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Keywords:

  • isotherms;
  • kinetics;
  • phosphamidon;
  • sorption;
  • zeolitic material

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULT AND DISCUSSION
  6. CONCLUSION
  7. LITERATURE CITED

Bagasse fly ash (BFA) a sugar industry waste had been collected and converted to zeolitic material (MZBFA) by combined conventional and microwave reflux method. The synthesized sorbent has been characterized using various techniques such as FTIR, XRD, and SEM. BFA and MZBFA were used for the removal of phosphamidon (PSM), an organophosphorus pesticide from aqueous solution. The PSM sorption capacities of MZBFA and BFA were determined by batch sorption technique. Langmuir, Freundlich, Dubinin Redushkwich, and Temkin isotherms were estimated for the sorption nature and efficiency of sorbents. The sorption kinetics is better reflected by pseudo second order model and thermodynamics parameters exhibited specified endothermic nature of sorption. The overall sorption was approximated well by film diffusion at lower concentrations and pore diffusion at higher concentrations simultaneously. From the results, it was observed that MZBFA exhibited higher removal capacity than BFA. © 2013 American Institute of Chemical Engineers Environ Prog, 33: 114–122, 2014


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULT AND DISCUSSION
  6. CONCLUSION
  7. LITERATURE CITED

Among different pollutants of aquatic ecosystems, especially organophosphorus pesticides are considered as priority pollutants since they are harmful to organisms even at µg L−1 levels [1]. Organophosphorus pesticides are used in agriculture, in home-gardens, and in veterinary practice. All apparently share a common mechanism of cholinesterase inhibition and can cause poisoning symptoms. Organophosphorus pesticides constitute a diverse group of chemical structures exhibiting a wide range of physicochemical properties.

The increased use of agricultural pesticides has meant that the mechanisms and magnitude of pesticide movement after their application continue to be an active area of research. Furthermore, their removal from contaminated lands is of paramount interest. Many pesticides which have been introduced to replace organophosphorus pesticides are so designed that they tend to break down fairly quickly in water. Thus, their relatively high solubility in water makes them quite mobile in the environment [2]. When the potential level of exposure to contaminants at a given site is unacceptable from the human health or ecological standpoints, remediation strategies become necessary.

Poison information center in National School of Occupational Health (NIOH), Ahmedabad, India, has reported that organophosphorus compounds are responsible for maximum number of poisoning (73%) among all agricultural pesticides [3]. Patients of acute organophosphorus poisoning have been reported to suffer from problems like vomiting, nausea, miosis, excessive salivation, blurred vision, headache, giddiness, and disturbance in consciousness [4].

Advance pesticide removal methods are usually needed to meet environment quality requirements. These include combinations of biological, chemical, and physical processes. Therefore, sorption has evolved into one of the most effective physical processes for pesticide removal. To date, the most common commercially used sorbent is activated carbon [5, 6], but it is relatively expensive. Recently, the potential of various economical alternative sorbents (such as natural materials [7, 8], biosorbents [9, 10], and waste materials [11, 12]) that are available from environment has received more attention as they perform well as good sorbent materials. For example, fly ash [13], montmorillonite [14], smectite [15], bentonite [16], and carbon cloth [17] have been used as a sorbent and have been successfully used as sorbent materials for sorption process for removal of pollutants because these materials are more economical and the method can be established with low cost.

Extensive work has been done to access the conversion of coal fly ash into zeolites [18, 19], except for our earlier study [20, 21], there are no reports on the conversion of bagasse fly ash (BFA) into zeolites. In this study, attempts have been made to develop a low-cost sorbent using BFA by converting it into zeolitic sorbent (MZBFA).From the available literature, it was found that there is no study related to the sorption capacity and application to organophosphorous pesticides for zeolitic BFA. It is unfair to expect BFA and zeolitic bagasse fly ash (MZBFA) to behave differently than many of those reported in the open literature, as far as its sorptive properties are concerned, because it depends on the origin of the sugarcane bagasse and the conditions of burning prevailing during its formation. The synthesis of low cost zeolitic sorbent from BFA and assessment of their ability in removal of Phosphamidon (PSM) from simulated water at different operational conditions was the aim of this research. The study was conducted to investigate factors that affect sorption, and determine the kinetics of the sorption process using virgin BFA and synthesized MZBFA.

