Adsorption Behavior of 2-Naphthalenesulfonate on Activated Carbon

Chiung-Fen Chang and Ching-Yuan Chang*. Graduate Institute of Environmental Engineering, National Taiwan University, Taipei 106, Taiwan. Wolfgang Ho1l...
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Ind. Eng. Chem. Res. 2003, 42, 6904-6910

Adsorption Behavior of 2-Naphthalenesulfonate on Activated Carbon from Aqueous Systems Chiung-Fen Chang and Ching-Yuan Chang* Graduate Institute of Environmental Engineering, National Taiwan University, Taipei 106, Taiwan

Wolfgang Ho1 ll Forschungszentrum Karlsruhe, Institute for Technical Chemistry, Section WGT, P.O. Box 3640, D-76021 Karlsruhe, Germany

The adsorption behavior of 2-naphthalenesulfonate (NS) on activated carbon Chemviron Filtrasorb 400 (F 400) in different aqueous systems was investigated in this study. Adsorption technology has high potential to remove NS from aqueous solutions due to the high adsorption capacity. The adsorption capacity of the NS by F 400 is effectively improved at lower pH value and higher ionic strength. The effective diffusion coefficients can be calculated by an approximate estimation method and further used to simulate the adsorption behavior of NS in a full-scale column when combined with linear driving force model (LDF model). Satisfactory results of simulating the adsorption breakthrough curve in the full-scale column can be obtained, which was further compared with the results of the rapid small-scale column tests (RSSCTs). The methods used in this study satisfactorily simulated the experimental results for investigating the performance of NS adsorbed by activated carbon F 400. Therefore, it can directly provide the parameters for designing and simulating the adsorber systems. Introduction Surfactants are common organic additives used in the metal surface treatment industries, and usually present at low concentrations in the system.1 According to the purpose of the utilization, surfactants may be divided into several categories, such as brightening, leveling, and dispersion agents. 2-Naphthalenesulfonate (NS) is an anionic surfactant and commonly used in the electroplating process as the leveling agent in order to increase the electroplating quality. Concerning the field of the chemical production, it is also used as the original material to produce naphthylamine sulfonic acid, semiproduct of dyes, etc. In addition, it has been found in the source of natural water as background organics. It is obvious that NS may be easily detected in the water body, process water, and wastewater. Because the shortage of water is one of the important environmental issues in the world, nowadays, the reuse of process water and materials is the trend at present to reach the goal of sustainable development. Granular activated carbon (GAC) is widely applied for organic compound removal in water/wastewater treatment with high removal efficiency. The lowest carbon dosage for the wastewater treatment to reach the economic benefit may be estimated from the adsorption isotherm. Due to the various demands, either of two systems (e.g., batch or fixed-bed adsorber systems) can be used to design the treatment system. A comparison of batch and fixed-bed adsorber systems indicates that the adsorption kinetics is ignored in the former, but important in the latter. In addition, the rapid smallscale column test (RSSCT) was successfully applied to * To whom correspondence should be addressed. Address: National Taiwan University, Graduate Institute of Environmental Engineering, 71 Chou-Shan Road, Taipei 106, Taiwan. Fax: 886-2-2363-8994. E-mail: [email protected].

simulate the long-term pilot studies and full-scale operation. The RSSCT possesses several primary advantages, compared to the full-scale test. These are (1) short time and small solution volume required to be conducted and (2) no need for the extensive isotherm and kinetic data.2 The goal of this study was to investigate the adsorption behaviors of NS, such as adsorption equilibrium and kinetics on activated carbon in different aqueous systems in order to provide an effective method to get the related kinetic parameters when the adsorption process is used as the recovery technology to recycle the aged process solution. The adsorption capacities in different aqueous system were also compared and discussed. The short fixed bed reactor (SFB-reactor) experiments were designed to obtain the liquid diffusivity, and then an effective method was employed to estimate the effective diffusion coefficient. Furthermore, the linear driving model combined with the diffusion was used to compute and predict the breakthrough curves of the full-scale column to examine the feasibility by means of comparing with the results of RSSCT experiments. Materials and Methods Adsorbent. Activated carbon, Chemviron Filtrasorb 400 (F 400), was used as the adsorbent with a particle size range between 0.42 and 1.68 mm. The mean particle size of F 400, dp ) 1.04 mm, was calculated from the sieve analysis of the representative samples obtained by using a rotating sample-splitting device by means of the weight percentages of particles in the different sieve sizes. The physical characteristics of F 400 are shown in Table 1. The pretreatment of the adsorbent comprised several steps. First, the adsorbent was washed by distilled water to remove the crushed

