Research: Science & Education
Free Radical Reactions in Aqueous Solutions: Examples from Advanced Oxidation Processes for Wastewater and from the Chemistry in Airborne Water Droplets N. Colin Baird Department of Chemistry, University of Western Ontario, London, ON, Canada N6A 5B7 The chemistry of the aqueous phase (especially acid– base processes) is dominated by reactions of closed-shell species, but free radicals do exist in small concentrations in water and their chemistry can be important. Although many of the principles recently developed to systematize free radical gas-phase reactions in air (1, 2) can be applied directly to radicals in aqueous solution, some extra considerations are necessary given the existence of radical ions as well as neutral molecules in water. Free radical reactions are of crucial importance in the so-called Advanced Oxidation Processes (AOPs) now used to purify wastewater that contains substances resistant to decomposition by chlorination and/or ozonation. The active ingredient in AOPs is the hydroxyl free radical, OH ?, which is an extremely strong and selective oxidizing agent in aqueous solution as well as in the gas phase (3, 4). Suspended airborne water droplets in clouds or fog also contain free radicals such as OH ?, and these are of importance in the oxidation there of sulfur dioxide and ions in which sulfur exists in the +4 oxidation state; these reactions result in the production of acid rain. In this paper, the reactions that play significant roles in the chemistry of free radicals in aqueous solutions are discussed. The principles derived in this discussion are then applied to the important chemistry that occurs during AOPs and in water droplets suspended in air. Acid–Base Chemistry of Hydrogen–Oxygen Systems (3, 4) The most important reactions during wastewater treatment involve not only OH?, but other oxygen and hydrogen– oxygen molecules and ions. These species include hydrogen peroxide, H2O2 , which is quite a weak acid (Ka about 10{11 ): H 2O2
H+ + HO2{
(1)
HO2{
Although is too weak an acid to consider further, its oxidized form, the hydroperoxy free radical, HO2?, is a weak acid having about the same strength (Ka about 10 {5 ) as acetic acid: HO2?
H+ + O 2·{
(2)
Thus at pH values greater than 5, the superoxide anion O2·{ predominates over its conjugate acid, the hydroperoxy radical. A somewhat weaker acid (Ka about 10{8) than HO2? is the corresponding species with a chain of three oxygen atoms: HO3?
H+ + O 3·{
(3)
Electron Transfer Reactions (3–5) A common free radical reaction in aqueous solution is the transfer of an electron from an anion to a neutral free radical; such reactions proceed readily, since the extra electron in many anions is not held strongly. For example, the hydroxyl free radical frequently reacts in aqueous solution by abstracting a loosely held electron from an anion: OH? + X { → OH { + X?
(4)
? OH? + X n{ → OH { + X(n{1){
(5)
or more generally
Indeed, the effectiveness of the hydroxyl radical as an oxidizing agent for organic substances in water is greatly diminished if the water contains high concentrations of bicarbonate ion, carbonate ion, or sulfate ion: for example, OH ? + HCO3{ → OH { + HCO3?
(6)
OH? + SO42{ → OH{ + SO 4·{
(7)
Hydroxyl radicals are not as stable in sea water as in freshwater because they abstract electrons from bromide ion, iodide ion (and at low pH from chloride ion): for example, OH? + Br { → OH{ + Br?
