Research article
Pillared graphene oxide composite as an adsorbent of soluble hydrocarbons in water: pH and organic matter effects

https://doi.org/10.1016/j.jenvman.2019.110044Get rights and content

Highlights

  • Low-molecular-weight chitosan optimized a pillared effect on graphene oxide.

  • An optimized chitosan/graphene oxide ratio of 0.1 achieved a surface area of 70 m2/g.

  • Medium and high molecular weight chitosan produced a little or no pillaring effect.

  • Composite had 2 to 4-fold the adsorption capacity of graphene oxide.

  • The removal decreased with increasing [H+] due to the amino groups protonation.

Abstract

Graphene oxide (GO) is a single-atom-thick sheet of carbon with oxygen-containing functional groups decorating its basal plane and edge sites. Most of its high surface area can be lost due to restacking of individual layers during the synthesis and drying of GO-based bulk sorbents. There is great interest to increase the specific surface area of graphene-based sorbents by introducing organic molecules as “pillaring agents” between GO sheets to hinder the stacking process and create sorbents with elevated surface area. This work synthesizes pillared GO by introducing chitosan (CS), a linear polysaccharide with various molecular weights. A composite of low molecular weight CS at a CS/GO ratio of 0.1 is shown to have the highest specific surface area (up to 70.5 m2/g) in comparison to the medium and high CS molecular weight, pristine GO, and the CS/GO composite materials. The affinity of the optimized GO/CS composites towards benzene, toluene, and naphthalene was evaluated at 19.3 mg/L of organic matter content while altering pH. Sips and Langmuir adsorption isotherm models well described the adsorption behavior, and benzene adsorption performance was reduced at low pH. Related to the presence of dissolved organic matter (DOM) in solution, lower diffusivity constants (k1) in hydrocarbon systems were recorded. Our results demonstrate the feasibility of CS as a potential pillaring agent in CS/GO composites to increase specific surface area and enhance the capture of soluble hydrocarbons from aqueous solutions.

Introduction

The mono-aromatic hydrocarbons benzene, toluene, and the poly-aromatic naphthalene are common pollutants that are constituents of gasoline and are widely used in numerous applications (Yakout, 2014). These pollutants are a matter of significant concern and have a relatively high water solubility compared to many others hydrocarbons, and thus a high bioavailability to aquatic organisms (Neff, 2002). Recent research on the remediation of water contaminated with mono-aromatic hydrocarbons uses a variety of physicochemical and biological approaches such as advanced oxidation, membrane filtration, and thermal processes (Bustillo-Lecompte et al., 2018; Jiménez et al., 2018; Ma et al., 2018; Xue et al., 2018). Adsorption-based technologies are the most widely applied treatment options due to their flexibility, high efficiency, and cost-effectiveness. Different types of organic (Tran et al., 2015) and inorganic (Wang and Peng, 2010) media have been evaluated as adsorbent materials for capturing aromatic compounds. Among them, activated carbons (AC) are particularly attractive due to comparatively high surface area, availability of adsorption sites, chemical stability, and low cost. New carbon-based nanomaterials have been considered as potential alternatives to conventional AC, and these include carbon nanofibers, carbon nanotubes, and fullerenes (Wang et al., 2014).

Recently graphene-based materials have been evaluated for removal of a wide range of pollutants from water (Wang and Zhao, 2016). Monolayer graphene and graphene oxide have ultra-high surface area, but upon drying often spontaneously restack to form ordered aggregates with greatly reduced area (Chen, 2015). Self-agglomeration of dried GO particles has restricted its application in large-scale adsorption processes (Kong, 2015). Hence, modification and functionalization of graphene sheets to prevent restakcing is a critical challenge in the synthesis of bulk sorbents from 2D nanosheet precursors.

One approach is to introduce organic molecules or polymers that spontaneously associate with the faces of graphene nanosheet and stearically hinder restacking. A cationic flocculating agent such as chitosan (CS) may be effective at the spontaneous association with negatively charged GO nanosheets through its abundant amine and hydroxyl groups, and also has its own high affinity for water-based pollutants such as metals and dyes that may contribute to the adsorption power of the composite (Crini and Badot, 2008; Wan Ngah et al., 2011). We hypothesized that such pillared graphene-based composites would have the potential for environmental capture of hydrocarbons with significant water solubility, such as benzene, toluene, and naphthalene.

