Abstract

Mineral-water partitioning provides a mechanism for the selection and concentration of simple organic compounds, such as amino acids, into the adsorbed phase prior to prebiotic evolution into more complex bio-macromolecules. Smectite and serpentine clays are the major alteration phases expected for early Earth and planetary systems. These clays vary in structure and composition, properties which influence their reactivity. Smectites, in particular, could have been effective at concentrating amino acids in early aqueous environments due to their high surface area (i.e., large number of reactive sites), permanent negative structural charge, and high cation exchange capacity. Serpentines, on the other hand, do not have hydrated interlayers and have negligible structural charge, so they should show distinct binding affinities based on adsorption to edge sites. However, the systematics of selective organic binding to clay minerals is poorly characterized in fluid compositions relevant to prebiotic systems. Therefore, the selective adsorption of amino acids to smectite and serpentine clays, as well as hydrothermally-altered rocks, was investigated through three experimental approaches. In the first study, the selective uptake and adsorption mechanisms of 10 amino acids (L-arginine, L-lysine, L-asparagine, L-tryptophan, L-serine, L-threonine, L-alanine, glycine, L-leucine, and L-glutamic acid) to the smectite montmorillonite were explored under varied fluid compositions (10 mmol/L NaCl or MgCl2) and pH (5 or 7). The amino acids studied had various R-groups (basic, acidic, amide, hydroxylic, aromatic, aliphatic, and none), which influenced their selectivity due to the amphoteric nature of amino acids in solution. The basic amino acids (L-arginine and L-lysine), which are predominantly cationic at pH 5 and 7, were selectively adsorbed to montmorillonite compared to other amino acids with their uptake minimally affected by changes in pH but substantially inhibited in the magnesium chloride fluid. This suggests that their adsorption was dominated by cation exchange into smectite interlayers. The adsorption mechanisms of the basic amino acids were characterized using attenuated total reflectance Fourier transform infrared spectroscopy and powder X-ray diffraction. Results indicated that L-arginine and L-lysine are intercalated within the montmorillonite at orientations parallel to the basal plane, with strong interactions between the protonated amino sidechain and the mineral surface. This observation further supports adsorption driven primarily through electrostatic interactions between amino acids and the negatively charged smectite surface. In the second study, the binding affinity of four amino acids with different chemical properties (L-arginine, glycine, L-threonine, and L-glutamic acid) to three smectites (montmorillonite, nontronite, and saponite), one serpentine (lizardite), and two altered rocks (diabase and dunite) was evaluated in 10 mmol/L NaCl at pH 5, 7, and 10. Adsorption varied among the clays and altered rocks due to varying properties of the single-phase clays and alteration products (i.e., mineral structure, composition, particle size layer charge, and location of the layer charge), as well as cations released from the clay and altered rock suspensions. Overall, the dominant clay (either smectite or serpentine) produced in the alteration assemblages controlled the adsorption of amino acids. The smectites and altered diabase showed strong affinity for L-arginine with the binding affinity weakly decreasing as the pH increased from 5 to 7 but strongly decreasing as the pH increased from 7 to 10. In contrast, the binding affinity of glycine, L-threonine, and L-glutamic acid weakly decreased as the pH increased. As a result, adsorption showed two co-occurring adsorption mechanisms universal to smectites: cation exchange of cationic amino acids within hydrated interlayers and baseline adsorption via surface complexation on edge sites. In contrast to the smectites, lizardite and altered dunite showed no relationship between the Langmuir constants and amount of cationic species in solution, instead displaying only baseline adsorption via surface complexation. Lizardite had higher affinity for L-arginine adsorption at pH 7 and 10, albeit much weaker than the smectites, whereas altered dunite showed binding affinities for L-glutamic acid equal to L-arginine at pH 7 and greater than L-arginine at pH 10. This deviation from lizardite likely occurred due to enhanced cation bridging to the octahedral sheet attributed to the release of Ca2+ from the dissolution of an alteration product in altered dunite. As a result, higher concentrations of divalent cations, especially Ca2+, could be important for enhanced adsorption of acidic (i.e., glutamic acid and aspartic acid) amino acids to serpentines in early environments, whereas competition with divalent cations for interlayer binding sites suppresses cation exchange of amino acids to smectites. In the third study, the selective uptake of 22 proteinaceous and non-proteinaceous amino acids (including four sets of L- and D-enantiomers) to seven smectites of varying Fe(III)-Mg-Al compositions and lizardite was evaluated in seawater. Two seawater fluids were used with initial amino acid concentrations of 210 and 730 nmol/L of each amino acid on average. Clay mineralogy affected selectivity patterns with adsorption normalized to glycine exhibiting selectivity that varied up to 100×, depending on the amino acid. The smectites showed strong selectivity for the cationic amino acids (arginine and lysine) in both seawater fluids. In contrast, lizardite showed strong selectivity for arginine and aspartic acid with weaker selectivity (i.e., lower adsorption) for lysine. Amino acid adsorption differed due to extrinsic and intrinsic properties of the minerals, such as clay-induced pH changes, layer charge, and particle size. In general, adsorption to smectites was controlled by the total layer charge and location of the layer charge for cationic amino acids, whereas clay-induced pH changes controlled adsorption of the anionic amino acids. This behavior demonstrates that cationic and anionic amino acids adsorb at different smectite binding sites consistent with adsorption through cation exchange within hydrated interlayers for cationic amino acids and surface complexation to edge sites for anionic amino acids. For the four sets of L- and D-enantiomers, chiral selectivity was negligible to weak among the clays, suggesting that clay mineral adsorption in realistic fluid matrices is unlikely to induce the substantial chiral selection needed to support the development of homochirality for terrestrial life. The results of this dissertation contribute to a fundamental understanding of the role of the smectite- and serpentine-water interface in the emergence of complex biochemistry on the prebiotic Earth and geochemical origin of life. These studies reveal that affinity and selectivity for basic amino acids is a consistent feature of prebiotically-relevant clay minerals and fluid compositions. The selectivity for basic amino acids to smectites (stronger) and serpentines (weaker) suggests that these amino acids are strongly retained by clays in prebiotic environments, likely skewing their detection in extraterrestrial samples and affecting amino acid migration, accumulation, and polymerization during fluid circulation in prebiotic environments. The affinity and selectivity of amino acids varied by up to 100× among different amino acids under varying conditions (i.e., fluid composition and pH), demonstrating that 100× variation in dissolved concentrations is required to create equal solid-phase concentrations of amino acids. This behavior may have enabled polymerization and biological utilization of arginine and lysine concentrated in the adsorbed phase despite their limited abiotic production. While fluid- and mineral-specific effects among other amino acids were weaker, amplification of these patterns during fluid flow provides a mechanism for generating distinct amino acid profiles in different environmental setting.

Committee Chair

Jeffrey Catalano

Committee Members

Alex Bradley; David Fike; Karyn Rogers; Kimberly Parker

Degree

Doctor of Philosophy (PhD)

Author's Department

Earth & Planetary Sciences

Author's School

Graduate School of Arts and Sciences

Document Type

Dissertation

Date of Award

12-17-2025

Language

English (en)

Author's ORCID

0009-0002-5075-943X

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Available for download on Tuesday, December 15, 2026

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