Fractionation of phosphate in marine aquaculture sediments: optimization of the ethylenediaminetetraacetic acid method and comparison with other procedures

The ethylenediaminetetraacetic acid (EDTA) method was compared with two other P-fractionation methods, Sedex and Hieltjes & Lijklema (H. & L.), in order to choose a suitable method that extracts better the inorganic (inorg-P) and organic (org-P) phosphate from marine aquaculture sediments. The EDTA method gave reliable results and did not change the P-composition of the sediments during the analysis. The Sedex method can be improved if the quantity of pre-extracted org-P is separately determined after digestion, and the org-P can thereby corrected. The Sedex method underestimates the org-P present in the sediments (-59%), whereas the calcium-bound phosphate (CaCO3P) is overestimated (+17%) in comparison with the EDTA method. The NaOH and HCl used in H. & L. method are not specific to extract inorg-P from the sediments. To provide optimal extractions of inorg-P in muddy sediments containing 1% org-C and 15% CaCO3, the EDTA method was optimized according to extraction times, dithionite concentration and solute/solid ratio. Five extractions of 2h duration each with Ca-EDTA are required to extract more than 95% of the iron-bound phosphate (Fe(OOH) P) while the extraction of CaCO3P with Na-EDTA takes more than 96 h. The concentration of dithionite up to 1% did not influence the amount of P and Fe extracted (P=0.098 and 0.174 respectively), whereas a solute/solid ratio of 40:1 was best suitable for the optimal extraction of Fe(OOH)P. These conditions can be applied to analyse P composition of other marine pond sediments having similar texture and chemical composition.