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Photodegradation behavior and mechanism of dibutyl phthalate in water under flood discharge atomization
Abstract
Flood discharge atomization is a prevalent hydraulics phenomenon in reservoir scheduling operations, however, its effect on the migration and transformation behavior of pollutants has not been examined. In this study, the behaviors and mechanisms of the direct photodegradation of dibutyl phthalate (DBP) in atomized water and the indirect photodegradation of DBP in the presence of ferric ions and nitrate were investigated. The results showed that the photodegradation rate of DBP was accelerated under atomization conditions by sunlight irradiation. The photodegradation efficiency of DBP in the presence of ferric ions and nitrate under atomization conditions was increased by 2.20 times and 1.82 times compared with no-atomization conditions, respectively. The quencher experiments indicated that the main active species for DBP photodegradation in the presence of ferric ions were hydroxyl radicals (·OH) and superoxide radicals (·O2-) with atomization, while the main active species in the presence of nitrate were ·OH, ·O2- and electrons (e-). In addition, the differences were found in the photodegradation products and pathways of DBP between with and without atomization treatment. In the presence of ferric ions, the benzene ring of DBP was opened to produce fumaric acid, while phthalic acid bis(4-hydroxybutyl) ester was produced in the presence of nitrate under atomization conditions. The results of this study provide a scientific basis for assessing the effect of water conservancy projects on the migration and transformation behaviors of pollutants, which is of great theoretical significance and scientific value.
... As a result, their release into the aquatic environment significantly raises ecotoxicological dangers and environmental hazards Xiang et al., 2019), making it critical and urgent to develop effective techniques to eradicate or change DBP, which is also one of the noble tasks of researchers. At present, there are studies on the degradation of DBP through adsorption (Jiang et al., 2018;Liu & He, 2020;Nozari et al., 2022), chemical oxidation Yao et al., 2020), biological methods (Sarathy et al., 2011), photocatalysis (Lin et al., 2023;Wang et al., 2019), and UV/H 2 O (Wang et al., 2016), which have achieved certain removal efficiencies and made inferences on the related reaction mechanisms and degradation pathways. However, these technologies have limitations such as high operating costs, low removal efficiency, and susceptibility to secondary pollution in actual operation. ...
This study aims to solve the discharge of printing and dyeing reverse osmosis concentrate (PDROC) from a local printing and dyeing factory in Hangzhou, China. Electrochemical technique was used to treat the organic content of the concentrated wastewater. At the same time, the concentration levels of the target chemical species were double-checked using liquid–liquid extraction-gas chromatography–mass spectrometry and the UV–Vis spectroscopy. The findings revealed that the contaminants in the wastewater were primarily consisted of semi-volatile organic compounds such as long-chain alkanes, aromatics, and phthalates, with the concentration of dibutyl phthalate (DBP) being 64.4 µg/L. Decent removal efficiencies of chemical oxygen demand (COD) and DBP were observed after 3-h electro-degradation, which reached 73.6% and 45.8%, respectively. Additionally, the increment of current density and initial pH showed positive effects on the DBP degradation. Additionally, high current density and low starting pH are beneficial to DBP degradation. Furthermore, the by-products were analyzed by their MS spectrums to better understand the electro-degradation process of DBP, which primarily includes phthalic acid, ethane-1,1,2-tricarboxylic acid, hydroquinone, and oxalic acid, implying that the hydroxyl radicals (·OH) produced by electrolysis oxidized DBP in this solution via three proposed degradation pathways.
Dibutyl phthalate (DBP), a recognized endocrine-disrupting contaminant, poses substantial environmental challenges due to its persistence, resistance to degradation, and potential for bioaccumulation. Hence, the development of efficient removal technologies capable...
Reactions at the interface between water and other phases play important roles in nature and in various chemical systems. Although some experimental and theoretical studies suggest that chemical reactions at water interfaces can be different from those in bulk water—for example, ‘on-water catalysis’ and the activation of photochemically inert fatty acids at the air–water interface upon photoexcitation—directly investigating these differences and generating molecular-level understanding has proved difficult. Here, we report on the direct probing of a photochemical reaction occurring at the air–water interface, using ultrafast phase-sensitive interface-selective nonlinear vibrational spectroscopy. The femtosecond time-resolved data obtained clearly show that the photoionization reaction of phenol proceeds 10⁴ times faster at the water surface than in the bulk aqueous phase (upon irradiation with photons with the same energy). This finding demonstrates that photochemical reactions at water interfaces are very different from those in bulk water, reflecting distinct reaction environments at the interface.
Photolysis and photocatalysis of typical phthalic acid esters (dimethyl phthalate, DMP; diethyl phthalate, DEP; dibutyl phthalate, DBP) were carried out in UV, UV/TiO2, and UV-Vis/Bi2WO6 systems. All of the selected phthalic acid esters and their decomposition byproducts were subjected to qualitative and quantitative analysis through HPLC and GC-MS. The results of 300 min of photolysis and photodegradation reaction were that each system demonstrated different abilities to remove DMP, DEP, and DBP. The UV/TiO2 system showed the strongest degradation ability on selected PAEs, with removal efficiencies of up to 93.03, 92.64, and 92.50% for DMP, DEP, and DBP in 90 min, respectively. UV-Vis/Bi2WO6 had almost no ability to remove DMP and DEP. However, all of the systems had strong ability to degrade DBP. On the other hand, the different systems resulted in various byproducts and PAE degradation pathways. The UV system mainly attacked the carbon branch and produced o-hydroxybenzoates. No ring-opening byproducts were detected in the UV system. In the photocatalytic process, the hydroxyl radicals produced not only attacked the carbon branch but also the benzene ring. Therefore, hydroxylated compounds and ring-opening byproducts were detected by GC-MS in both the UV/TiO2 and UV-Vis/Bi2WO6 photocatalytic systems. However, there were fewer products due to direct hole oxidation in the UV-Vis/Bi2WO6 system compared with the UV/TiO2 system, which mainly reacted with the pollutants via hydroxyl radicals.
Significance
Water is considered to be a stable and relatively inert molecule in bulk solution. We report an exceptional behavior of water: Water molecules are spontaneously oxidized to form hydrogen peroxide near the water−air interface of micron-sized water droplets. This process does not require any chemical reagent, catalyst, applied electric potential, or radiation. Only pure water in the form of microdroplets in air is necessary for the appearance of hydrogen peroxide. We suggest that this discovery opens various innovative opportunities including green and inexpensive production of hydrogen peroxide, green chemical synthesis, safe cleaning, and food processing.
