Phosphorus recovery from low phosphate-containing solution by electrodialysis

Author’s Accepted Manuscript Phosphorus recovery from low containing solution by electrodialysis

phosphate-

Eduardo Henrique Rotta, Carolina Scritori Bitencourt, Luciano Marder, Andréa Moura Bernardes www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(18)32610-3 https://doi.org/10.1016/j.memsci.2018.12.020 MEMSCI16703

To appear in: Journal of Membrane Science Received date: 18 September 2018 Revised date: 16 November 2018 Accepted date: 6 December 2018 Cite this article as: Eduardo Henrique Rotta, Carolina Scritori Bitencourt, Luciano Marder and Andréa Moura Bernardes, Phosphorus recovery from low phosphate-containing solution by electrodialysis, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2018.12.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Phosphorus recovery from low phosphate-containing solution by electrodialysis

Eduardo Henrique Rotta, Carolina Scritori Bitencourt, Luciano Marder, Andréa Moura Bernardes*

LACOR, PPGE3M, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, Setor 4, Prédio 43426, Campus do Vale, 91509-900, Porto Alegre, RS, Brazil. *Corresponding author: Phone: +55 51 3308 9427, E-mail: [email protected] (A. M. Bernardes)

Abstract: The use of electrodialysis to treat a low phosphorus-containing solution (0.011 g L-1 HxPO43-x) was evaluated aiming at the recovery of phosphorus from municipal wastewater. The study was held in galvanostatic mode using a five-compartment electrodialysis cell with two pairs of cation- and anion-exchange heterogeneous membrane. A two-stage electrodialysis setup was proposed to recover phosphorus from a low phosphate-containing municipal wastewater. At underlimiting current density conditions, a solution with a phosphate concentration higher than 0.100 g L-1 was obtained by consecutively changing the solution in the diluted reservoir after reaching a demineralization rate of 50.0 %. A concentration factor of 9.7 was obtained and no significant changes that could compromise the efficiency of the process were observed. The phosphate ions of the concentrated solution could be then separated from coexisting ions, such as sulfate, by the employment of overlimiting current density conditions, with its feasibility dependent on the use of alkali resistance anion-exchange membranes.

Keywords: Phosphorus recovery, electrodialysis, ion-exchange membranes, overlimiting current density. 1

1. Introduction Phosphorus (P) is inherent to all life forms, with no chemical or technological substitute [1,2]. The imminent scarcity of phosphate rock [3], along with the eutrophication problem due to excessive P loads on natural water bodies [4], instigate its recovery. One of the most studied and promising techniques to recover phosphorus from municipal wastewater, an abundant raw material [5], is precipitation/crystallization as struvite and hydroxyapatite [2]. According to Xie et al. [6], in order to obtain a good precipitation and ensure a satisfactory efficiency, the minimum phosphate concentration required at the beginning of the process is 0.100 g L-1. However, the concentration of phosphorus in municipal wastewater tends to be lower (from 0.004 to 0.040 g L-1) [7,8], being desirable a phosphorus enrichment at the solution prior to precipitation/crystallization. For this, different membrane-based processes have been proposed, such as electrodialysis [9]. Several authors have already studied and considered that electrodialysis is a promising technology to separate and recover phosphorus from wastewater streams [10–12]. However, the application of this technique to treat and recover phosphorus from wastewaters with low Pconcentration (< 0.100 g L-1) is little discussed [13]. In this context, the objective of this work is to evaluate the technical feasibility of an electrodialysis setup in the treatment of a low phosphorus-containing solution aiming the recovery of P from municipal wastewater. The ionic transport, the operational electrodialysis conditions to remove and concentrate phosphorus and the separation of sulfate from phosphate are studied. The investigation presented is particularly significant since it contributes to the mentioned scarcity of information on low phosphate containing solutions treatment. At the same time, one of the stages of this work will evaluate the use of overlimiting current density conditions, currently investigated to improve the transport of ions in electrodialysis systems. At this work, it will be applied to restrict the transfer of phosphate ions through the anion-exchange

2

membranes, allowing its separation from coexisting anions, like sulfate. This separation is notably interesting from the point of view of a subsequent precipitation/crystallization process.

