Ind. Eng. Chem. Res. 2007, 46, 5405-5411
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Adsorption of Dinitro Butyl Phenol (DNBP) from Aqueous Solutions by Fly Ash Hui-Long Wang and Wen-Feng Jiang* Department of Chemistry, Dalian UniVersity of Technology, Dalian, People’s Republic of China, 116023
Fly ash, which is a waste generated in local thermal power plants, has been collected and converted to a low-cost and efficient adsorbent. The prepared adsorbent has been characterized and used for the removal of a typical alkyl dinitro phenol compound, 2-sec-butyl-4,6-dinitrophenol (DNBP), from aqueous solution. Adsorption studies include the effect of contact time, temperature, initial solute concentration, adsorbent dosage, pH, and particle size on the uptake of DNBP. The results show that the high adsorption capacity and sufficient removal efficiencies can be achieved at an optimum pH of 4.0, using 5 g/L of adsorbent with a particle size of 160-200 mesh in 120 min of equilibration time. The adsorption of DNBP increased as the temperature increased, which indicated that the process was endothermic in nature. The thermodynamic parameters (such as the free energy, enthalpy, and entropy of adsorption) were calculated based on a statistical model. Interpretation of the results was given. Kinetic studies have been performed to understand the mechanism of adsorption. The adsorption occurred via a film diffusion mechanism at lower concentrations (e8.33 × 10-5 M) and via particle diffusion at higher concentrations (g1.25 × 10-4 M). The adsorption of DNBP followed pseudo-second-order rate kinetics. Some experiments have also been performed for the purpose of regenerating the used saturated fly ash. The results indicate that fly ash can be used for the efficient removal of DNBP from aqueous solutions. 1. Introduction Environmental pollution by toxic compounds can be detrimental to human health and the environment. Alkyl dinitro phenols are widely acknowledged to be a group of hazardous compounds. 2-sec-Butyl-4,6-dinitrophenol (DNBP) is a typical example of this class of toxic compounds. DNBP is widely used in petrochemical industry as polymerization inhibitor for vinyl aromatics1 and in agriculture as a herbicide.2 It is introduced into surface water from its manufacturing and application processes. Although much benefit is obtained from its uses, DNBP has some undesirable side effects, such as toxicity and carcinogenity, and it can be hardly destroyed in conventional wastewater treatment.3-5 Therefore, the removal of DNBP from aqueous solutions is necessary and very important. An adsorption technique that uses adsorbents is being applied extensively for the removal of organic and inorganic micropollutants from aqueous phases, because of its efficiency, flexibility, and economic feasibility. Currently, the most widely available adsorbent material in industry is activated carbon, which also has been studied for DNBP removal from wastewater.6 Despite the prolific use of activated carbon for wastewater treatment,7 carbon adsorption remains an expensive process, because of the relatively high operating costs and tedious procedure for the preparation and regeneration of activated carbon. Therefore, finding alternative low-cost materials that have comparable capacity to activated carbon is highly desired. Recently, the potential of various economical alternative adsorbents (such as nature materials,8-12 biosorbents,13-17 and waste materials18-22) that are available from industry has received more attention for this purpose. For example, fly ash,23-26 montmorillonite,27 smectite,28 bentonite,10 and carbon cloth29 have been used as an adsorbent for the adsorption of nitrophenolic compounds from aqueous solutions. Recently, many attempts have been made to convert wastes generated in some prime industries to a suitable adsorbent material. The utilization of waste materials in the preparation * To whom correspondence should be addressed. Tel.: +86-41184706295. Fax: +86-411-84708590. E-mail address: dlutjiangwf@ yahoo.com.cn.
