Tamara Polubesova

Clay-based formulations of metolachlor with reduced leaching

The current research in herbicide application aims to develop formulations that reduce leaching of the herbicide to deep layers of the soil and to concentrate its biological activity at the top layers. Adsorption of metolachlor on clay minerals, their organic derivatives or pillared forms provides the best possibility to develop slow-release formulations. Metolachlor is a selective pre-emergence herbicide widely used in irrigated crops to control annual weeds. It is adsorbed by bentonites and montmorillonites, but the amount adsorbed strongly depends on the type of bentonite and possible pretreatment reactions. Wyoming bentonites adsorbed considerable amounts of metolachlor but other bentonites did not bind this herbicide. An acid-activated pillared montmorillonite was also an effective adsorbent of metolachlor. Modification of this sample by preadsorbing different amounts of benzyl trimethylammonium ions did not influence the level of herbicide adsorption. The biological efficiency of the formulations was tested with bioassay soil columns. Slow-release formulations could be prepared with raw bentonites and the acid-activated pillared montmorillonite. A formulation, prepared by adsorbing metolachlor from aqueous solution on the acid-activated pillared montmorillonite, showed high herbicide activity at the top 10 cm, and did not diffuse significantly to greater depths. This formulation should allow a better weed control than the commercial formulations.

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.

Adsorption of benzyltrimethylammonium and benzyltriethylammonium on montmorillonite; experimental studies and model calculations

The adsorption of the monovalent organic cations benzyltrimethylammonium (BTMA) and benzyltriethylammonium (BTEA) to montmorillonite was studied as a function of their concentrations and ionic strength. At low ionic strength the adsorbed amounts of BTMA and BTEA reached values of the cation exchange capacity (CEC) of the clay. An increase in the ionic strength resulted in reduction in the adsorbed amounts of the organic cations, unlike the pattern observed previously with organic monovalent cationic dyes. The reduction in adsorbed amounts of BTMA and BTEA depended on the inorganic cations according to the sequence Cs+ > Na+ > Li+, which follows the sequence of binding coefficients of these inorganic cations added. The type of the anion (that is, Cl−, ClO4−, SO42-) had no effect on the adsorbed amounts. An adsorption model which considers the electrostatic Gouy-Chapman equations, specific binding and closeness of the system could adequately simulate the adsorbed amounts of BTMA and BTEA and yield predictions for the effect of the ionic strength and concentration of electrolytes. The binding coefficient employed was K = 5000 M−1 for the formation of neutral complexes of BTMA and BTEA. This value is larger than those found for the inorganic cations but is several orders of magnitude below those found for the monovalent dyes. The binding coefficients for the formation of charged complexes of BTMA and BTEA were 20 and 5 M−1, respectively. The basal spacing of the clay did not change significantly with the adsorbed amounts of both BTMA and BTEA up to the CEC.

Movement of metolachlor in soil: effect of new organo-clay formulations

The use of commercially available formulations of metolachlor has resulted in its leaching and migration to water sources. Formulations of metolachlor designed to reduce its leaching in soil have been prepared by adding the herbicide dissolved in an organic solvent or in water to organo-clay complexes. Best formulations were made when the organo-clay complex was formed by adsorbing the monovalent organic cations benzyltrimethylammonium (BTMA) or benzyltriethylammonium (BTEA) onto sodium montmorillonite (Mont) at 0.5 or 0.8 mole kg−1 clay. Adsorption of metolachlor to organo-clays followed the sequence Mont-BTMA 0.5 > Mont-BTMA 0.8 > Mont-BTEA 0.8 > Mont-BTEA 0.5 > Mont. Fourier transform infrared (FTIR) analysis demonstrated the occurrence of shifts of several peaks of adsorbed metolachlor relative to the free herbicide, indicating the existence of strong interactions between metolachlor molecules and the organo-clay surface. Leaching studies employing organo-clay and commercial formulations were carried out under greenhouse and field conditions. Metolachlor applied as organo-clay formulations leached less than the commercial formulation. Organo-clay formulations prepared by adding the herbicide as a water solution showed less leaching in the soil profile than those made by using organic solvent. Under greenhouse conditions, the herbicidal activity of organo-clay formulations was similar to that of the commercial one. Under field conditions, leaching from Mont-BTMA 0.5-metolachlor was less than that from the commercial formulation, demonstrating the environmental and agricultural advantages of the organo-clay formulations of metolachlor. © 1999 Society of Chemical Industry

Adsorption and oxidative transformation of phenolic acids By Fe(III)-montmorillonite.

