Copper Redox Transformation and Complexation by Reduced and Oxidized Soil Humic Acid. 1. X-ray Absorption Spectroscopy Study

Abstract

Natural organic matter (NOM) exerts strong influence on copper speciation and bioavailability in soils and aquatic systems. In redox-dynamic environments, electron transfer reactions between copper and redox-active moieties of NOM may trigger Cu(I) and Cu(0) formation. To date, little is known about Cu-NOM redox interactions and Cu(I) binding to NOM. Here, we present X-ray absorption spectroscopy results on copper redox transformations upon addition of Cu(II) or Cu(I) to untreated and electrochemically reduced soil humic acid (HA) under oxic and anoxic conditions. Both untreated and reduced HA mediated copper redox transformations. Under anoxic conditions, Cu(II) and Cu(I) added to reduced HA were primarily complexed and thereby stabilized as Cu(I)-HA at low loadings, whereas high copper loadings resulted in the additional formation of Cu(0) nanoparticles (16-64% of total copper). Cu(I) bound to HA was predominantly 2-fold coordinated and to a lower extent 3- to 4-fold coordinated, with a contribution of at least one nitrogen and/or sulfur ligand group. Under oxic conditions, Cu(II)-HA complexes prevailed, but smaller fractions of copper were also stabilized as Cu(I)-HA in a 3- to 4-fold coordination. Our results show that Cu-HA redox interactions are strongly affected by binding of Cu(II) and Cu(I) to HA and that HA contributes to the stabilization of Cu(I) against disproportionation.

Leaching potential of metallic elements from contaminated soils under anoxia

“…This may be due to the decreasing Cu solubility with increasing pH, as generally observed in acidic soils under reducing conditions. Moreover, recent studies by Biasioli et al, 12 Fulda et al 37 and Matocha et al 38 have shown that, under anoxic conditions, Cu(II) can be reduced to Cu(I) or Cu(0) and subsequently stabilized as insoluble Cu(I)-humic acids complexes or precipitated as Cu2O. Huang et al 18 presented similar results for OM-amended paddy soils.…”

Quantifying the total ionic release from nanoparticles after particle-cell contact

“…As XANES signals are highly sensitive to the elemental oxidation state (Fulda et al, 2013b), LCF analysis of the normalized XANES spectra was conducted for Ce speciation. Athena software was used to perform LCF analysis of reference spectra as described by Levard et al (2012).…”

“…More recently, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) assay, followed by liner combination fitting (LCF), were used to study the transformation of metals in complex environmental/biological matrix in a quantitative way (Fulda et al, 2013a(Fulda et al, , 2013b. These methods might also be suitable for the quantification of NPs dissolution in complex matrix without pre-separating the ions and particles.…”

Cu Isotopic Composition in Surface Environments and in Biological Systems: A Critical Review

“…Previous studies showed that Cu isotope fractionation could be induced by various low temperature processes, such as adsorption, mineral dissolution, redox reaction, complexation with minerals and/or organic matters, and biotic incorporation [38,39,40,41,42,43,44,45,46,47]. In these processes, the isotopic fractionation (expressed as Δ 65 Cu P-I ) was determined for describing the enrichment or depletion of Cu isotope ( 63 Cu or 65 Cu) in the product compared to the initial materials, and was calculated as following: Δ 65 Cu P-I = δ 65 Cu P − δ 65 Cu I where δ 65 Cu p and δ 65 Cu I were the isotope ratio in the product P and initial material I, respectively.…”