## 1 Introduction  
Heavy metal pollution is one of the most important environmental quality and human health problems today. Among the heavy metals, chromium is one of the most hazardous heavy metals, which is used extensively by several industries, including dyes, electroplating, textile and alloying [1, 2]. Chromium exists in two oxidation states, Cr(VI) and Cr(III) [3]. The Cr(VI) form is 500 times more toxic to human health than the trivalent one [4]. Soluble Cr(VI) pollutes the soil and water, which would cause lung cancer [5]. Hence, it is necessary to remove Cr(VI) from wastewater to prevent the tremendous threat of Cr(VI) on ecosystem and public health. A large number of treatment methods in Cr(VI) removal have been reported, including adsorption [6, 7], membrane separation [8, 9], coagulation and precipitation [10, 11], ion exchange [12, 13] and solvent extraction [14]. Compared with other methods, adsorption is generally recognized as the most promising and widely used feasible technique due to its low cost, high efficiency, regeneration ability and friendly to environment [15, 16]. A variety of adsorbents have been studied for Cr(VI) removal such as active carbon [17; 18], metal oxide nanoparticles [19; 20], synthesized polymer beads [4; 21] and agriculture waste [22; 23], _etc._ [PERSON] _et al._ reported that the adsorption capacity of rice husk carbon for the removal of Cr(VI) was evaluated to be 48.31 mg/g [24]. The adsorption of hexavalent chromium onto surfactant modified montmorillonitees [25] and zeolitic material [26] was studied, with the maximum adsorption capacities of 18.05 and 75.8 mg/g, respectively. Metal oxide nanoparticles such as ZnO were employed as adsorbents for Cr(VI) from aqueous solution, and the maximum adsorption capacities were 9.38 mg/g [19]. These studies shows that these materials have inferior performance for the removal of Cr(VI) from aqueous solutions. Thus, it is very significant to explore an adsorbent with high adsorption capacity. Synthesized polymer materials have been extensively used as adsorbents for removal of toxic heavy metal ions in recent years since the surface properties of the adsorbents can modified by the by the available functional groups to promote their adsorption ability. Several polymeric adsorbents have been used for removal of Cr(VI) such as glycine doped polypyrrole [27], 1,2-ethylenediamine-aminated macroporous polystyrene particles [28], magnetic poly-(MA-DVB) microspheres [29] and polyethylene glycolmethacrylate-co-vinylimidazole microspheres [30]. Glycidyl methacrylate (GMA) has a vinyl group and an epoxide group, which are possible for various material preparations through radical polymerization and ring-opening reactions. [PERSON] _et al._ reported the preparation of poly(GMA-co-PEGDA) microbeads modified with immodiacetic acid and their indium adsorption properties [31]. The epoxide group of GMA has a high reactivity with amine compounds, and amino-functionalized adsorbents showed an outstanding ability in the removal of Cr(VI) from wastewater [32]. Since amino groups are easily protonated under acidic conditions, the Cr(VI) can be adsorbed onto the adsorbent by electrostatic interaction and ion exchange. To improve the adsorption efficiency, adsorbents are usually made into a very small size. But it would be very difficult to separate them from solution after accomplishing adsorption [33]. Recently, magnetic separation technique has attracted many researchers' attentions because it makes the recovery of polymeric adsorbents from solution quite easy and simple under an external magnetic field [34]. Magnetic polymeric adsorbents are usually composed of the magnetic cores to ensure a strong magnetic responsibility and a polymeric shell to provide functional groups for various applications [35]. In the present work, Fe\({}_{3}\)O\({}_{4}\) nanoparticles modified by Sodium dodecyl sulfate (SDS) and Polyethylene glycol (PEG) were synthesized by co-precipitation of Fe\({}^{2+}\) and Fe\({}^{3+}\) salts.