Removal of Chromium from Electroplating Wastewaterby Aminated Polyacrylonitrile Fibers

Jing Wang,1 Xiang-Hua Lv,2 Liang Zhao,1,* and Jing-Ping Li3

1Institute of Chemistry, Henan Academy of Sciences, Zhengzhou, China.2School of Materials Science and Engineering, Zhengzhou University, Zhengzhou, China.

3School of Chemistry and Chemical Engineering, Shandong University, Jinan, China.

Received: November 22, 2010 Accepted in revised form: April 25, 2011

Abstract

Batch and column studies were conducted to evaluate the performance of a novel aminated polyacrylonitrile(TE-PAN) fiber for total chromium removal from electroplating wastewater. Results from batch sorption studiesshowed that sorption equilibrium of chromium on TE-PAN was achieved within about 10 min. Temperature hadno significant effect on sorption isotherms and estimated maximum uptake of total chromium was 133 mg/g.TE-PAN exhibited high removal efficiency over a wide pH range (2–7). Column studies demonstrated thatexhausted TE-PAN fibers could be completely regenerated by 0.1 M NaOH for repeated sorption of chromiumafter as many as 15 sorption-regeneration cycles. FTIR and SEM analyses further validated the efficiency andstability of TE-PAN for chromium removal. Findings of this study have significant practical applications fortreatment of chromium contaminated waters.

Key words: chromium removal; electroplating wastewater; aminated polyacrylonitrile fiber; column test

Introduction

Contamination of chromium has drawn much atten-tion in recent years due to both its severe threat to public

health and its wide application in processes like chromeplating, manufacture of dyes and pigments, leather tanningand wood preservation. Chromium is present in aqueousenvironments mainly in two oxidation states, Cr(III) andCr(VI), with Cr(VI) as the most soluble and toxic form. Thepermissible level of Cr(VI) in industrial wastewater is set bythe Chinese Environmental Agency to be 0.5 mg/L, while thetotal chromium is limited to 1.5 mg/L (GB 8978-1996).

Conventional technologies for treatment of aqueous Cr(VI)include reduction-precipitation (Almaguer-Busso et al., 2009),ion exchange (Tenorio and Espinosa, 2001), adsorption (Li etal., 2007), and membrane-based separation (Sachdeva andKumar, 2008). In the industrial application, chemical reductionof Cr(VI) to less toxic Cr(III) followed by Cr(OH)3 precipitationis the most popular method. However, a large amount ofsludge is generated in this process that should be safely dis-posed of. Ion exchange and adsorption are the most widelystudied because they are easy to operate and could be used foradvanced treatment. Moreover, when combined with an ap-propriate desorption step, they could recycle the valuablemetal and solve the problem of sludge disposal. Many adsor-

bents have been studied for chromium removal from aqueoussolutions, including ion exchange resin (Tenorio and Espinosa,2001), cellulose (Anirudhan et al., 2009), activated carbon (Fanget al. 2007; Acharya et al., 2009), silica particles (Li et al., 2007)and biomass (Boddu et al., 2003; Park et al., 2008; Kumar andChakraborty, 2009; Singh et al., 2009; Liu et al., 2010).

Ion exchange fibers have been widely studied as adsor-bents for heavy metal ions and other inorganic or organic ionsin recent years (Soldatov et al., 1999; Deng et al., 2003, 2008;Deng and Bai 2004). Compared with traditional adsorbents,these fibrous materials have smaller diameter and larger outerspecific surface area, making them more available for pollut-ant adsorption. Moreover, in a fixed-bed, they can be easilycompressed or loosened and the head loss is low. Deng andBai (2004) investigated the removal of Cr(VI) from syntheticwastewater and the associated sorption mechanisms throughbatch sorption tests. These studies have limitations to predictthe performances of the fibrous materials in practical situa-tions, where continuous reactors replace batch operation andactual industrial wastewaters are treated.

The objective of the current study was to explore the ap-plication of a novel aminated polyacrylonitrile (TE-PAN) fiberas a sorbent for chromium removal from a real electroplatingwastewater. Both batch and column sorption tests were con-ducted to elucidate the sorption kinetic, influence of solutionpH, sorption isotherms, desorption and regeneration perfor-mances, and stability of the fiber column. FTIR and SEM an-alyses were conducted to characterize the TE-PAN fiberbefore and after chromium sorption, thus providing more

*Corresponding author: Institute of Chemistry, Henan Academy ofSciences, Zhengzhou 450002, P.R. China. Phone: 086-371-65511536;Fax: 086-371-65511153; E-mail: wangj12@163.com

ENVIRONMENTAL ENGINEERING SCIENCEVolume 28, Number 8, 2011ª Mary Ann Liebert, Inc.DOI: 10.1089/ees.2010.0428

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information on stability of the sorbent in treatment of chro-mium contaminated water.

