Adsorption and Competitive Adsorption on Zeolites of Nitrophenol Compounds Present in Wastewater

Adsorption and Competitive Adsorption on Zeolites of Nitrophenol CompoundsPresent in Wastewater

Bachar Koubaissy, Guy Joly, and Patrick Magnoux*

UniVersite de Poitiers, Faculte des Sciences, Laboratoire de Catalyse en Chimie Organique,UMR CNRS 6503, 40 AVenue du Recteur Pineau, 86022 Poitiers Cedex, France

This work investigates the removal of various nitrophenolic compounds (ortho-nitrophenol (ONP), para-nitrophenol (PNP), meta-nitrophenol (MNP), and 2,4-dinitrophenol (2,4-DNP)) from aqueous solution usinghydrophobic FAU zeolites. The adsorption equilibrium of nitrophenols from aqueous solutions by FAU canbe well-described using the Fowler-Guggenheim equation. The relative affinity of nitrophenols toward theFAU is dependent on the pH solution and on the pollutant solubility in water. Their sorption capacity is inthe following order: ONP > 2.4-DNP > PNP > MNP. In binary mixtures, the most important parameter thatgoverns the adsorption in zeolites seems to be the solubility of pollutants in water. Thus, the less-solublecompound (in this case, ONP) was adsorbed more easily than the other components present in the binarymixture. Finally, hydrophobic FAU zeolite seems to be an efficient adsorbent; it is able to be easily regeneratedunder air or by solvent leaching, through retention of these initial adsorption properties.

Introduction

Organic pollution in industrial waste streams is becoming agrowing environmental concern. Numerous methods have beenused for the removal of pollutants in effluents, includingozonation, ozonation/UV or H2O2/UV, filtration, and adsorptionprocesses into activated carbon.1-5 Great research efforts onadsorption processes and adsorbent materials for the removingof organic pollutants from waste streams have been developed.Activated carbon is the most widely used adsorbent for watertreatment; however, regeneration is difficult and expensive.6

Therefore, inorganic materials, such as synthetic zeolites, havebeen widely investigated, to design efficient and recyclableadsorbents. However, the application of synthetic zeolites forthe removal of pollutants from wastewater has rarely beenreported; only some papers can be found.7-14 It has been shownthat, to adsorb pollutants selectively from water, zeolite adsor-bents must be hydrophobic (i.e., possess a high Si/Al ratio).9,14

The objective of this work was to investigate the adsorptiveproperties of hydrophobic zeolite adsorbents for the removalof nitrophenol compounds from aqueous solutions. Adsorptionwas conducted in batch and flow apparatuses, and the effect ofthe pH solution was studied, as well as the effect of the natureof the nitrophenol pollutants (monoisomers and dinitrophenol).Furthermore, the adsorption was studied for single-componentnitrophenol as well as when nitrophenols were in binarymixtures. Lastly, the regeneration and reuse of zeolite also waspresented.

2. Experimental Section

2.1. Zeolite. FAU zeolite (Si/Al ratio ) 100) and silicalite(MFI, Si/Al ) 500) were supplied by Zeolyst International. TheBEA zeolite (Si/Al ) ∞) was kindly synthesized by theLaboratoire des Materiaux Mineraux (UMR CNRS 7016 Mul-house, France).

HFAU zeolites are characterized by the presence of one typeof large cage (supercages), 13 Å in diameter and in ball form,

accessible through a 12-ring window with a free aperture of7.4 Å.15 These large cages are connected and a tridimensionalstraight channel is finally formed by the succession of thesecages. Zeolite BEA (Beta) has a three-dimensional intersectingchannel system: two mutually perpendicular straight channels,each with a cross section of 6.6 Å × 6.7 Å, and a sinusoidalchannel, with a cross section of 5.6 Å × 5.6 Å.15 This tortu-ous channel system is formed by the intersection of the twomain channels. The channel intersections of BEA zeolitegenerate cavities whose sizes are on the order of 12-13 Å.15

Zeolite MFI (ZSM-5 or silicalite with a high Si/Al ratio) is amedium-pore zeolite, presenting a three-dimensional intercon-nected channel system with 10-membered openings (withdimensions of 5.1 Å × 5.5 Å and 5.3 Å × 5.6 Å),15 and thesize of the channel intersections is ∼8.5-9.0 Å.

