Partial Fluxes of Phosphoric Acid Anions through Anion-Exchange Membranes in the Course of NaH2PO4 Solution Electrodialysis

Abstract

Electrodialysis (ED) with ion-exchange membranes is a promising method for the extraction of phosphates from municipal and other wastewater in order to obtain cheap mineral fertilizers. Phosphorus is transported through an anion-exchange membrane (AEM) by anions of phosphoric acid. However, which phosphoric acid anions carry the phosphorus in the membrane and the boundary solution, that is, the mechanism of phosphorus transport, is not yet clear. Some authors report an unexpectedly low current efficiency of this process and high energy consumption. In this paper, we report the partial currents of H₂PO₄^-, HPO₄^2-, and PO₄^3- through Neosepta AMX and Fujifilm AEM Type X membranes, as well as the partial currents of H₂PO₄^- and H⁺ ions through a depleted diffusion layer of a 0.02 M NaH₂PO₄ feed solution measured as functions of the applied potential difference across the membrane under study. It was shown that the fraction of the current transported by anions through AEMs depend on the total current density/potential difference. This was due to the fact that the pH of the internal solution in the membrane increases with the growing current due to the increasing concentration polarization (a lower electrolyte concentration at the membrane surface leads to higher pH shift in the membrane). The HPO₄^2- ions contributed to the charge transfer even when a low current passed through the membrane; with an increasing current, the contribution of the HPO₄^2- ions grew, and when the current was about 2.5 i_lim^Lev (i_lim^Lev was the theoretical limiting current density), the PO₄^3- ions started to carry the charge through the membrane. However, in the feed solution, the pH was 4.6 and only H₂PO₄^- ions were present. When H₂PO₄^- ions entered the membrane, a part of them transformed into doubly and triply charged anions; the H⁺ ions were released in this transformation and returned to the depleted diffusion layer. Thus, the phosphorus total flux, j_P (equal to the sum of the fluxes of all phosphorus-bearing species) was limited by the H₂PO₄^- transport from the bulk of feed solution to the membrane surface. The value of j_P was close to i_lim^Lev/F (F is the Faraday constant). A slight excess of j_P over i_lim^Lev/F was observed, which is due to the electroconvection and exaltation effects. The visualization showed that electroconvection in the studied systems was essentially weaker than in systems with strong electrolytes, such as NaCl.

Keywords: Fujifilm; Neosepta; ion-exchange membrane; limiting current density; phosphate transport; voltammetry.