Adsorption of Nitrate by a Novel Polyacrylic Anion Exchange Resin from Water with Dissolved Organic Matters: Batch and Column Study

Featured Application

The novelty of this work is that the prepared resin AEE-3 has a potential application in the removal of nitrate from real secondary treated wastewater containing dissolved organic matters.

Abstract

A novel anion exchange resin AEE-3 was synthesized by N-alkylation of a weakly basic polyacrylic anion exchanger D311 with 1-bromopropane to effectively remove nitrate (NO₃⁻-N) from aqueous solution. The related finding revealed that its adsorption isotherm obeyed the Langmuir model well, and the second-order model was more validated for the NO₃⁻-N adsorption kinetics study. Compared to commercially-available polystyrene-based nitrate specialty resin Purolite A 520E (A520E), AEE-3 resin has a higher adsorbed amount and better regeneration performance toward NO₃⁻-N in the existence of dissolved organic matter (DOM) using static and dynamic methods. Notably, a real secondary treated wastewater (STWW) obtained from a local municipal wastewater treatment plant was also assessed for NO₃⁻-N removal in fixed-bed columns. Observations from this study indicated that AEE-3 could effectively remove NO₃⁻-N from contaminated surface water.

1. Introduction

With the rapid development of industry and agriculture, excessively-used nutrients such as nitrogen (N) and phosphorous (P) in crop production have been infiltrated into rivers, lakes and ponds, leading to the eutrophication of aquatic systems [1,2]. Nowadays, nitrate (NO₃⁻-N) pollution in surface and ground water is a severe environmental problem due to its carcinogenicity, toxicity and potential hazards to human health and livestock. Therefore, the World Health Organization (WHO) set the NO₃⁻-N concentration limit of 50 mg/L, whereas the U.S. Environmental protection Agency (U.S. EPA) allows 10 mg/L in drinking water [3,4].

Current approaches like reverse osmosis, electrodialysis, biological denitrification, adsorption and ion exchange have been utilized to treat NO₃⁻-N polluted water [5,6]. In consideration of the merits of ion exchange, such as high efficiency, acceptable regeneration and cost-effective preparation, anion exchange resins (AERs) have been employed to adsorb NO₃⁻-N from contaminated water [7,8,9]. On the basis of their chemical structure, commercial AERs are normally categorized into two skeletons, namely, polystyrene and polyacrylic. Polystyrene AERs have been used to effectively adsorb NO₃⁻-N from ground water in the past decade [10,11]. However, high concentrations of dissolved organic matters (DOMs) always exist in surface water, including secondary treated wastewater (STWW), and can engender a strong affinity for resins via electrostatic attraction, hydrophobic interaction and so on [12,13,14]. Hence, AERs are more sensitive to fouling with DOMs, which would compete with NO₃⁻-N for the active sites and cause the decrease in NO₃⁻-N adsorption. More seriously, coexisting DOM could lead to an irreversible fixation to anion exchangers. Polyacrylic AERs with hydrophilic structures reveal a stronger resistance to organic fouling performance than polystyrene AERs [15]. Moreover, acrylic-based resins possess unique physiochemical properties, rapid adsorption rates and high ion-exchange capacities [16,17]. Accordingly, polyacrylic AERs could serve as a promising absorbent to effectively remove NO₃⁻-N from polluted water in the presence of DOMs. Nevertheless, few works have shown the feasibility of polyacrylic resins for NO₃⁻-N removal in real STWW containing NO₃⁻-N at high levels.

However, competitive anions such as chloride (Cl⁻) or sulfate (SO₄²⁻) exist widely in water, which can weaken NO₃⁻-N uptake efficacy of AERs [18]. To circumvent this limitation, studies have been devoted to preparing a novel anion exchanger with prominent selectivity toward NO₃⁻-N. The long alkyl chain addition at the exchange sites of resins could be of great benefit for the preferable selectivity toward monovalent anions because of its steric hindrance and hydrophobic effect [19,20]. Hence, it is intriguing for us to modify the weakly basic resin by introducing long alkyl groups around the active amine groups, which can easily form strongly basic functional groups (quaternary ammonium) which interact with NO₃⁻-N through strong electrostatic attraction.

