Quantification of the Enthalpy of Eu(III) and U(VI) Complexation to Graphene Oxide Using Microcalorimetry

We used isothermal titration calorimetry to measure the bulk heats for the deprotonation of the carboxylic functional group ([≡COOH]) on graphene oxide (GO) and those of Eu(III) and U(VI) sorption to GO at 25 °C. The enthalpies for the specific sorption reactions were calculated by fitting these bulk heat data sets. The GO speciation used for enthalpy calculation was simulated based on an electrostatic double-layer model for GO surface protonation/deprotonation and Eu(III)/U(VI) sorption to GO. The enthalpy for the deprotonation of [≡COOH] on GO is calculated to be 14.7 kJ/mol, by fitting the heat release data sets of NaOH titration to GO at variable concentrations (0.1, 0.5, and 1 g/L). This endothermic enthalpy for the deprotonation of the carboxylic site is consistent with that for bicarbonate. The enthalpy for Eu(III) and U(VI) complexation to GO was calculated to be 16.86 ± 1.83 and 33.8 ± 3.36 kJ/mol, by fitting multiple bulk heat data sets of EuCl3 or UO2(NO3)2 titration to GO. The enthalpies for Eu(III) and U(VI) sorption to GO were extrapolated from batch sorption data sets at variable temperatures based on the van’t Hoff relationship in a previous study, but these values are not reaction or site-specific, while this work presents an alternative method to quantify the enthalpy of reactions for GO associated with the specific functional group.