A hybrid adsorption/Donnan dialysis (DD) process for nitrate removal using PUROLITE A520E resin as an adsorbent was investigated in this work. PUROLITE resin was introduced into the DD process due to its good selectivity adsorption for nitrate and widespread availability. This study was conducted in order to benefit from each process, and it was an original and new combination. The retention efficiency of nitrate was discussed by considering the factors of adsorbent mass, nature of the receiver electrolyte and flow rate. The coupling was a solution to improve the resin mass and the amount of nitrate removed. The coupling was successfully performed, with a nitrate removal capacity of about 7 mg/g.

INTRODUCTION

Nitrate is mainly found in most natural waters at moderate concentrations (less than 1 mg/L), but is often enriched to over the contaminant levels by excessive use of fertilizers and uncontrolled discharges of raw wastewater (Peavy et al. 1985; Lin & Wu 1996; Sihrimali & Singh 2001). The most important environmental problems caused by nitrate are eutrophication in water supplies and infectious diseases (Barber & Stuckey 2000). Excess nitrate in drinking water may cause ‘blue baby disease’, called methemoglobinemia, in newborn infants as well as other illnesses (Arden 1994; Ozturk 2004). In order to protect public health from the adverse effects of high nitrate intake, the World Health Organization (WHO) set the standard as 50 mg/L to regulate the nitrate concentration in drinking water (WHO 2011). Conventional processes such as coagulation, filtration, chlorination, etc. for water treatment are not useful with regard to nitrate ion elimination from water (Sihrimali & Singh 2001). Therefore, traditional biological wastewater treatment (Jianping et al. 2003; Ergas & Rheinheimer 2004; Rabah & Dahab 2004; Reyes-Avila et al. 2004) which uses a membrane bioreactor (MBR), an anaerobic continuous stirred tank reactor, 12–1 gas-liquid-solid three-phase flow airlift loop bioreactor, and high performance fluidized bed biofilm reactors with sand as the biofilm carrier, respectively, showed very interesting nitrate removal capacities that are higher than 96%. Numerous researches have tested other methods to remove excessive nitrate from water such as adsorption (Ozturk 2004), ion exchange (Baes et al. 1997, 2002; Pintar et al. 2001; Kim & Benjamin 2004), Donnan dialysis (DD) (Schaetzel et al. 2004), electrodialysis (Salem et al. 1995; Elmidaoui et al. 2001, 2002; Sahli et al. 2004) methods have been applied to remove excessive nitrate from water.

In the DD process, an ion-exchange membrane (anion-exchange or cation-exchange) separates two solutions: the feed (containing the anions or cations that should be removed) and the receiver (an electrolyte with a relatively high concentration of the neutral driving anion or cation). The chemical potential gradient of the components on both sides of the membrane causes the flow of the driving counter ion from the receiver to the feed, and the resulting electric potential evokes the transport of counter ions in the opposite direction (Strathmann 2004) (Figure 1). As a result, water with a changed ionic composition is obtained; previously troublesome ions are replaced with neutral ions from the receiver. Water that is going to be desalinated contains easily soluble salts, and during this process it may avoid scaling problems and achieve a deep desalination effect.
Figure 1

Dialysis system analyzed.

Figure 1

Dialysis system analyzed.

DD with an anion-exchange membrane may be applied to the removal of harmful anions from drinking water, e.g., fluoride (Hichour et al. 2000) or nitrates. The process may be also combined with a bioreactor. An ion-exchange MBR removes nitrates from the water to the receiving solution, where denitrification takes place (Velizarow et al. 2002). Studies which use DD as a separation process to remove nitrate ions are rare. Therefore, the objective of this work is to eliminate nitrate ions from water by using anion-exchange membranes combined with a specific nitrate removal resin (PUROLITE A520E). To achieve this purpose, the AM3 anion exchange membrane (AEX) was used, the nature of the receiver electrolyte was examined, the flow rate was discussed and the effects of the mass of the PUROLITE A520E resin on the removal of nitrate from an aqueous solution are examined.