Herein, a weakly basic polyacrylic anion exchanger D311 was modified with 1-bromopropane (C₃H₇Br) to prepare a strongly basic anion exchanger AEE-3 with high efficiency and brilliant selectivity for NO₃⁻-N uptake. The adsorption kinetics, isotherms, influence of pH value and competitive anions were investigated. In addition, commercial nitrate-selective polystyrene resin Purolite A 520E (A520E) was chosen to make a comparison with AEE-3 in both batch and fixed-bed column experiments, further demonstrating that AEE-3 is more resistant to fouling by DOMs (humic acid, tannic acid and sodium dodecyl benzene sulfonate). Also, a dynamic experiment was studied to evaluate the adsorption performance of AEE-3 toward NO₃⁻-N in real STWW.

2. Materials and Methods

2.1. Material

A commercially-available weakly basic anion exchanger D311 and strongly basic anion exchanger Purolite A 520E (A520E) were obtained from Huizhu Resin Co., Ltd. (Shanghai, China) and Purolite Co., Ltd. (Huzhou, China), respectively. Tannic acid (TA), humic acid (HA) and sodium dodecyl benzene sulfonate (SDBS) were purchased from Sinopharm Chemical Reagent Company (Shanghai, China). Other chemicals in this work were of analytical reagent acquired from Nanjing Chemical Reagent Co., Ltd. (Nanjing, China). STWW samples with a high NO₃⁻-N concentration were obtained from a local municipal wastewater treatment plant in Nanjing, China.

2.2. Preparation and Characterization of AEE-3

D311 resin was firstly rinsed successively with 1 mol/L HCl and 1 mol/L NaOH in turn to remove impurities, and then washed with deionized water until neutral. Finally, resin was washed by ethyl alcohol and then dried at 323 K for 8 h.

The synthesis method of AEE-3 is the following (Scheme 1): 10 g of dried D311 resin was swollen in 100 mL of acetonitrile for 2 h. Afterwards, 20 mmol of KI and 20 mmol C₃H₇Br were added separately and the mixture maintained with stirring at 333 K for 48 h. After alkylation, the obtained resin beads were rinsed successively with dilute HCl, distilled water and dilute NaOH to attain a neutral pH. Finally, the product was extracted with anhydrous ethanol and dried at 323 K for 8 h. The obtained resin AEE-3 was characterized by Fourier transform infrared spectrometer (Nicolet 5700, Madison, USA) and N₂ adsorption analysis (ASAP-2010C, Micromeritics, Norcross, GA, USA).

2.3. Batch Adsorption Experiments

Isothermal adsorption experiments of NO₃⁻-N on adsorbents were performed when 0.05 g resin was introduced to each 150 mL conical flasks containing 50 mL of NO₃⁻-N in solution (10 to 60 mg/L concentration). To investigate the adsorption kinetics, 0.2 g resin was immersed into 200 mL NO₃⁻-N solution with the initial concentration of 60 mg/L, and samples were withdrawn at different time intervals. The pH (4.0–10.0) with 50 mg/L NO₃⁻-N solution in the presence of Cl⁻ (200 mg/L) was adjusted using 0.1 mol/L HCl or 0.1 mol/L NaOH solution, respectively, as required. A competing anion (SO₄²⁻ or Cl⁻) was added to the NO₃⁻-N solution to study the effect of negative ions on adsorption capacities. The static tests of D311 and AEE-3 were conducted in a bi-solute system (HA/NO₃⁻, SDBS/NO₃⁻ or TA/NO₃⁻) for four adsorption-desorption cycles. Analyses of NO₃⁻-N were obtained by measuring the UV1800 spectrophotometer at a wavelength of 220 nm. The adsorption amount Q_e (mg/g) was calculated using Equation (1):

$$Q_e=\frac{(C_0-C_{\mathrm{e}})V}{W}$$

(1)

where C₀ and C_e are the initial and equilibrium concentrations (mg/L) of NO₃⁻-N, V is the volume of solution (L) and W represents the weight of dry resin (g).

