Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 203211

Characteristics of simplified ferron colm

Ch ng ademyof G

arch 206

Abstract

Despite th plexno generally coloapplication l Nhydroxylamine and orthophenanthroline, the simplified ferron colorimetric solution can become more stable and is applicable in speciation for Alsolutions with no or low content of iron ions (i.e., [Fe]/[Al] 0.05). Polymeric aluminum species, especially Al13 ([AlO4Al12 (OH) 24(H2O)12]7+)may not react with ferron directly but via the process of Al decomposition. Al species of the similar form may have the same level of reactionrate with ferron. The Ala-ferron interaction time adopted to quantify Ala can greatly affect the speciation of hydroxy-Al solutions. The longer thetime adopted, the higher the content of oligomer Al species included in Ala (mononuclear and oligomer Al species). However, the adoption of theduration of fto ensure nocolloidal Alcomposed oto Al13. 2006 Else

Keywords: Fe

1. Introdu

Ferronhydroxy-Aity and relireaction kindifferentiatAl system,colloidal cquantitativereagent is aacetate andity. Hydroxto eliminat

CorresponE-mail ad

0927-7757/$doi:10.1016/jerron interaction with Ala + Alb has little effect on the speciation of Alb (soluble polymeric Al species) only if the time is long enoughfurther detectable absorbance increase. At least five kinds of Al species, such as mononuclear Al (Almon), Al2, Al6 polymers, Al13 and(Alc) exist in hydroxy-Al solutions. Generally, Ala includes Almon, Al2 and possibly part of Al6 polymers. In most cases, Alb is mainlyf Al13 and other species like Al6 polymers can also contribute. Only under certain conditions (B value ranges), Alb can be equivalent

vier B.V. All rights reserved.

rron colorimetric solution; Speciation; Hydroxy-Al solution; Al13; 27Al NMR spectroscopy

ction

assay has been widely used for the speciation ofl solutions because of its relatively high operabil-ability. The principle of this method is based on theetics of ferron with different hydroxy-Al species. By

ing the reaction kinetic behaviors of ferronhydroxythe Al monomers, oligomers, polymers and even

onstituents could be determined qualitatively andly [1,2]. The commonly used ferron colorimetriccomplicated solution system. Besides ferron, sodiumhydrochloric acid are added to provide buffer capac-ylamine and orthophenanthroline are always addede the disturbance of coexisting ions, especially fer-

ding author. Tel.: +86 10 62849144.dress: fengchenghong@hotmail.com (C. Feng).

ric/ferrous ions [3,4]. However, it has been reported that theexistence of hydroxylamine and orthophenanthroline has agreat influence on the stability of the colorimetric solution andconsequently reduces the credibility of this method to someextent [5,6]. In fact, with no addition of hydroxylamine andorthophenanthroline, ferron assay has been used for the spe-ciation of hydroxy-Al polymers in iron-free solutions [2,5,7,8].However, the application of this simplified colorimetric solutionin speciation of Al species has not been systematically evaluated.

The reactions of mononuclear Al species with ferron aremuch simpler and have been studied in depth by someresearchers [9]. However, the reactions of ferron with polynu-clear hydroxy-Al species are extremely complicated. On onehand, these kinds of Al species are metastable, and once mixedwith ferron colorimetric solution of high pH, they may undergoageing and evolutionary formation hydrolysis. On the otherhand, the complexation mechanisms have not been fully under-stood. It has been proposed that the reactions of ferron with

see front matter 2006 Elsevier B.V. All rights reserved..colsurfa.2006.03.053its application in hydroxy-aluenghong Feng a,, Baoyou Shi a, Dongsheng Wa

a State Key Lab of Environmental Aquatic Chemistry, RCEES, Chinese Acab School of Water Resources & Environment, China University

Received 22 January 2006; received in revised form 27 MAvailable online 7 April 20

e wide application of ferron assay in hydroxy-Al speciation, the comaccepted quantification stipulation. In this study, a simplified ferron

to the speciation of different hydroxy-Al samples with the aid of 27Aorimetric solution andinum speciation, Guohong Li b, Hongxiao Tang aof Sciences, Box 2871, Beijing 100085, China

eosciences, Beijing 100083, China006; accepted 31 March 2006

ation mechanism in function remains unclear and there isrimetric solution was suggested and characterized in its

MR spectroscopy. Results show that with no addition of

204 C. Feng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 203211

different hydroxy-Al species may exhibit different kinetic ratesand constants [10]. Based on this assumption, pseudo first-orderreactions atool to distparameterssent each Acharacterissuch kindlarge numbtechniquesThus, empchoice arethis methodand Al speis attributedimmediatelregarded americ specdefined as Atime is defispecies). Achoice to dbeen suggediscrepancby differensolution. Tof operatioit is necessthe operati

In thisplified ferrAnd then uwith 27Al nbehaviors oThe objectity and ophydoxy-Alcomplexatiand the effespeciation.

