The Photochemical Generation of Hydroxyl Radicals in the UV -vis/ Ferrioxalate/H 2 O 2 System KELLY A. HISLOP AND JAMES R. BOLTON* Photochemistry Unit, Department of Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 5B7 A reaction mechanism has been validated for the photochemical generation of hydroxyl radicals by ultraviolet or visible irradiation of oxalato iron(III) complexes (ferrioxalate) in the presence of hydrogen peroxide and 2-propanol as a model substrate. A kinetic simulation program incorporating the set of reactions was written to predict the behavior of this photochemical system. The program calculates the quantum yield of oxidation of 2-propanol used as a hydroxyl radical scavenger. The scavenger’s oxidation product, 2-propanone, was analyzed by gas chromatog- raphy after controlled exposure of the solution to a calibrated light source. The theoretical quantum yields of 2-propanol oxidation, Φ RH , agreed reasonably well with experimentally determined Φ RH values under a variety of initial reaction conditions. The value of Φ RH , which under appropriate conditions is directly proportional to the quantum yield for the generation of hydroxyl radicals, Φ OH , was considerably greater than unity in most cases (often Φ RH ) 3.0-4.0), indicative of a chain mechanism involving iron cycling between the II and III oxidation states. Introduction Much is known concerningthe nature ofthe Fenton reaction, first described in a paper that appeared in 1894 (1). In recent decades, attention has turned to its possible application in oxidative treatments of contaminated water because it produces the hydroxyl radical • OH, a powerful oxidizer capable of degrading many organic compounds (2), in a spontaneous dark reaction: Avariation ofreaction 1,called the photo-Fenton reaction, combines ultraviolet (and some visible) light, Fe(III), and hydrogen peroxide. It can also facilitate the production of hydroxyl radicals by a photochemical route (3-5) followed by reaction 1. More importantly, iron is cycled between the +2 and +3 oxidation states, so Fe(II) is not depleted, and • OH production is limited only by the avail- ability of light and H2O2. Fe(OH) 2+ , the dominant iron(III)- hydroxy complex in mildly acidic solutions (pH 2.5-5) photolyses in the UV -vis range (to ∼400 nm) but with a relatively low quantum yield (e.g., ΦFe(II) ) 0.14 ( 0.04 at 313 nm (3); ΦOH ) 0.195 ( 0.033 at 310 nm (6)). The quantum yield of Fe(II) production, ΦFe(II), greatly increases when Fe(III) is complexed with a carboxylic anion, such as oxalate (e.g., ΦFe(II) ) 1.24 at 300 nm, pH ∼2, and 6 mM ferrioxalate (7)). The ferrioxalate complex [Fe(C2O4)3] 3- is highly photosensitive and is used as the basis of a well- known chemical actinometer (8). The reduction of Fe(III) to Fe(II), through a photoinduced ligand to metal charge transfer, can occur over the ultraviolet and into the visible (out to ∼550 nm) (9): The overall rate of Fe(II) formed by reactions 3-5 is Φ Fe(II)N a/ V (M s -1 ),where ΦFe(II) isthe quantum yield ofiron(II) generation, N a is the absorbed photon flux (einstein s -1 ), and V is the volume (L) irradiated. One einstein is equal to one mole ofphotons.While the quantum yield ofthe primary photochemical step (reaction 3) is less than unity, the net value of ΦFe(II) can be greater than one as a result offormation ofthe oxalylradicalanion C2O4 •- ,which quicklydecomposes to the carbon dioxide radical anion CO2 •- [k 4 ) 2 × 10 6 s -1 (10)]. This reducing agent can produce Fe(II) via reaction 5 (11,12). When ferrioxalate is irradiated in the presence of H2O2, the Fenton reaction occurs to produce, under ideal condi- tions, one • OH for every Fe(II) generated (13). Here, Fe 2+ represents the sum of uncoordinated Fe(II) and [Fe II C2O4] 0 , both ofwhich are able to react with H2O2.Therefore, k 6 is the apparent second-order rate constant for the reaction between Fe 2+ andH2O2.In the presence ofa sufficient excessofoxalate, Fe(III)willcoordinate with either two or three oxalate ligands. As with the photo-Fenton reaction, iron cycles between oxidation states and so the production of hydroxyl radicals is limited only by the availability of light, H2O2, and oxalate, the latter two of which are depleted during the reaction. The combination of ultraviolet and/or visible light (out to 550 nm), ferrioxalate as the primary absorber, and H2O2 has recently been patented for use in the wastewater treatment industry as an Advanced Oxidation Technology (AOTs) (14,15). While the oxidation of a number of organic compoundsusingthismethod hasbeen demonstrated using both artificial and solar irradiation (16-20), detailed mecha- nistic studies ofsystems containing Fe(III)coordinated with oxalate have been generally limited to the very low reactant concentrations typical of natural surface and atmospheric waters (21-24). Here, the relative reactant concentrations are very different from those used in this investigation and so the reaction mechanism could differ. Ferrioxalate has been used to reduce compounds that are resistant to oxidation. Huston and Pignatello (25) sensitized the removal of halogen atoms from perchloro- alkanesusingcarbon dioxide radicalanionsCO2 •- generated from irradiated ferrioxalate (in the absence of H2O2). The reduction products of the perchloroalkanes, stable under *Corresponding author; phone: (519) 661-2170; fax: (519) 661- 3022; e-mail: jbolton@julian.uwo.ca. Fe 2+ + H 2 O 2 f Fe 3+ + • OH + OH - (1) Fe(OH) 2+ + h ν f Fe 2+ + • OH (2) Fe III (C 2 O 4 ) 3 3- 9 8 h ν Fe 2+ + 2C 2 O 4 2- + C 2 O 4 •- (Φ Fe(II) N a / V) (3) C 2 O 4 •- 9 8 k 4 CO 2 •- + CO 2 (4) CO 2 •- + Fe III (C 2 O 4 ) 3 3- 9 8 k 5 Fe 2+ + CO 2 + 3C 2 O 4 2- (5) Fe 2+ + H 2 O 2 + 3C 2 O 4 2- 9 8 k 6 Fe III (C 2 O 4 ) 3 3- + OH - + • OH (6) Environ. Sci. Technol. 1999, 33, 3119-3126 10.1021/es9810134 CCC: $18.00  1999 American Chemical Society VOL. 33, NO. 18, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 3119 Published on Web 07/31/1999