Photocatalytic degradation of N-nitrosodimethylamine: mechanism, product distribution, and TiO2 surface modification

Targeted electrochemical reduction of carcinogenic N-nitrosamines from emission control systems within CO2 capture plants

N-Nitrosamines are one of the environmentally significant byproducts from aqueous amine-based post-combustion carbon capture systems (CCS) due to their potential risk to human health. Safely mitigating nitrosamines before they are emitted from these CO₂ capture systems is therefore a key concern before widescale deployment of CCS can be used to address worldwide decarbonization goals. Electrochemical decomposition is one viable route to neutralize these harmful compounds. The circulating emission control waterwash system, commonly installed at the end of the flue gas treatment trains to minimize amine solvent emissions, plays an important role to capture N-nitrosamines and control their emission into the environment. The waterwash solution is the last point where these compounds can be properly neutralized before becoming an environmental hazard. In this study, the decomposition mechanisms of N-nitrosamines in a simulated CCS waterwash with residual alkanolamines was investigated using several laboratory-scale electrolyzers utilizing carbon xerogel (CX) electrodes. H-cell experiments revealed that N-nitrosamines were decomposed through a reduction reaction and are converted into their corresponding secondary amines thereby neutralizing their environmental impact. Batch-cell experiments statistically examined the kinetic models of N-nitrosamine removal by a combined adsorption and decomposition processes. The cathodic reduction of the N-nitrosamines statistically obeyed the first-order reaction model. Finally, a prototype flow-through reactor using an authentic waterwash was used to successfully target and decompose N-nitrosamines to below the detectable level without degrading the amine solvent compounds allowing them to be return to the CCS and lower the system operating costs. The developed electrolyzer was able to efficiently remove greater than 98% of N-nitrosamines from the waterwash solution without producing any additional environmentally harmful compounds and offers an effective and safe route to mitigate these compounds from CO₂ capture systems.

Efficient degradation of N-nitrosopyrrolidine using CoFe-LDH/AC particle electrode via heterogeneous Fenton-like reaction

With the rapid development of drinking water disinfection technology, extensive attentions are paid to the nitrogenous disinfection by-products (N-DBPs) that has strong carcinogenicity, thus their degradation becomes important for the health of human beings. In this work, for the first time, CoFe-LDH material used as particle electrode is proposed to treat trace N-nitrosopyrrolidine (NPYR) in a three-dimensional aeration electrocatalysis reactor (3DAER). The factors on the degradation efficiency and energy consumption of NPYR are systematically investigated, and the results of radical quenching experiments show that the degradation of NPYR is completed by combining with ·OH, ·O₂and direct oxidation together. CoFe-LDH particle electrode plays a vital role in generating ·OH via heterogeneous ‾Fenton-like reaction. Moreover, the adsorbed saturated CoFe-LDH particle electrode can be regenerated by electrochemical action to induce further recycle adsorption and form in-situ electrocatalysis. This work pave a way for the removal of NPYR with high efficiency, low energy conservation and environmental protection.

Photodegradation of N-nitrosodimethylamine under 365 nm Light Emitting Diode Irradiation

The photodegradation of NDMA has been extensively investigated under the irradiation of low-pressure or medium-pressure Hg lamps and xenon lamp. However, NDMA photolysis remains unknown under 365 nm ultraviolet light-emitting diode (UV-LED) irradiation. This study conducted a comprehensive investigation on NDMA photodegradation by 365 nm UV-LED illumination. The quantum yield of NDMA photolysis under 365 nm UV-LED irradiation was determined to be 0.0312 ± 0.0047. The influence of pH on NDMA photodegradation was found to be wavelength dependent. Compared with distilled and deionized water (DDW), tap water inhibited NDMA photodegradation, but secondary wastewater effluent did not. Based on the quantification of NDMA photolysis products and pH influence, the photooxidation of the excited NDMA in the nonprotonated form was proposed to be a major pathway for NDMA photodegradation under the irradiation of UV-LED lamp at 365 nm. This study further enhances our knowledge on NDMA photodegradation. PRACTITIONER POINTS: Quantum yield of NDMA photolysis at 365 nm was determined to be 0.0312 ± 0.0047. The influence of pH on NDMA photodegradation was wavelength dependent. NDMA photodegradation was inhibited in tap water compared with that in DDW. NDMA photodegradation in SWE was similar to that in DDW. Excited nonprotonated NDMA photooxidation is a major degradation pathway.

Changes in levels of N-nitrosamine formed from amine-containing compounds during chloramination via photocatalytic pretreatment with immobilized TiO2: Effect of source water and pH

We investigated the effectiveness of photocatalytic pretreatment (PCP) of precursors in minimizing the formation potentials (FPs) of carcinogenic nitrosamines, including N-nitrosodimethylamine (NDMA), N-nitrosodiethylamine (NDEA), and N-nitrosodiethanolamine (NDELA), during water chloramination. A steel mesh substrate with immobilized TiO₂ was highly efficient at mitigating nitrosamine formation and removing targeted precursors such as ranitidine, nizatidine, trimebutine, triethanolamine, and metoclopramide. Compared to UVC/H₂O₂, PCP under UVA irradiation (intensity of 0.67 mW cm^-2) was more effective for reducing nitrosamine-FPs during post-chloramination. However, the PCP efficacies varied with the water source, pretreatment pH, and irradiation time. For example, PCP of eutrophic water increased the NDMA-FPs, but produced notable reductions (up to 99%) for NDELA- and NDEA-FPs. Shorter irradiation times, up to 15 min, increased the NDELA-FP in triethanolamine, and the NDMA-FP in nizatidine and trimebutine. However, the nitrosamine-FP decreased by > 50% after PCP at a pH > 5.6, following irradiation for 120 min. Oxygen addition, N-de(m)ethylation, and N-dealkylation were responsible for decreasing nitrosamine-FPs via the destruction of key moieties; this has been elucidated by mass spectroscopy. This study suggests that PCP could be used as an alternative strategy for minimizing nitrosamine-FPs during water treatment.

