Adsorption of sulfometuron and other anions on pillared clay

The adsorption of several anions of agronomic and ecological importance, sulfometuron 2-[3-(4,6-dimethylpyrimidin'2-yl)ureidosulfonylbenzoic acid, sulfate, acetater and chloride on pillared clay was studied. Pillared clay (PC) is a very effective adsorbent of anions from aqueous solutions. The fractions ofsulfometuron adsorbed from aqueous solutions were more than 90% of added, but low solubility limited the added amounts. The maximal adsorbed amount of sulfate was 0.72 mol kg-1 and of acetate was 0.41 mol kg-1. Addition of 100 and 500 mM of NaCl as the background electrolyte did not affect the adsorption of sulfometuron, but drastically reduced the adsorption of sulfate and acetate. Acetate at 1000-fold larger excess competes with sulfometuron for the surface sites, but sulfate does not. Acetate decreased dramaticatly the adsorption of sulfate when both anions were added simultaneously but acetate did not adsorb on the clay with pre-adsorbed sulfate. The sequence of adsorption affinity of anions to the PC surface is: sulfometuron >> sulfate > acetate >> chloride. The binding coefficient was 12 000 M-1 for sulfometuron, 650 M-1 for sulfate, 350 M-1 for acetate, and 15 M-1 for chloride. The dominant mode of sulfometuron adsorption on PC is via strong electrostatic interactions. The proposed interactions of sulfate with PC are outer- and inner-sphere complexation. The pH is reduced with sulfate adsorption and Al is released. The proposed mode of acetate interactions with PC is mainly via inner-sphere complexation. In this case the pH increases with acetate adsorption, indicating exchange of acetate with surface hydroxyls, and Al is not released.This research was supported by Grant 8803-1-96 of the Ministry of Science and Arts, Israel, for French-Israeli collaboration, and partially supported by a Grant G-0405-95 from G.I.F., the German-Israeli Foundation for Scientific Research and Development. Dr. T. Undabeytia acknowledges the Spanish Government (ref. PF9633975424) and the European Community (Contract N. FAIR-BM-970892) for a postdoctoral fellowship at the Seagram Center.Peer Reviewe

Adsorption-desorption of chlordimeform on montmorillonite: effect of clay aggregation and competitive adsorption with cadmium

Effect of the aggregation state of montmorillonite clays of types SAz-1 and SWy-1 on the adsorption of the monovalent organic cation chlordimeform was studied. The shapes of the adsorption isotherms were related to the degree of dispersion of the clay, changing from S- to L-type by decreasing clay concentration. Unlike monovalent organic cations denoted dyes, chlordimeform adsorption did not exceed the cationic exchange capacity of the clay (CEC). At larger Ca2+/Na+ charge ratio (≃0.5), chlordimeform exhibited low apparent affinity for adsorbing to the SAz-1 clay, due to steric inhibition of its penetration between closely opposed clay platelets. The apparent affinity increased dramatically at smaller Ca2+/Na+ charge ratios (<0.06) for Ca2+-montmorillonite, or by switching to Na+- montmorillonite. The desorption process of chlordimeform shows an apparent hysteresis in Ca2+-montmorillonite. An adsorption model which combines electrostatic equations with specific binding in a closed system is able to account for part of this hysteresis by the reduction in the concentrations of the divalent cations Ca2+ and Mg2+ in the supernatant. Part of the hysteresis arises from a different state of aggregation of the Ca2+-clay in the adsorption and desorption experiments. The model also yields good predictions for the competition between chlordimeform and Cd in adsorption processes and their consecutive desorptions. | Effect of the aggregation state of montmorillonite clays of types SAz-1 and SWy-1 on the adsorption of the monovalent organic cation chlordimeform was studied. The shapes of the adsorption isotherms were related to the degree of dispersion of the clay, changing from S- to L-type by decreasing clay concentration. Unlike monovalent organic cations denoted dyes, chlordimeform adsorption did not exceed the cationic exchange capacity of the clay (CEC). At larger Ca2+/Na+ charge ratio (≈0.5), chlordimeform exhibited low apparent affinity for adsorbing to the SAz-1 clay, due to steric inhibition of its penetration between closely opposed clay platelets. The apparent affinity increased dramatically at smaller Ca2+/Na+ charge ratios (<0.06) for Ca2+-montmorillonite, or by switching to Na+-montmorillonite. The desorption process of chlordimeform shows an apparent hysteresis in Ca2+-montmorillonite. An adsorption model which combines electrostatic equations with specific binding in a closed system is able to account for part of this hysteresis by the reduction in the concentrations of the divalent cations Ca2+ and Mg2+ in the supernatant. Part of the hysteresis arises from a different state of aggregation of the Ca2+-clay in the adsorption and desorption experiments. The model also yields good predictions for the competition between chlordimeform and Cd in adsorption processes and their consecutive desorptions.Peer Reviewe

