A novel method for assessing chemical leaching from surface water–pavement interactions applied to recycled-tyre reuse products

Globally, recycling of otherwise waste materials into new products is desired. End-of-life tyres are increasingly incorporated into new pavement materials but leaching of entrained chemicals from such products is not well quantified. Chemical concentrations in runoff from pavements may pose environmental and human health risks. High liquid-solid ratio, batch-agitated leaching is standard practice for assessing leachability and hazards of chemicals-of-potential-concern in contaminated soil and wastes but is not reflective of important exposure scenarios and may mislead. A new static surface leaching procedure (SSLP) is introduced that is more representative of chemical leaching from pavement reuse materials whilst in contact with rainfall/runoff water. SSLP was evaluated over 2-14 d intervals against batch-agitated leaching for two rubberised pavement products containing 10-fold different proportions of crumbed end-of-life tyres. Although, batch leaching showed high mass removal of 1,3-diphenylguanidine (1,3-DPG, 34%) and hexamethoxymethylmelamine (HMMM, 30%), both batch- and SSLP-leached concentrations of 1,3-DPG, HMMM and <i>N</i>¹-(4-methylpentan-2-yl)-<i>N</i>⁴-phenylbenzene-1,4-diamine quinone (6PPD-Q) were below ECOSAR-predicted toxicity thresholds for fish and daphnids. SSLP highlighted differences in chemical leachability based on rubber content of pavement products and offers a method applicable to other scenarios, such as PFAS leaching from concrete/asphalt pavements. The SSLP was shown to approximate one-dimensional leaching from the surface of the pavement and to be dominated by diffusive processes, thus yielding a simple repeatable approach.