EXPERIMENTAL

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULT AND DISCUSSION
  6. CONCLUSION
  7. LITERATURE CITED

Materials

The raw material BFA used in this study was obtained from a local sugar mill, Shree Khedut Sahkari Khand Udhyog Mandali, located at Bardoli, Gujarat, India. Prior to its use, the procured BFA was washed repeatedly with double distilled water to avoid presence of foreign impurities and dried in sunlight for 8 h and then for 4 h in hot air oven at 353 ± 5 K. The washed and dried BFA was sieved through 200 μm mesh size sieve to eliminate the larger particles. All the reagents used for these studies were of analytical grade. High purity (99.2 %) PSM pesticide is obtained from United Phosphorous; India was used as sorbate in this study. PSM is a colorless, odorless fluid and is miscible in all properties with water, alcohols, esters, and aromatic hydrocarbons. The structure of PSM (Chemical formula: C10H19ClNO5P, Molecular Weight: 299.69, Boiling Point: 120°C, Chemical Abstracts Service No.: 13171-21-6) is illustrated in Figure 1. The aqueous solution was prepared by dissolving pesticide in double distilled water to produce a stock solution of 1000 mg L−1. Solutions of the required concentrations were prepared by successive dilution of the stock solution.

image

Figure 1. Molecular structure of PSM.

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Preparation of Sorbent

The dried BFA of 200 μm was mixed with 3M NaOH solution for (8:1 liquid/solid ratio) for partial microwave irradiation at earlier stages (15 min) followed by conventional heating (165 min) in a round bottom flask equipped with a reflux condenser and stirrer at a reaction temperature of 373 ± 5 K. Microwave experiments were carried out using a microwave oven (Q-ProM, Germany). The resultant precipitate was then repeatedly washed with double distilled water to remove excess sodium hydroxide, filtered and dried at 373 ± 10 K in hot air oven. The dried zeolitic material (MZBFA) was sieved through 75–90 µm mesh size sieve. The BFA and MZBFA were stored in air tight desiccators till utilized for the sorption process. The zeolization of BFA by microwave heating was also reported to be useful for shortening the reaction time.

Instrumental Techniques

The specific surface area and pore volume of the sorbents (BFA and MZBFA) were determined using BET and BJH N2 adsorption and desorption methods at 77 K (Micromeritics Gemini 2360, Shimadzu, Japan). Sorbents were chemically characterized by X-ray Fluorescence technique (X-ray XDL-B, Fischer scope, Japan). Fourier Transform Infrared (FTIR) spectra were brought by using FTIR spectrometer (Thermo Nicolet IS10, Thermo Scientific Ltd). The X-ray diffraction (XRD) patterns of the sorbents were obtained using Panalyticals X-Pert Pro (Netherlands) instrument employing nickel filtered Cu Kα (λ = 1.5406 A°) radiations. The surface morphologies of sorbents were analyzed by scanning electron microscopy (Leo 1430 VP, Zeiss, Cambridge, England). The moisture contents of the sorbents were carried out using Karl-Fischer (1204R of VMHI, Metrohm) instrument. A UV–visible spectrophotometer (UV-1601, Shimadzu, Japan) was used to detect PSM. pH measurements were made using a pH meter (PICO+, Labindia pH meter, India). The point of zero charge (pHpzc) values of BFA and MZBFA were ascertained by mass titration method [22].

