removal of the herbicide mcpa by commercial activated carbons: equilibrium, kinetics, and...

Removal of the Herbicide MCPA by Commercial Activated Carbons:Equilibrium, Kinetics, and Reversibility

Olga Gimeno, Pawel Plucinski, and Stan T. Kolaczkowski

Department of Chemical Engineering, University of Bath, Bath BA2 7AY, U.K.

Francisco J. Rivas* and Pedro M. Alvarez

Departamento de Ingenierıa Quımica y Energetica, Facultad de Ciencias, Universidad de Extremadura,06071 Badajoz, Spain

The adsorption of the herbicide 4-chloro-2-methylphenoxyacetic acid (CAS 94-74-6) has beenstudied using four commercial activated carbons (Norit 0.8, Aquacarb 207C, Aquacarb 208A,and Aquacarb 208EA). Adsorption equilibrium isotherms were obtained in the 293-358 Ktemperature range. The trend of adsorption for MCPA onto the four activated carbon was inthe order Norit 0.8 > Aquacarb 207C > Aquacarb 208A > Aquacarb 208EA. Among variousadsorption isotherm models, the Freundlich equation best fit the experimental data. Experimentsconducted at different temperatures allowed for the calculation of the isosteric heat of adsorption,revealing that the adsorption process was exothermic for three of the four adsorbents studied.The distribution of the herbicide into the pores of activated carbon Norit 0.8 was studied byconsidering the Dubinin-Radushkevich isotherm. Also, the adsorption kinetics were assessedby means of a simplistic mechanism based on the shrinking-core mass-transfer model. Finally,as a preliminary step in the study of the potential use of wet air oxidation (WAO) for theregeneration of exhausted activated carbon, the reversibility of the adsorption process was tested.Only partial desorption of MCPA was achieved under WAO conditions, which involves a likelydouble route of activated carbon regeneration, i.e., liquid and surface contaminant oxidation.

Introduction

The lack of clean water has always been an issue ofmajor environmental concern all over the world. Themain sources of water pollution are industrial (chemical,organic, and thermal wastes), municipal (largely sewageconsisting of human wastes, other organic wastes, anddetergents), and agricultural (herbicides, pesticides, andfertilizers).

The wide utilization of herbicides for agriculturepractices has already contributed to the increasingcontamination of surface and underground waters. Forthis reason, in recent years, emerging concerns aboutthe quality of water intended for human consumptionhave greatly increased. The toxicity of herbicides andtheir degradation products makes the contaminationcaused by these chemical substances a potential envi-ronmental hazard.1

MCPA (4-chloro-2-methylphenoxyacetic acid, CASnumber 94-74-6) is a post-emergence phenoxy herbicideextensively used in agriculture to control annual andperennial weeds in cereals, grasslands, trees, and turf.Similarly to other phenoxy herbicides, MCPA is an acid,but it is often formulated as a salt. It is very soluble(825 mg L-1 in water at 293 K), highly mobile, and canleach from the soil. This compound has been found inwell water in some countries and is classified by the U.S.Environmental Protection Agency (EPA) as a potentialgroundwater contaminant.2 The major metabolites ofthese phenoxy acid herbicides are phenols. The resulting

chlorophenols might pose an additional risk for ground-water pollution.

Several treatment processes are available for theremoval of such compounds from waters and waste-waters. Chemical oxidation with ozone;3,4 photo-degradation;5-8 combined ozone and UV irradiation;9,10

Fenton degradation,11 biological degradation;12 coagula-tion;13 and adsorption onto porous solids such as zeo-lites, clays,14,15 and fly ash16 have been investigated withvarying success.

Among the available adsorbents for the removal ofherbicides from water, activated carbon (AC) is thepreferred solid because of its high effectiveness. Acti-vated carbon is a meso-microporous adsorbent that canbe manufactured from a variety of carbonaceous materi-als, including wood, coal, lignin, coconut shells, andsugar. Its unique adsorption properties result from itshigh surface area, meso- and micropore structure, andbroad range of surface functional groups, which influ-ences its adsorption properties and reactivity. Adsorp-tion on activated carbon is the most widespread tech-nology used to deal with pesticides and other hazardouschemicals in drinking-water plants. Thus, a number ofstudies have been reported on the use of this methodfor the purification of water contaminated by herbi-cides.17-21 Additionally, adsorption on activated carbonis also an expensive technology because once AC isexhausted with the herbicide, a regeneration step isneeded that is able to eliminate the herbicide from thesurface without significant loss in adsorption capacity.An emerging technology that uses milder conditionsthan thermal regeneration, is wet air regeneration,22

(i.e., high temperature and pressure oxidation in aque-ous media).

* To whom correspondence should be addressed. Tele-phone: 34 924 289385. Fax: 34 924 271304. E-mail: [email protected].

