Removal of Fluoride from Contaminated Water Using Natural Calcium-Rich Attapulgite as a Low-Cost Adsorbent

In this study, natural calcium-rich attapulgite (NCAP) was used to develop a low-cost adsorbent for removing fluoride (F−) from contaminated water. The results showed that calcination can dramatically increase the F− sorption capacity of NCAP and that the maximum F− sorption capacity occurred at 700 °C. The sorption of F− on NCAP heated at 700 °C (NCAP700) followed pseudo-second-order kinetics and was described by the Langmuir equilibrium model. The estimated F− sorption capacity was approximately 140.0 mg/g at pH 8.0, which was comparable with the sorption capacities of some nanomaterials. The sorption of F− on NCAP700 performed well at pH values of 7 to 10. In addition, anions such as NO3− and SO42− did not affect fluoride removal, but PO43− and HCO3− moderately influenced fluoride removal. A column study conducted using NCAP700 with a particle size of 0.2–0.5 mm indicated that the adsorbent could effectively purify nearly 200 bed volumes (BV) of water containing 3.0 mg F/l at pH 8.5. The removal of F− from water mainly resulted from the formation of calcium fluoride precipitates and the complexation of fluoride with the –OH group of NCAP700, which was further confirmed by scanning electron microscopy–energy dispersive spectrometry (SEM-EDS) and X-ray photoelectron spectroscopy (XPS).