XAS analysis of copper binding in soils

Abstract

Ecological distribution of copper from soil is depended on its bioavailability and mobility. Copper binding on heterogeneous soil composition is a key information for understanding the long-term fate benefiting suitable management practice for copper in the environment. X-ray absorption spectroscopy (XAS) techniques were used to provide microscopic scale information of copper-binding on different soils. Soil samples containing 14.9-56.6% clay and 0.5-3.4% organic matter were studied. Dry-powder soil samples containing approximated 0.3% copper from batch equilibrated equilibrium experiments were analyzed. Copper K-edge spectrum were collected at the Synchrotron light research institute (SLRI), Thailand in X-ray absorption near edge structure (XANES) and Extended x-ray absorption fine structure (EXAFS) spectrum region. Linear combination fit (LCF) analysis of XANES spectrum resulted the phase fraction of copper were copper acetate (55-79%) and copper sulfate (9-46%) in all soils, while, copper hydroxide (24%) was exhibited in siliceous coarse-texture soil. EXAFS analysis using copper acetated model structure found four oxygen surrounded copper atom at 1.91-1.974 Ã… in the first shell which positioned in the equatorial plane of a Jahn-Teller distorted octahedral without the existing of weak binding of two axial oxygen atom. Carbon atoms presented in the second shell at 2.73-2.91 Ã… and its coordination number related to soil organic matter content. XANES and EXAFS studied on copper binding in soils result that the predominant copper in soil is in organic matter complex via carboxyl functional group and sulfated ligand. Copper-C binding occurred in local structure which corresponded to soil organic matter content.