1076 Ind. Eng. Chem. Res. 2003, 42, 1076-1086

10.1021/ie020424x CCC: $25.00 © 2003 American Chemical SocietyPublished on Web 01/31/2003

In the present study, the adsorption capacities of fourcommercial activated carbons have been examined forthe removal of dissolved MCPA. For this purpose, theadsorption equilibrium isotherms and kinetics of theprocess have been studied. Hence, in view of a potentialWAO regeneration stage, the stabilities of the activatedcarbons used in this study were investigated under wetair oxidation conditions. Also, MCPA desorption experi-ments under WAO conditions were carried out as apreliminary investigation on the feasibility of thistechnology for oxidation of the pesticide both in bulkwater and at the AC surface.

2. Experimental Section

2.1. Materials. Adsorbents. The four adsorbentsused in this study were the commercial activatedcarbons Norit 0.8 supplied by Aldrich (U.K.) and Aqua-carb 207C, Aquacarb 208A, and Aquacarb 208EA sup-plied by Waterlink Sutcliffe Carbons (U.K.). These fourcarbons were produced from different raw materials.Norit 0.8 and Aquacarb 207C were manufactured fromspecific grades of coconut shell, whereas Aquacarb 208Aand 208EA were manufactured from specific grades ofbituminous coal. General characteristics given by themanufacturers are listed in Table 1.

Prior to adsorption experiments, the activated carbonswere washed with distilled water to remove any pow-dered particulate. The ACs were then dried overnightat 110 °C to eliminate moisture from the porousstructure.

Sorbate. 4-Chloro-2-methylphenoxyacectic acid,MCPA, 95% purity, was supplied by Aldrich (U.K.).Aqueous solutions of MCPA were made by adding anexcess of the pesticide to distilled water and stirring themixture overnight. After saturation of water by MCPA,excess herbicide was removed by filtration throughnylon membrane filters. The MCPA initial concentrationwas determined by comparison to standard solutionsmade in HPLC-grade methanol.

2.2. Analysis. The concentration of MCPA in aqueoussolution was measured by HPLC (Gilson Dilutor 401,pressure controller 803c, dynamic mixer 811, pump 403,sample injection 231) using a Holochrome UV/vis detec-tor. The column used for the analysis was an Aqua 5µC18 200A column (Phenomenex). The mobile phasecomposed by acetonitrile (40)/water (58.8)/acetic acid(1.2) was pumped at a flow rate of 1 mL min-1; thewavelength used was λ ) 230 nm. Under these condi-tions, MCPA showed a retention time of ca. 9.5 min.All samples were filtered through nylon membranefilters (0.2 µm) before injection. Solid characterizationwas accomplished by means of a Quantachrome Auto-sorb 1 automated gas adsorption system (N2 adsorptionisotherm), a Pore Master 60 mercury porosimeter, anda Quantachrome stereopycnometer apparatus (helium).

2.3. Method. The bottle-point isotherm technique wasemployed to determine the equilibrium capacities forMCPA of the four activated carbon investigated. Experi-ments were conducted at an initial pH of 7 to keepMCPA in its anionic form (pKa ) 2.9). Accordingly,experiments were carried out in glass vials (100-cm3

capacity) sealed by Teflon caps. A constant initialconcentration of pH 7 MCPA solution was placed inthese test tubes (∼150 mg L-1), and different knownquantities of adsorbent mass were then added to thebottles. The samples were kept in a constant-tempera-ture water bath (Grant shaker, model OLS 200) andshaken continuously. Upon equilibration, the concentra-tion of MCPA remaining in the liquid phase wasmeasured, and the solid-phase concentration on theactivated carbons was calculated via mass balance.21

Prior to the equilibrium experiments, some adsorptiontests were completed to determine the minimum contacttime required to achieve equilibrium adsorption condi-tions for MCPA on the four activated carbons. Thiskinetic study, carried out at 293 K, showed that theadsorption equilibrium state was reached after contactperiods of ca. 72 h for Norit 0.8, 55 h for Aquacarb 207C,49 h for Aquacarb 208A, and 53 h for Aquacarb 208EA.Therefore, to ensure that equilibrium was reached, acontact time of 4 days was applied in the adsorptionexperiments of MCPA on Norit 0.8 and 3 days for therest of the studied carbons. Then, adsorption equilibri-um isotherms were obtained in the 293-358 K temper-ature range.

Kinetic experiments were conducted in a batch agi-tated reactor (1-L capacity) immersed in a thermostaticbath. Samples were steadily taken with time, and theMCPA remaining was measured by HPLC. All experi-ments were performed at 20 °C and a constant agitationspeed of 350 rpm.

3. Results and Discussion

3.1. Adsorption Isotherm Experiments. GeneralConsiderations. The adsorption results for MCPAobtained at 293, 313, 333, and 358 K are plotted inFigures 1-4. As seen in Figure 1, Norit 0.8 exhibits atypical type I isotherm profile indicative of monolayeradsorption or adsorption on microporous solids.23 If theclassification scheme of Giles et al. is considered,24 theadsorption isotherm can be defined as type L2 or H2,that is, adsorbate with a high affinity for the adsorbent,typical of a lack of competitiveness between solvent(water) and adsorbate (MCPA) or the presence ofinteractions between adsorbate molecules. Figures 2-4show the MCPA adsorption isotherms for Aquacarb207C, Aquacarb 208A, and Aquacarb 208EA, respec-tively. From these plots is inferred an L3 or H3 isothermprofile according to Giles et al.. These results suggestthat, near the point of inflection, a monolayer iscompleted, followed by further adsorption in successive

Table 1. General Characteristics of Activated Carbonsa

property Norit 0.8 Aquacarb 207C Aquacarb 208A Aquacarb 208EA

bulk density (g cm-3) 0.39 0.49-0.53 0.44-0.48 0.44-0.48moisture content (wt %) 2 <5 <5 <5specific surface area (m2 g-1) 1150 1050-1150 1000-1250 1100-1250ash content (wt %) 7 <3 <1 <1shape pellets granulate granulate granulateparticle size (mm) <0.6 1.70-0.6 1.7-0.425 1.7-0.425pH alkaline 9-11 7-8 7-8

a Information supplied by the manufacturers.

