The present invention relates to malathion oil-in-water emulsion
formulations comprising malathion as active ingredient and the usual formulation,
carrier and auxiliary agents. The invention further relates to the use of one
or more peroxide(s) as an additive to malathion oil-in-water formulations for reducing
the contents of isomalathion therein without a simultaneous reduction in the chemical
stability of malathion present therein.
It is well known that organophosphate pesticides, e.g. malathion,
methyl parathion, dimethoate and chlor-pyrifos when stored carry or develop an
obnoxious odour, which for the major part is probably due to the formation of
decomposition products of the mercaptan and sulphide type.
Inhibition of the development or removal of the unpleasant odour
in organophosphate pesticides or from organic solvent-based pesticide formulations
may at least partially be achieved by adding small amounts of oxidation agent,
e.g. peroxides, nitrites, nitrogen oxides or ozone, to the pesticides (US 3,714,301;
US 2,962,521; US 2,879,284; GB 960,013 and WO97/25076). The oxidation agents oxidize
the unpleasantly smelling mercaptans and sulphides into odourless compounds.
According to US 4,851,217 an addition of urea to oil-in-water emulsion
formulations of organophosphate pesticides will ameliorate the malodor problem
inherent in such formulations and also physically/chemically stabilize the emulsion.
But urea does not stabilize the proper active compound, such as malathion.
It is known to use peroxides for removal of bad odour from sewage
containing organophosphate, e.g. malathion (US 4,263,136). The patent leaves the
impression that malathion in the sewage water may be decomposed in connection
with the peroxide treatment.
Problems of destabilizing organophosphate pesticides by adding oxidation
agents are described in US 3,714,301 and US 2,879,284. In US 3,714,301 it is stated
that the known deodorizing treatment of organic thiophosphate products with peroxides
or ozone in many cases resulted in the sulphur atom being replaced with an oxygen
atom (i. e. forming oxone compounds such as paraoxon from parathion and malaoxon
from malathion). In US 3,314,851 it is mentioned how addition of ozone or hydrogen
peroxide and possibly a catalyst to a parathion oil-in-water emulsion leads to
formation of paraoxon. The conversion of parathion to paraoxon and, possibly because
the product is present as an oil-in-water emulsion, implicates a reduced toxicity
for mammals of said product.
According to said reference, a similar treatment of a malathion oil-in-water
emulsion with hydrogen peroxide leads to formation of malaoxon which, however,
toxicologically is unwanted. Malaoxon is considerably more toxic for mammals than
malathion, the oral LD50
value for rats being 158 mg/kg for malaoxon but
5500 mg/kg for malathion (NIOSH, Registry of Toxic Effects of Chemical Substances,
1981-1982, and Kyosh, S.R. , project No. 851341D/CHV/33/AC, 1986).
Grätzel, C.K. et al. (J.Mol.Catal., 60(1990) 375-387) proved that
addition of oxidation agent, e.g. hydrogen peroxide, to aqueous organophosphate
pesticide dispersions of e.g. malathion and parathion increased the decomposition
of the pesticide considerably, i.a. on account of a direct oxidation of the compounds.
Jensen-Holm, J. (Ugeskr. Læg. (Doctors' Weekly) 143(48) (1981) 3206-3211)
described the catalytic effect of hydrogen peroxide on the decomposition of organophosphate
pesticides in aqueous waste products.
Destruction of organophosphate pesticides in waste water by means
of oxidation agents is likewise described by Lion, C. et al. (Bull.Soc. Chim.Belg.
103(1994) 115-118), Egli, S. et al. (Chem.Oxid. 1992 (1994) 264-277), Hu, K. et
al. (Huanjing Huaxue 9(1990) 13-19) and Gomaa, H.M. et al. (Advan.Chem.Ser. (1972),
111, Fate of Org. Pestic. in the Aquatic Environ., 189-209).
According to Pilar Aires et al. (J.Photochem. Photobiol. A: Chem.
