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 π–π 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 ∼ 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 π–π interactions between the GA-GH and aromatic dyes.