Ind. Eng. Chem. Res., Vol. 42, No. 5, 2003 1077

layers. Adsorbents with a wide pore size distributionform this type of isotherm. This kind of shape has beenfound by other authors15 in adsorption studies of MCPAon layered double hydroxides. They claim that adsorp-tion proceeds in a two-step process: saturation of theexternal sites followed by an interlayer process occur-ring at high equilibrium concentration values.

Several models can be used to describe adsorptiondata. The two most frequently used for dilute solutionsare the Langmuir and Freundlich isotherms. The Lang-muir adsorption isotherm is given by

where Q represents the amount of solute adsorbed perunit weight of adsorbent; Qm is the amount of soluteadsorbed per unit weight of adsorbent required formonolayer coverage of the surface, also called themonolayer capacity; Ce is the concentration of adsorbatein solution at equilibrium conditions; and b is anequilibrium constant related to the heat of adsorption.The Freundlich adsorption equation is perhaps the mostwidely used for the description of adsorption in aqueoussystems. The Freundlich equation is of the form

where Q and Ce have the same definitions as previously

Figure 1. Adsorption isotherms of MCPA onto Norit 0.8 pellets at different temperatures. Conditions: pH ≈ 7, CMCPAo ≈ 150 mg L-1,AC ) 0.25-4.0 g L-1. Symbols: Experimental results. Lines: Model fitting (dotted, Langmuir; solid, Freundlich; dashed, Dubinin-Radushkevich).

Figure 2. Adsorption isotherms of MCPA onto Aquacarb 207C at different temperatures. Conditions: pH ≈ 7, CMCPAo ≈ 150 mg L-1, AC) 0.25-3.0 g L-1. Symbols: Experimental results. Lines: Model fitting (dotted, Langmuir; solid, Freundlich).

Q )QmbCe

1 + bCe(1)

Q ) KCe1/n (2)

1078 Ind. Eng. Chem. Res., Vol. 42, No. 5, 2003

presented for the Langmuir isotherm and K and 1/n arethe adsorption capacity and adsorption affinity, respec-tively.

The calculated adsorption isotherms of MCPA ob-tained for the four activated carbons studied are il-lustrated in Figures 1-4. The Langmuir constant b, themonolayer capacity Qm, and Freundlich parameters Kand 1/n are listed in Table 2.

As inferred from the statistical analysis in Table 2and theoretical isotherm profiles in Figures 1-4, it canbe concluded that Freundlich equation is more ap-propriate for modeling the adsorption process thanLangmuir expression. The latter gives acceptable resultsfor the case of Norit 0.8 activated carbon; however, forthe rest of AC considered, the values of R2 drop below

0.9 in some cases. By considering either Langmuir orFreundlich model parameters, the following order ofMCPA adsorption capacity for the four activated carbonsis deduced

The values of 1/n obtained for the adsorption of MCPAonto the four activated carbons were less than 1,indicating favored adsorption. Also, the lowest valuesof 1/n, obtained for Norit 0.8, are indicative of strongeradsorption affinity for MCPA than for the rest of thecarbons and consistent with adsorption in pores similarin size to MCPA.

Figure 3. Adsorption isotherms of MCPA onto Aquacarb 208A at different temperatures. Conditions: pH ≈ 7, CMCPAo ≈ 140 mg L-1, AC) 0.25-3.0 g L-1. Symbols: Experimental results. Lines: Model fitting (dotted, Langmuir; solid, Freundlich).

Figure 4. Adsorption isotherms of MCPA onto Aquacarb 208EA at different temperatures. Conditions: pH ≈ 7, CMCPAo ≈ 140 mg L-1,AC ) 0.25-3.0 g L-1. Symbols: Experimental results. Lines: Model fitting (dotted, Langmuir; solid, Freundlich).

Norit 0.8 > Aquacarb 207C > Aquacarb 208A >Aquacarb 208EA


Variations in the affinity of a specific adsorbatetoward distinct adsorbents of similar nature (i.e., AC)can be explained on the basis of their differences instructure, adsorption-site charges, and pore size distri-butions. When the adsorption of organic compounds byactivated carbon is governed by physical interactions,the adsorption capacity depends on acid-base, size-exclusion, and microporosity effects.25

Because MCPA was always in the anionic form inthese experiments, electrostatic attraction forces be-tween the adsorbate and the activated carbon wereenhanced through an increase in the electron affinityof the adsorption sites. In the case study, this occurredfor the alkaline activated carbons Norit 0.8 and Aqua-carb 207C, which exhibited the highest adsorptioncapacities.

