Interactions of pendimethalin with organo-montmorillonite complexes

Pendimethalin (PM) is a dinitroaniline herbicide, highly hydrophobic and with a very low solubility in water. It is used for pre-emergence weed control, is usually applied before sowing, and mechanically incorporated into the soil. We tested sorption of PM on montmorillonite and on two different organo-montmorillonites in a mixed water–chloroform environment, to determine the feasibility of developing an environmentally oriented formulation of the herbicide. FTIR measurements show that the interactions of PM with montmorillonite on which difenzoquat (DZ) or mepiquat (MQ) were previously adsorbed are via the nitro and methyl groups of the herbicide. The pre-adsorbed organocations cause a dehydration of the interlayer space, leading to hydrophobicity of the organoclay. Changes in the electrokinetic surface charge of the organoclay, measured using a particle charge detector, confirm these results. The high affinity to PM and the hydrophobic behavior of the used organoclays were also demonstrated in experiments using a mixed chloroform–water environment. The crude clay mineral stayed in the water phase, whereas DZ- and MQ-montmorillonites concentrated in the PM-chloroform phase. Amounts of PM adsorbed to the organoclays using this method were very high (up to 0.65 mol PM kg⁻¹ compound), which suggests a very efficient technology of preparation of herbicide-clay formulations, compared with techniques used in the past for similar herbicide-organoclay compounds. Plant experiments using the organoclay-herbicide formulation with higher amount of active ingredient (based on DZ-clay) demonstrate efficient herbicidal activity with 30% less active ingredient.     

Preparation of Berberine-montmorillonite-metolachlor formulations from hydrophobic/hydrophilic mixtures

Metolachlor which is a pre-emergent hydrophobic herbicide with moderate solubility, was detected in several water wells in agricultural and even urban areas. This study aims for the preparation of organoclay platforms which could release metolachlor in a more controlled pattern, thus — reducing the leaching to groundwater. The adsorption of metolachlor to organoclays based on difenzoquat, diquat or berberine added up to neutralizing the original charge of the clay mineral was tested. The adsorption loadings of metolachlor on berberine-montmorillonite were found to be more than 30% of the compound, whereas for difenzoquat- and diquat-montmorillonite the amounts were only 3.8% and 4.9%, respectively. Consequently, further experiments were conducted focusing on berberine-montmorillonite. Loading of preadsorbed berberine was crucial to the hydrophobicity of the platforms and it had also influence on the adsorption of metolachlor in mixed immiscible phases experiments. The exact procedure of preparation was also a key issue on determining final metolachlor amounts on the formulation: Preparation of the formulation from suspended organoclay which was not dried after preadsorbing the berberine, and was mixed with metolachlor dissolved in chloroform gave the highest percentages of adsorbed herbicide. Release from organoclay formulation was at a more controlled fashion than for commercial herbicide, or commercial herbicide mixed with non-modified montmorillonite, leaving a relatively constant concentration of herbicide after several desorption cycles.     

The lack of true cohesion in hydrate-bearing sands

The mechanical behavior of hydrate-bearing sands is affected by the hydrate quantity, hydrate morphology in the pores, soil skeleton characteristics, stress confinement and more. It has been traditionally assumed that bonding exists between hydrate and sand particles, which affects the global sediment strength. However, this paper shows, extending previous work, that the mechanical effect of hydrate in the sediment has kinematic rather than cohesive nature, based on comparison of mechanical and visual evidences with cemented sand. The visual analysis includes comparison between micro-scale X-ray images of sand containing either hydrate or cement agent. The mechanical analysis includes examining drained triaxial test results of hydrate-bearing sands with cemented sand results, using investigation through stress-dilatancy theories. The paper concludes that all mentioned hydrate-related effects should be interpreted by their influence on the sediment kinematics, rather than on strength characteristics. For a given kinematic response of hydrate-bearing sand, it is shown that the full mechanical behavior can be described using a single friction parameter.     

A new method to estimate adsorption energies between cations and soil particles via wien effect measurements in dilute suspensions and an approximate conductivity-activity analogy

Measurement of the Wien effect (increased electrical conductivity at strong electrical fields) in soil suspensions is proposed as the basis of a new method to characterize energy relationships between cations and soil particles. The simplified theory behind the method, the working principle of the short high-voltage pulse apparatus, and the measuring procedure are outlined briefly. The new method was used to evaluate the adsorption energies of two monovalent (Na, K) and two divalent (Ca, Cd) cations on yellow-brown soil and black soil particles, assuming an analogy between the activity of the cations and their contribution to the electrical conductivity of the suspension. Both the mean free bonding energies, deltaGbo, and the adsorption energies, deltaGad, of the cations for these two soils increased in the order: Na < K < Ca < Cd. Under the conditions of our experiments, estimated deltaGbo ranged from 4.7 to 6.4 kJ mol(-1) and from 7.0 to 8.2 kJ mol(-1) for mono- and divalent cations, respectively. The bonding energies obtained with the new method were similar to those determined previously by ionic activity measurement. The determined mean adsorption energies of cations desorbed at a field strength of 100 kV cm(-1), for example, ranged from 0.7 to 1.2 kJ mol(-1) and from 1.9 to 2.3 kJ mol(-1) for mono- and divalent cations, respectively.     

Adsorption of difenzoquat on montmorillonite: model calculations and increase in hydrophobicity

The adsorption of difenzoquat (DZ) on montmorillonite was studied at a wide range of concentrations and ionic strengths. Up to difenzoquat loadings of 0.4 mmol/g clay, all the added cation were adsorbed. Maximal adsorbed amounts exceeded slightly the cation exchange capacity (CEC) of the clay (0.8 mol_c/kg). The adsorbed amounts did not change upon increasing the concentration of NaCl in the medium to 500 mM. An adsorption model that combines electrostatic equations with specific binding in a closed system could adequately predict the adsorbed amounts of DZ, even at high ionic strength. Simultaneous adsorption of the divalent cationic herbicide diquat (DQ) and DZ was also determined and the predictions of the model were adequate for total loadings up to the CEC of the clay. At higher loads the model adequately predicts the DZ adsorbed, but underestimates the amounts of DQ adsorbed. The influence of adsorbed DZ on the hydrophobicity of montmorillonite was tested by using the hydrophobic herbicide pendimethalin (PM). The adsorption isotherm of PM on crude montmorillonite is of the S type, indicating very low adsorption at low added amounts, and increasing affinity after part of the surface is covered with the hydrophobic molecules. Adsorption of PM on montmorillonite saturated with DZ up to 80% of the CEC showed a C behavior, indicating a partition mechanism between the solvent and the adsorbent even at low added amounts. The enhanced hydrophobicity of DZ–montmorillonite was also demonstrated in qualitative experiments in a mixed chloroform–water environment: whereas the crude clay mineral stayed in the water phase, DZ–montmorillonite concentrated in the chloroform phase.