Experimental evaluation of sampling, storage and analytical protocols for measuring arsenic speciation in sulphidic hot spring waters

Highlights

• Evaluation of arsenic preservation during sampling, storage and speciation analysis in sulphidic, low iron hot springs.

• Liquid nitrogen freezing and oxygen-­scrubbing sachets in secondary containment prevent arsenic species artefact generation.

• Separation and quantification of thiomethylated arsenic species requires the use of an Atlantis C18 over an IonPac column.

Abstract

Recently developed analytical techniques allowed for the detection of a range of dissolved arsenic-sulphur species in sulphur-rich environments. These so called thioarsenates are unstable, however, and can degrade upon handling and storage. An experiment evaluating the effect of exposure to air on arsenic and sulphur-enriched geothermal waters demonstrated a near to complete loss of thioarsenate species to arsenite or arsenate during short oxidation times. In contrast, thioarsenic standards were stable for the duration of analysis in spite of exposure to air. For samples containing thio-methylated arsenic species, the extent of oxidation varied for different methylated arsenic species. This study recommends flash freezing of samples in liquid nitrogen immediately after recovery and further storage under anaerobic conditions at − 80 °C. A second experiment to test the efficiencies of different HPLC columns for separating arsenic species resulted in the preference for an IonPac column with NaOH as the mobile phase when analysing arsenic thioanions, over the commonly used PEEK PRP-X100 anion exchange and Atlantis C18 reverse phase column with ammonium phosphate mobile phases. Distinct separation of thio-methylated arsenic species with the IonPac column, however, was not successful potentially due to matrix components. Acceptable detection, separation and quantification of thio-methylated arsenic species were only achieved with the Atlantis C18 column. This study shows that preservation and analysis of samples is matrix dependent, which holds important implications for efforts to interpret arsenic speciation in geothermal waters, especially those of low pH (2–3), low oxygen (≤ 49% saturation), low iron (≤ 5 mg L^− 1) and high sulphur concentrations (≥ 91 mg L^− 1).

Introduction

Accurate measurement of arsenic speciation is critical for understanding the arsenic biogeochemical cycle in hydrothermal systems. Until recently the inorganic oxyanions arsenate and arsenite were believed to be the only dissolved arsenic species in hydrothermal waters [1], [8], [22], [25]. Arsenic bonds with free oxygen and forms arsenic oxyanions in different oxidation states: arsenite (As^IIIO₃^3 −) is stable under reduced geochemical conditions and arsenate (As^VO₄^3 −) under more oxidising conditions [20]. An increase in redox potential promotes the oxidation of arsenite to arsenate, while an increase in pH results in deprotonation of protonated arsenic species.

The majority of geothermal waters, however, contain sulphur (e.g., [6]), which has a higher affinity for arsenic than does oxygen [18]. In the presence of sulphide, low pH and redox conditions, As-S species can be formed (Fig. 1).

Arsenite can be transformed by reaction with either sulphide or elemental sulphur into dissolved arsenic-sulphur species referred to as thioarsenate species i.e. mono- di- and trithioarsenate, with sulphide or elemental sulphur [14], [21], e.g., as follows:

$${\mathrm{H}}_3{\mathrm{AsO}}_3+{\mathrm{HS}}^{-}\to {\mathrm{H}}_2{\mathrm{AsO}}_3{\mathrm{S}}^{-}+{\mathrm{H}}_2$$

$${\mathrm{H}}_3{\mathrm{AsO}}_3+{\mathrm{S}}^0\to {\mathrm{H}}_2{\mathrm{AsO}}_3{\mathrm{S}}^{-}+{\mathrm{H}}^{+}$$

In addition to abiotic thioarsenate transformations, microorganisms are also able to transform thioarsenate species directly via arsenic methylation, an arsenic resistance mechanism [7], [10], [11], [16], [19], or indirectly via sulphur metabolism [6].

