CHARACTERIZATION OF THE POROUS STRUCTURE OF CHILEAN VOLCANIC SOILS BY NITROGEN ADSORPTION AND MERCURY POROSIMETRY

MÓNICA ANTILÉN^*1, JUAN E. FÖRSTER², SYLVIE DEL CONFETTO³, ELIZABETH RODIER³, OLIVER FUDYM², ANNA M VENEZIA⁴, GIULIO DEGANELLO⁴, AND MAURICIO ESCUDEY².

¹Departamento de Química Inorgánica, Facultad de Química, Pontificia Universidad Católica de Chile, Santiago, Chile. Email: mantilen@uc.cl
²Departamento de Química de los Materiales, Facultad de Química y Biología, Universidad de Santiago de Chile, Santiago Chile.
³Ecole des Mines d'Albi, Route de Teillet, 81013, Albi France.
⁴Instituto per lo Studio dei Materiali Nanostrutturati (ISMN-CNR), Sezione di Palermo, Via Ugo La Malfa 153-90146 Italy.

ABSTRACT

Pore volume, specific surface area (SSA), and total intragranular porosity (TIP) of Chilean soils derived from volcanic materials were studied. Soil samples involving the 0-15 and 15-30 cm depth of virgin and cultivated Collipulli (Ultisol) and Diguillín (Andisol) soils at two particle size fractions (<1 mm and <2 µm) were considered.

From mercury porosimetry and N₂ adsorption, mainly mesopores (pore diameter, dpore, about 10 nm) were determined for <1 mm Collipulli samples. Diguillín <1 mm soil shows macroporosity with dpore from 70 nm to 7000 nm. The clay fraction of Collipulli has macropores (dpore from 2000 nm to 40000 nm) and mesopores (dpore from 3 nm to 23 nm), while for Diguillín clay-size fraction most of the porosity comes from macropores (dpore from 50 nm to 800 nm).

For all samples the SSA linearly correlates with the mesopore volume (r²=0.781; n=16) determined by N₂ adsorption, and with the mesopore + macropore volume (r²= 0.771; n=12) when Collipulli <1mm samples are excluded; an inverse relationship between SSA and organic carbon content was found (r²=0.854; n=14). Thus, the SSA defined mainly by mesopores and macropores is probably related to the soil organic matter content.

Mesopores and macropores mainly give the TIP, which increases as particle size decreases. No important changes in micropore and macropore volume, and in TIP were seen as result of cultivation. Mesopore volume is more important in samples dominated by kaolinite than in samples dominated by allophane (4 to 20 times). In general the soil pore distribution, its SSA and TIP are related to its mineralogy and organic matter content.

INTRODUCTION

In Chile most agricultural activity occurs on andisols and Ultisols, which are derived from volcanic materials. They have high organic matter content, high potential for crop production as they are easy to cultivate, conducive to root establishment, and posses ample water storage capacity.

Total porosity and specific surface area (SSA) are two characteristics of soils which contribute in a significant way to the relationship between the observed properties and behavior of volcanic soils.

The soil porosity, critical for its aeration (1), is a parameter related to the storage and plant availability of water (2). Porosity and pore size are considered as structural factors related to the thermal properties of the soil (3). SSA, extensively used in the interpretation of the soil physical characteristics, has been related to the clay and water content, the clay mineralogy, and the cation exchange capacity of the soil (4).

Pore size, specified as the pore width (dpore), is classified by IUPAC into three types: macropores (dpore >50 nm), mesopores (2 nm <dpore <50 nm) and micropores (dpore <2 nm).

In the soil, micropores are important in the physisorption processes and the physical sequestration of chemical pollutants from the environment (5). Micropores are also important in the soil physicochemical processes, such as the K-Ca cation exchange selectivity of volcanic soils (6), where the size and shape of micropores are involved (7).

Mesopores act as a transport system, and the soil mesoporosity is of special interest because it regulates accessibility to the micropores.

The proportion of micro- and macropores affects the soil's water holding capacity, and the higher the microporosity the higher the soil resistance to desiccation.

Soil pores provide the living space for the soil biota. Pores with a diameter smaller than 200 nm are inaccessible to soil microorganisms, from 800 nm to 6000 nm they are accessible to bacteria and protozoa, and pores larger than 30000 nm of diameter are needed by nematodes (8).

In Chilean volcanic soils only total porosity (defined as the ratio of the volume of voids and pores to the volume occupied by solid) has been studied. Reported total porosity ranges between 65 and 80%, with pores classified as large (dpore>10000 nm, 26-30%), medium (200 nm<dpore<10000 nm, 14-20%), and small pores (dpore<2000 nm, 25-30%) (9,10)

The aim of this work is to determine in two Chilean volcanic soils the total intragranular porosity and SSA, and to study if there is a relation between pore size distribution and particle size, organic matter content, mineralogy of the clay fraction, or agricultural management of the soils.