This paper summarizes the development of hydro-projects in China, blended with an international perspective. It expounds major technical progress toward ensuring the safe construction of high dams and river harnessing, and covers the theorization of uneven non-equilibrium sediment transport, inter-basin water diversion, giant hydro-generator units, pumped storage power stations, underground caverns, ecological protection, and so on.
By combining the results of prototype observation of flood discharge atomization at the Wujiangdu Hydropower Station, and by adopting the serial model test method, the model scale effect was examined, the influences of the Reynolds and Weber numbers of water flow on the rain intensity of flood discharge atomization were analyzed and a rain intensity conversion relation was established. It is demonstrated that the level of atomization follows the geometric similarity relations and it is possible to ignore the influence of the surface tension of the flow when the Weber number is greater than 500. Despite limitations such as incomplete data sets, it is undoubtedly helpful to study the scale effect of atomization flow, and it is beneficial to identify the rules of the model test results in order to extrapolate to prototype prediction.
Serious human health concerns have been recently raised from daily use of face masks, due to the possible presence of hazardous compounds as the phthalic acid esters (PAEs). In this study, the content of 11 PAEs in 35 commercial masks was assessed by applying a specific and accurate method, using Gas Chromatography/Mass Spectrometry. Surgical, FFP2 and non-surgical models, for both adults and children were collected from the Italian market. Analyses showed that four of the target analytes were detected in all tested samples with median total concentrations ranging between 23.6 mg/kg and 54.3 mg/kg. Results obtained from the experimental analysis were used in the risk assessment studies carried out for both carcinogenic and non-carcinogenic effects. Doses of exposure (Dexp) of PAEs ranged from 6.43 ×10-5 mg/kg bw/day to 1.43 ×10⁻² mg/kg bw/day. Cumulative risk assessment was performed for non-carcinogenic and carcinogenic effects. No potential risk was found for non-carcinogenic effects, yet the 20% of the mask samples showed potential carcinogenic effects for humans. A refined exposure assessment was performed showing no risk for carcinogenic effects. This paper presents a risk assessment approach for the identification of potential risks associated to the use of face masks.
Phthalate esters (PAEs) are ubiquitous environmental contaminants, arising growing public concern. Nevertheless, information on the exposure and risks of PAEs in wildlife remains limited. Here, we conducted the first investigation of the occurrences, spatiotemporal trends, and potential risks of twelve metabolites of PAEs (mPAEs) in 74 humpback dolphins from the northern South China Sea during 2005–2020. All twelve mPAEs (∑12mPAEs: 9.6–810.7 ng g⁻¹ wet weight) were detected in the dolphin liver, and seven major mPAEs showed increasing trends during the study period, indicating high PAE contamination in the coastal environment of South China. Monoethylhexyl phthalate accounted for over half of the ∑12mPAE concentrations. The accumulation of mPAEs in the dolphins was neither age-dependent nor sex-specific. Compared to parent PAEs, mPAEs generally induced higher agonistic effects on the dolphin peroxisome proliferator-activated receptor alpha/gamma (PPARA/G) as master regulators of lipid homeostasis. Although short-term in vitro assays revealed no significant activation of dolphin PPARA/G by tissue-relevant doses of mPAEs, long-term in vivo evidence (i.e., correlations between hepatic mPAEs and blubber fatty acids) suggested that chronic exposure to mPAEs might have impacted lipid metabolism in the dolphin. This study highlighted the potential health risks of PAE exposure on marine mammals.
Paired handwipe and urine samples were collected from adult (n = 130) and child (n = 82) residents of a typical urban community in southern China to examine relationships between external and internal exposure as well as the contribution of dermal absorption to the exposure of phthalates. The concentrations and composition profiles of phthalates were similar in handwipes from both adults and children, and contained mainly di-2-ethylhexyl phthalate (DEHP), di-n-butyl phthalate (DnBP) and di-iso-butyl phthalate (DiBP), consistent with profiles of phthalates in air and dust. The major metabolites of these phthalates, i.e., mono-n-butyl phthalate (mnBP) from DnBP, mono-iso-butyl phthalate (miBP) from DiBP and three metabolites of DEHP (namely mEHP, mEHHP and mEOHP) were widely detected in paired urine samples. Positive correlations were found between contamination levels of DiBP and DnBP in handwipes and their corresponding urinary metabolites, whereas no significant correlation was observed for DEHP. This suggests that dermal absorption might be an important exposure pathway particularly for low molecular weight phthalates. Our study shows that dermal absorption is a non-negligible exposure pathway for phthalates, to which children are particularly sensitive since the contribution of dermal uptake to the internal exposure of phthalates was higher in children than adults.
Fatty acids have been proposed to be a potential source of precursors for SOAs, but the autoxidation process was neglected in the oxidation studies. Here, the autoxidation of oleic acid was explored using microdroplet mass spectrometry. Bulk solution, concentration and solvent composition experiments provided direct evidences for that the autoxidation occurred at or near the air-water interface. The kinetic data showed an acceleration at this interface and was comparable to ozonation, indicating that autoxidation is an important pathway for SOAs formation. In addition, intermediates/products were captured and identified using tandem mass spectrometry, spin-trapping and quenched agents. The autoxidation mechanism was divided into addition intermediates (AIs) and Criegee intermediates (CIs) pathways mediated by hydroxyl radicals (OH). The CI chemistry which is ubiquitous in gas phase was observed at the air-water interface, and this leaded to the mass/volume loss of aerosols. Inversely, the AI chemistry caused the increase of mass, density and hygroscopicity of aerosols. AI chemistry was dominated compared to CI chemistry, but varied by concerning aerosol sizes, ultraviolet light (UV) and charge. Moreover, the MS approach of selectively probing the interfacial substances at the scale of sub-seconds opens new opportunities to study heterogeneous chemistry in atmosphere.
