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 gp 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 gp value of 2.35 [Cu(H2O)4L2. At a HS:Cu molar ratio of 0.6, a complex with a gp value of 2.31 [Cu(H2O)2]L4 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 gp 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.