6660 J. Agric. Food Chem., Vol. 55, No. 16, 2007
Table 1. AZS Physical and Chemical Properties
Pinna et al.
respectively, and the suspensions were then irradiated as described
above. At regular intervals, AZS aqueous suspensions (0.5 mL) were
collected and filtered through a 0.2 µm filter (Whatman) to remove
colloid particles. The filtrates were analyzed by HPLC.
Dark control experiments were carried out in conditions similar to
those described above, except that the photoreaction vessel was covered
by aluminum foil.
To study the kinetics of photodegradation, at appropriate times,
depending on the photolysis rate, each test solution was analyzed
directly by HPLC. All of the experiments were run in triplicate, and
the kinetic data reported under Results and Discussion are the average
values derived as the mean of three trials.
HPLC Analysis. AZS and its photodegradation products were
estimated by HPLC. For direct photolysis a Waters 1515 liquid
chromatograph equipped with a 250 × 4 mm i.d. Bondapak C18 (10
µm) analytical column, a multiwavelength Waters 2487 UV-vis
programmable detector operating at 240 nm, and a Breeze chromatog-
raphy workstation were used. Acetonitrile plus water (50 + 50 by
volume), previously brought to pH 2.7 with phosphoric acid, at a flow
rate of 0.5 mL min-1, was the eluant. The retention times were 9.8,
8.0, 6.9, 5.9, and 7.4 min for AZS, ADTA, DPA, DPU, and 1-methyl-
4-(2-methyl-2H-tetrazole-5-yl)-1H-pyrazole-5-sulfonamide (MPS), re-
spectively (Scheme 1).
For indirect photolysis a Waters 1515 liquid chromatograph equipped
with a 300 × 3.9 mm i.d. Bondapak C18 (10 µm) analytical column, a
multiwavelength Waters 2487 UV-vis programmable detector operat-
ing at 240 nm, and a Breeze chromatography workstation were used.
Acetonitrile plus water (40 + 60 by volume) previously brought to pH
2.7 with phosphoric acid, at a flow rate of 0.5 mL min-1, was the eluant.
Under the chromatographic conditions described previously the retention
times were 27.6, 12.3, 8.4, 7.2, 5.7, 4.9, and 4.3 min for the compounds
AZS, AZS-OH, MPS, DPA, acetic acid, malonic acid, and oxalic acid,
respectively. The photodegradation product AZS-OH ([M + H]+ m/e
441) was identified by liquid chromatography-mass spectrometry
analysis (LC-MS; Figure 3).
The concentration of AZS was calculated by using a calibration curve
obtained from HPLC measurements of AZS at five different concentra-
tions. The calibration curve, based on the average peak areas of the
external standard, was linear in the range of 2-10 µM (r2 ) 0.992).
The detection limit for AZS was 0.1 mg L-1, as calculated from the
concentration of the herbicide needed to obtain a detector response
approximately twice the background signal.
melting point
vapor pressure
170 °C
-
4 × °C)
10 9 Pa (25
solubility in water
72.3 (pH 5); 1050 (pH 7); 6536 (pH 9)
-
(mg L 1, 20
°
C)
solubility in organic solvents
acetone, 26.4; methanol, 2.1; hexane,
<0.2; acetonitrile, 13.9; methylene
chloride, 65.9; ethyl acetate,
-
(g L 1, 25
°
C)
13.0; toluene, 1.8
partition coefficient
(log Pow, 25 C)
4.43 (pH 5); 0.043 (pH 7); 0.008 (pH 9)
°
dissociation constant
surface tension
pKa
68.1
) 3.6
-
×
10 3 Nm (23.7
°C)
dioxide. The major products were identified, and degradation
pathways are proposed.
MATERIALS AND METHODS
Materials. AZS (99.7% purity) was supplied by DuPont de Nemours
France SA (Centre Europeen de Recherches), Nambsheim, France.
Some chemical and physical properties of AZS are reported in Table
1.
Its purity was checked by high-performance chromatography (HPLC).
The photoproducts 2-amino-4,6-dimethoxypyrimidine (DPA, 98.0%
purity, Scheme 1) and acetic, malonic, and oxalic acids were supplied
by Aldrich, Milano, Italy. All of the solvents were of HPLC grade
(Carlo Erba Reagenti, Milano, Italy) and were used without further
purification.
DOM was obtained from a sediment collected in a paddy field near
Novara (Italy). The sediment was air-dried and sieved to <2 mm. DOM
was extracted by shaking overnight a 1 kg sample of the sediment (<2
mm fraction) with 2 L of distilled water. Then the suspension was
filtered through a Durapore membrane filter 0.45 µm (Millipore), and
the filtrate was freeze-dried. Approximately 200 mg of DOM was
obtained from 1 kg of sediment.
