Partly reduced graphene oxide aerogels induced by proanthocyanidins for efficient dye removal
Introduction
Dyestuff wastewater from various industries, including printing and dyeing, leather, food and hairdressing and so forth, has received considerable attention due to its high toxicity, deep chromaticity and teratogenicity, etc (Dai et al., 2018a, Dai et al., 2018b, Song et al., 2016). On the other hand, most of the dye-containing effluents are stable and non-degradable in the natural environment, thus deteriorating the pollution of dyestuff. Many efforts have been devoted to developing various technologies for the treatment of effluents so far (Liew et al., 2018, Shen et al., 2015). Adsorption has been proved to be one of the most effective and economical methods for the removal of organic dyes from the effluents attributed to its simplicity and high efficiency. In recent years, many types of adsorbents have been developed to remove all kinds of dyestuffs from the wastewater (Li et al., 2019, Fan et al., 2013, Jia et al., 2019, Li et al., 2014, Chen et al., 2018a, Chen et al., 2018b, Liu et al., 2018a, Liu et al., 2018b, Liu et al., 2018c, Liu et al., 2017a, Liu et al., 2017b, Pan et al., 2017). In comparison with conventional adsorbents such as activated carbon, new adsorbents generally containing nano-fillers, graphene for example, have attracted enormous interest. Inspired by the porous structure of activated carbon, new strategies for preparing effective adsorbents are to focus on the fabrication of graphene reinforced composites with a sponge-like porous structure.
Graphene is a 2D carbonic material, exhibiting unique properties like large specific surface area, excellent mechanical property and outstanding gas barrier property. As a result, it has been employed as a versatile ingredient in preparing gas barrier materials, catalysts, electrically conductive materials and adsorbent materials, etc. (Du et al., 2017, Liu et al., 2017a, Liu et al., 2017b, Liu et al., 2018a, Liu et al., 2018b, Liu et al., 2018c). Graphene oxide (GO) is an important derivative of graphene. Importantly, GO sheet exhibits several kinds of oxygen containing groups, such as hydroxyl, epoxide, carboxyl and carboxide groups (Liu et al., 2013). These functional groups make GO compatible with water soluble polymer matrices. However, most of organic dyes possess aromatic structures. Hence, in order to administer the adsorption properties GO sheets in the composites should be reduced to graphene sheets for the purpose of adsorbing organic dyes. On the other hand, in consideration of polymer matrices are hydrophilic, some hydrophilic groups of GO should be remained for improving the compatibility of graphene sheets and the matrices. Therefore, GO sheets are desired to be partly reduced for these two factors.
Many reagents such as organic, inorganic and metallic materials have been employed to efficiently reduce GO to reduced graphene oxide (RGO) (Moon et al., 2010, Fan et al., 2011, Stankovich et al., 2007) The toxic hydrazine hydrate was the first reported reducer of GO (Stankovich et al., 2007). The hydrazine hydrate can almost completely remove the carbonyl, epoxide and carboxyl groups out of GO sheets, resulting in the incompatibility between hydrophobic RGO sheets and hydrophilic polymer matrices (Yang et al., 2010). The reductive Fe powder also exhibited strong reducing ability to convert GO into RGO completely (Fan et al., 2011). The biomass materials oligomeric proanthocyanidins (OPC) are a kind of naturally abundant polyphenols, which can be easily obtained from plant extract. It is currently recognized as the most effective natural antioxidant for scavenging free radicals in the human body. Hence, it is an ideal reducing agent to reduce GO to graphene sheets. Indeed, there were several papers reporting the efficient reduction of GO by OPC (Li et al., 2016, Wu et al., 2015). However, in the present paper, the partial reduction of GO by biomass OPC and the ternary functions of OPC in simultaneous partial reduction, surface modification and assembly of GO sheets to OPC-PRGO aerogels were highlighted. The as prepared OPC-PRGO areogels exhibited a porous structure and a marvelous adsorption property towards organic dyes, providing a solid basis for the design of graphene based adsorbent materials for wastewater purification. This is the first example of intently preparing partly reduced and surface modified graphene for the purpose of efficient adsorption property towards organic dyes. Therefore, partly reduced graphene oxide paves the way for preparing hydrophilic/graphene composite and exhibits potential application in dye adsorption.
Section snippets
Materials
Proanthocyanidins (C30H26O13) were purchased from Dostov biological reagent Co. Ltd., China. Natural flake graphite (>99%) was bought from Najing Pionerr Nano Co., Ltd., China. Concentrated sulfuric acid (H2SO4, 95%), hydrochloric acid (HCl), potassium permanganate (KMnO4), crystal violet (C25H30N3Cl), amino black (C22H14N6O9S2∙Na2), neutral red (C15H16N4∙HCl) and methylene blue (C16H18ClN3S·3H2O) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Hydrogen peroxide (H2O2, 30%),
The mechanisms of partial reduction and assembly of GO by OPC
When OPC was mixed with GO solution, OPC molecules were adsorbed onto the surface of GO sheets by the strong intermolecular forces such as hydrogen bonding and π-π interaction between OPC and GO sheets. Then, the hydroxyl groups of OPC reacted with the epoxy groups of GO sheets via a nucleophilic ring opening process (Liu et al., 2013), resulting in the chemical linkage of GO and OPC. When the temperature increased, the hydroxyl groups of OPC would induced a dehydration reaction between OPC and
Conclusions
OPC was employed as a gentle reducing and modifying reagent to realize the conversion of GO to OPC-PRGO. This OPC-PRGO provided a versatile platform for both improving the compatibility of PRGO with hydrophilic polymeric matrices and capturing dyes onto the adsorbent framework. The OPC-PRGO aerogel exhibited a honeycomb-like structure. The removal efficiencies of OPC-PRGO aerogels towards MB, CV, NR and RB were observed to be 97.5, 97.8, 94.5 and 95.1%, respectively. The enhanced adsorption
Acknowledgements
The authors express their gratitude to the financial supports of HAUST (No. 13480067, 13480051), National Natural Science Foundation of China (51675162, U1704144). The project was also supported by the Collaborative Innovation Center of Non-ferrous Metals, Henan Province, China, Henan Province Key Laboratory of Nonferrous Metal Material Science and Processing Technology, Luoyang, China.
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