2. Materia

In this sAnd all the

2.1. Prepasolution

Reagent(8-hydroxyCo., U.S.Aremove dis

Reagentsolved in 1

Reagentwas mixed

Each individual reagent mentioned above was filteredthrough pre-washed 0.45m membranes. The ferron colorimet-

utionat a rn of

ored

repa

er raas ne

osimof Nratiohoseumin, PACn top) wwit

can bn eve

S.A.

haran

Absiouscolo

H ofer Ton oave

is 85qu

AlCof pHed allowreme

Stabetritradhydratiofterns wance

Effeioussolue/Al, resnd nonlinear regression methods are put forward as ainguish Al species [1113]. However, several kinetic

deduced from fitting calculations may not repre-l species in solution. They only reflect the common

tics for a group of components [10]. Additionally,of mathematic approach to Al speciation requires aer of experimental data and complex data processing, which is not suitable for wide-range application.irical approaches based on operational protocol ofstill well acceptable by most researchers [1]. With, the duration of the overall reactions between ferron

cies is divided into different phases, and each phaseto a certain category of species. The species reacted

y in the initial short period is defined as Ala (usuallys mononuclear species); the low to medium poly-ies reacted in the following relatively longer period is

lb; the species that cannot react with ferron in limitedned as Alc (including high polymeric or colloidal Aln obvious problem with this method is the protocol ofivide each phase. Different operation protocols havested by different researchers [2,3,7,13]. As a result,ies often exist between the analytical results obtainedt researchers even for the test of the same hydroxy-Alo date, there is little documentation about the effectn protocol choosing on the Al speciation. Therefore,ary to investigate the inherent relationships betweenon protocol using and the sample speciation.paper, the characteristics and applicability of sim-on colorimetric solution were evaluated in detail.sing the simplified colorimetric solution combineduclear magnetic resonance spectroscopy, the reactionf ferron with different Al species were elucidated.

ive of this study is to further improve the reliabil-erability of the ferron assay in the application ofsolution speciation. Emphasis will be laid on the

on mechanisms of hydroxy-Al species with ferronct of different operation protocols adopted on the Al

ls and methods

tudy, deionized water was used to make all solutions.reagents were analytical grade chemicals.

ration of the simplified ferron colorimetric

A (0.2%, w/v, ferron): under rapid stirring, 2 g ferron-7-iodoquinoline-5-sulfonic acid, Sigma Chemical) was dissolved in deionized water (pre-boiled tosolved CO2), and then diluted to 1000 ml.B (20%, w/v, NaAc): 200 g sodium acetate was dis-

000 ml of deionized water.C (1:9, v/v, HCl): 100 ml hydrochloric acid (37%)

with 900 ml of deionized water.

ric soland Cadditiowas st

2.2. P

Undtion w(665 Dumes

molarwere c

polyalPAC05additioas PACPAC22dures(AlT) iCo. U.

2.3. Csolutio

2.3.1.Var

5.5 mlfinal p(Mettlvariatition, wUVva 1-cm0.1 Meffectobservwere a

measu

2.3.2.colorim

Theaddingone atThen asolutioabsorb

2.3.3.Var

AlCl3with Fand 1.0was obtained by mixing the above reagents A, B,atio of 2.5:2:1. The latter two were mixed prior to theferron. After preparation, the colorimetric solutionin refrigerator for later use.

ration of hydroxy-Al solutions

pid stirring, a calculated volume of 0.5 M AlCl3 solu-utralized slowly with 0.5 M NaOH with a titrometerat, Metrohm, Switzerland) at 25 0.1 C. The vol-

aOH added varied with the target B values (OH/Als). B values of 0, 0.5, 1.0, 1.5, 2.0, 2.2 and 2.5n in this study. The resulting solutions (usually calledum chloride, PAC) obtained were denoted as PAC0,l0, PAC15, PAC20, PAC22 and PAC25, respectively. In

the above solutions, purified Al13 solution (denotedas also used, which was separated and purified from

h sulfate precipitation method. The detailed proce-e found in [14]. The final concentration of total Alry sample was measured by ICP-OES (Perkin-Elmer).

