The removal of pesticide (ethoprophos) from aqueous solution using a natural biosorbent such as chitosan (CH) prepared from a biopolymer waste obtained from marine industry was studied. The Fourier Transform Infrared Spectroscopy (FTIR), Scanning electron microscopy (SEM), and X-ray diffraction spectroscopy (XRD) were used to study the structure of the adsorbent. The biosorption studies were carried out under various parameters, such as biosorbent dose, initial pesticide concentration and contact time. The experimental results show that the removal percentage of ethoprophos increased from 85.693% to 89.234%, as adsorbent dose (CH) increased from 0.02 to 0.1 g/100ml. The equilibrium uptake was increased with an increase in the initial pesticide concentration in solution. Biosorption kinetic data were fitted well with the pseudo-second order kinetic model. The experimental isotherms data were analyzed using Freundlich, Langmuir, Temkin and Dubinin-Radushkevich (D-R) isotherm equations. The best fit was obtained by Freundlich isotherm with high correlation coefficients. That the value of energy calculated from the D-R isotherm was 5.56 KJ/mol suggests the adsorption of ethoprophos on Chitosan is physical. All the results indicating CH was chosen as low-cost biosorbent could be applied for the removal of organophosphorous pesticide from aqueous solutions.
The contamination of surface and ground water by pesticides has become a serious environmental problem in recent years due to the extensive application of these agrochemicals in crop farms, orchards, fields and forest lands. This contamination arises from surface runoff, leaching, wind erosion, deposition from aerial applications, industrial discharges and various other sources. Consequently, pesticides have frequently been detected in water bodies in different countries of the world [1-3]. Pesticides are harmful to life because of their toxicity, carcinogenicity and mutagenicity [
Hydrochloric acid, oxalic acid, potassium permanganates, sodium chloride, potassium hydroxide, sodium hydroxide and O-ethyl S,S-dipropyl phosphorodithioate were purchased from Merck, Germany, and were used without further purification.
The pesticide used as adsorbate in the experiments is ethoprophos. Some properties and chemical structure of the pesticide is given in
The shrimp shells were collected from sea food shops and washed under running water to remove soluble organics, adherent proteins and other impurities. Then it dried in oven at 70˚C for 24 h or longer until completely dried shells were obtained.
The chitinous material (shells of the shrimp) was decalcificated with 1.0 M HCl (3.0%w/v) at room temperature with constant stirring for 1.5 hours. The decalcified product was filterated, washed and dried, then deproteinized with 4% NaOH solution at 50˚C with constant stirring for 5 hours. The deproteinized chitin was filtered and washed with de-ionized distilled water until the pH became neutral. It was dehydrated twice with methanol, and once with acetone, and dried. The dried chitin was added to boiling 0.1% potassium permanganate solution to remove the odor and to 15% oxalic acid solution to remove the color. The product chitin was filtered, washed
Data were obtained from [
with distilled water and dried. The chitosan was prepared by adding the dried chitin into a three-necked flask containing a solution of 40% (w/v) KOH, Scheme 1. It was refluxed under nitrogen atmosphere at 135˚C - 140˚C for 2 hours [
The adsorption experiments of ethoprophos onto CH were carried out in a set of 150 Erlenmeyer flasks. 100 ml of the pesticide solutions of various initial concentrations in the range 10 - 60 mg/L were added to separate flasks and a fixed dose of 0.1 g of CH was added to each flask covered with glass stopper at normal pH 5.48, room temperature (25˚C ± 2˚C), for contact time 24 h, with occasional agitation to reach equilibrium. The CH dose used is the optimum in the range of initial concentrations of pesticide studied and was obtained from preliminary studies. For kinetic studies of ethoprophos onto CH, 100 ml of the solution containing 10 - 60 mg/L with 0.1 g of CH for different time intervals from 5 to 300 minutes to determine the equilibrium time. From the triplicate flasks, 40 ml of filtrate was transferred to a separatory funnel and extracted successively three times with 20, 15 and 10 ml portions of dichloromethane. The combined extract was dried on anhydrous sodium sulfate to remove moisture content and evaporated using a rotary evaporator on a water bath at 40˚C. The extracted samples were analyzed using GC-FPD. Isothermal studies of ethoprophos were conducted with an adsorbent quantity of 0.1 g of CH with pesticide concentrations of 10 - 60 mg/L in identical conical flasks containing 100 ml of distilled water. Blank solutions were treated similarly (without adsorbent).
