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ISSN: 2161-0525
Journal of Environmental & Analytical Toxicology
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Synthesis, Characterization and Evaluation of Peanut Shells-Derived Activated Carbons for Removal of Methomyl from Aqueous Solutions

Magda A Akl1*, Abdel Fatah M Youssef1, Asaad H Hassan2and Heba Maher1
1Chemistry Department, Faculty of Science, Mansoura University, Mansoura, Egypt
2Chemistry Department, Faculty of Science, Damanhour University, Damanhour, Egypt
*Corresponding Author : Magda A Akl
Chemistry Department, Faculty of Science
Mansoura University, Mansoura, Egypt
Tel: 20502217833
E-mail: [email protected]
Received December 22, 2015; Accepted January 30, 2016; Published February 04, 2016
Citation:Akl MA, Youssef AFM, Hassan AH, Maher H (2016) Synthesis, Characterization and Evaluation of Peanut Shells-Derived Activated Carbons for Removal of Methomyl from Aqueous Solutions. J Environ Anal Toxicol 6:352. doi:10.4172/2161-0525.1000352
Copyright: © 2016 Akl MA, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Abstract

Peanut shells-derived activated carbons (ACPNS) with different chemical characteristics, appropriate for the removal of methomyl pesticide in aqueous solutions are prepared. The steam activated carbon is obtained from the carbonized peanut shells in the presence of nitrogen in the temperature range from 973 to 1173K. The carbonized peanut shells are, further, chemically activated using NaOH and ZnCl2. The structure of the ACPNS was characterized by N2 adsorption at 77K, scan electron microscopy and FTIR. B.E.T methods are used to deduce the effective surface areas. The parameters (such as initial pH, temperature, etc) affecting the adsorption capacity of peanut shells-derived activated carbons toward methomyl removal from aqueous solutions are investigated using batch experiments. The study of kinetic models including pseudo first order and pseudo second-order are carried out. Adsorption isotherms are investigated. Equilibrium adsorption data fitted the Langmuir adsorption isotherm well with R2> 0.9980. The maximum adsorption capacities of peanut shells-derived activated carbons for the removal of methomyl, are calculated. The thermodynamic parameters including ΔG°, ΔH° and ΔS° for the adsorption processes of methomyl on the ACPNS were calculated, and the negative value of ΔG° indicated the spontaneous nature of adsorption. The prepared peanut shells derived activated carbons are successfully applied to the removal of methomyl pesticide from different water samples with a recovery % > 95 and a RSD< 3%. The mechanism of adsorption is proposed.

Keywords
Methomyl; Pesticide; Peanut shells; Activated carbons; Adsorption
Introduction
Pollution of ground and surface water is considered as the most important problem can threaten human, animals, plants and aquatic organisms. One of the common reasons for water pollution is using pesticides which recently used in agriculture for killing insects, bacteria, fungus, and other things that feed on crops.
