Adsorption isotherms for describing heavy-metal retention in paper mill sludges

In this work the adsorption characteristics of paper mill sludge are evaluated for the purpose of removing metal ions from polluted waters. The Ag(I), Cd(II), Cu(II), Pb(II), and Cr(VI) (as chromate ion) adsorption mechanism on the sludge is studied by adsorption isotherms in batch conditions, after paper mill sludge characterization. The effect of several parameters (ionic charge of metals, pH, and ionic strength of metal solutions) and the competition between different metals are also considered. For Ag(l), Cd(II), Cu(II), Pb(II), and Cr(VI), the thermodynamic curves fit the Langmuir model best, whereas experimental data for Cu(II) fit the Freundlich model best. The nature of ionic charges influences the sorption process: cations are sorbed much more than chromate ion, and the affinity series for cations is Cu(II) > Pb(II) > Cd(II) congruent to Ag(I). The maximum amount of sorbed metal (q(max)) value is significantly influenced both by the liquid/solid ratio and by the pH values of metal solutions.