Immobilization of Arsenite and Arsenate Ions in Aqueous Sample Using Calcium Alumino-Zincate

摘要

Recently, a lot of problems concerning the contamination of the soil and groundwater by heavy metals such as chromium and lead have occurred, and the enlargement of the pollution area by diffusion of their ions has been also extremely worried. In addition, the arsenic pollution of well water in South Asia nations is a serious problem though it is not a metallic element. It was expected that the reaction of arsenic with cement compounds was similar to that of chromium, because both elements were dissolved in water as oxyanion. In general, large-scale facilities for chemical multi-staged process are necessary to remove the toxic anions such as chromate, arsenite and arsenate ions (denoted as Cr[VI], As[III] and As[V] respectively). The removal of those anions from the contaminated water by hydration of cement or cement compound is an advantageous method technically and economically because it needs neither large-scale facilities nor the elaborate setup. Many researches concerning the immobilization of Cr[VI] in mono-sulfate type hydrate (AFm) or tri-sulfate type hydrate (AFt) by hydration of calcium aluminates such as C3A have been reported. Moreover, coauthors are also planning to report on the immobilization of Cr[VI] by hydration of calcium alumino-zincates in this symposium. In this report, the immobilization process of As[III] and As[V] by calcium alumino-zincates, the influence of Cr[VI] coexisting in their arsenic solutions and the effect of Ca(OH)2 addition on the removal reaction were investigated. The compound 14CaO.5Al2O3.6ZnO (C14A5Z6 for short) was used as calcium alumino-zincate. The results obtained were summarized as follows: 1) Both As[III] and As[V] were removed from aqueous sample by addition of C14A5Z6, however, the removal rates of them were much lower than that of Cr[VI]. 2) The removal of As[III] from aqueous sample was accelerated by Cr[VI], though that of As[V] was hardly affected. 3) The addition of Ca(OH)2 to C14A5Z6 was effective to the removal of As[III] and As[V] from aqueous sample, and the removal rate of As[V] was considerably larger than that of As[III].