Bentonitic clay as adsorbent for the decolourisation of dyehouse effluents

Textile dyehouses generate high volumes of coloured wastewaters, which require convenient treatment in order to avoid water bodies' contamination, adverse effects to aquatic life and risks to human health. The complete decolourisation of such contaminated waters in a cost-effective manner is still a challenge nowadays. This work focuses on the use of a clayey material as a low-cost adsorbent for a basic textile dye (Red 46), commonly used in acrylic fibres dyeing. The bentonitic clay (49% of montmorillonite) presents a considerable (50 cmolc/kg) cationic exchange capacity and a porosity of 10%. Dye adsorption kinetics was successfully described by a pseudo-second order model, resulting an average kinetic constant of 0.23gdyegclay−1min−1. Adsorption equilibrium was reached in 12-24 h and data were successfully fitted to Freundlich and Langmuir equations. The maximum adsorption capacity predicted by the Langmuir model depends on the pH, varying between 217 mg/g at pH 8 and 584 mg/g at pH 9 (25 °C). Increasing the temperature from 25 to 35 °C also led to an increase in adsorbed amounts. In addition to the excellent adsorption performance, the clay showed a great affinity to the dye, indicating high propensity to complete decoulorization of textile dyeing wastewaters. The results obtained in the present work show that a natural, readily-available and cheap material i.e. a bentonitic clay can be used as an effective and environmentally-friendly adsorbent for basic dyes.