When bimetallic oxides and their complexes meet Fenton-like process

The heterogeneous Fenton-like reaction is an advanced oxidation process, which is widely recognized for its efficient removal of recalcitrant organic contaminants. In recent years, the construction of efficient and reusable heterogeneous Fenton-like catalysts has been extensively investigated. Recently, the use of bimetallic oxides and their complexes as catalysts for Fenton-like reaction has attracted intense attention due to their high catalytic performance and excellent stability over a wide pH range. In this article, the fundamental mechanisms of Fenton-like reactions were briefly introduced. The important reports on bimetallic oxides and their complexes are classified in detail, which are mainly divided into Fe-based and Fe-free bimetallic catalysts. We then focused in depth on the performance of their respective applications in Fenton-like reactions. Special consideration has been given to the respective contributions and synergistic mechanisms of the two metals in catalysts. Overall, it is concluded that synergistic effect of the two metals in the bimetallic catalyst can boost the utilization of hydrogen peroxide, provide adequate accessible active sites, which are all beneficial to improve catalytic performance. Finally, the current challenges in this field were proposed. Our review is expected to provide help for the application of bimetallic oxides and their complexes.

Cyclic coupling of photocatalysis and adsorption for completely safe removal of N-nitrosamines in water

The incomplete removal of N-nitrosamines in water through current degraded techniques and the carcinogenicity of N-nitrosamines call for alternative complete and safe removal approaches. Here, we describe a cyclic coupling process of photocatalysis and adsorption enabling N-nitrosamines in water thoroughly and safely removed. Among them, the immobilized TiO2/Ti photocatalyst degraded N-nitrosamines into primary and secondary amines up to 100% by attacking on nitrosyl nitrogen via •OH originated from its nanowire film morphology. Furthermore, the affinity of HY zeolite to primary and secondary amines led to efficient adsorption through corresponding to Lagergren adsorption rate equation of second order. And then the cyclic coupling process of photocatalysis and adsorption realized complete and safe removal of N-nitrosamines with various concentration ranging from 0.1 mM to 1 mM in water, significantly higher than the existing reports on the removal rate of N-nitrosamines and the formation potential of N-nitrosamines. This study will lead to new avenues for complete and safe eliminaton of hardly degradable hazardous substances in water.

Synthesis of carbon dots with high photocatalytic reactivity by tailoring heteroatom doping

The photocatalytic degradation of organic pollutant by carbon-based materials is still a challenge. Herein, xylose-derived carbon dots (X-CDs) and chitosan-derived CDs (C-CDs) were synthesized by heteroatoms-doping strategy. Although there is almost no difference in fluorescence emission behaviors, the two types of CDs demonstrated different advantages in photocatalysis and peroxymonosulfate (PMS) activation. Comparative research revealed that the X-CDs with doping of heteroatom S was superior in the separation of electron-hole pairs, resulting in a higher catalytic performance, while the S, N co-doped C-CDs can only exhibit high photocatalytic reactivity when they were coupled with PMS. The underlying reason is that the N-related functional groups with strong electron-donating property weakened the electron-trapping capacity of S-related energy level, but surface state resulting from this doping structures were conducive to promoting photo-generated electron transfer from C-CDs to PMS and played the primary role in organic oxidation. Thanks to the doping effect, both the X-CDs and C-CDs/PMS system displayed high photocatalytic performance for methylene blue removal under sunlight irradiation, showing almost 100% degradation efficiency in a 30 min period. The present study provides a valuable insight for the synthesis of CDs-based catalysts but also establishes a very promising catalytic oxidation system.

Study on the photoelectrical performance of anodized titanium sheets

Anodization is a widely used method to obtain multicoloured oxidized titanium sheets. However, most researchers paid great attention to the colour-related properties instead of photoelectrical properties of titanium oxide film obtained by anodization. In this work, to study their photoelectrical properties, a series of multicoloured oxidized titanium sheets were prepared by anodization method, and UV-vis absorption and photocurrents were tested. The relationship between anodization voltages/anodization durations and photocurrents of titanium sheets was studied. Results show that titanium sheets have excellent photoelectrical performance. With the increase of anodization voltage, the number of UV-vis absorption peaks increased under visible light which means increasing absorption. When anodization duration increased, absorption band edge also increased in the visible light region, which means the band gap needed to produce charge transfer transition decreased. Under simulated sunlight and applied voltage of +0.4 V, photocurrent increased with the increase of either anodization voltage or anodization duration, and can be expressed by linear equations. In addition, anodization currents were recorded during anodization. Morphology, crystal structure and photoelectrical properties of anodized titanium sheets were characterized. The anodized titanium sheets can not only be used as decorative material in jewellery and architecture fields etc. but also are supposed to be used as photoelectrical catalyst in further work.