2. Experimental

The experiments were carried out in a laboratory-scale electrodialysis cell with five compartments [14], showed in Fig. 1. The compartments were separated by four Chinese heterogeneous membranes alternately arranged: two cation- and two anion-exchange membrane, with 16 cm² of effective area and a thickness of 450 μm [15]. The cation-exchange membrane, named as HDX100, contains –SO3- as ion group attached, with water content of 35-50 %, an ionexchange capacity ≥ 2 mol kg-1 (dry), a permselectivity of ≥ 90% (0.1 mol KCl/0.2 mol KCl), a membrane surface resistance ≤ 20 Ω cm2 (0.1 mol NaCl), a burst strength ≥ 0.6 MPa, a dimension change rate ≤ 2 % and a water permeability of ≤ 0.1 mL h-1 cm-2 (< 0.2 MPa). The anion-exchange membrane, named HDX200, has –NR3+ as fixed ion-exchange sites, with a water content of 30-45 %, an ion-exchange capacity ≥ 1.8 mol kg-1 (dry), with ≥ 8.9 % of permselectivity (0.1 mol KCl/0.2 mol KCl), ≤ 20 Ω cm2 (0.1 mol NaCl) of membrane surface resistance, permselectivity ≥ 89 %, a burst strength of ≥ 0.6 MPa, a dimension change rate ≤ 2 % and a water permeability of ≤ 0.2 mL h-1 cm-2 (< 0.035 MPa) [14]. Commercial Ti/70TiO230RuO2 electrodes were used as anode and cathode. The cell was fed by centrifugal pumps with a flow rate of 85 L h-1, also ensuring the circulation of the solutions in the reservoirs.

3

(1)

(1)

(2)

(2)

+

(8)

-

(3)

(4)

(7)

(3)

(4)

(5)

(6)

Figure 1. Schematic representation of the electrodialysis cell set-up. (1) platinum wires. (2) electrodes. (3) cation-exchange membrane. (4) anion-exchange membrane. (5) diluted reservoir (6) cathodic concentrate reservoir. (7) anodic concentrate reservoir. (8) electrode reservoir.

Using synthetic solutions, prepared with a composition based on a real municipal wastewater (MWW) treated by macrophytes [16], the study was performed in four electrodialysis steps. In a first step, experiments were conducted aiming an evaluation of the phosphate transport by electrodialysis. A solution was prepared with the same MWW’s pH and conductivity, which was achieved by using a higher phosphate concentration than the one from MWW (Solution A, Table 1). The second step studied the determination of the electrodialysis operational conditions to phosphate recovery. Here, a solution with a low phosphate concentration, i.e., a solution with a phosphate concentration similar to the MWW, was used. Sodium sulfate was added to meet the MWW conductivity value (Solution B, Table 1). A third step was accomplished with the best operational conditions to concentrate phosphate. The experiments were also carried out with Solution B (Table 1). It is important to note that the solution to be treated in steps 1 to 3 was initially added to the diluted (Fig. 1, 4

reservoir 5) and concentrated (Fig. 1, reservoir 6 and 7) reservoirs. Also, to this and further studies, the cathodic and anodic concentrate reservoir (Fig. 1, reservoirs 6 and 7) were combined in a single one, named concentrate. A fourth step was then performed to separate phosphate and sulfate. The treated solution was similar to the concentrate obtained on the previous step (Solution C, Table 1). At these tests, a 5.2 g L-1 Na2SO4 solution initially fed the concentrate reservoir of the cell. To all experiments, Na2SO4 solutions were circulated in a close circuit on the electrodes compartments (Fig. 1, reservoir 8) in order to avoid variations of pH values.