Mass transfer and oxidant utilization are perhaps two of the most critical issues in sulfate radical (SO4•−) based advanced oxidation technologies (AOTs) and their scaled-up implementation. In this study, we propose using hydrodynamic cavitation (HC), considered a green, effective method, to promote both mass transfer and oxidant utilization in zero-valent iron (Fe⁰) activated persulfate (PS) system. Whilst the BET surface area of Fe⁰ was increased by 8 times after HC treatment, concentration of Fe²⁺ derived from Fe⁰ oxidation is greatly increased for effective PS activation. The reappearance of Fe⁰ and Fe²⁺ after cavitation ensured a good reusability of the catalyst. Likewise, the impact of pH revealed that TC adsorption on catalyst at acidic pH favored its degradation compared with that at higher pH. With respect to oxidant utilization, it is observed that PS even at a high dosage (2.8 mM) was completed converted within 30 min in the HC-Fe⁰/PS system. According to SEM, TEM, and BET analysis, we conclude that the microjets induced by cavitation bubbles or direct abrasion by HC agitation have contributed to the removal of hydroxide/oxide layers on the Fe⁰ surface, thus reactivating its catalytic activity. Given these reasons, we observed up to 97.80% removal of Tetracycline (TC), the model pollutant, with a synergistic coefficient as high as 2.62. After confirming SO4•− as the most dominant reactive species, five degradation pathways of TC were proposed given the intermediate evidence from LC-MS/MS analysis and density functional theory (DFT) calculations. Results from this study could provide new insights into the role of HC on PS activation and shed light on the potential implementation of the SO4•−-based AOTs for scaled-up wastewater treatments.
There has been a surge of interest in interfacial hypochlorous acid (HOCl) chemistry for indoor air quality and public health. Here we combined nanoelectrospray mass spectrometry (nESI-MS) and acoustic levitation (AL) techniques to study the chlorination chemistry of three model lipids (DPPE, POPG, DOPG) mediated by HOCl at the air-water interface of levitated water droplet. For DPPE with no CC double bonds, HOCl was insensitive to the alkane chains, and showed considerable delay directing to head amino groups compared to that in aqueous environment. Chlorination chemistry, for POPG and DOPG with CC double bonds, preferentially reacted with double bonds of one chain. The mechanism was discussed in light of these observations, and it is concluded that the increased hydrophilicity of the chlorinated chain disturbed the lipid packing and attracted it toward the water phase. In addition, the reaction rate constant and reactive uptake coefficient suggested that the chlorination of lipids exposed to HOCl at the air-water interface is likely to occur rapidly. These results gain the knowledge of HOCl mediated lipid interface reaction at the molecule level, and would better understand the adverse health effects associated with elevated indoor pollutants.
Oxygenation of the reduced species has been regarded as the major source for hydroxyl radical (HO•) generation in aquatic environments. Yet, the O2-induced formation of HO• in lake sediments during the flooding/drought transformation process remained largely unexplored. In this study, two types of sediments from Wucheng (WC) and Nanji (NJ) area in Lake Poyang, China, were collected, respectively, with the burst of HO• derived by flooding/drought transformation process exploring via the incubation experiments. Results showed that no obvious HO• can be detected for the two sediments during the flooding period, while the concentrations of HO• increased rapidly for the flooding/drought transformation process due to the enhanced dissolved oxygen contents. The highest concentrations of HO• in the surface sediment were 2.45±0.19 μmol kg⁻¹ for WC sediment and 0.69±0.25 μmol kg⁻¹ for NJ sediment, showing higher burst potential of HO• for the former. The contents of Fe(II) in the surface sediments for WC area (589.3±37.29 mg kg⁻¹) were about two times higher than those for NJ area (308.4±94.01 mg kg⁻¹) during the flooding period. Oxygenation of the surface Fe(II) contributed significantly to the burst of HO• in the flooding/drought transformation process. Moreover, the higher percentage of humic-like substances in WC sediment indicated that the dissolved humic fraction exhibited also important role in the HO• formation due to electrons transfer under redox conditions. This study highlighted the importance of reactive reduced species in manipulating the burst of HO• in lake sediment, which is essential for understanding the geochemical cycling of several major and trace elements as well as the behavior and fate of the contaminants in aquatic ecosystems.
Reservoirs play a crucial role in the drinking water supply for solving the uneven distribution of water resources. Reservoir bays, which serve as direct source areas and initial collection areas of pollutants, are crucial areas for drinking water safety. Systematic observations of heavy metal pollution in reservoir bays are required and urgent under different hydrological regulation. Water samples were collected from 65 bays in the Danjiangkou Reservoir which is the water source of the world’s longest trans-basin water diversion project during the storage and flood discharge period from 2015 to 2019. We investigated the temporal and spatial variation in eight dissolved heavy metals, chromium (Cr), manganese (Mn), nickel (Ni), copper (Cu), zinc (Zn), arsenic (As), cadmium (Cd), and lead (Pb), and assessed their human health risks. The driving factors of high-risk metals were identified by constructing a structural equation model (SEM). The results showed that the concentrations of heavy metals decreased after 5 years of reservoir operation and varied significantly (p < 0.05) between the storage period and flood discharge period. The concentrations of Cr, Mn, Cd and Pb were higher in the storage period, but the other four metals (Ni, Cu, Zn, As) had the reverse trend. Notably, the As, Mn, and Pb in individual bays exceeded the water quality standard threshold. The human health risk assessment indicated that only As posed a potential carcinogenic risk (1.29E-04) in the flood discharge period. Land use and water physicochemical properties were the main factors affecting As concentrations, and they explained 32% and 44% of the variability of As in the storage and flood discharge periods, respectively. This study provided critical insights into the periodic dynamics and driving factors of the concentration of heavy metals in reservoir bays, and formulated effective management measures for water quality management.
The inactivation mechanism of pathogenic microorganisms in water needs to be comprehensively explored in order to better guide the development of an effective and green disinfection method for drinking water safety. Here, metal-free modified g-C3N4 was prepared and used to inactivate two typical bacteria (namely, Gram-positive E. coli and Gram-negative S. aureus) in water under visible light from a comparative perspective. These two bacteria could be inactivated in the presence of modified g-C3N4 within 6 h of visible light, but their inactivation kinetics were quite different. E. coli were inactivated slowly in the early disinfection stage and rapidly in the later disinfection stage, whereas S. aureus were inactivated steadily during the entire disinfection process. Moreover, the impacts of important water parameters (pH, salt, temperature, and water matrix) on photocatalytic inactivation of E. coli and S. aureus were also distinct. In addition, scavenger experiments indicated that superoxide radicals played the most important role in E. coli inactivation, while both superoxide and hydroxyl radicals were important for S. aureus inactivation. Quantitative changes in fatty acids, potassium ions, proteins and DNA of the bacterial suspensions suggested that the higher resistance of E. coli in the early inactivation stage could be originated from the difference in the phospholipid repair system in cell membrane structures. This study can provide new insights into research and development of a safe and effective disinfection technology for drinking water.
