Heterogeneous photocatalytic ozonation of 2,4-D in dilute aqueous solution with TiO2 fiber

Photocatalytic ozonation (O(3)/UV/TiO2) is an emerging oxidation method for recalcitrant organic contaminants in water. However, immobilised TiO2 catalysts suffer from reduced photonic efficiency. Therefore, TiO2 catalysts with excellent mechanical and thermal properties and enhanced photonic efficiencies are sought. This paper aimed to elucidate the mineralisation of low concentration 2,4-D (45.0 microM) by O(3)/UV/TiO2 using the world's first high-strength TiO2 fibre in laboratory batch experiments. 2,4-D degradation and TOC removal followed pseudo first-order reaction kinetic. The removal rates for 2,4-D and TOC in O(3)/UV/TiO2 were 1.5 and 2.4-fold larger than the summation of the values for ozonation (O3)) and photocatalysis (UV/TiO2), respectively. O(3)/UV/TiO2 was characterised by few aromatic intermediates with low abundance, fast degradations of aliphatic intermediates and dechlorination as a major step. The significantly enhanced 2,4-D mineralisation in O(3)/UV/TiO2 was attributed to increased ozone dissolution and decomposition, and reduced electron-hole recombination resulting in large number of hydroxyl radical (*OH) formation from more than one parallel path. The discrepancies in the organic carbon mass budget were attributed to few apparently major unidentified intermediates, while chlorine mass balance was reasonably acceptable. The mineralisation efficiency of O(3)/UV/TiO2 with the TiO2 fibre can further be enhanced by optimisation of experimental design parameters. The new TiO2 fibre is very promising to overcome the problem of reduced efficiency of TiO2 catalyst in an immobilised state.     

Photodegradation of 4-chloronitrobenzene in the presence of aqueous titania suspensions in different gas atmospheres

The photocatalytic degradation of 4-chloronitrobenzene (4-CNB) was carried out using Degussa P25 TiO(2) as photocatalyst in three different gas atmospheres: nitrogen, oxygen, and ozone. The total organic carbon (TOC) and inorganic anions including chloride, nitrite, and nitrate anions were measured to monitor the mineralization processes, while the degradation of 4-CNB and the formation of intermediates were followed by liquid chromatography-mass spectrometry (LC/MS). Results showed that the photocatalytic efficiency followed the order of TiO(2)/UV/N(2) < TiO(2)/UV/O(2) < TiO(2)/UV/O(3), which was further proved by evaluating the reaction activities using electron paramagnetic resonance (EPR) spin trapping technique. Chlorine atom, nitro group and hydrogen atom of the benzene ring could be displaced by hydroxyl radical (?OH) leading to the formation of chloride, nitrite (III) anions and a variety of phenols, then the nitrite (III) anions were further oxidized to nitrate (V) anions. 4-Nitrophenol and 5-chloro-2-nitrophenol were identified and quantified in both of the TiO(2)/UV/N(2) and TiO(2)/UV/O(2) processes while no aromatic intermediates were monitored in the process of TiO(2)/UV/O(3).     

Gas-phase photocatalytic oxidation of motor fuel oxygenated additives.

Methyl tert-butyl ether (MTBE) and tert-butyl alcohol (TBA) were oxidized in the gas phase by photocatalytic oxidation (PCO). Transient PCO was carried out at room temperature on TiO2 (Degussa P25), 0.2% Pt-TiO2, and 2% Pt-TiO2 catalysts. Surface-adsorbed reaction by-products were characterized by temperature-programmed desorption (TPD) and oxidation (TPO). Continuous flow PCO was also carried out at 373 K on TiO2. Acetone, H2O, and CO2 were the gas-phase products for PCO of TBA and MTBE, and formic acid was adsorbed on the TiO2 surface. Temperature-programmed desorption of TBA and MTBE formed 2-methyl-1-propene, water (TBA), and methanol (MTBE). During continuous-flow PCO, acetone desorbed in molar amounts equal to the amount of decomposed TBA and MTBE. The Pt/TiO2 catalysts had higher rates of complete oxidation during PCO and TPO. Injection of water during transient PCO increased the rates of oxidation of adsorbed TBA, formic acid, and acetone. Photocatalytic oxidation of TBA proceeded faster in humid air than dry air, but MTBE oxidation was less sensitive to humidity. The TiO2 catalyst was stable for MTBE, TBA, and acetone PCO at 373 K. The PCO at low conversions followed the Langmuir-Hinshelwood model.     

Advanced oxidation of commercial textile biocides in aqueous solution: effects on acute toxicity and biomass inhibition.

