The influence of particle size and structure on the sorption and oxidation behavior of birnessite: I. Adsorption of As(V) and oxidation of As(III)

Abstract

Sorption and oxidation reactions in the environment may affect substantially the mobility of redox-sensitive toxic trace elements and compounds. Investigating the environmental factors that influence these reactions is crucial in understanding and predicting the geochemical fate of these environmental species, as well as to design appropriate engineered remediation schemes. Arsenic is a widespread contaminant of concern, especially in its oxidized forms, and Mn oxide minerals are some of the major contributors to its oxidation. The goal of this work was to investigate the influence of particle size and structural differences of environmentally-relevant Mn(IV) birnessites on the adsorption of As(V) and on the oxidation of As(III). An acid birnessite of 39m2/g and a δ-MnO2 of 114m2/g were used. Both birnessites sorbed a maximum Pb(II) of 0.3 Pb/Mn, indicating a significantly larger layer cationic vacancy content for acid birnessite, and a density of reactive edge sites for both of 12sites/nm2. As(V) forms a bidentate bridging complex on singly-coordinated surface sites at the birnessite particle edges regardless of loading, pH, birnessite type, and presence of pre-sorbed metals(II). Maximum As(V) adsorption, under repulsive electrostatic pH conditions did not yield adsorption congruency behavior between both birnessites at constant pH, presumably because the increase in internal vacancy content causes negative electrostatic repulsion towards external As(V) oxyanion binding.At pH 4.5 As(III) oxidation on birnessites was fast and quantitative at As/Mn ratios of 0.3–0.33, the reaction being largely driven by the proton concentration. At pH 6 δ-MnO2 oxidized As(III) faster and to a higher extent than acid birnessite, at equal masses; but the reverse at equal total surface areas. The oxidation driving force (independently from protons) was higher at pH 6 than at pH 4.5 because of Mn(II) product removal by sorption to interlayer vacancies, which overcomes reactive surface site blockage by this species, provided sufficient vacancies are present. Metals(II) pre-sorbed on birnessites always decreased the initial stages of As(III) oxidation rates as compared to the metal(II)-free systems presumably through site blockage. But after 24h the Pb(II)-equilibrated birnessites at pH 6 reached equal and sometimes higher oxidation extents through removal of As(V) from solution, by stabilizing its (binary) adsorption to edge sites through a decrease in electrostatic repulsion by the sorbed Pb(II).This work provides useful insights on the influence of particle size and structure (vacancy content) of birnessite minerals analogous to biogenic Mn oxides relevant to the environment, especially as it pertains to reactivity towards sorption of Pb(II), Zn(II), and As(V), and to oxidation of As(III), all of which are significant processes that dictate their transport and fate in aqueous geochemical environments.