concrete, photocatalytic oxidation, pavement, nitrogen oxides, sustainability, transportation
Emissions from motor vehicles cause the concentrations of nitrogen oxides (NOX) in the immediate vicinity of highways to be 30-100% higher than background concentrations. Photocatalytic pavements have been proposed to address this localized pollution problem. Bench-scale evaluation of environmental variables confirmed that nitric oxide (NO) oxidation rate positively correlated with influent NO concentration and irradiance and negatively correlated with relative humidity. However, comparison of the slope of these correlations found significant differences between this study and other published work. Slab water loss, a variable not investigated in prior work, positively correlated with NO oxidation rate at water losses of 0-2% of saturated mass, but negatively correlated at losses greater than 2%. A positive correlation was documented for slab temperature; this finding contrasts previous assertions which considered this variable insignificant. Overall, oxidation rates ranged from 6.2-57 nmole∙m-2∙s-1. This sensitivity demonstrates that selecting a field location requires exceptionally careful review of environmental conditions. Comparison of photocatalytic slabs of pervious concrete and mortar manufactured with the same photocatalytic cement and water-to-cement ratio found no evidence of a difference in NO oxidation rate at a confidence level greater than 90%. This finding contrasts previous speculation, which asserted superior pervious concrete performance due to increased specific surface area. In a second contrast to previous work, no evidence of a correlation was found between NO oxidation rate and photocatalytic layer depth in two-lift pervious slabs. Although pervious slabs were not superior to mortar slabs, they are another material that could address NOX pollution. Lab investigation of blinding by prevailing roadway pollutants did not find evidence of a substantial decrease in performance at normally observed field loadings. Reaction products did cause blinding; however, the performance decrease was asymptotic and a complete loss of activity was not observed. In disagreement with previous reports, preliminary results did not observe easy photoactivity regeneration. The first phase of an ongoing field study did not find evidence that NO concentration in a photocatalytic section was lower than NO concentration in a control section. The most probable explanations for this observation are excess vibration, which caused mixture segmentation, and insufficient NO oxidation rate.
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Sikkema, Joel K., "Photocatalytic Degradation of NOx by Concrete Pavement Containing TiO₂" (2013). Faculty Work: Comprehensive List. 98.