JOURNAL OF ENVIRONMENTAL SCIENCES 29 (2015) Available online at ScienceDirect

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1 Aville online t SieneDiret Flower-, wire-, n sheet-like MnO 2 -eposite itomites: Highly effiient sorents for the removl of Cr(VI) Yuheng Du 1,, Liping Wng 1, Jinshu Wng 1,, Gungwei Zheng 1, Junshu Wu 1, Hongxing Di 2, 1. Key L of Avne Funtionl Mterils, Ministry of Eution, College of Mterils Siene n Engineering, Beijing University of Tehnology, Beijing , Chin. E-mil: yhengu@jut.eu.n 2. Lortory of Ctlysis Chemistry n Nnosiene, Deprtment of Chemistry n Chemil Engineering, College of Environmentl n Energy Engineering, Beijing University of Tehnology, Beijing , Chin ARTICLE INFO Artile history: Reeive 3 April 2014 Revise 4 June 2014 Aepte 13 June 2014 Aville online 23 Deemer 2014 Keywors: Morphologil effet MnO 2 -eposite itomite Cr(VI) removl Asorption effiieny ABSTRACT Flower-, wire-, n sheet-like MnO 2 -eposite itomites hve een prepre using hyrotherml metho with Mn(A) 2,KMnO 4 n/or MnSO 4 s Mn soure n itomite s support. Physil properties of the mterils were hrterize y mens of numerous nlytil tehniques, n their ehviors in the sorption of hromium(vi) were evlute. It is shown tht the MnO 2 -eposite itomite smples with ifferent morphologies possesse high surfe res n unnt surfe hyroxyl groups (espeilly the wire-like MnO 2 /itomite smple). The wire-like MnO 2 /itomite smple showe the est performne in the removl of Cr(VI), giving the mximum Cr(VI) sorption pity of 101 mg/g The Reserh Center for Eo-Environmentl Sienes, Chinese Aemy of Sienes. Pulishe y Elsevier B.V. Introution Cr(VI) n Cr(III) ions re toxi n n le to mutgeniity n potentil rinogeniity ue to their long-time retention in the humn oy (Güly n Mehmet, 2008). The pollutnts re emitte from the inustril tivities of eletroplting, pigments, metl lening, lether proessing, n mining (Metin et l., 2008). It is well known tht toxi hromium minly exists in two oxition sttes (i.e., Cr(VI) or Cr(III)) (Mrtell n Hnok, 1996), of whih the hexvlent hromium shows muh higher toxiity thn the trivlent hromium euse of the stle vlene of the former (Frg et l., 2006). Cr(VI) is rinogeni n mutgeni for living orgnisms, esily sore y the humn oy, n umulte in the oy (Pgn et l., 2011). Therefore, it is highly esirle to tret Cr(VI)-ontining wstewter. Up to now, numerous methos suh s preipittion (Almei n Boventur, 1997), ion exhnge (Gzr n Dhhi, 2001), memrne seprtion (Kozlowski n Wlkowik, 2002), reution (Willifor et l., 2002), iosorption (Mohnty et l., 2006), n sorption (Mohnty et l., 2005) hve een utilize to remove Cr(VI) from wstewter. Among these methos, sorption (espeilly hemisorption) is onsiere to e prtiulrly fesile n effetive (Muriye n Irfn, 2007). In the pst yers, numer of works hve een fouse on the removl of Cr(VI) with sorents suh s surftnt-moifie tivte ron (Choi et l., 2009), mroporous polystyrene (Cui et l., 2013), tmrin sees (Gupt n Bu, 2009), n ette/sili omposite nnofiers (Th et l., 2012). However, these sorents re not effetive for the removl of Cr(VI). Hene, n lterntive n low-ost sorent nees to e evelope to purify Cr(VI)-ontining wstewter. Corresponing uthors. E-mils: yhengu@jut.eu.n (Yuheng Du), wngjsh@jut.eu.n (Jinshu Wng), hxi@jut.eu.n (Hongxing Di) / 2014 The Reserh Center for Eo-Environmentl Sienes, Chinese Aemy of Sienes. Pulishe y Elsevier B.V.