Size exclusion might control access of MCPA into thefine carbon pores. Hence, the molecular diameter ofMCPA was estimated and found to be ∼7-9 Å. How-ever, taking into account the broad pore size distributionof the four activated carbons used from <20 to >500 Å,size-exclusion effects were dismissed as a possibleexplanation. On the other hand, the micro-mesoporousnature of the activated carbons has a positive influencein the uptake of small organics molecules (i.e., smallorganic molecules having sizes in the range of 6-8 Åcan be adsorbed in micropores). It is likely that theadsorption energy increases in micropores as the porewidth approaches adsorbate dimensions. According tothe manufacturers, Norit 0.8 and Aquacarb 207C havea large fraction of micropores, important for the elimi-nation of dissolved small pollutants and pesticides,whereas Aquacarb 208A and Aquacarb 208EA have ahigh fraction of mesopores, important for the adsorptionof larger molecules. Therefore, taking into account theseresults, it is expected that Norit 0.8 presents a signifi-cant fraction of micropores whose size is not much largerthan the molecular diameter of the MCPA. This aspectwas investigated in more detail for the case of Norit 0.8by looking more closely at its structural characteristics.For instance,the surface area was calculated from theN2 adsorption isotherm by using the BET and Dubinin-Radushkevich (D-R) expressions. The true and appar-ent densities were also obtained. The pore size distri-

bution was measured by Hg porosimetry. Table 3 showsthe results of the Norit 0.8 characterization.

As inferred from Table 3, Norit 0.8 exhibits animportant fraction of micropores (around 40% of thepore volume) suitable for MCPA adsorption. The poresize distribution determined by the mercury penetrationmethod is shown in Figure 5. This plot reveals anonsymmetrical distribution, with most of the volumein pores with diameters of 1.7-10.0 nm, showing alsoan important contribution of secondary micropores(0.9-2 nm) and small mesopores to the total porevolume of Norit 0.8. Additionally, the distribution ofMCPA in the Norit 0.8 pores was determined by meansof the Dubinin-Radushkevich isotherm. This isothermexpressed as a function of the amount of MCPA per unitof mass of AC can be represented by

where R is the universal gas constant, T is the temper-ature, CS is the MCPA solubility, and E is the charac-teristic adsorption energy of the process. The rest ofparameters are similar to those previously cited. Fromregression analysis of experimental equilibrium data,the values of Qm and E were obtained for experimentsinvolving the adsorption of MCPA onto Norit 0.8 (seeFigure 1). The estimated parameters were Qm ) 159,149, 144, and 140 mg g-1 and E ) 13.7, 17.25, 18.06,and 19.72 kJ mol-1 for adsorption experiments con-ducted at 293, 313, 333, and 358 K, respectively. Asobserved, the values of Qm are slightly higher than thoseobtained from the Langmuir equation and follow asimilar trend with temperature, i.e., Qm decreasesslightly as the temperature increases. The E values arealways below 83 kJ mol-1, confirming the physicalnature of the adsorption.26 Similar results have beenreported for the adsorption of other herbicides ontoACs.27

From the Dubinin-Radushkevich isotherm param-eters, the MCPA distribution function J(r) can bededuced (distribution of MCPA into Norit 0.8 pores).This function is of the form

where the coefficients ν and ú are adjustable parametersobtained from regression analysis of the followingexpression given by Jaroniec and co-workers28

In this case, âJ is a coefficient dependent on theadsorbate29 with a value of 1, and kJ is anotherparameter (proportionality constant between the micro-pore radius and the characteristic energy of the solid)27

taking a value of 12 nm kJ mol-1. Finally, Γ(ν/2) is thegamma function

where γ ) 0.5772 is the Euler constant.

Table 2. Langmuir and Freundlich Parameters forMCPA Adsorption onto Activated Carbons

T(K)

Qm(mg g-1)

b(L mg-1)

-∆Gad(kJ mol-1) R2

K(mg g-1) 1/n R2

Norit 0.8293 133.61 0.337 31.00 0.901 63.209 0.169 0.954313 133.33 0.288 32.71 0.924 63.384 0.167 0.997333 130.06 0.270 34.62 0.951 60.763 0.172 0.995358 129.87 0.238 36.84 0.972 50.523 0.215 0.986

Aquacarb 207C293 117.65 0.314 30.83 0.967 35.276 0.254 0.961313 103.45 0.181 31.50 0.943 34.017 0.257 0.979333 93.96 0.164 33.24 0.885 27.011 0.285 0.982358 81.61 0.125 34.93 0.881 18.803 0.337 0.961

Aquacarb 208A293 106.167 0.188 29.58 0.907 29.512 0.286 0.905313 89.56 0.123 30.49 0.865 23.004 0.318 0.926333 85.68 0.089 31.55 0.856 11.665 0.478 0.923358 77.7 0.036 31.22 0.884 18.223 0.275 0.912

