The Photocatalytic Degradation Mechanism for Organic Pollutants in Seawater and the Fabrication of Catalyst-Loaded Floating Spheres for Effectively Removing Tetracycline

A visible-light-responsive chiral mesoporous TiO2 photocatalyst was employed for the photodegradation of tetracycline in seawater to avoid the potential biotoxicity of nanophotocatalysts. Compared with achiral mesoporous TiO2, the key structural parameters, including Ti3+ and oxygen vacancies, affecting the visible-light response and activity of the chiral mesoporous TiO2 photocatalyst were clarified. The influence of salt ions in seawater on photocatalytic degradation was systematically explored to reveal the photodegradation mechanisms in complex seawater. Based on the results of radical-trapping experiments and electron spin resonance spectroscopy, the active species generated by photocatalysts excited by visible light were the same in freshwater and seawater. However, the results of density functional theory calculations and the analysis of intermediate products showed that free radicals (•OH and •O2–) could not efficiently compete with Mg2+ and Ca2+ in seawater and therefore could not overcome the interference of salt ions to attack the nitrogen atom in tetracycline (TC). The main interference of the photocatalytic process in seawater was the enrichment around the atoms of high electronegativity in TC molecules with Mg2+ and Ca2+, which hindered the attack of holes and completely blocked the attack of •OH or •O2– radicals on TC molecules. Depositing Ag as acceptors for photogenerated electrons in chiral M-TiO2 could effectively enhance the production of holes, thereby significantly improving the catalytic degradation of seawater pollutants. Due to its high performance, the Ag-deposited chiral M-TiO2 catalyst was used for loading on polyurethane sponge cubes, which were placed in hollow porous plastic balls to prepare photocatalytic floating spheres for removing TC in seawater. The rate of removal for TC in simulated seawater by the floating spheres exceeded 85% after 10 h. This work not only identified the mechanism of photodegradation of antibiotic pollutants in different water environments but also has promising potential for the treatment of trace pollutants in seawater using heterogeneous photocatalysis.