Characterisation, validation and comparison of three methods for the extraction of phosphate from sediments

To determine the bioavailability/mobility of phosphate from aquatic sedimen ts chemical fractionation, involving sequential extraction procedures, has been widely applied. Among the several methods proposed, two different appr oaches have been used: the first one is based on the use of strong acids an d bases as extractants, and the second one based on the use of complexing a gents (such as NTA and EDTA). The lack of selectivity of the extractants im plies obtaining operational defined fractions and a high dependence of the results on the experimental conditions. Consequently, results cannot be com pared and the procedures cannot be validated. To validate them through inte rlaboratory exercises, first it is necessary that each laboratory validates the determination of phosphate in such matrices. This paper presents the v alidation of phosphate determination by spectrophotometric and inductively coupled plasma-atomic emission spectrometric techniques in the extracts obt ained applying the Williams and Golterman procedures, as well as the BCR me thod (a sequential extraction scheme designed for heavy metal distribution studies in sediments). The measurement conditions and the matrix effects of the reagents were studied using standard phosphate solutions, along with t he effects of the concomitant species in sediment extracts (Ca, Na, Na2S2O4 , K2S2O8). The analytical characteristics (linear range, accuracy, precisio n and limits of detection and quantification) of each technique were studie d. The optimum conditions were established and applied to study the extract able phosphate content in several sediment samples. Different distribution patterns were obtained when applying schemes, pointing out the need of harm onised procedures to study phosphate partitioning in aquatic sediments. (C) 1998 Elsevier Science B.V. All rights reserved.