Characteristics of Nitrate Ion Adsorption onto N-doped Activated Carbon Derived from Pinecone and Urea

Summary

Currently, ion exchange resins are widely used to remove nitrate ions from water, but they have problems such as high production cost and difficulty to reuse, so there is a demand for the development of alternative adsorbents. In this study, we attempted to introduce the quaternary nitrogen (N-Q) to the activated carbon using the biomass material to enhance the nitrate ion adsorption in aqueous solution. Then, the relationships between nitrate ion adsorption and equilibrium solution pH (pH_e), the elemental ratio, and the surface functional group contents of the sample were investigated to examine the amount of the N-Q and its effect on nitrate ion adsorption. As a result, the amount of N-Q in the sample increased with raising the ratio of urea to pinecone, and the optimal weight ratio that can introduce a large amount of N-Q into the sample and improve nitrate ion adsorption performance was Pc:Ur:Zn=1:1.5:0.5. The sample prepared with the optimum weight ratio showed high nitrate ion adsorption (0.5 mmol/g) as a carbon material and maintained the high adsorption performance in a wide pH range. These results indicated that the relatively easy preparation method in this experiment efficiently could introduce the N-Q into biomass AC, which promoted nitrate ion adsorption in aqueous solution. Also, it would imply that the influence of N-Q amount on nitrate ion adsorption performance was greater than the specific surface area, which is general indicator of activated carbon adsorption capacity.

INTRODUCTION

Recently, groundwater pollution by nitrate- and nitrite-nitrogen became a problem in recent years because of using large amount of chemical fertilizer and NO_x gas discharged by diesel engines^1,2,3). The pollution causes eutrophication in closed water body, which results in algal blooms and red tides in lakes and seas, respectively⁴⁾. Increase of algal blooms kills fish, and leads to odor problem^2,3). Also, when people intake nitrate- and nitrite-nitrogen by drinking groundwater, nitrate-nitrogen are reduced to nitrite-nitrogen. At the same time, hemoglobin is oxidized to methemoglobin in the human body. In this way, methemoglobinemia occurred^5,6,7). Moreover, nitrite-nitrogen is converted to nitrosamine by bonding to secondary amines, which is the cause of cancer in the human body^8,9).

To prevent these environmental problems, efficient removal methods of nitrate-nitrogen are needed^10,11). Currently, ion exchange method is one of the mainstreams¹²⁾. This method adsorbs the targeted ions using ion exchange resin, which has the functional groups favorable to targeted ions. However, ion exchange resin is expensive to prepare and difficult to reuse, because it cannot tolerate heat and strong acid/base treatment.

Against this background, currently development of new adsorbent for nitrate ion is required, which is the same adsorption performance as ion exchange resin and low cost. The purpose of this study was to develop the new adsorbent that replace ion exchange resin, focusing on activated carbon^13,14,15). The production cost of activated carbon is relatively low and it is reusable because it can tolerate heat and strong acid/base treatments. Also, environmental loading is low when it is discarded, which can be a combustion aid¹⁶⁾. Thus, activated carbon can be said to be low-cost and environmentally friendly adsorbent.

In recent years, the adsorption of nitrate by biomass waste-based adsorbents such as activated carbon has been studied widely^17,18,19). However, most of the activated carbons obtained from biomass waste had low adsorption capacity for nitrate. For example, the adsorption capacity of olive solid waste-based activated carbon²⁰⁾ was only 0.09 mmol/g. This is because the surface nature of activated carbon is non-polar, and activated carbon was mainly used for adsorption of organic pollutants. However, recent studies showed that quaternary nitrogen (N-Q) doped on activated carbon can adsorb nitrate ion depending on its content^{21,22,23,24,25)}. Also, it is known that the nitrogen, such as pyrrole nitrogen, pyridine nitrogen and pyridine-N-oxide, contained in activated carbon fiber turns into N-Q by heat treatment at high temperatures above 500°C, as demonstrated by Yoo et al., who revealed that the activated carbon fiber with N-Q introduced by thermochemical vapor deposition (t-CVD) could adsorb nitrate ion^26,27,28,29). Yuan et al. previously reported the adsorbent prepared by thermochemical deposition method to introduce quaternary nitrogen (N-Q). The adsorbent showed the maximum nitrate adsorption capacity of 0.74 mmol/g, which is 10 times higher than that of the biomass-based adsorbents mentioned above²⁵⁾.