Nitrogen adsorption measurements were performed at atemperature of -196 °C, using a gas adsorption system(Micromeritics, Model ASAP 2000). The characteristics ofzeolite samples are given in Table 1. It was concluded that high-Si/Al zeolites can be considered to be hydrophobic materials.9

2.2. Adsorption. Nitrophenols were obtained from Aldrich(98% purity), and their characteristics are summarized in Table2. The nitrophenol solutions were prepared in the concentrationrange of 1-500 mg/L in distilled water; the pH of the solutionwas adjusted using 1 M HCl or NaOH solutions. Adsorptionexperiments were performed using a batch equilibration tech-nique and a flow apparatus. For each equilibration technique,100 mg of the adsorbent was added to 200 mL of solution (e.g.,nitrophenol concentrations of 3, 7.5, 15, 30, 70, 100, and 500mg/L) and stirred for 24 h at 25 °C in a batch experiment. Foradsorption in a flow apparatus, 0.5 g of adsorbent was packedin a stainless steel column with an internal diameter of 4.6 mmand a length of 100 mm. This reactor was maintained attemperature with a thermostatic bath. Pollutant solutions (ni-

* To whom correspondence should be addressed. Tel.: +33(0)5494-53498. Fax: +33(0)549453779. E-mail address: [email protected].

Table 1. Characterization of the Zeolite Samples

pore volume (cm3/g)

zeoliteunit-cellformula

Si/Alratio micropores mesopores

NAl

(mmol/g)

BEA Si64O128 ∞ 0.197 0.043 0MFI H0.19Al0.19Si95.62O192 500 0.220 0.9FAU H1.9Al1.9Si190.1O384 100 0.285 0.056 4.4

Ind. Eng. Chem. Res. 2008, 47, 9558–95659558

10.1021/ie8001777 CCC: $40.75 2008 American Chemical SocietyPublished on Web 11/12/2008

trophenol concentration of 500 mg/L) were pumped throughthe column using a Gilson Model 307 pump, which allowedthe release of a constant flow rate of 2 mL/min. Whatever theprocess, samples were collected periodically and analyzed usinga high-performance liquid chromatography (HPLC) system(Varian Model Prostar chromatograph) that was equipped witha reverse phase column (Chromospher Pesticides) and an UVdetector (λ ) 254 nm, Model 340) with a mobile phase thatwas comprised of 70% methanol and 30% water (flow rate of1 mL/min).

The amount of adsorbed compounds was determined usingthe equation

m(Q))V(C0 - C) (1)

where m is the mass of zeolite (in grams), V the volume ofsolution (in liters), C the concentration after adsorption (givenin units of mg/L), C0 the initial concentration (mg/L), and Qthe amount adsorbed (given in units of mg/g of zeolite).

2.3. Adsorption Model. 2.3.1. Batch Reactor. The Lang-muir and Freundlich models were used but did not perform well,with regard to modeling isotherms; the Fowler-Guggenheimmodel,16 which is based on thermodynamic statistics and ex-hibits some interaction between the particles adsorbed, was usedin this study. The different theoretical curves were plotted fromthis model, following the equation

KC) θ1- θ

exp(2θWRT ) (linearized) (2)

ln[C(1- θ)θ ])-ln K+ (2θW

RT ) (3)

where K is the equilibrium constant for adsorption of theadsorbate on an active site (given in units of L/mol), C theconcentration at equilibrium adsorption (given in units of mol/L), W the empirical interaction energy between two moleculesadsorbed on nearest neighboring sites (given in units of J/mol),R the ideal gas constant (R ) 8.314 J mol-1 K-1), T thethermodynamic temperature (in Kelvin), and θ the fractionalcoverage of the surface.

This model16 is based on the fact that the energy of interactionis constant and independent of the recovery θ and, consequently,the number and distribution of adsorbed molecules.

2.3.2. Flow Reactor. The shapes of the obtained break-through curves were simulated using the theoretical model ofWolborska.17-19

ln( CC0

)) (�C0

Qads)t- (�

u )h (4)

where C0 is the initial concentration of pollutant (given in unitsof mol/g), Qads the concentration of pollutant in the solid (given

in units of mol/g), � the external mass-transfer kinetic coefficient(given in units of min-1), h the height of the adsorbent bed (incentimeters), and u the flow rate of a pollutant solution (givenin units of cm/min).

The values of the coefficient �, which is determined fromthe breakthrough curve and calculated from the Worlborskamodel, are identical for the same height of adsorbent bed andsize of the grains of adsorbent; however, in our case, variationof the coefficient � was observed, and this represents the internaltransfer (interaction between the adsorbate and the adsorbent,as well as the diffusivity of the adsorbed molecules). Here, � isgiven as

�)aQads

C0(5)

where a represents the slope of the curve (the interaction strengthbetween the adsorbate and the adsorbent), Qads the quantity ofpollutant adsorbed at the equilibrium, and C0 the initial pollutantconcentration.

