High efficient adsorption and photocatalytic removal of hexavalent chromium over three-dimensional grapheneoxide aerogel under visible light

Abstract

<p indent="0mm">Heavy metal ions such as hexavalent chromium (Cr(VI)) existed in wastewater have caused serious pollution problems.Cr(VI) is teratogenic and carcinogenic, while trivalent chromium (Cr(III)) is an essential nutrient for humans and animals. Adsorption method can transfer Cr(VI) from liquid phase to absorbent surface, but it needs follow-up treatment to remove Cr(VI). Photocatalysis technique can reduce Cr(VI) to Cr(III), so as to decrease its toxicity, but the adsorption capacity towards Cr(VI) is always small due to poor surface area of some traditional semiconductor oxide photocatalysts. Therefore, a new material with both good adsorption capacity and photocatalytic activity is needed for Cr(VI) removal. As an emerging kind of carbon materials, graphene oxide (GO) has attracted particular attention in the catalysis field. Graphene oxide aerogel (GOA), a three dimensional nanomaterial, possesses numerous adsorption sites and superior ability to accept and transport photogenerated electrons, would thus present good adsorption capacity and high photocatalytic performance simultaneously. So far, most studies on GO-base material are towards investigating its co-catalyst role (adsorption enhancement or electron shuttle) in improving efficiency of metal or semiconductor photocatalysts. Only a few studies have applied GO-base material as a sole photocatalyst. In this study, combined with the cross-linking method and the supercritical CO₂ fluid extraction method, three-dimensional porous and recyclable GOA with large specific surface area and defect sites was prepared by using GO as the precursor. The physicochemical properties were characterized by SEM, XRD, Raman spectroscopy and nitrogen adsorption methods. The adsorption behavior and photocatalytic reduction performance of GOA towards Cr(VI) under visible light were investigated. GO was hydrothermally reduced during the preparation process, and many defects were generated on GO sheets. Melamine and cyanuric acid were used as crosslinking agents to increase the interlayer spacing, and at the same time providing GOA a large specific surface area and a rich porous structure. When the mass ratio of GO, melamine, and cyanuric acid was 3:3:3 and the GO concentration was 6 mg L⁻¹, GOA exhibited the largest specific surface area and defect density. This kind of GOA, i.e., GOA_333-6 presented the best adsorption capacity and photocatalytic efficiency of Cr(VI). The adsorption behavior of GOA on Cr(VI) fits to the quasi-second-order kinetic model and the Langmuir adsorption isotherm model, indicating that the adsorption process was a single-layer chemical adsorption, in which the intra-particle diffusion in the initial adsorption stage was the rate-limiting step. The three-dimensional graphene oxide aerogel GOA obtained in this study exhibited large adsorption capacity, high photocatalytic removal efficiency for high concentration Cr(VI) under visible light, and good repeatability. Single GOA material has potential application in removing heavy metal ions from polluted wastewater.