Determination of ³⁵S in Radioisotope Wastes by a Wet Oxidation

The oxidation studies of a sulfur to a sulfate ion by various oxyhalide oxidants in organic (thiourea, methionine) and inorganic (sulfate, thiophosphate) compounds were carried out in an acidic solution. The optimized result of the oxidation reaction was obtained when a bromate compound (BrO₃⁻) as an oxidant and a 3 M HNO₃ solvent. The chemical yield for the oxidation of the organic and inorganic sulfur compounds to a sulfate ion was monitored as 80% for thiophosphate, 87% for methionine, and 100% for thiourea and sulfate within 5% RSD. The oxidation of thiourea required at least 1.6 equivalents of the bromate in an acidic solution. In the case of the oxidation of methionine and thiophosphate, the oxidation yield was above 80% if the bromate was used at 20 times that of the substrates. The chemical yield in the paper sample (WypAll) exceeded 100% because of its background sulfur contents (910 ppm). The sulfate ion was quantitatively measured by using GPC and/or LSC counting of ³⁵S followed by precipitates of BaSO₄. The interfering nuclides (¹⁴C, ³²P) were removed in an acidic condition. The minimum detectable activity (MDA) of ³⁵S was found to be 0.1 Bq/g.