Degradation of Emerging Plastic Pollutants from Aquatic Environments Using TiO2 and Their Composites in Visible Light Photocatalysis

Abstract

This review synthesized the current knowledge on the effect of TiO₂ photocatalysts on the degradation of microplastics (MPs) and nanoplastics (NPs) under visible light, highlighting the state-of-the-art techniques, main challenges, and proposed solutions for enhancing the performance of the photocatalysis technique. The synthesis of TiO₂-based photocatalysts and hybrid nanostructured TiO₂ materials, including those coupled with other semiconductor materials, is explored. Studies on TiO₂-based photocatalysts for the degradation of MPs and NPs under visible light remain limited. The degradation behavior is influenced by the composition of the TiO₂ composites and the nature of different types of MPs/NPs. Polystyrene (PS) MPs demonstrated complete degradation under visible light photocatalysis in the presence of α-Fe₂O₃ nanoflowers integrated into a TiO₂ film with a hierarchical structure. However, photocatalysis generally fails to achieve the full degradation of small plastic pollutants at the laboratory scale, and its overall effectiveness in breaking down MPs and NPs remains comparatively limited.

Keywords: TiO2; aquatic environment; degradation; micro/nanoplastic; visible light photocatalysis.

Figure 1

The number of papers published from 2015 up to now on degradation of MPs and NPs according to ScienceDirect platform. (a) Different degradation methods; (b) photocatalytic degradation. Key words: “degradation of micro and nanoplastics”, “advanced oxidation processes”, “biodegradation”, “photodegradation”, “magnetic degradation”, “electrochemical degradation”, “photocatalysis”, “photocatalysis by TiO₂”.

Figure 2

Degradation of MPs and NPs through photocatalysis under visible light. Reproduced from [77] with permission from Elsevier.

Figure 3

XRD patterns and FE-SEM micrographs of the sol–gel (a) and protein-derived N-TiO₂ semiconductors (b). The two sets of labels mean XRD and SEM, respectively. Reproduced from [89] with permission from Elsevier.

Figure 4

FEG-SEM micrographs of TS-ME (a–d) and TS-MG (e–h) photocatalysts. Reproduced from [93] with permission from Elsevier.

Figure 5

(a) XRD patterns of the samples. (b) Locally amplified XRD pattern of the sample. TEM images: (c) TiO₂/WPT-AC, (d) g-C₃N₄, (e) g-C₃N₄/TiO₂/WCT-AC. Red circles highlights the presence of TiO₂ and TiO₂/WCT-AC in composites. Reproduced from [90] with permission from Elsevier.

Figure 6

High-resolution XPS spectra of (a) g-C₃N₄/TiO₂/WCT-AC photocatalyst material, (b) C 1s, (c) N 1s, (d) O 1s, and (e) Ti 2p. Reproduced from [90] with permission from Elsevier.

Figure 7

The highest mass loss % for the degradation MPs/NPs by using TiO₂ and TiO₂ composite photocatalysts under visible light.

Figure 8

Degradation plots of MPs after photocatalytic experiments conducted at pH values of 3, 7, and 11, and temperatures of 2, 20, and 40 °C under 50 h irradiation. Degradation was expressed in terms of (a) initial concentration (C₀) and final concentration (C) of MPs at specific time, and (b) mass loss of MPs. Reproduced from [38] with permission from Elsevier.

Figure 9

The effect of light intensity and temperature on PS degradation using α-Fe₂O₃/TiO₂HNTAs catalyst. Reproduced from [95] with permission from Royal Society of Chemistry.

Figure 10

Proposed photocatalytic mechanism for doped TiO₂ semiconductor. Adapted from [106].

Figure 11

Efficiency of PET NP degradation using TiO₂-MIL-100(Fe) composite photocatalyst compared with TiO₂ photocatalyst under the optimum process conditions. Adapted from [92].

Figure 12

Heterojunction type II (a) and S-scheme (b). Adapted from [83].