Synthesis of nitrate selective anion exchange resin and denitrification of nitrate adsorbed on the resin

ABSTRACT

To purify groundwater contaminated with nitrate (NO₃-N) from manure and fertilizer used in tea fields, six anion exchange resins made with trimethylamine (1), triethylamine (2), tributylamine (3), trihexylamine (4), trioctylamine (5), or tridecylamine (6) were synthesized, and their performance was evaluated. The NO₃-N removal rates of resins 1–4 were very similar and were higher than those of resins 5 and 6. The nitrate adsorption by all resins was satisfactorily described by the Langmuir adsorption isotherm model. The groundwater originating from tea fields contains high concentrations of SO₄^2- in addition to NO₃-N. Resin 4 had the highest selectivity for NO₃-N adsorption in solutions containing SO₄²⁻. Therefore, resin 4 was selected for denitrification experiments, which demonstrated that NO₃-N adsorbed on resin 4 was denitrified and that resin 4 may be reusable. Biological denitrification was confirmed by measuring nitrous oxide (N₂O) in the headspace of the vial bottle containing 20 mL of the soil solution with NO₃-N removed, 0.1 g of glucose, and 0.1 g of NO₃-N-saturated resin 4. The denitrifying nitrogen calculated from the N₂O concentration was only 0.04% of the NO₃-N adsorbed on the NO₃-N-saturated resin 4. Moreover, when 1 g or more of glucose was added, approximately 10% of the NO₃-N in 10 mL of NO₃-N solution (100 mg-N/L) was absorbed on 0.05 g of resin 4 after denitrification. Further improvements in the efficiency of denitrification are necessary for their application in tea fields.

INTRODUCTION

Groundwater accounts for one quarter of annual domestic water use in Japan (Ministry of Health, Labour and Welfare, Japan, 2006). It is known that infants develop methemoglobinaemia when they ingest water with high concentrations of nitrate (NO₃-N) and nitrite (NO₂-N) (Downs, 1950). Therefore, the water quality standard for NO₃-N and NO₂-N was set to 10 mg/L by the Waterworks Act. However, in recent years, NO₃-N and NO₂-N contamination of groundwater has been reported in several places in Japan (Ministry of the Environment, Japan, 2004). Therefore, groundwater treatment and purification are necessary.

The sources of NO₃-N and NO₂-N contamination in groundwater include overfertilization, animal production, domestic wastewater, etc. (Pastén-Zapata et al., 2005; Wakida and Lerner, 2005; Tomiie et al., 2009; Obeidat et al., 2013; Wijayanti et al., 2013; Bourke et al., 2019; Nakagawa et al., 2021).

NO₃-N groundwater contamination is present in the southern part of Fukuoka prefecture in Japan. Its sources were found to be manure and fertilizers, such as ammonium sulfate [(NH₄)₂SO₄], used in tea fields (Matsuo et al., 2000; Ishibashi et al., 2004). Therefore, groundwater in this region contains high concentrations of sulfate (SO₄²⁻) in addition to NO₃-N (Ishibashi et al., 2015).

Techniques for the removal of NO₃-N include anion exchange resin filtration (Nujić et al., 2017; Nur et al., 2015). Jyo (2013) prepared several types of nitrate selective anion exchange fibers and evaluated their performance. The fibers prepared by the quaternization of polychloromethylstyrene-grafted polyolefin fiber with triamylamine and trihexylamine exhibited excellent kinetics and selectivity. In this study, several resins were prepared with reference to those prepared by Jyo (2013), and their adsorption characteristics were compared. The goal was to select a suitable anion exchange resin for use as a countermeasure against groundwater contamination caused by the use of manure and fertilizers in tea fields.

Spent resins are typically regenerated by exchange with chloride ions (Cl⁻) and biological denitrification. In this study, biological denitrification was used as the regeneration method considering the on-site nature of the experiment and the effect on the environment.

