Agriculture is a fundamental source of our food supply with the ideal objective to provide safe products with minimal adverse environmental impacts. Food needs are met by increasing food yields, which has been achieved by use of agrochemicals. Developing countries such as Pakistan have come to depend increasingly on the use of pesticides in order to meet national food requirements.
Additionally, Pakistan is known as the land of cotton, with farmed area increasing from 1.23 to 3.10 million hectares and production from 1.1 to 14.6 million bales. This phenomenal growth is attributed to the introduction of high-yield varieties with increased fiber quality, heat tolerance, and wider adaptability, combined with improved cotton production technologies. Therefore, about 4-5 million bales are being imported to meet the growing demand of the local textile industry. Limiting factors for cotton production include the potentials of crops varieties and changing climate conditions, although the. soils of the cotton belt are quite suitable for exploitation of full yield potential when properly irrigated.
Cotton is a very sensitive crop and attracts a large number of insect pests which in turn requires an increase use of insect repellents . In order to increase the yield both qualitatively and quantitatively, dependence on the use of pesticides has become imperative. Among all the agricultural crops, cotton consumes the largest proportion of pesticides. Some 60% of the total pesticide production in the world is used against the insect and mite pests of cotton. In Pakistan, cotton is grown during the hot summer monsoon season which aids in growth and irrigation for the crop. However, it also provides the most congenial environment for the reproduction and rapid increase of the various insect-pest complexes which attack the cotton crop at different growth stages and inflict heavy losses. Cotton is exposed to serious threats caused by an increase in the number of pests and their accompanying diseases. It is highly unlikely that an increase in cotton crop yield will come about without proper crop protection measures. Chemical pesticides are thus of the utmost importance since they literally save the crop from destruction. A variety of pesticides belonging to different chemicals groups, organochlorines, organophosphates, carbamates and synthetic pyrethroids are recommended for use. Some of these pesticides are broad-spectrum and can control more than one target. At the same time, some products containing the mixtures of two or more pesticides, generally organic phosphate plus synthetic pyrethroid (OP+SP), are available.
Cotton crop losses due to insect-pests are enormous and it is estimated that 80% of the total pesticides consumed in Pakistan are used to save the cotton crop from the ravages of the pest and diseases. Accordingly, the use of pesticides distinctly stands out as one of the major input factors for increasing crop yield. This was helped by the fact that Pakistan liberalized pesticide import, sale, and use by privatizing its trade in 1981. The result was that cotton production increased steadily year by year , with few pesticide formulations banned thereafter.
Continued indiscriminate application and the persistent nature of most of these pesticides poses a significant risk to the environment as well as to human health. This realization has caused attention to be focused on intensive research for the detection and identification of pesticide residues in cotton seed and the soil in which it is grown. The widespread use of various chemicals in agriculture, especially herbicides, has resulted in growing concern about soil and the whole natural environment . Adsorption of pesticides on soil particles influences other soil processes, thereby affecting the final destination of such compounds in soil . FAO/WHO has recommended the suitability of deltamethrin for application on field and foliar crops. However, its hydrophobic properties create a strong affinity to soil, thus accumulating pesticide remains , leading to environmental concerns of deteriorating soil quality due to slow release of xenobiotics . This has directed much recent research to understand the effects of deltamethrin metabolism in the food chain  stemming from its interaction with soil particle sand their importance from an environmental point of view. Pesticide sorption affects the transportaion, degradation, volatilization, bioaccumulation, and the final fate of these compounds in the soil environment  as well as surface and ground water contamination. Moreover, soils are a heterogeneous mixture of organic and inorganic compounds of varying composition and surface activity, which bind pesticides and reduce their efficiency on target pest species . The importance of organic matter, particle size, as well as pH of the soil for sorption has been well studied [9-10, 4]. However, limited data for the determination of safe dose of deltamethrin available.
The objectives of this study are:
To determine the concentration of Deltamethrin in cotton soil, to characterize the concentration and speciation of various heavy metals in soil, and make recommendations for commercial dosage use that is environmentally responsible.
The present study will be helpful in developing the baseline data for the adsorption of pesticides at variable pH and contact times.
