Enhanced Transport of TiO₂ in Unsaturated Sand and Soil after Release from Biodegradable Plastic during Composting

Biodegradable plastics can reach full degradation when disposed of appropriately and thus alleviate plastic pollution caused by conventional plastics. However, additives can be released into the environment during degradation and the fate of these additives can be affected by the degradation process. Here, we characterized TiO₂ particles released from a biodegradable plastic mulch during composting and studied the transport of the mulch-released TiO₂ particles in inert sand and agricultural soil columns under unsaturated flow conditions. TiO₂ particles (238 nm major axis and 154 nm minor axis) were released from the biodegradable plastic mulch in both single-particle and cluster forms. The mulch-released TiO₂ particles were fully retained in unsaturated soil columns due to attachment onto the solid-water interface and straining. However, in unsaturated sand columns, the mulch-released TiO₂ particles were highly mobile. A comparison with the pristine TiO₂ revealed that the mobility of the mulch-released TiO₂ particles was enhanced by humic acid present in the compost residues, which blocked attachment sites and imposed steric repulsion. This study demonstrates that TiO₂ particles can be released during composting of biodegradable plastics and the transport potential of the plastic-released TiO₂ particles in the terrestrial environment can be enhanced by compost residues.