Photochemistry of 4-chlorophenol on cellulose and silica

A review of pesticide phototransformation on the leaf surface: Models, mechanism, and influencing factors

Phototransformation is an important environmental fate of pesticides on plant leaves. This review found that the photodegradation rates of pesticides on leaves might be faster or slower than those in organic solvents or on glass because of the different spectral patterns and light fluxes on the model surface. Wax was found to play an important role in pesticide phototransformation because it has photosensitizing properties, which might be stimulated under light irradiation to produce reactive species, such as hydroxyl radicals, singlet oxygen, methyl radicals, alkyl radicals, and superoxide radicals. These reactive species could accelerate pesticide photodegradation by several times. Wax can also decrease the photodegradation rate of pesticides by quenching reactive species or light-shielding effects. The environmental conditions and phytochemical properties of leaves play important roles in pesticide phototransformation primarily because the composition of wax varies with plant species and environmental factors. The phototransformation of pesticides on leaves was promoted by a low dosage of adjuvant because they act as photosensitizers and improve the dispersity of pesticides, while it was inhibited at a high concentration of adjuvant because of their light shielding effect. Finally, recommendations for future research were discussed, including (1) distinguishing the direct and indirect photodegradation of pesticides; (2) developing model, molecular level visualization and analysis techniques; (3) conducting more field research; and (4) considering the effect of climate change, especially the interaction of climatic factors. This review gives a comprehensive overview of the current knowledge of pesticide phototransformation on leaves and provides suggestions for future studies.

Photochemical transformation of hexachlorobenzene (HCB) in solid-water system: Kinetics, mechanism and toxicity evaluation

As one of the first batch of persistent organic pollutants (POPs) included in Stockholm Convention, hexachlorobenzene (HCB) has attracted great attention because of its wide occurrence and great environmental risks. Considering the easy adsorption of HCB on solids and the complexity of natural particles, we systematically investigated the photodegradation of HCB on the surface of silica gel (SG) in aqueous solution in this work to reveal its fate in natural waters. Under mercury lamp irradiation, more than 90% of HCB loaded on SG could be removed after 240 min. Moreover, the effects of solution pH and water constituents were examined, and results showed that the presence of NO₂^-, NO₃^-, Fe³⁺ and humic acid (HA) significantly inhibited the reaction due to the scavenging of ROS and/or competitive absorption of light. According to radical quenching experiments and electron paramagnetic resonance (EPR) spectra, hydroxyl radicals and singlet oxygen generated on the surface of SG could participate in the transformation of HCB, but •OH played a dominant role. Based on products identified by high performance liquid chromatography-mass spectrometry (HPLC-MS) and gas chromatography-mass spectrometry (GC-MS), two main pathways were proposed for the removal of HCB, including dechlorination and hydroxylation which represent direct and indirect photodegradation, respectively, and the occurrence of these two reactions was further supported by density functional theory (DFT) calculations. From the quantitative analysis of penta-chlorobenzene, it was estimated that dechlorination and hydroxylation contributed to approximately 44.4% and 55.6% of initial HCB degradation, respectively. Furthermore, toxicity predictions by the ecological structure-activity relationship model (ECOSAR) suggested that the toxicity of HCB was decreased in the photodegradation process. This study would provide important information for understanding the photochemical transformation mechanism of HCB at the solid/water interface.

Effects of common inorganic anions on the ozonation of polychlorinated diphenyl sulfides on silica gel: Kinetics, mechanisms, and theoretical calculations

In this work, the ozonation properties of 2,2',3',4,5-pentachlorodiphenyl sulfide (PeCDPS) was systematically studied, with special emphasis on the underlying mechanism for the effects of inorganic ions. Kinetic experiments show that common ions can significantly reduce the oxidative properties of ozone, except for SO₃^2- and Cu²⁺. The inhibition effect of anions has been explained through the scavenging effect of free radicals and the generation of other free radicals with weaker oxidation potentials, but no research has reported on the effect of free radicals generated by anions on the degradation pathway. However, SO₃^2- and Cu²⁺ exerted a promoting effect through enhanced formation of ·OH via the hydrolysis effect and the catalyzed decomposition of O₃, respectively. According to the intermediate products identified by high performance liquid chromatography-mass spectrometry/mass spectrometry (HPLC-MS/MS) analysis, direct oxidation of S atom, substitution of Cl atom with -OH group, and hydroxylation of the benzene ring were commonly observed. The addition of NO₂^- and SO₃^2- produced new free radicals like ·NO₂, ·SO₃ and ·SO₄^-, which would attack the parent compound or its primary product, thus influencing the degradation efficiency and pathways. The radicals initiated reactions and the structures of the corresponding products were further rationalized by density functional theory (DFT) calculations. These findings provide new insights into the effects of common anions on ozone oxidation of organic compounds.