MATERIALS AND METHODS

REAGENTS FOR SYNTHESIS AND CHARACTERIZATION

The skeleton used to synthesize the anion exchange resin was chloromethylpolystyrene-divinylbenzene (Merrifield’s resin; 200–400 mesh, 3.5–4.5 mmol/g Cl), which was purchased from Sigma-Aldrich Japan (Tokyo, Japan). The reaction solvents were dehydrated N,N-dimethylformamide (DMF; purity≥99%) and dehydrated ethanol (EtOH; purity≥99.5%) purchased from Kanto Chemical (Tokyo, Japan). Special-grade nitrobenzene (purity ≥99.5%; Kanto Chemical) was dehydrated and dried using calcium hydride (purity ≥95.0%; Kanto Chemical) and distilled under reduced pressure to obtain a dehydrated solvent. The following were used as the tertiary amines (NR₃): trimethylamine (TMA; 31%–35% in ethanol solution, approximately 4.2 mol/L; Sigma-Aldrich Japan), triethylamine (TEA; purity ≥99.0%), tributylamine (TBA; purity ≥98.0%), trihexylamine (THA; purity ≥95.0%), trioctylamine (TOA; purity≥93.0%, tris (2-ethylhexyl) amine), and tridecylamine (TDA; purity≥98.0%, monotridecylamine, mixture of branched chain isomers), all purchased from Tokyo Chemical Industry (Tokyo, Japan). Methanol (MeOH; purity≥99.9%) and acetone (purity≥99.9%) purchased from Kanto Chemical were used as the solvents for washing the resin after the reaction.

Dilution and washing were performed using purified water obtained from an ultrapure water system (ADVANTEC RFU 665DA). Ion standards (Cl⁻, NO₂-N, NO₃-N, SO₄²⁻, sodium ion (Na⁺), ammonium nitrogen (NH₄-N), potassium ion (K⁺), magnesium ion (Mg²⁺), and calcium ion (Ca²⁺); 1,000 mg/L respectively) were purchased from FUJIFILM Wako Pure Chemical Corporation (Osaka, Japan). Mixed ion working solutions (anion1; NO₂-N and NO₃-N [10 mg/L], anion 2; Cl⁻ and SO₄²⁻ [100 mg/L], cation 1; NH₄-N [10 mg/L], and cation 2; Na⁺, K⁺, Mg²⁺, and Ca²⁺ [100 mg/L]) were prepared by diluting the ion standards with purified water. NO₂-N and NO₃-N solutions (0.1–4 mg/L), Cl⁻ and SO₄²⁻ solutions (0.1–40 mg/L), NH₄-N solutions (0.1–4 mg/L), Na⁺, K⁺, Mg²⁺, and Ca²⁺ solutions (0.1–40 mg/L) were prepared by diluting anion 1, anion 2, cation 1, and cation 2 with purified water, respectively, and were used to create the calibration line. In addition, NO₃-N and SO₄²⁻ standards were used to compare the adsorption characteristics of the synthesized anion exchange resins. A total organic carbon (TOC) standard solution (FUJIFILM Wako Pure Chemical Corporation) was used to measure TOC in soil solution for NO₃-N removal. D(+)-glucose (FUJIFILM Wako Pure Chemical Corporation) was used as organic matter for denitrification. Sodium nitrate (NaNO₃) (FUJIFILM Wako Pure Chemical Corporation) was used in the adsorption test after denitrification. Acetylene gas, high-purity nitrogen gas (N₂), and nitrous oxide (N₂O) gas were purchased from Fukuoka Oxygen (Fukuoka, Japan).

