Ghulam Ghaus Choudhry

Environmental Photochemistry of Herbicides

Herbicides are commonly used as preplant or preemergence and post-emergence for controlling several weeds in most major agricultural crops throughout the World (Worthing and Walker 1983). When utilized in agriculture they are likely to enter into various compartments of the environment. They may undergo leaching by water, volatilization into the atmosphere, sorption to various surfaces, and partitioning into organic films. Their environmental behavior in each of these compartments is of prime importance in the successful prediction of their fate. The environmental picture becomes even more complex by taking into account the varying conditions existing within each compartment. Moilanen et al. (1979) listed several of the most important variables encountered within each environmental compartment (Table 1).

Protocol guidelines for the investigations of photochemical fate of pesticides in water, air, and soils

Pesticides represent an increasingly important sort of synthetic chemical introduced into the environment through human activity. A compound is important in environmental chemistry provided it has (or shows evidence of having) the following characteristics (Choudhry et al. 1979a): produced or distributed in large quantity high likelihood of entry into the environment — dispersion tendency (transport) persistence (lack of degradation under biotic and abiotic conditions) bio-accumulation (concentration effects, e.g., in the food chain or by other mechanisms) significant toxicity and related biological effects

Environmental photochemistry of PCDDs. Part I. Kinetics and quantum yields of the photodegration of 1,2,3,4,7-penta- and 1,2,3,4,7,8-hexachlorodibenzo-p-dixin in aqueous acetontrile

For the determination of the quantum yields of 1,2,3,4,7-pentachlorodibenzo-p-dioxin (1) and 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin (2), ten Pyrex glass photochemical cells were constructed and their correction factors determined. Quantum yields at 313 nm for 1 and 2 in aqueous acetonitrile (4:6 v/v) were (9.781 ± 2.376) 10−5 and (1.096 ± 0.020) × 10−4, respectively. These quantum yields and the measured absorption spectra were used to estimate the environmental phototransformation first-order rate constants and photochemical half-lives of 1 and 2 in water under conditions of variable sunlight intensity during various seasons from solar intensity data available in the literature. In summer, typical half-lives for the phototransformation of 1 and 2, respectively, near the surface of water bodies at 40° north latitude would be 364 ± 88 and 151 ± 3 h; whilst such values in winter would be 1257 ± 309 and 518 ± 10 h.

Environmentally significant photochemistry of chlorinated benzenes and their derivatives in aquatic systems

The pollution of soil and aquatic environments by chlorinated aromatic pollutants (CAPs) such as polychlorobenzenes (PCBzs), polychlorophenols (PCPs), polychloro‐diphenyl ethers (PCDPEs), phenoxyacetic acids, etc., creates growing public anxiety. Phototransformation is an important process for pollutants in aquatic systems. This article extensively reviews the environmentally significant solution phase photochemistry of PCBzs as well as other CAPs derived therefrom. The paper includes photochemical fate of these CAPs at wavelengths >285 nm on the one hand and their photolysis in solution in aquatic systems on the other. In this article, photolytic reductive dechlorination and isomerization of PCBzs are reviewed together with the photoformation of several products including polychlorobiphenyls (PCBs) from PCBzs. Recently developed phenomena of photoincorporation of PCBzs into humic model monomers is also described. This review also describes the environmental photochemistry of chlorobenzene derivatives, na...

Environmental photochemistry of polychlorinated dibenzofurans (PCDFs) and dibenzo‐p‐dioxins (PCDDs): A review

Contamination of our water, air and soil environments by polychlorinated dibenzofurans (PCDFs) and dibenzo‐p‐dioxins (PCDDs) is of growing public concern. Among various environmental transforming processes of pollutants, photochemical transformation caused by sunlight is an important process. This paper reviews the potential photolytic source as well as the photochemical fate of hazardous PCDFs and PCDDs in our environment.

Photochemistry of halogenated benzene derivatives. X. Effects of sodium chloride on the aquatic photodegradation of bromoxynil (3,5-Dibromo-4-hydroxybenzonitrile) herbicide

Aqueous solution phase photochemistry of the herbicide bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) (1) in the presence of various concentrations of sodium chloride was extensively investigated with ultraviolet radiation near 313 nm. In the presence of 0.5 to 25.0x10−3 M NaCl, the quantam yields for the phototransformation of the herbicide bromoxynil (1) amounted to 0.045±0.005 to 0.017±0.007 vs. 0.052±0.004 in the absence of sodium chloride. These quantum yield data for the photolysis of1 followed the Stern-Volmer equation. The photolysis of the 7.8x10−6 M aqueous solution of the herbicide1 in the presence of sodium chloride (10.0x10−3 M) gave rise to the formation of 3-bromo-4-hydroxybenzonitrile (2), 3-bromo-5-chloro-4-hydroxybenzonitrile (3), 3-chloro-4-hydroxybenzonitrile (4), and 4-hydroxybenzonitrile (4A). The products3 and4 were formed via the photoincorporation of chloride ions into bromoxynil (1) and into the primary photoproduct2, respectively. In the case of this photoreaction of the chemical1, the percentages of maximum concentrations of the photoproducts3, 2, 4, and4A were obtained after 10.5, 20.0, 30.0, and 44.0-min exposures to UV light, respectively, the percentage disappearance of the starting material1 being 90% for 44-min photolysis. Photoproducts 2,3, 4, and4A were identified with the aid of GC-MS. The formation of 4-hydroxybenzonitrile (4A) decreased with the rise in concentration of NaCl. When a mixture of aqueous solutions of 3.0 mL bromoxynil (2.0x10−4M) plus 0.5 mL NaCl (0.5 M) was exposed to UV light for up to 3 hr, the photoproduct4A could not be observed; however, other products, namely, phenols2–4 were produced.

