Species resolved interaction mechanism between graphene oxide and Cu(II) in aqueous solution with implications on wastewater remediation

Heavy metals discharged into water by industrial activity give rise to severe environmental pollution. Herein, graphene oxide (GO) was prepared based on an improved Hummers method and utilized as adsorbent to remove aqueous Cu(II). The "species resolved" adsorption mechanism was deeply inspected via combining multiple explorations extracted from Cu(II) species distribution, GO surface charge, adsorption experiment and fitting, hardsoft acid-base (HSAB) theory, FTIR and XPS spectra. Different adsorption mechanism resolved by Cu(II) species was proposed, corresponding to different pH range. (1) When pH <8, bare Cu2+ is the dominant Cu(II) species, ion exchange and chemical complexation are the adsorption mechanism. (2) When pH=8, Cu(OH)(2) is the dominant Cu(II) species, precipitation, ion exchange and chemical complexation are the adsorption mechanism. (3) When pH>8, Cu(OH)(3)(-) is the dominant Cu(II) species, electrostatic attraction is the adsorption mechanism. Electron transfer and energy lowering calculated based on the HSAB theory demonstrates, OH is stronger than COOH regarding binding affinity towards Cu(II). As for adsorption efficiency, adsorption of Cu(II) onto GO equilibrated in 12 min, with adsorption percent and quantity 92.32% and 553.90 mg.g(-1), respectively. Findings of this work may shed light on the interaction mechanism of graphene oxide with heavy metals. Accordingly, these as clarified mechanisms may provide guidance for developing efficient adsorbent based on graphene for heavy metal scavenging.