The role of ferric ion in the photochemical and photocatalytic oxidation of resorcinol

The oxidation of resorcinol was investigated in homogeneous solutions of ferric nitrate (photochemical) and in heterogenous suspensions of titanium dioxide (photocatalysis). At pH ⩽ 2, synergetic effects were observed for both the photochemical and photocatalytic oxidation of resorcinol with varying ferric ion concentrations up to 5 mM. At pH⩾2.8, the presence of ferric ion enhanced the photochemical and photocatalytic oxidations of resorcinol at low ferric ion concentrations (⩽1 mM). Above 1 mM Fe3+, both the photochemical and photocatalytic oxidations of resorcinol were retarded. Spectrophotometric analysis and theoretical calculations showed that both pH and ferric ion concentration controlled the hydrolysis and polymerisation processes, and thus the Fe(III) speciation, for which the hydrolytic speciation of Fe(III) hydroxo-complexes is of vital influence on the oxidation of resorcinol. The enhancement observed in the presence of ferric ion was attributed to the generation of hydroxyl radicals by soluble Fe(III) hydroxo-complexes and their role as charge trapping species. On the other hand, the negative effect observed was due to the formation of Fe(III) as polynuclear complexes and the precipitation of amorphous hydroxide colloids that subsequently led to a decrease in the total concentration of photoactive iron species in solution.