Bio-adsorbent hydroxyapatite for drinking water defluoridation: column performance modelling studies

Waste marble powder (WMP) is a rich source of calcium and magnesium salts having an affinity for fluoride ions and therefore serves as a good defluoridation agent. Hydroxyapatite was synthesized from WMP generated by the marble processing industry to make an adsorbent for drinking water defluoridation. The synthesized marble hydroxyapatite (MA-Hap LR) powder was further formed into 2-3 mm pellets by extrusion spheronization technique using a polyvinyl alcohol binder. Continuous column defluoridation studies were conducted to obtain optimized column parameters such as input fluoride concentration, column inflow rates, optimum pellet size, and adsorbent bed parameters to obtain maximum fluoride adsorption capacity. The best breakthrough column performance was a maximum adsorption capacity of 1.21 mg/g, treating 10 mg/L fluoride concentration. The optimized column flow rate was at 1 LPH using an adsorbent bed height of 25 cm, which processed 28.5-bed volumes at an adsorbent exhaustion rate of 7.4 g/L. The column breakthrough performance data were fit into various kinetic models (Thomas model and Yoon-Nelson model) to describe adsorption kinetics and obtain correlation coefficients. Thomas's model fitted well with a high correlation coefficient value. Modelling studies indicate MA-Hap as a promising adsorbent for drinking water treatment, and optimum column design parameters were identified for scale-up for real applications.