EXPERIMENTAL

Soil samples from 0-15 cm and 15-30 cm depth of Collipulli (Ultisol; Fine, Mesic, Xeric, Paleumult) and Diguillin (Andisol; Medial, Mesic, Entic Dystrandept) collected from cultivated and uncultivated areas were used (Table 1). All samples were air-dried and sieved to 1mm. From each sample the <2 µm size fraction was also obtained by a sedimentation procedure based on Stokes law. Soil samples were characterized for organic carbon content (OC) by the Walkley-Black method, cation exchange capacity, and pH (11).

Mercury porosimetry measurements were carried out on a Micromeritics model Autopore III Win 9420 porosimeter. Soil samples of 0.300 g were used, with 30-second equilibration time during Hg introduction, pressure between 3.6x10^-3 and 413 MPa, and a 130 contact angle on soils. The relation between applied pressure and pore diameter, established by the Washburn equation (12), was used to calculate pore diameter. Macropores volume was calculated from plots of incremental intrusion of mercury (mL g^-1) vs pore diameter (nm).

SSA, micropores volume, and mesopores volume and diameter were determined from the N₂ adsorption-desorption isotherms, carried out in a Micromeritics model ASAP 2010 and in a Carlo Erba Sorptmatic 900 by the static volumetric method. Soil samples of 0.500 g were degassed at 483 K for 24 h, with a residual vacuum of 0.532 Pa. SSA was calculated from the nitrogen adsorption isotherm at 77 K by the Brunauer, Emmett, and Teller (BET) method (12); micropore volume was calculated from Nitrogen adsorption at 77 K by the t-plot method; mesopore volume and diameter were calculated from the nitrogen adsorption or desorption isotherm by the Barrett, Joyner and Halenda (BJH) method (12). All samples were analyzed in duplicate.

RESULTS AND DISCUSSION

Characterization

According to the characterization results (Table 1) the organic carbon content of Collipulli samples were lower than Diguillín samples. Andisols are rich in organic matter, and its accumulation is probably associated to its mineralogy dominated by low crystalline compounds; thus, in an observed property of soil the organic matter or the mineralogy or both may play a fundamental role. All samples were acidic; the <2 µm samples showed higher cation exchange capacity (CEC) at soil pH than the <1 mm soil samples. Both characteristics (CEC and OC) are probably mainly associated to mineralogical differences between the samples.

Collipulli soil is an Ultisol with kaolinite as the dominant clay mineral (crystalline aluminosilicate mainly with permanent charge), its OC range was 1.5-2.4 % (all size fractions), and the CEC range was 8.7-10.5 cmol₍₊₎ kg^-1 (<1 mm fraction) and 10.5-14.8 cmol₍₊₎ kg^-1 (<2 µm fraction). Diguillín soil is younger than Collipulli, with a clay fraction mineralogy dominated by allophane (aluminosilicate of low crystallinity and variable surface charge), its OC range was 4.3-10.7 % (all size fractions), and the CEC range was 11.0-12.0 cmol₍₊₎ kg^-1 (<1 mm fraction) and 29.8-37.3 cmol₍₊₎ kg^-1 (<2 µm fraction). As result of cultivation practices, the soil acidity increases slightly, the OC is reduced, and in most of the samples about the same or a slightly increased CEC is seen.

Through mercury porosimetry the volume of meso and macropores was obtained. Figures 1 and 2 show the pore diameter distribution and pore type in Collipulli and Diguillín <1 mm and <2 µm, and the existence of different pore types for each soil can be seen. In the <1 mm fraction of Collipulli soil, pore volume is given by mesopores, most of them with a pore diameter about 10 nm. For the <1 mm fraction of Diguillín soil macropores are the most important, with pore diameter in the range of about 70 nm to 7000 nm.

Fig. 1. Pore diameter distribution for <1 mm soil determined by incremental mercury intrusion.

Figure 2 shows that in the clay fraction of Collipulli samples macropores (dpore from 2000 nm to 40000 nm) and mesopores (dpore from 2 nm to 30 nm) are present.

Fig. 2. Pore diameter distribution for <2 µm soil samples determined by incremental mercury intrusion.

For Diguillín clay samples most of the porosity comes from macropores (dpore from 50 nm to 800 nm) which are more important than mesopores (dpore 4 nm). In relation to macro and mesopores determined through mercury porosimetry, clear differences due to soil mineralogy and organic matter content in both particle sizes are well established. Similar results to those presented in Figures 1 and 2 were obtained with cultivated and 15-30 cm depth samples; thus, mineralogy and OC apparently play the most important role in defining soil porosity associated with macro and mesopores.

Mesoporevolume can be calculated by mercury porosimetry and N₂ adsorption. In all samples the mesopore volume obtained by mercury porosimetry (0.1217-0.1951 cm³ g^-1 for <1 mm samples, and 0.1157-0.4011 cm³ g^-1 for <2 µm samples) was higher than that obtained by N₂ adsorption (0.0060-0.1281 cm³ g^-1 for <1 mm samples, and 0.0567-0.2227 cm³ g^-1 for <2 µm samples, Tables 2 and 3).

This result may be due to a deformation of the organic matter structure under the high pressures during mercury penetration (13), and the mesopore volume is probably overestimated by this method, and a more accurate estimation can be obtained from N₂ adsorption isotherms.

