Adsorption isotherm, kinetic and thermodynamic studies for adsorption of fluoride on waste marble powder

Abstract

Economically viable, efficient waste marble powder (WMP) for fluoride removal has been studied. A batch adsorption study has been carried out concerning contact time, initial concentration, adsorbent dosage, pH, and temperature. After using a batch adsorption mode, 97.13 % removal efficiency was observed with an adsorption capacity of 1.166 mg/g at the initial fluoride concentration of 6 mg/L, solution pH of 6, adsorbent dosage of 500 mg, temperature of 35 °C, and contact time of 60 min. According to kinetics studies, the experimental adsorption data fit well in a nonlinear pseudo-second order with the adjusted-R2 value 0.999, and the nonlinear Langmuir isotherm model fitted best with the adjusted-R2 value 0.988. The separation factor (RL) values were within the range of 0.178 - 0.086, suggesting a satisfactory uptake of fluoride ions. Thermodynamic analysis showed that the adsorption process was endothermic, and negative values of Gibbs free energy change (∆Go) indicated spontaneous fluoride adsorption. In contrast, a positive value of entropy change (∆So= 65.26) indicated arbitrary behaviour at the interface between the solid and the solution. Fluoride removal from aqueous solution was also studied using a continuous fixed-bed column. The Yoon-Nelson model, with a higher adjusted-R2 value of 0.9906, described the dynamics of the fluoride adsorption process better than the Adams-Bohart model because it had better predictions for the breakthrough curve. According to the regeneration analysis, 52 % of the adsorption rate with an adsorption capacity of 0.604 mg/g could be achieved even after five cycles of WMP regeneration.