Chapter 2

Potentialities of Graphene-Based Nanomaterials for Wastewater Treatment

Ana L. Cukierman1-3*, Emiliano Platero1, María E. Fernandez1,3, and Pablo R. Bonelli1,3

1Programa de Investigación y Desarrollo de Fuentes Alternativas de Materias Primas y Energía (PINMATE), Depto. de Industrias, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Buenos Aires, Argentina

2Cátedra de Tecnología Farmacéutica II, Depto. de Tecnología Farmacéutica, Facultad de Farmacia y Bioquímica, Universidad de Buenos Aires, Buenos Aires, Argentina

3Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Buenos Aires, Argentina

*Corresponding author: analea@di.fcen.uba.ar, anacuki@ffyb.uba.ar

Abstract

Due to its exceptional properties and potentialities for technological innovations in several fields, graphene - a single atomic layer of sp2-bonded carbon atoms densely arranged into a two-dimensional (2D) honeycomb lattice - has been the subject of intense research worldwide since its discovery in 2004. In particular, its remarkably high theoretical surface area has led to explore the feasibility of using nanomaterials related to this new carbon allotrope, such as graphene oxide (GO), reduced GO, and graphene-based nanocomposites, for wastewater treatment. Within this scenario, the present chapter provides an overview of recent research studies and advances concerned with the potential of graphene-based nanomaterials as novel promising adsorbents for the removal of various pollutant species, such as heavy metals, dyes, phenolic compounds, and emerging contaminants, from wastewater. A brief description of main routes to synthesize graphene, as well as attempts to regenerate and reuse the adsorbents loaded with the contaminants after saturation, are also included.

Keywords: Graphene, graphene oxide, reduced graphene oxide, pollutant adsorption, nanostructured adsorbents, regeneration

2.1 Introduction

The discharge of effluents containing pollutants generated from anthropogenic activities into water bodies constitutes a serious environmental problem that affects water quality. Different kinds of contaminants, including from conventional species, such as toxic metals, anions, and organics, to emerging contaminants, i.e. pharmaceuticals, steroid hormones, pesticides, and personal care products, may be present [1-2]. It is well recognized that concentrations of contaminant species above admissible thresholds in aquatic environments cause harmful effects on humans and a variety of other living species because of their toxicity, persistence, and accumulation in living tissues throughout the food chain [1]. Besides, growing water shortage in several regions worldwide, as a result from increasing population and rapid urbanization, is capturing attention to water recycling and reuse as a reliable, economically feasible, and environmentally sensitive means to maximize water resources [3].

Among the technologies proposed to remove low concentrations of pollutants from waste streams or drinking water, adsorption has demonstrated to be robust, economically favorable, and technically easy, although it requires highly efficient adsorbents [4-5]. Carbon-based materials, especially activated carbons, have been mostly used in wastewater treatment because of their high specific surface area, versatility, chemical and mechanical stabilities, and suitability for large-scale production [6].

In the search of novel, more efficient and selective alternative adsorbents, graphene, and related nanomaterials based on this new carbon allotrope have recently emerged as promising nano-platforms with great potentialities for environmental applications. Graphene - a single atomic layer of sp2-bonded carbon atoms densely arranged into a two-dimensional (2D) honeycomb crystal lattice - has been the subject of intense research worldwide since its discovery [7]. Its unique nanostructure confers graphene nanosheets exceptional properties that have attracted increasing attention due to their potentialities for technological innovations in different fields [8-10]. In particular, the large theoretical surface area of graphene nanosheets (2630 m2 g-1) makes this novel carbon allotrope and graphene-related materials, such as graphene oxide (GO), reduced GO (RGO), and graphene-based nanocomposites (GNCs), potentially suitable candidates for the removal of various environmental pollutants [11]. Compared to carbon nanotubes and its predecessor allotrope, main advantages of graphene are lower cost and content of metallic impurities [12].

