Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20740, USA.
Environ Sci Technol. 2012 Nov 6;46(21):11836-43. doi: 10.1021/es3029582. Epub 2012 Oct 23.
The mechanism(s) by which hydrogen peroxide (H(2)O(2)) is photoproduced by humic substances and chromophoric dissolved organic matter was probed by examining the dependence of the initial H(2)O(2) photoproduction rate (R(H(2)O(2))) and apparent H(2)O(2) quantum yields on dioxygen concentration for both untreated and borohydride-reduced samples. Although borohydride reduction substantially reduced light absorption, the R(H(2)O(2)) values were largely unaffected. Apparent monochromatic and polychromatic quantum yields thus increased following reduction. The results indicate that light absorption by charge-transfer states or by (aromatic) ketone/aldehydes does not lead to significant H(2)O(2) photoproduction. High concentrations of triplet quenchers relative to that of dioxygen produced only small decreases (sorbic acid) or small increases (Cl(-) and Br(-)) in R(H(2)O(2)), indicating that neither (1)O(2) nor excited triplet states of quinones contribute significantly to H(2)O(2) photoproduction. The dependence of R(H(2)O(2)) on O(2) concentration provides evidence that the intermediate(s) reacting with O(2) to produce superoxide are relatively long-lived (approximately tens of microseconds or more). Evidence of the photochemical formation of O(2)-reducing intermediates under anaerobic conditions was also obtained; these reducing intermediates appeared to be relatively stable in the absence of O(2). Our data suggest that these O(2)-reducing intermediates are generated by intramolecular electron transfer from short-lived excited states of electron donors to ground-state acceptors.
通过考察未经处理和硼氢化还原样品的初始 H2O2 光生产速率（R（H2O2））和表观 H2O2 量子产率对溶解氧浓度的依赖性，探究了腐殖质和发色溶解有机物将过氧化氢（H2O2）光生成的机制。尽管硼氢化还原大大降低了光吸收，但 R（H2O2）值基本不受影响。因此，还原后表观单色和多色量子产率增加。结果表明，电荷转移态或（芳族）酮/醛的光吸收不会导致显著的 H2O2 光生成。与溶解氧相比，三重态猝灭剂的浓度较高只会导致 R（H2O2）的微小降低（山梨酸）或微小增加（Cl（-）和 Br（-）），这表明既不是 1O2 也不是醌的激发三重态对 H2O2 光生成有显著贡献。R（H2O2）对 O2 浓度的依赖性为与 O2 反应生成超氧化物的中间产物提供了证据，这些中间产物具有相对较长的寿命（约几十微秒或更长）。还获得了在厌氧条件下光化学形成 O2 还原中间产物的证据；这些还原中间产物在没有 O2 的情况下似乎相对稳定。我们的数据表明，这些 O2 还原中间产物是通过电子供体的短寿命激发态到基态受体的分子内电子转移生成的。
