Fe₃O₄@SiO₂@CS-TETA functionalized graphene oxide for the adsorption of methylene blue (MB) and Cu(II)

Highlights

• The magnetic composites Fe₃O₄@SiO₂@CS-TETA-GO were synthesized.

• The composites exhibited a high capacity for the fast adsorption of MB and Cu(II).

• The composites can be easily recycled and reused as a novel absorbent.

Abstract

The graphene oxide (GO) functionalized by Fe₃O₄@SiO₂@CS-TETA nanoparticles, Fe₃O₄@SiO₂@CS-TETA-GO, was firstly fabricated in a mild way as a novel adsorbent for the removal of Cu(II) ions and methylene blue (MB) from aqueous solutions. The magnetic composites showed a good dispersity in water and can be conveniently collected for reuse through magnetic separation due to its excellent magnetism. When the Fe₃O₄@SiO₂@CS- TETA-GO was used as an absorbent for the absorption of MB and Cu(II), the adsorption kinetics and isotherms data well fitted the pseudo-second-order model and the Langmuir model, respectively. Under the optimized pH and initial concentration, the maximum adsorption capacity was about 529.1 mg g⁻¹ for MB in 20 min and 324.7 mg g⁻¹ for Cu(II) in 16 min, respectively, exhibiting a better adsorption performance than other GO-based adsorbents reported recently. More importantly, the synthesized adsorbent could be effectively regenerated and repeatedly utilized without significant capacity loss after six times cycles. All the results demonstrated that Fe₃O₄@SiO₂@CS-TETA-GO could be used as an excellent adsorbent for the adsorption of Cu(II) and MB in many fields.

Introduction

Dyes and heavy metal ions in water are deemed to be the most basic contaminants which are threatening to the ecological balance even the survival of human due to their non-biodegradability, toxicity, carcinogenicity and bioaccumulation [1], [2]. The sewages containing methylene blue (MB) or Cu(II), which affect the health of all creatures, are released from various industries, including the mining, electroplating, dyes manufacturing, textile, leather tanning, paper coloring as well as the preparation of cosmetics [3], [4], [5], [6]. Nowadays, multifarious techniques, including ion exchange [7], adsorption [8], membrane filtration [9], [10], electrochemical treatment [11], and photocatalytic degradation [12], [13], have been developed to remove the overmuch MB and Cu(II) from wastewaters. Among these methods, adsorption is frequently regarded as a preferred approach in wastewater treatment for the advantages such as workability, high efficiency, simple operation and low-cost over other methods.

Over the past decade, a variety of adsorbent materials have been widely applied to remove Cu(II) and MB from wastewater, including natural polymers such as cellulose [14] and chitosan (CS) [15], carbonaceous material represented by activated carbons [16], zeolite [17], graphene oxide (GO) and its derivatives [8], clay [18] and so forth. Among these various materials, GO and CS have attracted considerable attention for their unique properties. Graphene oxide nanosheets, equipped with a one-atom-thick 2D layer of sp²-hybridized carbon, have drawn widespread interest in adsorption of all kinds of organic and heavy metals contaminants due to fantastic physicochemical properties such as excellent mechanical strength, extraordinary large specific surface area, π-π interactions and relatively facile modification [19], [20], [21], [22]. However, poor solubility in water and less functional groups have limited their application in the wastewater treatment and thus a number of work have been carried out to modify graphene oxide with multifunctional materials to break these restrictions [3], [23], [24]. For example, Liu et al. [4] reported that the maximum adsorption capacity of β-cyclodextrin/poly(acrylic acid) grafted graphene oxide (β-CD/PAA/GO) was 247.9 mg g⁻¹ in the removal of MB from aqueous solutions. Though it can be simply prepared, the adsorption capacity of β-CD/PAA/GO for MB sounded still undesirable for industrial applications. Hu et al. [8] functionalized graphene oxide by chitosan (CS-GO) to adsorb MB, and reported its high adsorption capacity as 598.2 mg g⁻¹. But its preparation through self-assembly method as well as separation from wastewater seems complicated and impractical.

CS and its derivatives, a promising and extensive bio-adsorbent, have always been applied into the treatment of heavy metals and dye pollution due to their abundant functional groups [25]. For instance, Labidi et al. [26] prepared the ethylenediaminetetra acetic acid (EDTA) modified chitosan to adsorb Cu(II) and the maximum adsorption capacity was found to be 110 mg g⁻¹. Nevertheless, it is far from a desirable adsorbent because of its weak mechanical property. Recently an efficient way to improve its practicability is to form a rigid and magnetic core [27]. Sun et al. [28] synthesized a kind of magnetic chitosan microspheres with quaternary ammonium groups as adsorbents to remove Cr(VI) and the equilibrium time was determined to be 40–120 min which could be attributed to the large size (223.2 μm) of the adsorbent. Ren et al. [29] compounded magnetic Fe₃O₄@SiO₂@CS particles modified with EDTA (EDCMS), which were estimated to reach 10 μm as adsorbent to treat wastewaters containing Cu(II), the maximum adsorption capacity and equilibrium time were 44.4 mg g⁻¹ and 360 min respectively due to large size and low specific surface area. When graphene oxide grafted by xanthated Fe₃O₄-chitosan was used to remove Cu(II) from aqueous solutions, the maximum adsorption capacity was 426.8 mg g⁻¹ which was the highest reported [5]. The high adsorption capacity can be attributed to abundant functional groups. However, the time to reach adsorption equilibrium of the material was up to 45 min, and the reusability did not meet the requirement due to its bad acid resistance. Recent studies suggest that the materials with nanoscale structure and modified with amino groups could markedly increase not only the specific surface area but also the adsorption capacity and rate [4], [24].

In this paper, a new kind of adsorbent based on Fe₃O₄@SiO₂@CS-TETA functionalized graphene oxide sheets, Fe₃O₄@SiO₂@CS-TETA-GO, was successfully synthesized. As shown in Scheme 1, Fe₃O₄ magnetic cores were firstly synthesized by a convenient solvothermal method. Next, a thin layer of SiO₂ was coated on the Fe₃O₄ cores using revised Stöber method to improve the acid resistance of cores. And next, CS thin-film was coated on Fe₃O₄@SiO₂ by crosslinking with glutaraldehyde. After that, triethylenetetramine (TETA), possessing a good deal of amino groups, was grafted on the surface of Fe₃O₄@SiO₂@CS. Finally, the Fe₃O₄@SiO₂@CS-TETA nanocomposites were attached to the surfaces of GO by an amidation reaction between amino groups on TETA and carboxyl groups on GO. Thanks to the material and preparation process, the Fe₃O₄@SiO₂@CS-TETA-GO exhibited excellent hydrophily and magnetism which afforded it to easily separate from the wastewater for reuse. The performance of the adsorbent was evaluated through the adsorption isotherms and kinetics of MB and Cu(II). Strikingly, the maximum adsorption capacity was 529.1 mg g⁻¹ for MB and 324.7 mg g⁻¹ for Cu(II), respectively, and in particular, had no obvious loss after six cycles.