Selective removal of nitrate from water by a macroporous strong basic anion exchange resin

Desalination 296 (2012) 53–60

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Selective removal of nitrate from water by a macroporous strong basic anion exchange resin Haiou Song, Yang Zhou, Aimin Li ⁎, Sandra Mueller State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210046, PR China National Engineering Research Center for Organic Pollution Control and Resources Reuse, Nanjing 210046, PR China S. E. P. E. C. for Organic Chemical Industrial Waste Water Disposal and Resources Reuse, Nanjing 210046, PR China

a r t i c l e

i n f o

Article history: Received 1 January 2012 Received in revised form 1 April 2012 Accepted 2 April 2012 Available online 12 May 2012 Keywords: Anion exchange resin Nitrate Selectivity Absorption

a b s t r a c t An anion exchange resin (NDP-2) was synthesized for selective nitrate removal in the binary co-existence systems. The characterization of NDP-2 have been performed by FT-IR, SEM and BET surface area analyses. The results showed that the amounts of nitrate sorbed onto NDP-2 were the highest compared to D201 and Purolite A 300 (A 300) resin at equilibrium, and its sorption behavior followed the Langmuir adsorption isotherm model well. Furthermore, both the pseudo-first order and the pseudo-second order kinetic models showed well fitting about the process of nitrate sorbed onto NDP-2 resin. Attractively, NDP-2 resin demonstrated the more preferable absorption toward nitrate than the commercial D201 and Purolite A 300 in the presence of competing ions, such as SO42−, Cl − and HCO3−, in aqueous solution. Therefore, NDP-2 resin would be a promising adsorbent to improve removal of nitrate from contaminated drinking water resources. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.

1. Introduction Nitrate contamination due to the higher solubility is widely dissolved in surface and ground water. Toxic nitrate has a negative effect on the human's health [1], which could result in many diseases, such as birth defects, spontaneous abortion, increased infant mortality, diarrhea, abdominal pain, vomiting, diabetes, hypertension, respiratory tract infections, changes in the immune system, and methemoglobinemia [2–6]. With rapid development of industry and agriculture, more “three wastes” have been produced, and after their infiltrating into ground, the concentration of nitrate in ground water would be significantly increased [7–10]. Hence, the recommended standards (100 mg NO3 −/L) in drinking water have been established by European Community [11–14]. How to remove nitrate from water efficiently is always one of the challenging topics. Researchers have developed many approaches successfully, such as ion exchange, biological denitrification, reverse osmosis and electrodialysis etc. And they have to be employed for removing nitrate as much as possible in order to protect consumers from adverse effects associated with high nitrate concentration [10,11,15–24]. Biological denitrification method using degradation of microorganism offers the possibility of a very specific and selective reduction of nitrate to nitrogen. However, there are some limitations ⁎ Corresponding author. Tel.: + 86 25 8968 0568; fax: + 86 25 8968 0569. E-mail addresses: [email protected] (H. Song), [email protected] (A. Li).

due to contamination of drinking water with germs and metabolic substances. So, an extensive reconditioning of the drinking water by filtration and germicidal treatment is necessary [19,21]. Among them, ion exchange technology is usually more suitable for water decontamination and removal of inorganic ions due to its simplicity, effectiveness, selectivity, recovery and relatively low cost [18,25]. Therefore, the researches on novel adsorbents based on ion exchange technology are attracting the global concern considering practical application. In addition, it is understandable that some common competing anions, such as sulfate, bicarbonate and chloridion, widely exist in the ground water. Though the concentrations of the non-toxic ions are lower, their existence would significantly influence the completion of nitrate sorbed on the exchange sites. Therefore, the investigation of selective absorption of nitrate in the presence of competing anions is becoming more necessary for solving practical pollution problems better [1,26–28]. In this paper, a macroporous styrene–divinylbenzene copolymer due to its satisfactory mechanical strength and easy chemical modification had been employed as the host material, and the NDP2 resin would be prepared by chloromethylation and following quaternarization. The objective of this study was just to investigate equilibrium and kinetic parameters for nitrate removal from aqueous solutions by a macroporous strong basic anion exchange resin named as NDP-2. In addition, the effects of competing anion on the nitrate removal were also studied. Both Langmuir and Freundlich equations could be used to fit the equilibrium isotherms better.

