Titanium Pyrophosphate for Removal of Trivalent Heavy Metals and Actinides Simulated by ...

13 Pages·2017·3.25 MB·English

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e Scientific World Journal
Volume 2017, Article ID 2675897, 12 pages
https://doi.org/10.1155/2017/2675897
Research Article
Titanium Pyrophosphate for Removal of Trivalent Heavy Metals
and Actinides Simulated by Retention of Europium
HuemantzinBalanOrtiz-Oliveros,1,2RosaMaríaFlores-Espinosa,3
EduardoOrdoñez-Regil,3andSuilmaMariselaFernández-Valverde3
1Direccio´ndeInvestigacio´nTecnolo´gica,InstitutoNacionaldeInvestigacionesNucleares,A.P.18-1027,Col.Escando´n,
Delegacio´nMiguelHidalgo,CP11801,CiudaddeMe´xico,Mexico
2UniversidadAuto´nomadelEstadodeMe´xico,InstitutoLiterario100,CP50000,Toluca,EstadodeMe´xico,Mexico
3Direccio´ndeInvestigacio´nCient´ıfica,InstitutoNacionaldeInvestigacionesNucleares,A.P.18-1027,Col.Escando´n,
Delegacio´nMiguelHidalgo,CP11801,CiudaddeMe´xico,Mexico
CorrespondenceshouldbeaddressedtoHuemantzinBalanOrtiz-Oliveros;[email protected]
Received 6 January 2017; Revised 24 April 2017; Accepted 9 May 2017; Published 12 July 2017
AcademicEditor:Mu.Naushad
Copyright©2017HuemantzinBalanOrtiz-Oliverosetal.ThisisanopenaccessarticledistributedundertheCreativeCommons
AttributionLicense,whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedtheoriginalworkis
properlycited.
This work addresses the synthesis of titanium pyrophosphate, as well as the characterization and evaluation of the sorption
processofeuropium,forremovaloftrivalentheavymetalsandactinidessimulate.Theevaluationofthesurfacepropertiesof
titaniumpyrophosphatewascarriedoutdeterminingthesurfaceroughnessandsurfaceacidityconstants.Thevaluesobtained
fromthedeterminationofthesurfaceroughnessofthesynthesizedsolidindicatethatthesurfaceofthematerialpresentsitself
asslightlysmooth.TheFITEQLprogramwasusedtofittheexperimentaltitrationcurvestoobtainthesurfaceacidityconstants:
log𝐾+ =3.59±0.06andlog𝐾− =−3.90±0.05.Theresultsofsorptionkineticsevidencedthatthepseudo-ordermodelexplains
theretentionprocessofeuropium,inwhichtheinitialsorptionvelocitywas8.3×10−4mgg−1min−1andkineticconstantwas1.8×
−3 −1 −1
10 gmgmin .Themaximumsorptioncapacitywas0.6mgg .Theresultsobtainedfromsorptionedgeshowedtheexistenceof
twobidentatecomplexesonthesurface.
1.Introduction high toxicity, easy dispersion, bioaccumulation, and persis-
tency in the environment [5–9], particularly for actinides,
Therapidgrowthofourpopulationduringthelastdecades
whoselonghalf-lifeandhighradiotoxicityposeahealthrisk
hasledtoanecologicalimbalanceinourenvironmentwith
sincetheycanleadtovariousdiseasessuchaspoisoning,ner-
severe consequences and major risks both for our health
voussystemdamage,andcancer[10].Thetoxicityandbehav-
and our environment. An example for this is the cases of
iorofpersistentorganiccompoundsdependontheirconcen-
contaminationofhydricresources.Themostcommonwater
tration, molecular structure, and type of functional groups.
pollutantsarefecalresiduesfromanimalsandhumans,pesti-
Also, heavy metals can react with organic matter form-
cidesusedinagriculturalactivities,organicresiduesderived
ingorganometalliccompounds,whichmaybemoretoxicto
fromoil,heavymetals,andradionuclides,amongothers[1–
aquaticecosystems[11].
4]. In recent years, persistent organic contaminants such as
phenols,chlorophenols,chlorobenzenes,andpharmaceutical The contamination of hydric resources with metals and
drugs [2], as well as heavy metals like As, Cd, Cu, Cr, Pb, radionuclides is linked to manufacturing processes, the
Hg,andsoforth,orradionuclideslikeRaandactinides,are metal-mechanicindustry,oilprocessing,mining,transporta-
becoming more common in hydric resources. These have tionprocessesoferosionmaterial,infiltrationofsubterranean
caught the interest of the scientific community due to their waters,andnuclearaccidents,amongothers[12,13].
2 TheScientificWorldJournal
3+
Due to the chemical properties of these metals, their Eu ; and Maslova et al. [36] studied the synthesis and
removalrequiresspecifictreatmentprocesses.Theprocesses sorptionpropertiesofamorphoustitaniumphosphates.
most used are chemical precipitation, coagulation/floccu- Literaturereportsdifferentsynthesismethodsofpolyan-
lation, flotation (with dispersed air or dissolved air), elec- ions such as (P2O7)4−. The methods most used are (a)
trochemicalprocess,photocatalyticprocess,inverseosmosis, methods of coprecipitation where organic compounds are
ionexchange,andsorption[14–20].Manyoftheseprocesses usedasionsourcesandphosphoricacidsareusedasphos-
areusuallyefficientbutcanbelimitedbytheconcentration phatesources[33,34]and(b)hydrothermalmethodswhere
and the physicochemical form of the heavy metal as well metallic oxides are used as ion sources and water-soluble
asthecostsandthedifficultiesoftheoperationsuchasthe phosphate salts as phosphate sources in an acid medium
generationofresidualsludge. [32]. Rai et al. [33] used the method of coprecipitation
Sorption is one of the most used processes for the to obtain titanium pyrophosphate. During this method, an
removal of low concentrations (including trace levels) of aqueous solution of titanium isopropoxide is mixed with a
heavymetalsandradionuclidesinanaqueousmedium.The solution of phosphoric acid and diluted hydrochloric acid.