2 72 JOURNAL OF ENVIRONMENTAL SCIENCES 29 (2015) Ditomite-se mterils re unnt from nturl soures, lrge in surfe re, stle in hemil properties, n goo in sorption ility (Al-Ghouti n Al-Degs, 2011). In reent yers, Al-Degs et l. (2001), Khrisheh et l. (2004), Al-Ghouti et l. (2004), n Li et l. (2009) prepre mngnese oxie-loe itomite sorents, n oserve tht these mterils showe goo sorption pities for the removl of P(II), Cu(II), C(II), Zn(II), n Cr(III). Ditomite, however, oes not exhiit effetive sorption for the negtively hrge hevy metl oxie ions (e.g., HCrO 4,CrO 4 2,Cr 2 O 7 2,H 2 AsO 3,HAsO 3 2,AsO 3 3,H 2 AsO 4,HAsO 4 2, n AsO 4 3 ) in wter, n gve Cr(VI) removl effiieny of only 65% (Sungworwongpn n Pengpreh, 2011). Hene, it is neessry to moify itomite so tht its sorption pity of negtively hrge hevy metl oxies n e enhne sustntilly. In review rtile, Dnil e Nmor et l. (2012) summrize the extrtion results of hevy metls from wter using itomite n moifie itomite smples, in whih most of the moifie itomites were ompose of mehnil mixtures of itomite n moifiers (e.g., slts of iron, mngnese, n luminum, n orgni mtter). To the est of our knowlege, the use of orere nnostruturl metl oxie-moifie itomites for the removl of hevy metl ions from wter hs een rrely reporte. In this stuy, we prepre itomite-supporte MnO 2 with vrious morphologies, n exmine the ehvior in Cr(VI) sorption. It ws foun tht the MnO 2 -eposite itomite mterils possesse exellent Cr(VI) sorption performne. 1. Experimentl 1.1. Preprtion of MnO 2 -eposite itomites with ifferent morphologies Inustril-gre itomite (Jilin Chngi, Chin) ws use s reeive without further purifition. Anlytil gre mngnese sulfte monohyrte (MnSO 4 H 2 O), mngnese ette tetrhyrte (Mn(A) 2 4H 2 O), potssium permngnte (KMnO 4 ), potssium hyroxie (KOH), n mmonium persulfte ((NH 4 ) 2 S 2 O 8 ) were purhse from Beijing Chemil Regent Compny. The MnO 2 -ote itomite smples with ifferent morphologies were prepre using hyrotherml metho. The etile proeures re s follows: (1) g of Mn(A) 2 ws issolve in 20 ml of eionize wter, n the esire mount (Mn(A) 2 /KMnO 4 molr rtio = 3:2) of KMnO 4 ws issolve in 20 ml of eionize wter. The Mn(A) 2 queous solution ws e ropwise to suspension of itomite (0.1 g/ml) n stirre in thermosttte vessel for 0.5 hr. Then, the KMnO 4 queous solution ws e to the ove mixe solution. After eing stirre for 0.5 hr, the mixture ws trnsferre into Teflon-line stinless steel utolve (100 ml) for hyrotherml tretment t 80 C for 4 hr. The wet soli ws filtere, wshe with eionize wter three times, n rie t 60 C for 10 hr, thus otining the flower-like MnO 2 /itomite smple. (2) g of KMnO 4 ws issolve in 20 ml of eionize wter, n esire mount (KMnO 4 /(NH 4 ) 2 S 2 O 8 molr rtio = 1:1) of (NH 4 ) 2 S 2 O 8 ws issolve in 20 ml of eionize wter. The KMnO 4 queous solution ws e ropwise to suspension of itomite (0.1 g/ml) n stirre in thermosttte vessel for 0.5 hr. Then, the (NH 4 ) 2 S 2 O 8 queous solution ws e to the ove mixe solution. After eing stirre for 0.5 hr, the mixture ws trnsferre into Teflon-line stinless steel utolve (100 ml) for hyrotherml tretment t 90 C for 12 hr. The wet soli ws filtere, wshe with eionize wter three times, n rie t 60 C for 8 hr, thus otining the wire-like MnO 2 /itomite smple. (3) A MnSO 4 queous solution (0.2 mol/l) ws e ropwise to suspension of itomite (0.1 g/ml) n stirre in thermosttte vessel for 0.5 hr. Then, the esire mounts (MnSO 4 /(NH 4 ) 2 S 2 O 8 molr rtio = 1:1) of (NH 4 ) 2 S 2 O 8 n KOH queous solution (1.2 mol/l) were e to the ove mixe solution. After eing stirre for 0.5 hr, the mixture ws trnsferre into Teflon-line stinless steel utolve (100 ml) for hyrotherml tretment t room temperture for 12 hr. The wet soli ws filtere, wshe with eionize wter three times, n rie t 100 C for 8 hr, thus otining the sheet-like MnO 2 /itomite smple Smple hrteriztion X-ry iffrtion (XRD) ptterns of the smples in the 2θ rnge of were etermine on D/MAX-II X-ry iffrtometer with Cu Kα rition n nikel filter (λ = nm). The instrument ws operte t 35 ma n 35 kv with snning spee