By analogy with the moments’ theory, the final part of thebreakthrough curve (C/C0 > 0.5) is represented by a symmetricalpart of the initial curve described by the Wolborska model. Todescribe the final portion of the curve, we therefore use thefollowing equation:

CC0

) 1exp(b)

{2 exp(at1⁄2)- exp[a(2t1⁄2 - t)]} (6)

where t1/2 corresponds to the average time of stay, which isdefined as the time at which C/C0 ) 0.5 and b ) (�/u)h.

For competitive adsorption, the breakthrough of nondesorbedcompound has a similar shape, compared to a compound ab-sorbed without competition.

To describe the breakthrough curve of desorbed compounds,we propose the following model:

C2

Cm) [ exp(�aC2,0

a2,0)

exp(�hu ) ]

0<t<t1⁄2

+ 1

exp(�ah

u ){ 2[exp(�aC2,0

a2,0)t1⁄2] -

[exp(�aC2,0

a2,0)(2t1⁄2 - t)]}

t1⁄2<t<∞- (Cm - 1)

{ exp[(�aC1,0

a1,0)t]

exp(�hu ) }

0<t<t1⁄2′

(C2

C1)- (Cm - 1)

1

exp(�ah

u )×

{ 2[exp(�aC1,0

a1,0)t1⁄2

′ ] - [exp(�aC1,0

a1,0)(2t1⁄2

′ - t)]}t1⁄2′ <t

(C2

C1) (7)

Table 2. Characterization of Pollutants

structure

molecular formula o-C6H5NO3 p-C6H5NO3 m-C6H5NO3 2,4-C6H4N2O5

abbreviation ONP PNP MNP DNPmolecular weight (g/mol) 139.1 139.1 139.1 184solubility in water (g/L) 1.26 12.6 13.5 5.4pKa 7.17 7.15 8.4 4.07size (Å) 8.1 6.7 8.1 8.1dipole moment (D) 3.74 5.7 5.1 4.8

Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9559

where C1,0 is the initial concentration of nondesorbed compound,C2,0 the initial concentration of desorbed compound, Cm themaximal concentration of desorbed compound, a1,0 the concen-tration of adsorbent in the desorbed compound, a2,0 theconcentration of adsorbent in the nondesorbed compound, t1/2

the average time of stay of nondesorbed compound in theadsorbent, and t1/2

′ the average time of stay of desorbedcompound in the adsorbent.

2.4. Regeneration. Various techniques were used to regener-ate zeolites after adsorption, including thermal oxidation underair flow at 450 °C for 12 h and treatment by methanol (2 mL/min). For this last technique, the desorbed compounds were thensubjected to HPLC analysis and the methanol flow was stoppedwhen the organic pollutants disappeared.

3. Results and Discussion

3.1. Adsorption of ortho-Nitrophenol (ONP) over Vari-ous Zeolite Types. Figure 1 presents the adsorption isothermsof ortho-nitrophenol (ONP) from water (pH 4) onto the variouszeolites and confirms, as in the case of phenol adsorption,13

that BEA and FAU zeolites exhibit a high adsorption capacity(∼240 mg/g); this capacity had the same order of magnitudeas the results that have been reported in the literature for theadsorption of nitrobenzene over hydrophobic faujasite zeolite.14

However, for a low concentration of ONP, BEA presents ahigher adsorption capacity. At Ce ) 15 mg/L, BEA presents amaximal adsorption, whereas, on FAU, only 130 mg/g wereadsorbed. On the other hand, MFI zeolite seems effective atvery low concentration but its maximal adsorption capacity isonly ∼60 mg/g.

The various values determined from the Fowler-Guggenheimmodel16 are reported in Table 3.

A good correlation was found between these parameters andthe results obtained: the interactions constant K, between theadsorbent and the adsorbate, was more important over BEA andMFI and those zeolites were also efficient for FAU zeolite atlow ONP concentrations (see Figure 1). On the other hand, theinteraction energy between adsorbed molecules (W) is greateron the MFI zeolite. The lower adsorption capacity found over

MFI can be principally due to the size of the pore aperture (5.5Å, versus 6.7 and 7.5 Å, for BEA and FAU zeolite, respectively),but also to the diffusional limitation of the ONP in the narrowpores of this zeolites (5.5 Å, versus 8.1 Å for ONP). However,BEA zeolite presents an interesting behavior for ONP adsorptionas well as for phenol.9 But, because of its pore size (∼6.7 Å),the adsorption of bigger molecules, such as dinitrophenols(DNPs), was limited. That is the reason why, for the remainderof this work, FAU zeolite, which presents a larger pore aperture,will be selected.

3.2. Influence of pH on ONP Adsorption over FAUZeolite. The effect of pH was studied at pH 4, 7, and 9. ONPadsorption isotherms over FAU zeolite clearly show that thepH of the solution has a significant role on the adsorption (seeFigure 2); therefore, the adsorption has been shown to be morefavored for an acidic pH, the adsorption is preferential at pH 4(Qads ) 240 mg/g; see Table 4). These observations areconfirmed by the Henry’s constant (K) value, which representsthe equilibrium constant of interaction between the adsorbentand the organic compounds and calculated from the Fowler-Guggenheim model (see Table 4).