term sorption is used in a generic way to describe physic- The mixture is heated to 343K and agitated for 10-11h. In
ochemical processes in which the dissolved contaminant theend,theobtainedmaterialisheatedto1073Kfor3hin
(metal,radionuclide,etc.)istransferredfromthesolutionto a CO2 atmosphere. Other authors obtain TiP2O7 using the
thesolidphase[21,22],eitherbyprecipitatingonthesurface, sequenceproposedbyPatoux.ItconsistsinmixingTiO2and
diffusingontheoutsideandinsideoftheporesofthesolid (NH4)2HPO4andheatingthemixtureprogressivelyto1273K
phase,orbyformingcomplexesonthesurface[23]. usingintermittentgriddingsequences[45,46].
The phenomena of sorption are also used with success Thisworkaddressesthesynthesisoftitaniumpyrophos-
in the construction of engineering barriers to avoid the phate using an improved method. Titanium pyrophosphate
dispersion of heavy metals and radionuclides in the soil was prepared using high purity TiCl4 and concentrated
and subsoil, where the sorption process is carried out by H3PO4,throughprecipitationinanatmosphereofnitrogen.
interaction of chemical species (metals or radionuclides) in The precipitate obtained was heated to 1073K for 3h. With
solutionwithsurfacefunctionalgroupsofthecomponentsof titaniumpyrophosphatepreviouslycharacterized,thereten-
thesoil[24,25]. 3+
tioncapacityofEu wasevaluatedasachemicalanalogous
Duetotheimportancethatsorptionprocesseshaveinthe 3+ 3+ 3+
for heavy metals (Cr and As ) and actinides (Ac ) [47,
treatmentofindustrialwastewater,inindustrialapplications
48].Itiswellknownthattheeuropiumionicradiusissimilar
ortheremediationofsoilscontaminatedwithmetals,interest 3+ 3+ 3+
to Cr , As , and Ac radii, which results in a similar
has again risen during the last couple of years in carrying
physicochemical behavior [49]. Additional, modeling the
out investigations aimed at understanding the phenomena
sorption of the europium onto the titanium pyrophosphate
involved in sorption and also the development of different
asafunctionofthepHusingasurfacecomplexationmodel
materials or sorbents and in determining their retention
wasperformed.
properties.Inthatregard,severalinvestigatorshavestudied
low cost materials such as biosorbents, in which there is
2.Experimental
the biomass of residues of vegetable or animal origin or
bacteriaandfungi[8,26,27].Thesebiosorbentshaveproven 2.1. Synthesis of TiP O . The titanium pyrophosphate was
2 7
themselves efficient in the removal of metals in aqueous obtained by the described method by Ortiz-Oliveros et al.
solutions. In addition, new mineral sorbents have been [35]. Synthesis was performed mixing high purity liquid
synthesizedandtestedthatunlikethebiosorbentsmaycost titaniumtetrachloride(99.9%,Aldrich)astheionsourcewith
more but have a high retention capacity, low solubility, and concentratedphosphoricacid(85%,Baker)asthephosphate
high thermic and structural stability, even when submitted source,whilemaintainingaTi/Pstoichiometricratioof1:2
to intense fields of gamma radiation, an example of which underanitrogenatmosphereinaglovebox.TiCl4wasslowly
are the tests of sorption of metals and radionuclides in added to a stirred solution of phosphoric acid, and the
phosphatesofisomorphictetravalentmetals,perovskite,and precipitatewasrecuperatedandwashedwithdeionizerwater.
othersyntheticinorganicmaterials[28–31]. The precipitate was then centrifuged, separated, and dried
Asfarasphosphatesareconcernedsincethediscoveryof at room temperature. Finally, the dried solid was heated at
LiFePO4,newmaterialshavebeensynthesizedandidentified 1073Kfor3h.
3−
based on phosphates-based polyanions such as (PO4) ,
4− 5−
(P2O7) , or (P3O10) [32], which find their application 2.2.MaterialCharacterization. Thesolidobtainedwaschar-
in different fields of knowledge due to their major struc- acterized by X-ray powder diffraction (XRD) patterns in
tural anomalies, such as their anisotropic deformation, low a diffractometer (Siemens D5000). The XRD patterns were
redox potential, and low cost of synthesis [32, 33]. These obtained with Cu monochromatic K𝛼 rays at 35kV and
characteristics have motivated the study of the retention 25mA. The 2𝜃 diffraction angle (4∘–70∘) was scanned at a
−1
of metals in phosphates. The sorption of lanthanides and scanrateof1.83min .Theelementalandstructuralanalysis
uranylondifferentsyntheticphosphateshasbeenreported; ofthesolidwasperformedinascanningelectronmicroscope
forexample,Wangetal.[34]studiedthesorptionofU(VI) (PHILIPS model XL-30), coupled to a microprobe (EDAX
onto Zr1−𝑥Ti𝑥P2O7 and TiP2O7; Ortiz-Oliveros et al. [35] modelDX-4)forenergydispersiveX-rayspectroscopywith
studied the synthesis of 𝛼-Ti(HPO4)2H2O and sorption of aresolutionof140eV.Representativesampleswerefixedon
TheScientificWorldJournal 3
metalslideswithcarbontapeandwerecoveredwithathin nitrogenatmospherewith400mgoftitaniumpyrophosphate
filmofconductingmaterial(Au). for10mLofaqueoussolution(1×10−4Mofeuropiumnitrate,
Additional, samples of titanium pyrophosphate before Aldrich99.9%).Priortothesorptionexperiments,thesolid
and after europium sorption were investigated by Atomic was first hydrated with 0.5 KNO3 solutions for 24h and
ForceMicroscopy(CypherAsylumResearchMicroscope). stirredat45rpm,whichwasshowntobesufficienttoreach
Finally, the specific area (𝐴𝑠) was determined in a equilibrium. Afterward, the suspensions were centrifuged
GEMINI2360Micrometricssurfaceareaanalyzer. at 3500rpm for 15min, and all of the supernatant was
removed.