Previous adsorption studies with the CS/GO composites employed pre-established ratios of both precursors for the synthesis of the final adsorbent. Here we show that a specific amount of soluble CS polymer promotes an optimal pillaring effect for the GO sheets, resulting a noticeable increase in the final CS/GO composite product specific surface area and adsorption capacity for water-based aromatic pollutants. This study also examines the effect of chitosan molecular weight in the optimization of composite sorbents. The composites are applied to three key water pollutants such as benzene, toluene and naphthalene, and we study the effects of pH and the fundamental adsorption mechanisms.

Section snippets

Materials

Three chitosan (CS) samples of low (50–90 kDa), medium (190–310 kDa) and high molecular weight (>310 kDa) with a given degree of deacetylation of ≥75% were purchased from Sigma Aldrich Co., Ltd. The source materials, solvents and adsorbates were of analytical reagent grade.

Characterization of the composites

Different mass fractions and molecular weights of CS were evaluated to optimize the pillaring effect for surface area enhancement. After many tests with various CS/GO mass fractions, the composite with 10% mass fraction of unmodified low-molecular-weight (LMW) CS provided the highest specific surface area (~70 m2/g) (see Fig. 1a). In comparison, the specific surface areas of GO and CS precursors were below 2.6 m2/g, close to the detection limit of the vapor adsorption method. The 10% CS

Adsorption experiments

Fig. 4 shows the benzene, toluene, and naphthalene adsorption isotherms of the modified composite at the optimal surface area enhancement. The adsorption isotherms of GO were also recorded. The CS/GO_0.1 composite enhbited a noticeable affinity for hydrocarbons, being their adsorption capacity of 147, 60, and 6 mg/g for benzene, toluene and naphthalene, respectively. Three adsorption models known as Langmuir, Freundlich and Sips were proposed to describe the relation between the amount of

CS/GO composite as an adsorbent of aromatic hydrocarbons

Different pillaring agents for graphene and graphene oxide have been reported in the literature, which mainly included carbon-based materials like carbon nanotubes, nanocarbon fibers, carbon black and fullerenes (Guo et al., 2014). Another type of agents included polymers, metallic cations and in minor proportion organic polymers. For that reason, it is difficult to establish a clear comparison of the pillaring capability of CS. However, similar CS/GO composites reported in the literature, and

Conclusions

The present study showed out that a chitosan/graphene oxide composite prepared with a low-molecular-weight chitosan achieved an optimized specific surface area of 70 m2/g by pillaring the interlayers of graphene oxide (GO) nanosheets. Medium and high molecular weight chitosan molecules produce a little or no pillaring effect on GO registered at a CS/GO ratio between 0.2 and 0.4. For the optimized composite, CS/GO_0.1, a suite of characterization techniques verified the presence of chitosan

Author contributions

Composites characterization and adsorption isotherm measurements were conducted and analyzed by C Chaparro and C.J. Castilho, with assistance in analysis from I Külaots, R Hurt and J Rangel-Mendez. This study was conceptualized and designed collaboratively by I Külaots, R Hurt and J Rangel-Mendez. C Chaparro wrote the initial manuscript in collaboration with C.J. Castilho.

Acknowledgements

This work was supported by grant PDCPN-01-247032 (Mexico). Carlos E. Flores-Chaparro acknowledges a doctoral fellowship and a stay of research fellowship from CONACYT, Mexico, No. 424187. This research was also supported by the Superfund Research Program of the National Institute for Environmental Health Sciences under grant P42 ES013660. The authors express their gratitude to Z. Saleeba, R. Spitz, E. Isaacs, D.I. Partida, G. Vidriales, J.P. Rodas, and M.C. Rocha for their invaluable assistance

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    C.E. Flores-Chaparro and C.J. Castilho equally contributed as first authors.

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