WHAT
IS PESTICIDE?
Pesticides are
substances that protect plants against molds, fungi and insects; therefore
decreasing the percent of potential illness. This helps control shortages,
higher prices, and income loss and prevents unappealing blemishes. Most
pesticides are produced by plants naturally towards repelling-off their predators. Allowed pesticides are very low on the
list of risks hazardous to health. Large-scale studies suggest that pesticide
residues are unlikely to be an important risk factor for cancer to the general
public. Major risk factors include smoking, alcohol consumption, several
occupational chemicals and dietary imbalances. Another definition of pesticide is
any substance or mixture of substances intended for preventing, destroying,
repelling or mitigating any pest.
WHAT ARE THE TYPES OF PESTICIDE?
Pesticides are often referred to
according to the type of pest they control. Another way to think about
pesticides is to consider those that are chemical pesticides or are derived
from a common source of production method. Other categories include
biopesticides, antimicrobials and pest control devices.
1) Chemical pesticides
·
Organophosphate
pesticides - Most organophosphates are
insecticides. These pesticides such as parathion, malathion, dichlorvos and
dimethyldichlorovinylphosphate (DDVP) affect the nervous system by disrupting
the enzyme that regulates acetylcholine, a neurotransmitter. They are extremely
toxic to mammals, birds and fish (generally 10 to 100 times more poisonous than
most chlorinated hydrocarbons). They were developed during the early 19th
century, but their effects on insects, which are similar to their effects on
humans, were discovered in 1932. Some are very poisonous (they were used in
World War II as
nerve agents). However, they usually are not persistent in the environment.
·
Chlorinated
hydrocarbons such as
dichlorodiphenyltrichloroethane (DDT),
aldrin, dieldrin and lindane are synthetic organic insecticides that inhibit
nerve membrane ion transport and block nerve signal transmission. They are fast
acting and highly toxic.
·
Carbamates
or urethanes such as aldicarb, aminocarb, carbofuran
and Mirex share many organophosphate properties, including mode of action,
toxicity and lack of environmental persistence and low bioaccumulation.
Carbamates generally are extremely toxic to bees and must be used carefully to
prevent damage to these beneficial organisms.
·
Inorganic
pesticides include compounds of arsenic, copper,
lead and mercury. These broad-spectrum poisons are generally highly toxic and
essentially indestructible. Seeds are sometimes coated with mercury or arsenic
powder to deter insects. They are generally neurotoxins and even a single dose
can cause permanent damage.
2) Biopesticides
Biopesticides are certain types of pesticides derived from
such natural materials as animals, plants, bacteria and certain materials. For
example, baking soda has pesticidal applications and is considered
biopesticides. Biopesticides fall into three major classes:
· Microbial
pesticides consist of microorganisms (e.g. a
bacterium, fungus, virus or protozoa) as the active ingredient. Microbial
pesticides can control many different kinds of pests, although each separate
active ingredient is relatively specific for its target pest. For example,
there are fungi that control certain weeds and other fungi that kill specific
insects.
· Plant-Incorparated-Protectants
(PIPs) are pesticidal substances that plants
produce from genetic material that has been added to the plant. For example,
scientists can take the gene for the Bacillus
thuringiensis or Bt pesticidal
protein and introduce the gene into the plant’s own genetic material. Then the
plant, instead of the Bt bacterium,
manufactures the substances that destroy the pest.
· Biochemical
pesticides are naturally occurring substances that
control pests by non-toxic mechanisms. Conventional pesticides, by contrast are
generally synthetic materials that directly kill or inactivate the pest.
Biochemical pesticides include substances, such as insect sex pheromones that
interfere with mating as well as various scented plant extracts that attract
insect pests to traps.
FACTORS INFLUENCING PESTICIDE MOVEMENT TO ENVIRONMENT
1)
Properties
of the pesticide
2)
Properties
of the soil
3)
Site
conditions including rainfall and depth to groundwater
4)
Management
practices, including method rate of the application.
WHAT ARE THE PROPERTIES OF THE PESTICIDE?
1)
Persistence
Persistence
describes the staying power of a chemical. A pesticide that is persistent will
maintain in its structure, or stay a long time. Pesticides are broken down
(degraded) at different rates by soil microorganisms, chemical reactions and
sunlight. If the soil is moist and warm, microbes use the pesticide molecules
as a food source and turn them into harmless molecules such as carbon dioxide
and water. Breakdown processes occur mainly in the root zone. Breakdown is
considerably slower in deeper soils and sediments. Some pesticides form
intermediate substances during the breakdown process, which can be more toxic
than the original compound. Persistence is usually measured in terms of
half-life.