2.3.DeterminationofSurfaceProperties. Thesurfaceproper- A 10mL aliquot of the europium nitrate stock solution
tieswereevaluatedusinganalyticalgradereagents:potassium was added to the hydrated solid and then the suspensions
nitrate (Sigma-Aldrich ≥ 99%) and potassium hydroxide were adjusted to the required pH values and contact time,
(Baker≥85%).Allofthesolutionswerepreparedwithdeion- respectively.
izedwaterunderanitrogenflux,andpotassiumnitratewas The experiments carried out were (1) the evaluation of
chosen as the background salt for fix the ionic strength. At sorptionkineticsand(2)modelingthesorptionasafunction
thesametime,allpotentiometrictitrationswereperformed of the pH using a surface complexation model. For the
in a potentiometer ThermoOrion 720A+ with a combined 3+
sorptionkineticsofEu ,sorptionisothermswereperformed
Ag/AgClelectrode.
as a function of contact time by estimating the Eu(NO3)3
The evaluation of the surface properties of titanium
concentrations retained in the titanium pyrophosphate at
pyrophosphate was carried out determining (a) the surface
differentcontacttimes(1,3,5,7,16,18,20,22,24,48,and72h).
roughnessand(b)thesurfaceacidityconstants(𝐾+and𝐾−).
ThesorptionisothermsasfunctionofpHwereobtainedby
adjustingthesolutionstothedesiredpHvalue(1to7),which
Surface Roughness. The characterization of the superficial
wereleftfor24hat45rpm.
irregularity of the material (roughness) was determined
In both experiments, the suspensions were then cen-
accordingtoIsmailandPfeifer[50].Thismethodconsistsin
trifugedat3500rpmfor15minutesandseparated.Asolution
determining the surface fractal dimension by means of N2 without europium was used as the reference. Finally, the
adsorptionisothermsand(1):
europium uptake on solid was determined by measuring
𝑉 𝑃 the luminescence in a Fluorolog Jobin Yvon Horiba with
ln(𝑉 )=Γ+𝐴[ln(ln(𝑃𝑜))], (1) Xenonlamp(450W).Quartzcellswereloadedwith500𝜇L
𝑚
aliquots of the supernatant from each experiment and the
where 𝑉 represents the volume of adsorbed gas molecules stock solution (as a reference). The samples were excited
(mLg−1)attheequilibriumpressure(𝑃)andsaturationpres- at 397. The europium emission spectra were recorded from
sure(𝑃𝑜);𝑉𝑚isthevolumeofmonolayercoverageinmLg−1; 570nm to 650nm and the highest peaks were quantified
Γ is an exponential factor; and 𝐴 is a power-law exponent at 590 and 610nm [48]. An integration time 1.0s and a
dependent on the fractal dimension (𝐷) and adsorption wavelength increment of 0.5nm were used. The obtained
mechanism.TheN2adsorptionisothermwasdeterminedin spectrawereanalyzedinFluorologsoftware.
aGEMINI2360Micrometricssurfaceareaanalyzer.
2.4.1.ModelingSorptionKinetics. Therearedifferentsorption
Surface Acidity Constants. The surface acidity constants models in the literature which try to explain retention of
weredeterminedbyfittingtheexperimentalpotentiometric metals and radionuclides in solid substrates. These can be
titration curves using the FITEQL 4v program [51, 52]. dividedintoempiricandthermodynamicmodels.Theformer
Thiscomputerprogramdeterminesthechemicalequilibrium
isusefulwhenitcomestoestablishingkineticsandsorption
constantsfromexperimentaldataobtainedbypotentiometric
capacityasafirststepofestablishingtheinteractionmecha-
titrations.Theequilibriummodelmustbesolvedateachtitra-
nismbetweenthemetalandthesolidbutwithouttakinginto
tionpoint;therefore,theparametersareadjustedtominimize
considerationthatthesurfacesitesstronglydependonthepH
the difference between the experimental values and those
oftheaqueousmedium.Thelatterontheotherhandallows
calculated from the model. Several Surface Complexation
astudyofthedependencyofthepHfromthesurfacesitesof
Models have been used to simulate the experimental data,
thesolidfromtheknowledgeonthesurfaceacidityconstants
and in this case, the Constant Capacitance Model (CCM)
andbymeansoftheestimationoftheconstantsofformation
was used to fit the potentiometric titrations curves. This
ofsurfacecomplexes.
model has been used in many solids such as hydrous ferric
oxides, phosphates, and titanates. Our experimental condi- In the present work, four kinetic sorption models were
tions(0.5M)meettheionicstrengthrestrictioninthismodel, used with the aim of explaining the obtained experimental
and this model is preferred because of the relatively low data and determining the possible sorption mechanism of
3+
number of adjustable parameters. These parameters in the Eu in titanium pyrophosphate. The models used were
CCMare𝐾+ and𝐾− constants,theinner-layercapacitance pseudo-first-order (Lagergren’s equation), Elovich, pseudo-
(𝐶),thetotalconcentrationofsurfacessites,and𝐴𝑠. second-order,andintraparticlediffusionmodels.
