| dc.description.abstract | The presence of pesticide residues in water sources is of great environmental concern due
to the detrimental health problems to human and organisms in the aquatic environment
'-.
associated with pesticide toxicities. Among the widely used pesticides are tebuconazole
and clomazone, used in cultivation of numerous crops. Tebuconazole and clomazone
have been detected in rivers, streams and lakes at levels exceeding the set safe allowable
limits for drinking water. Conventional water treatment technologies that use activated
carbon singly have been demonstrated to be insufficient in pesticide removal from
wastewater. There is therefore need to evaluate other adsorbents for pesticide remediation
from water. Synthetic sodium-type faujasite X (NaX) has been reported to have the
capacity to abstract pesticides residues from water. However, the potential of NaX to
adsorb tebuconazole and clomazone has not been documented. The objective of this study
was to investigate the adsorption behavior of tebuconazole and clomazone pesticides
from aqueous solution onto NaX. The effects of pesticide concentration (100 - 1000 ug L-
'), contact time (15 - 390 min) and temperature (293 - 323 K) on the adsorption process
were investigated using batch shaking sorption experiments using HPLC-DAD and the
site of adsorption identified by FT-IR and XRD techniques. Removal efficiency varied
significantly (p<0.05) with the initial pesticide concentration for both pesticides.
Langmuir and Freundlich models gave the best fit for the experimental data for both
pesticides. The maximum adsorption capacity oftebuconazole onto NaX, calculated from
Langmuir isotherm, was 500 ug g-i. The adsorption reaction was found to follow pseudosecond
order kinetics for both pesticides. The sorption half-life was evaluated and the
adsorption rate of tebuconazole (0.0634 h) was faster than that of clomazone (0.1177 h).
The calculated thermodynamic parameters indicate that tebuconazole adsorption on NaX
is spontaneous (~G=-180.7 J mole") and exothermic (~H=-38.7 kJ mole") while
clomazone adsorption was non-spontaneous (~G=5719.6 J mole") and exothermic (~H=-
44.9 kJ mole") under the examined conditions. Intraparticle diffusion model showed that
the adsorption oftebuconazole and clomazone onto NaX was multi-mechanistic and was
not controlled by pore diffusion. FT-IR and XRD analysis indicated that adsorption of
tebuconazole and clomazone onto NaX is an external surface chemisorption process at the
single-four-rings (S4R) of the zeolite framework with involvement of the Na+ cations.
The results show that NaX is a better adsorbent for tebuconazole removal than clomazone
and is valuable for wastewater treatment plants for tebuconazole remediation. | en_US |
