Mercury Extraction from Contaminated Soils by l-Cysteine: Species Dependency and Transformation Processes

Abstract

A new approach in soil remediation washing techniques is the use of l-cysteine based on the formation of organic complexes. In this study, the applicability of l-cysteine for the mobilisation of different mercury species from contaminated soils was evaluated. Soils were treated with l-cysteine solutions with S–Hg molar ratios of 1, 2, 10, 20, 100 and 200. In 24 h batch experiments, leachates with water could mobilise 1% of Hg. The addition of l-cysteine led to an increase of Hg mobilisation of 42% for soils with inorganically bound Hg. In column experiments, the maximum Hg removal rate was 75%. For soils with organically bound Hg or HgS, only 1–5% of Hg was mobilised. Thus, the extraction of Hg from soils with l-cysteine is highly dependent on the Hg-binding form. Hg speciation analyses of leachates indicate that Hg–l-cysteine complexes are labile complexes which can be easily transformed. Soil samples speciation analysis revealed that reduction to elemental mercury takes place at low S–Hg ratios (1 to 10), assumingly by microbial activity. At higher S–Hg ratios of 10 and 100, precipitation of stable Hg–S complexes could be observed. These species transformation processes are limitations for considering l-cysteine leaching as a remediation strategy.