Effects of Salinity and soil improvers on Mercury Mobilization from polluted soils

Coastal soils in North-East Italy have a widespread mercury (Hg) contamination, due to past mining activity. These soils could become hotspots of Hg re-mobilisation, because of flooding and salinization caused by the sea level rise expected as a consequence of climate change. The aim of the present work was to assess changes in Hg solubility in contaminated coastal soils, after exposure to increasing salinity, flooding and soil improvers. A microcosm simulations of different salinity levels, flood events and soil improvers were carried out using soil columns filled with soil and the addition of 1% of biochar, gypsum and their mix. The soils were kept submerged with 25 mL of solutions of different salt concentrations ranging from 0 to 32.8 g L-1 for 30 days. After the flooding treatment, leached solutions were collected and the solubilized mercury was quantified. Mercury concentration in coastal soils was 34±6 μg g-1. Hg mobility increased with increasing salinity. With salt concentrations of 0 and 4.1 g L-1, the amount of solubilised Hg was negligible, but with 8.2 and 16.4 g L-1 soluble Hg increased significantly. The effect of salt concentration was more pronounced in the most concentrated solution (32.8 g L-1) reaching about 30 μg Hg L-1. The mobility of the contaminant do not change with the application of soil improvers. Our results show a potential risk of mercury re-mobilization from contaminated coastal soils as a consequence of sea level rise and an ineffectiveness of soil improvers in reducing its mobility.