cterization of the simplified ferron colorimetric

orbance spectra at different pH valuesvolumes of 0.5 M HCl or NaOH were mixed withrimetric solutions and then diluted to 25 ml. Theevery mixture was measured with MP220 pH meter

oledo, Switzerland). To analyze the effect of pHn the absorbance spectra of the colorimetric solu-length scan of each mixture was performed on a00 spectrophotometer (Tianmei Co., China) usingartz cell. Meanwhile, with the addition of 20ll3 solution to the above colorimetric solutions, the

on the absorbance of Al-ferron systems was alsot 364 nm (after the addition of AlCl3, the systemsed for five minutes of reaction prior to absorbancent).

ility comparison with traditional ferronc solutionitional ferron colorimetric solution was prepared byroxylamine and orthophenanthroline to the simplifieds of 0.24% (m/m) and 0.0056% (m/m), respectively.different ageing times, wavelength scans of the twoere performed to assess the effect of ageing time onspectra changes.

ct of ferric ions on Al speciationvolumes of 0.1 M FeCl3 solutions and 20l 0.1 M

tions were pipetted into 25 ml graduated glass tubesmolar ratios fixed at 0, 0.01, 0.025, 0.05, 0.1, 0.5pectively. Then 5.5 ml simplified ferron colorimetric

C. Feng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 203211 205

solutions were added, and finally diluted to 25 ml. The wave-length scan of each mixture was performed using the procedurementioned

2.4. Alumicolorimetri

The opetions and 5into a gradenized bymeasured aout as quictimed comferron were

2.5. 27Al N

Hydroxalso studieU.S.A). Thwere: NS =T = 298 K.The chemi34 and 62(Al2) and Awas determrespondingof the undesubtractingconcentrati

3. Results

3.1. Charasolution

Ferron,increased pgreatly affethe charact

Fig. 1ametric soluthe high va2, respectivextremely scorrespondwithin thehigher than5.49, correthe colorimues (curvespatterns. TpH rangesdistributionlations usintwo acid di

ffectc solu.0 (2)

pecincrea

. Mhe weciesnea

oiceer, o

solunce.wou

be dillu

iffereexespH ras re

d for the assay operation, which matches well with thetion of Hsu and Cao [5]. In this study, it was verified thatchanges caused by the hydroxy-Al solution (with pH of

5) addition could not lead the final pH to fall out of them range. Moreover, the content of HA in the colorimet-tion with this pH range could exceed 95%, which is alsor of Al speciation. This pH range is also suitable for Al

tion with the traditional ferron colorimetric solution [3].whether hydroxylamine and orthophenanthroline exist inmetric solution or not will have little effect on the solutiond ferron species distribution in these two kinds of ferron

etric solutions.ric/ferrous ions are the commonly encountered disturb-ctors for Al speciation using ferron assay. They couldroduced by the reagents adopted during the prepara-f hydroxy-Al solutions. Therefore, hydroxylamine andhenanthroline were often added to the traditional ferron

etric solutions. Hydroxylamine could reduce ferric ionsous ions and then the ferrous ions were complexed byabove.

num speciation using simplified ferronc solution

ration procedures are as follows: 20l test Al solu-.5 ml ferron colorimetric solutions were transferreduated glass tube; then diluted to 25 ml and homog-quick shaking; the absorbance of the mixture wast 364 nm immediately. The operations were carriedkly as possible to avoid any unnecessary delay. Theplexation reaction kinetic curves of Al species with

automatically recorded.

MR spectroscopy

y-Al solutions with different B values wered with 500 MHz 27Al NMR (Brookhaven Co.,e instrumental settings and experimental conditions128, P1 = 20s, PL1 = 3dB.Solvent: D2O, and

The inner standard was 0.05 M Al(OD)4 solution.cal shift of Al(OD)4 is 80 ppm. The signals near 0,.5 ppm represent mononuclear Al (Almon), dimeric Al

l13, respectively. The concentration of each speciesined by the ratio of the integrated intensity of the cor-peak to that of Al(OD)4 at 80 ppm [15]. The amounttectable species (denoted as Alun) was obtained bythe sum of the detected Al species from the AlT

on in the solutions.

and discussion

cteristics of simplified ferron colorimetric

a dibasic acid (H2A), dissociates to HA and A2 withH. The pH value of ferron colorimetric solution canct the distribution of ferron species and thus affect

eristics of the colorimetric solution.shows the wavelength scan curves of ferron colori-tions with different pH values. When the pH is atlue end (8.0 and 8.9, corresponding to curves 1 andely), the absorbance intensities around 364 nm aretrong; while the pH is at the low end (1.98 and 3.0,ing to curves 3 and 4, respectively), the absorbancesame wavelength band are much decreased, but stillthose associated with medium pH values (4.52, 5.25,

sponding to curves 5, 6, 7, respectively). Curves ofetric solutions associated with the medium pH val-5, 6, 7) exhibit much similar absorbance spectrum

he differences of absorbance spectra within differentcould be attributed to the changes of ferron species(data not shown), which can be obtained by calcu-g pH values of the colorimetric solutions and the

ssociation constants of ferron [16]. HA is the dom-

Fig. 1. Eorimetri(1) pH 85.49.