SYNTHESIS OF ANION EXCHANGE RESINS

The synthesis reaction of the anion exchange resins is shown in Fig. 1. The resins were synthesized as follows (Yamaguchi et al., 1971; Serita et al., 1976). Merrifield resin was placed in a pressure glass bottle covered with a septum cap under purging with N₂ gas. Then, the dehydrated solvent containing NR₃ was added to the bottle with a syringe. The type and quantity of the dehydrated solvent were determined according to the solubility of NR₃ in the solvent and the reaction rate. The reaction conditions are shown in Table 1. To prepare resin 1, 25.3 g (0.114 mol Cl) of Merrifield resin was used; 23.1 g (0.104 mol Cl) was used for resins 2–6. DMF was used in quantities of 450 mL for resin 1, 410 mL for resins 2 and 3, and 200 mL for resins 4 and 5. For resins 4 and 5, 370 mL of EtOH was used, and 110 mL of dehydrated nitrobenzene was used for resin 6. After the dehydrated solvent containing NR₃ was added, the septum cap was removed from the bottle and quickly sealed with a screw cap under purging with N₂ gas. Then, the mixture was stirred and heated at 60°C in an oil bath for 30 days. After the reaction, 300 mL of acetone and 100 mL of purified water were added, and the mixture was filtered through a quantitative paper filter (Advantec 5B) to obtain a white or light yellow solid. The solid was washed twice with purified water, twice with MeOH, and twice with acetone and then dried under reduced pressure (Yamaguchi et al., 1971; Jyo, 2013). After drying, it was crushed and sieved (Kenis, 235 mesh) to obtain an anion exchange resin. The resins containing TMA, TEA, TBA, THA, TOA, and TDA were labeled MPR-TMA, MPR-TEA, MPR-TBA, MPR-THA, MPR-TOA, and MPR-TDA, respectively, as shown in Table 1.

Fig. 1

Synthesis of anion exchange resin from Merrifield’s resin

Table 1 Alkyl group of tertiary amine and dehydrated solvent for synthesis of anion exchange resin

No.	NR3	R of NR3	Solvent	Anion exchange resin
1	TMA	CH3	DMF	MPR-TMA
2	TEA	C2H5	DMF	MPR-TEA
3	TBA	C4H9	DMF	MPR-TBA
4	THA	C6H13	DMF and EtOH	MPR-THA
5	TOA	C8H17	DMF and EtOH	MPR-TOA
6	TDA	C10H21	nitrobenzene	MPR-TDA

The water content in resins was measured using an evaporator (Kyoto Electronics Manufacturing, ADP-511) and a Karl-Fischer moisture titrator (Kyoto Electronics Manufacturing, MKC-510N). The nitrogen content was measured using an elemental analyzer (CHN Corder; Yanako MT-5). The yield of the obtained resin (Y [%]) was calculated using equation (1).   

$$Y=\left[W_{\mathit{2}}\times \left(100-x\right)/100\right]/W_{\mathit{1}}\times 100$$(1)

where,

W₁ (g): mass of anion exchange resin calculated assuming that all Cl⁻ in Merrifield resin reacted with NR₃, where the Cl⁻ content in Merrifield resin was 0.09 mol (0.1 mol for resin 1)

W₂ (g): mass of obtained anion exchange resin

x (%): water content of obtained anion exchange resin

NITRATE REMOVAL

The stock solution (1,000 mg-N/L) of NaNO₃ was prepared with purifying water. NO₃-N solutions (0–500 mg/L) were obtained by diluting the stock solution with purified water.

The resin (0.1 g) was added to 20 mL of NO₃-N solution (10, 50, 100, 200, 300, 400, or 500 mg/L), which was shaken for 1 h (200 spm) and allowed to stand for 1 day. The supernatant was filtered through a 0.20-μm membrane filter (Advantec, DISMIC 25CS020AS). The NO₃-N concentration of the filtrate was determined using ion chromatography (Thermo Fisher Scientific, eluent generator, ICS-1100 with AS20/AG20 and UV and electrical conductivity detectors).

