Advances in Water Treatment and Enviromental Management

ADSORPTION OF TRIHALOMETHANES ON TO ZEOLITES 183THM precursors are ill-defined macromolecular structures and their selective removalby adsorption can prove slow and difficult (Ram et al, 1986, p239). Disinfectantsother than chlorine may prove less effective and/or more expensive to use, eg ozone.Equally, precise control of the chlorination reaction is necessarily complex to copewith wide variations of feed conditions, without over or under dosing (Ram et al,1986, p46).The use of adsorption as a method of removing THMs from drinking water has anumber of advantages over the previous options. It can be applied as a discreteunit operation at the end of the treatment process, allowing all other operations toremain unchanged. The process is generally superior to other removal techniquessuch as air stripping, distillation or biological treatment, in that it usually haslower capital and operating costs, and is more effective (O’Brien et al, 1983, pp337–362). Adsorption using powdered activated carbon (PAC) or granular activatedcarbon (GAC) is already in widespread use for this application, especially in theUSA. The main problem is the cost of regenerating the carbon once it is saturated(O’Brien et al, 1983, pp337–362). This can either be done in situ, requiringmoderately expensive and complex plant, or the carbon is sent away forregeneration, and simply replaced with fresh stock.The purpose of this research project is to investigate the feasibility of using novelsynthetic crystalline pentasil-zeolites such as silicalite (Flanigen and Grose,1977) and similar materials to adsorb THMs from drinking water. Zeolites areporous crystalline aluminosilicates consisting of an assemblage of Si0 4 and Al0 4tetrahedra joined by oxygen atoms to form a crystal lattice with pores ofmolecular dimensions. The pore size is precise and uniform, giving them theirhighly selective nature. The Si and A4 atoms are bound together with oxygenatoms, and in the case of the pentasil-zeolites, having a structure based on adouble five ring unit (Figure 1) which, stacked in a variety of ways, form different,but related, structures. The ZSM-5 structure is characterised by a ring of tentetrahedra and may be prepared in a virtually aluminium-free form known assilicalite.By varying the ratio of Si to Al from one up to about 100, the nature of thesynthesised material is greatly affected. At low Si/Al ratios the zeolites have a highaffinity for polar molecules such as water, but at a Si/Al ratio of between eight andten a transition normally occurs, and a hydrophobic nature is introduced wherebythe material preferentially adsorbs organics rather than water. The uniform poresize of silicalite, at around 6 Angstrom units, limits this adsorption to lowmolecular weight organics only, making the material potentially attractive for theselective removal of THMs from water (Shultz-Sibbel et al, 1982). The secondadvantage of zeolitic adsorbents is their high thermal stability in air up totemperatures of about 1100°C, permitting regeneration by simply heating in air, ascompared to the inert atmosphere and closely controlled conditions necessary foractivated carbon regeneration.