Investigation of the adsorption/desorption equilibria of Cd(II), Zn(II) and Cu(II) ions on/from immobilized digested sludge using biosurfactants

The study investigated the adsorption characteristic of Cd(II), Zn(II) and Cu(II) ions and their mixture on activated sludge immobilized in 1.5 % sodium alginate with 0.5 % polyvinyl alcohol (ASAA), and the desorption of these metal ions from biosorbent using biosurfactants (saponin and JBR 515) and HNO3. Cadmium, zinc and copper were most effectively adsorbed by the immobilized sludge in a pH range of 5.0-6.0. Desorption of metals with saponin was the most effective at pH 1-5, with rhamnolipid JBR 515-at pH 5-6 and with HNO3 at pH 2.0. The results of adsorption and desorption were presented with the use of Freundlich, Langmuir, Redlich-Peterson, Sips and a double Langmuir adsorption isotherm equations. The coefficient of determination R-2 and the average relative error were used to evaluate whether the models fit to experimental data. The best fit of the adsorption isotherm equation to experimental data was demonstrated for the double Langmuir isotherm. The highest adsorption capacity from solutions containing single metals was obtained for Cd (108.83 mg/g dm). For solutions containing metal mixture, the highest adsorption capacity (37.21 mg/g dm) was obtained for Cu. The highest desorption of single metals was received with the use of saponin and HNO3 (over 90 %). Biosurfactants were shown to be the most effective in leaching of metals from their mixture. The desorption efficiency of saponin for each metal was over 99 %.