SULFATE ADSORPTION

(NH₄)₂SO₄ was used in the tea field at the study site (Ishibashi et al., 2004). Of the inorganic anions in the groundwater originating from the tea field, the SO₄²⁻ concentration was the highest (100–140 mg/L). SO₄²⁻ may particularly affect NO₃-N adsorption. Furthermore, organic acids and the like contained in the groundwater may affect adsorption. However, the groundwater contained little organic matter (TOC: 0.79–1.8 mg/L). Groundwater with the high concentration of SO₄²⁻ was investigated in this work.

The effect of SO₄²⁻ on NO₃-N adsorption was investigated as follows. A resin (0.1 g) was added to 20 mL of a solution containing NO₃-N (100 mg/L) and SO₄²⁻ (10, 50, 100, or 500 mg/L), which was shaken for 1 h and allowed to stand for 1 day. The supernatant was filtered through a 0.20-μm membrane filter. The NO₃-N concentration of the filtrate was determined using ion chromatography.

In addition, 0.1 g of a resin was added to 20 mL of a solution containing SO₄²⁻ (100 mg/L), and the SO₄²⁻ concentration of the filtrate was measured as described above to determine the SO₄²⁻ adsorption capacity.

CONFIRMATION OF DENITRIFICATION

Biological denitrification, a regeneration method, was confirmed by measuring N₂O in the headspace of the vial bottle containing the solution with denitrifying bacteria (soil solution), which was obtained by extracting the soil and NO₃-N-saturated resin. However, because the soil contained NO₃-N, the soil solution also contained NO₃-N. Determining whether NO₃-N adsorbed on resin is denitrified or not is not possible. Therefore, removing NO₃-N from the soil solution is necessary. The soil solution for NO₃-N removal was prepared as follows. First, 2 g of soil from the O horizon of the tea field and 4 g of dry IMAX HP555 resin (Ishibashi et al., 2015) used for removing NO₃-N from the soil solution were added to 200 mL of purified water. The mixture was shaken for 1 h (200 spm) and allowed to stand at 4°C for 1 day. The supernatant was collected to prepare the soil solution for NO₃-N removal. Table 2 shows the chemical species concentrations in the soil solution. The pH was measured with a pH meter (DKK-TOA Corporation, MM-50R), the TOC was measured with a TOC meter (Shimadzu Corporation, TOC-L) and the ion concentrations in the filtrate were measured using ion chromatography.

Table 2 Chemical species concentrations in soil solution

pH	TOC	Cl	NO2-N	NO3-N	SO42-	Na	NH4-N	K	Mg	Ca
5.5–5.8	5–10	3–5	<0.1	<0.1–0.1	<0.1	0.1–0.3	<0.1–0.1	0.4–2	0.3–0.6	0.6–1.0

Unit: mg/L (except for pH)

Resin 4 (2 g) was added to 100 mL of NO₃-N solution (500 mg N/L), which was shaken for 1 h (200 spm), filtered through a filter paper (Advantec 5B), and washed with purified water. The resin in which Cl⁻ was replaced with NO₃⁻ (a NO₃-N-saturated resin) was prepared by air-drying after washing. The NO₃-N-saturated resin 4 (0.1 g) and glucose (0–0.1 g) were added to 20 mL of soil solution for NO₃-N removal, which was degassed with N₂ gas for 2 h in a 45-mL vial bottle. The gas phase in the bottle was replaced with N₂ gas, and 1 mL of acetylene gas was added (Fig. 2). After the bottle was held at 30°C for 72 h, N₂O in the gas phase was measured via gas chromatography with electron capture detection (GC-ECD; Shimadzu Corporation, GC-2010).

Fig. 2

Confirmation of denitrification of NO₃-N adsorbed on anion exchange resin

NITRATE RE-ADSORPTION ON THE RESIN AFTER DENITRIFICATION

The NO₃-N-saturated resin (0.1 g) and glucose (0.5–10 g) were added to 20 mL of soil solution for NO₃-N removal degassed with N₂ gas. The resin was kept at 30°C for 1 week and then filtered through a filter paper and washed with purified water. Next, 0.05 g of the resin (resin after denitrification) was added to 10 mL of NO₃-N solution (100 mg N/L) after air-drying; the solution was shaken for 1 h (200 spm) and allowed to stand for 1 day. The supernatant was filtered through a 0.20-μm membrane filter. The NO₃-N concentration of the filtrate was determined using ion chromatography.

