Positive and negative effects of nanoscale zero-valent iron-enriched biochar on sulfamethoxazole remediation in contaminated soil

This study prepared surface-modified biochar, including acid washing biochar (HBC) and biochar supported with nanoscale zero-valent iron (nZVI-HBC). The surface-modified biochar was added to sulfamethoxazole (SMX)-contaminated soil with and without earthworms to examine the effects of surface-modified biochar and/or earthworms (Eisenia fetida) on the levels of SMX and its relevant genes (sul1, sul2, and intI1) in the soil. Additionally, the joint toxicity of these exogenous substances on earthworms was investigated. The results showed that although earthworms significantly enhanced the dissipation of SMX in the soils with and without HBC, this effect was not observed in the soil with nZVI-HBC. Among all treatments, nZVI-HBC most effectively accelerated SMX dissipation in the soil, regardless of coexisting earthworms. However, the presence of earthworms significantly increased the total relative abundances of sul1, sul2, and intI1 in the soil. A reasonable explanation for this is the shift in the bacterial community composition rather than the residual level of SMX. When earthworms coexisted, the richness of Proteobacteria evidently increased, which was the main host of the above genes. Both HBC and nZVI-HBC decreased these genes in the soil with earthworms, which was mainly due to the decrease in host genera from Proteobacteria, Actinobacteria, and Gemmatimonadetes. Although there was toxicity of single-surface-modified biochar or SMX on earthworms, the synergistic interaction of surface-modified biochar and SMX resulted in the most serious histopathological changes in earthworms and their highest superoxide dismutase activity. •Earthworms enhanced sulfamethoxazole (SMX) dissipation in soil without and with HBC.•nZVI-HBC most effectively decreased SMX in soil regardless of coexisting earthworms.•Earthworms increased the total relative abundance of sul1, sul2 and intI1 in soil.•Earthworms increased Proteobacteria that was mainly antibiotic resistance host in soil.•Joint toxicity of modified biochar and SMX to earthworms was stronger than single one.