inant sWith ior H2Ahave ttwo spgroundthe chHowevmetricdiffereminedshouldFig. 1bwith dcomplsmalland wadopteestimathe pH3.05.optimuric soluin favospeciaThus,colortipH ancolorim

Fering fabe inttion oorthopcolorimto ferrof acidity on the absorbance spectra of (a) simplified ferron col-tions at different wavelength; (b) Al-ferron complexes at 364nm.pH 8.92 (3) pH 1.98 (4) pH 3.0 (5) pH 4.52 (6) pH 5.25 (7) pH

es in colorimetric solutions when pH is around 5.2.sed or decreased pH, HA will be converted to A2oreover, the HA dominated colorimetric solutionseakest absorption around 364 nm, while the otherdominated solutions yield a strong absorption back-

r 364 nm. According to Beers law, as blank solution,of pH around 5.2 is favorable for the ferron assay.nce hydroxy-Al solution is added, the pH of colori-tion would be subject to variation due to their pHMoreover, the Al species distribution to be deter-ld also be affected. Therefore, an optimum pH rangeetermined to ensure the sensitivity of measurement.strates the absorbance changes of Al-ferron mixturent pH values at 364 nm. The absorbance of Al-ferronwas greatly influenced by pH, whereas within theange of 5.05.4 the absorbance reached maximumlatively stable. Therefore, this pH range should be

206 C. Feng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 203211

Fig. 2. Absorbance spectra of Al-ferron complexes using simplified ferron col-orimetric solution under the existence of ferric ions (Fe/Al molar ratios fixed at(1) 0 (2) 0.01 (3) 0.025 (4) 0.05 (5) 0.1 (6) 0.5 (7) 1.0, respectively).

orthophenanthroline. Consequently, the formation of Fe-ferroncomplex could be minimized and the disturbance from fer-ric/ferrous ions for Al speciation could also be avoided. Thus, itmay become an unnecessary operation to add these two reagentsto the colorimetric solution for the Al speciation in Fe-freesolution.

When the simplified colorimetric solution was adopted, thesignificancAbsorbancconcentratiwith low irtion maxim

364 nm, while at about 600 nm only the colored complex ofiron had relatively high absorption. Thus, these experimentalresults further proved that the existence of Fe ions in solutioncould affect the Al speciation. However, it can also be seen fromFig. 2 that no significant difference between curves 2, 3 and 4existed near 364nm. Consequently, if the ferric ion concentra-tion in solution is sufficiently low (i.e., [Fe]/[Al] 0.05), theaddition of hydroxylamine and orthophenanthroline to ferroncolorimetric solution is unnecessary and the simplified ferroncolorimetric solution can also be used for the speciation of thiskind of hydroxy-Al solution.

Additionally, without these two reagents, the preparation pro-cedure of ferron colorimetric solution could become simpler andthe reliability of measurement could also be improved. Fig. 3demonstrates the comparison of absorbance spectra betweenthe simplified ferron colorimetric solution and the traditionalone. With the addition of these two reagents, the absorbanceof the traditional ferron colorimetric solution changes signifi-cantly with ageing time, especially in the initial several daysafter preparation. This might be due to the slow reaction of fer-ron with hydroxylamine. Such kind of continuous absorbancechange could unavoidably cause experimental errors. Further-more, higher concentration of hydroxylamine and orthophenan-throline could result in higher instability of the colorimetric solu-tion and a shift of its maximum adsorption wavelength (max)after long ageing time [12]. Thus, the traditional colorimetricsolution is often used for Al speciation in a comparatively stable

(i.e.the

fiedt sho

wit

Fig. 3. Effectline.(1) 0 daye of disturbance from ferric ions was assessed (Fig. 2).e spectra of the Al-ferron mixtures with high ironon (curves 6, 7) are significantly different from thoseon concentration (curves 2, 3, 4). Moreover, absorp-a for both iron and Al complexes occurred at around

periodadds tosimplidid no364 nms of ageing time on the absorbance spectra of ferron colorimetric solutions in the pres(2) 3 days (3) 7 days (4) 11 days (5) 30 days., 525 days after the preparation), which inevitablycomplexity of ferron assay operation. In the case of

ferron colorimetric solution, the absorbance spectraw any appreciable change at the wavelength of nearhin the 30 days of observation duration, which indi-

ence (a) and absence (b) of hydroxylamine and orthophenanthro-

C. Feng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 203211 207

Fig. 4. Absohydroxy-Al so

cates the simstability thric acid anddistributionsimplified fafter preparmay have ano or low cof its simpl