This will help guide the remediation of contaminated soil with pesticides.
It will also provide valuable information on the ability of natural soil to adsorb the selected pesticide under different environmental conditions.
2 Materials and methods
The present study is aimed to investigate the adsorption behavior of the pesticide deltamethrin on soil collected from Dera Ghazi Khan, Punjab Pakistan. The soil was characterized for its total metal content and metal speciation. The residual fractions of deltamethrin in soil and water of cotton fields was analysed on GC-MS to assess the bound xenobiotic to soil particles, runoff to natural waters and potential effects on soil quality. The synthetic batch experiments were conducted at variable pH and dosage of the pesticide to study the adsorption behavior under these variables.
For the study of total metal and chemical speciation of six trace elements, soil samples were collected from Dera Ghazi Khan, city of Punjab (Figure 1). Top soil samples (4-5”) were collected from different fields of cotton. A total of 30-subsoil samples were collected in an X-pattern from at least three different fields. Composite samples were prepared by thorough mixing and blending of 30 sub soil samples representative of whole field was used for the further study and experimental work. The field temperature and pH of samples was also noted. Groundwater samples were collected in plastic bottles in triplicate from hand pumps close to cotton fields of Dera Ghazi Khan.
2.2 General procedure for extraction of deltamethrin
2.2.1 Extraction from water
The fortified samples were transferred to a 2 L separatory funnel and extracted three times vigorously with 25 mL dichloromethane. The extracts were mixed, dehydrated with anhydrous sodium sulfate, filtered and concentrated to a few milliliters in a rotary vacuum evaporator and injected to a GC-MS system. The calibration graph was constructed using known concentration solutions of deltamethrin.
2.2.2 Extraction from soil
Soil sample (50 g) and 150 mL of acetone: hexane mixture was treated in an ultrasonic bath for 1 hour. Mixture was filtered and extract was washed with distilled water. Lower aqueous layer was discarded and remaining hexane was dried with anhydrous sodium sulfate, filtered and concentrated to a few milliliters in a rotary vacuum evaporator and injected to the GC-MS system.
2.3 Soil metal speciation
Each digested soil sample was analyzed for total metal and chemical species content for six selective trace-metals i.e. Zn, Pb, Ni, Cd, Cu and Cr through a sequential extraction procedure using Flame Atomic Absorption spectrophotometer (SpectrAA220, Varian, Australia).
2.4 Batch experiment
The batch experiments were designed to determine the adsorption capacity of the cotton field soil samples towards the selected pesticide. For experimental purposes, the commercial pesticide deltamethrin was used, however pesticide standard obtained from (Ali Akbar Group, Pakistan) was used for the calibration. The active ingredient in commercial pesticide was determined experimentally from the calibration curve of the standard pesticide. It was found to be 3.17%. Nine separate sets of batch experiments were conducted for the adsorption studies at variable pH of 4, 7 and 10; different masses of soil (1, 3 and 5 g), varying concentration of the induced pesticide (1 mg/L, 2 mg/L and 3 mg/L). For each batch, samples were extracted at regular intervals of two minutes until equilibrium was attained. Each extract was run on the pre-calibrated UV-Visible spectrophotometer (UV-1601, Shimadzu, Japan) at 279 nm (λmax for pesticide) and absorbance was noted. Concentration of each extract was determined from the standard calibration curve.
3 Results and discussion
The present research is planned to cover three aspects: (I) to detect the residual concentration of deltamethrin in water and soil samples collected from cotton fields of Dera Ghazi Khan using GC-MS, (II) to determine the concentration of metal species in soil on FAAS, and (III) to determine the batch adsorption for decontamination of applied deltamethrin with natural material soil as adsorbent. All the experiments were performed from January to June, 2011 at applied chemistry Lab in Environmental Sciences department at Fatima Jinnah Women University, Rawalpindi Pakistan.
