Abstract
Manganese oxides are considered as one of the effective oxides capable of oxidizing arsenite and reduce the toxicity of arsenic. Since low molecular weight organic acids (LMWOAs) commonly found in nature can act as reducing and chelating agents for manganese oxides, it is particularly important to investigate how these organic acids with different numbers of carboxyl groups like citrate and EDTA affect oxidation and adsorption of arsenic by manganese oxides. In this study, low As(V) adsorption on manganese oxide is slightly enhanced by citrate and EDTA, which results from the increase in active sites via reduction of manganese oxide by LMWOAs. However, citrate and EDTA have different effects on the oxidation of As(III). MnIII/II citrate autocatalytic cycle as a manganese-based redox system decreases As(III) oxidation rate, but EDTA does not yield autocatalysis, which slightly increases the oxidation rate of As(III). Reduction of manganese oxide by EDTA and chelation between Mn(II) and EDTA lead to exposure of more active sites. Our research highlights the different effects of low molecular weight organic acids on the reactions between arsenic and manganese oxide.
Highlights
MnIII/IVoxides are highly reactive minerals, widely occurring in marine, riverine, and lacustrine systems [1]
For reactions involving Low molecular weight organic acids (LMWOAs), 0.03 g citrate or ethylenediaminetetraacetic acid (EDTA) was added in the beginning
Due to the reduction of manganese oxide by citrate, new active sites are created, and As(V) concentration continues to decrease after 30 min
Summary
MnIII/IV (hydr)oxides are highly reactive minerals, widely occurring in marine, riverine, and lacustrine systems [1] Due to their strong oxidation and adsorption capacities, manganese oxides are widely investigated for their controls on the transformation of different components in the environment, especially various trace metals and metalloids (As, Cr, Cu, Pb, etc). Many researchers have investigated competitive adsorption of dissolved organic matters, like humic acid, in the As-MnO2 system, but have not taken into account the redox roles of LMWOAs in the reaction between manganese oxides and As. The purpose of this study is to gain further insights into the effects of citrate and ethylenediaminetetraacetic acid (EDTA) on oxidation of As(III) and As(V) adsorption by manganese oxide and provide the theoretical basis for the transformation mechanism of arsenic in the environment
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