Conventional and non-conventional adsorbents for reducing dissolved heavy metal pollution in runoff from roads: A systematic review and meta-analysis

The amount of runoff from roads is increased due to the enhancement of impermeable surface. Among the great variety of pollutants which characterized this type of water, heavy metals (HMs) represent a serious source of concern due to their potential harmful effects on living organisms. In this work, the documents on the conventional and non-conventional adsorbents (CAds and NCAds, respectively) used for removing dissolved HMs from roads’ runoff have been systematically reviewed. The meta-analysis of the selected documents revealed that, despite the efforts, the specific area of NCAds remains on average lower than CAds (75.1 ± 35.8 m2/g vs. 226.7 ± 196.2 m2/g, respectively) but with lower pore diameter (6.1 ± 3.2 nm vs. 27.3 ± 12 nm) which can potentially help in the adsorption of dissolved HMs. In fact, NCAds showed on average an adsorption capacity of Cr and Cu one order of magnitude higher with respect to conventional ones, and a slightly higher adsorption capacity of Pb and Zn. In case of Cd, no strong differences in using NCAds or CADs have been highlighted, while for Ni more studies are suggested. Despite the encouraging results, some gaps remain as the low number of types of NCAds which have been tested (especially for Cr removal), and the initial conditions of the matrices which generally strongly differ from the real ones in terms of (i) HMs concentration (at least two orders of magnitude - in case of Pb - up to more than 4 order of magnitude - Cd), and (ii) competitive presence of other pollutants. The results of this work can be useful both for the scientific community, suggesting useful tips for optimizing future studies, and for technical stakeholders providing information about the use of CAds and NCads for the reduction of HMs’ pollution in roads’ runoff.