Aquacarb 208EA293 105.26 0.085 27.64 0.855 11.512 0.504 0.982313 - - - - 16.495 0.443 0.954333 105.88 0.046 29.71 0.978 16.719 0.365 0.980358 104.17 0.038 31.41 0.841 33.681 0.204 0.891

Q ) Qm exp[-(RT lnCS

Ce

E)2] (3)

J(r) ) 2úν

Γ(ν2)rν-1 exp[-(úr)2] (4)

Q ) Qm[1 + (RT lnCS

Ce

âJúkJ)2]-ν/2

(5)

Γ(x) ) xeγx[(1 + x1)e-x/1][(1 + x

2)e-x/2][(1 + x3)e-x/3] (6)


By considering eqs 3-6, the MCPA distribution intopores was plotted as a function of the radius (see Figure5). The J(r) profile was quite similar to the pore sizedistribution profile obtained from mercury porosimetry,confirming the asymmetry of both. As deduced from thisplot, most of the MCPA was adsorbed into pores withradii close to its molecular size 5.0-10.0 Å.

Thermodynamic Considerations. From the previ-ous study, some general conclusions can be deducedregarding the energetic changes occurring during theprocess. Thus, the b parameter from Langmuir equationcan be related to the adsorption free energy (∆Gads) asfollows30

where ω refers to the water concentration in solution(55.55 mol L-1). Table 2 presents the values of theadsorption free energy for the four ACs studied. As seenin Table 2, Norit 0.8 pellets show slightly higher valuesof the adsorption free energy than the rest of the ACsstudied. The higher affinity of MCPA for this kind ofcarbon is likely related to adsorption into pores close tothe size of the MCPA molecule (more contact points andhence more favorable adsorption energy).

Isothermal data at four different temperatures wereobtained to estimate the isosteric heat of the process.Consequently, enthalpy changes associated with ad-sorption processes could be estimated using the Clau-sius-Clapeyron equation. In this way, the isostericheats of adsorption, Qis, for various loadings werecalculated from eq 8 by means of the isosteres corre-sponding to each amount adsorbed (see Figure 6)

In eq 8, Qis is the isosteric heat of adsorption (kJ mol-1),a measure of the enthalpy change involved in the

transfer of solute from the reference state to the sorbedstate at a constant solid-phase concentration; Ce andQe are the equilibrium aqueous-phase and solid-phaseconcentrations, respectively; R is the universal gasconstant (8.314 J mol-1 K-1), and T is the temperaturein Kelvin.

Aqueous-phase solute concentrations (Ce) at differenttemperatures were calculated at constant solid-phasesolute concentration (Qe) using the calculated isothermparameters. A linear regression of ln Ce as a functionof 1/T yields a single value of Qis and its coefficent ofdetermination at a given Qe. The values obtained forthe isosteric heat of adsorption are listed in Table 4.

As seen in Table 4, the resulting values of Qis for Norit0.8 pellets were about -5 kJ mol-1 and almost constantregardless of the amount adsorbed in the interval of Qeexamined. These results suggest that Norit 0.8 exhibitsan energetically homogeneous surface. The isostericheat values for adsorption onto Aquacarb 207C (-24 to-14 kJ mol-1) and Aquacarb 208A (-26 to -30 kJmol-1) were found to vary as coverage increased, typicalof energetically heterogeneous surfaces. Variations inthe isosteric heat of adsorption with surface coveragehave been postulated to result from heterogeneous

Table 3. Characterization of Norit 0.8 Activated Carbon

parameter value expression

true density, FS 2000 kg m-3

particle density, FP 650 kg m-3

pore volume, VP 1.04 cm3 g-1 VP ) 1/FP - 1/FSporosity, εP 0.675 εP ) VPFS/(VPFS + 1)surface area, BET 730 m2 g-1 p/[v(po - p)] ) 1/vmc + [(c - 1)p]/vmcposurface area, D-R 1190 m2 g-1 v ) vm exp[-(RT ln(po/p)/E)2]micropore volume, BET 0.367 cm3 g-1 1.543 × 10-3 x adsorbed volume at P/Po ) 0.1micropore volume, D-R 0.421 cm3 g-1 1.543 × 10-3 x vm

Figure 5. Pore size distribution from mercury porosimetry (rightand bottom axes) and theoretical distribution of MCPA into pores(top and left axes). Data from Norit 0.8 isotherm at 293 K.

∆Gads ) -RT ln(bω) (7)

Qis ) R[d ln Ce

d(1/T) ]Qe

(8)

Figure 6. Clausius-Clapeyron plots of MCPA adsorption isos-teres for (A) Norit 0.8, (B) Aquacarb 207C, and (C) Aquacarb 208A.Qe (mg g-1): O, 40; b, 50; 0, 60; 9, 70; 4, 80.


