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 wasto 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 NO_x pollutants and 4-20% for SO₂ pollutants. The maximum NO_x 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, andultraviolet (UV) light intensity. In the field, NO_x concentrations were monitored for both the coated and uncoated sections to directly measure photocatalytic degradation. Furthermore, nitrates werecollected from the coated and uncoated areas for evidence of photocatalytic NO_x reduction. Results from both approaches show evidence ofphotocatalytic NO_x reduction. Further field evaluation is needed to determine the durability of the surface coating.