Journal ofMaterials Chemistry A

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View Article OnlineView Journal | View Issue

Bacteria cell tem

aBeijing Key Laboratory for Source Control T

Environmental Science and Engineering, Be

China. E-mail: qiubin2015@bjfu.edu.cnbIntegrated Composites Laboratory (ICL), D

Engineering, University of Tennessee, 1512

E-mail: zguo10@utk.educNational Engineering Research Center for A

Zhengzhou University, Zhengzhou 450002, C

† Electronic supplementary informa10.1039/c8ta06571c

Cite this: J. Mater. Chem. A, 2018, 6,16824

Received 9th July 2018Accepted 9th August 2018

DOI: 10.1039/c8ta06571c

rsc.li/materials-a

16824 | J. Mater. Chem. A, 2018, 6, 16

plated porous polyanilinefacilitated detoxification and recovery ofhexavalent chromium†

Kedong Gong,a Siyuan Guo,a Yue Zhao,a Qian Hu,a Hu Liu, bc Dezhi Sun,a Min Li,a

Bin Qiu*a and Zhanhu Guo *b

In this study, bacteria cells served as a novel green pore-forming agent

and template to fabricate porous polyaniline (Bac@PANI) by in situ

oxidative polymerization. The achieved specific surface area of 51.2m2

g�1 for Bac@PANI at a bacteria dosage ofOD600¼ 0.536was far higher

than that of the pristine PANI (27.5 m2 g�1). The Bac@PANI presented

an emeraldine form, hydrophilicity and decreased isoelectric point

(4.2) after being modified by bacteria. All these properties contributed

to an enhanced removal performance of hexavalent chromium (Cr(VI)),

with a high removal rate of 0.5 mg (min�1$g�1) and capacity of

835.06 mg g�1 of Bac@PANI. Efficient removal was achieved in both

acidic and neutral solutions due to its high H+ storage capacity. More

than 90% of Cr(VI) can be reduced to trivalent chromium (Cr(III)) by

oxidation of amide groups in both acidic and neutral solutions. These

Cr(III) ions can be adsorbed by Bac@PANI simultaneously, resulting

from the reversal of negative surface charge after treatment of Cr(VI);

then Bac@PANI can be easily recovered in an acidic solution.

1. Introduction

Cr(VI) is classied as one of the most toxic heavy metals forhuman health.1,2 Its existing anion forms in aqueous environ-ment, e.g., Cr2O7

2�, CrO42� and HCrO4

�, lead to its strongmobility.3,4 Reducing Cr(VI) anions to Cr(III) ions is recom-mended as an efficient protocol for purifying Cr(VI) contami-nants.5–7 Cr(III) has low toxicity, exists as Cr3+ ions in a solution,and can be easily immobilized. This reduction can be achievedby both biological processes and chemical reactions.8–12

echnology of Water Pollution, College of

ijing Forestry University, Beijing, 100083

epartment of Chemical and Biomolecular

Middle Dr, Knoxville, TN 37996, USA.

dvanced Polymer Processing Technology,

hina

tion (ESI) available. See DOI:

824–16832

Among the increasingly studied functional materials forrapid reduction of Cr(VI), PANI, as a conductive polymer con-taining both reductive amine groups (–NH–) and oxidativeimine groups (–N]) simultaneously, has attracted considerableinterests for reducing Cr(VI).13,14 PANI has three oxidation statesbased on the –NH–/–N] ratio: emeraldine (EB), pernigraniline(PB) and leucoemeraldine (LB).15 Electrons can be donatedwhen PANI is transferred from LB or EB state to PB state.16 Thisremarkable electron donation ability is the most importantreason for its application in Cr(VI) remediation. Cr(VI) waste-water of 1 mg L�1 can be completely removed within 15 minutesby PANI.17 In addition, PANI has a high resistance against acidicconditions, which coincides with the fact that the Cr(VI) is easilyreduced in acidic conditions.16–18 All these ndings imply thatPANI is a good candidate for the remediation of Cr(VI)contamination.

