Adsorption of metals onto graphene oxide: Surface complexation modeling and linear free energy relationships

Abstract

Abstract In this study, the adsorption behaviors of a range of metals onto multi-layered graphene oxide (GO) as a function of pH and metal:GO ratio were measured. The GO exhibited an affinity for the metals in the following order: Pb(II) > Cu(II) > > Cd(II) ≈ Ni(II) ≈ Zn(II) > Sr(II) ≈ Ca(II). A four discrete-site non-electrostatic surface complexation model was used to account for the observed adsorption behaviors. For most of the metals studied, we modeled the adsorption by invoking divalent cation binding onto two or more of the deprotonated GO surface sites. The stability constant for each metal-site complex was calculated from the modeling, and the resulting models in general yield good fits to the experimental data. The calculated stability constants exhibit a correlation to stability constants for the corresponding aqueous metal-acetate, or the corresponding metal-hydroxide complexes. The calculated stability constants and the derived relationships from this study enable preliminary but useful predictions of the extent of metal adsorption onto GO for metals which have yet to be studied experimentally, and we tested the accuracy of these predictions by estimating and measuring the extent of Nd(III) adsorption onto GO as a function of EDTA concentration. The predicted extent of Nd adsorption onto GO is within 5% of the measured extent of Nd adsorption in an EDTA-free solution, and the predicted Nd adsorption behavior with increasing EDTA concentration is also in reasonable agreement with the measurements. Our results demonstrate the high affinity that GO has for binding a wide range of aqueous metals, and the results can be used to estimate with a reasonable degree of certainty the distribution of metals in complex GO-bearing environmental or engineered systems.