Evaluating pore structures of soil components with a combination of “conventional” and hyperpolarised 129Xe NMR studies

Geoderma 162 (2011) 96–106

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Geoderma j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o d e r m a

Evaluating pore structures of soil components with a combination of “conventional” and hyperpolarised 129Xe NMR studies Svetlana Filimonova a,⁎, Andrei Nossov b, Alexander Dümig a, Antoine Gédéon b, Ingrid Kögel-Knabner a, Heike Knicker a,1 a Lehrstuhl für Bodenkunde, Department für Ökologie, Wissenschaftszentrum Weihenstephan für Ernährung, Landnutzung und Umwelt, Technische Universität München, 85350 Freising-Weihenstephan, Germany b Laboratoire Systèmes Interfaciaux à l'Echelle Nanométrique, CNRS UMR 7142, Université Pierre et Marie Curie, 75252 Paris, France

a r t i c l e

i n f o

Article history: Received 4 June 2010 Received in revised form 15 December 2010 Accepted 23 January 2011 Available online 22 February 2011 Keywords: Xenon Nuclear magnetic resonance Meso- and micropores Adsorption Charcoals Andosol

a b s t r a c t 129 Xe nuclear magnetic resonance (NMR) spectroscopic studies of xenon gas adsorbed on model systems representing soil porous components (Al (hyrd)oxides and charcoals) as well as natural soil materials (derived from a non-allophanic Andosol) were performed with the aim of characterising their micro- (b 2 nm) and mesopores (2–50 nm). Both conventional, i.e. thermally polarised (TP), and laser-polarised or hyperpolarised (HP) 129Xe NMR was applied. The latter technique significantly increased sensitivity of the measurements. Information on the pore size range was derived from the 129Xe resonance shifts, δ, monitored as function of Xe loading, whereas the temperature dependences of δ provided information on the nature of xenon–pore surface interactions in terms of effective adsorption enthalpies. Dissolved organic matter (DOM) sorption on the mesoporous Al2O3 was shown to proceed inhomogeneously indicative by the Xe adsorption enthalpies corresponding to the co-existing “empty” pores and pores coated with organic species. In AlOOH, an interconnected system of micro- and mesopores was tested. The enhanced sensitivity of HP 129Xe NMR allowed us detecting micropores in charcoals, where N2 adsorption method underestimated porosity due to the restricted N2 diffusion at 77 K. The interconnected pore structure of charcoals was attributed to the voids formed by both polyaromatic and aliphatic domains (evidenced by 13C NMR). The observed differences between the TP- and HP 129Xe NMR patterns were explained by the restricted xenon diffusion through charcoal particles caused by the constricted pore openings. Their suggested size is of the order of one or two diameters of the Xe atom. For the Andosol clay fractions, the large low-field 129Xe shifts (up to 175 ppm) increasing with Xe pressure indicated a developed porosity most obviously comprised by the interconnected micro- and mesopores. Such porous network may originate from the “multi-domain” structure of soil clay particles, i.e. particles formed by agglomerated nano-sized crystallites. The latter are assumed to be the polynuclear Alx(H2O)y(OH)z clusters formed by hydrolysis reactions of Al3+ species after the destroying of Alhumus complexes by the H2O2-oxidation. © 2011 Elsevier B.V. All rights reserved.

1. Introduction In soils, meso- and micropores (2–50 nm and b2 nm) are important for the formation of soil interfaces controlling pollutant binding as well as soil organic matter (SOM) stabilisation. The latter is thought to occur through limiting microbial access and controlling gas diffusivity and water availability (Thomsen et al., 2003). Kaiser and Guggenberger (2007) argued that organic molecules are preferentially sorbed at the mouths of micropores that reflect the surface roughness of minerals caused by defects in the minerals' composition. Mikutta et al. (2004) studied sorption of organic species on AlOOH oxide ⁎ Corresponding author. Tel.: +49 8161714186; fax: +49 8161714466. E-mail address: fi[email protected] (S. Filimonova). 1 Present address: Instituto de Recursos Naturales y Agrobiología de Sevilla (IRNASCSIC), 41012 Sevilla, Spain. 0016-7061/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2011.01.009

with the use of 1H NMR and showed that some organic molecules can even penetrate into the micropores. Evidence for the mesopore protection of humic compounds was provided by Zimmerman et al. (2004) by testing pure alumina and silica minerals that showed 3–40 times lower enzyme activity compared to that observed for humic substrates sorbed to nonporous minerals. Despite significant progress achieved in studying soil porosity by the use of adsorption methods (N2, CO2), the impact of specific pore properties such as connectivity or structure remains mostly unresolved. The limitation of the widely used N2 adsorption method is its inability to provide realistic specific surface areas (SBET) for soil samples rich in organic matter, due to the restricted N2 diffusion at 77 K (de Jonge and Mittelmeijer-Hazeleger, 1996; Echeverría et al., 1999). As a complementary technique, 129Xe NMR spectroscopy, being highly sensitive to pore dimensions and shape, was only recently introduced for characterising micro- and mesoporosity of soil materials (Magusin