Although some reviews on graphene-based materials targeted at the removal of pollutant species from wastewater have been recently published [11, 13-14], the dramatic growth of publications reported in the last few years deserves to examine, at least in a complementary way, the advances attained. Within this framework, the present chapter focuses on graphene-derived materials as nano-adsorbents for wastewater treatment. A brief overview of the two general routes adopted for graphene synthesis including representative methods of each one is first presented. Special attention on the mostly convenient methods for cost-effective, massive production of graphene, pristine, oxidized or reduced, is paid taking into account the potentially large demand of material required for full-scale tertiary wastewater treatment. Attempts to improve these methods are also included. Then, information concerned with potentialities of nanomaterials derived from graphene for the removal of heavy metals and different kinds of organic pollutants is revised. Main characteristics of the methods employed for preparation of the nano-adsorbents as well as their possible regeneration and reutilization after saturation are briefly summarized.

2.2 Graphene Synthesis Routes

The method adopted for graphene synthesis is important in connection with graphene properties and, consequently, with projected applications of this novel carbon allotrope. Graphene synthesis has been carried out through two major routes depending on whether it is derived from graphite or other carbon sources [15]. They are known as "top-down" and "bottom-up" routes, respectively. Figure 2.1 shows an illustrative scheme of representative methods of each of these routes and main applications of the graphene derived.

Figure 2.1 Schematic illustration of top-down and bottom up routes for graphene synthesis, representative methods, and applications.

In the bottom-up route, graphene is directly synthesized from small organic molecules or atoms by chemical processes. Epitaxial growth on electrically insulating surfaces such as silicon carbide, chemical vapor deposition, either thermal or plasma enhanced on various metal substrates, and solvo-thermal process are representative examples of the bottom-up route [15-18]. This route leads to highly defect-free graphene nanosheets, especially suitable for electronic applications, even though at the expense of low yields and high processing costs.

On the other hand, the top-down route involves graphite sources as starting material and physical or chemical methods to yield a mixture of single and few layer graphene nanosheets (Figure 2.1). Graphene was obtained for the first time through this route by applying micromechanical exfoliation from a graphite piece, known as the "Scotch" tape method [7].

The most commonly applied top-down method, with great potential for large-scale production due to its simplicity and high yield, is based on the reduction of highly oxidized GO nanosheets, a nonconductive hydrophilic carbon material. The Hummers method is first used to generate graphite oxide through the addition of KMnO4 to a solution of flake graphite, NaNO3, and concentrated H2SO4 acid [19]. The acid is used to intercalate graphite with the assistance of NaNO3, and KMnO4 to oxidize the acid- intercalated graphite [15, 20]. The strong oxidizing agents introduce functional groups that increase the distance between nanolayers and facilitate their isolation.

Graphite oxide is subsequently peeled off usually by ultrasonic exfoliation in water to obtain GO followed by centrifugation. Thermal expandable exfoliation, static exfoliation, and chemical exfoliation are other techniques applied [21-24]. The supernatant from water exfoliation is colloidal and contains few- and single-layer sheets of GO. Successive washing of the supernatant with water is performed to remove the oxidizing agents. H2O2 is often added to reduce the remaining KMnO4. Oxygen-containing functionalities in the resulting GO sheets include carboxyl and carbonyl groups at the sheet edges, and hydroxyl and epoxy (1,2-ether) functional groups on the basal plane, that can alter van der Waals' interactions leading to a range of solubility in water and organic solvents [25]. GO has attracted special attention not only as a precursor for large-scale production of graphene but also for adsorption applications because of its large theoretical surface area, oxygen surface groups, high water dispersibility, stability, and ease of synthesis [26].

Improvements to the Hummers method in order to make it more efficient and environmentally friendly have been explored. For instance, exclusion of NaNO3, increase in KMnO4 amount, and use of 9:1 mixture of concentrated H2SO4/H3PO4 led to...