Adsorption of textile dyes onto iron based waterworks sludge from aqueous solution; isotherm, kinetic and thermodynamic study

The use of iron based waterworks sludge as an adsorbent for the removal of cationic (Basic Violet 16), anionic (Direct Blue 71, Acid Blue 40, and Reactive Blue 29) and non-ionic (Disperse Brown 19) was investigated. Initial adsorption studies demonstrated that Direct Blue 71 and Disperse Brown 19 were adsorbed poorly by the sludge. Data obtained from further batch studies were applied to commonly used isotherm models. Among them, Langmuir isotherm model were found to be the best fitted one and based on Langmuir isotherm model adsorption capacities were 625 mg/g for Direct Blue 71, 833.34 mg/g for Acid Blue 40, and 3333.34 mg/g for Basic Violet Blue 16. Kinetic examinations were also performed for all dye tested and it was found that adsorption kinetic was best described by pseudo second-order kinetic model. Gibbs free energy values of the dye-sludge system were calculated and the negative values were found for Direct Blue 71, Acid Blue 40 and Basic Violet 16 indicating the spontaneity of the adsorption process.

► Removal of dyestuffs from aqueous solution with ferric sludge was confirmed.
► The sludge is a low-cost, efficient and rapid adsorbent for the removal of dyestuffs.
► The physical sorption is the basic mechanism for the removal of anionic and cationic dyestuffs.
► Clay, suspended solid and ferric-hydroxide in the sludge provide greater active surface area.