RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The adsorption and concentration of sugars onto mineral surfaces in geochemical environments, such as hydrothermal systems, may have influenced the evolution of early life on Earth. We conducted batch adsorption experiments between d-Ribose and brucite [Mg(OH)₂], a mineral produced from serpentinite-hosted hydrothermal systems, over variable initial ribose concentrations at four ionic strengths resulting from different Mg²⁺ and Ca²⁺ ion concentrations in the aqueous phase. Ribose adsorption generally increased with greater initial concentration and up to 0.3 μmol·m^–2 ribose attached onto brucite with 0.6 mM Mg²⁺ present. Ribose adsorption decreased over 6-fold (4.9 × 10^–2 μmol·m^–2) when the total Mg²⁺ ion concentration increased to 5.8 mM. Ribose adsorption increased to 0.4 μmol·m^–2 when 4.2 mM CaCl₂ was added to the system. Substantial amounts (over 21 μmol·m^–2) of dissolved Ca also attached to the brucite surface independent of ribose concentration. We characterized the interactions between ribose, Ca, and the brucite surface by fitting a surface complexation model to adsorption data. We propose three types of surface reactions that were consistent with the experimental data and involve (1) a bidentate outer sphere or a “standing” ribose surface species, (2) a monodentate Ca-ribose outer-sphere species, and (3) a monodentate Ca outer-sphere species. Our model predicts brucite particle surface charge is negative at low Mg²⁺ concentrations and further decreases upon the addition of MgCl₂, which may hinder our proposed surface complexation of the ribose species, Rib^–. We predict that brucite becomes positively charged with CaCl₂ addition, which may be a consequence of the significant extent of Ca adsorption. The increase in ribose adsorption with CaCl₂ is likely driven by Ca attachment and the formation of a positively charged, cooperative Ca–ribose species that our model predicts will predominate over the “standing” ribose species on brucite. Our model of the ribose–brucite system, established by a combination of batch adsorption experiments and surface complexation modeling, has enabled predictions of ribose adsorption over a wide range of pH and complex environmental conditions.