68(1992) 121-129) the presence of only 0,6% hydrogen peroxide in an aqueous malathion
solution increased the decomposition of pesticide by a factor 10, which meant
that practically all malathion had been decomposed in the course of a few days
of storing at room temperature, even though pH was near optimum for the stability
In contrast to the chemical destabilization of malathion which as
stated above occurs when peroxides or ozone are added to aqueous malathion products,
it is according to US 3,714,301, US 2,962,521 and US 2,879, 284 possible to obtain
a deodorization (primary) and, further, a stabilization against a development of
new mercaptane and sulphide smell, when ozone, peroxide or nitrite are added to
malathion products, if any aqueous phase present are removed from the active agent
malathion immediately after said addition of oxidation agent. If any aqueous phase
remains with the active agent the above mentioned decomposition reactions will
initiate. A subsequent drying of malathion, possibly under vacuum, is accordingly
elaborated upon in US 3,714,301 and US 2,962,521. If malathion is not effectively
dried the formation of the isomer isomalathion will entail toxicological problems,
isomalathion being more toxic to mammals than malathion proper.
The above mentioned organophosphate pesticides are often used as
solutions in organic solvent. The solutions are diluted with water immediately
before spraying. For both environmental reasons and for reasons concerning the
working environment there is a widespread desire to use, instead of solutions of
pesticides in organic solvent, oil-in-water emulsion formulations of the pesticides
without any content of organic solvent.
The chemical stability of malathion and its toxicity properties are
considerably worsened by the presence of water in formulations thereof. The formation
of isomalathion in aqueous malathion formulations is an example of decomposition
of an organophosphate pesticide to an isomer which is more toxic than the pesticide
itself. The problem is thus not only that isomalathion per se is the most toxic
compound, but further that iso compounds are to a considerable degree known for
potentiating the inherent toxicity of the active agents on humans and animals.
Consequently, both the initial content and the formation of isomalathion
in malathion formulations are to be limited as much as possible; moreover, authorities
do only allow very limited concentrations of the toxic isomalathion in commercial
It is possible to a certain degree to limit the conversion of malathion
to isomalathion by adequately choosing auxiliary compounds such as e.g. carriers
and emulsifying agents, and by adequately choosing the type of packaging, and
the value of pH in the formulations. Thus, malathion oil-in-water formulations
are mentioned in US 4,851,217 and EP 93/03165. In both publications malathion
oil-in-water formulation are said to possess physical-chemical stability, and US
4,851,217 further states that addition of urea, besides of the physical-chemical
stabilization of the emulsion also decreases an evolution of mercaptane and sulphide
However, a formation of isomalathion when storing such emulsions
has not been investigated. If oil-in-water emulsions of malathion are prepared
as described in US 4,851,217 and EP 93/03165, unacceptable amounts of isomalathion
are formed during their storage. This formation is found even if premium grade
malathion (i.e. malathion treated with peroxide or nitrite according to US 2,879,284,
US 2,962,521 or US 3,714,521 and then (vacuum-)dried) is used in the preparations.
Methods for effectively reducing the formation of isomalathion in
malathion oil-in-water emulsion formulations are to our knowledge not described
in the litterature.
It has most surprisingly been found that addition of peroxides to
malathion oil-in-water emulsion formulations considerably reduces the concentration
of the toxic isomeric compound isomalathion, even after a prolonged storage at
elevated temperatures. Likewize, it has surprisingly been found that said addition
of peroxides does not impair the chemical stability of malathion as it would be
expected in view of the above (US 4,263,136; US 3,714,301; US 2,879,284; J.Mol:
Catal. 60(1990) 375-387; Ugeskr.Læg. (Doctor's Weekly) 143(48) (1981) 3206-3211;
Bull.Soc.Chim.Belg. 103(1994) 115-118; Chem.Oxid. 1992(1994) 264-267; Huanjing
Auaxue 9(1980) 13-19; Advan.Chem:Ser. (1972) 111, Fate of Org. Pesic. in the Aquatic
Environ. 189-209; J.Photochem.Photobiol. A: Chem. 68(1992) 121-129). Moreover,
especially US 3,314, 851 should be mentioned, according to which even a short
time treatment of water-emulgated parathion or malathion with peroxide or ozone
leads to formation of paraoxon/malaoxon, respectively.
