Identification of dissolved nonreactive phosphorus in freshwater by precipitation with aluminum and subsequent 31P NMR analysis.

Little information exists on nonreactive phosphorus (nrP) in the water column, because the concentration is much lower than that in the sediment. Here we present a novel method for up-concentration and identification of nrP in lake water: nrP is precipitated with poly aluminum chloride and the precipitate is subsequently recovered and dissolved by NaOH. Additional up-concentration by rotary evaporation increases P concentrations up to 5500 times. Furthermore, there is only a low up-concentration of paramagnetic metals. The method is sensitive and easy to use. Bottom water from five Danish lakes was sampled in autumn 2008 and in four of the five lakes orthophosphate monoesters constituted the largest fraction of nrP (50-86%), whereas DNA-P was the largest fraction in the fifth lake (67%). The pyro-P/poly-P concentration varied between 0 and 33% of nrP in the lakes. Thus, most of the P compounds usually found in lake sediments were also found in the bottom water of these lakes.