Phosphorus recovery by Donnan dialysis: Membrane selectivity, diffusion coefficients, and speciation effects

Abstract

Donnan dialysis occurs when an electrochemical potential gradient exists for ions on either side of an ion-exchange membrane. We posit that this phenomenon can be leveraged to develop a sustainable process for nutrient recovery from wastewater. In this work, we conducted a fundamental study of the key parameters that control orthophosphate (P(V)) removal and recovery by Donnan dialysis. First, a new variable, namely the minimum draw ion concentration ratio between the draw and waste solutions (Rd/w), was established as the principal design parameter for Donnan dialysis. Then, the following variables were evaluated for their effects on P(V) removal and recovery: waste solution composition; draw anion type and concentration; and, membrane selectivity, thickness, and hydration. The waste solution pH controlled P(V) sorption to the anion-exchange membrane, with HPO42− exhibiting a higher affinity than H2PO4−. For an Rd/w of 10, 90.7% of H2PO4− and 98.4% of HPO42− were removed from a 10 mM P(V) waste solution using 100 mN and 218 mN NaCl draw solutions, respectively. The P(V) removal efficiency was dependent on draw solution concentration, and 77.1%, 95.3%, and 98.4% HPO42− removal was achieved with 48 (Rd/w = 2), 115 (Rd/w = 5), and 218 mM (Rd/w = 10) NaCl draw solutions, respectively. The rate of P(V) recovery was faster with HCOO− draw anions than with Cl− due to (i) the higher separation factor for P(V) over HCOO− (7.28) compared to Cl− (1.27) and (ii) the greater extent of membrane hydration with HCOO− draw solutions. Overall, this work established a new design parameter (Rd/w) and applied that parameter to optimize the draw solution chemistry for phosphorus recovery by Donnan dialysis.