Volcanic rock-based ceramsite adsorbent for highly selective fluoride removal: function optimization and mechanism

BACKGROUND While various granular adsorbents have been studied for fluoride removal from water, this study focuses on the optimization of mixture proportions of volcanic rock, bentonite, corn starch and aluminum sulfate using a four-component simplex-centroid mixture design (SCMD) method to achieve preferable fluoride removal ratio and ceramsite mechanical strength. To illustrate the adsorption mechanism, a series of experiments, including adsorption isotherms, adsorption kinetics and effects of pH value and co-existing anions, was carried out. RESULTS The synthesized adsorbent was mesoporous with 77.82% pore size distribution in the range 1.70-38.80 nm by detailed physicochemical characterization. A stable fluoride removal ratio (87.5%) was obtained in the wide pH range of 4.0-11.0, and the adsorption capacity was 10.17 mg g(-1) at 298 K. The adsorbent remained high selectivity for F- in the presence of sulfate (SO42-), chloride (Cl-) and nitrate (NO3-), while carbonate (HCO3-) posed a slight adverse effect on F- adsorption due to ion competition. This adsorption process was well fitted with the pseudo-second-order kinetic model and the Freundlich isotherm model. Electrostatic attraction and M+-H2O and M+-F ligand exchange were dominant in the adsorption mechanisms. CONCLUSION The synthesized ceramsite adsorbent will be competitive in the application for F- removal from aquatic environments. (c) 2019 Society of Chemical Industry