Sorption of As(III) and As(V) on chemically synthesized manganese dioxide

Abstract

Sorption of As(III) and As(V) on manganese dioxide was studied by batch equilibration method using ⁷⁶As radioactive tracer. Manganese dioxide was prepared by two different methods viz. reacting (a) KMnO₄ solution with MnSO₄ solution, and (b) KMnO₄ solution with concentrated hydrochloric acid. Manganese dioxide was characterized by zeta potential measurement, surface area measurement, thermogravimetry (TG), differential thermal analysis (DTA) and X-ray diffraction (XRD) techniques. Point of zero charge (PZC) for manganese dioxide was between pH 3 and 4. Radioactive tracer (⁷⁶As) was prepared by neutron irradiation of arsenious oxide in self serve facility of CIRUS reactor followed by conversion to As(III) and As(V), by appropriate chemical methods. Sorption of As(III) and As(V) were studied separately, between pH 1 to 11, using (i) freshly prepared, (ii) air-dried and (iii) aged manganese dioxide. Sorption of As(III) and As(V) on freshly prepared as well as aged manganese dioxide, from both the methods was greater than 98% between pH 1 to 9 and decreased above pH 9. Percentage sorption was comparable for manganese dioxide prepared by both the methods in different batches. Sorption capacity was ∼2 mg g⁻¹ for both As(III) and As(V). Arsenic was desorbed from the manganese dioxide by 0.1 M sodium hydroxide and oxidation state of desorbed arsenic was determined by solvent extraction method. It was found that the desorbed arsenic was present in As(V) oxidation state, independent of the initial oxidation states. This simple and direct chemical evidence, establishing that As(III) is converted to As(V) by manganese dioxide, is reported for the first time. Sorption of As(III) and As(V) on manganese dioxide did not cause an increase in manganese concentration above solubility limit confirming that Mn²⁺, formed during oxidation of As(III) to As(V), was re-adsorbed.

Keywords:

• Arsenic
• sorption
• MnO₂
• radioactive tracer

Acknowledgments

The authors are grateful to Dr. A. K. Tyagi and Mr. R. Shukla for XRD measurements, Dr. Sanjukta A. Kumar and Ms. Shailaja Pandey for ICP-MS analysis and Dr. R.K. Singhal for PZC measurement. The authors are also thankful to the staff members of CIRUS and Dhruva reactors for their support during irradiations.