0011-9164/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2012.04.003

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2. Experimental 2.1. Materials and methods A strong selective anion exchange resin, Purolite A 300, was provided by Purolite Int. Ltd. D201 and Rs-Ph-CH2Cl resin were purchased from China Nan & Ge Inc of China Int. Ltd. The NDP-2 resin was synthesized in our group. These macroporous strong basic anion resins were designed in order to remove anions from aqueous solution. Their physicochemical properties and specification were shown in Table 1 [28,29]. All the inorganic chemicals were purchased from Chemical Company in Beijing as analytical-grade reagents. The preparation methods of the entire model solutions were the following. A proper amount of Na2SO4, NaNO3, NaCl or NaHCO3 compounds was dissolved in double-distilled (D.I.) water respectively. The experiments were performed with initial mole ratio concentrations of 1/4 (N/N) for NO3 −/anions. Under the experimental conditions, the adsorption isotherm of nitrate was carried out. 2.2. Batch adsorption experiments Batch adsorption experiments were carried out in 150 mL conical flasks according to the detailed experimental procedure described by Milmile and Chen et al. [1,31]. The resin (0.1 g) was contacted with nitrate in solution (50 mL) with different concentrations (50, 100, 200, 400 and 600 mg/L, respectively) for adsorption isotherm study at 293 K separately. The flasks were then transferred to an incubator shaker and vibrated at 140 rpm for 24 h to ensure the equilibrium adsorption. Other competing anion (SO4 2−, Cl − or HCO3 −) was added into the nitrate solution while studying selective absorption behaviors. Without adjusting the pH of the solutions, the operating pH of the solution was 6.28–9.20 and did not change significantly with the dilution. All batch experiments were repeated three times under the same conditions, the data listed in the Tables and the Figures are the average values, and the relative error was less than 5%. Preliminary kinetic experiments indicated that sufficiently reaching adsorption equilibrium onto NDP-2 resin would take 4 h of time. For the kinetics experiments, 500 mL nitrate solution with the initial concentration of 100 mg/L was transferred into a 1000 mL flask, 1.00 g of a given adsorbent particle was subsequently added. A 1.0 mL solution was sampled from the flasks at various time intervals to determine nitrate concentration in solution.

of nitrate in the presence of SO4 2−, Cl − or HCO3 −. The model solutions were delivered down-flow to the column at a flow rate of SV (space velocity) of 0.67 mL/min using a peristaltic pump. The breakthrough curve was obtained by analysis of successive 50 mL fractions of the effluent. The elution of nitrate from the resin was performed using 0.6 M NaCl solution at SV (space velocity) of 0.67 mL/min. The elution profile was obtained by collecting 50 mL of fractions with test tubes [28]. 2.4. Preparation of NDP-2 resin The synthesis method of NDP-2 is the following (Scheme S1 in supporting information): 50 g of chloromethylation of macro porous styrene-divinylbenzene copolymers (Rs-Ph-CH2Cl) cross-linked (3%) was first swollen in 100 mL of triethylamine, and the pH value of mixture was adjusted to 11 [32]. Then, the mixture was continuously stirred at 303 K for 16 h and at 333 K for 12 h separately. Subsequently, polymer beads were poured into an ethanol bath containing 1% hydrochloric acid. After being filtered, polymer beads were extracted with ethanol for 9 h in a Soxhlet apparatus and dried under vacuum at 333 K for 8 h [33]. Finally, resin obtained will be washed by cycling with 0.1 M NaOH and 0.1 M HCl for 3 times. The resulting beads were rinsed with deionized water until the effluent pH approached to 7, and then they would be dried under reduced vacuum until reaching constant weight. Elemental analysis of the NDP-2 is 56.89 wt.% C, 9.09 wt.% H, 1.46 wt.% N. 2.5. Characterization Nitrate concentration was determined by the Shimadzu model ion chromatography equipment (IC). Surface topography of the macromolecules before and after quaternarization could be observed with a scanning electron microscope (LEO 1530VP, Germany). FT-IR spectra of the adsorbent beads were recorded using a spectrophotometer (Nexus 870 FT-IR), and the spectra are recorded in the wave number ranging from 4000 to 400 cm − 1. N2 adsorption–desorption tests were carried out at 77 K to determine surface area and pore size distribution based on BJH model using Micromeritics ASAP 2020 (U.S.). 3. Result and discussion 3.1. Characterization of NDP-2 resin

