Advanced Oxidation Processes in Triton X-100 and Wash-up Liquid Removal from Wastewater Using Modified TiO2/Al2O3 Photocatalysts
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Photocatalytic methods were applied to remove the recalcitrant or toxic pollutants from the water. The two models of wastewater containing either non-ionic surfactant Triton X-100 or commercially available wash-up liquid were tested in a self-constructed band reactor during the laboratory studies. The photocatalyst, being typed TiO2, was supported by porous Al2O3 and modified by the addition of Cu, Fe, Zn, Ni, Mo or Co. The photocatalysts were characterised by N2 adsorption–desorption, XRF, XRD, SEM-EDX, Raman and UV–Vis spectroscopy. All catalysts were efficient in the photocatalytic oxidation of surfactants, and they enabled at least 85 % COD reduction. TiO2/Al2O3 photocatalysts modified by the transition metals were efficient only for more complicated compositions of surfactants. The effect of H2O2 (0.01 vol.%) addition was also examined and compared with a type of compound and catalyst used—in this case a positive effect for Triton X-100 was only observed over the photocatalyst modified by Ni. When it comes to the wash-up liquid photoremoval, all studied photocatalysts seem to be slightly influenced by H2O2 addition. It was also observed that it is not economically justified to conduct such treatment for more than 2 h.
KeywordsPhotocatalysis TiO2/Al2O3 AOP Surfactants H2O2
Surfactants, because of their specific features, are widely used in the households as detergents or care products and in various industries: pesticides, chemicals, textiles or pharmaceuticals. They are common pollutants in the wastewater. It has been confirmed that generally non-ionic surfactants (at lethal concentrations depending on various tests from 0.0025 to 300 mg/dm3) are more toxic than others (anionic: 0.3 to 200 mg/dm3). Periods of long acclimation are required during the conventional treatment, and they typically result in a quite incomplete degradation (Liwarska-Bizukojc et al. 2005). Finally, introduced into water ecosystem, some of the pollutants such as alkylphenyl polyethoxylate-type non-ionic surfactants, e.g. Triton X-100 (TX), are suspected to be the endocrine disrupters in aquatic organisms (Saien et al. 2011). In order for the surfactants to be removed, the heterogeneous photocatalysis, as a one of the advanced oxidation processes (AOPs), can be applied successfully (Arslan-Alaton and Erdinc 2006; Fabbri et al. 2009). The observed progress in the field of AOPs has made them as alternatives or as a complement to conventional wastewater treatment. The main advantage of AOPs is that they can be used both independently or as pre-treatment technique for improving the biodegradability and efficiency of further treatments (Ioannou et al. 2011).
TiO2 is a widely tested photocatalyst in the studies concerning the photocatalytic removal of contaminants (Ollis et al. 1991; Friedmann et al. 2010). However, the recycling and recovery of typically used TiO2 powder photocatalysts in industry are quite troublesome and uneconomical, which becomes the main disadvantage of using the suspended systems. Therefore, many efforts are being made to immobilize TiO2 and construct reactors with an immobilized active phase (Chong et al. 2010).
Nowadays, photocatalytic studies are being conducted in three main areas: catalyst modifications, reactor construction and process modification (Lin et al. 2010). The catalyst modifications include metal and non-metal doping (Janus et al. 2009; Yalcin et al. 2010). Transition metal ion doping has been suggested to increase the response to solar spectrum over 380 nm and to develop the photocatalytic activity by introducing the defects into TiO2 lattice and reducing e−/h+ pair recombination. The result of the above has been the increased rate of *OH formation. The photocatalytic processes and their efficiency can also be enhanced by adding the auxiliary electron acceptors, such as H2O2 and O3 (Friedmann et al. 2010), but the results are not obvious and depend on many factors. H2O2 is one of the cheapest oxidants, and in the presence of UV irradiation at 254 nm, its photolysis into 2 *OH has been observed. However, the addition of too high H2O2 concentration hinders the process, since the excess of H2O2 captures the radicals (Daneshvar et al. 2004).
