Humic substances (HS) play an important role in the phototransformation of micro-pollutants in surface waters and a significant part of this reactivity is often attributed to oxidant triplet excited states (³HS*). The present study aims to understand why ³HS* were found to be negligibly involved in the HS-mediated phototransformation of β2-adrenoceptor agonist terbutaline (TBL) while this chemical shows a high reactivity towards triplet 3-carboxybenzophenone (³CBP*), a typical HS model sensitizer. To clarify this apparent discrepancy, several experiments were conducted. We first confirmed that TBL can be easily oxidized by triplet riboflavin (³RF*), another sensitizer (k = (1.7 ± 0.4) × 10⁹ M⁻¹ s⁻¹). Afterwards, we studied the effect of TBL on the HS-mediated phototransformation of 2,4,6-trimethylphenol (TMP), chosen for its high reactivity with ³HS*. TBL was found to enhance the rate of TMP loss. This result is rationalized by postulating that (i) TMP and TBL are both oxidized by ³HS* to yield the phenoxyl radicals TMP_-H^• and TBL_-H^•, respectively, and (ii) TBL_-H^• is further reduced by TMP which generates additional TMP_-H^•. Fitting the experimental data gives k = (1.8 ± 0.5) × 10⁹ M⁻¹ s^-1 for the reaction between ³HS* and TBL and DFT calculations further support the conclusion that TBL_-H^• can be reduced by a lot of phenols. Hence, in the case of TBL, the oxidant effect of ³HS* is counterbalanced by the antioxidant properties of HS.

腐殖质（HS）在地表水中微污染物的光转化中起重要作用，这种反应性的很大一部分通常归因于氧化剂三重激发态（³ HS *）。本研究旨在了解为什么³ HS *被忽略地参与了HS介导的β2-肾上腺素能受体激动剂特布他林（TBL）的光转化，而该化学品对三重态3-羧基二苯甲酮（³ CBP *）具有高反应活性。典型的HS型敏化剂。为了澄清这种明显的差异，进行了一些实验。我们首先证实，三联体核黄素（³ RF *）是另一种敏化剂（k  =（1.7±0.4）×10 ⁹ M ^-1  s ^-1）。之后，我们研究了TBL对2，4，6-三甲基苯酚（TMP）介导的HS介导的光转化的影响，TMP与³ HS *具有高反应活性。发现TBL提高了TMP的损失率。通过假设（i）TMP和TBL均被³ HS *氧化以分别生成苯氧基自由基TMP _-H ^•和TBL _-H ^•来合理化该结果，并且（ii）TMP将TBL _-H ^•进一步还原这会生成附加的TMP _-H ^•。拟合实验数据得出k  =（1.8±0.5）×10 ⁹  M ^-1  s对于³ HS *与TBL之间的反应，^-1表示，DFT计算进一步支持以下结论：TBL _-H ^•可以被许多酚还原。因此，在TBL的情况下，³ HS *的氧化作用被HS的抗氧化性能所抵消。