Pesticides are chemically and artificially synthesized compounds and it is considered as to be toxic bio accumulative agents [1]. There are several different types of pesticides such as herbicides, insecticides, nematocides and fungicides. Pesticides vary greatly in toxicity and they are potentially hazardous to living system and environment. Pesticides have harmful effects from dermal, oral and respiratory exposure. Such effects include carcinogenicity, liver damage, teratogenicity, oncogenicity, reproductive disorders, mutagenicity, nerve damage and allergenic sensitization [2]. So, pesticides have harmful effect and it constitute a threat for surface and ground water sources. This contamination occurs mainly through industrial wastewater discharges, leaching, surface run-off and deposition from aerial applications [3]. The maximum allowed concentration of total pesticides according to the European Union Directives and Regulations for drinking water hygiene is 0.1 μg/L [4]. Methomyl is one of different types of pesticides according to chemical structure. It is a highly toxic carbamate insecticide first registered in 1968 by the EPA as a "Restricted use pesticides" and is used on a wide variety of crops. It is a cholinesterase inhibitor and is often most effective against pests that have developed a resistance to organophosphates [5]. It dissolves rapidly in water and is not persistent in soil. The harmful effects of methomyl because its absorption through the skin or the lungs through oral exposure [6]. It is highly toxic when absorbed orally and moderately toxic when absorbed dermally and its chronic effects are similar to its acute effects [7,8]. Methomyl is highly toxic to fishes, aquatic invertebrates, birds and mammals [9]. It can be fatal to humans at relatively high doses. Because of its harmful effect and its toxic contamination there are different techniques for its removal from water supplies and for water purification. Some techniques include aerobic degradation [10], photo catalytic degradation [10,11]. Ultrasound combined with photo-Fenton treatment [12], electro dialysis membranes [13], filtration, ozonation [14], reactions with oxidants such as free radicals, disinfection, floatation [15], preconcentration [16] and adsorption [17]. Amongst these techniques, adsorption is considered as the most effective and widely used for control of taste and odor as well as removing synthetic organic compounds and wide range of pollutants from wastewater and water supplies [18,19]. There are many advantages for using adsorption technique in water purification. It is low cost, easy, and simple system. There is no formation of byproducts after treatment by adsorption process and free from or less generation of toxic substances. Nowadays, there are many different adsorbents can be used for water treatment and purification such as silica gel [20], activated alumina [21], zeolites [22], polymers [23] & resins [24], clay [25] and activated carbon [26].
Peanut shells derived activated carbons are rarely used for the removal of organic pollutants from aqueous solutions [27,28].
To the best of our knowledge, no reported data is available regarding the use of peanut shells derived activated carbons in the removal of methomyl pesticide from aqueous solutions.
The objectives of the present study are the preparation, characterization and modification of peanut shells derived activated carbons (ACPNS). Peanut is an oil plant which is extensively cultured in Egypt. Peanut shells have low cost and are considered as waste matter in the environment. The peanut shells are modified physically by steam and chemically using NaOH and ZnCl2. The modified peanut shells derived activated carbons are thoroughly investigated for the removal of the methomyl pesticide from aqueous solutions. The different experimental factors affecting the removal of methomyl are studied viz. the influences of initial concentration of pesticide, contact time, dose of activated carbon and temperature on the adsorption capacities of peanut shell activated carbon for methomyl were investigated. In addition, kinetic and thermodynamic studies were established to expect the adsorption behavior.
Experimental
Reagents and solutions
Methomyl (Scheme 1) was purchased from Kafr El-Zayat Pesticides & Chemicals Company, Kafr El Zayat, Gharbia, Egypt. A stock solution of 1000 ppm of methomyl was prepared by dissolving 1 g in 1 L distilled water. Peanut shells were obtained from local markets in Egypt. NaOH, ZnCl2 and HCl were purchased from Sigma-Aldrich.
Apparatus
The pH measurements were made using a pH meter (Hi 931401, HANNA, Portugal). The concentration of methomyl was analyzed using UV-Vis spectrophotometer (Chrom Tech-Co., Ltd., USA). The adsorbents were weighed using analytical balance. Water bath shaker was used for adsorption experiments.
Peanut shells were crushed, washed with 0.5% HCl to remove all dirt, dried in an oven at 378K for overnight, ground by a mill, sieved to mesh size of 1-4 mm and carbonized in a tabular furnace at 873K for 2 h.
Carbonized material was soaked in sodium hydroxide (NaOH) solution with two impregnation ratios of char: NaOH (w/w) equal to 1:1 (CN11) and 1:3 (CN13) for 24 h. The two mixtures were then dehydrated in an oven overnight at 378K to remove moisture and then activated at 1023K for 2 h. The resulting AC was washed with distilled water several times after cooling until pH of filtrate reached to (7-8). This was followed by drying of the AC overnight in an oven at 378K.
Zinc chloride activated carbon samples were prepared by soaking dried peanut shells in zinc chloride with two impregnation ratios viz. carbonized peanut shell: ZnCl2 (w/w) equal to 2:1 (CZ21) and 1:2 (CZ12). The two mixtures were dried in an oven at 378K overnight to remove moisture and then activated at 873K for 2 h. Then, the AC was washed with distilled water several times till filtrate from chloride ions. This followed by drying the two samples in an oven at 378K overnight.
