Dichloramine Hydrolysis in Membrane Desalination Permeate: Mechanistic Insights and Implications for Oxidative Capacity in Potable Reuse Applications

Dichloramine (NHCl2) naturally exists in reverse osmosis (RO) permeate due to its application as an antifouling chemical in membrane-based potable reuse treatment. This study investigated mechanisms of background NHCl2 hydrolysis associated with the generation of oxidative radical species in RO permeate, established a kinetic model to predict the oxidative capacity, and examined its removal efficiency on trace organic contaminants in potable reuse. Results showed that NHCl2 hydrolysis generated transient peroxynitrite (ONOO-) and subsequently dissociated into hydroxyl radical (HO•). The maximal HO• exposure was observed at an RO permeate pH of 8.4, higher than that from typical ultraviolet (UV)-based advanced oxidation processes. The HO• exposure during NHCl2 hydrolysis also peaked at a NH2Cl-to-NHCl2 molar ratio of 1:1. The oxidative capacity rapidly degraded 1,4-dioxane, carbamazepine, atenolol, and sulfamethoxazole in RO permeate. Furthermore, background elevated carbonate in fresh RO permeate can convert HO• to carbonate radical (CO3•-). Aeration of the RO permeate removed total carbonate, significantly increased HO• exposure, and enhanced the degradation kinetics of trace organic contaminants. The kinetic model of NHCl2 hydrolysis predicted well the degradation of contaminants in RO permeate. This study provides new mechanistic insights into NHCl2 hydrolysis that contributes to the oxidative degradation of trace organic contaminants in potable reuse systems.

Anti-inflammatory drugs degradation during LED-UV365 photolysis of free chlorine: roles of reactive oxidative species and formation of disinfection by-products

Light-emitting diode (LED) is environmentally friendly with longer life compared with traditionally mercury lamps. This study investigated the non-steroidal anti-inflammatory drugs (NSAIDs)- phenacetin (PNT) and acetaminophen (ACT)- removal during LED-UV (365 nm) photolysis of free available chlorine (FAC). Degradation of PNT and ACT during LED-UV₃₆₅/FAC treatment at pH 5.5-8.5 followed the pseudo-first order kinetics. The presence of hydroxyl radicals (·OH), reactive chlorine species (RCS), and ozone (O₃, transformed from O (³P)) were screened by using scavengers of ethanol (EtOH), tert-Butanol (TBA), and 3-buten-2ol, and 4-hydroxy-2,2,6,6-tetramethylpiperidine (TEMP), and quantified by competition kinetics with probing compounds of nitrobenzene (NB), benzoate acid (BA), 1,4-dimethoxybenzene (DMOB). Higher pH would lead to decrease of ·OH contribution and an increase of FAC contribution to PNT and ACT degradation. It has been determined that the contribution of O₃ to degradation of PNT and ACT was less than 5% for all pHs, and O³(P) reacts toward EtOH with second-order constant of 1.52 × 10⁹ M^-1s^-1. LED-UV₃₆₅/FAC system reduced the formation of five typical CX₃-R type disinfection by-products (DBPs) as well as the cytotoxicity and genotoxicity of water samples at pH 5.5 and 8.5, compared with FAC alone. The decrease of DBPs formation resulted from fast FAC decomposition upon LED-UV₃₆₅ irradiation. A feasible reaction pathway of DBPs formation in the LED-UV₃₆₅/FAC system was examined with density functional theory (DFT). For FAC decay during LED-UV₃₆₅/FAC with effluent from wastewater, the residual FAC in 15 min was 0.8 mg/L (lower than limit of 0.2 mg/L) once initial FAC was 2.0 mg/L. The results indicate that more tests on the balance of target pollutant removal efficiency, residual FAC and cost should be explored in LED-UV₃₆₅/FAC system for application.