Experimental Section

Materials

Commercial polyacrylonitrile fibers (PAN, HuangshanBrand; SINOPEC Anqing Co.) were used as the matrix fortriethylenetetramine (Tianjin Kermel Chemical Reagent Co.,Ltd.) grafting following a procedure modified from Deng et al.(2003, 2008) and Deng and Bai (2004). The weight gain cal-culated from the mass balance of the fibers before and aftermodification was 33.5%. Further analyses from FTIR andacid–base titration demonstrated that a great amount ofamine groups was successfully grafted onto the fiber, attain-ing an anion exchange capacity of 4.95 mmol/g.

The wastewater used in this study was collected from theelectroplating process in a machinery company in Zhengz-hou, China. Analyses from atomic absorption spectroscopy(ZEEnit700; Analytik Jena) and visible spectroscopy (722N;Shanghai Precision & Scientific Instrument Co., Ltd.) demon-strated that the total chromium concentration was 53.5 mg/Land the Cr(VI) concentration was 32.7 mg/L [i.e., 61% of chro-mium existed as Cr(VI)]. Details of the wastewater compositionwere demonstrated in Supplementary Table S1 (SupplementaryData are available online at www.liebertonline.com/ees).

Sample characterization

FTIR spectra of the TE-PAN samples [i.e., (a) original TE-PAN, (b) TE-PAN after chromium sorption, and (c) TE-PANafter NaOH regeneration] were collected on a FTIR spec-trometer (Thermo Nicolet IR 200; Thermo Electron Corp.)working in 4,000–400 cm - 1. Morphologies of the original TE-PAN and the TE-PAN after 15 cycles of sorption-regenerationwere examined by SEM ( JSM-6390LV, Japan).

Batch sorption experiments

Generally, 50 mg of sorbent were mixed with 50 mL ofchromium-containing wastewater in a 100 mL conical flask at25�C. After reaching sorption equilibrium, the supernatant wassampled and filtered, and the filtrate was analyzed by atomicabsorption spectroscopy for total chromium concentration.Sorption capacity (Qe, mg/g) and removal rate (R, %) of totalchromium at equilibriums were calculated using equations

Qe¼(C0�Ce) · V

M(1)

R¼ C0�Ce

C0· 100% (2)

where C0 and Ce (mg/L) are initial and equilibrium concen-trations of total chromium, M (g) is dry mass of sorbent, and V(L) is volume of wastewater. Batch sorption experiments wereconducted at different time intervals (1–60 min), various pHvalues (pH 2–9), and a series of Cr(VI) initial concentrations(10–300 mg/L) to evaluate sorption kinetic, effect of pH onchromium sorption, and sorption isotherms, respectively. Forsorption isotherms, synthetic wastewater was prepared withCr(VI) initial concentrations in the range of 10–300 mg/L.

Column sorption experiments

For the column experiments, a column (inner diameter13 mm, height 60 mm) packed with 1.5 g of TE-PAN was used,

and the empty bed contact time was controlled at 0.5 min forwastewater. Before operation, the bed was rinsed withdeioned (DI) water and then pretreated with 0.1 M HNO3 toconvert the sorbent to its protonated form. After being ex-tensively rinsed with DI water, the electroplating wastewaterinfluents were pumped upward through the column at a flowrate of 10 mL/min. The chromium breakthrough curves wereobtained by plotting the total chromium concentrations of thesamples versus the empty bed volume (BV). The break-through point was designated at the BV when total chromiumconcentration of effluents exceeded 0.5 mg/L.

After breakthrough, 0.1 M NaOH was used for elution ofthe sorbed chromium. The elution curve was obtained byplotting the total chromium concentration versus the BV ofNaOH solution. After regeneration, the column was washedsequentially with DI water, 0.1 M HNO3 and DI water prior to

0 10 20 30 40 50 600

20

40

60

80

100

Raw PAN

TE-PAN

Rem

oval

, %

Time, min

FIG. 1. Sorption kinetics of total chromium on both rawPAN and as-synthesized TE-PAN fiber; wastewater pH = 5.0(fixed by 0.01 M acetate buffer), sorbent dosage = 1 g/L.PAN, polyacrylonitrile.