RESULTS AND DISCUSSION

SYNTHESIZED ANION EXCHANGE RESINS

Table 3 shows the water content, nitrogen content, and synthesized anion exchange resin yield. The water content was 3%–11%, and the nitrogen content increased with decreasing side chain length. The yield exceeded 70%.

Table 3 Water content, nitrogen content, and yield of anion exchange resins

No.	Anion exchange resin	Water content (%)	Nitrogen content (%)	Yield (%)
1	MPR-TMA	2.9	4.9	82
2	MPR-TEA	11	4.0	81
3	MPR-TBA	3.9	3.1	87
4	MPR-THA	3.2	2.6	78
5	MPR-TOA	6.2	2.5	73
6	MPR-TDA	2.9	1.1	71

NITRATE ADSORPTION CAPACITY AND ADSORPTION ISOTHERM

NO₃-N concentration of NO₃-N solution, to which anion exchange resin was added, was measured as described in the Methods (nitrate removal). Fig. 3 shows the variation of NO₃-N removal rate with time when initial NO₃-N concentration in the NO₃-N solution was 50 mg/L. Adsorption kinetics results of resins 1–4 in Fig. 3 showed that the adsorbed NO₃-N reached equilibrium within 10 min and the NO₃-N removal rate exceeded 90%. The removal rates of resins 5 and 6 required a longer time to reach constant values than did those of resins 1–4. The NO₃-N removal rate of each resin was similarly obtained for other initial concentrations. Fig. 4 shows the NO₃-N removal rates from NO₃-N solutions of the resins. Fig. 4 shows the concentrations in NO₃-N solution on the x axis and removal rate of NO₃-N on the y axis, demonstrating the initial NO₃-N concentration and the constant value when the adsorbed NO₃-N reached equilibrium. The removal rates of resins 1–4 were very similar. The removal rate of resin 5 was lower than those of resins 1–4 when the initial NO₃-N concentration exceeded 100 mg-N/L, and the removal rate of resin 6 was lower than those of the other resins. The length of the side chain of the resin increased in order from resin 1 to resin 6. The lower removal rate of resin 6, which had the longest side chain, may be attributed to steric hindrance.

Fig. 3

Change in NO₃-N removal rate of anion exchange resin

Fig. 4

NO₃-N removal rate of anion exchange resin for NO₃-N solution

The adsorption capacity W (mg/g) was calculated as follows:   

$$W=\left(C_{\mathit{0}}-C_r\right)/m\cdot V$$(2)

where C₀ and C_r are the initial and residual NO₃-N concentrations (mg/L), respectively; V is the sample volume (0.02 L); and m is the resin mass (0.1 g).

The maximum adsorption capacity Ws (mg/g) was determined using the linearized Langmuir isotherm model, where the Langmuir adsorption isotherm was established following a previous report (Meng et al., 2014).   

$$W=a\cdot W_S\cdot C/\left(1+a\cdot C\right)$$(3)

where C (=C_r) is the NO₃-N concentration (mg/L) at equilibrium and a is the equilibrium constant.

Equation (4) can be rewritten as follows.   

$$C/W=\left(1/a\cdot W_S\right)+\left(1/W_S\right)C$$(4)

The relationship between C/W and C was established according to Fig. 4 and was almost linear, as shown in Fig. 5. Denoting the slope and intercept of equation (4) as b and c, respectively, we obtain   

$$W_S=1/b$$(5)

  

$$a=1/c\cdot W_S=b/c$$(6)

Fig. 5

Relationship between equilibrium concentration and adsorption capacity and the best model fitting

C: equilibrium NO₃-N concentration, W: adsorption capacity, ●: experimental value, –: Langmuir adsorption isotherm model

