The response of mercury (Hg) transformation in soil to sulfur compounds and sulfur-rich biowaste application

The mobility of mercury and its transformation as affected by different sulfur-rich amendments were investigated in a model laboratory incubation experiment. Two soils, Chernozem and Luvisol, differing in their physicochemical characteristics, were selected for the experiment. The soils were artificially contaminated with Hg by adding HgCl₂ solution to a final concentration of 12 mg kg⁻¹ of Hg in the soils. Subsequently, organic and inorganic amendments: (1) (NH₄)₂SO₄, (2) l-cysteine, and (3) digestate, a biowaste from a biogas station, were applied and the soils were incubated for 21 days in the dark. Soil samples were collected after 1, 7, 14 and 21 days of incubation. At the individual sampling times 30 g of each soil was collected for determinations of pH, the mobile Hg pool, carbon derived from microbial biomass, and dehydrogenase activity. The results confirmed the important role of digestate application leading to (1) improved nutrient status and microbiological activity in the contaminated soils and (2) an increased proportion of methylmercury in the soils as well as a decrease in mercury volatilization. These findings suggested that digestate could be applied to Hg contaminated soil for effective stabilization of this element in the soil. However, long-term experiments are necessary for an evaluation of further potential Hg transformations due to the decomposition of digestate-bearing organic matter.