layeredrare-earthhydroxideunilamellernanosheets...

Research ArticleLayered Rare-Earth Hydroxide Unilameller NanosheetsSynthesis Characterization and Adsorption

Solomon Omwoma 1 Adongo Stephen Odongo 1 Zablon Otieno 1 Silas Lagat1

and Joseph Owuor Lalah 2

1Department of Physical Sciences Jaramogi Oginga Odinga University of Science and Technology PO Box 210-40601Bondo Kenya2Department of Chemical Science and Technology Technical University of Kenya PO Box 52428-00200 Nairobi Kenya

Correspondence should be addressed to Solomon Omwoma solomwomayahoocom

Received 8 May 2019 Revised 7 November 2019 Accepted 16 November 2019 Published 28 January 2020

Academic Editor Philippe Trens

Copyright copy 2020 Solomon Omwoma et al +is is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work isproperly cited

Unilameller nanosheets with a lateral dimension of one nanometer have been isolated from a colloidal solution of europium-containing layered rare-earth hydroxide (LRH) material by the flocculation method +e nanosheets were achieved by changingpH of the colloidal solution from 67 to 115 +e resultant flocculated nanosheets show high efficiency in sorption of fluorideanions from aqueous media (40mmolg) providing a potentially useful sorbent material for water purification technology +esorbent material is demonstrated to be reusable for at least ten times without a significant loss of adsorption efficiency And theresults fit the Langmuir adsorption curve indicating the chemisorption nature of the nanosheets Most importantly the isolatednanosheets are expected to widen the applicability and flexibility in material synthesis using two-dimensional nanomaterials

1 Introduction

Synthesis of building blocks used in advancement of materialchemistry is highly affected by size surface shape defectproperties and anisotropy Anisotropy is determined bygeometry and structure of self-assembled materials and issometimes difficult to achieve in nanomaterials Howeverthere has been progress in synthesis of materials such as thefunctional oxide nanosheets and exfoliated layered doublehydroxides (LDHs) [1ndash4] +e synthesized nanosheets havebeen shown to display robust chemical-physical propertiessuch as lateral dimensions of less than 10 nm high ad-sorption rates great catalysts and high dispersion rates insolvents that make them useful building units in themanufacture of nanodevices [5ndash7]

Only LDH nanosheets exhibit a positive charge whereasall the other nanosheets are negatively charged [7] De-lamination of LDH nanosheets has been reported to occurmainly in formamide solution [8 9] It is noted that de-lamination of LDHs in aqueous media is difficult due to the

high charge density of the LDHs layers and the high anioncontents that result in strong interlayer electrostatic inter-actions between the sheets and the extensive interlamellarhydrogen bonding networks which lead to a tight stackingof the lamellae [2 10 11] +is limits the use of thesebuilding units especially for aqueous reactions

Layered rare-Earth hydroxides (LRHs) have a generalformula of Ln8(OH)20(Amminus )4mmiddotnH2O (Ln rare-earth ionsA intercalated anions) LRHs with one type of Ln3+ cationsoccupying the octahedral position on the layers can be easilydelaminated into individual nanosheets in aqueous media[12ndash17] which is in good contrast to that of LDHsMoreover the sonication of LRHs in the aqueous mediumresults in the formation of a colloidal solution whichconsists of a number of unilameller layers of the positivelycharged nanosheets of [Ln8(OH)20middotnH2O]4+ [12ndash14] +eselamellar positively charged nanosheets are vital buildingunits for the synthesis of LRH-containing functional ma-terials due to their positively charged surfaces Howeverthese unilameller nanosheets have not been isolated so as to

HindawiJournal of ChemistryVolume 2020 Article ID 8923871 11 pageshttpsdoiorg10115520208923871

extend the novel applicability of positively chargednanosheets

Herein a simple new method of isolating the unilamellernanosheets is reported Stable unilameller nanosheets wereisolated by changing the pH of the colloidal solution from 65 to115 +e flocculated unilameller nanosheets show good per-formance for fluoride sorption from aqueous media providinga potentially useful sorbent material for water purificationtechnology And we demonstrate the recyclability of this ma-terial in fluoride separation from the aqueous medium

It is important to note that fluoride has detrimentalhealth effects to aquatic life and humans such as cancerdental fluorosis bone resorption endocrine disruptionmutations and brain damage [18] Hence industries thatuse fluoride in their processes ought to separate it fromwastewater before disposal of the water to aquatic systems

In addition although nanosorbents present a great po-tential in advancingwater andwastewater treatment efficiencymost of the nanosorbents reported so far generate secondarywastes which are equally hazardous to the environment [19]As such development of recyclable nanosorbents is a big stepforward in wastewater treatment efficiency

2 Materials and Methods

21 Chemical Materials Analytically pure KOH NaOHEu2O3 HCl NaF KBr lacmoid ethanol and methanol werepurchased from Alfa Aesar and used without furtherpurification

22 Preparation of Wet LEuH-Cl Eu-containing layeredrare-Earth hydroxides (Eu2(OH)5ClmiddotnH2O) (LEuH-Cl) weresynthesized and characterized according to literaturemethods [12] In brief 01M KOH solution was added to005M EuCl3middot6H2O solution with stirring at 24degC +e re-sultant mixture was heated to 60degC for 12 h and thereafterrefluxed for 24 h as with magnetic stirring +e resultantslurry was washed with deionized water three times in acentrifuge +e solid sample was labeled as wet LEuH-Cl

23 Flocculation +e delamination of fresh prepared wetLEuH-Cl aggregate material was achieved by sonication(3min 100W) to obtain an aqueous colloidal solutionSonication beyond these specifications is noted to result intobroken delaminated nanosheets +e pH of the aqueouscolloidal solution was adjusted from 65 to 115 using 1MNaOH solution that led to flocculation of unilameller LEuHnanosheets (LEuH-flocs) +e contents were centrifuged at2000 rpm and the residue was washed with distilled waterfour times before a final wash with acetone followed byvacuum drying at 40degC