In this study, pinecone was used as a carbon precursor to prepare low-cost and environmentally friendly activated carbon. Urea was used as the source of nitrogen atoms. Chemical activation was chosen as the activation method, because it produces a higher specific surface area for activated carbon than gas activation. Zinc chloride was used as activator, and pinecone impregnated with urea was activated at high temperatures above 500°C to prepare activated carbon containing N-Q. It is expected that, when the amount of zinc chloride is too high, the collapse of the carbon-oxygen bond may be accelerated and nitrogen may be volatilized³⁰⁾. Therefore, the optimal ratio of raw materials was investigated based on the results of nitrate ion adsorption experiment. After that, the amount of nitrate ion adsorption and properties of adsorbent were measured in detail to evaluate nitrate ion adsorption performance.

MATERIALS AND METHODS

Reagents

All reagents were special grade and purchased from Kanto Chemical, Tokyo, Japan. Zinc chloride (ZnCl₂) and urea were used for the preparation of the activated carbon. Hydrochloric acid (HCl) was used for washing the sample and Boehm titration. Sodium nitrate (NaNO₃) was used to prepare nitrate ion solution. Sodium carbonate (Na₂CO₃) and sodium bicarbonate (NaHCO₃) were used for preparation of eluents in ion chromatography and Boehm titration. Methyl red as an indicator and sodium hydroxide (NaOH) was used for Boehm titration.

Preparation of sample

Pinecone collected on the Nishi-Chiba campus of Chiba University was used as a carbon precursor. After pinecone was loosened finely, it was washed by boiled pure water and dried.

It is clarified that high temperature heat treatment for the carbon precursor, which is impregnated with zinc chloride solution, makes the carbon precursor porous by dehydration reaction¹²⁾. In the experiment, pore formation and introduction of N-Q were performed by zinc chloride activation for the loosened pinecone with urea at high temperature. Loosened pinecone (Pc), urea (Ur) and zinc chloride (Zn) were weighed into a beaker in each weight ratio (Pc:Ur:Zn=X:Y:Z), and urea and zinc chloride were dissolved by 500 mL pure water. After that, the solution was dried in an oven at 110°C overnight to impregnate pinecone with the chemicals. The dried pinecone was activated in electric tube furnace under 100 mL/min nitrogen gas flow. The temperature was raised from room temperature to 600°C, and it was kept for 1 hour. After the inside of the tube furnace was cooled to room temperature, the sample was taken out. The sample was washed by 1 mol/L HCl solution and boiled pure water several times to remove remained zinc and urea. Washed sample was filtered under reduced pressure, and it was dried in an oven at 110°C overnight. Washed and dried sample was ground. The sample activated at 600°C was designated as PcUr-6Z (X:Y:Z) depending on the weight ratio.

Adsorption of nitrate ion

Thirty-milligram of the prepared sample and 15 mL of 200 mg-NO₃^-/L nitrate ion solution were placed in an Erlenmeyer flask, and it was stirred for 24 hours. The equilibrium pH (pH_e) was adjusted to 3 and 5 by 0.1 mol/L HCl solution. After that, the solution was filtered and diluted to appropriate concentration. The nitrate ion concentration of solution was measured by ion chromatography. The equilibrium adsorption amount (Q_e) was calculated from the difference between initial and equilibrium concentration using the following equation;

   ,

$$Q_{\mathrm{e}}=\frac{\left(C_0-C_{\mathrm{e}}\right)V}{W},$$(1)

where, Q_e is the equilibrium adsorption amount [mmol/g], C₀ and C_e are the initial and equilibrium concentration of nitrate ion [mmol/L]. V is the solution volume [L] and W is the weight of sample [g].