Half-life means
how long it takes 50% of the original amount applied to become biologically
inactive or broken down. The longer the half-life, the more persistent the
chemical. Residues of persistent pesticides may be long lasting in the root
zone or they may leach (move downward). Pesticides will continue to degrade
upon reaching groundwater. However, breakdown is generally much slower.
2) Adsorption
Persistence and
adsorption are the two most important characteristics of a pesticide, affecting
its potential to leach to groundwater. Adsorption describes how tightly a
compound becomes attached to soil particles. Pesticides that are strongly
adsorbed (tightly held) will be less mobile in soil that is leached with water
and will be less likely to reach groundwater.
Some pesticides
may be too tightly adsorbed to give proper pest control. Injury to sensitive
rotational crops may sometimes occur when a pesticide used on the previous crop
is later released (desorbed) from the soil particles in amounts great enough to
cause injury.
3) Solubility
Solubility is the
tendency of a chemical to dissolve in a solvent. It is another property that
affects the behavior of a pesticide in the soil. As water percolates through
soil, it carries water-soluble chemicals with it. This process is called
leaching. The higher the water solubility value, the more soluble the chemical.
For instance, a pesticide with a water-solubility value of 33,000 ppm at 800
Fahrenheit (270 C) is much more water-soluble than a pesticide with
a water-solubility value of 33 ppm at 800 Fahrenheit (270 C).
It is also more likely to leach.
4) Volatility
Volatility is the
tendency for a liquid or a solid to change into gas. Volatility describes how
quickly a liquid will evaporate when it is in contact with air. Highly volatile
chemicals are easily lost to the atmosphere. Some pesticides, such as
fumigants, must be volatile in order to move and provide uniform distribution
through the soil profile.
WHAT ARE THE PROPERTIES OF THE
SOIL?
1) Permeability
Permeability is a
measure of how fast water can move vertically through the soil. It is affected
by the texture and structure of the soil. Soils with coarse sandy textures are
generally more permeable. Soils with higher permeability have greater potential
for groundwater contamination than less permeable soils.
2) Texture
Soil texture is
an indication of the relative proportions of sand, silt and clay in the soil.
Coarse, sandy soils generally allow water to carry the pesticides rapidly
downward. Finer textured soils generally allow water to move at much slower
rates. They contain more clay and sometimes organic matter, to which pesticides
may cling.
3) Organic matter
Soil organic
matter influences how much water the soil can hold before it begins to move
downward. Soil containing organic matter has greater ability to stop the movement
of pesticides. Soils in which plants are growing are more likely to prevent
pesticide movement than bare soils.
4) Structure
Soil structure
describes how the soil is aggregated. Uncompacted soils allow more water flow.
Soils with a loose structure or soils with hollow channels such as dried root
channels or animal or worm tunnels, will also allow for increased flow of water
through the soil profile. Soils that permit rapid flow of water through the
soil profile present a higher potential for groundwater contamination than less
permeable soils. Sandy soils with their coarse textures and low water-holding
capacity will allow for greater infiltration than finer, heavier clay soils.
5)
Moisture
Soil moisture
affects how fast water will travel through the soil. If soils are already wet
or saturated before rainfall or irrigation, excess moisture will runoff. Soil
moisture also influences pesticide breakdown.
WHAT ARE THE SITE CONDITIONS?
1) Rainfall
Intense or
sustained periods of high rainfall may cause large amounts of water to move
through the soil, especially where little runoff occurs.
2) Depth to
groundwater
Depth to
groundwater is primary factor affecting the potential for pesticides to reach
groundwater. If the top of the water table is shallow, pesticides have less
distance to travel to reach groundwater.
3) Sinkholes and
bedrock
The presence of
sinkholes, cracked bedrock or confining layers in the bedrock significantly
affects vertical movement of water. Sinkholes, cracked bedrock and gravel soils
allow dissolved pesticides to freely move to groundwater as long as there are
no confining geologic layers. Sinkholes present a high risk for groundwater
contamination by pesticides if runoff from fields where they are applied
reaches them. Once water enters a sinkhole, it receives little filtration or
chance for degradation.
WHAT ABOUT MANAGEMENT PRACTICES?
1)
Select pesticides that are less likely to leach.
2)
Do not exceed recommended application rates.
3)
Calibrate application equipment to applied desired
rate.
4)
Mix and load pesticides carefully; prevent spills.