PANI in the powder/lm morphology has been widelysynthesized by the chemical oxidation polymerizationmethod.19,20 However, these PANI samples had low removalcapacity for Cr(VI) despite its rapid kinetics. For example,�15 and35 mg Cr(VI) was reduced by 1 g PANI and cellulose modiedPANI, respectively.8 The poor porosity of PANI in such forms wasfound to be responsible for this low capacity.21 Thus, increasingits porosity seems to be important, which can increase theaccessibility of the –NH– groups in PANI for Cr(VI) reduction.

Cr(VI) is easily reduced in acidic conditions (pH < 3) than inthe basic or neutral solutions, as H+ is an important reactantparticipating in the Cr(VI) reduction reaction.22,23 Thus, adjust-ing the solution pH to acidic is always needed in order to ach-ieve a fast reaction, which limits the application of manysynthesized materials on treating Cr(VI)-containing wastewater.Fortunately, the polar –N] groups in PANI can be easilyprotonated by H+, forming acid doped PANI.24 Therefore, PANIis supposed to have the ability to capture H+ ions from thesolution and store them on the polymer chains. These dopedchains can provide H+ required for Cr(VI) reduction, thus facil-itating the Cr(VI) reduction in versatile conditions, which maywiden the applications of PANI based materials.

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As documented, the surface of PANI is positively chargedwhen pH < 10.25 Moreover, Cr(VI) is present as negativelycharged anions, which can be easily adsorbed onto positivelycharged PANI. However, aer the reaction, the obtainedreduction product Cr(III) (Cr3+) is positively charged. Thus, thiselectrical charge transition of chromium in the reductionprocess prevented further adsorption of Cr3+ due to the repul-sion between PANI and Cr3+. For decreasing the isoelectric point(pI), PANI has been modied with many materials, such asmagnetite (Fe3O4) nanoparticles,18,26 sawdust,27 bers,28

biomass8 and carbon materials.8 However, these modicationscannot meet the requirements of low pI, high porosity and aciddoping simultaneously. Thus, a new method is needed tosynthesize PANI that is much more suitable for Cr(VI)remediation.

To achieve this goal, bacterial cells appear to be a goodcandidate to modify PANI. A bacterial cell is composed of a cellwall and an aqueous cytoplasm within the cell wall. The cell wallmainly contains abundant –NH2 and –OH groups,29 which canserve as active sites for PANI growth. Furthermore, bacterialcells can be easily broken, and cytoplasm will leak when the cellis dried, which causes the formation of a porous PANI.30 Addi-tionally, lower pI of the bacterial cell (pH¼�3)31 could decreaseoverall pI of the as-synthesized porous Bac@PANI.

In this study, a bacterial cell was used as a green poroustemplate to prepare porous PANI by surface initiated polymer-ization. This paper aims at elucidating the effects of bacteria onthe properties of porous PANI, such as porosity, pI, and capacityof H+ capture and storage. Performance of the porous PANI forCr(VI) reduction and further adsorption were also evaluated.Moreover, the mechanisms of enhanced performance bybacteria templated PANI were discussed.

2. Materials and methods2.1 Materials

Simulated contaminants of potassium dichromate (K2Cr2O7)and chromogenic agent 1,5-diphenylcarbazide were purchasedfrom Beijing Chemical Works, China for batch assays of Cr(VI)removal. Aniline monomers, p-toluene sulfonic acid (C7H8O3S),ammonium persulfate (APS) and acetone were purchased fromXiya Chemical Company Limited, China for PANI synthesis. Allreagents were used as received. Bacterial strain used in thisstudy was isolated from activated sludge in our lab andexhibited a spherical shape. It was incubated in a nutrientsolution at 35 �C for 24 h. The solution was centrifuged andwashed by normal saline 3 times to remove the nutrients.Bacteria cells were suspended in water to obtain an OD at 1.362(OD is the optical density value of bacteria solution measured at600 nm, which represents the amount of cells in the solution).This bacteria solution was used as stock solution for thesynthesis of porous PANI.

