The measured half-life ranged from 8 to 17 months at 20°C and 65 to 70 months at 4°C, when firmly adsorbed to sediments¹³. The medium pH also drastically influenced pyrethroids persistence, the half-life reaches 80 months in natural waters with high organic matters content¹⁶. Deltamethrin persistence depends also to the soil characteristics. In amended mineral soil (1mg/kg of active ingredient), 52% of the used deltamethrin was recovered after 8 weeks incubation^10,24,26-29; in organic soil, the recovery rate increased to 74%^10,24,26,27. Moreover, soil nutrients and related environment conditions influenced pyrethroid life time. In riparian wetland soil, the half-life time ranged from 27 to 291 days in absence and presence of amended nitrates³⁰. In an US sandy loam soil, half- lives are estimated to 25 and 36 days in aerobic and anaerobic conditions^26,27.

Pyrethroids are highly active contact insecticides which are 100 to 1000 times more toxic than organochlorine pesticides¹⁷. When pyrethroids will accumulate in environment, their high toxicity and activity may cause significant harm to ecosystems^2,17-25. Indeed, pyrethroids insecticides are found to be very toxic for non-targeted insects and downstream food chain aquatic animals. Therefore, the benefit in malaria vector control must not hide the environmental and human health potential risks.

Understanding, pyrethroids behavior in sub-Saharan Africa malarial zones will help to use them more efficiently and prevent their ecological impact. Pyrethroids abiotic degradation in sahelian environment was previously studied^16,31. In the present study, deltamethrin biodegradation in arid, semi-arid and soudano-sahelian environment conditions is investigated by simulating the sub-Saharan malarial zones.

MATERIALS AND METHODS:

Reagent:

Deltamethrin (99%, m/m) is purchased from Cluzeau Info Labo (CIL, France) and used as received. Deltamethrin is a highly toxic chemical that is slightly soluble in water (1 - 2 µg/L, 25°C)¹⁶. Unfortunately, pyrethroids strong affinity for the solid phase will allow their residues to move with runoff and to accumulate into surface streams where they are very toxics for aquatic animals^13,14,32.

Soil sampling:

Soils samples were taken in November 2008, at 0-30 cm depth from three long-term trials conducted on farms in Niamey (Niger) and Tillabery (Niger). The soil named “NY” was from Niamey (N 13° 30' 12''; E 02° 05' 55.2'') and soil named “TY” was from Tillabery (N 14° 48' 23.5''; E 01° 52' 51.8''). The above two sites are chosen in order to get sandy soil (poor clay soil) and clay soil to match arid, semi-arid and soudano-sahelian zone soils. Selected soils chemical and physical characteristics are given in Table 1. One hundred grams of each soil was amended with 0, 10, 20 and 40 mg insecticide kg^-1dry weight soil. The soil and the insecticide active substance were mixed into 100 mL polystyrene flask. This study is done in dark in order to avoid the insecticide photodegradation by solar UV-Visible irradiation^16,31.

Table 1: Selected chemical and physical characteristics of used soils

Parameters

soils

Clays (%)

Silt (%)

Sand (%)

Total organic carbon (%)

Na⁺ (cmol+/kg)

K⁺ (cmol+/kg)

Ca²⁺ (cmol+/kg)

Mg²⁺ (cmol+/kg)

pH(KCl)

Maximum water holding (mL)

CEC-Ag (cmol+/kg)

12.6

37.4

0.38

0.25

0.8

4.2

1.6

5.2

7.1

79.3

15.5

5.2

0.68

0.6

0.1

4.8

2.8

3.6

8.4

Rainfall simulation:

Arid, semi-arid and soudano-sahelian zones annual precipitation are respectively close to 0, 350 and 700 mm^33,34. One (1) mm of rain is defined as 1 L of rain equally distributed on 1m² of soil. Rainfall simulation was done at the “Laboratoire Sol Eau Plante” of Burkina Faso National Institute of Environment and Agriculture Research (INERA)³⁵^-39. Results gotten from our soil samples contained in flask (diameter = 5 cm), was respectively 0, 2.5 and 5 mL/week during three (3) months (January, February and March). Indeed, rainy season duration is around 3 months in the studied zones^33,34.

