Roles of Bromine Radicals and Hydroxyl Radicals in the Degradation of Micropollutants by the UV/Bromine Process

Abstract

The inevitable occurrence of Br– in natural water affects the degradation kinetics of micropollutants in the UV/chlorine process, the radical chemistry of which, however, is largely unclear. As Br– in the UV/chlorine process first forms free bromine (HOBr/OBr–), this study investigated the radical chemistry of the UV/bromine process for the degradation of selected micropollutants resistant to bromine, i.e., ibuprofen and benzoate, to focus on the roles of radicals. The actual quantum yields of HOBr and OBr– by UV photolysis at 254 nm are 0.43 (±0.025) and 0.26 (±0.025) mol Einstein–1, respectively. Br• and HO• are generated first, and then, Br2 •– is formed, with the signal detectable at 360 nm by laser flash photolysis. Compared with Cl• in the UV/chlorine system, Br• exists at higher concentrations (∼10–12 M) in the UV/bromine system while HO• exists at similar concentrations. In the UV/bromine process, reactive bromine species (RBS) dominates the degradation of ibuprofen, while HO• dominates the degradation of benzoate. Br• and Br2 •– are reactive toward ibuprofen which second-order rate constants (k) were determined to be 2.2 × 109 and 5.3 × 107 M–1 s–1, respectively, by laser flash photolysis. Br• was the major RBS for ibuprofen degradation by the UV/bromine treatment, whereas Br2 •– increasingly contributed to ibuprofen degradation with increasing free bromine or Br– concentrations. Br• could be scavenged by HCO3 – and natural organic matter (NOM), and the k with NOM was determined to be 2.6 × 104 (mg/L)−1 s–1. Both Br• and Br2 •– prefer to react with ibuprofen via electron transfer with activation energy barriers (Δ‡ G 0 SET) of 1.35 and 7.78 kcal mol–1, respectively. RBS promoted the formation of hydroxylated products. Then free bromine, rather than RBS, was responsible for the formation of brominated products, increasing the total organic bromine (TOBr) and tribromomethane yields in the UV/bromine system. This study demonstrates for the first time the roles of RBS and HO• in micropollutant degradation in the UV/bromine process.

Reactivities of hydrated electrons with organic compounds in aqueous-phase advanced reduction processes

“…Assuming an elementary reaction proceeds by the same reaction mechanism, the log of the rate constant and the log of the equilibrium constant are linearly related. 31 The natural log of the equilibrium constants has a linear relationship with the free energy reaction, ΔG react aq , which relates to the standard state reduction potential in eqn (5). Combining these two concepts enables the development of the LFER.…”

“…Free radical-based technologies are attractive and promising processes for destroying a wide variety of organic contaminants. Advanced oxidation processes (AOPs) that generate highly reactive oxygenated radical species (e.g., hydroxyl radicals) 1,2 and other reactive radicals (e.g., chlorine-, 3,4 bromine- 5 and nitrogen-derived radicals 6,7 and carbonate radicals 8 ) at ambient temperature and atmospheric pressure have been proven to degrade reduced forms of organic contaminants in water at full-scale treatment plants. Advanced reduction processes (ARPs) that generate reactive radicals (e.g., superoxide anion radicals) and electrons in homogeneous solution 9,10 and heterogeneous electrochemical [11][12][13] or catalytic 14 processes are effective in degrading the oxidized forms of organic and inorganic contaminants.…”

“…, hydroxyl radicals) 1,2 and other reactive radicals ( e.g. , chlorine-, 3,4 bromine- 5 and nitrogen-derived radicals 6,7 and carbonate radicals 8 ) at ambient temperature and atmospheric pressure have been proven to degrade reduced forms of organic contaminants in water at full-scale treatment plants. Advanced reduction processes (ARPs) that generate reactive radicals ( e.g.…”

Assessment of the UV/Chlorine Process in the Disinfection of Pseudomonas aeruginosa: Efficiency and Mechanism

“…17,18 However, HO • is nonselective and results in rapid consumption by cellular components before contacting intracellular DNA (iDNA). 18,19 In contrast, RCS (Cl • , Cl 2 •− , and ClO • ) selectively react with compounds containing electron-rich groups, 20 such as amino acids, pyrimidines, and purines. In addition, the concentrations of RCS, especially ClO • , are higher than that of HO • in the UV/chlorine process.…”

“…HO • has been demonstrated to play an important role in the inactivation of tetracycline-resistant Escherichia coli and in the removal of antibiotic resistance genes (ARGs) ( e.g. , sul 1 and blt ) by UV/chlorine treatment. − Additionally, HO • promotes the elimination of extracellular deoxyribonucleic acids (eDNA) in the UV/H 2 O 2 process and in the natural organic matter photosensitized system. , However, HO • is nonselective and results in rapid consumption by cellular components before contacting intracellular DNA (iDNA). , In contrast, RCS (Cl • , Cl 2 •– , and ClO • ) selectively react with compounds containing electron-rich groups, such as amino acids, pyrimidines, and purines. In addition, the concentrations of RCS, especially ClO • , are higher than that of HO • in the UV/chlorine process .…”