Activation of carbonized peanut shells with steam flow equal to 150 ml/min with different times to obtain 20% and 48% burn-off of activated carbon samples (CS20 and CS48).
Characterization of adsorbents
The BET surface area (SBET) measurements of the carbons were made by nitrogen adsorption at 77K using surface Area& Size Analyzer (QUANTACHROME- NOVA 2000 Series). The surface micrographs of the carbons were taken by using a scanning electron microscope JSE - T20 (JEOL, Japan). The instrument was operated at 40 kW. Prior to the analysis, the samples were dried at 383K for 4 h. A thin layer of gold was coated on the samples for sample metallization and charges dissipation.
Fourier transform infrared spectroscopy (FT-IR) technique was used for qualitative measurements of functional groups present on the activated carbon surface. By using Jusco instrument (Model 6100, Japan), Sample discs were prepared by mixing 1 mg of the samples with 500 mg of KBr (Merck) in an agate mortar and scanned in a range from 4000 to 400 cm<sup>-1</sup>.
Surface acidity and Boehm titration technique was applied for determination the type and amount of functional groups on the carbon surfaces by neutralization with bases of various strengths.
Adsorption experiments
Batch sorption experiments were done by shaking 0.02 g of activated carbon with 50 ml aqueous solution of methomyl in 250 ml- Erlenmeyer flasks placed in a temperature controlled shaking water bath at different concentrations (between 100 and 1600 mg/l), pHs (between 2 and 12), ionic strength (between 0.002 and 0.09 mole/l), temperatures (between 289 and 313K) and sorbent doses (between 0.01 and 0.02 g) at a constant shaking rate of 125 rpm. The amounts of pesticide removed by sorbents (qe) Eqn (1) and percent removed (R %) Eqn (2):
             (1)
             (2)
Where qe is the amount of pesticide adsorbed (mg/g). Co and Ce are the initial and equilibrium liquid-phase concentrations of pesticide (mg/g), respectively. V is the volume of the solution (L), and m is the weight of the sorbent used (g).
Results and Discussion
Characterization of the adsorbents
Textural properties
The nitrogen adsorption-desorption analysis: The surface area and porosity of carbons are prominent factors in determining their adsorption capacities [29,30]. The textural properties of solids are conventionally determined from the adsorption of nitrogen at 77K and the adsorption data are usually analyzed by the application of the BET equation [31,32].
The adsorption of nitrogen by the carbons investigated was found to be relatively rapid with the equilibrium attained within 25 min indicating that the adsorption is not controlled by activated diffusion encountered in ultrafine pores and meanwhile refers to the accessibility of the entire pore structure to the nitrogen molecules [33]. Figure 1 depicts the nitrogen adsorption isotherms measured at 77K for the carbons C, CS20, CS48, CZ21, CZ12, CN11 and CN13.
For carbon C, the isotherms is mainly type I according to the BDDT classification with no hysteresis loop [34]. The isotherms of ACPNS show mixed characteristics of type I and type IV; they are less steep and exhibited closed hysteresis loops.
The values of surface area of the ACPNS samples were calculated from the linear BET plots of N2 adsorption at 77K. The results are represented in Table 1 depicts:
(i) The surface area for non-activated carbon is very low compared to activated carbon due to the absorption of activating agent NaOH, ZnCl2 or steam. The SBET of CN13 is 35.5 times more than C, while CS48 is 20.8 times more than C and CZ12 is 26.8 times more than C.
(ii) The NaOH activated carbon (CN13) sample possesses higher surface area value than other ACPNS.
(iii)The SBET values of CN13 increased from 1242.86 m2/g to 2362.85 m2/g when the char: NaOH ratio increased from (1:1) to (1:3) because of increasing the impregnation ratio.
