Anne Louise Gimsing

Competitive adsorption and desorption of glyphosate and phosphate on clay silicates and oxides

Abstract Competitive adsorption of glyphosate and phosphate on goethite and gibbsite and on illite, montmorillonite and two kaolinites differing in surface area was evaluated. The results show that glyphosate and phosphate are competing for the adsorption sites, but the degree of competition depends on the adsorbent. On goethite the competition is very much in favour of phosphate, on gibbsite the competition is closer, but still phosphate is favoured, while on illite, montmorillonite and kaolinite the competition is almost equal. The amounts of glyphosate and phosphate, which can be adsorbed also depends on the adsorbent: the oxides adsorb more than the clay silicates. The amount adsorbed on kaolinite was dependent on the specific surface area. Changes in the surface area did not affect the competition between glyphosate and phosphate for adsorption sites. The results indicate that differences among soils of different mineralogical composition regarding the adsorption of glyphosate and phosphate can be expected.

Estimation of soil phosphate adsorption capacity by means of a pedotransfer function.

Abstract The capacity of soil to protect the aquatic environment against pollution is an important soil function. Thus, the phosphate adsorption capacity (PAC) is important in predicting the risk of phosphate pollution of the aquatic environment because of overfertilisation and because of reestablishment of former drained and cultivated wetlands. Moreover, PAC is an essential property in assessing the risk of drain and ground water contamination with the widely used glyphosate herbicide, which may compete with phosphate for adsorption sites on soil solids. As aluminium and iron oxides are the main phosphate adsorbents in many soils, PAC can be predicted by pedotransfer functions based on various aluminium and iron oxide fractions such as oxalate-extractable aluminium and iron (Al o , Fe o ) and dithionite-extractable iron (Fe d ). Accordingly, experimentally determined PAC taken as the sum of oxalate-extractable phosphate and the Langmuir maximum of the phosphate adsorption isotherm was found to be well predicted by the pedotransfer function P calc =0.22×Al o +0.12×Fe o +0.02×(Fe d −Fe o ) for a wide range of noncalcareous soils including Alfisols, Entisols, Inceptisols, Oxisols, Spodosols, and Ultisols from various parts of the world. A set of noncalcareous soil samples from Denmark, Ghana, and Tanzania was used in developing this pedotransfer function, while another set of soils from Canada and Tanzania was used in testing the function. While close relationships between experimentally determined and predicted PACs were obtained with these soils, the function failed, however, to predict PAC of two Tanzanian Andisols.

EFFECT OF KCl AND CaCl2 AS BACKGROUND ELECTROLYTES ON THE COMPETITIVE ADSORPTION OF GLYPHOSATE AND PHOSPHATE ON GOETHITE

Competitive adsorption between glyphosate and phosphate on goethite was evaluated. The influence of background electrolyte on the adsorption of glyphosate and phosphate was also investigated by using 0.01 M KCl, 0.1 M KCl and 0.01 M CaCl2 as background electrolytes. Experiments showed that phosphate displaced adsorbed glyphosate from goethite, whereas glyphosate did not displace phosphate. Results also showed that the background electrolyte had a strong effect on phosphate adsorption, but little effect on glyphosate adsorption. Thus, there are differences between the adsorption of glyphosate and phosphate. The study also showed that 0.01 M KCl caused dispersion of goethite, resulting in inefficient filtering, and that phosphate precipitated as calcium phosphates in 0.01 M CaCl2 background electrolyte solutions. The results suggest that 0.1 M KCl is a more suitable background electrolyte to determine competitive adsorption processes involving glyphosate and phosphate.

