Adsorption of Heavy Metal Ions from Aqueous Solution onto Sepiolite

Abstract

The adsorption capacity of sepiolite towards the removal of heavy metal ions from aqueous solution was studied in this work, with adsorption equilibrium data being obtained with a bath adsorber. The sepiolite sample was chemically stable over the pH range 2–12, but dissolved in acidic solutions with pH values less than 2. The Langmuir isotherm gave a better fit of the experimental adsorption equilibrium data, since the average absolute percentage deviation of the Langmuir isotherm was lower than that of the Freundlich isotherm. The adsorption capacity of sepiolite towards metal ions decreased in the following order: Cr(III) > Cd(II) > Cu(II) > Zn(II) > Ni(II) > Ag(I). Increasing the temperature from 15 °C to 35 °C led to an increase in the adsorption capacity. The capacity of sepiolite towards the adsorption of Cd(II) ions was diminished considerably on reducing the solution pH from 7 to 3. The effect of the solution pH was attributed to electrostatic interaction between the negatively charged sepiolite surface and the positively charged Cd(II) ions in the aqueous solution. The surface charge of sepiolite became less negative when Cd(II) ions were adsorbed onto the sepiolite surface. This result indicates that electrostatic interaction between the sepiolite surface and the Cd(II) ions in the aqueous solution played a very important role in determining the adsorption capacity of sepiolite. The adsorption of Cd(II) ions onto sepiolite at pH 7 was reversible when desorption was undertaken at pH 3, but not when desorption was undertaken at pH 7.