Removal of B, Cr, Mo, and Se fromWastewater by Incorporation intoHydrocalumite and EttringiteM I N Z H A N G * A N D E R I C J . R E A R D O N

Department of Earth Sciences, University of Waterloo,Waterloo, Ontario, Canada N2L 3G1

Boron, chromium, molybdenum, and selenium often occurin high concentrations in fly ash leachates. During theleaching of fly ash in alkaline environments, hydrocalumite(Ca4Al2(OH)12(OH)2‚6H2O) and ettringite (Ca6Al2(OH)12(SO4)3‚26H2O) form as secondary precipitates. In this study,the removal of B, Cr, Mo, and Se oxyanions from high pHwaters by incorporation into hydrocalumite and ettringitewas examined. Experiments were performed by precipitatingthese minerals in solutions containing B, Cr, Mo, and Seoxyanions at conditions relevant to lime-leaching of fly ashas well as to fly ash containing concrete. The uptake ofall four anions by hydrocalumite and ettringite was high.Anion uptake by hydrocalumite was larger than that byettringite, and hydrocalumite was able to reduce anionconcentrations to below drinking water standards. Ettringiteshowed an anion preference in the order of B(OH)4

- >SeO4

2- > CrO42- > MoO4

2-. In contrast, borate was leastpreferred by hydrocalumite. Coordination, size, andelectronegativity are likely the factors that result in theobserved differences among the oxyanions.

IntroductionBoron, chromium, molybdenum, and selenium are oftenenriched in solid wastes such as fly ash and spent oil shaleand occur in high concentrations in their leachates (1-3).High concentrations of these elements are also reported inindustrial discharges (4) and agriculture drainage waters (5).These elements are usually present as oxyanions and aremobile at the near-neutral to alkaline pH values typical ofmost hydrogeological environments. The solubilities of thecommon alkali and alkali earth salts of these anions areusually high, and adsorption onto negatively charged soilsurfaces is typically low. Because of their high solubilitiesand mobilities in the environment, mechanisms for removingthese oxyanions from wastewaters are of great importance.

Hydrocalumite and ettringite form as secondary precipi-tates during the hydration of fly ash and spent oil shale orcan be induced to precipitate in situ from the solid wastematerials. Both of them are also major hydration productsof ordinary Portland cement (6). Ettringite is commonlyidentified as a leaching product of alkaline fly ash (7), ofgasification ash (8), and in leached spent oil shale (9). Bothettringite and hydrocalumite were developed in neutral andacidic fly ashes when a simple lime treatment was applied(10). Decreases in anion concentrations observed in fly ashleachates have been related to the formation of these phases.Kumarathasan et al. (8) suggested that the formation of

ettringite was responsible for concentration reductions in Band Se in ash leachates. Reardon and Della-Valle (11) showedthat the concentrations of B, Cl, Mo, and SO4 decreasedmarkedly in a long-term leaching experiment of lime-treatedfly ash. Hydrocalumite, the most common secondary phaseidentified in the leached fly ash, was thought to be the hostmineral for these anions.

This study investigated the incorporation of B, Cr, Mo,and Se oxyanions into hydrocalumite and ettringite. Bycomparing the uptake behavior between these two phases,it is possible that the metal concentrations measured in thefly ash leachates can be correlated to the secondary pre-cipitates. It has been documented that extensive solidsolutions between different anions can form in both hydro-calumite and ettringite and that a conversion can occurbetween these two phases (6, 12). This study focused on theuptake of the oxyanions by hydroxyl hydrocalumite endmember (Ca4Al2(OH)12(OH)2‚6H2O) and sulfate ettringite endmember (Ca6Al2(OH)12(SO4)3‚26H2O), eliminating the influ-ence complicated by other anions. Ettringite precipitatedfrom fly ash leachates is predominantly enriched in sulfatebecause of the SO4

2- ions initially available in solutions. Asthe leachates become sulfate-depleted, ettringite convertsto hydrocalumite, in which OH- is the dominant anion. Inthe current study, we used a portlandite-saturated solutionto maintain a relatively constant pH. This solution simulatesthe conditions encountered in the lime treatment of fly ashand hydration of Portland cement.

