2.3. Zeta potential measurement
Zeta potentials of finely ground vermiculite were determined by
using Zeta-Meter System 3.0+ (Zeta Meter Inc., USA). The instrument
automatically measures the electrophoretic mobility of particles
in dilute dispersions and converts the electrophoretic mobility
into zeta potential for aqueous systems. A 1000 mL of 0.1% vermiculite
dispersion was prepared as stock dispersion in the absence or
presence of ions. For each measurement, a 60 mL of dispersions
was sampled from the stock dispersion and transferred into a
100 mL glass beaker. After adjustment of pH, the dispersion was
allowed to stand for 5 min in order to let the larger particles settle
to the bottom of the beaker. 40 mL of aliquot was taken from the
top and transferred into a GT-2 type electrophoresis cell. After placing
the electrodes and positioning the cell by using the ocular micrometer,
the zeta potential measurements were carried out. In
addition to heavy metal ions (Cr3+, Pb2+ and Zn2+), the influence
of Na+ ions on zeta potential was also investigated for comparison.
MP 220 Mettler-Toledo pH meter (Switzerland), coupled with a pH
combination polymer electrode, was used in pH measurements.