André Maes

Heterogeneity of charge density distribution in montmorillonite as inferred from cobalt adsorption

A comparison is made of the ion exchange behavior, towards the cobalt ion, of five sodium montmorillonite clays. The selectivity for the bivalent ion at low cobalt loading is correlated with the average dimensions of the particles in the various clays, as characterized by several methods. The data are interpreted in terms of a higher selectivity of the bivalent ions for the broken bonds located at the edges of the clay crystals. Using a model comprising two areas of different charge densities, the experimental differences in behavior can be predicted reasonably well.

Ion exchange of synthetic zeolite X and Y with Co2+, Ni2+, Cu2+ and Zn2+ ions

The ion exchange of the bivalent transition metal ions Co, Ni, Cu and Zn on synthetic NaX or NaY zeolite is studied at 0.01 total normality at 5, 25 and 45°C. In spite of the near identical crystal radius of the 4 ions, there is a marked difference in the exchange isotherms. The maximum loading limits are temperature dependent and are not consistent with the static model of a definite exchange capacity in the big cavities, i.e., 82 % and 68 % in X and Y respectively. The selectivity in the small cages depends on the occupancy and type of ion present in the big cavities and vice versa. The overall selectivity of both X and Y zeolite for bivalent transition metal ions increases in the order Ni < Co < Zn < Cu. In addition to the ion hydration characteristics and ionic radius the exchange is governed by the coordination ability of the transition metal ion.

Zinc and cadmium mobility in sand: effects of pH, speciation, cation exchange capacity (CEC), humic acid and metal ions

Abstract This study was undertaken to investigate the ways in which pH, speciation, cation exchange capacity (CEC), humnic acid and added metals influence Zn and Cd mobilities in porous media. The studies were conducted using columns of pure sand and sand in admixture with goethite. The mobile phases were based on aqueous NaCl solution. In all cases Zn and Cd were found to be more mobile at pH = 4.0 than pH = 6.5. Speciation calculations were used to rationalise all the observations. Introduction of a small amount of goethite in the column packing caused increased retardations. However the observed increases could not be explained simply in terms of an increased column CEC value, sorption dynamics were also involved. Increased retardations were also noted in the presence of HA. The effect reflected the amounts present in the stationary and mobile phases. Ca and Fe enhanced Zn and Cd mobilities but Al had no discernible effect.

Plant uptake of Cd and Zn in chelator-buffered nutrient solution depends on ligand type

The hypothesis that uptake of metals from solution by roots is controlled only by the activity of the free metal in solution is now being challenged. We examined the hypothesis that charge/binding strength of metal complexes determine their uptake by plant roots.

Influence of oxidation on sequential chemical extraction of dredged river sludge

Abstract The influence of oxidation on the speciation of transition metal ions in dredged sludge obtained from sequential extraction was investigated. The transition metals cadmium, copper, lead and zinc became more extractable after oxidation. This shift is related with the oxidation of their metal sulfides. Although iron has a similar affinity for sulfides, iron was less extractable from oxidised sediments. The conversion of Fe(II) to Fe(III) and the formation of iron(III)oxide minerals could account for this observation. The results emphasise the need of working under strictly anoxic conditions when studying anaerobic sediments. Three different extraction schemes were compared. Two extraction schemes (Oakley and Wallman) were found to give comparable results. The extraction scheme of Tessier differs largely from the two other methods used. This is probably due to the more severe extraction conditions. The influence of drying of the sediment was also investigated. Drying of sediments does not affect the...

Role of Organic Matter as a Geochemical Sink for Technetium in Soils and Sediments

The fate of a radionuclide when introduced (accidentally or otherwise) into a soil or sediment (marine or geological) is ruled by a combination of chemical, physico-chemical and biological factors (pH, redox conditions, sediment composition, complexing agents, microflora). The understanding of the behaviour of a radionuclide in such systems is needed on two main accounts: bioavailability and geochemical mobility.

