Abstract
In the present thesis, the adsorption οf four herbicides, amitrole, prometryn, alachlor and trifluralin, from their aqueous solutions, upon the polymeric resins Amberlites XAD-4 and XAD-7 was investigated. Both adsorbents possess high surface area and porosity. Particularly, hydrophobic resin XAD-4 is a non polar copolymer of polystyrene - divinylbenzene (porosity 0.35-0.50 ml.pore/ml bead-dry basis, surface area ³700 m2/g dry basis, average pore diameter 50 A-dry basis and mean particle size 40 mesh). Resin XAD-7 is a weak polar polyacrylic ester (porosity ³0.50 ml.pore/ml bead-dry basis, surface area ³400 m2/g dry basis, average pore diameter 300 A-dry basis and mean particle size 40 mesh). The effect of the following factors on adsorption effectiveness was examined: · Chemical structure of herbicides · Chemical structure of adsorbents · pH · Temperature · Ionic strength The experimental procedure included static adsorption experiments of herbicides amitrole, prometryn, alachlor and ...
In the present thesis, the adsorption οf four herbicides, amitrole, prometryn, alachlor and trifluralin, from their aqueous solutions, upon the polymeric resins Amberlites XAD-4 and XAD-7 was investigated. Both adsorbents possess high surface area and porosity. Particularly, hydrophobic resin XAD-4 is a non polar copolymer of polystyrene - divinylbenzene (porosity 0.35-0.50 ml.pore/ml bead-dry basis, surface area ³700 m2/g dry basis, average pore diameter 50 A-dry basis and mean particle size 40 mesh). Resin XAD-7 is a weak polar polyacrylic ester (porosity ³0.50 ml.pore/ml bead-dry basis, surface area ³400 m2/g dry basis, average pore diameter 300 A-dry basis and mean particle size 40 mesh). The effect of the following factors on adsorption effectiveness was examined: · Chemical structure of herbicides · Chemical structure of adsorbents · pH · Temperature · Ionic strength The experimental procedure included static adsorption experiments of herbicides amitrole, prometryn, alachlor and trifluralin at constant temperature for 4 hours. High Performance Liquid Chromatography (HPLC) was applied for the analysis. A great number of adsorption isotherms were conducted based on the experimental data. The efficiency and effectiveness of adsorption for each adsorbent and adsorbate were determined and compared under various conditions of pH, temperature and ionic strength. The adsorption behaviour was explained in accordance with the probable main mechanisms which could be developed. The mechanisms included the following interactions: · resin - herbicide · herbicide - solvent (water) · resin - solvent (water) · herbicides molecules Adsorption isotherms were simulated to Langmuir’s and Freundlich’s models and the parameters of adsorption, such as capacity and intensity, were estimated. In addition, the heat of adsorption was calculated by applying the van’t Hoff law in the case of normal adsorption behaviour with temperature. The experimental heat values were compared to those mentioned in the relevant literature and correlated with the developing interactions. Abnormal adsorption behaviour as a function of temperature (in alachlor case for the two resins and for amitrole adsorption on XAD-7 resin) was observed and discussed. The main conclusions are summarized as following: · More hydrophobic herbicides were adsorbed higher on the more hydrophobic resin (Traube’s rule). · Polar molecules adsorbed higher on the more hydrophilic resin under specific conditions. · The chemical structure of each herbicide - such as number of carbon atoms, type of substituents, (hydrophobicity), water solubility, polarity of groups or elements (Table 1) - determined the basic controlling mechanisms and develop polar interactions, increasing or decreasing the adsorption. · Generally, adsorption increases by pH decrease. · The effect of ionic strength was positive and significant on the effectiveness of adsorption. · Most of the herbicides showed adsorption decrease with temperature increase (normal behaviour). · Abnormal behaviour with temperature was observed for the organic molecules with high water solubility. · Water solubility was proved an important factor of adsorption. · The calculated heats of adsorption values were similar to those mentioned in literature for organic molecules, obtained by the same or other techniques. These values were in the range of 8 - 33 kcal/mol indicating that not only physical sorption but also chemical sorption should occur. The above conclusion was also confirmed by the mean heat values, the evaluation of which was based on the experimental data. · In some cases, a mathematical equation between the parameters lnKo - pH could be derived. Table 1. Chemical structure and physicochemical properties of herbicides Herbicides Structure /Properties Amitrole Prometryn Alachlor Trifluralin Chemical Structure Molecular form C2H4N4 C10H19N5S C14H20ClNO2 C13H16F3N3O4 Solubility in water (20-25 °C) 280g/l (23oC) 33mg/l (25oC) 242mg/l (25oC) 0.184mg/l (pH 5) 0.221mg/l (pH 7) (25oC) pKa (21 °C) 4.2 and 10.7 4.1 - - Mol.wt. 84.1 241.4 269.8 335.3
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