How toxic and hazardous pollutants influence the treatment performance of constructed wetlands: A review of pathways and mechanisms.

Abstract

With the growing global recognition of nature-based solutions, constructed wetlands (CWs) have been increasingly applied for advanced wastewater treatment. However, their design primarily targets conventional pollutants, whereas the implications of toxic and hazardous pollutants (ThPs), such as antibiotics, heavy metals, and microplastics, for long-term ecological stability remain poorly understood. Based on 111 publications, this review evaluates the removal characteristics of organic, inorganic, and particulate toxic pollutants in CWs, their effects on the removal of conventional pollutants, and the underlying responses of associated biotic and abiotic components. The highest median removal efficiency was generally observed for particulate toxic pollutants, followed by inorganic and organic pollutants. However, high removal efficiency does not necessarily eliminate the ecological risks due to pollutant accumulation in substrates and biofilms. TP removal was identified as a relatively sensitive functional indicator under complex pollution conditions, with declines of 14.0%, 16.3%, and 16.2% under organic, inorganic, and particulate toxic pollutant stresses, respectively. Mechanistic synthesis suggests that organic pollutants primarily disrupt microbial metabolism and nutrient transformation via bio-toxic stress and metabolic reallocation. In contrast, inorganic pollutants influence microbial functions through adsorption competition, shifts in pollutant speciation, and oxidative stress. Particulate pollutants alter pollutant migration by modifying pore structures, reorganizing biofilms, and limiting mass transfer. These multiscale interfacial disturbances reduce plant-substrate-microbial functional stability. Accordingly, for CW optimization, focus should shift from pollutant removal enhancement toward maintaining ecological function through multifunctional substrate utilization, microbial regulation, rhizosphere engineering, and intelligent operation strategies.