Effect of iron on membrane fouling by alginate in the absence and presence of calcium

• Interplay between Ca and Fe influences fouling by alginate assemblages on membranes.
• Low proportions of Fe to alginate result in gel-like, impermeable fouling layers.
• Increased dosing of Fe salts induces permeable iron oxide-alginate cake formation.
• Prior Ca binding by alginate induces aggregate growth by inhibiting Fe-binding.
• Materials properties of alginate assemblages enable prediction of permeate flux.

While the effects of calcium addition on membrane fouling by alginate have been extensively investigated, there exists little research into the impact of iron concentrations on the fouling behavior of this model polysaccharide. In this paper, the bulk solution characteristics of Fe-alginate assemblages such as size and zeta-potential were determined as were the properties of the materials deposited on flat sheet membranes at different iron and alginate concentrations under steady-state constant pressure filtration conditions. Biphasic filtration behavior of Fe-alginate was observed with highly impermeable gels formed at low iron concentrations, and reduced fouling at high iron concentrations with increased particle sizes observed in bulk solution. Any beneficial effects of iron addition, however, were strongly dependent on the concentration of alginate present. Increased alginate concentrations led to increased Fe-alginate complexation, stabilization of Fe-alginate aggregates and thus more severe fouling. Furthermore, calcium additions were found to strongly influence the properties of the Fe-alginate fouling layer with the presence of calcium inducing the formation of larger aggregates which formed fouling layers with greater surface roughness and lower specific cake resistance and porosity. The material properties of the Fe-alginate, Ca-alginate and Fe–Ca-alginate assemblages formed on flat sheet membranes were also compared and empirical expressions describing the hydraulic conductivity and compressibility of fouling layers developed as a function of trans-membrane pressure.

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