is Contribution to the DCO
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:
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.