Table 1. Composition of the solutions used in the study. Solution A B C

Na2HPO4.7H2O 0.650 0.022 0.116

Composition (g L-1) NaH2PO4.H2O 0.330 0.011 0.085

Na2SO4 0.481 5.2

The electrodialysis experiments were carried out at room temperature in galvanostatic mode. It was applied a current density corresponding to 75 % of the limiting current density (ilim) in steps 1, 2 and 3 [17]. In step 4, it was applied 125 % of the ilim [14]. The limiting current densities were obtained by current-voltage curves experiments, as described elsewhere [18]. The electric current density was gradually increased at a rate of 2 mA in 30 s, and the membranes potential values (Um) were determined by platinum wires electrodes (Fig. 1) connected to digital multimeters. Along the electrodialysis experiments, the conductivity and pH values of all solutions were measured, as well as the cell potential (Uc) and the potential of both membranes of the central compartment. At pre-established times, aliquots were collected from the reservoirs and sodium, sulfate and phosphate concentrations were determined by Ion Chromatography.

5

The electrodialysis performance was evaluated in terms of demineralization rate (dr) [19], percent extraction (pe) [14], concentration factor (cf) [10] and the ionic flux (J) [13] defined respectively by Eq. (1), (2), (3) and (4). dr %

(

where

i,D

i,

- t, i,

) 100;

(1)

is the initial conductivity in the diluted reservoir and

t,D

is the conductivity in the

diluted reservoir at time t, expressed in µS cm-1. (

pe %

i,

- t, i,

) 100;

(2)

where Ci,D is the initial concentration of the ion in the diluted reservoir (g L-1) and Ct,D is the concentration of the ion in the diluted reservoir at time t (g L-1). cf (

t, i,

);

(3)

where Ci,C is the initial concentration of the ion in the concentrated reservoir (g L-1) and Ct,C is the concentration of the ion in the concentrated reservoir at time t (g L-1). g m-2 h-1

(

t,

- i, t

);

(4)

where V is the initial volume of the diluted reservoir (L), N is the number of pairs in the electrodialysis cell, A is the effective membrane area (cm2) and t is the time (s). Fourier-Transform Infrared Spectroscopy spectra for virgin and used anion-exchange membranes were also obtained by Horizontal Attenuated Total Reflectance (FTIR-HATR). The membranes were dried for 5 h under vacuum at a temperature of 40 °C. The FTIR spectra were obtained by a Perkin Elmer® Spectrum 1000 spectrophotometer in the wavenumber range of 4000-400 cm-1 [20].

6

3. Results and Discussion

Figure 2 shows the current-voltage curves obtained to the solution with a conductivity and pH similar to the real municipal wastewater (0.7 mS cm-1 and 7.2, respectively), and a phosphate concentration of 0.457 g L-1 (Step 1, Solution A). This value of concentration corresponds to the amount of PO4 present in both Na2HPO4.7H2O and NaH2PO4.H2O salts. As it can be observed in Fig 2(a), the cation-exchange membrane, HDX100, presents a typical behavior [21], with three regions and one limiting state associated to ilim,CEM. The first region (I) is linear with a quasi-ohmic behavior, the second region (II) is characterized by an inclined “plateau” associated to the limiting state region), which is smoothly transformed into a third region (III) linked to overlimiting conditions attributed to water splitting and electroconvection mechanisms [22].

6.0

(a)

(b) III

4.0

-2

i (mA cm )

5.0

II

3.0

-2

2.0 1.0

2 ilim,AEM2 = 2.4 mA cm

1 -2

I

ilim,CEM = 1.4 mA cm

1 ilim,AEM1 = 0.7 mA cm-2

0.0 0.0

0.5

1.0

1.5

2.0

2.5 0.0

1.0

Um (V)

2.0

3.0

4.0

Um (V)

Figure 2. Current-voltage curves of the (a) HDX100 cation-exchange membrane and the (b) HDX200 anion-exchange membrane in contact with Solution A.

7

For the anion-exchange membrane, HDX200, (Fig. 2(b)), the current-voltage curve shape differs from the conventional one, presenting two plateaus and, consequently, two limiting state, ilim,AEM1 and ilim,AEM2. Pismenskaya et al. [23] and Belashova et al. [22] reported a similar behavior, being probably related to different phosphate species that can be formed in the bulk solution and membrane, as well as in the membrane/solution interface, according to the pH conditions, as can be observed in the speciation diagram (Fig. 3), calculated and plotted using the Hydra-Medusa® software [24]. The changes in the pH values may be associated to the availability of H+ and OH- ions at the anion-exchange membrane/diluted solution interface resulting from water splitting (Eq. 5) or from phosphate species dissociation (Eq. 6-8) [25]. The pK values for the reactions were taken at 25 °C [26]. -

2

2P

P

2 -

2-

P 2-

P P

(5)

-

(6)

; pK = 2.16

(7)

; pK = 7.21

-

(8)

; pK = 12.34

N [ a+ ]TO T =

7 .26mM H 3PO

10 .

