Surface modification of paddy husk ash by hydroxyl-alumina coating to develop an efficient water defluoridation media and the immobilization of the sludge by lime-silica reaction

Abstract

When fluoride content in the water crosses its permissible limit of 1.5ppm, it causes threat of fluorosis. The current defluoridation technologies are expensive and technologically complicated. Therefore, there is a need for cheap, socially acceptable methods of defluoridation. In this present approach, an improved adsorbent was prepared by surface modification of a locally available bio waste Paddy Husk Ash (PHA) with a soluble aluminium oligomer called as hydroxyl-alumina (OH-Al). Coating of PHA surface with OH-Al provides a good defluoridising composite adsorbent (A-PHA). To know the structure and property relationships of A-PHA, its characterisation was carried out by using techniques like XRD, SEM-EDS, particle size analysis. Also, the effects of various parameters like adsorbent dose, pH, initial fluoride ion concentration, contact time, presence of other interfering anions were studied. Freundlich adsorption isotherm and pseudo second order kinetic models were found to be the best fitted adsorption isotherm and kinetic data models respectively for the present process. Continuous mode column adsorption study was also carried out. More importantly the further scope of immobilising the spent adsorbent at low temperature by an economic route of reaction of the generated sludge with lime, to trap it further in the calcium-alumino-silicate hydrated matrix was also studied. Encouraging positive results on fluoride immobilization by sludge-lime reacted product was observed in both leaching as well as the hydration study.