Removal of Residual Aluminium-Dye Complex and Aluminium Ion from Spent Natural-Dye Solution Using Activated Carbons

Abstract

The crude dye extracted from the heartwood of Ceasalpinia sappan Linn. was reacted with alum to form a solution containing a cationic dye complex of [Al(brazilien)₂]⁺ and used for the dyeing of silk yarn. The dye solution left after dyeing was treated with coconut shell activated carbons with different pore structure and surface chemistry to remove the dye complex and with eucalyptus-based activated carbon to remove the residual aluminum ion (Al³⁺). Adsorption kinetics of this dye complex was well described by the pseudo-second order model and the time to reach equilibrium was relatively long, suggesting the intra-particle diffusion to be the controlling step. Isotherm data for dye adsorption of all tested carbons could be well fitted by the well-known Langmuir equation. Activated carbon containing mainly micropores was not effective in adsorbing the relatively large size of dye molecules. However, after oxidizing the carbon surface with HNO₃ to fix the oxygen surface functional groups, the adsorption was enhanced significantly by electrostatic attraction. Porous activated carbon constituting larger proportion of mesopores gave higher adsorption capacity than that of microporous carbon with the dispersive-force attraction being the dominant adsorption mechanism. The adsorption of Al³⁺ by eucalyptus activated carbon followed the linear isotherm equation with Henry's constant of 0.74 L/mg and about 80% removal of the initial Al³⁺ content (8.2 mg/L) could be achieved using a typical carbon dosage of around 6 g carbon/L of dye solution.