Adsorption of fluoride on clay minerals and their mechanisms using X-ray photoelectron spectroscopy

This research investigates the adsorptionmechanisms of fluoride(F)on four clay minerals(kaolinite,montmorillonite,chlorite,and illite)underdifferent F^(-)concentrations and reaction times by probingtheir fluoride superficial layer binding energies and elementcompositions using X-ray photoelectron spectroscopy(XPS).At high F^(-)concentrations(C_(0)=5-1000 mg·L^(-1)),the amount of F^(-)adsorbed(Q_(F)),amount of hydroxidereleased by clay minerals,solution F^(-)concentration,andthe pH increase with increasing C_(0).The increases areremarkable at C_(0)>50 mg·L^(-1).The QF increases significantlyby continuously modifying the pH level.At C_(0)<5-100 mg·L^(-1),clay minerals adsorb H+to protonatealuminum-bound surface-active hydroxyl sites in thesuperficial layers and induce F^(-)binding.As the C_(0)increases,F^(-),along with other cations,is adsorbed toform a quasi-cryolite structure.At C_(0)>100 mg·L^(-1),newminerals precipitate and the product depends on the criticalAl^(3+)concentration.At[Al^(3+)]>10^(-11.94)mol·L^(-1),cryoliteforms,while at[Al^(3+)]<10^(-11.94)mol·L^(-1),AlF_(3) is formed.At low C_(0)(0.3-1.5 mg·L^(-1)),proton transfer occurs,andthe F^(-)adsorption capabilities of the clay minerals increasewith time.