Batch Sorption Studies

Batch studies were carried out at 303 K (except temperature study). A weighed quantity of sorbents was placed in a 100 mL brown colored glass bottles with a lid containing 50 mL of a PSM solution. The sorbent loaded solution was stirred at 150 rpm until equilibrium was attained. The effect of different operational variables, namely, contact time (1–24 h), pH (2–12), dosage (1–6 g L−1), initial PSM concentration (50–200 mg L−1), and temperature (293–333 K) were examined for the sorption of PSM on both sorbents. After equilibrium, the concentration of the sorbate in the residual solution was determined spectrophotometrically at λmax of 220 nm. Consecutively, to minimize the error, the study of blank experiments was done for each condition without sorbent to check any sorption by experimental vessels and separating paper, which displayed no significant change in the residual PSM concentration. Each experiment was repeated three times and the mean values were taken. The initial pH of the PSM solutions was adjusted using 1M solution of either NaOH or HCL. The sorption uptake of PSM at equilibrium qe (mg g−1) was calculated using the following relationship (Eq. (1)).

  • display math(1)

where, C0 is the initial PSM concentration (mg L−1), Ce is the equilibrium PSM concentration (mg L−1), V is the volume of the solution (L), and m is the mass of the sorbent (g).

RESULT AND DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULT AND DISCUSSION
  6. CONCLUSION
  7. LITERATURE CITED

Characterization of Sorbents

The values of proximate analysis and physico-chemical properties show gain in amorphous character (Table 1). The formation mechanism of MZBFA from BFA has been proposed as follows: dissolution of glass phase (aluminum-silicate), into the alkaline solution, decomposition of aluminosilicate gel as zeolite precursor, and crystallization of zeolite. In this study, mechanism of zeolization is mainly separated into three terms. The first step is dissolution of SiO2 and Al2O3 into the alkaline solution at 0–15 min. After 15 min, the alumina and silicate ions are condensed to form an alumina-silicate gel, which is prematerial of zeolite crystal covering the outer surface of BFA particles. The intermediate gel begins to change into zeolite via dissolution-reprecipitation process at 40 min, and increase in Na2O contents of MZBFA is caused by captured of sodium ions to neutralize the negative charge on aluminate in zeolite structure when zeolite crystal is occurred [19]. The particle size of both sorbents was determined by Mastersizer 2000 particle size analyzer. The particle size distributions specify that majority of particles lies below 58.55 µm (95%) in case of MZBFA, while in BFA majority of particles lies below 150.62 µm (95%). BET surface area for MZBFA (328.30 m2 g−1) has been increased significantly after treatment as compared with virgin BFA (99.14 m2 g−1). Point of zero charge (pHpzc) for a given mineral surface is the pH at which surface has net neutral charge. The significance of this kind of plot show that a given mineral surface will have positive charge at solution pH values less than the point of zero charge and thus be a surface on which anions may adsorb. However, the mineral surface will have negative charge at solution pH values greater than point of zero charge. The pHpzc values obtained by mass titration method are 8.18 and 9.09 pH for BFA and MZBFA, respectively.

Table 1. Physico-chemical properties of BFA and MZBFA.
CharacteristicsObtained values
BFAMZBFA
Proximate analysis
Loss on drying (%)11.95 ± 0.213.74 ± 0.2
Moisture content (%)10.36 ± 0.312.25 ± 0.3
Ash content (%)72.85 ± 0.267.74 ± 0.2
Physico-properties
Specific density1.888 ± 0.022.036 ± 0.02
Bulk density (g cc−1)1.725 ± 0.021.983 ± 0.02
Dry density (g cc−1)1.081 ± 0.021.225 ± 0.02
Void ratio0.7470.662
Porosity, fraction0.4280.398
pHpzc8.18 ± 0.059.09 ± 0.05
BET Surface area99.14328.30
Chemical constituents
SiO2%46.3543.54
Al2O3%19.9918.90
Fe2O3%5.892.99
CaO%4.973.17
MgO%4.834.12
Na2O%4.177.24
K2O%3.742.35

Morphology of the Sorbents

SEM is one of the most widely used surface diagnostic tools. SEM photographs for sorbents shown in Figure 2. BFA mostly contains noncrystalline glass, with a loose structure and posses smooth surface particles with no pits because the surface is covered by an aluminosilicate glass phase. Contradictorily, MZBFA (Figure 2) shows clear crystalline forms with compact structures and honey comb aperture and holes. The particles of MZBFA appear to be more fluffy and porous. The SEM micrograph of MZBFA shows the enlargement of extended folded stands with deeper pits; with interior voids. Microscopy studies were conducted after sorption (Figure not shown) indicates that the layered strands is diminished showing that the pesticide sorption very intense on the surface throughout.

image

Figure 2. SEM micrographs of (a) BFA and (b) MZBFA at 5.0 KX magnifications.

