Mesoporous zirconium pyrophosphate for the adsorption of fluoride from dilute aqueous solutions

Mesoporous zirconium pyrophosphate(ZPP) adsorbents were synthesized, with six quaternary ammonium salts (Quats), namely, Octyltrimethylammonium (OTMA, C8), Decyltrimethylammonium (DCTMA, C10), Dodecyltrimethylammonium (DDTMA, C12), Tetradecyltrimethylammonium (TDTMA, C14), Hexadecyltrimethylammoium (HDTMA, C16), and Octadecyltrimethylammonium (ODTMA, C18), as template for studying fluoride adsorption. Quats of long carbon chain length significantly affected the specific surface area of mesoporous ZPP adsorbents. C18-ZPP exhibited the largest specific surface area and fluoride adsorption density. Fluoride adsorption density remained constant at pH < 7.0, decreased sharply as pH increased beyond 7, and became minimal at pH > 10–11. Langmuir adsorption isotherm described fluoride adsorption characteristics well. Results showed that mesoporous ZPP synthesized with C18 at Zr to C18 molar ratio of 0.57:1 exhibited the best fluoride adsorption capacity (32 mmol/g) among all ZPP adsorbents prepared at various Zr to C18 molar ratios. C18-ZPP exhibited two-fold increase in fluoride removal capacity compared to plain ZPP. The reusability of C18-ZPP was assessed by running fluoride removal at least five cycles. Mesoporous ZPP is a promising adsorbent for fluoride removal from water with much enhanced adsorption capacity, regenerability, and reusability.