Colloids and Surfaces A: Physicochemical and Engineering Aspects
Research PaperMolecularly imprinted TiO2 photocatalysts for degradation of diclofenac in water
Graphical abstract
Introduction
During recent years, the preservation of groundwater and surface have been focus of increasingly restrictive legislation [1]. One group of chemicals of concerns are the compounds that encompass personal steroids, pharmaceuticals, pesticides, health care products, surfactants, and dyes [2], [3], [4], [5]. These so-called emerging contaminants possess high persistence and low biodegradability [6]. As a consequence, pharmaceuticals and their metabolites have increasingly been found in several water bodies, including surface water and effluents from wastewater treatment plants. These compounds are of great concern because of their potential impact on human health and the environment even at low concentration levels [7].
Diclofenac (DIC) is one of the most widely used non-steroidal anti-inflammatory drugs used to reduce inflammation, and as an analgesic in conditions such as arthritis or acute injury. Previous investigations have shown that 15% of DIC was excreted unchanged after consumption [8]. Today it is still one of the most frequently detected pharmaceuticals in the water environment [9], and it has been detected in both the influents and effluents of wastewater treatment plants at concentrations up to mg/L level [6], [10]. As conventional wastewater treatment systems cannot efficiently remove DIC [11], it is necessary to develop alternative methods to effectively remove DIC from contaminated aquatic environment.
A condition that is necessary for complete removal is the mineralization of the contaminant, which can be achieved by using advanced oxidation processes (AOPs). Considering emerging contaminants, the heterogeneous photocatalysis is one of more promising strategies among AOPs. The use of TiO2 as catalyst encompasses several advantages, such as low cost and low toxicity [12] photocatalytic oxidation [10], [11].
Recently, several studies have explored the viability of employing photocatalysis for degradation of DIC [9], [13], [14], [15]. However, a serious shortcoming in heterogeneous photocatalytic oxidation is its low selectivity to the target contaminants, and the photocatalysts cannot differentiate highly toxic target pollutants from other organic compounds of low toxicity [16], [17]. In real scenarios, effluent streams may contain highly toxic organic pollutants (normally non-biodegradable) that coexist with less toxic and biodegradable molecules. It turns out that usually the former is present at a lower concentration [16], [18], and thus it is desirable preferential degradation toward the most toxic substance.
To overcome the problem of the co-existing non-target molecules, the molecular imprinting technique can be exploited to increase the selectivity of photocatalytic degradations. Molecular imprinting is well-known for its capability of creating template-defined molecular recognition sites in synthetic polymers [19]. With this technique, specific binding sites can be created in either organic or inorganic materials using template-directed radical polymerization or polycondensation [20], [21]. Although there are some reports in the literature showing the feasibility of preparing MIPs selective for DIC [22], [23], [24], [25], the combination of photocatalysis with molecular imprinting to enable selective removal of toxic pharmaceuticals is an emerging field [15], [17].
Besides the lack of selectivity, bare TiO2 have other intrinsic drawbacks that limit its applications, such as the difficulty to be recycled [26] and its limitation of relying on UV light for photocatalysis. The dependence of bare TiO2 on UV light for catalysis is due to the large bandgap of approximately 3.2 eV [27]. In order to improve the visible-light-active photocatalytic efficiency and to inhibit charge recombination, several research groups have doped TiO2 with special compounds that are able to decrease the band gap energy. As a result, the doped TiO2 can effectively be activated by visible light. In this sense, doping TiO2 with non-noble metals has been increasingly studied [28], [29], [30]. Among the non-noble metals, Cu2O has been found to be a good modifier for achieving both UV and visible-light responses [31], [32], [33], [34], [35], [36], [37].
Several papers have reported the use of relatively high loading of TiO2, from 200 to 1000 mg/L, to photochemically degrade DIC [13], [14], [38], [39], [40], [41]. Moreover, it has been shown that lower concentration of TiO2 might be used in combination of imprinting technology aiming to concentrate the target molecule before the photodegradation [42]. Based on the existing results, we envisaged that by introducing specific DIC binding sites into TiO2-based photocatalysts, it should be possible to use the new composite to concentrate target pollutants before they are photodegraded. In this way, it should be possible to lower the consumption of photocatalyst, realize higher selectivity and reduce the treatment cost.
The aim of this work was to develop a TiO2-MIP composite to realize target-selective photocatalysts. The inorganic-organic composite material was prepared via precipitation polymerization using a low TiO2 loading. Our main goal was to enhance the selectivity of the photocatalyst using a commercial sample (Degussa P25) as a bench mark. In addition, we also studied the combination of MIP with Cu2O-doped TiO2, and investigated the performance of the new, visible light-activated photocatalyst. To the best of our knowledge, modifying Cu2O-doped TiO2 with organic MIPs has not yet been reported.
Section snippets
Materials and methods
Acetonitrile (99.7%), methacrylic acid (MAA, 98.5%), azobisisobutyronitrile (AIBN, 98%), trimethylolpropane trimethacrylate (TRIM) and D-(+)-glucose were purchased from Merck (Darmstadt, Germany). AIBN was recrystallized from methanol before use. Fluoxetine (FLU), paracetamol (PARA), diclofenac (DIC) and titanium dioxide (P25) were purchased from Sigma-Aldrich (Steinheim, Germany). Hydrated copper (II) acetate was purchased from Acros Organic (Geel, Belgium). Diclofenac sodium was extracted
Characterization
The surface area and pore characteristics of the photocatalysts prepared in this work are shown in Table 1. For the sake of clarity, a brief description of each material is also given. The imprinted material containing non-extracted template molecules (NEP) had a surface area 19.2- and 4-folds of the non-imprinted material NIP25 and NIP, respectively. This dramatic difference of surface area indicates that the presence of the template perturbs the system and affects the formation of the pores.
Conclusions
In this work, molecularly imprinted photocatalysts containing a low loading of TiO2 and Cu2O-doped TiO2 were prepared using a precipitation polymerization method. The imprinted photocatalysts exhibited target-specific molecular binding and degradation for a model water pollutant, diclofenac. Compared to the commercially available TiO2, the molecularly imprinted photocatalysts displayed significantly higher selectivity and efficiency for degradation of the target pollutant due to the presence of
Acknowledgements
This project was partially funded by the Brazilian National Council for Scientific and Technological (CNPq) and the Swedish Research Council FORMAS (contract no. 212-2013-1305). C. Escobar is grateful for the grant provided by the Improvement of Higher Education Personnel (CAPES). The authors wish to thank the LNLS (Project D11A-SAXS1-8691) for the SAXS beamline measurements, and Tripta Kamra for carrying out the SEM analysis.
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