Here are some structures of chemicals that are commonly used as pesticides and herbicides:
Another important herbicide is Metolachlor, used in cotton, corn and bean crops in California, is shown below:
It is critical to find a biological process to accelerate the process of biodegradation of pesticides, as their accumulation can cause dramatic problems throughout the food chain. Atrazine, a common herbicide, is resistant to aerobic degradation, due to its triazine ring [Jessee]. Thus, it is resistant to biodegradation which increases the possibility for long-term contamination effects in the environment. Atrazine contamination in particular can cause serious repercussions, even at seemingly low concentrations. Twenty pounds of atrazine evenly dispersed throughout an aquifer is sufficient to contaminate 10 million liters (2.6 million gallons) of water at a level of 100 ppb (parts per billion) [Bode]. This level is significantly above the accepted maximum contamination level in drinking water of 3 ppb. [Nair]
Listed below are the structures of Atrazine and its metabolites via 3 different pathways (biological dealkylation, chemical hydrolysis, and biological hydrolysis):
Biological dealkylation is the most studied. The atrazine is first degraded through oxidative N-dealkylation. This process has been found to form three different metabolites: deisopropylatrazine (CEAT), deethylatrazine (CIAT), and desethyldesisopropylatrazine (CAAT). In order to simplify discussion of the atrazine metabolites, the names have been abbreviated based on the substituents on the triazine ring. Each metabolite takes a slightly different pathway to a common intermediate product, OOOT [Cook].
Abbreviations for metabolites based on the substituents on the triazine ring:
Abbreviation |
Substituent |
A |
amino |
C |
chloro |
E |
ethylamine |
I |
isopropylamino |
O |
hydroxy |
T |
triazine ring |
Deisopropylatrazine is first transformed into EOAT and then to EOOT before it reaches OOOT. Deethylatrazine is transformed to IOAT and then to IOOT before it reaches OOOT. Before forming OOOT desethyldesisopropylatrazine is first changed to COAT and then to COOT. These intermediate reactions before reaching OOOT have been tentatively identified as deamination reactions. The metabolite OOOT is then further degraded to biuret and then to urea. The urea is then broken down into carbon dioxide and ammonia.
FIGURE 2: Atrazine Metabolic Pathways in Microorganisms
Another commonly used insecticide is Diazinon, is an organophosate that functions to inhibit acetylcholinesterase, an enzyme needed for correct nervous system function. The insecticide primarily targets cockroaches, ants and fleas, but has shown to poison birds and is highly toxic to fish and bees. In soil and groundwater, the pesticide has a low persistence, and rarely migrates below the top half inch in soil (thus not usually contaminating groundwater). Bacterial enzymes can be used to treat emergency contamination situations. In water, the half-life of the pesticide can potentially be 6months if the water is highly acidic.
Aldicarb, a carbamate, is a systemic insecticide that also functions as a cholinesterase inhibitor. The main agricultural uses are in cotton, peanuts, sugar beets and citrus crops. Unlike Diazinon, Aldicarb is much more persistent in the food chain, and bioaccumulates fairly rapidly. It is very soluble in water and very mobile in soil, which increases its destructive effects beyond the target insects, contaminating ground and surface waters. Toxic to birds and fish, the chemical also causes mutagenesis in human white blood cells, and is toxic through dermal contact. See below for a cartoon of how a soluble pesticide would contaminate water more so than an adsorbed pesticide.