MATERIALS AND METHODS

Membrane

The AM3 AEX, produced by a Japanese firm Tokuyama Soda Co. Ltd, has quaternary ammonium as a functional group. The ion-exchange capacity of the membrane is 2.46 meq/g, with an active area of 4.52 cm2. The main characteristics of the AEX are presented in Table 1. The membrane is supplied in the chloride ion form. Before the membranes were used, they were treated with distilled water at 70 ± 1 °C for 1 h. Then, in order to remove the impurities, they were also treated with 0.1 M HCl and 0.1 M NaOH at 50 ± 1 °C for 1 h, respectively. The treated membranes were finally immersed into 1 M NaCl solution at 25 ± 1 °C for 24 h to transform the membrane into the chloride form.

Table 1

Properties of the studied membrane

Description Membrane properties 
Membrane AM3 
Mark Neosepta 
Rhone Poulenc Exchange capacity in Cl form (mmol/g) 1.13 
Thickness (mm) 0.17 
Water content (%) 22 
Description Membrane properties 
Membrane AM3 
Mark Neosepta 
Rhone Poulenc Exchange capacity in Cl form (mmol/g) 1.13 
Thickness (mm) 0.17 
Water content (%) 22 

Resin

Before use, the resin was washed in distilled water to remove the adhering dirt and then dried at 50 °C for about 2 hours. After drying, the resin was screened to obtain a particle size range of 0.3–1.2 mm.

The main characteristics of the PUROLITE A 520E, a macroporous anion-exchange resin, are given in Table 2.

Table 2

Physico-chemical properties of resin PUROLITE A 520 E

Skeleton Polystyrene cross-linked with DVB of the macroporous type 
Functional groupings Quaternary ammonium 
Physical aspect Opaque balls, beige color 
Granulometry 0.3–1.2 mm 
Ionic form Cl- 
Total exchange capacity 0.9 meq/mL min 
Humidity 45–52% 
Limit of temperature 100 °C 
Limits of pH 0–14 
Real density 1.06 
Skeleton Polystyrene cross-linked with DVB of the macroporous type 
Functional groupings Quaternary ammonium 
Physical aspect Opaque balls, beige color 
Granulometry 0.3–1.2 mm 
Ionic form Cl- 
Total exchange capacity 0.9 meq/mL min 
Humidity 45–52% 
Limit of temperature 100 °C 
Limits of pH 0–14 
Real density 1.06 

Adsorption

Adsorption is a highly effective process for a variety of applications such as removal of metal ions from wastewaters and it is considered one of the most efficient and fast methods to remove impurities. Adsorption experiments were carried out in a thermostatically bath, the beakers containing 250 mL of nitrate with an initial concentration of 200 mg·L−1 and with different amounts of PUROLITE A520E resin from 0.5 to 4 g 25 °C (Turki et al. 2012). The content was agitated with a constant stirring rate at 140 rpm. Samples were withdrawn after a definite time interval and filtered through Whatman N ° 1 filter paper (0.45 μm).

Reagents

KNO3, NaCl, NaHCO3, and Na2SO4 were of analytical grade obtained from Aldrich (Germany) and their solutions were prepared without further purification, with distilled water.

Model water application

The application of the hybrid process on a model water having the same composition as that of a natural tap water but with a nitrate content that exceeds the maximum acceptable value (50 mg/l) was performed under the following optimal conditions deduced from studies carried out in the previous part ([NO3-] = 100 mg/l; [Cl-] = 0.5 M; T = 25 °C; Q = 180 ml·h−1, mresin = 2 g).

The chemical composition of the model water is given in Table 3.

