Gallic acid-functionalized graphene hydrogel as adsorbent for removal of chromium (iii) and organic dye pollutants from tannery wastewater

The pollution caused by tannery wastewater containing high concentrations of trivalent chromium ions [Cr(iii)] and organic dyes has severely restricted the sustainable development of the leather industry. To address this problem, a three-dimensional (3D) porous graphene-based hydrogel with good mechanical strength and large surface area was fabricated by self-assembly of graphene oxide (GO) sheets reduced and modified by gallic acid (GA) through pi-pi interactions. As an adsorbent, this GA-functionalized graphene hydrogel (GA-GH) can effectively capture Cr(iii) by coordination complexation between Cr(iii) and deprotonated carboxylic groups of GA at pH similar to 4.0. Moreover, GA-GH could be easily regenerated by desorption of adsorbed Cr(iii) at pH 2.0 and maintained its high adsorption capacity after multiple adsorption-desorption cycles, which was also helpful for reusing desorbed Cr(iii) as tanning agent. In addition, compared with a graphene hydrogel (GH) without modification by GA, adsorption capacity of GA-GH for organic dye was significantly improved due to the enhanced pi-pi interactions between the GA-GH and aromatic dyes.