At pH 7, the phenolic compounds partially dissociate in water,leading to the dissociated species X-PHO-, whereas at pH 4,only the molecular species were formed (XPHOH). The

Figure 1. Adsorption isotherms for ONP obtained on ([) BEA, (9) FAU,and (]) MFI zeolites. Isotherms described by the Fowler-Guggenheimmodel.

Table 3. Parameter Values of the Fowler-Guggenheim Model fromONP Isotherms Obtained over Various Zeolites

value

parameter BEA MFI FAU

K (L/g) 40 54 13.4W (kJ/mol) -3.6 -4.8 -4.2Qads (mg/g) 240 66 240

Figure 2. Adsorption isotherms obtained on FAU zeolite for ONP at (9)pH 4, ([) pH 7, and (2) pH 9.

Figure 3. Adsorption isotherms obtained on FAU zeolite for ([) ONP,(9) DNP, (2) PNP, and (-) MNP. Isotherms described by the Fowler-Guggenheim model.

Table 4. Parameter Values from the Fowler-Guggenheim Model ofONP in Contact with FAU Zeolite

value

parameter ONP, pH 4 ONP, pH 7

K (L/g) 13.4 9.5W (kJ/mol) -4.2 -4.5Qads (mg/g) 240 210

9560 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008

dissociated species are more polar than molecular compoundsand interact preferentially with water rather than with zeolite.In this way, it is possible to explain that, in the case of ioniccompounds, repulsion between the oxygen sites of zeolite andthe negative group of the molecule occurs. Thus, at pH 9, whenthe compounds are totally dissociated, any adsorption thatoccurred was observed in zeolite (see Figure 2).

3.3. Adsorption of Nitrophenol Isomers over FAU Zeolite.Figure 3 reports the adsorption isotherms obtained at pH 4 overFAU zeolite of the three isomers of nitrophenol (ONP, para-nitrophenol (PNP), and meta-nitrophenol (MNP)), as well asthat obtained with dinitrophenol (DNP). Adsorption on zeolitewas favored for ONP; the maximal adsorption capacity andthe K-value were twice as great as that for PNP and MNP (seeTable 5). However, the adsorption capacity for DNP was almostthe same as that obtained for ONP and the K-values weresimilar. On the other hand, the energy W between adsorbedmolecules is similar for the three isomers but is lower with DNP.

Similar experiments were performed on benzene-derivedmaterials.20,21 Their different adsorptions were explained by theirpolarities, with the less-polar molecules being the most adsorbed.Thus, we have tried to establish a correlation between the dipolarmoments of nitrophenol compounds and their adsorption capaci-ties. The effect of polarity on adsorption on FAU is reported inTable 5, which shows that the polarity could have a significantrole in the adsorption, because ONP and DNP have the lowestdipole moments but the greatest capacity. However, DNP andMNP have similar dipole moments (4.8 and 5.1), but theadsorption capacity from DNP is twice as great as that for MNP.From these results, it is possible to conclude that the dipolemoment may not be the essential parameter that would explainour results.

Furthermore, we are also interested in the solubility in waterof the different compounds. Figure 4 shows a very good cor-relation between the adsorption capacity and the solubility inwater of these compounds. The less-soluble molecule (ONP)has the highest adsorption capacity. The other two compounds

(PNP and MNP) have much larger solubilities, which explainstheir low adsorption capacity. The DNP has an intermediatesolubility and, therefore, an intermediate adsorption capacity.

It is possible to conclude that the decrease of the pollutantinteractions with water causes an increase in interaction withthe adsorbent; the equilibrium moves in favor of the solid,which, here, is the zeolite structure.

As we consider that molecules are adsorbed in liquid form,and knowing that experiments conducted over siliceous meso-porous materials (MCM41) show that no significant adsorptionoccurs under our operating conditions, it was possible, fromthe density of nitrophenol molecules, to determine the volumeoccupied by the compounds at saturation. The calculated resultsobtained are higher than the experimental data observed. Thus,ONP and DNP occupy ∼85% of the micropore volume ofzeolite, whereas PNP and MNP occupy only 50%. It is possibleto conclude that the solubility of compounds in water is themore-important parameter, the more-soluble compound in water(PNP and MNP) remains preferentially in water. It can beunderlined that molecules were mainly adsorbed in the zeolitemicropores, which represent ∼85% of the total porous volume(recall Table 1).