Phenolic acids participate in various soil processes and are of great concern due to their allelopathic activity. The interactions of phenolic acids (ferulic, p-coumaric, syringic, and vanillic) with montmorillonite enriched with Fe(III) was investigated. Adsorption of the phenolic acids on Fe(III)-montmorillonite was accompanied by their oxidative transformation and formation of Fe(II). Oxidative transformation of phenolic acids was affected by their molecular structure. The order of maximal transformation at the initial acid concentration of 20 mg/L on the surface of Fe(III)-montmorillonite was ferulic (94%), syringic (60%), p-coumaric (35%), and vanillic (25%). Benzoic acid which was used as a reference aromatic compound exhibited only 5% transformation. Removal of the phenolic acids from solution increased with decreasing pH. LC-MS analysis demonstrated the presence of dimers, trimers, and tetramers of ferulic acid on the surface of Fe(III)-montmorillonite. Oxidation and transformation of ferulic acid were more intense on the surface of Fe(III)-montmorillonite as compared to Fe(III) in solution due to stronger complexation on the clay surface. The results of the current study demonstrate the importance of Fe(III)-clay surfaces for the abiotic formation of humic materials and for the transformation of aromatic (phenolic) pollutants.

DOM-Affected Transformation of Contaminants on Mineral Surfaces: A Review

This review analyzes the role and reactivity of dissolved organic matter (DOM) in oxidation, reduction, hydrolysis, and photochemical reactions of contaminants occurring on mineral surfaces. DOM affects transformation via competition for adsorption sites on the mineral surface, dissolution of minerals and exposing new reactive surface sites on the mineral surface, and by electron shuttling. Most of the data suggest that DOM reduces oxidation and hydrolysis, and increases reduction of contaminants by minerals. Alternatively, mineral surfaces can enhance redox transformations of contaminants due to interactions with DOM. DOM impact on transformation of contaminants varies as a function of its molecular composition and chemical properties. In some cases, the influence of dissolved small organic molecules on the transformation of contaminants by minerals may be opposite to the bulk DOM effect. In addition, fractionation of DOM on the mineral surface can also influence the contaminant-mineral interactions. Based on the vast reviewed data, we suggest that the evaluation of DOM effects on contaminant transformations needs to be based on the chemistry and concentration of the DOM functional groups and the overall physicochemical properties of DOM. Moreover, the self-fractionation of DOM upon interactions with minerals must be considered in order to elucidate the holistic effect of DOM in the contaminant-mineral system. In addition, we suggest that “natural DOM” should be used to elucidate DOM impact on the mineral surface reactions and not dissolved humic acids, which exhibit quite different chemical structure and properties.

Modeling of organic and inorganic cation sorption by illite

Sorption of several organic and inorganic cations on illite (Clay Minerals Society Source Clay Imt-2) was determined experimentally and results compared to model calculations. The cations studied were crystal violet (CV+), benzyltrimethylammonium (BTMA+), benzyltriethylammonium (BTEA+), Ca2+, Mg2+, K+, Na+, Cs+, and Li+. The adsorption-model calculations involved a solution of the electrostatic Gouy-Chapman equations. The model considered specific adsorption and sorption/exclusion in the double-layer region in a closed system. Model calculations considered the simultaneous presence of four to six cations in the system. The adsorption of CV included formation of neutral and charged complexes. The adsorption attained 0.37 mol kg−1 or 150% of the cation exchange capacity (CEC) of illite in aqueous suspension. The adsorption of BTMA and BTEA did not exceed the CEC and was reduced with an increase in ionic strength. The sorption of CV below the CEC was rather insensitive to the ionic strength because of the large binding coefficients and was only slightly reduced in NaCl, CsCl, or Na2SO4 solutions. When added in amounts exceeding the CEC in high ionic strength, 0.667 M NaNO3, NaCl, or CsCl solutions, the adsorbed quantities of CV increased to three times the CEC. At high sulphate concentrations (0.333 M Na2SO4), the adsorption was below the CEC. Model calculations yielded satisfactory simulations for the adsorption, particularly for cations added in amounts approaching or exceeding the CEC. The binding coefficients for formation of neutral complexes followed the sequence: CV > Ca > BTMA > BTEA > Cs > Mg > K > Na > Li. Model calculations also suggested that sites were present which bound exchangeable cations, particularly K+, Na+, and Mg2+, very tightly.

Mepiquat–acetochlor formulations: sorption and leaching

Our aim was to study the adsorption of the plant regulator mepiquat to montmorillonite and test the suitability of montmorilonite–mepiquat complexes in formulations to reduce leaching of the non-polar herbicide, acetochlor. Adsorption of the monovalent organic cation mepiquat to montmorillonite through a range of ionic strengths was determined and modeled. Mepiquat adsorption reached the cation exchange capacity (CEC) of the montmorillonite. The binding coefficients K=300 M−1 for neutral mepiquat complex formation and K=10 M−1 for positively charged complex formation were employed. Increased NaCl and CsCl electrolyte solution concentrations reduced mepiquat adsorption which was also affected by the inorganic cation binding coefficients. An adsorption model, which includes the electrostatic Gouy–Chapman equations and specific binding in a closed system, effectively simulated of the adsorption isotherms and yielded quantitative predictions for the effect of electrolyte concentration on mepiquat adsorption. Acetochlor adsorption to mepiquat–montmorillonite complexes was about 20% of the amount added, and increasing the mepiquat concentration from 0.5 to 0.8 mmol mepiquat g−1 clay made no difference. Acetochlor desorption from formulations was rather rapid with 38–67% released during 1 day. The rate of acetochlor release was reduced to 8.5% during 1 day after a brief pre-washing of the formulations. Despite the rate reduction in acetochlor release, current formulations are still not satisfactory for reduced acetochlor leaching.