S. Filimonova et al. / Geoderma 162 (2011) 96–106

et al., 1997; Filimonova et al., 2004, 2006). As compared with X-ray powder diffraction measurements, transmission electron microscopy, and gas adsorption isotherm measurements, methods which in principle provide information on pore size, pore volume and surface area, 129Xe NMR has an advantage of probing the connectivity and uniformity of pores not accessible by other techniques. A promising development in the recent years is the use of optical pumping devices allowing to produce laser-polarised, or hyperpolarised (HP) xenon, which increases sensitivity of the spectra by several orders of magnitude as compared to the conventional (i.e. using thermally polarised, TP) 129Xe NMR measurements (Driehuys et al., 1996). This innovation allowed enhancing the range of possible applications of 129Xe NMR (e.g. Moudrakovski et al., 2002, 2004; Nossov et al., 2003a, 2003b; Raftery, 2006; Pawsey et al., 2007; Springuel-Huet et al., 2007). A limitation of experiments using HP xenon in a batch mode is a profound depolarising effect of paramagnetic centres that are frequently present not only in natural porous solids but also in soils. With the use of a continuous flow (CF) apparatus, this drawback was shown to be reduced (Brunner et al., 1999). Soil is a multi-component heterogeneous system which contains— in various proportions—porous mineral phases and different forms of organic material. In such heterogeneous systems, Xe atoms experience fast exchange between the various adsorption zones, i.e. different soil phases, which causes significant broadening of the 129 Xe resonance peaks (Filimonova et al., 2004). However, by studying separated soil particle size fractions possessing more uniform pore sizes compared with bulk soils, we observed relatively narrow 129Xe signals (ca. 1–3 kHz width) (Filimonova et al., 2004, 2006) and assigned them to mineral pores falling into the mesopore range. In the investigated soils (Podzol, Luvisol and Gleysol), the micropores were not penetrated by xenon, neither within the minerals (possibly due to the pore blocking by organic molecules) nor within the soil organic matter (SOM). In this present investigation, we used both the HP- and TP 129Xe NMR studies with the aim of testing whether this combined approach can provide additional information on porous structure of soil components. Therefore, we investigated a few model samples representing soil constituents in order to evaluate the electronic factors responsible for the 129Xe resonance shifts detected in natural soils. The most interesting would be an understanding whether the absence of NMR signals from 129Xe atoms adsorbed in the natural SOM (Filimonova et al., 2006) relates to the pore inaccessibility or, alternatively, to the slow xenon diffusion within the polymeric structure of SOM that leads to the signal broadening beyond detection limit. In order to assess this, we used a less complex natural organic material such as charcoal whose porous structure is formed by thermally altered bio-macromolecules. Charcoals frequently occur in fire-affected soils being important carrier of organic pollutants, but little is known about their porosity and contribution of the latter to the measured porosity of bulk soils. On the other hand, our previous hypothesis (Filimonova et al., 2004) considered that the soil micropores may not have been observed due to interference with paramagnetic material, if a considerable part of them was present within the iron oxide phase. To avoid the impact of paramagnetic species, we have chosen for this present study two diamagnetic mineral (hydr)oxides, i.e. a mesoporous aluminium oxide Al2O3 and microporous hydroxide AlOOH in a form of boehmite (isomorphic with goethite) for assessing their behaviour in the sorption of dissolved organic matter (DOM). Soil materials represented the clay sized fractions of the Ah- and Bw horizons of a non-allophanic Andosol derived from the southern Brazilian highlands (Dümig et al., 2008). This soil is characterised by the high content of Al oxides and charred residues. N2 adsorption was used as a complementary method for estimating specific surface areas of the samples (SBET) and their micro- and mesopore volumes.

97

2. Theory In the pioneering work by Ito and Fraissard (1980), the resonance shift was presented as a sum of different variables: δ = δ0 + δXe + δs + δSAS + δE + δM

129

Xe

ð1Þ

where δ is the detected chemical shift, δ0 is the reference shift usually regarded to be zero, δXe is due to Xe–Xe collisions, δs is the shift caused by interaction of Xe with the surface of the solid, δSAS indicates the presence of strong adsorption centres, and δE and δM are caused by the electric or magnetic fields created by cations. For solids with negligible impact of the last three terms, the Eq. (1) can be re-written as: δ = δ0 + δXe−Xe ρXe + δs xads ;

ð2Þ

where ρXe is the local Xe density and xads is the relative population of Xe atoms in the adsorbed state. In micropores, ρXe is larger than the equilibrium Xe density and thus, Xe–Xe collisions cause an increase of δ with Xe pressure proportional to the pore size (Demarquay and Fraissard, 1987). In contrast, for bigger pores e.g. mesopores having larger volumes compared to the Xe atom diameter (4.4 Å), Xe–Xe collisions are rare (δXe−Xe = 0) that reduces Eq. (2) to δ = δ0 + δs xads ; where   xads = nads = nads + ngas :

ð3Þ

Here, the δs term can be a complicated function of the sorption energy, void space and temperature that excludes single correlation between the chemical shift and mesopore void space, although smaller δ generally corresponds to bigger pore diameters (Ripmeester and Ratcliffe, 1990). One possible model for quantitative interpretation of δ in mesopores is the model considering the volume-to-surface area ratio of the pore suggested by Terskikh et al. (1993) for silicabased materials. In this model, the case of Henry adsorption is assumed, in which the number of adsorbed Xe atoms, nads, varies directly proportional with the equilibrium PXe: nads = KPXe S. Here K is a Henry's constant, and S is a specific surface area. If the ideal gas law for Xe atoms adsorbed in the pore is valid (ngas = PV/RT), one obtains δ = δs = ð1 + V = SKRT Þ;

ð4Þ

where V/S is the local volume-to-area ratio of the mesopore which is proportional to the mean pore diameter D (V/S = D/η, η is geometric factor), K is the xenon adsorption constant, R is the universal gas constant, and T is the temperature of the measurement (Terskikh et al., 1993). Eq. (4) shows that in mesopores, the observed shift δ does not depend upon pressure, and it can be used to determine both the V/S and K values. Adsorption constant K depends, in turn, on temperature that may be used for determining Xe adsorption enthalpy by performing variable temperature experiments. In case of Henry adsorption K = K0 expðQ = RT Þ = √T;

ð5Þ

where K0 is the pre-exponent, and Q is the Xe adsorption heat for the pores considered (Terskikh et al., 1993). Combining Eqs. (4) and (5), and using logarithmic form, we obtain     ln ðδs = δ−1Þ√T = ln Dpore = RB −Q = RT;

ð6Þ

where δs is zero loading extrapolated 129Xe shift and B = K0η. Therefore, by plotting experimental δ versus T data in the ln[(δs/

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S. Filimonova et al. / Geoderma 162 (2011) 96–106

δ − 1)√T] vs 1/T coordinates, one obtains the Q/R values from the slope of the logarithmic plot (Eq. (6)).