The effect of equilibrium pH (pH_e)

The effect of equilibrium pH for nitrate ion adsorption performance was examined. The same experimental procedures without pH_e adjustment as in the previous sections were applied, and each pH_e was adjusted from 2 to 10. The effect of pH_e was examined from the obtained Q_e value.

Adsorption isotherm

The initial concentration of each nitrate ion solution was adjusted to 10-1,000 mg-NO₃^-/L, and pH_e was adjusted to 3. The other experimental conditions were the same as in the previous sections, and equilibrium concentration C_e [mmol/L] and the amount of nitrate ion adsorption at equilibrium Q_e [mmol/g] were calculated by using the equation (1). Each Langmuir adsorption parameter was calculated using the following linear equation;

   ,

$$\frac{C_{\mathrm{e}}}{Q_{\mathrm{e}}}=\frac{1}{X_{\mathrm{m}}}{C}_{\mathrm{e}}+\frac{1}{X_{\mathrm{m}}{K}_{\mathrm{e}}},$$(2)

where, C_e means the equilibrium concentration of nitrate ions [mmol/L], and X_m is the maximum adsorption capacity [mmol/g]. K_e represents the Langmuir isotherm constant representing adsorption affinity [L/mmol]^29,30,31,32). This equation can be obtained by substituting Q_e/X_m into the adsorption ratio θ³³⁾ and transforming it.

Elemental analysis

Elemental composition of each sample was examined using the elemental analyzer (PE2400II; PerkinElmer). About 1.4-1.5 mg of the sample dried at 110°C overnight was weighed into tin foil, and it was completely combusted. Afterwards, the amounts of carbon dioxide (CO₂), water (H₂O) and nitrogen compounds produced were measured by thermal conductivity detector (TCD), and that carbon (C), hydrogen (H) and nitrogen (N) composition of the sample was obtained. The oxygen (O) and other elements composition of the sample were calculated by subtracting the total value of weight ratio of C, H, N from 100 wt%.

Analysis of surface functional group

The amount of acidic and basic functional groups on the activated carbon surface was examined referring to the method of Boehm³⁴⁾. FIfty-milligram of the sample was added with 0.1 mol/L Na₂CO₃ solution, 0.1 mol/L NaHCO₃ solution, 0.1 mol/L sodium hydroxide solution and 0.1 mol/L HCl solution in each Erlenmeyer flask, and agitated for 3 days. Then, each solution was filtered, and 5 mL of it was neutralized and titrated by 0.05 mol/L HCl solution. The neutralization point was determined by the solution pH and change in methyl red color. For titration of HCl solution, firstly 10 mL of 0.1 mmol/L NaOH solution was added into each flask, and back titration was performed by 0.05 mol/L HCl solution. NaOH reacts with phenol groups, lactone groups, carboxy groups and/or quaternary nitrogen (N-Q). Na₂CO₃ reacts with lactone groups, carboxy groups and/or quaternary nitrogen (N-Q). NaHCO₃ reacts with carboxy groups and/or quaternary nitrogen (N-Q). Therefore, each acidic functional group was quantitatively determined using each titration value. Fig. 1 shows the neutralization relationship.

Fig. 1 Concept of Boehm titration for determination of acidic functional groups

X-ray photoelectron spectroscopy

The ratio of nitrogen species on the surface of sample was examined using an X-ray photoelectron spectrometer (JPS-9030; JEOL). The N1s spectral analysis of nitrogen species (binding energy [eV]) proposed by Pels et al. was used^26,30,35).

