70845664 reaction kinetics and mechanisms of zeolite dissolution in

7/22/2019 70845664 Reaction Kinetics and Mechanisms of Zeolite Dissolution In

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Reaction Kinetics and Mechanisms of Zeolite Dissolution in

Hydrochloric Acid

Ryan L. Hartman and H. Scott Fogler*

Department of Chemical Engineering, The University of Michigan, 2300 Hayward St.,Ann Arbor, Michigan 48109-2136

The kinetics of zeolite dissolution in aqueous hydrochloric acid was investigated over a stronglyacidic pH range (for pH e1 and [H+] )0.1-6 M) in a batch reactor. The apparent reactionorder with respect to hydrogen ion concentration decreased with increasing pH. Existing modelsdo not predict this trend. A fundamental rate model derived from an adsorption and surfacereaction mechanism is consistent with the experimental dissolution rates. The correlated modelparameters suggest that the hydrogen ion strongly adsorbs onto zeolite surfaces and that zeolitedissolution follows Langmuir-Hinshelwood kinetics. A comparison of the Langmuir-Hinshel-wood kinetic parameters reveals that the zeolite dissolution rates are dependent on the Si/Alratio in the zeolite framework. Nonstoichiometric dissolution is observed for analcime and typeY zeolites, indicating a unique mechanism in which aluminum atoms are selectively removed.The ratio of the measured silicon dissolution rate to the stoichiometric dissolution rate rangesfrom 0 to 1 and increases with decreasing Si/Al ratios in the range of 1 -2.6, suggesting that thezeolite dissolution phenomenon is controlled by the framework composition.

1. Introduction

Solid-liquid dissolution phenomena play a vital rolein processes throughout the pharmaceutical, petrochemi-cal, water treatment, and electronics industries. Thedissolution of zeolites has been investigated in a numberof different processes, including wastewater streamtreatment, radioactive waste immobilization, and pe-troleum reservoir acidization. This research is motivatedby the acidization of reservoirs in the Gulf of Mexicothat contain the zeolite analcime. Acidization is amineral dissolution technique used to enhance petro-leum recovery. Here, acid is injected into a porous, low-permeability, geologic formation in order to dissolvealuminosilicate minerals and, thereby, increase near-well bore permeability. Mineral precipitation can occurduring acidization and constitutes a significant risk ofpore blockage, which leads to production losses ofconsiderable economic magnitude.1A fundamental un-derstanding of zeolite dissolution science is crucial inorder to design acidization treatments, which maximizemineral dissolution while mitigating precipitation risks.

2. Zeolite Mineralogy

Oil-bearing geologic formations can contain clays,carbonates, quartz, feldspar, mica, and zeolites.2 Forma-tions containing zeolites are composed primarily of

quartz (50-90 wt %), which is considered to bevirtually inert, or undissolvable, to acid treatments. Theremaining 10 wt % is composed of zeolites, feldspar,clay, and mica, which are all dissolvable.3Acid formula-tions are selected on the basis of the reservoir mineral-ogy,2 and hydrochloric acid, hydrochloric/hydrofluoricacid mixtures, and organic acid mixtures are commonlyused as stimulation fluids.4-6 Severe mineral precipita-

tion has been documented when reservoirs composed ofzeolites were treated with hydrochloric acid.1 Hydro-chloric acid was, therefore, used as the primary solventin this study.

Naturally occurring zeolites exist in mineral forma-tions throughout the world.7 The zeolite of primaryeconomic concern is analcime because it is dissolvableand exists within oil-bearing formations in the Gulf ofMexico.1 Analcime can be found in geologic formationselsewhere, from Ireland to New Jersey and from Wyo-ming to Arizona, as well as in the deep sea floor,increasing the chance of encountering the zeolite duringreservoir stimulation treatments.3 Zeolites are a class

of minerals characterized by crystalline composition andstructure.3 The structure of zeolite minerals consists ofa network of silicon and aluminum tetrahedra. Eachoxygen atom within a tetrahedron shares bonds with aneighboring tetrahedron, resulting in a continuousframework of silicon and aluminum tetrahedra. Thecrystal structures of zeolites are microporous, whereasthose of clays and feldspars are not. Micropores inanalcime contain Na+ ions (dpore ) 2.6 ),8 and theSi/Al ratio in the framework may range from 1.8 to 2.8,as shown in Figure 1a.3

