Synthesis and characterization of Mn-slag based geopolymer for immobilization of Co

Growth of the nuclear industry has encouraged us to look for techniques to treat large volumes of nuclear waste. Co-60 is one of the most problematic radioactive wastes in the nuclear industry. In this study, a Mn slag-based geopolymer (MSG) was prepared, which exhibited better Co immobilization performance than the ordinary metakaolin-based geopolymer (MKG). Varying the mass ratio between water glass and NaOH (m(w)/m(n)) influenced the structure and chemical performance of the MSG samples, which consequently influenced their Co immobilization capacity. The optimized MSG was obtained at an m(w)/m(n) value of 1.5. After the 7-day leaching test, about 0.20% of the Co was released from MSG, which is less than two-thirds of the MKG sample. The chemical state of Co in the geopolymer matrix was characterized by X-ray photoelectron spectroscopy. Divalent Co remained in the MKG samples, while most of Co ions in MSG samples existed in the trivalent state. These results strongly suggest that divalent Co was oxidized to trivalent Co in the MSG matrix, resulting in enhanced Co solidification capacity compared to MKG. The results in this study indicate that the oxidation environment in the MSG played an important role in Co immobilization. (C) 2019 Published by Elsevier Ltd.