Biogeochemische Kopplung der Cd und Fe Zyklen in landwirtschaftlich genutzten Böden unter variierenden Redox- und geochemischen Bedingungen
Zusammenfassung der Projektergebnisse
In agricultural soils, the use of low-quality phosphate-based fertilizers can lead to the introduction of cadmium (Cd) into the environment. This toxic metal has several avenues for human consumption including, bioaccumulation in foodstuffs and mobilization in ground water. A deeper understanding of Cd biogeochemical cycling is therefore needed in order to limit adverse human health effects. In this project, we investigated aspects of Cd biogeochemical cycling that either had not been previously examined or had knowledge gaps, including: 1) the effects of initial speciation of Cd mobility during microbial metabolism, 2) the fate of Cd adsorbed to siderite subject to microbial Fe(II) oxidation, and 3) the role of natural organic matter in mediating Cd fate throughout microbial Fe(III)-reduction. First we demonstrated that the rate and extent removal of Cd from solution was dependent upon initial speciation during microbial metabolism, with the fastest and most extensive removal occurring when Cd was complexed with the small organic acid, cysteine. Fe mineral-adsorbed Cd remained primarily adsorbed throughout microbial Fe(III) reduction and using confocal scanning laser microscopy (CSLM), we showed that although exopolymeric substances (EPS) formed in the presence of Fe, Cd remained highly associated with Fe mineral phases. Also using CSLM, we further illustrated that when Cd was initially present as aqueous CdCl2, Cd was more highly correlated with cells compared to other initial species. Correspondingly, there was notably less microbial growth when aqueous Cd was the initial Cd species. This work highlights the importance of metal speciation on toxicity and mobility, even in the case of a non-redox active metal such as Cd. Next, the MSc work which aligns with WP2 will be completed in the Spring of 2021. This work will examine Cd solubility and speciation throughout microbial Fe(II) oxidation, giving a better understanding of Cd behavior under different redox regimes and the effects of Cd on different microbial species. Finally, we showed in a cell suspension setup (as opposed to continued growth), an increasing C/Fe ratio is correlated with increasing microbial Fe(III) reduction when Cd is initially adsorbed to ferrihydrite. This increasing microbial Fe(III) reduction however, did not correspond to more extensive mineralogical transformation in the presence of Cd, in that only ferrihydrite was detected in X-ray diffraction patterns for high C/Fe setups. In these same setups, a lower amount of Cd was desorbed and released into solution, demonstrating that while NOM may limit mineral transformation, it also in turn limits Cd mobility during microbial Fe(III) reduction. We further probed the interaction of Cd with NOM and Fe minerals via X-ray adsorption spectroscopy. Results show that the proportion of Cd-OM bond increased in Cd-NOM-ferrihydrite phase after microbial reduction, indicating Cd redistributed between ferrihydrite and NOM. These spectra are also evidence that tertiary Cd-NOM complexes may contribute to the decreased mobility of Cd during microbial Fe(III)-reduction. Overall, this work shows adsorbed NOM is an important sink for Cd, due to the formation of tertiary surface complexes that in turn limit mineral transformation and subsequent mobilization of Cd.
Projektbezogene Publikationen (Auswahl)
- (2020) “Complexation by cysteine and iron mineral adsorption limit cadmium mobility during metabolic activity of Geobacter sulfurreducens” Environmental Science: Processes & Impacts 22 (9), 1877-1887
Tomaszewski, E.J.; Olson, L; Obst, M; Byrne, J.M; Kappler, A; Muehe, E.M.
(Siehe online unter https://doi.org/10.1039/D0EM00244E) - (2020) “Effect of natural organic matter on fate of cadmium during microbial ferrihydrite reduction” Environmental Science & Technology 54 (15), 9445-9453
Zhou, Z; Muehe, M; Tomaszewski, E.J.; Lezama, P; Kappler, A; Byrne, J.M.
(Siehe online unter https://doi.org/10.1021/acs.est.0c03062)