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XRD Analysis

From the XRD pattern, the crystalline and mineralogical characteristics of sorbents have been accomplished using the database provided by “Joint Committee on Powder Diffraction Standards” [23]. The XRD pattern of BFA shows the presence of glass phase [19]. Figure 3 demonstrate that the BFA exhibits the presence of α-quartz (JCPDS 5–490) as a major part and other amorphous materials [23]. The XRD pattern of MZBFA exhibits several new and sharp diffraction peaks that are not present in BFA. The newly observed intense peaks at 2θ = 26.54° and 2θ = 43.09° can be the significance of zeolite formation in MZBFA. Zeolite P (Phillipsite, JCPDS 39–0219) and Zeolite X (JCPDS 28–1036) are found to be dominant in zeolite formation during hydrothermal treatment can be seen in Figure 3. The other crystalline phases identified in the MZBFA are Analcime (JCPDS 76–0901), Zeolite A (JCPDS 14–90), Chabazite (JCPDS 12–0194), and ZSM 12 (JCPDS 15–274).

image

Figure 3. PXRD patterns of BFA and MZBFA, P = Phillipsite, X = Zeolite X, A = Analcime, L = Zeolite A, Z = ZSM-12, C = Chabazite, Q = α-quartz.

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FTIR Analysis

FTIR spectra of BFA and MZBFA (Figure not shown) exhibit a broad band at about 3400 cm−1 indicating the presence of [BOND]OH group of the silanol (Si[BOND]OH). The band observed at 1033.97, 676.67, and 475.33 cm−1 can be ascribed to asymmetric, symmetric stretching vibration, and the bending vibration of internal tetrahedral, TO4 (where T = Si, Al), respectively. All these bands are more or less dependent on the crystal structure. The band at 1097.31 cm−1 of BFA was shifted to 1033.97 cm−1 in MZBFA confirms the tetrahedral coordination of aluminum in the zeolite framework. In MZBFA, the shifting symmetric stretching band 797.53 to 792.61 cm−1 of internal tetrahedral (TO4) of amorphous aluminosilicates formed by the reaction of dissolved Si+4 and Al+3 confirm the formation of zeolite phases [18, 24]. The amount of improved tetrahedral sites of the aluminosilcate framework of the zeolite can be enlightened by decreased in frequency of asymmetric stretching vibration of tetrahedral. The band at about 1646.21 and 1456.34 cm−1 are belongs to the bending vibration of water molecules [18].

Batch Sorption Studies

Effect of pH

The sorption of PSM was studied over a broad pH range of 2–12. As shown in the Figure 4, the percentage removal of PSM was maximum at acidic pH (pH = 2), decreased with further increase in pH of PSM (up to pH = 6), beyond which a sharp decline in sorption is observed. There are reports in the literature indicated that the sorption increases by increasing pH in acidic solution [7, 25-30]. In fact, this study exhibits good sorption capacity for PSM on BFA and MZBFA in acidic media. This can be explained by considering the point of zero charge (pHpzc) on the sorbent surface which is, in turn, influenced by the solution pH. The pHpzc of the MZBFA increased to 9.09 from 8.18 in case of BFA, due to alkaline hydrothermal treatment, which provide utilization of MZBFA for wide pH range, suitable for the sorption of pesticides. This means that when the solution pH < pHpzc, the sorbent surface is positively charged; otherwise, the surface is negatively charged. At pH > pHpzc, a repulsive force is raised between the sorbent surface and the sorbate molecule. This repulsive force is more operative between sorbent surface and hydrolyzed PSM ions. This makes interaction between the sorbent and the sorbate molecules weak. At a very low pH, this repulsive force gets diminished between sorbent and the sorbate molecules. Therefore, the optimum sorption occurred at pH 2.0 for PSM. Figure 4 indicate that the uptake efficiency of PSM on MZBFA is higher compared with BFA.

image

Figure 4. Effect of contact time and Effect of pH on the sorption of PSM by BFA and MZBFA.

