Copper redox transformation and complexation by reduced and oxidized soil humic acid. 1. X-ray absorption spectroscopy study.

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.