Anaerobic microbial degradation of diuron by pond sediment

Anaerobic degradation of the herbicide diuron, 3-(3,4-dichlorophenyl-1,1)dimethylurea, was studied. Enrichment cultures were established with seven different media in the presence of diuron (40 mg/liter). Media included combinations of sediment extract, mineral salts, and various organic amendments. Cultures were inoculated with aliquots of sediment collected from a pond previously treated with diuron and were maintained under an atmosphere of 95% N₂ and 5% CO₂. All enrichment cultures completely degraded diuron in 17–25 days. In all cultures showing diuron degradation, the product identified as 3-(3-chlorophenyl)-1,1-dimethylurea appeared in approximately stoichiometric amounts. Reinjection of diuron into each culture after 26 days resulted in rapid degradation of the parent herbicide with the appearance of proportionately more 3-(3-chlorophenyl)-1,1-dimethylurea. No other product was detected after 80 days in culture and the metachloro derivative was not degraded further during this time.     

Transformation of fenuron induced by photochemical excitation of humic acids

When neutral solutions containing the herbicide 3-phenyl-1,1-dimethylurea (fenuron) and a humic acid are irradiated at 365 nm, 3-(4-hydroxyphenyl)-1,1-dimethylurea and three biphenyl products are formed as main products. The apparent quantum yield of fenuron disappearance is evaluated as 6·2 × 10⁻⁵ mole E⁻¹. Upon irradiation of the same mixture at 253·7 nm, both direct and induced phototransformations of fenuron occur. Direct photooxidation yields 2- and 4-amino-N,N-dimethylbenzamide. The induced phototransformation leads to 2- and 4-hydroxylation of the aromatic ring in accordance with the fact that hydroxyl radicals are involved in the oxidation.     

Photochemical behaviour of dichlorprop [(+/-)-2-(2,4-dichlorophenoxy)propanoic acid] in aqueous solution

Aqueous solutions of dichlorprop were irradiated under different conditions of pH, wavelength and oxygenation. The photochemical behaviour was found to be complex and many photoproducts were formed. However, at low concentrations the main photoproducts were 4-chloropyrocatechol, 2-chlorophenol, 4-chlorophenol and 2,4-dichlorophenol. Some other photoproducts were identified, namely 2-(4-chloro-2-hydroxyphenoxy)propanoic acid, 2-(3,5-dichloro-2-hydroxyphenyl)propanoic acid and 2,4-dichlorophenyl acetate. From comparison with results previously obtained with mecoprop [2-(4-chloro-2-methylphenoxy)propanoic acid] it appears that the presence of a chlorine atom in position 2 on the ring strongly modifies the photochemical behaviour.     

The effect of the herbicide diuron on soil microbial activity

The inhibitory effect of the herbicide diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] on microbial activity in red Latosol soil was followed using microcalorimetry. The activity of the micro-organisms in 1.50 g of soil sample was stimulated by addition of 6.0 mg of glucose and 6.0 mg of ammonium sulfate under 35% controlled humidity at 298.15 (+/- 0.02) K. This activity was determined by power-time curves that were recorded for increasing amounts of diuron, varying from zero to 333.33 micrograms g-1 soil. An increase in the amount of diuron in soil caused a decrease of the original thermal effect, to reach a null value above 333.33 micrograms g-1 of herbicide. The power-time curve showed that the lag-phase period and peak time increased with added herbicide. The decrease of the thermal effect evolved by micro-organisms and the increase of the lag-phase period are associated with the death of microbial populations caused by diuron, which strongly affects soil microbial communities.     

