J.A. Herrera Melián

The photocatalytic disinfection of urban waste waters

In this paper we present the results of the photocatalytic disinfection of urban waste water. Two microbial groups, total coliforms and Streptococcus faecalis, have been used as indexes to test disinfection efficiencies. Different experimental parameters have been checked, such as the effect of TiO2, solar or UV-lamp light and pH. Disinfection of water samples has been achieved employing both UV-lamp and solar light in agreement with data shown by other authors. The higher disinfection rates obtained employing an UV-lamp may be explained by the stronger incident light intensity. Nevertheless no consistent differences have been found between TiO2-photocatalysis and direct solar or UV-lamp light irradiation at natural sample pH (7.8). At pH 5 the presence of TiO2 increases the relative inactivation rate compared with the absence of the catalyst. After the photocatalytic bacterial inactivation, the later bacterial reappearance was checked for total coliforms at natural pH and pH 5, with and without TiO2. Two h after the photocatalytic treatment, CFU increment was almost nill. But 24 and 48 h later an important bacterial CFU increment was observed. This CFU increment is slower after irradiation with TiO2 at pH 5 in non-air-purged samples.

Highly concentrated phenolic wastewater treatment by the Photo-Fenton reaction, mechanism study by FTIR-ATR

Phenol degradation by Photo-Fenton reaction has been studied in highly concentrated wastewaters and most intermediate species have been identified by Fourier Transform IR-Spectroscopy with ATR device. During the photodegradation of highly concentrated phenol solutions, the formation of dissolved and precipitate tannin has been observed. The possibility of a Fe3+-Pyrogallol complex formation, previous to the tannin formation, has been proposed too. The complex formation involving Fe3+ ions could be related to the observed Photo-Fenton activity decrease. Tannin formation inhibits the complete mineralization of phenol because *OH radicals attack will produce further condensation steps and the polymer size increase. This fact limits the applicability of the process for highly concentrated phenolic wastes mineralization. However, the tannin precipitation allows its separation from the solution by conventional filtration, and reduction of the corresponding dissolved organic carbon. These observations have been proved from the identification of primary degradation products, catechol and hydroquinone. Catechol is considered to be the first step for the formation of tannins. Degradation process for phenol, catechol and hydroquinone have been monitored by total organic carbon (TOC) measurements along the reaction time span. From these results, a global mechanism for the Photo-Fenton degradation of phenol is proposed.

Role of Fe3+/Fe2+ as TiO2 dopant ions in photocatalytic degradation of carboxylic acids

Abstract The photocatalytic degradation of organic carboxylic acids by Fe-doped TiO 2 has been studied. Results indicate that Fe is extracted by means of the formation of a [Fe-carboxylic acid] n + complex by which the organic molecule is degraded. Fe 2+ ions that remain in solution after the degradation go back to the catalyst surface due to their reaction with the photogenerated holes in the TiO 2 . This way the catalyst is reactivated becoming ready for a new degradation process. XPS studies have confirmed these results. Also, it has been observed that the catalyst preparation method affects the photoactivity of the formed complexes. The catalyst with lower Fe content and prepared from Fe(NO 3 ) 3 by the incipient wetness impregnation method degrades the formic acid more readily than the undoped catalyst and the catalysts prepared by other methods. Complexes formed by means of the iron interaction with formic and maleic acids are more photoactive than those formed with acetic or acrylic acids. The slower degradation of these last acids could be related with the photo-Kolbe reaction or other reduction processes. FTIR studies have been conducted with the goal of identifying the intermediates generated from the interaction of the different acids with the catalysts surface.

Maleic acid photocatalytic degradation using Fe-TiO2 catalysts: Dependence of the degradation mechanism on the Fe catalysts content

Abstract The photocatalytic degradation of maleic acid by using Fe-doped (0.15, 0.5, 2 and 5% w/w in Fe) TiO2 catalysts has been studied. Catalysts with the lowest Fe content (0.15 and 0.5%) show a considerably better catalytic behavior than non-doped TiO2 and catalysts with higher Fe contents. Maleic acid molecules interact with the surface of the lowest Fe-containing catalysts and as a consequence, iron atoms are extracted from the catalyst surface as photoactive Fe3+–maleic acid complexes. When this complex is degraded, the resulting Fe2+ ions react with TiO2 holes (h+) and the iron return to the catalyst surface as Fe2O3. In catalysts with low Fe content (0.15 and 0.5% w/w in Fe) the quite and return of the metal to the surface occurs in a fast way. On the contrary, in catalysts with high Fe content (2 and 5% w/w in Fe) the formation of less photoactive complexes seems to predominate. FTIR studies have proven the formation of such complexes in the doped catalysts.