24 Characterization +e OH content of solid samples wasdetermined titrimetrically by neutralization backtitrationafter dissolution of the samples in a 01N standard H2SO4Measurements of diffraction patterns of the 40degC driedsamples were achieved using a Rigaku XRD diffractometer

machine (6000) conditioned at 30mA 40 kV Cu-Ka(λ 0154 nm) and with a scan step of 001deg measured be-tween 3deg and 70deg Surface morphology was observed using aSEM (Zeiss Supra 55) machine simultaneously connected toan EDX detector In addition atomic forces were deter-mined using a Bruker AFM machine (A3A) in order todetermine lateral dimensions of nanosheets isolated Cross-sectional transverse morphological study was done using aHRTEM machine (Hitachi H-800) High sensitivity ele-mental analysis in aqueous solutions was performed usingICP-AES machine (ICPS-7500) Solid samples were firstdigested using aqua regia solutions before ICP-AES analysis+e pore volumes and specific surface area studies for solidsamples were done using BET and BJH methods withQuantachrome Autosorb1C VP machine Before such an-alyses were done the solid samples were first degassed at100degC for 6 hrs

25 Adsorption Experiments +e efficiency of fluoridesorption by LEuH-flocs nanosheets was compared to thesorption efficiency of LEuH-Cl aggregates To a knownconcentration of fluoride ions in aqueous media 04 g of testsample was introduced and then the test solution was keptstirring for 1 hour After that the nanocomposites werecentrifuged down and the supernatant solution was testedfor fluoride concentration colorimetrically [20] +e pelletedsample was dried and calcined at 500degC for 24 hours toremove the adsorbed fluoride and the material wasreconstituted in a NaCl solution to regenerate the LEuH-Clmaterial through the ldquomemory effectrdquo method of hydro-talcites [7] Direct adsorption of the fluoride ions by thecalcined LEuH-flocs was also determined

26 Kinetic Studies

261 Determination of Adsorption Rate and EquilibriumTen samples of fluoride ions dissolved in deionized water(100 ppm) were treated with 02 g of LEuH-flocs for 0 1 2 34 5 6 8 10 and 15 minutes After each experiment theLEuH-flocs were separated and the resultant supernatantwas tested for fluoride concentration A similar experimentwas repeated for the LEuH-Cl aggregate material

262 Adsorption Isotherms +e test samples (02 g) werestirred with 50ml solutions having different concentrations(1 5 10 20 50 100 200 300 400 500 and 1000 ppm) offluoride anions for 30 minutes to insure equilibrium +enanocomposites were centrifuged and the supernatant wastested for fluoride concentration+e amount of fluoride up-taken per gram of the LEuH-flocs (q) was determinedaccording to the following equation where C0 is the initialfluoride concentration C is the concentration of theequilibrated final solution V is the volume of the aqueousphase and m is the mass of the adsorbent in the system

q C0 minus C( 1113857V

m (1)

2 Journal of Chemistry

3 Results and Discussion

31 Characterization of the Nanocomposite MaterialsAdjusting the pH of a colloidal solution sonicated fromfreshly prepared wet LEuH-Cl aggregates from pH 65 topH 115 resulted into flocculation of unilameller LEuHnanosheets (LEuH-flocs) (Figure 1) During the pH ad-justment experiments there was no observable change be-tween pHsim45 and sim95 However below pHsim45 thenanosheets decomposed to individual elements Chemicalanalysis of LEuH-flocs reveal an elemental composition ofEu8(OH)20(CO3)067(H2O)44Cl315 which is simplified as[Eu8(OH)20(H2O)44]Cl4 +e composition resembles that ofthe starting material (LEuH-Cl aggregates)

SEM images of both LEuH-flocs nanosheets and LEuH-Cl aggregates show similarity in plate-like morphology withthe former exhibiting unilameller platelets while the latter isorganized in layers ranging from 20 nm to 65 nm (Figure 2)Energy dispersive X-ray spectroscopy (EDX) images providefurther evidence of composition similarity (Figure 2)However TEM images show the unilameller nature of theLEuH-flocs (Figure 2)

+e unilameller character of the LEuH-flocs can also beclearly distinguished from the layered character of LEuH-Claggregates by HRTEM images (Figure 3) And the selectedarea electron diffraction (SAED) patterns of LEuH-flocs aremuch clearer than that of LEuH-Cl aggregates indicating thehigh crystallinity of the LEuH-flocs nanosheets (Figure 3)Despite the crystallinity differences it is evidently clear fromthe SAED patterns that atomic arrangement remains thesame in both samples (Figure 3)+e atoms are arranged in apseudohexagonal symmetry with a unit fundamental cell ofaf 37 A It can be calculated that d100 2radic3af 128 A andd010 2af 74 A +ese particular atomic distances arefurther verified by XRD diffraction patterns shown inFigure 4

+e LEuH-Cl aggregate sample exhibits XRD patternsthat can be indexed as a single orthorhombic unit cell withlattice parameters of a 1290 A (d100) b 752 A (d010) andc 863 A (d001) +ese values clearly correspond to thecalculated values from SAED patterns in Figure 3 Since theLEuH-flocs exist as unilameller layers it is almost impossible

to distinguish the d100 and d010 reflections in the XRDpatterns however the d001 76 A reflection is clear with avery high intensity indicating the nanosheets to lie on their cplane Nevertheless there is no restacking because no d00lharmonics were observed And it is worth noting that theresults of LEuH-Cl aggregates are in good agreement withliterature [17]

+e AFM images of a colloidal solution for both LEuH-flocs and LEuH-Cl aggregates dispersed on wafer platesshow the width of LEuH-flocs to be approximately 1 nmwhile LEuH aggregates are approximately 26 nm (Figure 5)Dispersion of the LEuH-flocs nanosheets in water at pH 7results into a colloidal solution indicating the nanosheets tomaintain both their nanoscale size and their charge(Figure 5(c))

FTIR vibrations show an intense absorption band at3494 cmminus 1 attributed to O-H bond v(OH) stretching inboth the LEuH-flocs nanosheets and LEuH-Cl aggregates(Figure 6) +e vibration band at 1634 cmminus 1 is due to thebending mode of both surface and interlayer water (seeFigure 7 for water contents) Two sharp bands at 1511 and1454 cmminus 1 in LEuH-Cl aggregates are attributed to CO3

2minus

vibrations in a layered structure [13] However there exists ashift in these CO vibrations and a decrease in sharpnesswhich is indicative of the lack of stacking in the LEuH-flocsnanosheets [13] Two shoulder peaks at 848 and 818 cmminus 1 aredue to interlayer chloride anions while the bands at 638 and535 cmminus 1 are attributed to Eu-O stretching In addition theO-C-O bending mode may have shifted from 526 to535 cmminus 1 due to lack of stacking Furthermore the band at1374 cmminus 1 in LEuH-flocs and LEuH-flocs + fluoride mightarise from adsorbed CO3