The surface of each sample was irradiated with X-rays (MgKα), and photoelectron spectra were obtained capturing emitted photoelectrons. The nitrogen ratio contained in each sample was deconvoluted by separating of each peak of the obtained spectrum into N-6 (398.7±0.3 eV), N-5 (400±0.3 eV), N-Q (401.1±0.3 eV) and pyridine-N-oxide (N-X, 402-404 eV) for each binding energy.

BET specific surface area and pore distribution

The specific surface area and pore distribution of each sample were examined. The sample dried at 110°C overnight was degassed in vacuum at 150°C for 3 hours. The nitrogen adsorption and desorption isotherms were measured through BELSORP-mini II surface analyzer (Microtrac BEL Corp.) by passing nitrogen gas at a pressure between vacuum pressure and normal pressure into the sample in liquid nitrogen at -196°C. The BET specific surface area (S_BET [m²/g]) and the average pore diameter (D_avg [nm]) were determined from the BET method using the obtained isotherms²⁹⁾.

RESULTS AND DISCUSSION

Effect of raw material weight ratio on adsorption amount

Normally, activated carbon adsorbs nitrate ions only in the acidic range by protons (C-π adsorption sites) adsorbed on π electrons on activated carbon surface. However, the N-Q adsorption site is always positively charged, so it can adsorb nitrate ions in a wide pH range, and its adsorption amount would be enhanced. Therefore, if the sample has a large amount of positively charged adsorption sites such as N-Q, it shows the high adsorption of anions at any pH_e. And, if the sample does not have N-Q, the adsorption amount will decrease in the non-acidic range. In order to evaluate the sample that has a large amount of N-Q, the adsorption experiments in the two pH_e conditions, at pH_e 3 (acidic range) and at pH_e 5 (non-acid range), were performed.

The result of nitrate ion adsorption of samples prepared in various weight ratios was shown in Fig. 2. As a result, PcUr-6Z (1:1.5:0.5) and PcUr-6Z (1:0.3:1) showed the highest Q_e values of about 0.5 mmol/g at pH_e 3. The adsorption capacities of other biomass adsorbents such as palm-based biochar³⁶⁾, olive solid waste-based activated carbon²⁰⁾ and jackfruit peel-based biochar¹⁸⁾ were only 0.07 mmol/g, 0.09 mmol/g and 0.08 mmol/g, respectively. The adsorption capacities of PcUr-6Z (1:1.5:0.5) and PcUr-6Z (1:0.3:1) were about 5 times higher than that of the biomass-based adsorbents mentioned above. At pH_e 5, PcUr-6Z (1:1.5:0.5) and PcUr-6Z (1:0.3:1) maintained relatively high Q_e values compared to other samples. This result is similar to the adsorption behavior of the N-Q adsorption site.

Fig. 2 Effect of raw material weight ratio on nitrate ion adsorption performance

According to Pels et al., N-Q sites are formed by pyrolyzing nitrogen-containing carbon precursors at 500°C or higher, and nitrogen is incorporated into the heterocyclic 6-membered rings of the graphene structure of AC²⁶⁾. The activation temperature in our study was 600°C, so the temperature condition was appropriate. Also, in our study, the carbon precursor was pinecone, which contains almost no nitrogen, so it is important to incorporate nitrogen efficiently during activation. The amount of incorporated nitrogen is closely related to the weight ratio of pinecone, urea and zinc chloride. If the ratio of urea or zinc chloride to pinecone is too low, an insufficient nitrogen source is provided, and the effect of incorporated nitrogen into the six-membered heterocyclic ring on nitrate adsorption is not well demonstrated. Conversely, if the ratio of zinc chloride to pinecone is too high, the zinc chloride will excessively break carbon-carbon bond, resulting in a loss of nitrogen and carbon through volatilization.

Thus, in order to efficiently incorporate nitrogen into activated carbon, it is necessary to optimize the weight ratio of pinecone, urea and zinc chloride. It is considered that the weight ratio of Pc:Ur:Zn=1:1.5:0.5 and 1:0.3:1 was appropriate to maximize the effects of the urea and zinc chloride on nitrogen incorporation, which resulted in the maximized the amount of N-Q in the activated carbon.