3. Materials and methods 3.1. Samples Mesoporous aluminium oxide (Al2O3) was chosen as a model substance to follow changes within the mesopores after DOM adsorption. It was a product of KGaA Merck, Darmstadt, Germany (alumina form κ with parts of α). The particle size range was between 20 and 120 μm. The specific pore volume was determined from the adsorption branch of the N2 isotherm: V = 0.210 cm3 g−1. Hydraulic pore radius (2V/SBET) was 5.3 ± 0.1 nm and SBET was 80 m2/g. The OM/ Al2O3 samples were prepared by treating the Al2O3 with dithionite– citrate–bicarbonate (DCB) soluble dissolved organic matter derived from the Ah horizon of the non-allophanic Andosol. The DOM sorption was performed by stirring the mixed components for 5 h followed by repeated centrifuging with distilled water (3 times) to thoroughly wash out the samples. The resulting organic carbon concentrations (OC) are listed in Table 1, abbreviations OM1/Al2O3 and OM2/Al2O3 refer to the different OC-contents. A nanocrystalline boehmite powder AlOOH was a commercial sample of high purity grade (0.005–0.015 wt.% Fe2O3) distributed by Sasol North America Incorporation under the trade name Disperal®. The crystallite size was given as 4.5 nm and the particle size as measured by laser diffraction was d50 = 58 μm. The OM/AlOOH sample was prepared in a similar way as it is described above for the Al2O3, and its properties are listed in Table 1. The barbeque charcoal was a commercially available sample and its detailed description is given in Knicker et al. (2007). The wood char was obtained by charring of grained beech wood sawdust in a porcelain crucible in a muffle furnace at 350 °C for 12 min under oxic conditions. The C- and H-recovery after charring was 27 and 14%, respectively, resulting in an atomic H/C ratio of 0.8. The soil clay fractions were derived from a non-allophanic Andosol sampled in the southern Brazilian highlands with patchy forest vegetation (Dümig et al., 2008). For obtaining clay sized fractions, the bulk soil was suspended with deionised water and dispersed by ultrasound with an energy input of 1200 J ml−1 (Branson Sonifier 250). After sonication, the finer particles (b20 μm) were separated from the suspension by wet sieving and split up in a silt (2–20 μm) and clay fraction (b2 μm) by gravity sedimentation. The latter fractions were finally recovered from the suspensions by pressure filtration (cellulose nitrate, 0.45 μm) followed by freeze drying. Their characteristics are given in Table 2 and the properties of the bulk soils are available in Dümig et al. (2008, soil abbreviation “FP2”). The total carbon and nitrogen contents of the fractions were determined using dry combustion with a Vario EL elemental analyser (Elementar

Table 1 Comparative data of the N2 adsorption and

129

Analysen Systeme GmbH, Hanau, Germany). Since our soils did not contain carbonate, the total carbon concentrations were equal to the concentration of organic carbon, OC. The total amount of pedogenic Fe- and Al-oxides, Ald and Fed, was determined by extraction with the strong reductant sodium dithionite (Mehra and Jackson, 1960). The poorly crystalline fraction of Al- and Fe-hydroxides, Alo and Feo, was extracted using acid ammonium oxalate (Schwertmann, 1964). The extraction with pyrophosphate (Alp, Fep) was conducted according to Van Reeuwijk (2002) which is based on Blakemore et al. (1987). Superfloc was added to avoid flocculation in the pyrophosphate extracts. Both the dithionite, oxalate and pyrophosphate extracts were analysed for Fe and Al with an ICP-OES (Varian Vario-Pro). The procedure of the SOM oxidation with H2O2 was performed in the same way as it is described in detail in Filimonova et al. (2004).

3.2.

129

Xe NMR

129 Xe NMR experiments were performed with the use of both thermally- and laser polarised xenon. The term “thermally polarised xenon” (TP) refers to spin polarisation resulting from the equilibrium state population difference in an applied magnetic field, as defined by the Boltzmann conditions. For xenon at room temperature, this equates to 4.6∙10−6 T−1. Xe gas of 99.98% purity with a natural abundance of the 129Xe isotope (26.4%) was used for the adsorption. Before adsorption, the samples were dehydrated in glass tubes (10 mm outer diameter) under vacuum at temperatures of 55 °C (AlOOH) or 80 °C (Al2O3, charcoals and soil fractions) for 12–15 h. In the TP 129Xe experiments, the Xe gas was introduced into evacuated samples and allowed to equilibrate until no further drop in Xe pressure, PXe, was detected. Afterwards the sample tubes were flame-sealed. The equilibrium PXe ranged between 150 and 850 mbar. The TP 129Xe NMR spectra were measured on a Bruker DMX-400 spectrometer (Bruker Biospin, Rheinstetten, Germany) operating at a magnetic field 9.4 T and resonance frequency 110.68 MHz for 129Xe. The HP 129Xe NMR spectra were recorded on a Bruker AMX 300 spectrometer operating at 83.02 MHz and a continuous gas flow (CF) using a home-built optical pumping setup (Nossov et al., 2003a). Circular polarised light from a 60 W diode array laser (COHERENT) was used for optical pumping at the D1 transition of rubidium (794.7 nm) within a Pyrex pumping cell placed in the fringe field of the superconducting magnet and containing a small amount of rubidium vapour. The Xe–He mixtures containing 7–1300 mbar of Xe was purified with the help of an oxygen trap, polarised to ca. 1% at total pressure of 1300 mbar, and finally circulated at ca. 100 cm3 min−1 flow rate through the samples. The used glass tubes were equipped with two Young valves for flow experiments. For most of the spectra, 512 data points were accumulated, and a 20–30 Hz Gaussian line broadening was applied before Fourier

Xe NMR for the Al2O3, AlOOH and charcoal samples.

Sample

OC (mg g−1)

SBET (m2 g−1)

Vtotal⁎ (cm3 g−1)

Vmicro (cm3 g−1)

δ (129Xe) (ppm)

Al2O3 OM1/Al2O3 OM2/Al2O3 AlOOH OM/AlOOH Wood char Barb. charcoal

– 6.6 10.0 – 5.8 61.6 68.4

80.0 75.7 72.2 329 301 26 7

0.210 0.167 0.170 0.384 0.361 0.042 0.020

0 0 0 0.010 0.008 0 0

76.6 82 85 106 94 140–160 200; 130–160

⁎ Total pore volume was determined from the N2 adsorption–desorption isotherms at p/p0 = 0.99. ⁎⁎ Values for the barbeque charcoal were not determined due to the very low N2 adsorption.

