Factors affecting the roles of reactive species in the degradation of micropollutants by the UV/chlorine process

Photoactivation of aqueous chlorine could promote degradation of chlorine-resistant and photochemically stable chemicals accumulated in swimming pools. This study investigated the degradation of two such chemicals, N,N-diethyl-3-methylbenzamide (DEET) and caffeine, by low pressure ultraviolet (UV) light and simulated sunlight (SS) activated free chlorine (FC) in different water matrices. Both DEET and caffeine were rapidly degraded by UV/FC and SS/FC but exhibited different kinetic behaviors. The degradation of DEET followed pseudo-first-order kinetics, whereas the degradation of caffeine accelerated with reaction. Mechanistic study revealed that, under UV/FC, ·OH and Cl· were responsible for degradation of DEET, whereas ClO· related reactive species (ClOrrs), generated by the reaction between FC and ·OH/Cl·, played a major role in addition to ·OH and Cl· in degrading caffeine. Reaction rate constants of DEET and caffeine with the respective radical species were estimated. The imidazole moiety of caffeine...

Degradation of DEET and Caffeine under UV/Chlorine and Simulated Sunlight/Chlorine Conditions.

The photolysis of hypochlorous acid (HOCl) and hypochlorite (OCl−) produces a suite of reactive oxidants, including hydroxyl radical (˙OH), chlorine radical (Cl˙), and ozone (O3). Therefore, the addition of light to chlorine disinfection units could effectively convert existing drinking water treatment systems into advanced oxidation processes. This review critically examines existing studies on chlorine photolysis as a water treatment process. After describing the fundamental chemistry of chlorine photolysis, we evaluate the ability of chlorine photolysis to transform model probe compounds, target organic contaminants, and chlorine-resistant microorganisms. The efficacy of chlorine photolysis to produce reactive oxidants is dependent on solution and irradiation conditions (e.g., pH and irradiation wavelengths). For example, lower pH values result in higher steady-state concentrations of ˙OH and Cl˙, resulting in enhanced contaminant removal. We also present the current state of knowledge on the alteration of dissolved organic matter and subsequent formation of disinfection by-products (DBPs) during chlorine photolysis. Although the relative yields of DBPs during chlorine photolysis are also dependent on solution conditions (e.g., higher organic DBP yields at low pH values), there is conflicting evidence on whether chlorine photolysis increases or decreases DBP production compared to thermal reactions between chlorine and dissolved organic matter in the dark. We conclude the review by identifying knowledge gaps in the current body of literature.

Emerging investigators series: the efficacy of chlorine photolysis as an advanced oxidation process for drinking water treatment

The UV/chlorine process is an emerging advanced oxidation process (AOP) that produces various reactive species, such as hydroxyl radicals (HO) and reactive chlorine species (RCS). The effects of the treatment conditions, such as chlorine dosage and pH, and the water matrix components of natural organic matter (NOM), alkalinity, ammonia and halides, on the kinetics and reactive species in the degradation of four micropollutants, metronidazole (MDZ), nalidixic acid (NDA), diethyltoluamide (DEET) and caffeine (CAF), by the UV/chlorine process were investigated. The degradation of MDZ and CAF was primarily attributable to HO and ClO, respectively, while that of NDA was primarily attributable to both ClO and CO3-. HO, Cl and CO3- are important for the degradation of DEET. The second-order rate constants for ClO with CAF and CO3- with NDA were determined to be 5.1 (±0.2) × 107 M-1s-1 and 1.4 (±0.1) × 107 M-1s-1, respectively. Increasing chlorine dosage slightly changed the contribution of HO but linearly increased that of ClO to micropollutant degradation. Increasing pH decreased the contribution of either HO or Cl but not that of ClO. Both NOM and bicarbonate decreased the contributions of HO and Cl, whereas NOM but not bicarbonate significantly decreased that of ClO. The contribution of either HO or Cl first rose and then fell as the molar ratio of ammonia to chlorine increased from 0 to 1:1, while that of ClO decreased. The co-presence of high concentrations of Cl- and Br- enhanced the contribution of ClBr- and BrCl.

