Résumé:
The present work focuses on the development of effective, practical and inexpensive physico-chemical processes for the decontamination of water containing Cr (VI) ions.
Adsorption removal was carried out using commercial activated charcoal, prepared ferrihydrite and composites (activated carbon/iron) prepared by precipitation of Fe (III) ions in the presence of coal at mass ratios (1/10) and 1/5). These adsorbents are characterized by DRX, ATR-FTIR, BET analysis, thermal analysis and by
physicochemical analysis. The ion exchange removal is carried out using the Amberlite A21 resin (weak base type) and the Amberlite IRN78 resin (strong base type). The removal efficiency of Cr (VI) ions by precipitation after reduction is evaluated using ascorbic acid and sodium sulfite as reducing agents and soda and lime as precipitants.
In synthetic solutions, macroscopic experiments of Cr (VI) ions removal are carried out in the absence and in the presence of competitives ions in batch and in column (according the case). kinetics, equilibrium and thermodynamics studies are carried out.
The binding of Cr (VI) ions by adsorption is also studied through spectroscopic analysis. In chrome-plating waters, removal of Cr (VI) ions by these methods is evaluated as a function of pH
The synthesized composites are characterized by the predominance of mesopores. Their specific surfaces are similar to that of ferrihydrite. Increasing the amount of iron involves increasing the content of basic functions and the formation of hematite.
The results of macroscopic experiments show that whatever the method used, the maximum elimination of Cr (VI) ions is recorded at acidic pH. The effectiveness of sodium sulfite in reducing Cr (VI) ions decreases rapidly with increasing pH. The increase in temperature implies an increase in the elimination in the case of the resins
and a decrease in the case of the adsorbents. The presence of competitive ions implies a decrease in the optimum pH range of the removal and a slowing down of adsorption and exchange processes. The removal efficiency of Cr (VI) ions by ferrihydrite, composites and resins is strongly influenced by the presence of sulfate ions.
The spectroscopic analysis (ATR-FTIR) suggest that the mechanism of Cr (VI) uptake by ferrihydrite and composites depends on pH and the presence of competitive ions. At pH <6, Cr (VI) ions form inner-sphere surface complexes. Whatever the pH, the interaction of the Cr (VI) ions is essentially electrostatic in the presence of the competitive ions.
In chrome-plating waters, the efficiencies of Cr (VI) removal by the used solids at pH: 3 follow the order: IRN78> CAC> CACFe (1/5)> Ferrihydrite.
