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.

The nature of Cu bonding to natural organic matter

“…These chelates were never dominant in our experimental conditions, perhaps because the C to N ratio was too high, or the sorption pH (4.5 and 5.5) was not high enough as suggested by the inferred presence of N ligands using ESR in leaf litter at and above pH 6 (Sposito and Holtzclaw, 1988). Similarly, modeling results from Cu adsorption data presented by Karlsson et al (2008) suggested that monodentate CuL amino-Cu complex (log K = 9.2) dominated the complexation of Cu over Cu[6-O-ring] chelate (log K = 4.7) in soil peat above pH 6. The bidentate carboxyl complex was dominant at pH 4.5, but the amino complex was again the main species at lower pH, which is improbable.…”

“…Consequently, there is generally no correspondence between 'chemical species' derived from model-dependent titration and adsorption results (Lenoir and Manceau, 2010), and 'structural species' derived from spectroscopic and diffraction techniques. Reconciling results from the two approaches is sometimes difficult, as seen previously for the Ni chelates (Strathmann and Myneni, 2004), Cuphthalate, and the Cu-NOM complexes (Karlsson et al, 2008).…”

Thlaspi arvense binds Cu(ii) as a bis-(l-histidinato) complex on root cell walls in an urban ecosystem

“…90 Carboxyl-type O ligands from polysaccharides bind Cu 2+ with lower affinity than histidinerich glycoproteins 91 and therefore are expected to bind Cu 2+ only at high copper concentration after saturation of reactive amino sites. 92 The polyelectrolyte polymer character of the cell wall was demonstrated experimentally with Zn and Pb which have a reverse affinity to each other for reactive phosphate and carboxyl groups. 93 So, at low copper concentrations binding to histidine is preferred with a conformation in dynamic equilibrium with the typically mild acidic conditions of the cell wall (pH 4.5-6.0 [94][95][96] ).…”

Involvement of nitrogen functional groups in high-affinity copper binding in tomato and wheat root apoplasts: spectroscopic and thermodynamic evidence

“…ethylene diamine tetraacetic acid, histidine, nicotianamine) than for ligands with carboxyl groups only. 28,29 In line with these considerations, the two HA model was parameterized for wheat and tomato roots by considering that 34 to 47% of binding sites were low-pK a sites (i.e. type 1 and 2 in HA I ), presumably corresponding to carboxyl-like groups and having low-affinity Cu binding constants ranging from log K Cu 2.1 to 3.9.…”