Pyrethroid insecticide biodegradation study:

For each zone, the corresponding soil samples are kept in an incubator (25 – 30°C). A day is randomly chosen weekly to inject to the sample 0, 2.5 and 5 mL of distillated water. An incubated 0 and 40 mg/kg control samples were subjected to the same treatment (5 mL/week). The insecticide is added after a preliminary one week incubation of the soil microcosms (25 - 30°C). The soil is humidified to 2/3 of the water holding capacity corresponding to 2/3 ´ 24 mL and 2/3 ´ 60 mL, respectively for soils NY and TY^38,39.

Insecticide extraction from soil:

For arid zone sample (dry), 10 g of soil is taken after 0, 2 weeks, 1 month, 2 months and 3 months to extract the residual insecticide. For semi-arid, soudano-sahelian and preliminary incubated control samples, the corresponding soil quantity (m_i) is calculated based on the following equation Eq.(1): ; where is the sample weigh at t_i = 0, 2 weeks, 1 month, 2 months and 3 months;m₀ the empty flask weigh, determined before the sample preparation and i = 0, 1, 2 and 3.

The 10 g of dry soil or m_i of humidified soil were introduced into a 100 mL flask. Then 50 mL of a mixed hexane: isopropanol (v:v, 3:1) was added, vigorously shaken for 45 min and decanted for 30 min. Then 10 mL of the supernatant was taken and introduced into a separating funnel of 500 mL containing 15 mL of distillated water, agitated for 2 min and decanted for 30 min. The organic phase was collected, dried using sodium sulphate and filtered throw Whatman qualitative filter paper^38,39. The insecticide extract were kept in bottles which are protected from light irradiation with aluminum foil and stored in refrigerator at 4 °C until analysis.

RESULTS AND DISCUSSION:

Analytical measurements:

Biodegradation analysis was carried out using a spectrophotometer Helios Alpha (Thermo). Spectra displaying absorbance intensity variation versus deltamethrin concentration are shown in Figure 1. The blank NY (respectively TY) is obtained from soil without insecticide after using the described extraction procedure.

Figure 1: Variation of deltamethrin absorbance intensity versus concentration

A simple, accurate and reproducible spectrophotometric analysis method was first developed to follow deltamethrin biodegradation. The absorbance is measured at the maximum peak of the broad absorption band between 200 and 330 nm and a calibration curve displaying absorbance as a function of the insecticide concentration is plotted. All absorbance measurements were corrected for the solvent (background) signal with the appropriate blank. The results obtained are shown in Figure 2. The selected absorbance wavelength (λ_abs) is 257 nm; the linear dynamic range is 2.5 to 28.5 mg/L; the coefficient correlation (R²= 0.998); the limit of detection (LOD) defined as the amount of analytes giving a signal-to-noise ratio of 3 is 0.45 mg/L; the limit of quantification (LOQ) defined as the amount of analytes giving a signal-to-noise ratio of 10 is 1.5 mg/L; theabsolute limit of detection (ALOD), calculated using 2.5 mL sample is 1.125 µg and the relative standard deviation (%RSD, n=6) = ±3.2%.

Figure 2: Calibration curve for deltamethrin measurement

Deltamethrin biodegradation in arid, Semi-arid and soudano-sahelian zones

Deltamethrin is a broad spectrum insecticide^30,40. Its physico-chemicals properties are summarized in Table 2.

Table 2: Deltamethrin physico-chemical characteristics

Deltamethrin (C₂₂H₁₉Br₂NO₃)
IUPAC chemical name		[Cyano-(3-phenoxyphenyl)-methyl] 3-(2,2-dibromoethenyl)-2,2-dimethyl-cyclopropane-1-carboxylate
Solubility in water		0.2-2 µg/L (25°C)^3,8
Partition coefficient	K_ow	Octanol-Water partition coefficient (log K_ow) = 6.1⁴¹
	K_oc	Organic carbon-Water partition coefficient: 16344 cm³/g ⁴¹
Hydrolysis		pH 5 (buffered): negligible^16,42,43 pH 7 (buffered): negligible^16,42,43 pH 9 (buffered): half-life 2.5 days^16,42,43
Photolysis		Fast photolysis when exposed to high-intensity lamps (30,000 lux)^16,31 Photolysis under UV-Vis: half-life 48 days²⁵

Data in this table shows that deltamethrin is less soluble in water and has a very high adsorption capacity on organic matter (K_OC). While exposed to high intensity UV-Visible irradiation source, deltamethrin is photodegraded (half-life < 2 months). In dark, deltamethrin is relatively more stable in natural waters¹⁶. Thus, when firmly adsorbed to the soil or buried, deltamethrin is sheltered from hydrolysis and photodegradation processes. Biodegradation related to the soil microbial activity remains the only important degradation way. Here, the temperature and soil humidity play a key role in the microbial population development. The biodegradation of Deltamethrin in dark under arid (0 mm), semi-arid (350 mm) and soudano-sahelian (700 mm) annual rainfall conditions in sandy soil (soil NY) and clay soil (soil TY) is presented in Figure 3.