QIads (kJ mol−1)

QIIads (kJ mol−1)

4.8 4.4

12.1 11.2

Vmeso⁎⁎ (cm3 g−1)

Smeso⁎⁎ (m2 g−1)

0.210 0.167 0.170 0.374 0.353 0.024

80 75.7 72.2 310 215 22

S. Filimonova et al. / Geoderma 162 (2011) 96–106

99

Table 2 Characteristics of the Andosol clay fractions⁎. Sample

OC

ON

(mg g−1)

SBET (m2 g−1)

Vtotal

Vmicro

(cm3 g−1)

Alo

Feo

Ald

Fed

Alp

Fep

(mg g−1)

Ah b 2 μm Untreated H2O2-treated

110.7 11.0

5.2 2.1

45.7 114.9

0.035 0.275

0 0.004

19.3 ± 1.0 18.0 ± 0.9

17.4 ± 0.9 16.5 ± 0.8

24.0 ± 1.2 12.8 ± 0.7

36.6 ± 1.8 34.3 ± 1.7

57.6 ± 2.9 64.7 ± 3.2

41.4 ± 2.1 45.2 ± 2.3

Bw b 2 μm Untreated H2O2-treated

29.6 6.4

2.5 1.6

92.7 215

0.080 0.322

0 0.008

13.8 ± 0.7 12.2 ± 0.6

17.4 ± 0.9 16.4 ± 0.8

14.3 ± 0.7 14.8 ± 0.8

35.2 ± 1.7 39.5 ± 2.0

47.4 ± 2.4 50.8 ± 2.5

37.4 ± 1.9 28.1 ± 1.4

⁎ See text for the legend.

transformation. π/2 pulse length was 10–14 µs, delays between pulses were 2–5 s. A single-pulse sequence was used, and the number of accumulated scans varied between 2 × 103 and 5 × 104. Chemical shifts given with an accuracy of 1–2 ppm were referenced to the shift of gaseous xenon extrapolated to zero pressure. Downfield shifts were considered as positive. 3.3.

13

C NMR spectroscopy

The solid state 13C NMR spectroscopy (Bruker DSX 200) with the cross polarisation magic angle spinning (CPMAS) technique at a spinning speed of 6.8 kHz and a pulse delay of 300 ms was applied to the charcoal- and soil samples for analysing their chemical composition. A ramped 1H pulse was applied during a contact time of 1 ms and 2000 scans were accumulated. The 13C shifts were referenced to tetramethylsilane (=0 ppm). 3.4. N2 adsorption N2 adsorption measurements (77 K) were carried out by using an Autosorb 1 surface area analyser (Quantachrome Corp., Syosset, NY, USA). Prior to the measurements, the samples were outgassed under vacuum either at 55 °C (AlOOH and charcoals) or at 80 °C (Al2O3 and soil samples) for 20–24 h. The specific surface area, SBET, was calculated by applying the Brunauer–Emmett–Teller equation (Brunauer et al., 1938) to the data points in the relative pressure (p/p0) range between 0.05 and 0.30. A cross-sectional area of 16.2Å2 for the N2 molecule was assumed. The presence of microporosity was checked by the t-method, i.e. using a plot of the adsorbed N2 gas volume versus the statistical thickness of an adsorbed film, t (de Boer et al., 1966). The mesopore volumes and areas were determined as cumulative values in the range between 2 and 50 nm with a Barret–Joyner–Halenda (BJH) method (Barrett et al., 1951) using the desorption branches of the isotherms. 3.5. X-ray diffraction analysis X-ray powder diffraction (XRD) analysis was performed to control the mineral composition and the presence of expandable clay minerals in the studied clay-sized soil fractions. A Philips PW 1070 diffractometer with CoKα radiation (40 kV and 30 mA) equipped with a graphite monochromator was used for the analyses performed with steps of 0.02°2θ and with a counting time of 5 s for each step. 4. Results and discussion 4.1. Mesoporous Al2O3 129 Xe NMR spectra of hyperpolarised and thermally polarised xenon adsorbed on the pure Al2O3 were practically identical showing that in both cases the Xe atoms probed the same adsorption regions within this sample. The chemical shift, δ, did not depend upon Xe

pressure, PXe, indicating adsorption within the mesopores (Eq. (4)). Figure 1A illustrates the HP 129Xe NMR spectra for the pure Al2O3 sample and samples containing adsorbed OM. The used PXe was 15 mbar, i.e. 10–20 times lower than the Xe pressure needed for obtaining reasonable signal-to-noise ratios in the “conventional” 129 Xe NMR spectra (e.g. Filimonova et al., 2004). The advantage of using such low PXe is the negligible contribution of Xe–Xe collisions to the observed shift δ (δXe–Xe = 0) that allows interpretation of δ in terms of xenon–surface interactions (i.e. term δs in the Eq. (2)). The δs term, in turn, can be used to extract the local volume-to-surface area ratio of the pores (V/S), as well as the xenon adsorption constant (K) which reflects the chemical nature of pore surfaces (Eq. (4)). Spectra shown on Figure 1A contain a narrow intense peak around 0 ppm from gaseous-like xenon in inter-particles spaces, and signals at 78–90 ppm from xenon adsorbed within the mesopores. For the OM1/Al2O3 and OM2/Al2O3 samples, the peak of adsorbed xenon was shifted down-field by 10–12 ppm, as compared to the pure Al2O3, whereas its width did not change noticeably. Such an increase of δ points to the decreased V/S ratio in two OM/Al2O3 samples corresponding to their smaller averaged pore diameters, Dpore (Eq. (4)). Coincidently, the DOM sorption reduced the specific surface area (SBET) of Al2O3 by 5–8 m2 g−1 (6–10%) and also the total N2accessible pore volume (Table 1). The reduction in SBET normalised to the amount of carbon sorbed was 0.7–0.8 m2 mg−1 DOM-C. Spectra measured as a function of temperature for the OM/Al2O3 samples are illustrated on Figure 1B and C. This type of experiment provides information on the mobility of the adsorbate, its adsorption parameters, as well as on pore uniformity. The 129Xe resonance signal shifted from ca. 60 to 125 ppm when the temperature decreased from 353 to 163 K. This shift is due to the lowered mobility of Xe atoms associated with their longer contact time with the pore surfaces (i.e. increased xads term in the Eq. (2)) at lower temperatures. For the used PXe = 15 mbar, the adsorption isotherm is assumed to obey the Henry's law. In this case, the xenon adsorption constant can be described by the Eqs. (5) and (6). Therefore, by plotting our experimental δ versus T data in the ln[(δs/δ − 1)√T] vs 1/T coordinates, we observed two linear regions in the graph with different slopes (Fig. 2) which corresponded to the different adsorption heats for xenon (Q = 4.6 ± 0.2 and 11.7 ± 0.5 kJ mol−1, Table 1). Comparing these calculated Q values with those previously reported for various functionalised mesoporous oxides (e.g. porous silica and organic aerogels in Moudrakovski et al., 2002, 2004; metal-organic frameworks with carboxylate functionalised molecules in Pawsey et al., 2007), we assigned them to the different adsorption regions for xenon, i.e. the non-filled pores (I) and the pores coated with organic functional groups (II). The 129Xe resonance shifts for the coated pores (type II) corresponded to smaller δ values (Fig. 2) i.e. to larger pore size (Eq. (4)). Thus, upon their sorption, organic species seem to initially occupy larger mesopores that points toward a non-homogeneous character of the DOM distribution within the mineral oxide. Inhomogeneous distribution of SOM within the mineral soil phase

