3.7.1. Influence of RGO Loading on the Removal of Hg2<sup>+</sup> and Cr6<sup>+</sup>

In the GT-cl-poly(DMA) hydrogel matrix, different quantities of RGO (0.005 g, 0.01 g, 0.015 g, 0.02 g and 0.025 g) were incorporated to study the effect RGO loading on metal ions removal. The adsorption percentages for without RGO were 70.6% and 20.4% for Hg2<sup>+</sup> and Cr6<sup>+</sup> respectively. The adsorption percentages for Hg2<sup>+</sup> (Figure 6a) and Cr6<sup>+</sup> (Figure 6b) ions were enhanced on raising the concentration of RGO from 0.005 g to 0.020 g. The RGO contains carboxylic groups which boost interactions with metal ions resulting in high percentage adsorption [30]. The removal efficiency of Hg2<sup>+</sup> and Cr6<sup>+</sup> was 90.7% and 38.4% at RGO loading of 0.020 g. The development of tough three-dimensional networks was responsible for the decrease in adsorption percentage at higher RGO loading (>0.020). Therefore, 0.020 g was the optimized dose of RGO in the formation of GT-cl-poly(DMA)/RGO for removal of Hg2<sup>+</sup> and Cr6<sup>+</sup>.

**Figure 6.** *Cont*.

**Figure 6.** Influence of (**a**) RGO loading on Hg2<sup>+</sup> adsorption (**b**) RGO loading on Cr6<sup>+</sup> adsorption, (**c**) pH on adsorption of Hg2<sup>+</sup>, (**d**) pH on adsorption of Cr6<sup>+</sup>, (**e**) adsorbent dose on adsorption of Hg2<sup>+</sup>, (**f**) adsorbent dose on adsorption of Cr6+. Adsorbent: 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.

3.7.2. Influence of pH on Removal of Hg2<sup>+</sup> and Cr6<sup>+</sup> by GT-cl-poly(DMA) Hydrogel and GT-cl-poly (DMA)/RGO Hydrogel Composite

The impact of pH on adsorption percentage for Hg2<sup>+</sup> (Figure 6c) and Cr6<sup>+</sup> (Figure 6d) by 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 are shown in Figure 6. The removal percentage of Hg2<sup>+</sup> was first increased from pH 1.5 (77.6% for GT-cl-poly(DMA), 86.3% for GT-cl-poly(DMA)/RGO) to 5.5 (86.1% for GT-cl-poly(DMA), 97.6% for GT-cl-poly(DMA)/RGO) and then decreased from pH 5.5 (86.1%, 97.6%) to 9.5 (72.5%, 84.2%). The reported highest removal efficiencies for Hg2<sup>+</sup> were 86.1% and 97.6% by GT-cl-poly(DMA) hydrogel and GT-cl-poly(DMA)/RGO) hydrogel composite respectively at 5.5 pH. At low pH, the concentration of H<sup>+</sup> ions was high which could compete with Hg2<sup>+</sup> on GT-cl-poly(DMA)/RGO surface resulting in poor binding of Hg2<sup>+</sup> [36]. However, high pH was responsible for the decrease in the concentration of H<sup>+</sup> ions in the solution and improves the binding potential of Hg2<sup>+</sup> ions to the surface of GT-cl-poly(DMA)/RGO) (Scheme 2a). Hence adsorption of Hg2<sup>+</sup> was increased from pH 1.1 to 5.5. The dominant species were Hg(OH)2 and HgCl4 <sup>2</sup><sup>−</sup> [37] at pH above 5.5. The electrostatic repulsion among Hg(OH)2 or HgCl4 <sup>2</sup><sup>−</sup> [38] and negatively charged GT-cl-poly(DMA)/RGO) was responsible for low Hg2<sup>+</sup> adsorption percentage at pH above 5.5.

In the case of Cr6<sup>+</sup>, the recorded maximum adsorption percentages were 76.1% and 81.5% (Figure 6d) for GT-cl-poly(DMA) hydrogel and GT-cl-poly(DMA)/RGO hydrogel composite correspondingly at pH 3.5. The dominant Cr6<sup>+</sup> species [39] are as: H2CrO4 (pH < 3.5), HCrO<sup>−</sup> <sup>4</sup> (pH < 7), CrO2<sup>−</sup> <sup>4</sup> (pH <sup>&</sup>gt; 7). The Cr6<sup>+</sup> ions were exists in solution as negatively charged HCrO<sup>−</sup> <sup>4</sup> at pH 3.5. Therefore, electrostatic attraction of HCrO− <sup>4</sup> [40] took place at pH 3.5 with protonated positively charged group of GT-cl-poly(DMA)/RGO) (Scheme 2b). Hence, Cr6<sup>+</sup> exhibited maximum adsorption percentage at 3.5 pH. At pH < 3.5, electrostatic attraction for adsorption was reduced due to the dominance of H2CrO4. Also, with increasing pH from 3.5 to 7, the protonated group on GT-cl-poly(DMA)/RGO decreases which reduces the electrostatic attraction. At pH > 7, electrostatic repulsion between dominant CrO2<sup>−</sup> 4 species [40] and deprotonated GT-cl-poly(DMA)/RGO was attributed to low Cr6<sup>+</sup> adsorption.

**Scheme 2.** Possible interactions of (**a**) Hg2<sup>+</sup> at pH 5.5 and (**b**) Cr6<sup>+</sup> at pH 3.5 with GT-cl-poly(DMA)/RGO hydrogel composite adsorbent.