10.1021/ie030394d CCC: $25.00 © 2003 American Chemical Society Published on Web 11/13/2003

Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 6905 Table 1. Physical Characteristics of Chemviron Filtrasorb 400 (F 400) Activated Carbon property mesh sizea average particle diameter, dp, mm BET specific surface area,a m2/g specific external surface area,b as, m2/kg average true particle density,a Fs, kg/m3 apparent particle density,c Fp, kg/m3 filter bed layer density,d FB, kg/m3 particle porosity,e p filter bed porosity,d,f B

12-40 1.04 1026 5.76 2180 1000 530 0.54 0.47

a Data from ref 3. b Assumed as spherical particle and calculated using as ) 6/(Fpdp). c Data from the experiments of pycnometer. d In a water-filled bed (short-fixed bed, SFB). e Calculated using p ) 1 - (Fp/Fs). f Calculated using B ) 1 - (FB/Fp) for SFB.

carbon. Second, it was dried at 383 K in a vacuum oven overnight and then stored in the desiccator. Finally, it was wetted in the specific wetting solutions under vacuum, prior to both bottle-point and short fixed-bed reactor (SFB-reactor) experiments. Adsorbate and Standard Compounds. 2-Naphthalenesulfonate (C10H7SO3Na, denoted as NS), and p-nitrophenol (PNP) and anthraquinone-2-sulfonate (ACS) of reagent grade (provided by Merck) were used as the target adsorbate and standard compounds of SFB-reactor experiments, respectively. The initial concentrations of NS, PNP, and ACS for SFB-reactor experiments were around 10 mg dm-3, while that of NS for the adsorption isotherm experiments was between 0 and 500 mg dm-3. Prior to the analysis, all the samples were filtrated through a 0.45 µm membrane. Aqueous Systems. Several aqueous solvents were used to investigate the adsorption of NS by activated carbon F 400, as listed in Table 2. The design of SNS,elec and SNS,0.25, and SNS,0.25 and SNS,6 aqueous systems was used to investigate the effects of copper sulfate and the pH value, respectively. In addition, the aqueous systems of SPNP and SACS were designed on purpose to get the correction factor, fβ, for SFB-reactor experiments. Analytical Measurements. Analyzers, such as total organic carbon (TOC) analyzer (O.I.C. M-700/Carbon Analyzer Dohrmann DC-80), UV spectrophotometer (Perkin-Elmer UV/vis Lambda 3), and high performance liquid chromatograph (HPLC, HP1090), were used to determine the concentrations of organic adsorbates in this study. The analytical conditions of organic adsorbates are as follows. The wavelengths used in the UV spectrophotometer for NS, PNP, and ACS are 230, 315, and 253 nm, respectively. The column and detector used in HPLC are ODS hyperic and DAD diode array

detector. The HPLC method for NS is so-called ion-pair chromatography, which is suitable for the trace-level determination.3 The volume ratio of tetrabutylammonium hydrogen sulfate (5.42 m mol in H2O) to acetonitrile is 737 to 263, in which both solvents compose the eluent for HPLC analysis. Adsorption Equilibria. The bottle-point method was used to obtain the adsorption isotherms of different solutions. The final concentrations of adsorbates in various ranges were specially designed to yield suitable adsorption isotherms. Mixtures of liquid and solid with various ratios of NS and the adsorbent weights were prepared and shaken at constant 298 K until the concentrations of filtrate reach constant in one week. The influence of temperature was not investigated in this study since the temperature effect on the equlibrium is not significant in the range of 288-313 K.4 SFB-Reactor Experiments. The amount of F 400 used in the SFB-reactor (length of 20 cm and diameter of 2 cm) was about 3 g per experiment. The properties of F 400 and SFB reactor are those specified in Table 2. To obviate air bubbles in the carbon bed, the column was packed under specific wetting solutions using distilled water at the pH values of 0.25, 2, and 7 for NS, PNP, and ACS, respectively. The temperatures for the solutions containing NS and standard compounds were kept constant at 298 and 293 K, respectively, which were carefully controlled by noting that the effect of the temperature is significant for the kinetics of adsorption.5 The bed velocities uB in the experiments were about 5-20 m/h. RSSCTs Experiments. The column used in the RSSCTs experiments is at the length of 30 cm and diameter of 13.6 mm. The representative F 400 samples were ground, washed by distilled water, and then dried at 373 K with an average diameter of 183 µm. The properties of F 400 and RSSCTs column are those specified and designed in Table 3, which will be discussed later in detail. To obviate air bubbles in the carbon bed,a similar pretreatment procedure as in the SFB-reactor was also used prior to the adsorption process. Results and Discussion Adsorption Equlibria. The adsorption mechanisms of surfactants at the solid-liquid interface involve several functions, such as ion exchange, ion pairing, acid-base interaction, adsorption by dispersion forces, or polarization of π electrons.6 Activated carbon, F 400, is essentially nonpolar and hydrophobic. Its adsorption

