Diffusion and surface reaction processes of absorption on surfaces of soil particles in unsaturated soils Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/5m60qv25p

Descriptions

Attribute NameValues
Creator
Abstract or Summary
  • At present the adsorption kinetic of ions by soil particles is considered to be a surface reaction. When the transport of solutes in soils is described using the one-dimensional flow equation, it becomes a serious problem that lateral diffusion of ions from the bulk solution towards the soil surfaces is not considered. The objectives of this study were to re-examine the adsorption process on theoretical and experimental bases in an attempt to develop more appropriate descriptions. We considered the adsorption process to be a three-step mechanism, namely, diffusion of ions from the bulk solution to the "subsurface" layer around soil particles, diffusion across the "sub-surface" layer, and a surface reaction. Diffusion across the subsurface layer is assumed to be at a constant rate and may be the rate limiting process under certain conditions. Ions from the subsurface layer are adsorbed onto the surfaces of soil particles. This process is described by a surface kinetic reaction which is regarded another rate limiting process. A new procedure for measurement of adsorption isotherms was proposed and tested. The new procedure employs a soil sample packed in a short column so that the structure of the soil can be maintained. In our studies solution was added to bring the soil water potential to -0.05 bars. Other water contents could be used. After equilibration, solution was extracted using a pressure membrane apparatus. The resulting isotherms conformed to the Freundlich-type isotherm. For both iodide and cadmium ions, the soil column and the currently used shaking batch procedures produced isotherms that differed significantly from each other, with the magnitude of adsorption being much greater in the soil column. It was hypothesized that the arrangement of soil particles influences the attractive force fields and restricts the transitional freedom of ions to a greater extent than occurs in the shaking batch. It was concluded that the soil column procedure more realistically represents conditions as they exist in soil and therefore is preferable over the shaking batch method. The activation energies for adsorption and desorption were assumed to be logarithmic function of the amounts adsorbed. When the lateral diffusion process is also considered, the equation describing the adsorption kinetics for one-dimensional solute transfer in soil is as/at = [1/1/Kd+1/(k₁S⁻b)] (θ/pb)C - [1/(k₁S⁻²b/k₂)(1/Kd)+1/(k₂Sb)]S, where Kd (sec⁻¹) is the diffusive conductance of the subsurface layer, k₁ and k₂ (sec⁻¹) are surface reaction constants, b is a constant associated with the change of energy of adsorption as a function of surface coverage, (θ/pb)C (μg/g soil) is the amount of ions in the liquid phase and S is the nondimensional representation of S. The kinetic adsorption equation was evaluated for cadmium for four pore water velocities, v, ranging from 0 to 2. 7 cm/hr. The soil water potential was maintained at about -0.126 bars. The calculated values of Kd showed an increase with increasing v impling that the thickness of the subsurface layer decreased with increasing velocity. The magnitudes of Kd of about 0.07 to 0. 4 x 10⁻³ sec⁻¹ are very important. They are sufficiently low to indicate a rate limiting process especially when the system is far from equilibrium. The studies thus confirm our argument that lateral diffusion cannot be ignored in studies of adsorption kinetics. The ratio of the rate constants, k₁/k₂ also increased with v which led us to conclude that the rate of adsorption proceeds faster when there is solute flow in the soil system. This is attributed to the higher rates of lateral diffusion and rapid removal of ions originally present at the adsorption sites.
Resource Type
Date Available
Date Copyright
Date Issued
Degree Level
Degree Name
Degree Field
Degree Grantor
Commencement Year
Advisor
Committee Member
Academic Affiliation
Non-Academic Affiliation
Subject
Rights Statement
Peer Reviewed
Language
Digitization Specifications
  • File scanned at 300 ppi (Monochrome, 8-bit Grayscale) using ScandAll PRO 1.8.1 on a Fi-6670 in PDF format. CVista PdfCompressor 4.0 was used for pdf compression and textual OCR.
Replaces
Additional Information
  • description.provenance : Made available in DSpace on 2013-09-12T17:44:47Z (GMT). No. of bitstreams: 1 YingjajavalSuntaree1979.pdf: 1238755 bytes, checksum: 6bb9b5a3392492e7a753d185f1854337 (MD5) Previous issue date: 1979-01-25
  • description.provenance : Approved for entry into archive by Patricia Black(patricia.black@oregonstate.edu) on 2013-09-11T17:23:00Z (GMT) No. of bitstreams: 1 YingjajavalSuntaree1979.pdf: 1238755 bytes, checksum: 6bb9b5a3392492e7a753d185f1854337 (MD5)
  • description.provenance : Approved for entry into archive by Patricia Black(patricia.black@oregonstate.edu) on 2013-09-12T17:44:47Z (GMT) No. of bitstreams: 1 YingjajavalSuntaree1979.pdf: 1238755 bytes, checksum: 6bb9b5a3392492e7a753d185f1854337 (MD5)
  • description.provenance : Submitted by Katy Davis (kdscannerosu@gmail.com) on 2013-09-11T15:10:10Z No. of bitstreams: 1 YingjajavalSuntaree1979.pdf: 1238755 bytes, checksum: 6bb9b5a3392492e7a753d185f1854337 (MD5)

Relationships

Parents:

This work has no parents.

Last modified

Downloadable Content

Download PDF

Items