Effect of formation pH, molar ratio and calcination temperature on the synthesis of an anionic clay based adsorbent targeting defluoridation

Abstract

Calcined Ca Al (NO3) LDH, an anionic clay based adsorbent, was synthesized at different process conditions. The influence of formation pH, molar ratio, and calcination temperature on the physicochemical characteristics of this adsorbent was assessed. The response surface methodology (RSM) was applied to optimize the adsorption capacity by employing a 33 factorial design. Characterization of the adsorbent was conducted through XRD, FTIR, EDX, SEM and N2 adsorption desorption analysis. A significant variation in adsorption capacity (6.4 to 12.5mg/g) was observed for the experimental points designed. XRD results demonstrated that the increase in formation pH enhances the crystallinity of the LDH material. The defluoridation capacity was observed to decrease with increasing pH or molar ratio, as the aluminum content in LDH structure was found to be inversely correlated with both the factors. The fluoride adsorption capacity was found to attain optima at a calcination temperature of approximately 500°C. The reconstruction of the original structure from the calcined product of LDH through a ‘memory effect’ may enhance its adsorption capacity. The maximum adsorption capacity by the Langmuir isotherm model was estimated to be 108.69mg/g.