Mobile phosphorus stratification in sediments by aluminum immobilization.

There is a great heterogeneity in the distribution of mobile phosphorus (P) in natural sediments, while the assessment of P immobilization by amendment agents has mostly neglected this feature. In this study, the effects of aluminum sulfate (ALS) on P immobilization were investigated via a 60-day microcosm experiment using sampled sediment cores. The high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques were first employed to establish the profiles of soluble reactive P (SRP) and DGT-labile P in aluminum-amended sediments at 2 and 1 mm resolutions, respectively. Both concentrations of two mobile P forms decreased with increasing ALS dosages. The static layers with extremely low P concentrations (≤0.060 mg L for SRP and ≤0.024 mg L for DGT-labile P) were observed in the upper 6-16 mm sediment layers at 6-15 ALS/P treatments (mobile P is the sum of NHCl-P, BD-P, and NaOH-nrP; mol mol). The static layer was followed by an active layer in which the upward release potential (R) decreased from 33.4 to 21.9 for SRP and from 92.2 to 51.0 for DGT-labile P, respectively. As the formation of the static layer is a key to sustaining P immobilization in sediments, the minimal dosage of 9 ALS/P is required for ALS capping. Modeling with DGT Induced Fluxes in Sediments (DIFS) showed a greater increase in adsorption rate constant (k, maximum 7.2-fold) compared to adsorption rate constant (k, maximum 2.2-fold), demonstrating that the release of P from sediment solids became increasingly difficult after ALS amendment.