2.3. Column-mode sorption–elution studies The column tests were carried out with a column of glass with an internal diameter of 1.0 cm. The column was packed with 3.0 mL of wet-settled volume of resin with a particle size of 0.3–0.5 mm. All column sorption–elution experiments were performed with 100 mg NO3 −/L solution. Other ions were introduced to evaluate the removal

Table 1 Physicochemical properties of the resins. Property

Purolite A 300 [30]

D201 [30]

NDP-2

Skeleton

Gel polystyrene crosslinked with divinylbenzene R(CH3)2(C2H4OH)N+ Clear spherical beads

Macroporous styrene– divinylbenzene R(CH3)3N+ Opaque cream spherical beads 0.3–1.2 Cl− 3.8

Macroporous styrene– divinylbenzene R(C2H5)3N+ Opaque cream spherical beads 0.3–1.2 Cl− 4.8

Functional groups Physical aspect Granulometry (mm) Ionic form Total exchange capacity (meq g− 1)

0.3–1.2 Cl− 3.5–3.7

The structure changes of Rs-Ph-CH2Cl, NDP-2, NDP-2-NO3 − and NO3 − were investigated by FT-IR. The two strong characteristic bands at 1266 cm − 1 and 662 cm − 1 are due to \CH2\ bending vibration of \CH2Cl and C\Cl group in Rs-Ph-CH2Cl separately (Fig. 1d). However, in NDP-2 resin obtained, both peaks mentioned above disappeared completely; the band at 1450 cm− 1 which was attributed to the \CH2\ groups red-shifted to 1480 cm− 1; and a new absorption band at 1400 cm− 1 corresponding to \CH2\ of \CH2\N+R3 group was observed (Fig. 1c and d). These results strongly illustrated that the NDP-2 resin had been synthesized successfully. Attractively, the ability of removal nitrate for NDP-2 resin could be further investigated by FT-IR. After absorbing the nitrate, a distinct absorption peak at 1359 cm − 1 (Fig. 1b) attributed to anti-symmetry vibrating peaks of nitrate (Fig. 1a) appears, which strongly supported that nitrate could be sorbed onto NDP-2 resin. The surface topography of the resulting NDP-2 and Rs-Ph-CH2Cl resins was further observed by SEM, and micrographs were shown in Fig. 2. The particle size of NDP-2 resin had no difference with that of Rs-Ph-CH2Cl, and both were spherical. Specifically, more wrinkled and rough topography was presented in the latter (Fig. 2d), whereas only more uniform planar surfaces in the former (Fig. 2b). The other

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Table 2 Properties of the Rs-Ph-CH2Cl and NDP-2.

Fig. 1. FT-IR spectra of Rs-Ph-CH2Cl resin (d), NDP-2 resin (c), NDP-2-NO3− resin (b) and NO3− (a).

Property

BET surface area (m2/g)

Micropore area (m2/g)

Average pore diameter (nm)

Pore volume (ml/g)

Rs-Ph-CH2Cl NDP-2

29.41 10.39

5.70 0.97

14.12 8.21

0.0954 0.0034

where C0 and Ce are the concentrations of the nitrate in the solution at initial and equilibrium (mg/L), respectively; V is the volume of the solution (L); and W is the mass of the dry resin (g). From Fig. 3, the adsorption capacity of NDP-2 resin was the highest among the several chosen resins. It is understandable that Rs-Ph-CH2Cl resin could not remove nitrate. Compared to D201 and NDP-2, Purolite A 300 showed the lowest adsorption capacity (Fig. 3). However, the adsorption capacities of NDP-2 and D201 resins were almost the same, which is possibly due to their similar skeleton structures (Table 1). At the same time, the very small differences are attributed to the longer organic chains at the active sites [29].