Photochemical quantum yields and sunlight half-lives of polychlorodibenzo-p-dioxins in aquatic systems

Abstract The quantum yields ( ф tr ,λ ) for the direct phototrasformation of 1,2,3,7-tetrachlorodibenzo- p -dioxin (1,2,3,7-T4CDD) ( 1 ) and 1,3,6,8-T4CDD ( 2 ) in H2OCH3CN (2:3 v/v) measured at 313 nm were (5.424 ± 0.422) × 10−4 and (2.170 ± 0.135) × 10−3, respectively. The values of фtr,λ for dioxins 1 and 2 as well as those of 1,2,3,4,7-P5CDD ( 3 ), 1,2,3,4,7,8-H6CDD ( 4 ), 1,2,3,4,6,7,8-H7CDD ( 5 ) and O8CDD ( 6 ) (previously reported) were utilized for the estimation of the sunlight photolysis half-lives ( ( t 1 2 ) sp ) of these pollutants. For example, during summer, ( t 1 2 ) sp of the PCDDs 1 – 6 are expected to be 1.77 ± 0.14, 0.31 ± 0.02, 15.16 ± 3.67, 6.27 ± 0.11, 47.33 ± 5.11, and 17.85 ± 2.62 days, respectively.

Environmental photochemistry of the herbicide bromoxynil in aqueous solution containing soil fulvic acids

Photochemistry of the herbicide 3,5-dibromo-4-hydroxybenzonitrile (1) (bromoxynil) was investigated using a narrow band of ultraviolet radiation at 313 nm. UV absorption spectrometry of 7.8 μM 1 in water without and with soil fulvic acids (FAs) (50.0 mg L−1) showed that FAs exerted several fold additive effects on the spectrum of 1 Aqueous phase photoreactions of 7.8 μM of 1 in water in the absence (pH 6.0–7.0) and presence of 5, 10, 15, 20, 40, 45, 60 and 100 mg L−1 FAs (pH 5.5–6.5) were carried out at 313 nm. For instance, for 15 min photolysis time, as the amount of FAs increased, the rate of the photodestruction of 1 decreased. The first-order photolytic transformation rate constants (k p.λ) for 1 in the presence of 5-100 mg L−1 were in the range of (1.052 ± 0.011) × 10−3 to (0.083±0.006) × 10−3 sec−1 The intensity of light (Iλ) at 313 nm ranged from 3.70 to 3.80 μEL−1 sec−1. The light screening factor (S λ) at λ = 313 nm along with Iλ and k p.λ data were used to determine the spectral respon...

Quantum yields for the photodecomposition of polychlorinated dibenzo-p-dioxins (PCDDs) in water-acetonitrile solution

Abstract The quantum yields for the phototransformation of 1,2,3,4,7-penta-, 1,2,3,4,7,8-hexa-, 1,2,3,4,6,7,8-hepta-, and 1,2,3,4,6,7,8,9-octachlorodibenzo- p -dioxin in H 2 O-CH 3 CN (2:3 v/v), measured at 313 nm through the utilization of o -nitrobenzaldehyde in CH 3 CN as a chemical actinometer, were (9.781 ± 2.376) × 10 −5 , (1.096 ± 0.020) × 10 −4 , (1.526 ± 0.165) × 10 −5 , and (2.257 ± 0.331) × 10 −5 , respectively.

Environmental photoincorporations of polychlorobenzenes into several humic model monomers

Abstract Solution phase photoincorporations of six polychlorobenzenes (PCBzs), using UV light centered at 300 nm, into several humic model monomers such as benzoic acid, benzaldehyde, vanillic acid, vanillin, syringic acid, and syringaldehyde were investigated. All PCBzs except hexachlorobenzene gave minor yields of photoproducts formed by the reaction of the polychlorophhenyl radicals with the individual humic monomers. In the case of benzoic acid and benzaldehyde, chlorophenylation of the humic model monomers occurred; whereas when monomers having phenolic OH group were present, substituted polychlorodiphenyl ethers were also formed. Furthermore, the irradiation of 1,2,4,5-tetrachlorobenzene solution containing vanillic acid produced, in addition to other photoinduced products, monochloro- and dichloromethoxycarbohydroxydibenzofuran.