(8)
In aqueous solutions containing OH ? and other oxidizing agents such as molecular oxygen, hydrogen peroxide, and ozone, a competition exists for the loosely held electrons that convert neutral species into anions. In water, ozone has a higher electron affinity than does O 2 or HO2? , so it abstracts the extra electron from either O2·{ or HO2{ : for example, O3 + O2{· → O3{· + O 2
(9)
The order of electron affinity among the important species is O 3 > HO2? > O2. Thus the hydroperoxyl radical abstracts the extra electron from the superoxide anion, and ozone abstracts the electron from HO 2{. The Chemistry of Advanced Oxidation Processes (3–6) Although the hydroxyl radical is the most important oxidizing agent in the aqueous solutions of AOPs, it cannot be stored due to its high reactivity. Thus it must be generated on site when it is needed to react with intransigent organic pollutants. In the “O3 / H2 O2” process, OH ? is prepared via the fast decomposition of the HO3· free radical, which because of the presence of a chain having three oxygen atoms is expected (1, 2) to spontaneously decompose by splitting off diatomic oxygen: HO 3· → OH· + O2
Vol. 74 No. 7 July 1997 • Journal of Chemical Education
(10)
817
Research: Science & Education The HO3· radical is obtained from its conjugate base O3 {· by protonation of the latter: H+ + O 3·{ → HO 3·
(11)
(Decomposition of the HO3· occurs so quickly that this reaction does not achieve equilibrium.) The ozonide ion in turn is prepared by electron transfer (see above) to ozone from HO2{: HO2{ + O3 → HO 2· + O 3{·
(12)
{
The HO2 ion is obtained by proton removal from dissolved hydrogen peroxide under alkaline conditions: H2O2
H + + HO2{
(13)
The hydroperoxy radical produced in reaction 12 also can ionize, producing O2{· which in turn will reduce another ozone molecule to the ozonide ion (reaction 9) and result in the production of more hydroxyl radical once the O3{· ion becomes protonated (see reactions 10 and 11). It is an interesting exercise in the addition of reactions to deduce the overall reaction, which is the reaction of hydrogen peroxide with two ozones to give two hydroxyl radicals and three molecules of diatomic oxygen: H2O2 + 2 O3 → 2 OH· + 3 O2
(14)
The overall production of hydroxyl radicals occurs most rapidly at relatively high pH, presumably because such conditions promote the ionization of hydrogen peroxide and HO2{, even though they reduce the formation rate of HO3· from the ozonide ion owing to the small concentration of H+. A second technique, the “ozone / UV” process, that can be used to generate hydroxyl radicals in water involves the photochemical decomposition of ozone using ultraviolet light. This reaction produces hydrogen peroxide, which in turn becomes mainly photochemically decomposed to yield hydroxyl radicals: UV
O3 + H 2O → O2 + H2O2
(15)
H2O2 → 2 OH ?
(16)
UV
(Of course, some of the hydrogen peroxide can react with ozone to form OH? radicals by reaction 14.) Once formed, hydroxyl free radicals can initiate the oxidation of dissolved organic molecules in much the same way that they operate in air, namely, by hydrogen atom abstraction or by addition to a C= C double bond in an aromatic or other unsaturated molecule (1, 2). In aqueous solution, the alternative of oxidation of a molecule via electron abstraction by OH? also exists. The hydrogen abstraction mechanism involving attack by hydroxyl on an organic molecule HRH is thought to be followed, in solution as in the gas phase, by the addition of molecular oxygen to the new radical (3): HRH + OH? → HR ? + H2O
(17)
HR? + O2(aq) → HRO2?
(18)
In contrast to the gas phase, where the peroxy radical generally reacts with atmospheric nitric oxide, in solution it can lose H+ and O2 {· to produce a neutral, closed-shell molecule R: (19) HRO 2? → H + + R + O2{· The superoxide ion then is converted back into a hydroxyl radical via ozone and trioxide ion, as discussed above (reactions 9–11).
