Surfactant binding by humic acids in the presence of divalent metal salts

The binding of metal ions and surfactant to humic substances plays an important role in the distribution of toxic metal ions and toxic organic contaminants in the environment. We measured the binding of dodecyl- (C12TA+) and decyltrimethylammonium (C10TA+) surfactants to humic acid in the presence of heavy metal salts (Cu(II), Cd(II), Zn(II), Pb(II), and Ca(II)) at pH 8 using a surfactant ion-selective electrode. The binding curves of the surfactant cation were analyzed using the Langmuir equation to determine the binding constant K and capacity m. From the dependence of K on ionic strength, the hydrophobic interaction term is inferred to have Gibbs energy of −28 kJ/mol for the binding of C12TA+ ion to HA, and the dependence of K on the surfactant chain length revealed a 4 kJ/mol difference in Gibbs energy between C12TA+ and C10TA+ ion binding. In the presence of 0.5 mM divalent metal salts, K decreased significantly, partly because of territorial binding and partly because of the irreversible binding of metal ions. Aldrich humic acid had a smaller binding constant and capacity.