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Separation and Purification Technology 57 (2007) 250–256

Enhanced adsorption of p-nitroaniline from water bya carboxylated polymeric adsorbent

Kai Zheng a, Bingcai Pan a,∗, Qingjian Zhang a, Weiming Zhang a, Bingjun Pan a,Yuhua Han b, Qingrui Zhang a, Du Wei a, Zhengwen Xu a, Quanxing Zhang a

a State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210093, PR Chinab Jiangsu Tasly Diyi Pharmaceutical Co., Ltd., Huaian 223002, PR China

Received 10 November 2006; received in revised form 19 April 2007; accepted 19 April 2007

bstract

In the present study a carboxylated polymeric adsorbent ZK-1 was synthesized for enhanced removal of p-nitroaniline (PNA) from aqueousolution. A commercial polymeric adsorbent XAD-4 was selected for comparison purpose. Characterization of ZK-1 was characterized by infraredpectroscopy and pore size distribution analysis. Experimental results showed that PNA adsorption onto ZK-1 was greatly enhanced due to itsicropore structure and the carboxylic group introduced onto polymeric matrix. Different pH-dependent adsorption tendency of PNA onto XAD-4

nd ZK-1 was observed mainly due to the role of carboxyl group on the ZK-1 surface. Isotherms of PNA adsorption onto ZK-1 and XAD-4 could beepresented by Langmuir model reasonably. More favorable PNA adsorption onto ZK-1 than XAD-4 was further demonstrated by thermodynamic

nanlysis. Kinetic studies demonstrated that PNA uptake onto ZK-1 followed the pseudo-second order model, while that onto XAD-4 would beore suitably represented by the pseudo-first order model. Column adsorption runs indicated that PNA could be completely removed from aqueous

ystem by ZK-1. Moreover, efficient regeneration of the spent adsorbent ZK-1 was readily achieved by ethanol and water for its repeated use. 2007 Elsevier B.V. All rights reserved.

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eywords: Polymeric adsorbent; Carboxylation; 4-Nitroaniline; Adsorption en

. Introduction

Water pollution by chemical wastewater has attracted increa-ing concern particularly in developing countries (e.g., Indiand China) [1]. Accordingly, many disposal processes includingiodegradation, chemical oxidation, and adsorption were deve-oped to ensure its safely discharging into the receiving water2–4], among which adsorption by activated carbon is one of theost widely used technology in the specific field [5,6]. Howe-

er, its poor mechanical strength and challenging regenerationinders its wider application. Alternatively, low-cost adsorbentsased on red mud and other industrial wastes seems attractiveor removal of organic pollutants from waste streams [7,8], butheir feasibility for field application needs further demonstration.

n the past decades polymeric adsorbents have been emergings a potential alternative to activated carbon due to its bet-er mechanical strength and feasible regeneration under mild

∗ Corresponding author. Tel.: +86 25 8368 5736; fax: +86 25 8370 7304.E-mail address: bcpan@nju.edu.cn (B. Pan).

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383-5866/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.seppur.2007.04.017

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onditions [9–12]. Today, many industrial processes based onolymeric adsorbents are available to treat chemical effluents13–15]. Microporous adsorption polymers developed duringhe last years are characterized by very little pore diameters andigh specific surface areas (1400 m2 g−1). Their capacities arewo to five times higher than that of macroporous ones, andrequently exceed the adsorption capacity of activated carbon.n recent years chemical modification has been found to be anffective approach to further improve the adsorption capacity ofolymeric adsorbent [16,17]. For example, a hyper-cross-linkedolymeric adsorbent subject to amination would greatly enhancehe removal of aromatic acidic compounds such as phenols,romatic carboxylic acids and sulfonic acids through the spe-ific acid–base interaction or electrostatic interaction betweenhe amino group and acidic solutes [18–20]. Similarly, acidicroups once introduced to a polymeric matrix are intended tomprove adsorption of aromatic amines, which are ubiquitous in

hemical effluents and always pose a significant threat on humanealth due to their high toxicity.

