A novel technique using on-line microfiltration and HPLC analysis has been used to study sorption kinetics and sorption equilibrium of the herbicide atrazine with the clay minerals montmorillonite, kaolinite, and illite and the clay fraction of a soil. Fast and slow labile sorption have been observed for atrazine along with a reversible but kinetically slow sorption/desorption process that is consistent with diffusion of pesticide into the interior of the clay particles. Labile sorption capacity, mole fraction site coverage, labile sorption equilibrium function, and distribution coefficient were determined for the clay minerals in aqueous slurries with atrazine. The identification and quantitative descriptions of key chemical species avoid some of the commonly reported hysteresis problems. Distribution coefficients are derived from the equilibrium constants so that comparisons may be made with the results of other workers.
The effects of addition of ascorbic and oxalic acids on the atomic absorption signal for lead are explained by a gas-phase thermodynamic equilibrium model. The gas-phase composition of the graphite furnace was determined as samples containing ascorbic or oxalic acid were pyrolyzed.Hydrogen and carbon monoxide were identified as two of the major pyrolysis products. The amounts of these reducing gases, formed in the atomization cycle, varied with the modifier used, the charring temperature employed, and the condition of the surface of the pyrolytically coated graphite tube. The extent of the shift in the appearance temperature showed correlation with variations in concentrations of hydrogen and carbon monoxide produced by pyrolysis. Results for oxalic acid clearly showed that the appearance temperature shifts for lead in pyrolytically coated tubes cannot be explained by a surface reaction mechanism.
A thermodynamic (gas phase) equilibrium model of atomisation has been used t o explain the shift of the appearance temperature of the Pb atomic absorption signal to a lower value when ascorbic acid is used as a matrix modifier. Experimental results of the effect of gas-phase composition on the appearance temperature of the Pb atomic absorption signal are presented. The partial pressure of H2 was determined at various temperatures during the ashing and the atomisation cycle. The partial pressure of CO, COz and CH4 were found to be very low for the tube wall heating rate of 1.7 K ms-1; CO was found to be present in appreciable amounts for the tube wall heating rate of 1.5 K ms-1. The results are consistent with the hypothesis that H2 and CO released by the pryolysis of ascorbic acid decrease the partial pressure of O2 in the furnace and thereby cause the equilibrium position of the reaction PbO(,, Pb(,, + 4 O2 to shift to the right, favouring production of more Pb(,,, which results in a lowering of the appearance temperature of the Pb atomic absorption signal in agreement with the equilibrium model.
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