Kinetics and adsorption isotherm studies of Methylene blue and Direct red 81 onto post-coagulation sludge

Water treatment residuals generated during coagulation/flocculation, called also post-coagulation sludge, were used as adsorbents for Methylene blue (MB) and Direct red 81 (DR 81) adsorption. The adsorbent was characterized by specific surface area, scanning electron microscope, and Fourier-transform infrared spectroscopy measurement. Moreover, to characterize the adsorbent surface, the novelty approach of impedance measurement was applied in the studies. The Brunauer– Emmett–Teller of the sludge was 142.8651 m²/g. Moreover, the impedance studies showed a significant change in the normalized real component of the impedance after the adsorption process. For MB this dye deteriorates the electrical properties of the adsorbent surface, while DR 81 improves them. The batch adsorption experiments were conducted as a function of pH value, contact time, and initial concentration of the dyes. The results showed that the adsorption effectiveness of DR 81 radically decreased at higher pH than 2, while the efficiency of MB adsorption was increasing as the value of pH increased, and the highest uptake of MB was for pH 10. The required contact time for MB was 30 min whereas for DR 81 it was longer and equalled 90 min. The adsorption kinetics followed the Elovich and pseudo-second-order model for both dyes. That indicates the rate-limiting step involves the chemisorption. A much higher adsorption rate was obtained for MB than for DR 81. The equilibrium experiment result showed that the obtained data were well represented by the Jovanovic and Sips isotherm models. The maximum adsorption capacity according to the Jovanovic isotherm model was similar for both dyes and was equalled 871.6 and 823.1 mg/g for DR 81 and MB, respectively. Based on obtained results, it can be concluded that the water treatment residuals can be effectively used as adsorbent for MB and DR 81 removal from aqueous solutions.