(iv) Increasing the impregnation ratio of ZnCl2 led to an increase in SBET values from 1449.03 m2/g to 1783.52 m2/g with increasing of precursor: ZnCl2 from (1:2) to (2:1) ratio which is designed as CZ12 and CZ21 samples, respectively.
(v)The SBET of steam activated carbon increased from 801. 55 m2/g to 1388.00 m2/g, with increasing the burn off degree for CS20 and CS48 samples, respectively.
vi) The Total pore volume of non-activated sample is very low compared to activated ones and (ml/L) increased with activation and with increasing of % of activating agents.
(vii)The pore radius ranged between 0.389 and 0.514 nm for the activated samples.
Scanning electron microscopy (SEM): The scanning electron microscopy images gave insight into the peanut shells structure with respect to its shape. The dark areas are macro pores and the pale grey areas are due to the carbon matrix. Figure 2 represents the morphology of the resulting activated carbon prepared via physical (CS) and chemical (CN11, CN13, CZ12, CZ21) activation methods, respectively. It can be clearly seen that physical and chemical activations show some changes in the surface of the particles after activation. The different pore structures of the activated carbon prepared from either physical (using steam as activating agent) or chemical (using NaOH or ZnCl2) activation are observed, which depend upon different reaction mechanisms. Figure 2 depicts four pictures (a) carbonaceous precursor, (b) for chemically activated carbon using NaOH, (c) chemically activated using ZnCl2 and (d) physically activated carbon (CS). In this concern, the sample (d) activated by steam indicated that the carbon matrix are greater than the dark areas. This is referred to the nature of the micropores resulting from the activating steam. The dark areas are greater than the carbon matrix using NaOH or ZnCl2 as activators in chemical activation. This implies to the higher surface area of samples (b, c) due to mesopores as well as to micro-pores.
Also, this shows that the peanut shells activated by NaOH and ZnCl2 have irregular carbon matrix. i.e., of different shapes and sizes. This was proved by nitrogen adsorption isotherms of activated peanut shells which indicated that they were mainly micro pores only in physical activation and a mixture from micro- and mesopores in chemical activation. This may be explained by progressive changes in the surface of the cellular structure of the parent material with NaOH and ZnCl2 solution used as shown in Figure 2. The cellulose units are hydrolyzed or dehydrated by the chemicals used and thus the main components of the intercellular wall are broken down to smaller structure. Thus it is apparent that the external surface of peanut shells is quite rough, consisting of cavities, cracks, and irregular protrusions.
Chemical properties: The chemistry of the surface of a carbon is more important than its textural properties in determining its adsorption capacity of pests from aqueous solution particularly when the adsorption involves interaction with the surface functional groups via ion exchange and/or complex formation which is most probably the case in adsorption of pests on activated carbons.
FTIR: The chemistry of the carbon surface is attributed to the existence on the surface of carbon-oxygen functional groups of acid or basic character. The FTIR spectra of samples ACPNS (before and after adsorption of methomyl) have been recorded, Figure 3. The recorded IR spectra of CZ21, CN11 and CS20 (before adsorption of methomyl) depicts: the existence of a broad band around (3414-3446) cm-1 that is attributed to O-H stretching vibrations due to the presence of free hydroxyl groups, bonded O-H bands of carboxylic acids and adsorbed water [35,36]. The peaks observed around (1034-1116) cm-1 are attributed to the C-O group in carboxylic and alcoholic groups [35- 38]. The peaks observed around (610-875) cm-1 are attributed to in and out plane ring deformation in benzene derivatives [35,38,39].
The FTIR spectra of ACPNS after adsorption of methomyl show the following changes:
In CZ21-M : The appearance of a peak at 1630 cm-1 due to N-H bend denoting the presence of primary amines [40] and appearance of the peak at 1101 cm-1 due to C-O stretching due to presence of alcohols, carboxylic acids , esters and ethers [37].
For CN11-M: The disappearance of the peaks at 2116 cm-1 and peaks at 1391 cm-1. This indicates the absence of –C≡C– stretch of alkyne groups and absence of C-C stretch (in- ring) of aromatic rings, respectively.
