Precipitates in a Cr(VI)-Contaminated Concrete - American Chemical

University, P.O. Box 751, Portland, Oregon 97201-0751. Examination of Cr(VI)-contaminated concrete from a former hard-chrome plating shop revealed the...
0 downloads 0 Views 336KB Size
Download PDF
Environ. Sci. Technol. 2000, 34, 4185-4192

Precipitates in a Cr(VI)-Contaminated Concrete CARL D. PALMER† Environmental Science and Resources, Portland State University, P.O. Box 751, Portland, Oregon 97201-0751

Examination of Cr(VI)-contaminated concrete from a former hard-chrome plating shop revealed the presence of long, thin crystals varying from white to bright yellow. Many of the crystals examined by scanning electron microscopy (SEM) were acicular and 25-100 µm in length and 1-15 µm wide. The composition, determined by energydispersive X-ray spectroscopy, morphology, and d spacings, measured by electron diffraction, identifies these crystals as chromate enriched ettringite (Ca6Al2((S,Cr)O4)2(OH)12‚26H2O) with the mole fraction of CrO42- in the SO42- position being 0.41 and 0.72. A nearly pure CrO42- hydrocalumite (3CaO‚Al2O3‚CaCrO4‚nH2O) is also observed. Some of these crystals appear to be pseudomorphs of ettringite. The CrO42--hydrocalumite crystals are coated with smaller acicular crystals that are most likely solid solutions between Si-ettringite (Ca6Al2(SiO3)3(OH)12‚26H2O) and CrO42--ettringite. These crystals are bound together by an amorphous appearing SiO2 phase. Ca-silicate-hydrate gel (CSH) has precipitated around ettringite-group crystals containing chromium indicating that some CSH formation occurred after contamination of the concrete.

Introduction Chromium is a widely used, toxic industrial metal (1) and a common contaminant in soils and water (2). Chromium contamination at plating facilities often involves spills onto concrete and cement. Interactions between the chromiumbearing solutions and the concrete may require the removal, treatment, and disposal of the concrete as a hazardous waste. A better understanding of the forms of chromium within the concrete and cement will assist environmental scientists and engineers in designing better remedial actions and will help them to quantify the concentrations of Cr(VI) than can emanate from the concrete. In addition to chrome-plating facilities, there are many other situations where chromium-concrete interactions are important. In Hudson County, NJ, chromium emanating from chromite ore processing waste “muds” has been observed seeping through and forming precipitates in concrete basement walls (3). Scrapings from one basement wall had chromium concentrations of 37 000 mg/kg. There is interest in the potential use of cement mixtures for the stabilization of wastes (4, 5) including those that contain chromium (6-8). CrO3 added to monoliths of alite (3CaO‚SiO2), a common constituent of cement clinker, yielded CaCrO4‚2H2O in the first few minutes and Ca2CrO5‚ 3H2O within 24 h (7). The concentration of aqueous Cr(VI) was observed to decrease over a 14 month period, and it is † Current address: Idaho National Engineering and Environmental Laboratory, P.O. Box 1625, Idaho Falls, ID 83415-2107; phone: 208526-0875; email: [email protected].

