Facilitated transport of Cd(II) by plasma-modified microplastics: Influence of surface functional groups and interaction mechanisms

Discharge plasma is gaining increasing attention in environmental science due to its advanced oxidative properties. While discharge plasma is capable of degrading most of the pollutants in contaminated water, it is not effective in degrading the accompanying microplastics (MPs). Notably, residual MPs in plasma-treated water may become carriers of other pollutants in the environment. The adsorption and co-transport behaviors of two MPs with Cd(II), before and after Dielectric Barrier Discharge (DBD) plasma treatment, were examined via batch and column experiments. The results showed that plasma-treated MPs exhibited significantly enhanced adsorption capacity for Cd(II), attributed to grafted polar functional groups on their surfaces. Density Functional Theory (DFT) calculations revealed that the enhancement resulted from reduced steric hindrance, along with stronger hydrogen bonding and van der Waals interactions. Co-transport experiments revealed that plasma treatment significantly promoted the carrier role of MPs in facilitating Cd(II) transport. Specifically, plasma-treated MPs mitigated Cd(II) deposition at the column base under favorable conditions for transport. Derjaguin-Landau-Verwey-Overbeek (DLVO) theory provided a satisfactory explanation for the observed transport behaviors of plasma-treated MPs and Cd(II). These findings highlight the potential environmental risks posed by plasma-treated MPs as carriers of Cd(II) and the need for further studies on their long-term ecological impacts.