Insights into isotherm making in the sorptive removal of fluoride from drinking water

The defluoridation research has thrown up many technologies, with adsorption as a popular alternative, especially among fluoride endemic habitations of the developing world. In the endeavor to develop novel adsorbents for defluoridation, the adsorption potential of hardened alumina cement granules (ALC) were examined through isotherm fitting. Though the adsorbent showed enhanced adsorption capacity at higher fluoride concentration ranges, the errors associated with linearization in isotherm fitting were also found to be increasing. The propagation of these errors was more prominent in Dubinin-Radushkevich and Langmuir models but negligible in Freundlich. The χ2 analysis, used to correlate the equilibrium experimental data and the isotherm models, also suggested poor correlations at higher fluoride concentration ranges for all the models. The procedure of linear and nonlinear regression through optimization of error functions rendered the 'best-fit' model and optimum model parameters, through sum of normalized error (SNE) values. Though ALC exhibited maximum monolayer adsorption capacity of 34.36 mg g−1 in concentration variation studies of fluoride in the range of 2.5-100 mg l−1 in synthetic water, it got reduced to 10.215 mg g−1 in dose variation studies and further to 0.9358 mg g−1 in natural ground water. Though Langmuir appeared as the best-fit model in terms of R2 in synthetic studies of different fluoride concentrations, the procedure of linear and nonlinear regression demonstrated that Freundlich was the best-fit. The nonlinear χ2 analysis together with minimum SNE values convincingly demonstrated that the equilibrium studies with dose variations of ALC offers more reliable isotherm parameters than those with high fluoride concentrations. The sorption of fluoride by ALC appeared endothermic with Freundlich adsorption capacity parameter increased from 0.5589 to 0.9939 l g−1 in natural water and 3.980-7.5198 l g−1 in synthetic water systems for a rise in temperature from 290 to 310 K. The study deviates from conventional methodologies of relying solely on R2 values in selecting 'best-fit' isotherm model, and basically demonstrates how the optimum model parameters like 'adsorption capacity' evolves through linear and nonlinear regression using error functions.