Multi-phase fate and removal mechanism of acetamiprid in a planted coastal wetland mesocosm system: A 'dual-engine' perspective.

Acetamiprid (ACE), a widely used neonicotinoid insecticide, poses persistent ecological risks to coastal ecosystems. However, its multi-media transformation pathways and the underlying biological drivers in coastal wetlands remain poorly understood. Here, using high resolution mass spectrometry (HRMS) based non-target screening integrated with Partial Least Squares Path Modeling (PLS-PM), we systematically investigated the ACE migration, biotransformation, and removal mechanisms in a mesocosm wetland system planted with three salt-tolerant hydrophytes: Phragmites australis (PA), Iris pseudacorus (IP), and Scirpus tabernaemontani (ST). Integration of hydrophytes enhanced total ACE removal efficiency by 35.6% - 40.6% compared to the unplanted control, with ST achieving the highest total removal (70.0%). Mass balance analysis revealed that sediment acted as the primary reaction zone, whereas plants primarily functioned as biological mediators rather than direct pollutant sinks. A media-partitioned "dual-engine" transformation network was identified as the dominant mechanism driving ACE dissipation. In the water phase, hydrophyte-induced alkalinization coupled with solar radiation triggered initial side chain cleavage. Concurrently, in the rhizosphere sediment, the accumulation of low molecular weight organic acids (LMWOAs), particularly acetic acid, which reached 54.74 μg/g in the ST group, served as a key biochemical signal, selectively enriching bacterial taxa such as Flavobacterium and Lysobacter. Notably, while the N-demethylation product N-desmethyl-acetamiprid (IM-2-1) exhibited environmental persistence, the hydrophyte rhizosphere successfully facilitated further transformation along the cyano-hydrolysis pathway (N-methyl-(6-chloro-3-pyridyl)methylamine [IM-1-4] → 6-chloronicotinic acid [IC-0]). This integrated system achieved substantial parent compound dissipation coupled with declining intermediates, indicating deep biotransformation potential. Collectively, this study provides a comprehensive mechanistic framework for pesticide mitigation in coastal wetlands, offering theoretical guidance for plant selection and rhizosphere-based biostimulation in nature-based remediation.