Assessment of the Effect of Acid Activation of Kaolin from Malangali on Water Defluoridation

Abstract

Malangali kaolin clay was activated by means of an acid for defluoridation studies. The Acid Activated Malangali Kaolin (AAMK) clay and the Untreated Malangali Kaolin (UMK) were both characterized using spectroscopic and adsorption techniques. BET analysis of AAMK and UMK showed that both clays have mesoporosity characteristics with surface areas ranging from 55 to 58 m²/g. Spectroscopic studies showed that in both clays the major mineral forms are kaolinites and albites with high amounts of SiO₂ and Al₂O₃. AAMK and UMK are endowed with surface functional groups such as OH, Si–O–Si, and Al–O–Al all of which being suitable for adsorption and ion exchange processes. The SEM and TEM images of the clays showed that both internal and external morphologies are slightly modified by means of acid activation. Defluoridation studies carried out in both Artificially Fluoridated Water (AFW) and Natural Fluoridated Water (NFW) under optimized conditions of temperature and pH produced results for the AAMK clay that had a much better demonstrable percentage fluoride removal in AFW by 86.30% and in NFW by 84.68%. The improved defluoridation is associated with clay surface modification rather than the insignificant surface area changes. The Langmuir, Freundlich and Temkin isotherms were well correlated with R² values greater than 0.95 indicating the involvement of fluoride chemisorption during defluoridation. The quality of the AAMK clay defluoridated water met the permissible standard limits of acceptance of WHO, US-EPA and the Tanzania Bureau of standards (TBS) recommendations for drinking water. The use of AAMK clay based adsorbent which is sourced readily and cheaply, is therefore, considered a worthy undertaking, which must be followed by further studies in order to shed more light on the defluoridation mechanism.

Keywords: Kaolin clay; defluoridation; physico-chemical; UMK; AAMK; fluoride