Photocatalytic degradation of Acid Red 4 using a titanium dioxide membrane supported on a porous ceramic tube

A photocatalytic membrane supported on a porous ceramic tube was described, in which permeation of solutes through the membrane and tube and photocatalytic reaction occur simultaneously. In this photocatalytic membrane reactor, TiO(2) catalyst was coated on the surface of a porous ceramic tube and all experiments were conducted in one pass dead-end system. The objectives of this study are to demonstrate the predominance of dead-end operation and to determine the reaction kinetics model of the photocatalytic reaction. Acid Red 4 (AR 4) dye was used as a model pollutant. A detailed study of physical parameters including flow configurations (dead-end and cross-flow), flow rate, initial dye concentration, light intensity and catalyst loading has been performed to obtain the reaction kinetics. The simultaneous effect of light intensity and catalyst loading was also determined experimentally. Experiments were also conducted to compare the photocatalytic degradation of AR 4 in the dead-end and cross-flow system. The major findings of this study are: (1) the decomposition ratios for dead-end system were three and five times higher than cross-flow system at flow rates of 6.67x10(-8) and 4.00x10(-7)m(3)/s, respectively. (2) The decomposition ratio increased with increasing catalyst loading and light intensity, but remained constant at higher catalyst loading. (3) The decomposition ratio was found to be decreased with increasing flow rate.     

Photocatalytic degradation of methyl orange by TiO2-coated activated carbon and kinetic study

TiO2-coated activated carbon (AC) grain (TiO2/AC) was prepared through hydrolytic precipitation of TiO2 from Tetrabutylorthotitanate and following heat treatment. The TiO2/AC was characterized by BET, SEM, XRD and optical absorption spectroscopy. The samples were employed as catalysts for methyl orange photocatalytic oxidation degradation in aqueous suspension, used as probe reaction. The kinetics of methyl orange photodegradation was analyzed. The results indicate that BET surface area of TiO2-coated ACs decreased drastically in comparison with the original AC with increasing TiO2 coatings by more than 1 doped cycle. Nano-TiO2 particles were dispersed on the AC with the size of 20-40 nm. Crystalline TiO2 doped onto AC was from anatase to rutile with increase of heat-treatment temperature. The TiO2/AC was shown high photoactivity for the photodegradation of methyl orange (MO) dyestuff in aqueous solution under UV irradiation. The kinetics of photocatalytic MO dyestuff degradation was found to follow a pseudo-first-order rate law. It was observed that the presence of the AC enhanced the photoefficiency of the titanium dioxide catalyst. Different amount of TiO2 coatings induced different increases in the apparent first-order rate constant of the process. The kinetic behavior could be described in terms of a modified Langmuir-Hinshelwood model. The values of the adsorption equilibrium constants for the organic molecules, KC, and for the rate constants, kc, were certainly dependent on TiO2 content. At 47wt% TiO2 coatings with the highest rate constant, the KC and kc was 0.1116l mmol(-1) and 0.1872 mmol l(-1) min(-1), respectively. The mechanism of methyl orange degradation was discussed in terms of the titanium dioxide photosensitization by the AC.     

Photocatalytic oxidation of aqueous ammonia in model gray waters

This study investigated the TiO2 photocatalytic degradation of aqueous ammonia (NH4+/NH3) in the presence of surfactants and monosaccharides at pH approximately 10.1. Initial rates of NH4+/NH3 photocatalytic degradation decreased by approximately 50-90% in the presence of anionic, cationic, and nonionic surfactants and monosaccharides. Through correlation analysis, we concluded that scavenging of hydroxyl radical (.OH) by the products of surfactant/monosaccharide photocatalytic degradation, including carbonate and formate, could explain approximately 80% of the variance in initial rates of NH4+/NH3 removal in our system. Addition of a supplemental .OH source (H2O2) enhanced the rate of NH4+/NH3 degradation in the presence of the surfactant Brij 23 lauryl ether (Brij 35), further supporting the idea that .OH scavenging is the mechanism by which surfactants and monosaccharides decreased initial rates of NH4+/NH3 photocatalytic degradation. Despite slowed rates of NH4+/NH3 degradation, both surfactants/monosaccharides and NH4+/NH3 were removed by TiO2 photocatalysis, indicating that this process can effectively remove both carbonaceous and nitrogenous biochemical oxygen demand from gray water.     

