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 degrees C. The sorption of F- on NCAP heated at 700 degrees 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).