Nitrogen Isotopic Fractionation in Ammonia during Adsorption on

Feb 1, 2017 - Adsorption is a fundamental phenomenon that occurs at various interfaces; however, the isotopic fractionation in stable isotopes associa...
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Nitrogen Isotopic Fractionation in Ammonia during Adsorption on Silicate Surfaces Haruna Sugahara,*,† Yoshinori Takano,† Nanako O. Ogawa,† Yoshito Chikaraishi,†,‡ and Naohiko Ohkouchi† †

Department of Biogeochemistry, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima, Yokosuka, Kanagawa 237-0061, Japan ‡ Institute of Low Temperature Science, Hokkaido University, N19 W8 Kita-ku, Sapporo, Hokkaido 060-0819, Japan S Supporting Information *

ABSTRACT: Adsorption is a fundamental phenomenon that occurs at various interfaces; however, the isotopic fractionation in stable isotopes associated with this process has not yet been well documented for most molecules. In this study, we conducted ammonia adsorption experiments on two silicate minerals, montmorillonite and saponite, to determine the nitrogen isotopic fractionation during the process. Ammonia adsorbed on these minerals is up to +44‰ enriched in 15N relative to initial ammonia. The degree of 15N enrichment has a negative correlation with the adsorption ratio of ammonia. These enrichments are remarkably large compared to those reported in other physicochemical (e.g., evaporation) or biological (e.g., enzymatic reaction) processes. On the basis of these results, we can predict that preferential accumulation of 15NH3 occurs by adsorption on mineral surfaces, which may explain the heterogeneity of the 15N/14N ratio in the solar system. KEYWORDS: adsorption, isotopic fractionation, nitrogen, ammonia, silicate mineral, surface chemistry

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separation between the CH4 and CD4 systems.1,2 There are several reports of other deuterated small-molecular-weight hydrocarbons4,5 and molecular dinitrogen;6 however, methane and its deuterated isotopologues have been investigated most extensively. In addition, adsorption isotopic fractionation is of interest in geological studies. Isotopic fractionation as a result of the transport of natural gases (e.g., CH4 and CO2) through porous media, which is referred to as geochromatography, has been studied in particular.7,8 However, chemical and geological chromatographies involve diffusion and advection in addition to adsorption.9,10 Thus, the isotopic fractionation caused solely by an adsorption−desorption process has been poorly characterized. In this study, we examined the nitrogen isotopic fractionation of ammonia associated with adsorption on mineral surfaces under static conditions. Ammonia is a simple nitrogenous molecule observed on not only Earth (e.g., in the atmosphere, ocean, and terrestrial) but also other planets, asteroids, comets, etc. in the solar system and also in interstellar environments. Solar system objects show various degrees of 15N enrichment

INTRODUCTION Adsorption is a fundamental and ubiquitous phenomenon, in which gas or liquid components accumulate (positive adsorption) or are depleted (negative adsorption) on an interface layer (solid or liquid). Adsorption can be divided into two types based on the strength of the interaction between adsorbate and adsorbent: (1) physisorption by van der Waals interactions and (2) chemisorption by valence forces involving chemical bonds. The condensation of molecules on the interface layer may be a primary process in various physicochemical reactions, for which the isotopic fractionation of light elements (e.g., hydrogen, carbon, nitrogen, and oxygen) has often been documented. However, despite its importance, the isotopic fractionation caused by adsorption has not been well studied. There is a lack of studies characterizing the isotopic fractionation associated with adsorption, although the gas chromatographic separation of isotopologues in adsorption− desorption processes has been frequently investigated in analytical chemistry. Bruner and Cartoni1 and Bruner et al.2 showed that methane (e.g., CD4, 13CH4, and CH3D) and oxygen isotopologues (e.g., 16O2 and 18O16O) can be separated by gas chromatography using an adsorption glass capillary column. Van Hook3 later attempted to provide theoretical and statistical explanations of the reported gas chromatographic © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

November 9, 2016 February 1, 2017 February 1, 2017 February 1, 2017 DOI: 10.1021/acsearthspacechem.6b00006 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

Article

ACS Earth and Space Chemistry

Figure 1. Schematic of the experiments in the present study.