UV/sulfite chemistry to reduce N-nitrosodimethylamine formation in chlor(am)inated water

The disinfection by-product N-nitrosodimethylamine (NDMA) is a major concern in water quality management due to its carcinogenicity. Thus, a proper pretreatment is necessary to mitigate NDMA formation upon periodic chloramination by removing precursors, such as ranitidine (RNT). This study investigated the effect of UV/sulfite pretreatment on NDMA formation from an RNT-spiked tap and chloraminated synthetic swimming pool (SSP) water. At UVC intensity of 2.1 mW cm^-2 and 0.5 mM of sulfite, UV/sulfite chemistry showed complete degradation of 20 µM RNT within 30 min. It was found that SO₄^•- primarily reduced the NDMA formation potential (FP) of RNT, while hydrated electrons effectively mitigated the pre-formed NDMA in the SSP water. The UV/sulfite pretreatment alleviated NDMA formation during post-chloramination (24 h) by up to 82%, outperforming the commonly employed advanced oxidation processes such as UV/H₂O₂. However, in the presence of bromide ions, the effectiveness of UV/sulfite pretreatment was seriously deteriorated, although the bromide ion itself was found to inhibit the NDMA formation from RNT especially at pH < 8 during chloramination. Mass spectrometric analysis indicated that the NDMA-FP of RNT could be removed by UV/sulfite principally via N-methylation, dealkylation, and oxygen transfer pathways. Consequently, UV/sulfite could be used as an alternative unit process for water treatment with reduced NDMA formation.

Enhanced photocatalytic degradation of sulfamethoxazole by zinc oxide photocatalyst in the presence of fluoride ions: Optimization of parameters and toxicological evaluation

The presence of antibiotics in water bodies has received increasing attention since they are continuously introduced and detected in the environment and may cause unpredictable environmental hazards and risks. The photocatalytic degradation of sulfamethoxazole (SMX) by ZnO in the presence of fluoride ions (F-ZnO) was evaluated. The effects of operating parameters on the efficiency of SMX removal were investigated by using response surface methodology (RSM). Under the optimum condition, i.e. photocatalyst dosage = 1.48 g/L, pH 4.7, airflow rate = 2.5 L/min and the concentration of fluoride ions = 2.505 mM, about 97% SMX removal was achieved by F-ZnO after 30 min of reaction. The mechanism of reactions, COD removal efficiency and reaction kinetics were also investigated under optimum operating conditions. In addition, about 85% COD reduction was obtained after 90 min photocatalytic reaction. The pseudo-first-order kinetics rate constants for the photodegradation of SMX were found to be 0.099, 0.058 and 0.048 min^-1 by F-ZnO, ZnO and TiO₂ (P25), respectively. The figure-of-merit electrical energy per order (E_EO) was used for estimating the electrical energy efficiency, which was shown to be considerably lower than the energy consumption for the reported research on removal of SMX by photocatalytic degradation under UV irradiation. Toxicity assays were conducted by measuring the inhibition percentage (PI) towards E. coli bacteria strain and by agar well diffusion method. The results showed that after 30 min of reaction, the toxicity of the treated solutions by all photocatalysts fell within the non-toxic range; however, the reduction in toxicity by F-ZnO was faster than those by ZnO and P25. Despite the positive effects of surface fluorination of ZnO on the SMX and COD removal and reaction kinetics, its lower stability compared to ZnO and P25 in the repeated experiments gave rise to some doubts about its performance from a practical point of view.

Efficient photoreductive decomposition of N-nitrosodimethylamine by UV/iodide process

N-nitrosodimethylamine (NDMA) has aroused extensive concern as a disinfection byproduct due to its high toxicity and elevated concentration levels in water sources. This study investigates the photoreductive decomposition of NDMA by UV/iodide process. The results showed that this process is an effective strategy for the treatment of NDMA with 99.2% NDMA removed within 10min. The depletion of NDMA by UV/iodide process obeyed pseudo-first-order kinetics with a rate constant (k₁) of 0.60±0.03min^-1. Hydrated electrons (e_aq^-) generated by the UV irradiation of iodide were proven to play a critical role. Dimethylamine (DMA) and nitrite (NO₂^-) were formed as the main intermediate products, which completely converted to formate (HCOO^-), ammonium (NH₄⁺) and nitrogen (N₂). Therefore, not only the high efficiencies in NDMA destruction, but the elimination of toxic intermediates make UV/iodide process advantageous. A photoreduction mechanism was proposed: NDMA initially absorbed photons to a photoexcited state, and underwent a cleavage of NNO bond under the attack of e_aq^-. The solution pH had little impact on NDMA removal. However, alkaline conditions were more favorable for the elimination of DMA and NO₂^-, thus effectively reducing the secondary pollution.

Impact of Pore Size on Fenton Oxidation of Methyl Orange Adsorbed on Magnetic Carbon Materials: Trade-Off between Capacity and Regenerability

The economic cleanup of wastewater continues to be an active area of research. In this study, the influence of pore size on regeneration by Fenton oxidation for carbon materials with adsorbed methyl orange (MO) was investigated. More specifically three carbon supports, with pore sizes ranging from mainly microporous to half microporous-half mesoporous to mainly mesoporous, were impregnated with γ-Fe₂O₃ to make them magnetic and easy to separate from solution. The carbon samples were characterized before adsorption and after regeneration with hydrogen peroxide at 20 °C. In addition, adsorption kinetics and isotherms were collected, and the Weber-Morris intraparticle diffusion model and Freundlich isotherm model fit to the data. The adsorption capacity increased with increasing microporosity while the regeneration efficiency increased with increasing mesoporosity. Further experiments with varying regeneration and adsorption conditions suggested that the regeneration process may be kinetically limited. The MO adsorbed in the micropores was strongly adsorbed and difficult to remove unlike the MO adsorbed in the mesopores, which could be reacted under relatively mild conditions. Thus, there was a trade-off between adsorption capacity and regeneration.