Chemistry on water is a fascinating area of research. The surface of water and the interfaces between water and air or hydro-phobic media represent asymmetric environments with unique properties that lead to unexpected solvation effects on chemi-cal and photochemical processes. Indeed, the features of interfacial reactions differ, often drastically, from those of bulk-phase reactions. In this perspective, we focus on photoinduced oxidation reactions, which have attracted enormous interest in recent years because of their implications in many areas of chemistry, including atmospheric and environmental chemistry, biology, electrochemistry, and solar energy conversion. We have chosen a few representative examples of photoinduced oxidation reactions to focus on in this perspective. Although most of these examples are taken from the field of atmospheric chemistry, they were selected because of their broad relevance to other areas. First, we outline a series of processes whose photochemis-try generates hydroxyl radicals. These OH precursors include reactive oxygen species, reactive nitrogen species, and sulfur dioxide. Second, we discuss processes involving the photooxidation of organic species, either directly or via photosensitiza-tion. The photochemistry of pyruvic acid and fatty acid, two examples that demonstrate the complexity and versatility of this kind of chemistry, is described. Finally, we discuss the physicochemical factors that can be invoked to explain the ki-netics and thermodynamics of photoinduced
Discharge from high dam causes total dissolved gas (TDG) supersaturation, which dissipates slowly along the downstream river and induces fish bubble disease. The dissipation process is closely related to the hydraulic condition. In most TDG prediction models, the hydraulic condition is lumped in a parameter called the dissipation coefficient (k), whose determination usually contains large uncertainty. In this study, the property of k was experimentally investigated under three water depths and three flow velocities. The results demonstrated that the k value increased with v/H ratios. Based on the experimental results, an equation was proposed to calculate k value. The equation was validated by the observed data from field survey in Xiangjiaba Reservoir in the Jinshajiang River, the upstream of Yangtze River. Meanwhile, the proposed equation was compared to two typical equations, a dissolved oxygen (DO) reaeration equation and an empirical equation, using the same datasets. The comparisons indicated that DO reaeration equation was not suitable for k value estimation, particularly under shallow water conditions. The limitation of empirical equation was that the k value was determined by data from a specific case so that was not applicable to other conditions. The proposed equation showed best performance in calculating k value and hence estimating the TDG levels, especially under the condition of low sediment concentration. In addition, the proposed equation was more generically applicable compared to the existing formulas. Our findings could help to draw up governing measures to minimize the risk of supersaturated TDG and achieve river ecological restoration.
The nitrate pollution of surface water has attracted worldwide attention, and it is not optimistic in China. To identify the distribution, sources, and transformation mechanisms of nitrate in China's surface waters, the nitrate data of 71 major rivers from 7 regions were systematically collected. The spatial distribution of nitrate concentrations in surface water was analyzed, and the main nitrate pollution sources were revealed based on nitrogen and oxygen isotopes of nitrate. The results show that approximately 7.83% of rivers in China exceeded the Chinese drinking water standard for nitrate (45 mg·L-1). The concentrations of nitrate in Mudanjiang, Haihe River, and the estuary of Yangtze River was even more than 90 mg·L-1, which indicates a serious pollution phenomenon. The isotopic compositions of surface water in China ranged from -23.5‰ to 26.99‰ for δ15N-NO3 and from -12.7‰ to 83.5‰ for δ18O-NO3. The main sources of nitrate are domestic sewage in Northeast, Central, and East China, while those are inorganic fertilizers and domestic sewage in Southwest and South China. The nitrate sources of surface water in Northwest and North China are complex, mainly from domestic sewage, inorganic fertilizer, and nitration of soil organic matter. Through correlation analysis, it is found that the nitrate concentrations of surface water have a positive relationship with population, wastewater discharge, agricultural nitrogen fertilizer application, and GDP per capita in China. It is urgent to solve the problem of pollution and prevent the further pollution of China's surface waters. The new "10-Point Water Plan" issued by the Chinese government solved the previous problems, but it will take decades to control and repair the polluted surface waters. In Northwest, North China, Southwest, and South China, not only the scale of sewage treatment plants in cities and counties should be increased but also the use of agricultural fertilizers should be controlled and managed by the government. Northeast, Central, and East China need to further control point source pollution and reduce the discharge of industrial wastewater and domestic sewage into rivers.
We introduced superoxide as potassium superoxide (KO2) to artificial lake water containing dissolved organic matter (DOM) without or with introduced ferric iron complexes (DOM-Fe), and monitored the production rate of hydroxyl radicals as well as changes in the absorption and fluorescence properties of DOM. The introduction of KO2 decreased the absorption by DOM but increased the spectral slope coefficient of DOM more with complexed ferric Fe than without it. The introduction of KO2 increased the fluorescence of humic-like components in DOM without introduced ferric Fe but resulted in the loss of fluorescence in DOM with introduced ferric Fe. A single introduction of 13 μmol L⁻¹ KO2 produced 10 μmol L⁻¹ and 104 μmol L⁻¹ hydroxyl radicals during a week-long experiment without and with the introduced DOM-Fe complexes, respectively. The production rate of hydroxyl radicals decreased exponentially with time but levelled off and continued several days in DOM with introduced ferric Fe. These findings suggest that in the presence of DOM-Fe complexes, superoxide can trigger an autocatalytic Fenton reaction that produces hydroxyl radicals and breaks down DOM.
Rivers are considered the most important channel for microplastics traveling to the ocean from inland. The Yellow River is one of a main microplastics source for the Bohai Sea. As hydropower station operations—especially the artificial flood pulse—could significantly affect the transportation processes of microplastics, it is necessary to investigate the transportation regulation of microplastics from the Yellow River to the estuary. In this study, the distribution of microplastics (across 10 sections of the river from Xiaolangdi to the estuary) was detected at three stages (the pre-, inter-, and post-artificial flooding season). The result showed that the microplastic concentrations in the Yellow River ranged from 0.105 to 1.17 items m⁻². The main microplastic shapes found were foam and film, and the main components were expanded polystyrene and polyethylene. The average particle sizes were 1101.54, 484.93, and 509.46 μm at the three stages, respectively. The artificial flood accelerated the transport of microplastics from inland to the estuary and ocean; it also unified the overall particle size of microplastics along the river. The microplastics in the surface water of the study area likely originate mostly upstream of the Xiaolangdi Hydropower Station. The artificial flooding generated from water and sediment regulation transports a large amount of microplastics in the reservoirs and river channels to the estuary and offshore areas in a short time period via artificial flooding.
