Iron(III)-photoinduced degradation of 4-chloroaniline in aqueous solution

Abstract The degradation of a prototypical halogenoaromatic pollutant, 4-chloroaniline (4-CA), photoinduced by Fe(III) species, has been studied in acidic aqueous solutions (pH 2–4) of Fe(ClO 4 ) 3 by means of product analysis and nanosecond transient absorption spectroscopy. The degradation process is initiated by the attack of OH radicals on 4-CA, leading to radical cations, 4-CA + as the major transient species. At high photon fluences, the decay of 4-CA + is dominated by second-order reactions; an accelerating effect of Fe(III) could be evidenced at low fluence. A large number of minor reaction intermediates could be detected by product analysis following continuous irradiation at λ exc =365 nm of aerated solutions, arising from ring substitution, ring opening and oligomerization processes. At higher concentration of 4-CA, the specificity of the degradation process increases; the oligomerization pathway becomes predominant and leads to three major oligomeric products, one of which could be unequivocally identified, while probable structures were proposed for the other two. The chemical nature of these products, as well as the observation of the p -benzoquinone/hydroquinone couple as the major photoproduct in deaerated solution, led to the proposal that p -benzoquinone monoimine is a key intermediate in the degradation of 4-CA. Direct oxidation of 4-CA + by Fe(III) is put forward as a possible explanation for its formation. Prolonged irradiation at λ exc =365 nm in the presence of oxygen leads to complete mineralization; the process is, however, complex due to the formation of light-absorbing aromatic intermediates on the one hand, and to the behavior of the Fe(III)/Fe(II) redox couple during irradiation on the other. Fe(III)-photoinduced oxidation of 4-CA is a fairly efficient process as long as the photoinducing Fe(III) species are not depleted. The further progress of the reaction requires the reoxidation of Fe(II) into Fe(III), which is accomplished in processes requiring both light and oxygen, probably by reactions of Fe(II) with photoinduced oxidizing radicals.