Table 1. Silicate Minerals Used in the Experiments name

montmorillonite

saponite

empirical formulaa group crystal system BET surface area (m2/g)b median diameter of the particle (μm)c

(K0.02Na0.83)(Al3.15Mg0.65Fe0.20)(Si7.80Al0.20)O20(OH)4 smectite (clay minerals) monoclinic 10.8 1.312

Na0.71(Mg5.93Al0.07)(Si7.22Al0.78)O20(OH)4 smectite (clay minerals) monoclinic 201 0.034

Omotoso et al.17 based on analysis by X-ray fluorescence spectroscopy.18 bThe method of measurement is briefly explained in the Supporting Information. cReference data obtained by the water dispersion method with ultrasonication for 10 min.13 Miyawaki et al.13 reported that montmorillonite and saponite have strong cohesive and swelling properties, and thus, the obtained particle sizes were not stable. a

pressure was adjusted to ∼1 atm, and the samples were kept at room temperature (10−25 °C) for 7 days to reach a stable state (step 3 in Figure 1). The total amount of injected ammonia was calculated by weight (values are listed in Table S-1 of the Supporting Information). After 7 days of exposure to enclosed ammonia, 1−10 mg aliquots of the silicate minerals were sampled from each vial and wrapped in Sn foil to determine the nitrogen content and nitrogen isotopic composition. The analysis was performed immediately after sampling to minimize contamination by air. Multiple sampling runs (3−6 runs) and analyses were performed on each sample vial. The residual mineral sample in each vial was evacuated after initial analysis to examine the change in adsorbed ammonia under vacuum (step 4 in Figure 1). The nitrogen contents and isotope ratios were analyzed after 1, 2, 4, and 8 h of evacuation for each sample using the above-mentioned process. Stable nitrogen isotopic composition and nitrogen content of the mineral samples were analyzed with an elemental analyzer connected to isotope ratio mass spectrometer (EA/IRMS) using an EA/IRMS system (Finnigan DELTAplus XP, Thermo Fisher Scientific) coupled with an EA system (FlashEA 1112, Thermo Fisher Scientific) in continuous flow mode. This EA/ IRMS system was developed for ultrasensitive nanogram-scale nitrogen isotope analysis,16 which allowed for analysis with a minimum of 125 ng of nitrogen, which is 2 orders of magnitude lower than the conventional method. To avoid any effect during the injection to the EA/IRMS system, the sample wrapped in Sn foil was manually placed in the sampling well just before the injection to minimize the time exposed to highly pure oxygen gas (atmospheric pressure) in the injection port. The nitrogen isotope ratio is written in conventional δ notation as

compared to the sun (e.g., −400‰ for the sun, 0‰ for Earth, and +1200‰ for comets).11,12 The mechanism of this large variation is not fully understood and is one of the unsolved aspects of the formation of the solar system. Because ammonia is a polar, highly reactive compound, its adsorption is an important first step, particularly in solid surface chemistry. Our study will provide new insights into the adsorption−desorption processes that cause a large degree of nitrogen isotopic fractionation.

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EXPERIMENTAL SECTION We designed simple adsorption experiments using ammonia gas and powdered silicate minerals (Figure 1). Two common silicate minerals, montmorillonite (natural, JCSS-3101) and saponite (synthetic, JCSS-3501), were used as adsorbent materials modeling silicate minerals (Table 1).13 Both minerals belong to the smectite group of clay minerals and contain interlayer water and cations. These minerals have a monoclinic crystal system and are well-known for their adsorption capacity.14 Prior to the experiment, they were dried in an oven at 110 °C for more than 1 week to minimize the adsorbed water. A 50 mg aliquot of the pre-dried silicate mineral was enclosed in a 10 mL glass serum vial with a butyl cap and an aluminum seal, which was then evacuated to 17 Pa to eliminate contamination from atmospheric gases (step 1 in Figure 1). To examine the effect of water in the silicate minerals on the adsorption, a controlled amount of distilled water (0, 5, and 10 wt %, versus dry weight, Wako Pure Chemical Industries, Ltd., pH 5.0−7.5) was added to each sealed vial using a syringe (step 2 in Figure 1) and diffused in a thermostatic oven at 90 °C. The water was adsorbed on the mineral surface, although there may have been water molecules in the interlayer space. The amounts of water were small enough to expand the basal spacing of the minerals.15 After the vial was cooled to room temperature, ammonia gas (δ15N = +27.0‰) was injected into it, the