0 30 60 90 120 150 1800

20

40

60

80

100

120

140

25 °C

40 °C

Qe, m

g/g

Ce, mg/L

FIG. 2. Sorption isotherms of Cr(VI) on TE-PAN fiber un-der 25�C and 40�C; sorbent dosage = 1 g/L, pH = 5.0 (fixed by0.01 M acetate buffer). Synthetic wastewater was preparedfrom Na2Cr2O7 with Cr(VI) initial concentrations 10–300 mg/L. TE-PAN, novel aminated polyacrylonitrile.

586 WANG ET AL.

subsequent sorption. Multiple sorption-regeneration cycleswere conducted to demonstrate the sorbent stability in chro-mium removal from the real wastewater.

Results and Discussions

Sorption kinetics

As shown by the observed removal rate of Cr(VI) ver-sus sorption time (Fig. 1 and Supplementary Fig. S1), thesorption equilibrium was reached very fast and took onlyabout 10 min for the TE-PAN fiber (removal rate > 99%). Incontrast, the raw PAN had no significant uptake for Cr(VI),indicating that grafting of amine groups could substantially

improve its Cr(VI) sorption capacity. Deng and Bai (2004)also reported that the adsorption equilibrium took about 1 hfor Cr(VI) species on fiber material.

Sorption isotherms

As illustrated in Fig. 2, sorption capacities (Qe) of totalchromium increased with increasing equilibrium Cr(VI) con-centrations, and the two sorption isotherms at 25�C and 40�Chad no obvious difference. The estimated maximum uptake(Qm) of chromium from Langmuir fitting was 133 mg/g,much higher than the value (35 mg/g) reported by Deng andBai (2004) in their study of chromium removal with animatedPAN fibers, demonstrating a greater sorption capacity of theas-synthesized TE-PAN.

Effect of pH

Both batch (Fig. 3) and column (Fig. 4) sorption tests wereconducted to elucidate the effect of solution pH on chromiumremoval by the TE-PAN fiber. As illustrated in Fig. 3, theaqueous pH at sorption equilibrium (pHe) had a significantimpact on chromium removal by TE-PAN. The optimal pHe

was in the range of 2–7 where the chromium removal ratekept above 90%. Increasing the solution pHe to 8.7 decreasedthe removal rate to 80.3%. Under the studied conditions, pHe

was always higher than the initial pH (pH0). This proposedthat H + was apt to adsorb on the fiber, with resulting aminegroups protonated which was favorable for sorption of an-ionic Cr(VI) species (such as HCrO4

- , CrO42 - ). At high pH

values, the low solution H + concentration significantly in-hibited the amine protonation and consequently decreasedthe chromium removal rate. Similar pH patterns were re-ported for Cr(VI) sorption by aminated fibers (Deng and Bai,

2000

10

20

30

40

Ce, m

g/L

Cycle 1 Cycle 2

a b

c d

0 100 200

Effluent, BV0 100 200 300 400 500

0

10

20

30

40

Ce, m

g/L

Cycle 1 Cycle 2

0 100 200 300 400 500

Effluent, BV

100

1

2

Cde

, g/L

Cycle 2Cycle 1

10

Effluent, BV10 15 20

0

1

2

3

Cde

, g/L

Cycle 1 Cycle 2

0 100

0 5 0 5 0 5 0 5 10 15 20

Effluent, BV

FIG. 4. Breakthrough curves for chromium sorption by TE-PAN at pH 8.0 (a) and pH 5.0 (b) and their correspondingdesorption curves by 0.1 M NaOH (c, d). Sorption conditions: column inner diameter = 13 mm, height = 60 mm, TE-PANmass = 1.5 g, wastewater flow rate = 10 mL/min. Desorption conditions: NaOH flow rate = 2 mL/min.

2 3 4 5 6 7 8 90

20

40

60

80

100

pH0 Removal, %

Rem

oval

, %

pHe

2

3

4

5

6

7

8

9

pH0

FIG. 3. Impact of solution pHe (pH at equilibrium) onchromium removal by TE-APN fiber and the relationshipbetween pH0 (initial pH) and pHe; TE-PAN dosage = 1 g/L.