The effect of Diuron on the soluble nucleotide pool and growth of bean and corn seedlings

The effect of Diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] on the acid-soluble nucleotides and growth of bean and corn seedlings was examined. Seeds were germinated in sterile vermiculite and the seedlings fed with different concentrations of diuron through the roots. In the control plants there was a predominance of adenine and uridine nucleotides over guanine and cytosine nucleotides with ATP and UDP-glucose (UDPG) being the most abundant. Seedlings fed with 50 μM diuron showed a significant increase in ATP level and then a subsequent decrease with increasing diuron concentrations. There was a considerable decrease in the total nucleotide content of both types of seedlings fed with 125 and 250 μM diuron, the effect being much more pronounced in the ones treated with 250 μM. The decrease in the total nucleotide content resulted in reduced metabolic activity of the seedlings as was shown by the decrease in chlorophyll and in reduced fresh and dry weights.     

Residues of diuron and phytotoxic degradation products in aquatic situations. I. Analytical methods for soil and water

Thin-layer and gas-liquid chromatography, ultraviolet analysis and bioassay with Chlorella spp. have been used to investigate the pathway of degradation of diuron to phytotoxic derivatives when diuron was used as a soil-residual herbicide in irrigation canals. Observations suggest that 1-(3,4-dichlorophenyl)-3-methylurea and 1-(3,4-dichlorophenyl)urea make a contribution to total residues equivalent to a maximum of about 40 and 55%, respectively, of diuron concentrations. Application of a phyto-toxicity rating suggests that in this environment, measurement of diuron specifically would underestimate the total phytotoxicity of residues by a maximum of about 7%.     

Effect of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) on photosynthesis and respiration of Nitella dactyl cells

Long-term experiments with dactyl cells of Nitella flexilis showed that the herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) at a concentration of 1 × 10^?5M affected not only O₂ evolution in the light but also O₂ uptake in the dark. The inhibition of O₂ production was transitory, but dark respiration did not recover. DCMU induced the formation of giant mitochondria which disappeared before cell death. It was concluded that the algicidic effect of 1 × 10^?5M DCMU on N. flexilis, but not necessarily the elongation of mitochondria, was due to the inhibition of mitochondrial respiration and not of photosynthesis.     

The degradation of methazole in soil. II. Studies with methazole,methazole degradation products and diuron

[¹⁴C]-Labelled methazole, 1-(3,4-dichlorophenyl)-3-methylurea (DCPMU), 1-(3,4-dichlorophenyl)urea (DCPU), and diuron were incubated in soil at 20°C and field capacity soil moisture content. Decomposition followed first-order kinetics; half-lives for degradation of these four compounds were 2.4, 144, 30 and 108 days respectively. The amount of DCPMU and DCPU that could be extracted decreased with time and the decrease was accompanied by the generation of an equivalent amount of ¹⁴CO₂. This was not so in the studies with diuron and methazole, however, and the decrease in the concentrations of radioactivity extracted from soil treated with these compounds could not be entirely accounted for as carbon dioxide. It is concluded that the unextractable radiochemical that was present was DCPMU. Methazole appeared to be degraded through DCPMU to 3,4-dichloroaniline (DCA) with the production of only traces of DCPU.     

Degradation of Diuron Photoinduced by Iron(III) in Aqueous Solution

The degradation of diuron photoinduced by iron(III) in aqueous solution has been investigated with different iron(III) species (monomeric species Fe(OH)²⁺, dimeric species Fe₂(OH)₂⁴⁺ and water-soluble oligomeric species) under monochromatic excitation at 365 nm and under sunlight. The rate of degradation depends on the concentration in Fe(OH)²⁺, the most reactive species in terms of ^•OH radical formation. The major photoproduct is 3-(3,4-dichlorophenyl)-1-formyl-1-methylurea which represents more than 60% of diuron disappearance. The mechanism only involves the attack by ^•OH radicals arising from iron(III) excited species. The half-lives of diuron when submitted to such a process in the environment were estimated to be 1–2 h and a few days according to the concentration of Fe(OH)²⁺ (respectively 70% and <10% of total iron(III) concentration).     

