High-affinity metal binding by isolated humic acids has observed for both copper and calcium in metal titration. Results of titrations of humic acids a single metal (either calcium or copper) are consistent a discrete ligand site model of humate-metal. However, copper titrations in the presence of calcium do not show competitive effects predicted such a model. Hence, different ligand sites must be in calcium and copper binding, or a binding other than discrete ligand binding must be.
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Hering et al. (1988) studied this question.