Oxidative degradation of N-Nitrosopyrrolidine by the ozone/UV process: Kinetics and pathways

N-Nitrosopyrrolidine (NPYR) is an emerging contaminant in drinking water and wastewater. The degradation kinetics and mechanisms of NPYR degradation by the O3/UV process were investigated and compared with those of UV direct photolysis and ozonation. A synergistic effect of ozone and UV was observed in the degradation of NPYR due to the accelerated production of OH• by ozone photolysis. This effect was more pronounced at higher ozone dosages. The second-order rate constants of NPYR reacting with OH• and ozone was determined to be 1.38 (± 0.05) × 10(9) M(-1) s(-1) and 0.31 (± 0.02) M(-1) s(-1), respectively. The quantum yield by direct UV photolysis was 0.3 (± 0.01). An empirical model using Rct (the ratio of the exposure of OH• to that of ozone) was established for NPYR degradation in treated drinking water and showed that the contributions of direct UV photolysis and OH• oxidation on NPYR degradation were both significant. As the reaction proceeded, the contribution by OH• became less important due to the exhausting of ozone. Nitrate was the major product in the O3/UV process by two possible pathways. One is through the cleavage of nitroso group to form NO• followed by hydrolysis, and the other is the oxidation of the intermediates of amines by ozonation.

Adsorption and photocatalytic degradation behaviors of rhodamine dyes on surface-fluorinated TiO2 under visible irradiation

Surface-fluorinated TiO₂ (F-TiO₂) particles was synthesized by a simple fluorosilanization method to serve as a visible-light photocatalyst for rapid degradation of Rhodamine B (RhB) dyes.

Characterization of the microbial community structure and nitrosamine-reducing isolates in drinking water biofilters

Two biofilters were constructed using biological activated carbon (BAC) and nitrosamine-containing water from two drinking water treatment plants. The microbiome of each biofilter was characterized by 454 high-throughput pyrosequencing, and one nitrosamine-reducing bacterium was isolated. The results showed that nitrosamines changed the relative abundance at both the phylum and class levels, and the new genera were observed in the microbial communities of the two BAC filters after cultivation. As such, the genus Rhodococcus, which includes many nitrosamine-reducing strains reported in previous studies, was only detected in the BAC2 filter after cultivation. These findings indicate that nitrosamines can significantly affect the genus level in the microbial communities. Furthermore, the isolated bacterial culture Rhodococcus cercidiphylli A41 AS-1 exhibited the ability to reduce five nitrosamines (N-nitrosodimethylamine, N-nitrosodiethylamine, N-nitrosodi-n-propylamine, N-nitrosopyrrolidine, and N-nitrosodi-n-butylamine) with removal ratios that ranged from 38.1% to 85.4%. The isolate exhibited a better biodegradation ability with nitrosamine as the carbon source when compared with nitrosamine as the nitrogen source. This study increases our understanding of the microbial community in drinking water biofilters with trace quantities of nitrosamines, and provides information on the metabolism of nitrosamine-reducing bacteria.

Enhanced photocatalytic performance of N-nitrosodimethylamine on TiO2 nanotube based on the role of singlet oxygen

N-nitrosodimethylamine (NDMA) photocatalytic degradation performance and mechanism were investigated on the TiO2 nanotube prepared from anatase TiO2 nanopowder in terms of the production of reactive oxygen species including hydroxyl radical, singlet oxygen and superoxide radical. Significantly higher NDMA degradation efficiency was obtained on anatase TiO2 nanotube rather than anatase TiO2 nanopowder. The tubular morphology may be responsible for almost 100% NDMA removal on TiO2 nanotube, presumably due to its confinement effect leading to NDMA molecules within the nanotube being attacked by reactive oxygen species such as hydroxyl radical and singlet oxygen, and initiating reaction inside the nanotube. In particular, the ability of the nanotubular structure of TiO2 nanotube to promote a singlet oxygen oxidation pathway contributes much to the enhanced NDMA degradation efficiency and favors the formation of dimethylamine and NO3(-). Such function originating from nanotube morphology could bring new insights for the photocatalytic degradation of organic pollutants.

Iron–copper bimetallic nanoparticles supported on hollow mesoporous silica spheres: an effective heterogeneous Fenton catalyst for orange II degradation

Iron–copper bimetallic nanoparticles supported on hollow mesoporous silica spheres as a composite catalyst (FeCu/HMS) was synthesized.

Enhanced quantum yield of nitrogen fixation for hydrogen storage with in situ-formed carbonaceous radicals

N₂ can be reduced to NH₃, a green hydrogen storage material, using suitable photocatalysts and scavengers under UV irradiation.