Synthetic zeolites, which play important roles in anumber of industrial applications, exhibit propertiessimilar to those of naturally occurring zeolites. Type A

zeolites, shown in Figure 1b, are commonly synthesizedfor use as detergents and adsorbents. The characteris-tics of type A zeolites are dependent on the absorbedcation species,9 and the Si/Al ratio may range from 0.7to 1.2.3 For example, a Na+ exchanged type A zeolite,known as a type 4A, will permit the adsorption ofmolecules with a minimum effective diameter of about4.0 .9 Type 3A zeolites are prepared by exchanging Na+with K+, resulting in effective pore diameters of ap-proximately 3.0 .3,9 Similarly, type 5A zeolites, pre-pared by exchanging Na+ with Ca2+, exhibit effectivepore diameters of 4.9 .3,9 Type Y zeolites are manu-

* To whom correspondence should be addressed. Tel.: +1(734) 763-1361. Fax: +1 (734) 763-0459. E-mail: [email protected].

7738 Ind. Eng. Chem. Res.2005, 44, 7738-7745

10.1021/ie0504349 CCC: $30.25 2005 American Chemical SocietyPublished on Web 08/26/2005


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factured for use as reaction catalysts or molecular sievesin gas separations. The pore diameter of a type Y zeoliteis 7.4 , with Si/Al ratios ranging from 1.5 to 3, asshown in Figure 1c.3

3. Previous Work

Fundamental knowledge of the mechanism of zeolitedissolution in acidic solutions is limited as previousinvestigations have focused on dissolution in alkalineand near-neutral pH solutions.10-18 In one such study,Yamamoto et al.15 hypothesized that hydrogen andhydroxide ions diffuse into heulandite (a natural zeolite)crystals and remove the outermost aluminosilicate layerin dilute sulfuric acid and sodium hydroxide solutions,respectively. Measurement of the aqueous productconcentrations, however, was not performed, nor was arate law model developed to support this hypothesis.The research in the present study addresses zeolitedissolution from a fundamental perspective by develop-ing a rate law model based on surface adsorption andreaction phenomena.

While lacking fundamental derivation, an empiricalapproach has been taken with a number of empiricalrate expressions having been applied to describe zeolitedissolution,10,14,17 such as the one given by Gdanski:19

where, [HF] and [HCl] are the concentrations of molec-ular HF and HCl (H+), respectively. In the absence ofHF, the empirical rate law reduces to a power-lawrelationship with respect to the HCl concentration (i.e.,R ) 0). However, the reaction mechanism of zeolitedissolution in HCl must be accurately established beforesymbiosis between HF and HCl can be investigated.Other researchers have used empirical models to predictzeolite dissolution as well. Murphy et al.14 and Rag-narsdottir17 both assumed a power-law dependence ofthe rate law on hydrogen ion activity. Ragnarsdottir17demonstrated empirically that the reaction order, n,with respect to hydrogen ion activity is a linear functionof the Al/Si ratio within an aluminosilicate frameworkfor pHs from 2 to 7:

Cizmek et al.10-12 studied the dissolution kinetics of typeA, type X, and synthetic mordenite zeolites in aqueoussodium hydroxide solutions. A rate law model wasdeveloped to estimate zeolite dissolution in the near-equilibrium regime, which cannot be applied to theresults in this study. Rate models proposed by Gdan-ski,19 Murphy et al.,14 Ragnarsdottir,17 and Cizmek et

al.10 were empirically fit to experimental data over alimited range and cannot be accurately extrapolated tohigh H+ concentrations. In addition, no reaction mech-anism was applied to develop a rate model.

Kline and Fogler20 demonstrated that hydrofluoricacid attack on feldspar, kaolinite, and montmorillonitetakes place on crystalline, planar surfaces and edges

and applied a fundamental mechanism to develop a ratelaw model. The characteristic mechanism includes bothHCl (H+) and HF adsorption on the surface followed bya dissolution step.21 For the case when the surfacereaction is rate-limiting, the rate law was shown to beof the form

This approach from first principles is an example thatcan be applied to other systems in order to revealdissolution mechanisms.

