Mechanism of the Removal of AuCl − 4 Ions from Aqueous Solution by Means of Peat Soil Humin

Peat soil humin, obtained after the removal of fulvic and humic acid fractions according to the recommended procedure of the International Humic Substances Society (IHSS), has the ability to remove AuCl⁻₄ ions from aqueous solution. The removal was greatest at medium acidity, which was equivalent to pH 2.0, and it was governed by two different processes, i.e. attachment onto un-ionized –COOH groups through hydrogen bonding and reduction to Au metal by the action of the –OH group. Purification of humin through repeated immersion and shaking in a mixed solution consisting of 0.1 M HCl and 0.05 M HF led to the abrupt increase in the –COOH contents from 386 mmol/kg to 2510 mmol/kg, while the content of total –OH functional groups remained relatively constant at values slightly higher than 3000 mmol/kg. The increase in –COOH content improved the removal of AuCl⁻₄ ions from 23 mg/g to 27 mg/g, but it was unfortunately accompanied by a diminished reduction ability to Au metal. Hence, the role of the –COOH functional group in removing AuCl⁻₄ ions through hydrogen bonding was more significant for the purified humin, while that of the –OH group in removing AuCl⁻₄ ions through reduction to Au metal was better for the crude (unpurified) humin.