Temperature and pH dependence of neptunium(V) sorption on bentonite

Neptunium(V) sorption on the Japanese bentonite Kunigel V1 was experimentally investigated under various temperature and pH conditions. The batch sorption experiments, using Np-237 as a tracer, were conducted in 0.01N NaCl supporting electrolyte and nitrogen gas atmosphere for up to 14 days. Three temperature conditions, 26, 40, and 70 C were chosen. The results indicated that the sorption coefficient, Kd, drastically increased several orders of magnitude as the pH increased from 3 to 12. The temperature dependence was not pronounced in the narrow range of 26 to 70 C, but higher temperature seems to enhance the Np(V) sorption. The pH titration suggested that the more positively charged surface sites were developed as temperature increased. This may be one possible reason for the observed temperature dependence of Np(V) sorption. The geochemical speciation suggested that the neptunyl carbonate anions were dominant species in the solution over the pH range of the increase in Kd. The modeling by means of the constant capacitance model was made by the MICROQL code. The calculations generally gave acceptable fits to the observed values.