2.4. Sorption Experiments. All of the sorption experiments Pseudo-First-Order Model. First-order kinetic model is pro-
werecarriedoutinpolypropylenetubesat303Kandundera posedbyLagergren.Thismodeliswidelyusedtoexplainthe
4 TheScientificWorldJournal
sorption of solutes in aqueous solution [53]. The equation 3.ResultsandDiscussions
usedisgivenas
3.1. Material Characterization. The elementary analysis by
𝑘󸀠𝑡 EDS showed only oxygen, titanium, and phosphorous, and
log(𝑞𝑒−𝑞𝑡)=log(𝑞𝑒)− 2.303, (2) the Ti/P relationship was in agreement with the theoretical
ratiointitaniumdiphosphate.Withinthedetectionlimitsof
where𝑞𝑒isthesorptioncapacityatequilibrium(mgg−1),𝑞𝑡is thetechniqueused(𝑍>12),nocontaminantsweredetected
themassadsorbed(mgg−1),𝑡isthecontacttime(min),and inthesolidobtained.Additionally,Figure1showsadistribu-
𝑘󸀠isthefirst-ordersorptionconstant(min−1). tionmapofTi,P,andOofasamplerepresentativeofthesolid.
The map shows that the material has an elemental homo-
Elovich Model. It is a kinetic model used to describe the genous distribution. Figure 2(a) shows the micrograph of
kineticsofheterogeneouschemicalsorptionofgasesinsolids thesolidobtainedat5000x,inwhichpolymorphicparticles
[54]. Also, this model has been widely used to describe the of laminar appearance with a size of less than 3𝜇m can be
chemisorptionoftheadsorbatebyasolidinaqueousmedium observed. Figure 2(b) shows the micrograph of the solid
[55].Themodelpresentsitselfmathematicallyasfollows: obtained at 20000x, in which particles of spherical appear-
𝛿𝑞 anceandirregularsurfacewithanapproximatesizeof2𝜇m
𝑡 =𝛼exp(−𝛽𝑡𝑞), (3) canbeobserved.
𝛿𝑡 𝑡
Figure 3 shows XDR spectra of the sample prepared at
where𝛼istheinitialsorptionvelocity(mgg−1min−1)and𝛽 1073K,inwhichthreeintensereflectionslocatedattheangle
isthedesorptionconstant(gmg−1).Otherauthorsrelate𝛽to 2𝜃,22.55,25.25,and27.71,identifiedascubicTiP2O7 witha
Pa3(205)spacegroup(JCPD38-1468)canbeobserved.Fur-
theactivationenergyforthechemisorption[55].
thermore,aslightslippingofthediffractionpeakstotheright
Pseudo-Second-Order Model. Second-order kinetic model is wasobserved,indicatinglossofthesuperstructure.Itcanbe
widely used to explain the processes of chemisorption of explainedconsideringthat,inthiswork,synthesiswascarried
metals [56]. In (4), the general expression of the model is out at 1073K during 3h, and Norberg et al. [60] reported
presented: that at high temperatures the structure of pyrophosphate
group was disordered due to having unfavorable 180 bond
𝛿𝑞
𝑡 =𝑘 (𝑞 −𝑞)2, (4) angles.
𝛿𝑡 2 𝑒 𝑡 Theresultsobtainedfromtheanalysisby AFMarepre-
sented in Figure 4. This shows the surface images of the
where−1𝑘2 is−1the second-order sorption velocity constant titaniumpyrophosphatebeforeandaftertheeuropiumsorp-
(gmg min ).
tion. Figure 4(a) (before sorption) clearly showed various
small and larger particles in the surface morphology of
Intraparticle Diffusion Model. It is fractional-order kinetic
solid.Additionally,weobservedthatthesurfaceisapparently
model which permits establishing whether the sorption
smooth. Figure 4(b) (after sorption) shows the influence of
process is limited by intraparticle diffusion [57, 58]. In (5),
europium sorption on the surface morphology of titanium
themathematicalexpressionofthemodelisshown:
pyrophosphate. As can be clearly seen, the surface is fully
𝑞 =𝑘𝑡0.5+𝐶 , (5) coveredwithalayerofsorbedeuropium.
𝑡 𝑖 SL
Furthermore,theresultsfromtheinterpretationadsorp-
where 𝑘𝑖 is the intraparticle diffusion rate (mg g−1min−0.5) tion isotherm of N2 (by BET method) showed that the
and𝐶SL isaconstantrelatedtothethicknessofthesurface titanium pyrophosphate synthetized has an 𝐴𝑠 of 9.0 ±
layerformedontheadsorbed.Thehigherthevaluesof𝐶SL, 0.1m2g−1.
thegreatertheeffectofthesurfaceboundarylayer.
