Critical Review of UV-Advanced Reduction Processes for the Treatment of Chemical Contaminants in Water

Abstract

UV-advanced reduction processes (UV-ARP) are an advanced water treatment technology characterized by the reductive transformation of chemical contaminants. Contaminant abatement in UV-ARP is most often accomplished through reaction with hydrated electrons (e aq − ) produced from UV photolysis of chemical sensitizers (e.g., sulfite). In this Review, we evaluate the photochemical kinetics, substrate scope, and optimization of UV-ARP. We find that quantities typically reported in photochemical studies of natural and engineered systems are under-reported in the UV-ARP literature, especially the formation rates, scavenging capacities, and concentrations of key reactive species like e aq − . The absence of these quantities has made it difficult to fully evaluate the impact of operating conditions and the role of water matrix components on the efficiencies of UV-ARP. The UV-ARP substrate scope is weighted heavily toward contaminant classes that are resistant to degradation by advanced oxidation processes, like oxyanions and per-and polyfluoroalkyl substances. Some studies have sought to optimize the UV-ARP treatment of these contaminants; however, a thorough evaluation of the impact of water matrix components like dissolved organic matter on these optimization strategies is needed. Overall, the data compilation, analysis, and research recommendations provided in this Review will assist the UV-ARP research community in future efforts toward optimizing UV-ARP systems, modeling the e aq − -based chemical transformation kinetics, and developing new UV-ARP systems.

Quantifying Hydrated Electron Transformation Kinetics in UV-Advanced Reduction Processes Using theRe–,UVMethod

“…Note that k S, t ′ is the e aq – scavenging capacity of the water matrix and does not include the e aq – scavenging capacity of MCAA ( k MCAA,t ). All bimolecular rate constants were taken from literature values, with the exception of k DOC,e aq – , and adjusted for aqueous ionic strength impacts using the Brønsted-Bjerrum equation Section and is reported as 1.97 × 10 4 L mg C –1 s –1 .…”

“…Adjusted bimolecular rate constants accounting for ionic strength were used to calculate k S,0 ′ . Nonadjusted bimolecular rate constants (M –1 s –1 ) used are as follows: k SO 3 2– ,e aq – = 1.3 × 10 6 , k MCAA,e aq – = 1.0 × 10 9 , k CO 3 2– ,e aq – = 3.9 × 10 5 , k HCO 3 – ,e aq – = 1.0 × 10 6 , k DOC,e aq – = 1.97 × 10 4 L mg C –1 s –1 , k N O 2 – ,e aq – = 3.5 × 10 9 , and k NO 3 – ,e aq – = 9.7 × 10 9 . Formation of NO 2 – was observed in the Ohio River and LWC RBF source waters due to direct photolysis of NO 3 – (see SI Text S12).…”

“…Sulfite was selected as the e aq – source for these experiments because of its extensive use in prior UV-ARP studies. ,, UV/sulfite experiments were conducted under anaerobic conditions at 2 pH units above the p K a of HSO 3 – (p K a = 7.2) to minimize the e aq – scavenging impacts of HSO 3 – . ,, The initial pH of the groundwater and surface water experiments was adjusted to 9.4–9.7 using a 1.0 M sodium hydroxide solution. All ultrapure water experiments utilized a 1.0 mM borate buffer (pH 9.9) with the exception of one control experiment that used a 10.0 mM NaHCO 3 – buffer (pH 9.8).…”

Perfluorooctanoic acid Degradation by UV/Chlorine

“…Many approaches to degrade hazardous contaminants rely on advanced oxidation processes (AOPs), 14 such as photocatalysis, 15,16 sonolysis, 17,18 Fenton-based processes, 19,20 as well as advanced reduction processes. 21,22 Ultraviolet (UV) light is often used for AOPs, so UV-activated treatments could be integrated into water or wastewater treatment facilities that already have UV disinfection in place. One of the simplest AOPs is UV/H 2 O 2 , which generates hydroxyl radicals (HO • ) in solution to degrade many organic pollutants.…”

“…Various PFAS treatment methods have been considered over the past few years, often focusing on perfluorooctanoic acid (PFOA), one of the most abundant and widely studied PFAS. Many approaches to degrade hazardous contaminants rely on advanced oxidation processes (AOPs), such as photocatalysis, , sonolysis, , Fenton-based processes, , as well as advanced reduction processes. , Ultraviolet (UV) light is often used for AOPs, so UV-activated treatments could be integrated into water or wastewater treatment facilities that already have UV disinfection in place. One of the simplest AOPs is UV/H 2 O 2 , which generates hydroxyl radicals (HO • ) in solution to degrade many organic pollutants. − However, HO • is ineffective at initiating PFOA degradation …”