PHOSPHATE AND GLYPHOSATE ADSORPTION BY HEMATITE AND FERRIHYDRITE AND COMPARISON WITH OTHER VARIABLE-CHARGE MINERALS

Adsorption by synthetic 2-line ferrihydrite and hematite of glyphosate and phosphate, separately and together, was compared with adsorption results for goethite, gibbsite and two kaolinites in order to determine adsorption differences and similarities, in particular competition and phosphate preference, of these variable-charge minerals. Hematite rapidly adsorbed both compounds, while adsorption by ferrihydrite was slow, in particular of glyphosate, probably because of very slow diffusion of the bulky glyphosate molecules into interior sites in ferrihydrite particles. Accordingly, the Langmuir adsorption capacity of glyphosate (GAC) was considerably smaller (1.85 µmol m−2)than GAC for hematite (2.61 µmol m−2). The phosphate adsorption capacities (PAC) for ferrihydrite and hematite were more alike, 2.91 µmol m−2 and 2.85 µmol m−2, respectively. Differences between surface coordination (mono- or bidentate) may also contribute to the observed differences but conflicting information about the nature of the surface complexes makes this a difficult contributary factor to assess. The minerals were found to exhibit great variation in extent of competition and phosphate preference. Little competition and phosphate preference characterized hematite adsorption, while phosphate almost completely outcompeted glyphosate on goethite; ferrihydrite adsorption fell between these extremes. These differences may be attributed to different numbers of common (competitive) and specific (selective) adsorption sites on the three Fe oxides with a decreasing number of common sites in the order: goethite>>ferrihydrite>hematite, i.e. almost all goethite sites are common but with strong phosphate preference, while most hematite sites are specific for either glyphosate or phosphate. Alternatively, the result may be explained by adsorption in more planes, e.g. glyphosate adsorption onto the inner-Helmholtz-plane-adsorbed phosphate. For all six minerals compared, desorption of glyphosate following phosphate addition was found to be significantly correlated with the difference between the amounts of phosphate and glyphosate adsorbed indicating that this difference may be used as a competition index for predicting the influence of phosphate on glyphosate adsorption.

Degradation kinetics of glucosinolates in soil

Glucosinolates are compounds produced by all cruciferous plants. They can be hydrolyzed to several biologically active compounds and, as such, may serve as naturally produced pesticides. To optimize the pesticidal (biofumigation) effect and to assess the risk of glucosinolate leaching and spread in the environment, the degradation in soil of glucosinolates has been studied. The kinetics of degradation of four glucosinolates, two aliphatic (but-3-enyl and 2-hydroxy-but-3-enyl) and two aromatic (benzyl and phenethyl), in four soils was largely independent of the specific glucosinolate structure. Degradation followed logistic kinetics. Degradation was much faster in a clayey soil (half-life, 3.5-6.8 h) than in a sandy soil (half-life, 9.2-15.5 h). Degradation was much slower or nonexistent in the subsoil (<25 cm soil depth). The glucosinolates are not sorbed in the soil, and the degradation potential is, to a large extent, associated with the clay fraction. Measured activity in the soils of the enzyme myrosinase, which can catalyze the hydrolysis of glucosinolates, correlated well with the glucosinolate degradation kinetics. Autoclaving, but not sodium azide or gamma-irradiation, effectively blocked glucosinolate degradation, indicating that extracellular myrosinase is important for glucosinolate degradation.

Mineralization of the allelochemical sorgoleone in soil.

The allelochemical sorgoleone is produced in and released from the root hairs of sorghum (Sorghum bicolor). Studies have confirmed that it is the release of sorgoleone that causes the phytotoxic properties of sorghum, and sorgoleone has a potential to become a new natural herbicide, or the weed suppressive activity of sorghum can be utilized in integrated weed management. Since sorgoleone is released into soil, knowledge of the fate of sorgoleone in soil is essential if it is to be utilized as an herbicide. Fate studies will characterize the persistence and mobility of the compound. Three types of radioactively labelled sorgoleone were produced and used to study mineralization (complete degradation to CO(2)) of this lipid benzoquinone in four soils, two from the United States of America (Mississippi) and two from Denmark. The studies showed that sorgoleone was mineralized in all soils tested. The methoxy group of sorgoleone was readily mineralized, whereas mineralization of the remaining molecule was slower. Mineralization kinetics indicated that microorganisms in American soils were able to use sorgoleone as a source of energy.

