Phosphates Transfer in Pristine and Modified CJMA-2 Membrane during Electrodialysis Processing of NaxH(3-x)PO4 Solutions with pH from 4.5 to 9.9

Abstract

Phosphate recovery from different second streams using electrodialysis (ED) is a promising step to a nutrients circular economy. However, the relatively low ED performance hinders the widespread adoption of this environmentally sound method. The formation of "bonded species" between phosphates and the weakly basic fixed groups (primary and secondary amines) of the anion exchange membrane can be the cause of decrease in current efficiency and increase in energy consumption. ED processing of Na_xH_(3-x)PO₄ alkaline solutions and the use of intense current modes promote the formation of a bipolar junction from negatively charged bound species and positively charged fixed groups. This phenomenon causes a change in the shape of current-voltage curves, increase in resistance, and an enhancement in proton generation during long-term operation of anion-exchange membrane with weakly basic fixed groups. Shielding of primary and secondary amines with a modifier containing quaternary ammonium bases significantly improves ED performance in the recovery of phosphates from Na_xH_(3-x)PO₄ solution with pH 4.5. Indeed, in the limiting and underlimiting current modes, 40% of phosphates are recovered 1.3 times faster, and energy consumption is reduced by 1.9 times in the case of the modified membrane compared to the pristine one. Studies were performed using a new commercial anion exchange membrane CJMA-2.

Keywords: anion exchange membrane; bound species; boundary junction; current efficiency; current–voltage curve; electrodialysis; energy consumption; phosphates transfer; proton generation; weakly basic fixed groups.

Figure 1

Evolution of the number of publications in Scopus for the keywords “ion AND exchange AND membrane AND phosphates”.

Figure 2

The scheme of the laboratory-scale electrodialysis cell.

Figure 3

IR spectrum of the CJMA-2 pristine and the CJMA-2m modified membranes.

Figure 4

Possible interaction of the modifier, PQ-22, with strongly basic (a) and weakly basic (b) fixed groups of the anion-exchange membrane. Thin blue solid lines indicate the electrostatic interaction. Dotted blue lines indicate hydrogen bond.

Figure 5

Optical images of the surfaces and cross-section of the wet CJMA-2 membrane.

Figure 6

Droplets and contact angles on wavy surfaces of wet pristine (a) and modified (b) membranes. The membranes were pre-soaked in 0.02 M NaCl solution. The contact angles were measured 20 s after the drop was applied to the membrane surface.

Figure 7

Current–voltage curves of the CJMA-2 pristine and the CJMA-2m modified membranes in 0.02 M NaCl solutions with pH 4.5 ± 0.1 (a) and 9.9 ± 0.1 (b). The dashed line corresponds to the value i/i_lim^Lev = 1.0. The gray auxiliary lines show the procedure for i_lim^exp determining.

Figure 8

Electrochemical impedance spectra of the CJMA-2 pristine and the CJMA-2m modified membranes in 0.02 M NaCl solution with pH 4.5 ± 0.1. The spectra were obtained at i = 1.5i_lim^Lev. The letters denote the high-frequency arch (HF), as well as the Warburg (W) and Gerischer (G) arches. The frequency f_G is shown.

Figure 9

Schematic representation of the implementation of the “acid dissociation” mechanism in the system NaH₂PO₄ solution/anion exchange membrane with strongly basic fixed groups.

Figure 10

Current–voltage curves of the CJMA-2 pristine and the CJMA-2m modified membranes in 0.02 M Na_xH_(3−x)PO₄ solution with pH 4.5 ± 0.1 (a), as well as the pH difference of the solution at the inlet and outlet of the desalination compartment, obtained simultaneously with CVCs measurement (b). The dashed line indicates the value of the theoretical limiting current, i_lim^Lev = 1.62 mA cm⁻².

Figure 11

Electrochemical impedance spectra of the CJMA-2 pristine and the CJMA-2m modified membranes in 0.02 M Na_xH_(3−x)PO₄ solution with pH 4.5 obtained at i = 1.3 i_lim^Lev (a) and i = 2.0 i_lim^Lev (b). HF, G, and W letters denote the high frequency, Gerischer, and Warburg arches, respectively. The dotted lines are a guide to the eyes.

Figure 12

Current–voltage curves of the CJMA-2 and the CJMA-2m membranes in 0.02 M Na_xH_(3−x)PO₄ solutions with pH 9.9 ± 0.1 (a) and 6.9 ± 0.1 (b). Indexes 1 and 15 correspond to the duration (in hours) of membrane operation in an electric field before the measurements. The dashed line corresponds to i = i_lim^Lev.

Figure 13

Assumed schemes of ion transport and proton generation in the case of acidified (a) and alkaline (b) Na_xH_(3−x)PO₄ solutions. The anion exchange membrane contains a mixture of strongly and weakly basic fixed amino groups.

Figure 14

Electrolyte concentrations (a) and the number of protons coming from the membrane into the diluate stream (b), as well as potential drops measured by Luggin capillaries on the studied AEM and adjacent layers of the solution (c) vs. the duration of the ED desalination of 0.03 M Na_xH_(3−x)PO₄ with pH 4.5 ± 0.1. Electrodialysis was carried out at 1.63 mA cm⁻².

Figure 15

The degree of recovery of pentavalent phosphorus from the diluate stream vs. energy consumption during electrodialysis at current densities of 1.63 mA cm⁻² (a) and 3.75 mA cm⁻² (b).

Figure 16

Current efficiency for pentavalent phosphorus (a) and energy consumption (b). Given current density normalized to the theoretical limiting current. The data correspond to 40% the degree of phosphates recovery from the diluate stream.