3.4. Adsorption in a Flow Reactor. The adsorption ofsingle-component nitrophenol was studied from an aqueoussolution of 3.6 mmol/L of each constituent at pH 4. Figure 5presents the breakthrough curves obtained over FAU zeolite andtheir modelization from the Wolborska model.17-19

In agreement with the previous results obtained under batchconditions, the breakthrough volume of ONP is greater that thatof PNP and MNP, for which the breakthrough curves are veryclose. On the other hand, an initial plateau was observed forDNP; this can be explained either by a diffusional limitation ofthe molecule to enter the zeolite pores (because of the size ofthe DNP molecule) or by a small proportion of DNP beingpresent in dissociated form (because pKa ) 4.07, causing anelectrostatic repulsion between the dissociated compound andthe basic oxygen of the zeolite).

The experiment that was conducted at pH 3 (see Figure 6)demonstrates that this last hypothesis is correct. Indeed, at pH3, the plateau disappears, which is related to the decrease ofthe dissociated form of DNP molecules. Furthermore, thebreakthrough volume is slightly greater and was ∼200 mL.Moreover, the � coefficient, which represents the adsorbent-adsorbate interaction, passes from 1.96 at pH 4 to 3.3 at pH 3,showing that interactions between the adsorbent and DNPincrease as the solutions become more acidic (see Table 6).

For the nitrophenol isomers, the adsorption capacities fromONP to MNP were in good agreement for batch and flow

Figure 4. Quantity of nitrophenols adsorbed at equilibrium on FAU zeolite,as a function of the solubility of the compounds in water.

Table 5. Parameter Values of Nitrophenols Obtained from theFowler-Guggenheim Model for Adsorption over FAU Zeolite at pH4 (Batch Reactor)

value

parameter ONP PNP MNP DNP

K (L/g) 13.4 6.7 6.7 13W (kJ/mol) -4.2 -3.8 -4.1 -2.7Qads

(mg/g) 240 146 142 280(mmol/g) 1.72 1.06 1.02 1.50dipole (D) 3.74 5.7 5.1 4.8

Figure 5. Experimental and predicted breakthrough curves of (2) ONP,(9) DNP, ([) PNP, and (-) MNP on FAU zeolite at pH 4.


experiments (see Tables 5 and 6). Thus, by comparison of Tables5 and 6, the quantity of nitrophenols adsorbed in zeolite (Qads)from batch and flow experiments show similar results. Further-more, in the case of flow experiments, the values of � were ingood agreement with the adsorption capacity and decrease fromONP to MNP (see Table 6).

3.5. Competitive Adsorption. 3.5.1. ONP/PNP Mixture.In this study, the adsorption of a ONP/PNP mixture on FAUzeolite was studied. The breakthrough curves were obtained forequimolar concentration of 3.6 mmol/L at pH 4 and arepresented in Figure 7; the different capacities and selectivitiesevaluated from the breakthrough curves are given in Table 7.

These experiments lead to adsorption/desorption breakthroughcurves. In this case, the PNP was not adsorbed much in thezeolite, in comparison to ONP, which possesses the strongestinteraction with the zeolite. Therefore, after 100 mL of solution,FAU zeolite continues to adsorb ONP by eliminating PNP, anda desorption peak was observed that increased until C/C0 ≈1.4 (see Figure 7). Therefore, the preferential adsorption of theONP, in comparison with the PNP, confirms that more-favorableadsorption involves the compound presenting a lower solubilityin water.

The adsorbed quantity of ONP (225 mg/g) also is moresimilar to that observed in monoadsorption breakthrough (220mg/g), which shows that the presence of PNP does not influencethe adsorption of ONP. On the other hand, the adsorbed quantityof PNP is very weak (30.5 mg/g), which is five times less thanthat obtained in monoadsorption breakthrough (160 mg/g).Therefore, ONP, which has a greater interaction energy thanthat of PNP, also desorbs 65% of the adsorbed PNP.

On the other hand, the results show high selectivity of thezeolite for ONP compounds in the presence of PNP. Therefore,the selectivity R of ONP is, at high concentrations, ∼7.4 (seeTable 7). We can discern that the R value is more importantthat the adsorption capacity ratio for monocompounds (1.4);similarly, this R(ONP/PNP) value is higher than the ratio ofthe Henry’s constants (KHONP/KHPNP) at low concentrations (referto Table 7).

Therefore, the evolution from low concentrations to highconcentrations increases the adsorption affinity of the ONP,which is consistent with the type “S” isotherm, where theadsorbent-adsorbate interaction increases as the concentrationincreases.

3.5.2. ONP/DNP Mixture. The breakthrough curve of ONPand DNP in the competition adsorption onto FAU zeolite inthe same concentration (1.8 mmol/L) for each compound at pH3 is presented in Figure 8.

