The Molecular Scale Nature of Copper (II) and Arsenate Bonding with Goethite-Humate Complexes

Abstract

In soils, interactions between clay minerals and natural organic matter are known to affect the cycling of heavy metals and anions. Contaminant structures at mineral surfaces that contain adsorbed organic matter will have a significant influence on the bioavailability of the contaminant. The structure of copper(II) and arsenate complexes on goethite in the presence and absence of adsorbed humic acid were studied with extended x-ray absorption fine structure (EXAFS) spectroscopy and with x-ray absorption near edge structure (XANES) spectroscopy. The copper(II) and arsenate formed edge shared inner-sphere surface complexes with goethite. The copper(II) itself was present in a distorted octahedral configuration, and ternary complexes involving bonding with both the humic acid and the goethite (Type A complexes) or adsorbed humic acid (Type B complexes), occurred at pH 5.6. At pH 8, the copper(II) was found to display a stronger affinity for the goethite surface and the formation of ternary complexes did not occur. The EXAFS analyses demonstrated that arsenate was present in a tetrahedral configuration and was predominantly bonded to goethite surface sites. Regardless of the level of adsorbed humic acid, the arsenic coordination environment was similar and indicated that ternary complexes could not be distinguished.