Finally, CS20-M graph shows disappearance of peaks at 2103 cm- 1, 1430 cm<sup>-1</sup> and 1057 cm<sup>-1</sup> indicating the absence of –C≡C– stretch of alkyne groups , absence of C-C stretch (in- ring) of aromatic rings and C-H stretch of aliphatic amines, respectively. Also, new peak appeared at 1100 cm<sup>-1</sup> due to C-O stretch which indicates the presence of alcohols, carboxylic acids, esters or amines groups [41].
Surface acidity and Boehm titration: The pH of the aqueous slurry of the carbon material provides a convenient indicator of the type and concentration of the chemical parameters of the ACs investigated. Table 2 reveals that (i) the surface pH of carbon C is (7.9) indicating its surface basicity, i.e the basic functional groups on the surface of nonoxidized carbon are more dominating compared with those of the acid type. The same is also true for steam activated ACPNS of surface pH= 8.2 and 8.3. Steam activation at 1223K usually leads to the formation on the carbon surface of C-O groups of basic character [42]. Treatment with NaOH (CN11, CN12) increased the surface pH from 7.98 to 8.77 and 8.90 for CN11 and CN12, respectively. On the other hand, treatment with ZnCl2 increased the total acidity increase from 7.98 to 6.66.
The pH of zero point charge pH pzc of ACPNS ranged between 7.0 and 8.8 for activated samples due to the effect of basic function groups.
The surface acidic groups could be determined in (m.eq/g) by the selective neutralization with a series of bases of varying strength, viz. NaHCO3, Na2CO3, NaOH and NaOC2H5. NaHCO3 neutralizes carboxylic groups where as those neutralized by Na2CO3 but not by NaHCO3 are lactones. The weak acid groups neutralized by NaOH but not by Na2CO3 were postulated as phenols. The groups reacting with NaOC2H5, but not with NaOH, were suggested to be carbonyl groups [43]. Table 2 shows the surface acidity and the abundance of different acidic groups on the surface of non-activated and activated carbon samples.
The ash content in case of NaOH and steam activated carbon is high due to loss of carbon content by H2O (steam) or NaOH as activating agent and it is low in case of ZnCl2 sample due to volatilization effect of ZnCl2 on ash content.
Adsorption of methomyl
Effect of pH: The pH value of the solution, which affects the surface charge of the adsorbent and the degree of speciation of adsorbate was an important controlling parameter in the adsorption process. The chemical name of methomyl is S-methyl N-(methylcarbamoyloxy) thioacetimidate and the molecular weight is 162. According to the physicochemical properties of methomyl reported in Hazardous Substances Data Bank [29], the hydrolysis half-life of methomyl in aqueous environment depends on the pH value of the solution. Therefore, the effect of pH value of the solution on methomyl was investigated with the results presented in Figure 4. The results obtained revealed that, in the pH range of 2 to 8, the methomyl is rather stable. In contrast, when the pH value rises to 10, the residual amount of methomyl is only 12% of the original. Moreover, at high pH value of 12, methomyl is completely degraded to other compounds, interpreting that methomyl is degraded quite substantially in the basic solutions. Therefore, the effect of pH value on adsorption capacity was examined in the pH range of 2-8. As illustrated in Figure 4, the adsorption capacity decreases with increasing pH value, indicating that the lower pH value of solution enhances the adsorption of methomyl. ACPNS are weak base anions with pHpzc (point of zero charge) 7-8 as shown in Table 3. The adsorption of methomyl on ACPNS may be mainly due to dispersion forces and polarization of π electrons (electron-rich portion of the adsorbate).
On contrary, the lower pest removal at acidic pH(<2) is probably due to the presence of excess H+ ions competing with positively charged methomyl molecules for the sorption sites of sorbents. Furthermore, at lower pH values (below the pKa of carboxylic groups, approximate 4.6), the -COO- groups in ACPNS are protonated to -COOH groups and the hydrogen bonds between -COOH and -NH2 groups are formed resulting in a decrease of methomyl uptake. Also, the repulsion between positively charged amino group of ACPNS and cationic pesticide molecules resulted in decrease pest removal in acidic medium.