Table 2. The Identification and Compositions of the Aqueous Systems solution ID

composition

SNS,elec

Electroplating solution: containing concentrated sulfuric acid, H2SO4(conc), of 60 g dm-3, CuSO4‚5H2O of 200 g dm-3, and concentrated hydrochloric acid, HCl(conc), of 30 mg dm-3. The pH value of SNS,elec is around 0.25. The only organic compound in the solution SNS,elec is 2-naphthalenesulfonate (NS) at various concentrations. Distilled water containing HCl(conc) of 30 mg dm-3 and NS of various concentrations with the same pH value as electroplating solution adjusted by H2SO4(conc). Distilled water with dissolution of NS of 0-10 mg dm-3. The pH value of SNS,6 is around 6. Distilled water of pH ) 2 adjusted by HCl(conc) with dissolution of p-nitrophenol (PNP) of around 10 mg dm-3. Distilled water of pH ) 7 adjusted by NaOH with addition of buffer of 1 m mol dm-3 NaHCO3, and with dissolution of anthraquinone-2-sulfonate (ACS) of around 10 mg dm-3.

SNS,0.25 SNS,6 SPNP SACS

note Simulating the copper electroplating solution.

Simulating the general wastewater. Standard solution for obtaining the correct factor. Standard solution for obtaining the correct factor.

6906 Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 Table 3. Design Parameters of Rapid Small-Scale Column Test in This Study X)0

parameter particle diameter, dp,SC scale-up factor, dp,LC/dp,SC column diameter, dB,SC cross-sectional area, AB,SC bed height,b hB,SC bed volume, VB,SC filter bed velocity, uB,SC volumetric flow rate, QF,SC empty bed contact time,e tEB,SC mass of carbon dosage,f mB,SC

X)1 183 5.7a 13.6 1.45

7.01 10.2 45.6c 6.6 5.5 5.4

unit µm

8.3 12.0 9.5d 1.37 31.6 6.4

mm cm2 cm cm3 m/h dm3/h s g

a The particle diameter for full-scale column d P,LC is 1.04 mm taken as that used in short-fixed bed. b Calculated by (uB,SCtEB,SC). c Calculated by u d B,LC[dp,LC/dp,SC]. Calculated by uB,LC[(dp,LC/dp,SC)(ReSC,min/ReLC)], in which ReSC,min ) 1. ReLC ) [(dp,LCuB,LC)/(Bυ)] ) [(8 m/h) × (1.04 × 10-3 m)]/[0.47 × (1.02 × 10-2 cm2/s)] ) 4.82. uB,SC ) 8 m/h × 5.7÷(4.82) ) 9.5 m/h. e Calculated by tEB,LC[dp,SC/ dp,LC]2-X with tEB,LC ) 3 min. f Calculated by QF,SCtEB,SCFB,LC, in which X equals 0 and 1 for constant diffusion (CD) and proportional diffusion (PD), respectively.

of NS may mainly be due to dispersion forces and polarization of π electrons (e.g., the attraction between electron-rich aromatic nuclei of the adsorbate and positive sites on the adsorbent surface). When the pH value of the solution is lower than the value of zero point of charge (pHZPC) of F 400 (i.e., pHZPC ) 8.32), the surface of F 400 is positively charged and contains highly positive sites, due to the adsorption of protons onto the charged sites from the solution. Therefore, the electrostatic attraction caused by the positive sites on F 400 surface and the anionic surfactants also promotes the adsorption capacity of anionic NS on F 400. The empirical Freundlich isotherm relationship, which corresponds to the heterogeneous adsorbent surface, is listed below for the purpose of correlating the experimental data.