Infrared spectra were measured by a Ati Mattson FTIR spectrophotometer. The deacetylated chitin (chitosan) CH was subjected to infrared spectroscopy to calculate the degree of deactylation (D.D) %, by the relationship:
Where, A = absolute height of the absorption band of the amide group and hydroxyl group respectively [
Swelling is the most significant characteristic of hydrogels and it reflects the affinity of the chemical structure of hydrogels for water and other surrounding fluids.
Gel was prepared by dissolving 2% (w/v) chitosan in 1% (v/v) aqueous acetic acid with constant stirring at room temperature. The vicious solution prepared was filtered through a cheese cloth to remove any impurities and cast in petri dish to dry forming thin film. A known weight of the CH film was immersed in solutions of different pH (5, 7) at 40˚C and 25˚C until the swelling equilibrium was reached. The film was removed, dried with absorbent paper to get rid of excess water then weighed. The degree of swelling of these samples was calculated with the following equation:
Where m and denote the weights of sample and dried sample, respectively [
Molecular weight is also one of the significant characteristics that control the functional properties of CH. Viscosity is one of the simple techniques that is widely used for estimation of the molecular weights of polymers. The viscosity-average molecular weight was calculated using Mark-Houwink equation relating to intrinsic viscosity [
Where Km = 8.93 × 10−4 and a = 0.716 at 25˚C are the empirical viscometer constants that are specific for a given polymer, solvent and temperature.
The thermal stability of the chitosan was studied using a thermogravimetric analyzer (TGA). All TGA spectra were recorded under a nitrogen atmosphere up to 600˚C using a programmed rate of 10˚C/min.
XRD patterns were obtained on Siemens (Berlin, Germany) D500 diffractometer with a back monochromator and a Cu anticathode.
Phase morphology was studied using a JSM-T20 (JEOL, Tokyo, Japan) scanning electron microscope (SEM). For scanning electron observations, the surface of the sample was mounted on a standard specimen stub. A thin coating (~10−6 m) of gold was deposited into the sample surface and attached to the stub prior to SEM examination in the microscope to avoid electrostatic charging during examination.
The adsorption capacity was determined by using the following equation, taking into account the concentration difference of the solution at the beginning and at equilibrium [
Where C0 and Ce are the initial and the equilibrium ethoprophos concentration mg/L, respectively, V is the volume of solution (ml) and m is the amount of adsorbent used (g). The removal percentage can be calculated as:
Removal percentage = (5)
The concentration of ethoprophos was determined by HP 7890 A series Gas Chromatograph (GLC), equipped with Flame Photometric Detector (FPD) operated in the phosphorus mode (525 nm filter) under the following conditions. The used column was PAS: 1701 (30.00 m × 0.32 mm and 0.25 mµ film thickness. Detector temperature was 250˚C, injector temperature was 245˚C, and the column temperature was programmed so that reaches to 190˚C and hold on 2 minutes, then rose to 240˚C, at a rate of 10˚C min−1 and hold on 5 minutes. Nitrogen carrier gas flow rate was 4 ml∙min−1, hydrogen flow was 75 ml∙min−1 and air flow was 100 ml∙min−1.