Comparative Study of Phenolics Degradation Between Biological and Photocatalytic Systems

Phenol and phenol compounds are toxic organics that can be found in many industrial wastewaters. Biological wastewater treatments are considered to be the most convenient methods owing to their efficiency and low economic cost. Nonetheless, many organic pollutants are refractory to bacterial degradation. Photocatalytic methods can be an interesting alternative as pretreatment to improve biodegradability and reduce toxicity of industrial effluents. The goal of this study was to compare and combine TiO 2 -photocatalysis with constructed wetlands to obtain a low-cost method for the treatment of phenolic wastewater. The degradation of phenol was studied by means of Ti0 2 -Photocatalysis (solar and UV-lamp) in batch reactors. The degradations of phenol and two of its photocatalytic degradation intermediates, catechol and hydroquinone, were studied in wetland reactors with and without two wetland plants: common reed (Phragmites australis) and papyrus (Cyperus alternifolius). The application of pseudo first-order kinetics to the elimination of phenol in the wetland reactors provided high correlation coefficients (R 2 = 0.85 _ 0.99 ), allowing the comparison of the biological and photocatalytic methods. Although higher concentrations of phenol (250-400 mg/l ) could be treated, the elimination of 50 ppm was usually accomplished in batch experiments in less than 15 h, the time in which low or nil solar radiation is available for TiO 2 -photocatalysis. As a consequence, this concentration can be considered to be the upper limit for the wetland influent. The degradations of catechol and hydroquinone showed higher rate constants (0.2-0.4 h- 1 ) than that of phenol (about 0.15 h -1 ), particularly in the reactor with common reed (1-2 h -1 ). The degradation of phenol by the photocatalytic methods was three to four times faster than those obtained with the wetland reactors. By using solar TiO 2 -photocatalysis, concentrations of phenol up to 100 ppm were reduced down to 16 ppm and 27 ppm of phenol and hydroquinone, respectively, in about 7 h. However, it was toxic. When this sample was continuously (38 ml/min) added to wetland reactors with common reed, phenol and hydroquinone concentrations were below their detection limits (J ppm and 2 ppm, respectively). Solar Ti0 2 -Photocatalysis is a promising technique for the treatment of phenol but its application is limited to daytime periods with appropriate weather conditions. Constructed wetlands can also eliminate phenol and phenolic compounds without these limitations, but the toxicity of the influent must be as low as possible. The combination of both methods can provide a low-cost method for the treatment of phenolic wastewater.

Comparative Study of Photocatalytic Degradation Mechanisms of Pyrimethanil, Triadimenol, and Resorcinol

The photocatalytic degradation of an endocrine disruptor (resorcinol) and of two fungicides (pyrimethanil and triadimenol) has been studied and compared. The effect of pH, oxygen, and H 2 O 2 on the photocatalytic degradation of these compounds has been established. The three organics were analyzed by means of high pressure liquid chromatography (HPLC) and their mineralization by total organic concentration (TOC) measurements. The evolution of the toxicity to Lemna minor of the aqueous solutions of the three organics during their photocatalytic treatment has also been studied. The obtained results have been interpreted according to Fourier transform infrared studies on the interaction of the molecules with the catalyst surface and their reaction mechanisms by gas chromatographpy-mass spectrometry (GC-MS) analyses. The toxicity studies have shown that some intermediates acted as nutrients or toxicity antagonists as negative growth rate inhibitions were obtained. After 30 min of reaction, the resorcinol and pyrimethanil solutions were detoxified, although some amount of the organics still remained. In the case of triadimenol, a 92% detoxification was achieved after 60 min of reaction. The solar photocatalytic degradations of the pollutants have resulted to be comparable with those obtained with UV lamp. The obtained results suggest that the type of interaction of pyrimethanil and triadimenol with the TiO 2 surface decides their degradation mechanism by which the effect of pH, H 2 O 2 , and dissolved oxygen is determined. It has also been confirmed that the photocatalytic techniques are very efficient at the detoxification of wastewaters contaminated with these fungicides.

FTIR study of the photocatalytic degradation of NH4+ determination wastes

In this work we present the results of the application of photocatalytic techniques to organic pollutants in water. Such aqueous wastes are produced during the UV-Vis spectrophotometric determination of the NH 4 + ion in water. Processes taking place during the photocatalytic degradation of such organics have been studied by using FTIR, HLPC and TOC measurements. The obtained results suggest that in the presence of light, air and a catalyst (TiO 2 or Fe 2+ ) one of the existing organics, nitroprusside is decomposed to C≡N - , NO 2 - and NO 3 -. Free C≡N - ions react with other intermediates yielding nitrogen-containing compounds such as acetaloximes and acetamides. The photocatalytic degradation of this kind of N-containing compounds seems to be more complicated than the corresponding alcohols and carboxylic groups generated during the individual degradation of phenol, citrate or ethanol. A remarkable affinity of those nitrogenated species in adsorbing onto the TiO 2 surface has been observed. This may be the cause of the reduced catalyst activity observed in the process. It has been also determined that C≡N - ions tend to react with Fe 3+ ions to give complexes like [C≡N-Fe] 2+ and [R-C≡N-Fe] 2+ inhibiting this way the photo-Fenton reaction. In fact, no organic carbon reduction was observed during the application of the photo-Fenton reaction or TiO 2 photocatalysis. Toxicity of the samples was determined before and after the application of the photo-Fenton reaction and TiO 2 photocatalysis. Toxicity as determined by using the algal species Phaedactylum tricornutum after the photocatalytic treatment was higher. This suggests the possible above mentioned recombination of chemical species during the degradation of the samples.