qe ) kFCe(1/nF)

(1)

In eq 1, qe and Ce are the adsorbate concentrations in solid and liquid phases at equilibrium, respectively. The kF and nF are the Freundlich equilibrium constants, which represent the adsorption capacity and strength of adsorption, respectively. The results are shown in Table 4 and Figure 1. The Freundlich isotherm can well describe the adsorption behaviors of NS in three solutions (SNS,elec, SNS,0.25, and SNS,6) with the high correlation coefficients. Comparing the adsorption capacities of three solutions, one notes that the sequence of high adsorption capacity is SNS,elec > SNS,0.25 > SNS,6, interpreting that the lower pH value and higher ionic strength of solution promote the NS adsorption by activated carbon F 400. According to the Stern model of electrical double layers,7,8 the effective thickness of the diffuse layer (so-called Debye length), where the major portion of electrical interactions with surface occur, is inversely proportional to the square root of the concentrations of ionic species and to their valences.

Figure 1. The simulation of Freundlich isotherm (s) for the single adsorbate of 2-naphthalenesulfonate (NS) in the aqueous system on activated carbon F-400. [, 9, and 0: the experimental data at 298 K in the aqueous system of pH ) 6, aqueous system of pH ) 0.25 adjusted by concentrated H2SO4, and electroplating solution, respectively.

Therefore, the presence of electrolyte (i.e., increase of the counter- and co-ions) in the solution leads to the compression of the double layer and provides shielding, which shortens the range of electrical effects between NS and F 400. Furthermore, the hydration molecular size of NS is also reduced, which will be discussed later in detail, and then promotes the ability of NS to reach the surface of F 400. As a result, the adsorption capacity of NS by F 400 is effectively improved by the lower pH value and higher ionic strength of solution. Liquid Diffusivity. The Gnielinski correlations9 which are suitable for spherical particles in the range of Sc (Schmidt number) 500, are adopted in this study to calculate the liquid diffusivity as follows.

Shlam ) 0.664 Sc1/3 Re1/2 Shturb )

(2)

0.037 Re0.8 Sc 1 + 2.443 Re-0.1(Sc2/3 - 1)

(3)

ShE ) 2 + xSh2lam + Sh2turb ShB )

(4)

βLdP ) [1 + 1.5(1 - B)]ShE DL

(5)

Sh is the symbol of the Sherwood number. Shlam and Shturb represent the contributions of laminar and turbulent flows to the external mass transfer, respectively.

Table 4. Values of Adsorption Isotherm Parameters and Correlation Coefficients (rF2)a,b pH ) 6

electroplating solution (pH ) 0.25)

pH ) 0.25

compd

kF

nF

rF2 c

kF

nF

rF2 c

kF

nF

rF2 c

NS

0.454

7.46

0.9927

0.96

18.69

0.8860

1.56

8.375

0.9834

a

The units for Ce and qe are mol/m3 and mol/kg, respectively. b The rF are the correlation coefficients by fitting the experimental data to Freundlich isotherms. c The ranges of the equilibrium concentrations are 0-200 for NS.

Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 6907

Re () dpu/ν, u ) uB/B) is the Reynolds number. The values of ShE and ShB hold for the isolated single particle and the single particle in adsorption bed, respectively. Sc () ν/DL) is the Schmidt number. The quantity βL is the film mass transfer coefficient. The dp is the mean particle size. The B is the filter porosity. The quantities u and uB are the interstitial and bed velocities, respectively. The value of βLas obtained from the Gnielinski correlation is denoted as (βLas)cor. The film mass transfer coefficient, βL, may be determined by short fixed-bed (SFB) reactor experiments, which lead to obtaining the value of βL as (QF/mas) ln(C0/C)|t)0. The horizontal experimental development of the breakthrough curve yields the value of βL. The value of βLas thus computed from SFB-reactor experiments is denoted as (βLas)exp. The solution fed into SFB reactor is kept at constant initial concentration, which must be low enough to avoid the occurrence of the intraparticle diffusion of the solute. Due to the simplicity, the plugflow film diffusion is chosen in this study as the main model to obtain the βL value by neglecting the axial dispersion, which is inclusively considered in dispersedflow diffusion models. The DL values of PNP and ACS at 293 K were 7.8 × 10-9 and 6.4 × 10-10 m2/s obtained from the calculation by Wilke and Chang correlations10 and the determination by diaphragm cell experiment.11 By means of eqs 2-5, the values of (βLas)cor at different filter velocities for PNP and ACS can be calculated from the known DL values. As to the values of (βLas)exp, they can be determined by the SFB-reactor experiments in SPNP and SACS under the same filter velocities. The average value of correction factors, fβ ) ((βLas)exp/(βLas)cor), of different filter velocities was on behalf of the correction factor for activated carbon F 400. In this study, the fβ of activated carbon F 400 was 2.18, adopting the average value of two standard components in SPNP (fβ ) 2.11) and SACS (fβ ) 2.25). Note that, if the correction factor is neglected, it may cause about 40% error in the calculation of the liquid diffusivity. The horizontal portions of experimental data of breakthrough curves of solutions of SNS,elec and SNS,0.25 are presented in Figure 2. The calculated results of DL for the solutions of SNS,elec and SNS,0.25 with consideration of correction factor are shown in Table 5 and Figure 3. There is great consistence among the liquid diffusivities obtained from various bed velocities (uB) no matter what the solution system is. The values of DL in sequence are 6.87 × 10-10 m2/s of SNS,elec, 5.93 × 10-10 m2/s of SNS,0.25, and 3.76 × 10-10 m2/s of SNS,6. The DL values of SNS,elec and SNS,0.25 were determined by SFBreactor experiments, and that of SNS,6 was calculated by the Scheibel and Wilke-Chang correlations.12,13 Furthermore, if the Stokes-Einstein equation (DL ) (kT/6πµBR)) is adopted to calculate the hydrodynamic radius (R) of the solute in different solutions, the values of R in sequence are 1.94, 3.47, and 5.78 Å for solutions SNS, elec, SNS,0.25, and SNS,6, respectively. The results interpret that the existence of electrolyte and the reduction of pH value effectively compress the hydrodynamic radius and increase the liquid diffusivity of the solute, which may improve the adsorption capacity of the solute in the solution due to the shortened distance of the solutes and that between solute and the adsorbent. The intraparticle diffusion coefficients may be calculated by either the liquid diffusivity or experimental

Figure 2. Variations of dimensionless effluent concentrations (C* ) C(t)/C0) with time for the experiments using the short fixedbed reactor. 0, 9: experimental data of 2-naphthalenesulfonate (NS) in the electroplating solution (SNS,elec) and distilled water with the same pH value as SNS,elec (SNS,0.25), respectively. Table 5. The Properties of 2-Naphthalenesulfonate (NS) in Aqueous Solution Systems solution system

DL at 298 K calcd from SFB (10-10 m2/s)

hydrodynamic radius, Ra (10-10 m)

Dsb (10-12 m2/s)

SNS,elec SNS,0.25 SNS,6

6.87 5.93 3.76, 3.76c

1.94 3.47 5.78

1.38 1.92 2.61

a Calculated by Stokes-Einstein equation. b Calculated by the equation of DS ) RFSP[(DLpC0)/(q0Fp)]. c Calculated by Scheibel and Wilke-Chang correlations, respectively.

data from the completely stirred tank reactor (CSTR). However, the latter may need the complicated mathematical model to calculate the diffusion coefficient. Due to the high adsorption isotherm constant, NS is a favorable adsorbate for activated carbon F 400, so that the surface diffusion may be the dominant mechanism. Accordingly, an approximate estimation of the surface diffusion coefficient, DS, including both surface and pore diffusion contributions, can be calculated by eq 6,8

DLpC0 DS ) RFSP q0Fp

(6)

where RFSP is the surface to pore diffusion flux ratio. The value of RFSP is around 6.58 with a 95% confidence interval of 4.60 to 8.56 for GAC F 400(8). The results of DS are also shown in Table 5. A comparison of surface diffusion coefficients among the solutions indicates that the importance of this mechanism in sequence with larger values of DS decreases in the order SNS,6. > SNS,0.25 > SNS,elec, interpreting that the mobility of the adsorbed NS decreases with acidity of the lower pH value and the electrolytes. Therefore, it is obvious that the better the uptake of the solute larger value of kF, the poorer is mobility of the adsorbed NS. In addition, the surface diffusion coefficient is generally dependent on the initial concentration of the solution as well as the particle size of the adsorbent, in which

