Speciation and Mobility of Mercury in Soils Contaminated by Legacy Emissions from a Chemical Factory in the Rhône Valley in Canton of Valais, Switzerland

Abstract

Legacy contamination of soils and sediments with mercury (Hg) can pose serious threats to the environment and to human health. Assessing risks and possible remediation strategies must consider the chemical forms of Hg, as different Hg species exhibit vastly different environmental behaviors and toxicities. Here, we present a study on Hg speciation and potential mobility in sediments from a chemical factory site, and soils from nearby settlement areas in the canton of Valais, Switzerland. Total Hg ranged from 0.5 to 28.4 mg/kg in the soils, and 3.5 to 174.7 mg/kg in the sediments, respectively. Elemental Hg(0) was not detectable in the soils by thermal desorption analysis. Methylmercury, the most toxic form of Hg, was present at low levels in all soils (<0.010 mg/kg; <0.8% of total Hg). Sequential extractions and thermal desorption analyses suggested that most of the Hg in the soils was present as “matrix-bound Hg(II)”, most likely associated with soil organic matter. For factory sediments, which contained less organic matter, the results suggested a higher fraction of sulfide-bound Hg. Batch extractions in different CaCl2 solutions revealed that Hg solubility was low overall, and there was no Hg-mobilizing effects of Ca2+ or Cl− in solution. Only in some of the factory sediments did high CaCl2 concentrations result in increased extractability of Hg, due to the formation of Hg-chloride complexes. Additional experiments with soil redox reactors showed that even mildly reducing conditions led to a sharp release of Hg into solution, which may be highly relevant in soils that are prone to periodic water saturation of flooding.

Mercury in the terrestrial environment: a review

“…The content of methyl-Hg relative to THG was 1.9% in the roots of rice under background conditions and 0.55% in the leaves; in areas of Hg mining sites, the concentrations were 0.07% in the roots and 0.01% in the leaves; and in areas with artisanal Hg mining sites, the concentrations were 0.63% in the roots and 0.02% in the leaves [98]. Methyl-Hg in heavily contaminated soil in the Rhône Valley (Switzerland) accounted for < 0.8% of THg [99]. A number of factors that control microbial activity and/or the geochemical speciation of inorganic Hg 2+ govern MeHg formation in the environment [100].…”

“…Moreover, the natural/background Hg level was 0.08 mg/ kg in Brazil [121], 0.05 mg/kg in India [122], 0.23 mg/kg in New Zealand [123], 0.11 mg/kg in the Norwegian Arctic [124] and 0.4 mg/kg in Paris [125]. Most soil Hg was found as soil matrix-bound divalent Hg (Hg II ), whereas elemental Hg (Hg 0 ) was undetectable in soils [85,99]. Table 7 presents examples of the concentrations of Hg in the soils in the vicinity of industrial emission sources according to different authors, The highest Hg contents were found in soils near Hg mines: Almaden in Spain, with 2000 years of mining and ore processing (< 8889 mg kg −1 ) [126]; Idrija in Slovenia, with 500 years of mining activity (< 2759 mg kg −1 ) [97,127]; and in Alaska (5326 mg kg −1 ) [125].…”

Soil Hg Contamination Impact on Earthworms’ Gut Microbiome

“…Two experiments were performed in 5-L polypropylene pots (19 cm in diameter) with perforated lids, where soil and earthworms were incubated. For both experiments, soils were collected (in October 2015 for experiment 1 and October 2016 for experiment 2) in a field near the town of Raron in Canton Valais, Switzerland (46 • 18 10.6" N, 7 • 48 34.2" E), where long-term severe Hg pollution had been reported [24,[26][27][28]. The pollution originated from sediments of a canal that received a large amount of Hg (estimated at 50 to 250 tons) from ca.…”

Mercury in the terrestrial environment: a review

“…The content of methyl-Hg relative to THG was 1.9% in the roots of rice under background conditions and 0.55% in the leaves; in areas of Hg mining sites, the concentrations were 0.07% in the roots and 0.01% in the leaves; and in areas with artisanal Hg mining sites, the concentrations were 0.63% in the roots and 0.02% in the leaves [107]. Methyl-Hg in heavily contaminated soil in the Rhône Valley (Switzerland) accounted for < 0.8% of THg [108].…”