Wplyw pH i skladu fazy stalej na agregacje frakcji koloidalnej gleb

Relations among clay particle aggregation, clay composition and the pH of equilibrium solution were studied using sedimetrical scales and sodium forms of clay fractions separated from six acidic soils of different origin. Experimentally obtained sedimentation curves were numerically extrapolated to infinite time and the mass of sediment virtually deposited was taken as a basic factor characterizing aggregation. Aggregation of investigated clays was significantly influenced by the presence of particular solid phase components i.e. organic matter, iron and aluminium compounds, which are apparently connected with their surface charges (negative and positive) and their relative amounts. A pH increase lead to a decrease of aggregation for every case investigated.Badano wpływ pH i składu fazy stałej na agregację form sodowych frakcji koloidalnych wydzielonych z gleb kwaśnych o różnym pochodzeniu. Stosowano metodę wagi sedymentacyjnej przy czym do charakteryzowania agregacji stosowano ilość sedymentu, która potencjalnie osiadłaby po nieskończenie długim czasie osadzania. Zauważono, iż na agregację frakcji ilastych istotny wpływ ma skład fazy stałej, co wydaje się być związane z rodzajami i wielkościami ładunków powierzchniowych. Agregacja we wszystkich badanych przypadkach maleje ze wzrostem pH

Optimization of Adsorption of Hydrophobic Herbicides on Montmorillonite Preadsorbed by Monovalent Organic Cations: Interaction between Phenyl Rings

This study aimed to optimize organo-clay formulations for reduction of leaching of the herbicides alachlor, metolachlor, and norflurazon, which include a phenyl ring in the structure. The adsorbed amounts of herbicides increased severalfold when montmorillonite was preadsorbed by an organic cation; benzyltrimethylammonium (BTMA) was more effective than benzyltriethylammonium (BTEA). Fourier transform infrared studies indicated interactions between alachlor molecules and adsorbed BTMA. The adsorption affinity of the herbicides increased with BTEA loading up to the cation exchange capacity (CEC) of montmorillonite but reached a maximum at a BTMA loading of 5/8 of the CEC. The enhanced adsorbed amounts of herbicides are mainly due to interactions between the phenyl rings of herbicide molecules and organic cations, which are favored with the smaller cation, BTMA.BTMApreadsorbed on the clay up to the CEC forms a fraction (14-18%) of charged dimers so that less phenyl rings are available for interacting with herbicide molecules. This effect is small for preloading by BTEA, so that the amounts adsorbed increase with the degree of preloading. Thus, optimization of claybased herbicide formulations requires a selection of structurally compatible organic cations preadsorbed on the clay at optimal coverage.Peer Reviewe

Organo-clay formulations of pesticides: Reduced leaching and photodegradation

Adsorption of organic cations on several clay minerals is reviewed with an emphasis on the effect of ionic strength and modeling. The clay exchanged with suitable organic cations forms a basis for ecologically acceptable formulations of herbicides with reduced leaching, ground water contamination and enhanced weed control efficacy. Incomplete neutralization of the clay surface charge by an organic cation may be advantageous in achieving maximal adsorption of hydrophobic herbicides. One conclusion from these studies is that optimization of clay-based herbicide formulations requires a selection of structurally compatible organic cations preadsorbed on the clay at optimal coverage. New experimental results are presented for alachlor formulations, which significantly reduce herbicide leaching under conditions of heavy irrigation. We were able to demonstrate that organo-clay formulations of alachlor and metolachlor can increase crop yields in a 1-year field experiment. The photostabilization of pesticides is reviewed and improved organo-clay formulations of the herbicides trifluralin and norflurazon are described. A pillared clay, nanocomposite micro- and/or meso porous material, was effective in reducing leaching and in conferring photostabilization, without added organic cations.Peer Reviewe