Regular Article
Multi-carboxylic magnetic gel from hyperbranched polyglycerol formed by thiol-ene photopolymerization for efficient and selective adsorption of methylene blue and methyl violet dyes

https://doi.org/10.1016/j.jcis.2018.06.005Get rights and content

Abstract

Synthesis of adsorbents that simultaneously possess high adsorption performance and convenient separation characteristic remains a big challenge. Herein, we report a viable strategy to prepare hyperbranched polyglycerol (HPG)-based multi-carboxylic magnetic gel (MMG) based on the thiol-ene click chemistry. The obtained HPG-based MMG adsorbent with porous structure and multiple carboxylic groups not only exhibited fast adsorption rate, superior adsorption capacity, and favorable adsorption selectivity towards cationic dyes (e.g., methylene blue (MB) and methyl violet (MV)) but also presented convenient magnetic separation characteristic. The adsorption data showed nice correlation with pseudo-second-order kinetic model and Langmuir isotherm. The optimal adsorption capacity of HPG-based MMG for MB and MV can reach as high as 458.7 and 400.0 mg g−1, respectively. Moreover, the dye-adsorbed HPG-based MMG could be regenerated in ethanol for reuse without obvious decline of removal efficiency. On the basis of the superior adsorption property of HPG-based MMG, this work offers an effective strategy to prepare advanced adsorbent for dye removal from wastewater.

Introduction

Rapid industrialization not only contributes to significant improvement on economy but also results in serious environmental pollution problems such as water pollution. Organic dyes, which have been extensively utilized in manufacturing industries, are one class of the major water pollutants because trace amount of dyes may cause serious mutagenic and carcinogenic effects on ecosystem [1], [2], [3]. Accordingly, it is crucial to remove dyes from wastewater before discharging them into the aquatic environment. Until now, various techniques such as adsorption [4], [5], coagulation [6], photocatalysis [7] and membrane treatment [8], have been employed for removing dyes from water. Among them, adsorption with the merits of simplicity of design, ease of operation, low cost and wide suitability is regarded as the most promising technique [9], [10], [11], [12]. Conventional adsorbents (e.g., activated carbon), however, generally suffer from drawbacks of low adsorption capacity, slow adsorption rate, and poor adsorption selectivity and reusability. Therefore, much attention has been paid to the development of high-performance adsorbents that simultaneously possess fast adsorption rate, high adsorption capacity, superior adsorption selectivity, and favorable reusability.

Hyperbranched polymer (HP) with highly branched molecular structure has drawn increasing attention for utilizing as an adsorbent because it not only possesses unentangled molecular chains and a large number of functional groups but also can be synthesized by a facile one-pot reaction [13], [14], [15]. It is reported that the adsorption capability of an adsorbent strongly depends on the amounts of binding sites (e.g., charged functional groups) [15]. In this regard, the HP-based adsorbents contain much more binding sites that that of linear polymer-based adsorbents, and thus should exhibit better adsorption performance. To date, several groups have reported the employment of HPs as adsorbents for dye adsorption from water [16], [17], [18], [19], [20]. For instance, Liu and colleagues reported the use of hyperbranched polymeric ionic liquid with imidazolium backbones for removal of anionic dyes from water [19]. They found that the obtained adsorbent showed high removal efficiency for anionic dyes. Jiang’s group reported the employment of hyperbranched poly(ether amine) to prepare multi-responsive microgel for selective adsorption of hydrophilic dyes from water [20]. Although the reported HP-based adsorbents showed high adsorption efficiency towards dyes, they are generally difficult to recycle after adsorption especially for large scale treatment of dye solutions. This may not only cause the waste of adsorbents but also result in secondary pollution. To solve the separation problem, we have reported the grafting of HPs from the surface of magnetic nanoparticles as magnetic adsorbents [21]. However, the grafting reaction requires harsh experimental condition and complex operation, which significantly limited their practical applications. Recently, magnetic gel is emerging as a new kind of adsorbent for dye removal because it combines the features of magnetic nanoparticle and gel. Various linear polymers such as chitosan [22], poly(vinyl alcohol) [23], starch [24], poly(acrylic acid) [25] and cellulose [26] have been employed to prepare magnetic gel. However, the use of HPs to prepare magnetic gel adsorbent is still in its infancy.

Herein, we present the preparation of hyperbranched polyglycerol (HPG)-based multi-carboxylic magnetic gel (MMG) based on the thiol-ene click chemistry reaction (Scheme 1). HPG as a typical HP possesses multiple hydroxyl groups, high water solubility and favorable biocompatibility [27], [28], [29]. The employment of the obtained HPG-based MMG as an adsorbent for dye adsorption was studied in detail by considering the influences of a variety of adsorption parameters, including contact time, dye concentration, solution pH, ionic strength and temperature. In addition, selective separation of cationic dye from dye mixture and the regeneration of dye-adsorbed HPG-based MMG adsorbent in diverse eluting agents were also studied. Compared with the previously reported HPG-based adsorbents and magnetic gels, the HPG-based MMG adsorbent in this work was expected to exhibit combined merits, which not only presents superior adsorption behaviors towards cationic dyes but also shows facile magnetic separation performance.

Section snippets

Materials

Methylene blue (MB, 96%), tartrazine (TTZ, 89%), methyl orange (MO, 96%), methyl violet (MV, 99%), Pb(NO3)2 (99%), FeCl3 (98%), dimethyl sulfoxide (DMSO, 99.9%), 4-(dimethylamino) pyridine (DMAP, 99%), diethylene glycol (99%), NaOH (98%) and triethylamine (99.5%) were purchased from Aladdin Chemistry Co. Ltd. (Shanghai, China). Detailed information about the employed dyes is shown in Table S1. Succinic anhydride (>99%), glycidyl methacrylate (GMA, 97%) and 2,3-dimercapto-1-propanol (DCP, 98%)

Synthesis and characterization of HPG-based MMG

The synthetic process of HPG-based MMG is illustrated in Scheme 1. Magnetic Fe3O4@HPG nanoparticles with a mean diameter of 7.2 nm and saturation magnetization value of 54.5 emu/g were synthesized according to our previous report (Fig. S1) [31]. In the FTIR spectra of HPG and Fe3O4@HPG, the characteristic peaks at 3410 and 1110 cm−1 are assigned to the stretching vibration of hydroxyl group and Csingle bondO bond, respectively (Fig. 1a). The two bands located at 2975 and 2873 cm−1 corresponding to the

Conclusions

In conclusion, we have demonstrated an effective strategy to combine the features of multi-carboxylic HPG and magnetic Fe3O4 nanoparticles by fabrication of HPG-based MMG based on the thiol-ene click chemistry. The obtained HPG-based MMG possesses abundant carboxylic groups, porous microstructure and strong magnetic property. It simultaneously exhibited rapid adsorption rate, superior adsorption capacity, good adsorption selectivity, nice reusability, and favorable magnetic separation

Acknowledgements

This work was supported by National Natural Science Foundation of China (No. 21364003 and No. 51663007), Natural Science Foundation of Guangxi Province (No. 2017GXNSFFA198002), and the Project of Thousand Outstanding Young Teachers’ Training in Higher Education Institutions of Guangxi.

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