S. Staunton

Possible role of organic matter in radiocaesium adsorption in soils

The aim of this review is to examine the hypothesis that organic matter decreases the adsorption of radiocaesium on clay minerals. The factors that determine radiocaesium mobility and bioavailability in soil are briefly outlined to show why a relationship between soil organic matter content and enhanced Cs bioavailability is paradoxical. In all the investigations reviewed the ionic compositions of both the solid and the solution phases have been strictly controlled. We show that the addition of organic matter to reference clay minerals causes decreases of up to an order of magnitude in the distribution coefficient of radiocaesium. Similarly, the chemical removal of organic matter from the clay-sized fraction of soil usually leads to an increase in Cs adsorption. We suggest that the nature of the organic matter and its interaction with mineral surfaces are as important as the amount present.

Adsorption of 137 Cs on montmorillonite and illite; effect of charge compensating cation, ionic strength, concentration of Cs, K and fulvic acid

A thorough understanding of the capacity of clay minerals to adsorb radiocesium is essential in order to predict the fate of this pollutant in the environment and to interpret routine measurements, such as desorbability. We have investigated the adsorption of radiocesium onto 2 contrasting reference clay minerals in dilute suspensions after a 2-h contact period. The results have been expressed as the distribution coefficient, Kd, or the selectivity coefficient, Kc. The adsorption properties have been studied with respect to various parameters: nature of the charge compensating cation, ionic strength, concentration of Cs, concentration of K and concentration of a soil-extracted fulvic acid.A Freundlich isotherm has been found to fit experimental data well for levels of adsorbed cesium in the range 10−7 to 1 mol kg−1. Evidence of surface heterogeneity is discussed, but it was not possible to deduce the exchange capacities of the groups of exchange sites with differing affinities for Cs, which are thought to exist in illites. The concentration dependence of adsorption upon montmorillonite is postulated to arise from a modification of the exchange complex with increasing presence of Cs, rather than from heterogeneity of exchange sites. Increasing ionic strength caused a decrease in the relative affinity for a trace amount of cesium on both minerals, although mass action led to a fall in Kd. This is thought to indicate a high covalent interaction between Cs and the clay surface.Potassium caused a smaller decrease in adsorption than stable Cs, which suggests that neither K nor NH4 plays a decisive role in the immobilization of radiocesium by clay minerals. There was a small decrease in adsorption upon addition of soil organic matter, which may contribute to the poor fixation capacity observed for some soils with a high organic matter content.

Reduced adsorption of caesium on clay minerals caused by various humic substances

Abstract The effect of the addition of various humic substances on the adsorption of caesium on two mineral clays has been studied. All measurements were carried out in dilute suspension under controlled conditions of temperature and ionic strength. Only a small proportion of the humic substance was adsorbed on the clays (

Interpretation of the pH dependence of protein adsorption on clay mineral surfaces and its relevance to the understanding of extracellular enzyme activity in soil

Abstract The pH dependence of the adsorption of bovine serum albumin (BSA) and Aspergillus niger β- d -glucosidase on montmorillonite and of its structural consequences was studied by several methods in order to understand the mechanisms of interaction between extracellular enzymes and clay minerals in soils. The relative influence of electrostatic and hydrophobic interactions on adsorption was deduced from experiments involving the coadsorption of BSA and methylated BSA. Also, the surface coverage of the clay by the BSA was studied by following the release of a paramagnetic charge-compensating cation on adsorption of the protein. This method is based on the line broadening effect of the released cation on the 31P NMR signal from phosphate in the solution, and the specific interfacial area of the BSA was deduced from the ratio of the surface covered by the quantity of protein adsorbed. Information on the secondary and overall structure of adsorbed BSA was obtained by Fourier transform IR spectroscopy from the frequency range and line width of the amide I/I′ signal and from the intensity of the COOH band. Finally, the catalytic activity of the β- d -glucosidase adsorbed on the clay was compared with its activity in solution, and the pH dependence of the adsorption was measured. The following general conclusions could be drawn from these experiments. (i) Below the isoelectric point (IEP), proteins unfold on the clay surfaces in response to electrostatic attractive interactions, a phenomenon which inhibits enzyme activity. (ii) Near the IEP proteins are adsorbed with little modification of conformation and thus enzymes preserve their catalytic activities. (iii) Above the IEP the proportion of adsorbed proteins decreases due to electrostatic repulsive interactions, permitting the diffusion of enzymes in the liquid-filled pore network of the soil.

