Reaction of aqueous Cu–Citrate with MnO2 birnessite: Characterization of Mn dissolution, oxidation products and surface interactions

Abstract

Citric acid, a widespread soil rhizosphere plant/microbe carboxylic acid exudate can easily form chelates with heavy metals, increasing their availability in the environment. When Cu(II) from algal control in water bodies or reservoirs and fungicides, such as the Bordeaux mixture, and citrate interact, solubilization through chelation is a possible outcome. Manganese (hydr)oxides represent a significant portion of the subsurface environment and can affect the fate and transport of chemical species through adsorption and oxidation. This study explores the possible interaction between MnO2 and Cu–Citrate under ambient oxic conditions. The calculated Mn(II) dissolution rates during the initial 1h of reaction followed the series Cu(II)>Cu–Citrate 1:0.5>Cu–Citrate 1:1(oxic)>Citrate>Cu–Citrate 1:1(Anoxic), reinforcing the central role of (complexed or un-complexed) Cu(II) during the initial surface-coordination instead of following the s-shaped auto-catalytic curves of Mn(II) dissolution in citrate solution. The use of capillary electrophoresis allowed the detection of an intermediate Cu(II)Acetonedicarboxylate complex and the oxidation products acetonedicarboxylate, acetoacetate, acetone and acetic acid. The mass balance analysis of Cu–Citrate 1:1 suggests the partial adsorption of Cu–Citrate(ads) and catalytic degradation of acetonedicarboxylate through a MnO2–Cu surface sorbed complex. Lastly, XPS analysis confirmed the MnO2 surface Cu(II) reduction along with an outer-hydration layer at the MnO2 interface, where electron transfer and aquo ligand exchange may lead to the oxidation of Cu–Citrate.