6908 Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 Table 6. Design Parameters of Main Filter Bed in the Full-Scale Study parameter

common value

filter bed diameter, dB,LC cross-sectional area, AB,LC bed height,a hB,LC bed volume, VB,LC filter velocity, uB,LC volumetric flow rate, QF,LC empty bed contact time, tEB,LC

0.3 m 0.071 m2 2m 0.142 m3 8 m/h 0.568 m3/h 15 min

a The rapid small-scale column test (RSSCT) only simulates the adsorption behavior of the 40 cm bed height due to the highpressure drop in this study. With hB,LC ) 40 cm, tEB,LC ) hB,LC/ uB,LC ) 40 cm/(8m/h) ) 3 min.

If the surface diffusion is the controlling mechanism of intraparticle mass transfer, the LDF model can be written as eq 9. Therefore, the prediction of the adsorption behavior of NS on activated carbon F 400 in the full-scale column was calculated by eqs 6-9.

Figure 3. Variations of liquid diffusivity (DL) with filter velocity (uB) for the experiments using the short fixed-bed reactor experiments. 0, 9: experimental data of NS in the electroplating solution (SNS,elec) and distilled water with the same pH value as SNS,elec (SNS,0.25), respectively.

the former is already considered in eq 6. For the direct practical application of activated carbon F 400 to adsorb NS, the effect of initial concentration needs to be paid more attention than the particle size. Regarding the effect of initial concentration, Merk14 showed that the surface diffusion coefficient, DS, is not significantly influenced by the initial concentration of the adsorbate for the strongly adsorbed component PNP on activated carbon. A comparison of the adsorption capacities between PNP and NS under the same solvent/adsorbent ratio indicates that adsorption isotherm constants of NS in solutions SNS,elec and SNS,0.25 are comparable to those of PNP with aqueous concentrations ranged from 0.015 to 1 mol m-3, so that the DS values of SNS,elec and SNS,0.25 are closer than that of SNS,6. The DS obtained from eq 6 may be combined with linear driving force model (LDF model) to predict the adsorption behavior of NS in the full-scale column.15 The results may be compared with the simulating results of the rapid small-scale column test, as discussed in the following section. Rapid Small-Scale Column Test. The mass balance equation in the full-scale column is shown as eq 7. The LDF model postulated that the uptake rate of adsorbate by activated carbon is linearly proportional to a driving force, defined as the difference between the surface concentration (qs) and the average adsorbed-phase concentration (q j ),15 shown as eq 8.

B

∂C ∂C ∂2C ∂q j + uB - DZB 2 + FB ) 0 ∂t ∂z ∂t ∂z

(7)

6 dp/2 2 ∂q j j ) and q j) r q dr ) kp(qs - q ∂t dp 0 for a spherical pellet (8)

∫

DZ is the dispersion coefficient in axial direction. uB is the filter velocity. kp is the particle-phase transfer coefficient. q j and qs are the average solid-phase concentration and the solid-phase concentration at the exterior surface of the pellet, respectively.

kp )

15DS

(9)

(dp/2)2

To examine the performance of using fixed-bed adsorber systems to remove NS from aqueous solutions, the rapid small-scale column test was chosen in this study due to the advantages, such as smaller experimental volume, time savings, and convenience. The proper scaling between full-scale column (LC) and small-scale (SC) column is determined by eq 10.2

[ ]

dp,SC tEB,SC ) tEB,LC dp,LC

2-X

)

tSC tLC

(10)