2minus +ermogravimetric analysis of LEuH-flocs and LEuH-Cl

aggregates clearly show the difference in interlayer watercontent of the two samples (Figure 7) Interlayer water isused in stacking of layers together by creating hydrogenbonding networks with anions within the layers Howeverthe LEuH-flocs has very little of these water content indi-cating their unilameller property (Figure 7)

+e isolation of LEuH-flocs nanosheet is further verifiedby the difference in pore volume as compared to the LEuH-Cl aggregates (determined by BrunauerndashEmmettndashTeller

Delaminationwater

pH = 115

Unilameller LEuHnanosheets

LEuH-Cl aggregatesample LEuH-Cl Colloidal

Soln pH = 45ndash95

Figure 1 +e flocculation process generating LEuH-flocs nanosheets through pH adjustments

Journal of Chemistry 3

(BET) and BarrettndashJoynerndashHalenda (BJH) measurements)(Figure 8) whereas the pores in LEuH-Cl aggregates have anaverage pore volume of 034 ccg which is a characteristic of

interparticular porosity and the LEuH-flocs nanosheetsexhibit an average pore volume of 011 ccg that is a char-acteristic of lack of porosity In addition after the BET

200nm Cl

O

Eu

Eu Eu Pt ClEu

Eu

Eu

Si

0 1 2 5 6 7

(a)

200nm ClO Eu Eu

Eu EuEu Pt Cl

Si

0 1 2 5 6 7

(b)

200nm

O Eu

EuEu Pt

EuEu

Eu

Si

F

0 1 2 5 6 7

(c)

500nm

(d)

Figure 2 SEM and EDX images of (a) LEuH-Cl aggregates (b) LEuH-flocs nanosheets (c) LEuH-flocs + fluoride (Si and Pt come from thesilicon wafer used in SEM analysis) and (d) TEM image of LEuH-flocs


experiments the LEuH-flocs do not regain their originalposition as can be seen from the low pressure area in theisotherm where the two graphs (adsorptiondesorption)should be a single line if the original structure is regained[21] +is clearly indicates the unstacked nature of theunilameller LEuH-flocs which lie on their c axis and a changein pressure leads to an increase in their randomness

32 Application of the Unilameller LEuH-Flocs Nanosheets inFluoride Adsorption Experiments Ion exchange reaction ofunilameller LEuH-flocs nanosheets in a fluoride aqueoussolution is proposed to proceed via equation (2) +ereaction mechanisms are schematically represented inFigure 9 +e fluoride sorption reaction was monitored

titrimetrically using chloride concentration in the super-natant solution after the ion exchange reactions +e re-action was found to complete in 2 minutes

Eu8(OH)20 H2O( 1113857n1113858 1113859Cl4(s) + 4NaF(aq)

⟶ Eu8(OH)20 H2O( 1113857n1113858 1113859F4(s) + 4NaCl(aq)

(2)

+e sorption experiments of fluoride anions fromaqueous media were carried out with the unilameller LEuH-flocs nanosheets and compared to LEuH-Cl aggregates for 6minutes (Figure 10) Unilameller LEuH-flocs nanosheetshad the highest fluoride loading capacity of 40mmolg ascompared to sim200mmolg exhibited by LEuH-Cl aggre-gates (Figure 10) Note that LDH-Cl (MgAl with a molarratio of 067 033 respectively) materials exhibit adsorption

(a)

(b)

(c)

Figure 3 HRTEM and SAED images of (a) LEuH-Cl aggregates (b) LEuH-flocs nanosheets and (c) LEuH-flocs + fluoride ions


(a)

(b)

(c)

101

001

200

111 21

0 002

211

102

301

020 11

231

112

122

022

131

232

011

2 113

222

420 30

313

131

3 123

132

521

611 43

030

452

2

d100 = 129nm d010 = 075nmd001 = 086nm

20 402 eta (degree)

Figure 4 XRD patterns of (a) LEuH-Cl aggregates (b) LEuH-flocs nanosheets and (c) LEuH-flocs + fluoride ions

(a) (b)

(c)

(d) (e)

Vert dist = 265nm Vert dist = 11nm

50nm

25nm

0nm

50nm

25nm

0nm

Figure 5 AFM diagrams of (a) LEuH-Cl aggregate material (b) LEuH-flocs nanosheets (c) the dispersion of an incident light by an aqueouscolloidal solution of LEuH-flocs nanosheets (pH 7) demonstrating ldquothe Tindal effectrdquo of the dissolved powder (d) AFM analysis of LEuH-Cl aggregates and (e) AFM analysis of LEuH-flocs nanosheets


4000 3000 2000Wavenumber (cmndash1)

1000

1511 1454

3494

638

1634 818

848

578

535

1374

(c)

(b)

(a)

Figure 6 FTIR absorption patterns of (a) LEuH-Cl aggregates (b) LEuH-flocs nanosheets and (c) LEuH-flocs + fluoride ions

100

95

90

85

80

0 200 400 600 800Temperature (degree)

Sam

ple W

t (

)

Surface waterSurfacewater

Inter-layerwater

Dehydroxylationand

Dechlorination

Dehydroxylationand

Dechlorination

(b)

(a)

Water within the brucitesheet ([Eu8(OH)20nH2O]4+)

Figure 7 +ermogravimetric analysis of (a) LEuH-Cl aggregates and (b) LEuH-flocs nanosheets

200

150

100

50

000 02 04 06

Relative pressure (PP0)08 10

Av pore r = 1171nmAv pore vol = 034ccgSurf A = 5730m2g

Vol

ume (

cm3 g

)

(a)

00 02 04 06 08 10

60

48

36

24

12

0

Av pore r = 449nmAv pore vol = 011ccg

Vol

ume (

cm3 g

)

Relative pressure (PP0)

(b)

Figure 8 BET isotherms and BJH information of (a) LEuH-Cl aggregates and (b) LEuH-flocs nanosheets


capacities of 33mmolg [22] Hence the newly synthesizedunilameller LEuH-flocs provide an effective and fastchemisorption recyclable nanosorbent material reported sofar for fluoride separation from water