Table 4 shows the values of Ws and a obtained using equations (5) and (6). Ws values of the resins synthesized in this study were found to be 20–33 mg/g, whereas the Ws values of the resins with triethylamine functional groups (Purolite A520E, Nur et al., 2015), trimethylamine functional groups (SBG 1, Meng et al., 2014), quaternary ammonium functional groups (Relite A490 and Duolite A7, Nujić et al., 2017), and quaternary ammonium functional groups blended with hydrous iron oxide nanoparticles (Purolite FerrIX A33, Nur et al., 2015) provided by the manufacturers were 32.2, 29, 7.391, 5.068, and 8.77, respectively, according to other reports. The Ws value of SBG 1 was provided by the manufacturer. The Ws values of other resins provided by manufacturers, except SBG 1, were determined by batch experiments. The Ws values of resins 1 and 2 were similar to those of SBG 1 and Purolite A520E. The Ws values of the synthesized resins were better than those of the resins with quaternary ammonium functional groups. Ws increased with decreasing side chain length. Resin 1 had the largest Ws value among the synthesized resins. Resin 4 had the largest a value, followed by resin 5, and resin 6 had the smallest value.

Table 4 Maximum adsorption capacity and equilibrium constant

No.	Anion exchange resin	Ws (mg/g)	a (L/mg)
1	MPR-TMA	33	0.063
2	MPR-TEA	30	0.089
3	MPR-TBA	30	0.092
4	MPR-THA	21	0.28
5	MPR-TOA	20	0.20
6	MPR-TDA	21	0.047

Ws: Maximum adsorption capacity

a: Equilibrium constant

The Langmuir adsorption isotherm obtained using the Ws and a values in Table 4 is indicated by the black line labeled “Modeling” in Fig. 5. In addition, the experimental values are plotted in Fig. 5. The NO₃-N adsorptions by the synthesized resins were satisfactorily described by the Langmuir adsorption isotherm model.

EFFECT OF SULFATE

Fig. 5 shows the SO₄²⁻ removal rate of each resin. Most of the SO₄²⁻ was adsorbed on resins 1–3, and resins with longer side chains exhibited lower removal efficiency. The SO₄²⁻ removal rate was similar to that of NO₃-N (Fig. 4). For resin 4, 100-mg/L NO₃-N was almost adsorbed; however, the efficiency of 100-mg/L SO₄²⁻ adsorption was lower.

Fig. 6 shows the NO₃-N removal rate at each SO₄²⁻ concentration. The NO₃-N removal rates of resins 1–3 decreased with increased SO₄²⁻ concentration, and those of resins 4–6 were almost constant with increasing SO₄²⁻ concentration. The constant removal rate of resins 4–6, which have longer side chains than resins 1–3, may be attributed to steric hindrance. The NO₃-N removal rates of resins 5 and 6 were 71%–83%, whereas that of resin 4 was 95%–97% (Fig. 7). Therefore, resin 4 showed the highest selectivity for NO₃-N adsorption.

Fig. 6

SO₄²⁻ removals of anion exchange resins for 100-mg/L SO₄²⁻ solution

Fig. 7

Effect of SO₄²⁻ concentration on NO₃-N removal

Because resin 4 had a high NO₃-N removal efficiency and selectively adsorbed NO₃-N, it was chosen for denitrification testing for re-adsorption.

DENITRIFICATION AND RE-ADSORPTION OF NO₃-N on NO₃-N-SATURATED RESIN

Biological denitrification is a process in which denitrification bacteria convert NO₃⁻ to N₂. Several denitrification reactions have been reported (Payne, 1973; Suntharalingam and Sarmiento, 2000). An example is shown in equation (7).   

$${{\text{NO}}_3}^{-}\to {{\text{NO}}_2}^{-}\to \text{NO}\to {\text{N}}_2\text{O}\to {\text{N}}_2$$(7)

Some reactions do not produce N₂O, depending on the denitrifying bacteria. However, the N₂O emission rates of tea fields with acidified soil are much higher than those of other agricultural fields (Tokuda and Hayatsu, 2000; Akiyama et al., 2006), and the emission of N₂O from the soil of this tea field has been observed previously (Baba et al., 2000). Therefore, denitrification was confirmed by measuring N₂O in this study.