3.2.DeterminationofSurfaceProperties
2.4.2. Estimation of Surface Complex. The surface complex
SurfaceRoughness.Figure5(a)showstheadsorptionisotherm
constantsfortheeuropiumspeciessorbedontothetitanium
pyrophosphatewereobtainedfromthesorptionisothermsas ofN2.ItobeysareversibleisothermoftypeIIthatisconcave
function of pH using the FITEQL program and the CCM. at low pressure in relation to 𝑃/𝑃𝑜 and has originated from
Theinputdataare𝐾+,𝐾−,andinner-layercapacitancevalues the physisorption of nitrogen and is typical in nonporous
obtained. materials.Inthisfigure,twoperfectlydefinedregionscanbe
In the sorption modeling, it was assumed that (a) a observed. Region I (0 < 𝑃/𝑃𝑜 > 0.5) shows the beginning
monodentate complex is formed by the interaction of a of an almost linear section (starting from point B) which
europium species in solution with only a single surface site indicates the ending of the monolayer. Region II (0.5 <
and (b) a bidentate complex is formed when a europium 𝑃/𝑃𝑜 > 0.95) shows the beginning of the curvature of the
speciesinsolutioninteractswithtwosurfacesites. isothermandindicatestheinflexionpointoftheoverlapof
Theidentificationofthechemicalspeciesofeuropiumin the monolayer and the beginning of the multilayer, where
solution,tobeusedastheinitialinputinthesorptionsimu- 𝑃/𝑃𝑜 = 0.5 [61]. When 𝑃/𝑃𝑜 = 1, the thickness of the
lation,wascarriedoutbyMEDUSAprogramoftheSweden multilayerseemstoincreasewithoutlimitandthefillingof
RoyalInstituteofTechnology[59]. thevolumeoftheporesiscausedbycapillarycondensation.
TheScientificWorldJournal 5
SE/BSE, 255 None, 255 OKa, 15
PKa, 31 TiKa, 24
Figure1:Elementaldistributionofthesolidobtained,whereonlyTi,P,andOareobserved.
(a) (b)
Figure2:(a)Micrographobtainedat5000xmagnification,wherepolymorphicparticlesoflaminarappearancecouldbeobserved(3𝜇m).
(b)Micrographobtainedat20000xmagnification,wheresphericalparticlesofirregularsurfaceareobserved(2𝜇m).
Figure 5(b) shows the adjusted results of the isotherm canbeinferredthatthesurfaceofthesynthesizedsolidisnot
oftheadsorptionofN2 using(1).Thisfigurewasdrawnup rough.Inthesameway,IsmaelandPfeifer[50]suggestthat
plottingln(𝑉/𝑉𝑚)versusln(ln𝑃𝑜/𝑃).Inaccordancewiththe whenapplyingin(1)therelations𝐷−3/3and𝐷−3,both
isotherm analysis (Figure 5(a)), it can be deduced that two mechanisms function simultaneously and an intermediate
fractaldiameters(𝐷1and𝐷2)exist,whichareassociatedwith slope is obtained. According to Ismael and Pfeifer [50], the
twoadsorptionforces.AccordingtoIsmailandPfeifer[50], effect of the surface tension on the slope of the linearized
whenvanderWaalsattractionforcesaredominantbetween equation(1)canbeestablishedfromtheequality𝛿 = 3(1+
thesolidandtheabsorbentfilm,𝐷=3𝐴+3,thenthesurface 𝐴)−2 = −0.69,becausewhen𝐷 ≥ 2thevalueof𝛿should
tensionoftheliquid/gasisinsignificant.Whenthecapillary reach an absolute small value and the surface tension on
force is important, 𝐷 = 𝐴 + 3. From the equation of the the gradient would be negligible. When 𝛿 < 0, the surface
adjustmentoftheexperimentaldata,ithasbeenobtainedthat tensionisnotconsideredinsignificant.Whenconsideringthe
thevalueoftheslope𝐴=−0.56withastatisticalcorrelation magnitudeof𝐷2,itcanbeestablishedthatthesurfaceofthe
coefficient𝑟2 = 0.99allowsestablishingthat𝐷1 =1.31when synthesizedtitaniumpyrophosphateisslightlysmoothwhen
𝐷 = 3𝐴 + 3 and 𝐷2 = 2.43 when 𝐷 = 𝐴 + 3. Wang et al. multilayersareformedathigh𝑃/𝑃𝑜.Likewise,themagnitude
[44] and Cao et al. [62] suggest associating the surface of of𝐷canbeassociatedwiththesmallnumberofactivesites
the fractal diameter as an average value of 𝐷1 and 𝐷2, the (𝑁𝑠=7sitesnm−2)[37,38,63]whicharelinkedtothepolar
intervalfrom1.31to2.43equaling𝐷𝑚 =1.87.Fromthere,it groups(OH−)presentonthesurfaceofthematerial.
6 TheScientificWorldJournal
The differences among materials with the same functional
400 groups are attributed to impurities, crystal structures, the
synthesismethod,orthebackgroundsaltusedtodetermine
theionicstrength[52].
300
ps) 3.3.SorptionExperiments. Priortothesorptionexperiments,
c
y ( the europium chemical equilibrium was determined using
nsit 200 theMEDUSAprogramtodeterminetheeuropiumchemical
nte speciesatthesorptionconditions.Acomplimentarysearch
I
wasperformedtofindtheeuropiumspeciesreportedinthe
literature. The species and the formation constant obtained
100
are reported in Table 2. Additional, the results determined
using the MEDUSA program showed that, until a pH of 6,
+ 3+
0 twomajorspeciesarepresent:EuNO3 andEu .However,
10 20 30 40 50 60 70 Eu(OH)3cprecipitatedinthepHrangeof6to11.
