Removal of C.I. Reactive Red 2 by low pressure UV/chlorine advanced oxidation

Synergistic degradation of trimethoprim and its phytotoxicity via the UV/chlorine process: Influencing factors on removal and kinetic

Occurrence of trimethoprim (TMP), recalcitrant antibiotic, and its adverse effect on ecosystem have been reported in several countries. The study aims to remove the TMP and its phytotoxicity via a UV/chlorine process, compared with chlorination and UV irradiation alone. Various treatment conditions including chlorine doses, pHs, and TMP concentrations was conducted with synthetic waters and effluent waters. The UV/chlorine process exhibited a synergistic effect on the TMP removal, compared with chlorination and UV irradiation alone. The UV/chlorine process was the most effective in removing TMP, followed by chlorination. The UV irradiation slightly affected the TMP removal (less than 5%). The UV/chlorine process completely removed TMP by 15 min contact time, while chlorination for 60 min could achieve 71% of TMP removal. The TMP removal fitted well with the pseudo first-order kinetics, and the rate constant (k') increased with higher chlorine doses, lower TMP concentrations and low pH. HO• was the major oxidant affecting the TMP removal and its degradation rate, compared with other reactive chlorine species (e.g., Cl•, OCl•). The TMP exposure increased the phytotoxicity by decreasing a germination rate of Lactuca sativa and Vigna radiata seeds. The use of UV/chlorine process could effectively detoxify the TMP, resulting in the phytotoxicity level of treated waters equivalent or lower than those of TMP-free effluent water. The detoxification level depended on the TMP removal, and it was about 0.43-0.56 times of TMP removal. The findings indicated the potential use of UV/chlorine process in removing TMP residual and its phytotoxicity.

Analysis of degradation and pathways of three common antihistamine drugs by NaClO, UV, and UV-NaClO methods

Antihistamines (ANTs) are medicines to treat allergic diseases. They have been frequently detected in the natural water environment, posing potential threats to the ecological environment and human health. In this study, the degradation of three common antihistamines, loratadine, fexofenadine, and cetirizine, was estimated under different oxidation methods (NaClO, UV, and UV-NaClO). The results showed that UV-NaClO had the highest degree of degradation on the drugs under most conditions: 100% degradation for fexofenadine within 20 s at pH 7 and 10. Under UV irradiation, the degradation efficiencies of the three drugs during 150 s were all above 77% at a pH of 7. The drugs' removal by NaClO was much lower than that of the previous two methods. In addition, this study explored the contribution rates of active oxygen species in the photolysis process. Among them, the contribution of ¹O₂ to the fexofenadine and cetirizine removal rate reached 70%. Different aqueous matrices (HCO₃^-, NO₃^-, and humic acid) had varying degrees of influence on the degradation. Acute toxicity tests and ultraviolet scans of the degradation products showed that the drugs were not completely mineralized, and the toxicities of the intermediates were even higher than those of the parent drugs. There were 9, 8, and 10 chloride oxidation products of loratadine, fexofenadine, and cetirizine, respectively, and 8 photolysis products of cetirizine were identified. For cetirizine, it was found that there were three identical intermediates produced by photodegradation and NaClO oxidation.

Treatment of synthetic dye containing textile raw wastewater effluent using UV/Chlorine/Br photolysis process followed by activated carbon adsorption

