Growth of the nuclear industry has encouraged us to look for techniques to treat large volumes of nuclear waste. 60Co 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 (mw/mn) influenced the structure and chemical performance of the MSG samples, which consequently influenced their Co immobilization capacity. The optimized MSG was obtained at an mw/mn 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.

Introduction

Although nuclear energy exhibits many advantages over traditional energy types with regard to the increasing global energy crisis, treating large volumes of nuclear waste has become a critical factor for sustainable development of the industry. 60Co is formed by activation of the Co present as an impurity in the metals used in nuclear reactors. It is one of the most problematic radioactive wastes considering solubility, high yield, and long half-life. The best approach to retard mobilization of this radionuclide is to encapsulate it into highly stable low chemically reactive material. Cement-based materials are among the most widely used for encapsulating radioactive wastes. However, the thermal stability and acid corrosion resistance of cement-based materials are relatively low (Malviya and Chaudhary, 2006; Paria and Yuet, 2006; Zhou et al., 2006). Moreover, utilization of other materials, such as glass and resin, is limited by their high cost as well as complex preparation (Li and Wang, 2006). A geopolymer is a three-dimensional amorphous aluminosilicate inorganic polymer produced by reacting amorphous aluminosilicate with a highly alkaline activating solution. Dissolution and reorganization of the amorphous aluminosilicate occurs during activation. The SiO4 4
and AlO4
tetrahedron is randomly interlinked by shared O atoms and forms a rigid three-dimensional network. Because of its excellent mechanical performance, such as compressive strength, acid/alkaline resistance, and heat resistance, geopolymers have become ideal materials for solidifying toxic waste (Al-Zboon et al., 2011; Cheng et al., 2012; Ok et al., 2007).