Effect of TiO2–Pd and TiO2–Ag on the photocatalytic oxidation of diclofenac, isoproturon and phenol
Graphical abstract
Introduction
Concern is growing about the potential effects on health of a group of anthropogenic contaminants known as “emerging contaminants” (ECs) [1] which include, among others, the herbicide isoproturon (IP) [2], [3] and diclofenac sodium salt (DCF). IP, which is included on the EU list of priority substances in the field of water policy [4], is one of the most commonly used herbicides in the world, in particular on cereal crops. Its half-life is approximately 40 days in moderate climates and 15 days in tropical climates. DCF is a non-steroidal, anti-inflammatory drug commonly used as an analgesic, antiarthritic and antirheumatic agent. About 15% of the drug is excreted unmodified after human consumption [5].
There are various methods that can be used to eliminate organic pollutants including activated carbon or WWTPs (wastewater treatment plants). However, adsorption on activated carbon results in pollutant transfer and not in pollutant elimination and WWTPs are often unable to fully eliminate all pollutant types [6].
Advanced oxidation techniques can also be employed for organic pollutant removal. Among these techniques are AOPs (Advanced Oxidation Processes), which generate hydroxyl radicals that are able to mineralize the organic components to CO2 and water. TiO2 photocatalysis is one of the most promising AOPs in the removal of organic pollutants.
The principles of heterogeneous photocatalysis have been extensively discussed in the literature [7], [8]. Heterogeneous photocatalytic degradation of organic compounds begins with generation of electron–hole pairs (e−–h+) in the semiconductor particles by total or partial absorption of photons of light. If charge separation is maintained the e−–h+ pair will migrate to the TiO2 particle surface. Once on the surface, the e−– h+ pair can react with other species at the interface. The e−–h+ pairs which do not manage to separate and react with species on the surface recombine. This recombination can take place both on the surface of the particle and in its interior, with the latter situation being one of the problems of heterogeneous photocatalysis.
One way of improving efficiency by favoring separation of the e−–h+ pair comprises the deposition of metals on the titanium dioxide surface. This procedure is commonly used as a technique to enhance photocatalytic activity and impede e−–h+ recombination. As their Fermi level is lower than that of the TiO2, the metal deposits on the catalyst surface act as traps for the photogenerated electrons, providing sites for their accumulation. This enhances separation of the photogenerated e−–h+ pairs, as well as the separation of sites where reduction occurs (metal deposits) and the oxidation that takes place on the photocatalyst surface [9], [10]. In addition, the photoelectrons can enhance the rate of oxygen photoreduction and favor the generation of hydroxyl radicals [11], thereby contributing to more effective organic pollutant photoelimination.
The effects of silver and palladium on photocatalytic removal of organic substrates have previously been described in the literature [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. The activity of metal-modified materials depends on the nature of the organic compound as well as on other factors which include pollutant concentration, pH, metal type and load [10], [12].
Silver deposits on TiO2 have been shown to enhance the mineralization of mono-, di- and poly-carboxylic acids [12] and the removal of 2-propanol [17], chloroform and urea [19]. Meanwhile, palladium deposits have been shown to enhance removal of 2,4-dinitrofenol, formaldehyde and trichloroethylene [13], as well as the colorant methylene blue [20], [21].
The present work studies the effect of Ag and Pd deposits on a lab-made TiO2 (SG) synthesized following a sol–gel method in the photocatalytic removal of PHL, IP and DCF. This lab-made catalyst was chosen as it has been shown in previous studies to have a high degree of efficiency in the elimination of different pollutants from water [22], [23], [24].
The metals were photodeposited following a method similar to that described by Maicu et al. [25]. DCF and IP were chosen as they are examples of emerging contaminants [26], [27] and PHL was chosen as it is considered the reference molecule for use in photocatalysis studies [11], [28], [29], [30], [31], [32]. The tests were performed with the photocatalysts in suspension. The photodegradation intermediates of the pollutants were also identified.
Section snippets
Materials
Reagents: IP (99%) and methanesulfonic acid (99%) were supplied by Fluka; PHL (99%), DCF (99%), methanol (⩾99.9%), ethanol (⩾99.9%), titanium butoxide (97%), phosphoric acid (⩽85%), palladium (II) nitrate dihydrate (99%) and nitrate silver (99%) were supplied by Sigma–Aldrich; citric acid (99.5%), ammonium formate (⩾99%), sodium hydroxide (98%), and isopropanol (98.5%) were supplied by Panreac and acetonitrile (⩾99.8%) was used with an HPLC grade (Chromanorm).
