Photodegradation of microcystin-LR using graphene-TiO2/sodium alginate aerogels
Graphical abstract
Introduction
Increasing global water temperature, nutrient enrichment via anthropogenic runoff, droughts, and flooding lead to the eutrophication of fresh and coastal water bodies and result in toxicological cyanobacterial blooms. These blooms deteriorate the quality of water and are responsible for producing cyanotoxins. Microcystins are the most commonly detected cyanotoxins and are strong hepatotoxins that promote tumors by inhibiting protein phosphatase type 1 and PP2 A (Rastogi, Sinha, & Incharoensakdi, 2014). Microcystin-leucine arginine (MC-LR)—denoted as cyclo[-Adda-Glu-Mdha-Ala-Leu-MeAsp-Arg-]—is considered to be the most abundant, ubiquitous, and toxic variant of microcystins (Merel et al., 2013, Fotiou et al., 2013). MC-LR can cause cellular damage and liver cancer in humans (McLellan & Manderville, 2017). It also has damaging effects on kidney, heart, and the gastrointestinal tract (Q. Wang, Xie, Chen, & Liang, 2008). Li et al. found lethal effects of MC-LR on the reproductive systems of rats and testes were found to be the main target (Y. Li, Sheng, Sha, & Han, 2008). The World Health Organization has recommended 1.0 μg/L of MC-LR as a provisional safety guideline in drinking water (WHO, 1998), but at some places such as Sagar lake—India—the concentration was found to be as high as 0.67 μg/mL (Lone, Koiri, & Bhide, 2015). MC-LR cannot be easily removed using typical treatment processes due to its high stability, cyclic structure, and presence of distinctive amino acids (X. Wang et al., 2017).
Titanium dioxide (TiO2) has been employed to degrade MCLR (Andersen, Han, O’Shea, & Dionysiou, 2014). However, large bandgap of TiO2, high recombination of photogenerated electron-hole pairs, and possibility of secondary pollution owing to the difficult separation from the effluent make TiO2 nanoparticles less suitable option on a practical scale. These above limitations were met in this study by making a composite of TiO2 nanoparticles with graphene oxide (GO) in the presence of sodium alginate. Alginate—an inexpensive, non-toxic and biodegradable polymer—is derived from brown seaweeds and this anionic polysaccharide is composed of mannuronic acid (M) and guluronic acid (G) residues (Li, Yang, Li, Lan, & Peng, 2017). The binding of TiO2 with alginate is well established owing to the abundance of functional moieties present in alginate (Mihailović et al., 2010). However, alginate alone has not been a good support for photocatalysts due to high hydrophilicity, low thermal stability and poor mechanical strength (Liu, Li, & Li, 2016). GO has been used to improve the thermal stability and poor mechanical strength of alginate (Ionita, Pandele, & Iovu, 2013) and also for the effective charge separation when used as a photocatalyst in the form of graphene-TiO2/sodium alginate aerogel. Graphene receives attention because of its large specific surface area, excellent partitioning of photogenerated charges, and good thermal and electrical conductivities (Nawaz, Miran, Jang, & Lee, 2017). Pristine graphene, however, has limited chemical functionalities, which restrict its interaction with TiO2 and sodium alginate. Graphene oxide—a derivative of graphene—contains many functional groups such as carboxyl, epoxy, and hydroxyl groups, which are helpful in making bonds and composite materials with TiO2 nanoparticles. Moreover, graphene oxide also interacts with sodium alginate chains through hydrogen bonding (Zhao et al., 2014) and the subsequent chemical reduction of the blend of graphene oxide, TiO2, and sodium alginate makes a mechanically strong reduced graphene oxide/TiO2/sodium alginate (RGOTS) hydrogel, which upon freeze drying is converted to aerogel. Reduced graphene oxide can more efficiently suppress the charge recombination and expedite the transfer of electrons from the conduction band of TiO2 than graphene oxide. In addition, the introduction of reduced graphene oxide in photocatalysts can substantially inhibit the agglomeration and dissolution of nanoparticles. The three dimensional structural network and high surface to volume ratio of aerogel can further improve the stability and photocatalytic activity of the catalyst.
The aim of this study was to synthesize a robust graphene-TiO2/sodium alginate aerogel and to check its photocatalytic efficacy against MCLR under UVA light. The strong bonding between sodium alginate and graphene oxide was proposed for the increase in mechanical strength and recyclability of aerogel. The effect of graphene oxide reduction on MCLR degradation was expected which was checked by comparing the efficiency of RGOTS aerogel with graphene oxide-TiO2/sodium alginate (GOTS) aerogel. The study also included the effects of important parameters such as adsorption, pH, and optimal concentration of graphene oxide and TiO2 in graphene-TiO2/sodium alginate aerogels. Identification of degradation products was carried out to draw the degradation pathways of MCLR.
Section snippets
Materials
All reagents were of analytical grade and used without further purification. Microcystin-LR (≥ 95%) was purchased from Cayman Chemical Company, USA. TiO2 nanopowder anatase phase (product # 63725, average particle size = 23 nm, 99.7% trace metals basis) and L-ascorbic acid were purchased from Sigma-Aldrich (South Korea). Degussa P25 was obtained from Degussa AG (Germany). Sodium hydroxide, calcium chloride, hydrochloric acid, sodium nitrate, sulfuric acid, hydrogen peroxide, and ethyl alcohol
Role of sodium alginate in GOTS and RGOTS aerogels
In the fabrication of GOTS and RGOTS aerogels, the mixture of graphene oxide, sodium alginate and TiO2 nanoparticles was made at first stage. The electrostatic repulsion between graphene oxide and sodium alginate—due to the presence of negative ions—allowed graphene oxide to be well dispersed in sodium alginate aqueous solution (He et al., 2012) and formed a uniform solution. TiO2 nanoparticles had also been favorably dispersed and stabilized in alginate solution to form an efficient
Conclusions
A robust RGOTS aerogel was synthesized that showed a high synergy of adsorption and photodegradation—unlike other photocatalytic materials—in the photocatalytic degradation of MCLR. The strong bonding between sodium alginate and graphene oxide assisted in increasing the mechanical strength of RGOTS aerogel and the aerogel showed high recyclability without any rupture. The reduced form of aerogel was more efficient for MCLR degradation than the pristine form of graphene oxide. The effect of
Acknowledgments
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2018R1A6A1A03024962, NRF-2016R1A2B4010431, and NRF-2009-0093819).
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