Competitive complexation of trace metals to dissolved humic acid

Abstract

Effects of competing trace metals (10 µM) and Ca 2+ (5 mM) on Cd(II), Pb(II) and Cu(II) (0 - 100 µM) complexation by a groundwater humic acid were investigated in a series of 24 batch experiments at pH 6, 7, and 8 (I = 0.05 M). Both trace metals and Ca 2+ have little effect on Cd(II) complexation. No effect of Cu(II) on Pb(II) complexation is observed; Cd(II) appears to slightly enhance the binding between Pb(II) and humic acid. For Cu(II), the addition of Pb(II) decreases the amount of complexation, but Cd(II) also appears to slightly increase it. Calcium, however, decreases the amount of complexation in all cases. The effective binding constants determined show that the relative magnitudes of binding strengths are Cu(II) ,.., Pb(II) > Cd(II). These results indicate that in groundwaters, where more than one metal is present, the effect of other metals must be considered when predicting metal speciation.