Thermodynamic mechanisms of membrane fouling during filtration of alginate solution in coagulation-ultrafiltration (UF) process in presence of different ionic strength and iron(III) ion concentration

Abstract

Effects of iron(III) ion level and ionic strength on membrane fouling in coagulation-ultrafiltration (UF) process were explored in this study. Two interesting filtration phenomena during the filtration process were observed: extremely reduced specific filtration resistance (SFR) of alginate solution complexed with iron (III) ions level from 0.1 to 2.0 mM (phenomenon I), and different change trends of alginate SFR with ionic strength between two iron(III) ion levels (remarkable increase trend of alginate SFR with ionic strength at 0.1 mM ions level while almost remaining unchanged trend at 2.0 mM ions level) (phenomenon II). Experimental characterizations indicated that the two phenomena were closely linked to changes of zeta potential, viscosity and morphology of the related foulants. It was proposed that iron(III) ions at low level (0.1 mM) preferentially coordinate the terminal carboxyl groups of alginate chains and form gel layer. The chemical potential gap described by Flory-Huggins lattice theory is responsible for the extremely high SFR of gel layer. Coordinating the non-terminal carboxyl groups at high iron(III) ion level (2.0 mM) causes transition from gel layer to cake layer formation during filtration, and lots of interstices in cake layer correspond to low SFR, well interpreting phenomenon I. Meanwhile, electrostatic double layer compression caused by the increased ionic strength makes the gel layer shrunk and denser for low iron(III) ion level, but less affects the number and size of interstices in the cake layer for high iron(III) ion level, reasonably explaining phenomenon II. This study revealed essential and comprehensive thermodynamic fouling mechanisms and facilitated to optimize coagulation-UF/MF process.