Formation of hydroxylated derivatives and coupling products from the photochemical transformation of polyfluorinated dibenzo-p-dioxins (PFDDs) on silica surfaces

Polyfluorinated dibenzo-p-dioxins (PFDDs) are dioxin compounds that have been detected in industrial fluoroaromatic chemicals and can cause adverse effects to organisms. In this work, the photochemical behaviors of PFDDs congeners on silica was systematically investigated. The pseudo-first-order rate constants (k, h^-1) of surface photolysis changed with the substitution number and position of fluorine atoms, and the tetra-fluorinated PFDDs tended to degrade more efficiently. Octafluorinated dibenzo-p-dioxin (OFDD) was selected as a representative to explore the reaction mechanisms. Product analysis showed that OFDD was decomposed into hydroxylated PFDDs (OH-PFDDs) and hydroxylated polyfluorinated diphenyl ethers (OH-PFDEs) via hydroxyl substitution and (OH radical mediated or direct) C-O bond cleavage. Coupling elimination reaction was also observed, resulting in the formation of three-membered and four-membered ring compounds. According to the extracted peak areas in mass spectra and the energy barrier in potential energy surface, direct homolysis of C-O bond occurs as the dominant reaction pathway. This work could provide some new insights into the environmental fate of dioxin compounds.

Emerging investigator series: sunlight photolysis of 2,4-D herbicides in systems simulating leaf surfaces

Pesticides are commonly applied on foliage, forming dry deposits on the leaf cuticular wax. However, their photochemical transformation in this lipophilic environment is much less understood compared with that in surface water. In this work, sunlight photolysis of six chlorinated phenoxyacetic acid herbicides (i.e., 2,4-D and structural analogues) was evaluated in four organic solvents, on quartz, and on paraffin wax. In solvents of low polarity (i.e., n-heptane and 2-propanol), direct photolysis of 2,4-D herbicides was enhanced due to the relatively high quantum yields in these solvents. Photolysis on paraffin wax was slower than photolysis on quartz by a factor of 3-9, but was comparable with that in solvents of low polarity. With environmentally relevant irradiation and surface loading, the half-lives of 2,4-D herbicides on paraffin wax were 27-159 h, which are within the same range reported for biodegradation, the dominant dissipation pathway in the current 2,4-D fate model. Product analyses showed that photoreductive dechlorination is the dominant pathway in organic solvents, accounting for 68-100% of parent compound decay. On quartz and paraffin wax surfaces, however, photoreductive dechlorination products accounted for <60% of parent compound decay. Combining kinetic modeling and product analyses, it was shown that neither could the two additional putative pathways (photosubstitution of chlorine by hydroxyl group and cleavage of the ether bond) fully account for the total phototransformation on surfaces. These results suggest that rapid photolysis on surfaces can be attributed to unique pathways that are absent in the organic solvent phase.

Hydroxyl Radical Based Photocatalytic Degradation of Halogenated Organic Contaminants and Paraffin on Silica Gel

Photochemical materials are of scientific and practical importance in the field of photocatalysis. In this study, the photochemistry of several organic contaminants, including decabromodiphenyl ether (BDE-209), halogenated phenols (C₆ X₅OH, X = F, Cl, Br) and paraffin, on silica gel (SG) surface was investigated under simulated solar irradiation conditions. Photolysis of these compounds at the solid/air interface proceeds with different rates yielding various hydroxylation products, and hydroxyl radical was determined as the major reactive species. According to density functional theory (DFT) calculations, the reaction of physically adsorbed water with reactive silanone sites (>Si═O) on silica was indispensable for the generation of ^•OH radical, where the required energy matches well with the irradiation energy of visible light. Then, the BDE-209 was selected as a representative compound to evaluate the photocatalytic performance of SG under different conditions. The SG material showed good stability in the photodegradation process, and was able to effectively eliminate BDE-209 under natural sunlight. These findings provide new insights into the potential application of SG as a solid surface photocatalyst for contaminants removal.

Photochemistry of 4-chlorophenol in liquid and frozen aqueous media studied by chemical, compound-specific isotope, and DFT analyses

The photochemistry of 4-chlorophenol in liquid and frozen aqueous solutions and on the surface of ice grains yields substantially different photoproducts. Several complementary experimental and theoretical methods, such as trace analyses of the photoproducts, trapping experiments, compound-specific isotope analyses, and quantum chemical calculations, were used to study the reaction mechanism differences. A similar carbon kinetic isotope effect determined for the photolysis of 4-chlorophenol samples in the temperature range of 20 to -40 °C and the results of trapping experiments suggest that heterogeneous cleavage of the C-Cl bond in the excited state is probably a common key step that leads to the formation of carbene and hydroxyphenyl cation intermediates. We conclude that the subsequent specific reactions of these species under various conditions are responsible for the formation of different final photoproducts.

