Effect of the presence of phosphate on removal of metals from stormwater in bioretention cell

Demir, Veysel
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Urban stormwater runoff contains a wide variety of pollutants that are transported to many bodies of water. Removing heavy metals from urban stormwater runoff and improving the quality of urban stormwater runoff before discharge have been a great concern in recent years. An experimental and computer modeling approach has been put forward to understand the effect of the presence of phosphate from the application of phosphate based fertilizers in the watershed on the removal of heavy metals such as lead and copper that are typically found in the stormwater runoff in bioretention cells. Batch and column tests were conducted on five different low-cost bioretention media (hardwood mulch, juniper mulch, pine mulch, sawdust, and loam soil) to understand the effect of presence of phosphate on the removal of lead and copper from stormwater runoff.

Experimental results, geochemical and analytical models suggest that the presence of phosphate has a pronounced effect on metal immobilization. This is due to the formation of metal phosphates that have extremely low solubility. When phosphate based fertilizers are present in stormwater, a significant portion of metals precipitates. The remaining metals in aqueous phase are adsorbed on the media of the bioretention cell, with an overall removal rate of 99% for both the metals. The presence of phosphate immobilizes more than 93% of lead and 66% of copper due to precipitation prior to adsorption in a bioretention system. Batch and column studies show that in all samples addition of phosphate also decreases the leaching of lead and copper back to aqueous solution consistent with lead and copper immobilization. EPA's geochemical reaction model Visual MINTEQ, ver. 2.61, the Ogata-Banks (1961) analytical breakthrough model and Van Genuchten's (1981) analytical leaching model were also applied to understand conceptual reactions and adsorption mechanisms related to removal of heavy metals in stormwater runoff. All of these models also suggest that the presence of phosphate based fertilizers in bioretention cells significantly improves the removal of lead and copper in stormwater and it also increases the efficiency and lifetime of bioretention media.

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Civil and Environmental Engineering