Hydrocalumite is an anionic clay mineral. The structureof hydrocalumite is composed of portlandite-like principallayers, in which one-third of the Ca2+ sites are occupied byAl3+ (12). The substitution of Al3+ for Ca2+ generates netpositive charge to the octahedral layers, and anions areincorporated into the interlayers to balance these charges.Along with anions, water molecules also occupy the interlayerpositions (Figure 1A) (13). The structure of ettringite iscomposed of columns of positively charged chemical unitswith a composition of {Ca6[Al(OH)6]2‚24H2O}6+ (12). TheAl(OH)6 octahedra are linked with CaO8 polyhedra in thecolumns (Figure 1B). The columns are aligned parallel to thec-axis. Anions and water molecules reside between thecolumns (Figure 1C).

The objectives of this study were to compare the incor-poration of B, Cr, Mo, and Se oxyanions in hydrocalumitewith that in ettringite under high pH conditions; to examinethe anion uptake behavior in terms of the structural char-acteristics, anion coordination, size, and electronegativity;and to evaluate whether incorporation could be a potentialmechanism for removal of anions from wastewaters.

Experimental SectionMaterials. All commercial chemicals used in this study werereagent grade. Solutions were prepared using Nanopuredouble-deionized water with a conductance of less than 0.1µS/cm. Monocalcium aluminate (CaAl2O4) was prepared bycombusting a solution of calcium nitrate, aluminum nitrate,and urea at 500 °C for 5 min (14). Prepared batches wereroutinely analyzed using X-ray diffraction (XRD) analysis toensure purity. The solutions of B, Cr, Mo, and Se used inboth coprecipitation and control experiments were preparedfrom B2O3, K2CrO4, Na2MoO4, and Na2SeO4 compounds.

Experimental Design. The incorporation of B, Cr, Mo,and Se into hydrocalumite was studied by directly precipi-tating hydrocalumite from a solution containing low con-centrations of the oxyanions of these elements. A stocksolution containing approximately 10 ppm of each element

* Corresponding author telephone: (519)888-4567, ext 6489; fax:(519)746-0183; e-mail: mzhang@sciborg.uwaterloo.ca.

Environ. Sci. Technol. 2003, 37, 2947-2952

10.1021/es020969i CCC: $25.00 2003 American Chemical Society VOL. 37, NO. 13, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2947Published on Web 06/03/2003

was used in virtually all the samples. Early results indicatedthat borate was least preferred by hydrocalumite; therefore,an additional sample was prepared using a solution contain-ing 10 ppm B to determine if competition with other ionswas responsible for the low quantity of B uptake observed.

Samples for the hydrocalumite coprecipitation experimentwere prepared using a technique modified from Buttler etal. (15). Hydrocalumite was precipitated by adding 7.0 mmolof portlandite (Ca(OH)2) and 1.6 mmol of monocalciumaluminate (CaAl2O4) to 40 mL of stock solution. The resultingreaction can be described by

The amount of Ca(OH)2 added was in excess of thatrequired to form hydrocalumite to ensure that the solutionwas saturated with respect to portlandite during the reactionperiod. This simulates the conditions that exist in the flyash-lime-water and cement-water systems, where bothhydrocalumite and portlandite are present.

The uptake of B, Cr, Mo, and Se was also studied bycoprecipitation with ettringite. The method of Odler andAbdul-Maula (16) was adopted to prepare the ettringitesamples. Ettringite was precipitated by adding 7.0 mmol ofCa(OH)2 to 40 mL of 0.02 M Al2(SO4)3 solution, whichcontained the same concentrations of B, Cr, Mo, and Se asin the hydrocalumite coprecipitation experiment. The reac-tion is described by

When the results of this experiment indicated thatmolybdate was least preferred by ettringite, an additional

run was carried out using a 10 ppm Mo solution to determineif its uptake could be increased by removing the other anions.As in the hydrocalumite experiment, the amount of Ca(OH)2

added was in excess of that required to produce ettringite.To evaluate the possibility that Ca(OH)2 addition alone

could cause reductions in B, Cr, Mo, and Se concentrations,a control experiment was carried out. In the control samples,2.70 mmol of Ca(OH)2 was added to 40 mL of the stocksolution used in the coprecipitation experiments. Thisamount is slightly higher than the calculated excess Ca(OH)2

present in the hydrocalumite and ettringite coprecipitationexperiments.

