Vivianite formation and its role in phosphorus retention in Lake Orn, Denmark

Vivianite [(Fe-3(PO4)(2)center dot 8H(2)O)] may precipitate in anoxic lake sediments affecting the porewater orthophosphate concentration, and thereby the trophic status of lakes. We have investigated changes in lake diagenesis of Fe and P (1969-2009), with particular attention focused on vivianite formation with sediment depth (0-20 cm) in an iron-silica-carbon rich lake sediment (Ornso, Denmark). Porewaters were supersaturated for vivianite by two to five orders of magnitude (upper 10 cm) with porewater phosphate concentrations ranging between 0.69 and 10 mu mol l(-1), in winter, and summer concentrations ranging between 9.8 and 40 mu mol l(-1). Significant formation of vivianite was confirmed by X-ray diffraction while scanning electron microscopy and electron dispersive X-ray spectroscopy indicated an increase in vivianite crystal size with depth (similar to 20 to similar to 70 mu m across). Variations in elemental composition of vivianite crystals in relation to at.% P and Fe were especially seen going from 9.5 cm to 24.5 cm. The total sediment Fe pool was very large similar to 3000 mu mol g(-1) and total P increased from 200 mu mol g(-1) to 400 mu mol g(-1) descending down the sediment profile. Differential extraction experiments of P release at pH 3 estimated that vivianite amounts to between 3 and 5% of the total Fe pool. The total P burial fluxes estimate that similar to 38 mu mol cm(-2) yr(-1) or similar to 26% of sedimentary P in the lower sediments is sequestered as vivianite. There are seasonal variations in the porewater composition with lower Fe, orthophosphate and higher sulfate concentrations during winter (5 degrees C), than during summer (15 degrees C). This suggests that temperature modulates the rate of organic matter degradation which in turns affects the rate of Fe(III) phase reduction, release of phosphate, and thereby the porewater Fe2+ and orthophosphate concentrations and hence vivianite formation. This work highlights the role vivianite can play for P retention in a Si-Fe-C rich lake sediment. (C) 2015 Elsevier B.V. All rights reserved.