Atrazine discussion

A number of substituted triazines are used as herbicides, and their biodegradation has been discussed in Chapter 10, Part 1. Treatment of soil contaminated with atrazine (2-chloro-4-(ethylamino)-6-isopropylamino-l,3,5-triazine) illustrated a number of significant features. Although the soil that was used had the potential for degradation, a laboratory experiment with Pseudomonas sp. strain ADP that had an established potential for atrazine degradation revealed important limitations. There was a substantial decline in the numbers of Pseudomonas sp. strain ADP and only limited mineralization. Supplementation with citrate or succinate increased the survival of the strain, and successful mineralization was dependent on the preservation of a carbon/nitrogen ratio >10 (Silva et al. 2004). The last would apply generally to substrates with a low C/N ratio such as triazines. 

The isotope dilution gas chromatography-mass spectrometry method described by Lopez-Avila et al. [16] and discussed in section 5.3.1.3 has been applied to the determination of Atrazine in soil. In this method known amounts of labelled Atrazine were specked into soil samples before extraction with acetone-hexane. The ratio of the naturally abundant compound and the stable-labelled isotope was determined by high-resolution gas chromatography-mass spectrometry with the mass spectrometer in the selected ion monitoring mode. Detection limits of 0.1-l.Oppb were achieved. Accuracy was >86% and precision better than 8%. 

The persistence (half-life) of atrazine in the subsurface is governed by chemically and biologically mediated transformations. Because the solubility of atrazine is relatively high ( 30mg/L) compared to its toxicity level in water (5 Lig/L), atrazine has become a hazard to groundwater quality. Atrazine has been detected in groundwater more than any other crop protection chemical two examples of atrazine persistence-transformation in aquifer environments are discussed next. 

The basis of the selectivity and differences in type of activity of the triazines has been discussed thoroughly conjugation with glutathione, via displacement of the chlorine. 

Sorption mechanism of atrazine by SOM has been a subject of controversy. The early works (Weber et al., 1969 Hayes, 1970) showed that the sorption process is inhibited due to the low pKa value of herbicide, along with the proton transfer between carboxylic groups as well as the charger transfer at low pH values. These were discussed as probable retention mechanisms by organic colloids. However, Martin-Neto et al. (1994b, 2001) observed by FTIR (Figure 16.16) and UV-vis spectra that a charge-transfer mechanism was not operative in the HA-atrazine (HA-AT) interaction. FTIR showed that in pH <4, the carboxylate band (1610 cm-1) was observed in HA-AT spectrum, but a decrease in the wavenumber of C-H... 

The triazine herbicides can be divided into four different structural classes chlorotriazines, methylthiotriazines, methoxytriazines, and atypical or asymmetrical triazines. The chlorotriazine group includes atrazine, simazine, pro-pazine, terbuthylazine, and cyanazine. The methylthiotriazine group includes ametryn, prometryn, and terbutryn. The methoxytriazine group will include prometon and secbumeton. Hexazinone and metribuzin were chosen to represent the atypical triazine group. The plant metabolism of the most researched member of each triazine group will be discussed in detail to cover all major biological and chemical transformations reported in the literature. 

As new herbicides were introduced over the years, weed scientists and farmers looked for the best mixtures, rates, and ratios to determine where the new ones would fit. The objective was always to provide the grower with the most dependable and efficacious control of major weeds, with the least amount of herbicides and cost, and with little or no risk to the applicator, consumers, and environment. With corn, sorghum, sugarcane, and certain other crops, such mixtures most often included atrazine or other triazine herbicides. Many times as weed scientists or farmers would discuss the virtues and performance of new herbicides, they would state The new products performed well, but it sure helped to add a little atrazine. ... 

Worldwide there have been a total of 20 commercialized triazine herbicides. Of the 20 triazines, 7 are currently registered for land use within the United States ametryn, atrazine, metribuzin, prometryn, simazine, terbutryn, and prometon. For purposes of this discussion, only dietary estimates for the 5 most widely used domestic triazines are presented since the USEPA revoked cyanazine tolerances in 2004, prometon is not used for food crops and terbutryn has very limited use. Additionally, propazine was used under USEPA Section 18 registrations in the 1990s, and in 2007 was registered for weed control in sorghum (USEPA, 2007). 