2.2 Fabrication of porous PANI

Porous PANI was fabricated using a novel template methodcombined with the reported surface oxidation polymerization.5

This journal is © The Royal Society of Chemistry 2018

Specically, aniline (3.2 mL) was dispersed in 200 mL bacteriasolution with a desired OD and was cooled to 0–3 �C understirring at 600 rpm for 30 minutes. The OD value of bacteriasolution was adjusted by diluting the stock bacteria solution.Then, a precooled 100 mL solution of 36.0 mM APS and15.0 mM PTSA was added dropwise to the aniline-bacteriasolution at one milliliter per minute in an ice bath for poly-merization. The obtained deep dark green product was washedwith acetone and deionized water, in sequence, to remove anyoligomers and for further cleaning. Finally, the Bac@PANIcomposites were dried at 80 �C for 12 h and preserved untilapplication. As porous PANI was synthesized under acidic andstrongly oxidative conditions, the bacteria cells were destroyedand lost their activity. Thus, the bacteria template is safe forfurther synthesis of porous PANI.

2.3 Cr(VI) removal and Cr(III) recovery

As the key inuential factors for Cr(VI) removal performance ofBac@PANI composites, the kinetics, pH and initial concentra-tion, were studied by batch assays. Briey, for a kinetic study,10.0 mg porous Bac@PANI was employed to remove 5.0 mg L�1

Cr(VI) from wastewater (25.0 mL) for different removal timeintervals at room temperature. pH investigation was conductedat different pH values (1.0–11.0) adjusted with hydrochloric acidand sodium hydroxide for 30 minutes (based on kinetic exper-iment). For studying the effect of initial concentration of Cr(VI)while maintaining other parameters constant, the concentra-tion was adjusted from 1.0 to 1500.0 mg L�1.

For removal performance analysis, the concentration ofCr(VI) was measured using a spectrophotometric method.18

Removal percentage (R%) and removal capacity (Q, mg g�1) ofCr(VI) were obtained by eqn (1) and (2), respectively:

R% ¼ C0 � Ce

C0

� 100% (1)

Q ¼ ðC0 � CeÞVm

(2)

where C0 and Ce (mg L�1) represent initial and equilibriumconcentration (mg L�1), respectively; V (L) and m (g) are volumeof the solvent solution and dry mass of the Bac@PANI,respectively.

For the recovery of Cr(III), the used Bac@PANI was trans-ferred to acidic solutions with pH ranging from 1 to 5. Then, themixtures were stirred at 600 rpm for 12 hours to fully releaseCr(III) ions. The concentration of Cr(III) ions released to theacidic solution was measured using inductively coupled plasmamass spectrometry.

2.4 Characterizations of Bac@PANI

Micromorphology image of Bac@PANI was obtained usinga scanning electron microscope (JEOL, JSM-6301F). Fouriertransform infra-red (FT-IR) spectra were recorded using a FTIRspectrometer (Bruker, Vertex-70 with an ATR accessory). Porestructure and specic surface area (SBET) were recorded bya specic surface and micropore analyzer (Builder, SSA-7000).

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Pore distribution was measured using the current internationalBJH method. Elemental species of Bac@PANI and chromiumwere detected by X-ray photoelectron spectroscopy (XPS)(ESCALAB, 250Xi) with an Al Ka and Mg/Al dual anode lightsource. Isoelectric points of pristine polyaniline and Bac@PANIwere detected by a Malvern zeta potentiometer (Malvern, Zeta-sizer Nano ZEN2600). Total chromium concentration in eachbatch assay was analyzed using inductively coupled plasmamass spectrometry (ICP-MS, Agilent, 7900). Cr(III) concentrationwas calculated as the difference between total Cr and Cr(VI)concentrations.

