The effect of phosphate on heavy metal retention in bioretention cell from stormwater

Texas Commission on Environmental Quality (TCEQ) identified various BMPs to treat the stormwater runoff to protect the aquifer in Texas and recently included bioretention in the BMPs in the list of approved BMPs. A series of batch and column tests were conducted to evaluate the removal capacity of heavy metals for different local bioretention medias, such as hardwood mulch, pile mulch, cedar mulch, saw dust, and silty loam soil from the synthetic stormwater prepared in the laboratory by adding the pollutants to achieve the concentration of national average data. The removal capacity was compared with the removal capacity of silica sand. The effect of the presence of phosphate in stormwater on metal retention by different medias was also investigated.

All of the bioretention medias removed most of the lead and copper very effectively from the stormwater. Most of the cases, the removal rates were much higher than that of the silica sand. All the medias removed more that 90% of the metals from the stormwater except the sawdust, which did not remove copper significantly.

When phosphate was added to stormwater, the concentration of lead in stormwater decreased significantly due to the precipitation of phosphate salts of lead. However, this effect was not that pronounced for copper. Thus for lead, the retention method of metals in bioretention cells combines the effect of precipitation of lead after reaction with phosphate as well as the adsorption of lead in bioretention medias. Whereas, the retention method for copper is primarily the adsorption on bioretention medias.