Synthesis of the catalysts
The photocatalysts used in these
Characterization of the photocatalysts
Fig. 1(a) shows a comparison of the XRD patterns of the TiO2 (SG) and the TiO2 photodeposited with Pd and Ag (SG-1%Pd and SG-1%Ag). The peaks marked A and R correspond to the anatase and rutile phases, respectively. The majority phase is anatase in all samples with diffraction peaks at 25.35°, 37.78°, 47.5°, 53.90° and 62.72° (2θ). An A/R phase content ratio of 71/29 was found for the TiO2 (SG). Photodeposition of the noble metals did not alter the phase composition of the TiO2. The crystalline
Preliminary studies: photolysis and adsorption
The photolysis tests resulted in 55% DCF removal, while the results for IP and PHL were quantitatively insignificant. A period of 1 h was established to attain adsorption equilibrium for the IP and DCF with 25% removal of IP and 5% of DCF with all catalysts. However, PHL adsorption on the catalysts was quantitatively insignificant.
Photocatalytic experiments
Fig. 5 shows that photocatalytic degradation of PHL, IP and DCF seems to follow a first-order kinetic and, thus, the apparent reaction rate constants were determined from the slope of the plot ln [C/C0] vs. reaction time. Fig. 6 shows the apparent photodegradation rate constants (kapp) of the catalysts in suspension for the photodegradation of 50 mg·L−1 of PHL, IP and DCF in ultrapure water at pH 5.
An increase in SG photocatalytic activity after incorporation of the metals can be seen in the
Conclusions
The effects of silver and palladium metals on the photocatalytic degradation of isoproturon (IP), diclofenac (DCF) and phenol (PHL) in water over lab-made TiO2 (SG) were investigated. It was found that the addition of silver or palladium had a significant effect on the photocatalytic degradation of PHL, DCF and IP. The test conditions during the photodeposition of both metals determined their final oxidation state, with reduced particles of palladium and silver as well as silver oxides found on
Acknowledgements
We thank the Ministry of Economy and Competitiveness of the Government of Spain, for its financial support through the projects NANOBAC (IPT-2011-1113-310000) and 2010-3E UNLP10-3E-726. Rocio Espino-Estévez expresses her gratitude for the support of the FPI Grant Program of the Spanish Ministry of Education and Science BES-2010-036537. Finally, the authors also appreciate the contribution to this work of the lab technicians, Omayra Dominguez Santana and Ezequiel Henríquez Cárdenes.
References (68)
- et al.
Heterogeneous photocatalysed degradation of a herbicide derivative, isoproturon in aqueous suspension of titanium dioxide
J. Environ. Manage.
(2003) - et al.
Photochemical versus coupled photochemical–biological flow system for the treatment of two biorecalcitrant herbicides: metobromuron and isoproturon
Appl. Catal. B
(2000) - et al.
Factors affecting diclofenac decomposition in water by UV-A/TiO2 photocatalysis
Chem. Eng. J.
(2010) - et al.
Photocatalytic degradation of phenol in the presence of near-UV illuminated titanium dioxide
J. Photochem. Photobiol. A.
(1992) - et al.
Photocatalytic degradation of organic water contaminants: mechanisms involving hydroxyl radical attack
J. Catal.
(1990) - et al.
Effects of Ag and Pt on photocatalytic degradation of endocrine disrupting chemicals in water
Chem. Eng. J.
(2005) - et al.
Photocatalytic properties of surface modified platinised TiO2: effects of particle size and structural composition
Catal. Today
(2007) - et al.
Clarifying the role of silver deposits on titania for the photocatalytic mineralisation of organic compounds
J. Photochem. Photobiol. A
(2006) - et al.
Photocatalytic activities enhanced for decompositions of organic compounds over metal-photodepositing titanium dioxide
Chem. Eng. J.
(2004) The effect of the presence of Ag nanoparticles on the photocatalytic degradation of oxalic acid adsorbed on TiO2 nanoparticles monitored by ATR-FTIR
Mater. Chem. Phys.
(2014)