Hydrated ferric oxide was prepared by the addition of 500 mL of 1
N FeCl2 to 250 mL of 2 M KOH under rapid stirring (9). The fresh
precipitate was immediately washed with distilled water and dried under
vacuum. X-ray analysis showed it to be amorphous. In addition, it was
completely soluble in ammonium oxalate (pH 3).
Titanium dioxide (TiO2, purity ) 99.9%, density ) 3.9 g mL-1),
predominantly anatase, was supplied by Aldrich, Milano, Italy.
Photolysis. The UV spectrum of AZS exhibits an intense absorption
at 240 nm and a weaker band at 288 nm (Figure 2).
Direct photolysis experiments were carried out by irradiating 100
mL of an AZS aqueous solution (8 µM). For 254 nm experiments, the
device consisted of four low-pressure mercury lamps (RPR-2537 Å)
mounted in a circle in a merry-go-round Rayonet photoreactor (5). The
AZS solution, in a water-cooled quartz flask, was irradiated with an
average irradiation intensity of 110 mW/cm2.
Four black light fluorescent lamps (RPR-3500 Å) emitting in the
range 250-600 nm, with a maximum emission at 366 nm and an
average irradiation intensity of 175 mW/cm2, were used to simulate
sunlight irradiation. A water-cooled borosilicate flask, cutting the
radiation shorter than 290 nm to simulate a part of the solar radiation,
was the reaction vessel.
Isolation and Identification of Direct Photolysis Photoproducts
at λ ) 254 nm. A photolysis reaction was carried out to isolate
photodegradation products. AZS (250 mg) dissolved in 75 mL of
acetonitrile and 25 mL of water was irradiated at 254 nm for 24 h.
The crude reaction mixture was concentrated by evaporation of
acetonitrile under vacuum at room temperature. Unreacted AZS
precipitated off and was filtered. The mother solution, left in the dark
for 3 days at room temperature, yielded white crystals, which were
filtered, washed three times with cold water, and dried in a desiccator.
The compound was identified as the cyclic compound ADTA (Scheme
1). The spectral features for ADTA are as follows: IR (KBr), ν (cm-1
)
3470, 3357, 1687, 1629, 1584, 1545, 1368, 1193; 1H NMR (CD3CN),
δ 7.97 (1H, s), 7.33 (1H, m), 7.0 (1H, m), 5.69 (1H, s), 4.12 (3H, s),
3.95 (3H, s), 3.65 (3H, s), 3.37 (3H, s). The remaining aqueous solution
was extracted three times with diethyl ether. The combined extracts
were dried over anhydrous sodium sulfate and concentrated under
vacuum. The residue was separated by liquid chromatography (LC)
on a 60 × 1 cm i.d. glass column of silica gel (70-230 mesh, Merck)
using light petroleum distillate (bp 40-60 °C) plus ethyl acetate (3 +
4 by volume). Volumes of 25 mL were collected, and for each aliquot
thin layer chromatography (TLC) analysis was performed on Merck
silica gel F254 plates. Fractions showing similar chromatographic features
were grouped and evaporated to dryness. Two products were isolated:
the amine DPA and the amide 4,6-(dimethoxy)pyrimidin-2-ylurea
(DPU; Scheme 1). DPU was obtained as white crystals: IR (KBr), ν
(cm-1) 3357, 3198, 1698, 1606, 1576, 1370, 1342, 1221, 1114; 1H NMR
(CD3CN), δ 8.48 (1H, m), 7.71 (2H, m), 5.78 (1H, s), 4.89 (6H, s).
Infrared Analyses. Fourier transform infrared (FT-IR) spectra were
recorded at room temperature in the range of 4000-600 cm-1 using a
On the basis of preliminary trials, indirect photolysis experiments
were carried out by adding 5 mg samples of colloid (TiO2, Fe2O3, DOM)
to 100 mL of an AZS aqueous solution (10 µM). The suspension was
stirred in the dark until equilibrium was reached, that is, the AZS
concentration in the suspension became constant. After equilibration
was achieved (1 and 1.5 h for TiO2 and Fe2O3, respectively), the
suspensions were irradiated in a merry-go-round Rayonet photoreactor
with four black light fluorescent lamps. The suspension homogeneity
and the exchange with the gaseous phase (air) were ensured by magnetic
stirring. To trap hydroxyl radicals, further photocatalytic experiments
were carried out by adding traces of isopropanol, an OH• scavenger,
to the photolysis mixture. In this case, after the equilibration time, 10
µL of the alcohol was added to suspensions containing TiO2 or Fe2O3,