**1. Introduction**

Continuous industrialization leads to the excessive release of toxic pollutants into various sources of water. The heavy metal poisoning of water has now become a pandemic concern due to its dangerous impacts to human health because these pollutants are non-degradable, poisonous, cancer-causing agent and are hard to separate from water [1]. For metal uptake from water, several approaches have been established, among them, treatment via adsorption is the most appealing one. Adsorption technology is widely used for removing pollutants due to its simple operation, cost and easy implementation [2]. Specifically, hydrogels based on biopolymer have now become very useful in adsorptive wastewater treatment [3]. Gum tragacanth based hydrogel is highly adsorptive because of the presence of hydroxyl (–OH) and carboxyl (–COOH) groups [4,5]. It is a renewable, cost-effective and environmentally friendly polysaccharide that can be easily polymerized to form cross-linked structures [6–8].

Gum tragacanth is commonly found in the sap of different legumes in the Middle East. The biological source of gum tragacanth is a plant named *Astragalus gummifer*. It is a complex mixture of polysaccharides including bassorin and tragacanthin units. When mixed with water, gum tragacanth produces a colloidal hydrosol. The bassorin unit can (composed of 60–70% of the compound) swells to form a gel [9]. Mallakpour et al. reported the glutaraldehyde cross-linked gum tragacanth/CaCO3 hydrogel composite as an adsorbent for the abstraction of Pb2<sup>+</sup> ion [10]. Moghaddam et al. synthesized methoxyl gum tragacanth-glutamic acid/polyacrylamide hydrogel via electron beam radiations as an adsorbent for trapping uranium ions from toxic uranium solution [11].

The adsorption and stability of hydrogel can be improved by using reduced graphene oxide as filler in the hydrogel matrix. Reduced graphene oxide (RGO) can result in high C/O with better mechanical strength [12]. The reduced graphene oxide is partially decorated with an oxygen-rich functional group that acts as active sites for interaction. The high RGO surface, large porosity and defect sites are the features that help pollutants adsorption [13]. Sahraei et al. reported adsorption of Cr6<sup>+</sup> metal using chitosan/reduced-graphene oxide/montmorillonite composite hydrogel. The composite hydrogel showed maximum Cr6<sup>+</sup> absorption of 87.03 mg g−<sup>1</sup> [14]. Zhuang et al. synthesized molybdenum disulfide/RGO hydrogel as an adsorbent for mercury ions removal [15].

The synthesized gum tragacanth-cl-*N,N*-dimethylacrylamide (GT-cl-poly(DMA)) and reduced graphene oxide incorporated gum tragacanth-cl-*N,N*-dimethylacrylamide (GT-cl-poly(DMA)/RGO) hydrogel composite were efficient in adsorption of Hg2<sup>+</sup> and Cr6<sup>+</sup> as compared to previously reported adsorbents in the literature (Table 1 ). This was due to the perfect combination of reduced graphene oxide (RGO), gum tragacanth (GT), and *N,N*-dimethylacrylamide (DMA) led to the presence of many –OH, –NH2 and –COOH hydrophilic groups. The gum tragacanth-cl-*N,N*-dimethylacrylamide hydrogel and reduced graphene oxide incorporated gum tragacanth-cl-*N,N*-dimethylacrylamide hydrogel composite exhibited the highest removal capacity of 625 mg g−<sup>1</sup> and 666.6 mg g−<sup>1</sup> respectively for Hg2<sup>+</sup>. Similarly, for Cr6<sup>+</sup>, removal capacities were 401.6 mg g−<sup>1</sup> and 473.9 mg g−<sup>1</sup> respectively.

The previously reported works (Table 1) have not comprehensively considered the factors responsible for the high adsorption capability of the adsorbent. In this work, we achieved a better adsorption capacity of 666.6 mg g−<sup>1</sup> and 473.9 mg g−<sup>1</sup> for mercury and chromium ions within less time using a low adsorbent dose. Specifically, prepared reduced graphene oxide incorporated gum tragacanth cross-linked poly *N,N*-dimethylacrylamide hydrogel composite shows a very high adsorption percentage of 99% for mercury metal ion under optimal conditions (adsorbent dose = 0.035 g and time = 270 min, T = 25 ◦C, the concentration of mercury solution = 20 ppm) which means it is highly efficient for mercury adsorption. Also, compared to recently reported studies, we are able to synthesize our adsorbents in very short period (90 s) with high swelling percentage (Table 2) using microwave radiations. This is one of the key points where our synthesis part shows novelty. Hence, we developed the simple and fast synthetic route for the preparation of efficient, sustainable and eco-friendly graphene oxide incorporated gum tragacanth-cl-*N,N*-dimethylacrylamide hydrogel with high adsorption rate for heavy metal ions.


**Table 1.** Comparison of different adsorbents with gum tragacanth-cl-*N,N*-dimethylacrylamide (GT-cl-poly(DMA)) hydrogel and reduced graphene oxide incorporated gum tragacanth-cl-*N,N*-dimethylacrylamide (GT-cl-poly(DMA)/RGO) hydrogel composite for adsorption of Hg2<sup>+</sup> and Cr6<sup>+</sup> metal ions.

**Table 2.** Comparative analysis for swelling percentage of hydrogels.


In this work, we developed first-time gum tragacanth-cl-*N,N*-dimethylacrylamide hydrogel and reduced graphene oxide incorporated gum tragacanth-cl-*N,N*-dimethylacrylamide hydrogel composite for adsorption of Hg2<sup>+</sup> and Cr6+. The RGO was synthesized from graphite and incorporated in GT-cl-poly(DMA) hydrogel matrix to increase the adsorption efficiency. The sorption study was explained by kinetic and isotherm models. The effect of pH, adsorbent dose and RGO loading on adsorption were performed. The gum tragacanth-cl-*N,N*-dimethylacrylamide hydrogel was systematically designed based on swelling. The adsorbed samples were desorbed successfully by using 0.1 M HNO3 and used further for adsorption experiments.
