Mercury mobilization potential from highly contaminated alluvial soils

Mercury (Hg) is a global pollutant that affects human and ecosystem health. Alluvial soils of the Isonzo river basin are strongly contaminated by Hg transported from the Idrjia mine (SLO). Most of the area in the river basin is intensively cultivated with arable and horticultural crops.The mobility, bioavailability and potential biomagnification of Hg from these soils depends on its chemical and physical form.The overall aim of this research was to investigate the risks connected to agricultural activities, which may cause Hg mobilization.The first aim of this study was to assess the physical-chemical species of Hg. The second objective was to improve Hg speciation by combining sequential extraction procedures (SEP) with thermo-desorption (TD). The third aim was to investigate the effect of mineral fertilizers, organic amendments and root exudates on Hg mobility. This was combined with redox changes, since these soils are generally under the sea level.Total Hg concentration ranges from 7.32 to 50.6 mg Hg kg-1 soil. Texture fractionation showed an accumulation of Hg in the Fine Sand and particularly in the Silt size fractions. A six steps SEP on size-texture fractions showed that the Clay fraction had the highest percentage of volatile Hg, whereas the Fine Sand and Silt fractions had the highest percentage of Hg in the sulfide fractions.Hg contamination in the studied area is coherent with a long-term gradual deposition of contaminated particulate material from the Isonzo river.Application of different SEPs showed that most Hg is strongly retained in the sulfide and residual fractions.In order to improve the thermogram interpretation, TD was coupled to a seven steps SEP. A sediment contaminated by a different source of Hg, from a chlor-alkaly plant (Banduzzi channel), was also analyzed to validate the results obtained in the soil under study (Fossalon soil).Coupling TD measurements with SEP allowed the identification of two main Hg fractions in the Fossalon soil, the first peak at 220°C defined as “matrix-bound Hg” was attributed mainly to organic-Hg, and the second larger peak at 310 °C was attributed to HgS. In the Banduzzi sediment the organic-Hg was the highest peak, while the HgS peak was smaller. The assignment of the peaks were confirmed by analyzing standards and soil spiked with humic acids-Hg and with fulvic acids-Hg complexes.It can be concluded that both in the Fossalon soil and in the Banduzzi sediment the “matrix-bound” Hg peak can be ascribed to organic-Hg. This work also evidenced that TD cannot be a stand-alone tool for Hg speciation mainly due to peak overlapping. Whereas, SEP may allow a more specific identification of Hg compounds. The effect of several mineral fertilizers, organic amendments and root exudates were investigated on: Hg thermal stability, Hg solubility in water, Hg solubility and volatility in aerobic and anaerobic incubations.All tested inorganic and organic fertilizers did not affect Hg thermal stability. Water Hg extractability was also little affected by fertilizer addition.Aerobic and anaerobic soil incubations showed different results:Both Hg solubilization and volatilization increased in the native soil incubated under anoxic conditions with respect to oxic conditions.Mineral fertilizers had no significant effect on Hg mobility.Organic amendments enhanced Hg mobilization in aerobic conditions but not in anaerobiosis.Root exudates compounds decreased Hg solubility both in oxic and in anoxic soil incubation, with the exception of glutathione and malic acid. All tested root exudates increased Hg volatilization in anoxic soil incubation.Hypothesis on the mechanisms involved in the solubilization/volatilization have been formulated but the complexity of the system made it difficult to assign their relative importance.The overall conclusion is that although the alluvial soils under investigation are highly contaminated, Hg is practically immobile being represented mainly by HgS