Evaluation of Heavy Metal Removal Capacity of Bioretention Systems

Bioretention is one of the most common low-impact development (LID) types but there is lack of knowledge in the capacity and local behavior of bioretentions. In this study, laboratory-scale experiments are conducted in order to investigate the heavy metal removal capacity of bioretentions. Batch sorption experiments were first performed to determine the sorption parameters and retardation factor of copper (Cu), lead (Pb), and zinc (Zn) on various bioretention media, namely mulch, turf, vegetative soil, sand, and gravel. Reaction kinetics of Cu, Pb, and Zn were determined in order to assess the sorption equilibrium time of these metals for the five different bioretention media. The results of the batch tests show that turf has the highest sorption capacity followed by mulch and vegetative soil. For the range of concentrations considered in this study, linear sorption isotherm was found to best represent the metal sorption for all bioretention media. Metal removal percentages were highest for Pb and lowest for Zn. The time required to reach equilibrium ranged from 1 to 6 h depending on the type of bioretention media and metal. In addition to batch sorption experiments, column sorption experiments were also conducted in order to investigate effects of soil textures and organic content on removal of heavy metal in bioretention columns. For this purpose, four bioretention columns with different vegetative soil, turf, and sand ratios were prepared. The column tests were conducted for a period of 127 days under continuous boundary source, i.e., constant flow rate is supplied to each column with a concentration of 5 mg/L for each metal. Results show that different local soil types in bioretention design affect removal of heavy metal concentration considerably. Breakthrough analysis indicates that the removal of Zn reaches almost zero in about 127 days, while Cu and Pb are almost fully retained in all columns until the end of the experiment.