Evidence for vivianite formation and its contribution to long-term phosphorus retention in a recent lake sediment: a novel analytical approach

Vivianite, Fe3(PO4)2 · 8H2O, is a ferrous iron phosphate mineral which forms in waterlogged soils and sed- iments. The phosphorus (P) bound in its crystal lattice is considered to be immobilised because vivianite is stable un- der anoxic, reducing, sedimentary conditions. Thus, vivianite formation can make a major contribution to P retention dur- ing early diagenesis. Much remains unknown about vivianite in sediments, because technical challenges have rendered di- rect identification and quantification difficult. To identify vi- vianite and assess its significance for P burial during early di- agenesis we studied the consequences of a 1992/1993 in-lake application of FeCl3 and Fe(OH)3 aimed at restoring Lake Gros-Glienicke (Berlin, Germany). In a novel approach, we firstly applied a heavy-liquid separation to the iron-rich sur- face sediments which allowed direct identification of vivian- ite by X-ray diffraction in the high-density ( >