Vegetative biofiltration systems for heavy metal removal: a PRISMA-based systematic review of global performance and...

Vegetative biofiltration systems are increasingly promoted as nature-based solutions for wastewater treatment; however, their performance varies widely across system configurations, substrates, plant assemblages, and environmental conditions. No recent synthesis has simultaneously covered multiple system types, diverse wastewater contexts, and multiple performance determinants within a single quantitative framework. A PRISMA-based systematic review was conducted to analyze heavy metal removal across 214 experimental cases from 32 studies. Descriptive, inferential, and multivariate analyses were used to identify performance patterns and underlying determinants. Removal efficiencies differed markedly among metals, reflecting differences in physicochemical properties, redox behavior, and substrate affinity. Fe and Mn consistently showed high removal, driven by oxidative precipitation in root zone aerobic microsites. In contrast, Ni, Se, and Co remained poorly treated, owing to their high aqueous mobility and weak complexation with common substrates. System configuration and substrate composition emerged as the strongest predictors, with subsurface flow, vertical-flow, and hybrid systems consistently outperforming unstructured designs. Plant species contributed through radial oxygen loss, root biomass development, rhizosphere microbial stimulation, and metal bioaccumulation, amplifying efficiency within systems where hydraulic configuration and substrate chemistry already provided favorable conditions. Environmental factors including pH, climate, and metal speciation further modulated treatment outcomes. Experimental scale, influent concentration, and study duration were identified as contextual sources of methodological heterogeneity. This review provides an integrated, quantitative, and multi-determinant synthesis of heavy metal removal performance across the full diversity of vegetative biofiltration configurations and wastewater types, revealing the hierarchical structure of performance determinants and proposing evidence-based system archetypes to guide design and policy decisions.