Flood discharge atomization is a phenomenon of water fog diffusion caused by the discharge of water from a spillway structure, which brings strong wind and heavy rainfall. These unnatural winds and rainfall are harmful for the safe operation of hydropower stations with high water heads. Compared to the method of prototype observations, physical models and mathematical models, which are semi-theoretical and semi-empirical, numerical simulation methods have the advantage of being not limited by a similar scale and are more economical. A finite element model is presented to simulate flood discharge atomization based on water–air two-phase flow in this paper. Equations governing flood discharge atomization are composed of partial differential equations of mass and momentum conservation laws with unknowns for pressure, velocity and the water concentration. The finite element method is used to solve the governing equations by adopting appropriate solution strategies to increase the convergence and numerical stability. Then, the finite element model is applied to a practical project, the Shuibuya hydropower station, which experienced a flood discharge in 2016. Simulation results show that the proposed model can simulate flood discharge atomization with efficient convergence and numerical stability in three dimensions, and good agreement was observed between numerical simulations and prototype observational data. Based on the simulation results, the mechanism of flood discharge atomization was analyzed.
The present study aimed to model and optimize the dimethyl phthalate (DMP) degradation from aqueous solution using UVC/ Na2S2O8/Fe2+ system based on the response surface methodology (RSM). A high removal efficiency (97%) and TOC reduction (64.2%) were obtained under optimum conditions i.e. contact time = 90 min, SPS concentration = 0.601 mM/L, Fe2+ = 0.075 mM/L, pH = 11 and DMP concentration = 5 mg/L. Quenching experiments confirmed that sulfate radicals were predominant radical species for DMP degradation. The effect of CO3− on DMP degradation was more complicated than other aquatic background anions. The possible pathway for DMP decomposition was proposed according to HPLC and GC–MS analysis. The average oxidation state (AOS) and carbon oxidation state (COS) values as biodegradability indicators demonstrated that the UVC/SPS/Fe2+ system can improve the bioavailability of DMP over the time. Finally, the performance of UVC/SPS/Fe2+ system for DMP treatment in different aquatic solutions: tap water, surface runoff, treated and raw wastewater were found to be 95.7, 88.5, 80.5, and 56.4%, respectively.
Graphical abstract
CeO2-AgI, synthesized via depositing AgI nanoparticles onto CeO2 nanorods, was utilized for bacterial disinfection and organic contaminant degradation. Escherichia coli (E. coli) and Bisphenol A (BPA) were used as the model bacteria and emerging organic contaminant to test the photocatalytic activity of CeO2-AgI, respectively. Results showed that CeO2-AgI with the optimal AgI content exhibited superior photocatalytic activity over pure CeO2 or AgI for both inactivation of E. coli cells and BPA removal. However, the photocatalytic mechanisms for E. coli inactivation and BPA degradation were different. Specifically, the photo-generated holes (h+), photo-generated electrons (e-) and superoxide radicals (·O2-) were the dominated active species for E. coli inactivation, whereas, BPA degradation relied on the generation of ·O2- and e-. Cell membrane disruption was found to be the main disinfection mechanism. The decomposition of BPA was clarified by detecting the degradation intermediates by LC-MS and DFT calculation. The facile synthesized CeO2-AgI exhibited good photocatalytic stability in four reused cycles and thus could be potentially applied to purify water.
Iron and manganese are both redox-sensitive elements and important chemical indicators of drinking water quality. The anaerobic hypolimnion caused by thermal stratification results in the release of iron and manganese from sediments in water-supply reservoirs. The exceeding concentration of them may have a great impact on urban water supply. Little research has focused on the relationship between suspended sediment behaviors and the mobilization of iron and manganese in the water-sediment interface of reservoirs. To demonstrate the seasonal variation of iron and manganese and the influence of suspended matter on its behavior in the Biliuhe Reservoir, we comprehensively investigated and analyzed the vertical distribution characteristic of iron and manganese in water, suspended sediments, and sediments from February 2014 to February 2015. According to the results of Spearman correlation analysis, the concentration of iron is highly correlated with total suspended solids, total nitrogen, and total phosphorus, while, the concentration of manganese is significantly correlated with total suspended solids, dissolved oxygen, pH and total nitrogen. Further analysis results indicate that the thermal stratification, the anaerobic hypolimnion, and the resuspension of sediments significantly affect the concentration of iron and manganese in the Biliuhe Reservoir. The average dissolved manganese concentration accumulates on the bottom layer to 0.18 mg/L in the stratified season, while the dissolved iron releases very little from the sediment. The manganese concentration of the surface and middle layers increase to 0.07 mg/L in the mixing season. Because the average concentration of particulate iron in the bottom layer is approximately 0.3 mg/L, the resuspension of sediments may be the main iron source in the bottom layer of the Biliuhe Reservoir. In addition, the flocculated particulate matter and its adsorbed manganese could stay in the reservoir for a long time, so that the content of manganese in the suspended matter is about 7 times higher than that in the surface sediments. Thus, to reduce the impact of elevated iron and manganese concentrations on water supply in the Biliuhe Reservoir, the most effective management practices focused on multi-level intake, hypolimnetic aeration, and amalgamated waterworks, which could ensure the safety of urban water supply in Dalian.
Pyruvic acid (PA) exists in fogs, aerosols and clouds. The photochemistry-driven reaction pathways of PA in the aqueous phase are more elusive than the gas phase. The PA photochemical process may occur in the bulk liquid phase and at the air-liquid interface in ambient conditions. We conducted two sample preparation methods to simulate two possible scenarios: the air-liquid interface and the bulk liquid phase under photolysis. Time-of-flight secondary ion mass spectrometer (ToF-SIMS) was used to analyze samples because of its high sensitivity and mass accuracy in surface analysis. Both negative and positive ion mode mass spectra provide complementary information of the products under different reaction conditions. Spectral principal component analysis (PCA) is used to determine similarities and differences among various samples. The air-liquid interface facilitates more radical reactions and form higher molecular weight compounds (HMWCs) more quickly than the bulk liquid phase, which mainly has non-radical reactions such as anhydride reactions and decarboxylation reactions. Our results show that interfacial chemistry plays an important role in atmospheric scenarios. Moreover, different types of secondary organic aerosols (SOAs) are formed, suggesting the strong influence of interfacial photochemistry has on the earth atmosphere.