2 Figure 7 shows the variation in quantity of europium
retainedinthesolidovertime.Theseresultsindicatethatthe
JCPDS 38-1468
sorptionofeuropiumoccursintwophases,duringthefirst
Tip2O7obtained of which (𝑡 ≤ 10h) the retention velocity is fast and grows
Figure 3: XDR spectra of the obtained material; the lines corre- exponentially with time, reaching its maximum velocity at
spondtotitaniumpyrophosphate,patterntoJPCDS38-1468. timescloseto10h,duringwhichtimeapproximately50%of
the initial europium quantity (𝑞𝑡 = 0.28mgg−1) have been
retained. During the second phase the maximum sorption
percentage is reached at times exceeding 35h, where the
SurfaceAcidityConstants.Thepotentiometrictitrationcurve
can be fit to determine 𝐾+, 𝐾−, and𝐶. The experimental sorptionvelocitydiminishessignificantlyuntilreachingthe
thermodynamic equilibrium in which the solid is saturated
error was estimated to be ±0.2 pH units and 5% for the 3+
+ − andunabletoretainmoreEu .
concentrationsofH orOH added.Toreducethedegreesof
freedominthedeterminationofthesurfaceacidityconstants
3.3.1. ModelingSorptionKinetics. Theanalysisoftheexper-
oftitaniumpyrophosphateusingtheFITEQLprogram(IV)
[52],initialvalueswereusedfortheparameters𝑁𝑠 and𝐴𝑠, imental data of the sorption of europium according to
time was carried out using the empiric models previously
therebyreducingthenumberofparameterstobesolvedfor
described.
theequilibriummodelforthesystem.Thegoodnessofthefit
Figure 8(a) shows the results of the adjustment of the
canbedeterminedbytheratiobetweentheweightedsumof
experimental data using the linearized equation of the
squaresandthetotaldegreesoffreedom(WSOS/DF);forthe
Elovichmodel[48,49].Themodelparametersandthestatis-
analyzedsystem,thisratiowaslessthan20[51,64].
Theinner-layercapacitanceofthesurfaceacidityconstant ticalcoefficientwereobtainedplotting𝑞𝑡versuslog𝑡.Ascan
beseeninthefigure,theadjustmentoftheexperimentaldata
must be determined. This parameter is difficult to estimate
becausetheexperimentalcalculationisnoteasy.Theempiri- hasastatisticalcoefficient𝑟2of0.98.Theslopeandordinateof
−2 theadjustmentequationallowedanestimationofthemodel
calvalueisoftenchosenasapproximately1Fm formineral
parameters,duringwhichitwasdeterminedthat𝛼 < 0and
oxides,butinthesecases,theionicstrengthconsideredwas
nothigherthan0.1M[64].Phosphatecompoundsarehighly 𝛽 = 0.168mgg−1.TheseresultsshowthattheElovichmodel
−2 explains the experimental data mathematically as it counts
insulatingmaterialsandacapacitancevalueof3.08Fm has
onagoodcorrelation.Nevertheless,themodelassumesthat
already been used successfully for diverse phosphates with
theproductof𝛼𝛽 ≫ 1,sinceinourcase𝛼𝛽 < 0,itcanbe
ionic strengths greater than 0.1M [52]. For this work, we
3+
tested the previous values and found that the best fit was establishedthatthesorptionprocessofEu onsolidcannot
obtainedwithaninner-layercapacitancevalueof3.1Fm−2for beexplainedphenomenologicallywiththeElovichmodel.
anionicstrengthof0.5M. Figure 8(b) shows the results of the adjustment of the
The experimental and simulated curves of the solid experimental data using the pseudo-first-order model. This
are shown in Figure 6, and Table 1 reports the obtained analysiswascarriedoutplottinglog(𝑞𝑒−𝑞𝑡)versus𝑡.Ascan
acidity constants. The surface acidity constants determined beseeninthegraphic,thestatisticalcorrelationobtained(𝑟2)
for titanium pyrophosphate were log 𝐾+ = 3.59 ± 0.06 was0.91.Thekineticparameters𝑘󸀠 and𝑞𝑒,whosevaluesare
and log𝐾− = −3.90 ± 0.05; the goodness of the fit was 9. estimatedas1.68×10−3min−1 and0.64mgg−1,respectively,
The experimental data could only be fit when considering werecalculatedusingtheadjustmentequation.
onlyonesurfacesite;theseresultsagreewiththoseobserved Figure 8(c) shows the results of the adjustment of the
by other authors in phosphates with the ≡P-OH functional experimental data using the pseudo-second-order model
group[37,40].AsobservedinTable1,thereisnosignificance and using the linear adjustment equation proposed by Ho
betweenthevaluesobtainedforTiP2O7,ZrP2O7,andLaPO4. et al. [56], which was obtained by plotting 𝑡/𝑞𝑡 versus
Th(PO4)4P2O7hashighervaluesofsurfaceacidityconstants. 𝑡. With the adjustment equation, it was estimated that
TheScientificWorldJournal 7
500 500
87 87
400 400
86
86
300 300
85
m) m) m) m)
n n n n
( 84 ( ( 85 (
200 200
83
100 100 84
82
0 0
0 100 200 300 400 500 0 100 200 300 400 500
(nm) (nm)
(a) (b)
Figure4:SurfaceimagensoftitaniumpyrophosphateobtainedbyAMF:(a)beforeeuropiumsorptionand(b)aftereuropiumsorption.
2.4
35
2.2
30 2.0
25 1.8
1.6
20
)
g) Vm 1.4
L/ 15 V/
(m n( 1.2
V l
10
1.0
B
5 0.8
0 0.6 r2=0.99
0.4 A=−0.56
−5
0.0 0.2 0.4 0.6 0.8 1.0 −2.5 −2.0 −1.5 −1.0 −0.5 0.0 0.5 1.0
P/P0 ln[ln(P0/P)]
N2isotherm Experimental data Fitting data
(a) (b)
Figure5:AdsorptionisothermofN2ofthetitaniumpyrophosphate;(b)adjustedresultsofadsorptionoftheisothermN2.
