Sustainable Photocatalytic Asphalt Pavements for Mitigation of Nitrogen Oxide and Sulfur Dioxide Vehicle Emissions

The ability of titanium dioxide (${\mathrm{TiO}}_2$) photocatalytic nanoparticles to trap and decompose organic and inorganic air pollutants render them a promising technology as a pavement coating to mitigate the harmful effects of vehicle emissions. This technology may revolutionize construction and production practices of hot-mix asphalt by introducing a new class of mixtures with superior environmental performance. The objective of this study was to assess the benefits of incorporating ${\mathrm{TiO}}_2$ into asphalt pavements. To achieve this objective, the photocatalytic effectiveness and durability of a water-based spray coating of ${\mathrm{TiO}}_2$ was evaluated in the laboratory. This study also presents the field performance of the country’s first air-purifying photocatalytic asphalt pavement, located on the campus of Louisiana State University. Laboratory evaluation showed that ${\mathrm{TiO}}_2$ was effective in removing ${\mathrm{NO}}_x$ and ${\mathrm{SO}}_2$ pollutants from the air stream, with an efficiency ranging from 31–55% for ${\mathrm{NO}}_x$ pollutants and 4–20% for ${\mathrm{SO}}_2$ pollutants. The maximum ${\mathrm{NO}}_x$ and ${\mathrm{SO}}_2$ removal efficiencies were achieved at an application rate of $0.05\mathrm{L}/{\mathrm{m}}^2$. The efficiency of ${\mathrm{NO}}_x$ reduction is affected by the flow rate of the pollutant, relative humidity, and ultraviolet (UV) light intensity. In the field, ${\mathrm{NO}}_x$ concentrations were monitored for both the coated and uncoated sections to directly measure photocatalytic degradation. Furthermore, nitrates were collected from the coated and uncoated areas for evidence of photocatalytic ${\mathrm{NO}}_x$ reduction. Results from both approaches show evidence of photocatalytic ${\mathrm{NO}}_x$ reduction. Further field evaluation is needed to determine the durability of the surface coating.