Project

Discipline

calcium; phosphate; arsenic; precipitation; kinetics ; iron; dissolution; structure; oxidation

Lead
Die Oxidation von gelöstem Eisen(II) in Wasser führt zur Bildung schwerlöslicher Eisen(III)-Präzipitate welche auch andere Haupt- und Spurenelemente aufnehmen können. Dieser Prozess spielt eine zentrale Rolle im biogeochemischen Kreislauf des Eisens und beeinflusst auch das Verhalten kotransformierter Nähr- und Schadstoffe in der Umwelt stark, zum Beispiel in Böden und Seen. Zudem spielen Eisen-Präzipitate auch in technischen Systemen eine wichtige Rolle, wie zum Beispiel bei der Arsen-Entfernung aus Trinkwasser oder bei der Sanierung kontaminierter Grundwasserleiter. Die Struktur und die Eigenschaften solcher Eisen-Präzipitate hängen stark von der chemischen Zusammensetzung des Wassers ab in dem sie sich bilden. Trotz der zentralen Bedeutung von Eisen-Präzipitaten in aquatischen und terrestrischen Systemen ist bis anhin jedoch nicht ausreichend bekannt, wie verschiedene gelöste Stoffe die Struktur und die Eigenschaften von Eisen-Präzipitaten beeinflussen.
Lay summary
Ziel dieser Arbeit ist es zu bestimmen (i) wie Phosphat, Silikat und Kalzium die Zusammensetzung und Struktur durch Oxidation entstandener Eisen(III)-Präzipitate beeinflussen und (ii) wie sich strukturelle Unterschiede auf die Reaktivität von Eisen-Präzipitaten und die Kotransformation von Arsen auswirken. In kürzlich abgeschlossenen Laborexperimenten haben wir Eisen-Präzipitate in künstlichen Grundwässern unterschiedlichster Zusammensetzung synthetisiert und mit modernen spektroskopischen und mikroskopischen Methoden charakterisiert. Zudem haben wir untersucht wie effizient Arsen durch verschiedene Eisen-Präzipitate aus dem Wasser entfernt werden kann. Ziel weiterführender Experimente ist nun, die Kinetik und zeitliche Sequenz der Oxidation und Ausfällung des Eisens und die Kotransformation von Arsen quantitativ und mechanistisch besser zu verstehen. Des Weiteren ist vorgesehen, die Raten der reduktiven Auflösung verschiedener Eisen-Präzipitate zu bestimmen um deren Wichtigkeit für die Retention von Spurenelementen unter anoxischen Bedingungen besser abschätzen zu können, zum Beispiel in kontaminierten Grundwasserleitern oder überfluteten Auenböden. Mit den Erkenntnissen aus diesem Projekt wollen wir zu einem besseren Verständnis der Umwandlung des Eisens und anderer kotransformierter Elemente in der Umwelt beitragen. Zudem soll dieses Projekt Grundlagen liefern, welche zur Optimierung Eisen-basierter Methoden in der Wasserbehandlung benutzt werden können.

Direct link to Lay Summary

Last update: 02.04.2014

Lead
The oxidation of dissolved iron(II) in water leads to the formation of iron(III)-precipitates that also incorporate other major and trace elements. This process affects the cycling of Fe and co-transformed nutrients and contaminants in environments like soils or lakes and also plays a major role in technical applications such as removal of toxic arsenic from drinking water or remediation of contaminated groundwater. The formation, structure and properties of iron oxidation products depend on the composition of the water in which they form. Despite the importance of iron(III)-precipitates in a wide range of systems, however, the interdependent effects of major dissolved species on Fe precipitate structure and properties are still not fully understood.
Lay summary
In this project, we aim to determine how phosphate, silicate and calcium (Ca) affect the structure of Fe oxidation products and how these variations impact the reactivity of the formed solids and arsenic co-transformation. We recently completed laboratory experiments in which we formed Fe oxidation products in synthetic groundwater over a wide range of compositions and characterized the resulting Fe(III)-phases by advanced spectroscopic and microscopic techniques. Furthermore, we also investigated the extent of As removal by different types of precipitates. In continuing experiments, we aim to gain more detailed insight into the kinetics and temporal sequence of Fe oxidation and precipitation and its consequences for As co-transformation. We also plan to quantify the rates of reductive dissolution of different types of Fe(III)-precipitates to gain a better understanding of their role for trace element retention under anoxic conditions, for example in contaminated aquifers or flooded soils.

Direct link to Lay Summary

Last update: 02.04.2014

Active participation

Self-organised

Number	Title	Start	Funding scheme

The oxidation of dissolved Fe(II) in oxygenated water leads to the formation of structurally diverse Fe(III)-precipitates that determine the fate of a wide range of major and trace elements in environmental and technical systems. In an on-going SNF project, we showed for the first time that Fe(II) oxidation in phosphate-containing solution at near-neutral pH leads to initial precipitation of an amorphous Fe(III)-phosphate followed by its transformation into a phosphate-rich hydrous ferric oxide and concomitant precipitation of lepidocrocite during continuing Fe(II) oxidation. Furthermore, we assessed the interdependent effects of phosphate, silicate and Ca in near-neutral solutions on the structure of fresh and aged Fe oxidation products and on the solubility of co-precipitated arsenate. Using Fe K-edge X-ray absorption spectroscopy and electron microscopy for precipitate characterization, we were able to determine in unprecedented detail (i) the structure of fresh Fe(III)-precipitates as a function of the concentrations of dissolved phosphate, silicate and Ca, (ii) structural changes during aging, and (iii) implications for phosphate and arsenate solubility.In this two-year continuation project, we aim to further explore how different types of Fe(III)-precipitates that cover the structural range documented in our current work vary with respect to their reactivity and impact on co-transformed As. Specifically, in a first part, we plan to determine the reductive dissolution kinetics of different Fe(III)-precipitates to assess their susceptibility to reductive dissolution and associated release of co-precipitated phosphate and As under anoxic conditions. In a second part, we aim to determine the kinetics of heterogeneous Fe(II) oxidation and coupled As(III) co-oxidation during the formation of different types of Fe(III)-precipitates in order to quantify the importance of surface-catalyzed Fe(II) oxidation, its dependence on the type of precipitating phase, and implications for coupled As(III) co-oxidation and co-precipitation. This continuation project will enable the PhD student on the current project to continue and complete her PhD studies.Expanding on the findings from the on-going project, the results from this continuation project will allow us achieving a higher level of understanding of (i) how phosphate, silicate and Ca - via their effects on Fe(III)-precipitate formation - affect heterogeneous Fe(II) oxidation and As(III) co-oxidation in oxygenated water and (ii) how different types of amorphous to poorly-crystalline Fe(III)-precipitates vary with respect to reductive dissolution and As release. Such detailed knowledge of the relationships between the formation, structure and reactivity of Fe oxidation products and consequences for co-transformed As is essential for the quantification of nutrient and contaminant behaviour in redox-dynamic terrestrial and aquatic environments and for improved water resource use and water treatment.