818
Since the generation of OH? in solution is relatively expensive, it is economical to use AOPs to treat only the wastes that are resistant to the cheaper, conventional treatment processes. Thus, integrating AOP with pretreatment of the wastewater by biological or chlorination or ozonation processes to dispose of the easily oxidized materials is often appropriate. Another innovative technology for wastewater treatment involves the irradiation by sunlight of solid semiconductor photocatalysts such as TiO2, small particles of which are suspended in solution (6). Irradiation produces electrons, e{, in the conduction band and holes, h+, in the valence band of the metal oxide due to absorption of the UV component of the sunlight. The holes in the valence band of the semiconductor can react with water molecules or with surface-bound hydroxide ions, producing hydroxyl radicals in both cases: h+ + OH { → OH? (20) h+ + H2O → OH? + H+
(21)
The electron produced by the irradiated photocatalyst usually combines with dissolved molecular oxygen, producing O2·{. If present, hydrogen peroxide will react instead with the electron to form the anion radical. Since the extra electron in H2O2 {· occupies an antibonding σ* molecular orbital, the weak two-electron bond in the neutral molecule is thereby even further reduced in strength and the ion radical efficiently dissociates to hydroxyl radical and hydroxide ion: HO – OH + e{ → [HO…OH { ]? → OH? + OH { (22) Free Radical Reactions in Airborne Water Droplets (7, 8) It is known that the important oxidants in water droplets in clouds and fogs are ozone and hydrogen peroxide; for example they both efficiently oxidize dissolved sulfur dioxide to sulfate ion (2). However, dissolved free radicals also play a role in the chemistry of suspended droplets. For example, the superoxide anion destroys dissolved ozone, and thereby constitutes a significant loss process for tropospheric ozone: O2·{ + O3 → O2 + O 3{·
(23)
(The ozonide ion can go on to produce more hydroxyl radical.) One source of the superoxide ion is from reduction of dissolved molecular O 2 by an aquated electron that is photoionized from an organic chromophore C (such as an aldehyde or an aromatic molecule) dissolved in water, or photodetached from a nitrate or nitrite ion: C + light → C+ + e{ (aq) { (aq)
+ O2 → O2·{
(24)
(25) e · Some of the O2{ produced in the above reaction undergoes protonation to become HO 2?, two molecules of which can combine to produce hydrogen peroxide. Indeed, under some circumstances aqueous-phase photochemistry is a larger source of dissolved hydrogen peroxide and free radicals than is the gas-to-drop transfer of these species (1). In water droplets, OH? is produced from the ozonide ion (see reactions 10 plus 11) that results from electron photodetachment from a closed-shell species, and also from the indirect decomposition of hydrogen peroxide molecules. In the latter process, traces of Fe3+ dissolved in a droplet accept electrons photodetached from hydroxide ions that are complexed to them, thereby producing Fe 2+ and hydroxyl radicals:
Journal of Chemical Education • Vol. 74 No. 7 July 1997
Research: Science & Education Fe3+(OH {) + sunlight → [Fe2+(OH ?)] → Fe2+ + OH? (26) The ferrous ion is reoxidized to ferric by electron transfer to hydrogen peroxide; as discussed above, the intermediate H2 O2{· species efficiently decomposes to hydroxyl radical and hydroxide ion: Fe2+ + HO–OH → Fe3+ + [HO…OH {]? → OH? + OH { (27) The net reaction is the photochemical decomposition of a hydrogen peroxide molecule into two hydroxyl radicals. In the suspended water droplets, the hydroxyl radicals oxidize dissolved aldehydes, hydroxymethanesulfonate ion, dissolved sulfur dioxide, and probably polycyclic aromatic hydrocarbons and other organics as well.
Literature Cited 1. Baird, N. C. J. Chem. Educ. 1995, 72, 153–157. 2. Baird, N. C. Environmental Chemistry; Freeman: New York, 1995; Chapters 2 and 3. 3. Peyton, G. R.; Glaze, W. H. Environ. Sci. Technol. 1988, 22, 761– 767; Peyton, G. R.; Glaze, W. H. In Photochemistry of Environmental Aquatic Systems; Zika, R. G.; Cooper, W. J., Eds.; ACS Symposium Series 327; American Chemical Society: Washington, DC, 1987; Chapter 6. 4. Masten, S. J.; Davies, S. H. R. Environ. Sci. Technol. 1994, 28, 180A–185A and references therein. 5. von Sonntag, C.; Schuchmann, H-P. Angew. Chem. Int. Ed. 1991, 30, 1229–1253. 6. Aquatic and Surface Photochemistry; Helz, G. R.; Zepp, R. G.; Crosby, D. G., Eds.; Lewis: Boca Raton, FL, 1994; Part II. 7. Faust, B. C. Environ. Sci. Technol. 1994, 28, 217A–222A and references therein. 8. Lelieveld, J.; Crutzen, P. J. Nature 1990, 343, 227–233.
Vol. 74 No. 7 July 1997 • Journal of Chemical Education
819