The double aptitude of the photocatalyst to simultaneously adsorb reactants and to absorb photons efficiently is one of the primary factors affecting the photocatalysis. It has been observed that due to the pre-adsorption of reactants on the surface of TiO2 during the photocatalytic reaction, the process of electron transfer is more efficient (Popa et al. 2010). The supported porous catalysts with higher surface area are often being chosen, since, apart from easier disposal, they ensure a high density of active centers for photocatalytic reactions and an enhanced light harvesting because of light reflection and scattering by the pores (Herrmann 2010).
Since all strategies, surface deposition of transition metals on photocatalyst, porous materials as the support for TiO2 and H2O2 as additional source of *OH, can enhance photooxidation, it was proposed to study the synergistic effect of both support (Al2O3), H2O2 addition and dopant (Cu, Zn, Fe, Co, Ni, Mo) on the UV photocatalytic activity of TiO2. In the present investigation, we carried out a series of studies aiming for the determination of the catalytic activity of Al2O3 supported and modified TiO2 photocatalysts in the removal of the pollutants from wastewater. As photocatalysts TiO2/Al2O3 modified by Cu, Zn, Ni, Fe, Co or Mo were used. These dopants were chosen because of their ability to increase the solar spectrum response towards Vis or enhance the UV photoactivity. The aim of the research was also to investigate the role of dopant on UV photocatalytic activity of studied catalysts. The photocatalytic oxidation of TX, a non-ionic surfactant, and a mixture of common surfactants, containing both non-ionic and anionic compounds available as commercial wash-up liquid (WL) were investigated. The effectiveness of the removal was measured in the following configurations: TiO2/UV/O2 and TiO2/UV/O2/H2O2. TX was chosen because of its main features: it is a widely used non-ionic surfactant and it can be considered as an endocrine disrupting compound in the aquatic environment. Typically used in households, wash-up liquid was used to examine the photoremoval of commercially available products that are not analytical grade.
2 Material and Methods
2.1 Catalysts Preparation
All the chemicals used for the preparation of the photocatalysts were analytical grade and used without further purification. The modified TiO2 catalysts supported by γ–Al2O3 (INS Pulawy, Poland) were chosen for the studies. The catalysts were prepared using the classical impregnation method (CIM) according to the procedure described in (Pasieczna-Patkowska et al. 2010). For the impregnation of TiO2/Al2O3 (sample TiO2) were used 5 wt.% aqueous solutions of nitrates: zinc(II), copper(II), cobalt(II), iron(III), nickel(II) and molybdenum salt (NH4)6Mo7O24•4H2O (POCH Gliwice, Poland) resulted in samples CIM-Zn, CIM-Cu, CIM-Co, CIM-Fe, CIM-Ni and CIM-Mo, respectively. The effect of doping was compared with the sample TiO2 as reference.
2.2 Photoreactor and Photocatalytic Studies
The experiments concerning the photocatalytic degradation of organics in the wastewater were conducted in a band reactor of our own construction described in a previous paper (Czech and Cwikla-Bundyra 2007) applying the procedure described in the already mentioned paper (Pasieczna-Patkowska et al. 2010). The band reactor is equipped with the UV lamp (254 nm, 50 Hz) with a light intensity of 1.5–2.2 mW/cm2, measured by the Radiometer VLX254 (Vilber Lourmat, 254 nm). The employed UV lamp emitted light of more than 95 % within the UV light wavelength.
Triton X-100 (POCH Gliwice, Poland) and a commercially available wash-up liquid (“Ludwik”, Inco Veritas, Poland) were chosen to be the model contaminants. The solutions were prepared so as to have the COD value of ca. 3,000 mg O2/dm3, which was the concentration of 1.3 × 10−3 vol.% for TX, and the concentration of wash-up liquid was 0.65 × 10−3 vol.%. The wash-up liquid contained: 5–15 % anionic, <5 % non-ionic and <5 % amphoteric surfactants and many others (e.g. oxyethylene fatty alcohols, amine oxides, EDTA, acetic acid, polyethylene glycol, 2-bromo-2-nitropropane-1,3-diol and inorganic additions). Hydrogen peroxide (0.01 vol.%) (Standard, Lublin, Poland) was also applied.