of low-cost adsorbents has immense practical utility.30-32 Fly ash, which is a coal combustion byproduct from thermal power plants, consists of particles that contain SiO2, Al2O3, Fe2O3, CaO, etc.,33 and/or their complexes (such as quartz, mullite, hematite, spinel, etc.). Currently, most fly ash is used as an additive in cement, concrete, and construction materials.34-36 In addition, fly ash also may be used as a nonconventional, low-cost adsorbent to remove organic compounds, dyes, inorganic anions, and metal ions from wastewater.37 It is evident that fly ash is an interesting alternative to replace activated carbon for adsorption in water pollution treatment. The purpose of this paper is to assess the ability of fly ash generated from a local power plant to adsorb DNBP from aqueous solution. The study was conducted to investigate factors that affect adsorption and determine the kinetics of the adsorption process using fly ash. The thermodynamic parameters for adsorption process were calculated and discussed based on a statistical model that we proposed in the study of corrosion inhibition.38 2. Materials and Methods 2.1. Adsorption Materials. All the chemicals and reagents used were of analytical reagent (AR) grade. DNBP was used as an alkyl dinitro phenol pollutant and obtained from Retell Fine Chemical Co., Ltd. (Tianjin, PRC). A stock solution of DNBP (2.2 × 10-4 M) was prepared in deionized water. Further solutions of different concentrations were made using the same stock solution. The properties of DNBP are given in Table 1. A representative sample of the fly ash was collected from a local thermal plant in Dalian, Liaoning province, PRC. The plant was fueled with brown coal. The samples used in this research were first treated with 30% aqueous ethanol solution at 40 °C for 24 h to remove heterogeneously distributed organic matter and adhering dust. The resulting product was filtered, washed with deionized water, and dried in an oven at 100 ( 2 °C overnight. The sample then was powdered, ground, and sieved to the desired particle size before use. Finally, the product was stored in a vacuum desiccators for further use. 2.2. Equipment. The morphology of the fly ash particles was investigated via scanning electron microscopy (SEM), using an
10.1021/ie070332d CCC: $37.00 © 2007 American Chemical Society Published on Web 07/07/2007
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Table 1. Physicochemical Properties of DNBP physicochemical property
value
molecular weight, MW melting point, mp boiling point, bp pKa at 25 °C specific gravity
240.2 g/mol 38-42 °C 332 °C 4.62 d45 4 1.2647
electron microscope (model JSW-5600LV, Japan). The X-ray diffraction (XRD) pattern of the sample was obtained with a Bede D1 X-ray diffractometer, using nickel-filtered Cu KR radiation (40 kV, 30 mA). The surface area was measured with a model QS-7 Quantasorb surface area analyzer (Quanta-Chrome Corp., Boynton Beach, FL). The ζ-potential measurements of the adsorbent were performed using a Malvern Zetasizer NanoZS90 analyzer (Malvern Instruments Ltd., U.K.). The pH measurements were made using a Mettler-Toledo pH meter (model Delta 320-S). The density of the sample was determined using specific gravity bottles. The chemical constituents of the prepared absorbent were analyzed with a X-ray fluorescence spectrometer (model SRS3400, Bruker AXS, Inc., Germany). Optical absorbance measurements were recorded on a Rayleigh ultraviolet-visible (UV-Vis) spectrophotometer (model UV 1201, Beijing, PRC). A wavelength scan conducted at different pH levels indicated a wavelength of maximum absorbance of 375 nm at pH >8.0. Absorbance values were recorded at a maxiumum wavelength (λmax) of 375 nm. The absorbance was determined to be proportional to the concentration of DNBP at pH >8.0. The calibration curve was prepared in the concentration range of 0-6.67 × 10-5 M. 2.3. Adsorption Studies. The adsorption experiments were performed via the batch technique, using a series of 250-mL conical flasks capped with viton stops. Each conical flask was filled with 100 mL of DNBP solution of varying concentration and adjusted to the desired pH by a small addition of diluted hydrochloric acid or sodium hydroxide. A known amount of adsorbent was added to each conical flask and the flasks were agitated intermittently for 120 min. The effect of pH, concentration, adsorbent dose, contact time, and temperature was studied thoroughly. After the required experimentation was over, the solid phase was separated by centrifugation and the solution was spectrophotometrically analyzed at the corresponding λmax value for the concentration of DNBP remaining in solution. The extent of adsorption was quantified by calculating the amount of adsorbate adsorbed per mass of fly ash (qe), using eq 1 and the fraction DNBP adsorbed from aqueous solutions (η), using eq 2:
qe )
(C0 - Ce)V m
(1)
C0 - Ce C0
(2)
η)
where V is the volume of solution (given in liters); C0 and Ce are the initial and equilibrium concentrations (expressed in units of mg/L), respectively, of DNBP in solution; and m is the mass of the fly ash (given in grams). The percent removal efficiency (P%) of DNBP from solutions was obtained from the following equation:
P% ) η × 100
(3)
2.4. Kinetic Studies. The adsorption kinetics of DNBP on fly ash was investigated through the batch technique. Several stoppered 100-mL conical flasks that contained a definite
Figure 1. X-ray diffraction (XRD) pattern of fly ash.
