Pervious concrete as a nature-based filtering element for heavy metal removal from urban stormwater runoff: influence of mix design, hydraulic retention time and successive storm events.

In this study, six pervious concrete mixes were evaluated as passive filters for heavy metal removal (Cr, Cu, Zn, Pb) from urban stormwater runoff, examining the combined influence of mix design, hydraulic retention time (HRT) and repeated storm loading. Two laboratory campaigns used synthetic runoff spiked at representative urban concentrations. Campaign A applied single-event filtration at an HRT of about 1.7 min; Campaign B tested shorter HRTs (4 to 9 s, representative of in-situ pavements) across two and four consecutive storm events. Because the campaigns differed in ionic matrix and achieved influent concentrations, inter-campaign comparisons are qualitative. HRT governed removal more strongly than mix composition: within Campaign B, raising HRT from 4.4 to 9.0 s increased Cu removal from about 32% to 85%, Zn from 14% to 76% and Pb from 22% to 86%; Pb values are indicative, as the achieved Pb influent fell 3- to 12-fold below target. The precast CEM I element sustained the highest retention, and Cu, Zn and Pb removal remained broadly stable over four consecutive events. Chromium was HRT-dependent, shifting from net leaching at about 1.7 min (mean effluent concentration up to about six-fold above influent) to retention at 4 to 9 s (mostly 94-100%, minimum 71%), with a collapse of retention at the fourth event in the precast mix. Effluent pH reached about 12, above the pH 9.5 reference limit, calling for downstream neutralisation. These results identify HRT as a key hydraulic design variable for pervious concrete-based nature-based solutions.