+e sorption mechanisms in unilameller LEuH-flocsnanosheets are purely attributed to chemisorption as its datafit perfectly with the Langmuir model (Figure 11) [23] +isis further supported with the type iii isotherm recorded withits BET isotherms (Figure 8) In contrast LEuH-Cl aggre-gates sorption mechanisms could be attributed to bothphysisorption mechanism and chemisorption mechanisms

due to its type ii BET isotherm and the low R2 values of theLangmuir model [21] Specifically the adsorption in LEuH-Cl (hydrotalcite-like compounds) is initiated at the edges(physisorption mechanism) followed by ion exchange(chemisorption)

Anion adsorption behaviour of hydrotalcite-like com-pounds has been previously reported to occur at their edgesthrough physisorption mechanisms [24ndash26] However de-lamination of LEuH-Cl aggregate materials into unilamellerLEuH-flocs nanosheets provides a new pathway in which theexposed chloride ions are easily exchanged with fluoride ions

LEuH-F

LEuH unilamellerFlocs

LEuH-Claggregates

LEuH spinnels

(a)

(b)

(c)

(f)(g)

(e)

(d)

Figure 9 Schematic diagram representing the fluoride sorption process using layered rare-Earth hydroxides unilameller flocs as sorbents(a) flocculation (b) self-assembly (c) fluoride adsorption (d) calcination (e) fluoride adsorption (f ) chloride adsorption and (g)calcination

40

30

qe

qe

20

10

000 1 2

(a)

(b)

3 4 5 6Time (mins)

q (m

mol

g)

Figure 10 Kinetic studies for fluoride sorption from aqueous solutions by layered rare-Earth hydroxides (a) LEuH-Cl aggregates (b)LEuH-flocs nanosheets qe maximum adsorbed amount pH 7 temperature 298K and mass of sorbent 02 g


00

30

20

10

40

5 10 15 20 25 30C (mM times 10ndash1)

q (m

mol

g times

10ndash1

)qm

qm

A

B

(a)

0 5 10 15 20 25 30

30

25

20

15

10

05

00

Cq

C (mM)

y = 0092x +

0051 R2 = 099

y = 0017x + 0019 R2 = 081

A

B

(b)

Figure 11 (a) Optimal sorbed amounts of fluoride anions from aqueous media (b) Langmuir fit curves for (A) LEuH-Cl aggregates and (B)LEuH-flocs nanosheets

100

80

60

40

20

00 1 2 3 4 5 6 7 8 9 10

Number of cycle

q

max

Figure 12 Recycling of the synthesized LEuH adsorbent in fluoride adsorption from aqueous solutions 100 qmax 40mmolg pH 7temperature 298K mass of sorbent 02 g In cycle 1 we used LEuH-flocs and in the subsequent cycles we use LEuH-spinels obtainedafter calcination of LEuH-flocs at 500degC as sorbent materials

200nm

(a)

200nm

(b)

Figure 13 Continued


from aqueous media +e reaction is fast and effective andhas a higher fluoride loading rate than all other reportedfluoride adsorbents [27]

+e chemisorption process is further verified by theFHFminus covalent bond that shows FTIR vibrations at 578 cmminus 1

(Figure 6) [28 29] +e chemisorption process does notinterfere with the surface morphology of the unilamellerLEuH-flocs nanosheets as is depicted in the images takenafter the adsorption experiments by both SEM and HRTEM(Figures 2 and 3) In addition the XRD patterns of thefluoride-adsorbed unilameller LEuH-flocs nanosheets havesimilar peaks to the starting materials LEuH-flocs (Figure 4)

After calcination at 500degC to remove the adsorbedfluoride ions the resultant LEuH-spinels were reused influoride adsorption for at least ten times giving similaradsorption capacities (Figure 12) It also noted that thespinels could also be dispersed in a NaCl solution (1M) toreconstruct to the original LEuH-Cl starting material(Figure 13) It is however observed that when LEuH calcinedspinels are used directly to adsorb fluoride ions the resultantmorphology is different from LEuH-flocs material(Figure 13(d)) However this change in morphology doesnot affect adsorption capacity as can be seen from subse-quent reuse experiments (Figure 12)

4 Conclusions

In summary we have been able to isolate unilamellernanosheets from a colloidal solution of europium-con-taining layered rare-Earth hydroxide material (LEuH-Cl) by

changing its pH from 65 to 115 in an aqueous medium+eresultant unilameller LEuH-flocs nanosheets exhibit fluoridechemisorption ability of 40mmolmiddotgminus 1 as compared to LEuH-Cl aggregates of asymp200mmolmiddotgminus 1 and LDH-Cl aggregates of33mmolmiddotgminus 1 And the chemisorption reaction is complete intwo minutes with the resultant adsorbent being reusableafter calcination at 500degC

Data Availability

All the necessary information required for replication of thiswork andor conducting secondary analysis are includedwithin the article

Conflicts of Interest

+e authors declare that they have no conflicts of interest

Acknowledgments

+is study was financially supported fromNational ResearchFund of Kenya (201617) +e RSC Allan Ure Bursary fundof 2017 and Jaramogi Oginga Odinga University of Scienceand Technology are acknowledged

References

[1] T Sasaki Y Ebina T Tanaka M Harada M Watanabe andG Decher ldquoLayer-by-Layer assembly of titania nanosheetpolycation composite filmsrdquo Chemistry of Materials vol 13no 12 pp 4661ndash4667 2001

200nm

(c)

200nm

(d)

(e)

Figure 13 SEM images of (a) LEuH-flocs (b) LEuH-flocs + fluoride ions (c) LEuH-spinels obtained after calcination of LEuH-flocs at500degC (d) LEuH-fluoride adsorbed sample using LEuH-spinels as adsorbent and (e) self-assembly of LEuH-flocs with NaCl to reproducethe original LEuH-Cl aggregates


[2] T Sasaki and M Watanabe ldquoOsmotic swelling to exfoliationExceptionally high degrees of hydration of a layered titanaterdquoJournal of the American Chemical Society vol 120 no 19pp 4682ndash4689 1998

[3] T Sasaki M Watanabe H Hashizume H Yamada andH Nakazawa ldquoMacromolecule-like aspects for a colloidalsuspension of an exfoliated titanate Pairwise association ofnanosheets and dynamic reassembling process initiated fromitrdquo Journal of the American Chemical Society vol 118 no 35pp 8329ndash8335 1996