of 4 /min n step size of The rystl phses of the smples were ientifie y referring to the JCPDS Dtse. The snning eletron mirosopi (SEM) imges of the smples were reore on Hithi 570 instrument. The trnsmission eletron mirosopi (TEM) n high-resolution (HRTEM) imges of the smples were otine on Hithi H-9000 NAR pprtus (operting voltge = 250 kv). The speifi surfe res n pore-size istriutions of the smples were mesure using nitrogen sorption t 196 C on n ASAP 2020 pprtus. The elementl ompositions of the smples were etermine using inutively ouple plsm tomi emission spetrosopy (ICP-AES). Fourier trnsform infrre (FT-IR) spetr of the smples (1 wt.% smple + 99 wt.% KBr) were otine in the rnge of m 1 with resolution of 0.4 m 1 on Perkin-Elmer 1730 spetrometer. X-ry photoeletron spetrosopy (Ulv-PHI, PHI Qunter) ws employe to etermine the O 1s n C 1s ining energies of the smples using Al Kα (hv = ev) s exittion soure, n the C 1s signl t ev ws use to lirte the ining energies Cr(VI) sorption Cr(VI) queous 50 or 100 ml solution with onentrtion of 10, 25, 50, 75, 100, 125, 150, 175, 200, 225 or 250 mg/l ws ple in onil flsk, whih ws prepre using K 2 Cr 2 O 7 s Cr soure. The ph vlue of the Cr(VI)-ontining solution ws juste to 1,2, 3, 4, 6, 7, 8, 9 or 10 using queous solutions of HCl (2 mol/l) or NOH (2 mol/l) g of the MnO 2 /itomite ws e to the Cr(VI) solution, followe y osilltion for 10 min, 0.5, 1, 1.5, 2, 3, 4 or 5 hr t room temperture. After tht, the mixe solution ws entrifuge n the Cr(VI) onentrtion ws etete using UV spetrophotometer (SP-750, Shnghi). In the sorption experiment, 40 mg of the smple ws put into 100 ml of the Cr(VI) queous solution (250 mg/l) uner mgneti stirring. After Cr(VI) sorption for 4 hr, the smple solution ws llowe to settle. The Cr(VI) onentrtion in the

3 73 superntnt ws nlyze using the ICP-AES tehnique, n the mximl Cr(VI) sorption pity of the smple ws lulte oring to Eq. (2) elow. In the esorption experiment, the Cr(VI)-sore smple solution ws filtere n the soli ws put into 100 ml of H 2 SO 4 queous solution (20 wt.%) uner stirring for 2 hr. After Cr(VI) esorption, the smple solution ws filtere gin. The esorption opertion ws repete twie. The Cr(VI) sorption/esorption reyling ws onute 8 times. The Cr(VI) removl effiieny n equilirium Cr(VI) sorption pity were lulte oring to Eqs. (1) n (2), respetively. E ð% Þ ¼ C 0 C e 100% ð1þ C 0 ð Q e ¼ C 0 C e Þ V ð2þ m where, E, Q e, C 0, C e, V, n m re the Cr(VI) removl effiieny, equilirium Cr(VI) sorption pity (mg/g), initil Cr(VI) onentrtion (mg/l), equilirium Cr(VI) onentrtion (mg/l), solution volume (L), n sorent mss (g), respetively. 2. Results n isussion 2.1. Crystl phse omposition Fig. 1 shows the XRD ptterns of the smples. It n e oserve tht the XRD pttern of the itomite smple showe ro pek t 2θ = 22.6 (Fig. 1), ssignle to morphous SiO 2 ; the iffrtion peks orresponing to the (100) n (101) rystl plnes were ue to qurtz impurities. After loing of MnO 2 (Fig. 1,, n ), the iffrtion signl of itomite ws still retine, suggesting tht the morphous SiO 2 struture in itomite ws not hnge. Due to the ifferent onfigurtions of the [MnO 6 ] otherl unit, MnO 2 n show vrious polymorphs (Julien n Mssot, 2002). The flower-like MnO 2 /itomite smple showe three ro peks Intensity (.u.) SiO 2 (110) (200) (201) (100) α-mno 2 (310) (101) (211) γ-mno 2 (311) (111) (411) (600) (521) (002) (541) (312) (002) θ (egree) Fig. 1 XRD ptterns of (line ) itomite, (line ) flower-like MnO 2 /itomite, (line ) wire-like MnO 2 /itomite, n (line ) sheet-like MnO 2 /itomite. t 2θ = 21.9, 37.2, n 67.3 (Fig. 1), whih oul e inexe to irnessite-type γ-mno 2 (JCPDS PDF# ), similr to the γ-mno 2 rystllites (Yun et l., 2010,). Compre to the flower-like MnO 2 /itomite smple, the other MnO 2 /itomite smples ontine mixe MnO 2 rystl phses (Fig. 1 n ): irnessite-type γ-mno 2 in minority n tetrgonl α-mno 2 in mjority. The lttie prmeters of the ltter phse were: = = ± nm n = ± nm, in goo greement with the reporte t ( = = nm n = nm) of the stnr α-mno 2 smple (JCPDS PDF# ). The strong n shrp iffrtion