Effects of humus on the environmental activity of mineral-bound Hg: influence on Hg plant uptake

Abstract

Understanding the potential activation mechanisms and Hg transfer pathways is key to providing a basis for developing effective measures to prevent soil Hg from entering the food chain and dispersing into the environment. Besides acidification, the presence of natural complexing or chelating agents in the environment may also assist or intensify this process. The objective of this research is to understand the mechanisms behind the interaction of Hg with the complexity of humus material and other soil constituents with regards to plant uptake. Cultivation experiments with Basella alba were carried out to determine the effects of humus on the plant uptake of Hg bound by five pure mineral samples and five different soil types as well as ionic Hg in Hg(NO3)2 solution. Among all the tested forms of Hg uptake by B. alba, soluble inorganic Hg was found to be the most bioavailable. Without humus added, Hg bioavailability from the mineral substrates increased with the pHzpc of the mineral. The addition of humus can either suppress or promote Hg bioavailability depending on the mineral or soil type. Fulvic acid is the most active agent for Hg bound to ferric and manganese oxides or soils which contain high concentrations of these phases. Humic acids, particularly brown humic acid, tend to promote Hg bioavailability bound by pure bentonite and soils containing abundant 2:1 montmorillonite-type clays. The effect of gray humic acid is consistently weaker than brown humic acid for Hg bound by all mineral and soil forms examined in this study. The effect of a particular humus fraction on Hg bioavailability is related to its ability to convert Hg bound by solid phases into soluble complexes as well as the stability of the complexes that form. Soluble inorganic Hg is the most bioavailable form for plant uptake. A mineral’s pHzpc plays an important role in determining Hg bioavailability bound by pure mineral substrates. The addition of humus can either suppress or promote Hg bioavailability depending on the mineral or soil type. The effect of a particular humus fraction on Hg bioavailability is related to its ability to convert Hg bound by solid phases into soluble complexes as well as the stability of the complexes that form. The higher the complex capacity and the lower the complex stability of the humus with Hg, the greater is its ability to enhance the bioavailability of Hg bound by soil minerals.