Arsenite oxidation-enhanced photocatalytic degradation of phenolic pollutants on platinized TiO2

The effect of As(III) on the photocatalytic degradation of phenolic pollutants such as 4-chlorophenol (4-CP) and bisphenol A (BPA) in a suspension of platinized TiO2 (Pt/TiO2) was investigated. In the presence of As(III), the photocatalytic degradation of 4-CP and BPA was significantly enhanced, and the simultaneous oxidation of As(III) to As(V) was also achieved. This positive effect of As(III) on the degradation of phenolic pollutants is attributed to the adsorption of As(V) (generated from As(III) oxidation) on the surface of Pt/TiO2, which facilitates the production of free OH radicals ((•)OHf) that are more reactive than surface-bound OH radicals ((•)OHs) toward phenolic pollutants. The generation of (•)OHf was indirectly verified by using coumarin as an OH radical trapper and comparing the yields of coumarin--OH adduct (i.e., 7-hydroxycoumarin) formed in the absence and presence of As(V). In repeated cycles of 4-CP degradation, the degradation efficiency of 4-CP gradually decreased in the absence of As(III), whereas it was mostly maintained in the presence of As(III), which was either initially present or repeatedly injected at the beginning of each cycle. The positive effect of As(III) on 4-CP degradation was observed over a wide range of As(III) concentrations (up to mM levels) with Pt/TiO2. However, a high concentration of As(III) (hundreds of μM) inhibited the degradation of 4-CP with bare TiO2. Therefore, Pt/TiO2 can be proposed as a practical photocatalyst for the simultaneous oxidation of phenolic pollutants and As(III) in industrial wastewaters.

Assessing the photochemical transformation pathways of acetaminophen relevant to surface waters: transformation kinetics, intermediates, and modelling

This work shows that the main photochemical pathways of acetaminophen (APAP) transformation in surface waters would be direct photolysis (with quantum yield of (4.57 ± 0.17)⋅10(-2)), reaction with CO3(-·) (most significant at pH > 7, with second-order rate constant of (3.8 ± 1.1)⋅10(8) M(-1) s(-1)) and possibly, for dissolved organic carbon higher than 5 mg C L(-1), reaction with the triplet states of chromophoric dissolved organic matter ((3)CDOM*). The modelled photochemical half-life time of APAP in environmental waters would range from days to few weeks in summertime, which suggests that the importance of phototransformation might be comparable to biodegradation. APAP transformation by the main photochemical pathways yields hydroxylated derivatives, ring-opening compounds as well as dimers and trimers (at elevated concentration levels). In the case of (3)CDOM* (for which the triplet state of anthraquinone-2-sulphonate was used as proxy), ring rearrangement is also hypothesised. Photochemistry would produce different transformation products (TPs) of APAP than microbial biodegradation or human metabolism, thus the relevant TPs might be used as markers of APAP photochemical reaction pathways in environmental waters.

Degradation of typical N-nitrosodimethylamine (NDMA) precursors and its formation potential in anoxic-aerobic (AO) activated sludge system

N-nitrosodimethylamine (NDMA) is an emerging disinfection byproduct. Removal of its potential precursors is considered as an effective method to control NDMA. In this study, four typical NDMA precursors (dimethylamine (DMA), trimethylamine (TMA), dimethylformamide (DMFA) and dimethylaminobenzene (DMAB)) were selected, and their removal capacities by activated sludge were investigated. Batch experiments indicated that removal of NDMA precursors was better under aerobic condition than anoxic condition; and their specific degradation rates follow the order of DMA > TMA > DMFA > DMAB. In anoxic-aerobic (AO) activated sludge system, the optimal hydraulic retention time and sludge retention time were 10 h and 20 d, respectively, for the removal of both NDMA precursors (four selected NDMA precursors and NDMA formation potential (NDMA FP)) and nutrients. Our results also suggested that there was a positive correlation between NDMA FP and dissolved organic nitrogen (DON) in wastewater. The removal efficiency of NDMA FP was in the range of 46.8-72.5% in the four surveyed wastewater treatment plants except the one which adopted chemically enhanced primary process. The results revealed that the AO system had the advantage of removing NDMA FP. Our results are helpful for the knowledge of the removals of NDMA precursors during activated sludge treatment processes.

Reinvestigation on the ozonation of N-nitrosodimethylamine: Influencing factors and degradation mechanism

N-nitrosodimethylamine (NDMA), as a new disinfection byproduct, is a potential carcinogen. In this study, we focused on the role of ozone in NDMA degradation. We characterized the removal efficiency, influencing factors and degradation mechanism. Our results demonstrated that ozonation was an efficient process for NDMA degradation. The removal efficiency was affected by initial NDMA concentration; higher NDMA dosing required higher ozone utilization. NDMA oxidation was favored at high ozone dosage and high pH. NDMA ozonation under various pH as well as hydroxyl radical (OH) inhibition experiments verified that OH generated from ozone dominated NDMA oxidation. The main products of NDMA ozonation were methylamine (MA), dimethylamine (DMA), nitromethane (NM) and ammonium (AM). Their yields changed with the amount of ozone provided. A NDMA ozonation mechanism was proposed. It is suggested that NDMA degradation is induced by OH attacking through any of four pathways, with oxygen involving in the oxidation process. MA generation was due to OH attacking on amine nitrogen and methyl group. DMA formation was related to OH attacking on nitrosyl nitrogen via a parallel pathway. We speculate that supersaturated dissolved oxygen by ozone decomposition is responsible for NM generation by further oxidation of MA and DMA. AM formation may be favored due to MA degradation under OH exposure.