4. Experimental Methods

4.1. Materials. Research grade analcime was ob-tained from Mt. St. Hilaire, Quebec (Ward Science). Thecrystals were crushed for approximately 10 min usingan automated crusher and sieved into 0.2-38 mparticle sizes. The analcime crystals appeared to rangefrom colorless to white. Ultrapure type 4A and type Yzeolites were obtained from the PQ Corporation andZeolyst International, respectively. Type 4A zeolite wasdispersed into 0.5 M KCl and 0.5 M CaCl 2solutions forapproximately 20 min at room temperature in order toprepare type 3A and type 5A zeolites, respectively. Theparticles were subsequently filtered through a 0.2 mmembrane filter and washed with deionized water toremove excess Ca or K. Each sample was then dissolvedin acid and analyzed using an inductively coupledplasma spectrophotometer (ICP) in order to monitor theremoval of Na+ from the lattice. The process wasrepeated until the Na+ was completely exchanged withK+ or Ca2+. Reactant solutions were prepared withdeionized water and trace-metal-grade hydrochloricacid.

4.2. Dissolution Experiments. Zeolite dissolutionexperiments were carried out in concentrations ofhydrochloric acid ranging from 0.1 to 6 M using thebatch reactor shown schematically in Figure 2. A totalof 1 g of zeolite was dissolved in 300 mL of acid solutionat temperatures of 5.4, 8.0, and 10.3 C and a stirring

Figure 1. Crystal structures of (a) analcime, (b) synthetic type

A, and (c) synthetic type Y. (Modified from International ZeoliteAssociation,Atlas of Zeolite Structure Types: ANA, LTA, and FAU,2003. http://www.iza-structure.org/databases/.)

rproducts ) k[HF]R[HCl] (1)

n ) 0.25 + 0.75(AlSi) (2)

Figure 2. Experimental batch reactor.

rproducts )ka[HF]

1 + KA[HF](1 +kb[H

+]

1 + KB[H+]) (3)

Ind. Eng. Chem. Res., Vol. 44, No. 20, 2005 7739


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rate of approximately 500 rpm. Samples were obtainedusing micropipets and drawn through membrane filters(dpore ) 0.2m) at short time intervals. Filtration of thesuspended particles stopped the dissolution reaction.Hence, filtrate samples were representative of dissolvedmineral concentrations as a function of reaction time.The samples were analyzed for dissolved Al, Si, and Na(or Ca, K) using an ICP spectrophotometer. Dissolutionrates were quantified by the method of initial rates.Details of this method are described in a previousinvestigation.22

5. Results and Discussion

5.1. Zeolite Characterization. Two samples eachof analcime and type 4A, 3A, 5A, and Y zeolites weredissolved overnight in a 2.5 M HCl/0.5 M HF mixtureat 70 C in order to facilitate structural and composi-tional characterization. Each sample was drawn througha 0.2m membrane filter, diluted with deionized water,and analyzed using an ICP spectrophotometer. Table 1shows the Si/Al ratio, cation/Al ratio, density, cationexchange capacity, pore dimensions, and surface areasfor each zeolite. External surface areas for each samplewere calculated on the basis of particle shape factorsestimated from scanning electron microscope micro-graphs and from the particle size distribution. Theparticle shape factor is the ratio of the surface area ofa sphere to that of a nonspherical particle having the

same volume. Details of how to calculate the specificsurface area for a given shape factor and particle sizedistribution, as well as examples, are reported byGeankoplis.23 External surface areas of all samples wereestimated to be approximately 1 m2/g.

The specific surface area is an important propertythat can be used to compare dissolution rates of differentzeolites. This property is commonly measured by nitro-gen Brunauer-Emmett-Teller (BET) adsorption formacroporous zeolites and nonporous surfaces.24,25 Asshown in Table 1, nitrogen BET surface area measure-ments of analcime and type 4A and Y zeolites yieldedvalues of 1.33, 2.26, and 848 m2/g, respectively. Inter-nally accessible surface areas were theoretically esti-mated by constructing zeolite unit cells using molecularmodeling software26 for two cases: (1) N2adsorption and(2) H3O+ adsorption. Table 1 demonstrates that theo-retically predicted N2BET internal surface areas were2 orders of magnitude greater than the measured valuesfor type A zeolites, indicating that insufficient time wasallowed for adsorption because N2diffusion rates withintype A zeolites are very small, as reported by Karger.25In analcime, the effective window for diffusion is lessthan that required for N2adsorption, which resulted ina theoretically predicted internal surface area of 0 m2/g. The internally accessible surface areas for all zeoliteswere estimated to be on the order of 100 m2/g, assumingH3O+ adsorption. The results demonstrate conclusivelythat N2 BET measurements of type 4A and analcime

reflect external surface area measurements while mea-surements of type Y reflect internal and external areas,as shown in Table 1.