ℎ = 8.35 × 10−4mgg−1min−1, 𝑞𝑒 = 0.6mgg−1, and𝑘 2 = a𝑟2=0.92.Thiscorrelationshowsthatthesorptionprocess
1.8 × 10−3gmgmin−1. The obtained correlation coefficient ofEuisnotlimitedbyintraparticlediffusionalbeititsuggests
was 0.98. This correlation showed that the model explains that it can be influenced by diffusive phenomena: diffusion
the experimental data adequately; likewise the magnitudes in the liquid/solid interphase or diffusion on the surface.
of the kinetic parameters are congruent from a physical Differentauthorshavepointedoutthatwhentheadjustment
point of view, which indicates that the retention process of ishighandgivesastraightlineasaresultthisindicatesthat
europiumontitaniumpyrophosphateispossiblyachievedby theprocessisexclusivelycontrolledbyintraparticlediffusion;
chemisorption. otherwise,whentheadjustmentdatarespondtomorethan
Finally,Figure8(d)showstheadjustmentoftheexperi- oneadjustmentline,thisindicatesthatthesorptionprocessis
mentaldatausingamodeloffractionalorder(intraparticle influencedbymorethanonediffusivephenomenon[53,65].
diffusion model). In this case, the adjustment analysis was Fromtheanalysisoftheadjustmentresultsofthedifferent
carried out by plotting 𝑞𝑡 versus 𝑡0.5. The results showed models, itispossible to determinethatthepseudo-second-
that 𝑘𝑖 = 0.001mg g−1min−0.5 and 𝐶SL = 0.033mgg−1, with order model can be used to explain the sorption kinetics
8 TheScientificWorldJournal
Table1:Surfaceacidityconstantsoftitaniumpyrophosphate.
Solid Capacit−a2nce log𝐾+ log𝐾− Functionalgroup Ref.
(Fm )
TiP2O7 3.08 3.59±0.06 −3.90±0.05 P2O7 Thiswork
ZrP2O7 3.08 3.2 −4.2 P2O7 [37,38]
LaPO4 3.08 3.6 −5.4 PO4 [39]
Th4(PO4)4P2O7 3.08 6.5 −7.8 P2O7 [40,41]
12 0.6
100
0.5
10
80
0.4
8 g) 0.3 60 val
mg/ mo
pH 6 q(t 0.2 40 % re
0.1
20
4 0.0
0
−0.1
2 0 500 1000 1500 2000 2500 3000
Time (min)
−6.0 −4.0 −2.0 0.0 2.0 4.0 6.0
q
Concentration (mol/L) ×10−3 t
% removal
3+
Experimental data Figure 7: Sorption of Eu as a function of time in the solid
Simulate data synthetized, using 1 × 10−4M solution of EuNO3, in contact with
400mgofsolidatpH6.
Figure6:Acid/basetitrationsofTiP2O7in0.5Mpotassiumnitrate
solution.Experimental(circle)andCCMcalculatedcurve(line).
Furthermore,thetestedmodelsindicatethatthesorption
Table2:Equilibriumconstantsofthechemicalspeciesofeuropium of europium on the solid occurs through the diffusion of
in0.5MKNO3solution. the metallic ion in the solid/liquid interphase and on the
log𝛽,FI=0.5M, surface of the solid, subsequently through the interaction
Chemicalequilibriumequation 𝑇=298K Reference of the ion with the surface sites or functional groups of
Eu3++H2O↔Eu(OH)2+ −8.53 [42] tshtreontigtabnoiunmdspbyertowpeheonspthheateeu,rwohpiicuhmfaavnodrsththeemfoartemriaatli.oThn oisf
Eu3++2H2O↔Eu(OH)2+ −17.31 [42] retention mechanism has been observed in different heavy
Eu3++3H2O↔Eu(OH)3 −26.35 [42] metalssuchasCr,Cd,andPb[68].
Eu3++NO3−↔Eu(NO3)2+ 0.29 [43]
Eu3++CO32−↔Eu(CO3)+ 6.11 [42] 3.3.2.EstimationofSurfaceComplexSurface. Figure9shows
Eu3++2CO32−↔Eu(CO3)2− 10.45 [44] the europium sorption isotherm of Eu3+ onto titanium
pyrophosphate, and the sorption edge spreads between pH
= 2 and pH = 5.5, which indicated that the sorption pro-
for europium in the titanium pyrophosphate. It should be cess involves more than one surface complex. Under the
noted that the sorption capacity of the solid studied is like experimental conditions, 50% of the europium was sorbed
thatobservedinotherphosphateswithP2O7groups.Studies approximately at pH values between 2 and 3.5 and the rest
such as those carried out by Wang et al. [34] show that the oftheEu3+wassorbedintheintervalof3.5–6.