volume of solutions of DNBP of known concentrations were kept in a thermostatic water bath placed on a shaker. After attaining the desired temperature, a known amount of adsorbent was added to each conical flask and the conical flasks were allowed to agitate mechanically. At given time intervals, the solutions were centrifuged and the concentration of DNBP were determined spectrophotometrically. 3. Results and Discussion 3.1. Characterization of Adsorbent. The fly ash sample (1.0 g) was stirred with deionized water (100 mL, pH 6.9) for 2 h and the mixture was left in a stoppered conical flask for 24 h. An increase in the pH of fly-ash-leached deionized water was observed (pH 7.8). The different constituents of the fly ash used in these investigations were as follows: SiO2, 34.00%; Fe2O3, 18.60%; Al2O3, 13.50%; K2O, 2.94%; CaO, 2.01%; MgO, 1.34%; Na2O, 0.81%; TiO2, 1.07%; SO3, 0.96%; and P2O5, 0.50%. The loss on ignition (LOI) was determined to be 23.50%. The density was 1.42 g/cm3. The XRD pattern of the fly ash (Figure 1) showed the presence of quartz, mullite, magnetite, hematite, R-alumina, spinel, etc. Similar XRD peaks have been reported by Batabyal et al.50 The surface area of the adsorbent, as calculated by the Brunauer-Emmett-Teller (BET) method, was 24.56 m2/g. The surface area was also calculated theoretically, using methylene blue dye adsorption, assuming that the methylene blue dye molecule area was 197 Å2; the surface area was determined to be 15.18 m2/g via this method. Compared to the value determined by N2 gas adsorption, this value is slightly small and indicates that some pores of the fly ash are not accessible to methylene blue. SEM photomicrographs of the fly ash (Figure 2) clearly revel the surface texture and porosity of the sample. 3.2. Adsorption Studies. The effects of initial concentration of adsorbate, contact time, temperature, adsorbent dose, pH, etc. on the adsorptive reduction of DNBP are presented below. 3.2.1. Effect of Contact Time and Temperature. The effect of contact time and temperature on the removal of DNBP from its aqueous solution with an initial concentration of 33 mg/L, with a fixed pH (4.0) and adsorbent dose (5 g/L), is shown in Figure 3. It can be observed that the amount of DNBP adsorbed on the fly ash increases with time, up to 120 min, and, thereafter, it levels off at each temperature. This indicates that the equilibrium adsorption for DNBP is achieved after 120 min of contact time. The extent of the adsorption of DNBP was also observed to increase with temperature in the range of 20-40 °C, indicating the endothermic nature of the process. The increase in DNBP adsorption with increasing temperature might
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Figure 4. Effect of fly ash dose on the equilibrium adsorption of DNBP. Experimental conditions: pH 4.0; initial concentration, 33 mg/L; particle size, 160-200 mesh; contact time, 120 min; T ) 20 °C.
Figure 2. Scanning electron microscopy (SEM) photomicrographs of fly ash at different magnifications: (a) 1000× and (b) 200×. Figure 5. Adsorption isotherms of DNBP on fly ash at different temperatures. Experimental conditions: pH 4.0; particle size, 160-200 mesh; contact time, 120 min; adsorbent dosage, 5 g/L.
Figure 3. Effect of contact time and temperature on the adsorption of 2-secbutyl-4,6-dinitrophenol (DNBP). Experimental conditions: pH 4.0; initial concentration, 33 mg/L; particle size, 160-200 mesh; adsorbent dosage, 5 g/L.
