Cerium Loading Redistributes Tetracycline Removal between Dark Adsorption and the UV Stage on Hydrothermal Niobium Oxide

Photocatalysts that also adsorb are usually ranked by how much pollutant disappears at the end of a run, a number that says nothing about which of the two processes did the work. We separate the two on a single material family by running a dark stage and an illuminated stage in sequence and following the solution in-line. Niobium oxide with nominal Ce loadings of 1 %–5 % was prepared in one hydrothermal step from ammonium niobate oxalate and cerium nitrate, and tested against 20 mg L−1 tetracycline under 254 nm irradiation. Total removal after 180 min did not differ significantly across the three compositions, at 53.5 ± 0.7 %, 57.1 ± 1.9 % and 54.2 ± 3.2 % for Ce1%–Nb2O5, Ce3%–Nb2O5 and Ce5%–Nb2O5 respectively (Welch p≥0.19). The dark stage did separate them: uptake before illumination was 24.4 ± 0.7 mg g−1, 36.6 ± 0.8 mg g−1 and 27.5 ± 0.4 mg g−1 (p≤0.047 for every pair, n=2), and the apparent first-order constant of the UV stage fell as dark uptake rose (r=−0.90, p=0.014, n=6). Direct photolysis cannot produce that spread, because it would be fastest where the solution is most dilute, and dark uptake was close to its plateau by lamp-on, so the spread is attributable to the solids. Ce loading therefore redistributes removal between dark uptake and the UV stage rather than changing the total, so a fixed-time removal percentage cannot rank these materials, whereas dark-stage uptake separates them.