This study investigated the efficiency and feasibility of ultraviolet (UV)-assisted photolysis of synthetic dye containing textile raw wastewater effluent. For a said purpose, in-house developed UV/Chlorine/Br process was followed in the presence of activated carbon (AC) which additionally facilitate the dye adsorption. In UV/Chlorine process Cl^•, Cl₂^•-, and HO^• are generated in the solution and destroyed compounds that cannot be oxidized by the conventional oxidant. In this process, free bromine is formed and photolyzed by UV radiation and generate Br^• and Br₂^•- that can enhance the rate of pollutant degradation. In the present study, the dye removal efficiency was contributed by dark bromide (7.18%), UV irradiation (26.8%), dark chlorination (78.67%), and UV/Chlorine/Br (87.01%), respectively. With increasing pH from 3.0 to 8.30, the dye removal efficiency was enhanced but decreased by further increasing pH values. In addition, magnetized activated carbon from pomegranate husk using dual-stage chemical activation was used for post-adsorption of the residual dye and its degradation byproducts. The adsorption of the dye residues by AC followed the second-order kinetics with the rate constant of 1.7 × 10^-3. The phytotoxicity of the treated textile wastewater by UV irradiation, dark chlorination, and UV/Chlorine/Br was assessed by seed germination of Lepidium sativum seeds. The highest inhibition effect on seed germination was related to treated wastewater by UV irradiation (more than 90% inhibition) that alleviated to less than 10% when this effluent diluted to 5% v/v. The highest germination was observed when the seeds were irrigated by the effluent of the UV/Chlorine/Br process. The significant reduction in the toxicity of the treated wastewater revealed that the UV/Chlorine/Br process has a considerable potential to effectively detoxify textile wastewater. Graphical abstract.

Application of UV/chlorine processes for the DR83:1 degradation from wastewater: Effect of coexisting anions

In the present work, effect of coexisting anions on the degradation performance of UV/chlorine (UV/Cl) processes as an advanced oxidation treatment for the dye containing wastewater was investigated. The results showed that by increasing pH of the solution from 3 to 11, degradation efficiency of UV/Cl process was reduced from 96.2 ± 1.4% to 62.9 ± 3.1%. In the case of 100 mg/L of DR83:1, the removal efficiency was improved from 30.2 ± 1.5% to 93.3 ± 4.7% when the chlorine dose was increased from 100 to 1000 μM. The HCO₃- and Br^- ions were the main inhibitor and promotor anions responsible for the degradation of DR83:1. The relative contribution of Cl^• was higher than that for HO^• and UV, which was about three-times higher than that for HO^•. The lowest and highest amounts of trihalomethanes were generated at acidic and alkaline conditions as well as low disinfection by products at low pH due to the generation of more HO^• radicals at acidic pH in comparison to higher pH such that less intermediates were remained to react with chlorine.

Quantitative structure-activity relationship in the photodegradation of azo dyes

The photolysis characteristics of azo dyes are critically important in environmental pollution control, dye-sensitized solar cells, and dyeing-related industries. However, there is still lack of quantitative relationship between the structures of azo dyes and their photolysis characteristics. To address this issue, the photolysis of 22 azo dyes were conducted side by side at three pH (4.0, 6.0, 9.0). The obtained pseudo-first order photodegradation rate constants (k₁) were processed with meta-analysis. Statistically, the hydrazone tautomer had a smaller excitation energy and was easier to undergo photolysis than the azo tautomer. The ortho-substituted sulfonate groups had an obvious protective effect on the photostability of azo dyes. The softness (s), the most positive and negative partial charge on a carbon atom (qC⁺, qC^-) were found to be crucial descriptors in the establishment of QSAR models for the photostability of azo dyes. The QSAR model at pH 9.0 was robust for predicting the photostability of azo dyes under UV irradiation. N₂-purging experiments and quantum chemical computation verified that the cleavage of azo bond was not a result of direct photolysis but was caused by the attack of photoinduced reactive oxygen species. The results here are helpful for the design of more stable azo dyes or the selection of suitable approaches for the treatment of dye-contaminated water bodies.

