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

The ability of titanium dioxide (TiO2) 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 TiO2 into asphalt pavements. To achieve this objective, the photocatalytic effectiveness and durability of a water-based spray coating of TiO2 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 TiO2 was effective in removing NOx and SO2 pollutants from the air stream, with an efficiency ranging from 31–55% for NOx pollutants and 4–20% for SO2 pollutants. The maximum NOx and SO2 removal efficiencies were achieved at an application rate of 0.05 L/m2. The efficiency of NOx reduction is affected by the flow rate of the pollutant, relative humidity, and ultraviolet (UV) light intensity. In the field, NOx 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 NOx reduction. Results from both approaches show evidence of photocatalytic NOx reduction. Further field evaluation is needed to determine the durability of the surface coating.