Given that excess protons in water exist as hydratedcomplexes (e.g., H9O4+ or H5O2+), as described by Marxet al.,27 it is unlikely that the hydronium ion diffusesinto the porous framework of analcime or type A zeolitebecause the effective window for diffusion is smallerthan the effective diameter of a hydrated complex.Recent investigations by Ryder et al.28 and Franke andSimon29 demonstrate a Grotthus-type mechanism forproton transport within ZSM-5 zeolite frameworks.Ryder et al.28 showed that the presence of water withinZSM-5 micropores catalyzes proton migration rates via

a water-facilitated proton hopping mechanism. Excessprotons in the bulk migrate to the external surface ofzeolite particles where either a surface reaction takesplace or further migration into the porous frameworkoccurs. As a result, protons penetrate and attackinternal framework sites. Consequently, the surfacearea measured by N2BET adsorption is not representa-tive of the true reactive surface area.

In the current investigation, a reaction site is definedas either a Si-O or Al-O bond. In zeolites, there existtwo reaction sites per framework atom. If a constantreaction site surface density is assumed, then thedissolution rate can be normalized to m2 exposed surfaceand compared for different zeolites. The total number

of accessible reaction sites was estimated from theaccessible surface areas calculated using the molecularsimulations software package26 for two limiting cases:(1) accessible internal reaction sites and (2) only acces-sible external reaction sites. The results show that thereaction site density varies between zeolites, as shownin Figure 3. The analysis was, therefore, extended todescribe zeolite dissolution as a function of reaction sitedensity in addition to surface area.

5.2. Zeolite Dissolution.In the first series of experi-ments, initial dissolution rates of both Si and Al weredetermined as a function of H+ concentration. Figure 4shows dissolution rates of analcime as a function of acidconcentration. Under stoichiometric dissolution condi-tions, Si and Al dissolution rates expressed as molesdissolved per reaction sites per unit time are identical.The selective removal of Al atoms results in dissimilarSi and Al dissolution rates, as shown in Figure 4.Selective removal is defined as the removal of oneatomic species at a faster rate than another from thezeolite framework.

One observes from Figure 4 that the apparent reac-tion order with respect to the H+ concentration isapproximately 1 at low concentrations (


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able surface adsorption sites. Writing these observationsin terms of reaction steps we have

Assuming the surface reaction is rate limiting, theinitial rate law is

The rate law can be rearranged to take a form

where

and

Those familiar with enzymatics will recognize eq 5 asthe rate law for Michaelis-Menten kinetics, and k

3is

equivalent to the turnover number,kcat.32 The analogousMichaelis-Menten constant,Km, and the maximum rateof dissolution, Vmax, provide fundamental insight intothe reaction mechanism.

To test the suggested mechanism, the experimentaldata is linearized by the Hanes-Woolf method andplotted in Figure 5.32 The constant, Km, in eq 5 wasestimated to be on the same order as the H+ concentra-tion (Km,Al )1.75 and Km,Si ) 1.31 mol/dm3), which isconsistent with adsorption being a fundamental step inthe reaction mechanism of analcime (a natural zeolite)dissolution in hydrochloric acid.

Figure 6 shows an Eadie-Hofstee analysis32 of theanalcime dissolution data along with a comparison of

the empirical rate models proposed by Gdanski,19 Rag-narsdottir,17 and Murphy et al.14 One readily observesthe empirical models are not able to describe the data,while Langmuir-Hinshelwood kinetics provide an ex-cellent fit. Figure 7 shows a comparison of experimentaldata (pH


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dottirs empirical model, demonstrating that a powerrate law cannot be used to model the dissolution ofzeolites for pH


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4A zeolites, which exhibit the highest dissolution ratesunder all experimental conditions. One observes inTable 2 thatVmaxvalues decrease with increasing Si/Alratios. Silicon and aluminum dissolution rates of type

A zeolites are 1 order of magnitude greater thandissolution rates of analcime or type Y. Silicon dissolu-tion rates of analcime are 1 order of magnitude greaterthan that of type Y, while aluminum dissolution ratesare approximately on the same order of magnitude inboth zeolites. Analogous trends are observed for theturnover number, kcat, which is a ratio of frameworkatoms removed per framework atoms bound per unittime. Michaelis-Menten-type rate constants, shown inTable 2, are consistent with hydrogen ion adsorptiononto each zeolite.

