Triclosan (TCS) is an antimicrobial compound found in many household products used across the world. TCS is not completely removed in wastewater systems, resulting in trace-level concentrations present ubiquitously in surface waters. The direct photodegradation of TCS has been widely studied, with results indicating that TCS breaks down to chlorophenols and dioxins. To date, no studies have specifically investigated the effects of alkalinity on the photolysis of the acidic form of TCS. This study assessed the effect of carbonate/bicarbonate alkalinity, which is ubiquitous in natural waters, on the photolysis rate of TCS. Results indicate that bicarbonate enhances the photodegradation of TCS at pH values well below the pK_a of TCS (7.9), with direct photolysis reaction kinetics that are very slow in the absence of buffers, but significant in the presence of bicarbonate (0.711 h⁻¹ at pH 6.55). At pH values well above its pK_a, both unbuffered- and buffered-mediated photolysis increased dramatically (1.92 h⁻¹ for direct photolysis and 2.86 h⁻¹ in buffered water) and is attributable to the increased photoreactivity of TCS by its conjugate base. Photolysis of methyl triclosan (MeTCS), a non-acidic analog of TCS, demonstrated the importance of TCS’s acidic functionality as MeTCS did not degrade at any pH. The observed influence of alkalinity on the acidic form of TCS photolysis was attributed to both a decrease in its excited state pK_a, coupled with TCS deprotonation through an excited state proton transfer to a base (bicarbonate and to a lesser degree hydrogen phosphate) resulting in the more photo-labile conjugate base form of TCS.

三氯生 (TCS) 是一种抗菌化合物，存在于世界各地使用的许多家用产品中。 TCS 在废水系统中并未完全去除，导致地表水中普遍存在痕量浓度。 TCS 的直接光降解已得到广泛研究，结果表明 TCS 分解为氯酚和二恶英。迄今为止，还没有研究专门研究碱度对酸性 TCS 光解作用的影响。本研究评估了天然水中普遍存在的碳酸盐/碳酸氢盐碱度对 TCS 光解速率的影响。结果表明，在 pH 值远低于 TCS 的 p Ka (7.9) 时，碳酸氢盐增强了 TCS 的光降解_，直接光解反应动力学在没有缓冲剂的情况下非常缓慢，但在存在碳酸氢盐的情况下显着 (0.711 h ^{− 1}，pH 6.55）。当 pH 值远高于其 p Ka_时，无缓冲和缓冲介导的光解作用均显着增加（直接光解作用为 1.92 h ^-1，缓冲水中为 2.86 h ^-1），这归因于 TCS 的共轭碱增加的光反应性。甲基三氯生 (MeTCS)（TCS 的非酸性类似物）的光解证明了 TCS 酸性功能的重要性，因为 MeTCS 在任何 pH 值下都不会降解。观察到的碱度对 TCS 光解酸性形式的影响归因于其激发态 pKa 的降低，_以及通过激发态质子转移到碱（碳酸氢盐和较小程度的磷酸氢盐）而导致的 TCS 去质子化。 TCS 的光不稳定共轭碱形式。