sorption energies due to the differences in sorbatelateral interactions, particle geometry, and interactionenergies among sites on edge basal surfaces.31-33 Forthe case of Aquacarb 208EA, it was not possible tocalculate the isosteric heat of adsorption because theequilibrium concentration did not increase monotoni-cally with increasing temperature. This could be ex-plained if chemical interactions between the MCPAmolecules and the surface of this particular carbonoccurred to some extent, overwhelming physical interac-tions. Similarly, some aromatic compounds (i.e., phe-nols) have been reported to adsorb on activated carbonsurfaces through interactions of the aromatic ring withthe carbon surface.34 The adsorption force will thereforearise from the dispersion interaction of the π electronsin the respective aromatic system by a donor-acceptormechanism. It is well-known that the electron densityof an aromatic ring is strongly influenced by the natureof the substituent groups. MCPA, because of its chemi-cal structure, has one polar carboxylic group and onechloride group. The chloride group on the aromatic ringof MCPA acts as an electron-withdrawing group, there-fore reducing the overall electron density in the π ringsystem. Hence, MCPA can act as an acceptor in suchcomplexes. Also, the oxygen-group dipole moment is thedetermining factor in the strength of the donor-acceptorcomplex formed. Carbonyl oxygen has a larger dipolemoment than carboxylic acid oxygen, and hence, it actsas a strong donor.20 Consequently, it is possible thatMCPA molecules adsorb by a donor-aceptor complexmechanism involving carbonyl oxygen, with the surfaceof Aquacarb 208EA acting as the electron donor and thearomatic ring of MCPA acting as the acceptor.

3.2. Kinetic Experiments. As stated previously, interms of MCPA uptake, Norit 0.8 was demonstrated tobe the most efficient of the four activated carbonsstudied. Consequently, our kinetic study of the systemsMCPA-AC focused on the use of Norit 0.8.

To investigate the influence of adsorbent mass on theadsorption of MCPA, a series of experiments wasundertaken for a range of Norit 0.8 activated carbonmasses. The removal of MCPA using different massesof adsorbent is presented in Figure 7. As expected, anincrease in AC mass led to a higher depletion rate ofthe herbicide. Thus, with an initial concentration of 2 gL-1 of carbon, the MCPA removal reached 72% in 6 h,whereas 45% elimination of MCPA was observed for 1g L-1 of carbon.

In another experimental series, the influence ofvarying the initial herbicide concentration was assessed.The results are also shown in Figure 7 in terms of theMCPA accumulated in the solid phase. From Figure 7,it is obvious that the higher the initial MCPA concen-tration, the faster and the higher the amount of MCPAadsorbed. However, differences are significantly reducedif Q/Qe is plotted instead of Q.

Because the adsorption step is rapid, the overalladsorption rate of the herbicide molecules from theliquid phase to the solid phase will likely be controlledby both film diffusion and internal diffusion. It is notan easy task to elucidate the rate-limiting step inadsorption processes. However, some previous consid-erations can be adopted on the basis of simple correla-tions. Thus, the concentration dependence of the rateof adsorption is used to define the rate-limiting step inthe reaction.26 When intraparticle transport limits thekinetics, adsorption data linearly fit with the square rootof time. However, this linearity is normally observed atthe beginning of the process, when it is thought thatexternal mass transfer is rate-controlling.35

Usually, external transport is the rate-limiting stepin systems that have poor mixing, low concentrationsof adsorbate, small adsorbent particles sizes, and highaffinities of the adsorbate for the adsorbent.36 Thesystem under study can be considered as poorly mixed(350 rpm) compared to other systems. Actually, in anexperiment carried out at 500 rpm (CMCPAo ) 150 mgL-1, adsorbent mass ) 1 g L-1), after 400 min, a slightincrease in MCPA removal efficiency from 48 to 55%was experienced (results not shown). Also, MCPAexhibits a high affinity for Norit 0.8. Hence, it isexpected that both film diffusion and pore diffusioncould be rate-limiting steps for the studied system.

Process Modeling. Taking into account the previousconsiderations, a simple mechanism based on the shrink-ing-core mass-transfer model was applied 37 to describethe kinetics of the adsorption. According to this model,the adsorption rate is controlled by external and inter-nal mass-transfer resistances. By considering the masstransfer in the external liquid phase, the diffusion inthe pore liquid according to Fick’s law, the velocity ofthe concentration front, and the average solid-phaseconcentration in the particle, the following expression

Table 4. Isosteric Heats of Adsorption (Qis) as a Functionof Qe

Norit 0.8 pellets Aquacarb 207C Aquacarb 208A

Qea Qis

b Qea Qis

b Qea Qis

b

40 -5.02(0.995) 40 -23.94(0.918) 10 -25.56(0.964)50 -5.11(0.996) 50 -20.67(0.921) 20 -26.61(0.965)60 -5.25(0.997) 60 -18.24(0.924) 30 -27.05(0.967)70 -5.33(0.998) 70 -16.2190.928) 40 -28.60(0.969)80 -5.49(0.998) 80 -14.54(0.931) 50 -29.45(0.97)

a Qe in mg g-1. b Qis in mg g-1.

