Evaluation of Adsorption for the Removal of Heavy Metals in Stormwater Runoff

The focus of this project is the assessment of the potential for adsorption to reduce the concentration of dissolved heavy metals in runoff. One difficulty in evaluating adsorption in multicomponent systems is to capture the impacts of background organic matter and other complexing ions on adsorption behavior. The composition of the organic matter in stormwater runoff can be significantly different from natural organic matter typically used to assess the impact of background organics on metal ion adsorption. A compromise between using natural and synthetic storm water in this evaluation was therefore made by recognizing the importance of capturing the organic matter from natural storm water, but adding the flexibility of using laboratory chemicals to provide the inorganic constituents. To alleviate concerns associated with storing large volumes and aging of organic solutions, 1,000 gallons of storm water were collected and then concentrated by a factor of 150 using reverse osmosis. It was then freeze dried. The freeze-dried organic matter was reconstituted as needed at concentrations that mimic the initial total organic concentration of the stormwater when it was collected. A series of column experiments were then run using iron oxide, manganese oxide, Portland cement concrete, and crab shell waste as sorbents. Factors that were evaluated included organic matter concentration, competition between metal ion species (copper and zinc), pH, and ionic strength. The results of the column experiments indicate that iron oxide provides substantially better removal of dissolved metals than manganese. Portland cement concrete and crab shell waste were both found to be effective at controlling pH, which is the largest factor in determining dissolved metal removals. These experiments indicate that use of iron oxide augmented with either concrete or crab shell is potentially a cost-effective way to remove dissolved metals in highway runoff.