Degradation mechanism and properties of debris of photocatalytically degradable plastics LDPE-TiO2 vary with environments

Photocatalytically degradable plastics (PDP) are promising in mitigating plastic pollution due to their high degradability. Nevertheless, the unclear impact of environmental conditions on properties of debris for PDP and the knowledge gap in the mechanism from the perspective of photogenerated reactive species’ function hinder PDP's application. Here, low-density polyethylene (LDPE) containing TiO2 (LDPE-TiO2) was selected as representative PDP. The photocatalytic degradation performance and mechanism of LDPE-TiO2 in water without NaCl, water with NaCl, and air were studied. Compared with water, the air resulted in a higher carbonyl index and higher molecular weight of debris from LDPE-TiO2 whether the irradiation time was the same (120 h) or the mass loss of LDPE-TiO2 was the same (68 ± 2%). Chloride ions in water conditions resulted in CCl bond formation in the debris of LDPE-TiO2 suggesting generation of organochlorine substance. In the air the photogenerated electron (e−) contributed to LDPE-TiO2 film degradation, while in the water e− mainly underwent recombination with the photogenerated hole. In water with Cl−, HO‧ attacked LDPE via reaction with Cl− forming organochlorine substance. The effects of environmental conditions on the properties of debris and the functions of photogenerated reactive species for PDP degradation are reported for the first time in this study. The findings provide scientific support for the development of PDP technology.