CHROMIUM REMOVAL FROM WASTEWATER BY TE-PAN FIBER 587

2004), anion-exchange resins (Shi et al., 2009), amine-modifiedcelluloses (Anirudhan et al., 2009), etc. The high removal rate( > 90%) also suggested that both Cr(VI) and Cr(III) could beefficiently removed by TE-PAN.

To further evaluate the pH impact, two pH values werestudied in the column sorption experiments. For each test, twocycles were conducted. Breakthrough curves from thesestudies (Fig. 4a, b) demonstrated that at pH 8.0, about 220BVs of wastewater could be efficiently treated in each cycle.However, when pH of the wastewater was adjusted to 5.0,this volume was increased to about 360–380 BVs. This resultwas similar with the batch tests, demonstrating that thewastewater pH had a significant effect on chromium removal,and a weak acid condition was always beneficial. Moreover,there was no obvious difference for the volume of wastewaterbetween the two cycles.

Regeneration tests

0.1 M NaOH was used to regenerate the sorbent for itsfurther sorption. Results from the desorption curves (Fig.4c, d) showed that about 14 BVs of NaOH were needed fornearly complete desorption of the chromium sorbed on TE-PAN at pH 8.0. While the volume increased to about 20 BVsfor the TE-PAN exhausted at pH 5.0, due to more chromiumto be eluted. In addition, the eluted chromium was highlyconcentrated within 4–6 BVs (e.g., from 6th to 12th BV for testsat pH 5.0), enabling possible recovery or reuse of chromium.

Further studies demonstrated that after repeating thesorption-regeneration process for 15 cycles, only 5%–6%volume loss of the treated wastewater was observed. This

indicated that the synthesized TE-PAN was very efficient andstable in chromium-containing wastewater treatment.

FTIR and SEM

As indicated in Fig. 5 (top), the synthesized TE-PAN hadseveral characteristic peaks. Most of them were derived fromthe original PAN fiber which could be assigned as follows:broad band ranging from 3,100 to 3,700 cm - 1 (stretching vi-bration of OH and NH groups), 2,937, 2,875, 1,452 cm - 1 (C-Hstretching and blending), 2,243 cm - 1 (ChN stretching in ni-trile group), 1,072 cm - 1 (C-O stretching), and 538 cm - 1 (C = Otwisting). After triethylenetetramine grafting, a new broadband appeared at 1,550–1,680 cm - 1 (C = O group in amideand N-H group in amine), demonstrating that amine has beensuccessfully introduced in PAN fiber structure. After chro-mium sorption, two new bands appeared at 899 and 784 cm -

1, which disappeared after NaOH desorption, demonstratingthat these bands resulted from the sorbed chromium (Li et al.,2007; Toral et al., 2009; Sun et al., 2010). The complete disap-pearance of these two bands also suggested that NaOH coulddesorb chromium completely.

SEM patterns (Fig. 5, bottom) showed that the TE-PANfiber had a very smooth surface. After 15 cycles of sorption-regeneration, the fiber still kept its good fibrous morphology,indicating that the TE-PAN fiber was stable during the re-peated chromium sorption and base regeneration processes.

Conclusions

Grafting of amine groups on PAN fiber using triethylene-tetramine could substantially improve its chromium removal

FIG. 5. FTIR (top) and SEMphotographs (bottom) for (a)the as-synthesized TE-PANfiber, (b) TE-PAN after chro-mium sorption (photo notshown), and (c) TE-PAN after15 sorption-regenerationcycles.

588 WANG ET AL.

performance. The sorption equilibrium was achieved within10 min. Temperature did not have any obvious effect on thesorption isotherm and the estimated maximum uptake ofchromium was particularly high (133 mg/g). Efficient re-moval of total chromium was observed over a wide pH range(2–7). Results from column experiments demonstrated that1.5 g fiber could treat 220 BVs of wastewater at pH 8.0 or 380–390 BVs at pH 5.0. After regeneration by 0.1 M NaOH, thefiber could be repeatedly used for chromium removal evenafter 15 sorption-regeneration cycles. Findings of this studyhave significant implications for treatment of chromiumcontaminated waters.

Acknowledgment

Funding for this research was provided by the Major PublicWelfare Project in Henan province (grant no. 81100912100).

Author Disclosure Statement

The authors declare that no competing financial conflictsexist.

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CHROMIUM REMOVAL FROM WASTEWATER BY TE-PAN FIBER 589

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