Toxicity and fate of cadmium in hydroponically cultivated lettuce (Lactuca sativa L.) influenced by microplastics

Although more attention has been paid to microplastics (MPs) pollution in environment, research on the synthetic influence of microplastic and heavy metals remains limited. To help fill this information gap, we investigated the adsorption behavior of virgin polyvinyl chloride microplastics (PVCMPs) (≤450 µm white spherical powder) on cadmium (II). The effects on seed germination, seedling growth, photosynthetic system, oxidative stress indicators of lettuce, and changes in Cd bioavailability were evaluated under Cd²⁺ (25 μmol/L), PVCMPs (200 mg/L), and PVCMP-Cd combined (200 mg/L + 25 μmol/L) exposures in hydroponic system. The results demonstrated that the PVCMPs effectively adsorbed Cd ions, which validated by the pseudo-second-order kinetic and the Langmuir isotherm models, indicating the sorption of Cd²⁺ on the PVCMPs was primary chemisorption and approximates monomolecular layer sorption. Compared to MPs, Cd significantly inhibits plant seed germination and seedling growth and development. However, Surprising improvement in seed germination under PVCMPs-Cd exposure was observed. Moreover, Cd²⁺ and MPs alone or combined stress caused oxidative stress with reactive oxygen species (ROS) including H₂O₂, O^2- and Malondialdehyde (MDA) accumulation in plants, and substantially damaged to photosynthesis. With the addition of PVCMPs, the content of Cd in the leaves significantly (P<0.01) decreased by 1.76-fold, and the translocation factor and Cd²⁺removal rate in the water substantially (P<0.01) decreased by 6.73-fold and 1.67-fold, respectively in contrast to Cd²⁺ stress alone. Therefore, it is concluded the PVCMP was capable of reducing Cd contents in leaves, alleviating Cd toxicity in lettuce. Notably, this study provides a scientific foundation and reference for comprehending the toxicological interactions between microplastics and heavy metals in the environment.