The formation of reactive species having hydroxyl radical-like reactivity from UV photolysis of N-nitrosodimethylamine (NDMA): kinetics and mechanism

This study focuses on the detailed mechanism by which N-nitrosodimethylamine (NDMA) is photolyzed to form oxidized products, i.e., NO(2)(-) and NO(3)(-), and reveals a key reactive species produced during the photolysis of NDMA. Under acidic conditions, NO(2)(-) formed from the photodecomposition of NDMA was more prevalent than NO(3)(-). In this result, key species for the formation of NO(2)(-) are presumably N(2)O(3) and N(2)O(4) as termination products as well as NO and O(2) as reactants. Conversely, under alkaline conditions, NO(3)(-) was more prevalent than NO(2)(-). For this result, a key species for NO(3)(-) formation is presumably peroxynitrite (ONOO(-)). A detailed mechanistic study was performed with a competition reaction (or kinetics) between NDMA and p-nitrosodimethylaniline (PNDA) probe for hydroxyl radical (OH). It is fortuitous that the second-order rate constant for NDMA with an unknown reactive species (URS) was 5.13×10(8) M(-1) s(-1), which was similar to its published value for the reaction of NDMA+OH. Our study results showed that a key reactive species generated during NDMA photo-decomposition had hydroxyl radical-like reactivity and in particular, under alkaline conditions, it is most likely ONOO(-) as a source of nitrate ion. Therefore, for the first time, we experimentally report that an URS having OH-like reactivity can be formed during photochemical NDMA decomposition. This URS could contribute to the formations of NO(2)(-) and NO(3)(-).

Effects of visible and UV light on the characteristics and properties of crude oil-in-water (O/W) emulsions

The effects of visible and UV light on the characteristics and properties of Prudhoe Bay (PB) and South Louisiana (SL) emulsions were investigated to better understand the role of sunlight on the fate of spilled crude oils that form emulsions with a dispersant in the aquatic environment. Before irradiation, crude oil emulsions showed the presence of dispersed crude oil micelles in a continuous water phase and crude oil components floating on the surface. The crude oil micelles decreased in size with irradiation, but emulsions retained their high degree of polydispersity. UV irradiation reduced the stability of emulsions more effectively than visible light. The reduction of micelles size caused the viscosity of emulsions to increase and melting point to decrease. Further, irradiation increased acid concentrations and induced ion formation which lowered the pH and increased the conductivity of emulsions, respectively. Ni and Fe in PB emulsions were extracted from crude oil with UV irradiation, which may provide an efficient process for metal removal. The emulsions were stable toward freeze/thaw cycles and their melting temperatures generally decreased with irradiation. Evidence of ˙OH production existed when emulsions were exposed to UV but not to visible light. The presence of H(2)O(2) enhanced the photodegradation of crude oil. Overall, the changes in emulsion properties were attributed to direct photodegradation and photooxidation of crude oil components.

Effect of surface fluorination on the electrochemical and photoelectrocatalytic properties of nanoporous titanium dioxide electrodes

Titanium dioxide is a widely used photocatalyst whose properties can be modified by fluoride adsorption. This work is focused on the effect of surface fluorination on the electrochemical and photoelectrocatalytic properties of TiO(2) nanoporous thin films. Surface fluorination was achieved by simple addition of HF to the working solution (pH 3.5). Open circuit potential as well as ex situ XPS measurements verify that surface modification takes place. Fluorination triggers a significant capacitance increase in the accumulation potential region, as revealed by dark voltammetric measurements for all the TiO(2) samples studied. The photoelectrocatalytic properties (measured as photocurrents under white light illumination) depend on the substrate being oxidized and, in some cases, on the nature of the TiO(2) sample. In particular, the results obtained for electrodes prepared with a mixed phase (rutile + anatase) commercial nanopowder (PI-KEM) indicate that the processes mediated by surface trapped holes, such as the photooxidation of water or methanol, are accelerated while those occurring by direct hole capture from the adsorbed state (formic acid) are retarded. The photooxidation of catechol and phenol is also enhanced upon fluorination. In such a case, the effect can be rationalized on the basis of a diminished recombination and a surface displacement of both the oxidizable organic substrates and the poisoning species formed as a result of the organics oxidation. Photoelectrochemical and in situ infrared spectroscopic measurements support these ideas. In a more general vein, the results pave the way toward a better understanding of the photocatalysis phenomena, unravelling the importance of the reactant adsorption processes.

Performance evaluation of the UV/H2O2 process on selected nitrogenous organic compounds: reductions of organic contents vs. corresponding C-, N-DBPs formations

The presence of various organic contaminants in water sources is of concern due to their direct threats to human health and potential to react with disinfectants to form carcinogenic byproducts including trihalomethanes, haloacetic acids and nitrosamines in finished water. This study applied both medium-pressure and low-pressure ultraviolet light coupled with hydrogen peroxide (UV/H2O2) to evaluate its efficacy for degradation of selected nitrogenous organic compounds and corresponding disinfection byproduct (DBP) formation. Six organic compounds were chosen as target precursors based on their nitrogen contents and molecular structures. The results showed that higher oxidation capacity resulted in better reduction of organic matters and DBP formation potentials (DBPFPs). However, insufficient contact time and oxidant doses could lead to a rise of DBPFPs in the early stages of UV/H2O2 reactions. A greater percentage removal was achieved for organic carbon than organic nitrogen after UV/H2O2 treatment, especially for compounds with complicated structure such as diltiazem. During the UV/H2O2 treatment, the intermediate products include tertiary amine, dimethyl amine (DMA) or DMA-like structures, which are N-nitrosodimethylamine (NDMA) precursors after chlorination or chloramination. Furthermore, it was observed that using dissolved organic nitrogen and DMA to predict NDMAFP could lead to biased conclusions because of the complex nature of nitrogenous matters in aqueous environments.