Mechanistic and Kinetic Understanding of the UV254 Photolysis of Chlorine and Bromine Species in Water and Formation of Oxyhalides

This study investigated the UV₂₅₄ photolysis of free available chlorine and bromine species in water. The intrinsic quantum yields for ^•OH and X^• (X = Cl or Br) generation were determined by model fitting of formaldehyde formation using a tert-butanol assay to be 0.61/0.45 for HOCl/OCl^- and 0.32/0.43 for HOBr/OBr^-. The steady-state ^•OH concentration in UV/HOX was higher than that in UV/OX^- by a factor of 23.3 and 7.8 for Cl and Br, respectively. This was attributed to the different ^•OH consumption rate by HOCl versus OCl^-, while for HOBr/OBr^-, both the ^•OH formation and consumption rates were implied. This was supported by a k of 1.4 × 10⁸ M^-1 s^-1 for the ^•OH reaction with HOCl, which was >14 times less than the k for ^•OH reactions with OCl^-, HOBr, and OBr^-. Formation of ClO₃^- and BrO₃^- was found to be significant with apparent quantum yields of 0.12-0.23. A detailed mechanistic study on the formation of XO₃^- including a new pathway involving XO^• is presented, which has important implications as the level of XO₃^- can exceed the regulation (BrO₃^-) or guideline (ClO₃^-) values during UV/halogen oxidant water treatment. Our new kinetic models well simulate the experimental results for the halogen oxidant decomposition, probe compound degradation, and formation of ClO₃^- and BrO₃^-.

Kinetic mechanism of ozone activated peroxymonosulfate system for enhanced removal of anti-inflammatory drugs

Peroxymonosulfate (PMS) was employed as an activator of ozone (O₃) to degrade non-steroidal anti-inflammatory drugs (NSAIDs) (aspirin (ASA) and phenacetin (PNT)) in study. The combination of PMS in O₃ system promoted the O₃ decomposition and NSAIDs removal significantly. O₃ molecule, hydroxyl radical (OH) and sulfate radical (SO₄^-) were responsible for the removal of target pollutants in O₃/PMS system. The second-rate constants between O₃, OH and SO₄^- with ASA were determined to be 7.32, 4.18 × 10⁹ and 3.46 × 10⁸ M^-1·s^-1, and 37.3, 4.99 × 10⁹ and 5.64 × 10⁸ M^-1·s^-1 for PNT, respectively. The pattern of pollutant removal and contributions of oxidative species were fitted by experiments and two models. Nevertheless, the wide variety of two models suggested that a comprehensive model for O₃/PMS based on a first-principles approach was not yet possible, due to the number of radicals and subsequent chain reaction, such as SO₅^- or O₃^-. In addition, the formation of five typical CX₃R -type disinfection by products was evaluated from post‑chlorine tests and theoretically calculation by frontier electron density calculation. The calculated toxicity of typical CX₃R -type DBPs was found to decrease with the increase of pH. The results of this study provide a basis for exploring the mechanism of pollutant degradation in O₃ system.

Optimization of Boron Doped TiO2 as an Efficient Visible Light-Driven Photocatalyst for Organic Dye Degradation With High Reusability

No visible light activity is the bottle neck for wide application of TiO₂, and Boron doping is one of the effective way to broaden the adsorption edge of TiO₂. In this study, several Boron doped TiO₂ materials were prepared via a facile co-precipitation and calcination process. The B doping amounts were optimized by the degradation of rhodamine B (Rh B) under visible light irradiation, which indicated that when the mass fraction of boron is 6% (denoted as 6B-TiO₂), the boron doped TiO₂ materials exhibited the highest activity. In order to investigate the enhanced mechanism, the difference between B-doped TiO₂ and bare TiO₂ including visible light harvesting abilities, separation efficiencies of photo-generated electron-hole pairs, photo-induced electrons generation abilities, photo-induced charges transferring speed were studied and compared in details. h⁺ and ^·O2- were determined to be the two main responsible active species in the photocatalytic oxidation process. Besides the high degradation efficiency, 6B-TiO₂ also exhibited high reusability in the photocatalysis, which could be reused at least 5 cycles with almost no active reduction. The results indicate that 6B-TiO₂ has high photocatalytic degradation ability toward organic dye of rhodamine B under visible light irradiation, which is a highly potential photocatalyst to cope with organic pollution.

