Concomitant behavior of arsenic and selenium from the karst infillings materials of the fractured carbonate Dogger Aquifer (Hydrogeological Experimental Site, Poitiers, France)

Highlights

• Clayey karst-infillings of the mid-Jurassic aquifer are geogenic sources of Se in the HES of Poitiers.

• Pyritic minerals were initially the main carrier of As and Se in the clayey materials.

• After pyrite oxidation, As was primarily associated with iron oxyhydroxides and Se was associated with OM.

• In acidic and basic conditions, their release is controlled by partial dissolution of the carrier phases and desorption processes, respectively.

• At circumneutral pH, As and Se exhibit different behaviors, with a higher release of Se than As.

Abstract

Petrographic and mineralogical analyses combined with sequential extractions and leaching experiments as a function of pH were performed on black clayey sediments fulfilling karsts in the Hydrogeological Experimental Site (HES) of Poitiers (France) to investigate the behavior of arsenic and selenium in a fractured limestone aquifer.

Sequential extractions showed that arsenic is mainly associated with pyrite (about 35%) and secondary iron oxyhydroxides (around 13%), along with a substantial exchangeable fraction (about 13%). The soluble fraction and the fraction associated to organic matter are ∼2% and ∼5%, respectively. The distribution of selenium is mainly pyritic (around 39%) or associated with organic matter (about 18%). Its association to secondary iron oxyhydroxides minerals is low (around 2%), whereas its soluble fraction is around 5%.

SEM analyses revealed the presence of arsenic “hot spots” into euhedral pyrite crystals surrounded by a halo of iron oxyhydroxides resulting from their alteration, and both are enriched with arsenic. Selenium has a similar pyritic origin but after alteration, it is predominantly associated with organic matter.

Despite different distributions, the leaching experiment as a function of pH showed that the mobilization of arsenic and selenium overlapped below pH 2 and above pH 8. The main differences were observed between pH 2 and 8 with a plateau at 5% of released selenium, whereas the amount of mobilized arsenic continuously decreased. The pH-dependence of both elements is attributed to the partial dissolution of pyrite in acidic conditions combined with desorption processes at higher pH values.

Introduction

Arsenic (As) and selenium (Se) are trace elements normally present in scarce amounts in water bodies. However, their cumulative build-up can cause detrimental effects on human health, including several types of cancer (Chen et al., 1992; Tamoto et al., 2015). As a result, the European Union established standards at 10 μg/L in 1998 (European Union, 1998) for both elements as limits of consumption in drinking water. Both elements exist as oxyanions, and their speciation is controlled by different factors such as pH, redox conditions (Eh), water composition, and dissolved oxygen (DO) (Rosen and Liu, 2009). The inorganic species are usually more harmful than the organic ones, and the most common forms found in nature are arsenite (AsIII) and arsenate (AsV) for arsenic, and selenite (SeIV) and selenate (SeVI) for selenium (Smedley and Kinniburgh, 2002; Nakamaru and Altansuvd, 2014; Tabelin et al., 2017).

The mechanisms that control the mobility of these elements firstly depend on the solubility of the primary bearing-minerals, such as sulfides, arsenides, and selenides. The first steps of mobilization are related to dissolution/precipitation processes and/or sorption/desorption reactions (Wilkin et al., 2018). Geogenic sources of As and Se were reported in aquifers of many countries around the world (Raessler, 2018; Wilkin et al., 2018). Common secondary phases in sediments are responsible for the retention of both elements, such as carbonates, oxhydroxides, organic matter, sulfides, and clay minerals. Thus, to correctly assess the environmental behavior of the two elements, it is important to investigate their distribution and their geochemical associations in sediments, as well as their potential mobility under different environmental conditions.

Parallel and sequential extractions are simple and effective methods that provide insights into the bioavailability, mobility, carrier phases, and release mechanisms of trace elements into water bodies (Tessier et al., 1979). They have been extensively used to assess the various distribution of As and Se in different solid constituents (Bassil et al., 2016; Javed et al., 2014; Kim et al., 2014; Martens and Suarez, 1997; Wenzel et al., 2001). Javed et al. (2014) showed that arsenic was mainly present in sulfide-bearing minerals in shale deposits and specifically sorbed on the mineral and organic phases. Likewise, Kim et al. (2014) showed that arsenic was mainly retained by sulfide minerals and their weathering products, including iron oxides, in the studied soils. On the other hand, Kulp and Pratt (2004) reported that sulfide minerals, mainly pyrite, and organic compounds were the predominant carriers for selenium in organic-rich chalks and shales. Similar results obtained by Matamoros-Veloza et al. (2011) revealed that the primary hosts for Se in various shales in the world are pyrite and organic matter, but they considered that selenium had a higher affinity for pyrite at lower concentrations.

In addition to chemical extractions, the analysis of the pH effect on the leachability of As and Se helps to better identify the carrier phases and their release mechanisms into the environment (Jegadeesan et al., 2008; Tabelin et al., 2014; Tamoto et al., 2015; Wang et al., 2018).

Selective extractions and leaching experiments of previous studies in the Dogger aquifer highlighted the diversity of selenium distribution in continental sedimentary clay karst infilling material, inducing multiple release mechanisms (Bassil et al, 2016a, 2018). The presence of soluble selenium in different wells of the Hydrogeological Experimental Site (HES) of Poitiers was nevertheless mainly attributed to the relatively important easy-mobilizable Se fraction. The mobility of this fraction seemed to be mainly ruled by its association with the organic matter present in the argillaceous samples.

The groundwater from the Dogger aquifer of the HES exhibits soluble selenium concentrations ranging between 10 and 40 μg/L in different wells whereas no detectable presence of arsenic was quantified (Bassil et al., 2016a, 2018). These behaviors contrast with the chemical composition of the associated sedimentary materials, which contain about fourfold higher arsenic than selenium (Bassil et al., 2016a, 2018). Even though the karsts partially filled by clayey sediment are discontinuous in the mid-Jurassic limestone, they represent an important rock-source of As and Se at the regional scale for Dogger aquifer.

Many studies examined the leaching behaviors of arsenic and selenium concomitantly (Su and Wang, 2011; Tabelin et al., 2014; Wilkin et al., 2018b; Yang and He, 2016). However, studies with systematic comparisons between the geochemical behavior of As and Se are limited. In the present study, we focused on comparing the distribution and behavior of the two elements in dark argillaceous sediments found in karstic cavities of a mid-Jurassic aquifer. The primary objectives are: (1) performing petrographic investigations using optical microscopy, scanning electron microscope coupled with energy dispersive X-ray (SEM-EDX), and X-ray diffraction; (2) evaluating the distribution of As and Se using parallel and sequential extractions; (3) examining the effect of pH on their release; (4) correlating the occurrences of As and Se as well as the coexisting ions and geochemical parameters. Finally, all data have been statistically analyzed based on the null hypothesis i.e. by assuming that the behaviors of arsenic and selenium are different.