Root uptake and distribution of radiocaesium from contaminated soils and the enhancement of Cs adsorption in the rhizosphere

Uptake by roots from contaminated soil is one of the key steps in the entry of radiocaesium into the food chain. We have measured the uptake by roots of radiocaesium and its transfer to shoots of a heathland grass, sheep fescue (Festuca ovina L.) from two contrasting agricultural soils, a sandy podzol and a clayey calcareous soil. A culture device which keeps the roots separate from the soil was used thus allowing rhizosphere soil to be obtained easily and enhancing the effect of root action. Biomass production and 137Cs in shoots and roots were recorded. Cs adsorption was studied on both the initial, nonrhizosphere soil and on rhizosphere soil in dilute soil suspension. Cs desorption was measured by resuspending subsamples of contaminated soil in solutions containing various concentrations of stable Cs. The proportion of Cs fixed, i.e. not readily desorbable, was calculated by comparison of the adsorption and desorption isotherms. Uptake by roots varied considerably between soils and did not appear to be diffusion limited. Root-to-shoot transfer did not differ for the two soils studied. Root action considerably enhanced Cs adsorption on the soils, particularly the in sandy podzol with a low Cs affinity. The value of Kd was increased by up to an order of magnitude. A large proportion of adsorbed Cs was found to be fixed, the Kd was up to seven times greater on desorption than adsorption, indicating that up to 80% of adsorbed Cs was not readily exchangeable. Root action had little effect on the fixed fraction.

Adsorption and desorption of 85Sr and 137Cs on reference minerals, with and without inorganic and organic surface coatings.

The adsorption properties of reference minerals may be considerably modified by the presence of the inorganic and organic coatings that are ubiquitous in soils. It is therefore important to assess the effect of such coatings to evaluate the relevance of adsorption studies on pure minerals. The adsorption of trace amounts of (85)Sr and (137)Cs has been studied in dilute suspensions for various minerals that are common components of soils: quartz, calcium carbonate, kaolinite, montmorillonite and illite. We studied the effect of coatings with either Fe or Al oxide with varying additions of soil-extracted humic or fulvic acid. Both adsorption and desorption were measured and data presented as distribution coefficients, Kd. No adsorption was detected on quartz and it was not possible to coat this surface. Adsorption on calcium carbonate was small and not influenced by coatings. Adsorption of Sr on the three clay minerals was very similar, enhanced by the Al-coating, but not affected by Fe and organic coatings. The presence of organic coatings decreased Cs adsorption on illite. Similar but smaller effects were seen on montmorillonite and kaolinite. Aluminum coating enhanced Cs adsorption on illite, whereas both inorganic coatings caused decreases in adsorption on montmorillonite, and there was no effect on kaolinite. Effects were not additive with mixed, organic-inorganic coatings. Adsorption of both Cs and Sr on all minerals was strongly irreversible, with Kd (desorption) being up to four-times greater than adsorption Kd. The ratio of desorption and adsorption Cs Kd values (an assessment of irreversibility) was inversely related to adsorption Kd. This is consistent with a decreasing contribution of high-affinity adsorption as adsorption increases, but may also reflect the partial loss of organic coatings during desorption.

