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  • SZENT ISTVN EGYETEM

    THESIS OF (PhD) DISSERTATION

    Modelling of gaseous diffusion in undisturbed soil samples in function

    of soil structure

    Gyarmati Bernadett

    Gdll

    2013

  • 2

    Szent Istvn University - Doctoral School

    Name: Doctoral School of Environmental Sciences

    Discipline: Environmental Sciences

    Head: Dr. Gyrgy Heltai DSc.

    Szent Istvn University,

    Faculty of Agricultural and Environmental Sciences

    Institute of Environmental Sciences

    Department of Chemistry and Biochemistry

    Supervisors: Dr. gnes Ilona Mszrosn Blint PhD

    Szent Istvn University,

    Faculty of Agricultural and Environmental Sciences

    Institute of Environmental Sciences

    Department of Chemistry and Biochemistry

    Dr. Csaba Mszros PhD

    Szent Istvn University

    Faculty of Mechanical Engineering

    Department of Physics and Process Control

    Head of Schools approval

    Supervisors approvals

  • 3

  • 4

    BACKGROUND AND OBJECTIVES

    Transport processes in the Earths crust are very diverse. Crude oil exploration, processes of

    applying artificial fertilizers or organic manures are closely related to this topic. Without knowing

    the condition of the fertile upper layer of Earths crust it is difficult to answer successfully questions

    concerning complex energetic, agricultural and/or environmental challenges. Experimental and

    theoretic examination and computer simulation of transport processes in porous medium are crucial

    parts of revealing mass and energy flows occurring at the soil-atmosphere interface and under the

    surface. It is a completely open research area with several questions unanswered and with the

    possibility of developing and applying new modelling methods.

    In my present work I selected an environmentally important topic from the transport problems: I

    examined the gaseous diffusion in undisturbed soil samples in function of soil structure properties.

    First step as well as the first goal was the generation of adequate database for the further

    examination. In order to collect data I used my own designed experimental setup, which was

    compatible with the experimental setup built for theoretical (Mszros et al. 2011, Kirschner

    et al. 2007) and experimental (Nagy et al. 2011) examination of convection. The system was

    also compatible with the multistep outflow system (MSO) designed to measure the soil

    hydraulic conductivity and retention (Weller et al. 2011).

    In the first part of my work, I collected eight soil monoliths with different texture, 350 mm

    in height and 180 mm in diameter. After determination of the general soil properties,

    sterilization and sample preparation, they were connected to the (MSO compatible) gas

    diffusion experimental system with continuous gas supply. Thus the emission and changes

    of CO2, N2O, CH4 gas concentration were followed during ten day long experiments. Due to

    the experimental results the relative gas diffusion coefficients were determined for each gas.

    Afterwards samples were scanned in high resolution by computed tomography (CT).

    After the experimental part my aims were the followings:

    In this work segmentation of the CT images is the critical step, which is also true in medical

    data processing. There is a strict, standardized protocol for image segmentation in medical

  • 5

    practice which does not exist in case of geological samples. Thus I chose examination of

    three segmentation methods recommended for soil samples in the relevant literature

    (wherein the applied threshold by binarization were Th1, Th2, and Th3).

    By the help of advanced image processing software and based on the segmented CT data

    matrix, my further objects were the soil macropore space visualization and comprehensive

    pore space characterization. Characterisation of the soil structure covered the following

    properties: macroporosity, effective porosity, average macropore size and diameter, total

    surface area, macroporous network density, tortuosity, node density, and pore space

    skeleton. Most of the experiments designed for examining gaseous diffusion use undisturbed

    samples with volume 200 cm3 or less, which are not suitable to represent the heterogeneity

    of nature soils, thus it gives significance of this research task. Therefore, database

    originating from three-dimensional reconstruction of soil monoliths are particular valuable.

    Another goal was to analyse the relationship between the soil characteristics properties

    based on three-dimensional computer tomography reconstruction and the relative diffusion

    coefficient derived from the gas diffusion experiment. However, I would like to emphasize

    that this type of experimental combination is very rare, so this task includes only the

    elaboration and presentation of a new analyses possibility. Therefore, this objective excludes

    the presentation and analysis of soil transport properties in function of soil structure

    characteristic based on genetic soil classification (which is far beyond available budget of a

    doctoral research).

