An X-ray absorption fine structure spectroscopy study of metal sorption to graphene oxide

Remediation and prevention of environmental contamination by toxic metals is an ongoing issue. Additionally, improving water filtration systems is necessary to prevent toxic metals from circulating through the water supply. Graphene oxide (GO) is a highly sorptive material for a variety of heavy metals under different ionic strength conditions over a wide pH range, making it a promising candidate for use in metal adsorption from contaminated sites or in filtration systems. We present X-ray absorption fine structure (XAFS) spectroscopy results investigating the binding environment of Cd (II), U(VI) and Pb(II) ions onto multi-layered graphene oxide (MLGO). This study shows that the binding environment of each metal onto the MLGO is unique, with different behaviors governing the sorption as a function of pH. For Cd sorption to MLGO, the same mechanism of electrostatic attraction between the MLGO and the Cd⁺² ions surrounded by water molecules prevails over the entire pH range studied. The U(VI), present in solution as the uranyl ion, shows only subtle changes as a function of pH, likely due to the varied speciation of uranium in solution. The adsorption of the U to the MLGO is through a covalent, inner-sphere bond. The only metal from this study where the dominant adsorption mechanism to the MLGO changes with pH is Pb. In this case, under lower pH conditions, Pb is bound onto the MLGO through dominantly outer-sphere, electrostatic adsorption, while under higher pH conditions, the bonding changes to be dominated by inner-sphere, covalent adsorption. Since each of the metals in this study show unique binding properties, it is possible that MLGO could be engineered to effectively adsorb specific metal ions from solution and optimize environmental remediation or filtration for each metal.