4

[PO H 2PO

4

4

3 ] TO T

H PO

4

=

4 .81mM

2

PO

3 4

08 .

06 . F rac tion

P

; pK = 14.00

04 .

02 .

00 . 0

2

4

6

8

10

12

14

pH

8

Figure 3. Speciation diagram of phosphate species in solution at different pH values calculated and plotted using the Hydra-Medusa ® program.

Considering ilim,AEM1 as the limiting current density of the electrodialysis system, since that ilim,AEM1 is the first limiting state, it was applied 0.5 mA cm-2 (75 % of the ilim,AEM1). The electrodialysis experiments lasted for 15 hours, which was the time required for the solution in the diluted reservoir to reach a conductivity lower than the one measured in the local water supply (approximately 0.2 mS cm-1) [27]. The values of sodium and phosphate concentration, as well as the pH of the diluted, cathodic concentrate and anodic concentrate reservoirs along the treatment time are shown in Fig. 4. Phosphate ions may occur as differently charged species, thus HxPO43-x is used to express these ions [28]. As it can be seen in Fig. 4(a), the behavior of Na+ concentration values in the diluted and cathodic concentrate reservoir indicates its transport through the cation-exchange membrane. An increment in the values of this parameter is also observed in the anodic concentrate reservoir, related to the transfer of Na+ from the electrode reservoir (anode side). Regarding the HxPO43-x concentration, while its values remained practically unchanged in the cathodic concentrate reservoir, an increase in the anodic concentrate reservoir and a decrease in the diluted one in a practically mirrored design is observed. In addition to the removal of HxPO43x

ions from the solution of the diluted reservoir, this transport through the anion-exchange

membrane indicate the absence of membrane poisoning [29].

9

1.0 0.8

10.0

Anodic concentrate Cathodic concentrate Diluted HXPO43-x

(b)

Anodic concentrate Cathodic concentrate Diluted

8.0

Na+

0.6

pH

Concentration (g L-1)

(a)

6.0

0.4 4.0

0.2 0.0

2.0 0

4

8

Time (h)

12

16

0

4

8

12

16

Time (h)

Figure 4. Values of (a) concentration of HxPO43-x and Na+ and (b) pH of the solutions as a function of electrodialysis time.

The values of pH in the diluted reservoir started to decrease after 11 hours of electrodialysis, as presented in Fig. 4(b). Since it was applied a constant current density (0.5 mA cm-2) along the treatment, and considering the depletion of ions in the diluted reservoir solution, the system could have reached or exceeded the limiting current density values (ilim,AEM1 and ilim,AEM2). This behavior may leads to coupled effects of concentration polarization, namely the generation of OH- and H+ by water splitting (catalyzed by the HxPO43-x ions [22]) at the anionexchange membrane/diluted solution interface. The OH- ions may pass through the membrane to the concentrate side, where the values of pH remained nearly constant possibly due to the buffering capacity of phosphate species [2]. On the other hand, the H+ ions may shift the equilibrium to the formation of non-chargeable phosphate species H3PO4 (Eq. 6-8 and Fig. 3), acidifying the solution already depleted in HxPO43-x ions. The formation of H+ ions from the dissociation of phosphate species may also be considered to the changes in the pH values, in a different way compared to water splitting [22,23]. In phosphate-containing solutions, the H+ ions are excluded from the anion-exchange membrane as a result of the Donnan exclusion of the 10

