Feedbacks in eutrophication: the impact of phosphorus and silicon incorporation on iron oxide stability

Excess nutrient input and eutrophication of coastal marine systems are causing problems with water quality and ecosystem health on a global scale. Sedimentary iron oxides are an important temporary sink for nutrients such as phosphorus (P). Conversely, these nutrients can affect the process of iron mineral formation, indicating a potential for feedbacks between nutrient availability and iron mineralogy in marine systems. However, the impact of key nutrients such as P and silicon (Si) on the stability of iron oxides remains poorly understood.
Here, we study how P and Si affect the stability of the common iron oxide ferrihydrite. We show that exposing ferrihydrite to PO4 or Si(OH)4 either during (coprecipitation) or after (adsorption) mineral synthesis results in contrasting effects on mineral stability as determined in acidic and reductive dissolution experiments. Coprecipitation of PO4 and Si(OH)4 decreased ferrihydrite stability, while the opposite was observed for adsorption. We combine our laboratory findings with chemical modelling to explain the observed effects. Lastly, we discuss the environmental implications: can such changes in iron oxide stability affect the process of eutrophication?