In the present study, the decomposition of two biocides used in the textile finishing process with Advanced Oxidation Processes (AOPs) has been studied. Different AOPs, i.e O3/OH-, TiO2/UV-A and Fe2+/H2O2 have been used representing mutually combined components of the chemically and photochemically driven advanced oxidation systems. The course of reaction was examined by changes in chemical oxygen demand (COD), total organic carbon (TOC) and acute toxicity towards the water flea Daphnia magna (assessed in terms of the effective dilution ratio LD50). Particular attention has been paid to determine the inhibitory effect of raw and ozonated biocides on biological activated sludge consortium at concentrations typically encountered in textile finishing effluents. Significant oxidation and mineralization of both biocides could be achieved employing ozonation at pH = 11.5 and heterogeneous photocatalysis (TiO2/UV-A) at pH = 5.0, whereas Fenton's reagent appeared to be less effective in COD and acute toxicity abatement.     

Photocatalytic removal of cyanide with illuminated TiO(2)

Effects of TiO(2) dosage, pH and initial cyanide concentration on the removal efficiency of cyanide from aqueous solutions with illuminated TiO(2) have been investigated. Adsorption and oxidation were recognized as significant processes for the elimination of cyanide. From the Langmuir isotherm, the maximum adsorption capacity was determined as 17.24 mg/g at pH 7. Adsorbed amount of cyanide slightly increased as the TiO(2) dosage increased. However, as no significant increase was observed above 1 g/L TiO(2), an optimum TiO(2) dosage was determined as 1 g/L. Photocatalytic oxidation efficiency of cyanide was greatly affected by the solution pH. It increased as the solution pH decreased. The photocatalytic oxidation efficiency after 120 min was 80.4% at pH 3 while it was only 20.4% at pH 11. Photocatalytic oxidation of cyanide was well described by the second-order kinetics. Photocatalytic reaction with illuminated TiO(2) can be effectively applied to treat industrial wastewater contaminated with cyanide.     

Photocatalytical polishing of paper-mill effluents.

Photocatalytic oxidation (PCO) is a promising technology for purification of biological pretreated wastewater or destruction of non-biodegradable compounds. For this reason PCO has been investigated as a last step of purification of biologically pre-treated paper-mill effluents. The influence of the parameters pH, TiO2-modification, TiO2-concentration, catalyst re-use, concentration of substances to be oxidised (wastewater quality) has been determined. The TOC of the biologically pretreated wasterwater was up to 55 mg L(-1). This wastewater was treated with a previously presented aerated cascade photoreactor which was modified for batch experiments. A high specific oxidation rate of up to 0.76 g TOC m(-2) h(-1) as well as a complete TOC mineralization has been achieved after the optimisation of the process parameters. The complete destruction of recalcitrant compounds will offer the opportunity to reuse the wastewater in the production process. The increase of the BOD5/TOC ratio after a short irradiation period indicates the transformation of recalcitrant organic compounds to better biodegradable intermediates. The use of PCO as a pre-treatment step for the enhancement of the biodegradability of wastewater, containing recalcitrant or inhibitory compounds is an alternative to a long and energy-intensive total pollutant mineralization.     

Removal of humic acid foulant from ultrafiltration membrane surface using photocatalytic oxidation process.

The experimental results indicated that without the TiO2 particles and PCO treatment, the permeate flux of ultrafiltration (UF) membrane declined to 40% of the initial permeate flux after 8 hours filtration. Feeding the humic acid solution with TiO2 particles dosage of 1 g/L with calcium ions into UF membrane, after the same filtration time and PCO reaction at 120 minutes, the permeate flux was increased to about 90% of the initial permeate flux. At longer PCO reaction times, a better water quality of UF permeate was observed. It has been found that with the coexistence of calcium ions in humic acid solution, the smaller molecular fragments of humic acid (HA) generated by PCO reaction may be transferred to the surface of TiO2 by means of adsorption. The humic acid adsorption by TiO2 in the presence of Ca2+ is also pH dependent. The adsorption rates were 21.0, 14.9 and 10.8 ppmTOC/gTiO2 for pH value of 4, 7 and 10 respectively. The combination of effects of PCO mineralization of humic acid into CO2 and adsorption of humic acid by TiO2 through the forming of HA-Ca(2+)-TiO2 aggregate particles were responsible for the removal of humic acid foulant from UF membrane surface.     

Inorganic greywater matrix impact on photocatalytic oxidation: does flocculation of TiO2 nanoparticles impair process efficiency?