Table 3

Chemical composition of the model water to be treated

Components Concentrations (mg/L) 
Chloride (Cl340 
Sulfate (SO42−268,44 
Nitrates (NO3100 
Carbonate (HCO3145 
Sodium (Na+180 
Calcium (Ca2+122 
Magnesium (Mg2+47,42 
Potassium (K+129 
Components Concentrations (mg/L) 
Chloride (Cl340 
Sulfate (SO42−268,44 
Nitrates (NO3100 
Carbonate (HCO3145 
Sodium (Na+180 
Calcium (Ca2+122 
Magnesium (Mg2+47,42 
Potassium (K+129 

DD

The DD is an ion-exchange membrane separation process in which ions of the same electrical charge are exchanged between two solutions through an ion-exchange membrane. The DD is a continuous low energy process, requiring only a few simple chemicals and an unskilled workforce. The DD process is economical, and can easily be implemented in remote areas.

Experiments were carried out using a laboratory cell consisting of two compartments of equal volume separated by an AM3 anionic exchange membrane as shown in Figure 2. The transport of the nitrate ions from feed compartment (F) to receiver compartment (R) was performed by peristaltic pump (Masterflex® L/S series). Receiver and feed tanks with a 250 ml Erlenmeyer flask were used. DD experiments were carried out in a batch stirred cell.
Figure 2

The experimental device of the DD process.

Figure 2

The experimental device of the DD process.

We prepared one-component solutions containing 0.5 M NaCl (or 0.5 M Na2SO4 or 0.5 M NaHCO3) (the receiver). According to other studies that have been done in our laboratory, the optimal nitrate concentration is 100 mg/L, which is why we chose this concentration for the feed compartment.

The process was conducted in the laboratory set-up for dialysis, which comprised two compartments separated by AEX.

The concentration of residual nitrate ions was determined spectrophotometrically. Both external solutions were stirred at 700 rpm with a magnetic stirrer. All the measurements were carried out at 25 °C.

The removal rate of nitrate was calculated by Equation (1): 
formula
1
where C0 and Ce are the initial and equilibrium NO3- concentrations (mg·L−1).

Figure 2 shows the device used to study the nitrate removal by DD. It is composed of a thermoregulated water bath (25.0 °C ± 0.1 °C), containing a cell with feed and receiver compartments separated by an anion-exchange membrane. The solutions are pumped through the cell with a peristaltic pump fitted with a pair of identical heads and a speed variator allowing for variable flow rates. The hydrodynamic conditions on both sides of the membrane can be adjusted by two variable speed stirring rods. The dialysis cell consists of two detachable compartments made with polymethylmetacrylate (plexiglass). It is composed of four parts joined by three stainless steel treaded rods. The centring is assured by bolsters.

The two central compartments consist of two symmetrical tubes. Two threaded holes penetrate each compartment and serve as supports for introducing and circulating the solution in the compartment. The membrane is sandwiched between these two compartments, making a seal at the same time.

To supply the receiver compartment, a NaCl (Na2SO4 or NaHCO3) solution is used at a concentration of 0.5 M.

The principle of the process is illustrated in Figure 1 for the simple case of 100 mg/L KNO3 in the feed compartment (F) separated by an AEX from the receiver compartment (R) holding a 0.5 M NaCl (Na2SO4 or NaHCO3) stripping solution. Since the receiver compartment contains a high concentration, chloride ions will permeate to the feed compartment. As no cations can permeate through the AEX, electrical balance is maintained by the migration of an equivalent stream of nitrate ions from the feed compartment to the receiver.

RESULTS AND DISCUSSION

DD

First, DD experiments were performed with solutions circulating as a batch pass mode: a nitrate solution of 100 ppm and 0.5 M NaCl in the feed and receiver compartments, respectively. In Figure 3, the nitrate concentration at the outlet of the two compartments is plotted as a function of time. The outlet concentration decreased and increased in the feed (F) and receiver compartment (R), respectively, to reach a constant value corresponding to pseudo-equilibrium.
Figure 3

Time dependence of the nitrate concentration in the feed (F) and receiver (R) compartments during the DD.

Figure 3

Time dependence of the nitrate concentration in the feed (F) and receiver (R) compartments during the DD.

