Magnetic Separation of Cesium Ion Using Prussian Blue Modified Magnetite
Magnet i c Separat i on o f Ces i um Ion Us i ng Pruss i an B l ue Mod i?ed Magnet i te
Takah i ro Sasak i and Shun i tz Tanaka*
D i v i s i on o f Env i ronmenta l Mater i a l Sc i ence,Graduate Schoo l o f Env i ronmenta l Sc i ence,
Hokka i do Un i vers i ty,Sapporo,Hokka i do060-0810
(Rece i ved August9,2011;CL-110662;E-ma il:ch i cken9@ees.hokuda i.ac.jp,shun i tz@ees.hokuda i.ac.jp) Pruss i an-b l ue-mod i?ed magnet i te(PB-Fe3O4)was prepared
by a s i mp l e method.The Cs+sorpt i on ab ili ty o f PB-Fe3O4
was eva l uated by batch magnet i c separat i on.As a resu l t,
the max i mum sorpt i on amount o f Cs+w i th PB-Fe3O4was
16.2mg g11,and the sorpt i on ab ili ty was not great l y changed i n
the presence o f h i gh concentrat i on o f NaC l.The recovery
o f PB-Fe3O4f rom water by magnet pred i cted that i t was
su?c i ent l y app li cab l e to treatment o f water contam i nated w i th
rad i oact i ve Cs+.
Today,Japan i s f aced w i th a cr i t i ca l po ll ut i on prob l em w i th
rad i oact i ve spec i es such as ces i um(Cs),stront i um(Sr),and
i od i ne(I)sp ill ed f rom the Fukush i ma nuc l ear power p l ant (NPP).There f ore,easy and sa f e methods to treat and ana l yze rad i oact i ve mater i a l s i n env i ronments o f so il and water are requ i red.Pruss i an b l ue(PB)i s known as a l ow-cost adsorbent, wh i ch has a h i gh se l ect i v i ty f or ces i um(Cs)and a h i gh stab ili ty f or decompos i t i on.1,2There f ore,PB has been used as an sorbent f or the remova l o f Cs+uptaken i nto an organ i sm.3 The bas i c structure o f PB i s a three-d i mens i ona l po l ymer i c network w i th a f ace-centered cub i c ce ll cons i st i ng o f a l ternat i ng i ron(III)(coord i nated to n i trogen)and i ron(II)(coord i nated to carbon atom)i ons li nked v i a the br i dg i ng o f cyan i de li gands.4,5 Most o f sorbed Cs+i s i ntroduced i n the l att i ce o f PB.However, PB crysta l i s sma ll,and thus the m i crocrysta lli ne nature o f PB i s not adequate to co l umn l oad i ng,?l trat i on,and centr if ugat i on a f ter Cs+sorpt i on.A magnet i c separat i on us i ng magnet i c mater i a l s such as hemat i te(Fe2O3)and magnet i te(Fe3O4)i s an easy and qu i ck method to co ll ect and separate target compounds f rom water,and recent l y i t has been app li ed to var i ous?e l ds such as ana l yt i ca l b i ochem i stry,med i ca l sc i ence,and b i otech-no l ogy.6-8The magnet i c separat i on has an advantage i n the recovery o f adsorbent w i th harm f u l matter w i thout?l trat i on and centr if ugat i on a f ter adsorpt i on.Severa l stud i es on magnet i c separat i on have been reported about the treatment methods f or heavy meta l i ons and organ i c po ll utants by us i ng mod i?ed Fe3O4.9-12
The purpose o f th i s study i s to deve l op an easy and sa f e method f or treatment o f Cs+i n water us i ng PB-mod i?ed Fe3O4 (PB-Fe3O4).The co l umn method i s used w i de l y f or water treatment,and the treatment o f coo l ant conta i n i ng rad i oact i ve spec i es at Fukush i ma NPP i s a l so per f ormed by the co l umn method.However,use o f co l umns requ i res the cons i derat i on o f c l ogg i ng and water l eaks,and somet i mes generates back pressure.Furthermore,co l umns constructed w i th severa l p i eces o f equ i pment are o f ten comp li cated systems,and so there i s a h i gh probab ili ty o f mechan i ca l f a il ure.Our proposed method i s very s i mp l e and has f ew o f the prob l ems descr i bed above. Moreover,th i s method can be app li ed to turb i d so l ut i ons conta i n i ng many k i nds o f suspended matter.S i nce on l y magnet i c sorbents are recovered by the magnet eas il y,the sorbent can be used aga i n f or treatment.So th i s method can be app li ed to not on l y coo l ant so l ut i on but a l so wastewater i n var i ous cond i t i ons.Thus,we prepared PB-Fe3O4as f o ll ows and eva l uated the magnet i c method us i ng PB-Fe3O4.The i mage o f PB-Fe3O4i s shown i n F i gure1.
