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| Titel |
Multifractal characteristics of Nitrogen adsorption isotherms from tropical soils |
| VerfasserIn |
Eva Vidal Vazquez, Jorge Paz Ferreiro |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250039160
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| Zusammenfassung |
| One of the primary methods used to characterize a wide range of porous materials, including
soils, are gas adsorption isotherms. An adsorption isotherm is a function relating the amount
of adsorbed gas or vapour to the respective equilibrium pressure, during pressure increase at
constant temperature. Adsorption data allow easily estimates of specific surface area and also
can provide a characterization of pore surface heterogeneity. Most of the properties and
the reactivity of soil colloids are influenced by their specific surface area and by
parameters describing the surface heterogeneity. For a restricted scale range, linearity
between applied pressure and volume of adsorbate holds, which is the basis for current
estimations of specific surface area. However, adsorption isotherms contain also
non-linear segments of pressure versus volume so that evidence of multifractal
scale has been demonstrated. The aim of this study was to analyze the multifractal
behaviour of nitrogen adsorption isotherms from a set of tropical soils. Samples
were collected form 54 horizons belonging to 19 soil profiles in the state of Minas
Gerais, Brazil. The most frequent soil type was Oxisol, according to the Soil Survey
Staff, equivalent to Latossolo in the Brazilian soil classification system. Nitrogen
adsorption isotherms at standard 77 K were measured using a Thermo Finnigan
Sorptomatic 1990 gas sorption analyzer (Thermo Scientific, Waltham, MA). From the raw
data a distributions of mass along a support was obtained to perform multifractal
analysis. The probability distribution was constructed by dividing the values of the
measure in a given segment by the sum of the measure in the whole scale range. The
box-counting method was employed to perform multifractal analysis. All the analyzed N2
adsorption isotherms behave like a multifractal system. The singularity spectra,
f(α), showed asymmetric concave down parabolic shapes, with a greater tendency
toward the left side, where moments q >0. The width of the f(α) spectra ranged
from 1.167 to 2.741 for individual isotherms. Therefore, shape and width of the
singularity spectra suggest a high heterogeneity in the local scaling indices of the
measure. The mass exponent function, Ï(q), and the generalized dimension, Dq, also
corroborate this pattern. The capacity dimension, D0, was not significantly different
from 1.000, but the entropy dimension, D1, showed a wide range of values, from
0.317 to 0.749, as did the correlation dimension, D2, which oscillates between
0.157 and 0.675. In accordance with this parameter (D0- D2) ranged from 0.325 to
0.843. The value of D1 is also a good index of the degree of heterogeneity of a
measure. The closer the D1 value to the capacity dimension, the more homogeneous is
the distribution of the measure, whereas a D1 value close to zero is associated to
clustering , so that most of the measure concentrates in a small size domain of the study
scale. Because of the wide range of values obtained for D1, D2 and (D0 – D2),
these multifractal parameters provide a good characterization of N2 adsorption
isotherms and they appear to be appropriate to discriminate different soil types and soil
horizons.
Acknowledgement: This work was supported by Spanish Ministry of Education
(Project PHB2009-0094-PC) and Spanish Ministry of Science and Innovation (Project
CGL2009-13700-C02). |
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