Energy consumption, economic costs, and carbon footprint of the hierarchical black defective TiO₂ nanomaterials: From production to application in environmental remediation

Herein, a novel integrated assessment of the applied electrical energy (EE), economic costs, and carbon emissions (CE) associated with the synthesis and photocatalytic application of three distinct hierarchical structures of black defective TiO₂ nanomaterials (BD-TiO₂ NMs)—nanowires (BNWs), nanoflowers (BNFs), and nanonails (BNNs)—is presented. With only a small variation across morphologies, BNFs used more EE (28.607 kWh/kg) during synthesis than BNNs (24.945 kWh/kg) and BNWs (24.364 kWh/kg). EE costs and Scope 2-indirect CE showed major variation across geographic regions depending on the energy costs and the emissions factor (EF). EE costs ∼$9.414/kg in the United Kingdom (UK) compared to only $1.512/kg in Malaysia and $1.997/kg in China for producing BNFs. Similarly, CE produced in the UK during BNFs’ synthesis is 5.906 kg CO₂-eq/kg which is ∼4-fold lower than the CE in China (26.673 kg CO₂-eq/kg). At application stage, BNFs used average EE of 3.99 kWh/m3 to treat low concentration wastewater (<100 mg/L of phenol), which translated into CE of 2.57 kg CO₂-eq/m3 in Malaysia, 3.74 kg CO₂-eq/m3 in China, and 0.89 kg CO₂-eq/m3 in the UK. The t-costs varies greatly across developed and developing countries; in the UK, treating 1 m3 of wastewater contaminated with phenol costs $4.725, while in China and Malaysia it only costs $1.002 and $0.759, respectively. The findings presented here demonstrate that, in addition to their particular morphology, the energy context in which the BD-TiO₂ NMs are produced and employed determines their carbon footprint, economic feasibility and overall sustainability.