Mercury Isotope Fractionation in the Subsurface of a Hg(II) Chloride-Contaminated Industrial Legacy Site.

To understand the transformations of mercury (Hg) species in the subsurface of a HgCl2-contaminated former industrial site in southwest Germany, Hg isotope analysis was combined with an investigation of Hg forms by a four-step sequential extraction protocol (SEP) and pyrolytic thermodesorption. Data from two soil cores revealed that the initial HgCl2 was partly reduced to metallic Hg(0) and that Hg forms of different mobility and oxidation state coexist in the subsurface. The most contaminated sample (K2-8, 802 mg kg-1 Hg) had a bulk δ202Hg value of around -0.43 ± 0.06‰ (2SD), similar to published average values for industrial Hg sources. Other sample signatures varied significantly with depth and between SEP pools. The most Hg-rich samples contained mixtures of Hg(0) and Hg(II) phases, and the water-extractable, mobile Hg pool exhibited heavy δ202Hg values of up to +0.18‰. Sequential water extracts revealed slow dissolution kinetics of mobile Hg pools, continuously releasing isotopically heavy Hg into solution. This was further corroborated by heavy δ202Hg values of groundwater samples. Our results demonstrate that the Hg isotope signature of an industrial contamination source can be significantly altered during the transformations of Hg species in the subsurface, which complicates source tracing applications but offers the possibility of using Hg isotopes as process tracers in contaminated subsurface systems.

Comparing Modeled and Measured Mercury Speciation in Contaminated Groundwater: Importance of Dissolved Organic Matter Composition.

In addition to analytical speciation, reliable Hg species modeling is crucial for predicting the mobility and toxicity of Hg, but geochemical speciation codes have not yet been tested for their prediction accuracy. Our study compares analyses of Hg species in highly Hg-contaminated groundwater (Hgtot: 0.02-4 µmol·L(-1)) at three sites with predictions of Hg speciation obtained from three geochemical codes (WHAM, Visual MINTEQ, PHREEQC) with and without implementation of Hg complexation by dissolved organic matter (DOM). Samples were analyzed for chemical composition, elemental, inorganic, and DOM-bound Hg (Hg(0), Hginorg, HgDOM). Hg-DOM complexation was modeled using three approaches: binding to humic/fulvic acids, binding to thiol-groups, or a combination of both. Results of Hg(0) modeling were poor in all scenarios. Prediction accuracy for Hginorg and HgDOM strongly depended on the assumed DOM composition. Best results were achieved when weaker binding sites, simulated by WHAMs DOM submodel, were combined with strongly binding thiol groups. Indications were found that thiol-DOM ratios in groundwater are likely to be lower than in surface water, highlighting the need for analytical thiol quantification in groundwater DOM. This study shows that DOM quality is a crucial parameter for prediction of Hg speciation in groundwater by means of geochemical modeling.

Potential of Brass to Remove Inorganic Hg(II) from Aqueous Solution through Amalgamation.

Brass shavings (CuZn45) were tested for their efficiency to remove Hg(II) from contaminated groundwater through amalgamation. The study was focused on long-term retention efficiency, the understanding of the amalgamation process and kinetics, and influences of filter surface alteration. Column tests were performed with brass filters (thickness 3 to 9 cm) flushed with 1000 µg/L Hg solution for 8 hours under different flow rates (300 to 600 mL/h). Brass filters consistently removed >98% of Hg from solution independent of filter thickness and flow rate. In a long-term experiment (filter thickness 2 cm), Hg retention decreased from 96 to 92% within 2000 hours. Batch and column experiments for studying kinetics of Hg removal indicate ~100% Hg removal from solution within only 2 hours. Solid-phase mercury thermo-desorption analysis revealed that Hg(0) diffusion into the brass surface controls kinetics of mercury retention. Brass surface alteration could be observed, but did not influence Hg retention.

Mercury removal from contaminated groundwater: Performance and limitations of amalgamation through brass shavings.

Brass shavings have been proposed as a cost-effective filter material to remove Hg from contaminated groundwater. This method, which is based on the reduction of reactive Hg(II) and subsequent formation of amalgams, has been shown to be fast and effective in the short term. However, the effectiveness of brass filters and their stability over the long term, especially if used in passive filter systems such as permeable reactive barriers (PRB) under high flow conditions, is unknown. To evaluate the performance and limitations of brass shavings for Hg removal from contaminated groundwater, we performed long-term pilot scale filtration tests (6 and 28 months) at two former wood impregnation sites with severe groundwater contamination (up to 870 µg L(-1) Hg). The results showed that even under high flow conditions (>60 m d(-1)), 60-80% of the Hg was removed in the first 8 mm of the brass shavings filter bed. The kinetics of filtration, Hg total removal performance (>99.95%), and loading capacity (164 g L(-1)) surpassed those of a Hg-specific synthetic resin (LEWATIT(®)MonoPlus TP-214). However, under natural pH conditions (pH 6.4 and 6.7), Zn was leached from the brass and exceeded the threshold value (0.5 mg L(-1)) in the filter outflow by up to a factor of 40. Increasing pH (>8.5) decreased the Zn concentration (<0.05 mg L(-1)) but affected Hg removal due to the formation of Zn-hydroxide/carbonate coatings on the brass (up to 15% performance reduction). Thus, the use of brass shavings as an exclusive filter material in PRBs is restricted to aquifers with high pH. However, brass is ideal as a low-cost, thin-bed prefilter in onsite systems to remove the main Hg load from groundwater when Zn release is managed.

Mercury (II) reduction and co-precipitation of metallic mercury on hydrous ferric oxide in contaminated groundwater.

Mercury (Hg) speciation and sorption analyses in contaminated aquifers are useful for understanding transformation, retention, and mobility of Hg in groundwater. In most aquifers hydrous ferric oxides (HFOs) are among the most important sorbents for trace metals; however, their role in sorption or mobilization of Hg in aquifers has been rarely analyzed. In this study, we investigated Hg chemistry and Hg sorption to HFO under changing redox conditions in a highly HgCl2-contaminated aquifer (up to 870µgL(-1) Hg). Results from aqueous and solid phase Hg measurements were compared to modeled (PHREEQC) data. Speciation analyses of dissolved mercury indicated that Hg(II) forms were reduced to Hg(0) under anoxic conditions, and adsorbed to or co-precipitated with HFO. Solid phase Hg thermo-desorption measurements revealed that between 55 and 93% of Hg bound to HFO was elemental Hg (Hg(0)). Hg concentrations in precipitates reached more than 4 weight %, up to 7000 times higher than predicted by geochemical models that do not consider unspecific sorption to and co-precipitation of elemental Hg with HFO. The observed process of Hg(II) reduction and Hg(0) formation, and its retention and co-precipitation by HFO is thought to be crucial in HgCl2-contaminated aquifers with variable redox-conditions regarding the related decrease in Hg solubility (factor of ~10(6)), and retention of Hg in the aquifer.