The phenomenological mass transfer kinetics model for Sr²⁺ sorption onto spheroids primary microplastics

In this paper, the equilibrium and mass transfer kinetics of Sr²⁺ sorption onto 3 types of microplastics, including polyethylene terephthalate (PET), polyethylene (PE), and polyvinyl chloride (PVC) were investigated. A novel film-pore mass transfer (FPMT) model was developed and used to study the sorption kinetics and mechanisms. This model can be used to describe the external mass transfer (EMT) and the internal mass transfer (IMT) processes and to calculate the diffusion rate. The FPMT model could successfully predict the kinetics data of Sr²⁺ sorption onto microplastics. The maximum value of the EMT rate achieved at the beginning of sorption was 103 μg g^-1·h^-1 for PET, 247 μg g^-1·h^-1for PE, and 854 μg g^-1·h^-1 for PVC, and then it decreased dramatically with time. The IMT rate was far less than the EMT rate, and decreased slowly with time. The overall sorption rate of Sr²⁺ onto microplastics was controlled by the external mass transfer step.