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^-1 for SRP and ≤0.024 mg L^-1 for DGT-labile P) were observed in the upper 6-16 mm sediment layers at 6-15 ALS/P_mobile treatments (mobile P is the sum of NH₄Cl-P, BD-P, and NaOH-nrP; mol mol^-1). The static layer was followed by an active layer in which the upward release potential (R_AL) 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_mobile is required for ALS capping. Modeling with DGT Induced Fluxes in Sediments (DIFS) showed a greater increase in adsorption rate constant (k₁, maximum 7.2-fold) compared to adsorption rate constant (k₁, maximum 2.2-fold), demonstrating that the release of P from sediment solids became increasingly difficult after ALS amendment.