Augmented photocatalytic NOx abatement via hydroxylated recycled aggregates as hosting matrices for nano-TiO2: Waste upcycling for environmental improvement

Nitrogen oxides (NOx) are hazardous materials in the environment. Compared to other degradation methodologies, photocatalytic degradation, solely fueled by natural energy sources, holds immense potential for remediation, and more importantly, this study employed recycled waste to fulfill the green conversion using nano-titanium dioxide (TiO 2 , denoted as NT), which stands out due to its high reactivity and resistance to acid-base corrosion. However, the practical application of NT is hindered by poor dispersibility and proneness to agglomeration. The present study endeavors to mitigate these limitations by leveraging an innovative fabrication process and the porosity and high specific surface area of recycled aggregates (RAs) derived from construction waste as carriers for NT including recycled concrete aggregates (RCA) and recycled clay brick aggregates (RCBA)), which can provide ample sites for NT immobilization, thereby enhancing its dispersion and minimizing agglomeration. Given the polar nature of NT, the study initially allowed the RAs for hydroxylation modification first, intending to fortify the adhesion between the aggregates and NT. Subsequently, NT solutions at concentrations of 1 %, 3 %, 5 %, and NT hydrosol were prepared and utilized to impregnate the modified aggregates. Findings reveal that a 1 % immersion concentration yields the optimal result of 54.8 μmol/hm 2 NOx reduction per mg NT by treated RCA in 30 minutes that is superior to its hydrosol-prepared reference while 3 % immersion concentration in both RCA and RCBA still possesses comparable reduction performance as its hydrosol-treated counterpart. Microstructural and porosity analyses indicate that NT refines some macropores and reduces the porosity of aggregates due to the relatively uniform precipitation on the surface layer after process treatment increasing the photocatalytic NOx reduction performance. Lastly, the cost analysis presents that the aqueous dispersion of 3 % powdered NT demonstrates excellent economic advantage, offering a pragmatic solution for the substantial degradation of hazardous substances of NOx through the upcycling and reutilization of construction wastes contributing to a great potential for industrialization in sustainable development. The innovation of this study lies in utilizing the porous structure of recycled aggregates and the sol-gel method to enhance their functionality for environmental remediation purposes.