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Endosulfan Sulfate Sorption on Natural Organic Substances

September 21, 2008

By Bakouri, Hicham El Morillo, Jose; Usero, Jose; Ouassini, Abdelhamid

ABSTRACT: This work proposes a viable remediation method based on the use of natural organic substances (NOSs) that characterize the Mediterranean region to improve the ecological system. A series of experiments, including variable conditions, such as temperature, pH, contact time, and pesticide concentration, were performed to demonstrate the efficiency of endosulfan sulfate removal from water by NOSs. Experimental results showed that the pH and temperature of pesticide solutions negatively affect the adsorption process. The maximum adsorption capacity for a specific initial concentration of endosulfan sulfate (0.5 [mu]g/L) was achieved with Origanum compactum (75%), followed by Cistus ladaniferus and Raphanus raphanistrum (72 and 68%, respectively). The adsorption tests gave very satisfying results and point to the possible application of these supports as a remediation technique to prevent pesticide contamination of aquatic ecosystems.

Water Environ. Res., 80, 609 (2008).

KEYWORDS: natural organic substances, remediation, stir bar sorptive extraction, adsorption, endosulfan sulfate, pesticide.

doi: 10.2175/106143008X266733

(ProQuest: … denotes formulae omitted.)

Introduction

The heavy use of pesticides in agriculture can lead to major environmental problems, particularly when land is managed in a way that is aimed primarily at maximizing agricultural productivity. Because of the widespread use of these products, their fate remains an active area of research, as they pose a major threat to water resources. Numerous studies have reported contamination of aquatic ecosystems by these chemicals (Aga and Thurman, 2001; Cerejeira et al., 2003; Fava et al., 2005; Ilyas Tariq et al., 2004; Shukla et al., 2006; Younes and Galal-Gorchev, 2000). Even when used appropriately, pesticides entail major risks. Their persistence, mobility, and tendency to accumulate in living organisms when ascending the food chain can aggravate their toxic effects and have negative effects on the health and well-being of humans (Erdogrul et al., 2005; Krieger et al., 2001; Sudaryanto et al., 2006).

The transport and destination of pesticides involves complex mechanisms that are influenced by many processes, including volatilization, leaching, adsorption, and chemical and biological decomposition (Gao et al., 1998; Jarvis et al., 1991). The leaching of pesticides receives particular attention, because it directly influences the extent of surface water and groundwater pollution (Vighi and Funari, 1995). There are numerous factors that influence leaching, but the most important are the nature of the soil (clay and organic matter content), irrigation water flow, and temperature. Leaching increases with the frequency of irrigation (Frick et al., 1998); temperature accelerates the breakdown of pesticides, slowing their migration (Singh et al., 2005); and large amounts of clay and organic matter in the soil diminish pesticide mobility (Martinez Vidai et al., 1994). Moreover, it is generally accepted that adsorption of pesticides by soils is more closely related to the organic matter content of the soil than any other single property (Barriuso et al., 1992; Coquet, 2002; Huang and McKercher, 1984; Spark and Swift, 2002; Stevenson, 1972).

Chlorinated pesticides are considered the most dangerous products used in crop protection (Regitano et al., 2001). Endosulfan [6,7,8,9,10,10-hexachloro-1,5,5a,6,9,9a-hexahydro-6,9-methano-2,3, 4- benzo(e) dioxathiepin-3-oxide] is widely used throughout the world and is considered a very hazardous and toxic pollutant for living things and the environment (Golfinopoulos et al., 2003). It has been included on the list of priority substances in the area of water policy established by the European Community (Decision 2455/2001/ EC, 2001).

Commercial endosulfan is synthesized as a mixture of two isomers- approximately 70% alpha-endosulfan and 30% betaendosulfan. These have half-lives of only a few days in water, but the toxic biological metabolite, endosulfan sulfate, has a halflife of several weeks in water (Peterson and Batley, 1993). This metabolite persists longer in the soil and has bioaccumulation potential (Sutherland et al., 2002). Endosulfan contamination is frequently found in the environment, even at considerable distances from the point of its original application (Sethunathan et al., 2002; Siddique et al., 2003). It has also been detected in the atmosphere, soils, sediments, estuaries, surface and ground waters, and foodstuffs (Berrakat et al., 2002; Bhattacharya et al., 2003; Cerejeira et al., 2003; Golfinopoulos et al., 2003; Sujatha et al., 1999). As endosulfan is found in groundwater samples, it is apparent that there is significant mobility of this chemical through the soil (Ro et al., 1997; Spark and Swift, 2002). Therefore, it is of paramount interest to develop a simple remediation method to prevent and control contamination of groundwater by this pesticide.

