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COMPLEXATION OF RADIONUCLIDES WITH NATURALLY OCCURING ORGANIC COMPOUNDS IN GROUNDWATER

Objective

THE BROAD OBJECTIVE OF THE PROGRAMME IS TO STUDY THE ABILITY OF NATURAL ORGANIC COMPOUNDS PRESENT IN GROUNDWATER TO FORM MOBILE COMPLEXES WITH RADIONUCLIDES. THE WORK INVOLVES DEVELOPMENT OF TECHNIQUES TO EXTRACT NATURAL ORGANIC MATERIAL FROM GROUNDWATER WITH MINIMAL ALTERATION, THE SEPARATION OF THOSE ORGANIC FRACTIONS THAT COMPLEX WITH CATIONS, AND THE DEVELOPMENT OF GEOCHEMICAL SPECIATION MODELS TO DESCRIBE OR PREDICT METAL BINDING. THE MODELLING APPROACH WILL BE TESTED BY APPLYING IT TO THE STUDY OF THE MECHANISMS WHICH ENHANCE THE MIGRATION OF NICKEL FROM A DISPOSAL LAGOON INTO WHICH A WIDE RANGE OF ORGANICS AND HEAVY METALS HAVE BEEN DISCHARGED. LIAISON WITH LOUGHBOROUGH UNIVERSITY OF TECHNOLOGY HAS BEEN ESTABLISHED TO DEVELOP APPROPRIATE TECHNIQUES TO DETERMINE RADIONUCLIDE SPECIATION DIRECTLY AND TO SEPARATE ORGANIC FRACTIONS.
The complexation of radionuclides with humic materials has been studied using differential pulse polarography (DPP), membrane dialysis and ion exchange resins and the ability of these techniques to determine conditional association values for binding have been assessed.

Differential pulse polarography has been used to study the binding of nickel, cobalt and europium with sodium humate, humic acid and sodium fulvate. The half wave potentials of free metal and complexed metal for nickel and cobalt were found to be sufficiently separated to enable quantitative measurement of the binding of the metal with the organic materials. However, the half wave potentials for europium were not sufficiently separated to provide measurement of europium complexation. Conditional association values (log beta values) of 4.37 and 4.10 cubic decimetres per mol were determined for the interactions of nickel and cobalt with sodium humate at pH 6.5 in 0.005 M potassium sulphate. Conditional association values for humic acid were 4.30 cubic decimetres per mol for nickel and 4.03 cubic decimetres per mol for cobalt.

Dialysis using a nominal 1000 molecular weight cut off membrane was used to investigate the reactions of nickel, cobalt, europium and tin with sodium humate. The technique was found to be unsuitable for the quantitation of metal-humic interactions because of the continual migration of the humic material across the membrane. However, log beta values of 2.18 and 3.67 cubic decimetres per mol were obtained for nickel and cobalt interactions with sodium humate at pH 6.5 in 0.05 M sodium chloride solution.

Nickel, cobalt, europium and cerium complexation with sodium humate were investigated by the Schubert method using cation exchange resins. Log beta values of 5.17 cubic decimetres per mol for nickel, 5.03 for cobalt, 6.82 for europium and 7.29 for cerium were measured for sodium humate at pH 6.5 in 0.01 M sodium chloride solution and the method is valid provided that the slopes of the graphs produced are very close to one.

The use of conventional preparative scale chromatography (axial flow) and radial flow chromatography have been theoretically appraised for use in the extraction of humic substances from groundwater by di-ethyl-amino-ethyl (DEAE) cellulose. This appraisal has shown that the use of radial flow column technology allows a significant increase in sample throughput, ultimately leading to a larger sample extract. This form of extraction has the advantage over batch methods in the anoxic conditions can be maintained, that chromatography can be performed during the elution stage and that repetitive elution cycles may also be performed. The small size and weight of the radial flow column (approximately 2 kg) also makes it ideal for use in the field where it would be expected to outperform axial flow columns of commercially available dimensions.

Mechanisms of retention and elution of humic and fulvic acids from DEAE cellulose have been investigated in theory and by experiment. Theoretical and practical evidence from thin layer chromatography (TLC) and batch extraction studies show that humic and fulvic acids may be eluted either by neutralization of the cationic character of the resin or by neutralization of the anionic character of the bound material, provided that secondary interactions are reduced. Although the use of acidic eluents reduces the risk of oxidation it also releases bound iron, which has to be removed, necessitating extra purification steps, which may well lead to further fractionation and alteration. On balance it can be concluded that the use of moderately alkaline reagents such as 0.5 M sodium hydroxide produce the best yield of extracted material while minimizing further treatment and oxidation.

TLC also indicates that subtle differences exist between Aldrich sodium humate and the other test compounds and suggests that the purification process used to produce protonated Aldrich humic acids may remove components that are retained strongly by the anion exchange resin or alter its structure.

Natural dissolved organic matter present in groundwater can complex and increase the mobility of radionuclides in the aqueous phase. In order to study the possible complexation of nickel which as nickel-61 is a significant radionuclide in the low level radioactive waste inventory, a study has been made of the Villa Farm waste disposal site, near Coventry, where nickel together with a wide range of industrial organic compounds have been made of the Villa Farm waste disposal site, near Coventry, where nickel together with a wide range of industrial organic compounds have been deposited in a lagoon in hydraulic continuity with a shallow sand aquifer. The resulting pollution plume contains elevated nickel in association with dissolved organic carbon, and 4 groundwater samples were obtained under an atmosphere of air and of nitrogen.

Size exclusion and reversed phase high performance liquid chromatography (HPLC) techniques have been used to separate ultraviolet absorbing species present in groundwater samples. The chromatographically separated species were analysed for nickel content by graphite furnace atomic absorption spectrometry. The ultraviolet absorption spectra of the fractions associated with nickel were similar to those of natural organic compounds (humic and fulvic acids) which have a wide range of ultraviolet absorbing moieties and which absorb over a wide ultraviolet wavelength.

Ultrafiltration of Villa Farm groundwaters through membranes of various sizes also showed that the nickel was associated with relatively large molecular weight material. No nickel was associated with species having a relative molecular mass of more than 10000 Daltons, but 17% of the nickel was associated with fractions of molecular weight above 5000, 40% of the nickel was associated with fractions above 1000 Daltons, and 75% of the nickel was associated with fractions above 500 Daltons. Treatment of groundwater samples under an air or nitrogen atmosphere had negligible effect on the results of ultrafiltration. Added nickel equilibrated rapidly with the groundwater to provide similar size associations after ultrafiltration.
THE PROJECT IS DIVIDED INTO THE FOLLOWING RESEARCH AREAS:
2.1 ASSESSMENT AND DEVELOPMENT OF TECHNIQUES TO EXTRACT AND SEPARATE NATURAL ORGANIC MATERIAL FROM GROUNDWATER
2.2 THE SEPARATION, CHARACTERISATION AND QUANTIFICATION OF THOSE ORGANIC FRACTIONS WHICH COMPLEX WITH RADIONUCLIDES
2.3 DEVELOPMENT OF A MODEL TO DESCRIBE METAL BINDING CAPACITY BASED ON THE CHARACTERISATION OF NATURAL ORGANIC COMPOUNDS
2.4 INVESTIGATION OF THE MOBILITY OF NICKEL WHICH HAS APPARENTLY MIGRATED OVER 100 M IN GROUNDWATER ORGANICALLY CONTAMINATED WITH INDUSTRIAL WASTES, IN AN ATTEMPT TO APPLY AND VALIDATE THE MODELLING APPROACH.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Natural Environment Research Council
Address

NG12 5GG Nottingham
United Kingdom