3.1 Residue analysis
Fragmentation pattern and residue analysis of deltamethrin in soil and water samples run on GC-MS are tabulated in Table 1. The GC-MS analysis of deltamethrin showed that the pesticide is broken down in all soil and water samples. The relative intensity of the basal peak (m/z 253) in water and soil was 0.02% and 3.61%, respectively, with six additional fragments, showing breakdown of the parent compound. However, variation of intensity in the two compartments identifies the specific chemistry of water and soil towards deltamethrin. On the other hand, maximum relative intensity was found for the fragment (m/z 77), suggesting that deltamethrin is degraded mostly into this fragment and also revealing the stability of the fragment itself.
The Koc values were calculated with the following formula:
Where, Kd is amount of pesticide on adsorbent/ amount of pesticide in solution, OC is organic carbon content.
A previous study  reports an average Koc value of 704,000 for deltamethrin, indicating that the active ingredient is tightly bound to the soil and therefore considered as immobile. The present study calculations of Koc yields range from 50,000 to 4000,000. Significantly higher Koc values for soil sample in the present investigation indicates a much longer persistence and immobility, and hence the toxicity of the pesticide on cotton fields.
The large variation in Koc values may be attributed to the pH of soil. In general, for the adsorption of non-ionic molecules , soil pH has a minor effect. On the other hand, ionized and non-ionized molecular (adsorbate adsorbent) interactions and the ratio of species in solution changes with each unit increase in pH. As a result, Kd is expected to vary with changes in pH .
3.2 Soil metal speciation
Total metal content and identification of six metals (cadmium, chromium, copper, nickel, lead and zinc) were individually determined following sequential extraction for composite samples of three cotton field soils.
The average total metal content (mg kg-1) in soil samples from three different cotton fields were found to be:
The 2-tailed correlation between total metal content is found at 0.05 and 0.01 significance levels (Table 2). Significant positive and negative correlation is exhibited by Cd -Cr and Zn-Cd, Zn-Cr, which has also been reported previously . In addition, the positive correlation between Zn-Pb, Cu-Pb, Ni-Cu, Ni-Cd, Ni-Cr and Ni- Pb, matches a previous study .
The relative abundance of total metal content in three soil field samples is illustrated in Fig. 1(a). It is interesting to note that the relative abundance of chromium and nickel is correlated to their position in periodic table: the lower molecular weight elements show greater retention in soil. The highest total chromium concentration may be due to the sewage sludge used to treat the cotton fields of Dera Ghazi Khan. It has been reported in literature that chromium remains in soils as residual fraction for many years because of sewage sludge [16-19]. The high distribution of chromium in soil can also be attributed to its high sorption affinity in the solid phase , thus maintaining more chromium in soil. However, chromium concentrations showed variation in three different cotton fields. Soil samples DCF2 and DCF3 vary by four orders of magnitude higher and lower, respectively, compared to DCF1. On comparison of total metal content in three different cotton fields’ soil samples, it is noted that DCF3 showed the relatively highest concentration of four trace elements as per the sequence of Cr > Ni > Cu > Cd. It may be attributed to the relatively higher organic matter content and relatively more alkaline pH than the other two samples.
The anomalous findings seem to be influenced by factors other than organic matter, such as phosphate and sulfate. A negative correlation between zinc and phosphorus has been reported in literature . It is also confirmed by data that in the field sample (DCF2) where phosphate concentration is lower, Pb and Zn content is higher.
3.2.1 Cadmium species
The present study indicates the average values of cadmium in exchangeable, carbonate bound, Fe-Mn oxide and organic bound species following the sequence of Fe-Mn oxide ≈ Carbonate bound > Exchangeable > Organic bound. The smaller value of organic complexes of Cd  is likely the contributing factor preventing Cd from forming stable complexes with organic matter . There is no significant variation in chemical species concentration between the three different soil samples. However, DCF3 shows a relatively higher concentration. This may be due to the fact that DCF3 soil showed remarkably higher concentration of chlorides, as chlorides have more affinity for forming easily soluble complexes with cadmium .
3.2.2 Chromium species
It is interesting to note that chromium species exhibited the same sequence as cadmium. This may be related to the significant positive correlation of the Cd-Cr pair. However, Cd species show significant variation of species concentrations in 3 different soil samples that may be due variable contributing sources of Cr to soil . The relatively higher fraction of Fe-Mn oxide may have influenced this, as chromium is reported due to its labile nature through water and other supporting systems . Fig. 2(b) shows the percentage concentration of chemical species of chromium.
