Irradiation and DOM mediate lead release from polyvinyl chloride microplastics in natural surface water

Polyvinyl chloride (PVC) is a widely used plastic, but the potential risk of heavy metal additive release from PVC microplastics (MPs) has not been fully explored. This study evaluates the release of lead (Pb) from recycled PVC MPs under natural conditions. The released Pb concentration in the dark was 1079.5-1109.7, 551.4-571.6, and 374.1-433.0 μg/L in agricultural, sea, and river/lake water, respectively. In contrast, the Pb release was markedly inhibited by 34.1-59.1 % under irradiation. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), fluorescence emission-excitation matrix (EEM) spectroscopy, and Mantel test revealed that the dissolved organic matter (DOM), especially lignin/carboxyl-rich acyclic component (41.8-84.8 %), can complex with Pb to facilitate its release from PVC in the dark. However, the Pb released upon irradiation was first promoted and then inhibited. The promotion ascribed to the broken of MPs by reactive intermediates (RIs) including DOM* , O, and OH [(0.5-12.6) × 10, (1.2-10.9) × 10 and (0.1-8.9) × 10 mol/L, respectively]. The inhibition was attributed to two reasons: the photobleaching of DOM reduced Pb dissolution complexed by DOM; the increase of oxygen-containing functional groups enhanced the Pb adsorption on the surface of MPs. In addition, the Pb released from PVC MPs significantly inhibits the growth of Nannochloropsis sp. in seawater. These findings reveal the complicated release mechanism of Pb from MPs under environmental conditions.