Photodegradation of microcystin-LR using graphene-TiO2/sodium alginate aerogels

doi:10.1016/j.carbpol.2018.07.007

Carbohydrate Polymers

Volume 199, 1 November 2018, Pages 109-118

https://doi.org/10.1016/j.carbpol.2018.07.007 Get rights and content

Highlights

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Sodium alginate helped to synthesize a robust graphene-TiO₂ aerogel.
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Graphene-TiO₂/sodium alginate aerogel efficiently degraded microcystin-LR.
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Synthesized aerogel was highly recyclable without producing any secondary pollution.
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In microcystin-LR degradation pathway, significant role of OH radicals was found.

Abstract

In this study, sustainable graphene oxide-TiO₂/sodium alginate and reduced graphene oxide-TiO₂/sodium alginate aerogels were synthesized and the potential of these aerogels was investigated for microcystin-LR degradation in aqueous solution. Along with the role of alginate in the synthesis of aerogels, effects of different concentrations of photocatalyst, photolysis, pH, and combination of TiO₂ (anatase)/Degussa P25 with graphene were investigated in lieu of microcystin-LR photodegradation.The complete degradation of microcystin-LR was attained in case of reduced graphene oxide-TiO₂/sodium alginate aerogel—not in graphene oxide-TiO₂/sodium alginate aerogel case—by the synergistic effect of adsorption and photodegradation. The recyclability study of reduced graphene oxide-TiO₂/sodium alginate aerogel demonstrated high stability and photoactivity and the degradation efficiency was not much hampered during six consecutive cycles of degradation reaction. The possible fragmentation pathways were also proposed based on identified intermediate products. High adsorption and degradation synergy and ease of separation/recycling of reduced graphene oxide-TiO₂/sodium alginate aerogel can make it a suitable option for removing microcystin-LR from water systems.

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 MC single bond LR (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, TiO_2, 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 MC single bond LR 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 MC single bond LR.

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 MC single bond LR. 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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Photodegradation of microcystin-LR using graphene-TiO2/sodium alginate aerogels

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Role of sodium alginate in GOTS and RGOTS aerogels

Conclusions

Acknowledgments

Revealing the degradation intermediates and pathways of visible light-induced NF-TiO2 photocatalysis of microcystin-LR

Applied Catalysis B: Environmental

LC/MS/MS structure elucidation of reaction intermediates formed during the TiO2 photocatalysis of microcystin-LR

Toxicon

Adsorption of dyes on hierarchical mesoporous TiO2 fibers and its enhanced photocatalytic properties

Journal of Physical Chemistry C

Mechanically strong, electrically conductive, and biocompatible graphene paper

Advanced Materials

Photocatalytic degradation of microcystin-LR and off-odor compounds in water under uv‑a and solar light with a nanostructured photocatalyst based on reduced graphene oxide−TiO2 composite. Identification of intermediate products

Industrial and Engineering Chemistry Research

Alginate/graphene oxide fibers with enhanced mechanical strength prepared by wet spinning

Carbohydrate Polymers

Environmental applications of semiconductor photocatalysis

Chemical Reviews

Mechanisms underlying degradation pathways of microcystin-LR with doped TiO2 photocatalysis

Chemical Engineering Journal

Sodium alginate/graphene oxide composite films with enhanced thermal and mechanical properties

Carbohydrate Polymers

Detoxification of microcystins (cyanobacterial hepatotoxins) using TiO2 photocatalytic oxidation

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Chemical Society Reviews

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Applied Catalysis B: Environmental

Photocatalysis on TiO2 surfaces: Principles, mechanisms, and selected results

Chemical Reviews

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Journal of Materials Chemistry

Mechanistic studies of the photocatalytic oxidation of microcystin-LR: An investigation of byproducts of the decomposition process

Environmental Science and Technology

Hydrothermal etching fabrication of TiO2@graphene hollow structures: Mutually independent exposed {001} and {101} facets nanocrystals and its synergistic photocaltalytic effects

Scientific Reports

Enhanced stability and mechanical strength of sodium alginate composite films

Carbohydrate Polymers

An overview of the toxic effect of potential human carcinogen microcystin-LR on testis

Toxicology Reports

Photodegradation of microcystin-LR using graphene-TiO₂/sodium alginate aerogels

LC/MS/MS structure elucidation of reaction intermediates formed during the TiO₂ photocatalysis of microcystin-LR

Adsorption of dyes on hierarchical mesoporous TiO₂ fibers and its enhanced photocatalytic properties

Photocatalytic degradation of microcystin-LR and off-odor compounds in water under uv‑a and solar light with a nanostructured photocatalyst based on reduced graphene oxide−TiO₂ composite. Identification of intermediate products

Mechanisms underlying degradation pathways of microcystin-LR with doped TiO₂ photocatalysis

Detoxification of microcystins (cyanobacterial hepatotoxins) using TiO₂ photocatalytic oxidation

Removal of Cr(VI) by 3D TiO₂-graphene hydrogel via adsorption enriched with photocatalytic reduction

Photocatalysis on TiO₂ surfaces: Principles, mechanisms, and selected results

Highly dispersive {001} facets-exposed nanocrystalline TiO₂ on high quality graphene as a high performance photocatalyst

Hydrothermal etching fabrication of TiO₂@graphene hollow structures: Mutually independent exposed {001} and {101} facets nanocrystals and its synergistic photocaltalytic effects