Adsorption of N₂

From N₂ adsorption-desorption isotherms (Figure 3), meso and micropores can be calculated. A different behavior for <1 mm samples from Collipulli and Diguillín soils was observed.

Fig. 3. Nitrogen adsorption-desorption isotherms for <1 mm soil samples.

For Collipulli (<1mm), the isotherms fall within type IV IUPAC classification group, characteristic of capillary condensation in mesopores and with a type H2 hysteresis loop, with a complex and interconnected pore structure with pores of different size and shape, with which is in agreement with the mesopores found by mercury porosimetry and shown in Figure 1. Diguillín (<1 mm) showed a type II isotherm with a type H3 hysteresis loop, characteristic of aggregated particles with nonporous or macroporous adsorbents and unrestricted monolayer-multilayer adsorption.

The clay fraction of Collipulli and Diguillín showed a type II adsorption-desorption isotherm, with a type H3 hysteresis loop, similar to that seen for Diguillín <1 mm-size samples (figures not shown). The differences seen between <1 mm and <2 µm particle size for Collipulli soil samples agree with the differences in pore diameter established by mercury porosimetry for the same samples (Figures 1 and 2), where mainly mesopores in the <1 mm samples and macropores in the <2 µm particle size samples were seen.

The intensive soil washing during the clay extraction procedure may result in an expansion of kaolinite,(14) enough to allow the adsorption of N₂ or the mercury intrusion in the resulting <2 µm particle size sample. Thus, the results seen with particle size in both soils are the consequence of differences in the mineralogy of soils. Neither cultivation practices nor sample depth seem to influence the previously described pore size distribution.

Specific surface area

All samples from Collipulli soil present an SSA higher than the corresponding Diguillín samples (2.1 to 3.8 times for <1 mm samples; 1.3 to 2.3 times for <2 µm samples, Tables 2 and 3).

The SSA of all samples correlates linearly with the MEP volume determined by N₂ adsorption (Figure 4). With the exception of Collipulli <1mm samples, the MAP are also important in defining the SSA; this assertion can be demonstrated when MEP+MAP is plotted vs. SSA.

Fig. 4. Relationship between specific surface area and mesopore volume for <1 mm and <2 µm soil samples (p<0.01).

In this case the linear relationship SSA=-8.2+96.3x(MEP+MAP), r²=0.771, 12 samples, is obtained. MIP do not correlate with the SSA, and the MEP and MAP gives mainly the SSA of volcanic soil samples.

The SSA also had an inverse relationship with the organic carbon content (OC) of the samples, as shown in Figure 5, (SSA=82.23-10.64xOC, r²=0.854, 14 samples). Previous results had shown that organic matter might act as glue, hiding a part of the available SSA by binding together some of the clay particles (4, 15). In volcanic soils the OC content may play a similar role, and the disaggregation of particles during the clay fraction extraction procedure results in increasing values of SSA, especially from samples with high organic matter content, such as those from Diguillín. Thus the SSA, defined mainly by the mesoporosity, is controlled by the OC content of the samples, which is more important than particle size, mineralogy, and soil management.

Fig. 5. Relation between specific surface area and organic carbon content for <1 mm and <2 µm soil samples (p<0.01).

Total intragranular porosity

In all particle sizes micropore volume is the less important in defining the TIP of samples (Tables 2 and 3); thus, mainly mesopores and macropores give the TIP. Mesopore volume is more important in samples dominated by kaolinite than in samples dominated by allophane (4 to 20 times). Particle size is very important in observed TIP values; in both soil samples the TIP increases significantly when particle size decreases from <1 mm to <2 µm. Soil horizon and cultivation practices are less important or significant than particle size. While macropores are the most important in defining the TIP for all samples from Diguillín soil, mesopores are the most important for <1 mm Collipulli samples, but mesopores and macropores are important for all the <2 µm size samples. The differences may be associated to mineralogical differences and to organic matter content of the samples in a similar way to that discussed previously.

CONCLUSIONS

With respect to types of pores determined by mercury porosimetry, soil mineralogy and organic matter content in both particle sizes play the most important role in defining soil porosity associated to macro and mesopores.

Samples <1 mm from Collipulli and Diguillín present type IV and type II adsorption isotherms, respectively. For the clay fraction both soils exhibit a type II isotherm. This behavior was related to soil mineralogy and particle aggregation as a result of organic matter content.

In general, the SSA of volcanic soil samples, defined mainly by meso and macroporosity, is related with the organic matter content of the samples, which is more important than particle size, mineralogy and soil management.

Particle size is very important in the observed TIP values, which are mainly given by mesopores and macropores.

Pore size distribution, SSA, and TIP are related in an important way to factors such as soil mineralogy and organic matter content.

ACKNOWLEDGEMENTS

This study was supported by DICYT-USACH, FONDECYT 2003 N1030778. M Antilen Acknowledges the Financial Supports by CONICYT and Servicio Cultural de la Embajada de Francia en Chile.

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(Received: May 7, 2004 - Accepted: September 14, 2004)