10.1021/es991089f CCC: $19.00 Published on Web 09/01/2000

 2000 American Chemical Society

believed that some of the Cr(VI) was incorporated into the tetrahedral positions of a calcium-silicate gel. The ability of cement mixtures to bind chromium depends on the oxidation state of the chromium (6, 9). Cr(III) is not readily removed from cement, however, as much as 95% of added Cr(VI) can be leached (9). Cr(III) analogues of calcium aluminates Ca2Cr(OH)7‚3H2O, Ca2Cr2O5‚6H2O, and Ca2Cr2O5‚ 8H2O as well as hydrogarnet (3CaO‚(Al,Cr)2O3‚6H2O) have been synthesized (6). These phases can control the concentration of chromium in the pore water when Cr(III) is added to the clinker or Cr(VI) is added to clinker that also contains a reducing agent. Chromium has been used as an additive to cement clinker to produce a superhigh-early-strength portland cement (10). The formation of Cr(VI)-containing solid solutions of glaserite ((Na,K)2(S,Cr)O4) in these systems has been suggested (10). The addition of CaCrO4 to the clinker can accelerate the hydration of alite (3CaO‚SiO2), increase the growth of portlandite (Ca(OH)2), and result in the formation of larger and thicker acicular crystals of ettringite (11). Chromium additives have also been used in the manufacture of high refractory materials (12). Chromium-cement interactions can also serve as a model for the behavior of chromium in other alkaline systems. Waste disposal and leakage into naturally alkaline soils has occurred. Waste “muds” from chromite ore processing are highly alkaline. This waste material contains CaCrO4, tribasic calcium chromate (Ca3(CrO4)2), basic ferric chromate (Fe(OH)(CrO4)), and calcium aluminochromate (3CaO‚Al2O3‚ CaCrO4‚12H2O) (13). The sulfate analogue of this latter phase (3CaO‚Al2O3‚CaSO4‚12H2O) that is also referred to as sulfate hydrocalumite, is a common constituent of hydrated cement. These ore processing wastes have been used as fill around homes, schools, and playgrounds in Hudson County, NJ (3). Chromite ore-processing wastes have also been a problem in Baltimore, MD, England, and Japan (3). Fly ash and flue gas desulfurization waste are very alkaline materials that contain high levels of metals (14). These materials are often mixed with other types of wastes. The phases that control the concentrations of oxyanions such as CrO42- in concrete systems may also control concentrations emanating from these alkaline materials (15). We report here on the results mineralogical analyses of precipitates found in a sample of concrete contaminated with Cr(VI)-laden solutions. Knowledge of the identity and juxtaposition of these phases can be valuable in interpreting the chemical processes occurring as hexavalent chromium enters the concrete and can be useful in determining the potential for waste isolation by cementitious materials. A better understanding of the forms of chromium within the cement will also assist environmental scientists and engineers in designing better remedial actions and will help us to quantify the concentrations of Cr(VI) that can emanate from the concrete. Our observations will not only be useful in constraining models for the behavior of chromium in cement and concrete but can also provide insight into the behavior of chromium in other alkaline systems.

Experimental Section A piece of contaminated concrete was examined to identify precipitates that formed after long-term exposure to hexavalent chromium. The sample of contaminated concrete is from the United Chrome Products (UCP) site, in Corvallis, OR. UCP was a former hard-chrome plating facility operated from 1956 until 1985 (16, 17). Between 1956 and 1975 a dry well was used to dispose of floor drippings, washings, and product VOL. 34, NO. 19, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

4185

rinsates. Chromate solutions that were disposed in the dry well were also spilled onto a concrete pad behind the building. This pad was broken and eventually removed during site remediation. Geochemical characterization of the soils, sludges, and groundwater are described elsewhere (18). The sample of Cr(VI)-contaminated concrete was split open, and crystals were removed from interstices from the interior of the sample. Three subsamples of crystal clusters from the concrete were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM was a Zeiss 960 Digital microscopy equipped with a Link energy-dispersive X-ray spectrometer (EDX). One of the samples was coated with Au-Pd (∼200 Å) to reduce charging. The other two subsamples were not coated and were used only to obtain EDX spectra. The EDX spectra were obtained at a 31 mm working distance with an accelerating voltage of 20 kV. Most of the spectra were obtained for 100 s of live time with no window between the sample and the X-ray detector. A few spectra were obtained through a Be window. Samples for transmission electron microscopy (TEM) were prepared by sprinkling small amounts of the sample on copper grids. The samples were examined under a JEOL Fx TEM/STEM (scanning transmission electron microscope) with a 200-kV accelerating voltage. The chemical composition was obtained with a Noran EDX detector and a Noran 5500 analyzer. Selected area diffraction (SAD) and convergent beam electron diffraction (CBED) patterns of individual crystallites were obtained. The diffraction patterns were analyzed with the aid of Desktop Microscopist and methodologies outlined by Stanley et al. (19). Quantification with EDX was done using a standardless ZAF technique (20) using the ZAF-4 software package provided by the manufacturer of the X-ray unit (21).