This study was conducted in order to clarify whether photocatalyst flocculation--as observed in biologically pretreated greywater--contributes to photocatalytic oxidation (PCO) efficiency impairment. Aqueous solutions of tetraethyleneglycol dimethylether spiked with different inorganic salts in concentrations as found in biologically treated greywater were investigated with respect to TiO2 flocculation and PCO mineralisation kinetics. Flocculation of the photocatalyst primarily depended on pH (which was affected by the salts) and how close pH was to the point of zero charge (PZC). Photocatalyst agglomeration was maximum at pH 5.5. With salt concentrations >7 mmol L(-1), flocculation was strong even at pH far above PZC due to electric double layer compression. PCO rate constants were not unequivocally related to flocculation. Increasing pH was observed as the clearest factor deteriorating PCO efficiency. This was interpreted to result from impaired adsorbability of negatively charged oxidation intermediates as well as from enhanced CO2 absorption with increasing pH and subsequent formation of HCO3(-) anions which are OH radical scavengers.     

A bifunctionalized dye-sensitized TiO(2) film for efficient degradation of methyl orange under visible light irradiation

A new bifunctionalized TiO(2) film containing a dye-sensitized region and a degradation region was described. A similar structure of dye-sensitized solar cell (DSSC) was fabricated in the dye-sensitized region to accomplish separation of electrons from positive charges, and separation of dye from pollutants to avoid dye decomposition. The bifunctionalized TiO(2) film electrode and anode electrode can degrade methyl orange (MO) in reactors A and B, respectively. The degradation efficiency was enhanced remarkably by an external electrical potential. The decolorization of MO reaches as high as 95% after 2 h visible light irradiation at an external potential of 0.5 V along with a loss of 41% total organic carbon (TOC). The possible reason for the improvement of degradation by external DC potential was discussed. Effects of pH and inorganic salts on the decolorization are present.     

Advanced oxidation of bleached eucalypt kraft pulp mill effluent.

In this study a poorly biodegradable (BOD/COD = 0.3) industrial alkaline ECF bleaching filtrate was treated using different advanced oxidation processes to evaluate their use in combined chemical-biological treatment aimed at increasing recalcitrant COD removal and improving final effluent quality. Oxidative treatments included ozonation combined with hydrogen peroxide (2, 5, 10, 20 mmol L(-1) O3/0.7, 2, 5, 10 mmol L(-1) H2O2) and photocatalysis with hydrogen peroxide (UV/2, 4 and 8 mmolL(-1) H2O2) and with TiO2 (UV/TiO2/0.7 and 4 mmol L(-1) H2O2). The O3/H2O2 process increased effluent biodegradability by up to 68% as a result of increasing BOD and decreasing COD. Increasing the O3 dose had a greater effect on biodegradability improvement and lignin and colour removal efficiencies than increasing the H2O2 dose. A combined oxidant dose of 5 mmol L(-1) O3 and 2 mmol L(-1) H2O2 resulted in 75% lignin removal, 40% colour removal and 6% carbohydrate loss without mineralizing the organic carbon. The photocatalytic processes led to a decrease in effluent biodegradability through combined decrease in BOD and increase in COD and did not result in efficient lignin or colour removal. Photocatalytic oxidation was apparently inhibited by the high chloride and COD levels in the alkaline filtrate, and may be more efficient in recalcitrant COD removal if performed after biological.     

Quantum efficiencies of the photo-Fenton degradation of atrazine in water

An experimental work in a well-stirred batch recycling reactor for the photo-Fenton degradation of atrazine in water is presented. A study of the quantum efficiency is performed to assess the effectiveness of the photo-Fenton process on the atrazine degradation and total organic carbon (TOC) mineralization. Apparent and absolute quantum efficiencies of degradation and mineralization of an atrazine-based commercial herbicide are determined under different experimental conditions. Higher apparent efficiencies were found for both atrazine degradation and TOC mineralization when the ferric ion and hydrogen peroxide concentrations are increased. Because of the well known stability of the triazine ring, atrazine was not completely mineralized by the photo-Fenton process. However, a TOC reduction of 40% was achieved, being 62.5% of the maximum value that can be reached.     

Ozone catalysed with solids as an advanced oxidation process for landfill leachate treatment.

Heterogeneous catalytic ozonation (HCO) of wastewater is gaining both research and industrial interests. It is proved to be an advanced oxidation process since it involves hydroxyl radicals as oxidation species. Few studies have been carried out to test HCO in the treatment of landfill leachates. This work has been carried out to test three types of catalysts: activated carbon (AC), expanded perlite (EP) and titanium dioxide (TiO2) combined with ozone at 80 g/m3 gas concentration for the treatment of a leachate generated by Jebel Chakir landfill site near Tunis-capital of Tunisia. The work has shown a reduction in COD of about 45% and an increase in biodegradability (BOD5/COD) from 0.1 to 0.34. A catalyst concentration of 0.7 g/L was found optimal for the treatment of the leachate.     

