Field solar degradation of pesticides and emerging water contaminants mediated by polymer films containing titanium and iron oxide with synergistic heterogeneous photocatalytic activity at neutral pH

Photocatalytic degradation of phenol, nalidixic acid, mixture of pesticides, and another of emerging contaminants in water was mediated by TiO₂ and iron oxide immobilized on functionalized polyvinyl fluoride films (PVF^f-TiO₂-Fe oxide) in a compound parabolic collector (CPC) solar photoreactor. During degradation, little iron leaching (<0.2 mg L⁻¹) was observed. Phenol was efficiently degraded and mineralized at operational pH < 5 and nalidixic acid degradation was complete even at pH 7, but mineralization stopped at 35%. Pesticide mixture was slowly degraded (50%) after 150 min of irradiation. Degradation of the emergent contaminant mixture was successful for eight compounds and less efficient for six other compounds. The significant reactivity differences between tested compounds were assigned to the differences in structure namely that the presence of complexing or chelating groups enhanced the rates.PVF^f-TiO₂-Fe oxide photoactivity gradually increased during 20 days of experiments. X-ray photoelectron spectroscopy (XPS) measurements revealed significant changes on the catalyst surface. These analyses confirm that during photocatalysis mediated by PVF^f-TiO₂-Fe oxide, some iron leaching led to enlargement of the TiO₂ surface exposed to light, increasing its synergy with iron oxides and leading to enhanced pollutant degradation.     

Mechanism of phenol photodegradation in the presence of pure and modified-TiO(2): A review

In recent years, the application of heterogeneous photocatalytic water purification processes has gained wide attention due to its effectiveness in degrading and mineralizing the recalcitrant organic compounds as well as the possibility of utilizing the solar UV and visible-light spectrum. By far, titania has played a much larger role in this scenario compared to other semiconductor photocatalysts due to its costly effectiveness, inert nature and photostability. A substantial amount of research has focused on the enhancement of TiO₂ photocatalysis by modification with metal, non-metal and ion doping.This paper aims to review and summarize the recent works on the titanium dioxide (TiO₂) photocatalytic oxidation of phenol and discusses various mechanisms of phenol photodegradation (indicating the intermediates products) and formation of OH radicals. Phenol degradation pathway in both systems, TiO₂/UV and doped-TiO₂/Vis, are described.     

Comparison of photocatalysis and photolysis of malathion, isomalathion, malaoxon, and commercial malathion--products and toxicity studies

Malathion, one of the most widely applied insecticides, is still used in agriculture. There are many studies regarding its degradation under different experimental conditions, but few deal with its transformation products, i.e. malaoxon and isomalathion. Thus, malathion, malaoxon, isomalathion, and Radotion (one of its over 6000 commercial forms) were studied in terms of their degradation kinetics, identification of their transformation products, their toxicity, and their degree of mineralization, during UV photolysis (lambda = 254 nm) and TiO(2) photocatalysis (lambda = 355 nm). The degradation kinetics was similar for all four starting materials. More than 75% of theoretically expected sulfur in PS and P-S groups was oxidized after 240 min of photolysis and photocatalysis. On the other hand, less than 30% of stoichiometrically predicted amounts of phosphate was detected in the photolytic experiments, but more than 80% of expected phosphate was detected after photocatalytic treatment of all four organophosphorous materials. Several transformation products were identified by mass spectra of representative gas chromatographic peaks. Oxidation and isomerization were found as the main reactions of butenedioc acid diethyl esters and their analogs. The formation of malaoxon, isomalathion or trimethyl phosphate esters correlated well with the induced toxicity (inhibition of acetylcholinesterase), which was observed in photocatalysis of malathion and Radotion, and in photolysis of malaoxon and Radotion.     