Figure 7. (A) Effect of adsorbent mass on the adsorption of MCPAonto Norit 0.8 activated carbon. Conditions: T ) 20 °C, CMCPAo )150 mg L-1, pH ) 7.0. Adsorbent mass (g): O, 0.5; b, 1.0; 0, 2.0.(B) Effect of MCPA initial concentration on the adsorption rateonto Norit 0.8 activated carbon. Conditions: T ) 20 °C, adsorbentmass ) 0.5 g, pH ) 7.0. CMCPAo (mg L-1): 4, 14.0; 9, 26.0; 0, 43.0;b, 98.0; 0, 144.0 (3).


is finally obtained38

with the dimensionless parameters defined accordingto

where R is the particle radius; âL is the external mass-transfer coefficient; Deff is the effective diffusivity; C(t)is the MCPA concentration in water at time t; FS is thesolid density; and Co and Qe are the initial MCPAconcentrations in the aqueous phase and in the solid atequilibrium conditions, respectively.

From eq 9, the values of Deff and âL can be calculatedby means of the least-squares regression analysismethod. However, the external mass-transfer coefficienthas to be calculated from the term (1 - Bi-1), and it islikely that this term very close to unity (Bi > 100). Thepresumably inaccurate value calculated by the regres-sion analysis was overcome by determining âL directlyfrom experimental C(t) profiles. This parameter wascalculated by considering the adsorption rate at timezero. At the beginning of the process, the adsorption isassumed to be controlled exclusively by external masstransfer. Extrapolating the adsorbate abatement rateto time zero, the following expression is assumed27

where Fp is the particle density, V is the reactionvolume, and W is the mass of adsorbent used.

Evolution profiles of C vs t were fitted to a third-orderexponential decay expression and the correspondingvalues of -dC/dt were deduced. External mass-transfercoefficients were calculated in the proximity of 4 × 10-5

m s-1, with maximum and minimum values of 7.69 and1.67 × 10-5 m s-1, respectively (see Table 5).

Next, normalized Q values were plotted againstadsorption time and adjusted to a Fritz-Schlunderexpression of the type

From eq 15, values of dη/dt were obtained and thenplotted against η; the resulting curves were fitted to eq9 by considering the equatoion

All regression analyses gave correlation coefficientshigher than R2 ) 0.995. From the regression analysis,values of the effective diffusivity were therefore deter-mined.

Figure 8 shows the theoretical fitted profiles of dη/dtversus η for the kinetic experiments conducted withdifferent adsorbent masses, and Table 5 presents thevalues of Deff. As inferred from this table, the Deff valuesdid not remain constant, but rather, they depended onCo, which is typical of systems where pore and surfacediffusion play an important role. No clear trend wasfound when varying the adsorbent mass. It appears thatDeff does not depend on this parameter when Co is keptconstant; however, with just three experiments, it isdifficult to reach any consistent conclusion. For the caseof experiments carried out at different initial MCPAconcentration, a decreasing trend was observed forincreasing values of Co. Such results have already beenreported previously, and Chen and co-workers35 pro-posed power-law expressions to correlate the effectivediffusivity with Co.

3.3. Stability of Activated Carbon under WAOConditions. Reversibility of the Process. The re-sistance of the activated carbons to wet air oxidationregeneration conditions was first assessed by treatingthe adsorbents in an autoclave with 150 mL of waterat 180 °C and 38 bar. Figure 9 depicts the isothermsobtained for Norit 0.8, Aquacarb 207C, and Aquacarb208A obtained at 293 K after these activated carbonshad been subjected to WAO conditions for 2 and 6 h.Two distinct behaviors can be detected. After 6 h ofoxidizing conditions, the adsorption capacity of Norit 0.8

Table 5. External Mass-Transfer Coefficients andEffective Diffusivities in Kinetic Runs

runCo

(mg L-1)W(g)

Qe(mg g-1)

âL(cm s-1)

Deff(cm2 s-1)

1 150 0.5 128 7.69 × 10-3 1.27 × 10-8

2 150 1.0 120 1.33 × 10-3 0.66 × 10-8

3 150 2.0 75 1.67 × 10-3 1.59 × 10-8

4 14 0.5 28 5.48 × 10-3 10.4 × 10-8

5 26 0.5 50 3.48 × 10-3 3.33 × 10-8

6 43 0.5 80 5.9 × 10-3 2.76 × 10-8

7 98 0.5 105 1.84 × 10-3 0.8 × 10-8

8 144 0.5 128 5.4 × 10-3 1.41 × 10-8

3ê(1 - η)1/3

1 - (1 - 1/Bi)(1 - η)1/3) dη

dτ(9)

τ )CoDeff

FsQeR2t (10)

η ) QQe

(11)

ê )C(t)Co

(12)

Bi )âLRDeff

(13)

âL )RFpV(- dC

dt )t)0

3WCo(14)

Figure 8. Application of the shrinking-core mass-transfer model.Fitting of eq 18. Conditions: T ) 20 °C, CMCPAo ) 150 mg L-1, pH) 7.0. Adsorbent mass (g): b, 0.5; 2, 1.0; 9, 2.0.