The objective of the current study aims at preparing aarboxylated polymeric adsorbent for enhanced removal of

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K. Zheng et al. / Separation and Pur

-nitroaniline from aqueous solution. p-Nitroaniline is anmportant intermediate for manufacturing of azo dyes, phar-

aceuticals, gum inhibitors, etc. However, it is a well-knownriority pollutant in terms of its hematoxicity, splenotoxicity,epatoxicity, and nephrotoxicity [21,22], and its presence inater even at a very low level would be harmful to aquatic life

nd human health. Batch adsorption runs including isotherm,inetics and column experiments were performed compared to aommercially available polymeric adsorbent Amberlite XAD-4.

. Materials and methods

.1. Chemicals

Zinc chloride, 4-methylbenzoic acid, ethanol, o-dich-orobenzene, nitrobenzene, p-nitroaniline (A.R.) were purcha-ed from Shanghai Chemical Reagent Plant (Shanghai, China)nd used without further purification. p-Nitroaniline was dis-olved in double-distilled water for batch adsorption runs.he polymeric adsorbent Amberlite XAD-4 was obtained fromohm-Haas (Philadlphia, PA, USA). The chloromethylated St-VB copolymer (CSC) beads with chloro content of 17.3%

n mass were kindly provided by Langfang Resin Co. (Hebei,hina). A hyper-cross-linked polymeric adsorbent CHA-101as kindly provided by Jinxiang Chemical Plant (Jiangsu,hina).

.2. Preparation of a new adsorbent ZK-1

As shown in Scheme 1, ZK-1 was prepared by carboxyla-ion of the CSC beads followed by post-cross-linking of thearboxylated derivatives. First, 28.0 g of 4-methylbenzoic acidnd 30.0 g of CSC beads were swollen overnight by 250.0 g of-dichlorobenzene in a 500-ml flask. Under mild mechanicaltirring, 6.0 g of anhydrous zinc chloride was gradually addeds catalyst into the flask at 298 K and the carboxylation pro-ess lasted about 12 h. The reaction mixture was then filtered toemove the liquid moiety and the solid particles were extractedith ethanol for 4 h in a Soxlet apparatus. The carboxylated par-

icles (CCSC) were obtained and the chlorine content decreasedrom 17.3% to 12.6%.

In the post-cross-linking procedure 30.0 g of CCSC particles

ere also swollen overnight in 100 g of nitrobenzene in a flask.nder mechanical stirring, 6.0 g of anhydrous zinc chloride wasradually added into the flask at 298 K, followed by an increasef the reaction temperature from 298 K to 405 K within 1 h. After

amaw

Scheme 1. Conceptual illustration of p

on Technology 57 (2007) 250–256 251

h of the post-cross-linking reaction at 405 K, the mixture wasubjected to filtering and extracting in the similar manner forCSC particles and we got the new adsorbent ZK-1.

.3. Batch adsorption experiments

Batch adsorption experiments were performed as follows:.100 g of each adsorbent was introduced into a 250 ml coni-al flask containing 100 ml of the PNA solution ranging from0 mg/l to 500 mg/l. The flasks were sealed and shaken in a G25odel incubator shaker (New Brunswick Scientific Co. Inc.) atpreset temperature under 120 rpm for 24 h to ensure adsorptionquilibrium. The PNA uptake was calculated by conducting aass balance before and after adsorption using the following

quation:

e = V1(C0 − Ce)

W(1)

here V1 (l) is the volume of solution and W (g) is the massf a given dry adsorbent; C0 and Ce (mg/l) denote the initialnd equilibrium PNA contents in aqueous solution. As for theinetic study, the amount of adsorbent and solution was deter-ined as 0.50 g and 500 ml respectively, and 0.50-ml solutionas sampled at various time intervals to determine adsorptioninetics.

.4. Column adsorption experiment

Column adsorption experiments were carried out with a glassolumn (12 mm diameter and 230 mm length) equipped with aater bath to maintain a constant ambient temperature. A HL-B constant-flow pump (China) was used to assure a constantow rate. All column runs were performed under the identicalydrodynamic conditions: the superficial liquid velocity (SLV)nd the empty bed contact time (EBCT) were equal to 1.0 m/hnd 4 min, respectively.