In eq 10, tEB is the empty bed contact time, dp is the particle size, t is the corresponding elapsed time in the column tests, and X defines the dependence of the intraparticle diffusion coefficient on particle size. The subscripts SC and LC represent the small-scale and fullscale columns, respectively. If the intraparticle diffusivity is not varied with the particle size, then the design criterion with constant diffusivity (RSSCT-CD, i.e., X ) 0) is adopted to investigate the performance of full-scale column. In contrast, if the spreading in the mass transfer is mostly caused by the intraparticle diffusivity, which is proportional to the particle size, the design parameters with the proportional diffusivity (RSSCT-PD, i.e., X ) 1) are employed to examine the adsorption behavior of fullscale column. Both RSSCT-CD and RSSCT-PD were investigated to predict the adsorption behavior of the full-scale column, of which design parameters are shown in Table 6 with particle diameter dP,LC of 1.04 mm for full-scale column. The scaling factor () dp,LC/dp,SC) of this study is 5.7, and the related design parameters are shown in Table 3 with particle diameter dP,SC of about 0.183 mm for small-scale column. The related equations and the relationship used in this study may be referred to in ref 2 in detail. The results are illustrated in Figure 4. The breakthrough curves simulated by RSSCT-CD and RSSCT-PD show that the adsorption of NS onto activated carbon F 400 is strongly favorable as reflected by the sharpness of the curves, which is in agreement with the results of the adsorption isotherm. In addition, comparisons of the RSSCT and LDF results indicate that both constant diffusivity (CD) and proportional diffusivity (PD) describe the adsorption behavior well,

Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 6909

Figure 4. Variation of effluent concentration profiles with bed volumes for NS in the solution SNS,0.25 (pH ) 0.25). ], O, and s: experimental data of RSSCT-CD, RSSCT-PD, and simulating results of linear driving force model combined with surface diffusion coefficient.

although the RSSCT breakthrough curves appear earlier than the simulating one. This may be due to the higher pressure drop and slower kinetics of external mass transfer when smaller particles are used in RSSCT.2 The surface diffusion coefficient used in the LDF model obtained from eq 6 by assuming that surface diffusion is the dominant mechanism for NS adsorption shows that the satisfactory results of simulation can be obtained when the intraparticle diffusion coefficient is calculated from liquid diffusivity by means of the approximate estimation. The RSSCT method is much simpler than the normal complete stirred tank reactor method, which additionally needs a complicated mathematical model to get the intraparticle diffusion coefficients. Regarding of the sharpness of breakthrough curves with RSSCT-PD and RSSCT-CD, the former is very similar to the results predicted by the LDF model and steeper than the latter, which shows that the former has faster kinetics than the latter. Furthermore, because RSSCT-CD is controlled by both external and intraparticle mass transfers while RSSCT-PD is only controlled by the intraparticle diffusion, the intraparticle diffusion of NS on activated carbon may depend on the particle size of the adsorbent. However, there is not much difference between the results of these two RSSCT designs, and either RSSCT-CD or RSSCT-PD is suitable for the use of predicting the performance of the full-scale adsorption column in this study. Conclusions Equilibrium and kinetics of adsorption of 2-naphthalenesulfonate (NS) on activated carbon F 400 have been investigated in this study. The results demonstrate that the adsorption technology has a high potential to remove NS from aqueous solutions due to the high adsorption capacity. The Freundlich isotherm can describe the adsorption behavior of NS in three solutions (SNS,elec, SNS,0.25, and SNS,6) well with high correlation coefficients. The sequence of the adsorption capacity is SNS,elec >

