*3.3. FTIR*

The FTIR graphs of samples are presented in Figure 2. In spectrum of RGO (Figure 2a), broadband of nearly 3358 cm−<sup>1</sup> can be attributed to –OH stretching mode. The peak at 1421 cm−<sup>1</sup> corresponds to the carboxylic acid and peak at 1625 cm−<sup>1</sup> belongs to the –C=C group in the aromatic rings. The peak at 1128 cm−<sup>1</sup> is due to –C–O stretching in the C–OH functional groups of RGO [29]. The shifting of –C–O stretching from 1128 cm−<sup>1</sup> to 1125 cm−<sup>1</sup> in GT-cl-poly(DMA)/RGO hydrogel composite is related to the successful incorporation of RGO in GT-cl-poly(DMA) hydrogel. The bands at 1638 cm−<sup>1</sup> and 1748 cm−<sup>1</sup> correspond to asymmetric stretching of the carboxylate group and asymmetric stretching of C=O in galacturonic acid respectively [30], peak at 1142 cm−<sup>1</sup> ascribed to antisymmetric vibrations of C–O–C linkage in glycosidic groups [31]. The asymmetric stretching of C=O shows shifting of peaks from 1748 cm−<sup>1</sup> to 1750 cm−<sup>1</sup> after the crosslinking of poly(DMA). The GT-cl-poly(DMA)/RGO hydrogel composite (Figure 2a) shows shift related to asymmetric stretching of C=O from 1750 cm−<sup>1</sup> to 1758 cm−<sup>1</sup> suggesting interaction between RGO and GT-cl-poly(DMA) hydrogel. The peaks at 1608 cm−<sup>1</sup> and 1410 cm−<sup>1</sup> in GT-cl-poly(DMA) hydrogel ascribed to stretching vibrations of poly(DMA) amide group [32]. These stretching vibrations of poly(DMA) show peak shifting from 1608 cm−<sup>1</sup> to 1612 cm−<sup>1</sup> and from 1410 cm−<sup>1</sup> to 1403 cm−<sup>1</sup> in GT-cl-poly(DMA)/RGO hydrogel composite which confirms the changes in the structure of poly(DMA) after RGO incorporation. In GT-cl-poly(DMA)/RGO, the broadband of O–H stretching vibration shifted from 3403 cm−<sup>1</sup> to 3382 cm−<sup>1</sup> which may be attributed to the RGO interaction with GT-cl-poly(DMA) through intermolecular hydrogen bonds. The peaks intensity of GT-cl-poly(DMA)/RGO hydrogel composite is slightly lower than the GT-cl-poly(DMA) hydrogel, which also confirms the RGO dispersion in GT-cl-poly(DMA)/RGO hydrogel composite. The absorption band at 2910 cm−<sup>1</sup> was attributed to stretching vibrations of aliphatic C–H [10]. Also, peaks at 1048 cm−<sup>1</sup> and 1052 cm−<sup>1</sup> in the spectra of hydrogels correspond to the –C–O bending. After the adsorption of Hg2<sup>+</sup> and Cr6<sup>+</sup> on GT-cl-poly(DMA) hydrogel and GT-cl-poly(DMA)/RGO hydrogel composite, peak due to carboxylate groups was shifted from 1612 cm−<sup>1</sup> to 1621 cm−<sup>1</sup> and the intensity of the peaks decreases (Figure 2b). The peaks at 1410 cm−<sup>1</sup> and 1048 cm−<sup>1</sup> were shifted to 1403 cm−<sup>1</sup> and 1061 cm−<sup>1</sup> respectively, which was probably due to the interactions of metal ions to the active site of adsorbent. The peaks intensity of Hg2<sup>+</sup> loaded GT-cl-poly(DMA) hydrogel and GT-cl-poly(DMA)/RGO hydrogel composite was lower than the Cr6<sup>+</sup> loaded GT-cl-poly(DMA) hydrogel and GT-cl-poly(DMA)/RGO hydrogel composite, which supports higher Hg2<sup>+</sup> adsorption than Cr6<sup>+</sup> adsorption.

**Figure 2.** Fourier transform infrared of (**a**) gum tragacanth (GT), gum tragacanth-cl-*N,N*dimethylacrylamide (GT-cl-poly(DMA)) hydrogel, RGO and reduced graphene oxide incorporated gum tragacanth-cl-*N,N*-dimethylacrylamide (GT-cl-poly(DMA)/RGO) hydrogel composite, (**b**) Hg2<sup>+</sup> and Cr6<sup>+</sup> loaded GT-cl-poly(DMA) hydrogel and GT-cl-poly(DMA)/RGO hydrogel composite.