The environmental fate of diuron under a conventional production regime in a sugarcane farm during the plant cane phase

BACKGROUND: Field studies of diuron and its metabolites 3-(3,4-dichlorophenyl)-1-methylurea (DCPMU), 3,4-dichlorophenylurea (DCPU) and 3,4-dichloroaniline (DCA) were conducted in a farm soil and in stream sediments in coastal Queensland, Australia. RESULTS: During a 38 week period after a 1.6 kg ha(-1) diuron application, 70-100% of detected compounds were within 0-15 cm of the farm soil, and 3-10% reached the 30-45 cm depth. First-order t(1/2) degradation averaged 49+/-0.9 days for the 0-15, 0-30 and 0-45 cm soil depths. Farm runoff was collected in the first 13-50 min of episodes lasting 55-90 min. Average concentrations of diuron, DCPU and DCPMU in runoff were 93, 30 and 83-825 microg L(-1) respectively. Their total loading in all runoff was >0.6% of applied diuron. Diuron and DCPMU concentrations in stream sediments were between 3-22 and 4-31 microg kg(-1) soil respectively. The DCPMU/diuron sediment ratio was >1. CONCLUSION: Retention of diuron and its metabolites in farm topsoil indicated their negligible potential for groundwater contamination. Minimal amounts of diuron and DCMPU escaped in farm runoff. This may entail a significant loading into the wider environment at annual amounts of application. The concentrations and ratio of diuron and DCPMU in stream sediments indicated that they had prolonged residence times and potential for accumulation in sediments. The higher ecotoxicity of DCPMU compared with diuron and the combined presence of both compounds in stream sediments suggest that together they would have a greater impact on sensitive aquatic species than as currently apportioned by assessments that are based upon diuron alone.     

The effect of some heavy metal ions on photosynthesis in a freshwater alga

The effect of the following metals: copper, cadmium, lead, mercury, methyl mercury, and thallium, and the herbicide 3(3,4-dichlorophenyl)1,1-dimethylurea on (a) the light-induced oxygen evolution, (b) the Hill reaction (water → dichlorophenol-indophenol), and (c) a modified Mehler reaction (dichlorophenol-indophenol → methyl viologen) was studied with the freshwater alga Chlamydomonas reinhardii.The light-induced oxygen evolution of whole cells was found to be very sensitive to the cadmium ion, but this ion was almost without effect on the Hill reaction and the modified Mehler reaction. The light-induced oxygen evolution was also very sensitive to methyl mercury and lead.     

Photolytic demethylation of monuron and demethylmonuron in aqueous solution

Photochemical demethylation of monuron [3-(4-chlorophenyl)-1, 1-dimethylurea] and of demethylmonuron [ 1-(4-chlorophenyl)-3-methylurea] in aqueous solution was investigated. The photoproducts identified in this study were formaldehyde, formic acid and carbon dioxide. These products indicate that demethylation occurs either by oxidation of a methyl group to hydroxymethyl which is readily cleaved to yield formaldehyde, or by further oxidation to an N-formyl group, which is slowly hydrolysed under weakly acidic conditions to yield formic acid. Samples equilibrated either with air or oxygen (> 99 % purity) gave essentially the same yield of oxidation products. Experimental evidence indicated that extensive photodegradation of formaldehyde was occurring during photolysis, causing low recoveries of formaldehyde. Consequently, with [²H₃-methyl]demethylmonuron and deuterated water, only steady state concentrations of formaldehyde could be measured in an isotopic study.     

Phototransformation of metoxuron [3‐(3‐chloro‐4‐methoxyphenyl)‐1,1‐dimethylurea] in aqueous solution

UV irradiation of metoxuron in aerated aqueous solution at 254 nm or between 300 and 450 nm led initially to an almost specific photohydrolysis of the C–Cl bond, resulting in the formation of 3‐(3‐hydroxy‐4‐methoxyphenyl)‐1,1‐dimethylurea (MX3) and hydrogen chloride. The quantum yield was determined to be 0.020 (±0.005) in solutions irradiated at 254 nm. Five minor photoproducts were also identified, in particular the dihydroxydimethoxybiphenyl derivatives resulting from the phototransformation of MX3. Irradiation increased the toxicity of an aqueous solution of metoxuron to the marine bacterium Vibrio fischeri. © 2001 Society of Chemical Industry     