η ) QQe

) A1 x tA2

1 + A3 x tA4(15)

ê )C(t)Co

) 1 -ηWQe

VCo(16)


AC decreased significantly compared to the efficiencyof the fresh AC. After 2 h under WAO conditions, themain differences were found at low values of Ce;however, at the highest values of Ce tested in this work,the adsorption capacities of fresh and treated Norit 0.8were quite similar. Analogously, for the case of Aqua-carb 207C, a loss in adsorption capacity of this adsor-bent was experienced after 6 h of WAO treatment, butthe efficacy of treated Aquacarb 207C approached thatof the fresh AC as the value of Ce increased. However,if WAO conditions were applied for only 2 h, theadsorption potential was even slightly increased com-pared to that of the fresh carbon. Similar results wereobtained for Aquacarb 208A, for which the improvementafter 2 h of treatment was quite dramatic. The experi-mental data were fitted by the Freundlich isothermequation. The regression analysis showed an importantdecrease in the K parameter for Norit 0.8 as the timeof exposure to WAO conditions increased (63, 23, and7.8 mg g-1 for carbon treated for 0, 2, and 6 h,respectively). The value of 1/n followed an opposite trend(0.17, 0.38, and 0.55, respectively), indicating that a lossin adsorption efficiency was experienced. In the case ofAquacarb 207C, differences were found for the isothermobtained after 6 h of WAO treatment (K ) 3.78 mg g-1

and 1/n ) 0.76). However, the K and 1/n values werequite similar for fresh carbon (K ) 31.6 mg g-1 and 1/n) 0.28) and carbon treated under WAO conditions for2 h (K ) 35.2 mg g-1 and 1/n ) 0.29). The adsorptioncapacity for Aquacarb 208A also increased after 2 h ofWAO treatment compared to that of fresh carbon (K )

40.5 compared to 29.5 mg g-1), but in contrast, the valueof 1/n did not change significantly (0.33 and 0.28,respectively).

From these preliminary results, it can be suggestedthat Aquacarb exhibits a better predisposition towardWAO regeneration than Norit 0.8. Nevertheless, noconclusive statements can be made until proper regen-eration experiments have been carried out.

Also, the reversibility of the adsorption process wasinvestigated through desorption experiments carried outat high temperature and pressure. The extent of de-sorption can be used as an index of proclivity to liquid-phase oxidation of the adsorbate in the regenerationprocess. In any case, regeneration of spent AC can beeffected by surface oxidation of contaminants with noneed of previous desorption.21 Figure 10 illustrates theextent of MCPA desorption from Norit 0.8 and Aquacarb207C when these adsorbents were subjected to WAOconditions. In this figure, the extent of desorption isrepresented by the ratio between the equilibrium con-centration of MCPA in the liquid (Ce) and the concen-tration achieved if all MCPA were desorbed from theAC (CT) as a function of the desorption temperature.As observed from this figure, in the three ACs investi-gated, MCPA was partially desorbed from the adsor-bents, giving the possibility of its oxidation both in theliquid bulk and/or at the activated carbon surface.Partial desorption of the herbicide allows for the use ofexternal oxidation catalysts (not deposited in the car-bon), which might contribute to mineralization of thecontaminant. These features are being investigated, andthe results will be presented in a forthcoming paper.

4. Conclusions

From the present work, the following conclusions canbe deduced:

The equilibrium adsorption process can be acceptablydescribed by the Freundlich isotherm and, to a lesserextent, by the Langmuir isotherm. Thermodynamiccalculations confirm the exothermic nature of the ad-sorption of MCPA for three of the four ACs considered.The magnitude of the isosteric heat of adsorption ishigher for Norit 0.8 than for the rest of the adsorbentsstudied. This might be linked to the adsorption of MCPA

Figure 9. Effect of wet air oxidation on the adsorption capacityof MCPA onto commercial activated carbons. Isotherm condi-tions: T ) 20 °C, CMCPAo ) 150 mg L-1, pH ) 7.0. WAOconditions: T ) 180 °C, PT ) 38 bar. (A) Norit 0.8, (B) Aquacarb207C, (C) Aquacarb 208A. 0, Fresh AC; O, 2-h WAO; b, 6-h WAO.

Figure 10. Reversibility of MCPA adsorption onto commercialactivated carbons. pH ) 7.0, PT ) 38 bar.


into pores that have a diameter close to its molecularsize.

The kinetics of the nonequilibrium batch experimentscan be modeled by means of a mechanism based on theshrinking-core mass-transfer model. From this model,the effective diffusivity and the external mass-transfercoefficient can be calculated by fitting the model to theexperimental data.

Preliminary investigations as to the feasibility ofusing wet air oxidation as a method of carbon regenera-tion lead to the following two conclusions. First, theadsorption process is partially reversible. As a conse-quence, MCPA oxidation can be carried out in the bulkliquid phase or on the AC surface. In the first case,reactions can be enhanced by the use of externalcatalysts. In the second case, AC supported catalystsmight be an alternative option. Second, WAO operatingconditions do have an adverse effect on the adsorptioncapacity of Norit 0.8, although such conditions appearto exert a positive influence on Aquacarb 207 and 208Awhen pretreated for 2 h.

Acknowledgment

The authors express their gratitude to CICYT ofSpain (Project PPQ2000/04 12), and the Ministry ofEducation, Culture and Sports of Spain (Grant Ref.EX2001 32833176).

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Received for review June 7, 2002Revised manuscript received October 1, 2002

Accepted November 18, 2002

IE020424X


removal of the herbicide mcpa by commercial activated carbons: equilibrium, kinetics, and...

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