Photodegradation of (E)- and (Z)-Endoxifen in water by ultraviolet light: Efficiency, kinetics, by-products, and toxicity assessment

Endoxifen is an effective metabolite of a common chemotherapy agent, tamoxifen. Endoxifen, which is toxic to aquatic animals, has been detected in wastewater treatment plant (WWTP) effluent. This research investigates ultraviolet (UV) radiation (253.7 nm) application to degrade (E)- and (Z)-endoxifen in water and wastewater and phototransformation by-products (PBPs) and their toxicity. The effects of light intensity, pH and initial concentrations of (E)- and (Z)-endoxifen on the photodegradation rate were examined. Endoxifen in water was eliminated ≥99.1% after 35 s of irradiation (light dose of 598.5 mJ cm^-2). Light intensity and initial concentrations of (E)- and (Z)-endoxifen exhibited positive trends with the photodegradation rates while pH had no effect. Photodegradation of (E)- and (Z)-endoxifen in water resulted in three PBPs. Toxicity assessments through modeling of the identified PBPs suggest higher toxicity than the parent compounds. Photodegradation of (E)- and (Z)-endoxifen in wastewater at light doses used for disinfection in WWTPs (16, 30 and 97 mJ cm^-2) resulted in reductions of (E)- and (Z)-endoxifen from 30 to 71%. Two of the three PBPs observed in the experiments with water were detected in the wastewater experiments. Therefore, toxic compounds are potentially generated at WWTPs by UV disinfection if (E)- and (Z)-endoxifen are present in treated wastewater.

Treatments for color removal from wastewater: State of the art

It is evident from many recent papers that release of colored wastewater into the environment is source of pollution and this is a problem that particularly affect textile, dyeing and food industries. The review: (i) presents an analysis of various mechanisms involved in the different processes for color removal; (ii) describes conveniences and disadvantages that may exist in adopting one type of treatment in spite of another; (iii) reports the results of approximately 180 experimental tests. Both examples of treatments already widely applied to the real scale and still in the experimental phase are reported. This work focuses on different types of chemical/physical, chemical, electrochemical and biological processes applied in the field of color removal from industrial wastewater. Common chemical/physical treatments such as coagulation/flocculation, adsorption and membrane filtration as well as chemical-type processes are discussed, both those that exploit the traditional oxidizing chemical agents such as Ozone, H₂O₂ and reactive based on chlorine and those based on the principle of advanced chemical oxidation. In particular, both Hydroxyl radical based Advanced Oxidation Processes (AOPs) and Sulfate radical based AOPs are reported. The most commonly used Electrochemical processes for the removal of color are also presented as well as biological treatments. Based on more than 200 papers, this review provides important information on the use, effectiveness, advantages and downsides of the various treatments aimed at removing the color from the wastewater with a look at the technologies still under development.

Simultaneous decomplexation in blended Cu(II)/Ni(II)-EDTA systems by electro-Fenton process using iron sacrificing electrodes

This research explored the application of electro-Fenton (E-Fenton) technique for the simultaneous decomplexation in blended Cu(II)/Ni(II)-EDTA systems by using iron sacrificing electrodes. Standard discharge (0.3 mg L^-1 for Cu and 0.1 mg L^-1 for Ni in China) could be achieved after 30 min reaction under the optimum conditions (i.e. initial solution pH of 2.0, H₂O₂ dosage of 6 mL L^-1 h^-1, current density of 20 mA/cm², inter-electrode distance of 2 cm, and sulfate electrolyte concentration of 2000 mg L^-1). The distinct differences in apparent kinetic rate constants (k_app) and intermediate removal efficiencies corresponding to mere and blended systems indicated the mutual promotion effect toward the decomplexation between Cu(II) and Ni(II). Massive accumulation of Fe(Ⅲ) favored the further removal of Cu(II) and Ni(II) by metal ion substitution. Species distribution results demonstrated that the decomplexation of metal-EDTA in E-Fenton process was mainly contributed to the combination of various reactions, including Fenton reaction together with the anodic oxidation, electro-coagulation (E-coagulation) and electrodeposition. Unlike hypophosphite and citrate, the presence of chlorine ion displayed favorable effects on the removal efficiencies of Cu(II) and Ni(II) at low dosage, but facilitated the ammonia nitrogen (NH₄⁺-N) removal only at high dosage.