Accordingly, the malathion oil-in-water emulsion formulations of
the invention are characterized in that to the formulations is added 0.01-10, preferably
0.05-3 and in particular 0.3-1 percentage by weight, of one or more peroxide(s).
In the use according to the invention the above stated amounts of
peroxide(s) are employed.
Said addition will reduce the formation of isomalathion to toxicologically
If the reduction of the contents of isomalathion in malathion oil-in-water
emulsion formulations prompted by the inventive addition of peroxide(s) to such
formulations was due to the same mechanism as by the removal of smell from technical
malathion by an oxidation agent, it should be expected that addition of nitrite
to the formulations would also lead to formulations with preferred properties.
However, an addition of nitrite to malathion oil-in-water formulations leads to
the formation of a considerable amount of nitric fumes which are toxic and of an
evil stench. Thus, it is not possible for the skilled man from the- disclosures
of US 3,714,301; US 2,962,521; US 2,879,284; and US 3,314,851 to conclude how
the auxiliary compounds nitrite and peroxide will influence the properties of
malathion oil-in-water emulsion formulations.
Autooxidation of peroxide into water and oxygen may linit the effect
of the peroxide on the content of isomalathion in malathion oil-in-water emulsion
formulations. The autooxidation may, however, be considerably limited by addition
of stabilizing agents which stabilize the peroxide without limiting its effect
on the formation of the toxic isomalathion. Examples of peroxide stabilizing agents
are e.g. EDTA, salicylic acid, propylgallate, acetanilide, 8-hydroxyquinoline,
phenacetin and mixtures thereof.
In malathion oil-in-water emulsion formulations it may be advantageous
to use a combination of peroxides with different solubilities in the phases and
different distribution coefficients between the phases.
The addition of peroxide(s), alone or in combination with peroxide
stabilizing agents, makes possible a development of malathion oil-in-water emulsion
formulations with an acceptable low content of isomalathion.
The malathion oil-in-water emulsion formulations according to the
invention may besides of malathion also contain one or more pesticides of a different
The invention will now be further illustrated by the following examples.
The concentrations of pesticides and decomposition products thereof were determined
by gas and/or liquid chromatography and NMR spectroscopy.
Malathion 40% by weight oil-in-water emulsions were prepared
lege artis using an optimal combination of emulgators. The stirring velocity
during the emulsion formation was regulated such that the volume-surface middle
diameter was 10-12 µm. A suitable viscosity was obtained by addition of viscosity
increasing compounds. The value of pH was regulated so as to obtain optimal malathion
The concentration of hydrogen peroxide in the emulsions was varied.
The contents of isomalathion was analytically determined after storing of the formulations
for 6 and 14 days at 54°C (accelerated storing test). Likewise, the contents of
malathion was analytically determined before and after storing.
Fig. 1 shows the relationship between initial hydrogen peroxide concentration
and isomalathion concentration after 6 days of storing. A considerable reduction
of the isomalathion concentration was obtained by addition of hydrogen peroxide.
A pronounced reduction of the isomalathion concentration is obtained even after
14 days of storing at 54°C.
It appears from Table 1 (below) that the chemical stability during
a storing test at 54°C is improved by the addition of hydrogen peroxide.
Decomposition of malathion in 40% by weight oil-in-water emulsions at storing
for 14 days at 54°C
% hydrogen peroxide in formulation
% malathion decomposed relative to initial content
In selected malathion oil-in-water formulations the chemical stability
of malathion and the formation of isomalathion was investigated during storing
of said formulations for months at 23°C. The results appear from Table 2 (below).
Addition of hydrogen peroxide considerably reduces the isomalathion concentration
and improves the chemical stability of malathion.
Decomposition of malathion, and isomalathion concentration in malathion 40%
by weight oil-in-warter emulsions when stored for 4 months at 23°C.
% hydrogen peroxide in formulation
% malathion decomposed relative to initial content
ppm isomalathion after storing