Invited paperEnhanced dehydrochlorination of 1,1,2,2-tetrachloroethane by graphene-based nanomaterials☆
Graphical abstract
Introduction
Graphene-based nanomaterials, such as graphene oxide (GO) and reduced graphene oxide (RGO), have outstanding structural and electronic properties (Dreyer et al., 2010), and have proven to be great candidates for a range of applications, including catalysts (Huang et al., 2012), sensors (Chen et al., 2012), supercapacitors (Kim et al., 2010), to mention a few. The projected large-scale production and use of graphene-based materials make their environmental release highly probable, and the potential environmental impacts of these nanomaterials have received much attention (Hua et al., 2015, Lanphere et al., 2014, Liu et al., 2015, Pei et al., 2013, Wang et al., 2014, Zhao et al., 2014).
Many graphene-based materials contain abundant surface O-functionalities, such as carboxyl, carbonyl, and phenolic groups (Bagri et al., 2010, Dreyer et al., 2010). These surface O-functional groups not only render high colloidal stability and mobility in aquatic environments (Chowdhury et al., 2013, Qi et al., 2014b, Wu et al., 2013), but also make some of the materials highly reactive (Fu and Zhu, 2013, Gao et al., 2011, Kong et al., 2014, Perhun et al., 2013, Tan et al., 2013). It has been reported that GO and RGO can either directly participate or catalyze chemical or biological reactions (Fu and Zhu, 2013, Kong et al., 2014, Perhun et al., 2013, Zhang et al., 2015). For example, Fu and Zhu found that GO (at concentrations as low as 2 mg/L) can significantly facilitate the reduction of nitrobenzene by sulfide (Fu and Zhu, 2013). Zhang et al. reported that RGO improved the stability of enzyme up to 7 folds, by quenching superoxide radicals and serving as a redox mediator (Zhang et al., 2015).
Given their high surface O-contents, many graphene-based materials likely can affect the hydrolysis of organic contaminants, one of the most important abiotic transformation reactions controlling contaminant fate in the environment. Hydrolysis reactions are often acid-catalyzed or base-catalyzed (Schwarzenbach et al., 2003; Tinsley, 2004), and the surface O-functional groups—in particular, carboxyl and phenolic groups—may serve as Lewis acids or bases to catalyze such reactions. In our previous studies (Chen et al., 2014a, Chen et al., 2014b), we found that functionalized carbon nanotubes and activated carbons can catalyze the dehydrochlorination of 1,1,2,2-tetrachloroethane (TeCA), in that the deprotonated surface O-functional groups serve as conjugated bases to catalyze the reaction. Mackenzie et al. (Mackenzie et al., 2005) and Kopinke et al. (Kopinke et al., 2016) also reported that activated carbons can catalyze hydrolysis of TeCA and reductive dechlorination of trichloroethylene (TCE). Compared with carbon nanotubes and activated carbons, certain graphene-based materials are richer in surface O-functional groups (Dreyer et al., 2010), and will likely have even greater effects on hydrolysis reactions. Note that depending on the specific preparation methods graphene-based materials can vary remarkably in surface chemistry, including hydrophobicity, C/O ratio, and type and distribution of O-functional groups (Dreyer et al., 2010). It is likely that different materials will have different catalytic efficiency. To date, little is known about the effects of graphene-based materials on environmentally relevant hydrolysis reactions.
The main objectives of this study were to understand the catalytic effects of graphene-based materials on environmentally relevant hydrolysis reactions, and to link catalytic efficiency to key surface chemistry parameters of the materials. Seven GO and RGO materials that varied in C/O ratio, hydrophobicity, and type/distribution of surface O-functionality were obtained, and their effects on the dehydrochlorination of TeCA (which forms a sole transformation product, TCE) were examined. Additionally, the effects of divalent cations and dissolved organic matter (DOM) on the GO/RGO-catalyzed reactions were examined to further understand the underlying mechanisms controlling the catalytic efficiency of the GO/RGO materials.
Section snippets
Materials
Two GO products (referred to as GO1 and GO2) and an RGO product (referred to as RGO1) were purchased from Nano Materials Tech Co. (Tianjin, China). Based on the information provided by the supplier, GO1 and GO2 were synthesized using a modified Hummers method and RGO1 was obtained by reducing GO1 with hydrazine hydrate. Four more RGO materials were obtained by photochemically reducing GO2, using a XPA-7 photochemical reactor and a 500 W mercury lamp to provide UV light (Xujiang
Characteristics of GO and RGO materials
Selected physicochemical properties of the GO and RGO materials are summarized in Table 1. The XPS results (also see the spectra in Supplementary data Figs. S4 and S5) indicate that the two GO materials (GO1 and GO2) had different C/O ratios, whereas the types and distribution of their surface O-functional groups were comparable (e.g., both contained carbonyl and carboxyl groups). The chemically reduced product of GO1, i.e., RGO1, had a much higher C/O ratio (4.5) than GO1 (2.1), owing to the
Conclusions
Graphene oxide and reduced graphene oxide materials can catalyze the dehydrochlorination of TeCA under environmentally relevant conditions. The catalytic effects of graphene-based materials on the hydrolysis reaction originate from the deprotonated acidic groups of the material, which serve as conjugated bases to catalyze the base-catalyzed reaction. The adsorption affinity of a graphene-based material is also a determining factor controlling the catalytic efficiency of the material, in that it
Acknowledgment
This project was supported by the Ministry of Science and Technology of China (Grant 2014CB932001), and the National Natural Science Foundation of China (Grants 21237002 and 21425729).
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This paper has been recommended for acceptance by Baoshan Xing.