Metal-rich biochar as an asphalt modifier to improve sustainability and reduce VOC emissions

Asphalt-surfaced areas have been recently reported as non-combustion sources emitting organic chemical compounds. These emissions from asphalt not only accelerate its aging but also negatively affect air quality and the health of people who inhale the emitted compounds. Trying to mitigate the latter emission and inspired by the phytoremediation capacity of the acacia plant, we have developed a metal-rich carbonaceous adsorbent (biochar) from the acacia plant. Here, we evaluate the efficacy of our acacia-derived adsorbent to reduce the hazardous volatile organic compounds (VOCs) emitted from asphalt-surfaced areas. We further compare the efficacy of our acacia-derived adsorbent with an adsorbent made from a low-metal plant (silver grass). The higher metal content of the adsorbent made from acacia correlated with its higher efficacy compared to that of silver grass in reducing the release of VOCs from the corresponding asphalt, displaying emissions of 16.9% in asphalt binder containing acacia biochar compared to emissions of 21.2% using silver-grass biochar. Using density functional theory (DFT)-based molecular modeling, we specifically investigated the adsorption behavior of three prominent metals present in acacia biochar: Ca with ≈8 wt%, Al with 6.9 wt%, and Fe with 4.4 wt%. Using DFT, we demonstrated the individual contribution of each metal to adsorbing each of six chemical air pollutants. The DFT results revealed the superiority of Fe at interacting with VOCs across all N-containing zones of the biochar surface: pyridine, pyrrole, amine, and amide. Specifically, in the pyridine and pyrrole zones of the biochar surface, the trend of interaction energy for gas pollutants is as follows: Fe > Ca > Al > no metal. Considering the high concentration of Ca in acacia biochar (≈8 wt%), this trend of energy (Fe > Ca > Al > no metal) highlights the crucial role of Ca in enhancing biochar adsorption. Our findings highlight the potential of using inherently metal-rich biomass feedstocks to reduce volatile emissions from asphalt-surfaced areas, improving air quality and extending the service life of roadway infrastructure.