Photolysis of chlorsulfuron and metsulfuron-methyl in methanol

Photolysis of chlorsulfuron and metsulfuron-methyl was studied in methanol under UV light. Their rates of primary photolysis followed first-order kinetics. The main photoproducts were identified as 2-methoxy-4-methyl-1,3,5-triazin-6-amine, 2-chloro-benzenesulfonamide and methyl 2-(aminosulfonyl)benzoate, which entailed the cleavage of the two N–C ureic bonds. Further photolysis of benzenesulfonamide derivatives involved oxidation of −NH₂, cyclisation with loss of CH₃OH, and scission of the C–S bond A trace of methyl o-mercaptobenzoate was also detected. The corresponding photolysis pathways of chlorsulfuron and metsulfuron-methyl were tentatively proposed. © 1999 Society of Chemical Industry     

Metabolic fate of methazole in purple and yellow nutsedge

The mechanisms for the tolerance of purple nutsedge (Cyperus rotundus L.) and susceptibility of yellow nutsedge (Cyperus esculentus L.) to methazole [2-(3,4-dichlorophenyl)-4-methyl-1,2,4-oxadiazolidine-3,5-dione] were studied. Both species absorbed and translocated[¹⁴C]methazole and metabolites from nutrient solution; however, greater amounts of ¹⁴C per unit weight were detected in yellow than in purple nutsedge. Although intact plants and excised leaves of both species rapidly metabolized methazole to DCPMU [1-(3,4-dichlorophenyl)-3-methylurea], detoxification of DCPMU to DCPU [1-(3,4-dichlorophenyl) urea] occurred more slowly in yellow than in purple nutsedge. Compared to yellow nutsedge, a greater percentage of the radioactivity in purple nutsedge was recovered as polar products. Polar products were converted to the free forms of the parent herbicide and to phytotoxic DCPMU by proteolytic enzyme digestion. Based on the findings of this study, at least three mechanisms (differential absorption, metabolism, and formation of polar products) account for the differential tolerance of these two species to methazole.     

Photochemistry of parathion in the plant cuticle environment: Model reactions in the presence of 2-propanol and methyl 12-hydroxystearate

On UV-irradiation (λ > 280 nm), photodegradation of parathion dissolved in 2-propanol took place mainly by reduction of the phenylnitro group. The photoreduction intermediates so formed then combined, yielding primarily azoxyparathion which, upon further irradiation, rearranged into 2-hydroxyazoparathion. When parathion was irradiated in the presence of the cutin acid 12-hydroxystearic acid (as the methyl ester), azoparathion, azoxyparathion and 2-hydroxyazoparathion were the dominant photoproducts if the reaction was performed in thin-layer films of the 12-hydroxystearate. Irradiation of parathion dissolved in a solution of the 12-hydroxystearate in cyclohexane yielded mainly paraoxon. Furthermore, in all experiments, photolysis of the P—O ester bonds of the parent compound as well as of the photoproducts was observed at low levels.     

Binding characteristics of methyl parathion to photosynthetic membranes of Chlorella

When methyl parathion is added at a given concentration to exponentially growing Chlorella cultures containing different numbers of cells, the inhibition of cell number, packed cell volume, pigment content, or photosynthesis has been found to be a function of the cell number, the inhibition being decreased with increased number of cells. Inhibition of photosynthesis has been studied further with a view to characterizing the mechanism of inhibition with a simple assumption that methyl parathion binds to a specific component in Chlorella cells to form an inhibitory complex. The concentration of methyl parathion causing 50% inhibition (I₅₀) of photosynthetic O₂ evolution increases linearly with increasing concentration of chlorophyll in the culture medium. With whole cells the inhibition constant (K_i) is 15 times greater than that with cell-free photosynthetic membranes. This shows that the cell wall acts as a permeability barrier. The relation between the I₅₀ and K_i values and the analyses of the Hill plot of the inhibition curves reveal one binding site per 0.5 chlorophyll molecule and a cooperative binding of methyl parathion with at least three binding sites per binding molecule. Mild treatment of the photosynthetic membranes with trypsin makes the photosynthetic electron transport insensitive to the insecticide, suggesting that the binding component is proteinaceous in nature and the binding sites are located on the external surface of the membrane. The reversal of methyl parathion inhibition is parallel to that of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (diuron) inhibition during trypsin treatment suggesting that the binding proteins for these two inhibitors are similar.     

