Speciation-dependent bioaccessibility of lead in e-waste dismantling soils: Synchrotron spectroscopy reveals...

Lead (Pb) pollution is prominent in electronic waste (e-waste) disposal sites. While Pb bioaccessibility depends critically on its molecular speciation, the quantitative relationship between Pb speciation and bioaccessibility across different physiological compartments remains poorly understood. This study characterized soil Pb speciation and bioaccessibility in soil samples from an abandoned e-waste dismantling site through integrated assessment of synchrotron X-ray absorption near-edge structure (XANES) spectroscopy, compartment-specific bioaccessibility testing, sequential chemical extraction, and multivariable statistics. Results revealed extreme heterogeneity in total Pb concentration (28-9459 mg kg^-1) and gastric bioaccessibility (2.8-74.4%), yet total Pb concentration was a poor predictor of bioaccessibility. Synchrotron XANES spectroscopy, which directly identifies Pb molecular species and coordination environments, revealed the controlling factors: mineral-sorbed and organic-bound Pb were highly bioaccessible (r = 0.87, p < 0.01), while Pb phosphates and PbSiO₃ were essentially inert (r = -0.88, p < 0.01). By contrast, traditional sequential chemical extraction failed to distinguish these mechanistic differences and did not reliably correlate with bioaccessibility. Bioaccessibility differed dramatically across physiological compartments: the gastric phase (pH 1.2) showed the highest bioaccessibility (43.3% mean), while the pulmonary phase (pH 7.4) and intestinal phase (pH 6.3) showed negligible bioaccessibility (<0.2%), reflecting pH-dependent speciation solubility. During gastrointestinal digestion, Pb species underwent dynamic transformation: acidic gastric conditions dissolved oxide-bound and organic-complexed Pb while promoting pyromorphite formation, whereas neutral intestinal conditions induced re-deposition of hydroxides and phosphates, stabilizing Pb. These findings demonstrate that molecular-level speciation directly controls Pb bioaccessibility and that synchrotron spectroscopy provides mechanistic understanding essential for interpreting soil Pb behavior.