The structure and hydration status of attapulgite clay after heating at elevated temperatures and the stability of parathion on these clays was studied. Using infrared spectroscopy and scanning electron microscopy it was found that the bound water was lost in two steps, at 250° and 450° with the first step being largely reversible. At 650°C the structure began to dissolve releasing significant amounts of Mg, and a decrease in aggregate porosity was noted. At 850°C an amorphous phase was formed bearing little resemblance to the original attapulgite. Parathion was stable on all of the preheated clays when kept at 25°C for 190 days. The reactions of parathion on the preheated clays was studied at 110°C Hydrolysis of parathion was found to be minimal. Isomerization was the main reaction occurring on the Ca-attapulgite, whereas on an organo-clay no isomerization was observed. A mechanism for the isomerization reaction is proposed which entails a distortion of the phosphate moiety of the pesticide by the oxygen of the ligand water resulting in the conformational changes necessary for the isomerization to take place. On the organo-clay such a conformation was not possible; hence no isomerization occurred.

Results of adsorption studies of several pesticides on soils and clays show that the application of the reduced concentration concept to adsorption can either reduce the temperature effect on the isotherms, eliminate it altogether (e.g., parathion adsorption on Netanya soil), increase the temperature dependence (e.g., (β-BHC adsorption on Ca-bentonite) or even reverse the temperature dependence of the isotherms (e.g., parathion adsorption on Ca-attapulgite).

Literature and experimental data for the adsorption of parathion by Ca-attapulgite and by attapulgite with organic exchangeable cations of different sizes demonstrate that for many surface interactions the terms organophilic or hydrophilic are misleading. Organic compounds which are insoluble in water and soluble in apolar solvents will not necessarily adsorb preferentially on “organophilic“ surfaces. The specific interactions between adsorbate and adsorbent and steric considerations (in addition to the relative solubility of the organic molecule in water and apolar solvents and the organophilic or hydrophilic nature of the surface) will determine the partition between the adsorbed and the solution phase. An outstanding example is the order of adsorption of parathion on attapulgite: HDMA-attapulgite > Ca-attapulgite > TMA-attapulgite. This order is neither directly nor inversely related to the “organophilic” nature of the surfaces.

Parathion and methyl parathion adsorbed at high loads on clays and similar surfaces exhibited properties of a bulk phase, such as a well-defined solubility and a characteristic X-ray powder diffraction (XRD) pattern, different from those of the free substances. The aqueous solubility of parathion condensed at these surfaces was as low as 1.2 μg/ml as compared to 14 μg/ml for free parathion. The BET isotherm described the behavior of the investigated surface condensates even though these condensates were different from the multilayer adsorption for which the BET isotherm was originally developed. The XRD patterns of parathion and methyl parathion adsorbed on Na-bentonite exhibited reflections which were shifted towards higher angles as compared with the XRD patterns of the free compounds. This is in agreement with the lower solubility of the surface condensates. The properties of these condensates were easily controlled by the choice of adsorbent as well as by the choice of conditions under whch they were prepared. The existence of such surface-condensed phases and the manipulation of their properties can be utilized for the slow-release formulation of organophosphate esters which is important, for example, in pesticides application.

The organophosphate insecticides, malathion [S-1,2-di (ethoxycarbonyl)ethyl O, O-dimethyl phosphorodithioate], parathion (O, O-diethyl O-4-nitrophenyl phosphorothioate) and diazinon (O,O-diethyl O-2-isopropyl-6-methylpyrimidin-4-yl phosphorothioate) combined with bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide], caused severe injury to soybean [Glycine max (L.) Merr. ‘Corsoy′] and navy bean (Phaseolus vulgaris L. 'Seafarer′). Postemergence tank – mixture applications of bentazon with organophosphate or carbamate insecticides, or soil-applied organophosphate insecticides prior to bentazon treatments, did not interact with bentazon to injure corn (Zea mays L. ‘Great Lakes Hybrid 4122′). Technical grade malathion interacted with bentazon to the same extent as formulated malathion. Combination treatments of malathion with bentazon resulted in the same degree of injury whether they were applied as tank mixtures or split applications 48 h apart.