Enhanced adsorption of fluoride ion by polyethylenimine modified diatomite in aqueous solution: adsorption optimization, models, and mechanism

There is a growing problem of excessive fluoride ion (F – ) in mine water in western China and it is urgent to identify appropriate treatment strategies. In this work, we tested polyethylenimine-modified diatomite for adsorption of low concentration F – from aqueous solution. Fourier-transform infrared spectroscopy analysis suggested amino group (–NH 2 ) may be the main functional group promoting adsorption. Scanning electron microscopy and Brunauer–Emmett–Teller–Barrett– Joyner–Halenda analysis showed that the modified diatomite surface became smooth and the specific surface area decreased slightly with modification. The optimal pH, temperature, and dosage of polyethylenimine modified diatomite (PEI/DT) were determined as 2.0°C, 5°C, and 0.2 g/L, respectively. The whole adsorption process can be completed in 20–30 min. The adsorption data were better fit to the Langmuir model (average R 2 = 0.9962) to describe the adsorption process than the Freundlich model (average R 2 = 0.9413). The adsorption efficiency of F – by PEI/DT was highest at 5°C, with a maximum saturated adsorption capacity ( q max ) of 28.62 mg/g. The pseudo-second-order kinetic model (average R 2 = 0.9992) is more suitable to describe the overall adsorption process and the pseudo-first-order kinetic model (average R 2 = 0.7318) can only describe the initial stage of adsorption. Thermodynamic parameters confirm that the adsorption process is a spontaneous exothermic process. Overall, the results show that the prepared PEI/DT can potentially be used as an adsorbent to remove F – from mine water.