    The implementation of the study trying to open new approaches since computer

    tomographic studies mainly focus on fluid phase transport (hydraulic conductivity).

    Estimation on relative soil gas diffusion coefficient becomes more and more important

    because of greenhouse gas emissions originating from the soil and contributing to global

    climate change. Therefore it is very important to outline those soil properties (based on CT

    database) which have significant relationship with soil gaseous diffusivity. In addition, I

    would like to test the well-known, one and two parameter models predicting relative gaseous

    diffusion coefficient with CT input data from the three different segmentation techniques.

  • 6

    MATERIAL AND METHODS

    Soil sampling:

    I collected eight intact soil monoliths with different texture, 350 mm in height and 180 mm in

    diameter by the help of a special soil sample holder (DIN EN ISO 15493). The reason behind the

    non-standardised sampling method was to maintain the compatibility with other experimental

    system (MSO, multi-step-outflow (Weller et al., 2011); soil convection (Nagy et al., 2011). By

    sampling primary objective was to preserve the original structure.

    In addition, small samples (350 mm in height and 100 mm in diameter) were also collected just next

    to the soil monoliths in order to perform collateral examinations (particle size distribution, bulk

    density, organic content etc.).

    Table 1. contains the data on sampling (sampling location, depth) with the results of the collateral

    examination, wherein labels (capital letters) DU-T, EL-T, ID-T, KI-T, KO-T, LA-T, LI-T. MO-T

    belongs to the undisturbed samples, while small samples were marked with small letter

    analogously.

    Sample preparation:

    -Soil sterilization:

    Gamma radiation was used (30 kGy) (Izotp Intzet Kft.) to eliminate post sampling structural

    modification caused by soil flora and fauna and inhibits soil microbiological gas production.

    -Adjusting the soil moisture content:

    At first, full saturation was reached by wetting the samples gently based on the hydrostatic law of

    communicating vessels. Subsequently they were drained to -200 cm water potential (-19,6 kPa

    pressure potential) (hanging water column) by the help of sterilized column 2500 mm in height and

    400 mm in diameter, thus assumption of being approximately at field capacity could be drawn.

    Collateral examinations:

    -Bulk density, soil organic matter and particle size distribution:

    Bulk density was determined according to Blake and Hartke (1986). SEDIMAT 4-12 (Umwelt-

    Gerte-Technik GmbH, Mncheberg, Germany) laboratory apparatus was used for determination of

    particle size distribution in soil material with 4 fractions by DIN ISO 11277. Wet sifting was done

    to complete the partitioning beyond 0.063 mm. Hydrogen peroxide digestion method was used for

    organic matter determination described in detail by Schumacher (2002).

  • 7

    Table 1.: Sampling data and results of the collateral examinations

    Description of the diffusion experiment:

    -Sample preparation for the diffusion experiment:

    After adjusting the soil water content, samples were fixed with silicone

    adhesive (Ceresit Sanitr Silikon) into a plastic plate to ensure gastight

    connection in the entire system. 3x5 sampling ports (cut glass vials, original

    headspace 20 ml, WIC 43220, WICOM, Germany), capped with WICOM Figure 1.: Sample

    preparation

    Sample Subsample Sampling Sampling Bulk Particle size Organic Particle sizedistribution (%)

    ID ID place depth (mm) density density* matter (%) Clay Fine silt Medium silt Coarse silt Fine sand Medium sand Coarse sand Error (%)

    DU-T N 47 26' 0-350

    E 19 28'

    Du-a 250-350 1,53 2,43 4,5 23,34 4,44 13,54 30,24 19,91 2,52 3,26 2,75

    Du-b 150-250 1,50 2,65 2,4 24,74 2,16 9,38 29,77 29,59 1,20 0,57 2,59

    Du-c 50-150 1,49 2,48 3,6 33,59 8,19 9,85 22,26 20,53 1,62 0,56 3,39

    Du-d 0-50 1,46 2,57 3,6 25,94 6,71 6,91 20,56 24,52 12,29 1,03 2,04

    EL-T N 47 31' 0-350

    E 19 22'

    EL-a 250-350 1,45 2,52 2,2 15,10 2,53 11,29 36,76 29,82 1,13 0,94 2,44

    El-b 150-250 1,46 2,51 3,3 19,21 1,87 11,52 38,87 22,50 1,08 1,59

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