Mineral mesopore effects on nitrogenous organic matter adsorption

Abstract

The “mesopore protection hypothesis” [Chem. Geol. 114 (1994) 347; Geochim. Cosmochim. Acta 58 (1994) 1271] proposes that organic matter (OM) may be protected from enzymatic degradation by sequestration within mineral mesopores (2–50 nm diameter). This hypothesis is a leading, though controversial, theory in explaining both the preservation of some extremely labile OM compounds and observed correlations between OM content and mineral surface area in soils and sediments. To test this idea, we carried out batch experiments in aqueous suspensions to examine the adsorption/desorption of amino acid monomers and polymers onto fabricated mesoporous and nonporous alumina and silica. Each mineral pair was of similar surface chemistry and differed only in the presence or absence of intraparticle mesoporosity. All amino acid monomers and polymers smaller than about one-half the pore diameter exhibited significantly greater surface area-normalized adsorption to mesoporous alumina (8.2 nm mean pore diameter) and silica (3.4 nm mean pore diameter) compared to nonporous mineral analogues. Proteins larger than the mesopores exhibited greater adsorption to the nonporous phases indicating their exclusion from internal surfaces of mesoporous minerals. Greater desorption hysteresis for mesopore-sorbed OM indicates that desorption from pores was inhibited. The adsorption/desorption data, as well as Langmuir-Freundlich modeling and adsorption affinity distributions, suggest that capillary condensation, a ‘pore-filling’ mechanism, may explain the experimental observations. These results provide a potential mechanism for the selective sequestration and preservation of sedimentary OM as well as organic contaminants.