Adsorption and coadsorption of organic pollutants and a heavy metal by graphene oxide and reduced graphene materials

The adsorption and coadsorption of naphthalene, 1-naphthol and Cd 2+ onto graphene oxide (GO), chemically reduced graphene (CRG) and annealing reduced graphene (ARG) were compared to determine the unique adsorption properties of graphene nanosheets. The adsorption capability of organic pollutants followed the order CRG   >   ARG   >   GO, and all three adsorbents showed stronger adsorption to 1-naphthol than to naphthalene. In addition to a π–π interaction, the strong adsorption of 1-naphthol onto graphene nanosheets was mainly attributed to the n –π electron-donor–acceptor (EDA) interactions between the –OH groups of 1-naphthol and the electron-depleted sites on graphene nanosheets. The adsorption of 1-naphthol on reduced graphene materials increased with increasing pH and reached a maximum around its pKa, supporting the n –π EDA interaction mechanism. GO with more functional groups on the nanosheets over CRG and ARG exhibited a strong affinity with Cd 2+ . The adsorption of Cd 2+ onto GO and CRG facilitated the coadsorption of naphthalene and 1-naphthol via surface-bridging mechanisms, such as cation–π interactions. Notably, though ARG showed no significant Cd 2+ adsorption, the suppressed coadsorption of naphthalene onto ARG may be attributed to the sieving effect by hydrated Cd 2+ binding to the micropore edges on ARG.