Leaching and retention of dissolved metals in particulate loaded pervious concrete columns

Abstract

This study examined metal leaching and retention in pervious concrete with or without embedded particulate matter. Particulate matter was collected from an adjacent parking lot and from a nearby parking garage as examples of weathered and un-weathered particulate matter. Particle size distributions were similar, but metal content was 3-35-fold higher and organic matter content was 3-fold higher in the parking garage particulate matter compared to the parking lot particulate matter. Replicate columns were established with either no particulate added as the control, or 20 g of parking lot or parking garage particulate matter. Synthetic rainwater was passed through the columns at variable rainfall intensity or fixed intensity to assess leaching. Metals were leached at higher concentrations from the parking garage particulate amended column, but from all columns less than 1% of the metal mass leached. Rainfall intensity did not have a large effect on leached metal concentrations, only varying effluent by about 2-fold. Synthetic stormwater with elevated dissolved Cu, Zn, Cd and Pb concentrations was passed through the same columns and metal removal efficiencies were on the order of 85-95%, 30-95%, 60-90%, and 95+% for each metal, respectively. After loading the column with a year's worth of stormwater metal exposure, removal efficiencies in the no particulate and parking lot particulate amended columns decreased, while parking garage particulate amended columns performed similarly with a small drop in Cu and Pb removal efficiencies. Generally, columns with no particulate and parking lot particulate amendments performed similarly, suggesting the pervious concrete is responsible for the majority of the initial metal retention. The parking garage particulate amended columns retained more metals from stormwater, perhaps due to an increase in pH that promoted surface precipitation as hydroxides or carbonate species on the pervious concrete, or due to complexation in the higher concentrations of organic matter and iron oxides in the particulate matter. Overall, metal retention was aided by the presence of organic matter in the particulate matter, but the pervious concrete itself was more important than particulates for metal retention. A strategy to increase metal retention and removal from the environment could involve amending pervious concrete with mixtures of well-defined sorbents to enhance metal retention.

Keywords: Heavy metals; Leaching; Particulate matter; Pervious; Retention; Stormwater; Urban runoff.