Preparation of activated permeable brick and mechanism of removing nitrogen and phosphorus from stormwater runoff

Permeable bricks represent an effective strategy for mitigating stormwater-runoff pollution. However, traditional permeable bricks have poor nitrogen and phosphorus removal efficiency, and there is a lack of systematic research on improving the performance of permeable bricks. This study aims to prepare an active permeable bricks (APB) with high permeability, strong compressive resistance, and efficient nitrogen and phosphate removal efficiencies by selecting brick-making aggregates and optimizing of preparation parameters. The effect of APB on removing nitrogen and phosphorus under long-term rainfall was examined, and the mechanism of nitrogen and phosphorus removal by APB was elaborated. The results demonstrated that limestone exhibits a high adsorption capacity for phosphate and ammonia nitrogen, making it suitable as a brick-making aggregate. The optimal preparation parameters for APB were identified as a water-cement ratio of 0.26, design porosity of 15%, aggregate size of 3-6 mm, and fly ash content of 20%. The permeability coefficients and compressive strengths of APB achieved 2.06 × 10^-2 cm/s and 24.02 MPa, respectively, while the average removal rates for phosphate and ammonia nitrogen were 74.46% and 21.22%, respectively. The average removal rates of phosphate and ammonia nitrogen from the APB under long-term rainfall were 74.96% and 24.08%, respectively. Phosphate was predominantly immobilized as calcium-bound phosphorus, with stable forms of phosphorus in APB accounting for 98.28%. This indicated a low risk of phosphorus release. Phosphate was primarily removed through adsorption, retention, and chemical precipitation by APB, whereas ammonia nitrogen was predominantly eliminated through adsorption and retention.