The effetics of the(data not sabsorbanceorimetric s

3.2. Comphydroxy-al

The reaare showncurves can

by a graduareaction beied significfrom a muof gradualthe cases owas very lmuch longeincrease isron with mincreasinghydroxy-Aabove reacferron inter

Essentiainformationquantificati

7Al NMR spectroscopy of (a) 1. PAC05, 2. PACl0, 3. PAC15, 4. PAC205; (b) PACp and (c) PAC0.

reaction kinetics data. Unlike the ferron assay, the instru-technique of 27Al NMR has the advantage of revealingrbance spectra of simplified ferron colorimetric reagent withlutions (1) PAC0 (2) PAC05 (3) PACp (4) PAC25.

plified colorimetric solution possessed much higheran the traditional one (Fig. 3b). Besides, hydrochlo-

sodium acetate added as buffer had no effect on theof the ferron species with time elapsing. Thus, the

erron colorimetric solution can be used immediatelyation. Moreover, the simplified colorimetric solutionwider application for Al speciation in solutions withontent of iron ions than the traditional one becauseer preparation procedure and higher stability.ct of the temperature variation on the characteris-simplified colorimetric solution was also tracked

hown). Within the range of room temperatures, novariation was detected for the simplified ferron col-

olution.

lexation mechanisms of ferron withuminum solutions

ction kinetics of ferron with hydroxy-Al solutionsin Fig. 4. The common characteristics of the kineticbe seen: an initial rapid absorbance increase followed

Fig. 5. 25. PAC2

of thementall increase, and then ending in a plateau. However, thehaviors of different types of hydoxy-Al solutions var-antly. For PAC0 and PAC05, the absorbance startedch high level, and then experienced a short periodincrease before reaching the plateau. However, inf PACp and PAC25, the absorbance at the beginningow, and the gradual increase of absorbance lastedr before reaching the plateau. The initial absorbancebelieved to be associated with the reaction of fer-ononuclear Al species, and the following gradual

phases are related to reactions of ferron with variousl polymers. When all of the Al species mentionedt completely with ferron, the kinetic curve of Al-action would reach the plateau.lly, the colorimetric assay can only give indirectabout the speciation of hydroxy-Al solutions. The

on of specific species depends on the interpretation

part of the sAl monompatterns ofEach peakcific speciepeak is rela

The speNMR is predetected incomplexatiin the assa[4,13]. HowAl to reactexplanationcolorimetrifurther hydAl speciestructural information about some Al species, such asers and Al13 polymers. Fig. 5 shows the 27Al NMRdifferent hydroxy-Al solutions used in this study.

on the 27Al NMR spectroscopy corresponds to a spe-s as mentioned before. The integrated area of eachted to the relative abundance of that species.ciation of hydroxy-Al solutions determined by 27Alsented in Table 1. Only mononuclear Al species wasPAC0 (Fig. 5c). According to some researchers, theon reactions of mononuclear Al species with ferrony could be completed immediately after the mixing

ever, it actually took 35 min for all mononuclearcompletely with ferron (curve 1 in Fig. 4). A possible

is that after the PAC0 sample was transferred into thec solution, some of mononuclear Al might undergorolyzation and transform into dimeric or oligomericdue to the effect of pH environment change (the sam-

208 C. Feng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 203211

Table 1Speciation distribution of hydroxy-Al solutions determined by 27Al NMR spectroscopy and ferron assay

Solution 27 F

A

PAC0 8PAC05 6PAC25PACp

a AlT = Ala anceb

Undetec

ple pH was5.2). Andresulted incan be supobtained wof PAC05, bcertain amoriginal sametric soluConsequenreach the pron. The ra100 s is alstent of moPAC0.

As for Pciation stabmerizationAdditionalthat of ferrtion mighthydroxy-Aable to bettPACp and P

Due tooligomericof its Al spbeginning aConsequenascribed toAl13 is thedifferences4, which coily composindicated bThese Al sAl13, so the

PACp, namount oftent of Al1(Table 1). TThe resonaservable. Othat in PACferron goes

te ofdingouldundeays oeenorm

medferroike strallp w

echaerronses glainetioncom

a balancACp3 intrati

ron pas alnsfod a pg facrefoo rea

fastencer

n ofatomare spH Al NMR spectroscopy

Almon (%) Al13 (%) Alun (%)3.04 100 b 3.67 55.20 25.21 19.595.21 45.59 54.414.35 3.15 96.15 0.70

+ Alb + Alc; the fractions of Ala and Ala + Alb were calculated using the absorbtable.

