Birnessite-mediated simultaneous remediation of lead and benzo[a]pyrene co-contaminated soils

It is currently challenging to remediate soils co-contaminated by heavy metals and polycyclic aromatic hydrocarbons. Birnessite is a naturally ubiquitous manganese oxide mineral with strong oxidation and adsorption capacities, but its specific roles in pollutant transformation and interfacial interaction within co-contaminated systems remain elusive. This study investigated the simultaneous remediation of lead (Pb) and benzo[a]pyrene (BaP) in soils by birnessite through incubation experiments and density functional theory calculation. Birnessite treatment decreased CaCl₂- and toxicity characteristic leaching procedure-extractable Pb content by 64.3% and 86.6% and reduced the BaP content by 33.8%. Mechanistically, Pb immobilization was primarily driven by spontaneous adsorption, including ion exchange and surface complexation, which facilitated Pb transformation into Fe-Mn oxide-bound fractions. Concurrently, BaP removal occurred via a synergistic pathway involving reactive species and electron transfer processes. Increasing dosage of birnessite promoted Pb immobilization, but had little effect on BaP oxidation. Moreover, the co-existing Pb affected birnessite-mediated BaP adsorption and oxidation by promoting the formation of [BaP-Pb]²⁺ and [BaP-Pb(H₂O)]²⁺ complexes via cation-π interactions. These complexes were more preferentially adsorbed on birnessite compared with BaP molecules, but exhibited higher electron transfer barriers. The findings provide critical insights into the remediation of co-contaminated soils and the fate of co-existing contaminants.