Advanced pesticide removal methods are typically needed to meet environmental quality requirements and improve the ecological system. These include combinations of biological, chemical, and physical processes. Adsorption has evolved into one of the most effective physical processes for pesticide removal, because the technique uses equipment that is readily available, easy-to-use, and not energy-intensive, and also because the treatment is costeffective (Asian and Turkman, 2004; Boyd et al., 2001; Carrizosa et al., 2000; Shukla et al., 2002; Sudhakar and Dikshit, 1999).

This study proposes a new viable remediation technique based on the use of a wide array of natural organic substances (NOSs) that characterize the Mediterranean region as barriers to prevent the mobility of endosulfan from agricultural soil to groundwater resources. This tool can help farmers modify their fanning practices, with a view to preserving water quality. The adsorbents studied for this purpose were leaves of the following plants: Casuarina cunninghamiana, Eucalyptus gomphocephala, Populus nigra, Raphanus raphanistrum, Nerium oleander. Origanum compactum, and Cistus ladaniferus. The NOSs tested can be added to agricultural soil to increase the sorption of pesticides used for crop protection and can improve soil fertility, because they are organic soil amendments. The use of these adsorbents in fields can reduce pesticide leaching, so that the amount of pesticide applied by farmers can be reduced, which will afford a two-fold benefit- economic and ecological. The study evaluated the effects of several physicochemical parameters, such as pH of pesticide solution, contact time, temperature, adsorbent quantity, and pesticide concentration, on sorption processes. Finally, a comparative study of the adsorption capacity of each NOS was carried out using the Freundlich adsorption coefficient (K^sub f^ and n^sub f^) determined from sorption isotherms.

Materials and Methods

Chemicals. A standard stock solution of endosulfan sulfate dissolved in acetonitrile (10 mg/L) was obtained from Dr. Ehrenstorfer GmbH (Augsburg, Germany). Working solutions were prepared by diluting the stock solution first with methanol and then with ultrapure water. The percentage of solvent in the final pesticide solution was less than 0.1%. The standard stock and working solutions were stored at 4[degrees]C. They were used to prepare dilute solutions and spike water samples to the required concentrations. All the organic solvents were of analytical grade and were obtained from Merck (Darmstadt, Germany). Methylene blue was purchased from Fluka (Buchs, Switzerland). The water used was purified using a Milli-Q water-purification system (Millipore, Bedford, Massachusetts).

Equipment Termal desorption-gas chromatography with mass spectrometry (TD-GC-MS) analysis was performed using a Gerstel TDS 2 thermal desorption system equipped with a Gerstel MPS-2 autosampler and a Gerstel CIS 4 programmable temperature vaporization (PTV) inlet (Gerstel, Mullheim a/d Ruhr, Germany). Gas chromatography was performed with an Agilent 6890 gas chromatograph with a 5973 mass- selective detector (Agilent Technologies, Santa Clara, California).

The commercial Twister stir bar for sorptive extraction was obtained from Gerstel. It consists of a glass-encapsulated magnetic stir bar, 2 cm long, externally coated with polydimethylsiloxane (PDMS).

An ATI Unicam UV-visible spectrophotometer (Cambridge, United Kingdom) was used to determine the specific surface area of the NOS at 665 nm using standard solutions with different concentrations of methylene blue.

Adsorbent Preparation. The NOSs selected for this work were collected from Tangier province (northwest Morocco) and kept at ambient temperature (25 +- 2[degrees]C) in the laboratory for a week. They were then dried at 70[degrees]C in the oven for 3 days before crushing them in an electric mixer. The sieving of the various NOSs was carried out with a vibratory sieve (Fritsch, Idar- Oberstein, Germany). Particle sizes of less than 500 [mu]m were recovered for the extraction step.

