Browsing by Person "Stein, Mathias"

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Distribution of Al, Fe, Si, and DOC between size fractions mobilised from topsoil horizons with progressing degree of podzolisation
(2022) Krettek, Agnes; Stein, Mathias; Rennert, Thilo
Aluminium, Fe, Si, and dissolved organic C (DOC) accumulate in the subsoil of Podzols after mobilisation in the topsoil. We conducted laboratory experiments with topsoil horizons with progressing degree of podzolisation by irrigation with artificial rainwater at varying intensity and permanence. We monitored the concentrations and distribution of mobilised Al, Fe, Si, and DOC between size fractions (< 1000 Dalton, 1 kDa– < 0.45 µm, and > 0.45 µm). Total eluate concentrations were increased at the onset of the experiments and after the first irrigation interruption, indicating non-equilibrium release. There was no statistical effect of the degree of podzolisation on element concentrations. Release of Al, Fe, and DOC was mostly dominant in the fraction 1 kDa– < 0.45 µm, indicating metals complexed by larger organic molecules and colloids. Silicon released was dominantly monomeric silicic acid < 1 kDa. Particularly with the least podzolised soils, Al and Si concentrations < 1 kDa might have been controlled by short-range ordered aluminosilicates, while their transport in colloidal form was unlikely. Our study pointed to both quantitative and qualitative seasonality of element release during podzolisation, to decoupling of Al and Si release regarding size, and to different minerals that control element release as a function of the degree of podzolisation.
Formation and properties of inorganic Si-contaminant compounds
(2023) Stein, Mathias; Rennert, Thilo
Environmental contamination is the most pressing issue of our global society. Among others, contamination with potentially toxic elements (PTEs) such as cadmium (Cd), copper (Cu), and lead (Pb) threatens organisms, humans, and entire ecosystems. Silicon (Si) is known to benefit the resilience to such abiotic stresses and its application showed to alleviate PTE toxicity. These beneficial effects are predominantly attributed to in planta processes, but PTE immobilization in soil induced by Si addition has also been reported. However, interactions between silicic acid and Cd, Cu, and Pb at undersaturation of their silicates and other mineral phases remains elusive. Silicic acid, which is dissolved Si, may interact with cationic PTEs in soil, altering their environmental fate. At oversaturation, PTEs and silicic acid may precipitate forming metal silicates, whereas at undersaturation PTEs may be incorporated into the network of polymerized silicic acid or inner-spherically complexed on the negatively charged surface of polymeric silicic acid, forming particulate Si-contaminant compounds. Aiming to elucidate the extent and the mechanism of the potential PTE immobilization, long-term formation experiments in aqueous solution, a soil column experiment, and batch adsorption experiments including isothermal titration calorimetry (ITC) experiments were conducted. Long-term formation experiments in aqueous solution were conducted at undersaturation of PTE silicates and other mineral phases. Time-dependent particle size and charge changes were measured in between 211 days using dynamic light scattering and phase analysis light scattering. Solid phases were characterized by Fourier transform infrared (FTIR) spectroscopy and 29Si nuclear magnetic resonance (NMR) spectroscopy. Particle size measurements revealed a positive effect of cationic PTEs on silicic acid polymerization (Cu>Cd>Pb). However, only traces (2.1‰ Cd, 2‰ Cu and 1.4‰ Pb of the initially added PTEs) were bound during the polymerization of silicic acid. Copper was incorporated in the polymeric network of silicic acid during its polymerization as indicated by FTIR spectra and 29Si NMR relaxation experiments. Cadmium was only outer-spherically adsorbed. The long-term formation experiments revealed that particulate compounds form due to silicic acid/PTE interactions at undersaturation of other mineral phases. Soil column experiments were conducted to investigate the formation of Si-contaminant compounds in an acidic soil (pH 4.6). Therefore, a Haplic Phaeozem was preconditioned with Cu and Cd in the absence and presence of additional monomeric silicic acid and subsequently irrigated with artificial rainwater. Interactions of silicic acid and PTEs were investigated by monitoring the elemental composition of the eluates, and the size and charge of the particles eluted. After irrigation, total and exchangeable Si and PTE contents were analysed. Silicic acid application resulted in larger particles in the eluates, indicating silicic acid polymerization. The molar metal:Si ratios of the eluates and the significant correlation between Si and exchangeable metals indicated that particularly Cu formed Si-contaminant compounds in the soil, enhancing its retention. However, translocation of PTEs in particulate form, associated with polymerized silicic acid, was indicated. The negative charge and the very small size of the formed compounds may facilitate translocation from soil into groundwater. Batch adsorption experiments and ITC experiments were conducted to examine mechanism and extent of PTE adsorption to polymeric silicic acid. These experiments did not reveal any adsorption of the metals on polymeric silicic acid at pH 4 to 6 and after 24 h, which was underpinned by the results of the ITC experiments. However, zeta-potential measurements indicated weak electrostatic interactions between the negatively charged silanol groups and the PTEs. These electrostatic interactions may be the initial step of Si-contaminant compound formation. This thesis elucidates extent and mechanisms of silicic acid, either mono- or polymeric, and PTE interactions, showing the formation of particulate compounds from the reaction between silicic acid and cationic PTEs in aqueous solution and in an acidic soil. Particularly Cu formed stable compounds during silicic acid polymerization. However, the interactions showed a low extent and mainly weak electrostatic interactions, concluding that the addition of monomeric silicic acid to acidic soils may not be a quantitatively effective measure to reduce PTE mobility in soils. Aggregation effects, resulting from freezing/thawing or drying/rewetting, however, could alter the mobility of Si-contaminant compounds. These effects may be subject of future research, as well as the spectroscopic detection of Si-contaminant compounds in soils.