In order to identify and distinguish between abiotic and biotic arsenic transformation processes, it is essential to quantify all inorganic and organic arsenic oxy- and thioanions present. In this respect, thioarsenic species are more difficult to measure than arsenic oxyanions as a result of their lower stability with respect to potential oxidation. Previous studies have described thioarsenic species' stability under a range of sulphide concentrations, pH values and oxygen saturations [2], [4], [14]. Helz and Tossell [4] evaluated Gibbs free energy for the formation values of thioarsenate formation reactions (1 atm, 298 K) and concluded that, in the presence of sulphide, arsenate is the most stable arsenic species, followed by arsenite, MTA, DTA, TriTA and TetraTA.

$$\mathrm{\Delta }{\mathrm{G}}_{\mathrm{f}}^0\left({\mathrm{H}}_3\mathrm{As}{\mathrm{O}}_3\right)=‐640.03\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{‐1}$$

$${\mathrm{\Delta G}}_{\mathrm{f}}^0\left({\mathrm{H}}_3{\mathrm{AsO}}_4\right)=-766.75\mathrm{kJ}{\mathrm{mol}}^{-1}$$

$${\mathrm{\Delta G}}_{\mathrm{f}}^0\left({\mathrm{H}}_3{\text{AsSO}}_3\right)=-620.2\mathrm{kJ}{\mathrm{mol}}^{-1}$$

$${\mathrm{\Delta G}}_{\mathrm{f}}^0\left({\mathrm{H}}_3{\mathrm{AsS}}_2{\mathrm{O}}_2\right)=-411.5\mathrm{kJ}{\mathrm{mol}}^{-1}$$

$${\mathrm{\Delta G}}_{\mathrm{f}}^0\left({\mathrm{H}}_3{\mathrm{AsS}}_3\mathrm{O}\right)=-222.2\mathrm{kJ}{\mathrm{mol}}^{-1}$$

$${\mathrm{\Delta G}}_{\mathrm{f}}^0\left({\mathrm{H}}_3{\mathrm{AsS}}_4\right)=-27.7\mathrm{kJ}{\mathrm{mol}}^{-1}$$

Oxic conditions deplete thioarsenate species very quickly via conversion to less thiolated arsenic species and eventually arsenite [12]. Planer-Friedrich et al. [14] showed that the [SH⁻]:[OH⁻] activity ratio affects the stability of thioarsenic species. When [SH⁻] > [OH⁻] under anaerobic conditions, the most stable arsenic species is thioarsenite, whereas under aerobic conditions thioarsenate is more stable. When [SH⁻] < [OH⁻] under either aerobic or anaerobic conditions, the most stable arsenic species is arsenite, as thiolated arsenic species would be unstable at excess OH⁻. Arsenite oxidation, however, only starts to occur in significant levels when the thioarsenate species are almost completely transformed to arsenite.

$${\mathrm{H}}_2{{\text{AsOS}}_3}^{-}+2.5{\mathrm{O}}_2\to {\mathrm{H}}_2{\mathrm{AsO}}_2{\mathrm{S}}_2^{-}+{\mathrm{SO}}_4^{2-}$$

$${\mathrm{H}}_2\mathrm{As}{\mathrm{O}}_2{\mathrm{S}}_2^{-}+2.5{\mathrm{O}}_2\to {\mathrm{H}}_2\mathrm{As}{\mathrm{O}}_3{\mathrm{S}}^{-}+\mathrm{S}{\mathrm{O}}_4{{}^2}^{-}$$

$${\mathrm{H}}_2{\mathrm{AsO}}_3{\mathrm{S}}^{-}+{\mathrm{H}}^{+}\to {\mathrm{H}}_3{\mathrm{AsO}}_3+{\mathrm{S}}^0$$

$${\mathrm{H}}_3{\mathrm{AsO}}_3+{\mathrm{O}}_2\to {\mathrm{H}}_2{{\mathrm{AsO}}_4}^{-}$$

These artefacts produced by thioarsenate species oxidation, and to some extent also arsenite oxidation, can occur during sampling and storage [12]. Therefore, correct preservation of samples for analysis is critical. Acidification and storage at 4 °C is a commonly used preservation technique [25]. In sulphide- or sulphur-rich waters, however, acidification can potentially lead to total dissolved arsenic loss via induced arsenic-sulphur precipitation under low pH [12] and storage of samples at 4 °C could lead to thioarsenate degradation, due to the rapid oxidation of arsenic thioanions compared to oxyanions [12].

For analysis, high-performance liquid chromatography coupled to inductively coupled plasma mass spectrometry (HPLC–ICPMS) using an IonPac column and a sodium hydroxide gradient is a well-established laboratory method for thioarsenate speciation [13], [21], [23], while an Atlantis C18 column with phosphate buffer is commonly used to separate thio-methylated arsenic species [9]. These separation techniques, however, require an efficacy evaluation with different sample types. In this paper we describe our experiences in preserving geothermal samples and the analysis of thioarsenates and methylated thioarsenic species in arsenic-enriched hot spring waters by HPLC–ICPMS.