Degradation of bisphenol A by Fe-Al layered double hydroxides: A new synergy of homo- and heterogeneous Fenton systems.

Fe(II)-Al(III) layered double hydroxides (Fe-Al-LDHs) prepared by co-precipitation were used to degrade bisphenol A (BPA) in water. The LDH/H2O2 system proved to be effective in degrading BPA, even under high pH conditions. Influencing factors on the removal of BPA, such as LDH dosage, initial BPA concentration and H2O2 concentration, were studied. The adsorption results and kinetic analysis indicated that the degradation rate of BPA increased with increasing LDH and H2O2 concentration and decreasing the initial concentration of BPA. The reaction mechanism was deduced by using a number of analytical techniques, including XPS, XRD, EPR, SEM, TEM and SAED. It was found that the likely degradation mechanism of BPA by Fe-Al-LDHs is a synergy of the homo- and heterogeneous Fenton systems. In this process, when LDHs and H2O2 are present in the solution, Fe, Al and Cl− in the LDH are released into the solution due to the precipitation-solubility equilibrium. The dissolved Fe2+ reacts with H2O2 to form a homogeneous Fenton reaction system. In addition, the OH− produced during the Fenton reaction may induce ion-exchange through a dissolution-reprecipitation process. During the ion-exchange, the released Fe2+ also takes part in the homogeneous Fenton reaction and released ions reprecipitated into LDHs possibly by reaction with Al3+, Fe3+ and OH−. The undissolved and newly formed LDHs function as heterogeneous Fenton catalysts during the reaction due to the presence of Fe2+ in the structure, thus increasing the reaction efficiency. Moreover, the long term durability of the catalyst over several reaction cycles was also tested and it was found that the material only showed a slight decrease in efficiency after four cycles.