Hydrothermal-processed volcanic ash-based geopolymers for immobilization of sodium salt wastes containing Cs

Abstract

Volcanic ash-based sodium salts-containing geopolymer with target composition close to that of Pollucite Cs(AlSi2O6) was formulated at 200 °C by mild hydrothermal method to immobilize Cs as waste material. The impact of three salts, namely, Na2SO4, NaCl, and NaNO2 on mineral phases of geopolymer waste forms and immobilization of Cs+ was investigated, specifically, regarding salts/NaOH molar ratio. Results indicate that when salts/NaOH molar ratio was 1:9, major mineral phase of geopolymer waste was analcime, whereas major mineral phase of matrix samples with salts/NaOH molar ratio as 1:1 was sodalite, which is beneficial to accommodate amounts of anions. Pollucite was formed in all samples with the introduction of CsOH·H2O to matrix samples. Product Consistency Test (PCT) leaching results indicate that NaOH plays key role in immobilization of Cs+. Moreover, C–S–H gel exhibits functional adsorption on Cs+; therefore, it has positive effect on the immobilization of Cs+. In Cs-containing samples with high salts/NaOH molar ratio, PCT leaching results show that high salts/NaOH molar ratio leads to low leaching concentrations of salts. However, only partial amounts of NO2− can be immobilized by sodalite due to oxidation of NO2− to NO3−. Furthermore, the attachment of SO42− to the surface of C–S–H gel can prevent the destruction of structure of C–S–H gel by ion exchange of Al3+ and Si4+. As a result, SO42−-containing samples have the lowest Cs+ leaching concentration among all Cs-containing samples with high salts/NaOH molar ratios. The immobilization of Cs+ can be further improved by the adsorption of C–S–H gel on Cs+. Results suggest that besides the formation of Pollucite, the immobilization of Cs+ also depends on various salts in sodium salts wastes containing Cs.