Photochemical oxidation of arsenic by oxygen and iron in acidic solutions

The rate of oxidation of As(III) to As(V) by oxygen is increased by several orders of magnitude by the presence of dissolved Fe(III) and illumination with near ultraviolet light. The oxidation reaction has been studied in acidic conditions in the absence and presence of dissolved oxygen. A reaction mechanism has been postulated in which Fe(III)-hydroxide and -chloride complexes first absorb photons to produce highly oxidising hydroxyl and dichloro radicals. A series of rapid competing reactions involving free radicals then occur in which As(III) is oxidised via the As(IV) intermediate. The observed arsenic oxidation rate is controlled by the rate of photon absorption and the fraction of As(IV) which undergoes oxidising rather than reducing reactions. The use of iron compounds as a photo-oxidant is advantageous because Fe(III) hydroxide precipitate is an excellent adsorbent for As(V).