Stable graphene oxide/poly(ethyleneimine) 3D aerogel with tunable surface charge for high performance selective removal of ionic dyes from water

Highlights

• Porous and robust GO/PEI aerogel (GP) was fabricated via an optimized sol-gel method.

• Surface charges of GP can be tuned from positive to negative within a wide pH range.

• GP aerogels showed promising adsorption capacity for both anionic MO and cationic MB.

• GP aerogels performed excellent separation in mixed MO and MB aqueous solution.

• The adsorption of GP was dominated by the electrostatic attraction and π-π interaction.

Abstract

Graphene oxide (GO) with oxygen-containing functional groups views a promising adsorbent to remove cationic pollutants from water. To achieve high removal efficiency of both anionic and cationic pollutants and overcome recycling difficulty of GO, positively charged poly(ethyleneimine) (PEI) was associated with GO to fabricate GO/PEI (GP) 3D aerogels. Surface charges of the constructed GP aerogels were tuned from positive to negative within a wide range via changing solution pH. The constructed GP aerogels were stable in acidic and basic aqueous environments, and even in organic solvents (such as methanol, acetonitrile, methylbenzene, acetone and N,N-dimethyl formamide), which are easily recovered. Anionic methyl orange (MO) and cationic methylene blue (MB) were utilized as model pollutants to investigate the adsorption capability of the GP aerogels. The GP aerogels showed high adsorption capacity for anionic MO (331.0 mg/g) at pH 2.0 while for cationic MB (249.6 mg/g) at pH 10.5. The electrostatic attraction force and π-π interaction dramatically improved the adsorption capacity of the charge-tunable GP aerogels against both anionic and cationic dyes. In addition, the GP aerogels were able to selectively separate anionic and cationic dyes in designed aqueous environments. These aerogels may be used as versatile sorbents for various charged pollutants removal due to tunable surface charges.

Introduction

The advent of graphene oxide (GO) and its derivatives has opened a brand new era among carbon materials and are emerging as a new class for many potential applications [1], [2]. The presence of numerous oxygen-containing functional groups on aromatic scaffold of GO nanosheets, such as hydroxyl and epoxy on basal plane, and carboxyl and carbonyl at nanosheet edges, allows ionic and nonionic interactions with a wide range of molecules [3], [4], [5], [6], [7]. Although, the GO nanosheets have shown remarkable performance during adsorption for different environmental water pollutants such as dyes [8], [9], PAHs and their derivatives [10], antibiotics [11], [12], pesticides and heavy metal ions [13], [14], [15]. Nevertheless, the GO nanosheets in nano or micrometer range makes them difficult for recovery or recycling resulting quicker release in natural water [16]. Despite GO nanosheets, three-dimensional graphene-based macrostructures (3D GBM) are much easier to be recovered, enabling the reusability of GO nanosheets during practical applications and thus minimizing the potential risks of GO nanosheets directly from environmental media [16], [17], [18], [19]. In addition, 3D GBM can be shaped into different devices for various applications [20]. 3D GBMs could be prepared by different approaches, such as chemical vapor deposition [21], [22], self-assembly [23], [24], [25] and cross-linking [26], [27], [28], [29]. Although, the pristine GO nanosheets could be used to fabricate 3D GO aerogels with stochastic porous network through freeze-drying. Unfortunately, the stability of pristine 3D GO was hindered in practical applications and consequent dispersion in water [30]. Of these, additional reduction process was utilized to adjust the stability of 3D GO thereby removing negatively charged carboxyl groups and enhancing the π-π attraction, which are desirable for adsorption process as active sites [31]. Therefore, the fabrication of functional 3D GO aerogel with tailored macrostructure by controllable and scalable methods remains a significant challenge.

In order to preserve or minimize the consumption of oxygen-containing functional groups in 3D GBM during construction, different strategies such as additives like organic molecules [27], [32], [33], polymers [34], [35], [36], [37], multivalent ions [37], [38] and metal oxide nanoparticles [39], [40] have been presented to stabilize the 3D GBM without reduction for pollutants removal. Since, the abundant oxygen-containing functional groups of GO nanosheets dominate the negative charge, the resulting 3D GBMs indicated better adsorption capability to positively charged pollutants than negatively charged pollutants. Furthermore, surface charge of these 3D GBMs contains merely negative charge, which can only be adjusted in a narrow range, and limit the performance of 3D GBM during practical applications such as for pollutant abatement.

Poly(ethyleneimine) (PEI) is a typical poly-cation, which has been used commonly for surface charge tuning because of its high charge density [41] and isoelectric point (IEP)(>10) [42]. It seems that the incorporation of PEI with GO may fabricate desirable 3D GBMs with tunable surface charge and intensified stability. However, only limited research have rarely been reported for the fabrication of 3D PEI-GO aerogel as an adsorbent [43]. Even so, surface charge tuning, structural stability and adsorption mechanism of 3D PEI-GO aerogel have been studied on a limited scale.

Here in, PEI was mixed with GO in different ratios to fabricate GO/PEI 3D aerogel via an optimized sol-gel method. The as-prepared GP aerogel maintained intact macrostructure in water at different pH values as well as various organic solvents including methanol, acetonitrile, methylbenzene, acetone and N,N-dimethyl formamide. The GP aerogel indicated IEP at about 10, showing positive charge in a very broad range. The GP aerogel not only showed promising adsorption capacity for both anionic MO and cationic MB, but also performed excellent separation in mixed MO and MB aqueous solution. The prepared GP has great potential for charged pollutant removal, especially negatively charged molecules, highlighting the practical application for water treatment with easy scalability and durability.