peks inite the formtion of MnO 2 nnoprtiles with goo rystllinity Morphology, pore struture, n surfe re The ifferent shpes of MnO 2 were generte ue to the use of ifferent preprtion onitions (mngnese soures, retnts, retion temperture n retion time). The morphologies of the smples re shown in Fig. 2. The itomite smple possesse isk-like morphology with uniform orere porous struture, with the imeter of eh isk out 25 μm. There were lrge numer of flower-, wire-, n sheet-like MnO 2 nnoprtiles rnomly eposite on the surfe of the itomite. The TEM n high-resolution TEM imges further onfirm the morphologies of the MnO 2 -ote itomite smples. Well-grown flower-, wire-, n sheet-like MnO 2 nnoprtiles n itomite were oserve. From the high-resolution TEM imges (Fig. 3, f, i, n l), one n see tht (i) the lttie spings ( vlues) of the (111) n (201) rystl plnes of the flower-like MnO 2 /itomite smple were n nm, respetively, rther lose to those ( n nm) of the stnr γ-mno 2 smple (JCPDS PDF# ); (ii) the vlues (0.242 n nm) of the (211) n (310) rystl plnes of the wire-like MnO 2 /itomite smple were in goo orne with those of the stnr α-mno 2 smple (JCPDS PDF# ); n (iii) the vlue (0.490 nm) of the (200) rystl plne of the sheet-like MnO 2 /itomite smple ws lso not fr wy from tht ( nm) of the stnr α-mno 2 smple (JCPDS PDF# ). Furthermore, the reoring of multiple right eletron iffrtion rings in the SAED ptterns (insets of Fig. 3, f, n i) inites tht the MnO 2 -ote itomite smples were polyrystlline. Ditomite shows mesh-like struture with network of riging silion-oxygen tetrher. Due to the unertinty of the numer of silion toms, oorintion efets n oxygen rige efets exist in the mesh-like struture (Sheng et l., 2009). Hene, it is esy to form surfe silnols (Si OH) ue to the omintion of H with the surfe Si O ngling ons. Dissoition of surfe silnols usully ours in wter into Si O n H +, mking the surfe of itomite negtively hrge (Zhurvlev, 2000). Fig. 4 shows the N 2 sorption esorption isotherms n pore-size istriutions of the smples. The flower-, wire-, n sheet-like MnO 2 /itomite smples showe type II isotherm with hysteresis loop resemling H3 n H4 types, initing the presene of porous strutures (Yun et l., 2010,). The pore-size istriutions of the smples onfirm the presene of meso- n mropores. As shown in Tle 1, the BET surfe res of the flower-, wire-, n sheet-like MnO 2 /itomite

4 74 JOURNAL OF ENVIRONMENTAL SCIENCES 29 (2015) μm 100 nm 4 μm e f 500 nm 5 μm 200 nm Fig. 2 SEM imges of (, ) flower-like MnO 2 /itomite, (, ) wire-like MnO 2 /itomite, n (e, f) sheet-like MnO 2 /itomite. smples were 66.5, 144.2, n 49.5 m 2 /g, respetively, muh higher thn tht (25 m 2 /g) of the itomite smple Cr(VI) sorption property Influene of ph on the sorption of Cr(VI) Fig. 5 shows the effet of ph vlue on the Cr(VI) removl effiieny of the itomite n MnO 2 /itomite smples uner the onitions of Cr(VI) solution volume = 100 ml, initil Cr(VI) onentrtion = 50 mg/l, sorent mss = 0.04 g, sorption time = 30 min, n sorption temperture = room temperture. It ws oserve tht the mximl Cr(VI) removl effiieny rehe 99.98% t ph = 2 or 8. Cr(VI) speies in solution oul e present in the form of ihromte (Cr 2 O 7 2 ), hyrohromte (HCrO 4 )orhromte(cro 4 2 ). In n ii environment (lower ph vlues), Cr(VI) exists in HCrO 4 or Cr 2 O 7 2, wheres in n lkline environment (higher ph vlues), it exists preomintely in CrO 4 2 (Ajouye et l., 2010; Hn et l., 2008; Ajml et l., 1996). When ph < 2, the surfe of MnO 2 eme more positively hrge. Due to the high H + onentrtion in the solution, there ws ompetitive sorption of HCrO 4 with the Cr(VI) ions, thus eresing the Cr(VI) removl effiieny (Chen et l., 2010). When the ph vlue ws in the rnge of , the Cr(VI) removl effiieny erese rpily. The isoeletri point of itomite is 5, giving rise to positive surfe hrge in ii solution. The Cr(VI) removl effiieny rpily inrese t ph = 8, possily ue to the omine effet of oth hemil n eletrostti intertions etween MnO 2 n itomite. At higher ph vlues, however, the OH ions eme the ominnt speies, resulting in wekening