## 5 Environmental Aspects and Health Risks
### Leading Characteristics and Release of MPs from Plastic-Modified Asphalt Mixtures  
Plastic-modified asphalt mixtures have become increasingly favoured for their ability to diminish plastic waste and improve surface performance [71; 72]. Nonetheless, their environmental sustainability must be evaluated regarding leaching behaviour and MP emissions during their service life [14; 73]. Recent studies suggest that the integration of plastic into asphalt raises concerns about the emission of toxic chemicals and MPs as a result of environmental exposure, mechanical abrasion, and thermal degradation [14]. Leaching investigations of asphalt mixtures including waste plastics have predominantly concentrated on the discharge of heavy metals, nutrients, DOC, and other hazardous substances into aquatic environments [14]. For example, leachability tests conducted on asphalt pavement modified with plastic particles, utilizing Cantabro abrasion, demonstrate a restricted release of conventional water contaminants, including nutrients and heavy metals [25]. DOC, including many organic pollutants, is an essential parameter to monitor due to its potential to affect the mobility, bioavailability, and toxicity of other contaminants, as well as to stimulate microbial proliferation and algal blooms in aquatic ecosystems [25]. Recent findings by [25] demonstrated a strong correlation between particle size and DOC release, with smaller particles leaching significantly higher concentrations of DOC. In their study, asphalt specimens incorporating LDPE exhibited reduced DOC concentrations compared to unmodified mixtures, although particle sizes under 0.2 mm were associated with increased leaching intensity. A negative correlation (R\({}^{2}\) = 0.79) was observed between weighted average particle size and DOC levels, highlighting that finer abrasion products, which are more probable under intense traffic or ageing conditions, pose greater environmental leaching risks. Standardized leaching tests, including the US EPA Method 1311 (TCLP) [24], Method 1312 (Synthetic Precipitation Leading Procedure--SPLP) [74], have been utilized to assess the possible leachability of asphalt components [3; 25]. Nonetheless, discrepancies in testing conditions impede comparability among studies [3; 14]. Recent methodologies, such as utilizing particles from Cantabro abrasion instead of random crushing, provide a more authentic representation of field conditions [25]. The release of MPs from plastic-modified asphalt has gained considerable attention. MPs are predominantly produced through mechanical abrasion caused by vehicular traffic, environmental weathering, and UV exposure [14; 75]. Laboratory simulations utilizing Wet Track Abrason machines have shown that MPs can be released from both wet- and dry-incorporated plastics, but the mechanisms and degrees of release vary [76]. Wet methods generally integrate polymers more uniformly into the bitumen matrix, potentially enhancing surface exposure. Conversely, the dry method often utilizes bigger plastic particles acting as aggregates, exhibiting variable surface exposure depending on their placement inside the pavement [38]. Recent controlled laboratory simulations by [76] quantified microplastic (MP) emissions from asphalt mixtures containing rLDPE/LLDPE and rABS under varying environmental conditions. MP release increased with prolonged abrasion duration, elevated temperatures (from 5 \({}^{\circ}\)C to 40 \({}^{\circ}\)C), and high pH levels, demonstrating that environmental stresses substantially affect the release rate. Significantly, rABS-modified asphalt released substantially more MPs than rLDPE/LLDPE blends, with most emitted particles measuring below 40 um in size. This underscores the environmental hazard presented by high-friction polymer blends and the significance of material selection and binder integration in reducing microplastic pollution from road surfaces. Phase separation between plastic and bitumen has been recognized as a significant cause of both leaching and MP release [4; 14].