centration is 107.3 M) was added to 7 mL of zinc metal ion solution
of pH 3 (H+ concentration will be 103 M), the final H+ ion concentration
and the theoretical pH in the sample will be
approximately 7 104 M and 3.2, respectively. The adsorption
experiments were also carried out at various ionic strengths
(0.7–70.0 mM NaCl) and in the presence of competing cations
(Ca2+, Mg2+ and Fe2+, added as CaCl22H2O, MgCl26H2O and FeCl2-
4H2O, respectively). The zinc and competing ions were added
simultaneously at the beginning of the adsorption experiments.
Simulation by Visual MINTEQ software confirmed that more than
98% of the total initial zinc added was in the Zn2+ speciation in
all the batch adsorption experiments.
As the volumes used in the batch experiments were low, the use
of gravity settling for solid–liquid separation was difficult. The
samples were, therefore, filtered with a 0.45 lm syringe filter (cellulose
acetate, Sigma Aldrich) to be analyzed for the residual zinc
concentration. Control experiments were carried out to discard
the possibility of adsorption of Zn2+ ions to the filter material or
by precipitation (more details in Supporting Information (SI)). All
the experiments were carried out in duplicate. If the difference in
two measurements exceeded 10%, experiments were repeated.
The average values and errors of duplicate measurements are presented
in the figures.