The sorption of mercury(II) by humic acids

Uptake of Hg(II) (1–10 mg litre⁻¹) by two humic acids varied with pH, with >98% being sorbed at pH 4–5. Langmuir plots yielded adsorption capacity and bonding values of ∼50 mmol kg⁻¹ and 3 × 10⁶ litre mol⁻¹ respectively. The addition of increasing amounts of chloride reduced uptake (10–20%) and shifted the maximum sorption region to higher pH values. The presence of 10⁻² M levels of Ca²⁺, Mg²⁺ and NH₄⁺ promoted near total adsorption and reduced the pH effect; 10⁻⁴ levels slightly increased the amount of Hg(II) sorbed. Sulphate additions reduced Hg uptake. Much of the Hg(II) is considered to be chemisorbed, since while most of the sorbed species was extractable in 0·005 m EDTA, only half was released into 1 m HNO₃ and molar salt solutions retrieved less than a third.

The environmental significance of the results are discussed.