3.2.3 Copper species
Average percentage concentration of copper species is found to follow the pattern of Fe-Mn oxide > Carbonate > Organic bound > Exchangeable showing 48%, 24%, 22%, and 6%, respectively. It can be seen that most of the copper distribution is in Fe-Mn oxides followed by organic bound. In a similar study by Alina, 2001 , the greatest amount of Cu was found for the Fe-Mn oxides fraction. On the other hand, the exchangeable fraction of copper has the lowest concentration as this fraction does not hold much copper.
3.2.4 Nickel species
Results indicate that nickel species also follow the trend of copper speciation in soil samples. However, nickel species show wider concentration ranges, from 0.01-1.43 mg kg-1, than copper, from 0.01-0.50 mg kg-1. This is liekly due to the fact that nickel can be easily e-mobilized by changes in environment conditions such as pH, redox potential, salinity etc. [26-27]. The present study experimental results are comparable to a previous study [24,28].
3.2.5 Lead species
The distribution pattern of lead is comparable to Cd and Cr species (see Fig. 2b)). However, lead the Fe-Mn oxide fraction shows significantly higher concentration than the similar species of Cd and chromium. It has been investigated that Pb either co-precipitates with metal oxides or are adsorbed at metal oxide surface, thus retaining this chemical species into soil as long term source of contamination .
3.2.6 Zinc species
Metal speciation analysis shows that zinc is strongly associated with reducible (Fe-Mn oxide) fractions to the extent of 78% followed by carbonate (10%), confirming the fact that Zn-oxides in soil are easily transformed mainly to carbonates . This is in agreement with Zn associated [31-32]. The present study also shows a comparison of organic bound higher in sequence to exchangeable fraction with studies [27,33], as shown in Fig. 2(b). The cumulative response of all domains exhibits optimum of organic species of chromium .
3.3 Decontamination: batch adsorption
The experimental results indicate lower residual concentration of deltamethrin in cotton field soil compared to water. This gave an insight to the present research that soil can serve as a degradation agent. Therefore, soil was selected as adsorbent material for decontamination of deltamethrin. Based on this rationale, batch experiments were designed to determine the adsorption capacity of soil for deltamethrin. Nine sets of batch experiments were conducted in order to investigate the effect of varying pH, mass of soil and different concentration of pesticide on adsorption.
3.3.1 Effect of pH
The effect of change in pH on pesticide adsorption was studied. The experiment was conducted at pH of 4, 7 and 10 in order to study the acidic, neutral and basic range. As it is clear from the Fig. 2 that pH has a significant role on adsorption. An increase of 9% and 20% is noted in going from pH 4 to 7 and 7 to 10, respectively.
This behavior may be attributed to hydrolytic degradation of deltamethrin at pH (5-7) in water, surface functional group of the pesticide and nature of characteristics of surface soil . It is also observed that pH 10 is optimum for the adsorption of deltamethrin showing an increase of 29% order of magnitude in comparison to acidic pH (Fig. 3).
The alkaline pH of cotton field soil is found suitable for imidacloprid pesticide . A noteworthy dimension of the effect of pH is on the equilibrium time. The data show that both have a direct relation revealing more time is required for higher pH. Another look into the analysis reveals that pH effect on adsorption is influenced by adsorbent mass and adsorbate concentration as well (see Fig. 4).
3.3.2 Adsorption trends
Adsorption capacity of different field soil for deltamethrin is assessed in relation to its adsorbed concentration till equilibrium. Adsorption is a time dependent parameter as time is required for adsorbate to diffuse into the adsorbent .
A competitive to non-competitive behavior with increase in contact time is observed. The adsorption trend is examined at variable pH, concentration and mass of adsorbent (see Fig. 3).
The results indicate that % adsorption decreases with decreases in pH, mass and concentration. Contact time response depicts that equilibrium is attained in 20-70 minutes. Longer equilibrium time is attributed to higher mass (5 g), induced concentration (2 mg/L) and pH (10).