3.3. Effect of competing anions on adsorption properties of NDP-2 resin were listed in Table 2. The difference of topography also illustrated the successful synthesis of NDP-2. This selection of resins had either been previously used for nitrate sorption (D201, NDP-2 and Purolite A 300, which represented a variation of polymer backbone and size of the tertiary amine). D201 and NDP-2 are type I strong-base macroporous resins with a polystyrene backbone. Purolite A 300 is a gel-type polystyrene resin with a pendant (methyl)2(ethanol) group. The data of BET, pore diameter and pore volume have been added in Table 2. After the RsPh-CH2Cl was functionalized with triethylamine, the introduction of organic molecules would result in an understandable drop in pore volume, BET surface area, etc. 3.2. Equilibrium adsorption studies Equilibrium adsorption isotherms of nitrate onto D201, NDP-2, Purolite A 300 and Rs-Ph-CH2Cl were investigated (Fig. 3). The amounts sorbed at equilibrium, (Qe, mg/g) were further calculated by Eq. (1) [33]

Qe ¼

V ðC 0 −C e Þ W

Fig. 2. Microphotographs of Rs-Ph-CH2Cl (a and b) and NDP-2 resin (c and d).

ð1Þ

Generally, some ions dissolved in natural water sources are environmentally friendly. However, they as competing ions would strongly interfere with its adsorption at active sites during the process of nitrate removal, which would result in inefficiency. So, the selectivity of resin has to be evaluated for practical application besides considering the absorption ability. Thus, it was crucial to determine adsorption preference of NDP-2 resin toward nitrate in the presence of competing anions. Fig. 4 showed that the adsorption isotherms of nitrate using D201, NDP-2 and Purolite A 300 in the SO4 2−–NO3− co-existence system at 293 K in model solutions (NO3−/SO4 2− = 1/4, N/N). Attractively, the adsorption capacities of D201 and Purolite A 300 resin were less 16% and 42% than that of NDP-2 resin respectively, when original concentrations of NO3−–N was 120 mg/L. The selectivity of removal nitrate of NDP-2 resin with longer chains is still the best. This result was consistent to the equilibrium adsorption studies [29,34]. The variations of the chain length at the active sites could explain the phenomena better [31,35]. For example, variation of only one log units from the trimethyl to triethyl could increase selectivity of resin [29]. There must be some very deep reasons for this. It is well known that the resins with longer triethyl functional groups become hydrophobic, which would change the interfacial interactions between anions and resin by reducing hydration energy [29]. In all, the NDP-2 resin can be believed as the best one for selective removal of nitrate among the chosen ones in this paper according to the current results.

Fig. 3. Equilibrium adsorption isotherms of nitrate on NDP-2, D201, Purolite A 300 and Rs-Ph-CH2Cl from model solutions at 293 K.

