Mercury speciation analyses in HgCl_2-contaminated soils and groundwater-Implications for risk assessment and remediation strategies

摘要

Since the 19th century, mercury(II)chloride (HgCl_2) has been used on wood impregnation sites to prevent wooden poles from decay, leaving behind a legacy of highly contaminated soil/aquifer systems. Little is known about species transformation and mobility of HgCl_2 in contaminated soils and groundwater. At such a site the behaviour of HgCl_2 in soils and groundwater was investigated to assist in risk assessment and remediation. The soil is low in organic carbon and contains up to 11,000 mgHg/kg. Mercury (Hg) concentrations in groundwater decrease from 230 to 0.5 μg/l within a distance of 1.3 km. Hg species transformations in soil and aqueous samples were analysed by means of solid-phase Hg pyrolysis and CV-AAS. In aqueous samples, Hg species were distinguished between ionic/ reactive Hg and complex-bound Hg. Potential mobility of Hg in soils was studied through batch experiments. Most Hg in the soil is matrix-bound HgCl_2, whereas in the aquifer secondary formation to Hg~0 could be observed. Aqueous Hg speciation in groundwater and soil solutions shows that an average of 84% of soluble Hg exists as easily reducible, inorganic Hg species (mostly HgCl_2). The proportion of complex-bound Hg increases with distance due to the transformation of inorganic HgCl_2. The frequent occurrence of Hg~0 in the aquifer suggests the formation and degassing of Hg~0, which is, in addition to dilution, an important process, lowering Hg concentrations in the groundwater. High percentage of mobile Hg (3-26%) and low seepage fluxes will result in continuous Hg release over centuries requiring long-term groundwater remediation. Results of soluble Hg speciation suggest that filtering materials should be adapted to ionic Hg species, e.g. specific resins or amalgamating metal alloys.