Congwei Luo

Simulation and comparative study on the oxidation kinetics of atrazine by UV/H2O2, UV/HSO5− and UV/S2O82−

This study comparatively investigated atrazine (ATZ) degradation by irradiation at the wavelength of 254 nm in the presence of peroxides including hydrogen peroxide (H2O2), peroxymonosulfate (HSO5(-)), and persulfate (S2O8(2-)) at various initial ATZ concentrations and oxidant dosages. The effects of water matrix, such as carbonate/bicarbonate (HCO3(-)/CO3(2-)), chloride ions (Cl(-)), and natural organic matter (NOM), were evaluated on these three advanced oxidation processes. A simple steady-state kinetic model was developed based on the initial rates of ATZ destruction, which could well describe the apparent pseudo-first-order rate constants (k(app), s(-1)) of ATZ degradation in these three processes. The specific roles of reactive species (i.e., HO·, SO4(-·), CO3(-·), and Cl2(-·)) under various experimental conditions were quantitatively evaluated based on their steady-state concentrations obtained from this model. Modeling results showed that the steady-state concentrations of HO· and SO4(-·) decreased with the increase of CO3(2-)/HCO3(-) concentration, and the relative contribution of HO· to ATZ degradation significantly decreased in UV/H2O2 and UV/HSO5(-) systems. On the other hand, the scavenging effect of HCO3(-)/CO3(2-) on the relative contribution of SO4(-·) to ATZ degradation was lower than that on HO·. The presence of Cl(-) (0.5-10 mM) significantly scavenged SO4(-·) but had slightly scavenging effect on HO· at the present experimental pH, resulting in greater decrease of k(app) in the UV/S2O8(2-) than UV/H2O2 and UV/HSO5(-) systems. Higher levels of Cl2(-·) were generated in the UV/S2O8(2-) than those in the UV/H2O2 and UV/HSO5(-) systems at the same Cl(-) concentrations. NOM significantly decreased k(app) due to its effects of competitive UV absorption and radical scavenging with the latter one being dominant. These results improve the understanding of the effects of water constituents for ATZ degradation in the UV-based oxidation processes.

Degradation of sulfamethoxazole by UV, UV/H2O2 and UV/persulfate (PDS): Formation of oxidation products and effect of bicarbonate

The frequent detection of sulfamethoxazole (SMX) in wastewater and surface waters gives rise of concerns about their ecotoxicological effects and potential risks to induce antibacterial resistant genes. UV/hydrogen peroxide (UV/H2O2) and UV/persulfate (UV/PDS) advanced oxidation processes have been demonstrated to be effective for the elimination of SMX, but there is still a need for a deeper understanding of product formations. In this study, we identified and compared the transformation products of SMX in UV, UV/H2O2 and UV/PDS processes. Because of the electrophilic nature of SO4-, the second-order rate constant for the reaction of sulfate radical (SO4-) with the anionic form of SMX was higher than that with the neutral form, while hydroxyl radical (OH) exhibited comparable reactivity to both forms. The direct photolysis of SMX predominately occurred through cleavage of the NS bond, rearrangement of the isoxazole ring, and hydroxylation mechanisms. Hydroxylation was the dominant pathway for the reaction of OH with SMX. SO4- favored attack on NH2 group of SMX to generate a nitro derivative and dimeric products. The presence of bicarbonate in UV/H2O2 inhibited the formation of hydroxylated products, but promoted the formation of the nitro derivative and the dimeric products. In UV/PDS, bicarbonate increased the formation of the nitro derivative and the dimeric products, but decreased the formation of the hydroxylated dimeric products. The different effect of bicarbonate on transformation products in UV/H2O2 vs. UV/PDS suggested that carbonate radical (CO3-) oxidized SMX through the electron transfer mechanism similar to SO4- but with less oxidation capacity. Additionally, SO4- and CO3- exhibited higher reactivity to the oxazole ring than the isoxazole ring of SMX. Ecotoxicity of transformation products was estimated by ECOSAR program based on the quantitative structure-activity relationship analysis as well as by experiments using Vibrio fischeri, and these results indicated that the oxidation of SO4- or CO3- with SMX generated more toxic products than those of OH.