Effect of ferric ion added on photodegradation of alachlor in the presence of TiO2 and UV radiation

The effect of in situ photodeposited ferric ion onto TiO2 surface on the degradation of alachlor was investigated in the presence of the UV radiation. The photodegradation rate of alachlor could be described as an apparent first order. The rate constant (K(a)) of alachlor increased from 0.021 to 0.060 h(-1) as the number of coating times increased from 1- to 5-times in the absence of ferric ion, where the corresponding thicknesses of the TiO2 film were 67 and 174 nm. The rate constant (K(a)) increased from 0.030 to 0.060 h(-1) as pH value decreasd from pH 9 to 5 in the presence of only TiO2 immobilised with 5-times of coating. The rate constant increased slightly from 0.031 to 0.050 h(-1) as the concentration of ferric ion increased from 0.75 to 7.5 mg Fe3+ l(-1) in the absence of TiO2. However, those increased from 0.051 to 0.110 h(-1) in the presence of both TiO2 and ferric ion. In situ photodeposition of ferric ion onto the TiO2 surface enhanced the rate constant of photodegradation of alachlor by about 80% with an adding 7.5 mgFe3+ l(-1). During the alachlor photodegradation, three kinds of non-toxic organic compounds derived from alachlor were detected in 1 h.     

Photocatalytic degradation of non-steroidal anti-inflammatory drugs with TiO2 and simulated solar irradiation

The aim of this work is to evaluate and compare the degradation achieved for three non-steroidal anti-inflammatory drugs (NSAIDs) by heterogeneous TiO2 photocatalytic means in aqueous solution at laboratory scale. The selected pharmaceutical compounds were diclofenac (DCF), naproxen (NPX) and ibuprofen (IBP). These compounds were used in their sodium salt chemical form. Previous experiments (adsorption, photolysis and thermodegradation) were developed to evaluate non-catalytic degradation for each NSAID. Photocatalytic experiments were carried out in a Xe-lamp reactor in order to study the influences of different operational conditions (catalyst load, temperature and dissolved oxygen concentration). These results showed that the optimum amount of TiO2, to achieve maximum degradation, of IBP was 1g/L. In contrast, the maximum degradation for DCF or NPX was observed at a TiO2 loading of 0.1g/L. Temperature had a significant effect only for NPX degradation, achieving almost 99% phototransformation. No significant differences were observed for DCF and IBP at 20, 30 and 40 degrees C. Dissolved oxygen concentration was an important parameter to increase the degradation for NPX and IBP. However, it was observed that its rate of mineralization did not increase. Intermediate metabolites were detected in all cases. Hydroxyl metabolites were the most important residual compounds after the photocatalytic treatment of IBP. The inhibition percentage of bioluminescence from Vibro fischeri--as a toxicity parameter--increased during the irradiation time due to the residual concentration of the hydroxyl metabolites generated. However, after 120 min, in experiments with 40 mg/L of dissolved oxygen, a decrease of the % inhibition was observed. Only photocatalytic treatment of IBP drives to a satisfactory biodegradability index BOD5/COD (between 0.16 and 0.42) and, only in this case, a post-biological treatment could be suggested.     

Preliminary trial on degradation of waste activated sludge and simultaneous hydrogen production in a newly-developed solar photocatalytic reactor with AgX/TiO2-coated glass tubes

A solar fluidized tubular photocatalytic reactor (SFTPR) with simple and efficient light collector was developed to degrade waste activated sludge (WAS) and simultaneously produce hydrogen. The photocatalyst was a TiO₂ film doped by silver and silver compounds (AgX). The synthesized photocatalyst, AgX/TiO₂, exhibited higher photocatalytic activity than TiO₂ (99.5% and 30.6% of methyl orange removal, respectively). The installation of light collector could increase light intensity by 26%. For WAS treatment using the SFTPR, 69.1% of chemical oxygen demand (COD) removal and 7866.7 μmol H₂/l-sludge of hydrogen production were achieved after solar photocatalysis for 72 h. The SFTPR could be a promising photocatalysis reactor to effectively degrade WAS with simultaneous hydrogen production. The results can also provide a useful base and reference for the application of photocatalysis on WAS degradation in practice.     

