Desorption of adsorbed ametryn and diuron from soils and soil components in relation to rates, mechanisms, and energy of adsorption reactions

Typescript.Thesis (Ph. D.)--University of Hawaii at Manoa, 1976.Bibliography: leaves 162-177.Microfiche.177 leaves illThis study was designed to examine in detail the causes of hysteresis (irreversibility) in the adsorption-desorption process' for two herbicides and its impact under field conditions. An effort was made to relate rate, mechanism and energy of adsorption to this phenomenon. Adsorbents employed in this study were four soils which differed greatly in mineralogy and organic matter content, and two clay minerals, montmorillonite and kaolinite. Adsorption was measured by the batch (slurry) method, supplemented by a flow technique when necessary. Adsorption and desorption equilibrium times for each adsorbate and adsorbent combination were determined and the rate of adsorption was related to hysteresis. Desorption was generally slower than adsorption and both took comparatively longer times in the case of Kaipoioi soil (high in organic matter). Both adsorption and desorption proceeded rapidly during the first few hours and slowed down as the equilibrium time was approached. The faster adsorption equilibrium of ametryn was achieved, the larger was the hysteresis. This kind of relationship was absent in the case of diuron. The magnitude of hysteresis increased with the magnitude of adsorption (as expressed by the Freundlich adsorption constant, Kf) for the various systems. The Freundlich constant, l/n, did not exhibit any correlation with hysteresis. It was, however, observed that the constant increased with temperature and its values were relatively higher in the case of montmorillonite clay and Panoche soil (for which montmorillonite is the dominant clay mineral and organic matter is very low). Adsorption mechanisms appeared" to be related to hysteresis. The hysteresis was larger where ionic attraction was the adsorption mechanism (when ametryn is protonated at low pH) than where mainly physical forces were operative. Diuron exhibited more hysteresis with hydrophobic adsorption (on soils with high organic matter) than with the "water bridge" mechanism (on clay minerals and soils with very low organic matter content). Regardless of adsorption mechanisms, some of the adsorbate molecules were held strongly by the adsorbents, giving rise to hysteresis. Thermodynamic parameters were calculated from equilibrium adsorption measurements and were related to the observed hysteresis. In general, the higher the hysteresis, the larger was, the absolute value of the standard free energy change. The absolute values of standard enthalpy and entropy had a tendency to increase with an increase in magnitude of hysteresis. It can be deduced that the more negative the values of standard free energy change, the greater is the bonding strength, and thus the standard free energy change is related to adsorption mechanism. In the field-watershed portion of this study, the largest diuron concentrations in runoff water were found during the initial storms following herbicide application. After about one year the level of the chemical dropped to a few parts per billion and was almost constant thereafter. The bulk of pesticide was carried by runoff water rather than by suspended soil. The concentration of diuron in runoff was dependent on the concentration of the chemical in the soil. Laboratory adsorption-desorption measurements showed hysteresis in the adsorption-desorption process, which probably attenuated downward movement of the herbicide and contributed to its long persistence in the soil. Recently adsorbed diuron was released to a greater extent than residual diuron. Laboratory measurements assisted in predicting pesticide behavior in the field