Herbicides
Herbicides are applied more than any other chemicals to the landscape. Herbicides come in an enormous variety of chemical compounds. Weeds are known to cause enormous losses due to their interference in agroecosystems. Because of environmental and human health concerns, worldwide efforts are being made to reduce the reliance on synthetic herbicides to reduce weeds.[4] One of the most widely synthetically produced type of herbicide contains the triazine structure, which has a 6-membered ring in which C atoms alternate with N atoms ("triazine" denotes 3 nitrogen atoms). To understand this structure use this informative website on triazine herbicides.[14] Two of these triazine herbicides that are widely used in the industry are atrazine and simazine. These chemicals have been used as herbicides since they are effective at inhibiting the photosynthetic electron transport processes in annual grasses and broadleaf weeds. There are some ill effects of these herbicides since they are relatively mobile in soils and they degrade rather slowly. Also, the manner in which they are applied ensures that they are susceptible to being washed off with rainfall with the high potential to become water contaminants. These triazine herbicides are widely spread contaminants and are commonly found in drinking water supplies. Municipalities with high concentrations are required to use activated carbon filtration to remove these herbicides from municipal drinking water. Triazines also threaten our environment since they bioaccumulate in microorganisms, macroinvertebrates, worms, snails and fish.[14]
Green chemistry has helped develop more sustainable herbicides. In 1996, Monsanto received the Presidential Green Chemistry Challenge award for the manufacturing of glyphosate herbicides. Glyphosate is an isopropylamine salt and the active ingredient in Round-up herbicide. Round-up also contains water and a surfactant system which adheres the product to the leaves of the plant so the active ingredient can penetrate. Glyphosate kills weeds by interfering with the synthesis of some kinds of amino acids essential to plant proteins. Glyphosate moves throughout the plant deteriorating the plant tissue from the leaves to the root system. Glyphosate also binds tightly to most soils and is not available for uptake to other nearby plants. There is no risk to humans or animals since it disrupts an enzyme, EPSP synthase, that is not present in humans or animals.
Commonly used herbicides to use for review of the labels and MSDS sheets, to fill in the product profile worksheet at the end of the unit and to discuss with the students include Atrazine 90DF (Atrazine), Envoy Plus (Clethodium), Confront (Triclopyr), Lontrel (Clopyralid), Quicksilver (Carfentrazone-ethyl), and Round-up (Glyphosate). Students should contrast and compare the different herbicides and determine the use limitations of each of the chemicals. Their Excel sheet should be filled out for each of these chemicals with their half-life in the environment, water solubility, partition coefficient (K O C), and calculated Groundwater Ubiquity Score (GUS).
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