Modeling phosphorus transport in soil columns amended with polyaluminum chloride and anionic polyacrylamide water treatment residuals: implications for stormwater bioretention systems

Understanding phosphorus transport in soil columns amended with polyaluminum chloride and anionic polyacrylamide water treatment residuals (PAC-APAM WTRs) is crucial for the effective recycling of PAC-APAM WTRs into traditional soil-based stormwater bioretention systems. Phosphorus transport in columns containing three distinct soil types amended with PAC-APAM WTRs under saturated steady-state flow conditions was effectively modeled using three different models: the convection-dispersion equation (CDE) model with linear isotherm, the CDE model with Langmuir isotherm, and the chemical non-equilibrium two-site model (TSM). In Soils 1 and 2, amended with PAC-APAM WTRs, the primary mechanism governing phosphorus transport transitioned from instantaneous adsorption to two-site adsorption as flow rate increased. In contrast, Soil 3 amended with PAC-APAM WTRs was predominantly governed by two-site adsorption throughout the experiments. An increase in flow rate reduced the solid-liquid distribution coefficient and the fraction of equilibrium adsorption sites, resulting in decreased phosphorus adsorption and increased phosphorus mobility. This study strongly recommends the selection of soil amended with PAC-APAM WTRs that exhibits higher instantaneous phosphorus adsorption. Additionally, this study emphasizes the importance of appropriately designing the depth of the ponding layer, utilizing the TSM model, and refining modeling techniques to optimize phosphorus retention and mitigate pollution risks in stormwater management.