Abstract
Although calcium-based adsorbents are expected as effective arsenic adsorbents, the spent adsorbents which removed arsenic from contaminated water would contain a great amount of arsenic. If they were dumped into environment without an appropriate treatment, arsenic leached from them might cause a secondary environmental pollution. Also, silicic acid leached from soils or cementitious solidification materials may affect the environmental stability of the spent adsorbents.
In this study, the leaching tests with silicic acid solutions on the spent calcium-based adsorbents (CaO and Ca(OH)2) were carried out. The arsenic leaching ratios of spent Ca-based adsorbents were found to decrease as silicic acid concentration in liquid is higher. Calcium ions leached from the spent adsorbents can be considered to react with silicic acid components in the liquid to form calcium silicate species. Then, arsenic components leached from the spent adsorbents can be considered to be incorporated into the silicates during the above formation process. In addition, the average chemical compositions of the calcium silicates formed are considered to depend on the initial silicic acid concentration in the test solutions. In the cases of around 25 or 50～100 mg/L as the initial Si concentration, each of the average chemical composition formula was estimated to be Ca2SiO4 and CaSiO3. On the other hand, in the case that initial Si concentration was relatively low (around 5 mg/L), the reaction in which silicic acid components adsorb directly onto the spent adsorbent surface (ion-exchange or chemical adsorption reaction) was inferred to be predominant.
