Iron-Rich Biochar to Adsorb Volatile Organic Compounds Emitted from Asphalt-Surfaced Areas

This study introduces iron-rich biochar to selectively adsorb some volatile organic compounds (VOCs) emitted from asphalt-surfaced areas. Adsorption and retention of VOCs can improve the air quality while reducing asphalt’s mass loss which compromises asphalt’s durability. The iron-rich biochar was made from a hybrid feedstock of Cyanidioschyzon merolae (CM) algae and swine manure (SM) with a blend of 20:80 CM/SM, showing the highest biochar yield of 27 wt % and the highest metal content of Fe (21.8 g kg^-1). Comparison of VOC adsorption of iron-rich biochar and low-iron biochar shows the significant role of Fe content in adsorption of VOCs. When the iron-rich biochar was introduced to asphalt, a total emission reduction of 76% was observed; this number was only 59% when low-iron biochar was used. The superiority of iron-rich biochar over low-iron biochar in adsorbing volatiles is observed for some specific volatiles such as non-polar aromatics in our experiment. The dominant contribution of lightweight cyclic aromatics and alkanes in bitumen emissions indicates that minimizing the VOC emissions from saturate and aromatic fractions of bitumen could be a key factor in reducing the VOCs from asphalt. The hybrid biomass feedstock used here is also a nitrogen-rich substance containing abundant N sites to coordinate with Fe to form −C-N-Fe bonds. To understand the role of N-Fe functionals in the higher efficacy of Fe-rich biochar, density functional theory (DFT) calculations were performed on three O-containing compounds (benzoic acid, benzofuran, and hexanal) and three S-containing compounds (dibenzothiophene, 3-pentylthiophene, and hexanethiol). Based on DFT results, the adsorption energies of VOCs on active sites of Fe-biochar (containing −N-Fe functionals) are 8 to 10 times higher than those for pristine biochar (containing just N groups). It was also found that the presence of iron stimulates the catalytic performance of biochar, as evidenced by our DFT results, indicating degradation of dibenzothiophene and hexanethiol on the N-Fe active zones of iron-rich biochar. Application of iron-rich biochar can suppress the volatiles emitted from asphalt-surfaced areas; this in turn can improve the air quality and extend the service life of roadway infrastructures. Therefore, the study outcomes can promote built environment’ sustainability and public health.