Removal and Recovery of the Hazardous Azo Dye Acid Orange 7 through Adsorption over Waste Materials: Bottom Ash and De-Oiled Soya

An azo dye, Acid Orange 7 [p-(2-hydroxy-1 naphthylazo)benzene sulfonic acid] was removed by adsorption over two waste materials, namely, bottom ash, a power plant waste, and de-oiled soya, byproduct obtained during the processing of soybean in soya oil extraction mills. Both waste materials showed excellent adsorption abilities and can be treated as low-cost adsorbents. The adsorbents were characterized through IR spectroscopy and differential thermal analysis (DTA), and preliminary investigations were carried out by batch adsorption to examine the effects of pH, adsorbate concentration, sieve size, adsorbent dosage, contact time, and temperature. A plausible mechanism for the ongoing adsorption process and thermodynamic parameters were also obtained from Langmuir and Freundlich adsorption isotherm models. The kinetic measurements helped in determining the specific rate constant, confirming the applicability of the first-order rate expression. To identify whether the ongoing process is particle diffusion or film diffusion, the treatments given by Boyd et al. (Boyd, G. E.; Adamson, A. W.; Meyers, L. S. J. Am. Chem. Soc. 1947, 69, 2836) and Reichenberg (Reichenberg, D. J. Am. Chem. Soc. 1953, 75, 589) were employed. To assess the practical utility of the adsorbents, a fixed-bed column was designed, and necessary parameters were calculated by applying a mass-transfer kinetic approach. Experiments were also performed for the recovery of loaded dye through chemical regeneration of spent columns, and an estimate of the operating costs was also calculated.