Photocatalytic degradation of non-ionic, anionic, and cationic surfactants: from batch experiments through equilibrium/kinetic study to ecotoxicology analysis

Surface-active compounds constitute a group of important and widespread environmental pollutants. Among different methods used for their treatment, photocatalytic degradation is a promising and efficient solution. Nevertheless, the exact outcome of photodegradation of surfactants may be difficult to predict in case of concentrations used at the industrial scale and considering that the associated mechanism involves radicals which may potentially result in the formation of toxic by-products. In consequence, the aim of this study was to analyze the catalytic photodegradation of three groups of surfactants (three structurally diverse representatives of cationic, anionic, and non-ionic surfactants). Nanoparticles of zinc oxide were synthesized and used as a photocatalyst. The efficiency of photodegradation ranged from 93 to 100% at 10 mg/L, from 38 to 94% at 100 mg/L, and from 15 to 66% when the surfactants were supplied at 1000 mg/L, and the results depended on the structure of the compound subjected to treatment. Subsequent investigation of kinetics revealed that Freundlich, Langmuir or Dubinin-Radushkevich isotherms may be used to describe the adsorption of surfactants (1-65%) and allowed to establish the following order of reactivity: cationic > non-ionic > anionic. Subsequent LC-MS/MS studies of the residues were the basis for identification of photodegradation products for each respective compound. Ultimately, due to the fact that treated graywater is often used as potable water, the phytotoxicity of the degradation products was tested toward sorghum. It was established that in the majority of cases the treatment was efficient (in some cases stimulation of plant growth compared to control samples was observed), whereas an increase in toxicity was observed in case of two cationic surfactants (CTAB and benzalkonium chloride). Overall, the results presented in this study support the application of photocatalytic degradation as an efficient solution for treatment of surfactant-rich effluents.