Therefore, only these two samples and PcUr-6Z (1:0:1), which was prepared without urea, were used for further analysis.

Effect of equilibrium solution pH (pH_e) on adsorption amount

The results of nitrate ion adsorption at various pH_e using PcUr-6Z (1:1.5:0.5), PcUr-6Z (1:0.3:1) and PcUr-6Z (1:0:1) are shown in Fig. 3. As described in the previous sections, common activated carbon can adsorb nitrate ions in the acidic range by protonated π electrons (C-π adsorption sites), and the function of this adsorption site weakens as the solution pH increases. However, if there is a positively charged N-Q adsorption site on activated carbon, nitrate ions can be adsorbed regardless of the amounts of proton in the solution. In other words, the sample containing N-Q can adsorb a certain amount of nitrate ions regardless of pH_e, and also the adsorption onto protonated C-π site occurs in the acidic range.

Fig. 3 Effect of pH_e on nitrate ion adsorption performance

As a result, PcUr-6Z (1:0:1) showed a relatively high Q_e value of about 0.4 mmol/g in the acidic condition. As the pH_e increased, the Q_e value decreased. This result would imply the adsorption on only the C-π site³⁷⁾. Because pinecone contained only a little nitrogen atoms as shown in Table 1, and quaternary nitrogen is considered to be almost absent in PcUr-6Z (1:0:1)²²⁾. PcUr-6Z (1:1.5:0.5) maintained a higher Q_e value than 0.4 mmol/g at all pH_e conditions. This result suggests that PcUr-6Z (1:1.5:0.5) contains a relatively large amount of N-Q, and it is thought that nitrate ions of about 0.4 mmol/L could be adsorbed only at the N-Q site. PcUr-6Z (1:0.3:1) showed a high Q_e value in the acidic range. However, the Q_e value gradually decreased as the pH_e increased, and it decreased to about 0.2 mmol/g when the pH_e was 10. This result would show that the main adsorption site of PcUr-6Z (1:0.3:1) was C-π site. Also, PcUr-6Z (1:0.3:1) suggested the higher Q_e value than PcUr-6Z (1:0:1) at all pH_e ranges. From this result, it would imply that PcUr-6Z (1:0.3:1) contained more N-Q than PcUr-6Z (1:0:1).

Table 1 Elemental compositions and maximum adsorption capacity (X_m) of samples

*Calculated by difference

Adsorption isotherm

The adsorption isotherms of each sample are shown in Fig. 4, and each parameter value of the Langmuir adsorption isotherm calculated from the experimental data is suggested in Table 2. Some plots were significantly below the adsorption isotherm, which is probably due to air entering the pores of the activated carbon, preventing the adsorbate from approaching the adsorption sites. It was seen that the adsorption amount increases according to the amount of urea added at each equilibrium concentration.

Fig. 4 Langmuir adsorption isotherm of nitrate ion for each sample

Table 2 Langmuir parameters for adsorption of on the samples

When the adsorption isotherm fits the Langmuir model, it means monolayer adsorption at the surface of adsorbent. Previous study has revealed that N-Q on the surface of activated carbon adsorbs nitrate ions through ion exchange, and the isotherm can be depicted as the Langmuir type³⁸⁾. In this experiment, the isotherm curve was close to the Langmuir model, indicating that N-Q acted as an adsorption site in each sample. Furthermore, as shown in Table 2, each parameter also tends to increase depending on the amount of urea added. From this result, it was suggested that both the maximum adsorption amount (X_m) and adsorption affinity (K_L) increased depending on the N-Q content, indicating a high nitrate ion adsorption ability.

Elemental analysis

Table 1 shows the result of elemental analysis. The result suggests that the elemental composition of nitrogen tended to increase depending on the amount of urea. It indicates that adding urea to a carbon material with low nitrogen content, such as pinecone, and zinc chloride activation makes the activated carbon surface contain nitrogen after activation. Furthermore, since the maximum adsorption amount of nitrate ions increased with the amount of nitrogen introduced, the introduced nitrogen would become more quaternized.