Effect of initial methomyl concentration: The effect of initial concentration on the removal of methomyl by ACPNS was studied at different temperatures at normal pesticide pH. The results are graphed in Figure 5. It is seen that the equilibrium sorption capacities of the sorbents increase with an increase in initial pest concentration and the adsorption at different concentrations is rapid in the initial stages and gradually decreases with the progress of adsorption until the equilibrium is reached. This may be attributed to the fact that, the higher the initial methomyl concentration, the greater the driving force of the concentration gradient at solid-liquid interface which cause an increase of the amount of methomyl adsorbed on the adsorbent. At higher methomyl concentration adsorption capacity reached a plateau indicating saturation of the available binding sites on the adsorbents. The steep slope at initial methomyl concentrations is a desirable feature of the sorption system and the results indicates that the ACPNS are efficient adsorbents for methomyl.
Effect of adsorbent dosage: An experiment was made to study the effect of adsorbent dosage on methomyl adsorption. Various quantities of CN13 as selected ACPNS sample were added to a fixed initial pesticide concentration. The uptake of methomyl after a contact time of 24 h versus the adsorbent dosage (g/L) used is shown in Figure 6. It can be noticed that the methomyl uptake increased from 19 to 87% when CN13 dose was increased from 0.12 to 1.57 g/L, respectively. This is due to the fact that as the adsorbent dosage is increased, more adsorption sites are available for adsorbate enhancing the methomyl uptake [44]. Also, with increasing adsorbent loading, the quantity of methomyl adsorbed on to the unit weight of the adsorbent is reduced causing a decrease in qe (mg/g) values with increasing activated carbon loading.
Effect of contact time: Figure 7 shows the effect of contact time on the adsorption capacity of CN13, as an example, for methomyl. The adsorption capacity of CN13 increased rapidly with the increase of contact time from 0 to 20 min and more than 90% of the equilibrium adsorption capacity for methomyl occurred within 15 min. After 60 min, the adsorption capacity became constant and the adsorption reached equilibrium. Therefore, 60 min was selected as the contact time for the adsorption of methomyl onto the ACPNS under our experimental conditions.
As shown, the adsorption process was divided into three stages: (1) an initial stage with adsorption occurring instantly, (2) subsequently, slow adsorption, and (3) a final stage with adsorption reaching equilibrium and remaining constant. The first stage can be attributed to the rapid attachment of methomyl to the surface of the ACPNS by surface mass transfer. At this stage, more than 80% of methomyl adsorption was found in all cases. The second stage was slower, possibly because many of the available external sites was already occupied and because of the slow diffusion of methomyl molecules into the network of the ACPNS. An asymptotic trend was found after approximately 20 min regardless of the initial methomyl concentration applied to the adsorption system. The amount adsorbed did not vary significantly at contact times longer than the equilibrium time (60 min). The kinetics of the adsorption process indicates that the adsorption of methomyl onto ACPNS can be considered as a fast adsorption process because more than 80% of methomyl was adsorbed within 20 min especially at methomyl concentration lower than the maximum adsorbed. Such findings reveal the benefits of using this low-cost adsorbent or so-called eco-adsorbent for the treatment of aqueous solutions rich in pesticides in general and methomyl in particular.
Effect of ionic strength: The removal of pesticides from aqueous solutions depends greatly on electrostatic parameters such as surface charge, pH and ionic strength [45]. Ionic strength plays a major role in the electrical double layer (EDL) structure of a hydrated particle. The thickness of the DEL decreases as the ionic strength increasing, resulting in a decrease in adsorption [46]. In this work, the effect of the additional amounts of NaCl over a concentration range 0.002-0.05M was studied at the natural pH of methomyl (6.0). The results obtained revealed that that the adsorption capacities decreased with increasing NaCl concentration for the adsorbent. The ionic strength increases as NaCl concentration increases and more Na+ ions can screen the negative sites of the adsorbent, leading to the reduce of electrostatic attractive force, and accordingly, the amount adsorbed of methomyl decreases. The ionic strength may also lead to the dimerisation process of methomyl molecules, thereby reducing electrostatic force in the presence of high concentration of Na+ and Cl- ions.
