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Assessing exchangeable phosphate and related data in coastal sediments: Theoretical and practical considerations
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International audience. This article deals with the adsorption-desorption processes of phosphate on sediments according to the Langmuir theory. The theoretical developments are described and applied to the experiments which enable the determination of phosphate that is exchangeable with surrounding water (exch-P). Langmuir equation parameters such as the partition coefficient and the maximum adsorption capacity for exchange of phosphate between solid and water are assessed and discussed. The dataset consisted of contrasting sediments gathered over fifteen years from a large number of coastal and estuarine areas.The determination of exch-P was carried out with linear and asymptotic computations. The values, ranging from a few tenths of μmol g−1 in sandy sediments up to approximately 9 μmol g−1 in muddy sediments, are slightly more precise by using the linear computation. Partition coefficients (Kp: 0.03–3.8 L g−1), increasing with the proportion of fine particles, are likely related to the Fe content. It is proposed to use Kp as a criterion for the examination of the potential exchange of phosphate with the surrounding water.The maximum phosphate adsorption capacity of sediments (Qo) derived from the Langmuir equations exhibits large confidence interval. So, it is not found significantly higher than the exchangeable phosphate, as expected from published sediment saturation experiments.
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