Graphene oxides cross-linked with hyperbranched polyethylenimines: Preparation, characterization and their potential as recyclable and highly efficient adsorption materials for lead(II) ions

Highlights

• The cross linking agent-hyperbranched polyethylenimines can significantly improve the adsorption capacity for Pb²⁺.

• The adsorption capacity of the GO-HPEI gel for Pb²⁺ is as high as 438.6 mg g⁻¹.

• The GO-HPEI gel could be easily separated and regenerated.

• The GO-HPEI gels as promising adsorbents have great potential application in the removal of Pb²⁺ from wastewater.

Abstract

Three GO-HPEI gels were prepared by a cross-linking reaction of graphene oxide (GO) with hyperbranched polyethylenimines (HPEIs) with different molecular weight. The GO-HPEI gels are different from the most reported GO-based materials and have superior adsorption performance because that these gels have more adsorption sites over using the common cross-linking agent (especially double-functionality or tri-functionality monomer). The morphology and structure of the GO-HPEI gels were characterized by FTIR, XRD, SEM, TEM, BET, TGA and elemental analyses. The adsorption property of Pb(II) onto GO-HPEI gels and the factors that influence the adsorption, such as the Pb(II) concentration, contact time and pH value, were investigated. The GO-HPEI gel exhibited an adsorption capacity as high as 438.6 mg g⁻¹ for Pb(II) ions, and its insolubility in water makes the separation from wastewater easy. Adsorption experiments demonstrated that the amino groups and the cross-link density affect the adsorption capacity. In the Pb(II) adsorption process, the adsorption kinetic and equilibrium data were well simulated with the pseudo-second-order and Langmuir isotherm models. The adsorption capacity of the GO-HPEI gel for Pb²⁺ only exhibited a slight decrease after eight cycles of adsorption/desorption, indicating its good stability. Therefore, the GO-HPEI gels could serve as promising adsorbents for potential application in the removal of Pb(II) from wastewater.

Introduction

Lead(II) ions in water are of great concern due to their toxicity, non-biodegradability and accumulation in organisms or the food chain [1], [2]. With the rapid development of industries, the water pollution caused by Pb²⁺ contamination has become a serious issue. Long-term drinking of lead polluted water will cause serious disorders, such as kidney disease, anaemia and mental retardation. Due to their high toxicity, the elimination of Pb²⁺ from aqueous solution is crucial for the environmental pollution cleanup. Therefore, the efficient removal of Pb²⁺ from wastewater is very important and urgent. To date, several technologies and methods for Pb²⁺ removal from wastewater have been developed including solvent extraction, ion-exchange, membrane separation, reverse osmosis and adsorption [3], [4], [5]. Adsorption is one of the most promising and widely used approaches due to high efficiencies and economic feasibility [2], [6], [7]. A number of materials, such as activated carbon [8], zeolite [9], clay [10], kaolinite [11], microorganisms [12], manganese oxides [13] and carbon nanotubes [14], [15], have been successfully employed to adsorb Pb²⁺. However, the low removal efficiencies or low adsorption capacities of these adsorbents limit the engineering application. Therefore, it is necessary to develop new and more efficient materials for adsorption.

Carbon materials are significantly and heavily used for the removal of Pb²⁺ as an engineering adsorbent [16], [17]. Graphene oxide (GO), which is a new material in the carbon family, has attracted much attention for the adsorption treatment of lead-containing wastewater due to its huge surface area and a large number of functional groups [18]. Several researchers have applied GO or GO based materials for Pb²⁺ adsorption [19], [20], [21], [22], [23], [24], [25], [26]. However, the adsorption capacities are less than satisfactory. For example, Islam has reported Pb²⁺ adsorption using GO immobilized by polystyrene with an adsorption capacity of 227.92 mg g⁻¹ [19]. A nanocomposite LS–GO–PANI was prepared via in situ polymerization, and this nanocomposite exhibited an adsorption capacity of 216.4 mg g⁻¹ for Pb²⁺ [20]. Gui synthesized a magnesium silicate/reduced GO nanocomposite with a high adsorption efficiency for Pb²⁺ [21]. Recently, an aminosilanized GO was prepared for the selective adsorption of Pb²⁺, and the maximum adsorption capacity was determined to be 96 mg g⁻¹ [22]. Polyamidoamine dendrimer (PAMAM) modified GO can be easily prepared and has a high adsorption capacity [27], [28], [29]. In comparison to linear polymers with amino groups, the metal ion binding capacity of PAMAM was much larger due to the presence of the EDA core [30]. GO-PAMAM were prepared via the “grafting-from” [31] or “grafting to” [29] method, and the GO-PAMAM 2.0 total adsorption capacity for Fe³⁺, Cr³⁺, Zn²⁺, Pb²⁺ and Cu²⁺ reached 1.007 mmol g⁻¹.

Apparently, functionalization of GO is essential for improving its adsorption capacity [32], [33], [34], [35], especially by the dendritic polymers, such as PAMAMs. However, most reported GO and GO based materials cannot be directly applied as adsorbents because they are water soluble [25], [36] and small in size [23]. Assuming that adsorption occurs, the GO or GO based materials may be difficult to separate from the solution after the adsorption process using traditional separation methods due to its hydrophilic property. Therefore, the industrial application cost of GO or GO based composite materials will increase. In addition, the treated water may be re-polluted.

Therefore, the introduction of cross-linking with dendritic polymers could be more beneficial for enhancing the feasibility of GO based materials. Based on the specific spheroid-like shape, compact structure and multi-functionality, dendritic polymers (i.e., the dendrimers and hyperbranched polymers) have attracted tremendous interest, and these polymers are the most attractive compounds for the design of new structures and molecules [37], [38]. Unfortunately, the synthesis of dendrimers is a time-consuming and expensive process. Unlike dendrimers, hyperbranched polymers, which can be economically and easily prepared in one step, have gained increasing attention [38], [39], [40], [41], [42], especially in industrial applications. Commercially available hyperbranched polyethylenimine (HPEI) with a large number of primary and secondary amine groups has been widely used for encapsulation or adsorption of guest molecules [43], [44]. In addition, the number of the functional groups reacted as crosslink points will be reduced after the crosslinking reaction, which implies that the adsorption sites of these materials will be reduced. Differently and significantly, these gels have more adsorption sites over using the common cross-linking agent (especially double-functionality or tri-functionality monomer). Therefore, the objective of this work is to synthesize graphene oxide-hyperbranched polyethylenimine gels (GO-HPEI gels) that are recyclable and may possess a high adsorption capacity and be directly applied. These GO-HPEI gels could take full advantage of the characteristics of the specific physical, chemical and surface properties of GO including the specific spheroid-like shape, large number of amino groups of HPEIs, good stability and recyclability of the gels. Therefore, the GO-HPEI gel may have the potential to be a cost-effective adsorbent for the removal of Pb²⁺ from wastewater.