Polyamine-modified magnetic graphene oxide nanocomposite for enhanced selenium removal
Graphical abstract
Introduction
Selenium (Se) is an essential nutrient element for humans and animals, but is only required in trace amount and has a very narrow range between deficient and toxic levels [1]. In nature, selenium is a trace element that is found association with sulfur-containing minerals. It can also be produced as a byproduct during human activities such as petroleum refining, mining, fossil fuel combustion, and through other industrial processes. The major form of selenium in the drain water and industrial wastewater is selenate (SeO42−, Se(VI)) and selenite (SeO32−, Se(IV)) [2], both of which are soluble and have shown bioaccumulation [1]. Drinking water is one of the primary sources through which selenium can enter the human body. The maximum permissible concentration (MPC) for selenium in drinking water as regulated by US environmental protection agency is 50 ppb (50 µg/L) [3]. The World Health Organization currently set a more precautionary MPC of 10 ppb (10 μg/L) selenium for drinking water [4].
Various treatment technologies have been proposed for selenium removal from wastewater, including adsorption [5], [6], ion exchange [7], reverse osmosis [8], co-precipitation [9], [10], electrocoagulation membrane process [11], and biological treatments [12], [13]. Among them, adsorption is one of the most promising methods due to its simplicity, cost effectiveness, lack of harmful byproduct and great potential for regeneration and reuse. Up to now, numerous adsorbents have been studied including activated carbon [5], [6], chitosan-clay composites [14], iron and aluminum oxides [5], [15], modified silica [16], and other metal oxides [17]. However, there are still some problems, such as limited adsorption capacity, difficulty of efficient separation, that still limit their practical applications. As a result, it is critically important to develop new adsorbents with high separation efficiency, easy regeneration, and simple recycling.
Graphene oxide (GO), exhibiting large surface area and abundant oxidized functional groups on the surface, has shown strong adsorption abilities for heavy metal removal. GO and its composites with functional groups including carboxylic, hydroxyl, amine, pyrrole, have been reported to be a good adsorbent for the removal of heavy metals, such as Cr6+, Pb2+, Cu2+, Cd2+ [18], [19], [20], [21], [22]. However, the potential of GO for selenium adsorption is still under exploration. In our previous study, a magnetic GO (MGO) was developed for selenium removal and it can be separated effectively under an external magnetic field for recycle and reuse [23]. The MGO displayed the adsorption capacities of 23.8 mg/g for Se (IV) and 15.1 mg/g for Se (VI). The adsorption capacities of MGO for Se (VI) and Se (IV) may be further improved by grafting MGO with functional groups as above mentioned.
In this work, poly allylamine, a water-soluble polymer with a large number of positively charged amine groups on its macromolecular chain, is grafted to magnetic GO nanocomposite to enhance selenium removal. As shown in Scheme 1, the synthesis process has two steps. Firstly, GO was grafted with poly (allylamine) by an N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide/N-hydroxysuccinimde (EDC/NHS) reaction. Secondly, magnetic nanoparticles (NP) were co-precipitated and a polyamine-modified magnetic GO nanocomposite (PAA-MGO) was obtained. The composition and properties of PAA-MGO were determined by Fourier transform infrared spectroscopy (FTIR), thermo gravimetric analysis (TGA), zeta potential measurement, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and magnetic hysteresis. Then, the adsorption performance of PAA-MGO was tested for the removal of selenate and selenite from water. The adsorption experiments were conducted and the results indicate that PAA-MGO has much higher adsorption capacities for selenate and selenite than GO, MGO, and even most absorbents reported in literature. The adsorption kinetics and adsorption mechanism of PAA-MGO were also studied. It is demonstrated that PAA-MGO nanocomposite is a highly efficient and promising absorbent for selenium removal from water.
Section snippets
Materials
Graphite flake (7–10 µm) was purchased from Alfa Aesar. Poly(allyamine hydrochloride) (PAA) of average molecular weight (MW) ∼ 15,000 was purchased from AK Scientific, Inc. N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide (EDC), N-hydroxysuccinimde (NHS), FeCl2 and FeCl3 were purchased from Sigma Aldrich. All other chemical reagents were obtained from Fisher Scientific. All chemical reagents purchased were of analytical grade and used without further purification, and all aqueous solutions were
Characterization of PAA-MGO nanocomposite
The PAA-MGO nanocomposite was prepared as illustrated in Scheme 1. In brief, PAA-GO was firstly synthesized by grafting PAA to GO through an EDC-NHS reaction between carboxyl group and amine group. Then, iron oxide nanoparticles were formed on PAA-GO by a co-precipitation reaction of FeCl2 and FeCl3 in an alkaline solution. FTIR spectra of synthesized GO, PAA-GO, MGO and PAA-MGO were measured in Fig. 1.
On the spectrum of GO, the bands at 1733 cm−1 and 1618 cm−1 are assigned to stretching
Conclusions
A poly (allylamine) modified magnetic graphene oxide (PAA-MGO) with high adsorption capacities for selenium oxyanions was successfully synthesized by using the EDC/NHS reaction and co-precipitation method at room temperature. The chemical structure of PAA-MGO was characterized by FTIR and the thermal stability was tested by TGA. The surface charge and morphology were studied by zeta potential measurement and TEM. Magnetic properties were demonstrated by magnetic hysteresis tests. The adsorption
Acknowledgement
Financial supports from Natural Science and Engineering Research Council of Canada (NSERC), Canadian Centre for Clean Coal/Carbon and Mineral Processing Technology (C5MPT), and Teck Resources Ltd. are gratefully acknowledged.
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