Radiolytic Reactions of Monochloramine in Aqueous Solutions
22 Aug 2003-Journal of Physical Chemistry A (American Chemical Society)-Vol. 107, Iss: 38, pp 7423-7428
TL;DR: In this paper, the peroxyl radical exists in equilibrium NHClO 2 ¥ / ¥ NHCl + O2 with an estimated equilibrium constant of (3 ( 2) 10 -3 mol L -1.
Abstract: 30) L mol -1 cm -1 and 580 ) (56 ( 30) L mol -1 cm -1 . The ¥ NHCl radical undergoes self-decay and can react also with O2 to form a peroxyl radical. It is suggested that the peroxyl radical exists in equilibrium NHClO 2 ¥ / ¥ NHCl + O2 with an estimated equilibrium constant of (3 ( 2) 10 -3 mol L -1 . The reaction of chloramine with the carbonate radical is suggested to form a complex [CO3NH2Cl] ¥- with kf ) 2.5 10 5 L mol -1 s -1 and kr ) 4 10 2 s -1 , and this complex decomposes with k ) 7 10 2 s -1 to form ¥ NHCl.
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TL;DR: By combining radical scavengers and kinetic modeling, quantum yields for radical generation by the UV photolysis of HOCl, OCl-, and NH2Cl are derived, far below previous estimates that incorporated subsequent free chlorine or chloramine scavenging by the •Cl and •OH daughter radicals.
Abstract: Utilities incorporating the potable reuse of municipal wastewater are interested in converting from the UV/H2O2 to the UV/free chlorine advanced oxidation process (AOP). The AOP treatment of reverse osmosis (RO) permeate often includes the de facto UV/chloramine AOP because chloramines applied upstream permeate RO membranes. Models are needed that accurately predict oxidant photolysis and subsequent radical reactions. By combining radical scavengers and kinetic modeling, we have derived quantum yields for radical generation by the UV photolysis of HOCl, OCl-, and NH2Cl of 0.62, 0.55, and 0.20, respectively, far below previous estimates that incorporated subsequent free chlorine or chloramine scavenging by the •Cl and •OH daughter radicals. The observed quantum yield for free chlorine loss actually decreased with increasing free chlorine concentration, suggesting scavenging of radicals participating in free chlorine chain decomposition and even free chlorine reformation. Consideration of reactions of •ClO and its daughter products (e.g., ClO2-), not included in previous models, were critical for modeling free chlorine loss. Radical reactions (indirect photolysis) accounted for ∼50% of chloramine decay and ∼80% of free chlorine loss or reformation. The performance of the UV/chloramine AOP was comparable to the UV/H2O2 AOP for degradation of 1,4-dioxane, benzoate and carbamazepine across pH 5.5-8.3. The UV/free chlorine AOP was more efficient at pH 5.5, but only by 30% for 1,4-dioxane. At pH 7.0-8.3, the UV/free chlorine AOP was less efficient. •Cl converts to •OH. The modeled •Cl:•OH ratio was ∼20% for the UV/free chlorine AOP and ∼35% for the UV/chloramine AOP such that •OH was generally more important for contaminant degradation.
281 citations
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TL;DR: A mechanism of photodecay of monochloramine is proposed and nitrate formation was favored at low pH, while nitrite formation wasavored at high pH, and the effects of pH on formation of N2O and NH4+ were less clear.
Abstract: The ultraviolet (UV) photolysis of monochloramine (NH2Cl), dichloramine (NHCl2), and trichloramine (NCl3) in aqueous solution was investigated at wavelengths of 222, 254, and 282 nm. All three chloramines can be degraded by UV irradiation, and the quantum yields for these processes are wavelength-dependent. Stable photoproducts include nitrite, nitrate, nitrous oxide, and ammonium. Solution pH was observed to have little effect on the rate of photodecay; however, the product distribution showed strong pH dependence. Nitrate formation was favored at low pH, while nitrite formation was favored at high pH. The effects of pH on formation of N2O and NH4+ were less clear. On the basis of the results, a mechanism of photodecay of monochloramine is proposed.
180 citations
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TL;DR: In this paper, the fundamental radical chemistry involved in monochloramine (NH2Cl) photolysis and its efficiency in degrading 1,4-dioxane using a low-pressure Hg lamp (λ = 254 nm).