of the eletrostti fores etween the sorent n sorte, n hene leing to reue Cr(VI) removl effiieny (Xu et l., 2011; Chen et l., 2010). The Cr(VI) removl effiieny inrese in the orer itomite < sheet-like MnO 2 /itomite < flower-like MnO 2 / itomite < wire-like MnO 2 /itomite. The ft tht the Cr(VI) sorption performne of the wire-like MnO 2 /itomite smple ws est might e ue to it hving the lrgest surfe re n more expose tive (310) n (211) rystl plnes Influene of sorption time on the sorption of Cr(VI) Fig. 6 shows the effet of sorption time on the removl effiieny of the itomite n MnO 2 /itomite smples uner the onitions of Cr(VI) solution volume = 100 ml, initil Cr(VI) onentrtion = 50 mg/l, sorent mss = 0.04 g, ph = 2, n sorption temperture = room temperture. From Fig. 6, one n see tht the Cr(VI) removl effiieny inrese with the extension of sorption time, n rehe sturte vlue. The removl effiieny inrese rpily in the first 12 min n rehe stility within 15 min. Therefore, the pproprite sorption time ws 30 min. The Cr(VI) removl effiieny of the MnO 2 /itomite smples ws muh higher thn tht of the itomite smple, n the highest Cr(VI) removl effiieny (100% t n sorption time of 30 min) ws hieve over the wire-like MnO 2 /itomite smple. The rpi sorption of Cr(VI) y the MnO 2 /itomite smples might e ttriute to the externl surfe sorption (Hu et l., 2005), uring whih the sorte h n esy ess to the tive sorption sites of the itomite-ote MnO 2 nnoprtiles. The sturte Cr(VI) sorption pity inrese in the

5 nm (201) (111) nm 2 μm 200 nm 10 nm e (310) f nm (211) 1 μm 100 nm 5 nm nm g h nm i (200) 500 nm 50 nm 5 nm Fig. 3 TEM imges of ( ) flower-like MnO 2 /itomite, ( f) wire-like MnO 2 /itomite, n (g i) sheet-like MnO 2 /itomite. 500 A 0.8 B Volume sore (m 3 /g, STP) Reltive pressure (p/p 0 ) V/p (m 3 /(g.nm) Pore size (nm) Fig. 4 (A) N 2 sorption-esorption isotherms n (B) pore-size istriutions of (line ) flower-like MnO 2 /itomite, (line ) wire-like MnO 2 /itomite n (line ) sheet-like MnO 2 /itomite.

6 76 JOURNAL OF ENVIRONMENTAL SCIENCES 29 (2015) Tle 1 Preprtion onitions n texturl prmeters of the MnO 2 /itomite smples. Asorent Preprtion metho Mn soure BET surfe re (m 2 /g) Averge pore size (nm) Ditomite Flower-like MnO 2 /itomite Hyrotherml tretment t 80 C for 4 hr Mn(A) 2 n KMnO Wire-like MnO 2 /itomite Hyrotherml tretment t 90 C for 12 hr KMnO Sheet-like MnO 2 /itomite Hyrotherml tretment t room temperture for 10 hr MnSO 4 4H 2 O C 0 (mg/l). orer of itomite < sheet-like MnO 2 /itomite < flower-like MnO 2 /itomite < wire-like MnO 2 /itomite Influene of initil Cr(VI) onentrtion on the Cr(VI) sorption ehvior Fig. 7 shows the effet of initil Cr(VI) onentrtion on the sorption pity of the itomite n MnO 2 /itomite smples uner the onitions of Cr(VI) solution volume = 50 ml, sorent mss = 0.04 g, ph = 2, sorption time = 30 min, initil Cr(VI) onentrtion = 25, 50, 75, 100, 125, 150, 175, 200, 225 or 250 mg/l, n sorption temperture = room temperture, n the lulte C e, Q e,ne vlues for the smples re summrize in Tle 2. It ws oserve tht the sorption pity inrese with the rise in initil Cr(VI) onentrtion. This might e ue to the vilility of more Cr(VI) ions in the solution for sorption on the surfe tive sites of the smples (Yng et l., 2013; Lo et l., 2006). The Cr(VI) sorption pities of the MnO 2 /itomite smples were muh higher thn tht of the itomite smple, with the highest Cr(VI) sorption pity (101 mg/g t Cr(VI) onentrtion of 250 mg/l) eing hieve over the wire-like MnO 2 /itomite smple (Tle 2). At the sme initil Cr(VI) onentrtion, the Cr(VI) sorption pity inrese in the orer itomite < sheet-like MnO 2 /itomite < flower-like MnO 2 / itomite < wire-like MnO 2 /itomite. Usully, the Cr(VI) onentrtions of most wstewter re in the rnge of mg/l. Aoring to the Unite Sttes Environmentl Protetion Ageny (USEPA), the mximl permissile Cr(VI) limit is 0.1 mg/l for ishrging into inln surfe wter n 0.05 mg/l for potle wter. Uner the opte onitions, when the initil Cr(VI) onentrtion ws elow 25 mg/l, the removl effiieny ws mintine ove 96% (C e =0.02mg/L) over