Transformation of Iodide by Carbon Nanotube Activated Peroxydisulfate and Formation of Iodoorganic Compounds in the Presence of Natural Organic Matter

In this study, we interestingly found that peroxydisulfate (PDS) could be activated by a commercial multiwalled carbon nanotube (CNT) material via a nonradical pathway. Iodide (I-) was quickly and almost completely oxidized to hypoiodous acid (HOI) in the PDS/CNT system over the pH range of 5-9, but the further transformation to iodate (IO3-) was negligible. A kinetic model was proposed, which involved the formation of reactive PDS-CNT complexes, and then their decomposition into sulfate anion (SO42-) via inner electron transfer within the complexes or by competitively reacting with I-. Several influencing factors (e.g., PDS and CNT dosages, and solution pH) on I- oxidation kinetics by this system were evaluated. Humic acid (HA) decreased the oxidation kinetics of I-, probably resulting from its inhibitory effect on the interaction between PDS and CNT to form the reactive complexes. Moreover, adsordable organic iodine compounds (AOI) as well as specific iodoform and iodoacetic acid were appreciably produced in the PDS/CNT/I- system with HA. These results demonstrate the potential risk of producing toxic iodinated organic compounds in the novel PDS/CNT oxidation process developed very recently, which should be taken into consideration before its practical application in water treatment.

Oxidation Kinetics of Bromophenols by Nonradical Activation of Peroxydisulfate in the Presence of Carbon Nanotube and Formation of Brominated Polymeric Products

This work demonstrated that bromophenols (BrPs) could be readily oxidized by peroxydisulfate (PDS) activated by a commercial carbon nanotube (CNT), while furfuryl alcohol (a chemical probe for singlet oxygen (1O2)) was quite refractory. Results obtained by radical quenching experiments, electron paramagnetic resonance spectroscopy, and Fourier transform infrared spectroscopy further confirmed the involvement of nonradical PDS-CNT complexes rather than 1O2. Bicarbonate and chloride ion exhibited negligible impacts on BrPs degradation by the PDS/CNT system, while a significant inhibitory effect was observed for natural organic matter. The oxidation of BrPs was influenced by solution pH with maximum rates occurring at neutral pH. Linear free energy relationships (LFERs) were established between the observed pseudo-first-order oxidation rates of various substituted phenols and the classical descriptor variables (i.e., Hammett constant σ+, and half-wave oxidation potential E1/2). Products analyses by liquid chromatography tandem mass spectrometry clearly showed the formation of hydroxylated polybrominated diphenyl ethers and hydroxylated polybrominated biphenyls on CNT surface. Their formation pathway possibly involved the generation of bromophenoxyl radicals from BrPs one-electron oxidation and their subsequent coupling reactions. These results suggest that the novel nonradical PDS/CNT oxidation technology is a good alternative for selectively eliminating BrPs with alleviating toxic byproducts in treated water effluent.

Factors affecting formation of deethyl and deisopropyl products from atrazine degradation in UV/H2O2 and UV/PDS

In this study, the formation of deethyl products (DEPs) (i.e., atrazine amide (Atra-imine) and deethylatrazine (DEA)) and deisopropyl product (i.e., deisopropylatrazine (DIA)) from parent atrazine (ATZ) degraded in UV/H2O2 and UV/PDS processes under various conditions was monitored. It was found that SO4˙− displayed a more distinctive preference to the ethyl function group of ATZ than HO˙, leading to the higher ratio of DEPs/DIA in UV/PDS system than that in UV/H2O2 system in pure water. The effects of water matrices (i.e., natural organic matter (NOM), carbonate/bicarbonate (HCO3−/CO32−), and chloride ions (Cl−)) on ATZ degradation as well as formation of DEPs and DIA were evaluated in detail. The degradation of ATZ by UV/PDS was significantly inhibited in the presence of NOM, HCO3−/CO32− or Cl−, because these components could competitively react with SO4˙− and/or HO˙ to generate lower reactive secondary radicals (i.e., organic radicals, carbonate radicals (CO3˙−) or reactive chlorine radicals (RCs)). The yields of these DEPs and DIA products from ATZ degradation were not impacted by NOM or HCO3−/CO32−, possibly due to the low reactivity of organic radicals and CO3˙− toward the side groups of ATZ. Howbeit, the increase of DIA yield companied with the decrease of DEPs yield was interestingly observed in the presence of Cl−, which was attributed to the promotion of Cl− at moderate concentration (mM range) for the conversion of SO4˙− into HO˙. Comparatively, in the UV/H2O2 process, NOM and HCO3−/CO32− exhibited a similar inhibitory effect on ATZ degradation, while the influence of Cl− was negligible. Differing from UV/PDS system, all these factors did not change DEPs and DIA yields in UV/H2O2 process. Moreover, it was confirmed that RCs had a greater selectivity but a lower reactivity on attacking the ethyl function group than that of SO4˙−. These findings were also confirmed by monitoring the degradation of ATZ as well as the formation of DEPs and DIA in three natural waters.