A quantitative description of the selective removal ofAl is provided in order to further investigate thecontrolling mechanisms of zeolite dissolution. The selec-tive removal of Al atoms occurs when the resulting Si/

Al ratio in solution is less than the stoichiometric ratiowithin the zeolite framework, 0. When there is noprecipitation of Si, the ratio of the measured Si dissolu-tion rate to the stoichiometric Si dissolution rate (basedon Al) can be expressed in terms of the parameter

which can be used to quantitatively describe the selec-tive removal of Al.

Table 3 shows the selective removal parameter value for each zeolite. Figures 8-10 show that Si andAl dissolution rates of type A zeolites are nearly equal,

resulting in 1. For analcime, ranges from 0.770to 0.936, indicating an increased Al removal selectivity.Figure 11 and Table 2 show that rAl . rSifor type Yzeolites, resulting in , 1, as can be seen in Table 3.

For type Y zeolites, values range from 0.011 to 0.032,confirming that Al atoms are selectively removed. Theselective removal of Al has previously been observed byKerr,34 who estimated that 100% of framework Al intype Y zeolites may be removed by treatment withethylenediaminetetraacetic acid, a weak acid. The re-sults in Table 3 also show that the extent to which Alatoms undergo selective removal is strongly dependenton the zeolite Si/Al ratio. A comparison of the selectiveremoval parameter values in Table 3 shows thefollowing Al selective removal trend: type Y> analcime>type A. A decrease in with increasing 0indicatesvariations in the silicate framework stability. Zeoliteswith large values of the stoichiometric ratio of Si/Al, 0,(i.e., 2.6 > 0 > 2) are resistant to framework destruc-tion, which is consistent with the work of Carland andAplan.18

The general model for synthetic and natural zeolitedissolution rates can be applied to field operations,which can encompass a number of naturally occurringzeolites (e.g., analcime, clinoptilolite, heulandite, etc.).7Typical field operation formulations (e.g., 9 wt % HCl/1wt % HF) can incorporate pH conditions of less than 1,and the fluids are injected in stages. For example, anHCl preflush is generally followed by treatment withHCl/HF and the subsequent injection of an inert fluidas the last stage, all done in dynamic mode. To minimizethe shutdown period and raw material usage, it iscritical that the appropriate amounts and strength of

Table 2. Evaluation of the Kinetic Parameters

T(C) zeolite Si/Al

Vmax,Al(mol/1016sites min)

Vmax,Si(mol/1016sites min)

Km,Al(mol/dm3)

Km,Si(mol/dm3)

(Vmax,Al/Km,Al) 106(dm3/1016sites min)

(Vmax,Si/Km,Si) 106(dm3/1016sites min)

5.4

type 4A 1.06 4.51 4.32 0.296 0.283 15.2 15.2type 3A 1.03 4.14 3.80 0.382 0.337 10.8 11.3type 5A 1.02 3.77 3.61 0.330 0.337 11.4 10.7analcime 2.09 0.588 0.452 1.75 1.31 0.336 0.345type Y 2.56 0.209 0.007 0.093 0.121 2.26 0.057

T(C) zeolite Si/Al Vmax,Al/Vmax,4A Vmax,Si/Vmax,4A kcat,Al(min-1) kcat,Si(min-1) kcat,Al/Km,Al(dm3/mol/min) kcat,Si/Km,Si(dm3/mol/min)

5.4

type 4A 1.06 1.0 1.0 543 520 1830 1840type 3A 1.03 0.92 0.88 499 458 1300 1360type 5A 1.02 0.84 0.84 454 435 1370 1290analcime 2.09 0.13 0.10 70.9 54.5 40.4 41.5type Y 2.56 0.046 0.002 25.2 0.829 273 6.86

Table 3. Selective Removal Parameter Values for Each Zeolite

values ofa

[H+](mol/dm3) T(C)

type 5A0 ) 1.02

type 3A0 ) 1.03

type 4A0 ) 1.06

analcime0 ) 2.09

type Y0 ) 2.56

0.10 5.4 1.02 0.997 0.935 0.770 0.0278.0 X X 0.900 0.851 0.018

10.3 1.03 1.04 1.01 0.820 0.0110.25 5.4 1.04 1.07 0.960 0.828 0.032

8.0 X X 0.991 0.850 0.02810.3 1.03 1.08 0.988 0.860 0.018

0.50 5.4 0.956 1.07 0.929 0.854 0.0328.0 X X 0.945 0.885 0.028

10.3 1.01 0.980 0.955 0.891 0.0251.00 5.4 1.01 0.866 0.919 0.936 0.0112.00 5.4 X X X 0.928 X 4.01 5.4 X X X 0.858 X 6.00 5.4 X X X 0.787 X

aX) analysis not carried out.

