Carbon-based TiO2 materials for the degradation of Microcystin-LA
Graphical abstract
Introduction
Cyanobacteria (aka, blue-green algae) are a diverse group of photo-autotrophic organisms which can be found in aquatic systems, such as oceans, freshwater lakes, rivers and reservoirs throughout the world [1], [2]. These organisms are essential to the food chain in many ecosystems; however, some species of cyanobacteria can also produce toxic metabolites, which are harmful to human health and ecosystems [3], [4], [5]. Microcystins (MCs) are among the most common cyanobacterial toxins found in water and freshwater. The variant microcystin-LR (MC-LR) is the most regularly found and investigated [6]. Other frequently detected variants include MC-YR, MC-RR and MC-LA, which are less studied even though their toxicity has been frequently reported [7], [8]. The structure of MC-LA is similar to MC-LR since these toxins only differ in one amino acid group in their chain, i.e., arginine for MC-LR and alanine for MC-LA [9], [10]. The intensification and persistence of MCs in water represents an emerging environmental and health concern. Therefore, the development of efficient water treatment technologies is currently required.
Various studies reported that the application of heterogeneous photocatalysis, using titanium dioxide (TiO2) as a photocatalyst, may improve the removal of MCs from water [11], [12], [13]. In addition, coupling TiO2 with carbon materials, such as carbon nanotubes (CNT) [14], [15], [16] and graphene oxide (GO) [13], [17], have provided a synergistic effect, which can enhance the overall efficiency of the photocatalytic process in particular under visible light illumination.
In previous studies [14], [18], [19], we have found that the efficiency of the photocatalytic process depends on the nature and content of the carbon material used for the preparation of the carbon-based TiO2 composites. CNT-TiO2 composites with 20 wt.% of the carbon phase revealed to be very efficient for the removal of phenol, methylene blue, caffeine, and diphenhydramine [14], [19], [20]. In the case of GO-TiO2 composites, the highest efficiency for diphenhydramine degradation was achieved using composites containing 4 wt.% of GO [21]. However, only a few studies have been focused on the degradation of MCs with carbon-based photocatalysts. For instance, in a recent work, Fotiou et al. [22] studied the degradation of MC-LR under both UV-A and solar light irradiation in the presence of graphene-based composites (GO-TiO2) and proved that the combination of GO with TiO2 leads to an increase in the efficiency of MC-LR removal when compared with the bare material (TiO2).
Taking into consideration these previous findings, a composite containing 4 wt.% of GO (GO-TiO2-4) and others comprising 4 wt.% and 20 wt.% of oxidized CNT (CNT-TiO2-4 and CNT-TiO2-20, respectively) are compared in the photocatalytic degradation of MC-LA under visible and simulated solar light irradiation for the first time in this study. Insights on the identification of the resulting intermediate products are also given.
Section snippets
Carbon materials preparation
Natural graphite (purity > 99.9995%, Sigma–Aldrich) was oxidized using the modified Hummers’ method, as described elsewhere [21], [23]. Then the oxidized material was dispersed in water and exfoliated under sonication for 1 h. Finally, the non-exfoliated graphite oxide was removed by centrifugation and a GO aqueous dispersion was obtained.
Multi-walled carbon nanotubes (CNTs > 95% purity, purchased from Shenzhen Nanotechnologies Co., Ltd.) were oxidized with a 10 M HNO3 solution (purity > 65%, Fluka) at
Thermogravimetric analysis and surface area
The carbon content (wt.%) in the composite materials was determined by thermogravimetric analysis; both neat TiO2 and the composites were submitted to a thermal treatment under air and the weight loss was monitored. The carbon content in the composite corresponds to the difference between the weight loss observed for the composite and that of neat TiO2. The obtained results are in a good agreement with the nominal carbon content (i.e. 4 wt.% or 20 wt.% depending on the composite) which indicate
Conclusions
The photocatalytic degradation of the cyanotoxin MC-LA under simulated solar light and visible light irradiation was performed by using carbon-based TiO2 composites containing 4 wt.% and 20 wt.% of GO and CNT as carbon phase. Neat TiO2 was used as reference.
The efficiency of the photocatalytic process depends on the nature and content of the carbon material used. Among the photocatalysts used, GO-TiO2-4 exhibited the highest photocatalytic activity under simulated solar light and visible light
Acknowledgements
Financial support for this work was provided by project PTDC/AAC-AMB/122312/2010 co-financed by FCT (Fundação para a Ciência e a Tecnologia) and FEDER (ERDF - European Regional Development Fund) through Programme COMPETE (FCOMP-01-0124-FEDER-019503), and partially by PEst-C/EQB/LA0020/2013 (financed by FCT and FEDER through COMPETE) and NORTE-07-0162-FEDER-000050, NORTE-07-0124-FEDER-000015 and NORTE-07-0202-FEDER-038900 (financed by QREN, ON2 and FEDER). FCT is also acknowledged for the grants
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