Note, as in the case of the ONP/PNP mixture, that ONPpresents a traditional breakthrough curve, whereas the break-through DNP curve is much different: it increases up to a C/C0

value of ∼1.1. Figure 8 shows the remarkable effect of ONPon the decrease in DNP adsorption. ONP is more adsorbed thanDNP than itself desorbed lightly from the half-breakthroughvolume of the ONP, this desorption is lightly inferior to 5%.

Nevertheless, in the both cases, the breakthrough volumesare almost identical and are equal to 140 mL. The selectivityvalue (1.36) seems different than that reported by the Henry’sconstant at low concentrations (1.1) (see Table 8). Despite thehigh solubility of DNP (5.4 g/L), in comparison with ONP (1.26g/L), the selectivity is weak, in comparison with the selectivity

Figure 6. Breakthrough curve of DNP at pH 3.

Table 6. Parameter Values from the Wolborska Model of ONP,PNP, MNP, and DNP in Contact with FAU Zeolite (FlowExperiments)

value

parameter ONP PNP MNP DNP DNP (pH 3)

a (min-1) 0.0074 0.0077 0.0081 0.0035 0.005Qads (mmol/g) 1.6 1.15 0.94 1.52 1.78� (min-1) 3.26 2.37 1.97 1.96 3.3

Table 7. Parameter Values from the Breakthrough Curves of theONP/PNP Mixture in Contact with FAU Zeolite (pH 4)

value

parameter ONP PNP

maximal amount adsorbed, Qads,max (mg/g) 225 85.5amount desorbed, Qdes (mg/g) 55amount adsorbed, Qads (mg/g) 225 30.5selectivity of adsorption, R(ONP/PNP) 7.4KH(ONP)/KH(PNP)

a 2.4

a KH(ONP)/KH(PNP) ) ratio of Henry’s constants, calculated at lowconcentration (1 mg/L).

Figure 7. Breakthrough curves for the adsorption of the PNP/ONP mixtureon FAU zeolite at pH 4. Legend: ([) PNP and (9) ONP.

Figure 8. Breakthrough curves for the adsorption of the DNP/ONP mixtureon FAU zeolite at pH 3. Legend: (]) DNP and (9) ONP.


of the ONP/PNP mixture. This is probably due to the more-acidic character of the DNP molecule or to the hydroxyl groupof DNP (pKa ) 4.07), in comparison with ONP (pKa ) 7.17),because of the acid-base interaction between the hydrogen ofthe functional group and the basic oxygen of zeolites. Thegreater electron density in the hydrogen bond leads to a decreasein the hydrogen interaction with the adsorbent and accordinglydecreases the affinity of ONP to the adsorbent.

3.5.3. PNP/DNP Mixture. This study was realized from anequimolar DNP/PNP mixture (3.1 mmol/L) at pH 3. As in thecase of the ONP/PNP mixture, we can observe adsorption/desorption phenomena for the PNP molecule (see Figure 9).This result agrees with that previously reported and it is theDNP component that is preferentially adsorbed. The quantityof PNP adsorbed is very weak, in comparison with the DNP(see Table 8); the adsorption capacity of the DNP represents72% of that obtained in monoadsorption (1.78 mmol/L), whichshows that the presence of the PNP has a negative effect on theadsorption of DNP. Nevertheless, the selectivity remains R )5, in favor of the DNP. This high selectivity is probably due toa high solubility of the PNP in water (12.6 g/L), in comparisonto that of the DNP (5.4 g/L), but this, again, is more related tothe acidic character of the DNP.

This selectivity (recall Table 8) is twice as important as theratio of the Henry’s constants to the weak concentrations. Theseresults are consistent with those obtained previously. Also notethat R is greater for the study of the ONP/PNP mixture (7.4,versus 5.0 with the DNP/PNP mixture).

3.5.4. MNP/PNP Mixture. The adsorption isotherms of PNPand MNP are very similar; therefore, one study was realized incompetitive adsorption for these two compounds, at pH 4, forinitial concentrations of 3.6 mmol/L, with the intention ofcomparing these two compounds in competitive adsorption.

The breakthrough curves (not shown here), in contrast to theprevious case, are very similar and are almost confused; nodesorption has been observed for one compound in favor of

the other, and the adsorption capacities were ∼70 mg/g, versus∼100 mg/g in monoadsorption in batch experiments. Theseresults are consistent with the solubility parameter, because thesolubilities of these compounds in water are very similar (12.6g/L for PNP and 13.5 g/L for MNP), and the value of the ratioof their Henry’s constants (KH(PNP)/KH(MNP)) is ∼1.

In summary of this part, whatever the nitrophenol mixture,the less-soluble molecules in water are preferentially adsorbedin zeolite and desorb the compound that presents the greatersolubility in water. However, when solubilities of the compoundsare similar (e.g., the DNP/ONP mixture; see Figure 8), theeffects are less-pronounced.

