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Novel deep eutectic solvent-functionalized carbon nanotubes adsorbent for mercury removal from water

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Abstract

Due to the interestingly tolerated physicochemical properties of deep eutectic solvents (DESs), they are currently in the process of becoming widely used in many fields of science. Herein, we present a novel Hg2+ adsorbent that is based on carbon nanotubes (CNTs) functionalized by DESs. A DES formed from tetra-n-butyl ammonium bromide (TBAB) and glycerol (Gly) was used as a functionalization agent for CNTs. This novel adsorbent was characterized using Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, XRD, FESEM, EDX, BET surface area, and Zeta potential. Later, Hg2+ adsorption conditions were optimized using response surface methodology (RSM). A pseudo-second order model accurately described the adsorption of Hg2+. The Langmuir and Freundlich isotherm models described the absorption of Hg2+ on the novel adsorbent with acceptable accuracy. The maximum adsorption capacity was found to be 177.76 mg/g.

Introduction

It is well known that mercury (Hg) is one of the most toxic elements in nature. Hg usually exists in seawater, fresh water, and in soil [1]. In addition, Hg is a waste product associated with many industries, including production of chlor-alkali, fossil fuels, various switches and wiring devices, measuring and control devices, lighting, and dental work [2]. According to the World Health Organization (WHO), the maximum allowable concentration of Hg in water is 1 µg/l. This value is due to its extremely hazardous effects even at low concentrations [3].

Many conventional techniques have been utilized to reduce Hg concentrations in water, including solvent extraction, precipitation, ion-exchange, reverse osmosis, membrane separation, coagulation, and photoreduction [4]. However, most of these methods have drawbacks such as high energy requirements or their association with large quantities of environmentally hazardous chemicals [5]. As a result, Hg removal from water using the adsorption technique proved to be the most applicable on an industrial scale [6], [7].

Recently, nanoparticles have been introduced as extremely effective adsorbents for many pollutants due to their unique features, small size, catalytic potential, high reactivity, and large surface area [8]. Carbon nanotubes (CNTs) have attracted the most attention in the field of water remediation [9]. However, CNTs have yet to be fully optimized in term of solubility, aggregation, and difficulty in manipulation. On the other hand, CNTs have shown a great affinity for interaction with different compounds, especially after surface functionalization [10], [11], [12], [13], [14], [15], [16], [17]. Oxidative functionalization can enhance the surface charge of CNTs, but requires the use of strong acids and environmentally unfavorable chemicals. Consequently, the need for new types of economical and environmentally friendly functionalization agents is crucial for the development of novel applications [18], [19].

Recently, ionic liquids analogues, i.e., deep eutectic solvents (DESs) have utilized in many different scientific fields. DESs were first introduced by Abbot et al. in 2003 as a cheaper replacement for developed ionic liquids (ILs) [20]. Some researchers consider DESs to be the fourth generation of ILs, even though they are not entirely composed of ionic species [21]. Substantially, DESs can be simply defined as a mixture of two or more compounds that has a melting point lower than that of each individual compound [22]. Regarding environmental safety, DESs have met many of the criteria necessary to be considered environmentally friendly solvents, including availability, biodegradability, recyclability, flammability, and relatively low price compared to other conventional solvents [23]. Due to DESs’ physicochemical properties, they been used in a variety of applications [24], [25]. Most recently, DESs have achieved wide-spread use in nanotechnology-related fields, with uses such as media for synthesis of nanoparticles [26], [27], [28], [29], [30], [31], electrolytes in nanostructure sensors [32], and electrolytes in nanoparticle deposition [33], [34], [35], [36], [37], [38], [39], [40], [41]. Based on these applications, DESs have the potential to be used as economically and environmentally friendly functionalization agents.

In this study, an ammonium-based DES was synthesized using tetra-n-butyl ammonium bromide (TBAB) with glycerol (Gly) as the hydrogen bond donor (HBD). Later, this TBAB based-DES was utilized as a functionalization agent for CNTs, which were then used as an adsorbent agent for Hg2+ ions from water samples. In addition, the functionalized CNTs were fully characterized as the novel adsorbent to study the effect of TBAB based-DES on the CNT surface. This characterization includes Raman spectroscopy, XRD diffraction, FTIR, FESEM, EDX, BET surface area, and zeta potential. The optimal removal conditions for Hg2+ were determined using Response surface methodology (RSM). Moreover, kinetics and isotherm studies were also performed at the optimal conditions.

Section snippets

Chemicals and materials

Multi-wall carbon nanotubes with specifications of D × L 6–9 nm × 5 µm ˃95% (carbon), TBAB, Gly, sulfuric acid (95–97%), nitric acid (65%), potassium permanganate, sodium hydroxide pellets, and hydrochloric acid (36.5–38%) were all supplied by SIGMA-ALDRICH. A 1000 mg/L mercury standard solution was supplied by MERCK.

Functionalization of CNTs

The surface of the pristine CNTs (P-CNTs) was functionalized by oxidation through to different procedures. The first procedure used sonication with KMnO4 for 2 h at 65 °C [42]. The

Results and discussion

The screening studies showed that the KT-CNTs recorded the highest removal compared to other tested adsorbents. The significant effect of TBAB-based DESs on the removal percentage can be seen at both pH 2.2 and pH 6.0. Fig. 1 illustrates the screening studies for the adsorption of Hg2+.

For carbon-based materials, Raman spectroscopy is considered to be an essential characterization due its ability to indicate the degree of functionalization by comparing the intensity of the D band (ID) to that

Conclusion

The Hg2+ ion was successfully removed from an aqueous solution by CNTs functionalized with DESs. The novel adsorbent exhibited great potential for Hg2+ removal, where the maximum adsorption capacity was found to be 177.76 mg/g, which lies in the highest range compared to the published data. The optimal removal conditions were found to be at pH 6.4, adsorbent dosage of 6.0 mg, and contact time of 45 min. A TBAB-based DESs was synthesized and was a sufficient functionalization agent for CNTs. The

Acknowledgments

The authors express their thanks to the University of Malaya UMRG (RP017A-13AET) and the National Chair Of Materials Science And Metallurgy, University of Nizwa, Oman for funding this research.

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