Further increase in ionic strength above 0.05M decrease the adsorption capacities sharply until 0.15M where any further increase caused steep decrease in adsorption capacities because of salting out of pesticide molecules from aqueous solution. These results showed that ionic strength had an adverse effect on the pesticide adsorption, which may be induced by the competition between the cations and the pesticide for the electron rich sites.
Effect of temperature: The adsorption of methomyl on ACPNS was studied at different temperatures, and the results are shown in Table 3. The amount of methomyl adsorbed increased with a rise in temperature which means that methomyl adsorption from aqueous solutions on both modified and unmodified carbons is an endothermic process. The enhancement of adsorption capacity may be due to a chemical interaction between adsorbate and adsorbent, creation of some new adsorption sites, or an increased rate of intra particle diffusion into the pores of the adsorbant at higher temperature [38,39]. When the temperature increased from 289K to 313K, the maximum amounts of methomyl removed by ACPNS.
Adsorption isotherms
In equilibrium analysis, Langmuir [3] and Freundlich [4] models are the most frequently employed isotherm models for studying frequently employed isotherm models for studying the behavior and system of adsorption. Langmuir and Freundlich isotherms for the studied system at different temperatures are presented in Figures 8 and 9, respectively.
Table 3 shows also Langmuir’s and Freundlich adsorption parameters for different adsorption at 289 and 313K. Inspection of Table 3 depicts: (i) Adsorption of methomyl onto ACPNS adsorbents follow both of Freundlich and Langmuir models as proved by correlation coefficient (R2) which ranged between 0.996 and 0.999. The best fit model for adsorption of methomyl on all samples is Langmuir adsorption model. (ii) As temperature increased, the adsorption capacities increased which indicates an endothermic nature for the all process of methomyl adsorption;(iii) As the ratio of activating agent increased as in ZnCl2 or NaOH there was an increase in the adsorption capacities of the activated samples.
             (3)
             (4)
Adsorption kinetics
In order to understand the process of adsorption, four kinetic models were applied to analyze the experimental data. The linear form of pseudo first-order kinetic model of Lagergren [47], rthe linear form of Ho’s pseudo second-order kinetics [48], Intra-particle diffusion model [49] and Boyd model [5]. In this study, the four models were tested for methomyl on ACPNS. The best-fit model was determined depending on the linear correlation coefficient R2. The results are shown in Figure 10 a,b and represented in Table 4.
             (5)
The correlation coefficient of pseudo second-order which was close to unit was much higher than that of pseudo first-order and the theoretical q2e value computed from pseudo second-order was much closed to the experimental qexp value given in Table 4, indicating that the pseudo second-order model was more applicable for the system, while theoretical q1e value estimated from pseudo first-order kinetic did not give reasonable value, suggesting this model could not be used to depict the adsorption data of methomyl onto the adsorbent.
For determination of the adsorption mechanism and prediction of the rate limiting step in adsorption process to understand the phenomena of adsorption the intraparticle diffusion and Boyd models were applied. For intraparticle diffusion, Figure 10c, the results obtained indicated that the plot of qt against t0.5 of CN11 for methomyl is multilinear containing at least three linear segments which indicates that three steps occur during adsorption process. The first sharper portion is transport of pest molecules from the bulk solution to the adsorbent external surface by diffusion through the boundary layer (film diffusion). The second portion is the diffusion of the pest molecules from the external surface into the pores of the adsorbent. The third portion is the final equilibrium stage, where the pest molecules were adsorbed on the active sites on the internal surface of the pores and the intra-particle diffusion starts to slow down due to the solute concentration getting lower and lower in solution [50,51]. Also, the linear portion of the curve did not pass through the origin, suggesting that pore diffusion is not the step controlling the overall rate of mass transfer at beginning of adsorption. In Table 4, the correlation coefficients (R2) for the linear segment intra-particle diffusion model was 0.307 indicating that the intra-particle diffusion was not the only rate controlling step; other process could control the rate of adsorption.
