Degradation, mineralization and antibiotic inactivation of amoxicillin by UV-A/TiO₂ photocatalysis

The UV-A/TiO(2) photocatalytic decomposition of amoxicillin (AMX) in aqueous suspensions was investigated. Experiments were performed at antibiotic concentrations between 2.5 and 30 mg/L, eight commercially available TiO(2) catalysts at loadings between 100 and 750 mg/L, acidic or near-neutral conditions (pH 5 or 7.5) and two different matrices (ultrapure water and secondary treated effluent) at a photon flux of 8 × 10(-4) E/(L min). Of the various catalysts tested, Degussa P25 was highly active, i.e. complete AMX degradation and 93% mineralization could be achieved after 25 and 90 min of reaction, respectively at 10 mg/L AMX and 250 mg/L titania. In general, mineralization was slower than degradation due to the formation of stable transformation by-products. For the range of concentrations studied, initial degradation rates can be approached by a Langmuir-Hinshelwood kinetic model, while the reaction order with respect to AMX shifts from first to zeroth as initial concentration increases from 2.5 to 5 mg/L to higher values. Degradation in treated effluent was partly impeded compared to pure water due to the inherent presence of organic and inorganic constituents that compete for hydroxyl radicals. Although increasing solution pH from 5 to 7.5 had no effect on degradation, it retarded mineralization. The antibiotic activity of AMX prior to and after photocatalytic degradation was tested to three reference bacterial strains, namely Escherichia coli (ATCC 23716), Klebsiella pneumoniae (NCTC 5056) and Enterococcus faecalis (ATCC 14506). The first two were found to be highly resistant at AMX concentrations up to 25 mg/L, while the latter could partly be inactivated at lower AMX concentrations (i.e. 10 mg/L) and/or in the presence of photocatalytic by-products.