Radical Chemistry and Structural Relationships of PPCP Degradation by UV/Chlorine Treatment in Simulated Drinking Water

Abstract

The UV/chlorine process is an emerging advanced oxidation process (AOP) used for the degradation of micropollutants. However, the radical chemistry of this AOP is largely unknown for the degradation of numerous structurally diverse micropollutants in water matrices of varying quality. These issues were addressed by grouping 34 pharmaceuticals and personal care products (PPCPs) according to the radical chemistry of their degradation in the UV/chlorine process at practical PPCP concentrations (1 μg L) and in different water matrices. The contributions of HO and reactive chlorine species (RCS), including Cl, Cl, and ClO, to the degradation of different PPCPs were compound specific. RCS showed considerable reactivity with olefins and benzene derivatives, such as phenols, anilines, and alkyl-/alkoxybenzenes. A good linear relationship was found between the RCS reactivity and negative values of the Hammett ∑σ constant for aromatic PPCPs, indicating that electron-donating groups promote the attack of benzene derivatives by RCS. The contribution of HO, but not necessarily RCS, to PPCP removal decreased with increasing pH. ClO showed high reactivity with some PPCPs, such as carbamazepine, caffeine, and gemfibrozil, with second-order rate constants of 9.2 × 10, 1.03 × 10, and 4.16 × 10 M s, respectively, which contributed to their degradation. Natural organic matter (NOM) induced significant scavenging of ClO and greatly decreased the degradation of PPCPs that was attributable to ClO, with a second-order rate constant of 4.5 × 10 (mg L) s. Alkalinity inhibited the degradation of PPCPs that was primarily attacked by HO and Cl but had negligible effects on the degradation of PPCPs by ClO. This is the first study on the reactivity of RCS, particularly ClO, with structurally diverse PPCPs under simulated drinking water condition.

State of the Art of UV/Chlorine Advanced Oxidation Processes: Their Mechanism, Byproducts Formation, Process Variation, and Applications

“…Boxes depict primary radicals produced by photolysis of chlorine. [24] 6.0×10 8 [24] 1.0×10 9 [32] 1.7×10 9 [32] 1.5-3.2×10 9 [30] 6.2×10 8 [32] ---1.3×10 7 [31] 3.5×10 3 [25] 4.5×10 4 [25] 1.2-1.9×10 5 [25] 0 [26] Aldehyde formaldehyde [24] 5.9×10 9 [24] 5.0×10 9 [24] 7.0×10 9 [24] 7.0×10 9 [24] 6-12×10 9 [28] 0 that RCS show relatively high reactivity with olefins and benzene derivatives and electron-donating groups promote the attack of benzene derivatives [23]. HO˙ is known as a non-selective electrophilic reagent.…”

“…Although the standard electrode potential of ClO˙ is lower than others, the chlorine derivative radicals of Cl˙, ClO˙, ClOH˙−, Cl 2˙− , etc. are called "reactive chlorine species (RCS)" that are also responsible for advanced oxidation by UV/chlorine processes [20][21][22][23]. However, the reactivity of each radical species is thought to be different from others.…”

“…Some researchers applied this method for UV/chlorine AOPs to estimate the contribution of reactive species [20,23,31,37,40]. However, the addition of chemical probes affects both the degradation rate of pollutant and the steadystate concentration of reactive species through competitive consumption of reactive species.…”

“…In the case of trimethoprim, ClO˙ plays the most important role in the degradation [21], whose formation is enhanced under basic condition through reactions R 1 , R 2 , and R 3 in Table 1. In the case of phenolic compounds of salbutamol, ractopamine, and bisphenol A, these chemicals are transformed into the corresponding phenoxides with increasing pH, which are richer in electrons and more reactive with RCS [23]. Paracetamol degradation is also enhanced at pH higher than 7.0 due to higher reactivity of ClO − with paracetamol than that of HOCl [81].…”

“…Pan et al also pointed out that CO 3˙− produced contributed to the naproxen degradation to compensate for the loss of HO˙ [40]. Guo et al showed that steady-state concentrations of HO˙, Cl˙, and Cl 2˙− decreased in the presence of 1 mM bicarbonate, whereas CO 3˙− concentration increased and reached 3-6 orders of magnitude higher than those of HO˙ and Cl˙ [23]. These reports suggest the importance of CO 3˙− reactivity on understanding alkalinity effect.…”