products of protonation-deprotonation reactions of anions H2PO4- and/or HPO42- (Eq. 6-8) [25], enhanced with the demineralization of the solution or with the decrease of HxPO43-x concentration at the membrane surface. The possible occurrence of H3PO4 or PO43- may also have affected the percent extraction of HxPO43-x ions. The concentration of Na+ ions in the diluted reservoir decreased from (0.206 ± 0.003) g L-1 to (0.016 ± 0.002) g L-1, representing a percent extraction higher than 92.0 %. In contrast, the concentration of HxPO43-x ions decreased from (0.469 ± 0.003) g L-1 to (0.184 ± 0.021), corresponding to a percent extraction of approximately 61.0 %. Since PO43- ions have a high charge and ionic radius, and H3PO4 is a non-chargeable species [7,30], the transport of HxPO43-x ions to the concentrated side may be hampered. From these results, a better understanding of the phosphate transport by electrodialysis was obtained. Then, studies using a solution with HxPO43-x concentration similar to the real municipal wastewater (0.015 g L-1) and a conductivity adjusted with Na2SO4 were conducted (Step 2, Solution B). The obtained current-voltage curves are shown in Fig. 5, and the shape of the curves are similar to those shown in Fig. 2 (Step 1).

11

6.0

(a)

(b)

4.0

-2

i (mA cm )

5.0

3.0

2 -2

ilim,AEM2 = 2.4 mA cm

2.0

1 1.0

1

ilim,CEM = 1.4 mA cm-2

-2

ilim,AEM1 = 0.9 mA cm

0.0 0.0

0.5

1.0

1.5 0.0

Um (V)

1.0

2.0

3.0

Um (V)

Figure 5. Current-voltage curves of the (a) HDX100 cation-exchange membrane and (b) HDX200 anion-exchange membrane in contact with Solution B.

As well as it was considered to phosphate, this unconventional behavior observed for the anion-exchange membrane may correspond to the formation of different sulfate species (mostly SO42 and HSO4-) [31], in accordance to the pH in the bulk solution, inside the membrane or at the membrane/solution interface [23], as can be seen in Fig. 6 (calculated and plotted by HydraMedusa® software [24]). When NaCl was used to adjust the conductivity instead of Na2SO4, a conventional current-voltage curve shape was obtained (Supplementary material, Fig. S1). Unlike sulfate, chloride does not present a change of species as a function of pH, thus evidencing the influence of sulfate ions on the change of the shape of the current-voltage curve presented in Fig. 5(b).

12

3 4 ]TO T a+ ]TO T =

[PO N [

=

0 .16mM 7 .02mM

H SO

10 .

[SO

4

2 ] TO T

=

3 .39mM

4

SO

2 4

08 .

F rac tion

06 .

04 .

02 . N aSO 00 . 0

2

4

6

4

8

10

12

14

pH

Figure 6. Speciation diagram of sulfate species in solution at different pH values calculated and plotted using the Hydra-Medusa ® program.

So, considering the first limiting state value, it was employed 0.7 mA cm-2 (75 % of the ilim,AEM1) for 12 hours. Fig. 7 shows the values of demineralization rate, percent extraction, anion- and cation-exchange membrane potential and pH of the solution in the reservoirs. A demineralization rate of (91.2 ± 1.2) % was obtained at the end of the experiments (Fig. 7(a)), similar to that reported by Liu et al [13]. For this value of demineralization rate, a percent extraction of (96.9 ± 0.9) %, (86.8 ± 5.8) % and (83.6 ± 5.4) % were reported for Na+, HxPO43-x and SO42- ions, respectively. The profile of the percent extraction curves to HxPO43-x and SO42ions may indicate the preference in the transport of SO42- ions through the anion-exchange membrane, as noted by Tran et al [7]. An increase in the HxPO43-x extraction rate was observed after 9 h, when similar values to that reported in literature were reached [11,32].

13

100.0

14.0

dr

(a)

pe HxPO43-x

80.0

12.0

80.0

pe SO42-

60.0

40.0

40.0

pH

60.0

1.2

AEM potential CEM potential

10.0

pe (%)

dr (%)

pe Na+

1.4

Anodic concentrate pH Cathodic concentrate pH Diluted pH

(b)

1.0 0.8

8.0 0.6 6.0

20.0

20.0

0.0

0.0 0

2

4

6

8

10

12

14

Time (h)

Um (V)

100.0

0.4

4.0

0.2

2.0

0.0 0

2

4

6

8

10

12

14

Time (h)

Figure 7. Values of (a) demineralization rate and percent extraction of HxPO43-x, SO42- and Na+ ions and (b) potential of the cell, anion- and cation-exchange membrane and pH of the solution in the reservoirs of the electrodialysis setup.

