Anionic reactive dye removal from aqueous solution using a new adsorbent—Sludge generated in removal of heavy metal by electrocoagulation

The present work investigates the potential of electrocoagulated metals hydroxide sludge (EMHS) generated during removal of Cr3+ using Al electrode for adsorption of Congo Red (CR) from aqueous solution. The effects of contact time, stirrer speed (to make the external mass transfer effect negligible), pH of the solution, initial concentration of adsorbate and adsorbent dose on dye removal have been investigated. The adsorption is highly pH dependent due to formation of various charged hydroxylated species [Al(OH)2+, Cr(OH)2+, etc.] of EMHS and as resonance of CR occurs at alkaline pH through involvement of free lone pair of electrons. Comparative COD reduction with diminution of color (measured spectrophotometrically) confirms successful removal of the adsorbate. Preferable fitting of Langmuir isotherm over Freundlich isotherm suggests monolayer coverage of adsorbate at the surface of adsorbent. The maximum adsorption capacity (qm) increases from 271 to 513 mg/g when the initial pH is adjusted to 3.0 instead of 10.4. Comparison with literature reported values of qm and n (Freundlich constant) with either CR as adsorbate or metal hydroxide adsorbent, establishes EMHS as an attractive adsorbent. In the pH range of 3–10, concentration of Cr3+ in the leachate remains below the discharge concentration of the same. FT-IR spectrum of dye-loaded sludge suggests that the dye removal is due to chemical interaction of CR and EMHS. The SEM image before and after the adsorption indicates that, CR was adsorbed at the surface of the adsorbent.