Enhancing benzo[a]pyrene removal in constructed wetlands: A Ca-Fe layered double hydroxides/poplar sawdust-based...

CaFe-layered double hydroxides (LDHs) can effectively address the low degradation efficiency of polycyclic aromatic hydrocarbons (PAHs) in constructed wetlands (CWs). However, the lack of sustained electron donors often limits their long-term efficacy. To bridge this gap, CWs amended with pure CaFe-LDHs and CaFe-LDHs/poplar sawdust composite were established to assess their impact on benzo[a]pyrene (BaP) removal. Crucially, the poplar sawdust acted as a slow-release source of dissolved organic matter (DOM), providing sustained carbon and molecular bridges that accelerated interfacial electron transfer and strongly drove the Fe (III) and Fe (II) redox cycling of the LDHs. Driven by this synergistic mechanism, the composite substrate enhanced BaP removal efficiency by 7.3% and 16.6% compared to the pure LDHs and control groups, respectively. The continuous electron supply and DOM-mediated desorption reduced BaP retention in the solid substrate (11.5 ng/g) and facilitated plant interception, with belowground tissues (3.7 ng/g) accumulating significantly higher concentrations than aboveground parts (0.9 ng/g). Furthermore, alleviation of oxidative stress and sustained carbon supply reshaped the microbial community, evidenced by a 10.5% increase in species level and enrichment of genera associated with PAHs degradation and iron reduction (e.g., Pseudoxanthomonas, Geobacter, and Arthrobacter). Consequently, this optimized microenvironment significantly upregulated the expression of genes encoding ring-cleavage dioxygenases (catA, nahA) and stress-response enzymes (SOD). Our study elucidates the synergistic coupling of LDH-mediated iron cycling and DOM-driven metabolic processes promoted by lignin-based poplar wood within composite matrices, providing a low-cost and novel substrate-modification strategy for designing high-efficiency constructed wetlands targeting recalcitrant organic pollutants.