Solar Fenton and solar TiO2 catalytic treatment of ofloxacin in secondary treated effluents: Evaluation of operational and kinetic parameters

Two different technical approaches based on advanced oxidation processes (AOPs), solar Fenton homogeneous photocatalysis (hv/Fe²⁺/H₂O₂) and heterogeneous photocatalysis with titanium dioxide (TiO₂) suspensions were studied for the chemical degradation of the fluoroquinolone ofloxacin in secondary treated effluents. A bench-scale solar simulator in combination with an appropriate photochemical batch reactor was used to evaluate and select the optimal oxidation conditions of ofloxacin spiked in secondary treated domestic effluents. The concentration profile of the examined substrate during degradation was determined by UV/Vis spectrophotometry. Mineralization was monitored by measuring the dissolved organic carbon (DOC). The concentrations of Fe²⁺ and H₂O₂ were the key factors for the solar Fenton process, while the most important parameter of the heterogeneous photocatalysis was proved to be the catalyst loading. Kinetic analyses indicated that the photodegradation of ofloxacin can be described by a pseudo-first-order reaction. The rate constant (k) for the solar Fenton process was determined at different Fe²⁺ and H₂O₂ concentrations whereas the Langmuir-Hinshelwood (LH) kinetic expression was used to assess the kinetics of the heterogeneous photocatalytic process. The conversion of ofloxacin depends on several parameters based on the various experimental conditions, which were investigated. A Daphnia magna bioassay was used to evaluate the potential toxicity of the parent compound and its photo-oxidation by-products in different stages of oxidation. In the present study solar Fenton has been demonstrated to be more effective than the solar TiO₂ process, yielding complete degradation of the examined substrate and DOC reduction of about 50% in 30 min of the photocatalytic treatment.     

Concurrent filtration and solar photocatalytic disinfection/degradation using high-performance Ag/TiO2 nanofiber membrane

A facile polyol synthesis was used for the deposition of Ag nanoparticles on electrospun TiO₂ nanofibers for the subsequent fabrication of Ag/TiO₂ nanofiber membrane. The permeate flux of the Ag/TiO₂ nanofiber membrane was remarkably high compared to commercial P25 deposited membrane. The Ag/TiO₂ nanofiber membrane achieved 99.9% bacteria inactivation and 80.0% dye degradation under solar irradiation within 30 min. The Ag/TiO₂ nanofiber membrane also showed excellent antibacterial capability without solar irradiation. Considering the excellent intrinsic antibacterial activity and high-performance photocatalytic disinfection/degradation under solar irradiation, this novel membrane proved to have promising applications in water purification industry.     

Photocatalytic transformation and mineralization of 2,4,6-trinitrotoluene (TNT) in TiO2 slurries

An analysis of the photodegradation of TNT in a TiO₂ slurry reactor is presented. The rates and extent of TNT transformation and mineralization are compared for photocatalytic and direct photolytic reactions under conditions of varying light energies and in the presence and absence of oxygen. Certain initial organic transformation products are identified for both photocatalytic and photolytic reactions. Nitrate, nitrite, and ammonium ions are analyzed and the possibility of semiconductor sensitization by colored compounds is considered. TNT was transformed rapidly under each set of photochemical conditions but destruction was faster and more complete with TiO₂ photocatalysis. Transformation by-products were destroyed readily under oxygenated photocatalytic conditions and were observed to be more refractory under direct photolytic conditions. Mass balances performed on carbon and nitrogen revealed that when the TiO₂ photocatalyst was utilized in the presence of oxygen and near u.v. radiation (λ > 340 nm) approx. 90% of the TNT was mineralized and 35% of the total nitrogen was recovered as ammonium ion after 120 min. Among the large number of organic transformation products produced photocatalytically, trinitrobenzoic acid, trinitrobenzene and trinitrophenol have been identified as oxidative intermediate species and dinitroaniline as a reduction product. The photocatalytic transformation of TNT appears to involve both oxidative and reductive steps and sensitization by colored compounds plays no detectable role in degradation.     

