Adsorptive Removal of Direct Azo Dyes from Textile Wastewaters Using Weakly Basic Anion Exchange Resin

Direct dyes are still widely used for coloring a variety of materials due to their ease of use and the wide range of colors available at a moderate cost of production. In the aquatic environment, some direct dyes, especially the azo type and their biotransformation products, are toxic, carcinogenic and mutagenic. Hence the need for their careful removal from industrial effluents. It was proposed adsorptive retention of C.I. Direct Red 23 (DR23), C.I. Direct Orange 26 (DO26) and C.I. Direct Black 22 (DB22) from effluents using anion exchange resin of tertiary amine functionalities Amberlyst A21 (A21). Applying the Langmuir isotherm model, the monolayer capacities were calculated as 285.6 mg/g for DO26 and 271.1 mg/g for DO23. The Freundlich isotherm model seems to be the better one for the description of DB22 uptake by A21, and the isotherm constant was found to be 0.609 mg(1-1/n) L-1/n/g. The kinetic parameters revealed that the pseudo-second-order model could be used for the description of experimental data rather than the pseudo-first-order model or intraparticle diffusion model. The dye adsorption decreased in the presence of anionic and non-ionic surfactants, while their uptake was enhanced in the presence of Na2SO4 and Na2CO3. Regeneration of the A21 resin was difficult; a slight increase in its efficiency was observed using 1M HCl, 1 M NaOH and 1 M NaCl solutions in 50% v/v methanol.

Automated summary

Direct dyes are widely used for coloring various materials, but some of them, especially azo type dyes and their biotransformation products, exhibit toxicity, carcinogenicity, and mutagenicity in aquatic environment, thus their careful removal from industrial effluents is necessary. In this study, anion exchange resin Amberlyst A21 was proposed for the adsorptive retention of three direct dyes from effluents. The adsorption capacity of DO26 and DO23 was found to be 285.6 mg/g and 271.1 mg/g, respectively, according to the Langmuir isotherm model. The Freundlich isotherm model was better for describing the uptake of DB22, with an isotherm constant of 0.609 mg(1-1/n) L-1/n/g. The pseudo-second-order kinetic model was more suitable for describing the experimental data. The presence of anionic and non-ionic surfactants decreased dye adsorption, while Na2SO4 and Na2CO3 enhanced dye uptake. Regeneration of the A21 resin was challenging, but a slight increase in efficiency was observed using 1M HCl, 1 M NaOH, and 1 M NaCl solutions in 50% v/v methanol.