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S. Filimonova et al. / Geoderma 162 (2011) 96–106

A 90

OM2/Al2O3

OM1/Al2O3 78

Al2O3 200

160

120

80

40

0

δ (ppm)

C OM2/Al2O3

T, K

60

353 333

B

313 298

60

OM1/Al2O3

T, K

273

353

263

333

243

323 313

233 213

298

193

233 203

125

200

160

120

183

125

163

80

40

δ (ppm)

0

163

200

160

120

80

δ (ppm)

40

0

Fig. 1. HP 129Xe NMR spectra for the Al2O3 and OM/Al2O3 samples at PXe = 7 mbar: (A) room temperature; (B) variable temperature, OM1/Al2O3; (C) variable temperature, OM2/Al2O3.

ln[(δs/δ – 1)√T]

was reported elsewhere in the literature. Kaiser and Guggenberger (2007) evidenced that, at lower OC-contents, organic molecules were preferentially sorbed within or near to micropores. Our data show that, in bigger pores, the similar trends obviously exist.

3

pores coated with OM

OM1

QII OM2

2 empty pores

QI

1

2

4 1000/T (K-1)

6

Fig. 2. 129Xe chemical shift versus temperature data for the OM1/Al2O3 and OM2/Al2O3 samples plotted in the logarithmic coordinates.

4.2. AlOOH The structure of AlOOH is isomorphic to that of goethite, the mineral frequently occurring in soils. Therefore, the sorption properties of AlOOH can be used for predicting and modelling the sorption behaviour of goethite that cannot be directly investigated by NMR techniques due to its paramagnetic character. AlOOH in a boehmite form used in our study consisted of particles of ca. 58 μm size which are formed by aggregates of nano-sized (4–5 nm) crystallites. This “multidomain” particle structure gives a developed intraparticle porosity (SBET = 329 m2 g−1, Table 1). The TP 129Xe NMR spectrum for the pure AlOOH consisted of a single resonance shifting from 99 to 106 ppm upon increasing the equilibrium Xe pressure from 120 to 780 mbar (Fig.3). After the DOM contact with AlOOH, the 129Xe signal was peaking a few ppm up-field as compared to the pure AlOOH (Fig. 3). Typically, the presence of micropores is indicative of the marked chemical shift variations with Xe loading (Demarquay and Fraissard, 1987), while in mesopores the Xe shift does not vary upon changing the Xe loading (Terskikh et al., 1993). We observed only a slight variation of δ (ca. 7 ppm) that can be rationalised by a model of interconnected meso- and micropores. The up-field 129Xe shift following the DOM sorption points toward diminishing the averaged pore width probed by the Xe atoms.

S. Filimonova et al. / Geoderma 162 (2011) 96–106

101

A

94

PXe, mbar 780 106

AlOOH

N2-volume (cm3g-1)

OM/AlOOH

AlOOH

250 200

OM/AlOOH

150 Andosol Bw clay

100

780

50

100

Wood char

0 0.2

0.4

99

B

200

160

120

δ (ppm)

80

40

0

Fig. 3. TP 129Xe NMR spectra for the AlOOH and OM/AlOOH.

Concurrently, the SBET was reduced by ca. 9%, or 4.8 m2 mg−1 DOC being normalised to the amount of the carbon sorbed (Table 1). The N2 adsorption isotherms confirmed the existence of a mixed meso- and microposity in both pure- and OM covered AlOOH samples. Their isotherms (Fig. 4A) resembled a mixture of types I and IV isotherms according to Brunauer et al. (1940), i.e. the curve concave to the p/p0 axis at lower p/p0 which is typical for the presence of micropores, and a steep slope at higher p/p0 typical for capillary condensation in mesopores. The desorption branch exhibited a small slope at high relative pressures and a large slope where the part of the pores emptied. This can be explained by the presence of “ink-bottle” pores resulting in a type E hysteresis (de Boer, 1958; Mikutta et al., 2004). The DOM sorption changed neither the isotherm type nor the hysteresis loop but reduced the total pore volume (Table 1). The loss of specific micropore volume, as determined by the t-method, was ca. 20% (Table 1), i.e. nearly twice of the SBET loss upon DOM treatment. At the same time, the specific mesopore volume decreased only by ca. 5– 6%. Therefore, the DOM sorption on the examined AlOOH material obviously started within the micropores. We note that the pore volumes determined in our study are higher than those reported by Mikutta et al. (2004) for the AlOOH with adsorbed DOM and polygalacturonic acid. This difference can be explained by the higher pre-treatment temperature used in the present study that facilitated more efficient pore emptying. 4.3. Charcoals Two studied charcoals (barbeque- and wood char) exhibited only minor differences in their 13C NMR spectra (Fig. 5A): both were characterised by an intense signal of aromatic structures (130 ppm) with some intensity remaining in the chemical shift region of alkyl C (0–45 ppm) and O-alkyl-C (45–110 ppm, Knicker et al., 1996). The N2-specific surface areas comprised 7 and 26 m2 g−1 for these two samples, respectively, without microporosity input (Table 1). Their N2 adsorption isotherms were of type III (Brunauer et al., 1940) typical

1.0

AlOOH

N2-volume (cm3g-1)

240

0.8

120

120

280

0.6

p/p0

155

80

Andosol Bw clay

40

Wood char

2

4

Thickness (Å) Fig. 4. N2 adsorption-desorption isotherms (A) and corresponding t-plots (B) for the AlOOH, OM/AlOOH, wood charcoal and the H2O2-treated Andosol Bw clay fraction.

