Incorporation of initiation, promotion and inhibition in the Rct concept and its application in determining the initiation and inhibition capacities of natural water in ozonation

Ozonation is widely employed in water treatment to purify water. The R_ct concept, which is defined as the ratio of OH exposure to ozone exposure, has been commonly used to quantify the OH concentration generating from ozone decomposition and model the removal of organic pollutant. Due to its empirical nature, however, the R_ct concept is limited in quantitatively elucidating how initiator, promoter and inhibitor involved in the OH chain reactions affect its value. A new R_ct model was developed by integrating the R_ct concept and the transient steady-state OH concentration to evaluate the influences of these modes of reactions on the R_ct value. It was found that the R_ct value is not only the ratio of OH exposure to ozone exposure but also the ratio of the total initiation capacity to the total inhibition capacity in a system. The presence of promoter, however, does not affect the R_ct value although it does accelerate ozone decomposition leading to lower ozone and OH exposures and result in hindered removal of target pollutant. The hindered removal of ibuprofen by ozonation in the presence of methanol (promoter) can be quantitatively described by the new R_ct model. The model can also be used to quantify the initiation and inhibition capacities of an ozonation system via the addition of an external inhibitor. Its application in determining the initiation and inhibition capacities of natural water was demonstrated.     

Gamma irradiation of pharmaceutical compounds, nitroimidazoles, as a new alternative for water treatment

The main objectives of this study were: (1) to investigate the decomposition and mineralization of nitroimidazoles (Metronidazole [MNZ], Dimetridazole [DMZ], and Tinidazole [TNZ]) in waste and drinking water using gamma irradiation; (2) to study the decomposition kinetics of these nitroimidazoles; and (3) to evaluate the efficacy of nitroimidazole removal using radical promoters and scavengers. The results obtained showed that nitroimidazole concentrations decreased with increasing absorbed dose. No differences in irradiation kinetic constant were detected for any nitroimidazole studied (0.0014-0.0017 Gy⁻¹). The decomposition yield was higher under acidic conditions than in neutral and alkaline media. Results obtained showed that, at appropriate concentrations, H₂O₂ accelerates MNZ degradation by generating additional HO; however, when the dosage of H₂O₂ exceeds the optimal concentration, the efficacy of MNZ degradation is reduced. The presence of t-BuOH (HO radical scavenger) and thiourea (HO, H and e_aq⁻ scavenger) reduced the MNZ irradiation rate, indicating that degradation of this pollutant can take place via two pathways: oxidation by HO radicals and reduction by e_aq⁻ and H. MNZ removal rate was slightly lower in subterranean and surface waters than in ultrapure water and was markedly lower in wastewater. Regardless of the water chemical composition, MNZ gamma irradiation can achieve i) a decrease in the concentration of dissolved organic carbon, and ii) a reduction in the toxicity of the system with higher gamma absorbed dose.     

The effect of nitrate, bicarbonate and natural organic matter on the degradation of sunscreen agent p-aminobenzoic acid by simulated solar irradiation

Our experiments revealed that a model sunscreen agent, p-aminobenzoic acid (PABA), can be effectively transformed through reactions that are mediated by simulated solar irradiation. We systematically explored the effects of nitrate ions, bicarbonate and different types of natural organic matter (NOM) on the degradation of PABA by simulated solar irradiation. Experimental data suggest that these components ubiquitous in nature water have different influence on the rates of the photoinduced removal of PABA. Products were extracted and analyzed using LC/MS and a total of four products probably resulting from OH and NO₂ radicals attack were identified and the possible reaction pathways were proposed. The findings in this study provide useful information for understanding the environmental transformation of sunscreen agent in aquatic system.     

Degradation of selected pharmaceuticals in aqueous solution with UV and UV/H2O2

The degradation of four pharmaceutical compounds (PhACs), ibuprofen (IBU), diphenhydramine (DP), phenazone (PZ), and phenytoin (PHT) was investigated via ultraviolet (UV) photolysis and UV/H₂O₂ process with a low-pressure (LP) UV lamp. For each PhAC tested, direct photolysis quantum yields at 254 nm were found to be ranging from 6.32 × 10⁻² to 2.79 × 10⁻¹ mol E⁻¹ at pH 7. The second-order rate constants of the reaction between the PhACs and OH were determined to be from 4.86 × 10⁹ to 6.67 × 10⁹ M⁻¹ s⁻¹ by using a competition kinetic model which utilized para-chlorobenzoic acid (pCBA) as a reference compound. The overall effect of OH radical scavenging from humic acid (HA) and anions HCO₃⁻, NO₃⁻ was measured utilizing R_OH,UV method through examining the aqueous photodegradation of pCBA as a probe compound. Moreover, these fundamental direct and indirect photolysis parameters were applied in the model prediction for oxidation rate constants of the PhACs in UV/H₂O₂ process. It was found that the predicted oxidation rate constants approximated the observed ones. The results indicated that the new R_OH,UV probe compound method was applicable for measuring background OH radical scavenging effects in water treatment process of UV/H₂O₂. Furthermore, by GC-MS analysis, most of the intermediates created during the photodegradation of the selected PhACs in UV/H₂O₂ process were identified. For the photodegradation of PZ, a competition mechanism existed between the direct UV photolysis and the oxidation of OH. An appropriate dosage of H₂O₂ could hinder the occurrence of the direct photolysis.     