Surface photochemistry of pesticides containing 4-chlorophenoxyl chromophore

The surface photochemistry of pesticides containing 4-chlorophenoxyl chromophore was accessed on the model surfaces cellulose and silica, using diffuse reflectance and chromatographic techniques. 4-Chloroanisole was chosen as the model compound and its phototransformation was followed under lamp (254 nm) and sunlight irradiation. The photodegradation rates are faster on cellulose than on silica. The main photodegradation products on cellulose are anisole and dimerization compounds while on silica the formation of 4-hydroxyanisole and dimerization are the main reaction pathways. Transient absorption studies showed the formation of the 4-methoxyphenylperoxyl radical only on cellulose and in the presence of oxygen. The homolytic cleavage of the C-Cl bond, and consequent formation of a triplet radical pair, is the main primary reaction step on both surfaces. The radical pair then diffuses apart or undergoes electron transfer to form phenyl radicals and phenyl cations, respectively. These two reaction intermediates account for reduction, addition, and formation of bonded residues, the main reactions observed on the studied surfaces and also expected to prevail under natural conditions.

Uptake, removal, accumulation, and phytotoxicity of 4-chlorophenol in willow trees

4-chlorophenol (4-CP) is a well-known hazardous chlorinated compound and a precursor for the synthesis of the herbicide 2,4-dichlorophenoxyacetate. The relation between uptake, accumulation, toxicity, and removal of 4-CP in willow trees (Salix viminalis) was determined. In addition, the feasibility of implementing phytoremediation as a treatment method for 4-CP contamination was investigated. Willows were exposed to 4-CP levels < or =79.9 mg/L in hydroponic solution. The transpiration of the trees was used to determine toxic effects. Almost no inhibition of transpiration was detected at concentrations > or =15 mg/L. For concentrations > or =37.3 mg/L, transpiration decreased to < or =50%, and the trees wilted. Trees exposed to 79.9 mg/L wilted and eventually died. For concentrations of 79.9 mg/L, a significantly higher amount of 4-CP remained at the end of experiments in the test system compared with the amount remaining at all other concentrations. The loss of chemical from the system in experiments with trees was high, < or =99.5%. In treeless experiments, the mass loss of 4-CP was only 6% to 10%. The results indicated that degradation in the root zone is the main reason for the removal of 4-CP from the media. Phytoremediation of 4-CP in willow trees seems to be a remediation option, especially at concentrations <37.3 mg/L, at which point degradation of 4-CP is rapid and efficient, and the toxic effects on trees are not lethal.

Photodegradation of fenitrothion and parathion in tomato epicuticular waxes

Photodegradation of (14)C-labeled fenitrothion ([O,O-dimethyl O-(3-methyl-4-nitrophenyl) phosphorothioate]) and parathion ([O,O-diethyl O-(4-nitrophenyl) phosphorothioate]) was conducted on a series of solid surfaces including isolated tomato fruit and leaf cuticle waxes. The wax-coated glass plate gave the comparative degradation of fenitrothion observed for the intact plant but both surfaces of octadecyl-capped silica gel and poly(tetrafluoroethylene) enhanced its volatilization. Photoinduced desulfuration and ester cleavage were common to both pesticides in waxes, but formation of the azo derivative was found to be a major degradation pathway characteristic of parathion. The modified electronic states of the nitro group by introduction of m-methyl group accounted for this different photoreactivity based on molecular orbital calculations.

Surface photochemistry of pesticides: an approach using diffuse reflectance and chromatography techniques

The photochemistry of pesticides triadimenol and triadimefon was studied on cellulose and beta-cyclodextrin (beta-CD) in controlled and natural conditions, using diffuse reflectance techniques and chromatographic analysis. The photochemistry of triadimenol occurs from the chlorophenoxyl moiety, while the photodegradation of triadimefon also involves the carbonyl group. The formation of 4-chlorophenoxyl radical is one of the major reaction pathways for both pesticides and leads to 4-chlorophenol. Triadimenol also undergoes photooxidation and dechlorination, leading to triadimefon and dechlorinated triadimenol, respectively. The other main reaction process of triadimefon involves alpha-cleavage from the carbonyl group, leading to decarbonylated compounds. Triadimenol undergoes photodegradation at 254 nm but was found to be stable at 313 nm, while triadimefon degradates in both conditions. Both pesticides undergo photochemical decomposition under solar radiation, being the initial degradation of rate per unit area of triadimefon 1 order of magnitude higher than the observed for triadimenol in both supports. The degradation rates of the pesticides were somewhat lower in beta-CD than on cellulose. Photoproduct distribution of triadimenol and triadimefon is similar for the different irradiation conditions, indicating an intramolecular energy transfer from the chlorophenoxyl moiety to the carbonyl group in the latter pesticide.