5)
Do not apply a pesticide immediately prior to
irrigation or a heavy rain.
6)
Do not over irrigate.
7) Follow label
directions for pesticide storage and disposal.
FACTORS AFFECTING
PESTICIDE FATE AFTER APPLICATION
It is important to understand what happens to pesticides after they are
applied in the field. Not all of the applied chemical reaches the target site;
some may drift downwind and outside the intended application site, possibly to
non-target sites, including surface water. Three major processes determine its
fate: adsorption, transfer and degradation:
1)
Adsorption
Adsorption is a chemical process that results in a
pesticide being bound or adsorbed to a soil particle. For example, portions of
a pesticide molecule may bind electrically to clay minerals or organic matter.
Transfer refers to processes that move the pesticide away from the
application site and includes volatilization, runoff, leaching, absorption and
crop removal. Sometimes pesticide transfer is essential for pest control. For
example, certain preemergence herbicides must move within the soil to reach
germinating weed seeds. Volatilization occurs when a liquid or solid converts
to a gas and moves away from the initial application site. Runoff occurs when
water is added to a field faster than it can be absorbed into the soil.
Pesticides may move with runoff as compounds dissolved in the water or attached
to soil particles. Leaching is the downward movement of chemical through the
soil, eventually reaching the groundwater. Absorption is the uptake of
pesticides or other chemicals into the plant or animal. After absorption, the
pesticide residue may be broken down or remain in the plant or animal until harvest. Crop removal through harvest
or grazing may move pesticide residue.
Degradation
is the process of pesticide breakdown after application by either microbial
action, chemical action of photodegradation. This process may take hours, days,
weeks or years, depending on environmental conditions and the chemical
characteristics of the pesticide.
PESTICIDE MOVEMENT IN AIR, WATER,
SOIL AND LIVING ORGANISMS
1) Air
Pesticide
movement away from the release site in the air is usually called drift.
Pesticide particles, dusts, spray droplets and vapors all may be carried
offsite in the air. People who mix, load, and apply pesticides outdoors usually
are aware of the ease with which pesticides drift offsite.
Moving air easily
carries lightweight particles, such as dusts and wetable powders. Granules and
pellets are much heavier and tend to settle out of air quickly. Small spray
droplets also are easily carried in air currents. High-pressure and fine
nozzles produce very small spray droplets that are very likely to drift. Lower
pressure and coarse nozzles produce larger droplets with less drift potential.
Pesticide vapors
move about easily in air. Fumigant pesticides are intended to form a vapor when
they are released. Persons using fumigants must take precautions to make sure
the fumigant remains in a sealed container until it is released into the
application site, which also must be sealed to prevent the vapor from escaping.
2)
Water
Pesticide
particles and liquids may be carried offsite in water. Pesticides can enter
water through:
·
Drift, leaching and runoff from nearby applications
·
Spills, leaks and back-siphoning from nearby mixing,
loading, storage and equipment cleanup sites
·
Improper disposal of pesticides, rinsates and
containers.
Most pesticide movement in
water is across the treated surface (runoff) or downward from the surface
(leaching). Runoff and leaching may occur when:
·
Too much liquid pesticide is applied, leaked or
spilled onto a surface
·
Too much rainwater, irrigation water or other water
gets onto a surface containing pesticide residue.
Runoff water in the outdoor
environment may travel into drainage ditches, streams, ponds or other surface
water where the pesticides can be carried great distances
offsite. Pesticides that leach downward through the soil on the outdoor
environment sometimes reach the groundwater.
Runoff water in the indoor environment may get into
domestic water system and from there into surface water and groundwater. Runoff
can flow into floor, drains or other drains and into the water system.
Sometimes a careless pesticide handler washes pesticide
down a sink drain and into the water system.
Some pesticides can leach downwards in indoor
environments. In a greenhouse, for example, pesticides may leach through the
soil or other planting medium to floors or benches below. Some pesticides used
indoors may be absorbed into carpets, wood and other porous surfaces and remain
trapped for a long time.
3) Soil
It might seem
that a short half-life would mean a pesticide would not have a chance to move
far in the environment. This is generally true. However, if it is also very
soluble in water and the conditions are right, it can move rapidly through
certain soils. As it moves away from the surface, it moves away from the agents
that are degrading it, such as sunlight and bacteria. As it gets deeper into
the soil, it degrades more slowly and thus has a chance to get into
groundwater.
Figure 1: The fate of pesticides.
The pesticide
movement in soil will be discussed more detail in the case study later.
