The reaction of aquatic humic substances with copper(II) ions: an ESR study of complexation

The reaction of copper(II) ions with humic substances (HS) isolated from swamp water has been studied by electron spin resonance (ESR) spectroscopy at different HS:Cu molar ratios over the pH range from 3 to 13. Three distinct HS-Cu complexes have been detected at different HS:Cu molar ratios. At a HS:Cu molar ratio of 0.003, a complex with a g(//) value of 2.40 forms. This can be attributed to a complex [Cu(H2O)(5)]L, with one oxygen donor ligand. This complex is very sensitive to pH and a precipitate is formed at pH 7, suggesting that the complex is a mixed HS-Cu-H2O complex. Increasing the HS:Cu molar ratio to 0.03 leads to the formation of a complex with a g(//) value of 2.35 [Cu(H2O)(4)]L-2. At a HS:Cu molar ratio of 0.6, a complex with a g(//) value of 2.31 [Cu(H2O)(2)]L-4 forms. This HS-Cu complex is pH resistant up to pH 10 and an inner sphere bond complex must have formed. The stability constants of HS-Cu complexes formed at different molar ratios were estimated from the corresponding ESR parameter g(//) values. The ESR results are consistent with calculated species distributions of copper ions under similar conditions. This calculation shows that at a lower concentration of humic substances, the percentage of the complex formed decreases with an increase of pH and this complex does not exist at pH above 7, while at a higher concentration of humic substances, HS-Cu complexes with high stability are dominant in solution up to pH 10. (C) 1997 Elsevier Science B.V.