Adsorption of diazo dye direct red-28 and tetra-azo dye direct black-22 using calcined kaolin in aqueous solutions

ABSTRACT. In this study, the removal of diazo and tetra-azo dyes using calcined kaolin were compared. Direct red-28 (DR-28) and Direct black-22 (DB-22) was chosen as diazo and tetra-azo dyes, respectively. The kaolin obtained from Turkiye and calcined at 200 °C was found to be the most efficient adsorbent. Natural and calcined kaolins were characterized using SEM-EDS, FTIR, TGA, and XRD. The parameters such as, calcination temperature of the kaolin, adsorbent amount, temperature, initial pH, contact time, and initial concentration were studied. It was found that the removal rate of DR-28 was higher than that of DB-22 at the studied conditions. The removal of DR-28 and DB-22 were found to be 90.1% and 46.2% under the following conditions: adsorbent amount: 1.2 g/200 mL, initial concentration: 20 mg/L, original pH, temperature: 20 °C and contact time 120 min. The adsorption of the dyes was fitted to Langmuir isotherm model. The maximum adsorption capacity of calcined kaolin at 200 °C was determined as 5.39 and 3.5 mg/g for DR-28 and DB-22, respectively.  The adsorption of DR-28 and DB-22 followed a pseudo-second order model. The thermodynamic study showed that the adsorption of DB-22 onto calcined kaolin was endothermic while that of DR-28 was exothermic.   KEY WORDS: Adsorption, Diazo dye, Tetra-azo dye, Kaolin   Bull. Chem. Soc. Ethiop. 2023, 37(3), 593-610.                                                               DOI: https://dx.doi.org/10.4314/bcse.v37i3.5