Enhanced nitrogen removal using C/N load adjustment and real-time control strategy in sequencing batch reactors for swine wastewater treatment.

The laboratory-scale sequencing batch reactor (SBR) was used to study the effectiveness of an integrated strategy of real time control with C/N ratio adjustment for practical swine wastewater treatment. Swine waste was used as the external carbon source for continuous treatment in the SBR reactors. Oxidation-reduction potential and pH were used as parameters to control the continuous denitrification and nitrification process, respectively. A constant effluent quality could be obtained, despite drastic variations in the characteristics of influent wastewater. Also, a relatively complete removal of nutrients was always ensured, since the optimum quantity of the external carbon source could be provided for complete denitrification, and a flexible hydraulic retention time was achieved by the successful real-time control strategy. The average removal efficiencies of total organic carbon and nitrogen were over 94% and 95%, respectively.     

Bioaugmentation of a sequencing batch biofilm reactor by horizontal gene transfer.

Bioaugmentation by introduction of catabolic genes residing on mobile genetic elements into the microbial community of a soil or wastewater environment might be an alternative to bioaugmentation by addition of bacterial cells with chromosomally encoded catabolic genes. This study investigates the possibility to enhance degradation of the xenobiotic model compound 2,4-dichlorophenoxyacetic acid in a sequencing batch biofilm reactor (SBBR) by using the conjugative plasmid pJP4 carrying genes for 2,4-D degradation. After introduction of a plasmid donor strain to a lab-scale SBBR operated without 2,4-D, the number of plasmid-carrying cells first dropped, and then increased after switching to 2,4-D as the sole carbon source. The donor cells were unable to grow in the applied synthetic wastewater with 2,4-D as the sole carbon source. Transconjugants could be detected both by culture-dependent and culture-independent methods in the 2,4-D degrading biofilm. In contrast to 90% 2,4-D degradation in the bioaugmented reactor within 40 h, a control reactor which had not received the plasmid still contained 60% of the initial 2,4-D concentration after 90 h. This experiment clearly demonstrates the introduction of 2,4-D degradative genes into a microbial biofilm and indicates that horizontal gene transfer is a promising tool for bioaugmentation of reactors treating wastewater.     

Electrochemical treatment of Baker's yeast wastewater containing melanoidin: optimization through response surface methodology

Effluents from Baker's yeast production plant contain a high percentage of color and a large amount of organic load. In the present study, Baker's yeast wastewater (BYW) is treated with the electrocoagulation (EC) process using Al electrodes. Operating parameters (pH, current density, color intensity and operating time) are optimized by response surface methodology (RSM). Quadratic models are developed for the responses which are removal efficiencies of color, chemical oxygen demand (COD) and total organic carbon (TOC) and operating cost (OC). Optimum operating parameters and responses are determined as initial pH 5.2, current density of 61.3 A/m(2) and operation time of 33 min, and 71% of color, 24% of COD, 24% of TOC removal efficiencies and OC of 0.869 €/m(3), respectively. The quadratic model fits for all responses very well with R(2) (>0.95). This paper clearly shows that RSM is able to optimize the operating parameters to maximize the color, COD and TOC removal efficiencies and minimize the OC.     

Advanced treatment of benzothiazole contaminated waters: comparison of O3, AC, and O3/AC processes.

Benzothiazole (BT) is a toxic and poorly biodegradable contaminant, usually found in wastewater from rubber related applications. This compound could be effectively eliminated using advanced treatment processes. This paper compares experimental results on detoxification systems based on ozone oxidation, activated carbon adsorption, and simultaneous adsorption-oxidation using ozone in the presence of activated carbon. The effect of pH (2-11), and the presence of radical scavengers (tert-butyl alcohol and sodium carbonate) on process rates and removal efficiencies are assessed at laboratory scale. The experimental system consisted of a 1 L differential circular flow reactor and an ozone generator rated at 5 g O3/h. Results show that ozone oxidation combined with activated carbon adsorption increases the overall BT oxidation rate with respect to the ozonation process and activated carbon adsorption. In the presence of free radical scavenger, only a 44% reduction in BT removal rate is observed in the simultaneous treatment, as compared with 72% when ozonation treatment is used, suggesting that BT oxidation reactions mainly take place on the activated carbon surface.     

