Assessing exchangeable phosphate and related data in coastal sediments: Theoretical and practical considerations

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 mu mol g-1 in sandy sediments up to approximately 9 mu mol 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.

Automated summary

This article explores the adsorption-desorption processes of phosphate on sediments using the Langmuir theory. It describes the theoretical developments and applies them to experiments for determining exchangeable phosphate. The results show that partition coefficients and maximum adsorption capacity vary depending on sediment type and content, and can be used as indicators for examining the potential exchange of phosphate with surrounding water. However, the maximum phosphate adsorption capacity derived from the Langmuir equations has a large confidence interval and is not significantly higher than the exchangeable phosphate.