Kinetics of fluoride removal from spent pot liner leachate (SPLL) contaminated groundwater

Groundwater contaminated with spent pot liner leachate (SPLL) was treated with calcite (CaCO3) and CO2 in a series of free drift batch reactor kinetic experiments with the aim of optimizing fluoride removal. In order to better understand the reaction kinetics for SPLL, the pseudo-second order (PSO), four and five parameter Hill (Hill 4 & Hill 5) models were applied to observed fluoride removal as a function of CO2 partial pressure and stirring rate. To account for differences in the number of model parameters, model fitting was compared using the Akaike information criterion (AIC), the Schwartz Bayesian information criterion (BIC) and the statistical F-test. In every case, the best fitting model order was found to be Hill 5 > Hill 4 > PSO due to the ability of the Hill models to accommodate sigmoidal kinetics. Removal of fluoride from SPLL under atmospheric CO2 conditions was found to be inhibited with no removal occurring for the first 3000 min with the inhibition times, based on the reaction half-life, reducing with increased CO2 partial pressure and stirring rate. X-ray diffraction (XRD) analysis revealed that the residual solid filtrate consisted of trona (Na3(CO3)(HCO3)·2H2O) a potentially valuable mineral kogarkoite (Na3SO4F) and fluorite (CaF2) accounting for fluoride removal, and unreacted calcite (CaCO3).