3. Results and discussion3.1 Hexavalent chromium removal

A rapid removal process was achieved by the bacteria-templatedporous PANI (Fig. 1A). In total, 1000 mg L�1 Cr(VI) was removedby Bac@PANI (dosage of 500 mg L�1) in 10 minutes, which wasmuch faster than raw PANI without using bacteria template (1h). This result indicates that the bacteria template signicantlyenhanced Cr(VI) removal by PANI. It was noted that a fasterremoval was achieved by Bac@PANI using bacteria templatewith a dosage at OD600 ¼ 0.536. Thus, OD600 ¼ 0.536 of bacteriatemplate was demonstrated as the optimized dosage forsynthesizing porous PANI, which was suitable for remediationof chromium pollution. Moreover, increased initial concentra-tion of Cr(VI) would enhance the removal capacity of porousPANI (Fig. 1B). Nevertheless, exorbitant initial concentrations(>1200 mg L�1) have almost no impact on the removal capacity.

Fig. 1 (A) Cr(VI) removal rate of porous PANIs using different template d

Table 1 Cr(VI) removal performance by PANIs modified by various subst

Materials pHRemoval capacity(mg g�1)

HCl/PANI 4.0 180.0DI/PANI 4.0 92.0PAN/PANI 2.0 53.4H-TNB/PANI 2.0 156.9MC/PANI 2.0 172.33MnO2/PANI nanosheet 2.0 263.2PTSA/PANI 1.0 184.0Bac/PANI 1.0 835.06

16826 | J. Mater. Chem. A, 2018, 6, 16824–16832

Accordingly, 835.06 mg g�1 was calculated to be the maximumremoval capacity of porous PANI, indicating a 3.5-fold increaseas compared with that of pristine PANI doped with hydrochloricacid (�180 mg g�1). Moreover, compared with PANI modiedwith other substrates (Table 1), Bac@PANI synthesized in thisstudy has a higher removal capacity.

3.2 Characterization of Bac@PANI

The used bacterial templates exhibit a spherical form witha particle size of about 50 nm (Fig. S1A†). As shown in Fig. 2A,aniline was attached evenly on the surface of bacterial cell, andthen polymerized and grown around the spherical bacteria dueto surface energy of the cells,32 thus forming small core–shellbacteria@PANI. Then, the small PANI pellets were stacked intoa large sphere during further polymerization of aniline.Following this, the bacterial cell was broken and cytoplasmicsubstances were released out of the cell when the composite wasdried, forming the porous structure of PANI.33 Bac@PANI alsopresents as spherical particles with an average diameter of�400 nm (Fig. 2B and C), which was different from the pristinePANI nanobers (Fig. S1B†). Spherical shape and protein-richenvironment of the bacteria contributed to the sphericalmorphology and the surface roughness of the as-synthesizedPANI, respectively.

BET surface area (SBET) and pore structure were inuencedby the bacterial template. In detail, SBET initially increased withincrease in dosage of bacterial template, and then decreasedwhen the bacteria dosage became higher than OD600 ¼ 0.536(Fig. 3A). A maximum SBET of �51 m2 g�1 was obtained when

osages, and (B) Cr(VI) removal capacity of porous PANI.

ances

Modied substance Reference

Hydrochloric acid (HCl) 43Deionized water (DI) 43Polyacrylonitrile (PAN) 38Hydrogen-titanate (H-TNB) 44Magnetic porous carbon (MC) 45MnO2 46P-Toluene sulfonic acid (PTSA) This studyBacterial (Bac) This study

This journal is © The Royal Society of Chemistry 2018

Fig. 2 (A) Proposed synthesis pathway of porous PANI using bacteria as the template, SEM images of porous PANIs with template dosages of (B)OD600 ¼ 0.276 and (C) OD600 ¼ 0.833.

