Cyclodextrin functionalized 3D-graphene for the removal of Cr(VI) with the easy and rapid separation strategy

Highlights

• The CDGF was synthesized with the one-step facile hydrothermal treatment.

• The mass, volume of CDGF and the sorption capacity increased with initial β-CD mass.

• The CDGF possessed good selectivity to Cr(VI) compared with other metal ions.

• CDGF could be separated easily after adsorbing, which reduced the process and time.

Abstract

As a useful heavy metal ion, chromium has seen its applications in various fields. While it is also a toxic contaminant in water and may cause serious threats to the environment and human health. To develop a novel material with good adsorption capacity and easy solid-liquid separation strategy was necessary and significant. In this paper, the β-cyclodextrin (β-CD) functionalized three-dimensional structured graphene foam (CDGF) was successfully synthesized with the facile and one-step hydrothermal method. The SEM, BET, XRD, FT-IR and XPS analysis were carried out and the results confirmed the successfully grafting of β-CD onto GF. The batch adsorption of Cr(VI) was also taken out and the CDGF possessed good selectivity compared with other metal ions at pH = 3. The adsorption capacity reduced gradually as the initial pH of the Cr(VI) solution grew higher, which was because the anionic species of Cr(VI) were partial to the positively charged surface of CDGF. The easy separation strategy of the CDGF was also demonstrated and the CDGF could be taken out easily with a tweezer after the adsorption of Cr(VI), which significantly simplified the separation procedure and reduced time. By comparing the FT-IR and XPD analysis results, the adsorption mechanism was explored and the hydroxyl groups on CDGF played the main role in the adsorption process. This work brings a novel material for the adsorption of Cr(VI) from water and provides an innovative direction for the easy and fast solid-liquid separation strategy in the adsorption and other application fields.

Introduction

Chromium (Cr) is a kind of versatile metal ion and the applications have been witnessed in various areas including catalyst, chrome plating, electroplating, leather tanning, glass and textile industries, etc. (Jiang et al., 2018) However, the improper leaching of Cr to water and soil makes it the highly toxic contaminant and may bring serious threat to the environment as well as human health (Zou et al., 2016; Zhao et al., 2018). The existing form of chromium contains Cr(III) and Cr(VI), and Cr(VI) is more poisonous for living organisms. It can cause serious health problems such as skin irritation, pulmonary congestion, hepatopathy, and lung cancer (Bai et al., 2018; Ghosh et al., 2018; Du et al., 2019). Several techniques like adsorption, precipitation, solvent extraction and membrane filtration have been applied to remove Cr(VI) from water and soil. Up to now, adsorption has become the rising star among the techniques due to its simple designation, easy operation and environmentally friendly strategy (Wang et al., 2018). Various materials, such as graphene (Samuel et al., 2019), carbon nanotube (Huang et al., 2019), cyclodextrin (Wang et al., 2014) have been employed for the adsorption of Cr(VI) from water (Tan et al., 2018; Wu et al., 2018). While, the low adsorption capacity and the complex solid-liquid separation process still restrict the further application of the adsorbents. To simplify the separation process, the magnetic materials were emerged and have been extensively applied in the adsorption field (Fan et al., 2012a,b; Filik and Avan, 2019; Geng et al., 2019). Li et al. (2013) developed the graphene oxide modified with magnetic cyclodextrin-chitosan for the removal of Cr(VI) in wastewater and with an external magnetic attraction, the Cr(VI) could be easily and rapidly isolated from water. Zhang’s group (Zhang et al., 2013) synthesized the magnetic Fe₃O₄@C nanospheres and the Fe₃O₄@C could be easily extracted by an extra magnetic field after adsorption of Cr(VI). Seeking for novel, easy and rapid separation strategy is urgent and needs further research.

Graphene oxide (GO) has hold great promise to be used as adsorbents for metal ions because of the high surface area, abundant binding sites and easy functionalization properties (Novoselov et al., 2012). While, the easy self-stacking and the aggregation of the GO sheets were discovered in water due to their two-dimensional (2D) layer structure and the hydrophilia property, which decreased the effective surface binding sites and impaired the potential applications. Recently, the three dimensional (3D) structured GO was a significant advancement in catalysts and environment applications (Bi et al., 2012; Song et al., 2016; Lu et al., 2018). Under the hydrothermal treatment, the 2D GO sheets could self-assemble to form the reduced 3D porous graphene foam (GF) and there are three advantages for the GF: (1) it could offer large accessible surface area thus exposing active sites for further functionalization; (2) its interconnected micro, meso and macro porous structure could enhance the chances of combining with the functional materials as well as the diffusion of the metal ions; (3) the 3D structured foam composed of the nanoscale sheets was easy to take out from the solutions which was benefit for the separation after the adsorption of metal ions (Cong et al., 2012; Nguyen et al., 2012; Wang et al., 2017; Li et al., 2018; Li et al., 2016). Up to now, few study was taken out to employ the modified GF for the adsorption of Cr(VI) with the easy-separation strategy and further research was needed.

Herein, the β-cyclodextrin (β-CD) was chosen as the modified material for the adsorption of Cr(VI). The super cavities of β-CD are benefit for the formation of host-guest complexes after encapsulating with metal ions and the β-CD has shown great potential in the Cr(VI) separation field. (Sikder et al., 2014; Yu et al., 2018; Sun et al., 2019). In this paper, the β-CD modified graphene foam (CDGF) with the characteristic 3D interconnected porous structure was synthesized through a facile and one-step hydrothermal method. The CDGF with various amount of β-CD was synthesized and the morphology, structure and elemental composition of CDGF were characterized with SEM, XRD, BET, FT-IR and XPS analysis. A series of Cr(VI) adsorption experiments, including the effect of initial amount of β-CD, the effect of pH and other metal ions, the adsorption kinetics and the adsorption isotherm were carried out with CDGF. The easy-separation strategy was designed with a demonstration experiment and the adsorption mechanism was further explored with the FT-IR and XPS analyzing results. According to our knowledge, this is the first time to apply the β-CD functionalized 3D structured CDGF for the adsorption of Cr(VI) with this facile and easy separation mode. This paper could provide a new material for the Cr(VI) separation and also introduce a novel strategy for the easy and fast separation area.