The behavior of the solutions pH may suggest the occurrence of water splitting after 5 h of electrodialysis (Fig. 7(b)). As it can be observed, the pH values increased after 8 h of treatment in the anodic concentrate reservoir and decreased after 5 h in the cathodic concentrate reservoir, probably as a consequence of the transport of H+ and OH- ions by water splitting. The cathodic and anodic pH variations agree with Um increments to both anion- and cation-exchange membranes, which may indicate the scarcity of ions at the membranes/diluted solution interfaces [5]. Furthermore, values of pH < 5 are reported in the diluted reservoir solution after 5 h, refusing the water release on the environment according to national standards in Brazil [33]. Also, up to this time (dr = 50.0 %), the cell potential do not shown severe increase, but after this period one should expect a high energy consumption and a reduction in the treatment efficiency [19]. So, to keep the system operating under favorable conditions, with no significant changes that could compromise the efficiency of the process, the next concentration experiments were conducted at underlimiting current density condition and up to a demineralization rate of 50.0 % 14

(Step 3). The solution in the diluted reservoir was successively changed after reaching the predetermined value of demineralization rate (being this procedure called “cycle”), while the solution in the concentrated reservoir was maintained throughout the experiments. The experiments were conducted for 75 h, corresponding to 15 cycles of 5 h. The values of demineralization rate (Fig. 8(a)) remained nearly constant and close to the pre-established value (dr = 50.0 %, dotted line), presenting an average value of (52.0 ± 2.9) %, indicated by the solid line. For this average value of demineralization rate, a percent extraction of (49.8 ± 3.0) %, (46.7 ± 3.5) % and (42.6 ± 12.2) % was obtained for Na+, SO42- and HxPO43-x ions, respectively. As it can be seen in Fig. 8(b-d), the values of percent extraction for these ions presented a steady performance during the operational time. The average values of cell potential and pH of the solutions in the diluted and concentrated reservoir, monitored at the beginning and at the end of each cycle of 5 h, also agreed with the pre-determined values in Step 2 (Table 2). These facts indicate the absence, along the 75 h of treatment, of any significant changes that could compromise the efficiency of the concentration process.

15

100.0

100.0

80.0

80.0

60.0

60.0

40.0

20.0

0.0 100.0

0.0 100.0

Na+

80.0

80.0

60.0

60.0

40.0

40.0

20.0

20.0

0.0

0.0 20

40

60

SO42-

(d)

pe (%)

pe (%)

(c)

H3PO4

40.0

20.0

0

3-x

(b)

pe (%)

dr (%)

(a)

80

Time (h)

0

20

40

60

80

Time (h)

Figure 8. Values of (a) demineralization rate and percent extraction of (b) HxPO43-x, (c) Na+ and (d) SO42-. Solid lines indicate the average value of the parameter after 75 h of concentration experiments. Dotted lines indicate the average value reported in Step 2.

Table 2. Values of cell potential and pH of the solutions in the diluted and concentrated reservoir at the beginning and at the end of each cycle of 5 h. Parameter

Average value

Reported in Step 2

Uc,initial

(5.0 ± 0.4) V

Uc,final

(5.6 ± 0.2) V

pHdiluted,initial

6.7 ± 0.1

pHdiluted,final

5.9 ± 0.4

pHconcentrated,initial

7.2 ± 0.2

6.9 ± 0.1 (anodic concentrate)

pHconcentrated,final

7.3 ± 0.1

6.2 ± 0.3 (cathodic concentrate)

(6.3 ± 0.3) V 5.2 ± 0.1

As already mentioned, the precipitation/crystallization process is effective when the HxPO43-x concentration is higher than 0.100 g L-1 [6]. The increment in HxPO43-x concentration of 16

the solution in the concentrated reservoir is demonstrated in Fig. 9, showing that the minimum value required to ensure satisfactory conditions in precipitation/crystallization (dotted line) was reached after 65 h of electrodialysis, when a HxPO43-x concentration of (0.103 ± 0.004) g L-1 was reported.

