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Sorption of Cd to colloidal ferric hydroxides: impact of pH and organic acids
Örebro University, Department of Natural Sciences.
1997 (English)In: Chemical speciation and bioavailability, ISSN 0954-2299, E-ISSN 2047-6523, Vol. 9, no 1, p. 3-14Article in journal (Refereed) Published
Abstract [en]

The sorption (both ad- and desorption) of Cd to or from colloidal ferric hydroxides as a function of pH, the concentration of organic acids (acetic acid (AA), EDTA, fulvic acid (FA) and malonic acid (MA)), the precipitation procedure for the ferric hydroxide as well as the contact time were investigated in the present study. The concentrations of Cd, colloids and organic acids were chosen to be representative of natural surface waters. The ferric hydroxide were either precipitated in presence of FA or the acid was mixed with the colloids after the purification steps. The presence of FA during the precipitation of ferric hydroxide, had an impact on the size distribution of the solid phase, but it did not have any influence on the sorption behaviour of Cd. The adsorption process was found to be relatively fast and largely pH-dependent. It was fully reversible when the pH was decreased, however, with somewhat slower kinetics. The addition of up to 1 mmol L-1 AA or MA did not cause any measurable desorption of Cd, probably due to the relatively weak complexes formed between Cd(II)-ions and the anions of the acids. Thermodynamic calculations of the distribution between different soluble Cd-species indicated that EDTA and FA would form stronger complexes with Cd. This was indicated in the experiments as enhanced adsorption at low pH-values due to the adsorption of Cd(II)-organic species, and an increase in the soluble fraction at high pH-values caused by competition for Cd(II)-ions between complex forming groups on the surfaces and the soluble organic ligands.

Place, publisher, year, edition, pages
1997. Vol. 9, no 1, p. 3-14
National Category
Natural Sciences Chemical Sciences
Research subject
Chemistry
Identifiers
URN: urn:nbn:se:oru:diva-3131OAI: oai:DiVA.org:oru-3131DiVA, id: diva2:137081
Available from: 2004-04-28 Created: 2004-04-28 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Metal distribution and mobility under alkaline conditions
Open this publication in new window or tab >>Metal distribution and mobility under alkaline conditions
2004 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The adsorption of an element, expressed as its distribution between liquid (aquatic) and solid phases in the biogeosphere, largely determines its mobility and transport properties. This is of fundamental importance in the assessment of the performance of e.g. geologic repositories for hazardous elements like radionuclides. Geologic repositories for low and intermediate level nuclear waste will most likely be based on concrete constructions in a suitable bedrock, leading to a local chemical environment with pH well above 12. At this pH metal adsorption is very high, and thus the mobility is hindered. Organic complexing agents, such as natural humic matter from the ground and in the groundwater, as well as components in the waste (cleaning agents, degradation products from ion exchange resins and cellulose, cement additives etc.) would affect the sorption properties of the various elements in the waste. Trace element migration from a cementitious repository through the pH- and salinity gradient created around the repository would be affected by the presence and creation of particulate matter (colloids) that may serve as carriers that enhance the mobility.

The objective of this thesis was to describe and quantify the sorption of some selected elements representative of spent nuclear fuel (Eu, Am) and other heavy metals (Zn, Cd, Hg) in a clay/cement environment (pH 10-13) and in the pH-gradient outside this environment. The potential of organic complexing agents and colloids to enhance metal migration was also investigated. It was shown that many organic ligands are able to reduce trace metal sorption under these conditions. It was not possible to calculate the effect of well-defined organic ligands on the metal sorption in a cement environment by using stability constants from the literature. A simple method for comparing the effect of different complexing agents on metal sorption is, however, suggested.

The stability in terms of the particle size of suspended colloidal matter (clays and an iron hydroxide) was measured by photon correlation spectroscopy. In the absence of large amounts of organic material, the present colloids will, in most cases, not enhance metal migration in a cementitious environment due to agglomeration of the colloidal phase at high pH and salinities.

Place, publisher, year, edition, pages
Örebro: Örebro universitetsbibliotek, 2004. p. 60
Series
Örebro Studies in Chemistry, ISSN 1651-4270 ; 4
Keywords
Kemi, Adsorption, Colloids, Nuclear waste, Cement, Montmorillonite, miljökemi
National Category
Chemical Sciences
Research subject
Chemistry
Identifiers
urn:nbn:se:oru:diva-66 (URN)91-7668-388-5 (ISBN)
Public defence
2004-05-19, Filmsalen, Forumhuset, Örebro universitet, Örebro, 10:00
Opponent
Supervisors
Available from: 2004-04-28 Created: 2004-04-28 Last updated: 2017-10-18Bibliographically approved

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