2.4. Column Adsorption and Desorption

The experiments were carried out using a glass column (Φ18 × 200 mm) with 5 mL of dry fresh resin and a peristaltic pump to ensure the constant flow rate under 298 K. The initial concentration of NO₃⁻-N was 20 mg/L, containing 1 mg/L TA, 1 mg/L HA and 2 mg/L SDBS. The superficial liquid velocity (SLV) was 30 bed volume per hour (BV/h), and the breakthrough curve was obtained by analysis of different time intervals of the effluent. After adsorption, the elution of NO₃⁻-N from the resin was performed using 0.6 mol/L NaCl solution. The adsorption–desorption cycle was repeated three times to evaluate the regeneration efficiency of resins. The desorption rate (D) of NO₃⁻-N was defined by Equation (2):

$$D(\%)=\frac{C_{\mathrm{d}}}{C_{\mathrm{a}}}\times 100$$

(2)

where C_a is the amount of NO₃⁻-N adsorbed on resins and C_d is the amount of NO₃⁻-N desorbed from resins by NaCl solution.

3. Results and Discussion

3.1. Characterization

The adsorption-desorption isotherms and the pore size distribution curve of D311 and AEE-3 are displayed in Figure 1. The isotherm plot of two resins can be considered type IV with a hysteresis loop in the relative pressure range of 0.8–1.0, hinting at the existence of both mesopores and macropores in these resins.

Table 1. Physicochemical properties of two resins.

Resin	Strong Base Ion-Exchange Capacity (mmol/g Dry Resin)	Average Pore Diameter a (nm)	BET Surface Area (m2/g)	Pore Volume (cm3/g)
D311	0	40.92	31.88	0.36
AEE-3	3.42	47.44	23.46	0.20

^a BJH Desorption average pore diameter.

lists the essential properties of D311 and AEE-3.

The FT-IR spectra of D311 and AEE-3 samples are shown in Figure 2. Notably, two bands with peaks at 1366 cm⁻¹ and 2930 cm⁻¹ are ascribed to asymmetric bands of the alkyl groups (–CH₂–and‒CH₃), and the peak at 1478 cm⁻¹ is assigned to C–N group. The appearance of a new band at 1407 cm⁻¹ is associated with –CH₂–of–CH₂–N⁺R₃ group [7]. On the other side, the fact that the strong base ion-exchange capacity of the resin increased from 0 to 3.42 mmol/g implied that propyl groups were successfully introduced onto the primary amine after alkylation. These observations proved AEE-3 had been synthesized successfully.

3.2. Adsorption Kinetics

Figure 3 describes the effect of contact time on NO₃⁻-N uptake by AEE-3 and A520E at 298 K, and all the data were simulated using two commonly used kinetic models [21,22]:

$$\mathrm{Pseudo}-\mathrm{first}-\mathrm{order}:Q_{\mathrm{t}}=Q_{\mathrm{e}}(1-e^{-k_{1}t})$$

(3)

$$\mathrm{Pseudo}-\sec \mathrm{ond}-\mathrm{order}:Q_{\mathrm{t}}=\frac{k_2{Q}_{\mathrm{e}}^{2}t}{1+k_2{Q}_{\mathrm{e}}t}$$

(4)

The initial adsorption rate was calculated by Equation (5):

$$h_0=k_2{Q}_{\mathrm{e}}^2$$

(5)

where Q_e (mg/g) is the equilibrium adsorption capacity of NO₃⁻-N, and Q_t (mg/g) refers to the amounts of NO₃⁻-N in resins at time t (min). k₁ (1/min) and k₂ (g/(mg·min)) represent the rate constants for the pseudo-first- and pseudo-second-order kinetic models, respectively. h₀ is the initial adsorption rate (mg/(g·min)). The kinetic parameters, together with the coefficient of determination (R²), are listed in

Table 2. Kinetic parameters of NO₃⁻-N adsorption by two resins.

