Silica gel-enhanced oxidation of caffeine by ferrate(VI)

The present paper demonstrates the employment of simple solid silica gel (SiO2) to remarkably enhance the oxidative transformation of caffeine (CAF) by ferrate(VI) (FeO42-, Fe(VI)) in water under environmentally relevant pH conditions (pH similar to 8.0). Complete oxidative transformation of CAF was achieved at Fe(VI) to CAF molar ratio of 6.0 in the presence of 4 g SiO2/L under mild alkaline conditions (pH similar to 8.0). Comparatively, only 53.0% CAF removal was achieved at the same Fe(VI) to CAF molar ratio of 6.0 in the absence of SiO2 gel at similar pH conditions (pH similar to 8.0). Variation of particle size and pore volume of SiO2 gel showed no impact on the oxidation of CAF by Fe(VI). In the presence of SiO2 gel, Na+, Mg2+, Ca2+, Cl-, HCO3-, and SO42- ions had no significant effect on the removal of CAF by Fe(VI). Natural organic matter (NOM) in water decreased the removal of CAF by Fe(VI) in the presence of SiO2 gel. The influence of NOM on the removal efficiency of CAF by Fe(VI) could be decreased by increasing the amount of SiO2 gel in water. Identification of three products of CAF by Fe(VI)-SiO2 system suggests a reaction pathway initiated by the attack on the C-4=C-5 double bond of CAF, which is supported by the frontier molecular orbitals calculations of CAF.