RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The interaction of biomolecules at the mineral–water interface could have played a prominent role in the emergence of more complex organic species in life’s origins. Serpentinite-hosted hydrothermal vents may have acted as a suitable environment for this process to occur, although little is known about biomolecule–mineral interactions in this system. We used batch adsorption experiments and surface complexation modeling to study the interaction of l-aspartate onto a thermodynamically stable product of serpentinization, brucite [Mg(OH)₂], over a wide range of initial aspartate concentrations at four ionic strengths governed by [Mg²⁺] and [Ca²⁺]. We observed that up to 1.0 μmol of aspartate adsorbed per m² of brucite at pH ∼ 10.2 and low Mg²⁺ concentrations (0.7 × 10⁻³ M), but surface adsorption decreased at high Mg²⁺ concentrations (5.8 × 10⁻³ M). At high Ca²⁺ concentrations (4.0 × 10⁻³ M), aspartate surface adsorption doubled (to 2.0 μmol m⁻²), with Ca²⁺ adsorption at 29.6 μmol m⁻². We used the extended triple-layer model (ETLM) to construct a quantitative thermodynamic model of the adsorption data. We proposed three surface reactions involving the adsorption of aspartate (HAsp⁻) and/or Ca²⁺ onto brucite:

2 > SOH + H⁺ + HAsp⁻ → SOH₂⁺ > SAsp⁻ + H₂O,
> SOH + HAsp⁻ + Ca²⁺ → SO⁻_Ca(HAsp)⁺ + H⁺, and
> SOH + Ca²⁺ + 2H₂O → SOH₂⁺_Ca(OH)₂ + H⁺.

We used the ETLM to predict that brucite particle surface charge becomes more negative with increasing [Mg²⁺], creating an unfavorable electrostatic environment for a negatively-charged aspartate molecule to adsorb. In contrast, our addition of Ca²⁺ to the system resulted in Ca²⁺ adsorption and development of positive surface charge. Our prediction of surface speciation of aspartate on brucite with Ca²⁺ revealed that the calcium–aspartate complex is the predominant surface aspartate species, which suggests that the increase in aspartate adsorption with Ca²⁺ is primarily driven by calcium adsorption. The cooperative effect of Ca²⁺ and the inhibitive effect of Mg²⁺ on aspartate adsorption onto brucite indicate that serpentinite-hosted hydrothermal fluids provide an ideal environment for these interactions to take place.