nanostructured titanium pyrophosphates and the isomor- Tofitthesorptioncurve,𝐾+,𝐾−,and𝐶valueswerefixed
4+ 4+
phoussubstitutionofZr byTi resultinanenhancement atthevaluesdeterminedabove.Duringtheexperimentaldata
ofsorptioncapacityofactinides.Othermaterials,suchasthe analysis,theerrorwasestimatedtobe±0.2pHunitsand5%
novelSiO2-ZrO2-calciumalginateaerogels[66]orgraphene forthetotaleuropiumconcentration.Weusedthisinforma-
[67], have a higher actinide retention capacity than the tion to identify the surface complexes that are involved in
solid studied, but their structural stability has not been theretentionofeuropiumbytitaniumdiphosphate.Figure9
demonstratedwhensubjectedtointensefieldsofthermaland presents the sorption modeling. The results obtained from
ionizingradiation. the experimental data show the probable existence of two
TheScientificWorldJournal 9
0.6
−0.2
−0.4
0.5
−0.6
−0.8
0.4
) −1.0
−qt −1.2 g/g) 0.3
qe m
log( −1.4 q(t
Where
−1.6 0.2
Where <0
−1.8 k=1.68×10−3min−1 =0.168gmg−1
−2.0 qr2e==00..9614mgg−1 0.1 r2=<01.98
−2.2
0.0
0 500 1000 1500 2000 2500 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6
t(min) log(t)
Experimental data Experimental data
Data fitting Data fitting
(a) (b)
0.6
5500
5000 0.5
4500
0.4
4000
g)
qt 3500 g/ 0.3
/ m
t (
3000 qt
0.2
2500 Where Where
ℎ=8.35×10−4mgg−1min−1 ki=0.001mgg−1min0.5
2000 qe=0.6mgg−1 0.1 CSL=0.033mgg−1
1500 r2=0.98 r2=0.92
0.0
0 500 1000 1500 2000 2500 3000 10 20 30 40 50 60
t(min) t0.5
Experimental data Experimental data
Data fitting Data fitting
(c) (d)
Figure8:(a)Experimentaldatafitofthepseudo-first-ordermodel(Lagergren’sequation);(b)experimentaldatafitusingthelinearized
equationoftheElovichmodel,𝑞𝑡=𝛼+2.303𝛽log(𝑡);(c)experimentaldatafitusingthelinearizedequationofpseudo-second-ordermodel
proposedbyHoetal.(2005),𝑡/𝑞𝑡=𝑞/ℎ+(1/𝑞𝑒)𝑡;(d)experimentaldatafitusingthelinearizedequationoftheintraparticlediffusionmodel.
bidentate complexes on the surface. In the pH range of 2 europiuminthesolutionisthechemicalprecipitationofthe
to 4.5, two surface sites are required to form the bidentate condensatespeciesontothesurface.
complex (≡XOH)2EuNO32+ with the elimination of one Table 3 shows the values for the sorption constants
3+
hydrogen;inasimilarway,inthepHrangeof4.5to7,two obtained from this research of Eu on titanium pyrophos-
surfacesitesarerequired(≡XO−)toformabidentatecomplex phateandalsothosefortheEu3+complexformedinsimilar
withEuNO32+((≡XO)2EuNO3). phosphates. The result obtained in this work for the first
Therefore, the europium sorption mechanisms in tita- surfaceeuropiumcomplexissimilartothatreportedbyFinck
niumpyrophosphateresultfromtheinteractionofa≡XOH2+ [38]fortheZrOcomplexedwitheuropiumat308K,which
surface site (pH < 3.3) and a ≡XO− surface site (pH > couldbeattributedto(≡TiOH)EuNO32;however,thevalue
2+
3.3) with EuNO3 and the formation of the inner-sphere fortheP2O7 complexisdifferent.Finck[38]reportsavalue
bidentate surface complexes. For pH values, greater than of−3.2±0.3,butDrotetal.[40]reportavalueof0.94±0.14
7, the mechanism that reduces the concentration of the for the P2O7 complex in the Th(PO4)4P2O7. Drot’s value is
10 TheScientificWorldJournal
Table3:Eu(III)sorptionequilibriaontotitaniumpyrophosphateandothercompoundsandassociatedconstants.
Surfacecomplex log𝐾 Compound Reference
≡XOH+Eu3++NO3−↔(≡XOH)EuNO32+ 5.8±0.1 TiP2O7 Thiswork
≡XOH+Eu3++NO3−↔(≡XO)2EuNO3 1.2±0.5 TiP2O7 Thiswork
2(≡ZrOH)+Eu3+↔(≡ZrOH)2Eu3+ 6.25±0.3 ZrP2O7 [38]
2(≡POH)+Eu3++NO3−↔(≡PO)2Eu+2H+ −3.2±0.3 ZrP2O7 [38]
2≡XOH+Eu3++NO3−↔≡(XOH)2EuNO32+ 7.49±0.05 ZrP2O7 [40,41]
2≡XOH+Eu3++NO3−↔≡(XO)2EuNO3+2H+ 0.94±0.14(P2O7) Th(PO4)4P2O7 [37,40,41]
2≡XOH+Eu3++NO3−↔≡(XO)2EuNO3+2H+ −2.23±0.13(PO4) Th(PO4)4P2O7 [37,40,41]
2≡XOH+Eu3++NO3−↔≡(XO)2EuNO3+2H+ −3.0±0.3(PO4) Zr2O(PO4)2 [37,40]
2≡XOH+Eu3++NO3−↔≡(XO)2EuNO3+2H+ 0.31±0.5(oxo) Zr2O(PO4)2 [37,40]
×10−5 and surface diffusion and second due to chemisorption
relatedtothepossibleformationoftwotypesofinner-sphere
10
bidentate surface complexes. The stability constants of the
formed complexes show the feasibility of using titanium
8 pyrophosphateasanengineeringbarrierinthecontainment
L) ofradioactivewaste.
ol/
m 6
n ( ConflictsofInterest
o
ati
ntr 4 Theauthorsdeclarethattherearenoconflictsofinterest.
e
c
n
Co Acknowledgments
2
The technicians of DRX, MEB, and the Chemistry Depart-
0 mentofININareacknowledged.ThisworkispartofProjects
0 1 2 3 4 5 6 7 8 DR-001andCB-907.
pH
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(XO)2EuNO3 Simulated [1] S. E. Manahan, Environmental Chemistry, Lewis Publishers,
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Figure9:Eu sorptionmodelingontitaniumpyrophosphateand
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