Structural changes and denitrification performance evaluation of bioretention cells with collapsible loess modified by coal gangue

Bioretention cells are considered to be promising low-impact development facilities and are widely used in urban areas. Common bioretention cells are filled with planting soil, gravel, and sand. In collapsible loess areas, gravel and sand are expensive and difficult to obtain. However, the abundant loess in these regions can be used to build bioretention cells. Collapsible loess has excellent adsorption and water retention properties and is suitable for use as filling in bioretention cells. Because the collapsibility of this soil type leads to poor water permeability, it is necessary to modify the collapsible loess used as bioretention cell filler. Modification with coal gangue can significantly improve the permeability coefficient of collapsible loess and reduce its settlement, allowing modified loess filling to meet the parameter requirements for bioretention cells. This experiment shows that when the volume ratio of collapsible loess to coal gangue was less than 2:1, the permeability coefficient was greater than 1.35 cm/h. Variations in the pollutant removal effect under varying initial filler water content were studied using columns that met the parameter requirements. It was found that the removal efficiencies of ammonia-nitrogen (NH₄–N), nitrate-nitrogen (NO₃–N), and total nitrogen (TN) in the experimental columns were as high as 98%, 97%, and 98%, respectively. When the proportion of collapsible loess in the filler was higher, the column was more likely to recover its pollutant removal capacity after new rainfall events. The results show that the initial water content of the filler before each rainfall event had a significant influence on the pollutant removal efficiency. When the initial moisture content was lower than 40%, a large number of pollutants were leached from the bioretention cell during the early stage of water inflow, and the recovery time was long. Therefore, maintaining bioretention cells during dry periods is very important for their performance.