3.04 and the colorimetric solution pH was aroundconsequently the in situ formed oligomer speciesa lower reaction rate with ferron. This assumptionported by the comparison of the PAC0 speciationith ferron assay and 27Al NMR (Table 1). In the caseecause its B value is higher than PAC0, there existed

ount of oligomeric and polymeric Al species in themple, and once PAC05 was introduced into colori-tion, more of such species were supposed to exist.tly, it took much more time for curve 2 in Fig. 4 tolateau and such species to react completely with fer-pid absorbance increase of curve 2 during the initialo lower than that of curve 1, which indicates the con-nonuclear Al species in PAC05 is less than that in

ACp and PAC25, it has been proved that their spe-ility is relatively high because of their higher poly-degree and the stable Keggin structure of Al13 [2].

ly, the pH values of PACp and PAC25 are close toon colorimetric solution so that the mixing opera-not lead to significant further hydrolysis of these twol solutions. Thus, curves 3 and 4 in Fig. 4 should beer represent the actual reaction kinetics of ferron withAC25, respectively.the high B value of PAC25, its mononuclear orAl species should be at a very low level and most

ecies exist as Alb and Alc (Table 1). As evidence, thebsorbance of curve 4 in Fig. 4 is approximately zero.tly, the absorbance increase of curve 4 was mainlythe reaction of ferron with Alb. Generally, in PAC25,primary species of Alb. Fig. 4 shows no significantof the slopes in the initial 500 s between curves 3 andnfirms that the Alb component in PAC25 was primar-ed of Al13. After 500 s, the much slower reaction ratey curve 4 is attributed to the other Al species in Alb.

tion raAccorrate shferronpathwIt has bin the fbe forsibly,cage-lthe cenof PACthis mwith fdecreabe expinteracferronexistedThis bwith Pcurve

concen

Al-ferance w

and trareachelimitin

Theliable tmuchare co

positiothe Alatomspecies may have higher polymerization degree thanrates of their reaction with ferron are much lower.

amely the purified Al13 solution, contained very smallmononuclear and colloidal Al species, and the con-3 accounted for 96% of the total Al concentrationhe 27Al NMR pattern of PACp is shown in Fig. 5b.

nce peak of mononuclear Al species is almost unob-bviously, the content of Al13 in PACp is higher than25. Accordingly, the reaction curve of PACp withhigher than that of PAC25. The relatively rapid reac-

different fo

3.3. Effectspeciation

A genehydroxy-Arecognizedat a very fto ferron cerron assay ICP-OES

la (%) Alb (%) Alc (%) AlTa (M)8.1 11.9 0.1020.97 37.56 1.47 0.1032.87 62.86 34.27 0.1031.08 95.35 3.57 0.095

measured in the initial 30 s and 7200 s of Al-ferron interactions.

PACp with ferron kept constant for more than 500 s.to the conclusions of Jardine and Zealzy [12], thisbe the eigenvalue of the reaction between Al13 andr the experimental conditions. However, the reactionf Al13 with ferron have not been fully studied as yet.suggested that Al13 does not react with ferron directlyof a whole polymer unit, and complexation productsonly by mononuclear Al species with ferron. Pos-n firstly attacks octahedrally coordinated Al in thetructure of Al13, and thus distorts the symmetry ofy located tetrahedral Al [17]. The reaction behaviorith ferron observed here seems to be in support ofnism. During the initial phase, the reaction of Al13is going on at a very high rate, thereafter, the rate

radually (curve 3 in Fig. 4). This phenomenon couldd as follows: at the initial stage of the Al13-ferron

, the decomposition of Al13 and the formation of Al-plexes were running at a relatively high rate. Therealance between Al13 and its decomposed products.e guarantees the same level of reaction rate of ferronand the same level of slope in the initial section of

Fig. 4. However, with the reaction time elapsing, theon of Al13 decreased and its decomposition and theroduct formation rates also became slower. The bal-so gradually broken. After all Al13 were decomposedrmed into Al-ferron complexes, curve 3 eventuallylateau. The decomposition rate of Al13 might be thetor for the ferron interaction with Al13.

re, mononuclear and oligomeric Al species are morect with ferron and their reaction rates with ferron are

r. However, as far as the higher polymeric Al speciesned, since most Al atoms are located in the innertheir corresponding structures, ferron would attacks in the outer spheres first. Only after these outer Al

eparated, ferron can react with the inner atoms. Thus,

rms of Al often react at varying rates with ferron.

