Synthesis and physicochemical characterization of MnO2 coated Na-bentonite for groundwater defluoridation: Adsorption modelling and mechanistic aspect.

An efficient fluoride adsorbent was synthesized by coating Na-activated bentonite with MnO2 through in-situ reduction of KMnO4. Physicochemical and mineralogical compositions of the adsorbent were determined using X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM) and Fourier Transform Infrared spectroscopy (FTIR), Brunauer Emmett Teller (BET) and Barrett Joyner Halenda (BJH) techniques. Batch experiments were conducted to evaluate the adsorption kinetics, adsorption isotherm, effect of pH and the effect of co-existing anions on fluoride removal. The data showed a better fit to Langmuir adsorption isotherm while the kinetics data fitted better to Pseudo second order. Sorption of fluoride occurred via electrostatic attraction and ion exchange adsorption mechanisms The regeneration study showed that MnO2 coated Na-bentonite (Mn-NaB) can be successfully regenerated up to 5th cycle with 0.01 M NaOH. The chemical stability assessment showed that Mn-NaB leaches out traces of Mn2+ ions at low pH levels. The study recommends that Mn-NaB be used for defluoridation of groundwater in areas where no alternative sources of safe clean water are available. Moreover, further studies should be conducted to improve the stability of Mn-NaB.