Arsenite Depletion by Manganese Oxides: A Case Study on the Limitations of Observed First Order Rate Constants

Abstract

Arsenic (As) contamination of drinking water is a threat to global health. Manganese(III/IV) (Mn) oxides control As in groundwater by oxidizing more mobile As III to less mobile As V. Both As species sorb to the Mn oxide. The rates and mechanisms of this process are the subject of extensive research; however, as a group, study results are inconclusive and often contradictory. Here, the existing body of literature describing As III oxidation by Mn oxides is examined, and several potential reasons for inconsistent kinetic data are discussed. The oxidation of As III by Mn(III/IV) oxides is generally biphasic, with reported first order rate constants ranging seven orders of magnitude. Reanalysis of existing datasets from batch reactions of As III with δ-MnO 2 reveal that the first order rate constants reported for As depletion are time-dependent, and are not well described by pure kinetic rate models. This finding emphasizes the importance of mechanistic modeling that accounts for differences in reactivity between Mn III and Mn IV , and the sorption and desorption of As III , As V , and Mn II. A thorough understanding of the reaction is crucial to predicting As fate in groundwater and removing As via water treatment with Mn oxides, thus ensuring worldwide access to safe drinking water.

Biogenic and Synthetic MnO2 Nanoparticles: Size and Growth Probed with Absorption and Raman Spectroscopies and Dynamic Light Scattering

“…This phenomenon is a constraint on evaluating the role of MnO 2 in environmental oxidation and sequestration processes. For example, arsenic in groundwater is a serious health problem of global proportions, , and there is great interest in the potential of MnO 2 to remediate arsenic in situ, or in drinking water filters. − Arsenate (As V ) is strongly adsorbed on MnO 2 , as well as Fe oxides surfaces, and arsenite (As III ) can be oxidized by MnO 2 . ,− This is important as As III is the more toxic form of arsenic, and is also more mobile. , Thus, MnO 2 oxidation of As III to the strongly adsorbed As V is an important determinant of groundwater arsenic contamination, and of remediation strategies. Surface passivation limits the oxidation capacity, ,, and further mechanistic studies may open routes to circumventing passivation.…”

Modelling heavy metals contamination in groundwater of Southern Punjab, Pakistan

“…Manganese oxides have strong sorption capacities. The oxides of manganese can have an impact on the geochemistry of arsenic (Schacht and Ginder-Vogel 2018). The manganese starts at 1.82e-7 mol/l (10 µg/l) and it is constant for first 10,000 years and then it increases with time until it reaches the value of 2.87e-6 mol/l (158 µg/l) at 100,000 year.…”

Highly Sensitive As3+ Detection Using Electrodeposited Nanostructured MnOx and Phase Evolution of the Active Material during Sensing

“…Manganese oxides exist as MnO 6 octahedra that form tunnels and/or layered structures. , The tunnels and layers are connected by corner or edge sharing. Cations and water molecules can occupy these tunnels or interlayers . The nanostructured MnO x provides active sites for arsenic adsorption during sensing.…”

“…The nanostructured MnO x provides active sites for arsenic adsorption during sensing. Sorption of As on manganese oxide (MnO x )-based materials due to their selective adsorption property is widely reported. − Mn oxides are known to be one of the strongest naturally occurring oxidizing agents . Also, the surface areas of different oxides of manganese may vary from 3 to ∼200 m 2 /g .…”