The influence of environmental conditions on kinetics of arsenite oxidation by manganese-oxides

Abstract

BackgroundManganese-oxides are one of the most important minerals in soil due to their widespread distribution and high reactivity. Despite their invaluable role in cycling many redox sensitive elements, numerous unknowns remain about the reactivity of different manganese-oxide minerals under varying conditions in natural systems. By altering temperature, pH, and concentration of arsenite we were able to determine how manganese-oxide reactivity changes with simulated environmental conditions. The interaction between manganese-oxides and arsenic is particularly important because manganese can oxidize mobile and toxic arsenite into more easily sorbed and less toxic arsenate. This redox reaction is essential in understanding how to address the global issue of arsenic contamination in drinking water.ResultsThe reactivity of manganese-oxides in ascending order is random stacked birnessite, hexagonal birnessite, biogenic manganese-oxide, acid birnessite, and δ-MnO2. Increasing temperature raised the rate of oxidation. pH had a variable effect on the production of arsenate and mainly impacted the sorption of arsenate on δ-MnO2, which decreased with increasing pH. Acid birnessite oxidized the most arsenic at alkaline and acidic pHs, with decreased reactivity towards neutral pH. The δ-MnO2 showed a decline in reactivity with increasing arsenite concentration, while the acid birnessite had greater oxidation capacity under higher concentrations of arsenite. The batch reactions used in this study quantify the impact of environmental variances on different manganese-oxides’ reactivity and provide insight to their roles in governing chemical cycles in the Critical Zone.ConclusionsThe reactivity of manganese-oxides investigated was closely linked to each mineral’s crystallinity, surface area, and presence of vacancy sites. δ-MnO2 and acid birnessite are thought to be synthetic representatives of naturally occurring biogenic manganese-oxides; however, the biogenic manganese-oxide exhibited a lag time in oxidation compared to these two minerals. Reactivity was clearly linked to temperature, which provides important information on how these minerals react in the subsurface environment. The pH affected oxidation rate, which is essential in understanding how manganese-oxides react differently in the environment and their potential role in remediating contaminated areas. Moreover, the contrasting oxidative capacity of seemingly similar manganese-oxides under varying arsenite concentrations reinforces the importance of each manganese-oxide mineral’s unique properties.

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

“…Additionally, Mn III edge sites react more slowly than Mn IV sites, resulting in decreased As(III) oxidation rates [48]. The reactivity of Mn oxides, with respect to As III oxidation, may increase with increasing vacancies, decreasing crystallite size, and increasing AMON [48,50,51]. However, in the context of As III oxidation, the exact relationship between Mn oxide reactivity, and these parameters remains poorly understood [38,50,52].…”

“…To further complicate the comparison of rate data across studies, the nomenclature used to describe synthetic minerals is inconsistent. For example, acid birnessite has been used to describe both triclinic and hexagonal birnessite [4,50]. The commonly used δ-MnO 2 is also referred to by more general terms, including hydrated Mn oxide (HMO) and birnessite [63,64].…”

The Hazardous Status of High Density Sludge from Acid Mine Drainage Neutralization

“…Mn(IV) oxides will not only help buffer ferric oxide reduction and dissolution but also actively oxidize (or re-oxidize) ferrous iron and Mn 2+ . If As occurs in the waste, the presence of Mn will result in its oxidation to the less soluble As(V) arsenate [45].…”

Enhanced oxidation of arsenite to arsenate using tunable K+ concentration in the OMS-2 tunnel

“…However, an increase in its specific As(III) oxidation rate was very low. The second strategy involved controlling reaction conditions by changing the system pH, temperature, initial As(III) concentration, and manganese oxide dosage (Fischel et al, 2015;Li et al, 2010). Nevertheless, manipulating pH, temperature, and As(III) concentration is very difficult to control in actual arsenic polluted groundwater, and the effect of reducing coexisting ions still remains unclear.…”

“…Manganese oxides are ubiquitous in terrestrial environments and possess high redox potentials (Fischel et al, 2015;Villalobos et al, 2014), being highly efficient oxidants of As(III), potentially promoting total As removal (Chakravarty et al, 2002;Chen et al, 2018;Deschamps et al, 2005;Maliyekkal et al, 2009;Neumann et al, 2013;Zhang et al, , 2007. For example, observed that synthetic Fe-Mn binary oxides had a high removal capacity for As(III), which was attributed to the oxidation ability of manganese oxides for As(III), but also As(V) adsorption by iron oxides (Zhang et al, 2014(Zhang et al, , 2007.…”