[4] L Li RMa Y Ebina N Iyi and T Sasaki ldquoPositively chargednanosheets derived via total delamination of layered doublehydroxidesrdquo Chemistry of Materials vol 17 no 17pp 4386ndash4391 2005

[5] A Takagaki T Yoshida D Lu et al ldquoTitanium niobate andtitanium tantalate nanosheets as strong solid acid catalystsrdquo9e Journal of Physical Chemistry B vol 108 no 31pp 11549ndash11555 2004

[6] DW Kim A Blumstein and S K Tripathy ldquoNanocompositefilms derived from exfoliated functional aluminosilicatethrough electrostatic layer-by-layer assemblyrdquo Chemistry ofMaterials vol 13 no 5 pp 1916ndash1922 2001

[7] S Omwoma W Chen R Tsunashima and Y-F SongldquoRecent advances on polyoxometalates intercalated layereddouble hydroxides from synthetic approaches to functionalmaterial applicationsrdquo Coordination Chemistry Reviewsvol 258-259 pp 58ndash71 2014

[8] D-L Long and L Cronin ldquoPushing the frontiers in poly-oxometalate and metal oxide cluster sciencerdquo DaltonTransactions vol 41 no 33 pp 9815-9816 2012

[9] XWangW Chen and Y-F Song ldquoDirectional self-assemblyof exfoliated layered europium hydroxide nanosheets andNa9EuW10O36middot32H2O for application in desulfurizationrdquoEuropean Journal of Inorganic Chemistry vol 2014 no 17pp 2779ndash2786 2014

[10] M Adachi-Pagano C Forano and J-P Besse ldquoDelaminationof layered double hydroxides by use of surfactantsrdquo ChemicalCommunications vol 91 no 1 pp 91-92 2000

[11] M Meyn K Beneke and G Lagaly ldquoAnion-exchange re-actions of layered double hydroxidesrdquo Inorganic Chemistryvol 29 no 26 pp 5201ndash5207 1990

[12] B-I Lee and S-H Byeon ldquoHighly enhanced photo-luminescence of a rose-like hierarchical superstructure pre-pared by self-assembly of rare-earth hydroxocationnanosheets and polyoxomolybdate anionsrdquo Chemical Com-munications vol 47 no 14 pp 4093ndash4095 2011

[13] K-H Lee and S-H Byeon ldquoExtended members of the layeredrare-earth hydroxide family RE2(OH)5NO3middotnH2O (RE SmEu and Gd) synthesis and anion-exchange behaviorrdquo Eu-ropean Journal of Inorganic Chemistry vol 2009 no 7pp 929ndash936 2009

[14] H Jeong B-I Lee and S-H Byeon ldquoDirectional self-as-sembly of rare-earth hydroxocation nanosheets and para-dodecatungstate anionsrdquo Dalton Transactions vol 41 no 46pp 14055ndash14058 2012

[15] B-I Lee J-S Bae E-S Lee and S-H Byeon ldquoSynthesis andphotoluminescence of colloidal solution containing layeredrare-earth hydroxide nanosheetsrdquo Bulletin of the KoreanChemical Society vol 33 no 2 pp 601ndash607 2012

[16] F Geng Y Matsushita R Ma et al ldquoGeneral synthesis andstructural evolution of a layered family of Ln8(OH)20Cl4middotnH2O (LnNd Sm Eu Gd Tb Dy Ho Er Tm andY)rdquo Journal of the American Chemical Society vol 130 no 48pp 16344ndash16350 2008

[17] F Geng H Xin Y Matsushita et al ldquoNew layered rare-earthhydroxides with anion-exchange propertiesrdquo Chemistry-AEuropean Journal vol 14 no 30 pp 9255ndash9260 2008

[18] M Ando M Tadano S Yamamoto et al ldquoHealth effects offluoride pollution caused by coal burningrdquo Science of 9eTotal Environment vol 271 no 1ndash3 pp 107ndash116 2001

[19] X Qu P J J Alvarez and Q Li ldquoApplications of nano-technology in water and wastewater treatmentrdquo Water Re-search vol 47 no 12 pp 3931ndash3946 2013

[20] Z Barghouthi and S Amereih ldquoSpectrophotometric deter-mination of fluoride in groundwater using resorcin bluecomplexesrdquo American Journal of Analytical Chemistry vol 3no 9 pp 651ndash655 2012

[21] K S W Sing D H Everett R A W Haul et al ldquoReportingphysisorption data for gassolid systems with special referenceto the determination of surface area and porosity (Recom-mendations 1984)rdquo Pure and Applied Chemistry vol 57 no 4pp 603ndash619 1985

[22] T Kameda J Oba and T Yoshioka ldquoRecyclable Mg-Allayered double hydroxides for fluoride removal kinetic andequilibrium studiesrdquo Journal of HazardousMaterials vol 300pp 475ndash482 2015

[23] S Azizian ldquoKinetic models of sorption a theoretical analysisrdquoJournal of Colloid and Interface Science vol 276 no 1pp 47ndash52 2004

[24] A Ookubo K Ooi F Tani and H Hayashi ldquoPhase transitionof ClndashIntercalated hydrotalcite-like compound during ionexchange with phosphatesrdquo Langmuir vol 10 no 2pp 407ndash411 1994

[25] M Jobbagy and A E Regazzoni ldquoComplexation at the edgesof hydrotalcite the cases of arsenate and chromaterdquo Journal ofColloid and Interface Science vol 393 pp 314ndash318 2013

[26] M Jobbagy and A E Regazzoni ldquoAnion-exchange equilib-rium and phase segregation in hydrotalcite systems inter-calation of hexacyanoferrate(III) ionsrdquo9e Journal of PhysicalChemistry B vol 109 no 1 pp 389ndash393 2005

[27] P Miretzky and A F Cirelli ldquoFluoride removal from water bychitosan derivatives and composites a reviewrdquo Journal ofFluorine Chemistry vol 132 no 4 pp 231ndash240 2011

[28] F A Miller and C H Wilkins ldquoInfrared spectra and char-acteristic frequencies of inorganic ionsrdquoAnalytical Chemistryvol 24 no 8 pp 1253ndash1294 1952

