No abstract

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 [Mg 2+ ] and [Ca 2+ ]. We observed that up to 1.0 µmol of aspartate adsorbed per m 2 of brucite at pH~10.2 and low Mg 2+ concentrations (0.7x10-3 M), but surface adsorption decreased at high Mg 2+ concentrations (5.8x10-3 M). At high Ca 2+ concentrations (4.0x10-3 M), aspartate surface adsorption doubled (to 2.0 µmol•m-2), with Ca 2+ 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 Ca 2+ onto brucite: 2>SOH + H + + HAsp-= >SOH 2 + >SAsp-+ H 2 O, >SOH + HAsp-+ Ca 2+ = >SO-_Ca(HAsp) + + H + , and >SOH + Ca 2+ + 2H 2 O = >SOH 2 + _ Ca(OH) 2 + H +. We used the ETLM to predict that brucite particle surface charge becomes more negative with increasing [Mg 2+ ], creating an unfavorable electrostatic environment for a negativelycharged aspartate molecule to adsorb. In contrast, our addition of Ca 2+ to the system resulted in Ca 2+ adsorption and development of positive surface charge. Our prediction of surface speciation of aspartate on brucite with Ca 2+ revealed that the calcium-aspartate complex is the predominant surface aspartate species, which suggests that the increase in aspartate adsorption with Ca 2+ is primarily driven by calcium adsorption. The cooperative effect of Ca 2+ and the inhibitive effect of Mg 2+ on aspartate adsorption onto brucite indicate that serpentinite-hosted hydrothermal fluids provide an ideal environment for these interactions to take place.