Reductive transformations of layered manganese oxides by small organic acids and the fate of trace metals

Manganese oxides are often major controls on the fate of trace metals in the environment. Exposure to dissolved Mn(II) alters the mineral structure and metal binding mechanisms. Such direct interaction of reduced and oxidized Mn species is largely limited to redox interfaces, but indirect Mn(II) generation may occur more widely. Small organic acids common in soils and organic matter-bearing sediments reduce Mn oxides, generating dissolved Mn(II), but the impact of such reduction on mineral structure and trace metal fate is largely unknown. This study investigates the reaction of three organic acids (oxalate, citrate, and 4-hydroxybenzoate) with two phyllomanganate minerals at pH 4, 5.5, and 7. Aging these phases for four weeks with each organic acid caused partial Mn reduction, generating solid-phase Mn(II) and Mn(III) as well as dissolved Mn(II), with the greatest concentration observed at pH 4. Citrate and 4-hydroxybenzoate produced substantially greater reduction than oxalate, with the net electron transfer indicating that oxidation products of the initial organic acids also reacted with the minerals. In all cases the phyllomanganate sheet structure was preserved after reaction and no phase transformations were observed. Metal uptake was not substantially altered by the organic acids at pH 7, despite these molecules causing substantial solid-phase Mn reduction, whereas at pH 4 adsorption of Ni and Zn decreased compared to organic acid-free systems. Ni adsorption mechanisms transitioned from binding above vacancy sites to at sheet edges in the presence of citrate and 4-hydroxybenzoate, whereas oxalate addition favored Ni binding to vacancy sites. In addition, at pH 7 oxalate and 4-hydroxybenzoate enhanced incorporation into Mn(III)-poor phyllomanganates sheets. For Zn, oxalate had little effect on binding mechanisms but citrate and 4-hydroxybenzoate increased the fraction of tetrahedral species and binding to edge sites. For both metals, changes in uptake and adsorption mechanisms were likely caused by both vacancy filling with Mn(II) and Mn(III) as well as the formation of Mn(III)-rich edges. This study shows that reduction by organic acids alters the reactivity of Mn oxide minerals which enhances trace metal micronutrient and contaminant availability in soils and aquatic systems. (C) 2019 Elsevier Ltd. All rights reserved.