Removal of congo red from aqueous solutions by biogas waste slurry

Removal of Congo Red was carried out using biogas waste slurry as adsorbent at different concentrations of dye, adsorbent dosage, agitation time and pH. The process follows the first‐order rate expression. The equilibrium data fit well in the Freundlich model of adsorption. Maximum removal of dye, 95%, was observed in the pH range 2.3–9.4. Desorption of Congo Red showed that it is solubilised in 50% acetic acid to the extent of 6% and the remainder appears to be chemically complexed irreversibly to the adsorbent. Low desorption of dye from the adsorbent surface in water indicates that the process may not be, essentially, a reversible one.