Exploring and comparing the roles of Ca2+ and Mg2+ in small-sized natural organics-induced charged nanofiltration membrane fouling

This study was conducted to systematically investigate how Ca2+/Mg2+ interacted with the preserved humic acid components after UF filtration (PHACUF) to cause fouling of nanofiltration (NF) membrane. Inherent characteristics including molecular weight distribution, charge, wettability, hydrodynamic size of PHACUF and its fractions under predetermined conditions and differences in PHACUF-induced NF membrane fouling behaviors, surface characteristics in the absence/presence of Ca2+/Mg2+ were analysed and explored, respectively. Microinteraction forces between PHACUF and Ca2+/Mg2+ were also compared. The results manifested that contrary to many typical reports about HA foulant, Mg2+ exhibited superior charge screening effect with PHACUF, and performance in accelerating hydrodynamic colloid size, dense membrane fouling compared to Ca2+. O 1s bonding energy shift and ITC data provided powerful and quantified evidences that Mg2+ could coordinate with PHACUF more effectively and cause more serious fouling deposits on NF membrane surfaces than Ca2+, presumably due to its favorable specific charge and binding ability. In terms of its own components, HOA fraction seems destined to preferentially bond with most of Ca2+ and Mg2+ ions and constitute a high proportion of membrane fouling loading because it possesses the highest charge density and content compared with other fractions in near-actual natural surface water circumstance.