Photochemistry of vinclozolin in water and methanol–water solution

The photolysis of the fungicide vinclozolin in aqueous and methanol–water (50 + 50 by volume) solution has been examined. Irradiation at λ = 254 nm for 10 minutes resulted in <90% and ≤95% substrate transformation respectively. The dissipation of vinclozolin both in water and methanol + water was linear and the calculated half-lives were 1.01 and 2.0 min respectively. Irradiation (8 h) with UV light (λ ≥ 290 nm) resulted in 10% degradation of the chemical, which is of the same magnitude as that of the control (not irradiated). Irradiation (8 h) under artificial sunlight (Suntest) in the presence of commercially available humic acid (K-salt) resulted in 55% degradation of the chemical. Photolysis leads to the opening of the 2,4-oxazolidine-dione ring, forming 3,5-dichlorophenyl isocyanate and 3,5-dichloroaniline. In addition, dechlorination and elimination of the  CHCH₂ moiety takes place, and one or both the chlorine atoms are replaced by a methoxy group. © 1999 Society of Chemical Industry     

Direct and induced phototransformation of mecoprop [2‐(4‐chloro‐2‐methylphenoxy)propionic acid] in aqueous solution

Mecoprop was irradiated under various conditions of pH, oxygenation and wavelengths in order to study the reactions involved in the phototransformation. Four main photoproducts were identified: 2‐(4‐hydroxy‐2‐methylphenoxy)propionic acid ( I ), o‐cresol ( II ), 2‐(5‐chloro‐2‐hydroxy‐3‐methylphenyl)propionic acid ( III ) and 4‐chloro‐o‐cresol ( IV ). When the anionic form of mecoprop was irradiated between 254 nm and 310 nm (UV‐C or UV‐B), I was the main photoproduct. At 254 nm its formation initially accounted for more than 80% of the transformation. It has not previously been reported in the literature. The reaction results from a heterolytic photohydrolysis. Product II accounted for only a low percentage of the transformation. The stoichiometry was different with the molecular form: the main photoproduct, III , resulted from a rearrangement after a homolytic scission. Products I, II and IV were also formed as minor photoproducts. Some other minor photoproducts were also identified. In contrast, IV was the main photoproduct under sunlight irradiation or when solutions were irradiated in near‐UV light (UV‐A). This wavelength effect is attributed to the involvement of an induced phototransformation; IV is also the main photoproduct when the phototransformation is induced by Fe( III ) perchlorate or nitrite ions. In usual environmental conditions the excitation of the molecular form is negligible and the phototransformation is mainly due to induced photoreactions. © 2000 Society of Chemical Industry     

An experimental approach to the photolysis of pesticides adsorbed on soil: Thidiazuron

An experimental procedure has been developed for conducting photolysis studies with pesticides adsorbed by soil surfaces. In these experiments, solutions of thidiazuron were sprayed on segments of a 0.5-mm layer of soil covering a glass plate. The soil surface was irradiated using simulated sunlight. The light source was a xenon arc in combination with glass filters that absorbed light of wavelength <290 nm. During irradiation, one half of the sprayed areas was covered to obtain nonirradiated samples. After various time intervals, groups of three irradiated and three non-irradiated segments of the soil were scraped off and extracted; the extracts were analysed. Information was obtained on the photoproducts formed and on the kinetics of photodegradation of thidiazuron.