Investigation of Adsorption Kinetics and Isotherms of Synthetic Dyes on Biochar Derived from Post-Coagulation Sludge

An activated biochar was produced from post-coagulation sludge (also called water treatment residuals or water treatment sludge) in the pyrolysis process at 800 °C in a nitrogen atmosphere and chemical activation using NaOH. The produced adsorption material was characterised by an S_BET surface area of 439 m²/g, a total volume of pores of 0.301 cm³/g, and an average pore size of 1.4 nm. FTIR analysis reveals the presence of primarily C-H, C-O, N-H, C-N, and O-H groups on the activated biochar surface. The batch adsorption process was conducted for three dyes: Acid Red 18, Acid Green 16, and Reactive Blue 81. In the study, the effect of pH, contact time, adsorption kinetics, and adsorption isotherm was determined. The studies showed that, for all dyes, the highest efficiency of the process was achieved at a pH of 2. The results indicate the occurrence of a chemical adsorption process, as evidenced by the best fit to the experimental results obtained with the pseudo-second-order kinetics model and the Elovich model. In the case of the adsorption isotherm, the SIPS model best describes the adsorption for Acid Red 18 and Reactive Blue 81, and the Jovanovic model describes the adsorption of Acid Green 16.