Residual mulch-film characteristics affect heavy metal migration of different soil layers in the subtropical croplands of China.

In recent years, as the abundance of residual mulch film (RMF) in agricultural soil continues to increase, whether the adsorption capacity of its surface affects the migration of heavy metals is a topic of current interest for scholars. Herein, this study investigated the distribution of RMF abundance and metal concentration in different soil layers of 75 plastic-mulching croplands in subtropical China; meanwhile, we also explored the associations of RMF characteristics with metal concentration. The results showed that land type, film mulching amount, and film mulching time were the main factors affecting RMF abundance, distribution, and particle size composition. The highest abundance of RMF was found in the garden soils (910 n·kg-1) with more than 15 years mulching period and more than 19.5 kg hm-2 of annual mulch amount. The lowest abundance of RMF was occurred in the group of field and conservation agricultural land (237 n·kg-1). Moreover, the concentrations of metals in soil, especially Cd, Cr, Cu, and Pb, were closely related to the extent of RMF contamination in the soil environment. In the 0-10 cm and 10-20 cm soil layers, microplastic abundance exhibited a negative correlation with Cr and Cu concentrations and a positive correlation with Pb concentration. Based on the above findings, it is demonstrated that RMF significantly influences the mobility of metals in soil via adsorption processes, with potential synergistic effects between RMF and heavy metals posing a heightened risk to the soil environment.

The types of microplastics, heavy metals, and adsorption environments control the microplastic adsorption capacity of heavy metals.

Anthropogenic development has released large amounts of microplastics (MPs), which are carriers of migratory heavy metals, into the environment, and heavy metal adsorption by MPs may have strong combined toxic effects on ecosystems. However, until now, a comprehensive understanding of the factors influencing these adsorption capacities of MPs has been lacking. Thus, we used 4984 experimental data points to systematically assess the factors influencing the adsorption strength of 8 types of MPs on 13 types of heavy metals. We found that (1) the types of MPs, heavy metals, and adsorption environments significantly impacted the heavy metal adsorption capacities of MPs; (2) polyvinyl alcohol (PVA) showed a higher adsorption capacity for lead (Pb) and cadmium (Cd) than did other MPs, by 2810.62 mg/kg and 2732.84 mg/kg, respectively; (3) the adsorption capacities of MPs for heavy metal were regulated by multiple variables, with heavy metal concentration, MP quality, solution amount, adsorption time, and pH being the most important; and (4) MPs had a higher adsorption capacity in aquatic environments (except for seawater) than which in soil environments. Overall, our study clearly showed that the types of heavy metals, adsorption environments, and MPs influenced the heavy metal adsorption capacities of MPs and may exacerbate their combined environmental toxicity, which would help better characterize the severity of MP pollution.