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Effect of Dosage and Initial PSM Concentration

The sorption increases with sorbent dose due to the availability of plenty of unoccupied sorbent sites by PSM molecules. It will continue till the sorbent becomes saturated by the sorbate (PSM) then after the removal slowed down as the dose concentration increased and attains a constant value (Figure 5). Beyond the dosage of 3 g L−1 for both sorbent, the percent removal increases slowly, which indicates that 3 g L−1 is the optimum sorbent dosage.

image

Figure 5. Effect of sorbent dosage and PSM concentration on the sorption of PSM by BFA and MZBFA

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The removal of PSM is dependent upon the concentration of PSM in the solution. Sorption of the total amount of pesticide increases with the initial concentration of PSM in the solution (Figure 5). This is due to high concentration gradient between the solution and the solid phase [31]. It can be also observed that the percent removal efficiency of the solute decreases as the concentration of PSM increases. The uptake of PSM by the sorbent at low sorbate concentration is higher than that at high concentration. MZBFA could remove 89 ± 1% and 49 ± 1% of PSM and BFA could remove 67 ± 1% and 41 ± 1% of PSM at 50 mg L−1 and 200 mg L−1 of initial pesticide concentration respectively at pH 2.0, temperature 298 K, contact time 5 h, and sorbent dose of 3 g L−1. The curves indicate that the uptake efficiency of PSM on MZBFA is higher compared with BFA.

Effect of Contact Time

A sorption capacity of PSM was measured as a function of time (24 h) keeping the initial concentration of PSM (125 mg L−1) and sorbent dosage of 3 g L−1. The effect of contact time on sorption of PSM is shown in Figure 4. The sorption equilibrium was obtained in 240 min. The major sorption was done in initial 90 min. This may be due to the fact that number of active sorption sites on the surface of sorbent was more initially. With sorption time the repulsive force is exerted on surface of the sorbent due to accumulation PSM molecule on the sorbent surface and further consumption of PSM molecules take more time. Saturation of BFA occurs for 47 ± 1% sorption while for MZBFA it occurs at 73 ± 1% of sorption. The resulting data shows that MZBFA has more potential to adsorb PSM than BFA.

Effect of Temperature–Thermodynamic Parameters

When the sorption was carried out at five different temperatures from 293 to 333 K with an interval of 10 K, the extent of sorption improved steadily with an increase in sorption temperature, and the most pronounced change occurring between 293 and 303 K (Figure not shown). The temperature has distinct effect on the removal of PSM, as the rise in temperature causes more dissociation of the PSM molecules. The PSM sorption on both the sorbents were definitely endothermic in nature requiring some amount of activation.

The thermodynamic parameters for the sorption process were evaluated from Von't Hoff plot (Figure not shown). The change of enthalpy (ΔH0) and change of entropy (ΔS0) evaluated from the slope and intercept (Eq. (2)).

  • display math(2)

where, Kc is the sorption equilibrium constant, ΔH0 is the enthalpy change (kJ mol−1), ΔS0 is the entropy change (J K−1 mol−1), R is the gas constant (8.314 J mol−1 K−1) and T (K).

The negative values of ΔG0 indicate the sorption process is spontaneous without any induction period and more favorable at higher temperatures. The positive values of ΔH0 were in the range of 11.83–9.52 kJ mol−1, lower than 20 kJ mol−1 confirms the endothermic nature of the overall sorption process and the sorption to be physical rather than chemical involving weak attraction forces [32, 33]. The positive value of ΔS0 (70.76–72.93 J K−1 mol−1) corresponds to an increase in the degree of freedom of the sorbed species, suggesting weak interaction between PSM and sorbents, which is thermodynamically favorable [33].