For both solutions, the nitrate concentration at the pseudo-equilibrium remained lower than the upper permitted level (50 mg/l).

The amount of nitrate removed by the DD was determined on the basis of the following parameters: nature of the receiver electrolyte, Cl- concentration, flow rate, temperature and magnetic stirring. The study allowed us to find the best conditions to remove nitrate by DD. These conditions were obtained in a previous study ([Cl-] = 0.5 M; T = 25 °C; Q = 180 ml·h−1 and at a high speed of magnetic stirring (Turki et al. 2015).

Adsorption onto PUROLITE A520E resin

Anion exchange for nitrate removal is similar to a water softener, with nitrate ions removed rather than hardness ions. Nitrate is removed from the treatment stream by displacing chloride on an anion exchange resin. The mechanism of nitrate removal by Purolite A520E resin is shown in Figure 4.
Figure 4

Mechanism of nitrate adsorption on Purolite 520E resin.

Figure 4

Mechanism of nitrate adsorption on Purolite 520E resin.

In order to remove nitrate quantitatively from aqueous solution, the optimum amount of resin was found to be 1.68 g, which is the intersection of the two slopes of the adsorption percentage and the loading capacity (Figure 5) (Das et al. 2006).
Figure 5

Loading capacity and adsorption percentage of nitrate as a function of PUROLITE A520 E resin dose.

Figure 5

Loading capacity and adsorption percentage of nitrate as a function of PUROLITE A520 E resin dose.

The quantity of nitrate adsorbed at equilibrium (mg/g) was calculated by the following expression: 
formula
2
where m is the mass of resin (g), V is the volume of the solution (L), C0 is the initial concentration of nitrate (mg/L) and Ce is the equilibrium nitrate concentration (mg/L).
As can be seen from Figure 6, the nitrate removal capacity corresponding to the WHO guideline value for nitrate of 50 mg/L is about 15 mg/L. Similar results were also reported by other researchers (Hafshejani et al. 2016).
Figure 6

Effect of resin mass on adsorption capacity.

Figure 6

Effect of resin mass on adsorption capacity.

It can be observed from Figure 6 that the retention capacity increases with an increase in the equilibrium nitrate concentration.

DD coupled to adsorption onto PUROLITE A520E resin

In order to improve the amount of nitrate removed by DD and reduce the experimental time, we have tested for the first time the DD process coupled to adsorption onto PUROLITE A520E resin. First, experiments were performed with 0.5 mol·L−1 of NaCl solution (Na2SO4 or NaHCO3) only in the receiver compartment, and 100 mg·L−1 of initial nitrate concentration. The influence of the flow rate was investigated, and then the varying dose of PUROLITE A520E resin was added.

Influence of the nature of the receiver electrolyte

DD operations were performed with 0.5 M NaCl, 0.5 M Na2SO4 or 0.5 M NaHCO3 receiver solution. Figure 7 shows that the equilibrium concentration of the outlet feed solution, , was higher or lower than the norm with the sulfate, hydrogenocarbonate and chloride ion, respectively.
Figure 7

Time dependence of the feed removal efficiency of the NO3- ions. F: 100 mg/l KNO3; R: 0.5 M NaCI; 0.5 M Na2SO4; 0.5 M NaHCO3; QR = 180 ml/h, mresin = 2 g.

Figure 7

Time dependence of the feed removal efficiency of the NO3- ions. F: 100 mg/l KNO3; R: 0.5 M NaCI; 0.5 M Na2SO4; 0.5 M NaHCO3; QR = 180 ml/h, mresin = 2 g.

The lower efficiency of the sulfate ion could be explained by its higher affinity for the ionized sites of the membrane. Some authors have studied the role of the receiver electrolyte in optimizing the DD of feed monovalent cations and anions (Saracco 1997). They have demonstrated that the receiver electrolyte would be selected to minimize the association between the fixed sites and the driving ions.

Influence of the flow rate

The flow rate was one of the important parameters of the DD process. It was studied in order to improve the efficiency of the system and thus of the nitrate removal yield.

