the decreasing sediment As(V) and Fe(III) (hydr)oxides concentrations with depth.

Introduction

Elevated levels of geogenic arsenic (As) in groundwater used for drinking water supply have been detected in over 70 countries, causing the exposure to As from groundwater to be a major threat to public health (Smedley and Kinniburgh, 2002). A recent machine-learning based geostatistical model has estimated that 94–220 million people are at risk of exposing to unsafe level of As in groundwater that is above the World Health Organization’s provisional guideline value of 10 μg/L for drinking water, with 94% of the affected being in Asia (Podgorski and Berg, 2020). Because As also contributes significantly to soil-derived health risks (Antoniadis et al., 2019), understanding the coupling among biogeochemical cycles of As, Fe (O’Day et al., 2004a) and S (Bostick et al., 2004, Du Laing et al., 2009, O’Day et al., 2004a) in bio-hydro-lithospheres is crucial (Hussain et al., 2021, Kumarathilaka et al., 2018), and the need to mitigate the health risks has led to research on low cost and environmental friendly treatments (Shakoor et al., 2019, Amen et al., 2020). Especially relevant is the sediment iron (Fe) mineralogy due to the high affinity of As for the surface of Fe(III) (hydr)oxides (Bowell, 1994). Further, reductive dissolution and transformation of As-bearing Fe(III) (hydr)oxides is widely accepted as the key mechanism behind the As-tainted groundwater in reducing aquifers (e.g. Fendorf et al., 2010, Harvey et al., 2002, Natasha et al., 2021, Wallis et al., 2020, Smedley et al., 2005). Yet this prevailing paradigm is established mostly on the basis of aqueous geochemical characterization of the As-rich groundwater (Zheng et al., 2004), correlations between bulk and sequentially-extracted As and Fe concentrations in sediment depth profiles (Berg et al., 2001), or from ex situ incubation (Van Geen et al., 2004). Despite intense research on the groundwater As issues (e.g. Harvey et al., 2002, Frohne et al., 2011, LeMonte et al., 2017), the direct evidence for that reduction of sediment As associated with Fe(III) (hydr)oxides causes elevated As concentration in groundwater remains rare, especially lacking is the paired speciation analysis of As and Fe in aqueous and solid phases to support the active reduction of both As and Fe.

To understand the current and future distribution of groundwater As, it is critical to identify the minerals hosting and releasing As in aquifers and their redox transformations. Despite this realization, sediment Fe mineralogy is seldom investigated relative to more abundant studies of groundwater aqueous chemistry (Fendorf et al., 2010, Zheng et al., 2005). One of the reasons is that high-As groundwater occurs frequently in fluvial-deltaic aquifers where sediment deposits are notoriously heterogenous (Smedley and Kinniburgh, 2002, Goodbred and Kuehl, 2000, Anderson et al., 1999), contributing to a large degree of spatially heterogenous groundwater As distributions from local to basin scales (Berg et al., 2001, van Geen et al., 2003, Yang et al., 2015). In addition, quantifying Fe mineral composition in heterogenous and complex sediments and soils is analytically challenging (Postma et al., 2010, Sun et al., 2018). To date, most evidences supporting that As-bearing sediment Fe(III) (hydr)oxides is the culprit for high-As groundwater are based on sequential extraction, such as the Bengal Basin (Nickson et al., 2000, Reza et al., 2010b), the Mekong delta (Kocar et al., 2008, Kocar and Fendorf, 2009), the Red River delta (Berg et al., 2001), the Hetao Plain (Qiao et al., 2020, Shen et al., 2018) and the Datong Basin (Xie et al., 2008) in the Yellow River corridor like the Yinchuan Plain in this study. However, interpreting these extraction data is not only ambiguous but also limited by its capability of delineating speciation. Anaerobic phosphate extraction has been used to evaluate sediment As speciation in Bangladesh, with only 2 of 4 samples showing consistent As(III)% with XANES data (Jung et al., 2012). A bigger problem is iron because even the hot-HCl extracted Fe(II)/Fe ratio used to evaluate redox condition (Horneman et al., 2004), is only a portion of the bulk Fe (Aziz et al., 2017).