In all experiments, high-density polyethylene bottles (60mL) were used as reaction vessels. Once the solutions wereprepared, the bottles were loaded on a wheel, immersed ina 25 ( 0.1 °C water bath, and rotated continuously to ensurecomplete mixing during the reaction period. Both the solutionand solid phases were sampled at various times over a periodof 30 d. Sufficient replicate samples were prepared so thateach reaction vial was opened and sampled only once toavoid contamination.

Sampling and Analysis. Samples of both solution andsolids were taken after 1, 3, 7, and 30 d of reaction. Theadditional runs conducted with pure B or Mo solution weresampled after 7 d of reaction. To prevent potential con-tamination from atmospheric CO2, filtration was conductedin a CO2-free glovebox. All solution samples were filteredthrough 0.22-µm cellulose acetate filter paper and thenacidified with 1:1 HCl to preserve them for chemical analyses.Solid samples were dried in a desiccator at a relative humidityof 37%.

B, Cr, Mo, and Se were analyzed using a Jarrell-Ash ICPspectrophotometer. The analytical uncertainties are within(5%. Phase identification of solid samples was performedby X-ray powder diffractometry (XRD) using Cu KR radiationgenerated at 30 mA and 50 kV. Specimens were step-scannedas random powder mounts from 5 to 55° 2θ at 0.05° 2θ steps;integrated at 1 s per step. The recorded data were interpretedusing powder diffraction files for ettringite from the JointCommittee on Powder Diffraction Standards and the Inter-national Center for Diffraction Data (JCPDS-ICDD) and usingXRD patterns reported in the literature (15) for hydrocalumite.Selected solid samples were examined for morphology andcrystallinity using a JEOL JSM-840 scanning electron mi-croscope (SEM).

ResultsThe XRD results of the solid samples collected from thehydrocalumite coprecipitation experiment are shown inFigure 2. Hydrocalumite had formed after 1 d of reactionand was the principal phase produced in all samples. Thepeak intensities of this phase increased with time, especiallybetween 7 and 30 d, reflecting an increase in the amount ofhydrocalumite present. The XRD results confirm the presenceof portlandite (Ca(OH)2) in all samples. Low-intensity peaksfor hydrogarnet (3CaOAl2O3‚6H2O) were also identified inthe 1-d sampled precipitates, and the peak intensities of thisphase increased with time. Hydrogarnet is a reaction productof hydrocalumite (10). This phase was not present in thepreliminary study, where the same reaction was monitored.It is possible that difference in the conversion rate ofhydrocalumite to hydrogarnet in these two studies was aresult of differences in the grain size or reactivity of themonocalcium aluminate used. More importantly, the finalsolution concentrations measured in both studies were verysimilar, suggesting that the presence of hydrogarnet has hadno significant effect on the uptake results.

The XRD patterns of the ettringite coprecipitation samplesare shown in Figure 3. Ettringite was identified as the principalphase in all samples. With reaction time, the peak intensities

FIGURE 1. (A) Schematic representation of the structure ofhydrocalumite, consisting of portlandite-like layers intercalated byhydrated anions, modified from Constantino and Pinnavaia (13). (B)Single column of ettringite projected on (112h0). Unmarked circlesrepresent H2O molecules. (C) Projection of the columns on (0001).The polygons represent the columns, whereas the triangles representthe anions and groups of H2O molecules, modified from Taylor (12).

3Ca(OH)2 + CaAl2O4 + 10H2O f

Ca4Al2(OH)12(OH)2‚6H2O (1)

6Ca(OH)2 + Al2(SO4)3 + 26H2O f

Ca6Al2(OH)12(SO4)3‚26H2O (2)

2948 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 13, 2003

of ettringite increased only slightly, indicating that theformation of ettringite was nearly complete after 1 d ofreaction. The presence of portlandite was also confirmed inthese samples.

