Browsing by Person "Lenhardt, Katharina Raphaela"

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Associations of short-range ordered aluminosilicates and organic matter: formation, properties and stabilization of organic matter
(2023) Lenhardt, Katharina Raphaela; Rennert, Thilo
Short-range ordered aluminosilicates (SROAS) typically form during the weathering of volcanic ejecta by polymerization of released aluminium (Al) and silicon (Si). These minerals exhibit variable chemical composition and crystallinity. Tubular imogolite is a SROAS with long-range order; its locally defined Si configuration occurs also in poorly ordered SROAS. Interactions of SROAS and organic matter (OM) promote carbon (C) accrual by protecting OM from microbial degradation in the long term; however, the fundamental processes are poorly understood. Stable mineral-organic associations may form by adsorption of dissolved OM (DOM) on SROAS surfaces and by co-precipitation of DOM with SROAS during mineral formation. The objective of this study was to elucidate the chemical interactions between DOM and SROAS by both processes, and to assess the stability of OM sorbed by SROAS. Therefore, the impact of SROAS composition on DOM adsorption, partitioning of OM moieties by adsorption and co-precipitation, the structure of co-precipitates, and the degradability of co-precipitated OM was investigated. A method to synthesize SROAS at ambient conditions was developed to mimic the surface properties of natural weathering products. Characterization of SROAS structure by solid-state 27Al and 29Si nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared (FTIR) spectroscopy evinced a close similarity of synthetic SROAS to their natural analogues. Aluminium-rich SROAS (molar Al:Si>2) resembled proto-imogolite, with Al mainly in octahedral coordination and ≥38% of Si nuclei exhibiting an imogolite-like configuration. Silicon-rich SROAS (molar Al:Si = 1.4) contained tetrahedral Al and Si existed largely in ill-defined environments. Analyses of the specific surface area by nitrogen adsorption revealed marked aggregation of Al-rich SROAS, which was less pronounced in Si-rich SROAS. These results show that the poor crystallinity of Al-rich SROAS permits a very dense spatial arrangement of mass at the submicron scale, while Si incorporation restricts aggregation. Interactions of SROAS and DOM were studied using natural DOM with heterogeneous composition collected in situ from a Dystric Cambisol at two depths, and by water extraction of litter. Solid-state 13C-NMR and FTIR spectroscopy revealed a major contribution of oxidized aromatic moieties to soil DOM, likely originating from lignin degradation, while litter DOM was predominantly composed of carbohydrates. Soil DOM adsorption was driven by surface accessibility and was thus larger for Si-rich SROAS than for Al-rich SROAS, showing the importance of SROAS physical properties for OM retention. Aromatic products of lignin degradation preferentially adsorbed on SROAS, inducing relative enrichment of aliphatic substances, particularly carbohydrates, in the residual DOM. Topsoil DOM adsorption depended more strongly on contact time (1–168 h) than subsoil DOM adsorption, possibly due to qualitative differences of the aromatic fraction. Associations formed by co-precipitation contained more C than adsorption complexes. As Al interacted preferentially with oxidized aromatic compounds, co-precipitation of DOM increased as a function of aromatic C. Nevertheless, marked sorption of carbohydrates from litter DOM evinced possible retention of substances with low affinity for Al by co-precipitation, in contrast to adsorption. Time-dependent (1–72 h) structural evolution of SROAS in the absence of DOM was examined to resolve the mechanisms of SROAS formation. Irrespective of the initial molar Al:Si ratio, amorphous precursors formed by olation during the first hour. After 72 h, up to 50% of Si nuclei exhibited imogolite-like configuration, showing rapid development of short-range order. Dissolved OM interfered in condensation of Al and Si, causing partial exclusion of Si, and slowed crystallisation of the octahedral Al sheet, promoting ill-defined Si species in the co-precipitates. Hence, DOM likely impedes assembly of precursors into structurally ordered particles, in particular, oxidized aromatic DOM in topsoils. The binding strength of DOM to SROAS surfaces may be affected by Si incorporation due to structure modifications. Thus, a mechanistic adsorption study was conducted with oxalic, salicylic and octanoic acid as models of functional moieties in DOM. Adsorption of oxalic and salicylic acid was up to 80–90% lower for Si-rich SROAS than for Al-rich SROAS. Rapid (<1 min) release of hydroxyls, indicating ligand exchange, was observed only for oxalic and salicylic acid, suggesting octanoic acid interacted electrostatically with SROAS surfaces. Chelate complexes of oxalic acid and partial inner-sphere binding of salicylic acid on both SROAS were identified by FTIR spectroscopy. Fast adsorption kinetics were retraced by changes in electrical conductivity using a stopped-flow technique. Ligand exchange by oxalate proceeded at a similar rate as complexation of monomeric Al3+, showing its binding to octahedral Al. Hence, the much lower susceptibility of Si-rich SROAS to ligand exchange with carboxyl groups is caused by tetrahedral Al. Consequently, little OM may be stabilized by chemical bonds with Si-rich SROAS. The degradability of co-precipitated carbohydrates was tested by addition of b-glucosidase, a microbial extracellular enzyme, at optimal concentration and quantification of the reaction product glucose. Glucose release was analysed for initial DOM, co-precipitated OM and for residual DOM to account for compositional changes by co-precipitation. As a result of carbohydrate enrichment in residual DOM, its degradability by b-glucosidase increased. Minor amounts of glucose were released from co-precipitated carbohydrates, showing their restricted accessibility for enzymes due to occlusion. Formation of SROAS in soils likely induces preferential association of lignin degradation products with the mineral matrix and alters the composition of OM introduced to the subsoil. Otherwise easily degradable OM with low affinity for SROAS surfaces can be effectively protected from mineralization by co-precipitation. As the structure of SROAS reflects formation processes and affects their reactivity, future research characterizing natural SROAS will give mechanistic insights into C sequestration and potentially other vital soil functions.