From FTIR analysis was shown in
powder shows characteristic hydroxyl group and stretching vibrations of C-O group at 1787 cm−1. The bending vibrations of C-H bond in -CH2 are located at 2926 cm−1 and in -CH3 group are located at 2876 cm−1. The band at 1651 cm−1 is related to the stretching vibrations of amide group carbonyl bonds C-O and the band at 1597 cm−1 is related to the stretching vibrations of amine group. Bending vibrations of methylene and methyl groups are located at 1322 cm−1 and 1450 cm−1, respectively. The spectrum in the range from 1150 to 1000 cm−1 is attributed to stretching vibrations of C-O groups. The characteristic bands needed to confirm complexation between chitosan and Ethoprophos are shifted to the higher frequency.
Characteristic amide group from chitosan located at 1651 cm−1 is shifted to higher frequencies, to 1720 cm−1. The peak located at 1540 cm−1, which corresponds to the NH2-group form chitosan, is shifted to higher frequency,1590 cm−1, indicating that the acetic acid residue (CH3COO-) is attached to amine group in the chitosan chain. The shift of amide and amine group can be related to the electrostatic interaction between these groups and the negatively charged sites in the Ethoprophos structure, which confirm complexation between chitosan and Ethoprophos. The changes in the FTIR spectrum of the powders after binding with the Ethoprophos are significant. The -OH stretching, observed as strong broad band at 3445 cm−1 in unloaded CH, shifts to 3430 cm−1 after binding Ethoprophos. These results confirm that -CONH2, -NH2 and -OH groups from chitosan, are involved in binding of Ethoprophos.
It is evident from the results of the swelling properties represented in
These properties decreased at pH = 5, but increased at pH = 5 at 40˚C, pH = 7 at 25˚C and pH = 7 at 40˚C. These results are attributed to the increase in the mobility of the molecules with increase of temperature; this leads to increase in the porosity of the surface of the chitosan molecules and increase in its swelling properties. At Ph < 7, the decrease in the ratio of -OH groups leads to decrease in the swelling properties. The chitosan polymer which contains more amide groups cannot be easily protonated in acidic solutions. Since the electron withdrawal by carbonyl groups makes the nitrogen of the amide groups a much poorer source of electrons than that of the amino groups, and so the electrons are less available for sharing with hydrogen ions.
The viscosity-average molecular weight (Mv( of the prepared chitosan sample CH was determined by using an Ostwald viscometer at 25˚C in 0.3 M acetic acid and 0.2 M sodium acetate buffer solution and were calculated by using equation [
The previous investigations have shown that the decomposition of chitosan has 2 endothermic processes, the first one around 60˚C for water evaporation, and the second one starts around 225˚C and reaches a maximum 260˚C. Thermal degradation of pure chitosan, as Shown in
SEM is a widely used technique to study morphology
and surface characteristics of the adsorbent.
The XRD pattern of chitosan prepared from shrimp shells waste at
Adsorbent dose is an important parameter influencing adsorption processes since it determines the adsorption capacity of an adsorbent for a given initial concentration of the adsorbate at the operating conditions. The effect of CH dose on removal of ethoprophos was studied in range of 0.02 - 0.1 g/100ml.
The effect of ethoprophos concentration by CH was studied at different initial pesticide concentrations of 10, 20,
30, 40, 50 and 60 mg/L.
The effect of contact time was first investigated to deter-
mine the equilibrium time for ethoprophos adsorption onto CH at 25˚C ± 2˚C.
The equilibrium adsorption isotherm has the importance in the design of adsorption systems [
The Langmuir equation, which is valid for monolayer adsorption onto a completely homogeneous surface with a finite number of identical sites and with negligible interaction between adsorbed molecules, is represented in the linear form as follows [
Where qe (mg/g) and Ce (mg/L) are the amount of adsorbed ethoprophos per unit mass of adsorbent and ethoprophos concentration at equilibrium, respectively. Qm is the maximum amount of the ethoprophos per unit mass of adsorbent to form a complete monolayer on the surface bound at high Ce and b is the Langmuir constant (L/mg). The plot of specific adsorption (Ce/qe) against the equilibrium concentration (Ce)
the adsorption obeys the Langmuir model. The Langmuir constants qm and b were determined from the slope and intercept of the plot and are presented in
Where C0 is the highest initial concentration of adsorbate (mg/L), and KL (L/mg) is Langmuir constant. The RL values between 0 and 1 indicate favorable adsorption.
