Terephthalate as a probe for photochemically generated hydroxyl radical.

Hydroxyl radicals (OH) are known to play a key role in natural water systems by influencing pollutant degradation and nutrient cycling. Researchers have developed various chemical probes to detect and quantify OH in these environments. However, many of these probes face limitations in terms of sensitivity or selectivity for OH. Terephthalate (TPA) is one such compound that reacts with OH to form a fluorescent product, hydroxyterephthalate (hTPA). Despite its known reactivity, TPA's suitability for studying photochemically generated OH has not been fully evaluated. This gap motivated further investigation into TPA's behavior under photochemical conditions and its potential as a reliable OH probe.

The aim of this study was to assess the suitability of terephthalate (TPA) as a probe for detecting photochemically generated hydroxyl radicals (OH) in natural waters. The researchers sought to determine TPA's reactivity with OH and its selectivity against other reactive species like singlet oxygen ((1)O(2)). Additionally, they aimed to evaluate the photostability of TPA and its reaction product, hTPA, under ultraviolet (UV) irradiation. This work was driven by the need for a more stable and sensitive OH probe that could function under shorter photo-exposure times than existing alternatives. The study also aimed to compare TPA's performance with similar probe molecules, such as benzoate.

The researchers conducted a series of experiments to characterize the photochemical behavior of terephthalate (TPA) and hydroxyterephthalate (hTPA). They measured the rate constants of TPA and hTPA reacting with hydroxyl radicals (OH) and singlet oxygen ((1)O(2)). These measurements were performed using spectrophotometric and fluorescent techniques. Additionally, the team tested whether TPA undergoes direct photolysis under UV light. They also evaluated the direct photochemical degradation of hTPA at 365 nm. TPA was applied to monitor OH production from nitrate, nitrite, and dissolved organic matter (DOM). The study compared TPA's performance with other probe molecules and analyzed the photostability of hTPA under UV exposure.

Terephthalate (TPA) and hydroxyterephthalate (hTPA) reacted with hydroxyl radicals (OH) at rate constants of (4.4 ± 0.1) × 10⁹ M⁻¹ s⁻¹ and (6.3 ± 0.1) × 10⁹ M⁻¹ s⁻¹, respectively. Their reactions with singlet oxygen ((1)O(2)) were significantly slower, with rate constants of <<10⁴ M⁻¹ s⁻¹ and (5.0 ± 0.1) × 10⁴ M⁻¹ s⁻¹. TPA did not undergo direct photolysis, but hTPA degraded under UV irradiation with a direct photolysis quantum yield of (6.3 ± 0.1) × 10⁻³ at 365 nm. TPA was effective in detecting OH produced by nitrate, nitrite, and dissolved organic matter (DOM), with quenching rate constants for DOM matching prior studies. TPA outperformed similar probes like benzoate in terms of stability and sensitivity under shorter photo-exposure times. However, hTPA's photoinstability limited TPA's use under UV wavelengths shorter than 360 nm.

The study found that terephthalate (TPA) is a selective and sensitive probe for hydroxyl radicals (OH) in photochemical systems. Its high reactivity with OH and low reactivity with singlet oxygen ((1)O(2)) suggest that TPA can distinguish OH from other reactive species. TPA's stability and performance under shorter photo-exposure times make it a better probe than benzoate. However, the photodegradation of hTPA under UV light limits TPA's utility in systems exposed to wavelengths shorter than 360 nm. The researchers propose that TPA is suitable for monitoring OH produced by nitrate, nitrite, and dissolved organic matter (DOM). The findings suggest that TPA could be a valuable tool in environmental and photochemical studies where OH detection is needed.