Adsorption of heavy metals onto alkali-activated pervious concrete: Mechanism, long-term performance, and leaching...

This study investigates the capacity of alkali-activated pervious concrete (AAPC) and Portland cement pervious concrete (PCPC) for the competitive adsorption and potential leaching of heavy metals commonly found in stormwater, including Fe⁺³, Pb⁺², Ni⁺², Cu⁺², Cr⁺³, Mn⁺², Cd⁺², and Zn⁺². Precipitation, co-precipitation, complexation, and ion exchange were found to be the primary mechanisms of AAPC to remove heavy metals. Irrespective of the aluminosilicate precursors (slag, fly ash, and metakaolin), the AAPC specimens showed a comparatively consistent order of adsorption capacity: Cu⁺² > Cr⁺³ > Fe⁺³ > Zn⁺² > Pb⁺² > Cd⁺² > Ni⁺² > Mn⁺². During the adsorption experiments, the AAPC specimens, which showed a higher pH during the adsorption isotherm experiment, exhibited a greater capacity to adsorb heavy metals (18-50 %) but lower compressive strength (22-58 %). Continuous flow column experiment with accelerated loading revealed the long-term efficiency of AAPC in removing heavy metals. At the initial stage, the AAPC pavement maintained high removal efficiency (>90 %) for most heavy metals; however, the removal efficiency gradually declined over time. For example, after 25 equivalent years of application, the adsorption efficiency of AAPC in removing Fe⁺³ and Zn⁺² reduced to 69 % and 35 %, respectively. The PCPC also exhibited a heavy metal removal behaviour similar to that of AAPC; however, depending on the heavy metal, its removal efficiency was approximately 10-40 % lower than that of AAPC. Less than 2.5 % of adsorbed heavy metals leached from AAPC, while for the PCPC, the leaching of adsorbed heavy metals was approximately 6.0 %.