Notes
Adsorption Capability of Fe-HT3.0 for Nitrite and Nitrate Ions in a Binary Solution System
2019 Volume 67 Issue 10 Pages 1168-1170
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2019 Volume 67 Issue 10 Pages 1168-1170
In this study, the adsorption capability of Fe-HT3.0 for nitrite and nitrate ions in a binary solution system was evaluated. It was found that the amount of nitrite and nitrate ions adsorbed in a single solution (1.19 and 1.27 mmol/g, respectively) was higher than that in a binary solution (0.36 and 0.90 mmol/g, respectively). Equilibrium adsorption was attained within 6–24 h. The adsorption data were fitted to a pseudo-second-order model (correlation coefficient: 0.999), and indicated that the adsorption of both nitrite and nitrate ions is controlled by chemical sorption. Additionally, the binding energies before and after the adsorption of nitrite and nitrate ions in the binary solution system were measured. After adsorption, new nitrogen peaks (approx. 399 and 403 eV) were detected. The results of this study show the potential of Fe-HT3.0 for the removal of nitrite and nitrate ions from aqueous solution systems.
Nitrites and nitrates (nutrients) have caused widespread concern for surface and groundwater safety around the world due to their impacts on human health (cyanosis in children and cancers of the alimentary canal) and ecological systems (eutrophication).1,2) Agricultural overapplication of natural and synthetic fertilizers, aquaculture, municipal wastewaters, detergent manufacturing, and mineral processing industries are the main sources of nutrient release into the aquatic environment.3,4) The WHO guideline for nitrite (NO2−-N) in drinking water is 0.91 mg/L. The maximum contaminant level of nitrite and total level of nitrite and nitrate have been set at 0.04 mg/L and 10 mg/L, respectively, for drinking water in Japan. Therefore, it is very important to remove nitrite and nitrate from the water environment, specifically drinking water.
Anionic clay, also known as hydrotalcite or layered double hydroxide, is a favorable sorbent material, with a great anion sorption capability.5) A previous study reported that Fe-Mg-type hydrotalcite showed high adsorption capability for nitrite and nitrate ions in a single solution system. Additionally, the study elucidated part of the adsorption mechanism of the nitrite and nitrate ions.6) In order for Fe-Mg-type hydrotalcite to be used in the removal of nitrite and nitrate ions in the field, further study is required. Additionally, there are no reports on the adsorption of nitrite and nitrate ions in complex solution systems using Fe-Mg-type hydrotalcite. Therefore, the objectives of this study are to reveal the capacity of nitrite and nitrate ion adsorption to Fe-Mg-type hydrotalcite in a binary solution, analyze the kinetic data, and evaluate the adsorption mechanism of the nitrite and nitrate ions.
The Fe-Mg-type complex hydroxide (Fe-HT3.0) (molar ratio Mg2+ : Fe3+ of 3 : 1) was obtained from Tomita Pharmaceutical Co., Ltd., Japan. Nitrite and nitrate ion solutions were prepared from potassium nitrite/nitrate obtained from FUJIFILM Wako Pure Chemical Corporation, Japan.
Adsorption Capability of Nitrite and Nitrate IonsFe-HT3.0 (0.05 g) was added to 10 mmol/L nitrite and/or nitrate ion solutions prepared as either single or binary systems (50 mL). The suspensions were shaken at 100 rpm for 24 h at 25°C. The samples were then filtered using 0.45 µm membrane filters (Toyo Roshi Kaisha, Ltd., Japan) and the filtrates were analyzed using a Dionex ICS-900 ion chromatograph (Thermo Fisher Scientific Inc., Japan). Measurements were performed using an IonPac AS12A column (4 × 2 mm, Thermo Fisher Scientific Inc., Japan). The mobile phase and regenerant were composed of 2.7 mmol/L sodium carbonate +0.3 mmol/L sodium hydrogen carbonate, and 12.5 mmol/L sulfuric acid, respectively. The flow rate used was 1.0 mL/min at ambient temperature. The micromembrane suppressor used was an AMMS 300 system (4 mm, Thermo Fisher Scientific Inc., Japan), and the sample volume was 10 µm. Elution times of nitrite and nitrate ions were 4.0 min and 7.2 min, respectively, under the experimental conditions. The amount of nitrite/nitrate ions adsorbed was calculated using Eq. (1):
 | (1) |
where q is the amount of ions adsorbed (mmol/g), C0 is the initial ion concentration (mmol/L), Ce is the equilibrium ion concentration (mmol/L), V is the solvent volume (L), and W is the weight of the adsorbent (g).
The effect of contact time on the adsorption of nitrite/nitrate ions onto Fe-HT3.0 was also evaluated. Briefly, Fe-HT3.0 (0.05 g) was added to 10 mmol/L nitrite or nitrate ion solution prepared as a single solution system (50 mL). The suspensions were shaken at 100 rpm for different time intervals (0.2, 0.5, 0.75, 1, 1.25, 1.5, 2, 6, 12, and 24 h) at 25°C. The amount of nitrite/nitrate adsorbed was then calculated using the same chromatography conditions previously mentioned. The data are expressed as the mean ± standard deviation (S.D.) of the mean (n = 3−5).
To evaluate the adsorption mechanism of nitrite/nitrate ions in a complex solution system, the amount of chloride ions released before and after adsorption was measured by ion chromatography. The measurement conditions were kept the same as before. Under these conditions, the elution time of the chloride ion was 10.5 min. Chemical analyses using electron spectroscopy and scanning electron microscopy (SEM) were also performed on the Fe-HT3.0 surface before and after the adsorption of nitrite/nitrate ions in the complex solution system. Instruments used for these analyses were an AXIS Nova X-ray photoelectron spectrometer (XPS) (Shimadzu Co., Ltd., Japan) and a Hitachi VP-SEM SU1510 (Hitachi, Ltd., Japan), respectively.