Photocatalytic degradation of benzenesulfonate on colloidal titanium dioxide

Titanium dioxide-mediated photocatalyzed degradation of benzenesulfonate (BS) was investigated by monitoring chemical oxygen demand (COD), total organic carbon (TOC) content, sulfate concentration, pH as well as the absorption and emission spectral changes in both argon-saturated and aerated systems. Liquid chromatography-mass spectrometry analysis was utilized for the detection of intermediates formed during the irradiation in the UVA range (λ_max = 350 nm). The results obtained by these analytical techniques indicate that the initial step of degradation is hydroxylation of the starting surfactant, resulting in the production of hydroxy- and dihydroxybenzenesulfonates. These reactions were accompanied by desulfonation, which increases [H⁺] in both argon-saturated and aerated systems. In accordance with our previous theoretical calculations, the formation of ortho- and meta-hydroxylated derivatives is favored in the first step. The main product of the further oxygenation of these derivatives was 2,5-dihydroxy-benzesulfonate. No decay of the hydroxy species occurred during the 8-h irradiation in the absence of dissolved oxygen. In the aerated system much more efficient desulfonation and hydroxylation, moreover, a significant decrease of TOC took place at the initial stage. Further hydroxylation led to cleavage of the aromatic system, due to the formation of polyhydroxy derivatives, followed by ring fission, resulting in the production of aldehydes and carboxylic acids. Total mineralization was achieved by the end of the 8-h photocatalysis. It has been proved that in this photocatalytic procedure the presence of dissolved oxygen is necessary for the cleavage of the aromatic ring because hydroxyl radicals photochemically formed in the deaerated system too alone are not able to break the C-C bonds.     

Photocatalytic oxidative degradation of acridine orange in aqueous solution with polymeric metalloporphyrins

In the presence of polymeric M [meso-tetra (4,4'-biphenybisulfony) phenylporphyrin] (PMTBPBSOPP, M = Co, Mn, Cu, Zn, Fe), the photocatalytic oxidative degradation of acridine orange (AO) was investigated under conditions of irradiation with high-pressure mercury lamp (HPML), iodine tungsten lamp (ITL) and natural sunlight (NSL), respectively. The results indicated that PCoTBPBSOPP had remarkable effects on photodegradation of AO under NSL irradiation. The experiments also proved that the photocatalytic degradation of AO in aqueous solutions could be enhanced when hydrogen peroxide 0.4 g/L was added. When the initial concentration of AO was 43.8 mg/L, the degradation and the decolorization rates of AO under HPML irradiation were up to 98% and 97%, respectively. A number of factors affecting photocatalytic degradation effects, such as the initial concentration of AO, light sources, pH etc., were studied in detail. The experimental results confirmed that polymeric metalloporphyrins could fully decompose AO by using artificial light source such as HPML and ITL, moreover could degrade AO rapidly under NSL in 3 h. Definitely, polymer of metalloporphyrins in the heterogeneous photocatalysis phase had a potential application     

Carbon-sensitized and nitrogen-doped TiO2 for photocatalytic degradation of sulfanilamide under visible-light irradiation

A novel carbon-sensitized and nitrogen-doped TiO₂ (C/N-TiO₂) was synthesized by a facile sol-gel method using titanium butoxide as both titanium precursor and carbon source, and nitric acid as nitrogen source. The calcination temperature had a great effect on the crystal phase structure, nitrogen incorporation into the TiO₂ lattice and content of carbonaceous species. The incorporated carbonaceous species could serve as photosensitizer, while the nitrogen doping could lead to the remarkable red shift of absorption edge of C/N-TiO₂. The C/N-TiO₂ calcinated at 300 °C (T300) exhibited the highest photocatalytic activity for sulfanilamide (SNM) degradation under irradiation of visible-light-emitting diode (vis-LED). The SNM photocatalytic degradation and mineralization were more efficient in acidic conditions due to the carbon photosensitizing effect. Insignificant inhibitory effects were observed in the presence of chloride, nitrate and sulfate, while bicarbonate, phosphate and silica could inhibit the SNM mineralization to different degrees. Acetate, ammonium and sulfate were released during SNM mineralization. T300 exhibited good photochemical stability and could be reused for 5 times with less than 10% decrease in the SNM removal efficiency. The acute toxicity of SNM solution could be reduced over prolonged photocatalysis according to the Microtox assay.     

