Elsevier

Journal of Hazardous Materials

Volume 373, 5 July 2019, Pages 232-242
Journal of Hazardous Materials

Adsorption and oxidation of arsenic by two kinds of β-MnO2

https://doi.org/10.1016/j.jhazmat.2019.03.071Get rights and content

Highlights

  • To the best of our knowledge, it is the first report about the adsorption kinetics of arsenic on β-MnO2 surface.

  • Based on the new discovery in the present study, a new oxidation mechanism has been suggested.

  • Two kinds of β-MnO2 with high potential of decrease arsenic are suggested in the present study.

Abstract

MnO2 is one of the most widespread and cheapest materials in nature that can both adsorb arsenic and oxidize arsenite [As(III)] to arsenate [As(V)]. In this study, column β-MnO2 [CM] with the main facet of {110} and pincer β-MnO2 [PM] with the facets of {110} and {101} are synthesized and used to remove arsenic in water under different conditions. For the adsorption process, the experimental data are fitted well with the pseudo second-order kinetic model; the Langmuir model is better than the Freundlich model to describe the adsorption equilibrium isotherms. Furthermore, the As(III) oxidation rate can be denoted by the pseudo zero-order kinetic model and is related to the O2 concentration, the pH value, the light source and the initial concentration of As(III). Finally, the oxidation mechanism is investigated, and the oxidant should be related to O2. It is interesting to find that these two kinds of β-MnO2 exhibit different pH effects for both adsorption and oxidation. For As(III), the adsorption and oxidation abilities of CM follow the order pH 9 > pH 7 > pH 4, whereas the adsorption and oxidation orders of PM are pH 4 > pH 7 > pH 9.

Introduction

Arsenic is a toxic element that is widely distributed in water, soil and rock. Increasingly, highly populated Asian countries, such as China, Bangladesh, Vietnam and India, have reported that the concentration of arsenic is higher than the World Health Organization (WHO) recommended concentration of 10 μg/L [1,2]. In nature, arsenic exhibits four major oxidation states: -3, 0, +3 and +5. The toxicity of arsenic depends strongly on its chemical form [3,4]. As(III) is more toxic and more difficult to remove than As(V), so there have been many publications reporting how to transform As(III) into As(V) [[5], [6], [7]]. Among the treatments for removing arsenic from drinking water, adsorption and/or oxidation processes are the most useful methods.

As a nontoxic and inexpensive material, differently structured manganese oxides (MnO2) have been extensively used for arsenic removal due to their adsorption and oxidation capacities [[8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]]. As is commonly known, β-MnO2 is widespread and stable in nature. However, only a limited amount of data is available on arsenic adsorption and oxidation by β-MnO2 because β-MnO2, with a low specific surface area (SBET), did not show an excellent ability to decrease arsenic in water [15,16]. In 2012, we reported a kind of β-MnO2, namely, column β-MnO2 (CM), with the main facet of {110} [25]. In 2014, we reported another kind of β-MnO2, namely, pincer β-MnO2 (PM), with the facets of {110} and {101} [26]. These two kinds of β-MnO2, with large SBET values, exhibit high capacities to remove arsenic. To the best of our knowledge, no papers presenting the adsorption kinetics of arsenic on β-MnO2 surfaces are available in the literature. In the present paper, both adsorption and oxidation are investigated in detail. Based on the adsorption and oxidation experimental results under different conditions, CM and PM present different tendencies especially for the pH effect. In the present study, it is shown that the oxidation of As(III) is related to the concentration of O2.

Section snippets

Preparation of β-MnO2

Column β-MnO2 (CM) has been prepared with the following method [25]: 7 mmol KClO3, 4 mmol MnSO4·H2O and 69.6 mmol CH3COOH were dissolved in 60 mL distilled water with magnetic stirring. Transferring the solution into a Teflon-lined stainless steel autoclave of 100 mL after the solution became clear and kept it at 433.15 K for 12 h. After the reaction, the precipitates were washed and filtered with distilled water, then CM was obtained from the Buchner funnel. At last, CM was dried at 333.15 K

Morphology and structure

The crystallinity and surface of CM and PM are obtained by XRD, as shown in Fig. 1. The patterns fit well with the standard β-MnO2 (JCPDS 24–0735). The peaks for CM and PM are narrow and sharp, indicating that both compounds have good crystallinities, and the crystallinity of PM is better than that of CM. It is worth noting that the peak of the {101} facet for PM is high, which implies a large {101} surface area for the crystal. The FT-IR spectra of CM and PM at room temperature are shown in

Conclusion

Systemic experiments have been carried out to investigate the absorption and oxidation abilities of CM and PM. The pseudo second-order kinetic model is more suitable to describe the adsorption kinetics, and the Langmuir model is better to describe the adsorption isotherms. The adsorption orders of As(III) and As(V) for CM are pH 4 < pH 7 < pH 9 and pH 4 > pH 7 > pH 9, respectively. However, for PM the adsorption orders of As(III) and As(V) are pH 4 > pH 7 > pH 9 and pH 4 < pH 7 < pH 9,

Acknowledgements

The financial supports of this work are the National Natural Science Foundation of China (21373104, 21173022, 20803014), the National Natural Science Foundation of Guangdong Province (2016A030313704), and the Guangdong University funding program (xj201611845170, xj201711845034).

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