be due to the enhanced rate of intraparticle diffusion of the adsorbate, because diffusion is an endothermic process. It also suggests that the number of active surface centers available for adsorption increases as the temperature increases.39 3.2.2. Effect of Solute Concentration and Adsorbent Dose. Isothermal adsorption experiments also were performed to determine the amount of adsorption of DNBP by contacting the fly ash with solutions for 120 min. The temperature was controlled at 20 °C, and the pH was 4.0. Figure 4 shows the effect of fly ash dosage on the percent removal of DNBP at
equilibrium at 20 °C in the presence of 33 mg/L DNBP. Figure 4 shows that, for fly ash dosages up to 5 g/L, the percent removal efficiency was proportional to the amount of fly ash. This is due to the availability of a higher surface area for adsorption with increasing fly ash dosage. Beyond a fly ash dosage of 5 g/L, the percent removal increases slowly, which indicates that 5 g/L is the optimum adsorbent dosage. The adsorption experiments were conducted using different concentrations of DNBP at pH 4.0 with a particular fly ash dosage of 5 g/L and a shaking time of 120 min at temperatures of 20, 30, 40, and 50 °C to determine the adsorption capacity at various temperatures and thermodynamic parameters, which are depicted in Figure 5. The curves are regular and concave to the concentration axis, thereby indicating a positive adsorption. It is observed that the adsorption capacity of fly ash increases as the concentrations of DNBP increase, up to 33 mg/L, and, thereafter, the uptake of DNBP does not increase significantly with increasing initial concentration over the range of temperatures investigated. It can be also observed that the percent removal efficiency of the solute decreases as the concentration of DNBP increases. The uptake of DNBP by fly ash at low adsorbate concentration is higher than that at high concentration. In fact, when all of the available monolayer sites are occupied, some fresh internal surface can be created.40,41 In reality, this process cannot continue indefinitely without irreparable damage to the adsorbent. The high adsorption capacity and sufficient removal
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Ind. Eng. Chem. Res., Vol. 46, No. 16, 2007 Table 2. Effect of Particle Size on the Adsorption of DNBP
Figure 6. Effect of pH on the adsorption of DNBP and the ζ-potential on the surface of the fly ash particles. Experimental conditions: initial concentration, 33 mg/L; particle size, 160-200 mesh; contact time, 120 min; adsorbent dosage, 5 g/L; T ) 20 °C.
efficiencies must be fulfilled when fly ash is applied as an adsorbent in industry. Therefore, during all the experiments discussed in this paper, an initial solute concentration of 33 mg/L was applied. 3.2.3. Effect of pH. The adsorption of DNBP was studied over a broad pH range of 1-12 (see Figure 6). As shown in the figure, there is almost no change in the uptake of DNBP at pH e4.0, beyond which a sharp decline in adsorption is observed. This can be explained by considering the point of zero charge (PZC) on the adsorbent surface which is, in turn, influenced by the solution pH. The pH at the point of zero charge (pHPZC) of the adsorbent was determined according to the procedure described in Babic´ et al.42 Experimental results have shown the value of pHPZC of the fly ash to be 3.2, using 0.01 M KNO3 solutions as the supporting electrolyte, to keep the ionic strength constant. This means that when the solution pH is 4.0, the uptake of DNBP was less and decreased sharply, because of electrostatic repulsions between the negative adsorbent surface charge and the DNBP anions and between dinitrophenolate-dinitrophenolate anions in solution. Also, Figure 6 clearly shows that the ζ-potential on the surface of the fly ash particles changed slightly at pH e4.0 and decreased quickly as the pH of the medium increased. The results account for the fact that an increase in pH promotes an increase in the negative charge on the surface of the fly ash and a lower uptake of DNBP that exists as anions. Therefore, the optimum removal occurred at pH 4.0 for DNBP. 3.2.4. Effect of Particle Size. The adsorption experiments were performed using four different sizes at pH 4.0 with a dosage of 5 g/L and contacting time 120 min at 20 °C. The results are presented in Table 2, which reveals that the uptake of DNBP increases as the adsorbent particle size decreases. The increase in DNBP uptake with decreasing fly ash particle size might be due to the enhanced surface area of the adsorbent, because the number of active surface centers available for adsorption increase as the adsorbent particle size decreases. The
particle size
surface area (m2/g)
qe (mg/g)
40-100 mesh 100-160 mesh 160-200 mesh 200-250 mesh
12.84 21.32 24.56 31.16
3.76 4.03 5.10 5.18