.5. Analysis

Concentrations of PNA in solution were determined spectro-hotometrically by using a Helious Betra UV–vis spectrometerUnicam Co., UK) at wavelength of 381 nm (Zhengkai). Surface

rea and pore size distribution of a given adsorbent were deter-ined by N2 adsorption using a Micromertics ASAP 2010C

utomatic analyzer. Infrared spectra of polymeric adsorbentsere taken from Nexus 870 FT-IR spectrometer (USA) with a

reparation procedures of ZK-1.

2 urification Technology 57 (2007) 250–256

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ellet of powered potassium bromide and adsorbent in the rangef 500–4000 cm−1.

. Results and discussion

.1. Characterization of adsorbent ZK-1

Some important characteristics of ZK-1 are listed in Table 1ompared to XAD-4. In spite of similar matrix structure andET surface area of both adsorbents, ZK-1 presents a specificicroporous structure, which was further confirmed by their dif-

erent pore size distributions (Fig. 1). The presence of carboxylroups on ZK-1 was proved by the strong band at 3446 cm−1 inhe IR spectrum (Fig. 2).

.2. Effect of solution pH on adsorption

Effect of solution pH on PNA adsorption onto XAD-4 andK-1 was depicted in Fig. 3 including PNA dissociation curveependent upon solution pH. Meanwhile, the dissociation curvef 4-methylbenzoic acid was used to simulate the carboxyl

roups on the polymeric matrix of ZK-1. ZK-1 exhibits largerNA adsorption capacity, and pH-dependent adsorption trendsre different for both adsorbents. PNA adsorption onto XAD-4radually increases as pH increases to about 3, and then keeps

able 1haracteristics of polymeric adsorbents used in the present study

Adsorbent

XAD-4 ZK-1 JX-101

atrix Polystyreneolarity Nonpolar Moderate NonpolarET surface area (m2 g−1) 908 1000 1040icropore surface area (m2 g−1) 115 587 610

ore volume (cm−3 g−1) 1.33 0.58 0.71icropore volume (cm3 g−1) 0.033 0.27 0.28verage pore diameter (nm) 5.64 2.31 2.50unctional group (mmol g−1) None –COOH (1.32) None

ig. 1. Pore size distribution of two polymeric adsorbents XAD-4 and ZK-1.

AvPag

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Fig. 2. The IR spectra of two polymeric adsorbents XAD-4 and ZK-1.

onstant when solution pH values are larger than 3. In general,NA adsorption onto XAD-4 is mainly driven by van der Waals

nteraction [23] and the protonated PNA cannot be effectivelyoaded on the nonpolar inner surface of XAD-4. It is consistentith the dissociation curve of PNA versus solution pH (in Fig. 1).s for ZK-1, a maximum capacity of PNA adsorption was obser-ed at pH about 4, and less or larger pH was unfavorable forNA adsorption. Apparently, effect of carboxyl group on aminedsorption should not be negligible, which can be inferred asiven below [20,24]:

–C6H4COOH + R′NH2 ⇔ RC6H4COOH·NH2R′ (2)

–C6H4COO− + R′N+H3 ⇔ RC6H4COO−·N+H3R′ (3)

here R represents the polymeric matrix, and R′ is the-nitrobenzyl group. For the PNA adsorption onto ZK-1, elec-rostatic interaction shown in Eq. (2) may be negligible because

NA and carboxyl group cannot be simultaneously chargednder a given solution pH, as deduced from their dissocia-ion curves. Consequently, effect of carboxyl groups on PNAdsorption onto ZK-1 mainly resulted from the formation of

ig. 3. Correction of solution pH with adsorption capacity in equilibrium (Qe)f XAD-4 and ZK-1 (left) and the proportion of molecular state of 4-nitroanilinend 4-methylbenzonic acid (right).

ification Technology 57 (2007) 250–256 253

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he acid–base complex. A intersection point of pH about 4 bet-een the dissociation curves was observed, where the largest

mount of the acid–base complex would be formed theoreti-ally and therefore results in a maximum adsorption capacity.ote that the micropore structure of ZK-1 was also favo-

able for PNA adsorption enhancement, which results fromhe micropore filling mechanism [25]. In fact, the synergisticffect between hydrophobic and hydrogen-bonding interactionay occur to favor the enhanced removal of PNP on ZK-1

26,27].