for nonporous carbons, i.e. convex to the p/p0 axis over its entire range (Fig. 4A, case of the wood char). The absence of any hysteresis indicated that mesopores were not tested by the adsorbed N2 in both charcoals. The 129Xe NMR spectra evidenced the resonances between ca. 130 and 200 ppm (Fig. 5B). Surprisingly, the TP- and HP 129Xe NMR patterns were not identical for both charcoals, and the more pronounced differences were observed for the barbeque char. The TP 129Xe NMR spectrum for this latter sample consisted of a broadened line at ca. 140 ppm (PXe = 400 mbar), while its HP 129Xe spectrum exhibited a narrower but less intense signal at 203 ppm. For the wood char, the chemical shift difference between the HP- and TP 129 Xe patterns comprised ca. 30 ppm, and their signal widths were comparable (Fig. 5B). Unlike zeolites and silica-based materials, no general correlation of 129 Xe NMR parameters with porous characteristics of coals was reported largely because of complexity of these materials. The large value of the 129Xe chemical shift (130–200 ppm) can be rationalised in terms of the (i) Xe interaction with strong adsorption centres, e.g. acidic sites (Suh et al., 1991; Simonov et al., 1999); (ii) adsorption within micropores (Tsiao and Botto, 1991; Romanenko et al., 2006); or (iii) preferential orientation of graphitic layers (Simonov et al., 1999; Romanenko et al., 2005). The latter explanation can be

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A

B

130

130

wood char HP Xe

160 TP Xe 30

wood char

*

*

203

barb.char 65

barbeque char

*

HP Xe

*

140 TP Xe

300

220

140

60

δ (ppm)

-20

-100

300

260

220

180

140

100

60

δ (ppm)

Fig. 5. 13C CP MAS spectra for the barbeque- and wood charcoals (A); 129Xe NMR spectra for the barbeque- and wood charcoals recorded respectively at PXe = 400 and 770 mbar (TP Xe) and 7 mbar (HP Xe) (B). Spinning sidebands are marked with asterisks.

immediately rejected since our charcoals are amorphous in nature and thus, no regular orientation of graphitic planes on the surface could be expected. The idea of Xe adsorption on strong adsorption

centres is compatible neither with the independence of the HP 129Xe shift on Xe loading (Fig. 6A) nor with a positive correlation of the TP 129 Xe shift with Xe loading (from 130 to 160 ppm upon Xe pressure

B

A

PXe, mbar

T, K

1022

213 213

243

623 207.3 273 413 203.1

203

296 8

250

200

150

100

δ (ppm)

50

0

250 200 150 100 δ (ppm)

50

0

C

Xe

Fig. 6. HP 129Xe NMR spectra for the barbeque charcoal recorded at varying pressure (A) and varying temperature at PXe = 7 mbar (B) together with the suggested pore model showing constricted pore openings formed by the polyaromatic units (C).

S. Filimonova et al. / Geoderma 162 (2011) 96–106

change between 250 and 940 mbar, spectra not shown). If strong adsorption centres would be responsible for these observed 129Xe shifts, an opposite trend of the δ variation should be detected, or we must assume that all the surface consists of such strong centres. Thus, it is very probable that both the HP- and TP 129Xe resonances correspond to the Xe atoms adsorbed within confined spaces, i.e. pores. In accordance with previous studies (e.g., Tsiao and Botto, 1991), the signal at ca. 200 ppm can be tentatively assigned to the tight pores within aromatic domains, while the resonance at 130– 160 ppm may corresponds to aliphatic regions of the charcoal. The non-identity of the HP- and TP 129Xe patterns for both charcoals means that these two techniques probe different environments within these samples that might be caused by the specific compositional or structural features of charcoals. Typically, there are no intrinsic differences in the properties of HP- and TP Xe; moreover, the HP Xe atoms immediately become “thermally polarised” after relaxation. However, the use of the gas flow conditions can cause differences in relaxation-, adsorption- and diffusion issues in the HP 129 Xe NMR, as compared to the “conventional” 129Xe NMR, which is typically conducted under equilibrium Xe pressures. Therefore, in the HP 129Xe NMR spectrum we can only detect adsorption sites (e.g. pores) whereto the laser-polarised Xe atoms have time to go without loosing their polarisation. In contrast, in the conventional 129Xe NMR experiments, the time limitation problems do not exist. The signal at 140 ppm is missing in the HP 129Xe spectrum of the barbeque char (Fig. 5B), and this is most tentatively due to the fast depolarisation of Xe atoms undergoing adsorption within the pores corresponding to δ = 140 ppm. Such depolarisation can occur on the way of the HP Xe atoms into the pore volumes. Generally, the observed signal intensities in the HP 129Xe NMR spectra are determined not only by the T1 relaxation time of xenon in different phases but also by the exchange rate between depolarised 129Xe (the hyperpolarisation is destroyed by a radio frequency pulse) and hyperpolarised 129Xe. This exchange itself depends on the gas flux inside the hyperpolarisation cell and on the Xe diffusion rate inside the pore network (SpringuelHuet et al., 2007). In our case, one possible explanation of the polarisation loss by some Xe atoms could be their longer diffusion time required to enter particular charcoal pores caused e.g. by the spatial hindrances of pore openings due to specific pore geometries. To verify this assumption, we compared the data of the variable Xe pressure- and variable temperature 129Xe NMR measurements (Fig. 6A and B, case of the barbeque charcoal). The first type of the experiment can be used to estimate the micropore size, while the second one gives the enthalpies of Xe adsorption. As mentioned above, the signal with δ=203 ppm did not shift noticeably upon Xe loading variation (Fig. 6A). This is not a typical behaviour for micropores since Xe–Xe collisions should cause an increase of δ with Xe pressure (Eq. (2)). However, to allow such collisions to occur, the pore sizes must be sufficiently large to possess two or more Xe atoms, i.e. the condition Dpore N 2dXe =8.8 Å should be fulfilled. In our case, the lack of any pressure dependence combined with the large δ value points towards a micropore size of less than 8.8 Å. The low intensity of this peak is a sign of rather low concentration of corresponding pores that makes it clear why they were not detected by the conventional (TP) 129Xe NMR which possesses lower sensitivity. Quite similar spectral behaviour (large chemical shift that did not depend on Xe pressure) was observed for xenon adsorbed in side pockets of mordenite (Springuel-Huet and Fraissard, 1992) that could only accommodate a single Xe atom. From the temperature dependence of the 129Xe chemical shift (Fig. 6B), we extracted the xenon adsorption heat, Q, by treating the δ vs T data points in the logarithmic coordinates according to the Eq. (6). The calculated value of Q = 18 kJ mol−1 is comparable with that previously reported for coals (e.g. Glass and Larsen, 1993) that also confirms the assignment of the signal at 203 ppm to the Xe atoms adsorbed within the aromatic domains The latter were manifested by the 13C NMR moiety at 130 ppm (Fig. 5A).

