Interaction of Th(IV) with graphene oxides: Batch experiments, XPS investigation, and modeling

Abstract

The high surface area, the pH-dependent highly negative charge, and the abundant surface functional groups make graphene oxides (GOs) efficient for cation immobilization. In this study, the performance of GOs in Th(IV) immobilization as a function of contact time, pH, ionic strength, GO concentration, humic acid (HA) and temperature was investigated by batch experiments. The sorption data of pH effect at different ionic strengths and HA concentrations were fitted by diffusion layer model (DLM). The results showed that the whole sorption process was dependent on pH and independent of ionic strength, suggesting that surface complexation was the dominant sorption mechanism over ionic exchange. Ionic strength did not affect the macroscopic sorption of Th(IV) on GOs but did influence the microscopic distribution of the adsorbed Th(IV) species on GOs. The strong acidic groups played a dominant role in Th(IV) sorption on GOs. The adsorbed Th(IV) mainly presented as ≡XOHTh4+ and ≡XOTh(OH) 2+ on GO surfaces. XPS analysis further provided evidence that the oxygen-containing functional groups on GO surfaces participated in the sorption of Th(IV). HA enhanced the sorption of Th(IV) on GOs mainly by the increase of the stability of GOs rather than the increase in the negative surface charge of GOs or the formation of Th(IV)–HA–GO surface complexes. Based on the Langmuir isotherm model, the maximum sorption capacity of GOs was 252.5μmol/g, which is higher than those of the common sorbents. This work highlights the possible application of GOs in Th(IV) contamination management.