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the bacteria dosage was OD600 ¼ 0.536. This is about twotimes higher than SBET of pristine PANI (27.5 m2 g�1). More-over, the pore diameter increased when the bacteria dosagewas higher than OD600 of 0.536 (Fig. S2†). During polymeri-zation, the bacteria cell was damaged, releasing gelatinouscytoplasm, which formed the porous structure of Bac@PANIupon drying.21 However, excess cytoplasm blocked the poresof PANI, leading to a decrease in SBET. The porous structureexposes more amine groups, thus increasing the availableactive sites for Cr(VI) reduction (Fig. 3B),34 which determinedthe rapid Cr(VI) and high removal capacity of the Bac@PANIsynthesized with suitable dosage of bacteria templates(Fig. 1A).

Fig. 3 (A) BET surface area of porous PANI using different dosages of bacporous PANI.

This journal is © The Royal Society of Chemistry 2018

Hydrophilic property of PANI is important for the masstransfer between solution and solid PANI.35 Pristine PANI pre-sented a hydrophilic surface with a contact angle of 68.6�

(Fig. 4A). Bacteria cell mainly contains protein, which isa hydrophobic substance.36 Hence, the hydrophilicity of PANIdecreased with an increase in the dosage of bacterial templates.However, the bacterial templates with low dosage (OD600 <0.536) have no signicant impact on the hydrophilicity ofporous PANI. Identical result was obtained as the removal rateof Cr(VI) decreased signicantly for the porous PANI witha template dosage higher than OD600 ¼ 0.536 (Fig. 1A). Thus,OD600 ¼ 0.536 was determined as the optimal bacteria templatedosage for fabrication of Bac@PANI. As shown in Fig. 4B, upon

teria templates; (B) schematic diagram of the exposed amino groups in

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Fig. 4 (A) Contact angles, (B) zeta potentials and (C) FT-IR spectra of pristine and bacteria-templated porous PANIs.

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increasing the solution pH, the surface charge of both pristineand bacteria-templated porous PANI (template dosage of OD600

¼ 0.536) gradually decreased. pI of the bacteria-templatedporous PANI was observed at pH ¼ �4.2, which was reducedby half compared to that of pristine PANI (�8.0). Bacterial cellshave a low pI at pH ¼ �3;31 thus, residual bacteria materialcontributed to the signicantly decreased pI of the as-synthesized porous PANI. Bac@PANI tends to attract negativesubstances in acidic solutions (pH < 4.2). In addition, thehydrophilic and positively charged properties facilitate fastCr(VI) anion adsorption on the porous PANI surface, which isa vital step before Cr(VI) reduction.

FTIR spectrum of Bac@PANI (Fig. 4C) indicated theappearance of N]Q]N (1566 cm�1) and N–B–N (1483 cm�1),where Q and B were quinoid ring and benzenoid ring,8,37

respectively. The pristine and bacteria-templated porous PANIhave almost the same characteristic peaks, while the charac-teristic peaks of the porous PANI shied to higher wave-numbers compared to those of the pristine PANI. This indicatedthat functional groups of bacterial cells participated in thepolymerization reaction. The peaks at 1566 and 1483 cm�1

represent the amine (–NH–) and imine (–N]) groups in PANI,respectively,6 while their intensity ratio indicated the presenceof EB form in both pristine PANI and bacteria-templated PANI.The –NH– group has high electron donating ability, and isconsidered as a good electron donor for Cr(VI) reduction.34 Theabundant –NH– groups (electron donor) and large surface areadetermined the rapid reduction and high Cr(VI) removalcapacity of the bacteria-templated porous PANI.