SEM examination of the 1-d sample from the hydro-calumite coprecipitation experiment showed anhedral tosubhedral hydrocalumite crystals of 1-10 µm in size. After7 d, the crystals were similar in size but appeared as well-formed hexagonal plates (Figure 4A). In the 30-d sample, thehydrocalumite crystals were substantially larger, reachingsizes greater than 50 µm. There were also abundant smallhexagonal crystals, ranging from 1 to 5 µm, which occurredon the surfaces of the larger crystals (Figure 4B). These smallcrystals likely formed at later reaction times, supporting theXRD results, which suggest that the quantity of hydrocalumiteincreased with time. Ettringite occurred typically as needle-like crystals in the 7-d sample (Figure 4C). The crystal sizeof ettringite increased slightly with time (Figure 4D).

The solution compositions as a function of time from thehydrocalumite coprecipitation experiment are shown in Table1. The concentrations of Cr, Mo, and Se dropped to belowdetection after 1 d of reaction or a decrease of over 2 ordersof magnitude. The B concentration decreased by a factor of10 in the first day and by a factor of 2 over the next 30 d.Because three of the anion concentrations were broughtbelow detection limits, it is not possible to establish a specificorder of preference of these anions by hydrocalumite.However, it is evident that borate is the least favored. TheB uptake results for the borate solution were similar to thesolution containing all four anions. This suggests that anioncompetition, at least at these low solution concentrations,is not an important factor influencing the extent of uptakeof the individual anions.

In comparison to hydrocalumite, anion uptake by ettring-ite is much lower (Table 2). Only borate, which was the least

preferred by hydrocalumite, showed substantial concentra-tion reductions. After 1 d of reaction, the B concentrationdropped at least 2 orders of magnitude to below detectionand remained below detection for the duration of theexperiment. In contrast, the concentration reductions forCr, Mo, and Se were much less. After 1 d, the concentrationswere reduced by 63, 47, and 84%, respectively. Theirconcentrations remained relatively constant with reactiontime. The order of preference for all four elements by ettringiteis B . Se > Cr > Mo. The Mo uptake results for the solutioncontaining only molybdate were similar to the solutioncontaining all four anions. This suggests, as with hydro-calumite, that under these experimental conditions anioncompetition does not influence the extent of uptake of theindividual anions by ettringite.

The concentrations of B, Cr, Mo, and Se with time arereported in Table 3 for the control samples, in which onlyportlandite was present. A comparison of the results for theoriginal stock solution (top row entry) and the 1-d sample(second row entry) shows that only a slight concentrationreduction occurred for each element upon addition ofCa(OH)2. After this initial decrease, the concentrationsremained stable for the next 30 d. These results verify thatthe reductions in anion concentrations observed in thehydrocalumite and ettringite coprecipitation experiments area result of incorporation into these phases rather than uptakeby portlandite or precipitation as a calcium salt.

DiscussionTwo mechanisms are possible for oxyanion uptake byhydrocalumite and ettringite: (i) adsorption onto the surfacesand (ii) direct substitution for OH- in the interlayer regionsof hydrocalumite or for SO4

2- in the intercolumn regions ofettringite. Myneni et al. (17) conducted adsorption andcoprecipitation experiments to examine the uptake of As(V)

FIGURE 2. XRD patterns of the solids recovered from the hydro-calumite coprecipitation experiment. H, hydrocalumite; P, port-landite; G, hydrogarnet.

FIGURE 3. XRD patterns of the solids recovered from the ettringitecoprecipitation experiment. E, ettringite; P, portlandite.

