Sorption mechanisms of metals to graphene oxide

Experimental Measurements and Surface Complexation Modeling of U(VI) Adsorption onto Multilayered Graphene Oxide: The Importance of Adsorbate–Adsorbent Ratios

“…At the lowest tested MLGO concentration (20 mg L –1 MLGO), ionic strength influenced the extent of U­(VI) adsorption at the MLGO surface, but the extent of this change within a range of ionic strengths that span over two orders-of-magnitude was relatively small. These observations are consistent with U­(VI)-GO adsorption and XAS , studies in the literature that suggest inner-sphere complexation for U­(VI) at the GO surface.…”

“…23 We always assumed a 1:1 stoichiometry between discrete MLGO surface sites and adsorbed U(VI) species because the X-ray absorption spectroscopy (XAS) measurements we previously performed on similar systems implicated monodentate binding as the most likely adsorption mechanism. 32 However, these same XAS measurements could not define the stoichiometries of the adsorbed U(VI) species because the number/identity of deprotonated MLGO surface sites and the speciation of the adsorbed U(VI) both change with pH. In addition, using XAS, it is extremely difficult (if not impossible) to distinguish between the C atom from the MLGO structure and the C atom from the uranyl carbonates that are likely to represent one or more of the adsorbed U species (as discussed later) because each C atom could presumably exist in the second shell around U. Consequently, we chose to assume model reactions between the most abundant aqueous U(VI) species present within a given pH range and MLGO surface sites that are deprotonated in the same pH range.…”

“…Unlike the amphoteric surface sites used in some previous GO surface complexation models, , our models assumed that MLGO surface sites are not capable of being doubly protonated, which is consistent with findings that near-surface charge values (i.e., ζ potential) remain negative throughout our tested range of experimental pH . We always assumed a 1:1 stoichiometry between discrete MLGO surface sites and adsorbed U­(VI) species because the X-ray absorption spectroscopy (XAS) measurements we previously performed on similar systems implicated monodentate binding as the most likely adsorption mechanism . However, these same XAS measurements could not define the stoichiometries of the adsorbed U­(VI) species because the number/identity of deprotonated MLGO surface sites and the speciation of the adsorbed U­(VI) both change with pH.…”

“…Stability constants for the aqueous uranyl complexes in these models are found in SI Table S1 and all models exhibit the same pH and background electrolyte conditions as those from the batch adsorption experiments. We assume that the precipitation of solid U­(VI) species is negligible in our systems, as our previous XAS measurements on U-MLGO systems found none of the U–U bonds that would be indicative of U precipitation, even at U concentrations that are significantly higher than the concentration used here.…”

Trace determination of uranium preconcentrated using graphene oxide by total reflection X‐ray fluorescence spectrometry

“…This observation showed that adsorption of uranium slightly decreased with increasing ionic strengths when complete adsorption was not achieved. These observations and X-ray absorption spectroscopy (XAS) [39,40] studies in the literature that suggest inner-sphere complexation for uranium at the GO surface demonstrate that uranium was preferential toward GO relative to sodium. The absence of matrix effects for uranium adsorption in the high levels of ionic strength (1 mol L −1 ) is remarkable, thus showing that our approach is well suited for the detection of uranium in high salinity water.…”