At?rst,Fe3O4was prepared i n the f o ll ow i ng manner.135.2g o f FeC l3￠6H2O and2.0g o f FeC l2￠4H2O were d i sso l ved i n 25mL o f de i on i zed water(DW),and then0.85mL o f12M HC l was added i n the so l ut i on.The so l ut i on was added to250mL o f 1.5M NaOH so l ut i on w i th v i gorous st i rr i ng at80°C.A f ter 60m i n w i th st i rr i ng,the m i xed so l ut i on was coo l ed by water, and then the Fe3O4co ll ected by magnet was washed w i th DW unt il the pH o f supernatant was around7.DW f or preparat i on o f so l ut i on was used a f ter purg i ng w i th N2.The sur f ace area o f Fe3O4was measured by Autosorb6(YUASA),and the resu l t was 7.82m2g11.The mod i?cat i on o f PB on Fe3O4was carr i ed out w i th f ormat i on o f PB on the Fe3O4sur f ace us i ng potass i um hexacyan i do f errate(II)(K4[Fe(CN)6]).14
100mg o f Fe3O4was added i nto100mL o f 2.0mM K4[Fe(CN)6]so l ut i on(pH2w i th HC l),and then the m i xture was st i rred f or3h.The PB-Fe3O4co ll ected by magnet was washed w i th water three t i mes and then dr i ed i n an oven at40°C f or24h.A ll reagents were prov i ded by Wako,Japan.Scann i ng e l ectron m i croscopy(SEM,JSM-7400F JEOL)and transm i ss i on e l ectron m i croscopy(TEM,JEM-2100F JEOL)i mages o f PB-Fe3O4are shown i n F i gure2.Core-she ll structure o f PB-Fe3O4 and crysta l o f PB on Fe3O4can be observed i n F i gures2b and 2c,respect i ve l y.The s i ze o f PB crysta l gra i n on Fe3O4was about100nm,and the th i ckness o f PB l ayer on Fe3O4was200-400nm f rom TEM i mages.The presence o f PB mod i?ed on Fe3O4was con?rmed by powder X-ray d i?ract i on(XRD)and Four i er trans f orm i n f rared spectroscopy(FT-IR).The XRD pattern measurements were per f ormed by M i n i?ex(R i gaku) w i th N i?l tered Cu K?rad i at i on(-=0.154nm)at35kV, 15mA.As shown i n F i gure3a,the peaks o f(220),(311),(222), (400),(422),(511),and(440)were detected as the character i st i c
Fe3+Fe2+C N
Prussian
blue
F i gure1.Schemat i c representat i on o f PB-Fe3O4and l att i ce structure o f PB.
Pub li shed on the web December24,2011
32
doi:10.1246/cl.2012.32
peaks o f Fe 3O 4.15A f ter PB mod i?cat i on,some new peaks,marked peaks o f F i gure 3b,co i nc i ded w i th the character i st i c peaks o f PB.16The FT-IR measurements were curr i ed out w i th FT /IR-4100(JASCO)i n KBr pe ll ets.In F i gure S2,20peaks at around 1610cm 11observed i n curve (a)and (b)were re l ated to the -OH de f ormed v i brat i on,and the peaks around 3400cm 11i n curve (a)and (b)were ass i gned to the -OH stretch i ng v i brat i on on the sur f ace o f Fe 3O 4.17,18The peak o f 2082cm 11i n curve (b)was ass i gned to the CN stretch i ng i n the f ormed [Fe II -CN -Fe III ]structure.19The s i ze and |potent i a l o f Fe 3O 4and PB-Fe 3O 4were measured by us i ng photon corre l at i on spectroscopy (PCS)and e l ectrophoret i c li ght scatter i ng (ELS),respect i ve l y (De l sa ·nano HC,Beckman Cou l ter).As a resu l t,the average part i c l e s i zes o f Fe 3O 4and PB-Fe 3O 4were 6.4and 6.7ˉm,respect i ve l y.The |potent i a l s o f Fe 3O 4and PB-Fe 3O 4were 20and 179mV at pH 5.5(F i gure S1).20The amount o f mod i?ed PB est i mated by grav i metry was 0.29mmo l g 11(F i gure S2and Tab l e S1).20The sorpt i on ab ili ty o f PB-Fe 3O 4f or Cs +was eva l uated by sorpt i on exper i ments.The batch exper i ments were carr i ed out us i ng 10mL o f Cs +so l ut i on and 10mg o f PB-Fe 3O 4at pH 5.5(uncontro ll ed)and 25°C,and then PB-Fe 3O 4was co ll ected by Nd -Fe -B magnet (0.4T,25?10?5mm 3)a f ter sorpt i on.Cs +concentrat i on i n supernatant was measured by ICP-MS (SPQ 6500,SEIKO).F i gure 4showed the resu l ts o f sorpt i on i n var i ous concentrat i ons o f Cs +(0.01-0.5mM).The resu l ts i nd i cated that the sorpt i on amount o f Cs +onto PB-Fe 3O 4was i ncreased w i th i ncrease i n the concentrat i on o f Cs +i n the so l ut i on.The sorpt i on resu l ts were ?tted to a Langmu i r mode l and Freund li ch mode l to obta i n the max i mum sorpt i on amount o f Cs +onto PB-Fe 3O 4.Both ?tt i ng resu l ts were f avorab l e to these mode l s,but the resu l t o f Freund li ch was s li ght l y better than that o f the Langmu i r (the corre l at i on coe ?c i ent w i th Freund li ch mode l :0.977and w i th Langmu i r mode l :0.975accord i ng to Tab l e S2).20The max i mum sorpt i on amount f rom the ?tt i ng data o f Langmu i r mode l was 16.2mg g 11.Freund li ch mode l i s genera ll y sa i d to we ll ?t the sorpt i on on porous mater i a l such as act i vated carbon.On the other hand,the Langmu i r mode l i s we ll ?tted to mono l ayer adsorpt i on.S i nce the l ayer o f the l att i ce structure o f PB f ormed on Fe 3O 4was re l at i ve l y th i n,the property o f PB-Fe 3O 4as porous mater i a l was not s i gn i?cant l y pronounced.As a resu l t,the sorpt i on behav i or m i ght a l so ?t we ll f or the Langmu i r mode l .F i gure 5i nd i cated the resu l ts o f sorpt i on rate o f Cs +onto PB-Fe 3O 4and i n ?uence o f coex i st i ng sa l t on sorpt i on o f Cs +,where concentrat i ons
were
F i gure 2.SEM (a)and TEM (b,c)i mages o f PB-Fe 3O 4.