Conventional and photocatalytic degradation of aromatic amines from nitrite determination wastes

Different methods such as KMnO4 oxidation, TiO2-photocatalysis and photo-Fenton reaction have been employed for the degradation of wastes from the spectrophotometric determination of in seawater. The optimisation of the degradation process was determined by total organic carbon (TOC) measurements. Experimental parameters such as pH, TiO2, H2O2, Fe3+ or Fe2+ concentrations have been checked. The highest TOC reduction was achieved by applying the photo-Fenton reaction with 0.02 g of FeSO4 and 3 mL of H2O2 for every 100 mL of sample. Mean TOC reductions of about 78 and 47% were achieved by photo-Fenton reaction and TiO2-photocatalysis, respectively. The addition of potassium oxalate to the photo-Fenton reaction enhanced the mineralisation efficiency. The mineralisation of individual components of the samples by photo-Fenton reaction was also attempted. Sulphanilamide was degraded to a large extent since TOC reductions of about 83% were achieved. In contrast N-(1-naphtyl-)-ethylenediamine degradation was negligible and TOC reduction was never larger than 6%. Degradation of N-(1-naphtyl-)-ethylenediamine by KMnO4 yielded a TOC reduction of 40.89%. The application of a KMnO4 pretreatment enhances the oxalate--photo-Fenton reaction efficiency by greatly reducing the irradiation time. The solar photo-Fenton reaction of the sample gave a TOC reduction of about 70%. After the treatment wastes lose their characteristic pink colour becoming nearly transparent or showing a very subtle yellowish colour.

Adsorption and Photocatalytic Degradation of Phthalic Acid on TiO2 and ZnO

Abstract The photocatalytic degradation of phthalic acid (H2Ph) with TiO2 (Degussa P-25) and ZnO (Aldrich) at different pHs and substrate concentrations (10-150 ppm) has been studied. The concentration and mineralization of H2Ph has been monitored. Results in optimal conditions are: ko = 6.63 · 10-4 s-1, %TOC = 86.02 for TiO2 and ko = 12.7 · 10-4 s-1, %TOC = 81.90 for ZnO. Dissolved zinc was monitored during reaction. Zinc concentration in solution showed a maximum when that of the organic compound was at its minimum. At this step zinc in solution decreases. Catalyst solubility, catalyst photocorrosion and extraction by the substrate, could be main processes explaining dissolved zinc concentration along the reaction. Adsorption and rate constants have been obtained by using the Langmuir-Hinselwood model. Adsorption constants in the dark and adsorption constants obtained from Langmuir-Hinselwood kinetic model have been compared. The adsorption constant values in TiO2 studies are greater than in ZnO ones. Considering the relationship between degradation rate and surface bonding FTIR studies have been performed. FTIR results show that H2Ph molecule interacts with ZnO and TiO2 surfaces. In both cases, H2Ph interaction occurred by means of the formation of a phthalate.

Treatment of Phenol, Formaldehyde and their Mixtures by Photocatalytic and Biological Methods

Abstract Phenol and formaldehyde are toxic compounds present in many industrial wastewaters. Particularly, in synthetic resin producing industries the concentrations of one or both of these organics can reach concentrations of grams per litre. In the present work, the degradation of phenol, formaline (formaldehyde: methanol mixture) and their mixtures with photocatalytic and biological methods has been studied to determine the best strategy for the treatment of wastewaters containing those organics. Photocatalytic studies in batch and in continuous (by dosing) have been performed. Experiments in batch showed that formaline slows down phenol degradation. Another set of experiments with phenol-methanol mixtures revealed that the presence of this alcohol also slows down the degradation of phenol but less than the mixture formaldehyde: methanol. FTIR studies indicated the formation of 4-hydroxybenzaldehyde as a result of phenol-formaldehyde interaction with the catalyst surface. Thus, the lower phenol degradation rate is attributed to the formation of this aldehyde on the surface of the catalyst in addition to the ·OH scavenging effect of methanol. This inhibitory effect in the degradation was overcome by dosing the influent into the photocatalytic reactor. In the present work the biological systems have been employed to optimise and speed up the degradation process of phenol-formaldehyde mixtures. These systems have shown lower detoxification efficiency of the samples in comparison with that of the photocatalytic method with dosing. Taking into account the obtained results of degradation and toxicity, a novedous process consisting of a biological treatment followed by a photocatalytic one with dosage for the treatment of formaldehyde-phenol containing wastewaters is proposed.