The Growing Concern of VOC Emissions in Asphalt Pavement
Asphalt pavement has established itself as the primary choice for highways, celebrated for its smooth driving experience and rapid construction times (Fu et al., 2021; Tran et al., 2022). However, this popular material brings with it a hidden danger—volatile organic compounds (VOCs) produced during various phases of asphalt handling. These emissions occur during mixing, transportation, paving, and even while the pavement is in service, raising potential ecological and health concerns (Xiu et al., 2020).
Understanding VOCs and Their Impact
VOCs are a diverse group of gaseous compounds, including alkanes, olefins, aldehydes, and esters (Boczkaj et al., 2014). While they may seem innocuous in small quantities, their impact on both the environment and human health is profound and alarming (Cui et al., 2020a; Sánchez-Monedero et al., 2019). These compounds trigger significant atmospheric pollution, exacerbating issues like the greenhouse effect and acid rain. The oxidation of sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) is notably accelerated in the presence of VOCs, facilitating the rapid formation of strong acids that contribute to acid rain (Zavala et al., 2020; Lee et al., 2024).
Asphalt VOCs are not just harmful to the environment; they pose direct health risks as well. Prolonged exposure can lead to respiratory ailments, skin issues, and neurological disorders, even increasing the likelihood of cancer (Cui et al., 2020b; Benson et al., 2021). Research indicates that construction workers dealing with asphalt may have a heightened risk of lung cancer (Boffetta et al., 2002). Ultimately, the health hazards linked to asphalt VOC emissions manifest in various forms, from headaches to more severe conditions like leukemia.
Mitigating VOC Emissions: Strategies and Solutions
Recognizing the detrimental effects of asphalt VOCs, it is imperative to control these emissions right from their source. One effective method is the use of adsorption, classified into chemical and physical categories. Chemical adsorbents interact with VOCs through chemical bonding, while physical adsorbents utilize their pore structures and large surface areas to bind VOCs within their network (Wang et al., 2021).
In this context, trans-cinnamaldehyde has been identified as an effective agent for diminishing the toxicity of polycyclic aromatic hydrocarbons (PAHs) found in asphalt. Its chemical reactions substantially reduce the presence of single PAHs (Feng et al., 2018). Similarly, ammonium phosphomolybdate serves as another noteworthy inhibitor, leveraging heat-reducing properties to inhibit VOC emissions during asphalt application (Tang et al., 2020).
While these chemical innovations are promising, they often fail to achieve a significant overall reduction in VOC concentrations. Therefore, the rise of physical adsorbents has garnered attention. For instance, mesoporous silica (MSHS) has demonstrated superior capabilities in suppressing asphalt VOCs through physical adsorption mechanisms (Shu et al., 2019). Furthermore, biochar derived from diverse organic sources has emerged as an environmentally friendly alternative for VOC suppression, achieving commendable results in reducing various VOCs groups (Zhou et al., 2024).
Unique Characteristics of Asphalt Binder vs. Asphalt Mixture
The emission characteristics of VOCs significantly vary between asphalt binders and asphalt mixtures. External factors such as temperature and wind direction can influence VOC emissions, while the composition of aggregate materials can further modify these characteristics (Li et al., 2021). Insights into the behavior of asphalt mixtures can enhance the understanding of VOC emissions during pavement construction (Long-Sheng, 2018).
Previous studies on tourmaline anion powder (TAP) revealed a 50% reduction in VOC emissions from asphalt mixtures upon TAP incorporation (Zhang et al., 2021). Other additives like alumina trihydrate (ATH) and organic montmorillonite (OMMT) have shown potential to diminish VOC emissions while also enhancing thermal properties (Yang et al., 2020). In particular, organic modifications have improved both VOC suppression and the overall performance of asphalt mixtures.
Additionally, advancements in treating materials such as crumb rubber (CR) through acidic washing have highlighted their favorable impact on the VOC emissions of asphalt mixtures. Such modifications have potential implications for storage stability and workability within pavement applications (Bueno et al., 2021). Novel approaches including the utilization of oil sludge (OS) as a rejuvenator exhibit promising results in reducing air emissions and total VOCs (Dalhat et al., 2022).
The Role of Porous Materials in VOC Treatment
Porous materials like carbon black, biochar, and zeolite are increasingly utilized due to their effective physical adsorption properties. These materials boast extensive surface areas and a well-developed pore structure, allowing for high efficacy in VOC absorption (Cong et al., 2014; Wang et al., 2022; Zhu and Zheng, 2016). Despite carbon black’s commonality in anti-aging formulations for asphalt, its unexplored potential in VOC suppression should not be underestimated. Conversely, biochar serves a dual purpose, providing long-lasting carbon sequestration alongside VOC absorption (Ringsby et al., 2024).
Zeolite compounds, recognized for their ion-exchange capabilities, are particularly adept at adsorbing polar VOCs, presenting an environmentally beneficial alternative (Maia et al., 2024). The economic viability of biochar and zeolite renders them attractive candidates for large-scale application in VOC management efforts.
A Need for Comprehensive Evaluation Parameters
Most current research evaluates VOC emissions based solely on total concentration, overlooking the important nuances of individual VOC groups and components. This generic approach neglects significant differences in health and environmental impacts posed by various VOC emissions. It is essential to prioritize a more detailed analysis that considers total, group, and component concentrations (Li et al., 2020b).
For a more accurate assessment of environmental consequences and health hazards linked to asphalt VOCs, a multi-faceted evaluation methodology is key. Each component harbors distinct implications, warranting tailored approaches for mitigation and safety.
Exploring Innovative Solutions
In addressing the challenge of asphalt VOC emissions, this study sets out to evaluate the effectiveness of various inhibitors such as carbon black, biochar, and zeolite in asphalt mixtures. Through experimental analyses using techniques such as gas chromatography-mass spectrometry (GC-MS), the research aims to pinpoint VOC reduction levels across multiple dimensions. The ecological advantages tied to these emissions will also be scrutinized, ensuring a comprehensive understanding of the environmental and health implications.
This work extends an invitation to reevaluate the methodologies applied in managing asphalt VOCs, suggesting that emerging strategies and inhibitors can reshape our approach to environmentally conscious pavement practices. By systematically investigating the road performance of these mixtures, the study highlights not only the practical implications but also the exciting potential of developing robust asphalt applications that contribute positively to both health and ecological sustainability.