Solar photocatalytic treatment of synthetic municipal wastewater

The photocatalytic organic content reduction of a selected synthetic municipal wastewater by the use of heterogeneous and homogeneous photocatalytic methods under solar irradiation has been studied at a pilot-plant scale at the Plataforma Solar de Almeria. In the case of heterogeneous photocatalysis the effect of catalysts and oxidants concentration on the decomposition degree of the wastewater was examined. By an accumulation energy of 50 kJL(-1) the synergetic effect of 0.2 gL(-1)TiO(2) P-25 with hydrogen peroxide (H(2)O(2)) and Na(2)S(2)O(8) leads to a 55% and 73% reduction of the initial organic carbon content, respectively. The photo-fenton process appears to be more efficient for this type of wastewater in comparison to the TiO(2)/oxidant system. An accumulation energy of 20 kJL(-1) leads to 80% reduction of the organic content. The presence of oxalate in the Fe(3+)/H(2)O(2) system leads to an additional improvement of the photocatalytic efficiency.     

Hybrid photocatalysis/membrane treatment for surface waters containing low concentrations of natural organic matters

Since the mid-1990s, numerous studies on the treatment of drinking water by photocatalysis have been reported. Once optimised, the photocatalytic process can completely degrade numerous natural and artificial organic compounds. In this study, a hybrid photocatalysis/membrane process was used as a polishing treatment of surface water containing a small concentration of natural organic matters (i.e. total organic carbon (TOC) concentration of around 3mg/L) which may be difficult to remove using conventional filtration or coagulation. An optimum pH of 4.5 and a TiO(2) concentration of 0.1g/L were found to lead to the highest removal efficiencies. The relative effect of the individual processes featuring in the hybrid system (UV radiation, TiO(2) adsorption and membrane filtration) was also assessed for different pH values. The membrane separation process was accounted to remove around 18% of the initial TOC concentration, while TiO(2) adsorption alone was generally responsible for less than 5% of TOC removal during the 120 min of the experiments. However, when the natural water was only radiated by UV light, up to 70% of TOC was removed. A synergetic effect was observed when the three processes (TiO(2), UV and membrane) were used together. Comparison of removal efficiencies obtained during real and model (International Humic Substance Society) waters treatment by photocatalysis is also presented, revealing the importance of the nature of the feed in this type of treatment.     

Hydrogen peroxide-assisted low pressure UV photodegradation of atrazine in aqueous solution

Aqueous solutions of atrazine (ATZ, 2-chloro-4-ethylamino-6-isopropylamino-1, 3, 5-triazine) were photolysed (λ = 254 nm) by low pressure UV/H₂O₂ process (LP/UV/H₂O₂) under a variety of parameters including hydrogen peroxide, the initial concentration of ATZ, pH, and humic acid. The results show that the most favourable reaction condition appears to be a moderate concentration of H₂O₂ from 100 mg/L to 120 mg/L. The presence of humic acid in the solution has a negative impact on the LP/UV/H₂O₂ treatment because of scavenging effects. Ninety percent of ATZ is destroyed in one hour under the optimum conditions. In this study, LP/UV/ H₂O₂ treatment of ATZ yielded several organic by products which are identified, including DIA, DEA, OHDIA, OHDEA, DAA and OAAT. They are quantified over the range of treatment tested and the ATZ degradation scheme is proposed combined with by products information.     

Optimization of capacity and kinetics for a novel bio-based arsenic sorbent, TiO2-impregnated chitosan bead

The optimization of TiO₂-impregnated chitosan beads (TICB) as an arsenic adsorbent is investigated to maximize the capacity and kinetics of arsenic removal. It has been previously reported that TICB can 1) remove arsenite, 2) remove arsenate, and 3) oxidize arsenite to arsenate in the presence of UV light and oxygen. Herein, it is reported that adsorption capacity for TICB is controlled by solution pH and TiO₂ loading within the bead and enhanced with exposure to UV light. Solution pH is found to be a critical parameter, whereby arsenate is effectively removed below pH 7.25 and arsenite is effectively removed below pH 9.2. A model to predict TICB capacity, based on TiO₂ loading and solution pH, is presented for arsenite, arsenate, and total arsenic in the presence of UV light. The rate of removal is increased with reductions in bead size and with exposure to UV light. Phosphate is found to be a direct competitor with arsenate for adsorption sites on TICB, but other relevant common background groundwater ions do not compete with arsenate for adsorption sites. TICB can be regenerated with weak NaOH and maintain full adsorption capacity for at least three adsorption/desorption cycles.     

