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

Abstract. 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 clay minerals and Fe/Mn-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 HgCl₂ solutions (Experiment 1 [EXP1]: 46.1 ± 1.1 mg L^-1; and Experiment 2 [EXP2]: 144 ± 6 mg L^-1) in low OM (0.16 ± 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. 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 ± 100 and 2320 ± 60 mg kg^-1 for the EXP1 and EXP2, respectively, and is similar to those determined for high OM soils. Desorption fits exponential decay models and 46 ± 6 % and 58 ± 10 % of the sorbed Hg is removed from the solid-phase materials at the termination of desorption in EXP1 and EXP2, respectively. This desorption profile is linked to the initial release of easily exchangeable Hg(II) species physically sorbed to Fe/Mn-oxides and clay mineral surfaces 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. Hg stable isotope data support preferential sorption of lighter isotopes to the solid-phase materials with results indicating isotopically heavy liquid-phase and isotopically light solid-phase. The divergence of δ²⁰²Hg (describing mass dependent fractionation (MDF)) between liquid- and solid-phase continues into desorption and we attribute this to lighter isotopes being favoured in secondary processes occurring after initial sorption to the solid-phase materials (i.e., matrix diffusion, change in Hg(II) speciation, elemental Hg (Hg(0)) production) that lead to less exchangeable forms of Hg. Consequently, heavy isotopes are preferentially released during desorption. These observations agree with data from HgCl₂ contaminated sites. 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 <175 °C, which again agree with field observations. Retardation (R_D) and distribution (K_D) coefficients are 77.9 ± 5.5 and 26.1 ± 3.0 mL g^-1 in EXP1, respectively, and 38.4 ± 2.7 and 12.4 ± 0.6 mL g^-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.