Degradation Investigation of Selected Taste and Odor Compounds by a UV/Chlorine Advanced Oxidation Process

Abstract

Taste- and odor-causing (T&O) compounds are a major concern in drinking water treatment plants due to their negative impacts on the safety and palatability of water supply. This study explored the degradation kinetics and radical chemistry of four often-detected T&O compounds, geosmin (GSM), 2-methylisoborneol (MIB), benzothiazole (BT), and 2-isobutyl-3-methoxypyrazine (IBMP), in the ultraviolet/chlorine (UV/chlorine) advanced oxidation process. All experiments were carried out in a 700 mL photoreactor and the process effectively degraded the investigated T&O compounds in a slightly acidic environment. The degradation of T&O decreased with increasing pH but slightly with decreasing chlorine dosage. When the pH increased from 6 to 8, the pseudo-first-order rate constants of GSM, MIB, BT, and IBMP dropped from 2.84 × 10−3, 2.29 × 10−3, 3.64 × 10−3, and 2.76 × 10−3 s−1 to 3.77 × 10−4, 2.64 × 10−4, 6.48 × 10−4, and 6.40 × 10−4 s−1, respectively. Increasing the chlorine dosage slightly accelerated the degradation of the investigated T&O compounds, but excessive hypochlorous acid and hypochlorite scavenged the HO• radicals and reactive chlorine species (RCS). Generally, HO• primarily contributed to the degradation of all of the investigated T&O compounds as compared to RCS. The degradation by RCS was found to be structurally selective. RCS could not degrade GSM, but contributed to the degradation of MIB, BT, and IBMP. The results confirmed that the proposed oxidation process effectively degraded typical T&O compounds in aqueous phase.

Removal of Algae, and Taste and Odor Compounds by a Combination of Plant-Mineral Composite (PMC) Coagulant with UV-AOPs: Laboratory and Pilot Scale Studies

“…For the case of the UV/Cl 2 process, it is known that there are formation of reactive chlorine species (i.e., Cl•, Cl 2 − •, and ClO•), but it was reported that OH• is a primarily radical species to the degradation of taste-and-odor compounds as compared to reactive chlorine species which are formed in the UV/Cl 2 process [36]. Treatment in the presence of algae species using oxidation processes has drawbacks such as undesirable toxins and taste and odor compounds [37,38]; therefore, the AOPs were applied after the coagulation process with the optimum PMC dose, i.e., 20 mg/L.…”

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

“…However, some researches show that contributions of both HO˙ and RCS increase with a decrease in pH [31,37,40,79]. This discrepancy can be explained by the radical scavenger effect of ClO − [5,16,40,77,79]. Since reaction rate constants of ClO − with HO˙, Cl˙, and Cl 2˙− are higher than those of HOCl (Table 1), the radical scavenging effect of free chlorine is stronger in basic pH than in acidic pH.…”

“…Thus, it is expected that the relative contribution of HO˙ decreases at acidic pH, if pollutants are reactive with RCS. However, some researches show that contributions of both HO˙ and RCS increase with a decrease in pH [31,37,40,79]. This discrepancy can be explained by the radical scavenger effect of ClO − [5,16,40,77,79].…”

“…Many researchers have observed that acidic pH is better for UV/chlorine AOPs on removing organic pollutants such as amitriptyline hydrochloride [75], benzalkonium chloride [53], caffeine [31], chloramphenicol [22], diethyltoluamide [31], 1,4-dioxane [5], iodoform [76], methyl salicylate [75], metronidazole [31], nalidixic acid [31], naproxen [40], neonicotinoid insecticides [74], nitrobenzene [77], 2-phenoxyethanol [75], ronidazole [78], and taste and odor compounds [79]. Three mechanisms, namely a change in quantum yield of chlorine photolysis, equilibrium shifts of radical species, and radical scavenger effects of HOCl and ClO − , are mainly responsible for the pH dependence of the performance of UV/chlorine AOPs.…”

Insight into the Degradation of Two Benzophenone-Type UV Filters by the UV/H2O2 Advanced Oxidation Process

“…The potential risks resulting from the occurrence of BP and HBP must not be overlooked.Advanced oxidation processes (AOPs) are a series of processes that can generate hydroxyl radicals to efficiently degrade organic pollutants in contaminated water [4,14,15]. The UV/H 2 O 2 system is a type of AOP, in which hydrogen peroxide (H 2 O 2 ) is added in the presence of UV light to generate hydroxyl radicals [16][17][18]. It is found that the UV/H 2 O 2 process can remove a large range of organic pollutants, including cyanobacterial toxins [19], diclofenac [20], amoxicillin [21], and other emerging contaminants [22][23][24].Benzophenone-type UV filters are usually the hardly biodegradable substances [25].…”