Among many mineralogical techniques, synchrotron X-ray absorption spectroscopy, especially EXAFS, has proven to be advantageous in quantifying Fe mineralogy in sediments and soils through linear combinations of reference spectra or theoretical shell fitting (O’Day et al., 2004b, Sun et al., 2016). To date, only a handful of studies have characterized Fe mineralogy in geogenic high-As aquifers using EXAFS, with far less sediment Fe and As mineralogy data than bulk Fe and As concentrations (Table S1) (Aziz et al., 2017, Schaefer et al., 2017, Quicksall et al., 2008, Stuckey et al., 2015, Gnanaprakasam et al., 2017). For example, only 8 samples from one 30-m core in the Jianghan Plain were included for As XANES (n = 9) and Fe EXAFS (n = 8) analyses while the sampling resolution for bulk chemistry was roughly one sample per 1.5 m (Schaefer et al., 2017). High-resolution sampling of both sediment and water offers a unique opportunity to study redox transformations across chemical gradients that are otherwise difficult to characterize, and in doing so, help to develop an unambiguous understanding of which phases host As, and which are sources of groundwater As in reducing aquifers, and the relative roles of Fe and As reduction in the formation of groundwater As.

The main objectives of this study were therefore (i) to characterize sediment Fe and As mineralogy in As-rich reducing aquifers at high depth resolution, (ii) to delineate the carrier phases of As and the transformations of those phases that affect As release into groundwater, and (iii) to identify geogenic processes that result in the enrichment of As in heterogeneous sediments. This research focuses on two sediment cores, YCA and YCB, from the Yinchuan Plain, with semi-confined Quaternary aquifers consisting of late Pleistocene and Holocene unconsolidated alluvial-lacustrine sediments. It has been shown that shallow (< 40 m) groundwater in the Yinchuan Plain has As concentration between 3 and 177 μg/L (n = 142) (Han et al., 2013). Both cores were sampled at high-spatial resolution (each m or higher, to 30 m for YCA and to 40 m for YCB) to determine As speciation via XANES and Fe mineralogy using EXAFS. Additional measurements on the sediments included chemical extraction and X-ray Fluorescence (XRF). The redox zonation of the aquifer was described using mineralogical data in combination with changes in groundwater composition and sedimentology as a function of depth. In addition, metagenomic analysis of microbial community was conducted within the Fe/Mn-reducing zones to identify specific microbial processes affecting the redox chemistry of As and Fe. These data provide a window into the processes that control groundwater As levels through both the release and accumulation of As in sediments.

Section snippets

Hydrogeologic setting

The study area is located in the northern Yinchuan Plain, northwestern China (Fig. 1), where a high subsidence rate of 0.22 cm per year has resulted in substantial accumulation of Quaternary sediments up to 2000 m thick (Han et al., 2013, Wang et al., 2015). The northern plain consists of multiple layers of alluvial and lacustrine deposits forming an unconfined shallow aquifer with depths between 10 and 40 m and two confined aquifers with depths between 25 and 60 m and > 140 m (Han et al., 2013

Groundwater and sediment redox condition

The depth profiles of redox-sensitive parameters suggest sequential reduction of NO3-, followed by overlapping Mn and Fe reduction and finally SO42- (Fig. 2), with sulfate reducing conditions dominating at > 20 m depth at YCA and YCB (Fig. 1). All groundwater are likely anoxic as indicated by low DO (Table S3) with non-detectable nitrate (<0.01 mg L–1), except the shallowest well YCA-3 m with 7.8 mg L–1 NO3. Groundwater from YCA-3 m is also the least reducing and is undergoing active nitrate

Evidence for As mobilization by active reductive dissolution of As(V) bounded Fe(III) (hydr) oxides

Mobilization of As by reductive dissolution of its host in the sediments, i.e., Fe(III) (hydr)oxides has been widely accepted as the mechanism for elevated levels of geogenic As in groundwater of South and Southeast Asia, supported primarily by groundwater chemistry data (Smedley and Kinniburgh, 2002, Zheng et al., 2004). Laboratory adsorption experiments have shown that As adsorbed on ferrihydrite, goethite or complexed with Fe(III) nano-particles can possess a very high As:Fe molar ratio of

CRediT authorship contribution statement

Yuqin Sun: Investigation, Methodology, Formal analysis, Writing - Original draft preparation, Writing - Review & Editing; Jing Sun: Methodology, Writing - Review & Editing; Athena Nghiem: Investigation, Writing - Review & Editing; Benjamin Bostick: Investigation, Methodology, Writing - Review & Editing, Funding acquisition; Tyler Ellis: Investigation; Long Han: Investigation, Methodology; Zengyi Li: Investigation, Methodology; Songlin Liu: Investigation; Shuangbao Han: Investigation; Miao Zhang

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

Funding for this work was provided by the National Natural Science Foundation of China Grant 41772265 and 41831279 and Shenzhen Science and Technology Innovation Commission Grant GJHZ20180411143520274 to Y.Z., and National Science Foundation (NSF) grant EAR 15-21356, National Institute of Environmental Health Sciences grant ES010349 to B.C.B., and an NSF Graduate Research Fellowship to A.A.N. We thank Tingwen Wu from CGS and Yunjie Ma, Meng Ma from Y.Z.’s lab for their help with the fieldwork;

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