δ15 N (‰) = (R sample/R standard − 1) × 1000 B

(1)

DOI: 10.1021/acsearthspacechem.6b00006 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

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ACS Earth and Space Chemistry where R is the 15N/14N ratio and atmospheric N2 (air) is the standard. The analytical error was less than 0.5‰ based on replicate measurements of an authentic reference material (IAEA-N-2, ammonium sulfate, +20.3‰). The adsorption ratio of ammonia to the silicate mineral is expressed as the weight ratio (wt %) of each analyzed sample portion as adsorption ratio (wt %) = (ammonia content of the analyzed sample portion (weight) /weight of the analyzed sample portion) × 100

(2)

The nitrogen content and δ N value of blank samples were also analyzed; they only had a minor effect on the results of the adsorption experiments (Table S-1 of the Supporting Information). 15

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RESULTS AND DISCUSSION Montmorillonite and saponite showed ammonia adsorption ratios of 0.18−1.00 and 0.24−1.32 wt %, respectively, and they exhibited a negative correlation between the adsorption ratio and δ15N value (Figure 2; the original data are shown in Table

Figure 3. Results for the non-evacuated and evacuated samples of (a) montmorillonite and (b) saponite.

Table S-2 of the Supporting Information). The samples that were evacuated for 1 h were the most enriched in 15N, with the highest δ15N value being +91‰. The samples that were evacuated for 2 h or more showed lower δ15N values (from +60 to +70‰) than those evacuated for 1 h, without a large change in the adsorption ratio. These δ15N values may represent the adsorbed ammonia (0.1−0.2 wt %) that formed a strong bond to montmorillonite, most likely by chemisorption, which is resistant to vacuum conditions (2−8 h). In contrast, evacuated saponite showed an overall independent trend compared to that observed for the non-evacuated samples. In fact, the adsorption ratios decreased for evacuated saponite, and some of the 1 h evacuated samples showed high δ15N values up to +87‰ (Figure 3b). However, all other samples showed a decreasing δ15N trend, with values ranging from +44 to +74‰, although the adsorption ratios remained almost constant. The present results suggest that the adsorption of ammonia on silicate minerals is controlled by more than one process. Several potential processes have been suggested in the literature: (1) replacement of water by ammonia in the interlayer space, which generates a coordination complex with the exchangeable cations (e.g., Na+ and Ca2+), (2) protonation of ammonia to form the ammonium ions (NH4+) on the mineral surface and in the interlayer water, and (3) physical trapping of ammonia in the interlayer space (Figure 4).19−21 Salam et al.22 showed with Fourier transform infrared spectroscopy (FTIR) analysis that ammonia gas was strongly adsorbed by chemisorption in adsorption−desorption experi-

Figure 2. Results of the adsorption experiments of ammonia on montmorillonite and saponite before evacuation. The percentages in the legend show the water content of the samples. Note that we removed three to six portions of the samples from each glass vial for analyses. Thus, the variation observed within a single vial (i.e., the same silicate mineral and same water content) is due to the varying degree of adsorption within the samples.

S-1 of the Supporting Information). At a low adsorption ratio, the δ15N value reached +67‰ for montmorillonite and +71‰ for saponite, with adsorption ratios of 0.18 and 0.24 wt %, respectively. The δ15N value decreased with an increasing adsorption ratio and eventually dropped to the initial value of ammonia gas (+27‰) at an adsorption ratio of ∼1.2 wt %. There was no correlation between the δ15N value and water content of the minerals. This result demonstrates that the water content of the silicate minerals plays a negligible role in controlling the nitrogen isotopic fractionation and adsorption ratio during the adsorption of ammonia. In addition, the surface area of saponite is 20 times larger than that of montmorillonite (Table 1); however, there is no clear difference in the results for the two silicate minerals, which demonstrates that the surface area has no effect on the adsorption of ammonia. The evacuated montmorillonite samples showed higher δ15N values at the low adsorption ratio ( +1000‰) observed in primitive solar system materials (e.g., comets and chondrites)12 is photodissociation of N2;31 however, the adsorption of ammonia on the mineral surface might partly explain the 15N enrichment. If we extrapolate the plot in Figure 2 by assuming Rayleigh fractionation, we obtain the plot in Figure 5. The equation of Rayleigh fractionation is

