Modeling Copper(II) Complexation in a Peat Soil Based on Spectroscopic Structural Information

Abstract

The speciation of Cu in soils and surface waters is largely influenced by complexation reactions with natural organic matter (NOM). In this study, ion selective electrode data for the binding of Cu2+ to a forest peat soil were collected as a function of equilibration time, pH (2.4–6.6), and total Cu(II) concentration (1–54g Cu kg−1 dry soil). As a first step, a one‐site Langmuir isotherm was successfully fitted to the Cu adsorption data for the complete concentration range at pH 4.6. In a second step, structural information extracted from extended x‐ray absorption fine structure (EXAFS) spectroscopy, showing that Cu(II) forms five‐membered rings with possible combinations of amine, carboxyl, and carbonyl functional groups in NOM, were used as input for chemical speciation calculations (using the chemical equilibrium model MINTEQA2). In agreement with the EXAFS results, a model consisting of one RNH2, forming monodentate complexes (Cu2+ + RNH2 ↔ RH2NCu2+; log stability constant KRH2NCu2+ = 9.2; −log acid dissociation constant [pKa] = 9.0 for RNH3+), and two adjacent RCOO− groups, forming bidentate complexes (Cu2+ + 2RCOO− ↔ Cu(OOCR)2; log stability constant β(RCOO)2Cu = 4.7; pKa = 4.5 for RCOOH), gave the best fit to the experimental data. Determined stability constants for Cu(II)–amine and Cu(II)–carboxyl complexes were in good agreement with well‐defined Cu complexes with amino acids and carboxyls, respectively.