I n t e n s i t y (a r b u n i t )
2θ/degree
F i gure 3.XRD
patterns o f Fe 3O 4(a)and PB-Fe 3O 4(b).05101520Equilibrium Concn/mg L ?1
S o r b e d a m o u n t o f C s +/m g g ?1
F i gure 4.Adsorpt i on i sotherm o f Cs +onto PB-Fe 3O 4.
Time/min
S o r p t i o n r a t i o o f C s +/%
F i gure 5.K i net i cs o f Cs +sorpt i on onto PB-Fe 3O 4on 0.001(c l osed c i rc l e),0.01(c l osed tr i ang l e),and 0.1mM (c l osed square)o f Cs +w i th NaC l and w i thout NaC l (open square).33
0.001,0.01,and0.1mM.The concentrat i on o f coex i st i ng NaC l
was3wt%,the same as that o f seawater.Accord i ng to these
resu l ts,most o f Cs+sorbed onto PB-Fe3O4w i th i n the?rst
60m i n i n a ll cases.In0.01and0.1mM Cs+so l ut i on,Cs+
sorbed s l ow l y a f ter60m i n.The presence o f two d i?erent
sorpt i on rates o f Cs+suggests that the sorpt i on s i te o f Cs+i n the sorbent i s not on l y the sur f ace o f PB l ayer but a l so
the i ns i de o f PB l att i ce.Moreover,the i n?uence o f coex i st i ng
sa l t on the sorpt i on o f Cs+by PB-Fe3O4was eva l uated by
compar i son between sorpt i on behav i ors o f Cs+i n0.1mM Cs+ so l ut i on w i th and w i thout NaC l.The amount o f sorbed Cs+f rom water conta i n i ng NaC l was91%o f that w i thout NaC l.It was f ound that the ab ili ty o f Cs+sorpt i on w i th PB-Fe3O4was present even w i th l ow concentrat i on Cs+under the h i gh matr i x cond i t i ons.
The recovery o f PB-Fe3O4f rom water by a magnet was eva l uated by measur i ng we i ght o f co ll ected PB-Fe3O4.The recovery test was per f ormed i n the f o ll ow i ng way.F i rst,100mg o f PB-Fe3O4was d i spersed i n1L o f water.Then,the PB-Fe3O4 was recovered by magnet at var i ous t i mes w i th st i rr i ng.The recovered PB-Fe3O4was dr i ed at40°C f or24h i n an oven and then the we i ght measured.Accord i ng to the resu l ts,the recovery rat i o o f PB-Fe3O4was a l most100%at6m i n(F i gure S3).20 Thus,the good recover i ng ab ili ty o f PB-Fe3O4was use f u l i n the water treatment process.I f the part i c l e s i ze o f PB-Fe3O4i s sma ll er than the case i n our study,the sorpt i on amount o f Cs+w ill be i ncreased because o f i ncreas i ng sur f ace area and mod i?ed amount o f PB.However,the decrease i n the part i c l e s i ze o f PB-Fe3O4may l ead to the reduct i on o f recovery because o f weaker magnet i c strength.
Based on our study,a f u ll automat i on system f or treatment
o f contam i nated water may be f eas i b l e if a hanged e l ectr i ca l
magnet i s used f or co ll ect i on o f PB-Fe3O4a f ter sorpt i on o f Cs+.
It w ill make poss i b l e to treat the contam i nated water w i th
rad i oact i ve Cs+w i th f ew exposure.
We thank Pro f essor F.Matsuda(Hokka i do Un i vers i ty)f or
the IR measurement,Assoc i ate Pro f essor Y.Kam i ya(Hokka i do
Un i vers i ty)f or the XRD measurement and Dr.H.W.Yu
(Hokka i do Un i vers i ty)f or the TEM.The ana l ys i s o f sur f ace
area,part i c l e s i ze d i str i but i on,and|potent i a l were carr i ed out
w i th Autosorb6and De l sa·nano HC at the OPEN FACILITY,
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