Adsorptive infiltration of metals in urban drainage--media characteristics

Urban pavement drainage often contains significant quantities of anthropogenic metal elements, including Cd, Cu, Pb and Zn that exceed surface water discharge standards. In many urban areas low rainfall pH, results in predominately dissolved metal element mass. Such partitioning has critical implications for the selection of in-situ treatment. One such category of treatment is engineered infiltration systems. To be effective, such systems must adsorb dissolved metal elements to their fixed media while also acting as filters for particulate-bound fractions. One such strategy is called a partial exfiltration trench (PET). The PET contains oxide-coated sand (OCS); an amphoteric media of high surface area (5-15 m2/g) as compared to uncoated silica sand (0.01-0.05 m2/g). OCS was generated through heating a mixture of silica sand and ferric nitrate solution to dryness. This paper presents results of both media characterization and bench scale PET simulations. Media tested were OCS and plain silica sand. Media testing was carried out until capacity was exhausted, using both synthetic and actual stormwater loadings. Testing was conducted for pH levels of 6.5 and 8.0. Results indicated that OCS had greater capacity than silica sand for all dissolved fractions. As the pH was raised from 6.5 to 8.0, OCS capacity was improved. A PET configuration with porous pavement resulted in the highest in-situ treatment capacity for metal element bearing storm water.