Electro-photocatalytic degradation of acid orange II using a novel TiO2/ACF photoanode

A novel photoanode was prepared by immobilizing TiO₂ film onto activated carbon fibers (TiO₂/ACF) using liquid phase deposition (LPD) to study the electro-photocatalytic (EPC) degradation of organic compounds exemplified by an azo-dye, namely, Acid Orange II (AOII). Results demonstrated that by applying a 0.5 V bias (vs. SCE) across the TiO₂/ACF electrode, the AOII degradation rate was increased significantly compared to that of photocatalytic (PC) oxidation. The application of an electric field promotes the separation of photogenerated electrons and holes as confirmed by electrochemical impedance spectroscopy (EIS) measurements. The structural and surface morphology of the TiO₂/ACF electrode was characterized by field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD). SEM images showed that TiO₂ was deposited on almost every carbon fiber with an average thickness of about 200 nm with the inner space between neighboring fibers being maintained unfilled. The morphological features of the photo-anode facilitated the passage of solution as well as UV light through the felt-form electrode and created a three-dimensional environment favorable to EPC oxidation. Both the large outer surface area of the 3D electrode and the good organic adsorption capacity of the ACF support promoted high contact efficiency between AOII and TiO₂ surface. Anatase was the major crystalline TiO₂ deposited. UV-vis spectrophotometry, TOC (total organic carbon) analysis, and HPLC technique were used to monitor the concentration change of AOII and intermediates as to gain insight into the EPC degradation of AOII using the TiO₂/ACF electrode.     

Photocatalytic degradation of Acid Red 88 using Au-TiO(2) nanoparticles in aqueous solutions

Metal loaded semiconductors in general possess greater photocatalytic activity than pure semiconductors. Hence, with an attempt to achieve higher photocatalytic activity, Au-TiO₂ photocatalysts were prepared by deposition-precipitation method and used for the photocatalytic degradation of an azo dye (Acid Red 88; AR88). The materials were characterized by different analytical techniques. A possible mechanism for the photocatalytic degradation of AR88 by Au-TiO₂ in the absence and presence of other oxidizing agents (peroxomonosulfate (PMS), peroxodisulfate (PDS) & hydrogen peroxide (H₂O₂)) has been proposed. The extent of mineralization of the target pollutant was also evaluated using Total Organic Carbon (TOC) analysis.     

Degradation of paracetamol in aqueous solutions by TiO2 photocatalysis

In this study, photo/photocatalytic oxidation of common analgesic and antipyretic drug, paracetamol (acetaminophen), was investigated to determine the optimal operating conditions for degradation in water. UVA (365 nm) radiation alone degraded negligible amount of paracetamol, whereas paracetamol concentration decreased substantially under an irradiation of UVC (254 nm) with marginal changes in total organic carbon (TOC). In the presence of TiO2, much faster photodegradation of paracetamol and effective mineralization occurred; more than 95% of 2.0mM paracetamol was degraded within 80 min. The degradation rate constant decreased with an increase in the initial concentration of paracetamol, while it increased with light intensity and oxygen concentration. The degradation rate also increased with TiO2 loading until a concentration of 0.8 g L(-1). The degradation rate slowly increased between pH 3.5 and 9.5, but significantly decreased with increasing pH between 9.5 and 11.0. Based on the experimental data, a kinetic equation describing paracetamol photocatalytic degradation with various process parameters is obtained.     

Photoassisted reduction of metal ions and organic dye by titanium dioxide nanoparticles in aqueous solution under anoxic conditions

The photoassisted reduction of metal ions and organic dye by metal-deposited Degussa P25 TiO₂ nanoparticles was investigated. Copper and silver ions were selected as the target metal ions to modify the surface properties of TiO₂ and to enhance the photocatalytic activity of TiO₂ towards methylene blue (MB) degradation. X-ray powder diffraction (XRPD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) were used to characterize the crystallinity, chemical species and morphology of metal-deposited TiO₂, respectively. Results showed that the particle size of metal-deposited TiO₂ was larger than that of Degussa P25 TiO₂. Based on XRPD patterns and XPS spectra, it was observed that the addition of formate promoted the photoreduction of metal ion by lowering its oxidation number, and subsequently enhancing the photodegradation efficiency and rate of MB. The pseudo-first-order rate constant (k_obs) for MB photodegradation by Degussa P25 TiO₂ was 3.94 × 10^− 2 min^− 1 and increased by 1.4-1.7 times in k_obs with metal-deposited TiO₂ for MB photodegradation compared to simple Degussa P25 TiO₂. The increase in mass loading of metal ions significantly enhanced the photodegradation efficiency of MB; the k_obs for MB degradation increased from 3.94 × 10^− 2 min^− 1 in the absence of metal ion to 4.64-7.28 × 10^− 2 min^− 1 for Ag/TiO₂ and to 5.14-7.61 × 10^− 2 min^− 1 for Cu/TiO₂. In addition, the electrons generated from TiO₂ can effectively reduce metal ions and MB simultaneously under anoxic conditions. However, metal ions and organic dye would compete for electrons from the illuminated TiO₂.     

