Effects of pH, organic acids, and competitive cations on mercury desorption in soils

The effects of pH, organic acids, and competitive cations on Hg2+ desorption were studied. Three representative soils for rice production in China, locally referred to as a yellowish red soil (YRS), purplish clayey soil (PCS), and silty loam soil (SLS) and classified as Gleyi-Stagnic Anthrosols in FAO/UNESCO nomenclature, were, respectively, collected from Jiaxin County, Deqing County, and Xiasha District of Hangzhou City, Zhejiang Province. Most of the added Hg2+ was adsorbed at low initial concentrations (<2 mg l−1). Desorption of the adsorbed Hg2+ in 0.01 M KCl (simulating soil solution) was minimal, but was significantly enhanced by the change of pH, and the presence of organic acids or competitive cations. The desorption of Hg2+ in the soils decreased with pH from 3.0 to 5.0, leveled off at pH 5.0–8.0, but increased with pH from 7.0 to 9.0. The presence of organic ligands enhanced Hg2+ desorption in the soils except for YRS, in which the addition of tartaric, malic, or oxalic acid reduced Hg2+ desorption at low concentrations (<10−4 M), but Hg2+ desorption generally increased with organic acid concentration. Citric acid was most effective in increasing Hg2+ desorption, followed by tartaric acid and malic acid; and oxalic acid was the least effective. Desorption of adsorbed Hg2+ increased with increasing concentrations of added Cu2+ or Zn2+. Applied Cu2+ increased Hg2+ desorption more than Zn2+ at the same loading rate.CapsuleThe effects of organic acids and competitive cations on Hg desorption in soil–water system are related to their concentrations, basic chemical properties, and soil properties.