High capacity aluminium substituted hydroxyapatite incorporated granular wood charcoal (Al-HApC) for fluoride removal from aqueous medium: Batch and column study

Abstract

In this study, aluminium substituted hydroxyapatite (Al-HAp) was synthesized in-situ on granular commercial wood charcoal at room temperature for fluoride removal from groundwater. Granular size (0.5–3.0 mm) of the carbon particles was selected considering the practical application of the final adsorbent in a packed-bed column filter. The adsorbent was characterized thoroughly. The Al-HAp phase was precipitated inside the pore channel as well as on the surface of highly porous wood charcoal matrix. The adsorbent had surface area of 442 m²/g and pH_zpc value was 5. The maximum fluoride adsorption capacity at 303 K was achieved as 105 mg/g which is the highest for any granular adsorbent reported till date for adsorption of fluoride. Batch studies were used to deduce the effective role of pH, temperature and ion selectivity on adsorption efficiency. The reusability of the media was tested upto five cycles after consecutive regeneration with 1 M NaOH. Using the developed media, continuous column run experiments were conducted at different operating conditions, viz., bed volume, flow rate and feed concentration. The maximum number of bed volume was obtained as 1415 for a 30 cm bed height with flow rate 10 L/day and feed concentration of 3 mg/L. A fundamental mass transport model based on pore-diffusion was used to simulate breakthrough behavior in fixed-bed column runs with the synthetic and real contaminated groundwater. The relevant model parameters, namely, fixed bed mass transfer coefficient (kƒb), effective pore diffusivity (D_p) and the coefficient of axial dispersion (E_z) were estimated.