4.0 -1

HxPO43-x

Na and SO4 concentration (g L )

SO42Na+

3.0

0.2

2.0

0.1

1.0

0.0

0.0

2-

0.3

+

HxPO43-x concentration (g L-1)

0.4

0

20

40

60

80

Time (h)

Figure 9. Values of concentration of HxPO43-x, Na+ and SO42- ions in the concentrated solution. Dotted line indicates HxPO43-x = 0.100 g L-1.

At the final of the experiments, the mean value of HxPO43-x concentration was (0.118 ± 0.001) g L-1 in the concentrated reservoir. Since the initial concentration of HxPO43-x in this reservoir was (0.012 ± 0.001) g L-1, a concentration factor of HxPO43-x of 9.7 ± 0.4 was reached, higher than the one reported by Wang et al. [11] and Zhang et al. [12]. In fact, this concentration factor was only obtained after a long period of operation, which is associated to the use of a laboratory-scale electrodialysis cell with a low effective membrane area (16 cm2 per ionexchange membrane) [12]. Considering the maintenance of the calculated value of ionic flux, , J = (0.4 ± 0.2) g m-2 h-1, the operating time may be considerably reduced by employing, for example, an electrodialyzer with 100 paired chambers and a effective membrane area of 1600

17

cm2. In this case, to treat 1 m³ of solution, the operating time would be approximately 1 hour per cycle. As HxPO43-x, sodium and sulfate ions were also concentrated throughout the experiments, as shown in Fig. 9. The final concentration of both ions in the solution of the concentrated reservoir was (1.931 ± 0.085) g L-1 of Na+ ions and (3.720 ± 0.165) g L-1 of SO42- ions. The presence of these coexisting ions may negatively affect a subsequent crystallization/precipitation process by increasing the induction time [34], which may delay the recovery of phosphate in struvite and/or hydroxyapatite. Given this, it is convenient to separate the HxPO43-x ions from other coexisting ones, such as Na+ and SO42-. The separation of sodium and phosphate could be effectively achieved by the application of electrodialysis at underlimiting current density conditions, since they present opposite charges. However, this condition would not be efficient to separate two ions of the same charge, such as sulfate and phosphate. Taking into account the restriction in transport of HxPO43-x ions observed in Step 1, the use of overlimiting current density conditions was studied (Step 4). Such current density condition is commonly studied to improve the transport of the ions in an electrodialysis system [35]. In contrast, in this study it could promote an intense concentration polarization, leading to severe water splitting at the anion-exchange membrane/diluted solution interface. Although the generation of H+ by HxPO43-x species protonation-deprotonation occurs even in underlimiting current density conditions [22], the use of overlimiting density currents should also enhance this phenomenon. By the formation of H3PO4 or PO43- ions, the transport of HxPO43-x ions may be suppressed, promoting their separation from sulfate ions. So, considering the current-voltage curves obtained for the concentrated solution (Supplementary material, Fig. S2), the electrodialysis experiments were performed with a current density of 25.0 mA cm-2 (125 % of ilim,AEM2) for 4 h. 18

As it can be seen in Fig. 10(a), the concentration of Na+ ions in the diluted reservoir decreased throughout the treatment, from (1.947 ± 0.026) g L-1 to (0.042 ± 0.010) g L-1, representing an average removal value of (97.7 ± 0.1) %. The concentration values of SO42- ions presented a similar behavior, with a decrease in its values from (3.630 ± 0.239) g L-1 to (0.201 ± 0.080) g L-1, reporting an average removal value of (94.2 ± 0.1) %. At the end of the treatment, a HxPO43-x removal of only (18.7 ± 0.1) % was obtained, decreasing the values of concentration from (0.127 ± 0.009) g L-1 to (0.105 ± 0.001) g L-1. The final concentration of HxPO43-x reported in the concentrated reservoir was (0.019 ± 0.001) g L-1 (Fig.10(a)), indicating that the amount removed from the diluted reservoir was transferred to the concentrated one and no membrane poisoning was observed. In addition, the concentration of HxPO43-x ions on the diluted compartment declined only in the first half hour of electrodialysis, remaining approximately constant after this period. When compared to Fig 10(b), it can be seen that this behavior accompanies the concentration polarization, reflected by the increase of the anion-exchange membrane potential and the alkalization of the concentrated solution, possibly due to the water splitting at the interface of this membrane and the diluted solution.