Photodegradation of the antibiotics nitroimidazoles in aqueous solution by ultraviolet radiation

The objective of this study was to analyze the efficacy of ultraviolet (UV) radiation in the direct photodegradation of nitroimidazoles. For this purpose, i) a kinetic study was performed, determining the quantum yield of the process; and ii) the influence of the different operational variables was analyzed (initial concentration of antibiotic, pH, presence of natural organic matter compounds, and chemical composition of water), and the time course of total organic carbon (TOC) concentration and toxicity during nitroimidazole photodegradation was studied. The very low quantum yields obtained for the four nitroimidazoles are responsible for the low efficacy of the quantum process during direct photon absorption in nitroimidazole phototransformation. The R₂₅₄ values obtained show that the dose habitually used for water disinfection is not sufficient to remove this type of pharmaceutical; therefore, higher doses of UV irradiation or longer exposure times are required for their removal. The time course of TOC and toxicity during direct photodegradation (in both ultrapure and real water) shows that oxidation by-products are not oxidized to CO₂ to the desired extent, generating oxidation by-products that are more toxic than the initial product. The concentration of nitroimidazoles has a major effect on their photodegradation rate. The study of the influence of pH on the values of parameters ? (molar absorption coefficient) and k′_E (photodegradation rate constant) showed no general trend in the behavior of nitroimidazoles as a function of the solution pH. The components of natural organic matter, gallic acid (GAL), tannic acid (TAN) and humic acid (HUM), may act as promoters and/or inhibitors of OH· radicals via photoproduction of H₂O₂. The effect of GAL on the metronidazole (MNZ) degradation rate markedly differed from that of TAN or HUM, with a higher rate at low GAL concentrations. Differences in MNZ degradation rate among waters with different chemical composition are not very marked, although the rate is slightly lower in wastewaters, mainly due to the UV radiation filter effect of this type of water.     

The photocatalytic degradation of dicamba in TiO2 suspensions with the help of hydrogen peroxide by different near UV irradiations

The direct photolysis and the photocatalytic degradations of dicamba in TiO2 suspensions with and without the use of hydrogen peroxide were studied using two different monochromatic UV irradiations (300 and 350 nm). Both the direct photolysis and photocatalytic degradations of dicamba follow pseudo-first-order decay kinetics. Photolysis reactions were slow but the corresponding photocatalysis rates were increased by about 3 and 5 times in the presence of TiO2 at 300 and 350 nm of UV, respectively. Photocatalytic rates were increased with the pH at acidic to neutral ranges because of the increase of hydroxide ions, but the reaction was gradually retarded at the alkaline medium due to the effect of charges repulsion. The different proton sources causing various degrees of rate retardation were due to the presence of the corresponding counter anions. The results of H2O2-assisted photocatalysis experiments showed that a low H2O2 dosage in photocatalysis using UV 300 nm would enhance the decay rate of dicamba by 2.4 times, but an overdose of H2O2 will retard the rate because of the expenditure of hydroxyl radicals. However, this process was found impracticable at UV 350 nm due to the absorption characteristic of H2O2. A neutral initial pH level was found to favour the H2O2-assisted photocatalysis at UV 300 nm. The reactions were highly retarded at the alkaline medium due to the unstable properties of H2O2.     