Photocatalytic degradation of nitrobenzene using titanium dioxide and concentrated solar radiation: chemical effects and scaleup

Photocatalytic degradation of nitrobenzene (NB) using titanium dioxide (Degussa P-25) as photocatalyst and concentrated solar radiation has been studied. The effects of various factors, such as the presence of anions common in the industrial wastewater, the initial pH and the addition of FeSO(4), were investigated. The intermediates detected were o, p,m-nitrophenols and dihhydroxy derivatives indicating that degradation proceeds via z.rad;OH radicals. Degradation of NB was studied in three reactors of different diameters to ascertain the effect of photon penetration depth on the degradation.     

Arsenic sorption on TiO2 nanoparticles: Size and crystallinity effects

Single solute As (III) and As (V) sorption on nano-sized amorphous and crystalline TiO₂ was investigated to determine: size and crystallinity effects on arsenic sorption capacities, possible As (III) oxidation, and the nature of surface complexes. Amorphous and crystalline nanoparticles were prepared using sol-gel synthesis techniques. For amorphous TiO₂, solute pH in the range of 4-9 had a profound impact on only As (V) sorption. As (III) and As (V) sorption isotherms indicated that sorption capacities of the different TiO₂ polymorphs were dependent on the sorption site density, surface area (particle size) and crystalline structure. When normalized to surface area, As (III) surface coverage on the TiO₂ surface remained almost constant for particles between 5 and 20 nm. However, As (V) surface coverage increased with the degree of crystallinity. X-ray absorption spectroscopic analysis provided evidence of partial As (III) oxidation on amorphous TiO₂ rather than crystalline TiO₂. The data also indicated that As (III) and As (V) form binuclear bidentate inner-sphere complexes with amorphous TiO₂ at neutral pH.     

Effect of dissolved organic compounds on the photodegradation of the herbicide MCPA in aqueous solution

This work shows that the addition of phenol and 2-propanol as model organic compounds significantly decreases the direct photolysis quantum yield of 4-chloro-2-methylphenoxyacetic acid (MCPA) upon UVB irradiation in aqueous solution. Laser flash photolysis data suggest that 2-propanol is able to decrease the formation of the MCPA excited states under irradiation. A decrease from 0.54 to 0.34 of the photolysis quantum yield of the anionic form of MCPA (which prevails over the undissociated one in surface waters) could have a considerable impact on the MCPA lifetime in ecosystems where the direct photolysis is the main phototransformation pathway. In surface water bodies where the direct photolysis has comparable kinetics as the reaction with OH, a decrease of the quantum yield would enhance the relative importance of the OH pathway, which yields considerably less toxic intermediates than the direct photolysis.     

Solar photocatalytic thin film cascade reactor for treatment of benzoic acid containing wastewater

A solar photocatalytic cascade reactor was constructed to study the photocatalytic oxidation of benzoic acid in water under various experimental and weather conditions at HKUST. Nine stainless steel plates coated with TiO(2) catalyst were arranged in a cascade configuration in the reactor. Photolytic degradation and adsorption were confirmed to be insignificant total organic carbon (TOC) removal mechanisms. A turbulent flow pattern and, hence, improved mixing in the liquid film were achieved due to the unique cascade design of the reactor. The photoinduced consumption of oxygen during reactions was demonstrated in a sample experiment. The proposed rate equations provided good fits to 90 data points from 17 experiments. The regression results showed that the TOC removal rates averaged over 30 min intervals did not illustrate significant dependence on TOC(0) and that I(mean) was more important in affecting the photocatalytic process within the ranges of the data examined. The percentage removal of TOC in 7 l of 100 mg/l (or 100 ppm) benzoic acid solutions increased from 30% to 83% by adding 10 ml of hydrogen peroxide solution (30 wt%). Hydrogen peroxide was also shown to enhance the efficiency of the degradation process at elevated temperatures. Ortho-, meta- and para-hydroxybenzoic acids were identified by HPLC analysis as the intermediates of benzoic acid during reactions without the addition of hydrogen peroxide solutions.     