Despite being endothermic in nature, the spontaneity of the sorption process was ensured by a decrease in the Gibbs energy of the system. The ΔG0 value varied in a narrow range with the values showing a gradual increase from −8.67 to −11.54 kJ mol−1 and −11.62 to −14.64 kJ mol−1 for BFA and MZBFA, respectively, in the temperature range of 293−333 K in accordance with the endothermic nature of the sorption process. The parameters, ΔH0, ΔS0, and ΔG0, for the sorbate-sorbent interactions changed in a way that made the sorption thermodynamically feasible with a high degree of affinity of the pesticide molecules for the sorbent surface.

Sorption Isotherm Studies

The efficiency and nature of the sorption on the sorbent can be evaluated from the sorption isotherms. To obtain experimental equilibrium sorption data were then compared with the sorption isotherm models. Four models were used: Langmuir (Eq. (3)), Freundlich (Eq. (4)), Dubinin-Redushkwich (Eq. (5)), and Temkin (Eq. (8)) sorption isotherm. The models for characterization of equilibrium distribution relate the quantity qe (mg g−1) as a function of concentration at a fixed temperature.

  • display math(3)

where qm is the amount sorbed (mg g−1), Ce is the equilibrium concentration of the sorbate (mg L−1), and Q0 and B are the Langmuir constants related to maximum sorption capacity and energy of sorption, respectively.

The linear regression lines obtained for Langmuir isotherm graphs of Ce/qe against Ce, gave highly significant correlation coefficient values closer to unity (Table 2). The value of dimensionless parameter, RL, was less than unity (Table 2) which manifest that the sorption is favorable under the applied conditions. The comparatively smaller value of RL for sorption by MZBFA than BFA indicates sorption to be more feasible.

  • display math(4)

where qe is the amount sorbed (mg g−1), Ce is the equilibrium concentration of the sorbate (mg L−1), and Kf and n are Freundlich constants related to sorption capacity and sorption intensity, respectively.

Table 2. Isotherm parameters for the sorption of PSM on BFA and MZBFA.
IsothermsSorbentsParameter values
Langmuir qm (mg g−1)B (dm3 mg−1)RLR2
BFA34.4830.0230.2610.968
MZBFA35.7140.1120.0670.991
Freundlich Kf (dm3 g−1)n R2
BFA14.3552.262 0.993
MZBFA206.5383.922 0.927
D-R Xm (mg g−1)β (mol2 J−2)E (kJ mol−1)R2
BFA14.2252.08 × 10−40.4900.915
MZBFA27.7161.64 × 10−51.7460.903
Temkin KT (dm3 mg−1)B1 R2
BFA0.2287.757 0.965
MZBFA2.8215.646 0.950

The Freundlich isotherm is derived by assuming a heterogeneous surface with a nonuniform distribution of heat of sorption over the surface. The Freundlich constants Kf and n were calculated from the slope and intercept of the linear plot ln qe versus ln Ce. The values of heterogeneity factor, n, obtained from the slope were >1 would indicate conformity of the data to multilayer formation at the sorbent surface. Table 2 shows that the value of n for the sorption of PSM was higher for MZBFA than BFA demonstrate the higher sorption of PSM on MZBFA.

  • display math(5)

where qe is the amount sorbed (mg g−1), Xm is D-R monolayer capacity, inline image is the activity coefficient related to mean sorption energy, and ε is Polanyi potential, which is equal to

  • display math(6)

where R is gas constant (J K−1 mol−1), T is temperature (Kelvin), and Ce is the equilibrium concentration of the sorbate (mg L−1). When ln qm is plotted against E2, a straight line is obtained. The slope of the plot gives the value of inline imageand the intercept yields the value of sorption capacity, Xm. The value of inline imageis related to sorption energy, E, via following relationship:

  • display math(7)

The Dubinin Redushkwich isotherm model applied to test a pore filling mechanism in micropores of the sorbent, rather than layer-by-layer formation of a film on the walls of the pores. The sorption energy, E, of the process was calculated using the value of inline image, from that it can be deduced that the sorption mechanism is either ion-exchange or physical in nature. The sorption process follows ion-exchange process when the magnitude of E is between 8 and 16 kJ mol−1, while it is of a physical nature if the values of E < 8 kJ mol−1. The observed values of sorption energy, E, were < 8 kJ mol−1 (Table 2), which manifests the sorption of PSM in the studied sorbate-sorbent systems is to be physical in nature.

