Unravelling the photocatalytic degradation of polyethylene microplastics with TiO2 under UV light: evidence from kinetic studies

Abstract

The accumulation of plastic debris in water bodies requires urgent remediation. Significant research efforts are being made for the effective capture of microplastics (MPs), but a technology to achieve the complete degradation is still a must. Photocatalysis of MPs mediated by the action of semiconductor materials under light irradiation deserves attention due to its advantageous environmental and energetic characteristics. Here, we aim to contribute to this field by a detailed experimental analysis of the photocatalytic degradation of polyethylene (PE) microspheres, selected as target MPs, with TiO2 P25 as the photocatalyst. Dark and photolysis experiments do not achieve significant degradation yields. Additionally, under the experimental conditions and after 8 h of treatment the photocatalytic degradation is evidenced by: i) gravimetric analysis that resulted in a 34.0 ± 1.4 % of mass loss, ii) chemical transformation quantified through a 58.5 % increase in the carbonyl index, and iii) particle size reduction, from an initial size of 234 ± 12 μm to an average diameter of 185 ± 13 μm. Furthermore, changes in the dissolved organic carbon, detection of short-chain acids in solution and CO2 in the gas phase, confirm the degradation of the initial MPs. It is noteworthy that the implementation of consecutive cycles contributes to increase the degradation yield, increasing the mass loss to 54 % after 5 cycles. Photocatalysis phytotoxicity analysis in different water matrices demonstrates that treated water is suitable for irrigation. This research represents a step forward in the analysis of the photocatalytic degradation of MPs in water.