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longer chains (triethylamine) exhibits higher selectivity for nitrate, while D201 resin having shorter chains (trimethylamine) showed enhanced exchange capacity. So, the NDP-2 resin was suitable for removal nitrate in such binary competing systems. Furthermore, keeping the concentration of nitrate constant, the effects of increased competing anions (Cl −, HCO3− and SO42−) on nitrate retention have been investigated by NDP-2 at 293 K (Fig. 6). The aqueous solutions dissolved NO3− and any one of Cl −, HCO3− or SO42− respectively according to the assigned ratios were regarded as model ones. The corresponding adsorption isotherms were shown that the competing ions could reduce the adsorption capacities of nitrate on NDP-2 resin in the binary co-existence system. HCO3− had the least impact on the removal of nitrate compared to Cl− and SO42−. The results suggested that the affinity sequence NO3− >SO42− > Cl−>HCO3− appears to be dependent of the initial total ionic ratio at b20. The conclusions obtained above demonstrate that removal nitrate utilizing NDP-2 resin was most effective again [29]. The mechanism of selective removal by ion exchange approach could be explained with the hydration energy of the target ion. Lower hydration energy is more favorable for ion exchange [36]. For example, the hydration energy of nitrate (ΔG°, −314 kJ/mol) is lower than Cl− (ΔG°, −363 kJ/mol) and SO42− (ΔG°, −1103 kJ/mol) [29]. Therefore, the NDP-2 resin preferentially absorbed NO3− and was commonly referred as a NO3−-selective resin in removing NO3− in aqueous solution at relatively low concentrations. The ammonium type resins have a significantly stronger affinity for nitrate over either chloride or bicarbonate but a lower affinity for nitrate as compared to sulfate at ionic concentrations typical of potable waters. 3.4. Effect of different pH value The pH of the aqueous solution is identified as one of the most important variable in the batch adsorption studies. The effect of different pH has been studied and the results were shown in Fig. S1. Before the addition of the resin, the initial pH of solution was 6.28, and then solution pH was varied from 5 to 10 by adding 0.01 M HCl or 0.01 M NaOH. As depicted in Fig. S1 removal of nitrate from the solution was in the range of 39.84 to 40.58 mg/g for pH varying from 5 to 10, which indicated that the considerable adsorption of nitrate independent of the pH for the NDP-2 resin [1]. Most attractively, at different locations where the pH of the water varied, the NDP-2 resin would exhibit almost the identical adsorption activity due to the independent of the pH. Considering the complex and variable polluted water systems, the NDP-2 resin has broad prospects for practical application [1]. 3.5. Equilibrium adsorption studies

Fig. 4. The adsorption isotherms of nitrate using D201 (a), NDP-2 (b) and Purolite A 300 (c) when SO42− ions appear and absent at 293 K. The concentration of SO42− was 4 times of nitrate as equivalent concentration.

Nitrate adsorption isotherms onto NDP-2 resin were determined in the single-component solution and the experiment data were further correlated by the Langmuir (Eq. (2)) and Freundlich adsorption isotherm models (Eq. (3)) [1] Qe ¼

In order to broaden the application range of the NDP-2 resin, the selectivity had been tested in binary co-existence systems. Fig. 5 showed the effects of Cl −, HCO3 − and SO4 2− on NO3 − uptake by NDP-2 adsorbent comparing with two commercial resins. Every competing anion resulted in a dramatic decrease in nitrate adsorption onto three adsorbents with the variation of the molar ratios from 0 to 30. Interestingly, the decrease speed of equilibrium capacity for NDP2 resin was obviously slower than that of Purolite A 300 and D201 resin with the increase of concentration ratios. In another word, the extent of competing anions influencing equilibrium capacity of resin was different in the binary co-existence system. NDP-2 resin with the

Q 0 bC e 1 þ bC e ð1=nÞ

Q e ¼ K f Ce

ð2Þ

ð3Þ

where Q0 is the maximum amount of nitrate adsorbed (mg/g); Qe is the amount of nitrate adsorbed in equilibrium (mg/g); Ce is the nitrate concentration in equilibrium (mg/L); b is the Langmuir constants related to energy of adsorption. Kf and 1/n are Freundlich constants related to adsorption capacity and adsorption intensity, respectively. Fig. 7 showed that behavior of nitrate adsorption on NDP-2 resin can be represented by Langmuir isotherm (Fig. 7a, red

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Fig. 6. Effect of the competing anions (Cl−, HCO3− and SO42−) on nitrate retention by NDP-2 at 293 K. (Initial nitrate 100 mg/L; S/L ratio 2.00 g/L).

