Anodic TiO₂ nanotube layer directly formed on the inner surface of Ti pipe for a tubular photocatalytic reactor

We demonstrated the successful water treatment performance of a continuous-flow tubular TiO₂ nanotube reactor made of Ti pipe where a TiO₂ nanotube (TNT) layer was directly formed on the inner wall by an electrochemical anodization process. A cylindrical UV lamp was inserted into the pipe to irradiate the inner TNT surface as a light source. To optimize the thickness of TNT layer on Ti pipe, TNT layers with 1, 5 and 10 μm thickness were prepared and their photocatalytic activities were tested for the degradation of various organic substrates (acid orange 7 (AO7), dichloroacetate (DCA), and 4-chlorophenol (4-CP)) under UV illumination. The TNT layer with 10 μm thickness showed the highest photocatalytic activity, with first-order removal rate constants (k:min^-1) of 0.019, 0.039, and 0.025 for AO7, 4-CP, and DCA, respectively. The directly formed TNTs (10 μm thickness) on Ti pipes in the continuous-flow tubular TNT reactor, having a strong adhesiveness and a high surface area, enabled the successful degradation of organic compounds and to have kept the activity without decrease during the repeated cycles and long-term experiments. The photocatalytic performance of the tubular TNT reactor was tested with a higher concentration, smaller volume ([4-CP]₀ = 100 μM, solution vol. = 15 mL) and a lower concentration, larger volume ([4-CP]₀ = 20 μM, solution vol. = 100 mL) in the circulation-flow mode (20 mL min^-1). A good performance was achieved under the low intensity UV illumination (≈2 mW cm^-2) with a half-life of ca. 50 min and ca. 11 h with 80% and 79% removal of TOC for the above two test conditions, respectively. The proposed tubular TNT reactor can be an effective option in the design of continuous flow-type photocatalytic reactors for practical water purification.