This section discusses the occurrence of the degradation products of atrazine, cyanazine, simazine, propazine, prom-etryn, and prometon, as well as their degradation pathways from soil into surface water. More detailed discussion of triazine degradation can be found in several other chapters of this book. 

This chapter presents a quantitative probabilistic risk assessment for atrazine and simazine conducted for Syngenta Crop Protection, Inc. The risk of an effect is the likelihood that an individual will develop the effect as a result of that individual s exposure to atrazine and/or simazine. The risk assessment is quantitative because it characterizes the likelihood in numerical terms. The risk assessment does include some qualitative discussion of the uncertainties associated with the quantitative characterization of the likelihood. It also assumes relevance of an animal effect in humans even when a lack of human relevance has been established, as is the case with atrazine and simazine [US Environmental Protection Agency (USEPA), 2006]. 

The likelihood that an individual will develop a specified response depends on the dose the individual receives as a result of exposure and the relevance of the effect to humans. The dose of atrazine and/or simazine is measured as the intake in milligrams of herbicide per kilogram of body weight per day (mg/kg/day). The way in which the likelihood that an individual will develop a specified response is characterized depends upon the dose-response relationship (defined and discussed in the next three paragraphs). 

In relation to this article, those chlorinated pesticides which are rapidly degraded in the environment, and considered as non-persistent, will not be discussed. Compounds such as atrazine, chlorpyrifos and the phenoxy acid herbicides (2,4-D), mecoprop, MCPA, etc.) all contain organic chlorine and may... 

Kelter, P. B. Grundman, J. Hage, D. S. Carr, J. D. Castro-Acuna, C. M. A Discussion of Water Pollution in the U.S. and Mexico, with High School Laboratory Activities for Analysis of Lead, Atrazine, and Nitrate, / Chem. Educ. 1997, 74, 1413-1421. 

Most of the ways in which natural water/solid interfaces influence the rates of transformation of pesticide compounds, however, are consistent with the various effects of related chemical species on these reactions in homogeneous solution, discussed in preceding sections. Russell et al. (1968), for example, observed that the hydrolysis of atrazine at low pH was greatly accelerated in suspensions of montmorillonite clay, relative to the rate ofreaction in homogeneous solution at the same pH. The authors attributed this effect to the acid-catalysis of the reaction by the clay surface, which exhibited a pH that was considerably lower (by as much as 3-4 pH units) than that of the bulk solution. As noted previously, the sorption of pesticide... 

The toxic effects of some pesticide mixtures are additive, particularly when their toxic mechanisms are identical. The additive effects of the organophosphates chlorpyrifos and diazanon were demonstrated in one study. T Another study found the s-triazine herbicides atrazine and cyanazine to show additive toxic effects. Not all mixtures of similar pesticides produce additive effects, however. In one study, mixtures of five organophos-phate pesticides (chlorpyrifos, diazinon, dimethoate, acephate, and malathion) were shown to produce greater than additive effects when administered to laboratory animals. Another article discusses nonsimple additive effects of pyrethroid mixtures. Despite the similarities in their chemical structure, pyrethroids act on multiple sites, and mixtures of these produce different toxic effects. 10 ... 

Clearly terbufos, the first entry in the Table, is the most persistent with a T. <- of more than 35 days. Alkyl substitution on the carbon between the sulfur atoms of those molecules containing a nitrile group increases the soil persistence. Still, none are as persistent as terbufos. Therefore, the presence of a nitrile in these molecules, as was previously discussed for the Atrazine/ Bladex-triazine relationship, decreases the soil persistence of these organophosphorodithioates compared to terbufos. 

Atrazine, a triazine herbicide, blocks electron transport between QA and QB in PSD. DCMU (3-(3,4-dichlorophenyl)-l,l-dimethylurea) also blocks electron flow between the two molecules of plastoquinone. Paraquat is a member of a family of compounds called bipyridylium herbicides. Paraquat is reduced by PSI but is easily reoxidized by 02 in a process that produces superoxide and hydroxyl radicals. Plants die because their cell membranes are destroyed by radicals. Of the herbicides just discussed, determine which, if any, are most likely to be toxic to humans and other animals. What specific damage may occur ... 

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