Fate of adsorbed Pb(II) on graphene oxide under variable redox potential controlled by electrochemical method

Lead removal using graphene oxide (GO) and GO based adsorbents has attracted increasing attention worldwide, whereas the potential release of previously adsorbed Pb(II) from GO surfaces induced by exposure to variable redox conditions is presently underappreciated. The current study revealed that reduction of GO to r-GO (the reduced form) was coupled with a decrease of oxygen-containing groups (OCGs) under reductive potential, and the maximum adsorption capacity of GO for Pb(II) decreased from 931.66 to 714.78 mg g^-1 after electrochemical reduction. The release of adsorbed Pb(II) from GO-Pb(II) increased gradually when the potential dropped from 0 to -600 mV. The content of released Pb(II) decreased when the potential reached -700 mV because of the reduction of Pb(II) to insoluble Pb(0). Cyclic voltammetry (CV) analysis demonstrated that there are three reductive potentials, e.g. -760, -400, and -120 mV, related to the reduction of OCGs. X-ray photoelectron spectroscopy indicated that the reducing sequence of three OCGs, namely C-O, CO and OCO groups, depended on the applied potential. This application of an electrochemical method to investigate adsorbed Pb(II) from spent GO absorbent provides valuable information about heavy metal transportation in environments containing GO under varying redox conditions.