3.6. Adsorption of ONP in the Presence of SalicylicAcid (SA). The salicylic acid (SA) molecule, which presents akinetic diameter of 8.4 Å and an acidic functionality, was chosenas a model compound representative by its acidic function ofthe organic matter present in water.

3.6.1. Influence of pH on SA Adsorption over FAUZeolite. The adsorption properties of SA in aqueous solutionwere studied at different pH values. The obtained isotherms arerepresented in Figure 10. We can conclude that the SA adsorbsonly in FAU at very acidic pH. This is due to its physicochem-ical properties (pKa ) 2.97), because, at pH >3, these moleculesare dissociated (change of structural characteristics), whichcreates electrostatic repulsion forces with the basic oxygen ofthe zeolite decreasing the interaction manners of SA andfaujasite zeolite.

However, at pH 3, the adsorption capacity increases signifi-cantly, because the proportion of anionic forms decreases andthe neutral molecules dominates. For an equilibrium concentra-tion of 50 mg/L, we can observe a 50-fold increase in theadsorption capacity.

3.6.2. Effect of SA in the Adsorption of ONP. Theadsorption of ONP in the absence and presence of SA wasperformed at pH 4 and a SA concentration of 15 mg/L. Figure11 summarizes the isotherms obtained. We notice that theamount of ONP adsorbed increases significantly in the presenceof SA. This increase is especially more important at lowconcentrations. We notice, for example, that, for an equilibriumconcentration of 10 mg/L, the amount of ONP adsorbed is ∼50mg/g in the absence of SA, versus 175 mg/g in the presence ofSA.

However, when the concentration of ONP increases,the effectof the SA becomes more limited. This is due to the approachingsaturation of micropores, which prevents new molecules of ONPfrom adsorbing.

An increase in the amount of pollutants adsorbed was alsoobserved by Xu et al.22 One of the explanations offered is theinteraction of pollutants with functional groups of organicmaterials adsorbed creating new sites for adsorption of these

Table 8. Parameters from the Breakthrough Curves of the ONP/DNP and DNP/PNP Mixture in Contact with FAU Zeolite at (pH 3)

ONP/DNP DNP/PNP

parameter ONP DNP DNP PNP

maximal amount adsorbed, Qads,max(mmol/g)

1.01 0.77 1.25 0.54

amount desorbed, Qdes (mmol/g) 0.025 0.29amount adsorbed, Qads (mmol/g) 1.01 0.745 1.25 0.25R(I/II) (ONP/DNP) 1.36 (DNP/PNP) 5KH(I)/KH(II)

a 1.1 2.8

a KH(ONP)/KH(PNP) ) ratio of Henry’s constants, calculated at lowconcentration (1 mg/L).

Figure 9. Breakthrough curves for the adsorption of the PNP/DNP mixtureon FAU zeolite at pH 3. Legend: (9) PNP and (]) DNP.

Figure 10. Adsorption isotherms obtained on FAU zeolite for SA at differentpH values: ([) pH 3, (2) pH 4, and (9) pH 7.


compounds. However, in our case, we have demonstrated thatthe SA molecule is not adsorbed on the zeolite at pH 4. Theincrease in the amount of ONP adsorbed in zeolite is probablydue to the decrease in solubility of ONP in the presence ofmolecules that present no affinity for the zeolite, because ofthe presence of its anionic form. Another hypothesis can berelated to a bridge combination between SA and ONP molecules,which also decreases the apparent solubility of ONP.

3.7. Adsorption Mechanism over Faujasite Zeolite. Thesorption mechanism of nitrophenol on FAU zeolite has beeninvestigated at pH 7 and pH 4, using Raman spectroscopy. TheRaman spectra of PNP in solution in water at pH 4 and a PNPoccluded state in FAU zeolite are presented in Figure 12. Thebands assigned near 1595 cm-1 correspond to the CdC vibrationof the aromatic ring, the wavenumber at 1338-1342 cm-1

corresponds to the band of NO2, and the wavenumber at 1110cm-1 corresponds to the band of C-H; therefore, some of theresults presented in Figure 12 mark wavenumbers that changedfrom PNP in solution to the PNP occluded state.

These results show the presence of interactions between theNO2 group, the aromatic ring, and FAU (a decrease of 4 cm-1

in the vibration bands), but it does not confirm if interactionoccurs between the zeolite and the organic composite directlyor the organic compounds is surrounded by water; for thatdetermination, a second experiment was realized, using dichlo-

romethane instead of water. The result of adsorption of PNP indichloromethane presented in Figure 12 shows a spectrumidentical to those observed in the case of the adsorption on FAUwith water; therefore, the nitrophenol does not hydrated inadsorption on FAU zeolite. We have conducted studies identicalwith ONP, which confirms this hypothesis.