The influence of the flow rate in the feed compartment was studied with a 0.5 M NaCl solution as receiver electrolyte, the feed solution being 100 mg/L KNO3.

Figure 8 shows that nitrate removal efficiency decreases with the increase in the flow rate, in the concentration range studied from 180 to 450 ml/h.
Figure 8

Time dependence of nitrate removal efficiency for different flow rates. F: 100 mg/l KNO3; R: 0.5 M NaCl; mresin = 2 g.

Figure 8

Time dependence of nitrate removal efficiency for different flow rates. F: 100 mg/l KNO3; R: 0.5 M NaCl; mresin = 2 g.

It was observed that at higher values of flow rate, the nitrate removal efficiency decreases. This can be explained by the residence time of ions inside the cell compartment. In fact, the nitrate ions have more time to be transferred from the membrane when the flow rate is lower. This result was in accordance with those of Dieye et al. (1998).

Influence of the adsorbent mass

The effect of the adsorbent mass in the receiver compartment on the nitrate removal efficiency through the AM3 AEX is shown in Figure 9, which indicates that the transport of nitrate was maximum at m = 2 g. At a mass above 1 and 3, a decrease in nitrate transport was observed.
Figure 9

Time dependence of nitrate removal efficiency for different adsorbent masses. F: 100 mg/l KNO3; R: 0.5 M NaCl; QR = 180 ml/h, mresin = 1, 2, 3 g.

Figure 9

Time dependence of nitrate removal efficiency for different adsorbent masses. F: 100 mg/l KNO3; R: 0.5 M NaCl; QR = 180 ml/h, mresin = 1, 2, 3 g.

Removal of nitrates from model water by hybrid process

We realized this study by following the optimal conditions deducted from studies carried in the previous part ([NO3-] = 100 mg/l; [Cl-] = 0.5 M; T = 25 °C; Q = 180 ml·h−1, mresin = 2 g). The application of the hybrid process to the treatment of a model water having the same composition as that of a natural tap water but with a nitrate content that exceeds the maximum acceptable value (50 mg/l) (Table 3) gives a rate of elimination of 49.77%.

Figure 10 shows the amount of nitrate removal in the feed compartment in two cases: model water (▴) and nitrate solution (▪). It is noteworthy that there was an imminent decrease of nitrate removal when we treated the model water by the hybrid process. This profit can be explained by the influence of the presence of the other anions (Cl-, SO42-, HCO3-) in the model water, which prevent the passage of the nitrate ions through the AEX first of all, which reduces the return on the hybrid process.
Figure 10

Amount of nitrate removed in the feed compartment for natural tap water and for nitrate solution.

Figure 10

Amount of nitrate removed in the feed compartment for natural tap water and for nitrate solution.

CONCLUSION

The combination of adsorption and the DD process permits denitrification in a batch mode. Moreover, it avoids putting adsorbents directly in contact with drinking water. This work indicates that nitrate transport depends on the nature of the receiver electrolyte, flow rate and mass of resin. In fact, the use of NaCl as a counter anion increases the nitrate removal efficiency. By decreasing the flow rate from 450 to 180 ml/h, we can increase the nitrate removal efficiency through the membrane. Under the above conditions, the highest nitrate removal capacity is obtained (7 mg/g).

ACKNOWLEDGEMENTS

The authors would like to acknowledge CERTE, for providing research facilities.

