Synthesis, characterization, and potential application of Mn2+-intercalated bentonite in fluoride removal: adsorption modeling and mechanism evaluation

The study synthesizes a low-cost adsorbent made from Mn2+-modified bentonite clay for groundwater defluoridation. The clays were characterized using X-ray diffraction, X-ray fluorescence, scanning electron microscopy, and Fourier transform infrared techniques. The fluoride adsorption capacity of the modified clay was evaluated using batch experiments. The adsorption kinetics results showed that the optimum fluoride (F−) uptake was achieved within the 30 min’ contact time. The data fitted well to pseudo-second-order of reaction kinetics indicating that adsorption of F− occurred via chemisorption. In addition, the adsorption isotherm data fitted well to Langmuir isotherm model indicating that adsorption occurred on a mono-layered surface. Maximum F− removal of 57% was achieved from groundwater with an initial F− concentration of 5.4 mg L−1 and natural pH of 8.6 using adsorbent dosage of 1 g/100 mL. Fluoride adsorption occurred through ligands and ion exchange mechanisms. The synthesized adsorbent was successfully regenerated for up to five times. The study shows that Mn2+-intercalated bentonite clay has potential for application in defluoridation of groundwater.