Marble waste derived hydroxyapatite: Low-cost adsorbent for the defluoridation of drinking water

Abstract

Synthesis of hydroxyapatite (Hap) powder using waste marble powder resource was conducted in a pilot plant scale setup for its application in the defluoridation of drinking water. During synthesis, process parameters such as temperature, process time and pH were optimized so as to obtain good quality hydroxyapatite (MA-Hap LR) products. MA-Hap LR powder was characterised using FTIR, XRD which confirmed formation of purer form of Hap when synthesized at 72 °C having a crystallite size of 43.87 nm and surface area of 24.66 m2/g. SEM and TEM studies of MA-Hap LR indicated rod like morphology similar to pure Hap. The powder was pelletized via the extrusion-spheronization technique in a pelletizer using Polyvinyl alcohol as the binder. Pellets of varying size ranges (2–3 mm) were studied for their fluoride adsorption capacity. Continuous column experiments were conducted to optimize column operational parameters like inlet flow rate, input fluoride concentrations, pellet size, and bed height for maximum fluoride removal. The column fluoride adsorption capacity was obtained at 1.21 mg/g for 10 mg/L input fluoride concentration, bed height 25 cm, 1 LPH flow rate, and processed 28.5-bed volumes at a maximum adsorbent exhaustion rate of 7.4 g/L. Adsorbent regeneration studies indicated that MA-Hap LR72 pellets could be reused for three cycles. The eluent concentration required for the regeneration of exhausted pellets was optimized at 1 M NaOH, confirmed by FTIR and SEM studies of the pellets. The treated water quality was within the acceptable limits for parameters such as pH, alkalinity, hardness and TDS according to WHO and BIS regulations which describe the utility of hydroxyapatite prepared from waste marble powder as a good defluoridation agent for practical drinking water treatment applications.