Adsorption of PCBs on microplastics mitigated greenhouse gas emission by changing C/N metabolism in freshwater sediment

Microplastics (MPs) and polychlorinated biphenyls (PCBs) might coexist in freshwater sediments. However, the effect of the adsorption of PCBs on MPs on greenhouse gas emissions and microbial communities is still not clear. In this paper, incubation experiment was conducted by pre-treating different particle sizes of MPs with and without adsorption of PCBs into the sediments for 30 days. The results showed that the coexistence of MPs and PCBs suppressed carbon dioxide (CO2) emissions. The treatment group without adsorption promoted methane (CH4) emissions by increasing the functional gene of mcrA, mcrB, mcrG, mtbC, and mtmC, while the adsorption attenuates this effect. At the same time, the coexistence of the two pollutants reduced alpha diversity in the sediment, especially when the MPs were small in size and in that with adsorbed PCBs. The principal component analysis showed that the pre-adsorption of PCBs on MPs reduced the differences among microbial communities as compared to that without pre-adsorption. It was also found that the coexistence of MPs and PCBs affected carbon (C) and nitrogen (N) metabolic pathways in the sediment, especially, CH4 metabolism, and increased norC content with the potential to increase nitrous oxide (N2O) emissions. The results suggest that sediment pollution with MPs and PCBs should be paid attention, which is important in affecting greenhouse gas emission and achieving sustainability.