ADSORPTION OF SOIL-DERIVED HUMIC ACID BY SEVEN CLAY MINERALS: A SYSTEMATIC STUDY

Humic acid (HA)-clay complexes are well known for their contribution to soil structure and environmental processes. Despite extensive research, the mechanisms governing HA adsorption are yet to be resolved. A systematic study was conducted to characterize the adsorption of a soil-derived HA to seven clay minerals. Clay surfaces affected HA adsorption directly due to structural differences and indirectly by altering solution pH. The following order of HA removal was obtained for the clay minerals at their natural pH: illite ≫ palygorskite > kaolinite > sepiolite > montmorillonite = hectorite ≫ talc. Removal of HA (precipitation and adsorption) by kaolinite and illite was attributed to the low pH they induce, resulting in protonation of the clay and HA surfaces. In spite of the low pH, the zeta potential for HA remained negative, which promoted HA adsorption to the protonated clay surfaces by ligand exchange. Ionic strength did not affect HA adsorption to clay minerals with low zeta potentials, indicating that charge screening is not a major mechanism of HA adsorption for these minerals, and supporting the suggestion that ligand exchange is the main adsorption mechanism to pH-dependent sites. The increase in ionic strength did, however, promote HA adsorption to clay minerals with high zeta potentials. At pH 8–9 the order of HA affinity for clay minerals was: palygorskite >>sepiolite > montmorillonite = hectorite > kaolinite > illite > talc, emphasizing strong HA interactions with the fibrous clays. This strong affinity was attributed to their large surface areas and to strong interactions with OH groups on these clay surfaces. Results indicated that HA did not enter the intracrystalline channels of the fibrous clays but suggested that their macro-fiber structure facilitates HA adsorption. The sorption of HA to kaolinite further increased in the presence of Cu2+, and the sorption of Cu2+ increased in the presence of HA, due to a number of synergistic effects. This study emphasizes the diverse effects of clay structure and solution chemistry on HA adsorption.

Chapter 5.2 - Clays, Clay Minerals, and Pesticides

Abstract Design and test of clay-based formulations of pesticides for solving environmental and economical problems are described. Organoclays were mainly designed to promote the adsorption of neutral and hydrophobic pesticides and slow their release. Adsorption of organic cations modifies the nature of the clay mineral surface, transforming it from hydrophilic to hydrophobic. The modified clay mineral surface can have enhanced affinity for adsorbing neutral organic molecules of hydrophobic characteristics. The adsorption of the hydrophobic herbicides alachlor, metolachlor norflurazon, and acetochlor, which include a phenyl ring, was maximal for montmorillonite preadsorbed by a small cation, for example, phenyl trimethylammonium at a loading corresponding to 5/8 of the cation-exchange capacity (CEC). Loading of the organic cations above the CEC of the clay can promote the adsorption of certain anionic herbicides, such as imazaquin. Reduction of volatilization and photodegradation of herbicides was also achieved by certain organoclay formulations. In certain cases, an organic cation adsorbed on the clay mineral can act as an energy acceptor of the photoexcited molecule of the pesticide, which returns to its ground state before its photodecomposition occurs, thus becoming photostabilized. Clay mineral–micelle and –liposome formulations were introduced for obtaining slow release formulations of certain anionic herbicides, which could not be achieved by organoclay ones. The procedure involves incubation of the clay mineral with organic cations, which are mostly in micelles or liposomes. The complex formed between ODTMA (octadecyl trimethylammonium) and montmorillonite in the presence of excess of micelles is very different from the complex formed in the exclusive presence of ODTMA monomers, as shown by electron microscopy, XRD, and adsorption measurements. Unlike the monomer–clay mineral complex, which was not efficient for the adsorption of anionic organic molecules, such as sulfometuron, the micelle–clay mineral complex was highly efficient. Liposome–clay mineral formulations were prepared by employing the positively charged didodecylammonium and the neutral and EPA approved phosphatidylcholine. Efforts to develop slow release formulations also focused on encapsulation of herbicides in clay mineral polymer nanocomposites. The efficacy of the bypiridil herbicides paraquat (PQ) and diquat (DQ), which are divalent organic cations, used for post-emergence weed control was enhanced by addition to the herbicide formulation of monovalent organic cations which could compete for adsorption to the dust with DQ and PQ, thus making them more available for herbicidal activity.