The degradation of micropollutants by the UV/chlorine process

The kinetics of the disproportionation reactions of OH and OD radicals, OH + OH → O + H2O (1), OD + OD → O + D2O (3), have been studied by the mass spectrometric discharge-flow method at temperatures between 233 and 360 K and at total pressure of 1 Torr of helium. The following Arrhenius expressions were obtained:  k1 = (7.1 ± 1.0) × 10-13exp[(210 ± 40)/T] and k3 = (2.5 ± 0.5) × 10-13exp[(170 ± 60)/T] cm3 molecule-1 s-1. Reaction 4 between OH and OD radicals was also investigated and the same rate constant as for reaction 1 was measured. Both the observed temperature dependence of k1 and the measured kinetic isotopic effect (k1/k3) are compatible with the mechanism proposed in a previous theoretical study (Harding, L. B.; Wagner, A. F. 22nd International Symposium on Combustion, 1988). In addition, a temperature-independent rate coefficient of (1.20 ± 0.25) × 10-10 cm3 molecule-1 s-1 was measured for the reaction D + NO2 → OD + NO in the temperature range 230−365 K.

Kinetic Study of OH + OH and OD + OD Reactions

Gas phase UV/VIS-absorption spectra are reported for HOBr and CH 3 OBr in the wavelength range (230<λ<400) nm. The quantum yield for Br( 2 P 3/2 ) atom generation was determined to be ≥0.95 in the laser photolysis of HOBr at λ=363 nm.

UV/VIS-absorption Spectra of HOBr and CH3OBr; Br(2P3/2) Atom Yields in the Photolysis of HOBr

Nascent products formed in the laser photolysis of the two hypochlorites CH3OCl and HOCl at wavelenths close to the maxima in their absorption spectra have been determined by resonantly enhanced multiphoton ionisation (REMPI) using a time-of-flight (TOF) mass analyser. Cl-atoms in the states 2P32 and 2P12 were found to be the predominant photodissociation products for both hypochlorites. In the case of HOCl, the nascent OH was also recorded with rotational resolution. Formation of other possible photofragments from CH3OCl i.e. HCl, CH3 and ClO was clearly excluded to < 10−2 and < 10−3, respectively.

The absolute quantum yields for Cl-atom production in the photolysis at the atmospherically relevant wavelength of λ = 308 nm were found to be φ = 0. 95 ± 0.05 and φ = 1.00 ± 0.05 for CH3OCl and HOCl, respectively. Relative spin-orbit state population of the nascent chlorine (2Pj) atoms varied strongly with photolysis wavelength.

The kinetic energy released to the Cl-atoms in the photolysis at two wavelengths was determined for CH3OCl and HOCl for the first time. In the photolysis at λ = 308 nm, Cl-atoms are formed with Etrans of about 1 eV. The atmospheric implications of this finding are briefly outlined.

Identification of nascent products formed in the laser photolysis of CH3OCl and HOCl at 308 nm and around 235 nm. Total Cl-atom quantum yields and the state and velocity distributions of Cl(2Pj)

The reactions of HOBr with atomic chlorine (1), with oxygen (2) and with hydroxyl radicals (3) have been investigated at 300 K using the discharge flow technique with mass-spectrometric detection. The rate constants based on HOBr consumption have been determined to be k 1 = (8.0±0.4).10 -11 and k 2 = (3.1±0.2).10 -11 . For the reaction with OH radicals k 3 < 5.10 -13 was obtained. All rate constants are given in units cm 3 molecule -1 s -1 . The reactions of HOBr with Cl and with O are shown to proceed via bromine abstraction with formation of BrCI and BrO, respectively. The predominant channel in the reaction of OH with BrCl was identified to be step (4a) OH+BrCl → HOBr+Cl (4a) Based on the present kinetic investigation the heat of formation for HOBr at T= 300 K was evaluated to be ΔH f 0 (HOBr) = - (60.22 ± 2) kJ.mol -1 .

U. Kirchner

Papers

UV/VIS-absorption Spectra of HOBr and CH3OBr; Br(2P3/2) Atom Yields in the Photolysis of HOBr

Identification of nascent products formed in the laser photolysis of CH3OCl and HOCl at 308 nm and around 235 nm. Total Cl-atom quantum yields and the state and velocity distributions of Cl(2Pj)

A Gaskinetic Investigation of HOBr Reactions with CI(2P), O(3P) and OH(2II). The Reaction of BrCl with OH(2II)

Gaskinetic Investigation of HOBr Reactions with Cl(2P), O(3P), and OH( 2π). The Reaction of BrCl with OH(2π).