Remediation of traffic related heavy metals, microplastics and 6PPD-Quinone with nature-based stormwater infrastructure

This work reviews (i) reported concentrations of three types of emerging pollutants of concern (EPOCs) in stormwater originating from vehicular traffic activities, and (ii) the capacity for bioretention cells and permeable pavements for treating these EPOCs. Reported concentrations in the literature for microplastics (MPs), 6PPD-Quinone (6PPD-Q), and heavy metals lead (Pb) and cadmium (Cd) in urban stormwater runoff from 2010 to the present day were either from directly measured runoff samples, indirectly obtained from some other measure/surrogate, cited from known sources and information, or modelled. Levels of each EPOC concentration in stormwater were highly variable in magnitude and were affected by location, climate and traffic intensity. The literature contains many remediation studies on the efficacy of bioretention cells and permeable pavements for Pb and Cd, but only a handful on treating MPs and 6PPD-Q that have mostly arisen in the last few years. The studies show that these infrastructure are promising tools for treating these EPOCs but research into the mechanisms for their removal were primarily inferred by comparing inlet versus outlet concentrations as opposed to being directly studied. Insufficient detection limits, and insufficient knowledge of how MPs degrade or how chemical transformations occur within the infrastructure, continue to plague advances in knowledge and design. Designing these stormwater infrastructure types for mitigating these EPOCs is also hindered by a lack of threshold criteria and the lack of a standard assessment for MPs in terms of form and quantity.