Iron (Fe) and manganese (Mn) are redox-sensitive elements that are both important chemical indicators of water quality. To study their vertical distribution characteristics in south-subtropical reservoirs during summer, Fe and Mn concentrations in the water column, as well as the physical and chemical conditions of the water, were investigated in the lacustrine zones of nine reservoirs in the eastern of Guangdong Province, in July 2016. These results showed that Fe and Mn exhibited a strong concentration gradient in deep reservoirs due to thermal stratification. The total Fe (TFe), total Mn (TMn), dissolved Fe (DFe), and dissolved Mn (DMn) concentrations in the surface water are significantly lower than in these in bottom zones (TFe, F=6.708, P=0.032; TMn, F=9.720, P=0.014; DFe, F=8.129, P=0.029; DMn, F=11.125, P=0.016). The average concentrations of TFe and TMn are 0.399 mg·L⁻¹ and 0.422 mg·L⁻¹, respectively at the bottom five of the deep reservoirs, and the dissolved ions accounted for more than 70% of this concentration. In shallow reservoirs without thermal stratification, TFe, TMn, DFe and DMn concentrations in surface water are lower than those measured at bottom of the reservoir, no significant difference was detected (TFe, F=0.135, P=0.726; TMn, F=0, P=1; DFe, F=0.006, P=0.943; DMn, F=0.007, P=0.936). The average concentrations of TFe and TMn are 0.110 mg·L⁻¹ and 0.089 mg·L⁻¹, respectively at the bottom of the four shallow reservoirs, and they were present predominantly in particulate form. The concentrations of TFe and DFe correlated significantly (P<0.05) with the dissolved oxygen (DO) concentration, pH and depth, but did not significantly correlate with TN or TP concentration in the deep reservoirs. The gradient distributions of DO and pH levels is critical to the gradient distribution of Fe and Mn in deep reservoirs because of water stratification. Low DO concentrations and acidic conditions in these areas contributed to the release of Fe and Mn from sediment and result in high dissociation of these elements in the bottom of deep reservoirs. Simultaneously, high concentrations of Fe and Mn at the bottom of reservoirs may also attribute to the characteristic red soil of this region. The results from this study imply that the high concentrations of Fe and Mn common in the bottom of deep reservoirs is associated with the stable stratification of these reservoirs in the summer. The concentrations of Fe and Mn may be exceed water standards when deep water is taken for public water provision. The problem may be avoided with surface water supplying.
Nano-α-Fe2O3 can accelerate the degradation of diethyl phthalate ester (DEP) in the presence of citric acid under UV (300–400 nm) irradiation, but the underlying mechanism remains unclear. In this study, based on ultraviolet photoelectron spectra (UPS) and density functional theory (DFT) analyses, it was found that thermodynamically, a lower bandgap energy (Ebandgap = ECB-EVB < 5.8 eV) of the complex of CA-α-Fe2O3 predicted its higher catalytic ability than citric acid (Ebandgap = ELUMO-EHOMO = 7.0 eV). X-ray photoelectron spectroscopy (XPS) study indicated that citric acid was chemically adsorbed on the surface of α-Fe2O3 through its carboxyl group by filling the surface oxygen vacancies of α-Fe2O3 (disappearance of Fe2p3/2 peak at 711.5 eV). UV light induced a succession of reactions which resulted in heterogeneous photo-Fenton-like reactions that produced ·OH for DEP degradation. The dissolved oxygen was proposed to be the initial oxidant forming the reactive oxygen species. The solution pH, citric acid and α-Fe2O3 concentration substantially influenced DEP degradation. The method in this study can be further applied to study other organic acids and metal oxides catalysts.
After the impoundment of the Three Gorges Reservoir (TGR), the hydrological situation of the reservoir has changed greatly. The concentration and distribution of typical persistent organic pollutants in water and sediment have also changed accordingly. In this study, the concentration, distribution and potential sources of 16 polycyclic aromatic hydrocarbons (PAHs) and 6 phthalic acid esters (PAEs) during the water drawdown and impoundment periods were investigated in water and sediment from the TGR. According to our results, PAHs and PAEs showed temporal and spatial variations. The mean ΣPAH and ΣPAE concentrations in water and sediment were both higher during the water impoundment period than during the water drawdown period. The water samples from the main stream showed larger ΣPAH concentration fluctuations than those from tributaries. Both the PAH and PAE concentrations meet the Chinese national water environmental quality standard (GB 3838-2002). PAH monomers with 2-3 rings and 4 rings were dominant in water, and 4-ring and 5-6-ring PAHs were dominant in sediment. Di-. n-butyl phthalate (DBP) and di-2-ethylhexyl phthalate (DEHP) were the dominant PAE pollutants in the TGR. DBP and DEHP had the highest concentrations in water and sediment, respectively. The main source of PAHs in water from the TGR was petroleum and emissions from coal and biomass combustion, whereas the main sources of PAHs in sediments included coal and biomass combustion, petroleum, and petroleum combustion. The main source of PAEs in water was domestic waste, and the plastics and heavy chemical industries were the main sources of PAEs in sediment.
Photodegradation of nonylphenol tri-ethoxylate (NPEO3) in aqueous solution, and the effects of Fe(III) or Fe(II) were studied. The increasing degradation kinetics of NPEO3 were observed when 500µM Fe(III) or Fe(II) was present in the solutions. Altered formation of NPEO oligomers with shorter EO chains, including nonyphenol (NP), NPEO1 and NPEO2, was observed in water and in solutions containing Fe(III) or Fe(II). The molar percentage yields of NP and NPEO1,2 production from NPEO3 photodegradation were approximately 20% in NPEO3 solution, while NPEO3 solution with Fe(III), this percentage increased to approximately 50%. In solution with Fe(II), the molar balance between the photodegradation of NPEO3 and the production of NP and NPEO1,2 was observed. A luminescent bacterium, Vibrio fischeri, was used to identify changes in the toxicity of NPEO3 solutions during the photodegradation process under different conditions, while dose addition (DA) model was used to estimate the toxicity of products. Toxicity of NPEO3/water solution increased significantly following the irradiation of UVA/UVB mixture. In contrast, obviously decreasing toxicity was observed when NPEO3 underwent photodegradation in the presence of Fe(III).