N₂O emission was not confirmed when the NO₃-N-saturated resin was added to the soil solution and cultivated. Because the TOC concentration in the soil solution was low (Table 2), the soil solution may not contain sufficient organic matter for denitrification. Therefore, glucose was added to the cultivation system. Fig. 8 shows the N₂O concentration in the headspace of the vial bottle after glucose addition, which indicates that glucose addition promoted denitrification. Although denitrification occurred, the denitrifying nitrogen calculated from the N₂O concentration when 0.1 g of glucose was added was only 0.04% of the NO₃-N adsorbed on the NO₃-N-saturated resin. The low efficiency of NO₃-N removal may result from the use of an oligotrophic soil solution to remove NO₃-N.

Fig. 8

Effect of glucose addition on denitrification

The amount of NO₃-N adsorbed on the resin after denitrification was calculated from the NO₃-N concentrations before and after the NO₃-N solution containing the resin was shaken to determine whether the resin was reusable. Fig. 9 shows the amount of adsorbed NO₃-N when glucose was added. When 1 g or more of glucose was added, approximately 10% of the NO₃-N in 10 mL of NO₃-N solution (100 mg-N/L) was absorbed on the resin after denitrification. The result indicated that the synthesized resin may be reusable. Nitrate selective resins were bioregenerated and reused for at least five or six cycles in previous studies (Ebrahimi and Roberts, 2013; Ye et al., 2019). Furthermore, Meng et al. (2014) evaluated the bioregeneration conditions and determined the optimal pH, conductivity, biomass content, etc. The resin synthesized in this study was bioregenerated and reused for only one cycle, and only 10% of the NO₃-N was absorbed on the resin despite the use of clean water. Further improvements in the denitrification efficiency will be needed for application on tea filed.

Fig. 9

Re-adsorption of NO₃-N on the resin after denitrification

CONCLUSION

Six anion exchange resins made with trimethylamine (1), triethylamine (2), tributylamine (3), trihexylamine (4), trioctylamine (5), or tridecylamine (6) were synthesized. Maximum adsorption capacities of resins 1–6 were 33, 30, 30, 21, 20, and 21 mg/g, respectively. The nitrate adsorption of all resins was satisfactorily described by the Langmuir adsorption isotherm model. The NO₃-N removal rates of resins 1–4 were very similar. The removal rate of resins 5 and 6 were lower than those of resins 1–4. The NO₃-N removal rates of resins 1–3 decreased with increased SO₄²⁻ concentration in solution containing NO₃-N and SO₄²⁻, and those of resins 4–6 remained constant with increasing SO₄²⁻ concentration. The NO₃-N removal rates of resins 5 and 6 were 71%–83%, whereas that of resin 4 was 95%–97%. Because resin 4 had a high NO₃-N removal efficiency and selectively adsorbed NO₃-N in solutions containing high concentration of SO₄²⁻, it was chosen for denitrification testing for re-adsorption. The denitrifying nitrogen was only 0.04% of the NO₃-N adsorbed on the NO₃-N-saturated resin 4 in the vial bottle containing 20 mL of the soil solution with NO₃-N removed, 0.1 g of glucose, and 0.1 g of the resin. When 1 g or more of glucose was added, approximately 10% of the NO₃-N in 10 mL of NO₃-N solution (100 mg-N/L) was absorbed on 0.05 g of resin 4 after denitrification. Further improvements in the denitrification efficiency will be necessary for application in tea fields.

ACKNOWLEDGMENTS

This study was supported by JSPS KAKENHI (grant number 25340081).

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