Approximately 10 g of every NOS were extracted two times with ultrapure water (75 mL) at ambient temperature (25 +- 2[degrees]C) with stirring for 24 hours. The adsorbent was then placed in methanol (25 mL) in an ultrasonic bath for 5 hours (frequency 35 kHz, 32OW) to increase the extraction recovery of molecules soluble in this solvent, such as chlorophyll (driver of the physiological function of leaves) and other aromatic organic compounds (Latasa et al., 2001). The adsorbents were additionally washed in a classic soxhlet apparatus, in which 10 g of each NOS were placed in the cartridge with 200 mL of acetone. The extraction was carried out for 12 hours, at the end of which, it can be assumed that a large part of the organic compounds that may have a matrix-enhancement effect have been eliminated (Mourabit et al., 2002; Stajnbaher and Zupancic Kralj, 2003).

Specific Surface Area Determination. The methylene blue adsorption method was used to determine the specific surface area of each adsorbent. This method is used widely for solids of various kinds, such as oxides, graphite, yeast, activated carbons, and calcium carbonate (Graham, 1955; Savitsky et al., 1981; Tanada et al., 1980). Methylene blue was dried at 110[degrees]C for 2 hours before use, and the concentrations of the methylene blue solutions were analyzed by measuring their absorbance at 665 nm using the UV- visible spectrophotometer. This wavelength corresponds to the maximum absorption peak of the methylene blue monomer (Bergman and O’Konski, 1963).

First, a kinetic adsorption study was carried out to find the adsorption equilibrium time, which was determined by a series of measurements extending from 30 minutes to 24 hours at 25[degrees]C. In the presence of the solid, adsorbing solutions reached complete equilibrium in approximately 8 hours, and this was chosen as the time for measuring the adsorption isotherm of all samples.

Adsorption measurements for all samples were carried out as follows: 1 g of each adsorbent was put in 100 mL of methylene blue solution of known concentration and stirred with a stirring rod until the adsorbent was mixed into the dye solution. The mixture was maintained at 25[degrees]C and continuously shaken at 800 rpm for 8 hours. After this time, the methylene blue uptake onto each adsorbent was calculated by the difference between the methylene blue concentration before and after adsorption.

A calibration curve of optical densities plotted against methylene blue concentrations was obtained by using standard methylene blue solutions of known concentrations at pH values between 7.5 and 8. The experimental data were fitted by a straight line with a high determination coefficient (R^sup 2^ = 0.998).

Different concentrations of methylene blue (10, 20, 40, 60, 80, and 100 mg/L) were used to obtain an adsorption isotherm, with three replications for each adsorbent. The Langmuir equation was used to calculate the specific surface area of the natural organic substances. The general form of the Langmuir isotherm is as follows (Langmuir, 1918):

C/N = C/N^sub m^ + 1/K . N^sub m^ (1)

Where

C = equilibrium concentration of the methylene blue solution (mg/ L),

K = a constant,

N = number of moles of methylene blue adsorbed per gram of adsorbent at equilibrium concentration, and

N^sub m^ = number of moles of methylene blue per gram of adsorbent required to form a monolayer.

The specific surface area was calculated by the following equation (Gregg and Sing, 1982):

S = N^sub m^ . a . N . 10^sup -20^ (2)

Where

S = specific surface area (m^sup 2^ g^sup -1^),

a = surface area occupied by one molecule of methylene blue (197.2 A^sup 2^) (Graham, 1955), and

N = Avogadro’s number (6.02 x 10^sup 23^ mol^sup -1^).

Adsorption Tests. In this study, endosulfan sulfate (insecticide) was used for determining the adsorption efficiency of each NOS. The adsorption tests were carried out by adding 0.5 g of each adsorbent to 50 mL of pesticide solutions of different concentrations (solid/ liquid ratio =10 g/L). After stirring for 5 hours (800 rpm), the solution was centrifuged at 5000 g for 10 minutes, and the supernatant was recovered.