4) Living organisms
Living organisms
may also play a significant role in pesticide distribution. This is
particularly important for pesticides that can accumulate in living creatures.
An example of accumulation is the uptake of a very water-insoluble pesticide by
a creature living in water. Since this pesticide is stored in the organism, the
pesticide accumulates and levels increase over time. If this organism is eaten
by a higher organism, which also stores this pesticide, levels can reach much
higher values in the higher organism that is present in the water in which it
lives. Levels in fish, for example, can be tens to hundreds of thousands of
times greater than ambient water levels of the same pesticide. This type of
accumulation has a specific name. It is called “bio-accumulation”.
In this regard,
it should be remembered that humans are at the top of the food chain and so may
be exposed to these high levels when they eat food animals that have
bio-accumulated pesticides and other organic chemicals. It is not only fish but
also domestic farm animals that can be accumulators of pesticides and so care
must be taken in the use of pesticides in agricultural situations.
Further
information of how pesticide enters fish also will be discussed in the case
study later.
CASE STUDY
1) Uptake of
herbicides by leaves or by roots
The upper part of
the plant is hydrophobic by virtue of the waxy cuticle of the leaves whereas
the lower part is essentially hydrophilic since the major function of roots is
to take in water and various water-soluble substances. When a toxic substance
with a measure of oil solubility is applied to leaves, it will tend to
penetrate through the waxy cuticle. If a leaf-applied herbicide is sufficiently
soluble in both oil and water, it may enter the waxy cuticle, penetrate it and
then leave on its inside.
Photosynthetic
products (assimilates) move through the inter-communicating plasmadesmata of
mesophyll cells and thence to the companion cells of the sieve tubes. It is
assumed that leaf-applied herbicides move with photosynthetic products.
Movement of solutes in the phloem reflects the general metabolic activity of
the plant and the majority of systemic foliage-applied herbicides are
translocated most readily when the weeds are growing rapidly. Similarly,
herbicides usually move out of the leaves rather slowly if poor light intensity
restricts photosynthesis. However, some herbicides damage leaf tissue in such a
way to reverse the natural flow of water in the xylem. When this happens, xylem
transportation of herbicides from foliage can occur.
Figure2: Water transportation in plant.
Water is absorbed
from soil by roots and other epidermis cells in or near the roots area. Then,
it moves via the cortex tissue, across an endodermic layer and perisicle. Then
it gets into xylem. Xylem in root is connected to xylem in trunk and allows
water to move out from root to the trunk. In the trunk xylem, water moves into
leaves by transpiration. Then, it gets into mesophyl cell and evaporates. The
vapor is out to atmosphere via stoma.
The root does not
have waxy cuticle and water is absorbed directly from soil. Water is absorbed
into the root by epidermis cells via osmosis process. Some may move in
passively, others may masquerade as natural substances; some may be actively
pumped out as they enter, while others may destroy cells and so nullify the
plant’s normal protective system. The roots absorb water, which contains
nitrate, sulphate and also herbicides.
The rate of
upward movement of herbicide will be influenced by factors such as elevated
temperature or reduced humidity, which affect transpiration. Movement in the
xylem normally carries most of the herbicides to the leaves, a process that
enables those that are photosynthetic inhibitors to reach their site of action.
However, a proportion of the total dose absorbed usually escapes from the xylem
to the phloem on the way up.
2) Pesticides in
soil and water
When a pesticide
is applied to a field, certain reactions follow. Foliar-applied pesticides
stick to leaves, where they are absorbed. But rainfall inevitably washes some
of the chemical off the leaf surface onto the soil below and some may be
transformed by sunlight. The pathway and reactions of pesticides in soils are
shown in Figure 3. There are three mechanisms of pesticide adsorption soils,
shown in Figure 4.
Figure 3: The
pathway and reactions of pesticides in soils.
Sorption is a
transfer process by which pesticides are dispersed in soil. The transfer called
‘partitioning’ of a pesticide into organic matter in soil is a somewhat
nonspecific mechanism. Much organic matter (humus) generally consists of two
systems: a hydrophilic surface and a hydrophobic interior. Nonionic pesticides
escape from soil solution into the hydrophobic interior. Pesticides move
between organic matter and water in soil. Also, pesticides may undergo an aging
process, whereby the chemical moves deeper into organic matter and becomes
unavailable to move back into soil solution. Pesticides that are water-soluble
tend to remain at the surface of soil organic matter, while those that are
insoluble will penetrate to the hydrophobic interior. Sorption to soil
particles is also dependent on soil water content because water is necessary
for chemical movement and water molecule will compete with pesticide molecules
for attachment sites on clay and organic matter. Therefore, pesticide sorption
tends to be greater in dry soils than in wet soils. Besides entering the plants
and soil, pesticides also can enter water through drift, leaching, runoff and
other sources. Now we mention about how
pesticides enter water from soil.