Flotation as a remediation technique for heavily polluted dredged material. 1. A feasibility study

The flotation behaviour of highly polluted dredged material was investigated at different pH values by mechanical agitated (Denver) flotation. Up to 80% of cadmium, copper, lead and zinc could be concentrated in the froth layer which represented only 30% of the total mass. The maximum specificity for heavy metals, defined as the concentrating factor, was obtained at pH 8-9. The maximum recovery of heavy metals on the other hand was found to be reached at elevated pH values (pH 12). In addition the specificity of the flotation process for the transition metals could be assigned to their presence as metal sulphides in the dredged material. However, the interaction with organic matter is an important factor in determining their flotability. The carbonate fraction was irrelevant for the flotation behaviour of heavy metals.

Evidence for the existence of Tc(IV) - humic substance species by X-ray absorption near-edge spectroscopy

Summary The redox–sensitive fission product technetium–99 has been investigated in systems containing different reducing solid phases (pyrite, magnetite, ironsulphide and Gorleben sand) on the one hand and Gorleben groundwater, which contains a high amount of humic substances, on the other hand. Initially, technetium–99 was added to these systems as pertechnetate (Tc(VII)), which was reduced in presence and absence of humic substances with the aid of the different reducing surfaces (neutral to alkaline pH). Both in absence and presence of humic substances, Tc concentrations were observed which exceeded the TcO2 solubility limit, whereby the presence of humic substances had a 100 fold higher Tc concentration compared to its absence. Using the La–precipitation method, it was shown that Tc(IV) inorganic colloids and organic colloids were quantitatively precipitated. It was demonstrated for the first time by a combination of chemical speciation methods (La–precipitation method and gel permeation chromatography) and XANES spectroscopy of the humic substance containing supernatant solutions, that Tc(IV) species were formed in these systems, indicating an association between Tc(IV) and humic substances.

Flotation as a remediation technique for heavily polluted dredged material. 2. Characterisation of flotated fractions.

The particle size distribution and the metal speciation of the heavy metals were investigated on dredged sediment and on the fractions obtained by mechanical agitated (Denver) flotation. The transition metal ions (cadmium, copper, lead and zinc) were flotated specifically independent of the particle size. Particle size analysis, EDTA extraction and sequential extracts indicated that during flotation a redistribution of metals occurred due to the oxidation of metal sulphides. This oxidation process was more pronounced when the flotation was performed at higher pH values and resulted in a decrease in flotation specificity.

Charge reduction of zeolite Y through exchange with lanthanum ions: intracrystalline oxide and hydroxide formation

Abstract La 3+ exchanged zeolite Y (68% exchanged or 13 ions/u.c.) heated to temperatures between 150° and 750°C for 24 h results in irreversible lanthanum migration into the small cavities. The residual Na + cation exchange capacity (CEC) obtained after thorough Na + -exchange, decreases with pretreatment temperature from 2.7 (at 150°C) to 1.9 (at 750°C) meq g −1 backbone. The apparent lanthanum charge decreases to 2.5 (at 400°C). Irreversible lanthanum-oxide or oxihydroxide formation occurs upon heating beyond 200°C. The residual charge in the small cages, obtained from the difference between the Cs +  and Na + CEC, decreases from 1.5 meq g −1 backbone (Na + CEC original Y = 4.7 meq g −1 backbone) to about 0.2 meq g −1 backbone (at Na + CEC of 3 meq g −1 backbone). La 3+ reexchange of the samples heated to 300°C and reheated to different temperatures results in a maximum irreversibly blocked lanthanum content of 16 atoms/u.c, with an apparent charge of 2 and corresponding to a full occupation of all cubooctahedra by a lanthanum oxi(hydroxi)de complex. The data demonstrate that lanthanum incorporation in the small cages offers the possibility of preparing zeolites of variable exchangeable charge which is almost exclusively located in the big cavities.