.3. Adsorption isotherm

Adsorption isotherms of PNA onto both adsorbents at 303 Kre present in Fig. 4a. A hyper-cross-linked polymeric adsorbentX-101 with similar pore structure to ZK-1 and no functionalroup was also included to elucidate the role of functional group.t can be seen from Fig. 4a that adsorption capacites of threedsorbents are listed in order XAD-4 < JX-101 < ZK-1. Takennto account their similar surface area, adsorption capacity of JX-01 higher than XAD-4 may result from its micropore structure,nd that of ZK-1 higher than JX-101 may be attributed to thedded carboxyl group.

Effect of ambient temperature on PNA adsorption onto bothdsorbents are also shown in Fig. 4b. The Freundlich equationnd Langmuir equation were employed to describe the adsorp-ion isotherms of PNA on ZK-1 and XAD-4.

e = KFCne (4)

e = QmKLCe

1 + KLCe(5)

here KF and n is the Freundlich parameter for a heterogeneousdsorbent, Qm (mmol g−1) the maximal adsorption capacity, andL (mmol l−1) is a binding constant. The constant KF is always

aken as a relative indicator of adsorption capacity, n is rela-ed to the magnitude of the adsorption driving force and to thedsorbent site energy distribution. All the associated parameteralues were listed in Table 2. It can be seen that the Lang-uir model fit the adsorption isotherms of XAD-4 and ZK-1

etter than the Freundlich model. Another observation is thatNA adsorption on both adsorbents is an exothermic process

n nature because lower temperature is more favorable for PNAdsorption.

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able 2sotherm parameters of 4-nitroaniline adsorption onto XAD-4 and ZK-1

dsorbent T (K) Freundlich model

KF 1/n R2

AD-4 288 1.08 1.81 0.992303 0.58 1.72 0.955318 0.36 1.40 0.979

K-1 288 2.32 2.72 0.962303 2.11 3.26 0.983318 2.00 2.07 0.994

ig. 4. Adsorption isotherms of 4-nitroaniline onto polymeric adsorbents (a) at03 K; (b) at different temperatures.

.4. Thermodynamic aspects

The free energy change for adsorption process is given by

G = �H − T�S (6)

t low solute concentration, �G can be determined as follows

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G = −RT∫ Ce

0 Q(Ce) d ln Ce

Q(Ce)(7)

Langmiur model

KL (mmol l−1) Qm (mmol g−1) R2

0.97 2.31 0.9900.69 1.52 0.9900.24 1.88 0.985

8.60 2.47 0.9855.85 3.20 0.9902.96 2.84 0.992

254 K. Zheng et al. / Separation and Purification Technology 57 (2007) 250–256

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ig. 5. Determination of isosteric enthalpy of adsorption of 4-nitroaniline onAD-4 adsorbent (ln KL vs. 1/T).

hen the adsorption capacity Q (Ce) follows the Langmuir equa-ion, incorporating Eq. (5) into Eq. (7) yields

G = −RTQm

Qe[ln(1 + KLCe]Ce

0 (8)

f isosteric adsorption enthalpy change (�H) can be assumedo be approximately constant, the van’t Hoff equation can betilized to calculate the �H values of a specific adsorptionrocess.