Evaluation of enhanced coagulation pretreatment to improve ozone oxidation efficiency in wastewater

Enhanced coagulation (EC) using ferric chloride was evaluated as a pretreatment process to improve the efficiency of ozone (O₃) for the oxidation of trace organic contaminants in wastewater. At the applied dosages (10-30 mg/L as Fe), EC pretreatment removed between 10 and 47% of the dissolved organic carbon (DOC) from the three wastewaters studied. Size exclusion chromatography (SEC) showed that EC preferentially removed higher apparent molecular weight (AMW) compounds. Subsequent O₃ testing was performed using an O₃:DOC ratio of 1. Results showed that O₃ exposures were similar even though the required doses were reduced by 10-47% by the EC pretreatment process. Hydroxyl radical (HO) exposure, measured by parachlorobenzoic acid (pCBA), showed 10% reduction when using a FeCl₃ dose of 30 mg/L, likely due to the lower O₃ dose and decreased production of HO during the initial phase of O₃ decomposition (t < 30 s). The oxidation of 13 trace organic contaminants (including atenolol, carbamazepine, DEET, diclofenac, dilantin, gemfibrozil, ibuprofen, meprobamate, naproxen, primidone, sulfamethoxazole, triclosan, and trimethoprim) was evaluated after EC and O₃ treatment. EC was ineffective at removing any of the contaminants, while O₃ oxidation reduced the concentration of compounds according to their reaction rate constants with O₃ and HO.     

Evaluation of UV/H2O2 treatment for the oxidation of pharmaceuticals in wastewater

Advanced oxidation treatment using low pressure UV light coupled with hydrogen peroxide (UV/H₂O₂) was evaluated for the oxidation of six pharmaceuticals in three wastewater effluents. The removal of these six pharmaceuticals (meprobamate, carbamazepine, dilantin, atenolol, primidone and trimethoprim) varied between no observed removal and >90%. The role of the water quality (i.e., alkalinity, nitrite, and specifically effluent organic matter (EfOM)) on hydroxyl radical (OH) exposure was evaluated and used to explain the differences in pharmaceutical removal between the three wastewaters. Results indicated that the efficacy of UV/H₂O₂ treatment for the removal of pharmaceuticals from wastewater was a function of not only the concentration of EfOM but also its inherent reactivity towards OH. The removal of pharmaceuticals also correlated with reductions in ultraviolet absorbance at 254 nm (UV₂₅₄), which offers utilities a surrogate to assess pharmaceutical removal efficiency during UV/H₂O₂ treatment.     

Solar photo-Fenton degradation of nalidixic acid in waters and wastewaters of different composition. Analytical assessment by LC-TOF-MS

This work assessed the solar photo-Fenton degradation of nalidixic acid (NXA), a quinolone antibacterial agent, in several different aqueous solutions. It has been proven that the composition of the water clearly affects the efficiency of the photo-Fenton process. The presence of chlorine ions induces the concurrence of different mechanisms involving Cl and Cl₂⁻ radicals, which slow down the process. Up to 35 transformation products (TPs) were identified and their structures characterized by accurate LC-TOF-MS mass measurements during treatment of the different model waters. Photocatalytic degradation was thus observed to proceed mainly through the attack of the hydroxyl radicals on the double bond C₍₂₎C₍₃₎ which induce further ring opening. All the TPs identified persisted after total degradation of NXA. NXA in real pharmaceutical effluent was treated by photo-Fenton as a first stage before biological treatment. As NXA has been demonstrated to be recalcitrant to biological treatment, photo-Fenton treatment of the effluent was continued until its total degradation. Although NXA was efficiently degraded, LC-MS analyses demonstrated that some of the TPs identified after the photo-Fenton treatment were also recalcitrant to biological treatment, persisting after the combined treatment. These results show that analytical assessment of photocatalytic water treatments is essential to assure they are functioning as intended.     