[29] R S C Smart and N Sheppard ldquoInfrared and far infraredspectroscopic studies of the adsorption of water molecules onhigh-area alkali halide surfacesrdquo Journal of the ChemicalSociety Faraday Transactions 2 vol 72 p 707 1976


extend the novel applicability of positively chargednanosheets

Herein a simple new method of isolating the unilamellernanosheets is reported Stable unilameller nanosheets wereisolated by changing the pH of the colloidal solution from 65 to115 +e flocculated unilameller nanosheets show good per-formance for fluoride sorption from aqueous media providinga potentially useful sorbent material for water purificationtechnology And we demonstrate the recyclability of this ma-terial in fluoride separation from the aqueous medium

It is important to note that fluoride has detrimentalhealth effects to aquatic life and humans such as cancerdental fluorosis bone resorption endocrine disruptionmutations and brain damage [18] Hence industries thatuse fluoride in their processes ought to separate it fromwastewater before disposal of the water to aquatic systems

In addition although nanosorbents present a great po-tential in advancingwater andwastewater treatment efficiencymost of the nanosorbents reported so far generate secondarywastes which are equally hazardous to the environment [19]As such development of recyclable nanosorbents is a big stepforward in wastewater treatment efficiency

2 Materials and Methods

21 Chemical Materials Analytically pure KOH NaOHEu2O3 HCl NaF KBr lacmoid ethanol and methanol werepurchased from Alfa Aesar and used without furtherpurification

22 Preparation of Wet LEuH-Cl Eu-containing layeredrare-Earth hydroxides (Eu2(OH)5ClmiddotnH2O) (LEuH-Cl) weresynthesized and characterized according to literaturemethods [12] In brief 01M KOH solution was added to005M EuCl3middot6H2O solution with stirring at 24degC +e re-sultant mixture was heated to 60degC for 12 h and thereafterrefluxed for 24 h as with magnetic stirring +e resultantslurry was washed with deionized water three times in acentrifuge +e solid sample was labeled as wet LEuH-Cl

23 Flocculation +e delamination of fresh prepared wetLEuH-Cl aggregate material was achieved by sonication(3min 100W) to obtain an aqueous colloidal solutionSonication beyond these specifications is noted to result intobroken delaminated nanosheets +e pH of the aqueouscolloidal solution was adjusted from 65 to 115 using 1MNaOH solution that led to flocculation of unilameller LEuHnanosheets (LEuH-flocs) +e contents were centrifuged at2000 rpm and the residue was washed with distilled waterfour times before a final wash with acetone followed byvacuum drying at 40degC

24 Characterization +e OH content of solid samples wasdetermined titrimetrically by neutralization backtitrationafter dissolution of the samples in a 01N standard H2SO4Measurements of diffraction patterns of the 40degC driedsamples were achieved using a Rigaku XRD diffractometer

machine (6000) conditioned at 30mA 40 kV Cu-Ka(λ 0154 nm) and with a scan step of 001deg measured be-tween 3deg and 70deg Surface morphology was observed using aSEM (Zeiss Supra 55) machine simultaneously connected toan EDX detector In addition atomic forces were deter-mined using a Bruker AFM machine (A3A) in order todetermine lateral dimensions of nanosheets isolated Cross-sectional transverse morphological study was done using aHRTEM machine (Hitachi H-800) High sensitivity ele-mental analysis in aqueous solutions was performed usingICP-AES machine (ICPS-7500) Solid samples were firstdigested using aqua regia solutions before ICP-AES analysis+e pore volumes and specific surface area studies for solidsamples were done using BET and BJH methods withQuantachrome Autosorb1C VP machine Before such an-alyses were done the solid samples were first degassed at100degC for 6 hrs

25 Adsorption Experiments +e efficiency of fluoridesorption by LEuH-flocs nanosheets was compared to thesorption efficiency of LEuH-Cl aggregates To a knownconcentration of fluoride ions in aqueous media 04 g of testsample was introduced and then the test solution was keptstirring for 1 hour After that the nanocomposites werecentrifuged down and the supernatant solution was testedfor fluoride concentration colorimetrically [20] +e pelletedsample was dried and calcined at 500degC for 24 hours toremove the adsorbed fluoride and the material wasreconstituted in a NaCl solution to regenerate the LEuH-Clmaterial through the ldquomemory effectrdquo method of hydro-talcites [7] Direct adsorption of the fluoride ions by thecalcined LEuH-flocs was also determined

26 Kinetic Studies

261 Determination of Adsorption Rate and EquilibriumTen samples of fluoride ions dissolved in deionized water(100 ppm) were treated with 02 g of LEuH-flocs for 0 1 2 34 5 6 8 10 and 15 minutes After each experiment theLEuH-flocs were separated and the resultant supernatantwas tested for fluoride concentration A similar experimentwas repeated for the LEuH-Cl aggregate material

262 Adsorption Isotherms +e test samples (02 g) werestirred with 50ml solutions having different concentrations(1 5 10 20 50 100 200 300 400 500 and 1000 ppm) offluoride anions for 30 minutes to insure equilibrium +enanocomposites were centrifuged and the supernatant wastested for fluoride concentration+e amount of fluoride up-taken per gram of the LEuH-flocs (q) was determinedaccording to the following equation where C0 is the initialfluoride concentration C is the concentration of theequilibrated final solution V is the volume of the aqueousphase and m is the mass of the adsorbent in the system

q C0 minus C( 1113857V

m (1)










2minus





Delaminationwater

pH = 115



Soln pH = 45ndash95





200nm Cl

O

Eu

Eu Eu Pt ClEu

Eu

Eu

Si

0 1 2 5 6 7

(a)

200nm ClO Eu Eu

Eu EuEu Pt Cl

Si

0 1 2 5 6 7

(b)

200nm

O Eu

EuEu Pt

EuEu

Eu

Si

F

0 1 2 5 6 7

(c)

500nm

(d)








(2)


(a)

(b)

(c)



(a)

(b)

(c)

101

001

200

111 21

0 002

211

102

301

020 11

231

112

122

022

131

232

011

2 113

222

420 30

313

131

3 123

132

521

611 43

030

452

2

d100 = 129nm d010 = 075nmd001 = 086nm

20 402 eta (degree)


(a) (b)

(c)

(d) (e)


50nm

25nm

0nm

50nm

25nm

0nm




1000

1511 1454

3494

638

1634 818

848

578

535

1374

(c)