Leaching
Leaching is the term for the
transport process of downward movement (infiltration) of pesticides in water.
Two kinds of phenomena are associated with leaching: preferential flow and
matrix flow. Preferential flow allows pesticide molecules to move rapidly through
a section of the soil profile, with reduced likelihood that the molecules will
be retained by soil particles or degraded by microbes. Preferential flow is
characterized by water that flows rapidly through wormholes, root channels,
cracks and large structural voids in soil. Matrix flow results in a slow
migration of water and chemical through the soil structure; the pesticide moves slowly with water into small pores in soil and has
more time to contact soil particles.
Runoff and erosion
Runoff is a movement of
water across the soil surface at a rate faster than it can infiltrate the soil.
As rain falls, small soil particles become dislodged and are carried laterally
by water in a process known as erosion. Because some pesticides are applied
directly to the soil, large amounts eventually end up there and as water runs
off and soils erode, dissolved and sorbed pesticides go along. Runoff and
erosion have the potential to move more pesticide off site than leaching, due
to the fact that runoff is a surface phenomenon.
3) Pesticides enter living organisms
·
Bioaccumulation and biomagnification
Living organisms may also
play a significant role in pesticide distribution. This is particularly
important for pesticides that can accumulate in living creatures. Cells have
mechanisms for bioaccumulation, the selective absorption and storage of a great
variety of molecules. This allows them to accumulate nutrients and essential
minerals, but at the same time, they also may absorb and store harmful
substances like pesticides through these some mechanisms. Pesticides that are
rather dilute in the environment can reach dangerous levels inside cells and
tissues through this process of bioaccumulation.
The effects of pesticides also are magnified in the environment
through food webs. Biomagnification occurs when the pesticide burden of a large
number of organisms at a lower trophic level is accumulated and concentrated by
a predator in a higher trophic level. Phytoplankton and
bacteria in aquatic ecosystems, for instance, take up heavy metals or the
pesticide molecules from water or sediments. Their predators- zooplankton and
small fish, collect and retain the pesticides from many prey organisms,
building up higher concentrations of pesticides. The top carnivores in the food
chain- game fish, fish-eating birds and humans can accumulate such high toxin
levels that they suffer adverse health effects. One of the first known examples
of bioaccumulation and biomagnification was DDT, which accumulated through food
chains so that by 1960’s it was shown to be interfering with reproduction of
peregrine falcons, brown pelicans and other predatory birds at the top of their
food chains.
Figure 5:
Bioaccumulation and biomagnification. Organisms lower on the food chain take up and store toxins
from the environment. They are eaten by larger predators, who are eaten, in
turn, by even larger predators. The highest members of the food chain can
accumulate very high levels of the toxin.
4)
Pesticides enter air
- volatilization
Volatilization is the
process whereby a solid or liquid evaporates into the atmosphere as a gas. The
process provides a significant pathway of transfer for some pesticides. In
principle, volatilization is an escape mechanism. Compounds with high vapor
pressure and low water solubility have a tendency to volatilize. The tendency
of a pesticide to volatilize from water is approximated by the ratio of its
vapor pressure to its aqueous solubility. The same is partially true for soils,
but the tendency for a pesticide to volatilize from
soil also can be inversely proportional to its
potential to bind to soil.
Specific environmental
factors that tend to increase volatilization include high temperature, low
relative humidity and air movement. A pesticide that is tightly sorbed to soil
will have a lower solution concentration and be less likely to volatilize. That
is, less volatilization occurs from drier soils because the lack of water
allows the pesticide to sorb onto soil particles. Volatile pesticides usually
are incorporated (plowed into the soil) after application to reduce loss into
the atmosphere. However, it has also been shown that pesticide volatilization
from soil is complex and highly dependent on the movement of water to and from the
soil surface.
Once a pesticide enters the
atmosphere as a gas, it can become diluted in water droplets and as a result,
highly susceptible to long range transport from
the application site. Within the atmosphere, the pesticide may undergo
reactions with light (photolysis) and water (hydrolysis) and sorb to suspended
materials such as dust particles. Pesticides in a gaseous state may dissolve in
atmospheric water.