Trimethoprim: kinetic and mechanistic considerations in photochemical environmental fate and AOP treatment

Trimethoprim (TMP), a bacteriostatic antibiotic, has recently been detected in wastewater and surface waters. In this study the sunlight mediated photochemical fate, and treatment using advanced oxidation and reduction (free radical) processes, have been investigated with respect to their effect on TMP. Photochemical fate, in the presence of humic acid, and advanced oxidation treatment both involve the hydroxyl radical (OH) as one of the reactive species of interest. Another reactive oxygen species, singlet oxygen (¹O₂), may also be important in the photochemical fate of TMP. The bimolecular reaction rate constants of TMP with ¹O₂ and OH were evaluated to be (3.2 ± 0.2) × 10⁶ M⁻¹ s⁻¹ and 8.66 × 10⁹ M⁻¹ s⁻¹, respectively. The reaction kinetics for the sub-structural moieties of TMP, 1,2,3-trimethoxybenzene (TMBz) and 2,4-diaminoprimidine (DAP), was evaluated to facilitate an understanding of the loss mechanisms. For TMBz and DAP the reaction rate constants with ¹O₂ were <1.0 × 10⁴ and (3.0 ± 0.1) × 10⁶ M⁻¹ s⁻¹, while with OH they were 8.12 × 10⁹ and 1.64 × 10⁹ M⁻¹ s⁻¹, respectively. The data suggests that the ¹O₂ attacks the DAP and the OH radical attacks the TMBz moiety. However, for TMP, ¹O₂ and OH reactions accounted for only ∼19% and ∼6%, of its total photodegradation, respectively. Therefore, the reaction of TMP with excited state natural organic matter is postulated as a significant degradation pathway for the loss of TMP in sunlit waters containing natural organic matter. There was no effect of pH on the direct or indirect photolysis of TMP. To complete the study for reductive treatment processes, the solvated electron reaction rates for the destruction of TMP, TMBz and DAP were also evaluated. The absolute bimolecular reaction rates obtained were, (13.6 ± 0.01) × 10⁹, (6.36 ± 0.11) × 10⁷ and (10.1 ± 0.01) × 10⁹ M⁻¹ s⁻¹, respectively.     

Degradation of atrazine in aqueous medium by electrocatalytically generated hydroxyl radicals. A kinetic and mechanistic study

Oxidative degradation of atrazine by hydroxyl radicals (OH) was studied in aqueous medium. OH were formed in situ from electrochemically generating Fenton's reagent by an indirect electrochemical advanced oxidation process. Identification and evolution of seven main aromatic metabolites and four short-chain carboxylic acids were performed by using liquid chromatography analyses. Total organic carbon (TOC) and ionic chromatography were used in order to evaluate the mineralization efficiency of treated aqueous solutions. A high mineralization rate of 82% (never reported until now) was obtained. The oxidative degradation of cyanuric acid, the ultimate product of atrazine degradation, was highlighted for the first time. The absolute rate constant of the reaction between atrazine and hydroxyl radicals was evaluated by competition kinetics method as (2.54 ± 0.22) × 10⁹ M⁻¹ s⁻¹. Considering all oxidation reaction intermediates and end products a general reaction sequence for atrazine degradation by hydroxyl radicals was proposed.     

Photochemical fate of atorvastatin (lipitor) in simulated natural waters

Cholesterol-lowering statin drugs are among the most frequently prescribed for reducing human blood cholesterol and they have been detected as contaminants in natural waters. In this study the photochemical behavior of atorvastatin (lipitor) was investigated at two different concentrations of 35.8 μM (20 mg L⁻¹) and 35.8 nM (20 μg L⁻¹) using a solar simulator and a UV reactor. Photochemical fate in natural waters can be described in most cases by the sum of the loss due to hydrolysis, direct photolysis, and, reaction with hydroxyl radical (OH), singlet oxygen (¹O₂) (or O₂ (¹D)), and excited state dissolved organic matter (DOM). The absolute bimolecular reaction rate constant with OH was measured, using pulsed radiolysis, (1.19 ± 0.04) × 10¹⁰ M⁻¹ s⁻¹. The reaction rate constant of ¹O₂ was determined to be (3.1 ± 0.2) × 10⁸ M⁻¹ s⁻¹. Under the experimental conditions used, at high atorvastatin concentration (35.8 μM) the contribution of singlet oxygen (¹O₂) to the photodegradation of atorvastatin in natural waters was higher than that of hydroxyl radical, and accounted for up to 23% of the loss in aqueous solutions. Whereas, at a concentration of 35.8 nM, ¹O₂ (and OH) both played a minor role in the removal of this compound. Lastly, it also appears that atorvastatin reacts with ³DOM* contributing to its loss in simulated natural waters.