So, Boyd kinetic model (Figure 10d) was applied to predict the slow step involved in the adsorption process. For Boyd kinetic model, the results obtained indicated that the linear plot of B(t) against (t )of CN11 for methomyl doesn’t pass through the origin indicating that the adsorption process is controlled by film diffusion.
Thermodynamic studies
To determine the effect of temperature on methomyl adsorption, adsorption experiments were also conducted at 16 and 40°C, respectively. As shown in Figure 5, the adsorption capacity of methomyl onto ACPNS clearly increases with rising temperature, indicating an endothermic reaction. The thermodynamic parameters such as the change in Gibbs free energy (ΔGo, kJ/mol), enthalpy (ΔHo, kJ/mol) and entropy (ΔSo, J/(mol•K)) are calculated [52].
The thermodynamic parameters for methomyl adsorption onto ACPNS are shown in Table 5. The positive value of ΔH° reflects an endothermic nature of methomyl adsorption ACPNS and indicates that the adsorption is favored at high temperature, which is supported by the increase of methomyl adsorption onto ACPNS with rising temperature. The adsorption is more favorable at higher temperature and methomyl was strongly adsorbed on the surface of both ACPNS.
The negative ΔG° indicates that the adsorption process is spontaneous. The magnitude of ΔG° increased with increasing temperature, which also indicated that better adsorption was actually obtained at higher temperatures, because a more negative ΔG° implied a greater driving force of adsorption, resulting in a higher adsorption capacity.
The positive ΔS° suggests increased randomness at the solid/ solution interface during lead adsorption [53,54].
Mechanism of adsorption
Modification of the carbon surface via steam, NaOH or ZnCl2 introduces a variety of carbon oxygen functional groups of acidic nature. These groups dissociate in aqueous solutions and participate in the metal binding process. It is likely that other surface oxygen atoms (some of them may not be a component of protonogenic functional groups) may participate in complex formation. Thus, one would expect an ion exchange between methomyl with the H+ released from the carboxylic and phenolic surface groups and therefore these groups contribute significantly to the enhancement of pesticide uptake. Carbonyl and/or lactonic groups, on the other hand are more likely involved in a complex formation with methomyl [55].
Analytical application
The prepared ACPNS were successfully applied for the removal of known amounts of methomyl spiked to different natural water samples. The recovery % ranged between and with a relative standard deviation (RSD, %, <3).
Conclusion
The results of the present study reveal that ACPNS may be extremely viable adsorbent for application in the removal of methomyl from aqueous solutions. Modification of ACPNS by steam, NaOH and ZnCl2 significantly increased the oxygen-containing groups on the surface of activated carbons with noticeable change in the surface morphology and textural properties. The adsorption of methomyl was dependent on initial concentration, reaction temperature and pH. The methomyl adsorption capacity increased with the increase of pH in the range of 3-8, where ionization of carboxylic group occurs. The adsorption of methomyl onto the activated carbons reached equilibrium within about 60 min. The adsorption equilibrium could be well described by Langmuir adsorption isotherms, namely monolayer adsorption on a homogenous surface. The adsorption kinetics followed a pseudo-second order kinetic model and intra-particle diffusion was involved in the adsorption process. Thermodynamic results indicated that The Gibbs free energy change (ΔG°) was negative as expected for a spontaneous process under the applied conditions. The prepared ACPNS were successfully applied to the removal of methomyl from natural water samples.
References

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  1. Izz Udeen
    Posted on Sep 22 2016 at 1:57 pm
    Research article narrates the preparation, characterization and modification of peanut shells derived activated carbons. In general, this article is informative and provides useful and comprehensive information.
 

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