Complexation of cadmium(II) with water- and soil-derived fulvic acids: effect of pH and fulvic acid concentration

Abstract

We studied the conditional stability constants of cadmium(II) bound to fulvic acid derived from water and soil, and found that (1) stability constants increased with increasing pH, and (2) stability constants decreased as we increased the fulvic acid concentration toward 70 mg/L. The second effect does not occur for the copper(II)–fulvate system. Conformational changes that occur when a fulvic acid solution becomes more concentrated apparently weaken sites that are otherwise more accessible to weak-binding cadmium. From pH 4 to 8, the overall conditional stability constant increases from 1.4 to 12 × 10³ for water-derived fulvic acid and from 1.7 to 43 × 10³ for soil-derived fulvic acid. Increases in fulvic acid concentration from 20 mg/L to 70 mg/L halve the conditional stability constant at a given pH.