Combining sequential extractions with bulk and micro X-ray spectroscopy to elucidate iron and phosphorus speciation in sediments of an iron-treated peat lake

In shallow lakes, mobilization of legacy phosphorus (P) from the sediments can be the main cause for persisting eutrophication after reduction of external P input. In-lake remediation measures can be applied to reduce internal P loading and to achieve ecosystem recovery. The eutrophic shallow peat lake Terra Nova (The Netherlands) was treated with iron (Fe) to enhance P retention in the sediment. This treatment, however, intensified seasonal internal P loading. An earlier study suggested that Fe addition led to increased P binding by easily-reducible Fe(iii) associated with organic matter (OM), which readily releases P when bottom waters turn hypoxic. In this complementary study, bulk and micro Fe K-edge and P K-edge X-ray absorption spectroscopy and micro-focused X-ray fluorescence spectroscopy were applied to characterize the P hosting Fe(iii) pool. Combined with sequential extraction data, the synchrotron X-ray analyses revealed that a continuum of co-precipitates of Fe(iii) with calcium, phosphate, manganese and organic carbon within the OM matrix constitutes the reducible Fe(iii) pool. The complementary analyses also shed new light on the interpretation of sequential extraction results, demonstrating that pyrite was not quantitatively extracted by nitric acid (HNO₃) and that most of the Fe(ii) extracted by hydrochloric acid (HCl) originated from phyllosilicate minerals. Formation of an amorphous inorganic-organic co-precipitate upon Fe addition constitutes an effective P sink in the studied peaty sediments. However, the high intrinsic reactivity of this nanoscale co-precipitate and its fine distribution in the OM matrix makes it very susceptible to reductive dissolution, leading to P remobilization under reducing conditions.