الفهرس 
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Abstract
Recently, there have been an increasing interest of trace metals entering into our environment. Many of them are hazardous to human health and to the ecosystem. Indeed, the problem of environmental protection and pollution control has become one of modern man’s preoccupations. Therefore, in the present thesis we present a detailed survey on the essential background information and analytical methodology on the preconcentration, speciation and determination of trace chromium (III &VI) and selenium (IV & VI) species in natural water. The overall work can be summarized as follow. l-In chapter one. a brief review on the recommended methods for the preconcentration, separation and determination of toxic metals is undertaken. Special attention is given to liquid-liquid, liquid-solid separation employing polyurethane foam as a solid sorbent and potentiometric measurements using ion-selective membrane electrodes. The reported literature survey revealed that few studies on chromium (III& VI) and selenium (IV &VI) species employing polyurethane foams and membrane sensor are known. Hence, there is a need for reliable and rapid techniques for minimizing, removal and simultaneous determination of chromium (III&VI) and selenium (IV &VI) species in natural and industrial wastewater. 2- The second chapter is concerned with: i-Developing a simple and accurate extractive spectrophotometric procedures for the chemical speciation of chromium (III) and (VI) species in aqueous media without separation. The method is based upon the extraction of the complex ion-associate formed between the hydrogen chromate and the reagent (C6H5)4P+.Br- or (C6H5)4As+.Cl- at pH ≤ 0 in chloroform followed by direct spectrophotometric measurement at 355 nm. The optimum concentration range evaluated by Ringbom’s plot, the molar absorptivity, the Sandell’s sensitivity and the stiochiometry of the formed complex ion-associates were determined. chromium (III) was also determined by the proposed procedure after prior oxidation to chromate with H2O2 in alkaline solution. The method was also applied successfully for the analysis of chromium (VI) in industrial wastewater. ii- The kinetics of chromium (VI) sorption by the polyurethane foams (PUFs) loaded with the reagent (C6H5)4P+.Br- or (C6H5)4As+.Cl- were found fast, reached equilibrium in few minutes and followed a first-order rate equation with an overall rate constant k of 0.0076 and 0.007 min-1, respectively. The dependence of the chromium (VI) uptake by the reagents loaded foam on pH, salt effect, metal ion concentration, etc., was explained in a manner consistent with a ”solvent extraction” mechanism. The sorption data also followed Langmuir, Freundlich and Dubinin-Radushkevich (D-R) type sorption isotherms. Thus, a dual-mode sorption mechanism involves both absorption related to solvent extraction and an added component for surface adsorption seems a more likely model. The thermodynamic characteristics of chromium (VI) uptake by the reagent loaded foams have been studied. The positive values of ∆H and ∆S may be interpreted as the endothermic of chromium (VI) sorption onto PUFs. The sorption and recovery percentages of chromium (VI) from fresh. natural and industrial wastewater by the proposed loaded foam columns were achieved quantitatively. The height equivalent (HETP). the number (N) of the theoretical plates , the breakthrough capacity and the critical capacity of chromium (VI) uptake onto the loaded foam column were found in the range 0.92-0.97; 129-131; 19-19.8 and 14.8-15 mg/g.