Resin	Qe, exp (mg/g)	Pseudo-First-Order				Pseudo-Second-Order
		Qe (mg/g)	k1 (1/min)	R2	Qe (mg/g)	103k2 (g/(mg·min))	R2	h0 (mg/(g·min))
AEE-3	32.75	30.85	0.327	0.9371	33.66	6.06	0.9819	7.13
A520E	30.22	29.19	0.100	0.9897	30.14	1.45	0.9987	1.16

. It is evident that the adsorption behavior of NO₃⁻-N onto both resins was preferable to fit the pseudo-second-order based on the R² values. Obviously, AEE-3 exhibited a greater capacity for NO₃⁻-N than A520E, illustrating that the introduction of alkyl chains would form quaternary ammonium groups which can easily interact with NO₃⁻-N via electrostatic attraction. Finally, a larger h₀ value of AEE-3 was obtained, and the difference in the adsorption rate is related to the difference of the resin matrix.

3.3. Effect of Coexisting Anions

In general, some coexisting anions in water would strongly compete with NO₃⁻-N to occupy active sites via electrostatic interaction. Thus, the selective absorption of resin is of vital importance in evaluating the practical application, as well as in considering the absorption performance. A520E is known for its good selectivity toward NO₃⁻-N, and its adsorption capacities are still relatively high in the present of Cl⁻ or SO₄²⁻ [20]. As displayed in Figure 4, a slightly higher adsorption amount of AEE-3 was detected compared to A520E with an identical addition of anions. The phenomena may be rationalized by the fact that the long alkyl chain on AEE-3 is conducive to the adsorption selectivity for NO₃⁻-N with lower hydration energy than Cl⁻ or SO₄²⁻.

3.4. Effect of pH

The primary mechanism for NO₃⁻-N uptake onto a strongly basic anion exchanger can be explained by electrostatic interaction or columbic force [23]. Figure 5 displays the influence of solution pH varying from 4.0 and 10.0; the adsorption capacity of NO₃⁻-N on AEE-3 was in the range of 25.5 to 26.0 mg/g under a Cl⁻ concentration of 200 mg/L. It shows a considerable amount of NO₃⁻-N uptake independent of the pH for the strongly basic anion exchanger. It may be concluded that the quaternary tripropylamine groups of AEE-3 could remain stable, thereby causing steady adsorption activity for NO₃⁻-N under both acidic and alkaline conditions [24].

3.5. Adsorption Isotherms

The isotherms of NO₃⁻-N adsorption by AEE-3 are displayed in Figure 6. The experimental data were fitted by both Langmuir and Freundlich equations which are represented as follows [25,26]:

$$Q_{\mathrm{e}}=\frac{Q_{\mathrm{m}}{K}_{\mathrm{L}}{C}_{\mathrm{e}}}{1+K_{\mathrm{L}}{C}_{\mathrm{e}}}$$

(6)

$$Q_{\mathrm{e}}=K_{\mathrm{F}}{C}_{\mathrm{e}}^{1/\mathrm{n}}$$

(7)

where C_e (mg/L) is the concentration at equilibrium state, Q_e (mg/g) is the adsorption capacity at equilibrium state, Q_m (mg/g) is the maximum adsorption capacity, K_L is the Langmuir constant and K_F and n are the Freundlich constants. The R² of the isothermal fit are presented in

Table 3. Isotherm parameters for NO₃⁻-N adsorption at 288, 303, and 318 K.

T (K)	Langmuir Model			Freundlich Model
	KL	Qm (mg/g)	R2	KF	n	R2
288	0.12	40.32	0.9915	7.18	2.21	0.9816
303	0.10	40.00	0.9977	6.39	2.13	0.9810
318	0.09	39.45	0.9973	5.66	2.06	0.9721

. In view of the R² values, the equilibrium adsorption of NO₃⁻-N onto AEE-3 was better described by the Langmuir isotherm, which coincided with the prevalent ion-exchange mechanism [7,23]. Moreover, the Q_m values of AEE-3 dropped with the increase of temperature, revealing that the NO₃⁻-N adsorption by the resin was an exothermic process, and therefore, reducing the temperature benefited NO₃⁻-N uptake.

The maximum NO₃⁻-N adsorption amount of AEE-3, as evaluated according to the Langmuir model, was compared with other materials including chitosan, activated carbon and commercial anion exchange resins (A520E and D201), etc. As shown in

Table 4. Comparison of NO₃⁻-N maximum adsorption capacity (Q_m) by different materials.

