Incorporating hard-soft acid-base theory in multi-aspect analysis of the adsorption mechanism of aqueous heavy metals by graphene oxide

Abstract

Water contamination by heavy metals is a serious environmental concern. To unravel the “species resolved” interaction mechanism of heavy metals with graphene oxide (GO), a multi-aspects analyses incorporating metal species distribution, GO surface charge, adsorption behavior and fitting, hard-soft acid-base (HSAB) theory, Fourier transformation infrared spectroscopy and X-ray photoelectron spectroscopy were performed. For pH 7 and T = 25 °C, adsorptions of aqueous Cd(Ⅱ) and Pb(Ⅱ) with equal initial concentrations of 300 mg L−1 by GO at 500 mg L−1 equilibrated in 15 min. The corresponding adsorption percentage and quantity for Cd(Ⅱ) were 98.04% and 588.22 mg g−1 and those for Pb(Ⅱ) were 92.10% and 552.56 mg g−1. When pH ≤ 7, bare Cd2+ and Pb2+ were the dominant metal species adsorbed by GO via ion exchange and chemical complexation. When pH ≥ 7, hydroxyl-complexed species were the dominant ones adsorbed by GO via precipitation, ion exchange, and chemical complexation. Electron transfer and energy lowering induced by the GO–heavy metal interaction, calculated based on HSAB theory, illuminated two points: (1) OH had a stronger binding affinity than COOH toward either Cd(Ⅱ) or Pb(Ⅱ) and (2) either OH or COOH had a stronger binding affinity toward Cd(Ⅱ) than Pb(Ⅱ). Explorations of this work may provide guidance in developing efficient graphene-based adsorbents for wastewater remediation.