Elimination de colorants de l'industrie textile par des sciures de bois Dyes removal from textile effluents by wood sawdusts

Résumé

L'élimination de la couleur de rejets de petites industries textiles peut être réalisée à l'aide de sciures d'essences de bois diverses dont le choix est fait après étude de leur relargage dans l'eau (couleur et DCO); les sciures de chêne, de châtaignier, de pin Douglas conduisent à des relargages de substances fortement colorées correspondant à des DCO d'environ 60 mg d'oxygène par gramme de sciures sèches et ceci en réacteur discontinu. Par contre, les sciures de hêtre, bouleau, sapin et peuplier relarguent nettement moins de produits colorés ou non et conduisent à des DCO plus faibles, excepté pour le bouleau (30 mg d'O₂.g^-1).

Le pH de l'eau est également modifié par addition de sciures et l'on obtient des valeurs de pH d'équilibre atteint en 10 min., de 4,3; 5; 5,8 et 7 pour le bouleau, le sapin, le hêtre et le peuplier respectivement.

L'élimination de colorants à 25 mg.l^-1 dans l'eau par des doses de sciures de 50 g.^-1 pour 2 h de temps de contact, montre qu'une meilleure décoloration (98 à 100 %) est obtenue pour le colorant cationique Basic Red 22 avec les quatre sciures sélectionnées, qu'avec les colorants anioniques (ou acides) dont les valeurs de rétention, variables selon les sciures (de 4 à 8 %) peuvent être très bonnes également.

Les isothermes d'adsorption permettent de déterminer les capacités d'adsorption à saturation; à titre d'exemple, des valeurs de 0,1 à 0,2 mmol.g^-1 sont obtenues sur des sciures de peuplier et de hêtre pour le colorant BR 22 et de 4 à 8.10- mmol.g^-1 pour le colorant anionique Acid Blue 25, sur ces mêmes sciures. Contrairement à d'autres matériaux, on observe que les résultats obtenus ne dépendent que très peu de la température (entre 15 et 35 °C).

Une étude dynamique sur colonne confirme les écarts pour les deux types de colorants, mais les résultats dépendent de la vitesse de passage : des pourcentages d'élimination de 0,4 à 1 % en masse peuvent être obtenus pour l'AB 25 le BR 22 sur la sciure de hêtre. Il faut cependant signaler que les produits minéraux et organiques contenus dans les effluents colorés industriels diminuent l'efficacité des sciures par rapport à leur efficacité sur des solutions de colorants purs.

Mots-clés:

• Sciures de bois,
• colorants textiles,
• adsorption

Abstract

Anticipated water pollution regulations require textile dye industries to reduce substantially the amount of colour in their effluents. One possible method of colour removal is through adsorption techniques. The most commonly used adsorbent for treatment of textile effluents is activated carbon. The capability of sawdust for removing colour was recognised some time ago (POOTS et al. 1978; ABO-ELELA and EL-DIB, 1987; ASFOUR et al., 1985) as were those of peat and of charred woollen fiber treatments (PERINEAU et al., 1983). Sawdust has recently received more attention (ASFOUR et al., 1985) owing to its economic advantages when the sawmills are near the textile mills. In order to measure the real efficacy of some types of sawdust, evaluation of the released coloured products and their influence on the effluent water pH are never described. The aim of the present work is to study and evaluate the behaviour of types of sawdust from Limousin woods (released COD and pH of solutions) and the discolouring power of the selected sawdust (beech, poplar, birch and fir trees) with respect to textile dyes in dilute solutions and industrial effluents.

MATERIALS AND METHODS

Experiments were conducted using sawdust from beech, chestnut, poplar, oak, birch, fir and Douglas pine, in doses of 50 g.l^-1 of water. The industrial sawdust particle site was between 0.5 - 1.5 mm.

The dyestuffs used in all the experiments are reported in table 1. The initial concentration of colouring matter was 25 mg.l^-1 and determined spectrophotometrically at maximum absorbance wavelength. In a batch system the time required for equilibrium was 2 hours (or less).

RESULTS AND DISCUSSION

Kinetic rejections of the sawdust are shown in figure 1; four types of sawdust are suitable : beech, poplar, birch, and fir were selected for further experiments. The absorbance depends on the wavelength and becomes maximum near UV range (as shown in fig. 2 and 3). The COD of the released products is particularly important for chestnut, oak, Douglas pine (60 mg.O2.g^-1; table 2) and also for birch (31 mg.O2.g^-1). These data have never been mentioned in published works where the wood was not submitted to any form of pre-treatment.

Sawdust contains some acidic or basic groups that modify the pH of water (fig. 6, 7) which becomes more acidic; the phenomenon can be used to neutralize textile fixing effluents loaded with carbonate ions (pH : 10). Batch results (table 3) indicate that the tour selected types of sawdust give good colour removal for the 8 dyes; particularly, the cationic dye : Basic Red 22 is discoloured with very good yield (96 to 99.5 %) on beech and on birch sawdust.

A variation of temperature between 15 and 35 °C does not change significantly the adsorption results and at pH = 2, the release decreases (table 2) and the adsorption increases (table 4).

Equilibrium conditions of adsorption of basic and acidic dyes on four types of dust were studied using the Langmuir equation; this equation was also used for the determination of the « ultimate capacity » Г_∞ and the equilibrium constant K (STUMM and MORGAN, 1981). The resulting Г_∞ and K for some dyes (AB 25, BR 22, NLB) on fir, beech, poplar, and birch sawdust are given in table 5. The best results were obtained for the cationic dye BR 22 which showed an ultimate capacity of 0.210 and 0.06 mmol.g^-1 on beech and on birch sawdust respectively. [The result for other dyes is in the order of 0.005 mmol.g^-1]. The results obtained by extrapolation of the linearized Langmuir equation are somewhat different from those obtained by the experimental saturation curve (fig. 8) for BR 22 on polar sawdust. For AB 25, the saturation curve shows a great increase of the adsorption capacity for concentrated solutions (2 - 2.25 g.l^-1), this is probably due to a micellary process as shown on figure 9 : the critical micellary concentration is between 1.5 and 3.3 g.1^-1.

DYNAMIC STUDY

Flow tests using small columns (3 - 7 cm of height) are used with beech sawdust for determining the breakthrough curves (rate : 1.03 m.h^-1). The concentrations of AB 25, BR 22 and their mixture are 25 mg.l^-1 (close to industrial waste-water concentration). The adsorption capacity deduced from the « Bed Depth Service Time » model is 0.4 % and 0.8 % for each dye respectively.

Glass columns (4 cm diameter) containing 20 g or 30 g of beech sawdust were also used. The values of adsorption for a rate of 3.1 m.h^-1 are 0.45 mg.g^-1 (0.04 %) for AB 25 and 13 mg.g^-1 (1.3 %) for BR 22. Approximately 650 litres of this dye can be treated by this sawdust (fig. 10 and 11).

Industrial effluents (unknown composition) are less discoloured, probably due to the presence of many other compounds.

Batch experiments adsorption of dyes on wood sawdust can lead to different measurements of the efficiency of the sawdust depending on rejection by the adsorbent of chemical species in the bulk solution. These substances modify the pH, the adsorbance of the solution and the amount of adsorbed dyes by competitive reaction.

Keywords:

• Wood sawdust,
• dyestuff,
• adsorption