Abstract: Although chloramines are ubiquitously present during ultraviolet-driven advanced oxidation processes (UV/AOP) that are becoming increasingly important for potable water reuse, the photochemistry of chloramines in treated wastewater, and the associated effects on trace chemical contaminant degradation, are unknown. This study investigated the fundamental radical chemistry involved in monochloramine (NH2Cl) photolysis and its efficiency in degrading 1,4-dioxane using a low-pressure Hg lamp (λ = 254 nm). These results showed that the UV fluence-normalized rate of 1,4-dioxane degradation in UV/NH2Cl ranged between 1.1 × 10–4 and 2.9 × 10–4 cm2·mJ–1. The photolysis of NH2Cl produced NH2• and Cl•, which further transformed to a series of reactive radical species. An optimal NH2Cl dosage for 1,4-dioxane degradation was observed at a NH2Cl/1,4-dioxane concentration ratio of 8.0, while excess NH2Cl scavenged reactive radicals and decreased the treatment efficiency. Scavenging experiments and probe compound calcula...
107 citations
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TL;DR: In environmental water matrices, the performance and radical contributions in UV/NH2Cl and UV/H2O2 systems were taken into comparison, which showed faster degradation of OMPs and a more significant contribution of CO3•- in the UV/ NH2Cl process.
Abstract: Monochloramine (NH2Cl) can be irradiated by UV to create an advanced oxidation condition (i.e., UV/NH2Cl) for the elimination of organic micropollutants (OMPs) from source water. However, informati...
101 citations
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TL;DR: Results from this study suggest that the presence of chloramines can be beneficial to persulfate photolysis in the removal of 1,4-D; however, the treatment efficiency depends on a careful control of an optimal NH2Cl dosage and a minimal chloride residue.
Abstract: A sequential combination of membrane treatment and UV-based advanced oxidation processes (UV/AOP) has become the industry standard for potable water reuse. Chloramines are used as membrane antifouling agents and therefore carried over into the UV/AOP. In addition, persulfate (S2O82-) is an emerging oxidant that can be added into a UV/AOP, thus creating radicals generated from both chloramines and persulfate for water treatment. This study investigated the simultaneous photolysis of S2O82- and monochloramine (NH2Cl) on the removal of 1,4-dioxane (1,4-D) for potable-water reuse. The dual oxidant effects of NH2Cl and S2O82- on 1,4-D degradation were examined at various levels of oxidant dosage, chloride, and solution pH. Results showed that a NH2Cl-to-S2O82- molar ratio of 0.1 was optimal, beyond which the scavenging by NH2Cl of HO•, SO4•-, and Cl2•- radicals decreased the 1,4-D degradation rate. At the optimal ratio, the degradation rate of 1,4-D increased linearly with the total oxidant dose up to 6 mM. The combined photolysis of NH2Cl and S2O82- was sensitive to the solution pH due to a disproportionation of NH2Cl at pH lower than 6 into less-photoreactive dichloramine (NHCl2) and radical scavenging by NH4+. The presence of chloride transformed HO• and SO4•- to Cl2•- that is less-reactive with 1,4-D, while the presence of dissolved O2 promoted gaseous nitrogen production. Results from this study suggest that the presence of chloramines can be beneficial to persulfate photolysis in the removal of 1,4-D; however, the treatment efficiency depends on a careful control of an optimal NH2Cl dosage and a minimal chloride residue.
99 citations
References
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TL;DR: In this article, the rate constants for over 3500 reaction are tabulated, including reaction with molecules, ions and other radicals derived from inorganic and organic solutes, and the corresponding radical anions, ⋅O− and eaq−, have been critically pulse radiolysis, flash photolysis and other methods.
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TL;DR: In this article, rate constants have been compiled for reactions of various inorganic radicals produced by radiolysis or photolysis, as well as by other chemical means in aqueous solutions.
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2,958 citations
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TL;DR: In this paper, a fundamental equation of state has been formulated for heavy water in the form Ψ = Ψ(p,T) in which Ω = Helmholtz free energyp = density T = thermodynamic temperature.
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1,161 citations