Pesticide solutions of various concentrations (0.1, 0.25, 0.5, 1, 2, 5, 10, and 25 [mu]g/L) were used to quantify the adsorption capacity of each NOS. Data obtained were adjusted to the Freundlich equation, as follows:

… (3)

Where

Q^sub e^ = amount of pesticide adsorbed per gram of adsorbent (mg/ g);

C^sub e^ = equilibrium concentration of pesticide in the solution (mg/L); and

K^sub f^ and n^sub f^ = adsorption coefficient and adsorption constant, respectively.

This relation implies an energy distribution that can be explained by the heterogeneity of the adsorbent surface (Osma et al., 2007).

To determine the endosulfan sulfate concentration, we applied a stir bar sorptive extraction technique, which consisted of the adsorption of pesticides contained in the recovered solution by using a stir bar (Twister) coated with PDMS. The Twister was placed in an Erlenmeyer flask containing 40 mL of the supernatant solution. The extraction was performed over a period of 12 hours, with a stirring speed of 800 rpm, at room temperature (25 +- 2[degrees]C) using a 15-position magnetic stirrer. After extraction, the Twister was placed in an empty glass thermal desorption tube (187 mm x 4 mm internal diameter). All glass tubes containing a Twister were placed on the MPS-2 injection system tray and successively inserted to the thermal desorption module by the autosampler.

The thermal desorption system was programmed to increase at 60[degrees]C/min from 40 to 280[degrees]C, and remained at this temperature for 7 minutes. In the meantime, the desorbed analytes were trapped on a liner (Tenax and quartz wool filling) in the CIS 4 PTV injector (Gerstel) at 30[degrees]C. Finally, the CIS 4 was set to increase the temperature from 30 to 300[degrees]C (held for 7 minutes) at 12[degrees]C/s to inject the trapped analytes into the gas chromatography column. Injection was performed in the solvent vent mode. Separation was accomplished on a DB-5 MS fused silica column (30 m x 0.25 mm internal diameter, 0.250-[mu]m film thickness, Agilent Technologies). After completion of the desorption stage, the oven temperature was kept at 70[degrees]C for 2 minutes and, after that, set to rise from 70 to 150[degrees]C at 25[degrees]C/min, then to 200[degrees]C at 3[degrees]C/min, and finally to 300[degrees]C at 8[degrees]C/min. This temperature was maintained for 10 minutes. The flow of helium (carrier gas) was adjusted using Agilent Retention Time Locking (RTL) software (Agilent Technologies), so that chlorpyrifos-methyl was eluted at a constant retention time of 16.59 minutes. The mass spectrometer was operated in selected ion monitoring mode with electron ionization. The amount of pesticide adsorbed was considered to be the difference between that initially present in solution and that remaining after equilibration. The endosulfan sulfate detection limit (DL) was estimated by using the following equation:

DL = y^sub B^ + 3 . S^sub D^ (4)

Where

y^sub B^ = blank signal (signal of water without target analyte), and

S^sub D^ = standard deviation of the blank (Miller and Miller, 1994).

The detection limit was verified by using the U.S. Environmental Protection Agency (Washington, D.C.) (U.S. EPA) method (U.S. EPA, 2003). A volume of spiked water was prepared to yield seven replicate aliquots, with a concentration equal to the estimated detection limit. The detection limit was then determined by multiplying the standard deviation by 3.143 (the Student’s i- statistic at a 99% percentile for n – 1 degrees of freedom). The resulting detection limit was 0.001 [mu]g/L.

Statistical Analysis. Statistical evaluation of differences among the experimental results was performed using analysis of variance (ANOVA). In the tests, the hypotheses of variance normality and homogeneity (tested with Cochran and Bartlett tests) were successful. Post-hoc comparisons (Fisher’s least significant difference [LSD] p

Results and Discussion

Specific Surface Area Determination. For all the isotherms of methylene blue adsorption onto NOSs, the plot of CIN versus C gave a straight line, with a slope equal to 1/N^sub m^ and intercept equal to 1/K . N^sub m^. Therefore, the Langmuir isotherm is an adequate description of the adsorption of the mthylene blue onto NOSs.