the wire-like MnO 2 /itomite smple, whih meets the USEPA stnr for the rinking wter; the C e vlue hieve over the flower-like MnO 2 /itomite smple ws 0.1 mg/l, whih meets the USEPA stnr for inustril wstewter; with the rise in initil Cr(VI) onentrtion from 25 to 250 mg/l, however, the removl effiieny erese. Tle 3 summrizes the Cr(VI) removl effiieny n sorption pities of our smples n ifferent sorents reporte in the literture. Clerly, the Cr(VI) removl effiieny over our wire-like MnO 2 / itomite smple ws muh higher thn those over the EDA-PSt (Cui et l., 2013) n ADAS-se tivte ron (Gorzin n Ghoreyshi, 2013) smples, n slightly higher thn C Al LDHs (Li et l., 2013,), TAC/Fe (Liu et l., 2012), Eihhorni rssipes root iomss-erive tivte ron (Giri et l., 2012), oeylmine-moifie soium montmorillonite (Kumr et l., 2012), n HDTMA-moifie nturl re ly (Głysz-Płsk et l., 2012) smples. Sine the Cr(VI) removl effiieny is ssoite with the Cr(VI) onentrtion n sorent mss, Removl effiieny (%) ph vlue Removl effiieny (%) Asorption time (min) Fig. 5 Effet of ph vlue on the Cr(VI) removl effiieny of (line ) itomite, (lien ) flower-like MnO 2 /itomite, (line ) wire-like MnO 2 /itomite, n (line ) sheet-like MnO 2 /itomite. Fig. 6 Effet of sorption time on the Cr(VI) removl effiieny of (line ) itomite, (line ) flower-like MnO 2 /itomite, (line ) wire-like MnO 2 /itomite, n (line e) sheet-like MnO 2 /itomite.

7 77 Asorption mount (mg/g) Initil Cr(VI) onentrtion (mg/l) Fig. 7 Effet of initil Cr(VI) onentrtion on the Cr(VI) sorption mount of (line ) itomite, (line ) flower-like MnO 2 /itomite, (line ) wire-like MnO 2 /itomite, n (line ) sheet-like MnO 2 /itomite. the pproprite prmeter for evluting the Cr(VI) sorption ehvior of n sorent is the Cr(VI) sorption pity. It is oserve from Tle 3 tht the Cr(VI) sorption pities of the MnO 2 /itomite smples otine in the present work were omprle with those of the C Al LDH smple (Li et l., 2013,), n higher thn those of ADAS-se tivte ron (Gorzin n Ghoreyshi, 2013) n the other nonmetlli minerl sorents. Although the EDA-PSt (Cui et l., 2013) n PA6@Fe x O y (Li et l., 2013,) smples showe etter Cr(VI) sorption performne thn our smples, the former re muh more expensive (limiting their engineering pplitions). In ition, the heper nturl sorents (e.g., ly, TA-AC, montmorillonite, n so on (Kumr et l., 2012; Wen et l., 2011; Li et l., 2012; Thnos et l., 2012)) i not possess high Cr(VI) sorption pities. Therefore, our inexpensive MnO 2 /itomite smples re promising for prtil pplitions in the removl of Cr(VI) from inustril wstewter Asorption stility of the smples Fig. 8 shows the Cr(VI) sorption pity versus the numer of Cr(VI) sorption/esorption yles on the itomite, flower-like MnO 2 /itomite, wire-like MnO 2 /itomite, n sheet-like MnO 2 / itomite smples. It n e seen tht fter 8 yles of Cr(VI) sorption/esorption, the mximl Cr(VI) sorption pities of the itomite, flower-like MnO 2 /itomite, wire-like MnO 2 / itomite, n sheet-like MnO 2 /itomite smples erese y 18.4%, 27.6%, 18.3%, n 26.4%, respetively. Clerly, the itomite n wire-like MnO 2 /itomite smples showe etter Cr(VI) sorption stility thn the flower-like MnO 2 /itomite n sheet-like MnO 2 /itomite smples. Beuse the Cr(VI) sorption pity of wire-like MnO 2 /itomite ws muh lrger thn tht of itomite, the former ws fr superior to the ltter in soring Cr(VI) Cr(VI) sorption mehnism To onfirm the ove-mentione eution, we reore the FT-IR spetr of the wire-like MnO 2 /itomite smples efore n fter the sorption of Cr(VI) t ph = 2; t the sme time we lso reore the FT-IR spetr of the rw itomite smples efore n fter the eposition of wire-like MnO 2,s shown in Fig. 9. It is seen tht the sorption peks entere t 3450 n 1637 m 1 were ue to the strething n ening virtions of the OH group in itomite (Fig. 9 ). The presene of SiO 2 ws verifie y the etetion of sorption peks entere t 1100 n 465 m 1, whih were ue to the symmetril strething n ening virtions of the Si O Si on in itomite, respetively. The sorption pek entere t 798 m 1 ws ttriutle to the Si O Al on (Hung et l., 