As seen in Table 2, KL in the Langmiur equation changes withhe temperature and �H, as illustrated in the following relation29]:

n KL = ln K0 − �H

RT(9)

The enthalpy change can be determined from the slope of then KL versus 1/T plot. The enthalpy change, entropy change andree energy change for PNA adsorption on XAD-4 and ZK-1 areresented in Fig. 5 and Table 3. The enthalpy values for PNAdsorption onto ZK-1 and XAD-4 are all negative, implying anxothermic nature of PNA adsorption. Moreover, PNA adsorp-ion onto both adsorbents are spontaneous as indicated by the

egative �G values, and larger absolute values of free energyhanges for ZK-1 than XAD-4 was consistent with their differentdsorption capacities.

able 3hermodynamic parameters of 4-nitroaniline adsorption onto ZK-1 and XAD-4nder different conditions

dsorbent T (K) �H (kJ mol−1) �G (kJ mol−1) �S (J mol−1 K−1)

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ig. 6. Effect of contact time on 4-nitroaniline adsorption onto ZK-1 and XAD-4t 303 K.

.5. Adsorption kinetics

The influence of contact time on the removal of PNA byAD-4 and ZK-1 is illustrated in Fig. 6. Adsorption equilibriumf PNA on XAD-4 was achieved in a shorter time than ZK-1ue to its larger average pore diameter (in Table 1). The widelysed pseudo-first-order and pseudo-second-order model weremployed to fit the experimental data [29–31]:

seudo-first order model : lnQe

Qe − Qt

= k1t (10)

seudo-second order model :1

Qt

= 1

k2Q2e t

+ 1

Qe(11)

here Qe is the equilibrium adsorption capacity (mmol g−1), Qt

he adsorption capacity (mmol g−1) at the contact time t (min), k1he pseudo-first order rate constant (min−1), and k2 is the pseudo-econd order rate constant (g mmol−1 min−1). Results in Table 4ndicated that the pseudo-second order model could represent theNA adsorption kinetics onto XAD-4 well, while PNA adsorp-

ion onto ZK-1 followed the pseudo-first order model moreeasonably.

.6. Column adsorption tests

Fig. 7 depicted a complete effluent history of a fixed-bedolumn packed with either XAD-4 or Zk-1 for a feeding solutionontaining 500 mg/l PNA at pH 4.0. Earlier breakthrough occur-ed expectedly in case of XAD-4 than ZK-1 further demonstrated

able 4he kinetic parameters of 4-nitroaniline adsorption onto ZK-1 and XAD-4 for

wo kinetic models

dsorbent Pseudo-first order model Pseudo-second order model

k1 (min−1) R2 k2 (g mmol−1 min−1) R2

AD-4 0.023 0.926 0.65 0.991K-1 0.020 0.987 0.38 0.797

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ig. 7. A complete breakthrough curve of p-nitroaniline ZK-1 at 303 K compa-ed to XAD-4.

dsorption enhancement of PNA onto the carboxylated sorbent.fter column adsorption the spent ZK-1 was subjected to a sepa-

ate elution using 1 bed volume (BV) of ethanol (2 M) followedy 1.5 BV of deionized water at 333 K. An entire elution effi-iency of more than 99% indicated that ZK-1 could be employedor repeated use after regeneration. The satisfactory adsorptionnd regeneration behavior of ZK-1 promoted us to believe its a potential candidate for treatment of chemical wastewaterontaining aniline pollutants.

. Conclusions

A carboxylated polymeric adsorbent (denoted ZK-1) wasrepared for enhanced removal of p-nitroaniline (PNA) fromqueous phase compared to a commercial adsorbent AmberliteAD-4. Enhanced removal of PNA was observed onto ZK-, which was possibly attributed to its micropore structure asell as carboxylic group introduced onto the polymeric matrix.dsorption isotherms of PNA onto ZK-1 followed Langmuirodel well and its kinetic behavior could be represented by the

seudo-first order model. Column adsorption results indicatedhat a complete PNA removal from aqueous solution would beealized by ZK-1. The spent ZK-1 adsorbent could be readilyegenerated by ethanol and water for repeated use. All the aboveesults suggested that ZK-1 is a potential adsorbent for PNAemoval from industrial wastewater.

cknowledgements

This research was financially funded by National Naturalcience Funding of P.R. China (Grant No. 20504012) and 863rogram of China (2006AA06Z383).

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