These results show that the adsorption occurs, in this instance,via the formation of a complex donor-acceptor between theelectrons of the nitroaromatique compounds and the oxygenelectrons of the silanol groups.

3.8. Regeneration of the FAU Zeolite. Different techniquescould be used to regenerate zeolite after adsorption (thermaloxidation, regeneration by ozone, steam washing, solventextraction, etc.). In this part, regeneration was studied by thermaloxidation with air and by solvent extraction using methanol.

3.8.1. Thermal Regeneration by Air. After adsorption,zeolite was regenerated under an air flow at 450 °C for 12 h.Examinations were made to confirm that, after this treatment,no residual carbon remained in the adsorbent, which means thatthe pollutant was completely oxidized. Regeneration was studiedafter ONP adsorption and successive adsorption-regenerationsequences were performed (see Figure 13).

After 10 cycles, the initial adsorption capacity of the zeolitewas totally restored (240 mg/g of ONP can be adsorbed in thepore structure of FAU zeolite). These results indicate that theregeneration is efficient and that the zeolite retains all of itsproperties after this treatment, contrary to the activated carbon,which presents a decrease in adsorption capacity after thermaloxidation.6 However, the disadvantage of this technique is theenergy used during regeneration. For this, solvent extractioncan be more appropriate.

3.8.2. Regeneration by Extraction Using Methanol. Thisregenerate was performed in the case of a flow reactor in thefollowing steps.

After the adsorption of a pollutant in the zeolite, methanolwas introduced at a constant flow rate of 2 mL/min. Afterdesorption of the pollutant (followed by HPLC analysis), themethanol flow was stopped and pure water was introduced (2mL/min) for 30 min. After that step, the new adsorption of thepollutant can be realized.

In the case of nitrophenol isomers whose present a solubilityof ∼100 g/L in methanol, a total desorption of pollutants wasobtained after 20-30 mL of methanol flow. The PNP compoundwas more easily desorbed that the other isomers; only 15 mLof methanol is required to regenerate the zeolite after PNPadsorption.

Five adsorption-desorption cycles were conducted on FAUzeolite after PNP adsorption. Results are presented in Figure14, and they show that the zeolite retains its adsorption

Figure 11. Adsorption isotherms of ONP in the (9) presence and ([)absence of SA at pH 4.

Figure 12. Fourier transform-Raman spectra in the 0-2000 cm-1 rangeat room temperature of (s) PNP in water solution, (- - -) PNP occludedwith water in FAU zeolite, and ( · · · ) PNP occluded with CH2Cl2 in FAU.

Figure 13. Effect of thermal regeneration on the maximal adsorptioncapacity of ONP on FAU zeolite.


properties after this methanol treatment. Therefore, it seemspossible to regenerate and reuse the hydrophobic faujasite zeoliteafter methanol extraction treatment using a very small volumeof solvent.

4. Conclusion

The removal of nitrophenolic compounds from waste streamsvia adsorption on zeolitic adsorbents is enhanced with hydro-phobic zeolites that possess a high Si/Al ratio and large poreapertures (especially FAU zeolites). The adsorption capacitiesare strongly dependent on the pH of the solution. Thus, theneutral form of the pollutants is more easily adsorbed into zeolitepores than the dissociated form, in which anionic charges ofthe component were in wrong interaction with the negativecharge of framework oxygen of zeolite. Also, the adsorptioncapacity is dependent on the polarity of the molecule and ofthe pollutant solubility in water. For the same type of compounds(here, nitrophenols), the lower the solubility of the compound,the more important the adsorption.

The binary competitive adsorption that is conducted in a flowreactor is consistent with the results obtained under batchconditions. ONP, which is the less-soluble compound, possessesthe strongest interaction with the zeolite and desorbs the othercompounds, such as PNP and DNP molecules. In the presenceof salicylic acid, the adsorption of ONP increases, most certainlybecause of the decrease in ONP solubility in the presence ofthese acidic molecules, which are strongly soluble in water.

In conclusion, the advantages of using zeolites are their highadsorption capacities and especially their stability after regenera-tion, while retaining the initial properties after severaladsorption-regeneration cycles.

Acknowledgment

B.K. gratefully acknowledges the “Region Poitou-Charentes”for her scholarship.

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ReceiVed for reView January 31, 2008ReVised manuscript receiVed September 19, 2008

Accepted October 9, 2008

IE8001777

Figure 14. Effect of regeneration by methanol of FAU zeolite afteradsorption of PNP: ([) fresh zeolite, (9) after one cycle, (2) after twocycles, and (/) after five cycles.


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Adsorption and Competitive Adsorption on Zeolites of Nitrophenol Compounds Present in Wastewater