Adsorption experiments
Adsorption experiments were obtained with a batch equilibration
procedure using 15-mL polypropylene centrifuge tubes covered
with polypropylene caps immersed in a thermostatic shaker
bath. Before starting the experiment, a stock PQ2+ solution
(1 103 M) was prepared by adding the corresponding solid to
pH buffer solutions. The pHs used in these studies were 4.4, 7.0
and 9.5 by using 0.1 M acetate/acetic acid, HPO2
4 =H2PO1
4 and
CO2
3 =HCO1
3 buffer solutions, respectively. Fifty milligrams of
adsorbent was introduced into the tubes and mixed with varying
quantities of PQ2+ and KCl (used as supporting electrolyte) solutions.
The range of initial PQ2+ concentration was 4–500 lM, and
the final volume was 8 mL. The mixture was stirred for 4 h at
90 rpm and centrifuged to separate the phases. An aliquot of the
supernatant was analyzed to quantify the PQ2+ remaining in the
supernatant and to calculate the adsorbed amount of PQ2+. In most
experiments, no supporting electrolyte was used and the working
temperature was 25 C (except when effects of ionic strength and
temperature were investigated). For the batch kinetic studies, the
initial PQ2+ concentration was 5 104 M. The mixtures were shaken
for different reaction times in order to achieve complete
adsorption or to gather enough data points.
Quantification of PQ2+ was performed by UV–vis spectroscopy
at 258 nm using an Agilent 8453 UV–vis diode array spectrophotometer
equipped with a Hellma 1-cm quartz cell. The supernatant
of the withdrawn aliquot was placed into the cell and the spectrum
was recorded in the 200–900 nm wavelength range. Calibration
curves at the working pH were also constructed with several
PQ2+ solutions having concentration that ranged between 2 and
150 lM. Very good linearity was found in all cases.
The adsorption isotherms were fitted using the Freundlich
equation, which was commonly used in the adsorption of herbicides
on several adsorbent systems [23,24]. The linear form of this
equation is displayed as follow:
ln PQ2þ