Organic matters, but inorganic matters too: column examination of elevated mercury sorption on low organic matter aquifer material using concentrations and stable isotope ratios

McLagan, David S.; Esser, Carina; Schwab, Lorenz; Wiederhold, Jan G.; Richard, Jan-Helge; Biester, Harald

Sorption of mercury (Hg) in soils is suggested to be predominantly associated with organic matter (OM). However, there is a growing collection of research that suggests that clay minerals and inline-formula M1inlinescrollmathml chem normal Fe / normal Mn 36pt14ptsvg-formulamathimg90fe3a1452cc00b92ffa3e2e5adb0578 soil-10-77-2024-ie00001.svg36pt14ptsoil-10-77-2024-ie00001.png oxides are also important solid phases for the sorption of soluble Hg in soil–groundwater systems. We use a series of (60 mL syringe based) column experiments to examine sorption and subsequent desorption of HgClinline-formula2 solutions (experiment 1 (EXP1): 46.1 inline-formula± 1.1 mg Linline-formula−1; experiment 2 (EXP2): 144 inline-formula± 6 mg Linline-formula−1) in low-OM (0.16 inline-formula± 0.02 %) solid-phase aquifer materials. Analyses of total Hg concentrations, Hg speciation (i.e. pyrolytic thermal desorption (PTD)), and Hg stable isotopes are performed on both solid- and liquid-phase samples across sorption and desorption phases of the experiments.

page78The sorption breakthrough curve best fitted a Freundlich model. Despite the very low-OM content, the Hg equilibrium sorptive capacity in these columns is very high: 1510 inline-formula± 100 and 2320 inline-formula± 60 mg kginline-formula−1 for EXP1 and EXP2, respectively, and it is similar to those determined for high-OM soils. Data from the experiments on mass-dependent Hg stable isotope fractionation data from these experiments (described by inline-formulaδ202Hg) support preferential sorption of lighter isotopes to the solid-phase materials with results indicating an isotopically heavier liquid phase and an isotopically lighter solid phase. Desorption fits exponential decay models, and 46 inline-formula± 6 % and 58 inline-formula± 10 % of the sorbed Hg is removed from the solid-phase materials at the termination of desorption in EXP1 and EXP2, respectively. The divergence of inline-formulaδ202Hg values between liquid and solid phases also continues into desorption. This desorption profile is linked to the initial release of easily exchangeable Hg(II) species physically sorbed to inline-formula M15inlinescrollmathml chem normal Fe / normal Mn 36pt14ptsvg-formulamathimg324a1a6e466a287c68fb215644f27bf3 soil-10-77-2024-ie00002.svg36pt14ptsoil-10-77-2024-ie00002.png oxides and clay mineral surfaces (liquid phase enriched in heavy isotopes) and then slower release of Hg(II) species that have undergone secondary reaction to more stable/less-soluble Hg(II) species and/or diffusion/transport into the mineral matrices (processes favouring lighter isotopes; solid phase enriched in lighter isotopes). The secondary production of Hg(0) within the columns is confirmed by PTD analyses that indicate distinct Hg(0) release peaks in solid-phase samples at inline-formula<175inline-formulaC, which again agrees with field observations. Retardation (inline-formulaRD) and distribution (inline-formulaKD) coefficients are 77.9 inline-formula± 5.5 and 26.1 inline-formula± 3.0 mL ginline-formula−1 in EXP1, respectively, and 38.4 inline-formula± 2.7 and 12.4 inline-formula± 0.6 mL ginline-formula−1 in EXP2, respectively. These values are similar to values derived from column experiments on high-OM soil and provide the basis for future Hg fate and transport modelling in soil–groundwater systems.

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McLagan, David S. / Esser, Carina / Schwab, Lorenz / et al: Organic matters, but inorganic matters too: column examination of elevated mercury sorption on low organic matter aquifer material using concentrations and stable isotope ratios. 2024. Copernicus Publications.

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