Purification of Waste Water Using Alumina as Catalysts Support and as an Adsorbent

Abstract

Alumina is one of the most widely used adsorbent for removal of dissolved pollutants from waste water. Various chemical species, especially ions, are known to be adsorbed onto alumina. On the other hand, alumina is a typical support for catalysts. Many kinds of metal supported catalysts are prepared using alumina as their support. These two roles of alumina, i.e., as adsorbent and support, are closely related to each other due to two reasons. Firstly, most supported catalysts are prepared by impregnation. Impregnation is the process in which solid alumina is contacted with liquids which contain various metal precursors. Thus, adsorption phenomena play a crucial role in this process. Since a new phase is formed on the surface of the support after the impregnation process, alumina has been reported to have an essential role in the formation of catalysts active sites. Secondly, alumina used as a catalysts support can adsorb reactants in the course of catalytic reaction. Such adsorption onto support must be taken into account when the conversion and selectivity of the catalyst are calculated. In this chapter, the two roles of alumina and their relationship are discussed in the light of experimental result. Former part of this chapter, behavior of alumina in the process of impregnation is examined and its relationship with the formation of active sites is discussed. Alumina is one of a very active adsorbent for heavy metal ions. This feature is used both in purification of waste water containing heavy metal and preparation of metal supported catalysts. In order to prepare Cu/Al2O3 catalyst, alumina was suspended in aqueous solution of CuSO4. This is a typical impregnation process and in the meantime alumina had been thought to be inactive. However, after long contacting time, alumina was found to dissolve even in the neutral pH range. Because alumina did not dissolve in the same experimental condition in the absence of Cu2+ ions, the dissolution was found to be induced by contacting with Cu2+ ions. Alumina dissolution was observed during impregnation with PdCl42in acid pH range, too. These facts mean that the active site formed on alumina after impregnation can contain significant amount of aluminum, and thus the catalytic performance may different from pure metal particles supported on alumina. In order to examine this possibility, Ru/Al2O3 catalyst was prepared by both impregnation and colloidal Ru methods. In the colloidal method, it is possible to minimize support effect by preparing metal particles in the absence of the support, and then deposit them onto the support. Although the two catalysts prepared had same composition, their catalytic