REFERENCES

REFERENCES
Arden
T. V.
1994
New World Water
, p.
59
.
Baes
A. U.
Okuda
T.
Nishijima
W.
Shoto Okada
E. M.
1997
Adsorption and ion exchange of some groundwater anion contaminants in an amine modified coconut coir
.
Water Sci. Technol.
35
(
7
),
89
95
.
Baes
B. U.
Jung
Y.-H.
Han
W.-W.
Shin
H.-S.
2002
Improved brine recycling during nitrate removal using ion exchange
.
Water Res.
36
(
13
),
3330
3340
.
Das
J.
Patra
B. S.
Baliarsingh
N.
Parida
K. M.
2006
Adsorption of phosphate by layered double hydroxides in aqueous solutions
.
Appl. Clay Sci.
32
,
252
260
.
Dieye
A.
Larchet
C.
Auclair
B.
Mar-Diop
C.
1998
Elimination des fluorures par la dialyse ionique croisée
.
Eur. Polym. J.
34
,
67
75
.
Elmidaoui
A.
Elhannouni
F.
Sahli
M. A. M.
Chay
L.
Elabbassi
H.
Hafsi
M.
Largeteau
D.
2001
Pollution of nitrate in Moroccan ground water: removal by electrodialysis
.
Desalination
136
(
1–3
),
325
332
.
Elmidaoui
A.
Elhannouni
F.
Taky
M.
Chay
L.
Sahli
M. A. M.
Echihabi
L.
Hafsi
M.
2002
Optimization of nitrate removal operation from ground water by electrodialysis
.
Sep. Purif. Technol.
29
(
3
),
235
244
.
Ergas
S. J.
Rheinheimer
D. E.
2004
Drinking water denitrification using a membrane bioreactor
.
Water Res.
38
(
14–15
),
3225
3232
.
Hafshejani
L. D.
Hooshmanda
A.
Naseri
A. A.
Mohammadi
A. S.
Abbasi
F.
Bhatnagar
A.
2016
Removal of nitrate from aqueous solution by modified sugarcane bagasse biochar
.
Ecological Engineering
95
,
101
111
.
Hichour
M.
Persin
F.
Sandeaux
J.
Gavach
C.
2000
Fluoride removal from waters by Donnan dialysis
.
Sep. Purif. Technol.
18
,
1
11
.
Peavy
H. S.
Rowe
D. R.
Tchobanoglous
G.
1985
Environmental Engineering
.
McGraw-Hill Book Company
,
New York
,
696
pp.
Reyes-Avila
J.
Razo-Flores
E.
Gomez
J.
2004
Simultaneous biological removal of nitrogen, carbon and sulfur by denitrification
.
Water Res.
38
(
14–15
),
3313
3321
.
Sahli
M. A. M.
Tahaikt
M.
Achary
I.
Taky
M.
Elhanouni
F.
Hafsi
M.
Elmghari
M.
Elmidaouia
A.
2004
Technical optimisation of nitrate removal from ground water by electrodialysis using a pilot plant
.
Desalination
167
(
15
),
359
.
Salem
K.
Sandeaux
J.
Molenat
J.
Sandeaux
R.
Gavach
C.
1995
Elimination of nitrate from drinking water by electrochemical membrane processes
.
Desalination
101
(
2
),
123
131
.
Saracco
G.
1997
Transport properties of monovalent-ion-permselective membranes
.
Chemical Engineering Science
52
,
3019
3031
.
Sihrimali
M.
Singh
K. P.
2001
New methods of nitrate removal from water
.
Environ. Pollut.
112
,
351
359
.
Strathmann
H.
2004
Ion-exchange Membrane Separation Processes
.
Elsevier
,
Amsterdam
.
Turki
T.
Ben Hamouda
S.
Hamdi
R.
Ben Amor
M.
2012
Nitrates removal on PUROLITE A 520E resin: kinetic and thermodynamic studies
.
Desalination and Water Treatment
41
,
1
8
.
Turki
T.
Hamdi
R.
Tlili
M.
Ben Amor
M.
2015
Donnan dialysis removal of nitrate from water: effects of process parameters
.
American Journal of Analytical Chemistry
6
,
569
576
.
Velizarow
S.
Reis
M. A.
Crespo
J. G.
2002
Integrated transport and reaction in an ion-exchange membrane bioreactor
.
Desalination
149
,
205
210
.
WHO
2011
Guidelines for drinking-water quality. Fluoride. World Health Organization (http://www.who.int/water sanitation 1th/GDWQ/Chemicals/fluoridefull/html
).