Results and Discussion The results and discussion are presented in three key sections. In the first section, the general observations that were made are presented. In the second section, the morphologies, chemistry, and crystallography from our electron microcopy work are described. In the third and final section, a conceptual model for chromic acid-concrete interactions that is consistent with our observations is presented. General Observations. The concrete sample had a yellow appearance indicating the presence of Cr(VI) crystals. Closer examination of the sample with an optical microscope revealed acicular crystals varying from white to bright yellow in some of the interstices within the concrete. In other areas of the sample, a yellow color was observed, but no distinct crystallites were observed. Examination of the subsamples using SEM revealed many acicular crystals scattered on the stub. In addition, an aggregate of crystals comprised of several different morphologies was observed. Finally, a particle consisting of acicular crystals embedded in a matrix was also observed. Electron Microscopy. Acicular Crystals. A small mass of yellow crystals was removed from the concrete and examined using SEM/EDX to determine the general chemical composition of the crystals and to ascertain if more than one chromium-containing phase were present. The yellow crystals were euhedral, prismatic, and acicular. Many of the crystals removed were approximately 25-100 µm in length and 1-5 µm in diameter (Figure 1a). At 8000×, the broken tip of the largest crystal in Figure 1b clearly shows ridges parallel to the c-axis of the crystal. A naturally terminated crystal (Figure 1c) shows that the crystal is twinned and may be terminated with a tetragonal pyramid. The EDX spectrum (Figure 1d) for the crystal in Figure 1b indicates that it is comprised of Ca, Cr, S, and Al. Since a Be window was used, the oxygen peak is very weak. 4186

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 19, 2000

Twelve acicular crystals were randomly chosen for EDX analysis. In addition, six crystals from each of two other groups of crystallites were examined to determine the variability in their composition. The concentrations (atom %) of Ca, Al, O, S, and Cr were obtained using ZAF-4, a standardless quantitation method. One of the crystals from the group of 12 is not included because it had a large carbon peak thus making it chemically distinct from all of the other crystals. Sulfate is a common constituent of concrete; therefore, we assumed that S is present as SO42-. The yellow color suggests that Cr is present as CrO42-. SO42- and CrO42have identical charges, similar structures, and nearly the same thermochemical radii; therefore, it is likely that the chromate ion is substituting in the sulfate position within the crystallite. Several sulfate minerals including barite (22, 23), jarosite (24), and ettringite (25-30) are known to form solid solutions with their chromate analogues. We therefore consider the sum (S+Cr) in our analyses. The average atom percentages of O, Ca, Al, and (S+Cr) for the 23 particles (Table 1) are close to the theoretical composition of ettringite, a common constituent of concrete. Ettringite is a hydrated calcium-alumino-sulfate with an idealized formula of Ca6[Al(OH)6](SO4)3‚26H2O. The coefficient of variation in the measured Ca, Al, and (S+Cr) concentrations for the 23 crystallites are relatively large. We can normalize the concentrations to the sum of these three less volatile components, rather than the sum of all of the atoms in a unit formula. If we use this type of normalization, the relative standard deviations of Ca, Al, and (S+Cr) are much smaller (Table 1), and the data plot are very close to an idealized ettringite composition on a ternary diagram (Figure 2). The large coefficients of variation obtained when oxygen is included in the analyses can be explained by the loss of water of hydration as the sample is heated in the beam under high vacuum. Thermogravimetric analysis shows the loss of 23 water molecules from ettringite as it is heated from 40 to 180 °C at 1 atm pressure (31). The loss of three additional water molecules occurs between 200 and 280 °C. If water is lost during observation, there are two potential effects that can affect the elemental concentrations. First, the percentages of the less volatile elements (Ca, Al, S+Cr) should increase with decreases in the oxygen and hydrogen from the crystal structure. Second, the released water may condense on the LaB6 detector causing positive errors in the measured O content. Both of these effects were seen in our data. An estimate of the average water content of the crystals was made by assuming the excess negative charge is balanced by protons associated with the water. The average calculated number of water molecules, 32.4 ( 2.2, is close to the theoretical value of 32 for ettringite. Additional errors result from the approximate ZAF factors used in the quantitation program, deviations from a perfectly flat sample surface, and substitution of other elements (e.g. C, Si) in the ettringite crystal structure. To provide further evidence that the acicular crystals are from the ettringite group, electron diffraction patterns were obtained in the TEM. Seven crystallites were chosen based on their morphology and general chemistry. For each particle, diffraction patterns were obtained at one to as many as five different angles. For the seven crystallites, a total of 26 different d spacings were obtained (Table 2). Of these 26 d spacings, 24 could be matched with ettringite (32, 33) or Ca6[Al(OH)6]2(CrO4)3‚26H2O ((31) and PDF 41-0218) within the expected error confirming that the acicular crystals belong to the ettringite group (hexagonal, space group P31c(159)). There are two d spacings (7.7 and 6.0 Å) that do no match the reported d spacings for these phases. Within the uncertainty, the 6.0 Å peak is the same as the 6.12 Å peak for {hkl} ) {3 0 1} reported for a natural sample of ettringite

