Degradation of microplastics by hydroxyl radicals generated during microbially driven humus redox transformation

The production of center dot OH during transformation of redox active substances has been increasingly documented, and it causes the ageing or degradation of microplastics (MPs) in natural systems. However, the contribution of the humus redox cycle to center dot OH generation and MPs transformation has previously been overlooked, even though it is ubiquitous in alternating anoxic-oxic environments. In this work, the integrated pathways of center dot OH generation during the redox transformation of humic acids (HAs) and the contribution of this center dot OH to the transformation of MPs were investigated for the first time. It was found that center dot OH could be produced continuously during successive cycles of redox transformation of HAs mediated by Bacillus thermotolerans SgZ-8 through exogeneous HAs dependent and independent pathways. O-2(center dot-) and H2O2 were identified as the key intermediate species, which were produced by both microbial aerobic respiration and HA oxidation. The center dot OH generated by HA redox cycles could lead to a weight loss of PS-MPs of 18.1% through oxidative degradation during a period of 8 weeks of anoxic-oxic incubation. The EDC of HAs is closely related to center dot OH production, which could have a large influence on the effectiveness of oxidative degradation of PS-MPs during various HAs redox cycles in temporarily anoxic environmental systems. These findings provide new insights into center dot OH formation and MPs transformation through microbially driven humus redox cycles in alternating anoxic-oxic environments.

Automated summary

The formation pathways of ·OH during the redox transformation of humic acids (HAs) and its contribution to the transformation of microplastics (MPs) were investigated for the first time in this study. It was found that ·OH could be continuously produced through two pathways during the redox cycles and lead to oxidative degradation of MPs. Furthermore, the electrocatalytic activity of HAs was closely related to ·OH generation.