Cs adsorption on the clay-sized fraction of various soils : Effect of organic matter destruction and charge compensating cation

Abstract The association of organic matter with clay minerals may decrease their affinity for Cs and thus enhance its bioavailability. We have investigated this hypothesis by comparing Cs adsorption on several soils, both topsoils and the corresponding subsoils, before and after organic matter destruction with H 2 O 2 . The clay-sized fractions were homoionic in either K, Na or Ca, to avoid artefacts due to variable composition of the exchange complex. All experiments were carried out in dilute suspension under controlled conditions. The affinity of the clay-sized fractions for Cs and the value of the Freundlich b parameter are typical of illites. This supports the hypothesis that the adsorption properties of soils are dominated by small amounts of illite. However, if this is the case, the affinity of soil illites is higher than that of reference illites. The destruction of organic matter has a variable effect. In some cases, a marked enhancement is observed, in others there is no significant effect, or a small decrease. There is no clear pattern relating the effect of organic matter destruction and either dominant clay mineralogy or organic matter content.

Copper concentration in plants and in the rhizosphere as influenced by the iron status of tomato (Lycopersicon esculentum L.)

Changes of metal concentration that occur in the rhizosphere may arise from several processes including variation in the concentration of complexing ligands, pH or redox potential that can be influenced by the Fe status of the plant. The aim of this study was to assess for both acidic and calcareous, Cu-contaminated soils how Cu concentration in plants and in the rhizosphere was affected by the Fe status of a strategy I plant species. The change of soil solution pH, total solution Cu concentration and soil redox potential was monitored for 8xa0days in the rhizosphere of tomato (Lycopersicon esculentum L.) in response to contrasting Fe supply. The concentration of Cu in roots was enhanced under Fe deficiency in the acidic soils. Shoot Cu however did not vary with the Fe status of the plant. The plant Fe status had little effect on rhizosphere pH, redox potential or Cu concentration in solution in either acidic or calcareous soils. Marked differences in pH and solution Cu concentration were observed between rhizosphere and uncropped soils. Roots induced an increase in pH of acidic soils and a decrease in solution Cu concentration in all soils. The decrease in solution Cu concentration in acidic soils may be explained by the increase in rhizosphere pH. The proposed device provided new data on the fate of Cu in the rhizosphere and showed a positive correlation for the four soils considered together between the total Cu concentration in soil solution and root Cu concentration.

ROLE OF IRON OXIDES IN THE PHOSPHATE ADSORPTION PROPERTIES OF KAOLINITES FROM THE IVORY COAST

The phosphate adsorption properties of three clay samples, with kaolinite as the dominant mineral, from different deposits in the Ivory Coast have been investigated. The clays contain varying amounts of crystalline Fe oxides and kaolinite with structural Fe. All measurements were made in dilute suspension under controlled conditions of temperature, pH, ionic strength and saturating cation. Data have been fitted to Langmuir adsorption isotherms. Both P adsorption and surface area measurements have been made on samples before and after chemical removal of Fe oxides. The samples have large P adsorption capacities, which are not entirely explained by their large specific surface areas. The presence of Fe oxides makes a strong contribution to the surface area and enhances the adsorption capacities. There is little evidence that structural Fe makes a strong contribution to the enhanced P adsorption capacity.

Adsorption of Alexa-Labeled Bt Toxin on Mica, Glass, and Hydrophobized Glass: Study by Normal Scanning Confocal Fluorescence

We studied the kinetics of adsorption of alexa-labeled Bt toxin Cry1Aa, in monomer and oligomer states, on muscovite mica, acid-treated hydrophilic glass, and hydrophobized glass, in the configuration of laminar flow of solution in a slit. Normal confocal fluorescence through the liquid volume allows the visualization of the concentration in solution over the time of adsorption, in addition to the signal due to the adsorbed molecules at the interface. The solution signal is used as calibration for estimation of interfacial concentration. We found low adsorption of the monomer compared to oligomers on the three types of surface. The kinetic adsorption barrier for oligomers increases in the order hydrophobized glass, muscovite mica, acid-treated hydrophilic glass. This suggests enhanced immobilization in soil if toxin is under oligomer state.