)rSi

0rAl) ( measured Si dissolution ratestoichiometric Si dissolution rate) (8)



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the acid are determined prior to injection at the wellhead. First-order rate law models with respect to theH+ concentration overpredict dissolution rates in the lowpH regime, as shown in Figure 6, resulting in overes-timated amounts of dissolved formation and inaccuratepermeability/porosity estimates. Empirical rate modelsdeveloped by Gdanski,19 Ragnarsdottir,17 and Murphyet al.14 also fail to predict the dissolution rate. Figure 5indicates accurate predictions of the dissolution rate bya Michaelis-Menten-type kinetic analysis, which canlead to more accurate models for acidization treatmentformulation. To design effective acidization treatments,the appropriate rate model and kinetic parameters forzeolite dissolution must be applied.

6. Conclusions

Experiments were performed to elucidate the mech-anism of zeolite dissolution in hydrochloric acid solu-tions at pH conditions e 1 (or [H+] ) 0.1-6 M). Previousempirical models cannot be used to accurately predictthe reaction rate as a function of pH. In this work, afundamental kinetic model was developed and appliedto the experimental results, demonstrating that zeolitedissolution rates can be estimated using a form of theLangmuir-Hinshelwood rate law analogous to theMichaelis-Menten equation. A mechanism involvingadsorption of the hydrogen ion followed by a surfacereaction, which is rate-limiting, is consistent withexperimental data. Empirical zeolite dissolution modelsare inconsistent with experimental data and should notbe used as the basis for the design of reservoir acidiza-tion treatments.

Quantitative analysis of the selective removal of Aldemonstrates that zeolite dissolution exhibits a strongdependence on the Si/Al framework ratio. The ratio ofmeasured Si dissolution rate to stoichiometric Si dis-solution rate increases with decreasing Si/Al ratio. TypeA zeolites dissolve stoichiometrically, whereas type Y

and analcime dissolve nonstoichiometrically at a tem-perature of 5.4 C. Nonstoichiometric dissolution canresult in undissolvable silicate particles; consequently,one cannot assume that treating zeolites with hydro-chloric acid at 5.4 C will always result in completestructural dissolution.

Acknowledgment

The authors acknowledge financial support from thefollowing members of the Industrial Affiliates Programon Flow and Reaction in Porous Media at the Universityof Michigan: Baker Petrolite Corporation, Chevron-Texaco Energy Technology Company, ConocoPhillipsCompany, Schlumberger Ltd., Shell International Ex-

ploration & Production, and Total.

Nomenclature

aH+ ) hydrogen ion activitydpore ) pore diameter

KA) equilibrium constant for HF adsorptionKB ) equilibrium constant for H+ adsorptionKm ) analogous Michaelis-Menten constantka ) specific reaction rate for dissolution by HFkb ) specific reaction rate for dissolution by H+kcat ) turnover number[H+] ) hydrogen ion concentration[HF] ) molecular HF concentration[HCl] ) molecular HCl concentration

n ) empirical reaction order with respect to hydrogen ionactivity

-r* )initial dissolution raterproducts ) rate of product formationVmax ) maximum rate of dissolution

Greek Letters

R ) empirical reaction order with respect to molecular HF ) empirical reaction order with respect to molecular HCl ) ratio of measured Si dissolution rate to stoichiometric

Si dissolution rate0 ) Si/Al ratio in zeolite before dissolution

Subscripts

0 ) denotes initial dissolution rates4A) type 4A zeolite

Al ) aluminum speciesi ) framework atom species or zeoliteSi ) silicon species

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Received for review April 8, 2005Revised manuscript received July 22, 2005

Accepted July 29, 2005

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70845664 reaction kinetics and mechanisms of zeolite dissolution in

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