of reaction duration adoption protocols on theof hydroxy-Al solutions

ral description of determining the speciation ofl solutions is illustrated in Fig. 6. Ala, commonlyas the mononuclear Al species, reacts with ferronast rate immediately after the addition of sampleolorimetric solution. If the time of k is adopted as

C. Feng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 203211 209

Fig. 6. Schematic diagram of the speciation of hydrox-Al solutions with ferronassay.

the reaction end point of mononuclear species, the content ofAla is denoted as Ala (k) for simplicity. Then the subsequentabsorbance increase with time till the kinetic curve reaches aplateau is ascribed to the reaction of Alb species with ferronand the content of Alb is marked as Alb (k) here. Since the Alcspecies is rather inert in terms of its reaction with ferron [6], itscontent can(k) and the

The reactors, such aand compo

quently, the adoption of end time for Alb reaction may bringsome influences on the quantification of Alb content. No con-sensus on the end time has been reached as yet. Most researcherssuggested 2 h as the end time of the Alb-ferron interaction [2,18],while others believed that 30 min or 1 h should be the bestchoice [13,10]. As a principle, the end time adopted to quan-tify the content of Ala + Alb should ensure the completion ofreaction between Alb and ferron, which can be indicated by thenegligible absorbance increase of the kinetic curve. Therefore,if this principle is followed, the end time adoption could notbecome an important factor of interfering with the quantifica-tion of Ala + Alb. According to the reaction pathways discussedabove, once the kinetic curve reaches a plateau, polymeric Alspecies consisting of Ala and Alb have been decomposed com-pletely into mononuclear Al species and reacted with ferron.Solution pH and further ageing might not affect their existingstatus any more. Longer time adopted to quantify Ala + Alb hasno effect on the speciation of Alb. Thus, the adoption of twohours for determination of Ala + Alb is safe and rational.

Unlike the adoption of end time for the quantification ofAla + Alb, there is currently no general principle to determinethe reaction duration of k for Ala quantification. The adoptionof different k value may greatly affect the speciation of Alaand Alb in a hydroxy-Al solution. Various k values have alsobeen proposed in literature, such as 30 s, 40 s, 60 s and 90 s, etc.[2,3,7,13]. However, it is still unclear which one is the most rea-sonable choice for the Ala-ferron interaction. Fig. 7a gives the

risonla (3nt Bined) are

Fig. 7. Distribrespectively;be obtained by the difference between Ala (k) + AlbAlT value.tion of Alb with ferron can be affected by many fac-s the polymerization degree, ferron/Al molar ratiossitions of ferron colorimetric solution [13]. Conse-

compa30 s [AdifferedetermAla(90ution of various Al species in hydroxy-Al solutions with different B values (a) Almo(c) Alb (30) and Al13.of Ala contents determined by choosing k value of0)] and 90 s [Ala (90)] in hydroxy-Al solutions withvalues. The corresponding contents of Al monomerswith 27Al NMR are also shown. Both Ala(30) andhigher than the contents of Almon, which indicates

n, Ala (30) and Ala (90); (b) Ald (k), k is 30 s, 40 s, 60 s and 90 s,

210 C. Feng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 203211

that not only mononuclear Al species such as Al3+, Al(OH)2+and Al(OH)2+, but possibly some oligomeric Al species (Ald)might be innot be simp

The distshown in F(k) (k is 30calculatedsured withassay [Aldsuggestedsolutions, ionly the fothe existenstudy, dimewhich wasin Fig. 5a.values rangFig. 7b shothose of olB values. Atents of Al2B values. Imight be asuch as Alhas been bAl and thetime. Unfo[7].

With theand this trethe differen90 s, respechigher B vaof oligomeB values woligomericand Al6 poadopted covalue of Ato the specevery hydroto Almon. Treaction dusolutions w

Generaland other hsoluble poparison ofAlb (30) inAlb (30) varoughly wiin the hydr(30) values27Al NMRdue to theTherefore,

Alb could be reasonably regarded as a surrogate of Al13. Fromthe comparison of Al speciation obtained by ferron assay and27 MR

s, sust hy

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n andlloidscluded in the Ala component. Therefore, Ala couldly regarded as mononuclear Al species.ribution of oligomeric Al species (Ald) in samples isig. 7b. Here, the contents of Ald were denoted as Alds, 40 s, 60 s and 90 s, respectively), and they were

by subtracting the mononuclear species content mea-27Al NMR (Almon) from Ala (k) obtained by ferron(k) = Ala (k) Almon]. Baes and Mesmer [19] have

