Mobile phosphorus stratification in sediments by aluminum immobilization

Area of Science:

• Environmental Chemistry
• Aquatic Sediment Science
• Water Quality Management

Background:

• Natural sediments exhibit heterogeneous mobile phosphorus (P) distribution, often overlooked in P immobilization studies.
• Amendment agents are used to control P release, but their efficacy can be affected by sediment heterogeneity.

Purpose of the Study:

• To investigate the effects of aluminum sulfate (ALS) on P immobilization in sediments.
• To determine the optimal ALS dosage for creating a sustained P-immobilizing layer.

Main Methods:

• A 60-day microcosm experiment using sediment cores treated with varying ALS dosages.
• High-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques to measure soluble reactive P (SRP) and DGT-labile P profiles.
• DGT Induced Fluxes in Sediments (DIFS) modeling to analyze P release dynamics.

Main Results:

• ALS amendment significantly decreased mobile P concentrations (SRP and DGT-labile P) in sediments.
• A static layer with extremely low P concentrations formed in the upper sediment layers at ALS dosages of 6-15 ALS/P_mobile.
• The upward P release potential (R_AL) decreased significantly across the active layer following ALS treatment.
• A minimum dosage of 9 ALS/P_mobile was identified as sufficient for effective ALS capping.
• ALS amendment increased the adsorption rate constant (k₁) more than the release rate constant (k_-1), hindering P release from sediment solids.

Conclusions:

• Aluminum sulfate is an effective agent for immobilizing mobile phosphorus in aquatic sediments.
• The formation of a P-depleted static layer is crucial for sustained P immobilization.
• A specific minimum ALS dosage is required to establish this static layer and achieve effective P capping.
• ALS amendment alters sediment P dynamics by increasing adsorption and reducing P release, improving water quality.