3.6. Adsorption kinetic studies Sorption kinetic experiments of nitrate on D201, NDP-2 and Purolite A 300 were also performed and the results were presented in Fig. 8. It can be seen that initial adsorption of nitrate was very quick, followed by a slow adsorption approaching equilibrium within 100 min. Kinetic data for three adsorbents were then represented by the pseudo-first-order model (Eq. (4)) and the pseudo-secondorder model (Eq. (5)) [38,39] logðQ e −Q t Þ ¼ logðQ e Þ−

K ad t 2:303

ð4Þ

Fig. 5. Effect of different nitrate levels by NDP-2, D201 and Purolite A 300 on Cl− (a), HCO3− (b) and SO42− (c) retention at 293 K. (Initial nitrate 100 mg/L; S/L ratio 2.00 g/L).

lines) and Freundlich isotherm (Fig. 7b, blue lines) adsorption isotherm models well, and the regression coefficient (R 2) were ≥ 0.98. The Langmuir adsorption isotherm plots (Qe vs. Ce) (Fig. 7a) and the Freundlich adsorption isotherm plots (Fig. 7b) showed the excellent applicability of Langmuir and Freundlich adsorption isotherms. The maximum capacity of NDP-2 resin was 174.20 mg/g, and energy of sorption value (b) was 0.052 L mg− 1. The results (Fig. 7 and Table 3) indicated that nitrate removal by NDP-2 can be represented by the Langmuir and Freundlich models reasonably. The major mechanism of sorption was electrostatic interaction [29]. The use of large alkyl quaternary ammonium groups sacrifices the anion-exchange capacity due to a limited bead-surface area [37].

Fig. 7. Equilibrium Langmuir isotherm (red line, a) and Freundlich isotherm (blue line, b) for nitrate sorbed using NDP-2, D201 and Purolite A 300 resin.

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Table 3 Values of the parameters for nitrate from water onto different adsorbents at 293 K as obtained from the Langmuir and Freundlich equation. Adsorbent

NDP-2 A 300 D201

Langmuir equation

Freundlich equation

Adsorbent

Q0 (mg/g-dry resin)

b (L/mg)

R2

n

Kf (mg/g)

R2

174.20 147.41 173.80

0.052 0.035 0.037

0.99 0.99 0.99

3.22 3.04 2.89

31.01 21.87 24.91

0.98 0.99 0.99

t 1 t ¼ þ Q t K t Q 2e Q e

Table 4 Kinetic parameters of adsorption for nitrate on NDP-2, D201 and Purolite A 300 resin at 393 K.

ð5Þ

where Qt is the nitrate concentration at time t and Qe is the nitrate concentration in equilibrium; Kad is the first-order adsorption kinetic constant (min − 1) and Kt is the intra-particle diffusion constant (min − 1). Kad values for nitrate reduction onto three resins are calculated through iterative algorithm in Fig. 8a (red lines). High correlation coefficients (R 2 > 0.99) indicated that uptake of nitrate onto three adsorbents can be approximated favorably by the pseudofirst-order and the pseudo-second-order models. As suggested by the Kad and Kt values presented in Fig. 8 and the equilibrium time required for three adsorbents, functional groups do not pose any negative role in adsorption kinetics. The data fitted and evaluated by application of equations were summarized in Table 4. The relatively high correlation coefficients (R 2 ≥ 0.98) indicated that both intra-particle-diffusion process and membrane diffusion process are dominant processes to control

NDP-2 D201 Purolite A 300

Pseudo-first order equation

Pseudo-second order equation

Kad

Qe (mg/g)

R2

Kt

Qe (mg/g)

R2

0.056 0.045 0.044

48.36 47.13 44.75

0.99 0.99 0.99

0.0014 0.0011 0.0011

53.93 53.55 50.86

0.98 0.98 0.98

sorption rate [17]. All of the other kinetics parameter constants were showed in the supporting information.

3.7. Column adsorption and regeneration The column study was performed using a 100 mg NO3 −/L model solution. Fig. 9 illustrated an effluent history of a separate fixed-bed column packed with NDP-2 for a feeding solution containing nitrate and competing anions (Cl−, HCO3 − and SO4 2−). Ce/C0 in excess of unity on breakthrough curves for nitrate adsorption onto NDP-2 was caused by the elution effect of the competing anion. As seen in Fig. 9, the breakthrough point was about 294.8 bed volumes (BV) with a breakthrough capacity of 111 mg NO3 −/g resin with simple nitrate model water. Different competing anions have different influence about the removal of nitrate. The ordering follows the sequence SO42− > Cl− > HCO3−. The nitrate sorbed on the resin was contacted with various concentrations of NaCl solutions. A quantitative stripping of nitrate from the resin was obtained with 0.6 M NaCl solution. So, the NDP-2 resin was subjected to in situ regeneration by using the 0.6 M NaCl solution. Fig. 10 showed that the elution profile. Nitrate loaded onto NDP-2 resin was quantitatively eluted with 60 BV of 0.6 M NaCl solution with the corresponding desorption efficiency about 98%.

