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

To determine the bioavailability/mobility of phosphate from aquatic sediments chemical fractionation, involving sequential extraction procedures, has been widely applied. Among the several methods proposed, two different approaches have been used: the first one is based on the use of strong acids and bases as extractants, and the second one based on the use of complexing agents (such as NTA and EDTA). The lack of selectivity of the extractants implies obtaining operational defined fractions and a high dependence of the results on the experimental conditions. Consequently, results cannot be compared and the procedures cannot be validated. To validate them through interlaboratory exercises, first it is necessary that each laboratory validates the determination of phosphate in such matrices. This paper presents the validation of phosphate determination by spectrophotometric and inductively coupled plasma-atomic emission spectrometric techniques in the extracts obtained applying the Williams and Golterman procedures, as well as the BCR method (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 the effects of the concomitant species in sediment extracts (Ca, Na, Na2S2O4, K2S2O8). The analytical characteristics (linear range, accuracy, precision and limits of detection and quantification) of each technique were studied. The optimum conditions were established and applied to study the extractable phosphate content in several sediment samples. Different distribution patterns were obtained when applying schemes, pointing out the need of harmonised procedures to study phosphate partitioning in aquatic sediments. (C) 1998 Elsevier Science B.V. All rights reserved.