The Freundlich isotherm, on the other hand, assumes a heterogeneous sorption surface with sites that have different energies of sorption. The Freundlich model can be represented as:
Equation (9) can be linearized in the logarithmic form (Equation (10)) and the Freundlich constants can be determined:
Where Kf is the relative adsorption capacity of adsorbent and nf is a constant related to adsorption intensity. The plot of log qe versus log Ce should give a straight line with a slope of 1/nf and intercept of ln Kf
Temkin adsorption isotherm model was used to evaluate the adsorption potentials of the ethoprophos on CH. The derivation of the Temkin isotherm assumes that the fall in the heat of sorption is linear rather than logarithmic, as implied in the Freundlich equation. The Temkin isotherm
has commonly been applied in the following form [63- 65];
The Temkin isotherm, Equation (11) can be simplified to the following equation:
Where β = (RT)/b, T is the absolute temperature in Kelvin and R is the universal gas constant, 8.314 J (mol/K). The constant b is related to the heat of adsorption qe (mg/g) and Ce (mg/L) are the amount adsorbed at equilibrium and the equilibrium concentration, respectively. A and B are constants related to adsorption capacity and intensity of adsorption. Plots of lnCe against qe for the adsorption of ethoprophos onto CH are given in
D-R model was applied to estimate the porosity apparent free energy and the characteristic of adsorption [
Where K is a constant related to the adsorption energy, qe (mg/g) is the amount of pesticide adsorbed per g of adsorbent and qm represents the maximum adsorption capacity of adsorbent, β (mol2/J2) is a constant related to adsorption energy, while ε is the Polanyi potential can be calculated from Equation (14):
The values of β and qm can be obtained by plotting ln qe vs. ε2. The mean free energy of adsorption (E, J/mol), defined as the free energy change when one mole of ion is transferred from infinity in solution to the surface of the sorbent, was calculated from the K value using the following relation (Equation (15)) [
The calculated value of D-R parameters is given in
The kinetic studies provide useful data regarding the efficiency of adsorption process and feasibility of scale-up operations [
The pseudo-first order kinetic model can be expressed in a linear form as:
Where qe and qt are the amount of ethoprophos adsorbed (mg/g) on the adsorbent at the equilibrium and at time t, respectively, and k1 is the rate constant of adsorption (min−1). Values of k1 were calculated from the plots of log (qe − qt) versus t. The application of this equation to the data of ethoprophos on CH (data not shown) indicated the inapplicability of the model.
The Pseudo-second order kinetic model can be represented as:
Where K2 is the rate constant for the pseudo-second order kinetics (g/mg min). The linear plot of (t/qt) versus t is shown in
The parameters calculated for the pseudo-second order kinetic model are listed in
In order to identify the diffusion mechanism, the intraparticle diffusion model can be represented as:
Where Ki is the intraparticle diffusion rate constant and C is a constant which gives information about the thickness of boundary layer. The intraparticle diffusion
rate constants values are shown in
In this study, the biosorption removal of ethoprophos from aqueous solution by Chitosan as a low-cost and natural available adsorbent was investigated. The results show that the increase in mass biosorbent leads to increase in pesticide biosorption due to increase in number of biosorption sites. The equilibrium uptake was increased with increasing the initial concentration of pesticide in solution. The equilibrium data could be well interpreted by Freundlich isotherm. The value of activation energy
calculated from the D-R isotherm model 5.56 KJ/mol suggest that the ethoprophos adsorption on CH is physical. Different kinetic models were used to fit experimental data. The biosorption process could be best fitted by the pseudo-second order kinetic model. The results revealed that prepared chitosan can be used as an effective natural economical biosorbent for the treatment of water containing synthetic pesticide.