Blue mineral grains at the bottom of Lake Müggelsee: blue vivianite is an iron phosphate that forms in sediments under oxygen-free conditions. For better visibility, the relatively heavy vivianite particles were enriched by density separation. | Photo: Lena Heinrich
When phosphorus is released into a lake, it can be bound in the sediment, for example to iron-hydroxide. This happens under oxygen-rich conditions at the interface of water and sediment. In the presence of oxygen, iron forms rust-coloured particles, to whose surface the phosphorus can adsorb. However, if there is a depletion of oxygen or anoxia, the iron hydroxides that bind the phosphorus are not stable long-term. This can happen when organic matter (e.g. from dead algae) accumulates at the bottom of a lake, where it decomposes. Then, oxygen is consumed in the process.
Michael Hupfer and IGB doctoral student Lena Heinrich have conducted laboratory experiments to investigate what happens to the iron hydroxides and the phosphorus adsorbed to them under anoxia. They simulated the conditions at the bottom of a lake in a self-contained deoxygenated box. Their observation: the iron hydroxides with adsorbed phosphorus, which they had added to natural sediments, were transformed into vivianite. Since under laboratory conditions, the transformation occured within weeks, the researchers assume that vivianite may also be formed under seasonal anoxia.
Binding iron and phosphorus to form vivianite is particularly interesting for long-term phosphorus immobilisation in lake sediment. After all, not only does the mineral form under oxygen-free conditions, it is also stable under such conditions. Iron and phosphorus remain bound in the sediment in particle form; the phosphorus is unable to return to the water column and is unavailable for algae growth long-term. When vivianite forms, lake sediment therefore acts as a long-term phosphorus sink, even under anoxic conditions. The study shows that the availability of oxygen at the sediment-water interface, at least seasonally, promotes the formation of vivianite. Under this condition, iron precipitates can form, and phosphorus can adsorb to the precipitated iron. When anoxia occurs, these compounds are then transformed into vivianite.
Scientists are aware that this process does not occur in or affect all lakes. One reason for this could be a lack of iron. In iron-poor eutrophic lakes, adding iron may promote the formation of vivianite and long-term phosphorus immobilisation in sediment, helping improve the condition of the lake. However, this must be considered carefully, the scientists point out: After all, even if large amounts of iron are available, competing reactions (such as with sulphur) may bind the iron first, leaving none for the formation of vivianite. These competing reactions will be explored in the next step.
