Photochemical decomposition of perfluorooctanoic acid mediated by iron in strongly acidic conditions

Abstract The performance of a ferric ion mediated photochemical process for perfluorooctanoic acid (PFOA) decomposition in strongly acidic conditions of pH 2.0 was evaluated in comparison with those in weakly acidic conditions, pH 3.7 or pH 5.0, based on iron species composition and ferric ion regeneration. Complete decomposition of PFOA under UV irradiation was confirmed at pH 2.0, whereas perfluoroheptanoic acid (PFHpA) and other intermediates were accumulated in weakly acidic conditions. Iron states at each pH were evaluated using a chemical equilibrium model, Visual MINTEQ. The main iron species at pH 2.0 is Fe 3+ ion. Although Fe 3+ ion is consumed and is transformed to Fe 2+ ion by photochemical decomposition of PFOA and its intermediates, the produced Fe 2+ ion will change to Fe 3+ ion to restore chemical equilibrium. Continuous decomposition will occur at pH 2.0. However, half of the iron cannot be dissolved at pH 3.7. The main species of dissolved iron is Fe(OH) 2+ . At pH 3.7 or higher pH, Fe 3+ ion will only be produced from the oxidation of Fe 2+ ion by hydroxyl radical produced by Fe(OH) 2+ under UV irradiation. These different mechanisms of Fe 3+ regeneration that prevail in strongly and weakly acidic conditions will engender different performances of the ferric ion.