Dimethyl phthalate (DMP) as one of the most important and extensively used Phthalic acid esters (PAEs) is known to likely cause dysfunctions of the endocrine systems, liver, and nervous systems of animals. In this paper, the degradation and behavior of DMP were investigated in a laboratory scale anaerobic/anoxic/oxic (AAO) treatment system. In addition, a degradation model including biodegradation and sorption was formulated so as to evaluate the fate of DMP in the treatment system, and a mass balance model was designed to determine kinetic parameters of the removal model. The study indicated that the optimal operation condition of HRT and SRT for DMP and nutrients removal were 18 h and 15 d respectively, and the degradation rates of anaerobic, anoxic and aerobic zones for DMP were 13.4%, 13.0% and 67.7%, respectively. Under the optimal conditions, the degraded DMP was 73.8%, the released DMP in the effluent was 5.8%, the accumulated DMP was 19.3%, and the remained DMP in the waste sludge was 1.1%. Moreover, the degradation process of DMP by acclimated activated sludge was in accordance with the first-order kinetics equation. The model can be used for accurately modeling the degradation and behavior of DMP in the AAO system.
Hydroxyl radicals (•OH) play a crucial role in the fate of redox-active substances in the environment. Studies of the •OH production in nature has been constrained to surface environments exposed to light irradiation, but is overlooked in the subsurface under dark. Results of this study demonstrate that abundant •OH is produced when subsurface sediments are oxygenated under fluctuating redox conditions at neutral pH values. The cumulative concentrations of •OH produced within 24 h upon oxygenation of 33 sediments sampled from different redox conditions are 2‒670 μmol •OH per kg dry sediment or 6.7‒2521 μM •OH in sediment pore water. Fe(II)-containing minerals, particularly phyllosilicates, are the predominant contributor to •OH production. This production could be sustainable when sediment Fe(II) is regenerated by the biological reduction of Fe(III) during redox cycles. Production of •OH is further evident in a field injection-extraction test through injecting oxygenated water into a 23-m depth aquifer. The •OH produced can oxidize pollutants such as arsenic and tetracycline, and contribute to CO2 emissions at levels that are comparable with soil respiration. These findings indicate that oxygenation of subsurface sediments is an important source of •OH in nature that has not previously been identified, and •OH-mediated oxidation representing an overlooked process for substance transformations at the oxic/anoxic interface.
Lake circulation is an important phenomenon that ensures oxygenation of the water column. Here we report that aeration of anoxic hypolimnion water causes production of highly reactive hydroxyl radicals (OH), which are also produced photochemically in the epilimnion. Model calculations suggest that the dark process of OH generation can be comparable with photochemical reactions in some lake environments, provided that the hypolimnion is a significant fraction of the whole lake volume. In these cases, lake overturn could significantly contribute to the yearly OH budget of the lake water and might cause significant degradation of some pollutants, for which the reaction with OH is an important removal process from surface waters.
Copyright © 2015 Elsevier B.V. All rights reserved.
Photodegradation of dimethyl phthalate (DMP) in aqueous solutions by Fe(III)-pyruvate complex system was preliminarily investigated. The influences such as light sources, initial pH value, initial concentration of Fe(III), pyruvate and DMP on photodegradation efficiency of DMP were discussed in detail. The result indicates that DMP could be decomposed efficiently in Fe(III)-pyruvate system. The degradation efficiency of DMP are dependent on initial pH value, Fe (III) initial concentration and pyruvate initial concentration. The optimum pH for photodegration of DMP is 3.0. The degradation efficiency of DMP increases with increase of the initial concentrations of Fe(III) or pyruvate, whereas decreases with increase of the initial concentrations of DMP. Various light sources including metal halide lamps, daylight lamps, UV disinfection lamps and sunlight can be adopted in the system.
Significance
Ozone is one of the most important atmospheric trace gases in Earth's atmosphere. It can interact with water in the gas phase as well as with water in cloud droplet surfaces. This work identifies unique spectral signatures for ozone adsorbed at the surface of cloud water droplets in the UV and visible light domains. The adsorption process itself is thermodynamically spontaneous. With this information, it is found that the photochemistry of ozone at the air–water interface is a significant previously unidentified source of OH radicals generated at the surface of clouds. The broader implication is that the surface of cloud water droplets can be an active chemical reactor that contributes to the oxidizing capacity of the troposphere on a global scale.
This paper gives an overview of the main reactive transient species that are produced in surface waters by sunlight illumination of photoactive molecules (photosensitizers), such as nitrate, nitrite, and chromophoric dissolved organic matter (CDOM). The main transients (.OH, CO3-., 1O2, and CDOM triplet states) are involved in the indirect phototransformation of a very wide range of persistent organic pollutants in surface waters. Shine a light on your chemistry! This paper gives an overview of the reactive transient species that are produced in surface waters by sunlight irradiation of photoactive molecules (photosensitizers), such as nitrate, nitrite, and chromophoric dissolved organic matter (CDOM) (see scheme). The main transient species (.OH, CO3-., 1O2, and CDOM triplet states) are involved in the indirect phototransformation of a very wide range of organic pollutants in surface waters.
The esters of phthalic acid are considered as hazardous pollutants due to their mutagenicity, carcinogenicity and are also classified as endocrine disruptor chemicals. Several compounds of this class of substances for decades, and probably even now, were used as softeners in water-based synthetic paintings. Surfaces and structures, such as house walls painted with phthalates based paintings, can be a concern to construction workers engaged in demolition, restore and paint removal activities if they are not protected from hazardous dust inhalation. In this paper we report the results of an investigation about phthalate esters degradation by direct UV irradiation at 254 nm. The results of kinetic parameters for PAEs photodegradation are reported and it shows that k values for the single PAEs ranged from 0.221 to 0.737 h- 1.
Moreover, the results indicate that photolysis is a successful way to remediate PEAs contaminated on mural painting.
The degradation of chlortoluron photoinduced by Fe(III) aquacomplexes has been investigated under continuous irradiation at 365 nm.
The determined quantum yields of chlortoluron disappearance give evidence for the involvement of the most photoactive species Fe(OH)2+,
which leads to the formation of hydroxyl radicals and Fe(II); the importance of oxygen in the degradation process was also demonstrated.
