Defluoridation of wastewaters using waste carbon slurry

Adsorption of fluoride on waste carbon slurry was investigated. Waste carbon slurry was obtained from fuel oil based generators of a fertilizer industry. The work involves batch experiments to investigate the effects of contact time, pH, temperature and adsorbent dose on the extent of adsorption by carbon slurry. The contact time and pH for maximum fluoride uptake were found 1 h and 7.58, respectively. Maximum adsorption capacity (4.861 mg g−1) of fluoride on carbon slurry was observed at 15.00 mg L−1 initial fluoride concentration using 1.0 g L−1 adsorbent dose. Among four applied models, the experimental isotherm data were found to follow Langmuir equation more closely. Thermodynamically, adsorption was found endothermic with values 7.348 kJ mol−1, −25.410 kJ mol−1 and 0.109 kJ mol−1 K−1 for enthalpy, free energy and entropy, respectively showing the feasibility of adsorption process. From kinetic analysis, the adsorption was found to follow second-order mechanism with rate constant 49.637 g mg−1 min−1. The rate-controlling step of the adsorption was found pore diffusion controlled. In order to investigate the potential of this adsorbent on industrial scale, column and desorption experiments were carried out. The breakthrough capacity of column was calculated 4.155 mg g−1 with at a flow rate of 1.5 mL min−1. The proposed adsorbent has been used to remove fluoride from groundwater and wastewater. Desorption has been achieved under alkaline conditions (pH 11.6) from exhausted carbon slurry. The performance of carbon slurry was compared with many other reported adsorbent for fluoride removal and it was observed that proposed adsorbent is effective in terms of performance and cost especially.