Nitrosamines and water

This paper provides an overview of all current issues that are connected to the presence of nitrosamines in water technology. N-nitrosodimethylamine (NDMA) is the most frequently detected member of this family. Nitrosamines became the hottest topic in drinking water science when they were identified as disinfection by-products (DBPs) in chloraminated waters. The danger that they pose to consumer health seems to be much higher than that from chlorinated DBPs. This review summarizes our contemporary knowledge of these compounds in water, their occurrence, and precursors of nitrosamines in drinking and wastewaters, in addition to attempts to remove nitrosamines from water. The paper also reviews our knowledge of the mechanisms of nitrosamine formation in water technology. The current, commonly accepted mechanism of NDMA formation during chloramination of drinking waters assumes that dichloramine reacts with dimethylamine, forms unsymmetrical dimethylhydrazine and further oxidizes to NDMA. The question to answer is which precursors are responsible for delivering the DMA moiety for the reaction since the presence of DMA in water cannot explain the quantities of NDMA that are formed. There are also reports that other oxidants that are commonly used in water technology may generate NDMA. However, the mechanisms of such transformations are unknown. Methods for the removal of nitrosamines from water are described briefly. However, the research that has been undertaken on such methods seems to be at an early stage of development. It is predicted that photolytic methods may have the greatest potential for technological application.

Cellular uptake and mutagenic potential of metal oxide nanoparticles in bacterial cells

Extensive production and consumption of nanomaterials such as ZnO and TiO(2) has increased their release and disposal into the environment. The accumulation of nanoparticles (NPs) in ecosystem is likely to pose threat to non-specific targets such as bacteria. The present study explored the effect of ZnO and TiO(2) NPs in a model bacterium, Salmonella typhimurium. The uptake of ZnO and TiO(2) bare NPs in nano range without agglomeration was observed in S. typhimurium. TEM analysis demonstrated the internalization and uniform distribution of NPs inside the cells. Flow cytometry data also demonstrates that both ZnO and TiO(2) NPs were significantly internalized in the S. typhimurium cells in a concentration dependent manner. A significant increase in uptake was observed in the S. typhimurium treated even with 8 and 80 ng mL(-1) of ZnO and TiO(2) NPs with S9 after 60 min, possibly the formation of micelles or protein coat facilitated entry of NPs. These NPs exhibited weak mutagenic potential in S. typhimurium strains TA98, TA1537 and Escherichia coli (WP2uvrA) of Ames test underscoring the possible carcinogenic potential similar to certain mutagenic chemicals. Our study reiterates the need for re-evaluating environmental toxicity of ZnO and TiO(2) NPs presumably considered safe in environment.

Electrochemical destruction of N-nitrosodimethylamine in reverse osmosis concentrates using Boron-doped diamond film electrodes

Boron-doped diamond (BDD) film electrodes were use to electrochemically destroy N-nitrosodimethylamine (NDMA) in reverse osmosis (RO) concentrates. Batch experiments were conducted ito investigate the effects of dissolved organic carbon (DOC), chloride (Cl(-)), bicarbonate (HCO(3-) and hardness on rates of NDMA destruction via both oxidation and reduction. Experimental results showed that NDMA oxidation rates were not affected by DOC, Cl(-), or HCO(3-) at concentrations present in RO concentrates. However, hydroxyl radical scavenging at 100 mM concentrations of HCO(3-) and Cl(-) shifted the reaction mechanism of NDMA oxidation from hydroxyl radical mediated to direct electron transfer oxidation. In the 100 mM Cl(-) electrolyte experimental evidence suggests that the in situ production of ClO(3)(.)also contributes to NDMA oxidation. Density functional theory calculations support a reaction mechanism between ClO(3)(.) and NDMA, with an activation barrier of 7.2 kJ/mol. Flow-through experiments with RO concentrate yielded surface area normalized first-order rate constants for NDMA (40.6 +/- 3.7 L/m(2) h) and DOC (as C) (38.3 +/- 2.2 L/m(2) h) removal that were mass transfer limited at a 2 mA/cm(2) current density. This research shows that electrochemical oxidation using BDD electrodes has an advantage over other advanced oxidation processes, as organics were readily oxidized in the presence of high HCO(3-) concentrations.

Fabrication of porous TiO2 film on Ti foil by hydrothermal process and its photocatalytic efficiency and mechanisms with ethyl violet dye

Most of commercial dyes and pigments have rather complicated polyaromatic chemical structures with prolonged lifetime surviving in the Mother Nature. However, TiO(2) has been reported as one of the best photocatalytic candidates for degrading dye pollutants. In this report, TiO(2) film/Ti foil was prepared by hydrothermal reaction in alkali solution, the porous TiO(2) film with microcrystalline structure has been obtained. The porous structure of TiO(2) film was analyzed and characterized by XRD, FE-SEM and XPS. This is the first report that demonstrates that TiO(2) film/Ti foil has an excellent commercial application potential for photocatalytic degradation of Ethyl Violet (EV). Especially, because of refluxing at 100 degrees C, the porous TiO(2) film structure remained undisturbed, and EV decomposed in the period of 20 h. In addition, porous TiO(2)-mediated EV photo degradation mechanism has been proposed, as intermediates are isolated and clearly identified by GC-MS and HPLC-PDA-ESI-MS.