Effect of Phosphate on the Adsorption of Glyphosate on Soils, Clay Minerals and Oxides

The effect of phosphate (ortho-phosphate) on the adsorption of the widely used glyphosate herbicide was evaluated with three typical Danish agricultural soils as well as pure oxides (goethite, FeOOH and gibbsite, Al(OH) 3 ) and silicates (illite and montmorillonite), which are considered the most important glyphosate and phosphate adsorbents in soils. Batch experiments where made in order to find out how phosphate affects adsorption of glyphosate and how glyphosate affects adsorbed phosphate. Solution glyphosate was quantified by liquid scintillation counting of 14 C-taggered herbicide and the concentration of phosphate by the molybdenum blue method. All experiments showed competition between phosphate and glyphosate for adsorption sites but the various adsorbents exhibited great variation in affinity for glyphosate and phosphate. Thus, gibbsite and, in particular goethite strongly prefer phosphate, whereas the competition on the silicates is more equal. The current studies showed that the competition in soils is almost equal, but still phosphate affects the sorption of glyphosate in soil. The amount of glyphosate and phosphate adsorbed by the various kinds of adsorbents was found to decrease in the order: oxides > silicates > soils. For the soils tested aluminium oxides, and to a lesser extent iron oxides seem the most important components in determining a soils ability to adsorb phosphate and glyphosate, whereas the clay content and clay type seem of minor or little importance for adsorption of these species.

Degradation and sorption of 2-propenyl and benzyl isothiocyanate in soil.

Isothiocyanates of natural origin produced by the hydrolysis of plant-produced glucosinolates have the potential to control soil pests, but getting sufficiently high isothiocyanate concentrations in soil is difficult. Furthermore, the isothiocyanates have proven toxic to a wide range of organisms and hence may also harm nontarget organisms. Knowledge of the sorption and degradation of the isothiocyanates is essential to optimize the use of natural isothiocyanates for pest control while minimizing the environmental impact. We have conducted studies on the sorption and degradation of two isothiocyanates of natural origin, 2-propenyl isothiocyanate and benzyl isothiocyanate. The experiments show the isothiocyanates degrade very quickly (t(1/2) = 0.93-4.25 h) in a 1:1 soil water slurry at 25 degrees C and they are sorbed by the organic matter in soil. From an environmental point of view, a fast degradation is desirable, but if the natural isothiocyanates are to be utilized for pest control, a fast degradation may imply they are not present long enough to have the desired effect on pests.

The toxic effects of benzyl glucosinolate and its hydrolysis product, the biofumigant benzyl isothiocyanate, to Folsomia fimetaria

Natural isothiocyanates (ITCs) are toxic to a range of pathogenic soil-living species, including nematodes and fungi, and can thus be used as natural fumigants called biofumigants. Natural isothiocyanates are hydrolysis products of glucosinolates (GSLs) released from plants after cell rupture. The study investigated the toxic effects of benzyl-GSL and its hydrolysis product benzyl-ITC on the springtail Folsomia fimetaria, a beneficial nontarget soil-dwelling micro-arthropod. The soil used was a sandy agricultural soil. Half-lives for benzyl-ITC in the soil depended on the initial soil concentration, ranging from 0.2 h for 67 nmol/g to 13.2 h for 3,351 nmol/g. For benzyl-ITC, the concentration resulting in 50% lethality (LC50) value for F. fimetaria adult mortality was 110 nmol/g (16.4 mg/kg) and the concentration resulting in 50% effect (EC50) value for juvenile production was 65 nmol/g (9.7 mg/kg). Benzyl-GSL proved to be less toxic and consequently an LC50 value for mortality could not be estimated for springtails exposed to benzyl-GSL. For reproduction, an EC50 value was estimated to approximately 690 nmol/g. The study indicates that natural soil concentrations of ITCs may be toxic to beneficial nontarget soil-dwelling arthropods such as springtails.