A combined process of non-thermal plasma with sludge stabilization for the simultaneous reduction of odor and organic waste

Odor emissions and sludge disposals have become most challenging issues in many sludge treatment plants, and separate chemical oxidation processes have been commonly employed for these problems. It is, therefore, necessary to develop a combined process that simultaneously reduces the odor emissions and the amount of waste sludge. In this study, a non-thermal plasma reactor using a dielectric barrier discharge type was employed to treat a gas stream contaminated with 10 ppm(v) of H(2)S, and then the ozone-laden gas stream was supplied to a sludge stabilizer to reduce the organic constituents. The specific energy input to the plasma was varied to investigate the performance of the combined process. When the specific energy inputs were 200, 260, and 360 J/L, the rates of ozone generation from the plasma reactor were 0.036, 0.070, 0.144 g-O(3)/h, respectively. At these experimental conditions, the removal efficiencies of H(2)S were 60, 75, and >99%, and total chemical oxygen demand (TCOD) reduction efficiencies of 33, 61 and 67% were observed. In addition, a soluble organic fraction of the sludge increased at ratios of 1.9, 8.9, and 10.7%, respectively. Consequently, the combined process effectively and simultaneously reduced the odor compounds as well as the organic constituents and particles in the waste sludge.     

Activation of solgel titanium nanofilm by UV illumination for NOM removal.

The control of natural organic matter (NOM) in drinking water treatment plants is required in order to control (i) the formation of potentially harmful disinfection byproducts (DBPs), (ii) the regrowth of bacteria and (iii) pipe corrosion in the distribution system. Photocatalysis is a promising advanced oxidation technology due to its ability to mineralise chlorinated byproduct precursors such as humic acids (HAs) to carbon dioxide and water. In this study, the efficiency of HAs and NOM removal in terms of UV absorbance at 254 nm (UV254) was tested by means of a new photocatalytic reactor made of stacked polymethylmethacrylate (PMMA) rings coated by TiO2 nanofilm. Three different sets of rings were coated with TiO2 gel one, two and three times respectively to optimise the coating thickness according to UV254 removal efficiency. The titania sol was immobilised on the substrate by a low temperature procedure and after 8 months the reactors were reactivated by means of UV radiation before the experiments. The photocatalytic removal efficiency of humic acid in terms of UV254 was significantly higher after 1 hour for the reactor employed with high thickness TiO2 nanofilm (around 20%) compared to middle and low thickness reactors (6 and 1.4%, respectively). However, during the same reaction time only 10% of UV254 was removed with high thickness TiO2 nanofilm using raw surface water, probably owing to ionic species naturally occurring in the raw water sample. Finally, the activation of the TiO2 nanofilm may be effectively accomplished by means of UV radiation where calcination cannot be applied (e.g. thermally sensitive substrates).     

Degradation of wine industry wastewaters by photocatalytic advanced oxidation.

Wine industry wastewaters contain a high concentration of organic biodegradable compounds as well as a great amount of suspended solids. These waters are difficult to treat by conventional biological processes because they are seasonal and a great flow variation exists. Photocatalytic advanced oxidation is a promising technology for waters containing high amounts of organic matter. In this study we firstly investigated the application of H2O2 as oxidant combined with light (artificial or natural) in order to reduce the organic matter in samples from wine industry effluents. Secondly, we studied its combination with heterogeneous catalysts: titanium dioxide and clays containing iron minerals. The addition of photocatalysts to the system reduces the required H2O2 concentration. Although the H2O2/TiO2 system produces higher efficiencies, the H2O2/clays system requires a H2O2 dosage between three and six times lower.     

Preliminary studies using hybrid mediated electrochemical oxidation (HMEO) for the removal of persistent organic pollutants (POPs).

This study investigates the hybrid mediated electrochemical oxidation (HMEO) technology, which is a newly developed non thermal electrochemical oxidation process for organic destruction. A combination of ozone and ultrasonication processes to the mediated electrochemical oxidation (MEO) process is termed as hybrid mediated electrochemical oxidation. The electrochemical cell was developed in this laboratory. In the present study, several organic compounds, such as phenol, benzoquinone and ethylenediaminetetraacetic acid (EDTA), were chosen as the model organic pollutants to be destructed by the hybrid process. The organic destruction was monitored based on the CO2 generation and total organic carbon (TOC) reduction. The HMEO process was found to be extremely effective in the destruction of all the target organics chosen in this study. The information obtained from this study will provide an insight in adopting this technique for dealing with more recalcitrant organics (POPs).