Authors' response to comments on "Inhibiting the regeneration of N-nitrosodimethylamine in drinking water by UV photolysis combined with ozonation" by F. Xiao

Previous published paper "Inhibiting the regeneration of N-nitrosodimethylamine (NDMA) in drinking water by UV photolysis combined with ozonation" by our research group, was commented by Dr. Xiao. This comment was criticized and doubted from several aspects including the research topic, experimental design and interpretation of data. We thanked Dr. Xiao for the useful suggestion for our future research. However, we do not fully agree with other comments. In this letter, we now take the opportunity of responding, some issues are discussed here in more detail.

Comparison of photodegradative efficiencies and mechanisms of Victoria Blue R assisted by Nafion-coated and fluorinated TiO2 photocatalysts

The purposes of this research were to study the effects of two modified photocatalysts, Nafion-coated TiO(2) and fluorinated TiO(2), and photocatalytic degradation of Victoria Blue R in aqueous solution. Photocatalytic degradation of Victoria Blue R was accelerated by the modified photocatalysts. Bulk and surface characterizations of the resulting powders were carried out. Attachment of the anions to the TiO(2) surface using the Nafion-coated-TiO(2) possibly results in increased adsorption of the cationic dye, and the degradation rate is larger for the cationic dye. It was found that Victoria Blue R on the two illuminated TiO(2) surfaces underwent very different changes. To obtain a better understanding on the mechanistic details of this modified-TiO(2)-assisted photodegradation of the Victoria Blue R dye with UV irradiation, a large number of intermediates of the process were separated, identified, and characterized by a high-performance liquid chromatography-mass spectrometry technique. Several probable photodegradation pathways were proposed and discussed.

Electrochemical oxidation of N-nitrosodimethylamine with boron-doped diamond film electrodes

This research investigated NDMA oxidation by boron-doped diamond (BDD) film electrodes. Oxidation rates were measured as a function of electrode potential, current density, and temperature using rotating disk and flow-through reactors. Final NDMA reaction products were carbon dioxide, ammonium, and nitrate, with dimethylamine and methylamine as intermediate products. Reaction rates were first-order with respect to NDMA concentration and surface area normalized oxidation rates as high as 850 +/- 50 L/m(2)-hr were observed at a current density of 10 mA/cm(2). The flow-through reactor yielded mass transfer limited reaction rates that were first-order in NDMA concentration, with a half-life of 2.1 +/- 0.1 min. Experimental evidence indicates that NDMA oxidation proceeds via a direct electron transfer at potentials >1.8 V/SHE with a measured apparent activation energy of 3.1 +/- 0.5 kJ/mol at a potential of 2.5 V/SHE. Density functional theory calculations indicate that a direct two-electron transfer can produce a stable NDMA((+2)) species that is stabilized by forming an adduct with water. The transfer of two electrons from NDMA to the electrode allows an activation-less attack of hydroxyl radicals on the NDMA((+2)) water adduct. At higher overpotentials the oxidation of NDMA occurs by a combination of direct electron transfer and hydroxyl radicals produced via water electrolysis.

Change of adsorption modes of dyes on fluorinated TiO2 and its effect on photocatalytic degradation of dyes under visible irradiation

Surface-fluorinated TiO2 (F-TiO2) particles were prepared via the HF etching method. The surface characteristics of fluorinated TiO2, the adsorption modes of dyes, and the reaction pathways for the photocatalytic degradation of dye pollutants under visible light irradiation were investigated. It was found that, in the treatment of TiO2 by HF etching, F(-) not only displaces surface HO(-) but also substitutes some surface lattice oxygen. Using zwitterionic Rhodamine B (RhB) dye as a model, the change of the adsorption mode of RhB on F-TiO2 relative to that on pure TiO2 was validated by adsorption isotherms, X-ray photoelectron spectroscopy (XPS), and IR techniques for the first time. RhB preferentially anchors on pure TiO2 through the carboxylic (-COOH) group, while its adsorption group is switched to the cationic moiety (-NEt 2 group) on F-TiO2. Both the photocatalytic degradation kinetics and mechanisms were drastically changed after surface fluorination. Dyes with positively charged nitrogen-alkyl groups such as methylene blue (MB), malachite green (MG), Rhodamine 6G (Rh6G), and RhB all underwent a rapid N-dealkylation process on F-TiO2, while on pure TiO2 direct cleavage of dye chromophore ring structures predominated. The relationship between surface fluorination and the degradation rate/pathway of dyes under visible irradiation was also discussed in terms of the effect of fluorination on the surface adsorption of dyes and on the energy band structure of TiO2.

Detoxification of phytotoxic compounds by TiO2 photocatalysis in a recycling hydroponic cultivation system of asparagus

TiO 2 photocatalytic decomposition and detoxification of phytotoxic compounds released by the roots of asparagus ( Asparagus officinalis L.) were investigated from the viewpoint of conservation-oriented cultivation. The phytotoxically active fraction was extracted either from dried asparagus roots or from the recycled nutrient solution of an asparagus hydroponic cultivation system. We found that the phytotoxic activity gradually decreased in the fraction with TiO 2 powder under irradiation with ultraviolet (UV) light at an intensity of 1.0 mW/cm (2). The growth of asparagus plants under actual cultivation conditions was also investigated by comparing asparagus grown in a hydroponic system where recycled waste nutrient solution was photocatalytically treated with solar light and a system with untreated recycled waste nutrient solution. The results showed, as measured by growth indices such as stem length and stem thickness, that asparagus growth in the photocatalytically treated system was superior to the untreated one. Furthermore, the yield of asparagus spears was 1.6-fold greater in the photocatalytically treated system, demonstrating the detoxification effect on the phytotoxic compounds and also the killing effect on pathogenic microorganisms.