2.3 Catalysts Examination
Physicochemical properties of the support and the photocatalysts
Ti content [wt.%] Surface compositiona (ab:bc)
Dopant content [wt.%] Surface compositiona (ab:bc)
Total surface area SBET [m2/g]
Mean pore diameter [nm]
3 Results and Discussion
TiO2 (AEROXIDE® TiO2 P25, Evonik Degussa GmbH formerly Degussa P25), being widely tested (Herrmann 2010) and commercially available, is used as a standard catalyst in various studies, but the application of powder photocatalysts creates great technological problems in recycling, management and disposal. The powdered catalysts are inconvenient to study in the band reactor, as it is a flow-type device. In order to avoid these problems, the supported catalysts were used. The results obtained using suspended and immersed supported TiO2 photocatalysts are thought to be very difficult to compare directly.
3.1 Photocatalysts Characterization Results
3.1.1 Physicochemical Properties
The physicochemical properties of the catalysts determined by BET and XRF are presented in Table 1. From that data, it is shown that a similar amount of dopant −2.7–3.2 wt.%, was introduced to all photocatalysts, except CIM-Mo. During further impregnation and calcination, the BET surface area changed from 150 to 130 m2/g.
3.1.2 SEM and SEM-EDX Studies
The modification of previously prepared TiO2/Al2O3 has resulted in the uniform distribution of dopants. They are present in the same areas of all studied photocatalysts, namely dark points, and their amount in bright and dark areas is remarkably different. They are observed mainly in dark areas with the concentration even threefold increased (for example CIM-Co and CIM-Ni) in comparison with the content in dark areas, which may suggest that their adsorption occurred easier on Al2O3 than TiO2.
3.1.3 XRD Studies
3.1.4 Raman Spectroscopy Results
The characteristic peaks for ZnO, (388, 430, 461 and 498 cm−1), NiO (~570, ~730 and ~1,090 cm−1) modes are not clearly seen in the picture. The strong signal obtained from anatase in the Raman spectra of all studied catalysts indicates that Zn, Ni or Fe are incorporated into the structure of some alumina or alumina–titania forms. They are probably hidden and superimposed in the spectra though the deconvolution of the peaks was necessary.
The curve-fitting analysis was performed using the Peak Fit programme (Version 4.12). In the Raman spectra of studied photocatalysts, the deconvolution revealed the presence of peaks: CIM-Zn at 447, 597, 680 cm−1; CIM-Ni at 212, 325, 453, 583, 691 cm−1; CIM-Co at 482, 596, 689 cm−1; CIM-Mo at 211, 330, 380, 436, 824, 893, 952, and 1,005 cm−1. The peaks in CIM-Mo Raman spectra at 824 cm−1 indicate the presence of MoO3; in CIM-Co at 689 cm−1, CoTiO3; and at 496 cm−1, Co (Brundle and Morawitz 1982).
The Raman studies results have confirmed the data obtained by SEM/EDX and XRD. The surface of all photocatalysts looks similar. There are some observed agglomerates of TiO2, and dopants are introduced and adsorbed over Al2O3 creating some mixed alumina–titania or alumina–titania–dopant oxides.
3.1.5 UV–Visible Spectroscopy Results
However, the visible light response of the Cu-, Co- or Ni-modified photocatalysts did not pose any significant effect on their UV photocatalytic activity during the removal of studied pollutants because the employed light source emitted light of more than 95 % within the UV light wavelength but do not exclude their higher activity in Vis.
3.2 Photocatalytic Tests Results
The correlation between the surface areas of studied photocatalysts and the effectiveness of treatment is presented in Fig. 6. Generally, the removal of TX was independent from the surface area and the observed slight increase towards higher values is, however, not justified economically. The activity of Cu-modified TiO2 catalysts is strongly connected with the amount of a dopant by which Cu effectively inhibits the recombination of photo-induced charge carriers. The observed decrease in the removal of TX can be attributed to the excess of Cu in comparison to TiO2. It has been stated (Carvalho et al. 2010) that too high Cu values in the photocatalysts result in the more excessive oxygen vacancies, and thus Cu species become the recombination centers of photo-induced e–/h+. Cu covering the TiO2 surface can be noticed. The effect of Ni can be also attributed to the doping type (Yu et al. 2006), since uniform Ni doping can improve the photocatalytic activity only a little.
