Design of a pesticide: the formulation of a pesticide, environmental effects and degradation
Design of a pesticide
A pesticide is composed of a set of molecules comprising:
one (or several) active material that is due, in whole or in part, the toxic effect.
A diluent is a solid or a liquid (solvent) incorporated into a preparation and to lower the concentration of active ingredient. These are mostly vegetable oils in the case of liquids, the clay or talc in the case of solids. In the latter case the diluent is referred to as charge.
Adjuvants are substances that lack of biological activity, but capable of altering the quality of the pesticide and to facilitate its use.
There are around the world nearly 100,000 specialties authorized for commercial sale, comprising from 900 different active ingredients. 15 to 20 new active ingredients are added every year.
The properties of a pesticide derived for most of the structure of its active ingredient. It comprises 3 parts (the division is artificial, no party can be literally separated):
active structure, which provides power pesticide.
chemical functions of ensuring the degree of solubility in water.
some support for the other two conditions the solubility in oil.
This concept of solubility is important because it is the affinity of a pesticide in water or fats, which will condition its penetration into the target organ.
The other components: the formulation of a pesticide
The formulation of a pesticide is intended to present the active ingredient in a form that its application by adding substances to improve and facilitate its work. These are the adjuvants. These include surfactants, adhesives, emulsifiers, stabilizers and antiperspirants, dyes, materials repulsive and emetics (emetic) and sometimes antidotes.
The formulation of a pesticide must meet 3 key objectives:
ensure optimum efficiency in the active: the active ingredient must reach under the best conditions for its biochemical target, ie to achieve as quickly as possible with minimum loss. It restricts its dispersal into the environment (environmental cost) and the dosage required per hectare (economic cost). For this purpose improves contact with the target organism by the addition of wetting agents (the "wetting" are additives that enhance the spread of the pesticide on the surface. They reduce the contact angle of droplets with support the plant (or animal), with two consequences: increased membership and greater surface contact and action. For systemic products, we try to improve the speed and balance of penetration and transportation of products in the plant. The formulation may also improve the biological efficacy of the active molecule by the effects of synergy, additives that retard its degradation, thereby prolonging its duration of action. Conversely, other additives may accelerate its removal by plants to protect or in the soil.
reduce the risk of poisoning for the handler: Seeking minimal toxicity by contact and inhalation, preventing accidental ingestions by adding dyes, repellent, antidote or vomitive (case of Paraquat in Japan is blue and with an emetic). In the case of liquids, solvents are less toxic selected. Dilution of the active ingredient is much stronger that it is highly toxic.
return the active ingredient: the solvent used by the user is generally inexpensive and readily available. Various additives to improve the conservation storage and / or prevent the corrosion of materials application.
An international code of 2 capital letters, placed after the name indicates the type of formulation. The main formulation types are:
Presentations solid:
Wettable powders (WP): The active ingredient is finely ground (solid) or fixed (liquid) on a porous or adsorbent (silica). Of surface-active agents (dodecylbenzene, lignosulphonate Ca, Al or Na) and expenses dilution (kaolin, talc, chalk, aluminum silicate and magnesium carbonate or Ca) are added and the antiredépositions agents, anti-static or anti-foam. Stabilizer (anti-oxygen and pH buffer) are included to make them compatible with other preparations. These powders must be dispersed in water at the time of use.
Dispersible granules (WG): granules obtained by the town with a little water of active ingredient, and load binders and dispersing agents, followed by drying. These powders must be dispersed in water at the time of use.
The microgranules (MG): identical to the WG but a smaller size (0.1 to 0.6 mm).
Presentations liquids:
Soluble concentrates (SL) is a solution of active ingredient to be diluted in water, with added surfactants.
The suspension concentrates (SC): the active solid material, insoluble in water are maintained in suspension in water concentrated in the presence of wetting agents, dispersants, thickeners (bentonite, silica) or anti-redeposition , antifreeze (ethylene glycol, urea) of anti-foaming and sometimes bactericidal (Methanal or formalin). These preparations are diluted in water at the time of use.
The emulsifiable concentrate (EC): the active ingredients are concentrated solution in an organic solvent and emulsifiers added responsibility of stabilizing emulsions obtained at the time of use by dilution with water.
Concentrated emulsions (EW): the active ingredient is dissolved in an organic solvent. The solution added emulsifiers is dispersed in a small amount of water. This presentation is less toxic and less flammable than emulsifiable concentrates.
Origin of the environmental effects
The dispersion of pesticides in soils
During a treatment, more than 90% of the quantities of pesticides used do not reach the target pest. The main result of pesticides in soils where they undergo the phenomenon of dispersion. Environmental risks are even greater than these toxic products are used on surfaces and the dose / high frequency and they are persistent and mobile in soils.
The soil contains organic and mineral elements and living organisms. In soil, pesticides are subjected to simultaneous action phenomena transfers, immobilization and degradation.
The phenomena of transfer
Transfers to the surface only a small proportion of products applied (usually less than 5%). They contribute to pollution of surface waters where they are trained, the state is dissolved, or retained on soil particles themselves trained.
Transfers in the soil are most important. They are caused by rainwater and are moving along the flow of water. These movements vary greatly depending on the water regime, soil permeability, the nature of the product. For example, silty soil, aldicarb is a very mobile and that lindane does not migrate (the limit of use of aldicarb has been set for 31/12/07 and will be forbidden to use this past date, as well as lindane, which has a ban from the 20/06/02 date of implementation)
The phenomena of immobilization
This phenomenon is due to adsorption, resulting from the attraction of molecules active in the gas phase or in solution in the liquid phase of soil surfaces of mineral and organic soil. Many factors influence on the adsorption capacity of soil, is related to the characteristics of the molecule, or those of the soil (mineral and organic components, pH, amount of water). Similarly, the phenomena of desorption corresponding to the release of the molecule in the ground (opposite of adsorption).
Pesticides are mostly rapidly adsorbed by soil humic substances (minerals and organic colloids). Adsorbed molecule is dissolved in the liquid or gas. No longer available, its biological effects are eliminated, it is degraded by soil micro-organisms which increases its persistence. It is no longer driven by the water, preventing pollution of the latter. Its desorption him Biotox all its capabilities. More strongly retained in general or clay soils rich in organic matter.
The degradation
Soil is an ecosystem that has a capacity of detoxification very high. The degradation process of active ingredients ultimately lead to obtaining mineral molecules such as H 2 O, CO 2, NH 3
The degradation is accomplished mainly by biological organisms of soil microflora (bacteria, actinomycetes, fungi, algae, yeast), which up to 1 t of dry matter per hectare. Its action is mostly in the first centimeters of soil. There are also physical or chemical degradation, such as photodecomposition. These actions help reduce the amount of active ingredient in the soil and thus reduce the risk of pollution. The kinetics of degradation of a molecule is determined by estimating the persistence of the product. To do so, determine its half life is the time at which its initial concentration in soil was reduced by half. This half-life may vary with temperature, soil type, sunlight, etc. Thus, the DDT is about 30 months in temperate regions and 3 to 9 months under tropical climate.
The lindane, DDT and the endrin degrade in a few weeks in flooded rice fields, in contrast to the aldrin, the dieldrin and chlordane.
Soils behave as an active filter ensuring the degradation of pesticides, and selective, since they are able to retain some of these products.
For example, we cite the case of the copper oxychloride, which accumulate in the soil, which resulted in the sterilization of some 50,000 ha of land for banana plantations in Costa Rica.
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