An authigenic iron phosphate phase in estuarine sediments: composition, formation and chemical reactivity

Reductive dissolution experiments and microprobe analyses reveal the existence of a discrete iron-phosphorus phase (Fe-P) in intertidal freshwater sediments from the eutrophic Scheldt estuary (Belgium and The Netherlands). This mineral phase is found at all depths sampled (0-50 cm) and exhibits a remarkably uniform composition and chemical reactivity. It accounts for more than 70% of total phosphorus in the sediments, and is quantitatively extracted by ascorbate at near-neutral pH. High molar P/Fe ratios (0.7-0.8), and identical dissolution kinetics of phosphorus and iron in pH7.5 ascorbate solution, point to a hydrous ferric phosphate mineral. Bulk sediment Fe(III)/Fe(II) ratios measured with Mossbauer spectroscopy are also consistent with a predominantly ferric, rather than ferrous, iron phosphate phase. Mineral precipitates collected in situ on Teflon strips reveal active formation of ferric phosphate phases in the upper oxidized sediment layer at the freshwater sampling site, but also at a downstream brackish site. Oxidative precipitation and subsequent burial of iron phosphate explain the large-scale removal of nutrient phosphorus from the water column in the freshwater portion of the estuary. In the downstream brackish and marine portions of the estuary, this sedimentary phosphorus sink is limited by the increased availability of sulfate, as sulfide produced by sulfate reduction converts the reactive ferric phosphate pool into iron sulfides. (C) 2004 Elsevier B.V. All rights reserved.