Following heavy metal pollution caused by thallium in watersheds, people typically employ emergency treatment methods such as water sampling and transfer for dilution or in situ coagulation and precipitation. However, the thallium that is adsorbed by the precipitates in the sediment persists for a long time and is gradually released, posing a significant threat to the ecosystem. In this study, the 16S rRNA sequencing method was used to simulate the effects of water dilution or in situ coagulation and precipitation on microbial communities through thallium impact loading and thallium-containing iron floc shaking bottle experiments. The emendation of Fe(III) floc led to an increase in the relative abundance of Actinobacteriota. Meanwhile, Nitrospira and Proteobacteria exhibited distinct tolerances to Tl shock and Tl floc stress, respectively. Thallium pollution inhibited the reduction in nitric oxide and nitrogen fixation while increasing the relative abundance of the napA/B genes and decreasing the relative abundance of narG/H genes involved in nitrate reduction. This study offers new insights into the effects of various emergency treatment measures on river ecosystems following sudden thallium pollution, particularly from the perspective of microbial community composition and biogeochemical cycles.

In situ coagulation is regarded as the most effective measure in response to the frequent metal spills in China. Excessive coagulant is often used in pursuit of extremely high removal rates of contaminants. Yet the secondary ecological impact of the iron-containing coagulation flocs left on the river sediments after emergency response is still unclear. In the current study, we investigated the impact of flocs derived from three different iron-based coagulants, polymeric ferric sulfate (PFS), polymeric ferric chloride (PFC), and ferric chloride (FeCl₃), on microbial communities in sediment based on microcosm experiments. Metagenomics, quantitative PCR, and determination of ammonia oxidation potential were adopted to elucidate community shifts. The results indicate that the community structure and function of microorganisms in sediments have been affected, especially processes and species related to nitrogen cycling, and the effect was coagulant-specific. Flocs retrieved from FeCl₃ caused a more pronounced decline in diversity, shifts in community composition, and decreased potential ammonia oxidation. Ammonia-oxidizing archaea (AOA) was more sensitive to iron-containing flocs than ammonia-oxidizing bacteria (AOB), while PFS-flocs tended to reduce multiple genes involved in nitrate reduction. This indicates that the pre-polymerization of inorganic coagulants may be the primary factor leading to different microbial ecological effects. Sulfate, on the other hand, may affect specific biogeochemical processes due to its competition for electron donors. Our results confirmed that even without heavy metals as contaminants, coagulant flocs alone could present an effect on nitrogen cycling in sediments. The results will provide a scientific basis for environmental emergency decision-making: in emergency response to metal pollution incidents, the use of coagulants should be limited to only the necessary level.