Functional group analysis, X-ray photoelectron spectroscopy (XPS)

Table 3 shows the result of functional group analysis. PcUr-6Z (1:0.3:1) and PcUr-6Z (1:1.5:0.5) contain more amount of carboxy groups or N-Q than PcUr-6Z (1:0:1). It was also found that the maximum adsorption amount increased depending on the amounts of carboxy groups or N-Q in Boehm titration. However, the carboxy group releases protons into the solution and becomes negatively charged, so it cannot adsorb nitrate ions well^39,40). Therefore, it would imply that the increase of the maximum adsorption amount was caused by N-Q, and that the increased carboxy groups or N-Q of samples were mostly N-Q.

Table 3 Surface functional groups by Boehm titration of the samples

Table 4 and Fig. 5 show the ratio of nitrogen species in each sample measured by XPS. N-Q increases with raising the ratio of urea to pinecone, and the maximum adsorption amount also increases with the amount of N-Q.

Table 4 N species (N1s), N-Q ratio and equilibrium adsorption amount (X_m) of samples

Fig. 5 The nitrogen single deconvolution (N1s) characterization of the samples by XPS analysis

BET specific surface area and pore distribution

Table 5 shows the specific surface area of each sample. It is found that the specific surface area decreases in the order of PcUr-6Z (1:0:1), PcUr-6Z (1:0.3:1) and PcUr-6Z (1:1.5:0.5). However, PcUr-6Z (1:0:1) possessing the largest specific surface area suggests the lowest maximum adsorption amount, and PcUr-6Z (1:1.5:0.5) shows the largest maximum adsorption amount in 3 samples. In the acidic range, PcUr-6Z (1:0:1) had a high adsorption capacity, which would be related to the C-π sites on the activated carbon. The amounts of C-π site are increased with those of basic sites, which are almost the same in all three samples as shown in Table 3. From these results, it would imply that the influence of the amount of N-Q on the activated carbon for nitrate ion adsorption performance was greater than the specific surface area, which is general indicator of activated carbon adsorption capacity. This is also described on the relevant literature, which means that the introduction of suitable functional groups for the adsorption of specific ions is more advantageous than increasing the specific surface area⁴¹⁾.

Table 5 Porous structural parameters and maximum adsorption capacity (X_m) of samples

CONCLUSION

In this study, experiments and analyzes were examined to prepare a new low-cost and environmentally friendly nitrate ion adsorbent. First, samples were prepared by impregnating pinecone with urea and zinc chloride at various weight ratios and activation, and the weight ratios were optimized by conducting adsorption experiments. Various adsorption experiments and physical property analyzes were then conducted on the prepared samples and reference. From the results, the influence of properties of each sample on the nitrate ion adsorption performance was considered. The conclusions obtained from the experiments are as follows.

1) By activating pinecone using zinc chloride with urea, it was possible to introduce a large amount of nitrogen into activated carbon by adding urea.

2) The amount of N-Q in the sample increased with raising the ratio of urea to pinecone.

3) The amount of N-Q had a greater effect on nitrate ion adsorption than the specific surface area.

4) The optimal weight ratio that could introduce a large amount of N-Q into the sample and improved nitrate ion adsorption performance was Pc:Ur:Zn=1:1.5:0.5.

ACKNOWLEDGMENTS

This study was supported in part by Grants-in-Aid for Scientific Research (C) from the Japan Society for the Promotion of Science (KAKENHI Grant No. JP20K05187). The authors thank the center for analytical instrumentation Chiba University for supporting elemental analysis and TAKAGI Co, Ltd. for the measurement of nitrate ion. It was also grateful to Prof. Dr. Reiko Uruma, the head of Safety and Health Organization, Chiba University, for her financial support on our study.

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