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by ettringite and found that substitution of arsenate wasdominant over surface adsorption. From a structural pointof view, substitution of anions should be favored becausethe structural columns of ettringite are positively chargedand require anions for charge balance. In contrast, the particlesurfaces of ettringite possess net negative charge (17), which

should repel anions and reduce the importance of surfaceadsorption. These findings agree with molecular dynamicssimulations of Kalinichev and Kirkpatrick (18), which showedthat Cl- ions were repelled near ettringite surfaces. Both linesof evidence indicate that direct substitution is likely thedominant mechanism for uptake of oxyanions by ettringite.For hydrocalumite, the principal layers have a net positivecharge. In this case, the particle surfaces are dominated bythe interlayer cleavage face, which is positively charged.Therefore, surface sorption of anions is possible, and thishas been shown to occur for Cl- on hydrocalumite surfacesby NMR spectroscopic studies (19). However, unless particlesizes are in the nanometer range, the bulk of the anion uptakecapacity is within the interlayer regions. Lattice substitutionof borate, chromate, molybdate, and selenate for hydroxylhas been confirmed in the synthesized hydrocalumite solidsolutions using XRD analysis (10).

The discussion will focus on the characteristics of theoxyanions (coordination, size, and electronegativity) and thestructures of hydrocalumite and ettringite to address whythe oxyanions are selectively incorporated and the factorsthat account for the observed difference in selectivity betweenhydrocalumite and ettringite.

Speciation and Coordination of Borate, Chromate,Molybdate, and Selenate. The aqueous speciation of an

FIGURE 4. Micrographs of the solid recovered from the coprecipitation experiments: (A) hexagonal platy crystals of hydrocalumite after7 d of reaction; (B) well-formed hexagonal plates of hydrocalumite after 30 d of reaction; (C) needle-like ettringite crystals after 7 d ofreaction; (D) needle-like ettringite crystals after 30 d of reaction.

TABLE 1. Temporal Changes in B, Cr, Mo, and Se SolutionConcentrations in the Hydrocalumite CoprecipitationExperimenta

traceelement

reactionperiod (d)

B(ppm)

Cr(ppm)

Mo(ppm)

Se(ppm)

B, Cr, Mo, Se original 9.44 10.11 9.81 11.57B, Cr, Mo, Se 1 0.94 <0.02 <0.08 <0.1B, Cr, Mo, Se 3 0.48 <0.02 <0.08 <0.1B, Cr, Mo, Se 7 0.40 <0.02 <0.08 <0.1B, Cr, Mo, Se 30 0.39 <0.02 <0.08 <0.1B original 9.40B 7 0.31

a Solution/hydrocalumite mass ratio was 44:1.

TABLE 2. Temporal Changes in B, Cr, Mo, and Se SolutionConcentrations in the Ettringite Coprecipitation Experimenta

traceelement

reactionperiod (d)

B(ppm)

Cr(ppm)

Mo(ppm)

Se(ppm)

B, Cr, Mo, Se original 9.44 10.11 9.81 11.57B, Cr, Mo, Se 1 <0.08 3.77 5.22 1.81B, Cr, Mo, Se 3 <0.08 3.80 5.39 1.99B, Cr, Mo, Se 7 <0.08 4.23 5.61 2.34B, Cr, Mo, Se 30 <0.08 3.99 4.55 2.04Mo original 9.86Mo 7 4.09

a Solution/ettringite mass ratio was 40:1.

TABLE 3. Temporal Changes in B, Cr, Mo, and Se SolutionConcentrations in the Control Experiment.

traceelement

reactionperiod (d)

B(ppm)

Cr(ppm)

Mo(ppm)

Se(ppm)

B, Cr, Mo, Se original 9.44 10.11 9.81 11.57B, Cr, Mo, Se 1 9.21 8.23 7.56 11.02B, Cr, Mo, Se 3 9.38 8.47 7.61 11.34B, Cr, Mo, Se 7 9.19 8.44 7.72 10.11B, Cr, Mo, Se 30 8.79 8.06 7.56 10.54

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element determines the extent to which it is incorporatedinto precipitating mineral phases. For borate, chromate, andmolybdate ions, polymerization is a common phenomenon.Extensive formation of polynuclear species, however, onlyoccurs with these ligands at low pH and high total elementalconcentrations. Under the experimental conditions (pH≈12.5; total elemental concentrations ≈10 ppm), onlymononuclear hydrolysis species are important, and the mostdeprotonated or hydrolyzed species are the dominant species(i.e., B(OH)4

-, CrO42-, MoO4

2-, and SeO42-).