adsorbent with a particle size of 160-200 mesh was selected for further adsorption studies, because of its sufficient adsorption capacity and its ease of preparation, as well as it being less labor intensive in centrifugation. 3.3. Thermodynamic Parameters. Basic information on the interaction between the adsorbate and the adsorbent can be provided by the thermodynamic parameters for the adsorption process. The adsorption of pollutant organic compounds (POCs) from a liquid phase to a solid phase (referred as S) is considered to occur in the process of adsorption, and this process can be depicted by the following reversible reaction:
S + POCs h S(POCs)ads Let S in the aforementioned reaction be the system in the ensemble and the solvent that contains POCs as donor particles be the medium. The adsorption process can be regarded as the course of distribution of donor particles to the system. Therefore, it is justifiable to extend the model of a variable number of particles in statistical physics for the adsorption process. According to the statistical physics,43 the probability of distribution of a variable number of particles, for systems of such a type, is given by
ω(i,) ) A exp
(
)
iµ - i θ
(4)
where A is the normalizing coefficient, µ the chemical potential (which is dependent on the temperature and concentration of the donor particles), i the number of donor particles distributed in each system, θ the distribution modulus, i the energy of the system that contains i donor particles (which is assumed to be approximately equal for the systems that contain the same number i of donor particles), and i ) 0 at i ) 0. The normalizing condition is n
ω(i,) ) 1 ∑ i)0 or
[
( )]
n
A 1+
(5)
exp ∑ i)1
iµ - i θ
)1
(6)
The average number of donor particles accepted by each system is n
nj )
n
iω(i,) ) ∑ iA exp ∑ i)1 i)1
( ) iµ - i θ
(7)
Eliminating A from eqs 6 and 7, we obtain n
nj )
i exp ∑ i)1 n
1+
( ) ( ) iµ - i
exp ∑ i)1
θ
iµ - i θ
(8)
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Figure 7. Curvefitting of data for the adsorption of various concentrations of DNBP at different temperatures, using a statistical model. Table 3. Thermodynamic Parameters for Adsorption of DNBP on Fly Ash temperature (°C)
∆G°ads (kJ/mol)
∆H°ads (kJ/mol)
∆S°ads (J mol-1 K-1)
20 30 40 50
-20.11 -20.93 -21.91 -22.63
4.91
85.40
For the adsorption process, the S(POCs)ads formation reaction corresponds to the distribution with i ) 0 or 1. The condition for which i ) 0 is taken to correspond to a state of having no adsorption at all (the fraction of solute adsorbed from solutions is η ) 0 and the corresponding percent removal efficiency of solute P% ) 0), i ) 1 to a state of complete adsorption (fraction solute adsorbed from solutions η ) 1 and the corresponding percent removal efficiency of solute P% )100). The actual occurred process of adsorption is a random distribution between i ) 0 and 1. Therefore, the corresponding actual percent removal efficiency of adsorbate (P%) is a value between 0 and 100, and the actual fraction adsorbate adsorbed from solutions (η) is equal to the statistical average value for such a (0,1) distribution. For the adsorption process, eq 8 then is reduced to
1
nj ) η )
1 + exp
(
-µ θ
)
(9)
Here, 0 e nj e 1. Considering the fact that η is related to the concentration of adsorbate for a fixed dosage of adsorbent and is associated with the change of free energy of adsorption G°ads,44 the following equation can be derived from eq 9:
ln
(1 -η η) )
∆G°ads RT ln C θ θ
(10)
The curvefitting of the adsorption data in Figure 5 to the statistical model at different temperatures is presented in Figure 7. The values of θ and ∆G°ads, at different temperatures, can be calculated using the slope and the ordinate axis intercept of the straight line. Considering the fact that the values of enthalpy and entropy changes of the adsorption process have no distinct changes in the temperature range studied, the thermodynamic parameters ∆H°ads and ∆S°ads for the adsorption of DNBP on fly ash can be calculated using the thermodynamic equation; these values are given in Table 3. The negative free-energy values are characteristic of the feasibility and spontaneous nature of DNBP adsorption onto fly ash in the temperature range
investigated. The positive value of ∆H°ads suggests that the adsorption of DNBP onto fly ash is an endothermic process. This value is consistent with the experimental results presented in Figure 3. A positive entropy change indicates the increased randomness at the solid/solution interface and also reflects the affinity of the adsorbent material toward DNBP.24,39 The adsorption process of a solute onto a solid surface includes, at least, two steps: the adsorption of the solute molecules by the adsorbent, and in company with the desorption of the solvent molecules from the solid surface; therefore, the traditional adsorption entropy change may be divided into two independent fractions, according to the stoichiometric displacement model for adsorption (SDM-A).45 Because of the fact that the temperature is given, the two fractions of ∆S°ads include the entropy change of adsorption affinity for DNBP onto the fly ash surface and the desorption entropy changes of water from the adsorbent surface. The volume of DNBP molecule is ∼10 times that of water; therefore, the attachment of a solute molecule to the fly ash (entropy decrease) is expected to cause more water desorption from the adsorbent (entropy increase), thus resulting in the large positive value of ∆S°ads. The large positive value of ∆S°ads for the given adsorption system also suggests that the action of the adsorption affinity should be attributed to the entropy driving. 3.4. Adsorption Dynamics. To understand the practical application of adsorption, it is necessary to determine the steps involved in the process of adsorption, which govern the overall rate of removal. The adsorption dynamics and other mechanistic aspects is represented by the following expressions:
