Treatment Technology of Hazardous Water Contaminated with Radioisotopes with Paper Sludge Ash-Based Geopolymer—Stabilization of Immobilization of Strontium and Cesium by Mixing Seawater

Zhuguo Li,Ko Ikeda,Mariko Nagashima

doi:10.3390/ma11091521

Abstract

Long-term immobilization ratios of strontium (Sr2+) and cesium (Cs+) in paper sludge ash-based geopolymer (PS-GP) were investigated in one year. PS-GP paste specimens were prepared in the conditions of 20 °C and 100% R.H., using two kinds of paper sludge ash (PS-ash). Two kinds of alkaline solution were used in the PS-GP as activator. One was prepared by diluting aqueous Na-disilicate (water glass) with seawater. Another was a mixture of this solution and caustic soda of 10 M concentration. When seawater was mixed into the alkaline solution, unstable fixations of Sr2+ and Cs+ were greatly improved, resulting stable and high immobilization ratios at any age up to one year, no matter what kind of PS-ash and alkaline solution were used. Element maps obtained by EPMA exhibited nearly even distribution of Cs+. However Sr2+ was biased, making domains so firmly related to Ca2+ presence. The mechanism that seawater stabilizes immobilization of Sr2+ and Cs+ was discussed in this study, but still needs to further investigation. Chemical composition analyses of PS-GP were also conducted by SEM-EDS. Two categories of GP matrix were clearly observed, so called N-A-S-H and C-A-S-H gels, respectively. By plotting in ternary diagrams of SiO2-(CaO + Na2O)-Al2O3 and Al2O3-CaO-Na2O, compositional trends were discussed in view of ‘plagioclase gels’ newly found in this study. As a result, it is suggested that the N-A-S-H and C-A-S-H gels should be strictly called Na-rich N-C-A-S-H and Ca-rich N-C-A-S-H gels, respectively.

Highlights

By using minerals as immobilization media, various attempts have been made so far to remove hazardous elements from water contaminated with heavy metals
The immobilization principle of geopolymer is the polycondensation of silicic acid monomers, in which the foreign ions are incorporated into the siloxane bonds of tetrahedra to promote polymerization of monomer [SiO4 ]-complex associated with other supplemental coordination sites
XRD results are represented in Figure 1 for seawater-mixed and non-seawater-mixed PS-ash-based geopolymer (PS-GP) at the age of 52 weeks

Summary

Introduction

By using minerals as immobilization media, various attempts have been made so far to remove hazardous elements from water contaminated with heavy metals. Cementitious materials are generally used to treat contaminated sludge with less water. Portland cement paste or concrete has incomplete immobilization capabilities, especially for lead and sometimes zinc [2,3]. Geopolymer as other kind of immobilization media has attracted attention in recent years [3,4,5,6,7,8,9,10,11]. The immobilization principle of geopolymer is the polycondensation of silicic acid monomers, in which the foreign ions are incorporated into the siloxane bonds of tetrahedra to promote polymerization of monomer [SiO4 ]-complex associated with other supplemental coordination sites

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Conclusion