Transparent exopolymer particles-associated membrane fouling analyses of systems containing sodium alginate, calcium, iron, alum and their combination during dead-end ultrafiltration

Abstract

In this work, a serious of filtration experiments were designed and implemented to compare the effects of different metal cations (Ca2+, Fe3+, and Al3+) on fouling by sodium alginate. As indicated by the experimental results, these three types of cations all induced biphasic filtration behavior by 50 mg/L sodium alginate, that addition of these cations initially aggravated the fouling but then mitigated it upon further addition. The maximum specific filtration resistance (αav) increased in the following order: Al (34.5 ± 1.2 1014 mg/kg) < Ca (52.4 ± 2.5 1014 mg/kg) ≈Fe (54.1 ± 1.2 1014 mg/kg), but the corresponding critical concentration of the cation was in the order: Al3+ (0.02 mM) = Fe3+ (0.02 mM) < Ca2+ (0.05 mM). Increasing the metal cation concentration induced a higher transparent exopolymer particle concentration with the size being initially decreased but then gradually increased via the cation bridging effect, until the cation concentration reached up to 2 mM with the formation of metal hydroxides. So the fouling resistance mainly resulted from the dynamic equilibrium between concentrations and properties of transparent exopolymer particles with different sizes (0.05–0.1 μm, 0.1–0.2 μm, 0.2–0.4 μm and >0.4 μm). Similar biphasic fouling behavior also observed in the co-existence of Ca2+ and Fe3+/Al3+, but the comparable maximum αav values were obtained at much lower ion concentrations. Besides changes in the transparent exopolymer particle concentration, the hindered back transport of different sodium alginate molecules and steric effect also contributed to the synergistic fouling. Furthermore, there was little competition between the binding sites for divalent and trivalent ions, but the coiling behavior could be limited by changing the cation dosing sequence, leading to a weakened synergistic effect. This study links cation types, transparent exopolymer particle formations and fouling layer structures, with practical relevance for the hydraulic performance of low-pressure membranes, hopefully leading to their wider application in water treatment.