The Pearson correlation coefficient is a measure of linear association between two variables i.e., adsorption at variable pH 4, pH 7 and pH 10. A correlation matrix is shown in Table 3. A significant negative correlation is found for the removal of deltamethrin on soil at pH 7 and pH 10. It can be deduced from the positive relationship between pH 4 and pH 7 that these pHs play a significant role for the removal of deltamethrin.
The effect of varying concentration (1 mg/L, 1.5 mg/L and 2 mg/L) on adsorption shows a direct relation, suggesting rate of adsorption depends on the induced concentration. The same behavior has been previously reported by Spark and Swift .
A significant increase in adsorption in moving from lower to higher concentration is observed, defining the sequence as: 2 mg/L > 1.5 mg/L > 1 mg/L.
It is clear that concentration also affects the equilibrium time and percentage adsorption. The % increase in adsorption was 2.8% and 0.75 % respectively for 1 mg/L; 1.5 mg/L and 2 mg/L at equilibrium time (see Fig. 3). As reported in the literature, the maximum loading capacity of the adsorbent and the rate of adsorption were found to increase with increases in the pesticide initial concentration . Adsorbate concentration reveals a concentration to mass ratios of 1:1, 1.5:3 and 2:5, suggesting that concentration is not influenced significantly by mass.
Adsorbent mass is another contributing component of adsorption. Three different masses of soil i.e.; 1 g, 3 g, and 5 g were investigated in the present study. An increase of 23% and 27.8% adsorption with increase in mass is observed in going from 1g through 3 g to 5 g. However, a cumulative jump of 50.80% occurs between lowest and highest mass. This increase in adsorption relates to an increase in surface area of adsorbent, which leads to an increase in binding sites thus facilitating the reaction between adsorbent and adsorbate . Adsorption increase with increase of soil content is also reported .
On close observation of a complete batch, sorption-desorption pattern is visible that signifies that adsorbent-adsorbate interaction in a solution is either towards or away from the soil surface.
It is generally observed that the rate of uptake of deltamethrin is rapid in the beginning followed by a zigzag pattern. This behavior is reasonably approximated by an instantaneous equilibrium . It can be explained on the basis that initially there is a rapid reversible sorption of the solute adhering to accessible sites on the soil surface .
A substance like bactericide or fungicide pesticides that kill or retards the growth of microorganisms and is considered to have minimal or no harm on the environment is termed a green biocide or eco-friendly biocide . However, soils as natural resources can be an effective remediate for pesticides and provide an environment friendly resource management.
Major conclusions drawn from present investigation:
Cotton field soil is found to have alkaline pH, and the most abundant total residual metal is Cr followed by Ni and Zn. Two-tailed significant positive correlation is depicted by Cr-Cd.
pH has a significant role on adsorption. pH 10 is optimum for the adsorption of deltamethrin.
Adsorbent mass seems to be independent of change in pH as similar trend of adsorption increase. Adsorbent to adsorbate ratio of 1:1, 3:1.5 and 5:2 show subsequent increases in adsorption depicting that adsorbent mass is independent of adsorbate dose.
Adsorbent-adsorbate interaction in closed-batch removes 95% deltamethrin in 54 minutes.
Time is not significantly affected by mass, dose and pH mass of 5 g, pH 10 and 2 ppm takes relatively slight more time to attain equilibrium.
High Koc in soil samples offer soil as efficient degradation agent proposing remediation model for deltamethrin also indicates its immobility, persistence and toxic nature.
The study recommends effective remediation of contaminated cotton field and provides an environment friendly and natural (soil) resource management.
The authors thank Applied Chemistry Lab at Fatima Jinnah Women University, Rawalpindi Pakistan, for providing the necessary techniques for carrying out the experimental work.
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About the article
Published Online: 2016-12-05
Published in Print: 2016-01-01
Conflict of interest: The authors report no conflict of interest.
Citation Information: Open Life Sciences, Volume 11, Issue 1, Pages 417–426, ISSN (Online) 2391-5412, DOI: https://doi.org/10.1515/biol-2016-0055.
© 2016 Uzaira Rafique et al.. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License. BY-NC-ND 3.0