Ababneh Fuad A, Scott Susannah L, Al-Reasi Hassan A, Lean David R S
Department of Chemistry, P.O. Box 450 Station A, University of Ottawa, Ottawa, Canada ON K1N 6N5.
Sci Total Environ. 2006 Aug 31;367(2-3):831-9. doi: 10.1016/j.scitotenv.2006.02.018. Epub 2006 Mar 6.
In this study, ferric oxalate is used to represent the photosensitive Fe(III) complexes as well as the diacid compounds which are at significant concentrations in cloud and rain droplets. Because of the common carboxylate functional group; ferric oxalate is also used as a model to represent humic substances found in natural water. UVA irradiation of aqueous acidic mercuric chloride (pH 1-4) in the presence of an excess of ferrioxalate results in partial reduction of the mercuric ion to elemental mercury. The pseudo-first-order rate constant "kobs" for the photoreduction reaction is pH-dependent as is the yield of residual Hg(II). When exposed to visible irradiation the rate is about 10 times slower and no reaction was observed in the dark. The inferred mechanism of photoreduction involves the reaction of Hg(II) with a secondary photoproduct, the strongly reducing radical anion CO2-. In the presence of dissolved oxygen, competition for CO2- between Hg(II) and O2 reduces the rate and efficiency of mercuric ion reduction. The O2-*/HO2 products do not reduce Hg(II). On the contrary, their disproportionation leads to the formation of H2O2 which causes a re-oxidation of Hg(0) at pH values of <or=4. Chloride ion decreases the rate of the reduction of Hg(II).
在本研究中，草酸铁用于代表光敏性铁（III）配合物以及在云雾和雨滴中浓度较高的二酸化合物。由于存在常见的羧基官能团，草酸铁还被用作代表天然水中腐殖质的模型。在过量草酸铁存在下，对酸性氯化汞水溶液（pH 1 - 4）进行紫外光A照射会导致汞离子部分还原为元素汞。光还原反应的准一级速率常数“kobs”与残余汞（II）的产率一样，都与pH值有关。当受到可见光照射时，反应速率约慢10倍，且在黑暗中未观察到反应。推测的光还原机制涉及汞（II）与二级光产物——强还原性自由基阴离子CO2-*的反应。在溶解氧存在的情况下，汞（II）和O2之间对CO2-的竞争降低了汞离子还原的速率和效率。O2-/HO2产物不会还原汞（II）。相反，它们的歧化反应会导致形成过氧化氢，而过氧化氢会在pH值≤4时使汞（0）重新氧化。氯离子会降低汞（II）的还原速率。