SNS,0.25 > SNS,6. The adsorption capacity of NS by F 400 is effectively improved by lower pH values and a higher ionic strength of solution. The existence of the electrolyte and the reduction of pH value effectively compress the hydrodynamic radius and increase the liquid diffusivity of the solute. The effective intraparticle diffusion coefficients including both surface and pore diffusion contributions can be calculated by an approximate estimation method and further used to simulate the adsorption behavior of NS in a full-scale column when combined with linear driving force model (LDF model). The importance of surface diffusion with larger coefficient DS for NS transfer in the solutions decreases according to SNS,6 > SNS,0.25 > SNS,elec, indicating that the mobility of the adsorbed NS decreases with the acidity at a lower pH value and the electrolytes. The satisfactory results of simulating the adsorption breakthrough curve in the full-scale column can be yielded by applying the surface diffusion coefficient obtained from the approximate estimation combined with the LDF model, also in good agreement with the rapid small-scale column tests (RSSCTs). The two RSSCT designs, either RSSCT-CD (RSSCT with constant diffusivity) or RSSCT-PD (RSSCT with proportional diffusivity), are adequate for the application of predicting the performance of the full-scale adsorption column in this study. The intraparticle diffusion of NS on activated carbon may depend on particle size of the adsorbent by noting the better simulating results of RSSCT-PD. The results of this study are very useful to the treatment of related industrial wastewater and directly offer parameters for the designing the adsorber systems. Nomenclature ACS ) anthraquinone-2-sulfonate AB ) cross-sectional area of SFB-reactor or filter bed, cm2 as ) specific external surface area, m2/kg C ) value of Cb at outlet in SFB-reactor or at t ) t in CSTR, mg dm-3 Cb ) bulk concentration of solution Ce ) asdsorbate concentration in the liquid phase at equilibrium with qe, mg dm-3 or mol m-3 C0 ) value of Cb at inlet in SFB reactor or at t ) 0 in CSTR, mg dm-3 CSTR ) completely stirred tank reactorreacotr C* ) completely stirred tank reactor, C/C0 DL ) liquid diffusivity, liquid diffusion coefficient, m2/s DS ) surface diffusion coefficient, m2/s DZ ) dispersion coefficient in axial direction dB ) bed diameter dp ) mean particle size, mm F 400 ) Chemviron Filtrasorb 400 fβ ) correction factor for particle shape (ratio of experimental to correlation values of βLas, ) (βLas)exp/(βLas)cor) GAC ) granular activated carbon hB,LC ) height of large-scale bed k ) Boltzmann constant, 1.381 × 10-23 J K-1 kF ) Freundlich isotherm constant as specified in eq 1, mole (1-1/nF) kg-1 m3/nF LC ) large-scale or full-scale column LDF model ) linear driving force model m ) mass of adsorbent NS ) 2-naphthalenesulfonate, C10H7SO3Na nF ) Freundlich isotherm constant as specified in eq 1 PNP ) p-nitrophenol

6910 Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 QF ) volumetric flow rate of fluid in SFB-reactor QF,LC, QF,SC ) QF in large-scale and short columns q ) adsorbate concentration in solid phase, g kg-1 q j ) average solid-phase concentration qe ) adsorbate concentration in solid phase at equilibrium with Ce, mg/g or mol/kg q0 ) q at equilibrium with C0 qs ) solid-phase concentration at the exterior surface of the pellet, mol/kg R ) solute radius, hydrodynamic radius, equivalent radius, Å Re ) Reynolds number, (udp)/ν RFSP ) surface to pore diffusion flux ratio, with value of 6.58 in this study RSSCT ) rapid small-scale column test RSSCT-CD ) RSSCT with constant diffusivity with X ) 0 RSSCT-PD ) RSSCT with constant diffusivity with X ) 1 r2 ) correlation coefficient SC ) small-scale column SC ) Schmidt number, ν/DL SNS,elec, SNS,0.25, SNS,6, SPNP, SACS ) aqueous solutions with different compositions as specified in Table 2 SFB ) short fixed-bed Sh ) Sherwood number Shlam ) Sh in eq 2 represents contribution of laminar flow to external mass transfer Shturb ) Sh in eq 3 represents contribution of turbulent flow to external mass transfer ShE ) Sh in eq 4 for single particle ShB ) Sh for single particle in adsorption bed, (βLdp)/DL T ) absolute temperature, K TOC ) total organic carbon t ) adsorption time or elapsed time, h tEB ) contact time of empty bed u ) interstitial velocity, m/h uB ) filter velocity in SFB reactor or full-scale column, QF/ AB m/h uB,LC ) filter velocity in large scale column, m/h VL ) volume of solution in CSTR, dm-3 VB,LC ) volume of large-scale bed X ) defines the dependence of the intraparticle diffusion coefficient on particle size as in eq 7 ZPC ) zero point charge βL ) film mass transfer coefficient gained from SFB reactor, m/s βLas ) specific value of βL, m3/s/kg (βLas)exp, (βLas)cor ) βLas obtained by experiments and correlation p ) adsorbent porosity B ) filter bed porosity µ ) dynamic viscosity, cp µB ) µ of solvent, cp µBG,Cu ) µ of solution BG,Cu, cp µw ) µ of water, cp

υ ) kinetic viscosity, µ/F, m2/s F ) density, kg/m3 FB ) filter bed layer density, kg/m3 Fp ) apparent particle density, kg/m3 Fs ) average true particle density, kg/m3

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Received for review May 1, 2003 Revised manuscript received September 15, 2003 Accepted October 1, 2003 IE030394D