The Mechanism of Copper(II) Binding by Humic Acid: An Electron Spin Resonance Study of a Copper(II)—Humic Acid Complex and Some Adducts with Nitrogen Donors

Abstract

AbstractElectron spin resonance (ESR) spectroscopy was used to study the mechanism of Cu2+ binding by humic acid (HA) extracted from a Chalmers silt loam soil. The Cu‐HA complex and some adducts with nitrogen donors were examined. The ESR data presented showed that the addition of pyridine (py) and 1,10‐phenanthroline (phen) to the Cu‐HA complex resulted in the substitution of coordinated H2O ligands by the nitrogen donor ligands without displacement of HA oxygen donor ligands also coordinated to the Cu2+ center. The addition of a more basic nitrogen donor, ethylenediamine (en), resulted in the displacement from Cu2+ of both coordinated H2O ligands and HA oxygen donors, forming Cu(en)22+. The g‖ and A‖ values obtained for the py and phen adducts revealed that there were two HA oxygen donors coordinated equatorially to the Cu2+ center in the original Cu‐HA complex. The characterization of the adducts with phen and en was consistent with the formation of a Cu2+ chelate in the original Cu‐HA complex, where the two equatorial Cu‐HA oxygen bonds would occupy cis‐ positions. The crystal field strength of HA oxygen donor ligands coordinated to the Cu2+ center was stronger than that of H2O.