The physicochemical properties of Fe-HT3.0 have already been evaluated in a previous study.6) SEM images showed that Fe-HT3.0 has a rounded morphology and lacks a perfect crystal boundary. X-ray diffraction patterns of Fe-HT3.0 showed several diffraction peaks that could be indexed to the crystal structure of hydrotalcite. The specific surface area measured was 22.4 m2/g.7)
In our current study, the amount of nitrite and nitrate ions adsorbed onto Fe-HT3.0 is shown in Fig. 1. The amount of nitrite or nitrate ions adsorbed (1.19 mmol/g and 1.27 mmol/g, respectively) in a single solution was higher than that in a binary solution (0.36 mmol/g and 0.90 mmol/g, respectively). These results indicate that nitrite and nitrate ions compete with each other under the experimental conditions. Additionally, the total amount of nitrite and nitrate ions adsorbed in the binary solution system was 1.26 mmol/g. This suggests that the adsorption sites of Fe-HT3.0 became saturated, and the affinity between the adsorption sites of Fe-HT3.0 and the nitrate ions was higher than that of the nitrite ions. This result attributes to the higher charge density of nitrate ion compared to that of nitrite ion.1,8)
Initial concentration: 10 mmol/L, adsorbent: 0.05 g, solvent volume: 50 mL, contact time: 24 h, agitation speed: 100 rpm, temperature: 25°C
Figure 2 shows the effect of contact time on the adsorption of nitrite and nitrate ions. The amounts of nitrite and nitrate ions rapidly increased during the first 2 h. Equilibrium was attained within 6–24 h under the experimental conditions.
Initial concentration: 10 mmol/L, adsorbent: 0.05 g, solvent volume: 50 mL, contact time: 0.2–24 h, agitation speed: 100 rpm, temperature: 25°C
To investigate the reaction pathway and the rate-controlling mechanism of the adsorption process, adsorption kinetic data were analyzed in terms of pseudo-first-order (Eq. (2)) and pseudo-second-order (Eq. (3)) kinetic models.9,10)
 | (2) |
 | (3) |
where qe and qt are the amounts of nitrite and nitrate ions adsorbed at equilibrium at time t (mmol/g), respectively; k1 is the pseudo-first-order rate constant (h−1); and k2 is the pseudo-second-order rate constant (g·mmol−1·h−1).
The correlation coefficient of the pseudo-second-order model for nitrite and nitrate ions (>0.999) was higher than that of the pseudo-first-order model (0.849–0.874). Additionally, the value of qe obtained experimentally was closer to that of qe obtained from the pseudo-second-order model (Table 1). These results suggest that the adsorption of nitrite and nitrate ions is controlled by chemical sorption or chemisorption involving valency forces.10,11)
| Fitting models | Samples | k1 (h−1) or k2(g/mmol/h) | qe.cal (mmol/g) | r |
|---|---|---|---|---|
| Pseudo-first-order model | Nitrite ion | 0.59 | 0.66 | 0.849 |
| Nitrate ion | 0.52 | 1.52 | 0.874 | |
| Pseudo-second-order model | Nitrite ion | 2.18 | 1.29 | 0.999 |
| Nitrate ion | 2.36 | 1.21 | 0.999 |
Previous studies have reported an adsorption mechanism for nitrite and nitrate ions in a single solution system (correlation coefficient: 0.981–0.998).6) The adsorption of nitrite and nitrate ions was related to ion exchange with chloride ions, which also involved the interlayer of Fe-HT3.0. Therefore, we evaluated the relationship between the amount of nitrite and nitrate ions in a binary solution system and the amount of chloride ions released from Fe-HT3.0. A correlation coefficient of 0.997 (positive correlation, data not shown) was measured under the experimental conditions. Additionally, the binding energies before and after adsorption of nitrite and nitrate ions were evaluated (Fig. 3). After adsorption, nitrogen peaks (approx. 399 and 403 eV) were clearly detected, which was not detected before adsorption. Conversely, chlorine peaks (approx. 195 and 265 eV) present before adsorption, disappeared after adsorption. These results indicate that ion exchange with chloride ions occurred during this study. Finally, SEM images of before and after adsorption of nitrite and nitrate ions were measured (Fig. 4). We cannot confirm the clearly changes of Fe-HT3.0 surface before and after adsorption. Thus, further studies are needed to elucidate the adsorption mechanism of nitrite and nitrate ions and the potential of Fe-HT3.0 for field application and eventual commercialization.
A dotted line and solid line shows after adsorption and before adsorption, respectively.
We evaluated the potential of Fe-HT3.0 for the removal of nitrite and nitrate ions from a binary solution system. Fe-HT3.0 shows adsorption capability for nitrite and nitrate ions. The affinity of nitrate ions to adsorption sites on Fe-HT3.0 was greater than that of nitrite ions in the binary solution system. Batch kinetic studies indicated that a pseudo-second-order model for the adsorption kinetic curves of nitrite and nitrate ions is a good fit (correlation coefficient: 0.999). Moreover, binding energies before and after adsorption were measured, and a new nitrogen peak was detected after adsorption. The positive correlation coefficient (0.997) between the amount of nitrite and nitrate ions in the binary solution and the amount of sulfate ions released from Fe-HT3.0 was confirmed under the experiment conditions. The results indicate that Fe-HT3.0 could be useful for the removal of nitrite and nitrate ions from aqueous solution systems.
The Research Foundation for Pharmaceutical Sciences.
The authors declare no conflict of interest.