Electrocatalytic characterization and dye degradation of Nano-TiO2 electrode films fabricated by CVD

A 20-40 nm anatase-titania film on a titanium electrode was fabricated using chemical vapor deposition (CVD). The film was characterized using field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and atomic force microscopy (AFM). The CVD deposition time and number of deposition coatings were evaluated to establish the appropriate film fabrication parameters. Results indicate that two coatings at a deposition time of 6 h each produced the best nano-TiO₂ electrode films (NTEFs) with an even distribution of ca. 20 nm diameter nanoparticles in the anatase lattice. The NTEF was tested as an electrocatalytic anode to investigate the degradation efficiency in treating methyl orange dye wastewater. A high removal efficiency of methyl orange dye and total organic carbon (TOC) of 97 and 56%, respectively; was achieved using a current density of 20 mA cm^− 2 for 160 min. Cyclic voltammetry showed that the electrochemical degradation reaction rate at the NTEF surface was predominately driven by molecular diffusion. The electrocatalytic decomposition rate of organic pollutants at the NTEF is controlled by mass transport, which was associated with the nanostructure of the electrocatalytic electrode.     

Degradation of H-acid and its derivative in aqueous solution by ionising radiation

The mechanism of high-energy radiation induced degradation of H-acid (4-amino-5-hydroxynaphthalene-2,7-disulphonic acid, (I)) and its derivative, 4-hydroxynaphthalene-2,7-disulphonic acid (II) (central parts of a large number of azo dyes), was investigated in aqueous solutions. These compounds can be efficiently destroyed by the (*)OH and hydrated electron intermediates produced during water radiolysis. As the first step of degradation mainly cyclohexadienyl-type radicals form, however, with I H-atom elimination from the NH(2) group is also observed yielding anilino-type radicals. Both the cyclohexadienyl and the anilino-type radicals decay on the millisecond timescale. In the (*)OH reactions as stable products hydroxylated molecules and quinone-type compounds form. These molecules by further decomposition of the ring structure transform to open chain molecules. In the case of hydrated electron, the primarily formed products have absorption spectra shifted to the low-wavelength region indicating the destruction of at least one of the aromatic rings.     

Decomposition of 2-naphthalenesulfonate in aqueous solution by ozonation with UV radiation

This study investigates the ozonation of 2-naphthalenesulfonate (2-NS) combined with ultraviolet (UV) radiation. Naphthalenesulfonic acids are of importance as dye intermediates for the dye and textile auxiliary industries. Its derivatives, such as 2-NS, have been found in rivers and tannery effluents causing pollution problems. Thus, the 2-NS is of concern for the aquatic pollution control especially in the surface and waste waters. Ozonation combined with UV radiation is employed for the removal of 2-NS in the aqueous solution. Semibatch ozonation experiments were proceeded under different reaction conditions to study the effects of ozone dosage and UV radiation on the oxidation of 2-NS. The concentrations of 2-NS and sulfate are analyzed at specified time intervals to elucidate the decomposition of 2-NS. In addition, values of pH and oxidation reduction potential are continuously measured in the course of experiments. Total organic carbon is chosen as a mineralization index of the ozonation of 2-NS. The mineralization of 2-NS via the ozonation is remarkably enhanced by the UV radiation. These results can provide useful information for the proper removal of 2-NS in the aqueous solution by the ozonation with UV radiation.     

Photocatalysis of p-chlorobenzoic acid in aqueous solution under irradiation of 254 nm and 185 nm UV light

The photolytic and photocatalytic degradation of p-chlorobenzoic acid (p-CBA) in aqueous solution were investigated using two kinds of low-pressure mercury lamps: one emitted at 254 nm and the other emitted at 254 nm and 185 nm. Both the photolytic and photocatalytic degradations of p-CBA followed pseudo first-order reaction rate form. The rate constants decreased with increase of initial concentration in VUV and TiO2/VUV. The TiO2/VUV was the most efficient process, in which rate constants was 3.0-6.5 times as that in TiO2/UV depending on the initial concentration. The gas bubbling efficiently improved the mass transfer and the kind of bubbling gas had evident influence on the reaction rate except in UV photolysis. TOC reduction rate was greatly reduced in VUV and TiO2/VUV processes when bubbled with nitrogen gas, it was no more than one third of that when bubbled with oxygen, however, the degradation rate of p-CBA was not affected by bubbled nitrogen so greatly.     

