Abstract: The presence of dissolved organic matter (DOM) affects the stability of zinc oxide nanoparticles (ZnO NPs) in water. By constructing vertical subsurface constructed wetlands, taking humic acid (HA) as a representative substance of DOM, we examined the effects of coexistence of 1 mg·L^-1 ZnO NPs and different concentrations of HA (1, 5, 10, 20 and 30 mg·L^-1) on the treatment performance, clogging and microbial communities of constructed wetlands. The results showed that low HA concentration (≤1 mg·L^-1) could promote the agglomeration of ZnO NPs, while the mediumhigh concentrations (1-25 mg·L^-1) showed opposite effect. The coexistence of ZnO NPs and DOM would reduce the purification effect of constructed wetlands. At the concentration of 1 mg·L^-1  HA, the removal efficiencies of COD, ammonia nitrogen and total nitrogen decreased by 18.07%, 19.94%, and 26.37%, respectively. Under the condition of 1 mg·L^-1 DOM and 1 mg·L^-1 ZnO NPs, the constructed wetland was most seriously blocked, and the permeability coefficient decreased by 67.5% after 60 days. The coexistence of HA and ZnO NPs could affect the secretion of extracellular polymeric substance (EPS). The EPS concentration was as high as 140.30±0.29 mg·g^-1 at 1 mg·L^-1 ZnO NPs and 1 mg·L^-1 HA, while that in the blank group was only 78.67±0.32 mg·g^-1, explaining the clogging of constructed wetlands. In addition, the addition of ZnO NPs and HA reduced the richness and diversity of microbial community and promoted the growth of EPS-secreting bacteria (Thauera). The findings may provide a new explanation for the clogging mechanism of constructed wetlands.

Key words: dissolved organic matter, zinc oxide nanoparticles, constructed wetland, clogging.