Temkin isotherm contains a factor that explicitly takes into account sorbate species-sorbent interactions. This isotherm assumes that: (i) the heat of sorption of all the molecules in the layer decreases linearly with coverage due to sorbate species–sorbent interactions and (ii) sorption is characterized by a uniform distribution of binding energies, up to some maximum binding energy.

  • display math(8)

where B1 = RT/b and KT are the constants. KT is the equilibrium binding constant (L mol−1) corresponding to maximum binding energy and constant B1 is related to the heat of sorption. A plot of qe versus ln Ce enables the determination of the isotherm constants KT and B1.

Temkin considered the effects of indirect sorbate-sorbate interactions on sorption isotherms. The values of KT and B1 obtained from the Temkin plots of qe versus ln Ce, are shown in Table 2. The heat of sorption of all the molecules on the sorbent surface layer decreases linearly with coverage due to sorbent-sorbate interactions. So, the sorption of PSM on sorbent can be characterized by a uniform distribution of the binding energies, up to some maximum binding energy. The sorption equilibrium data of PSM on BFA follows the order of Freundlich > Langmuir ≥ Temkin > D-R and on MZBFA the order is Langmuir > Temkin > Freundlich > D-R.

Kinetic Studies

To define the sorption kinetics of PSM, the parameters for the sorption process were studied for contact times ranging between 0 and 24 h by monitoring the percent removal of the PSM by the sorbent. The data were then regressed against the Lagergren equation, which represents a pseudo-first-order kinetic (Eq. (3)) and against a pseudo-second-order kinetic (Eq. (4)).

  • display math(9)
  • display math(10)

where, qt (mg g−1) is amount of PSM sorbed at time t (min), kf is the rate constant (min−1) and ks is the corresponding kinetic constant (g mg−1 min−1) are the rate constants of the pseudo-first-order and pseudo-second-order kinetics equations, respectively. The slopes and intercepts of these curves were used to determine the values of kf and ks, as well as the equilibrium capacity (qe). The calculated value of qe (Table 3) from the pseudo-first-order kinetics model were well below the monolayer capacities found by Langmuir equilibrium isotherm model, suggesting the sorption process is not a true first order reaction. However, the linearized pseudo-second-order kinetics model (Figure 6 and Table 3), provided much better R2 values than those for the pseudo-first-order model. The results obtained reveal that the initial sorption rate (“h” value) was highest for sorption of PSM by MZBFA than that by BFA and a somewhat complex mechanism of sorption instead of single step process. As a result, the sorption system appears to follow pseudo-second-order kinetics.

Table 3. Kinetic parameters for the removal of PSM by BFA and MZBFA.
Pseudo-first order
 qe (mg g−1)kf (min−1) R2
BFA7.2616.610 × 10−3 0.912
MZBFA8.3186.425 × 10−3 0.921
Pseudo-second order
 qe (mg g−1)ks (g mg−1 min−1)h (mg g−1 min−1)R2
BFA21.2772.110 × 10−30.0450.999
MZBFA32.2582.155 × 10−30.0700.999
Bangham
 αk0 (g) R2
BFA0.2460.813 0.933
MZBFA0.2311.855 0.944
Intraparticle diffusion
 kid,1 (mg g−1 min−1/2)I1 (mg g−1) R2
BFA0.9977.032 0.962
MZBFA1.24315.698 0.979
Intraparticle diffusion
 kid, 2 (mg g−1 min−1/2)I2 (mg g−1) R2
BFA0.18516.339 0.903
MZBFA0.23126.744 0.951
image

Figure 6. Pseudo-second-order kinetic model (t/qt vs. time) and Intraparticle diffusion plots (qt vs. t1/2) on the sorption of PSM by BFA and MZBFA.

