Removal of heavy metals from urban stormwater runoff using different filter materials

Heavy metals wash from tires, automobile exhausts, road asphalt, fuel combustion, parking dust, and recreational land into urban stormwater runoff and its subsequent discharge into surface and subsurface water sources can create public health and environmental hazards. An in-ground permeable reactive filter system is proposed to treat contaminated urban stormwater. However, the filter materials should be carefully selected. Several series of batch experiments were conducted with synthetic stormwater containing individual metal contaminants at different concentrations to determine the adsorption and removal behavior of four potential permeable inorganic filter materials (calcite, zeolite, sand, and iron filings) for six common toxic heavy metal contaminants (Cd, Cu, Pb, Ni, Cr, and Zn). The adsorbed metals, pH, oxidation–reduction potential and electrical conductivity of batch samples were determined. Isotherm modeling was performed to assess the mechanisms and quantify the adsorption of each filter material for the contaminants. The extent of adsorption and removal of metals was found to depend on the type and concentration of metal as well as the filter material. Langmuir or Freundlich isotherm proved best to describe the metal adsorption behavior. The maximum removal rates achieved for individual metals were: 95–100% Cd, Cu, Pb and Zn by calcite, zeolite and iron filings, 90% Ni by zeolite, and 100% Cr by iron filings. Sand produced low results with maximum levels of 8–58%. Based on the maximum adsorption capacity of each filter material, the typical filter size and volume of stormwater that can be treated were estimated. No single filter material was capable of removing all metals to the maximum extent; therefore, a combination of filter materials should be investigated for the simultaneous removal of multiple heavy metals.