Alkaline aging significantly affects Mn(II) adsorption capacity of polypropylene microplastics in water environments: Critical roles of natural organic matter and colloidal particles

Most microplastic particles may undergo various aging in water environments. In this work, surface physico-chemical properties were firstly compared among pristine polypropylene (PP-pris) microplastics, and two aged ones obtained after pretreated with HCl (PP -acid) and NaOH (PP-alka). When compared with PP-pris and PP -acid, PP-alka had a much stronger Mn(II) adsorption capacity. The results regarding the role of natural organic matter and colloidal particle concentrations on adsorption demonstrated that for water solutions either containing kaolin or not, humic acid (HA) had significantly negative influence on Mn(II) adsorption capacity of PP-alka due to their complexation and competition effects, and its negative influence became enhanced with increasing kaolin concentrations. Besides, established conceptual models of adsorption were applied to comprehensively explore adsorption mechanisms of PP-alka for Mn(II) in the coexistence of HA and kaolin. An important sug-gestion was that in complicated adsorption-reactor system, great numbers of microplastics-kaolin hetero-aggregates might be formed via ion bridging of Mn(II) and/or polymer bridging of HA. So these formed aggregates were possible to re-organize themselves, under pre-set vibration-speed conditions, for achieving a more stable structure. As a consequence, Mn(II) adsorption behaviors would be affected by changes in steric-hindrance effects of HA molecules and surface charge distribution of resultant heteroaggregates.

Automated summary

The study compares the surface properties of different treated polypropylene microplastics and finds that NaOH-treated microplastics have a stronger manganese adsorption capacity. The study also investigates the effects of natural organic matter and colloidal particles on adsorption and establishes a conceptual model to understand the adsorption mechanism of microplastics in complex environments.