Borate occurs in both trigonal and tetrahedral coordina-tions, with B-O bond length of 1.37 and 1.48 Å, respectively(20). Tetrahedral CrO4

2- (Cr-O of 1.66 Å) is the onlycoordination found for hexavalent chromium (20). Molybdatealso shows a 4-fold coordination; however, the molybdatetetrahedron could be distorted (21). The bond length of M-Ois the largest among the anions studied (1.76 Å) (22). Incontrast to molybdate, SeO4

2- occurs as a regular tetrahedronwith little distortion in mineral phases. The characteristicsof SeO4

2- are similar to SO42-, but the bond length (Se-O)

is larger than that of sulfate (1.61 vs 1.49 Å) (20).The coordination of borate in hydrocalumite and ettringite

is different. Using IR spectroscopy, Wenda and Kuzel (23)concluded that boron was trigonally coordinated in thesynthetic borate hydrocalumite. On the basis of this coor-dination and the measured composition, HBO3

2- was sug-gested as the species present in the structure of hydro-calumite. In the ettringite structure, however, borate wastetrahedrally coordinated and present as B(OH)- (8, 24).

Oxyanion Substitution in Hydrocalumite. Hydrocalumitecan accommodate anions of various sizes. The thickness ofits interlayer regions varies as different anions are incorpo-rated (e.g., 7.6 Å for CO3

2- and 8.9 Å for SO42-) (12). Studies

on the uptake of anions by hydrotalcite (Mg6Al2(OH)16X‚4H2O,X represents a divalent anion) revealed that ion selectivityis determined by the mobility of anions between the principallayers (25). Hydrotalcite compounds are close analogues ofhydrocalumites, and the results from hydrotalcite studiesare therefore useful for understanding the uptake of anionsby hydrocalumite. The mobility of anions in the interlayersof hydrotalcite is likely dependent on the strength of theirbonding to the hydroxide layers (25), which in turn isdependent on ionic radius, charge, and geometry. Highselectivity is associated with anions of higher charge andsmaller ionic radius (26). Miyata (26) also documented thattrigonal planar carbonate is more preferred in hydrotalcitesthan tetrahedral sulfate. This is because carbonate anionsare oriented parallel to the principal layers (27), whereassulfate tetrahedra are randomly distributed with apicaloxygens of the tetrahedron pointing either up or down (28).

In this study, the oxyanions occupy less than 2% of theanionic sites in the interlayers, the rest being occupied byOH-. Consequently, competition among oxyanions is not afactor in the degree of uptake, only their relative preferencefor an interlayer site as compared to OH- is important. Underthese conditions, trigonal planar borate might not show asite preference over tetrahedral coordinated oxyanions. Infact, CrO4

2-, MoO42-, and SeO4

2- all showed a concentrationdecrease of over 2 orders of magnitude in this study, whereasborate showed much lower uptake.

Borate occurs as trigonal HBO32- in the structure of

hydrocalumite. However, B(OH)4- is the predominant borate

species in solution at pH above 9.2. Thus, a change in chargemust have occurred during the incorporation of B(OH)4

-.Dutta and Puri (29) also observed a similar change duringthe incorporation of sulfate and phosphate by a hydrotalcite-like phase. The species SO4

2- was determined in the hydro-talcite-like phase although the molar ratio of HSO4

-/SO42-

was 4.5 in the solution. Such a change was attributed to thepreference of divalent anion by hydrotalcite. The incorpora-

tion of borate into hydrocalumite also involves a change inits coordination from tetrahedral B(OH)4

- to trigonal HBO32-.

For this conversion to occur, an energy barrier must beovercome. This may explain the lower uptake of borate byhydrocalumite as compared to the other oxyanions. Hem-ming et al. (30) used a similar argument to explain greaterborate substitution for carbonate in the aragonite structureas compared to calcite. In that case, a coordination changeto trigonal was required for borate incorporation in calcitebut not in aragonite.

Oxyanion Substitution in Ettringite. In contrast tohydrocalumite, variations in the lattice dimensions ofettringite are more limited. However, some change is stillallowable (e.g., the dimensions of the unit cell of ettringiteincrease gradually as selenate replaces sulfate in the solidsolution) (31). In addition, anion substitution for SO4

2- inettringite does not appear to require an exact match in theanion geometry. Solem-Tishmack et al. (32) observed thatpyramidal selenite was more competitive for the sulfate sitesthan tetrahedral selenate. This indicates that anion geometryis not the major factor controlling uptake. In ettringite, thesize difference between sulfate and the substituting anion isprobably more important.