The specific surface areas of the different adsorbents are shown in Table 1. We observe that Cistus ladaniferus gave the highest value for specific surface area (311.42 m^sup 2^/g), and Casuarina cunninghamiana gave the lowest (233.47 m^sup 2^/g). The ANOVA analysis of the results showed a significant difference among specific surface areas of different NOSs tested (p

Effect of pH. The pH of the solution may affect the adsorption process (Gao et al., 1998; Roy and Krapac, 1994). Studies on the influence of pesticide solution pH on the adsorption efficiency of each NOS were carried out in the pH range 2 to 10, using 50 mL of pesticide solution (0.5 [mu]g/L) and 0.5 g of each adsorbent. The results obtained were expressed in terms of percentage of pesticide adsorbed by each NOS (A^sub d^) and calculated as follows:

A^sub d^ = [(C^sub o^ - C)/C^sub o^] x 100 (5)

Where

C^sub o^ and C = initial and final concentrations of pesticide in the water sample, respectively.

Table 2 shows that the quantity adsorbed by each NOS decreased with increasing pH. The highest adsorption efficiency was obtained at pH 2, which is in agreement with the results of other studies of pesticide adsorption (Al-Qodah et al., 2007; Kyriakopoulos et al., 2005; Zuhra Memon et al., 2007). The pH affects the surface properties of the sorbent. At a very low pH, the surface of the sorbent would be surrounded by hydronium ions, which can enhance the sorbate interaction with binding sites of the sorbent because of greater attraction forces, and thus increase its uptake on polar adsorbents (Zuhra Memon et al., 2007). Effect of Temperature. The effect of temperature on the adsorption of endosulfan sulfate by using different NOSs was studied in the range 10 to 35[degrees]C. For this, 0.5 g of each adsorbent was added to 50 mL of pesticide solution (0.5 [mu]g/L) (solid/liquid ratio = 10 g/L). This study showed a slight decrease in the removal efficiency of endosulfan sulfate with temperature (Table 3). The ANOVA analysis of the results showed that temperature had a significant effect on the adsorption of endosulfan sulfate (p

Kinetic Study. The study of the kinetics of endosulfan sulfate adsorption on NOSs was carried out using 0.5 g of each adsorbent and 50 mL of pesticide solution (0.5 [mu]g/L), with stirring from 30 minutes to 24 hours (800 rpm). Figure 1 shows that the removal efficiency increased with longer contact time for all the NOSs studied. The endosulfan was adsorbed on the various NOSs in a 3- stage process. In the first phase, more than 50% of the endosulfan was transported into the macropores after 1.5 hours of contact with Origanum compactum, Cistus ladaniferus, and Raphanus raphanistrum, as opposed to only 25% for Casuarina cunninghamiana. During the second phase, a slight decrease in the adsorption kinetics was noted, most likely because of slow diffusion of the pesticide into the smaller pores and irregularities on the adsorbent surface (Kaoua et al., 1987). The final stage took place after 7 hours of contact and remained even after 10 hours. Origanum compactum showed the maximum capacity of endosulfan sulfate adsorption (75%), followed by Cistus ladaniferus and Raphanus raphanistrum (72 and 68%, respectively).

The rate of the pesticide adsorption reaction (r) was determined in the range of time between 30 minutes and 1.5 hours. The kinetic constant (K^sub c^) and the order of reaction (n^sub c^), respectively, were determined by the values of intercept and slope of the logarithm of the rate equation of the following adsorption reaction:

ln r = In K^sub c^ + n^sub c^ ln (C^sub o^ – C^sub t^) (6)

Where

C^sub o^ and C^sub t^ = equilibrium concentration of pesticide at the start and at time t, respectively.

The experimental values of K^sub c^ and n^sub c^ (Table 4) showed that the adsorption reaction was consistent with first-order kinetics (n^sub c^ [asymptotically =] 1). The statistical ANOVA confirmed that K^sub c^ was significantly affected by the specific surface area (p

Adsorption Model. The importance of mathematical modeling of adsorption isotherms is related to the possibility of obtaining characteristic coefficients of the adsorption process, which are generally compared with values obtained for various pesticide/ adsorbent systems. According to the results of the kinetic study and to achieve a compromise between removal efficiency and duration of the full analysis, 5 hours was chosen as the time for obtaining the adsorption isotherm.

