Removal of fluoride ions from aqueous solutions on unmodified and iron-modified hydrogels

Unmodified and iron-modified hydrogels were synthesized from polyvinyl alcohol and polyvinylpyrrolidone; these materials were used for the adsorption of fluoride ions from an aqueous solution. The structural and morphological characteristics of the unmodified (HG) and modified (HG-Fe) hydrogels were determined by means of hydration percentage, point of zero charge (PZC), infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and dispersive energy spectroscopy (EDS). The parameters considered in the adsorption processes were contact time, pH, fluoride concentration and dosage. The kinetic data were obtained at an initial pH of 6.7 and were best fitted to the Elovich model (R² = 0.994); the adsorption capacity was higher for HG-Fe than HG. The data obtained at pH 5 were adjusted to the Lagergren equation indicating physical adsorption. The capacities of both materials (HG and HG-Fe) were similar, and the equilibrium time was shorter for HG than HG-Fe. The isotherms were best fitted to the Langmuir model, indicating homogeneous materials. The maximum adsorption capacities were similar for both adsorbents (HG-Fe and HG), and the thermodynamic parameters showed that the processes are exothermic.