BIOTECHNOLOGY IN BIOREMEDIATION OF
PESTICIDE-CONTAMINATED SITES
Biotechnology has been highly
touted as a potential source of safe, inexpensive and effective methods for the
remediation of sites heavily contaminated with agrochemicals and for the direct
treatment of agrochemical wastes. One area in which much was envisioned of
biotechnology but little has yet been realized is the remediation of chemically
contaminated sites. An agricultural researcher described which has shown that
microorganisms are capable of degrading agricultural chemicals and the use of
microbes or microbial systems in waste disposal or remediation of contaminated
sites.
Microbial Degradation of Pesticides
Microbial degradation is a
transformation process that results when soil microorganisms (bacteria and
fungi) either partially or completely metabolize (break down) a pesticide.
Microorganisms can cause changes in a pesticide when this activity occurs; in
the presence of oxygen it is termed aerobic metabolism and in the absence of
oxygen, anaerobic metabolism.
Most microorganisms
inhabiting the soil profile where oxygen is plentiful degrade pesticides via
aerobic metabolism. As a pesticide undergoes aerobic metabolism, it is normally
transformed into carbon dioxide and water. Under anaerobic metabolic
conditions, microorganism degradation may produce additional end products such
as methane. Those microorganisms using anaerobic metabolism for breaking down pesticides are typical of the microbes inhabiting waterlogged soils in
terrestrial systems or living in the bottom sediments of ponds, lakes and
rivers. These organisms are also present in groundwater and to some extent, in
the soil profile and enter surface water as runoff. Pesticides along with many other naturally occurring organic molecules may serve as a source
of food or energy for soil microbes. A pesticide in soil solution has to move
to these microbial colonies and cross the microbial cell membrane into the cell
to metabolize. Some microbes produce enzymes, which are exported from the cell
to predigest pesticides that are poorly transported. Once inside an organism, a
pesticide can metabolize via internal enzyme systems. Any energy derived from
the breakdown of the chemical can be used for growth and reproduction; any
portion not fully degraded to carbon dioxide or incorporated into cells is
released back into soil solution as intermediate chemical metabolites.
Degradation of Pesticides by Lignin-peroxidase-producing Phanerochaete
sp..
The white-rot
fungus, Phanerochaete chrysosporium was one of the first organisms shown
to degrade lignin through the action of a potent lignin peroxidase enzyme. This
enzyme converts hydrogen peroxide into hydroxy radical, which can attack
lignin, breaking it into smaller components resulting in eventual decomposition
of the ligneous material. It has been shown that lignin-peroxidase producing
cultures of Phanerochaete chrysosporium can degrade a number of
pesticide or pesticide-like compounds, including DDT and methoxychlor, lindane,
chlordane and dieldrin and pentachlorophenol. In addition to Phanerochaete
there are other white-rot fungi and lignin-degrading actinomyces, which may
produce lignin peroxidases that attack pesticides. Through the systematic
isolation of such organisms and the characterization of the pesticide degrading
ability of the lignin peroxidases they produce, it may be possible to assemble
an arsenal of enzymatic weapons for use in remediation of pesticide
contaminated soils.
Degradation of Pesticides by
Specific Enzymes and Pathways
A large number of
microorganisms, which produce specific pesticide degrading enzymes or that
carry entire pathways for the mineralization of pesticides have been
characterized. Parathion hydrolase can degrade coumaphos, methyl parathion and
a number of other related compounds while N-mrthylcarbamate hydrolases can
degrade aldicarb, carbofuran and carbaryl and a number of other related
compounds. These enzymes are stable and the genes have been cloned so that
their production for use in detoxification of pesticides is feasible.
TREATMENT OF PESTICIDE WASTE
Cleaning pesticide
applications equipment produces rinse water that contains pesticide residues
proper management of this pesticide rinse water is necessary to avoid the
contaminations of soil, ground water and surface water that can occur when this
material is improperly discharged. Concentrations of pesticide in the rinse
water range from 1 to 1000 mg/l. Contaminations of soil and water has been
documented at a number of sites in the United State where pesticide have been
improperly managed. Management options for this rinse water include:
Re-applications of the
material as a
·
Dilute
pesticide
·
Re-use
as a diluents for subsequent batches of pesticide
·
Disposal
as a waste
·
Treatment
The first two options are the most widely used by pesticide applicators
who are properly managing rinse water. The last two are much less widely used
because of the expense of these methods and the difficulties encountered in
complying with the regulatory requirements that apply to these management
methods.
What is treatment?
