Phosphate Transport Through Homogeneous and Heterogeneous Anion-Exchange Membranes: A Chronopotentiometric Study for Electrodialytic Applications

Abstract

This study investigates the behavior of phosphate ion transport through two structurally distinct anion-exchange membranes-AMV (homogeneous) and HC-A (heterogeneous)-in an electrodialysis system under both static and stirred conditions at varying pH levels. Chronopotentiometric and current-voltage analyses were used to investigate the influence of pH and hydrodynamics on ion transport. Under underlimiting (ohmic) conditions, the AMV membrane exhibited simultaneous transport of H₂PO₄^- and HPO₄^2- ions at neutral and mildly alkaline pH, while such behavior was not verified at acidic pH and in all cases for the HC-A membrane. Under overlimiting current conditions, AMV favored electroconvection at low pH and exhibited significant water dissociation at high pH, leading to local pH shifts and chemical equilibrium displacement at the membrane-solution interface. In contrast, the HC-A membrane operated predominantly under strong electroconvective regimes, regardless of the pH value, without evidence of water dissociation or equilibrium change phenomena. Stirring significantly impacted the electrochemical responses: it altered the chronopotentiogram profiles through the emergence of intense oscillations in membrane potential drop at overlimiting currents and modified the current-voltage behavior by increasing the limiting current density, reducing electrical resistance, and compressing the plateau region that separates ohmic and overlimiting regimes. Additionally, both membranes showed signs of NH₃ formation at the anodic-side interface under pH 7-8, associated with increased electrical resistance. These findings reveal distinct ionic transport characteristics and hydrodynamic sensitivities of the membranes, thus providing valuable insights for optimizing phosphate recovery via electrodialysis.

Keywords: electroconvection; electrodialysis; membrane potential drop; nutrient recovery; phosphorus; water dissociation.

Figure 1

Representation of the electrochemical cell used.

Figure 2

Speciation diagram of (a) phosphate, (b) ammonium, and (c) phosphate and ammonium compounds in aqueous solution. The dashed lines indicate the specific pH values examined in this study (4.4, 7, and 8).

Figure 3

Chronopotentiometric curves obtained with the AMV membrane in a 0.05 mol/L solution at pH 7 (a) without stirring and (b) with stirring.

Figure 4

(a) Current–voltage curve for the AMV membrane in a 0.05 mol/L solution at pH 7, recorded under both unstirred and stirred conditions. Panels (b,c) show the magnified view of the ohmic regions of the unstirred and stirred systems, respectively.

Figure 5

Chronopotentiometric curves obtained with the AMV membrane and solutions at pH (a) 4.4, (b) 7, and (c) 8 under stirring.

Figure 6

Amplitude of E_m recorded in the ChPs obtained at each current density using the AMV membrane and the three tested solutions.

Figure 7

(a) Current–voltage curve for the AMV membrane in a 0.05 mol/L solution at different pH values under stirring. Figure (b–d) show the magnified view of the ohmic regions of the solutions at pH 4.4, 7, and 8, respectively.

Figure 8

Chronopotentiometric curves obtained with the HC-A membrane and solutions at (a) pH 4.4 and (b) pH 8 under stirring.

Figure 9

Amplitude of E_m recorded in the ChPs obtained at each current density using the HC-A membrane and the two tested solutions.

Figure 10

Current–voltage curve for the HC-A membrane in a 0.05 mol/L solution at different pH values under stirring.