A water-dispersible, carboxylate-rich carbonaceous solid: synthesis, heavy metal uptake and EPR study

Thermal oxidation of Na-cholate hydrate at 300 °C in air results in a carbonaceous solid (SC-30) nanomaterial bearing a steroid interior and a significant fraction of carboxylate sites. Electron Paramagnetic Resonance spectroscopy reveals that SC-30 bears a significant concentration of stable C-based radicals located at the interior of the steroid carbonaceous matrix. H-binding, determined by potentiometric acid–base titrations show that the SC-30 contains three types of H-binding sites. One type with pKa = 4.2 corresponds to surfacial metal-binding COOH groups. A second type of sites with pKa = 6.2 corresponds to COOH buried at the interior of the SC-30 carbon matrix, which are inactive in metal binding. A third type with pKa = 8.5—is also inactive in metal binding—originates from aggregated stacked-states like those observed for cholate in solution. The surfacial COO⁻ carboxylate groups, confer the solid hydrophilic character, therefore it can be easily dispersed in water at high concentrations providing clear aqueous colloids. pH-edge metal uptake experiments and Surface Complexation Modelling show that SC-30 is an efficient heavy metal adsorbent in aqueous solution for Pb²⁺ and Cu²⁺ ions at pH 5–8. A structural/functional model is discussed based on the heterogeneous character of the SC-30 material.