Bhupesh B. Kalekar

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

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.

Thermophysical studies on the binary system UO2(NO3)2·6H2O – Sr(NO3)2

Abstract Thermoanalytical (TG-DTA-EGA) and X-ray diffraction measurements have been used to study the reaction between uranyl nitrate hexahydrate and strontium nitrate. The results confirmed the absence of a direct interaction between the two compounds. The presence of strontium nitrate, however, ensured that the extent of hydrolysis and polymerisation of uranyl nitrate hexahydrate during its dehydration and decomposition to UO 3 is significantly reduced. DTA curves recorded in both heating and cooling modes gave evidence to the occurrence of a reaction between molten strontium nitrate and uranium trioxide to form nitrato-complexes of uranium and strontium. X-ray diffraction data on reaction residues obtained at different temperatures and cooled to room temperature also showed evidence for the formation of such complexes. The results obtained indicated an increase in thermal stability of these nitrato-complexes with increase in Sr/U ratio. The complex with an Sr/U ratio of 2.0 is stable up to 660 °C and the complex with Sr/U ratio of 4.0 is stable up to 680 °C. These complexes decompose at higher temperatures to give strontium uranates.