Design of TiO2-poloxamer stabilized Pickering emulsions for the photocatalytic degradation of 4-propylbenzoic acid

Organic contaminants in wastewater, including dyes, pesticides, and pharmaceutical waste, pose significant environmental and health hazards. Photocatalysis has emerged as a cost-effective method capable of degrading pollutants with nanoparticles like titania. Among approaches to optimize photocatalytic titania in wastewater treatment, Pickering emulsions have shown great promise. Non-ionic surfactants like tri-block poloxamers promote nanoparticle adsorption to the oil-water interface by altering their wetting properties. The study addressed two objectives: balancing emulsion stability with high photodegradation rates and identifying the degradation products and pathways of 4-propylbenzoic acid to assess the toxicity of the intermediate products.

Five poloxamers (P84, F127, F108, F68, PEG8000) with varying hydrophilicity and chain lengths were selected. Adsorption studies showed increased adsorption with higher molecular weight and 2PEO/PPO ratio, with F68 achieving the highest adsorption due to its intermediate chain length and small PPO block. Emulsion stability tests revealed that TiO2-F68, F127, and F108 emulsions were resistant to pH and salinity changes, while P84 emulsions coalesced at high salinity due to poor adsorption and lack of electrostatic stabilization.

Photocatalytic experiments using 4-propylbenzoic acid (4-pb) as a model pollutant showed that poloxamer chain length influenced degradation rates, with longer chains introducing diffusion limitations for analyte adsorption on TiO2. TiO2-F127 emulsions achieved the best balance of high stability and effective photocatalytic activity. Demulsification during photodegradation was linked to degradation of the adsorbed poloxamer layer. Analysis of 4-pb degradation products revealed pathways involving hydroxylation and alkyl chain termination. Complete mineralization was found to be necessary to mitigate the toxicity of intermediate products.