Filter Materials for Sorption of Cu and Zn in Stormwater Treatment: A Batch Equilibrium and Kinetic study

Sammanfattning: Urban stormwater is today recognized as a significant source of pollution that has contributed to the deterioration of water quality in lakes and streams. Of the pollutants, metals are commonly occurring in stormwater and can cause major damage when released into the receiving waters. A promising treatment method for removing these metals before it reaches the receiving waters is to filter the water in various filter technologies such as catch basin inserts. In this study, the potential of five materials to remove dissolved copper (Cu) and zinc (Zn) from stormwater was investigated. The study was initiated with batch equilibrium tests to assess the sorption capacity of the materials at different metal concentrations relevant for stormwater. This was done in both single and binary batches to investigate whether or not any competitive sorption occurred between the metals. Langmuir, Freundlich and Sips isotherm models were adapted to the obtained data. Through geochemical modelling, it was also possible to investigate whether precipitation of metals was likely. Finally, kinetic studies were conducted to investigate whether the reaction rate of Cu and Zn was relevant in the context of catch basin inserts. The results showed that the biochar had the highest sorption capacity followed by peat, bark, milkweed, and polypropylene. It could also be noted that Cu competed with Zn binding sites at high concentrations and that the sorption capacity of biochar, peat and bark, at concentrations between 50-100 μg/l Cu and 50-500 μg/l Zn, was sufficient to meet the limits set for stormwater emissions. For these sorbents, the reaction rate was also significant. Kinetics tests showed that at an initial concentration of 1,000 μg/l, 83% of the metals were sorbed after 5 minutes and after 10 minutes this number was 93%. Bark and peat also showed low effluent pH and leaching of dissolved organic carbon (DOC). The Langmuir and pseudo-second order equation could be well adapted to the data while geochemical modelling showed that precipitation of metals was unlikely. This suggests that chemical adsorption may be the mechanism that largely accounted for the removal of Cu and Zn. The results of this study can hardly be used to estimate the field performance of stormwater filters, but can be used as a basis for comparing and selecting sorbents for subsequent column tests.