2007). The wire-like MnO 2 /itomite smple showe six sorption peks entere t 528, 622, 1110, 1400, 1637, n 3186 m 1 : The peks t 528 n 622 m 1 oul e srie to the Mn O ening virtions (Tng et l., 2010), those t 1637, 1400, n 1110 m 1 were ttriute to the ening virtions of OH group tthe to the Mn toms (Yun et l., 2010,), n the ro one t 3186 m 1 oul e ssigne to the strething virtion of the OH group in sore wter (Li et l., 2007). After the sorption of Cr(VI), new sorption peks entere t 581 n 860 m 1 ppere, srile to the strething virtions of the Cr O or Cr O Cr on (Lkshmipthirj et l., 2013). The new sorption pek t 2071 m 1 orrespone to the strething virtion of the Cr_O on, whih suggests tht the Cr(VI) speies h interte with the surfe hyroxyl groups of the smple. Tle 2 Cr(VI) removl effiieny (E) of the MnO 2 /itomite smples t ifferent initil Cr(VI) onentrtions (C 0 ). Smple C 0 (mg/l) Wire-like MnO 2 /itomite C e (mg/l) Q e (mg/g) E (%) Flower-like MnO 2 /itomite C e (mg/l) Q e (mg/g) E (%) Sheet-like MnO 2 /itomite C e (mg/l) Q e (mg/g) E (%) Asorption onitions: Cr(VI) solution volume = 50 ml, sorent mss = 0.04 g, ph = 2, sorption time = 30 min, n sorption temperture = room temperture.

8 78 JOURNAL OF ENVIRONMENTAL SCIENCES 29 (2015) Tle 3 Cr(VI) removl effiieny n sorption pities of vrious sorents. Asorent Optiml ph vlue Removl effiieny (%) Asorption pity (mg/g) Referene Flower-like MnO 2 /itomite 2 or This work Wire-like MnO 2 /itomite 2 or This work Sheet-like MnO 2 /itomite 2 or This work EDA-PSt Cui et l. (2013) ADAS-se tivte ron Gorzin n Ghoreyshi (2013) C Al hyrotlite (C Al LDHs) Li et l. (2013, 2013) TAC Liu et l. (2012) TAC/Fe Liu et l. (2012) Eihhorni rssipes root iomss-erive tivte ron Giri et l. (2012) Doeylmine-moifie soium montmorillonite Kumr et l. (2012) HDTMA-moifie nturl re ly Głysz-Płsk et l. (2012) PA6@Fe x O y Li et l. (2013, 2013) Chitosn-ote fly sh Wen et l. (2011) TA-AC (tnnin-immoilize tivte ly) Li et l. (2012) Zeolite Vermiulite Bentonite Attpulgite The shifts in sorption peks from 1100 to 1137 m 1 n from 1400 to 1439 m 1 were possily ue to the intertion of Si O Si n roxyli groups with Cr(VI), respetively. Another shift ws oserve from 1637 to 1537 m 1, orresponing to the omplextion of hyroxyl groups with Cr(VI). The results inite tht the struture of the wire-like MnO 2 / itomite smple ws not signifintly hnge fter Cr(VI) sorption; for the wire-like MnO 2 /itomite smple, the Mn O ons, hyroxyl groups tthe to the Mn toms, n roxyl groups my ply mjor role in the sorption of Cr(VI). Fig. 10 n shows the full-sn XPS spetr of the wire-like MnO 2 /itomite smples efore n fter the sorption of Cr(VI) t ph = 2. It n e seen tht Si, Mn, Cr, O, n C were present on the surfe of the wire-like MnO 2 / itomite smple; furthermore, two well-resolve Cr 2p XPS Asorption pity (mg/g) Cr(VI) sorption/esorption reyle Fig. 8 Cr(VI) sorption pity versus the numer of Cr(VI) sorption/esorption reyle on (line ) itomite, (line ) flower-like MnO 2 /itomite, (line ) wire-like MnO 2 /itomite, n (line ) sheet-like MnO 2 /itomite. peks t BE = n ev (ttriutle to the Cr 2p 3/2 n Cr 2p 1/2 finl sttes (Co et l., 2012), respetively) were oserve s shown in Fig. 10. This result inites the ourrene of Cr(VI) sorption. From Fig. 10, one n oserve three O 1s XPS peks t BE = 529.8, 531.1, n ev, ssignle to the surfe lttie oxygen speies on MnO 2, surfe lttie oxygen speies on itomite, n sore hyroxyl speies on itomite, respetively. After Cr(VI) sorption over the wire-like MnO 2 /itomite smple (Fig. 10), three O 1s XPS peks t BE = 529.8, 531.3, n ev were oserve, whih oul e ttriute to the surfe lttie oxygen speies on MnO 2 n sore Cr(VI) like CrO 3 (Fig. 10), surfe lttie oxygen speies on itomite, n physilly sore wter speies on itomite, respetively. Clerly, the surfe OH onentrtion of wire-like MnO 2 /itomite efore Cr(VI) sorption ws higher thn tht fter Cr(VI) sorption, initing tht the hyroxyl Intensity (.u.) Wvenumer (m -1 ) Fig. 9 FT-IR spetr of (line ) rw itomite n wire-like MnO 2 /itomite (line ) efore n (line ) fter Cr(VI) sorption.