According to the nature of the obtained photoproducts, the attack of •OH radicals on two sites of chlortoluron are favoured: the aromatic
ring and the methyl group of the urea function. A homogenous photocatalytic process based on the oxidation of Fe(II) into Fe(III) is
responsible for the continuous production of hydroxyl radicals in such a system. This photocatalytic cycle allows the total mineralisation
of compounds like chlortoluron using conditions compatible with a safe aquatic environment.
Experiments are conducted to determine the effect of a cage of water molecules on the photolysis quantum yields of nitrate, FeOH2+, and H2O2. Results suggest that the quantum yields of nitrate and FeOH2+ are decreased by the recombination of photo-fragments ( OH + NO2 and Fe2+ + OH, respectively) before they leave the surrounding cage of water molecules. However, no evidence is found for an enhanced quantum yield for H2O2. Therefore, the photolysis of nitrate and FeOH2+ could be enhanced if the cage of the solvent molecules is incomplete, as is the case at the air–water interface of atmospheric droplets. The photolysis rate constant distribution within nitrate, FeOH2+, and H2O2 aerosols is calculated by combining the expected quantum yield data in the bulk and at the interface with Mie theory calculations of light intensity. The photolysis rate constant of nitrate and FeOH2+ would be significantly higher at the surface than in the bulk if quantum yields are enhanced at the surface. In the case of H2O2, the photolysis rate constant would be enhanced by surface accumulation. The results concerning the expected rates of photolysis of these photoactive species are applied to the assessment of the reaction between benzene and OH in the presence of OH scavengers in an atmospherically relevant scenario. For a droplet of 1 μm radius, a large fraction of the total OH-benzene reaction (15% for H2O2, 20% for nitrate, and 35% for FeOH2+) would occur in the surface layer, which accounts for just 0.15% of the droplet volume.
Juvenile chinook salmon, Oncorhynchas tshawytscha, were exposed to selected levels of dissolved atmospheric gas ranging from 100 (control) to 120% of saturation, and surviving fish were then tested for maximal swimming performance.Decreased swimming capability resulted from exposure to concentrations ranging from 106 to 120% of saturation if the fish were tested immediately; other tests indicated that recovery of swimming capabilities occurred within 2 hr if the fish were returned to equilibrated water (100% of atmospheric saturation) before testing.
Dimethyl phthalate, DMP, and diethyl phthalate, DEP. are rather photoslable ( ( -phthalate) ≤ 0.03). Exposure of neat DMP and DEP in the presence of air to high absorbed doses of UV light yielded a variety of products, many of them identifiable by GC-MS analysis. The products were divided into two fractions by ether extraction. Typical products contained in the ether-soluble fraction of irradiated DMP and DEP are: The ether-insoluble fraction consists of oligomeric compounds (molar mass: several 103 g mol−1) that are characterized by a high degree of substitution at the aromatic rings and by the existence of conjugated double bonds that act as chromophoric groups absorbing light in the visible wavelength range. Reaction pathways based on the primary cleavage of bonds next to the carbonyl groups and leading to the following radicals are proposed:
The rigin and fate of six phthalate esters (dimethyl phthalate (DMP), diethyl phthalate (DEP), di-n-butyl phthalate (DnBP), butyl benzyl phthalate (BBP), di (2-ethylhexyl) phthalate (DEHP) and di-n-octyl phthalate (DnOP)), were investigated during 2005 and 2006 in the densely populated Seine river estuary. Four compounds, DMP, DEP, DnBP and DEHP were detected at all the stations with DEHP (160–314 ng L−1), followed by DEP (71–181 ng L−1) and next DnBP (67–319 ng L−1), except at la Bouille, where DnBP was the second most important compound. BBP and DnOP concentrations remained low and were not found at all the stations. Considering all six phthalates, Caudebec-en-Caux (beginning of the salinity gradient) was the least polluted station (464 ng L−1), whereas Honfleur (771 ng L−1) and La Bouille (716 ng L−1) displayed the highest contamination levels, probably related to important industrial plants. From Caudebec-en-Caux to Honfleur (maximum turbidity), variation of DEHP concentration was related to that of suspended matter. In addition, the salinity rise in that area might have facilitated DEHP sorption upon particles. A significant correlation between flow magnitude and DEHP concentration was found (P < 0·01, n = 12), supporting the influence of the hydrological cycle upon contamination. Runoff contribution (56·9 kg d−1) to river contamination was confirmed by the annual evolution of phthalate concentrations in the Seine river at Poses. Concentrations of DEHP in the tributaries were in the same range as those of the Seine River (100–350 ng L−1), except for two in densely populated and industrialized areas: Robec (800 ng L−1) and Cailly (970 ng L−1). The treatment plant discharge fluxes were in the same range as those of tributaries (30·4–250 g d−1). During high flow periods, the influence of tributaries and of treatment plants seemed to play a minor part in the contamination level of the Seine river estuary. Copyright © 2009 John Wiley & Sons, Ltd.
Keywords: Review Phthalic acid esters PAEs in the environment PAEs removal Water treatments Endocrine disrupting chemicals a b s t r a c t This article describes the most recent methods developed to remove phthalic acid esters (PAEs) from water, wastewater, sludge, and soil. In general, PAEs are considered to be endocrine disrupting chemicals (EDCs), whose effects may not appear until long after exposure. There are numerous methods for removing PAEs from the environment, including physical, chemical and biological treatments, advanced oxidation processes and combinations of these techniques. This review largely focuses on the treatment of PAEs in aqueous solutions but also reports on their treatment in soil and sludge, as well as their effects on human health and the environment.
The degradation of dibutyl phthalate (DBP) photoinduced by Fe(III) in aqueous solution has been investigated under monochromatic irradiation and sunlight. Hydroxyl radicals OH, responsible of the degradation, are formed via an intramolecular photoredox process in excited Fe(III) aquacomplexes. The concentration in Fe(OH)2+ in the starting Fe(III) solution appears to be a controlling parameter of the degradation rate, as already stated in our previous works. The first step of the decomposition of DBP involves the hydrogen abstraction on the butyl chain mainly on the carbon in α-position to aromatic ring. The major primary photoproducts are hydroxy, dihydroxy and carboxylic derivatives. For prolonged irradiations, DBP and its photoproducts are completely mineralized due to the regeneration of the absorbing species and the continuous formation of OH radicals that confers a catalytic aspect to the process. Consequently, the degradation photoinduced by Fe(III) could be an efficient method of DBP removal from water.