Microplastic contaminants and their adsorption behaviors of coexisting metal ions have received increasing attention. In this work, the interaction effects of initial pH, temperature, particle size and humic acid concentration on Mn(II) adsorption of aged PSMPs were comprehensively explored. The analysis results of Box–Behnken design (BBD) revealed that temperature and initial pH should be preferentially controlled. The pseudo-second-order model was more suitable for describing Mn(II) adsorption kinetics than pseudo-first-order one. The Langmuir model was found to better fit adsorption isotherm data. Accordingly, a hydrated functional zone was thought to be formed surrounding the microplastic surface; this zone was characterized as a sandwich-like structure involving inner and outer films together with their enclosed space. As adsorption proceeded, free Mn(II) ions first passed through the outer film from the bulk liquid via Brownian motion, and were further driven by the force field, arising near the inner film, to approach/occupy active adsorption sites on the microplastic surface. When no more adsorption sites were available, the adsorption process became slow until reaching equilibrium. Eventually, part of Mn(II) ions were bound within the enclosed space. This newly-established conceptual model was successfully applied to elucidate the influencing mechanisms of representative factors on Mn(II) adsorption.