The applications of a
process that alters the chemical characterizes of the wastewater to the extent
that the rinse water can be managed as a non-pesticide or non-hazardous
material. The process used may be physical, chemical, biological or
combinations of these.
Treatment system that can be used
successfully to manage pesticide rinse water must have the following
characteristic:
·
Technology
appropriate for the pesticide applicator
·
Economic
practicality
·
Acceptable
treatment capability
·
In
compliance with applicable regulatory requirements.
Researchers developing treatment
systems must take these four characteristics into account if they hope to see
their systems successfully used. Of these four, the regulatory requirements can
be the most difficult to accommodate.
Existing treatment systems
The pesticide rinse water treatment systems in used legally in the United State can be divided into two general classes:
- Carbon
filtration treatment systems
- Evaporation/degradation
treatment systems.
Carbon filtration treatment systems
In United State, the
treatment systems employ granulated activated carbon (GAC) to remove the
pesticide residue from the rinse water. GAC filtrations systems function by the
exposing of the pesticide contaminated rinse water to carbon particles. The
organic pesticide are adsorbed onto the carbon and thus removed from the rinse
water. Figure 6 is a simplified schematic of this process.
Figure 6: Schematic drawing of simplified carbon
filtration system for treatment of pesticide rinse water.
There are two treatment
products to be managed in these systems- the treated rinse water, which may
still contain low concentrations of pesticide and the exposed carbon, which now
contains pesticide residues. There are a number of firms that produce GAC
filtration systems for industrial use. Two that have targeted the treatment of
pesticide rinse water are the Wilbur-Ellis Company and Imperial Chemical
Industries, Ltd. (ICI). Figure 7 is a schematic of the Wilbur-Ellis system that
illustrates these features - a solid particle filter, a settling tank, an oil
filter and an ozonation chamber.
treatment of pesticide rinse water.
An ultra-violet light has also been added to
the system downstream of the carbon filters to further degrade any bacteria or
organics. The solid particle filter and settling tank remove soil particles
that can clog pores in the filters downstream. The oil filter removes oil and
greases that interfere with the ozonation process and carbon adsorption. The
ozonation unit exposes the rinse water to ozone and oxidizes organics in the
rinse water as well as any bacteria, algae or other organisms that may act to
foul the carbon filters. Oxidation of the organic pesticides in the rinse water
may also enhance their adsorption to the carbon. The carbon filters remove any
residual ozone from the rinse water.
Currently, all spent
filters and sludges generated through the use of the system are to be disposed
of as hazardous wastes. The first carbon filter in the filter series is changed
out after treating 50000 gallons of rinse water. The second filter is then
rotated into the first position, the third into the second position and a new
filter placed into the third position. The water treated is stored for reuse in
subsequent cleaning operations and then recycled through the system.
The ICI system is
simpler and is designed to be portable. Pesticide rinse water is treated in
discrete batches of about 265 gals (1000 liters). Rinse water is treated first
with a flocculation agent and then put through a sand filter and two carbon
filters. The carbon filters are supposed to be replaced after 20 batches or
5300 gallons. The flocculation chemicals added during the treatment process
contain a dye that serves to indicate when the filters are no longer
functioning properly. Sludge settling from the flocculation step and spent
filters are to be disposed as solid waste and would have to be disposed of as
hazardous waste.
The GAC filtration
system described here has the characteristics necessary for successful
treatment systems. The technology used is compatible with the abilities of
pesticide users. The systems treat the rinse water to produce a product (the effluent) that is less toxic and more rapidly degradable than the
original rinse water. The effluent has
lower concentrations of toxic materials and degradation products. With
sufficient treatment, it should be possible to meet water quality standards and
obtain a discharge permit for the treated water. In most cases in the United
States, however, the treated water is reused as cleaning water for subsequent
cleaning operations. The spent carbon and filters are managed as hazardous
waste and there is thus no contaminated material
released to the environment through this treatment process.
Evaporation/ Degradation Systems
The other system type used
to legally treat pesticide rinse water in the United States is the
evaporation/degradation system. The original systems consisted of a lined pit
filled with a soil matrix into which rinse water was placed. The liquid portion
of the rinse water evaporated and the pesticide residues were adsorbed onto the
soil matrix and eventually degraded by microorganisms in the soil. There was no
discharge from the system.
The systems in use now
have been modified to eliminate concerns about possible groundwater
contamination. These systems now typically use aboveground tanks
to contain the matrix. A secondary containment system is also provided. The
replacement of the pit with a tank allows the systems to be operated without
the need for groundwater monitoring. Leaks can be detected by inspection and
corrected. The secondary containment systems provide extra assurance against
groundwater contamination should a leak occur. Figure 8 is a drawing of a
system design used in Florida.
