Secondary pollution of microplastic hetero-aggregates after chlorination: Released contaminants rarely re-adsorbed by the second-formed hetero-aggregates

In urban waters, microplastics (MPs) usually form hetero-aggregates through adsorption of organics and microbes. However, the effects of hetero-aggregates on water quality are rarely reported. In this study we found that the hetero-aggregates, which accumulated contaminants, were like a ???time bomb???. Chlorination was able to trigger the ???time bomb??? through destruction of hetero-aggregates, lysis of microbial cells and elevation of the concentration of low-molecular-mass organics. Thereupon previously adhered organics desorbed from MPs, intracellular metabolites were released from lysed cells, and re-formation of hetero-aggregates was limited. This process rapidly increased the concentration of organics but prevented the re-adsorption of organics, which leads to secondary pollution. Thus, to alleviate the risks of secondary pollution caused by hetero-aggregates, the choice of oxidant species and dose should be optimized based on the characteristics of existent hetero-aggregates when purifying urban waters containing MPs.

Automated summary

In urban waters, microplastics form hetero-aggregates through adsorption and accumulation of contaminants. This study reveals that these hetero-aggregates act as a "time bomb" due to their vulnerability to chlorine treatment. The exposure to chlorine leads to the destruction of hetero-aggregates, release of previously adhered organics, and release of intracellular metabolites from lysed cells. This process rapidly increases the concentration of organics but prevents their re-adsorption, resulting in secondary pollution. To mitigate the risks of secondary pollution, the choice of oxidant species and dosage should be optimized based on the characteristics of the existing hetero-aggregates in urban waters containing microplastics.