Samples	Qm (mg/g)	Reference
Chitosan/PVA	35.00	[27]
Cationic polymer-modified granular activated carbon (CPMG)	27.56	[28]
Modified commercial activated carbon	21.51	[29]
Magnetic anion-exchange resin MD217	30.40	[30]
A520E	36.15	[31]
D201	38.46	[31]
AEE-3	40.32	This study

, the Q_m of AEE-3 for NO₃⁻-N uptake was higher than that of these commercially available materials, signifying that the novel anion exchange resin can act as a potential material for the technological application of NO₃⁻-N removal from water.

3.6. Effect of DOMs on Static Adsorption and Desorption

Figure 7 shows the adsorption amounts of NO₃⁻-N using AEE-3 and A520E in a bi-solute system (HA/NO₃⁻, SDBS/NO₃⁻ or TA/NO₃⁻) for four adsorption-desorption cycles. It was seen that the adsorption capacities of A520E for NO₃⁻-N distinctly declined over the four cycles. DOMs can occupy more active sites through hydrophobic and electrostatic interactions on the A520E resin with a polystyrene matrix [12]. By contrast, AEE-3 exhibited greater NO₃⁻-N adsorption in bi-solute systems, and the reduction of NO₃⁻-N uptake was slight with increasing the number of adsorption-desorption cycles. This result validated the hypothesis that AEE-3 has a strong resistance to interferential organics due to its hydrophilic matrix. Besides, over 94% of the adsorbed NO₃⁻-N on AEE-3 was desorbed, while the desorption rates of A520E in the presence of organic matters declined gradually with the number of adsorption-desorption cycle (Figure 8). In addition, Figure 9 presents the desorption rates of DOMs regenerated by a NaCl solution, and the low values for A520E resin can be attributed to the strong affinity between the polystyrene matrix and the hydrophobic organics, causing the DOMs to not be removed fully by regeneration. Hence, these results demonstrate an improved performance on adsorption/regeneration of AEE-3 compared to A520E.

3.7. Effect of DOMs on Column Mode Experiments

It is imperative to test the utilization efficiency of adsorbents in a dynamic fixed-bed experiment related to real operating systems [32]. The influence of three organics on NO₃⁻-N adsorption was studied using an influent with constant NO₃⁻-N concentration of 20 mg/L in the presence of HA, TA and SDBS. As can be seen in Figure 10, the breakthrough curves (Run 2 and Run 3) for NO₃⁻-N adsorption onto AEE-3 have a close coincidence with the original breakthrough curve (Run 1). Nevertheless, a large separation between the three breakthrough curves was observed for NO₃⁻-N adsorption onto A520E, which is indicative of a limited reusability of A520E towards NO₃⁻-N solution containing DOMs.

3.8. Assessments of Practical Application

Conventional heterotrophic denitrification techniques are severely restricted by the low C/N ratio of wastewater, thereby causing the high NO₃⁻-N content in STWW [33,34]. To further assess the practical application of AEE-3, the dynamic column test of NO₃⁻-N uptake onto AEE-3 was performed using a real STWW effluent which was collected from a local sewage treatment plant, containing the following quality parameters: pH, 6.54; NO₃⁻-N, 20 mg/L; COD, 39 mg/L; TDS, 436 mg/L. The breakthrough curves of AEE-3 are shown in Figure 11. An inconspicuous deviation between Run 1 and Run 3 breakthrough curves was found, further confirming its potential application in the removal of NO₃⁻-N from real STWW effluent.

4. Conclusions

The strong base polyacrylic AER AEE-3 could be developed and utilized to absorb NO₃⁻-N selectively from aqueous solutions. Batch adsorption studies exhibited that a pseudo-second-order model was appropriate for depicting the kinetic process, and that the best fit for the isotherms data was Langmuir model. In addition, AEE-3 displayed a greater adsorption capacity and better regeneration capability for NO₃⁻-N uptake than the commonly-used, nitrate-selective AER A520E in the presence of interferential DOMs. Notably, the effective adsorption of NO₃⁻-N in real STWW by AEE-3 packed in fixed-bed columns revealed its promising potential in the removal of NO₃⁻-N from actual complex wastewater.