16828 | J. Mater. Chem. A, 2018, 6, 16824–16832

3.3 Wide pH range

As documented, H+ plays a vital role in this reduction (eqn (3)–(6)).38 Fig. 5A shows removal performance of Cr(VI) by thebacteria-templated porous PANI at different initial pH valuesfrom 1.0 to 13.0. It is noteworthy that although Cr(VI) reductionis highly pH dependent,42 more than 90% Cr(VI) can also bereduced by porous PANI at initial pH as high as 9. For pristinePANI, 90% of removal percentage could be achieved only whenthe pH was lower than 3 (Fig. 4A). This indicated that thebacteria templates widened the application of PANI for Cr(VI)reduction. This as-synthesized porous PANI can reduce Cr(VI)under a wider pH range than a traditional modication by othersubstrates (1.0 to 3.0), such as Fe3O4,18 cellulose,8 and carbonber.17

Cr2O72� + 14H+ + 6e� / 2Cr3+ + 7H2O (3)

CrO42� + 8H+ + 3e� / Cr3+ + 4H2O (4)

H2CrO4 + 6H+ + 3e� / Cr3+ + 4H2O (5)

HCrO4� + 7H+ + 3e� / Cr3+ + 4H2O (6)

It is clear that H+ is a crucial component for Cr(VI) reduc-tion.14 The bacteria templates have been found to improve theH+ storage capacity of PANI. Fig. 5B shows that pH increasedsignicantly in 3 minutes when PANI was added to the acidicsolutions, which implies that PANI could adsorb H+ quickly,

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Fig. 5 (A) Cr(VI) removal percentage of Bac@PANI at different initial pH; (B) H+ adsorption of porous PANI using different template dosages; (C) N1s XPS spectrum of porous PANI; (D) proposed H+ storage by synthesized porous PANI.

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forming an acidic micro-environment for Cr(VI) reduction. H+

storage capacity increased signicantly with an increase indosage of bacteria cells, which indicated the capture of H+ bythe PANI from the solution and storage of H+ on its chains dueto the electronegativity of –N] and –NH–. It must also bementioned that both the maximum adsorption rate and H+

storage capacity were obtained when the dosage of bacteriatemplates was controlled at OD600 ¼ 0.536 (Fig. S3†). Four peaksobserved in the N 1s XPS spectrum (Fig. 5C) were ascribed to–N] (399.0 eV), –NH– (399.6 eV) and N+ in doped imine andamine group (400.3 and 401.4 eV, respectively).39,40 This indi-cated that H+ was captured and stored on the porous PANI(Fig. 5D). The H+ stored on the –N] in PANI can provide anacidic microenvironment for Cr(VI) reduction. Moreover, storedH+ can also release into the environment (Fig. S4†), whichmakes it available for Cr(VI) reduction. As reported, chromicions primarily exist in the form of HCrO4

� and H2CrO4 in acidicsolutions (pH < 6.8). Moreover, HCrO4

� holds a higher redoxpotential (1.33 eV),41 which will contribute to easier reduction.

3.4 Removal mechanisms of Cr(VI)

Bacteria-templated porous PANI has been proved as a promisedmaterial for chromium removal due to its high porosity andabundant –NH– groups. The removal mechanisms of Cr(VI) wererevealed using FT-IR and XPS techniques. Fig. S5A† shows thatthe intensity ratio of imine group (1580 cm�1) to amine group(1482 cm�1) slightly increased aer treatment of Cr(VI), indi-cating that a fraction of –NH– groups in the composite have

This journal is © The Royal Society of Chemistry 2018

been oxidized to –N].21 XPS spectra of the porous PANI(Fig. S5B†) show the binding energy peak positions at 576.0 and585.2 eV, ascribed to the trivalent chromium (Cr(III)).42 More-over, no Cr(VI) was detected on the porous PANI, implying thatall the adsorbed Cr(VI) was completely reduced. The signicantlyincreased ratio of peak intensities of –N] (397.6 eV) and –NH–

(399.0 eV) in N 1s spectra (Fig. S5C†) of the treated porous PANIalso conrmed the reductive effect of –NH– in PANI in theremoval of Cr(VI). Thus, the removal mechanism for Cr(VI) wasproposed, as shown in Scheme 1. First, due to availability ofa large number of binding sites (–N]) in the porous structure,the H+ in the solution was captured and stored on the chains ofthe porous PANI, forming acidic microenvironment due to theprotonation of –N] groups. Then, Cr(VI) anions were adsorbedon surface of Bac@PANI via electrostatic attraction. The porousstructure provided suitable conditions for Cr(VI) reduction,including an acidic microenvironment and abundant electrondonors (–NH–). Finally, Cr(VI) was puried to Cr3+ by oxidizing–NH– to –N].