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Combining experimental findings described above, we suggest a pore model for the charcoals which implies the so-called “bottle neck” effect (Fig. 6C). The micropores (δ = 203 ppm) are assumed to be the narrow pore entrances which are easily reachable for both the HPand TP Xe atoms, but they cannot be registered with the TP 129Xe NMR due to the low concentration of Xe atoms caught within these “bottle necks.” The xenon access to bigger pores (δ = 130–160 ppm) is supposed to be hindered by those constrictions. The latter could be e.g. the points of closest approach of the polyaromatic hydrocarbon units (Radovic et al., 1997) which have a natural tendency to align parallel to each other but may be prevented from doing so by the heteroatoms at their peripheries. Within the time limit needed to overcome the energy barrier associated with the narrow pore openings, the HP Xe atoms become depolarised. The slightly differing results for the wood charcoal, i.e. smaller δ value for the HP 129Xe possibly advises to the less constricted xenon diffusion e.g. due to the smaller particle size of this sample making pore openings wider so that some “closed” porosity became accessible. The model of interconnected pore network which includes numerous ultra-microporous constrictions (presumable size 4.4–8.8 Å) also explains the low SBET for both charcoals. 4.4. Soil materials Figure 7A compares the spectra for the original (untreated) and H2O2-treated clay fractions of the Andosol Ah- and Bw horizons. We note that the signal intensities of the shown spectra are arbitrary and they were chosen to emphasise the signals of adsorbed species. The 129 Xe resonances for the untreated Ah- and Bw clay fractions appeared at ca. 40 ppm (patterns a, c). The micropores were not observed there, most probably, due to their clogging with organic species. In contrast, for the clay fractions oxidised by the H2O2 (accelerating mineral pore emptying; Kaiser and Guggenberger, 2003), a down-field shift of the 129Xe signals was detected (patterns b, d–f) that might indicate the pore emptying concurrently with the SOM removal. Coincidently, the N2-measured specific surface areas as well as the total- and micropore pore volumes increased for the H2O2treated clay fractions (Table 2). For the Ah horizon, the observed 129Xe resonance was markedly broader (pattern b) that might suggest the restricted xenon diffusion within small pores originating from organic species that resist the H2O2-oxidation. Those oxidation-resistant species can be e.g. the charred residues that were evidenced by the large proportion of aromatic carbon resulting in the 13C NMR signal peaking at ca. 130 ppm (Fig. 7B, pattern b) combined with small contents of lignin-derived phenols (Dümig et al., 2009). On the other hand, the essential signal broadening might also show an enhanced paramagnetic influence of Fe oxides due to their de-shielding after the SOM removal. The unusually large 129Xe chemical shift (155–175 ppm) detected for the Andosol Bw clay fraction can be considered as a new interesting finding for this soil type. Increasing of δ with Xe loading (Fig. 7, patterns d–f) can be rationalised in terms of Xe–Xe interactions within extended micropores that are, more probably, combined with the mesopore network. The relatively small increase of δ (ca. 20 ppm) favours the latter idea. We note that 129Xe peaks in this chemical shift range have not been observed for previously studied soil types like Luvisol, Gleysol and Podzol (Filimonova et al., 2004). Concurrently, this fraction is characterised by a large SBET (215 m2 g−1), which is only ca. 1.5 times lower than the SBET value for the pure AlOOH hydroxide (329 m2 g−1) having a high micropore content (Table 1) and nearly twice lower than the SBET of poorly crystalline (hydr)oxides like ferrihydrite or goethite (400–500 m2 g−1, Eusterhues et al., 2005). The latter compounds are known to be composed by nano-sized (4–5 nm) crystallites which are agglomerated into the particles. Such structure provides large specific surface area making those (hydr)oxides important porosity sources in natural soils.

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A

B

0

160 8

char

g

175

Bw

170

f e

155

Bw

c

d 40

c Ah

130

Ah

b

b a

a 320

240

160

80

0

δ (ppm)

550

350

150

δ (ppm)

C

-250

b

a

Fe

-50

Al Al

Al Al

Al

Alx(H2O)y(OH)z

Fe Fig. 7. (A) TP 129Xe NMR spectra of xenon adsorption on the Andosol clay fractions untreated (a and c), H2O2-treated (b and d–f), and natural Andosol char particles (g). Equilibrium Xe pressure was chosen to be comparable for all spectra (PXe = 650–670 mbar), except for the patterns d–f detected at a varying Xe pressure of 270, 560 and 940 mbar; (B) 13C NMR spectra for the Andosol clay fractions untreated (a and c) and H2O2-treated (b); (C) suggested structure of the Andosol clay particles untreated (a) and subjected to the SOM removal (b).

Enhanced porosity of the Andosol Bw clay fractions, inferred from the large 129Xe shift increasing with Xe pressure, originates not from the clay minerals which are non-swelling illite and chlorite (XRD analysis data). Likewise, it comes also not from the poorly crystalline Fe (hydr) oxides since Xe adsorption within these compounds possessing paramagnetic character would make the 129Xe resonances undetectable. Moreover, the soil clay-sized fractions with the comparable Fe contents of ca. 25–30 mg g−1 (e.g. Luvisol, Gleysol and Podzol investigated by Filimonova et al., 2006) whose pores were attributed to poorly crystalline Fe oxides exhibited SBET = 70–80 m2 g−1, i.e. ca. 3 times lower than the presently studied Andosol materials. We assume that the charred residues resisting the H2O2-oxidation can contribute to the observed 129Xe resonance in the Andosol clay samples. Indeed, the 129Xe NMR signal of the natural char derived from the examined Andosol appeared in the same chemical shift region (ca. 160 ppm, Fig. 7A, pattern g). The concentration of the charred residues in the H2O2-treated Andosol fractions is, however, quite low (the resistant OC-content is ca. 0.6 wt.%, Table 2) that did not allow obtaining a reasonable signal-to-noise ratio in the 13C NMR spectrum of the clay fraction of the Bw horizon. Therefore, the main porosity source in our examined soil materials obviously originates from their specific morphological features. The latter most probably relate to the unique chemical properties of the non-allophanic Andosol associated with the formation of the Al- and Fe-humus complexes (Shoji et al., 1985; Aran et al., 2001). The total Al content reached 8–10 wt.% in our studied soil clay fractions, and the pyrophosphate extractable Al- and Fe contents (corresponding to the Al- and Fe-humus complexes) approached ca. 4–6 wt.% (Table 2). This might indicate that the entire clay particle is composed by the Al-humus and Fe-humus fragments. The Al3+- and Fe3+ cations might be located at

