Download photooxidative degradation of phenyl

PHOTOOXIDATIVE DEGRADATION OF PHENYL-UREA HERBICIDE
IN AQUEOUS CATALYST SUSPENSION OF TITANIUM DIOXIDE
Laoufi N. A., S., Bentahar F.
Laboratoire des Phénomènes de Transfert. Département de Génie Chimique et de Cryogénie.
U.S.T.H.B. Faculté de Génie Mécanique et de Génie des Procédés Bp 32, El Alia, 16111 Bab
Ezzouar. Algiers. Algeria
[email protected], [email protected]
Abstract
Chlortoluron, a phenyl-urea herbicide, is used in large quantities as herbicide in Algeria. Because
of its higher toxicity, this product is an urgent problem for which remedial measures need to be
found. In spite of its high herbicidal activity, the use of this urea herbicide is no without danger
due to its fairly high water solubility, and becomes as persistent water pollutants. This herbicide
being a biorecalcitrant compound, its destruction needs a modern technique as a photocatalytic
oxidation process (an advanced oxidation process). This method based on photocatalysis using
TiO2 is particularly attractive for removing organic pollutants from water because it can destroy
toxic and hazardous contaminants, not simply transfer them to another phase. The TiO2/UV
process has many important advantages, and, the end products of this process are carbon dioxide,
water and inorganic mineral salts. In this process, titanium dioxide is the most widely used
photocatalyst for organic photodestruction due to its high catalytic activity, stability in acidic and
basic media and nontoxicity and has been demonstrated to be active in the presence of sunlight.
The photocatalytic degradation of chlortoluron has been studied in a helical reactor. The titanium
dioxide was exposed to UV radiation to generate reactive OH· radicals which induce the
herbicide destruction. The photodegradation of chlorotoluron studied in various operating
conditions with an irradiation lamp emitting at 365 nm (18 Watts) was reported. The effects of
the recirculation flow, initial chlortoluron concentration, amount of catalyst, suspension pH and
the amount of Ca2+ on the chlortoluron photodegradation were investigated. These parameters
were studied to find the optimal conditions for a complete oxidation of this herbicide.
Kinetic experiments were performed at 23 ºC over a range of herbicide concentrations from 10 to
100 mg L-1; a range of TiO2 concentrations from 0,2 to 1,4 g L-1; a range of flowrate recirculation
from 0,074 to 0,36 L min-1; a range of suspension pH from 3 to 11; and a range of Ca2+
concentration from 0,625 to 5 mg L-1. Results showed that the efficiency of the photocatalytic
process depends strongly on the experimental conditions. All studied parameters play a
significant role in the degradation process, thus, the chlortoluron degradation was more effective
with a TiO2 concentration of 0,9 g L-1, a flowrate recirculation of 0,23 L min-1, a free pH, in the
absence of Ca2+ ions and a long enough irradiation time. The rate constants for the different
parameters were evaluated. Kinetic studies showed that titanium dioxide photocatalyst P25 was
very active in chlortoluron degradation; we observed that 70 % of pollutant was degraded after 7
hours of UV irradiation; furthermore, we observed that chlortoluron degradation was more
effective under acidic conditions (near pH = 6) and free pH. The kinetics were described by the
Langmuir-Hinshelwood (L-H) kinetic model. The pseudo-first order kinetic constant values
obtained in acidic pH were higher than those calculated for basic pH media.