4. Conclusions A novel NDP-2 resin with higher selective adsorption ability toward nitrate was synthesized successfully. In contrast to the commercial D201 and Purolite A 300 resins, NDP-2 showed a higher adsorption capacity for nitrate removal, which is possibly due to the variation of chain length on the exchange sites. And most importantly, this resin could remove selectively nitrate in the model aqueous solutions containing competing ions. The higher selectivity is possibly due to the longer alkyl chains at the exchange sites of NDP-2. So, it would widely be applied in purification of practical polluted drinking water resources in the near future.

Fig. 8. Influence of contact time on nitrate removal by NDP-2, D201 and Purolite A 300 resin of Pseudo-first-order (a) and Pseudo-second-order (b) equation fit of data.

Fig. 9. Breakthrough curves of nitrate sorbed on NDP-2 resin using model solution.

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Fig. 10. Elution curves of nitrate obtained by NDP-2 resin using model solution of NO3− (a), Cl−/NO3− (b), HCO3−/NO3− (c), SO42−/NO3− (d).

Acknowledgments We gratefully acknowledge the generous support provided by Program for Changjiang Scholars Innovative Research Team in University, NSFC (50938004, 50825802 and 51178215), Jangsu Nature Science Fund (BK2010006 and BK2011032) P.R. China and the Scientific Research starting fund for postdoctors, Nanjing University (No. 0211003046). We thank Purolite Int. Ltd, China Nan and Ge Inc of China Int. Ltd for supplying resins. Thanks to Y. Zhou for his efforts on synthesis of NDP-2 resin. Appendix A. Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.desal.2012.04.003. References [1] S.N. Milmile, J.V. Pande, S. Karmakar, A. Bansiwal, T. Chakrabarti, R.B. Biniwale, Equilibrium isotherm and kinetic modeling of the adsorption of nitrates by anion exchange Indion NSSR resin, Desalination 276 (2011) 38–44. [2] D.D. Weisenburger, Human health-effects of agrichemical use, Hum. Pathol. 24 (1993) 571–576. [3] A. Azizullah, M.N.K. Khattak, P. Richter, D.P. Häder, Water pollution in Pakistan and its impact on public health — a review, Environ. Int. 37 (2011) 479–497. [4] M.H. Ward, J.D. Brender, Drinking water nitrate and health, Encycl. Environ. Health (2011) 167–178. [5] J.A. Camargo, A. Alonso, Ecological and toxicological effects of inorganic nitrogen pollution in aquatic ecosystems: a global assessment, Environ. Int. 32 (2006) 831–849. [6] J. Colman, G.E. Rice, J.M. Wright, E.S. Hunter, L.K. Teuschler, J.C. Lipscomb, R.C. Hertzberg, J.E. Simmons, M. Fransen, M. Osier, M.G. Narotsky, Identification of developmentally toxic drinking water disinfection byproducts and evaluation of data relevant to mode of actio, Toxicol. Appl. Pharmacol. 254 (2011) 100–126. [7] B.J.S. Singh, G.S. Sekhon, Nitrate pollution of groundwater from farm use of nitrogen fertilizers—a review, Agric. Environ. 4 (1979) 207–225. [8] B. Basso, J.T. Ritchie, Impact of compost, manure and inorganic fertilizer on nitrate leaching and yield for a 6-year maize-alfalfa rotation in Michigan, Agric. Ecosyst. Environ. 108 (2005) 329–341. [9] M. Maeda, B. Zhao, Y. Ozaki, T. Yoneyama, Nitrate leaching in an Andisol treated with different types of fertilizers, Environ. Pollut. 121 (2003) 477–487.

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