Phenol chlorination and photochlorination in the presence of chloride ions in homogeneous aqueous solution.
Phenol chlorination was studied in the presence of dissolved Fe(III) and chloride under irradiation and of hydrogen peroxide and chloride in dark acidic solutions. In the former case phenol photochlorination is most likely due to the formation of Cl2*- as a consequence of Fe(III) irradiation in the presence of chloride. The most efficient pathway is the photolysis of FeOH2+ producing hydroxyl, which oxidizes chloride to Cl*. The latter finally yields Cl2*- upon further reaction with chloride. The importance of the pathway involving FeOH2+ is higher at higher pH and moderately low chloride concentration. At pH 2.0 and [Cl-] > 0.03 M chlorophenol generation rate decreases with increasing [Cl-], due to the formation of the much less photoactive species FeCl2+/FeCl2+. The photolysis of FeCl2+/ FeCl2+ yielding Cl* is likely to play an important role at pH 0.5 and high chloride, but under such conditions chlorophenol formation rates are about an order of magnitude lower than at pH 2.0. Due to pH and kinetic constraints, under most environmental conditions the photochemistry of FeCl2+/FeCl2+ can be expected to play a minor role toward chlorination when compared with the one of FeOH2+, which leads to hydroxyl-mediated chloride oxidation. Hydrogen peroxide and chloride react in dark acidic solutions to yield HClO, involved in electrophilic chlorination processes. Chlorophenol formation rates under such conditions are directly proportional to [H+]. The described chlorination and photochlorination processes can take place in acidic aerosols of marine origin, naturally rich in chloride and Fe(III). Antarctic aerosol is also rich of hydrogen peroxide and often strongly acidic due to the presence of sulfuric acid of biogenic origin.