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The kinetic data were further used to learn about the slow step occurring in the present sorbent system using Bangham's equation [34]. The applicability of Bangham's equation to the present pesticide sorption studies was tested. The values of constants α (<1) and k0 are given in Table 3. The double logarithmic plot according to above equation did not yielded perfect linear curves, indicate that the diffusion of sorbate into pores of the sorbent is not the only rate controlling step.

Intraparticle diffusion was characterized using the relationship between specific sorption (qt) and the square root of time (t1/2). This relation is expressed as follows:

  • display math(11)

where qt is the quantity of PSM sorbed at time t (mg g−1), kid is the intraparticle diffusion rate constant (mg g−1 min−1/2) and I is the intercept (mg g−1). Value of I (Table 3) give an idea about the thickness of the boundary layer, that is, the larger the intercept, the greater is the boundary layer effect [35]. The deviation of straight lines from the origin (Figure 6) may be because of the difference between the rate of mass transfer in the initial and final stages of sorption. Further, such deviation of straight lines from the origin indicates that the pore diffusion is not the sole rate-controlling step [36] as shown earlier by Bangham's equation. From Figure 6, it may be seen that there are two separate regions–the initial portion is attributed to the bulk diffusion and the linear portion to intra-particle diffusion [37]. The lower values of kid, 2 than kid, 1 (Table 3) signify that PSM diffuses into the pores of the sorbents. As the diffusion resistance increases with time the diffusion rate decreases, thus, sorption is a multistep process involving transport of PSM to the surface of the sorbents followed by diffusion into the interior of the pores.

Removal of Pesticide from Wastewater

To test the efficiency of removal organophosphorus pesticide from real environmental samples, an agrochemical wastewater sample was collected from the effluent of a rice field, near Ankleshwar city, India. This agro-wastewater was mainly contaminated with PSM and the concentration of pesticide in the original water was measured to be 147 mg L−1. The contaminated water sample (50 mL) was placed in a 100 mL brown colored glass bottles. To study the application of the developed sorption system batch experiments, within optimized conditions were carried out with both the sorbents. It has been observed that 41 ± 1% and 69 ± 1% of PSM were removed by BFA and MZBFA, respectively. However, the amount of PSM removed from wastewater was less than the amount sorbed from aqueous solutions, which is due to competitive sorption of possible contaminants in wastewater. Briefly, the reported method is effective, selective, and sensitive for the removal of PSM, and it can be used for their removal from wastewater.

CONCLUSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULT AND DISCUSSION
  6. CONCLUSION
  7. LITERATURE CITED

A sugar industry waste (BFA) was converted to Zeolitic material (MZBFA) by combined conventional and microwave reflux method for its use as sorption of PSM pesticide. Both the adsorbents were characterized by FTIR, XRD, SEM, and XRF techniques. Synthesized zeolitic material MZBFA show higher sorption capacity for PSM than the original BFA. The XRD analysis data confirm the formation of Zeolite P and Zeolite X as major minerals.

Batch experiments revealed that sorption is a pH dependent process and the maximum sorption occurs at pH 2. Equilibrium was attained in 240 min for 3 g L−1 dose and 125 mg L−1 concentration of PSM. Batch studies indicated adsorption followed Langmuir monolayer adsorption isotherm, was endothermic in nature, and followed a pseudo-second-order kinetic model. Intraparticle diffusion study showed that both film diffusion and intraparticle diffusion were simultaneously operating during the process of the sorption of PSM by both the sorbents.

Use of such waste material (BFA), which has a very low economic value, may be used effectively for the synthesis of zeolitic materials (MZBFA) and represents an effective and environmentally clean way of utilization of wastewater treatment process.

LITERATURE CITED

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULT AND DISCUSSION
  6. CONCLUSION
  7. LITERATURE CITED
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