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

The production of ·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 ·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 ·OH generation during the redox transformation of humic acids (HAs) and the contribution of this ·OH to the transformation of MPs were investigated for the first time. It was found that ·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. O2·− and H2O2 were identified as the key intermediate species, which were produced by both microbial aerobic respiration and HA oxidation. The ·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 ·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 ·OH formation and MPs transformation through microbially driven humus redox cycles in alternating anoxic-oxic environments.