3.5 Cr(III) recovery

As discussed above, Cr(VI) anions were completely reduced toCr3+. The further removal of Cr3+ was oen difficult by PANI dueto the charge transition of chromium. As shown in Fig. 6A, onlyhalf of Cr3+ was adsorbed by porous PANI, indicating that�50%of chromium has not been safely disposed when the solutionpH is 1.0. The electrostatic repulsion between PANI and Cr3+

was reduced on increasing the pH, as evidenced by the

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Fig. 6 (A) Distribution of Cr(III) after treatment by Bac@PANI; (B) Cr(III) recovery efficiency with different initial pH of recovery solution.

Scheme 1 Proposed mechanism involved in Cr(VI) removal by the porous PANI.

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signicantly decreased Cr3+ concentration. More than 90% ofCr(III) can be removed by the porous PANI at pH 3.0, while only�40% of Cr3+ was removed by the pristine PANI at pH 5.0(Fig. 6B). It was demonstrated that the bacteria-templatedporous PANI better adsorbs Cr(III) compared to pristine PANI.Cr(III) exists as positively charged ions in the acidic solution,41

and electrostatic repulsion between bacteria-templated porousPANI and Cr(III) led to high Cr3+ aqueous concentration.

Moreover, when the initial solution pH was 3.0, the nal pHreached �4.5 aer the reaction due to the consumption of H+

during Cr(VI) reduction. When the pH of the wastewater excee-ded pI of Bac@PANI (pH ¼ 4.2), its surface charge was reversedto negative. This property was consistent with the chargereversal of chromium (from Cr(VI) anions to Cr3+). Thus, thesurface charge of Bac@PANI facilitated the adsorption of Cr(VI)anions, assuring their reduction and further Cr3+ ions removalaer the reduction reaction by the electrostatic attraction(Scheme 1). Then, the adsorbed Cr3+ on the surface of bacteria-

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templated porous PANI can be desorbed easily in an acidicsolution (pH # 3) (Fig. 6B), regenerating the Bac@PANI.Recovery efficiency increased with the decrease in pH, andalmost 99% Cr(III) was recovered when the solution pH wasmaintained at 1.

4. Conclusions

A bacteria cell induced porous structured PANI was synthesizedusing a chemical polymerization method. The porous PANIexhibits a 2-fold increase in SBET compared to the pristine PANI.The porous PANI synthesized with the bacteria cell as templatesdemonstrated excellent Cr(VI) removal performance with a fastremoval rate of 0.5 mg (min�1 g�1) and maximum removalcapacity of 835.06 mg g�1. The formed porous structure,decreased pI, and increased H+ storage capacity were respon-sible for the enhanced chromium removal performance. Cr(VI)reduction and adsorption of Cr(III) can be achieved

This journal is © The Royal Society of Chemistry 2018

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simultaneously by the bacteria templated PANI. Moreover, theadsorbed Cr(III) can be easily recovered in an acidic solution.Thus, porous PANI was demonstrated as an efficient materialfor Cr(VI) remediation and Cr(III) recovery. Furthermore, withunique electrical conductivity, low density and high specicsurface area, these PANI nanostructures can have other poten-tial applications, such as those in sensors, anticorrosion coat-ings, multifunctional materials, and energy storage.47–53

Conflicts of interest

There are no conicts to declare.

Acknowledgements

This project was nancially supported by the FundamentalResearch Funds for the Central Universities (No. 2018ZY09) andthe National Natural Science Foundation of China (No.51608037).

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