the peripheries of the metal-humus associates binding them by multiple bonds as it is shown on Figure 7C-a. After their destroying by the H2O2treatment, the hydrolysis reactions of Al3+ and Fe3+ might lead to the formation of various Al- and Fe-hydroxyl species of the general form Al − n)+ − n)+ (H2O)6 − n(OH)(3 , Fe(H2O)6 − n(OH)(3 (Bertsch and Bloom, n n 1996). The former have a tendency to form polynuclear species Alx (H2O)y(OH)z, i.e. clusters of a few nanometer size resulting in a multidomain particle structure that provides a mixture of micro- and mesopores accessible for the adsorbed molecules like Xe or N2 (Fig. 7C-b). The exact composition of those polynuclear species is difficult to establish. Particular Al13 species [AlO4Al12OH24H2O12]7+ were reported for the Oa horizon of an acid Spodosol (Bertsch and Bloom, 1996). The N2 adsorption data coincide with this suggested model. The isotherm for the H2O2-treated Bw clay fraction (Fig. 4A) was assigned to the type IIb adsorption isotherms which are generally observed for aggregates of plate-like particles (Rouquerol et al., 1999). The marked hysteresis loop on desorption is associated with capillary condensation in mesopores having slit-like shapes (type B by de Boer, 1958). On the other hand, the N2-accessible micropore content in the Andosol Bw clay fraction was comparable with that for the AlOOH sample as evidenced by the identical intercepts of their t-plots (Fig. 4B; Tables 1 and 2), being considerably larger than the micropore contents determined for the previously studied soils (e.g. Kaiser and Guggenberger, 2003; Filimonova et al., 2006). This observation favours the hypothesised similarity of the porous structures of these two materials, i.e. the pores formed by the aggregated nano-sized domains. Thus, the combined 129Xe NMR and N2 adsorption investigation confirmed the specific morphological properties of the non-allophanic

S. Filimonova et al. / Geoderma 162 (2011) 96–106

Andosol, i.e. developed micro- and mesoporosity originating from the (well known) unique chemical properties of this soil type. 5. Conclusions Pore environments of a series of samples representing porous soil constituents have been studied using conventional (TP) and hyperpolarised (HP) 129Xe NMR spectroscopy. Xenon gas behaved as an efficient probe for interrogating their pore structures through i) higher sensitivity for probing micropores within polymeric organic structures, e.g. charcoals, as compared to the N2 adsorption; ii) possibility to evaluate not only the pore size range but also adsorption enthalpies for xenon that reflect the nature of Xe–pore surface interactions; and iii) assessing the extent of pore attainability since the latter affects relaxation phenomena which finally determine appearance of the 129Xe NMR patterns. For the charcoals, an existence of an interconnected but highly constricted pore system was inferred based on the observed differences between the TP- and HP 129Xe NMR patterns. The narrow micropores (presumable size 4.4–8.8 Å) were considered to be those constricted pore openings. The limited pore availability for the adsorbed N2 observed for both charcoals coincides with this model. Specific pore structure might determine high sorptive capacity of charcoals. Further studies are necessary to confirm this relevance. The unusually large 129Xe NMR chemical shifts detected for the clay fractions of the non-allophanic Andosol coincided with their large N2-specific surface area which is, in turn, comparable with that for microporous hydroxides like AlOOH or ferrihydrite. The interconnected micro- and mesopores were attributed to the volumes formed by the agglomerated nano-sized Alx(H2O)y(OH)z clusters, with a possible contribution from the charred residues. Briefly, the following pore structures were identified: (i) well defined meso- or micropores, or their mixture (Al (hydr)oxides); (ii) interconnected pore network with ultra-microporous constrictions (charcoals); (iii) multi-domain particle structure formed by aggregated nano-sized species (non-allophanic Andosol). Acknowledgements We are indebted to Dr. Werner Häusler for the XRD-analyses. The German Science Foundation (DFG, KN 463/8-1) is gratefully acknowledged for the financial support. Charcoal materials were taken from the DFG project KN 463/5-2. References Aran, D., Gury, M., Jeanroy, E., 2001. Organo-metallic complexes in an Andosol: a comparative study with a Cambisol and Podzol. Geoderma 99, 65–79. Barrett, E.P., Joyner, L.G., Halenda, P.P., 1951. Determination of pore volume and area distribution in porous substances. (I) Computation of N2 isotherms. J. Am. Chem. Soc. 73, 373–379. Bertsch, P.M., Bloom, P.R., 1996. Methods of Soil Analysis. Part 3. Chemical Methods – SSSA Book Series No. 5. Madison, WI 53711, USA. Blakemore, L.C., Searle, P.L., Daly, B.K., 1987. Methods for chemical analysis of soils. N. Z. Soil Bur. Sci. Rep., vol. 80. New Zealand Society of Soil Science, Lower Hutt. Brunauer, S., Emmett, P.H., Teller, E., 1938. Adsorption of gases in multimolecular layers. J. Am. Chem. Soc. 60, 309–319. Brunauer, S., Deming, L.S., Deming, W.S., Teller, E., 1940. On a theory of the van der Waals adsorption of gases. J. Am. Chem. Soc. 62, 1723–1732. Brunner, E., Haake, M., Kaiser, L., Pines, A., Reimer, J.A., 1999. Gas flow MRI using circulating laser-polarized 129Xe. J. Magn. Reson. 138, 155–159. de Boer, J.H., 1958. The Structure and Properties of Porous Materials. Butterworths, London, p. 68. de Boer, J.H., Lippens, B.C., Linsen, B.G., Broekhoff, J.C.P., van der Heuvel, A., Osinga, T.J., 1966. The t-curve of multimolecular N2-adsorption. J. Coll. Interf. Sci. 21, 405–414. De Jonge, H., Mittelmeijer-Hazeleger, M.C., 1996. Adsorption of CO2 and N2 on soil organic matter: nature of porosity, surface area and diffusion mechanisms. Environ. Sci. Technol. 30, 408–413. Demarquay, J., Fraissard, J., 1987. 129Xe NMR of xenon adsorbed on zeolites. Relationship between the chemical shift and the void space. Chem. Phys. Lett. 136, 314–318. Driehuys, B., Cates, G.D., Miron, E., Sauer, K., Walter, D.K., Happer, W., 1996. Highvolume production of laser-polarized 129Xe. Appl. Phys. Lett. 69, 1668.

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