Yasser El-Nahhal

Adsorption of phenanthrene on organoclays from distilled and saline water

Isotherms of phenanthrene adsorption on different organoclay complexes were obtained using the HPLC technique to understand the adsorption behavior and to characterize the effect of sodium chloride (NaCl) on the adsorption. The adsorbed amounts of phenanthrene on montmorillonite exchanged by organic cations such as tetraheptylammonium, benzyltrimethylammonium, hexadecyltrimethylammonium, or tetraphenylphosphonium were several times higher than those obtained using montmorillonite clay without surface modification. At the same equilibrium concentration, the adsorbed amount of phenanthrene is higher on clay modified with benzyltrimethylammonium than on clay modified with hexadecyltrimethylammonium or other cations. Adsorption of phenanthrene on clay modified with benzyltrimethylammonium increased dramatically as the concentration of NaCl increased up to 150 g/l in the aqueous solution. The shape of the curves obtained can be classified as S-type. The adsorption data obtained from salinity experiments support a mathematical model that links the Langmuir constant with the salinity constant. This model may be useful to predict the equilibrium concentration of a contaminant in saline solution. FTIR studies showed strong interactions between the aromatic rings of phenanthrene and the preadsorbed benzyltrimethylammonium on clay surfaces.

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.

Reduction of photodegradation and volatilization of herbicides in organo-clay formulations

The use of commercially available emulsifiable concentrate (EC) formulations of alachlor and metolachlor may be an environmental hazard because of their volatilization to the atmosphere and photodecomposition, which requires increased applied amounts. The objectives of this study were to develop organo-clay based formulations which would be less volatile and better protected from photodegradation. Bioassays have shown that the use of organo-clay formulations improves photoprotection, reduces volatilization and maintains herbicidal activity in the soil under laboratory and field conditions. Largest adsorption of herbicides by organo-clays correlates with an optimal reduction of photodecomposition and volatilization. It appears that the role of the organic cation, e.g., benzyltrimethylammonium (BTMA) is to enhance the adsorption of the non-polar herbicides to the organo-clay complex, whereas the actual photoprotection is mainly provided by the clay.

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 mechanism of chloroacetanilide herbicides to modified montmorillonite.

This study was undertaken to characterize the adsorption mechanism of alachlor and metolachlor on montmorillonite modified with cationic surfactants. Adsorbed amounts of cationic surfactant on montmorillonite surfaces were determined by CNHSO analyzer. Equilibrium concentrations of alachlor and metolachlor were determined by GC and adsorption results were fit to a linear regression equation. The slope of the isotherms (Kd) was normalized to the fraction of organic carbon on montmorillonite complexes to produce corresponding Koc. Adsorption of surfactants fit very well to Langmuir equation. Increased basal spacing indicates that surfactant molecules could penetrate through the interlayer spacing and arrange themselves in different ways. Equilibrium data of alachlor and metolachlor suggest that adsorption may occur via physical or chemical bonds. Koc values of alachlor or metolachlor decreased as the fraction of the organic carbon increased in montmorillonite complexes indicating independent adsorption process. Changes of the molar free energy of the adsorption reactions were in the range of physical adsorption, indicating that adsorption reactions are spontaneous and the molecules either adsorb on the surface or penetrate into the inter‐layers of montmorillonite‐surfactant complex. Careful investigation of the adsorption data suggests that interaction may occur via the active groups such as carbonyl group (–C˭O), anilidic (C–N) group and/or phenyl rings. This information may provide better understanding on adsorption mechanism and be useful in designing ecologically acceptable herbicide formulations.

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.