Sorption and Release of Dissolved Pollutants Via Bioretention Media

Abstract

The removal and retention of cadmium, copper, and zinc is investigated with batch and column experiments of compost and sand using synthetic stormwater. The maximum sorption capacities found using the Langmuir isotherm equation for Cd and Zn are 2.13 mg/g and 3.82 mg/g for compost and 0.02 and 0.07 mg/g for sand. Copper precipitates at the solution pH and could not be modeled. Column studies using three different ratios of compost (50, 30, 10, and 0%, by bulk volume) in sand were conducted to develop breakthrough curves. The sorption capacities for 15 cm of filter media found using the Thomas Model are 0.78, 0.37, 0.23, and 0.07 mg Cd/g for 50%, 30%, 10% and 0% compost, respectively. The column study results were used to predict bioretention lifespan. At stormwater concentrations, 15 cm of filter media composed of 30% compost and 70% sand will last 95 years until breakthrough, when the effluent concentration is 10% of the influent concentration. In both batch and column studies, effluent concentrations of phosphorus exceeded the influent concentrations indicating phosphorus export from the filter media to receiving waters. The release of phosphorus was initially high, but then decreased to a steady state value of 0.29 mg/L (C0=0.13 mg/L) and remained constant throughout the test. The total yearly load exported from a typical bioretention practice containing 30% compost is estimated at 347 mg dissolved phosphorus per year. Lastly, the results are discussed in relation to stormwater management operation and maintenance. Over time, toxic metals will continue to migrate down through the filter media. Influxes of new toxic metals and the release of toxic metals by degrading compost will cause the concentration in the filter media to increase. The removal of phosphorus is dependent on the background level of phosphorus on the chosen filter media and, based on the column study results, may continue to release from the filter media at a constant rate. Bioretention practices composed of MNDOT Grade 2 compost and C‐33 sand may be a source of phosphorus to receiving waters. Thus, a novel recommendation for bioretention redesign is made that will enable us to better design infiltration practices for protection of receiving ground and surface waters by retaining both dissolved toxic metals and phosphorus.