Photolytic and photocatalytic decomposition of aqueous ciprofloxacin: Transformation products and residual antibacterial activity

Previous work demonstrates that widely used fluoroquinolone antibacterial agents, including ciprofloxacin, are degraded by means of aqueous ultraviolet photolytic and titanium dioxide (TiO₂) photocatalytic (using both ultraviolet-A (UVA) and visible light (Vis) irradiation) treatment processes. In this study, we investigate the effects of photolytic and photocatalytic treatment processes on the antibacterial activity of ciprofloxacin solutions under controlled laboratory conditions. In agreement with earlier work, rates of ciprofloxacin degradation under comparable solution conditions (100 μM ciprofloxacin, 0 or 0.5 g/L TiO₂, pH 6, 25 °C) follow the trend UVA-TiO₂ > Vis-TiO₂ > UVA. Release of ammonia and fluoride ions is observed and a range of organic products have been identified with liquid chromatography-tandem mass spectrometry. However, the identified organic products all appear to retain the core quinolone structure, raising concerns about residual antibacterial potency of the treated solutions. Quantitative microbiological assays with a reference Escherichia coli strain indicate that the antimicrobial potency of ciprofloxacin solutions track closely with the undegraded ciprofloxacin concentration during photolytic or photocatalytic reactions. Quantitative analysis shows that for each mole of ciprofloxacin degraded, the antibacterial potency of irradiated solutions decreases by approximately one “mole” of activity relative to that of the untreated ciprofloxacin solution. This in turn indicates that the ciprofloxacin photo(cata)lytic transformation products retain negligible antibacterial activity relative to the parent compound. The energy demands for achieving one order of magnitude reduction in antibacterial activity within the experimental system are estimated to be 175 J/cm² (UVA-only), 29 J/cm² (Vis-TiO₂), and 20 J/cm² (UVA-TiO₂), which indicates that the UVA-TiO₂ photocatalysis is the most energy efficient process for achieving ciprofloxacin inactivation under laboratory conditions.     

Photolytic reaction mechanism and impacts of coexisting substances on photodegradation of bisphenol A by Bi2WO6 in water

Bi₂WO₆ displayed great photolytic degradation efficiency to bisphenol A (BPA) under simulated solar light irradiation but its reaction mechanism and the impacts of coexisting substances on the degradation remain unclear. In present study, the reaction mechanism was investigated using DMPO spin-trapping ESR spectra and experiments with scavengers of hydroxyl radicals (OH) and holes. The results supported that hole oxidation mainly governed the photodegradation process. As a common humic substance in natural water, humic acid accelerated the degradation of BPA when its concentration was 1 mg/L, while the photodegradation was impeded with the increase of humic acid concentration in the range of 5-20 mg/L. Almost all anions, including NO₃⁻, HCO₃⁻, Cl⁻, SO₄²⁻ inhibited the degradation of BPA by Bi₂WO₆ and their inhibition effects followed the order of SO₄²⁻ > Cl⁻ > HCO₃⁻ > NO₃⁻. Cations of Na⁺, K⁺, Ca²⁺ and Mg²⁺ displayed slight suppressing effect on BPA degradation mainly due to the impact of Cl⁻ coexisting in the solution. However, Cu²⁺ hindered the BPA photodegradation heavily. Fe³⁺ and H₂O₂ affected the photodegradation in a complicated way: they suppressed or promoted the photodegradation depending on their concentrations. This could be the result of competition between photolyitc hole generated by Bi₂WO₆ and OH produced by Fe³⁺ or H₂O_2.