E)1-
6
∞
∑ 2 n)1
π
B)
1 n
2
exp[-n2Bt]
π2Di r02
(11)
(12)
where E is the fractional approach to equilibrium at time t, Di the effective diffusion coefficient of adsorbate in the adsorbent phase, and r0 the radius of the adsorbent particle (which is assumed to be spherical). The fractional approach to equilibrium E can be obtained using eq 13:
E)
qt qe
(13)
where qt is the amount adsorbed at time t and qe is the maximum equilibrium uptake. For every calculated value of E, the corresponding Bt values, as derived from eq 11, have been obtained from Reichenberg’s table.46 Linearity tests of the Bt versus time t plots (not given) are applied to distinguish between controlled rates of exchange. It can be observed that the plots did not pass through the origin at the lower concentration (e8.33 × 10-5 M), which indicated that the adsorption occurred via film diffusion, whereas at higher concentrations (g1.25 × 10-4 M), the same passed through the origin, which signified the nature of the adsorption to be occurring via particle diffusion.47 Similar results have been reported by Gupta et al.48 The rate constant for the adsorption of DNBP on fly ash was determined using the pseudo-second-order kinetic model:49
1 1 t ) + t qt k q 2 qe ad e
(14)
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constants for temperatures in the range of 20-40 °C at constant concentration. The studies revealed that fly ash can be fruitfully used as an adsorbent for DNBP removal. Acknowledgment Li-Juan Han, a M.Sc. student, is acknowledged for her contribution during the experiments. Literature Cited
Figure 8. Pseudo-second order adsorption kinetics of DNBP onto fly ash at different temperatures.
where qt and qe are the amount of DNBP adsorbed (expressed in units of mg/g) at any time t and at equilibrium, respectively, and kad (expressed in units of g/(mg min)) is the second-order adsorption rate constant. A plot of t/qe vs t gives a straight line, as can be seen in Figure 8, confirming the applicability of the second-order rate expression. The adsorption rate constants for temperatures in the range of 20-40 °C at an initial concentration of 33 mg/L of DNBP were calculated from the intercepts of the respective linear plots, and the values were determined to be 0.02582 g/(mg min) at 20 °C, 0.1402 g/(mg min) at 30 °C, and 0.1426 g/(mg min) at 40 °C, respectively. 3.5. Regeneration of Adsorbent. Regeneration of the adsorbent is an important process in wastewater treatment. To assess the practical utility of the adsorbent, some experiments were also conducted for the regeneration of used saturated fly ash with 2% aqueous H2O2 solution at 45 °C for 60 min and 6% NaOH solution in succession at 30 °C for 40 min. The solution then was filtered. The regenerated fly ash was washed, dried, cooled, and used for further adsorption at 30 °C. It was observed that the adsorption capacity was restored almost to the original value. The aqueous H2O2 can decompose to water and nascent oxygen in the presence of fly ash as a catalyst. The adsorbed DNBP may be oxidized and decomposed by the available nascent oxygen and then be desorbed from the fly ash surface.50 The NaOH solution may be helpful for the formation of sodium salt of the residual adsorbate on fly ash, which could be readily removed. 4. Conclusions The adsorbent developed from thermal power plants fly ash was determined to be effective for the removal of 2-sec-butyl4,6-dinitrophenol (DNBP) from wastewater. The high adsorption capacity and sufficient removal efficiencies of DNBP from solutions can be achieved at pH 4.0 with an adsorbent dosage of 5 g/L of particles with a size of 160-200 mesh at 20 °C and a contacting time of 120 min. The adsorption of DNBP increases as the temperature increases, thereby indicating that the removal is endothermic in nature. The results indicate that the statistical model is suitable to fit the experimental data of the adsorption of DNBP and estimate the corresponding thermodynamic parameters. The rate of adsorption of DNBP on fly ash seems to be controlled by film diffusion at lower concentrations, whereas at higher concentrations, the rate of adsorption is dominated by a particle diffusion mechanism. The pseudosecond-order kinetic model can be applied to determine the rate
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ReceiVed for reView March 4, 2007 ReVised manuscript receiVed May 27, 2007 Accepted June 7, 2007 IE070332D