19

SO42-

0.04

HXPO43-x concentrated

1.0

0.03

C/C0

0.8 0.6

0.02

0.4 0.01 0.2 0.0

0.00 0

1

2

3

Time (h)

4

5

14.0

8.0

(b) 12.0 6.0

10.0 Diluted pH Concentrated pH AEM potential CEM potential

8.0 6.0 4.0

4.0

Um (V)

Na+

pH

HxPO43-x

(a)

HxPO43-x concentration (g L-1)

1.2

2.0

2.0 0.0

0.0 0

1

2

3

4

5

Time (h)

Figure 10. Values of (a) decrease in the concentration of HxPO43-x, SO42- and Na+ ions in the diluted reservoir and HxPO43-x concentration in the concentrated reservoir and (b) pH of the solution in the diluted and concentrated reservoir and membrane potential.

It is important to highlight that, at the end of the experiment, the side of the anionexchange membrane in contact with the concentrated compartment presented a significant change in its coloration (Fig. 11). The used membrane FTIR spectra (Fig. 11(b)) shows an absorption peak at a wave number of 1050 cm-1, which is not observed in the FTIR spectra for the virgin one (Fig 11(a)), suggesting a transformation of the quaternary amine into tertiary amine groups in the structure of the anion-exchange membrane. This may be possibly due to the alkalization of the concentrated solution (Fig. 10(b)), resulting from the water splitting at the diluted solution/anion-exchange membrane interface [36], and/or associated to the phosphate species protonation-deprotonation mechanism. Thus, the pH of the membrane internal solution may be much higher than the pH of the external solution, reflecting in an intense process of degradation of anion-exchange materials in contact with phosphate-containing solutions [25]. In this case, the feasibility of this process depends on using alkali-resistance anion-exchange membranes. 20

Transmitance

(a)

(b)

1050 cm

3900

3250

2600

1950

-1

1300

650

-1

Wavelength (cm )

Figure 11. Image of (a) virgin and (b) employed anion-exchange membrane, as well as the FTIR spectra.

In view of the results obtained, a two-stage electrodialysis setup to phosphorus recover from a low phosphate-containing municipal wastewater may be suggested. The first stage consists in the use of underlimiting current density conditions to obtain a phosphate concentrated solution, changing the diluted solution (respecting a demineralization rate of 50.0 %) until a concentration of HxPO43-x ≥ 0.120 g L-1 is reached in the concentrated reservoir. Employing alkali resistance anion-exchange membranes, the second step is based on the use of overlimiting current density conditions, allowing the separation of phosphate from the coexisting anions, such as sulfate, in a municipal wastewater.

4. Conclusions

The employment of an electrodialysis system in the treatment of a low phosphatecontaining solution (0.015 g L-1 of HxPO43-x) to recover phosphorus from municipal wastewater 21

was investigated. At underlimiting current density conditions (i < ilim), it was possible to obtain a solution with a phosphate concentration of approximately 0.120 g L-1 and concentration factors of Na+, SO42- and HxPO43-x higher than 9.7 were reached. By applying overlimiting current density conditions (i > ilim), commonly employed to improve the transport of the ions, the phosphate transfer through the anion-exchange membrane was hampered, allowing its separation from the coexisting ions, such as sulfate.

Acknowledgments

The authors wish to thank the Brazilian funding agencies CAPES, CNPq and FAPERGS. Besides

the

financial

support

from

FINEP/CT-HIDRO/TECSANTA

and

from

FINEP/SIBRATEC/RESETRA is acknowledged.

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24

Highlights: 

Electrodialysis concentrates phosphorus from a low phosphate-containing solution;



No significant operational changes were observed up to a dr = 50.0 %;



A phosphate concentration factor of 9.7 ± 0.4 was obtained;



Overlimiting current density conditions allowed separate phosphate and sulfate;



Structural changes in the anion-exchange membrane were observed.

25