Influence of thickener on uptake? leach processes of glyphosate and its major metabolite on bentonite sodica.

ASTORECA P.A.; MONGE M.P.,; BERGESIO M.V.; MAGNOLI A.P.,; MAGNOLI C.E.; CHIACCHIERA S.M.

Carlos Paz

Congreso; 13ª Conferencia Latinoamericana de Físico-Química Orgánica; 2015

Glyphosate or N - phosphonomethyl glycine (PMG) is a broad-spectrum and non-selective herbicide, widely used to eliminate unwanted weeds either in agricultural environments (mainly soybean, maize, sunflower and wheat cultivars) or non-agricultural environments. Soil, aquatic sediments and water are affected by the microbial biodegradation of glyphosate being aminomethylphosphonic acid (AMPA) its main metabolite. Polar soil components, such as Fe and Al oxides, clays and organic matter (humic acids) strongly bind both PMG and AMPA protecting the environment against their lixiviation to water. However, herbicide formulations include adyuvants and extrusion aid which increases the phytotoxicity of glyphosate or change the physical properties of commercial product. These additives are able to modify the herbicide agronomic effects, and have environmental consequences. Among other things, these compounds can modify natural remediation processes, affecting the binding interaction of PMG and AMPA to soil components, favoring leaching of the toxic to natural aqueous environments. The aim of this work was to study the influence (polyoxyethylene bis (amine), POEBA) among the adsorption isotherms of PMG and AMPA on bentonite. A fully characterized bentonite from Mendoza was used. A highly expansible clay used as adsorbent have CIC (197meq/100g); specific surface adsorption of water vapor (591m2/g), specific surface area by N2 adsorption (26.3 m2/g), isoelectric point (IEP was used: 6.2), structural formula ([(Si3.87Al0.13) (Al1.58Fe0.27 Mg0.15)] M+0.28) and DRX mineralogical composition: Montmorillonite: 70%; feldspar: 10%; Quartz: 20%. Adsorption isotherms of PMG and AMPA in presence and absence of POEBA were performed. The effect of the amount of POEBA in the experiments was also investigated. PMG and AMPA working solutions were prepared from a stock solution, covering an appropriated range of adsorbate concentrations. The dilutions were prepared in water at pH 6, with sodium chloride addition to achieve ionic strength control. Two replicates of each solution were left contacted with a bentonite suspension stabilized at the working pH inside of capped vials and were allowed to keep in contact 24h with orbital shaking at room temperature. All solutions were centrifuged after the incubation period. The concentration of PMG or AMPA in the supernatant was analyzed by HPLC with UV-visible detection of previously derivatized samples by reaction with p-toluenesulfonyl chloride in 0.4 M K2HPO4 buffer (pH 11) for 10 minutes at 50 ° C. Controls of the adsorbent were included in each assay. The adsorbed amount was calculated by measuring adsorbate depletion in the supernatant after absorption. Langmuir or S-shaped isotherms were observed. The presence of an inflection point at the monolayer coverage could be attributed to a multilayer adsorption or to the presence of more than one equivalent types of surface sites for the adsorption. For both adsorbates, the adsorption was greatly decreased in the presence of POEBA. On the other hand the saturation of the interface takes place at lower adsorbate equilibrium concentration when POEBA concentration increases. These results are consistent with the decrease in accessibility of sites at the interlayer of the montmorillonite fraction of adsorbent material. The results demonstrate that polyoxyethylene based additives, can significantly modify the clay ability to retain glyphosate. Besides the PMG adsorption process was reversible independently of the presence POEBA while AMPA showed an irreversible adsorption process which became reversible in the presence of POEBA. This fact is particularly important because it could indicate that under ambient conditions, the polyoxyethylene aditives might favor the leaching of this metabolite to environment.