Internal interaction between chemically-pretreated polypropylene microplastics and floc growth during flocculation: Critical effect on floc properties and flocculation mechanisms

• The type and concentration of chemical for pretreatment altered PP properties. • Different floc growth features were observed for various chemically-pretreated PP. • PP-containing floc growth was partially controlled by charge neutralization. • Chemical pretreatment affected the involvement of HA into PP-containing flocs. • Microplastic removal process is sensitive to their physicochemical properties. Microplastic pollution in drinking water resources has attracted increasing concerns due to the potential harm of microplastic particles to the health of either human or other living creatures. To better understand how the presence of microplastic contaminants would affect floc morphological properties and associated flocculation mechanism, various chemically-pretreated polypropylene (PP) microplastics were firstly produced by exposing pristine ones to HCl or NaOH solution at two concentrations of 1 and 5 mol/L. SEM and FTIR analyses demonstrated that microplastic physicochemical properties (including surface morphological details and oxygen-containing functional groups) were related to the type and concentration of chemical reagent for pretreatment. When flocculating model PP-containing water samples with coagulant of polymeric ferric sulfate (PFS), the involvement of PP microplastics after NaOH-based pretreatment was found more beneficial for the formation of larger floc aggregates during flocculation than those after HCl-based pretreatment at a fixed chemical-reagent concentration, whereas for either chemical pretreatment, PP microplastics produced at a higher solution concentration seemed to facilitate the process of larger PP-containing floc growth. Also, the structure, strength and recovery ability of resultant floc aggregates at steady state of flocculation appeared quite different, possibly attributed to distinct interaction patterns between preformed floc aggregates and microplastic particles, under all circumstances of chemically-pretreated PP microplastics investigated. Furthermore, it was suggested that the growth of PP-containing flocs during flocculation was partially determined by not only charge neutralization, but also the incorporation of humic acid (HA) molecules into PP-containing floc aggregates via the formation of PFS-HA and/or PFS-HA-PP complexes.