The synergistic effect of ZnO and TiO2 semiconductors has not been observed during the removal of TX. It can be, however, noticed during the removal of more complicated compositions and compounds, e.g. WL (Fig. 6b). That may be in agreement with the literature data, methyl orange photoremoval was slightly enhanced over ZnO-TiO2 (Kim et al. 2008).
In the WL removal (Fig. 6b), despite its more complicated composition, deeper photooxidation of organics has been caused by the doping. The effects have been better than over TiO2 for all the photocatalysts except CIM-Cu. The effectiveness of removal over CIM-Co or CIM-Zn has been similar—(97 %). It indicates that TiO2 may be involved in the oxidation of the non-ionic surfactants, and the dopants have facilitated photooxidation of the other WL components, even inorganic. However, about 5–8 % of the deepened photooxidation using modified photocatalysts has not been economically justified. The decreased efficiency of WL treatment observed for the photocatalysts with enhanced surface area may be connected with the increased adsorption of the components.
The effect of H2O2 addition has varied depending on the type of the pollutants used, or the photocatalysts (Fig. 6a, b). The photooxidation of TX over TiO2 and CIM-Ni has been deepened by H2O2. Moreover, CIM-Ni seems to be the most sensitive to H2O2 presence during the removal of TX in the band reactor, as the reduction of COD has increased from about 89 % to 96 %. When it comes to CIM-Co and CIM-Zn, no effect has been observed. For the other photocatalysts, namely CIM-Cu, CIM-Mo and CIM-Fe, the addition of H2O2 has hindered photooxidation. H2O2 addition seems not to influence the wash-up liquid photooxidation over any studied catalysts—the obtained results have been almost similar to photocatalytic oxidation.
The photooxidation of TX by the UV/TiO2 process can be described by the modified Langmuir-Hinshelwood-type kinetic equation. It is assumed that the reaction rate has a linear relation to light intensity for the experiments conducted at constant pH value and TiO2 dosage.
Pseudo-first-order reaction rate constants (k1) for photocatalytic oxidation of studied pollutants
k1 [×10−2 min−1]
k1 [×10−2 min−1]
TX: CIM-Co > CIM-Ni > TiO2 > CIM-Fe > CIM-Zn > CIM-Cu > CIM-Mo
WL: CIM-Ni > CIM-Zn > CIM-Co > CIM-Mo > CIM-Fe > CIM-Cu > TiO2
However, the highest values (over 0.9) of the coefficient of determination (R2) were obtained only for Ni-, Fe- or Co-modified photocatalysts which may indicate that only for this photocatalysts the kinetics follows the L-H model.
The observed loss of photocatalytic activity during TX removal using the transition metal doped TiO2 is suggested to be caused by the increase of the photogenerated e−/h+ recombination rate (Popa et al. 2010). Therefore, it can be implied that the results of doping catalysts depend on the type of the dopant, its amount and the treated compound.
All the modified TiO2/Al2O3 photocatalysts are efficient in the removal of the studied pollutants from water;
- 2.The effect of dopants depends on the type of treated pollutant:
The results indicate that the non-ionic surfactants were oxidized mainly by TiO2, whereas photooxidation of the other components of the wash-up liquid has been facilitated by dopants;
In generally, however, the addition of dopants to the TiO2/Al2O3 during TX and WL removal in the band reactor is not economically justified;
The addition of H2O2 (0.01 vol.%) variously influences the photooxidation of the photocatalysts under study but generally it is not economically justified;
Conducting the treatment for more than 2 h is not economically justified.
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- Brundle, C.R., Morawitz, H. (1982). Vibrations at surfaces (studies in surface science and catalysis, vibrations at surfaces: proceedings of the third international conference, Asilomar, California, U.S.A., 1-4 September, 1982, Elsevier.Google Scholar