that two oligomeric Al species exist in hydroxy-Al.e., dimeric Al (Al2) and trimeric Al (Al3). However,rmer can be detected by 27Al NMR spectroscopy andce of the latter has not been identified [20]. In thisric Al (Al2) was also the only oligomeric Al speciesdetected at 34 ppm on the 27Al NMR spectroscopyHowever, only in some hydroxy-Al solutions with Bing from 0.5 to 1.5, dimeric Al have been detected.ws the comparison of the contents of dimeric Al withigomeric Al in hydroxyl-Al solutions with differentld (k) values were significantly higher than the con-, particularly in the hydroxy-Al solutions with lowert can be inferred that the difference between themscribed to the existence of other oligomeric species,6 polymers. Al6, with a six-member ring structure,elieved to play a crucial role in the hydrolyzation offormation of solid Al hydroxide for a long period ofrtunately, Al6 species is undetectable by 27Al NMR

increase of B value, Ald (k) exhibits a decrease trendnd is more obvious with higher k values. In addition,ces between Ald (k) values (k is 30 s, 40 s, 60 s andtively) become smaller in hydroxy-Al solutions withlues (Fig. 7b). The results indicate that the contentsric Al species in hydroxy-Al solutions with lowerere higher than those with higher B values, and thespecies contained at least two kinds of species: Al2lymers. For a hydoxy-Al solution, a bigger k valueuld result in a higher value of Ald (k) and a lowerlb (k). Therefore, the choice of the k value is vitaliation of Al-polymer using ferron assay. Ald (30) inxy-Al solution is less than 10% and relatively closerhus thirty seconds of k value might be the rational

ration of Ala with ferron, particularly for hydroxy-Alith high B values.ly, soluble polymeric Al species is composed of Aldigher polymerized Al species. Al13 is just one of thelymeric Al species. Fig. 7c demonstrates the com-Al13 content and the ferron colorimetric determinedsamples. Clearly, the contents of Al13 are equal to

lues when the B values are 1.5 and 2.0, which agreesth the conclusions of Bertsch and Park [13]. However,oxy-Al solutions with lower and higher B values, Albare higher than the contents of Al13 determined usingspectroscopy. The differences between them might beexistence of some Al6 polymers as discussed above.only within certain B value ranges (i.e., 1.52.0),

Al Nspeciein momationsophis

4. Co

In tron co

The emtions uinvestitraditiomuchthe simspecia(i.e., [Fwith haccordbe asslimitattion enadoption thetial 30the reanique,betterbe sele

Ackno

Thethe Anfor theNation

Refere

[1] D.Rtroalu

[2] D.Sof

[3] J.Lalu

[4] R.Wous

250[5] P.H

nat[6] S.J

hyd195

[7] S.LtioCo, it can be concluded that at least five kinds of Alch as Almon, Al2, Al6 polymers, Al13 and Alc, existdroxy-Al solutions. However, more detailed infor-ut their speciation needs further studies using moreed analytical methods in the future.

sions

aper, the comparison of the traditionally used fer-etric solution and the simplified one was conducted.

cal approaches to the speciation of hydroxy-Al solu-the simplified ferron colorimetric solution were

d with the aid of 27Al NMR. Compared with theone, the simplified ferron colorimetric solution hader stability. Within a suitable pH range of 5.05.4,ed ferron colorimetric solution can be used for Alif the samples have no or low content of iron ionsAl] 0.05). Theoretically, the kinetic curve of ferronxyl-Al solutions can be divided into different phaseso the evolution of reaction rates, and each phase can

to certain specific Al species. However, due to theof actual operation, it is hard to distinguish the reac-es of mononuclear and oligomeric Al species. The

f Ala-ferron reaction durations had significant effectiation of hydroxy-Al solutions. Even within the ini-species other than mononuclear one had involved in

ns with ferron. Combined with the 27Al NMR tech-speciation results obtained from ferron assay can bepreted and more reasonable operation protocols can.

gements

hors thank gratefully professor Deng and Wang ofical and Testing Center, Beijing Normal Universityable help in 27Al NMR Spectrum. The support from3 program under 2002AA601290 is acknowledged.

s

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Characteristics of simplified ferron colorimetric solution and its application in hydroxy-aluminum speciationIntroductionMaterials and methodsPreparation of the simplified ferron colorimetric solutionPreparation of hydroxy-Al solutionsCharacterization of the simplified ferron colorimetric solutionAbsorbance spectra at different pH valuesStability comparison with traditional ferron colorimetric solutionEffect of ferric ions on Al speciation

Aluminum speciation using simplified ferron colorimetric solution27Al NMR spectroscopy

Results and discussionCharacteristics of simplified ferron colorimetric solutionComplexation mechanisms of ferron with hydroxy-aluminum solutionsEffect of reaction duration adoption protocols on the speciation of hydroxy-Al solutions

ConclusionsAcknowledgementsReferences