Elsevier

Environmental Pollution

Volume 214, July 2016, Pages 341-348
Environmental Pollution

Invited paper
Enhanced dehydrochlorination of 1,1,2,2-tetrachloroethane by graphene-based nanomaterials

https://doi.org/10.1016/j.envpol.2016.04.035Get rights and content

Highlights

  • Graphene-based materials catalyze hydrolysis of 1,1,2,2-tetrachloroethane.

  • Catalytic effects originate from deprotonated surface O-functional groups.

  • Adsorption affinity of a material plays important role in catalytic efficiency.

  • Catalytic effects of nanomaterials may significantly affect contaminant fate.

Abstract

Graphene oxide (GO) and reduced graphene oxide (RGO) materials contain a variety of surface O-functional groups that are chemically reactive. When released into the environment these materials may significantly affect the abiotic transformation of organic contaminants, and therefore, may alter their fate and risks. We found that two GO and five RGO materials that varied in C/O ratio, hydrophobicity, and type/distribution of surface O-functionality all had catalytic effects on the dehydrochlorination of 1,1,2,2-tetrachloroethane (TeCA). Even though the catalytic effects of the materials originated from their deprotonated surface O-functional groups, which served as conjugated bases to catalyze the reaction, the catalytic efficiencies of the materials did not correlate strongly with their surface O contents. The spectroscopic evidence (X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy), surface charge data, and adsorption experiments demonstrated that the catalytic efficiencies of the GO/RGO materials were controlled by a complex interplay of the type and distribution of surface O-functionality, as well as adsorption affinity of the materials. Both Ca2+ and Mg2+ inhibited the catalytic efficiency of the materials by binding to the surface O-functional groups, and consequently, decreasing the basicity of the functional groups. At an environmentally relevant concentration of 10 mg/L, Suwannee River humic acid (used as a model dissolved organic matter) alone had little effect on the dehydrochlorination of TeCA. However, it could inhibit the catalytic efficiency of the GO/RGO materials by coating on their surface and thus, decreasing the adsorption affinity of these materials for TeCA. The findings further underline the potentially important impacts of nanomaterials on contaminant fate and effects in the environment.

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).

References (48)

  • F. Wang et al.

    Effects of sulfide reduction on adsorption affinities of colloidal graphene oxide nanoparticles for phenanthrene and 1-naphthol

    Environ. Pollut.

    (2015)
  • F. Wang et al.

    Sorption of humic acid to functionalized multi-walled carbon nanotubes

    Environ. Pollut.

    (2013)
  • C. Zhang et al.

    Reduced graphene oxide enhances horseradish peroxidase stability by serving as radical scavenger and redox mediator

    Carbon

    (2015)
  • A. Bagri et al.

    Structural evolution during the reduction of chemically derived graphene oxide

    Nat. Chem.

    (2010)
  • C.H. Bolster et al.

    Spatial distribution of deposited bacteria following miscible displacement experiments in intact cores

    Water. Resour. Res.

    (1999)
  • D. Chen et al.

    Graphene oxide: preparation, functionalization, and electrochemical applications

    Chem. Rev.

    (2012)
  • W. Chen et al.

    Adsorption of hydroxyl- and amino-substituted aromatics to carbon manotubes

    Environ. Sci. Technol.

    (2008)
  • W. Chen et al.

    Catalytic effects of functionalized carbon nanotubes on dehydrochlorination of 1,1,2,2-tetrachloroethane

    Environ. Sci. Technol.

    (2014)
  • I. Chowdhury et al.

    Colloidal properties and stability of graphene oxide nanomaterials in the aquatic environment

    Environ. Sci. Technol.

    (2013)
  • W.J. Cooper et al.

    Abiotic transformations of halogenated organics. 1. Elimination reaction of 1,1,2,2-tetrachloroethane and formation of 1,1,2-trichloroethene

    Environ. Sci. Technol.

    (1987)
  • D.R. Dreyer et al.

    The chemistry of graphene oxide

    Chem. Soc. Rev.

    (2010)
  • H. Fu et al.

    Graphene oxide-facilitated reduction of nitrobenzene in sulfide-containing aqueous solutions

    Environ. Sci. Technol.

    (2013)
  • Y. Gao et al.

    Reduced graphene oxide as a catalyst for hydrogenation of nitrobenzene at room temperature

    Chem. Commun.

    (2011)
  • W.C. Hou et al.

    Photochemical transformation of graphene oxide in sunlight

    Environ. Sci. Technol.

    (2015)
  • Cited by (18)

    View all citing articles on Scopus

    This paper has been recommended for acceptance by Baoshan Xing.

    View full text