Elimination de l'ion chrome hexavalent dans l'eau par les celluloses greffées Chromium removal in water by modified cellulose

Résumé

L'étude concerne le chrome (VI) et son élimination dans l'eau par les celluloses greffées (échangeuse d'anions) AE₂ sous forme OH¯, Cl¯, S0₄⁼.

La capacité d'échange correspond à 0,5-0,6 meq g^-1 de cellulose. Les cinétiques d'échanges du Cr (VI) avec les ions CL¯ OH¯ et SO₄ ont été déterminées en fonction du pH du milieu, de la température, de ta force ionique, de La quantité de détergents et des formes de celluloses AE₂. Les résultats expérimentaux montrent que les formes de celluloses sont efficaces dans l'ordre OH¯ > Cl¯ > SO₄⁼ vis-à-vis du Cr (VI) et que l'échange décroît pour les ions étudiés dans l'ordre suivant : CrO₄⁼ > Cr₂0₇⁼ > HCrO₄¯.

Mots-clés:

• Cellulose greffées,
• échange d'ions

Abstract

Ion exchange is used extensively for water and waste water treatment for the removal of a variety of ion species. It is known that at acidic pH nome anion-exchange resins can selectively remove and recover chromate from cooling water (SENGUPTA, 1986) where chromate is used as a corrosion inhibitor. This method is employed for plating waste treatment and to recover hexavalent chromium. The waste stream is first passed through a cation exchanger to remove metal ions such as iron, zinc, nickel and trivalent chromium. The hexavalent chromium passes through as CrO₄⁼, and is subsequently removed in a anion exchanger.

Hexavalent chromium may exist in the aqueous phase in different ionic forms with total chromate concentration and pH dictating which particular chromate species will predominate. The equilibrium reactions for different Cr (VI) species are :

H₂CrO₄ ⇌ HCrO^-₄ + H⁺

HCrO^-₄ ⇌ CrO^-₄^- + H⁺

Cr₂O^-₇^- + H₂O ⇌ 2HCrO^-₄

Cr₂O^-7- + H₂O ⇌ 2CrO^-₄^- + 2H⁺

In the present study a description is given of the extraction of small quantities of Cr (VI) ion in water by modified cellulose (anionic form, (AE₂) counterions OH¯, Cl¯, S0₄⁼ ; 0,5-0,6 meq l^-1 ion exchange capacity) which is represented by the following chemical formula : (CHATELIN, 1982 ; WATTIEZ, CHATELIN, 1981).

Cell - O - CH₂ - CH - O - CH₂ - CH -

                          |                         |

                        CH₂                    CH₂

                          |                         |

                         N⁺    CL^-             N⁺    CL^-

                       /  |  \                   /  |  \

             CH₃  CH₃  CH₃         CH₃  CH₃  CH₃

Kinetics of exchange and effects of pH, competiting ions, concentration, temperature, ionic strength, surfactants and types of cellulose (Cl¯, OH¯, SO₄⁼) have been studied.Chromate only, chromate-pH, chromate-chloride, chromate-sulfate and chromate-surfactant kinetics (20 ± 1 °C) were determined for different anionic forms of modified cellulose. Kinetics data were generated by a batch equilibration technique where a weighed amount (5 g) of modified cellulose was shaken gently for 0.5-1 h with a fixed volume of solution (500 ml) containing Cr (VI) with successively H⁺, Cl¯, SO₄⁼, a surfactant of known initial composition. A the end of the equilibration solution, the composition was determined again to calculate the Cr (VI) uptake. Equilibrium was achieved in 0.5 h, but 75 % of Cr (VI) was removed in 5 min.

RESULTSSome specific aspects of the ion-exchange process for chromate recovery from industriel waste have been reported.It was observed that the more efficient types of cellulose for chromium removal where OH¯ > Cl¯ > SO₄⁼ respectively and that ion exchange decreased as follows

CrO₄⁼ > Cr₂0₇⁼ > HCrO₄¯.

Keywords:

• Modified cellulose,
• ionic exchange