9 79 Intensity (.u.) Intensity (.u.) Mn2s Cr2s Mn2p Bining energy (ev) ev Cr2p O1s ev ev Bining energy (ev) groups h een exhnge with Cr 2 O 7 2 or HCrO 4 in the Cr(VI) solutions. Therefore, hyroxyl groups plye ritil role in Cr(VI) sorption. The Lngmuir n Freunlih isotherm moels re most ommonly use in effetively esriing sorption moes. The former ssumes the monolyer sorption of metl ions on homogenous surfe, wheres the ltter esries multilyer sorption of metl ions on heterogeneous surfe. The Lngmuir n Freunlih isotherm equtions n e expresse s follows: Intensity (.u.) C1s ev Si2p ev Mn3s Si2s Mn3p ev Bining energy (ev) Fig. 10 () Full-sn XPS spetrum of wire-like MnO 2 /itomite efore n fter Cr(VI) sorption, () O 1s XPS spetrum of wire-like MnO 2 /itomite, n () O 1s spetrum of wire-like MnO 2 /itomite fter Cr(VI) sorption. mximl sorption pity, Lngmuir onstnt, n Freunlih onstnts, respetively. Tle 4 summrizes the min sorption isotherm prmeters. It n e seen tht the sorption t otine over the MnO 2 /itomite smples were more onsistent with the Lngmuir isotherm moel thn the Freunlih isotherm moel, wheres the itomite smple ws more fit etter y the Freunlih isotherm moel thn the Lngmuir isotherm moel. Furthermore, the sorption of Cr(VI) on the surfe of the MnO 2 /itomite smples ws fvorle ue to the 1/n vlue rnge of , n the sorption pity followe the sequene of wire-like/itomite > flower-like MnO 2 /itomite > sheet-like MnO 2 /itomite > itomite. The ifferent sorption pities of these smples might result from the vrious strutures n funtionl groups s well s their surfe istriutions. 3. Conlusions By using hyrotherml metho, we generte flower-, wire-, n sheet-like MnO 2 -ote itomite sorents with high surfe res ( m 2 /g). The sorption of Cr(VI) ws gretly influene y the ph of the Cr(VI) queous solution. The highest Cr(VI) removl effiieny (96.0% 99.98% t initil Cr(VI) onentrtion of 10 mg/l n ph 2.0 or 8.0) ws oserve over the MnO 2 -ote itomite smples with ifferent morphologies, n the mximl Cr(VI) sorption pity rehe 101 mg/g over the wire-like MnO 2 /itomite smple. The kineti t revels tht the Cr(VI) sorption over the MnO 2 /itomite smples ws fit etter y the Lngmuir moel. Aknowlegments This work esrie ove ws finnilly supporte y the Siene n Tehnology Progrm of Jilin Provine (Nos GX, SF), n the Beijing Nturl Siene Fountion (No ), n the Ntionl Nturl Siene Fountion of Chin (No ). Lngmuir isotherm eqution : 1=Q e ¼ 1= ðq 0 C e K L Þ þ 1=Q 0 Freunlih isotherm eqution : logq e ¼ 1=n logc e þ logk F where, C e (mg/l), Q e (mg/g), Q 0 (mg/g), K L, n, nk F re the equilirium onentrtion, equilirium sorption pity, Appenix A. Supplementry t Supplementry t to this rtile n e foun online t Tle 4 Lngmuir n Freunlih equtions for the sorption of Cr(VI) on the itomite n MnO 2 /itomite smples. Asorent Lngmuir eqution Freunlih eqution K L Q 0 (mg/g) R 2 logk F 1/n R 2 Ditomite Flower-like MnO 2 /itomite Wire-like MnO 2 /itomite Sheet-like MnO 2 /itomite

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