Effect of microplastic aging degree on filter cake formation and membrane fouling characteristics in ultrafiltration process with pre-coagulation

Microplastic aging has gained more and more attentions in the field of environmental studies. However, it is not clear yet how aged microplastics possessing different aging degrees would affect membrane filterability and associated fouling mechanisms during ultrafiltration of pre-coagulated microplastic-containing waters. Here, four chemically-aged polypropylene (PP) microplastics were individually prepared by immersing pristine PP into either HCl or NaOH solution at two prearranged concentrations (1 and 5 mol/L). The aging degree of each chemically-aged PP was reflected by microplastic surface physicochemical changes after artificially-accelerated aging. All pre-coagulation and ultrafiltration experimental results demonstrated that microplastic aging degree indeed had a critical effect on permeate flux decline and membrane resistance distribution, along with microscopic characteristics of fouled cake layer. These expected effects were mainly induced by the involvement of different chemically-aged PP in microplastic-containing floc growth and subsequent filter cake formation. For HCl-based aging, aged microplastic particles produced at the higher concentration (5 mol/L) appeared to play a stronger skeleton role in filter cake formation than those obtained at 1 mol/L, giving an enhanced filterability of ultrafiltration membrane. In contrast, for the two cases of NaOH-based aging, the lower reagent-solution concentration corresponded to a rather slighter permeate flux decline during ultrafiltration, possibly arising from the predominated role of humic acid molecules in membrane pore blocking/narrowing fouling and steric hindrance effects.