Table 2. Basal Spacing of Blank Samples and AmmoniaAdsorbed Silicates Determined from XRD Dataa d001 (Å), blank d001 (Å), adsorbed NH3 Δd001 (Å) by NH3 adsorption

adsorption ratio of sample evacuated for 8 hb (wt %)

a

The XRD analysis method, including XRD patterns of the samples, is shown in the Supporting Information.

process in the adsorption of ammonia. In addition, the replacement of interlayer water by ammonia progresses rapidly when silicate minerals are exposed to ammonia gas, and the rereplacement of ammonia by atmospheric water progresses fairly rapidly, even without evacuation.14 Therefore, the variation in the δ15N value on the adsorption curve probably reflects the variance in the replacement ratio of ammonia in the silicate minerals. Replaced ammonia is highly enriched in 15N during this process, but the enrichment gradually reduces and ultimately approaches the initial value (+27‰) when the adsorption ratio increases to 1.2 wt %. In contrast, when the sample was evacuated, only chemisorbed ammonium ions (process 2) remained and the δ15N value decreased to approximately +60‰ for montmorillonite and +44‰ for D

DOI: 10.1021/acsearthspacechem.6b00006 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Haruna Sugahara: 0000-0002-8388-3245 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank anonymous reviewers for critical and constructive comments that helped to improve the manuscript. The authors also thank Dr. H. Imachi and Dr. T. Yoshimura for kindly providing the technical tools and sample materials for the experiments. This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas (Research Project on Evolution of Molecules in Space, 25108006) and a Grant-inAid for Scientific Research (16K13916) from the Japan Society for the Promotion of Science (JSPS).

Figure 5. Extrapolated diagram of the adsorption results in Figure 2 by assuming Rayleigh fractionation.

⎡ (1000 + δ15 N ⎤ sample) ⎥ = (α − 1)ln(1 − f ) ln⎢ ⎢⎣ (1000 + δ15 N0) ⎥⎦

(3)

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where α is a fractionation factor and f is defined as f = (total amount of introduced NH3 gas − amount of adsorbed NH3 on clay mineral)/total amount of introduced NH3 gas. The graph shows that 15N enrichment of a few hundred per million can be observed when a tiny amount of ammonia is adsorbed in silicate minerals.

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CONCLUSION

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ASSOCIATED CONTENT

ABBREVIATIONS USED XRD, X-ray diffraction; BET, Brunauer−Emmett−Teller; EA/ IRMS, elemental analyzer connected to isotope ratio mass spectrometer; FTIR, Fourier transform infrared spectroscopy

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We have shown that substantial 15N enrichment is caused by the adsorption−desorption process of ammonia on silicate minerals by model experiments that focus on the isotopic fractionation associated with adsorption. Adsorption is a complex phenomenon that involves several processes on the molecular scale. The rationale for the observed isotopic fractionation in our study has yet to be proven; however, the current findings will provide new perspectives in various fields. The isotopic fractionation associated with adsorption, especially that of nitrogen, has not been investigated but has broad potential applicability, including analytical chemistry and astrochemistry.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsearthspacechem.6b00006. Analytical methods for determining the δ15N value of initial ammonia gas, measurement of the BET surface area, determination of the lattice spacing by XRD analysis and XRD patterns (Figure S-1), ln(1 − f) versus ln[(1000 + δ15Nsample)/(1000 + δ15N0)] diagram (Figure S-2), summary of sample information and analytical results of unevacuated samples and blanks (Table S-1), summary of sample information and analytical results for evacuated samples (Table S-2), calculation results based on the Rayleigh fractionation model (Table S-3), and fractionation factors obtained by the Rayleigh fractionation model (Table S-4) (PDF) E

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DOI: 10.1021/acsearthspacechem.6b00006 ACS Earth Space Chem. XXXX, XXX, XXX−XXX