According to the Freundlich equation, the quantity adsorbed increases with increasing concentrations of pesticide solution. In general, although it is not very satisfactory for high concentrations, it gives a good description of the adsorption of contaminants at very low levels, which is the case of pesticides in water (Gicquel et al., 1997). The Freundlich isotherm can also be expressed in linear form after logarithm linearization, as follows:

ln Qe = ln K^sub f^ + 1/n^sub f^ ln C^sub e^ (7)

A statistical analysis was performed by linear regression to calculate the isotherm coefficients K^sub f^ and 1/n^sub f^ (Figure 2). The relevant parameters for fitting the Freundlich equation are summarized in Table 5.

By comparing the adsorption behavior of each adsorbent, it was observed that endosulfan sulfate had a higher K^sub f^ for Origanum compactum (7.31) and that the Freundlich isotherm exponent 1/n^sub f^ was similar for all the NOSs, with a value close to 1, which illustrates a linear increase in the adsorption concentration to the equilibrium concentration. Analysis of the results showed that there was not a significant correlation between K^sub f^ values and the specific surface area of the adsorbents (p > 0.05), which indicates that the adsorption of endosulfan sulfate depends on the structure and texture of the NOS tested.

We can use the K^sub f^ values to differentiate these adsorbents. The adsorption capacity is highest for Origanum compactum and decreases in the following order: Cistus ladaniferus, Raphanus raphanistrum, Nerium oleander, Populus nigra, Eucalyptus gomphocephala, and Casuarina cunninghamiana.

Previously published data on organic compound adsorption using different materials show that the Freundlich isotherm model is the one that seems to describe the adsorption process best (Estevinho et al., 2006). Information on pesticide removal by NOSs was not found in the available literature. Studies of endosulfan metabolite adsorption using four Indian soils showed K^sub f^ values between 0.033 and 0.521 (Kumar and Philip, 2006). Thus, the obtained K^sub f^ values are higher than the K^sub f^ values on soil, which proves the efficiency of NOSs in retaining endosulfan. The results of this study are of interest and point to the possible use of these adsorbents as an ecological barrier to prevent pesticide contamination of groundwater resources.

Conclusions

The removal efficiency of endosulfan sulfate demonstrates that this remediation technique may be an effective management practice to prevent pesticide contamination of aquatic ecosystems, because 0.5 g of Cistus ladaniferus was able to remove more than 75% of endosulfan sulfate from a pesticide solution (0.5 [mu]g/L). This high adsorption percentage was found, even though the surface area of the NOSs studied was less than other substances used as adsorbents (activated carbon, clay, etc.), which revealed the physicochemical affinity of NOSs for adsorbing pesticides strongly on their surface.

The removal efficiency of endosulfan sulfate increased with decreasing pH and temperature, and it was confirmed that adsorption increases with the quantity of NOSs used. According to the kinetic adsorption data, the equilibrium time for deriving the adsorption isotherm is 90 minutes. The Freundlich isotherm was the best approach for adsorption equilibrium data correlation. The Freundlich constant (K^sub f^) values showed that adsorption capacity depends mainly on the nature of each adsorbent. The removal efficiency was highest for Origanum compactum and decreased in the following order: Cistus ladaniferus, Raphanus raphanistrum, Nerium oleander, Populus nigra, Eucalyptus gomphocephala, and Casuarina cunninghamiana.

Credits

The authors are grateful to the Ministry of Science and Technology (Spain) for financial support this research through the National Plan of Scientific Research, Development, and Technological Innovation (Project Ref. CGL2006-11646/HID). Helpful proofreading by Eva Vanderlinden and Martha Hobart-Burela is greatly appreciated.

Submitted for publication June 25, 2007; revised manuscript submitted November 30, 2007; accepted for publication December 11, 2007.

The deadline to submit Discussions of this paper is October 15, 2008.

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Hicham El Bakouri1*, Jose Morillo1, Jose Usero1, Abdelhamid Ouassini2

1 Department of Chemical and Environmental Engineering, University of Seville, Seville, Spain.

2 Department of Chemical Engineering, University of Abdelmalek Essaadi, Tangier, Morocco.

* Department of Chemical and Environmental Engineering, University of Seville, 41092, Seville, Spain; e-mail address: elbakouri@us.es.

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