The effect of pH on the binding of heavy metal ions with humic substances and hymatomelanic acids of peats

Abstract

The values of the dissociation constants (–COOH and –OH) of humic substances and hymatomelanic acids have been determined by the potentiometric titration method. It has been shown that hymatomelanic acids are enriched in phenolic groups compared with the original humic substances. The solubility of humic substances and hymatomelanic acids at various pH values is not influenced by their origin. The maximum concentration of humic substances in the absence of metal ions has been observed at pH 4–5. The reduction of pH inhibits dramatically the solubility of humic substances; complete subsidence of macromolecules of humic substances occurs at рН 3 ? 1. The content of humic substances and hymatomelanic acids in the presence of metal ions in the solution decreases due to the formation of humic substance – metal complexes. The maximum binding of metal cations with humic substances of black-alder fen peat and sphagnum transition peat has been found at рН 4–7 and 3.5–4, respectively. The binding of humic substances with lead ions takes place in the sludge phase due to the precipitation of insoluble lead humates, and the binding of humic substances with zinc ions takes place preferentially in the solution phase with the formation of soluble humates. Additional deprotonation of phenolic groups contained in hymatomelanic acids with the increasing of pH enhances their binding capacity in the solution phase in relation to zinc ions.

Keywords

• humic substances
• hymatomelanic acids
• heavy metals
• sorption
• binding capacity
• effect of ph