Trace Metal Complexation by Fulvic Acid Extracted from Sewage Sludge: II. Development of Chemical Models

Two recent chemical models of trace metal complexation by fulvic acid extracted from sewage sludge were incorporated into the computer program GEOCHEM and were compared in illustrative speciation calculations. These calculations were performed for 21 saturation extracts obtained from three acid soils that had been amended with sewage sludge. One of the models studied, the Mixture Model, consisted of a set of nine organic acids whose metal‐complexing functional groups were expected to simulate closely the distribution and complexation behavior of the groups in the ligands of the fulvic acid fraction of sewage sludge. The other model, the Fulvate Model, comprised 18 metal‐fulvic acid complexes whose stability constants had been either measured in the laboratory or estimated by linear correlation analysis. The speciation calculations that incorporated the Fulvate Model were used as a basis for evaluation of those involving the Mixture Model. The two calculations disagreed in their estimates of the extent of organic complexation of Cu(II) and Cd(II), but were in general accord on the overall speciation of Cd(II). The reasons for the differences could be understood in terms of competition effects and the values of the stability constants employed in each calculation. The good general correspondence in the case of Cd(II) speciation suggested that a direct comparison with experiment of the two computer estimates of Cd2+concentration would be worthwhile. The two sets of predictions for the concentration of the species Cd2+in the 21 saturation extracts were found to be in good agreement with measured values obtained with a Cd electrode. These results indicated that the Mixture Model could be used in conjunction with computer estimates of Cd(II) speciation in soil solutions, but should not be applied to help speciate metals, such as Cu(II), which are expected to form strong fulvic acid complexes, but evidently do not.