The preference of ettringite for borate that was observedin this study is likely due to its similar size to sulfate.Kumarathasan et al. (8) has suggested some possible chargebalance mechanisms that allow substitution of monovalentB(OH)4

- for SO42-, such as coupled substitution of Na+ or

some other monovalent cation for Ca2+. The order ofpreference of uptake for the anions that are divalent insolution is SeO4

2- > CrO42- > MoO4

2-, correlating with anincrease in metal-oxygen bond length in the same sequence.Molybdate is least preferred by ettringite because the sizedifference between molybdate and sulfate is the largest.However, the size difference between SeO4

2- and CrO42- may

be too small to account for their difference in uptake. Becausethe incorporation of both SeO4

2- and CrO42- in ettringite is

virtually a substitution for SO42-, the difference in electro-

negativity between sulfur and selenium or chromium is alsoa possible factor influencing the substitution. Compared withsulfur, selenium has a very similar electronegativity (2.4 vs2.5), whereas chromium has a much lower electronegativity(1.6). The similarity in electronegativity between seleniumand sulfur would lead to a higher preference of selenate tosubstitute for sulfate in the structure of ettringite.

Environmental Implications. This study shows thathydrocalumite and ettringite are capable of greatly reducingthe concentrations of borate, chromate, molybdate, andselenate from solutions. Hydrocalumite in particular canreduce the oxyanion concentration levels to below drinkingwater standards. According to the U.S. EPA, the maximumcontaminant levels for B, Cr, Mo, and Se in drinking waterare 0.6-1, 0.1, 0.04, and 0.05 mg/L, respectively. Theconcentrations of B, Cr, Mo, and Se measured in thehydrocalumite coprecipitation experiment are 0.94, <0.02,<0.08, and <0.1 mg/L after 1 d of reaction. Ettringite wasalso able to reduce boron, but not the other elements, tolevels in solution that are below drinking water standard.

Both hydrocalumite and ettringite have a large capacityto incorporate borate, chromate, molybdate, and selenateinto their structure. Although the metal ion uptakes in thisstudy were only several hundred milligrams per kilogram,the maximum uptake capacities for both mineral phases aremuch higher. The end members of borate, chromate, andmolybdate hydrocalumite have been synthesized (23, 33, 34).For these compounds, the uptake capacities of B, Cr, andMo are 18 700, 74 700, and 133 000 mg/kg, respectively.Substitution of borate up to two-thirds of the total anionicsite capacity in ettringite was reported by Pollmann et al.(24). Complete substitution of chromate and selenate for

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sulfate in ettringite was achieved by Kumarathasan et al. (8).These correspond to potential total uptake capacities of B,Cr, and Se in ettringite of 33 800, 119 000, and 170 000 mg/kg, respectively.

The most important limitation in using these calciumaluminate phases to control contaminant oxyanion levels inwater is pH. High pH conditions must be maintained in adisposal or treatment environment because both hydro-calumite and ettringite are unstable at low pH. Gabrisova etal. (35) indicate that pH values greater than 10.7 are requiredto stabilize ettringite and greater than 11.6 to stabilizehydrocalumite. Although the competition between the anionswas not a factor influencing the uptake under the conditionsused in this study (e.g., less than 2% of the anionic sites wereoccupied by the oxyanions in hydrocalumite), this influencemay be considerable at high anion concentrations. Anionssuch as SO4

2- and CO32- in natural environments could

compete for the sites in the structures of these mineral phases,leading to lower uptake.

AcknowledgmentsThe authors thank Dr. M. Hobbs for her critical review of thismanuscript. Funding for this research was made availablethrough grants from the National Science and EngineeringResearch Council of Canada (NSERC).

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1023-1027.

Received for review October 4, 2002. Revised manuscriptreceived April 18, 2003. Accepted April 18, 2003.

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