In-situ synthesis of calcium/magnesium phosphate system for water de-fluoridation: Clay ceramic materials

Abstract

Clay ceramic water filters impregnated with silver colloids have been used to purify water in many parts of East Africa disregarding the removal of fluoride, which is a health-threatening ion in water supplies. This paper presents a synthesis of clay ceramic materials containing in-situ generated calcium/magnesium phosphate system as an adsorbent for water de-fluoridation. The materials were prepared in varied ratios of clay (K) to heat-pretreated dolostone (R) (K:R = 0:100, 90∶10, 85∶15, 75∶25 and 100:0), sintered at temperatures between 500 °C and 900 °C, and characterized using XRF, AAS, CHNS–O analyzer and X-ray PDF analysis. The materials improved in de-fluoridation performance with increase in sintering temperature at 500–700 °C, before exhibiting a sharp drop at 800 °C. The adsorption performance of K:R = 75:25 was higher than that of K:R = 0:100, signifying the effect of clay. The adsorption data obeyed Freundlich isotherm model, suggesting multi-layer adsorption. Fluoride uptake follows pseudo-second-order and intra-particle diffusion models, which suggests a chemisorption process involving rate-limiting intra-particle diffusion.