*2.7. Adsorption of Hg2*<sup>+</sup> *and Cr6*<sup>+</sup>

The batch adsorption analyses were performed in 150 mL beaker using adsorption shaker (200 rpm) at pH of 5.5 and 3.5 for Hg2<sup>+</sup> and Cr6<sup>+</sup> removal respectively. For more illustration, 0.010–0.070 g of adsorbents were used in Hg2<sup>+</sup> and Cr6<sup>+</sup> ions solution (50 mL, 20 ppm) at 25 ◦C for fixed period. After adsorption, the mixture was filtered to determine the concentration of Hg2<sup>+</sup> and Cr6<sup>+</sup> ions using 1,3-diphenylcarbazide method [14]. The concentration of adsorbed Hg2<sup>+</sup> and Cr6<sup>+</sup> was calculated by evaluating the absorbance of heavy metal ion solution using UV-Vis spectrophotometer at 532 nm and 370 nm respectively. The amount of adsorbed Hg2<sup>+</sup> and Cr6<sup>+</sup> was calculated by Equation (2) [28]:

$$\mathbf{q}\_{\mathbf{c}} = \frac{(\mathbf{C}\_{\mathbf{o}} - \mathbf{C}\_{\mathbf{c}})\mathbf{V}}{\mathbf{M}} \tag{2}$$

where qe = equilibrium gum tragacanth-cl-*N,N*-dimethylacrylamide and reduced graphene oxide incorporated gum tragacanth-cl-*N,N*-dimethylacrylamide adsorption capacity, Co = initial Hg2<sup>+</sup> and Cr6<sup>+</sup> concentration (mg L−1), Ce = equilibrium Hg2<sup>+</sup> and Cr6<sup>+</sup> concentration (mg L−1), V = volume (L) of Hg2<sup>+</sup> and Cr6<sup>+</sup> ion solution, M = weight (g) of gum tragacanth-cl-*N,N*-dimethylacrylamide and reduced graphene oxide incorporated gum tragacanth-cl-*N,N*-dimethylacrylamide hydrogels. Adsorption-desorption analyses were conducted by using 0.1 M HNO3. We have optimized the HNO3 concentration (0.02 M, 0.04 M, 0.06 M, 0.08 M, 0.1 M, 0.12 M) for desorption experiments. The maximum adsorption-desorption rate was found at optimized 0.1 M HNO3. The Hg2<sup>+</sup> and Cr6<sup>+</sup> loaded GT-cl-poly(DMA) hydrogel and GT-cl-poly(DMA)/RGO hydrogel composite were desorbed by using 100 mL of 0.1 M HNO3 followed by neutralization with 0.1 M NaOH. Finally, the desorbed adsorbent was washed by distilled water and dried at room temperature for further adsorption of Hg2<sup>+</sup> and Cr6<sup>+</sup>.
