Dye abatement in textile industry wastewater with bentonite and aluminum salts
The textile industry uses synthetic dyes, most of them being toxic. In Algeria, the agricultural reuse of treated wastewater, even of industrial origin, is becoming commonplace. It is therefore compulsory to drastically reduce pollutant fluxes. The presently operated conventional processes cannot meet the water quality requirements: bioelimination of dyes is negligible and flocculation with iron salts, as currently carried out in the SOITEX plant located in Tlemcen, Algeria, is not effective enough. The use of aluminum salts in the flocculation of such wastewaters is well known (FIESSINGER AND BERSILLON, 1977; LAHAV et al., 1978) but the resulting microflocs are not easily settleable. Bentonite, locally available at a low cost, can also eliminate micropollutants (LAHAV et al., 1978). Associated with polyhydroxyaluminum, it can reduce such compounds as benzene or toluene, favoring simultaneously the liquid-solids separation. This paper evaluates the treatability of dyes by bentonite associated with aluminum salts. All the runs were carried out in a 200 cm3 batch reactor, mechanically stirred and thermoregulated at 20·C. The main physico-chemical characteristics of the bentonite are given in Table 1. The flocculant was aluminum chloride, previously neutralized with sodium hydroxide (mass ratio OH-/Al=1.85). The solutions were used immediately or left to polymerize during 6 days leading to polyhydroxyaluminum PHAl (LAHAV et al., 1978). When the reactor was operated with bentonite and aluminum, the mass ratio Al/bentonite was maintained at 53.10-3 (KACHA, 1994). Four dyes belonging to two main families were tested: Supranol Yellow 4GL and Nylomine Green (acid dyes) and Foron Red RDGL and Foron Violet S3RL (dispersive dyes). Their concentrations were obtained by spectrophotometry. Bentonite alone does not induce a significant abatement excepted for low pH values around 4 (Figs. 1 and 2). Dye elimination appears to require a previous protonation step followed by cation exchange. The equilibrium can be modeled by a Freundlich equation (Fig. 3 and Table 2). The dyes can also be eliminated by aluminum salts alone (Fig. 4). The efficiency is then better with polyhydroxyaluminum, i.e. more than 90 % of the initial concentration is removed. Nevertheless, the dyes abatement probably results from an adsorption or chemical reaction on microflocs which are not easily settleable. By assuming that all the aluminum ions are precipitated as aluminum hydroxide, the equilibrium is modeled by the Langmuir equation which would indicate a monolayer adsorption (Fig. 5). When the reactor is operated with bentonite and aluminum salts, dye abatement is nearly complete and the liquid-solids separation is particularly efficient (Figs. 6 and 7). The best results are obtained with PHAl but the use of the monomer can be sufficient. The required concentrations are relatively low and the process is then economically feasible (Table 3). However, the experimental data can no longer be modeled by the Freundlich equation nor by the Langmuir equation. When the aluminum salts react alone with the dyes, the conductance displayed against the aluminum concentration shows two straight lines of different slopes (Fig. 8). The abscissa of the points where the slopes change are proportional to the initial dye concentration, suggesting a chemical reaction between the dye and the aluminum salts (Fig. 9). However, the final pH value lies at the limit value of aluminum hydroxide precipitation; an adsorption on aluminum hydroxide or an aluminum salt precipitation cannot then be assumed. In presence of bentonite, such changes of slope are not observed and, moreover, the final pH value does not correspond either to a precipitation value (Figs. 11 and 12). At this stage, a comprehensive mechanism cannot thus be proposed. However, the process using bentonite/PHAl is particularly efficient and easy to operate (Fig. 13 and Table 3). The results were confirmed with a true industrial effluent, the Chemical Oxygen Demand (COD) of which was reduced from 770 mg/l to less than 30 mg/l (Fig. 14). As a matter of comparison, the actual process, which includes an activated sludge treatment followed by an iron sulfate/lime flocculation, leads to an effluent containing only 140 mgCOD/l.