(b)

(a)


100

95

90

85

80


Sam

ple W

t (

)


Inter-layerwater

Dehydroxylationand

Dechlorination

Dehydroxylationand

Dechlorination

(b)

(a)



200

150

100

50

000 02 04 06



Vol

ume (

cm3 g

)

(a)

00 02 04 06 08 10

60

48

36

24

12

0


Vol

ume (

cm3 g

)


(b)







LEuH-F


LEuH-Claggregates

LEuH spinnels

(a)

(b)

(c)

(f)(g)

(e)

(d)


40

30

qe

qe

20

10

000 1 2

(a)

(b)

3 4 5 6Time (mins)

q (m

mol

g)



00

30

20

10

40

5 10 15 20 25 30C (mM times 10ndash1)

q (m

mol

g times

10ndash1

)qm

qm

A

B

(a)

0 5 10 15 20 25 30

30

25

20

15

10

05

00

Cq

C (mM)

y = 0092x +

0051 R2 = 099

y = 0017x + 0019 R2 = 081

A

B

(b)


100

80

60

40

20

00 1 2 3 4 5 6 7 8 9 10

Number of cycle

q

max


200nm

(a)

200nm

(b)

Figure 13 Continued






4 Conclusions



Data Availability




Acknowledgments


References


200nm

(c)

200nm

(d)

(e)








































2minus





Delaminationwater

pH = 115



Soln pH = 45ndash95





200nm Cl

O

Eu

Eu Eu Pt ClEu

Eu

Eu

Si

0 1 2 5 6 7

(a)

200nm ClO Eu Eu

Eu EuEu Pt Cl

Si

0 1 2 5 6 7

(b)

200nm

O Eu

EuEu Pt

EuEu

Eu

Si

F

0 1 2 5 6 7

(c)

500nm

(d)








(2)


(a)

(b)

(c)



(a)

(b)

(c)

101

001

200

111 21

0 002

211

102

301

020 11

231

112

122

022

131

232

011

2 113

222

420 30

313

131

3 123

132

521

611 43

030

452

2

d100 = 129nm d010 = 075nmd001 = 086nm

20 402 eta (degree)


(a) (b)

(c)

(d) (e)


50nm

25nm

0nm

50nm

25nm

0nm




1000

1511 1454

3494

638

1634 818

848

578

535

1374

(c)

(b)

(a)


100

95

90

85

80


Sam

ple W

t (

)


Inter-layerwater

Dehydroxylationand

Dechlorination

Dehydroxylationand

Dechlorination

(b)

(a)



200

150

100

50

000 02 04 06



Vol

ume (

cm3 g

)

(a)

00 02 04 06 08 10

60

48

36

24

12

0


Vol

ume (

cm3 g

)


(b)







LEuH-F


LEuH-Claggregates

LEuH spinnels

(a)

(b)

(c)

(f)(g)

(e)

(d)


40

30

qe

qe

20

10

000 1 2

(a)

(b)

3 4 5 6Time (mins)

q (m

mol

g)



00

30

20

10

40

5 10 15 20 25 30C (mM times 10ndash1)

q (m

mol

g times

10ndash1

)qm

qm

A

B

(a)

0 5 10 15 20 25 30

30

25

20

15

10

05

00

Cq

C (mM)

y = 0092x +

0051 R2 = 099

y = 0017x + 0019 R2 = 081

A

B

(b)


100

80

60

40

20

00 1 2 3 4 5 6 7 8 9 10

Number of cycle

q

max


200nm

(a)

200nm

(b)

Figure 13 Continued






4 Conclusions



Data Availability




Acknowledgments


References


200nm

(c)

200nm

(d)

(e)


































200nm Cl

O

Eu

Eu Eu Pt ClEu

Eu

Eu

Si

0 1 2 5 6 7

(a)

200nm ClO Eu Eu

Eu EuEu Pt Cl

Si

0 1 2 5 6 7

(b)

200nm

O Eu

EuEu Pt

EuEu

Eu

Si

F

0 1 2 5 6 7

(c)

500nm

(d)








(2)


(a)

(b)

(c)



(a)

(b)

(c)

101

001

200

111 21

0 002

211

102

301

020 11

231

112

122

022

131

232

011

2 113

222

420 30

313

131

3 123

132

521

611 43

030

452

2

d100 = 129nm d010 = 075nmd001 = 086nm

20 402 eta (degree)


(a) (b)

(c)

(d) (e)


50nm

25nm

0nm

50nm

25nm

0nm




1000

1511 1454

3494

638

1634 818

848

578

535

1374

(c)

(b)

(a)


100

95

90

85

80


Sam

ple W

t (

)


Inter-layerwater

Dehydroxylationand

Dechlorination

Dehydroxylationand

Dechlorination

(b)

(a)



200

150

100

50

000 02 04 06



Vol

ume (

cm3 g

)

(a)

00 02 04 06 08 10

60

48

36

24

12

0


Vol

ume (

cm3 g

)


(b)







LEuH-F


LEuH-Claggregates

LEuH spinnels

(a)

(b)

(c)

(f)(g)

(e)

(d)


40

30

qe

qe

20

10

000 1 2

(a)

(b)

3 4 5 6Time (mins)

q (m

mol

g)



00

30

20

10

40

5 10 15 20 25 30C (mM times 10ndash1)

q (m

mol

g times

10ndash1

)qm

qm

A

B

(a)

0 5 10 15 20 25 30

30

25

20

15

10

05

00

Cq

C (mM)

y = 0092x +

0051 R2 = 099

y = 0017x + 0019 R2 = 081

A

B

(b)


100

80

60

40

20

00 1 2 3 4 5 6 7 8 9 10

Number of cycle

q

max


200nm

(a)

200nm

(b)

Figure 13 Continued






4 Conclusions



Data Availability




Acknowledgments


References


200nm

(c)

200nm

(d)

(e)





































(2)


(a)

(b)

(c)



(a)

(b)

(c)

101

001

200

111 21

0 002

211

102

301

020 11

231

112

122

022

131

232

011

2 113

222

420 30

313

131

3 123

132

521

611 43

030

452

2

d100 = 129nm d010 = 075nmd001 = 086nm

20 402 eta (degree)


(a) (b)

(c)

(d) (e)