Comparing the UV/Monochloramine and UV/Free Chlorine Advanced Oxidation Processes (AOPs) to the UV/Hydrogen Peroxide AOP Under Scenarios Relevant to Potable Reuse

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.

Photochemistry in the mixed aqueous solution of nitrobenzene and nitrous acid as initiated by the 355 nm UV light

The 355 nm photon-initiated microscopic reaction mechanisms of the mixed aqueous solution of nitrobenzene and nitrous acid in the presence or absence of O(2) were studied by the laser flash photolysis technique. The main transient absorption peaks in the recorded spectra were assigned and the growth/decay trends of several transient species were investigated. It was found that the OH radical formed from the photolysis of nitrous acid triggered most of the subsequent radical reactions. The rate constant of the reaction between OH radical and nitrobenzene was measured to be (3.4 +/- 0.1) x 10(9) l mol(-1) s(-1). The product from this reaction, namely C(6)H(5)NO(2)-OH adduct, was found to react with O(2) to yield C(6)H(5)NO(2)-OHO(2) adduct with a rate constant of (1.6 +/- 0.2) x 10(9) l mol(-1) s(-1). Final steady-state products were identified by GC/MS analysis and were in accordance with the transient spectroscopic results. The possible reaction pathways were proposed.

Reaction of the Hydroxyl Radical with Phenol in Water Up to Supercritical Conditions

The rate constants for the reactions of phenol with the hydroxyl radical (OH*) in water have been measured from room temperature to 380 degrees C using electron pulse radiolysis and transient absorption spectroscopy. The reaction scheme designed to fit the data shows the importance of an equilibrium, giving back reactants (OH* radical and phenol) from the dihydroxycyclohexadienyl radical formed by their reaction, and the non-negligible contribution of the hydroxycyclohexadienyl radical absorption from H* atom addition. The accuracy of the reaction scheme and the reaction rate constants determined from it have been determined by the analysis of two different experiments, one under pure N2O atmosphere and the second under a mixture a N2O and O2. We report reaction rates for the H* and OH* radical addition to phenol, the formation of phenoxyl, the second-order recombination, the reaction of dihydroxycyclohexadienyl with O2, and the decay of the peroxyl adduct. Nearly all of the reaction rates deviate strongly from Arrhenius behavior.

Radiation-induced hydroxylation of thymine promoted by electron-affinic compounds

The effect of 20 electron-affinic compounds including nitroimidazoles, nitrofurans, nitrobenzenes, and quinones on the radiation-induced reaction of thymine in aqueous solution was studied under deaerated and N2O-saturated conditions. The radiolysis of thymine in aerated aqueous solution was also performed for comparison. Thymine decomposition was depressed to some extent by the addition of electron-affinic compounds in both deaerated and N2O-saturated solutions, while promoted in aerated solution. The radiolyses with varying concentration of misonidazole indicated that the depression of thymine decomposition can be attributed to a competition between thymine and electron-affinic compounds for the reactions with .OH. Among the radiolysis products, the formation of thymine glycol was remarkably promoted by the addition of electron-affinic compounds. Irrespective of structures of the electron-affinic compounds, the G-value of thymine glycol increased in sigmoidal form with increasing one-electron reduction potential of the electron-affinic compounds and attained the ultimate values of ca. 1.1 and 1.8 in deaerated and N2O-saturated solutions, respectively. The results are in accord with one-electron oxidation of the hydroxythymyl radical, produced by the reaction of thymine with .OH, to the corresponding cation by electron-affinic compounds. The so-formed hydroxythymine cation undergoes solvolytic substitution to give thymine glycol. Based on the ultimate G-values of thymine glycol, the difference in reactivity between hydroxythymine-5-yl and 6-yl radicals toward electron-affinic compounds is discussed.