Figure 3: Deltamethrin rate in soil NY (Fig.3.a) and Soil TY (Fig.3.b) in arid, semi-arid and soudano-sahelian zones

Under arid, semi-arid and soudano-sahelian zones, deltamethrin biodegradation patterns are similar in the two soils. Deltamethrin is more biodegraded in humid zones (semi-arid and soudano-sahelian) compared to dry arid zone. Indeed, at local ambient temperature (25 – 30 °C) humidity is favorable for microbial population activities. Therefore, the highest biodegradation rate is observed after one week preliminary incubation in the two soils. Besides, pyrethroid insecticide presence in the media before incubation has probably induced the inhibition of the microbial population development. After three months, the remaining insecticide rates in the sandy soil (Fig.3.a) are 41, 31 and 16% respectively under arid (0 mm), semi-arid (350 mm) and soudano-sahelian (700 mm) climate conditions. In the clay soil (Fig.3.b), the corresponding rates are respectively 48, 35 and 21%. Deltamethrin biodegradation is more important in sandy soil (soil NY) compared to clay soil (soil TY). This is probably due to its less content in clay (12.6% vs 79.3%) and organic carbon (0.38% vs 0.68%). Indeed, pyrethroid insecticides are hydrophobic; they are strongly adsorbed to soil particles (clays, organic matters and sediments) which may drastically affect their biodegradation processes.

Impact of soil deltamethrin content on the biodegradation rate

The above study has shown that, (i) the humidity in the medium influenced deltamethrin biodegradation and (ii) the insecticide presence in the medium before incubation inhibit the biodegradation processes. Other studies have shown that, insecticide such as endosulfan was also seen as a factor affecting the respiratory activity and the cultivable aerobic microorganisms of Sahel sandy soils^38,39. In order to understand, the impact of the deltamethrin content on the biodegradation processes, soil NY (respectively soil TY) was amended with 10, 20 and 40 mg insecticide kg^-1dry weight soil. In the two used soils, under arid, semi-arid and soudano-sahelian zones conditions, the degradation rate decreased when the deltamethrin content in soil increased. The obtained results are summarized in Table 3.

Tableau 3: Impact of deltamethrin concentration in soil on the biodegradation rate in arid, semi-arid and soudano-sahelian zones.

Nature of soil	Soil NY			Soil TY
Pluviometry (mm)	0	350	700	0	350	700
Degradation rate (%) of 40 mg.kg^-1amended soil	59%	69%	84%	53%	65%	78%
Degradation rate of 20 mg.kg^-1 amended soil	73%	81%	86%	67%	74%	84%
Degradation rate of 10 mg.kg^-1 amended soil	>80%	>80%	>80%	24%	>80%	>80%

CONCLUSION:

Because of pyrethroids recent widespread uses in community health in Sub-Saharan Africa, they will probably contribute significantly to the contamination of environment in the coming years. Thus, it’s essential to understand their persistence in the sub-saharan region in order to prevent large scale pollution. Biodegradation is generally the principal degradation way for organic molecules in soil. Pyrethroids biodegradation depends to soil physico-chemical characteristics and humidity. Hence, deltamethrin is more biodegraded in relatively humidified soudano-sahalian and semi-arid zones compared to arid zone. Under the three studied climatic zones, deltamethrin appeared to be less biodegraded in soil with high clay and organic matter content. Moreover, the soil bioremediation capacity decreases when the level of insecticide increase.

ACKNOWLEDGEMENT:

This work was supported by Abdou Moumouni University 2008 - 2009 Research grant and Belgium Technical Cooperation (CTB-Niger) 2009 - 2010 toxico-chemistry training grant.

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Received on 21.08.2011

Modified on 14.10.2011

Accepted on 28.10.2011

Research J. Science and Tech. 3(6): Nov.-Dec. 2011: 318-323