Analysis of copper and calcium–fulvic acid complexation and competition effects

The binding of Cu2+ and Ca2+ to a fulvic acid (FA), simulating naturally occurring conditions, was studied. Furthermore, copper–FA complexation in the presence and absence of calcium was compared. For this, potentiometric titrations were carried out using a solution of FA at a concentration of 100 mg L-1, and of ionic strength 0.1 M. The Ca2+–FA complexation reaction was carried out at pH 6.5, 7.5 and 8.5 and the Cu2+–FA complexation reaction at pH 5.5 and 6.5, in both the presence and absence of calcium ion. The calcium ion had a significant effect on copper binding at [Ca2+]>=2.5×10^-3 M. The experimental binding curves were analyzed using non-electrostatic discrete site and continuous distribution models. Competition by the calcium ion mainly affected the maximum binding capacity of the Cu2+–FA complexation, whereas the slight effect observed on the binding constant appears to indicate that calcium only competes for specific copper sites, thus modifying the binding sites distribution function. It was also found that the number of sites occupied by the copper ion represented only 20% of the total concentration of acid groups ionized on FA at the pH values studied.