Figure 8: Drawing
of evaporation/degradation system design used in Florida for treatment of pesticide rinse water.
Evaporation/degradation
systems are very simple to operate. Rinse water is collected on a wash down
slab for transfer to the tank or emptied directly into the tank. Solar
radiation evaporates the water and pesticide residues are adsorbed to the soil
matrix. Pesticides are degraded in the tank by bacteria or other mechanisms
(such as hydrolysis or photolysis). There is no discharge of liquid from the
system. The matrix in the tank is left undisturbed for the life of the system.
When the system is dismantled, the matrix can be tested for residues and
disposed of as hazardous waste if necessary.
These systems treat
rinse water through the degradation of the pesticide component by
microorganisms in the soil matrix. Pesticides are adsorbed by the soil matrix
and may be only slowly degraded in these systems.
SUMMARY
Pesticides are
substances that protect plants against molds, fungi, insects or any substance
or mixture of substances intended for preventing, destroying, repelling or
mitigating any pest. The categories of pesticide are chemical pesticides and
biopesticides. The examples of chemical pesticides are organophosphate
pesticides, chlorinated hydrocarbons, carbamates and inorganic pesticides. The
examples of biopesticides are microbial pesticides,
Plant-Incorporated-Protectants (PIPs) and biochemical pesticides. The movement
of pesticide to environment is influenced by properties of the pesticides,
properties of the soil, site conditions including rainfall and depth to
groundwater and management practices, including method rate of the application.
The properties of the pesticide include persistence, adsorption, solubility and
volatility. The properties of the soil include permeability, texture, organic
matter, structure and moisture. The site conditions refer to rainfall, depth to
groundwater and sinkholes and bedrock. The best management of pesticides are
select pesticides that are less likely to leach, do not exceed recommended
application rates, calibrate application equipment to applied desired rate, mix
and load pesticide carefully; prevent spills, do not apply a pesticide
immediately prior to irrigation or a heavy rain, do not over irrigate or follow
label directions for pesticide storage and disposal. It is important to
understand what happens to pesticides after they are applied in the field.
Three major processes determine its fate such as adsorption, transfer and
degradation. Pesticide movement away from the release site in the air is
usually called drift. Pesticide particles, dusts, spray droplets and vapors all
may be carried offsite in the air. Pesticides also can enter water through
drift, leaching and runoff from nearby applications, spill, leaks and back
siphoning from nearby mixing, loading, storage and equipment cleanup sites, improper
disposal of pesticide rinsates and containers. Runoff and leaching may occur
when too much liquid pesticide is applied, leaked or spilled onto a surface and
too much rainwater irrigation water or other water gets onto a surface
containing pesticide residue. It also can move rapidly through certain soils.
Living organisms may also play a significant role in pesticide distribution.
Pesticides can accumulate in living creatures. The type of accumulation is
called bioaccumulation. Microorganisms are capable of degrading agricultural
chemicals. Microbial degradation is a transformation process that results when
soil microorganisms either partially or completely breakdown a pesticide.
Cleaning pesticide application equipment produces rinse water that contains
pesticide residues proper management of these pesticides rinse water is
necessary to avoid contamination of soils, groundwater and surface water.
Treatment system that can be used successfully to manage pesticide rinse water
must have the following characteristics; technology appropriate for the
pesticide applicator, economic practicality, acceptable treatment capability
and in compliance with applicable regulatory requirements. The pesticide rinse
water treatment systems in used legally in the United States can divided into
two general classes; carbon filtration treatment systems and
evaporation/degradation treatment systems.
REFERENCES:
- http://pested
.unl.edu
- http://vm.cfsan.fda.gov
- http://www.organicconsumers.org/Toxic/braindamage.cfm
- http://www.ianr.unl.edu/PUBS/water/gll82.htm
- http://www.agnic.org.
- http://edis.ifas.ufl.edu
- http://www.deq.state.or.us/wmc/hw/pesticidedispose.html.
- Pesticide
Waste Management. John B. Bourke, Allan S.Felsot, Thomas J.Gilding, Janice
King Jensen and James N.Seiber. American Chemical Society, Washington,
1992.
- The Chemistry of Pesticides. Kenneth A.Hassall, Verlag Chemie, Florida.
- Pesticide Transformation Products. L.Somasundaram and Joel. R.Coats. American Chemical Society, Washington.
Thank you for sharing. Nice articles.
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