A comparison of fenuron degradation by hydroxyl and carbonate radicals in aqueous solution

A comparative study of the transformation of the herbicide fenuron (1,1-dimethyl-3-phenylurea) by hydroxyl radicals and carbonate radicals in aqueous solution (pH 7.2-phosphate buffer) has been undertaken. Hydroxyl radical was generated by the well-known photolysis of hydrogen peroxide at 254 nm and carbonate radical was formed by photolysis of Co(NH₃)₅CO₃⁺ at 254 nm. Competitive kinetic experiments were performed with atrazine used as the main competitor for both processes. Accordingly, the second-order rate constant of reaction between fenuron and carbonate radical was found to be (7−12±3)×10⁶ M⁻¹ s⁻¹ [(7±1)×10⁹ M⁻¹ s⁻¹ for hydroxyl radical]. The formation of degradation products was studied by LC-MS in the two cases and a comparison has been performed. The reaction with carbonate radical leads to the formation of a quinone-imine derivative which appears as the major primary product together with ortho and para hydroxylated compounds. These two compounds represent the major products in the reaction with hydroxyl radicals. The reaction of both radicals also leads to the transformation of the dimethylurea moiety.     

Photocatalytic degradation of 2,4-dinitrophenol (DNP) by multi-walled carbon nanotubes (MWCNTs)/TiO2 composite in aqueous solution under solar irradiation

Nanosized multi-walled carbon nanotubes (MWCNTs)/TiO₂ composite and neat TiO₂ photocatalysts were synthesized by sol-gel technique using tetrabutyl titanate as a precursor. The as prepared photocatalysts were characterized using XRD, SEM, FTIR and UV-vis spectra. The samples were evaluated for their photocatalytic activity towards the degradation of 2,4-dinitrophenol (DNP) under solar irradiation. The results indicated that the addition of an appropriate amount of MWCNTs could remarkably improve the photocatalytic activity of TiO₂. An optimal MWCNTs:TiO₂ ratio of 0.05% (w/w) was found to achieve the maximum rate of DNP degradation. The effects of pH, irradiation time, catalyst concentration, DNP concentration, etc. on the photocatalytic activity were studied and the results obtained were fitted to the Langmuir-Hinshelwood model to study the degradation kinetics. The optimal conditions were an initial DNP concentration of 38.8 mg/L at pH 6.0 with catalyst concentration of 8 g/L under solar irradiation for 150 min with good recyclisation of catalyst. The degree of photocatalytic degradation of DNP increased with an increase in temperature. The MWCNTs/TiO₂ composite was found to be very effective in the decolorization and COD reduction of real wastewater from DNP manufacturing. Thus, this study showed the feasible and potential use of MWCNTs/TiO₂ composite in degradation of various toxic organic contaminants and industrial effluents.     

Electrochemical degradation of perchloroethylene in aqueous media: An approach to different strategies

An approaching study to the electrochemical degradation of perchloroethylene (PCE) in water has been carried out using controlled current density degradation electrolyses. The different electrochemical strategies to degrade perchloroethylene in aqueous media (i.e. cathodic, anodic and dual treatments) have been checked using divided and undivided configurations. The influence of the initial concentration, pH and current density on the general behavior of the system has been studied, and special attention was paid to the nature of the byproducts formed and to the analysis of the closed mass balance at the end of the reaction. Results from several analytical techniques have been compared. Undivided configuration provides the best results in these experimental conditions, with degradation percentages higher than 50% and with only 6% of the initial perchloroethylene concentration remaining in the system.     

Study of pH effects on the evolution of properties of brown-water natural organic matter as revealed by size-exclusion chromatography during photocatalytic degradation

This study shows the effect of pH on the photocatalytic degradation of natural organic matter (NOM). The experiments were carried out in batch reactor (a solar UV-light simulator) with Degussa P-25 titanium dioxide (TiO₂). The NOM degradation was followed by size-exclusion chromatography for dissolved organic carbon (DOC), ultraviolet absorption and fluorescence-detection (SEC-DOC, SEC-UV₂₅₄ and SEC-Fl_254/450). Changes in pH values affected the adsorption of NOM onto TiO₂, but did not affect the photodegradation sequence of NOM. For high or low pH values, the degradation of the NOM preferentially removed the larger molecular size fraction in comparison to the middle and small molecular size fractions, resulting in the relative increase of these smaller fractions. This sequence of NOM degradation leads to the evolution of the formation potential for disinfection by-products (DBPs). Specifically, the trihalomethanes and halogenated organic compounds formation potential (THMF and AOXFP) decreased steadily.