50nm

25nm

0nm

50nm

25nm

0nm




1000

1511 1454

3494

638

1634 818

848

578

535

1374

(c)

(b)

(a)


100

95

90

85

80


Sam

ple W

t (

)


Inter-layerwater

Dehydroxylationand

Dechlorination

Dehydroxylationand

Dechlorination

(b)

(a)



200

150

100

50

000 02 04 06



Vol

ume (

cm3 g

)

(a)

00 02 04 06 08 10

60

48

36

24

12

0


Vol

ume (

cm3 g

)


(b)







LEuH-F


LEuH-Claggregates

LEuH spinnels

(a)

(b)

(c)

(f)(g)

(e)

(d)


40

30

qe

qe

20

10

000 1 2

(a)

(b)

3 4 5 6Time (mins)

q (m

mol

g)



00

30

20

10

40

5 10 15 20 25 30C (mM times 10ndash1)

q (m

mol

g times

10ndash1

)qm

qm

A

B

(a)

0 5 10 15 20 25 30

30

25

20

15

10

05

00

Cq

C (mM)

y = 0092x +

0051 R2 = 099

y = 0017x + 0019 R2 = 081

A

B

(b)


100

80

60

40

20

00 1 2 3 4 5 6 7 8 9 10

Number of cycle

q

max


200nm

(a)

200nm

(b)

Figure 13 Continued






4 Conclusions



Data Availability




Acknowledgments


References


200nm

(c)

200nm

(d)

(e)
































(a)

(b)

(c)

101

001

200

111 21

0 002

211

102

301

020 11

231

112

122

022

131

232

011

2 113

222

420 30

313

131

3 123

132

521

611 43

030

452

2

d100 = 129nm d010 = 075nmd001 = 086nm

20 402 eta (degree)


(a) (b)

(c)

(d) (e)


50nm

25nm

0nm

50nm

25nm

0nm




1000

1511 1454

3494

638

1634 818

848

578

535

1374

(c)

(b)

(a)


100

95

90

85

80


Sam

ple W

t (

)


Inter-layerwater

Dehydroxylationand

Dechlorination

Dehydroxylationand

Dechlorination

(b)

(a)



200

150

100

50

000 02 04 06



Vol

ume (

cm3 g

)

(a)

00 02 04 06 08 10

60

48

36

24

12

0


Vol

ume (

cm3 g

)


(b)







LEuH-F


LEuH-Claggregates

LEuH spinnels

(a)

(b)

(c)

(f)(g)

(e)

(d)


40

30

qe

qe

20

10

000 1 2

(a)

(b)

3 4 5 6Time (mins)

q (m

mol

g)



00

30

20

10

40

5 10 15 20 25 30C (mM times 10ndash1)

q (m

mol

g times

10ndash1

)qm

qm

A

B

(a)

0 5 10 15 20 25 30

30

25

20

15

10

05

00

Cq

C (mM)

y = 0092x +

0051 R2 = 099

y = 0017x + 0019 R2 = 081

A

B

(b)


100

80

60

40

20

00 1 2 3 4 5 6 7 8 9 10

Number of cycle

q

max


200nm

(a)

200nm

(b)

Figure 13 Continued






4 Conclusions



Data Availability




Acknowledgments


References


200nm

(c)

200nm

(d)

(e)

































1000

1511 1454

3494

638

1634 818

848

578

535

1374

(c)

(b)

(a)


100

95

90

85

80


Sam

ple W

t (

)


Inter-layerwater

Dehydroxylationand

Dechlorination

Dehydroxylationand

Dechlorination

(b)

(a)



200

150

100

50

000 02 04 06



Vol

ume (

cm3 g

)

(a)

00 02 04 06 08 10

60

48

36

24

12

0


Vol

ume (

cm3 g

)


(b)







LEuH-F


LEuH-Claggregates

LEuH spinnels

(a)

(b)

(c)

(f)(g)

(e)

(d)


40

30

qe

qe

20

10

000 1 2

(a)

(b)

3 4 5 6Time (mins)

q (m

mol

g)



00

30

20

10

40

5 10 15 20 25 30C (mM times 10ndash1)

q (m

mol

g times

10ndash1

)qm

qm

A

B

(a)

0 5 10 15 20 25 30

30

25

20

15

10

05

00

Cq

C (mM)

y = 0092x +

0051 R2 = 099

y = 0017x + 0019 R2 = 081

A

B

(b)


100

80

60

40

20

00 1 2 3 4 5 6 7 8 9 10

Number of cycle

q

max


200nm

(a)

200nm

(b)

Figure 13 Continued






4 Conclusions



Data Availability




Acknowledgments


References


200nm

(c)

200nm

(d)

(e)




































LEuH-F


LEuH-Claggregates

LEuH spinnels

(a)

(b)

(c)

(f)(g)

(e)

(d)


40

30

qe

qe

20

10

000 1 2

(a)

(b)

3 4 5 6Time (mins)

q (m

mol

g)



00

30

20

10

40

5 10 15 20 25 30C (mM times 10ndash1)

q (m

mol

g times

10ndash1

)qm

qm

A

B

(a)

0 5 10 15 20 25 30

30

25

20

15

10

05

00

Cq

C (mM)

y = 0092x +

0051 R2 = 099

y = 0017x + 0019 R2 = 081

A

B

(b)


100

80

60

40

20

00 1 2 3 4 5 6 7 8 9 10

Number of cycle

q

max


200nm

(a)

200nm

(b)

Figure 13 Continued






4 Conclusions



Data Availability




Acknowledgments


References


200nm

(c)

200nm

(d)

(e)
































00

30

20

10

40

5 10 15 20 25 30C (mM times 10ndash1)

q (m

mol

g times

10ndash1

)qm

qm

A

B

(a)

0 5 10 15 20 25 30

30

25

20

15

10

05

00

Cq

C (mM)

y = 0092x +

0051 R2 = 099

y = 0017x + 0019 R2 = 081

A

B

(b)


100

80

60

40

20

00 1 2 3 4 5 6 7 8 9 10

Number of cycle

q

max


200nm

(a)

200nm

(b)

Figure 13 Continued






4 Conclusions



Data Availability




Acknowledgments


References


200nm

(c)

200nm

(d)

(e)




































4 Conclusions



Data Availability




Acknowledgments


References


200nm

(c)

200nm

(d)

(e)
































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