Efficient decolorization of typical azo dyes using low-frequency ultrasound in presence of carbonate and hydrogen peroxide

The aims of this study as to evaluate and understand the decolorization of azo dyes using carbonate and hydrogen peroxide under low-frequency ultrasonic irradiation. Under optimal conditions, the decolorization ratio of acid orange 8 (AO 8), a typical azo dye, was > 90% after 2 h of irradiation. The decolorization rate of AO 8 was 0.023 min^-1 under ultrasonic irradiation, which was about two times that without ultrasound. Different from the results of other published studies, OH played a minor role, while CO₃^- played the most important role in AO 8 ultrasonic decolorization in the presence of CO₃^2- and H₂O₂, with a contribution of 56.52%, followed by CO₄^2- (32.61%) and ¹O₂ (10.87%). Another difference is that CO₃^- formed through the cleavage of peroxymonocarbonate or peroxydicarbonate under ultrasonic irradiation rather than through reaction between hydroxyl radical and carbonate. Investigations for different azo dyes revealed that the decolorization rate decreased in the order AO 8 ≈ orange II > acid red 9 > acid yellow 11, probably because of molecular differences among the azo dyes.

Degradation of polyvinyl alcohol (PVA) by UV/chlorine oxidation: Radical roles, influencing factors, and degradation pathway

Polyvinyl alcohol (PVA) is widely used in industry but is difficult to degrade. In this study, the synergistic effect of UV irradiation and chlorination on degradation of PVA was investigated. UV irradiation or chlorination alone did not degrade PVA. By contrast, UV/chlorine oxidation showed good efficiency for PVA degradation via generation of active free radicals, such as OH and Cl. The relative importance of these two free radicals in the oxidation process was evaluated, and it was shown that OH contributed more to PVA degradation than Cl did. The degradation of PVA followed pseudo first order kinetics. The rate constant k increased linearly from 0 min^-1 to 0.3 min^-1 with increasing chlorine dosage in range of 0 mg/L to 20 mg/L. However, when the chlorine dosage was increased above 20 mg/L, scavenging effect of free radicals occurred, and the degradation efficiency of PVA did not increase much more. Acidic media increased the degradation efficiency of PVA by UV/chlorine oxidation more than basic or neutral media because of the higher ratio of [HOCl]/[OCl^-], higher free radical quantum yields, and the lower free radical quenching effect under acidic conditions. Results of Fourier Transform Infrared Spectroscopy showed that carbonyl groups in degradation products were formed during UV/chlorine oxidation, and a possible degradation pathway via alcohol to carbonyl was proposed.

Theoretical investigation on the mechanism of the OH-initiated degradation process of reactive red 2 azo dye

The dual descriptor (Δf) data of azo form (a, RR2) and hydrazone form (b, HRR2) of RR2 dianion. For Δf> 0 (green), the site is favorable for nucleophilic attack, for Δf< 0 (blue), the site is favorable for electrophilic attack. Key bond lengths in Å.

UV-Enhanced NaClO Oxidation of Nitric Oxide from Simulated Flue Gas

A wet de-NOxtechnique based on an UV-enhanced NaClO oxidation process was investigated for simulated flue gas of a diesel engine using a bench-scale reaction chamber. The effects of UV irradiation time, initial pH value, and available chlorine concentration of NaClO solution were studied, respectively. The results showed that when the UV irradiation time was 17.5 min and the initial pH value of NaClO solution was 6, NO removal efficiency of UV/NaClO solution was increased by 19.6% compared with that of NaClO solution. Meanwhile, when the available chlorine concentration of NaClO solution decreased from 0.1 wt% to 0.05 wt%, the enhancement in NO removal efficiency of UV/NaClO solution increased from 19.6% to 24%, compared with that of NaClO solution. The reaction pathways of NaClO solution photolysis and NO removal by UV/NaClO process were preliminarily discussed. The results suggested that HOCl might be the most active species that released many UV-induced photooxidants through photolysis reactions, which played an important role in NO removal process.