FIGURE 1. SEM images of acicular crystals removed from a piece of Cr(VI)-contaminated concrete: (a) general view of dispersed crystals showing general morphology and distribution of sizes (∼875×), (b) closeup of broken crystal (∼8000×), (c) naturally terminated crystal, and (d) EDX spectrum of the particle in b.

TABLE 1. Mean Composition of 23 Acicular Particles atom % element:

O

average for particles ideal ettringite ideal SO4 hydrocalumite

82.59 ( 2.52 81.97 75.86

average for particles ideal ettringite ideal SO4 hydrocalumite

Al

Ca

Concentration 3.12 ( 0.33 9.23 ( 1.52 3.28 9.84 6.90 13.79 Normalized to Al, Ca, Cr+S 18.1 ( 1.8 52.9 ( 1.8 18.2 54.5 28.6 57.1

(PDF 31-0251) that may contain CO32- and SiO2. The 7.7 Å peak may also be the result of substitution within the ettringite structure, or alternatively, it could be a d spacing for secondary products formed by dehydration of the ettringite during observation. The ratio of Cr:S varied from 0.7 to 2.6 (mole fraction of CrO42- in the SO42- position ) 0.41-0.72) for the 23 crystals for which spectra were obtained. No coatings were observed on the crystallites in either the SEM or the TEM. We have synthesized a continuous solid solution between ettringite and its chromate analogue in our laboratory (25), and complete solutions have been reported by others (26-30). It is highly likely, therefore, that these phases are part of a solid solution. The concentrations of elements within the samples normalized to the nonvolatile elements (Ca+Al+Cr+S) match

S

Cr

S+Cr

2.35 ( 0.49 4.92 3.45

2.71 ( 0.67

5.05 ( 0.83 4.92 3.45

15.6 ( 2.7 27.3 14.3

13.4 ( 2.0

29.0 ( 1.3 27.3 14.3

very well with Ca6[Al(OH)6](SXCr1-XO4)3‚26H2O. An estimate of the amount of water in the crystallites is close to the value for ettringite. Further, the morphology observed for the crystallites is similar to that reported by others (4, 26, 34). Last, the d spacings obtained in the TEM are in agreement with those for ettringite and its chromate analogue. Therefore, we are confidant that these acicular crystals belong to a solid solution between ettringite and its chromate analogue. Crystal Aggregates. Rather than individual crystals, the particles occur as aggregates within the concrete (Figure 3a). These aggregates contain more than one type of crystal morphology. For example, the aggregate in Figure 3 contains acicular crystals (D and F), plates (A, B, and C), and thin coatings (F). The thin plates on the left side (A) and right side (B) of Figure 3a are comprised of Ca, Al, Cr, and O (Figure 4A,B). VOL. 34, NO. 19, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

4187

FIGURE 2. Ca-Al-(S+Cr) ternary diagram. Shaded squares labeled E and M denote the ideal compositions for ettringite and monosulfate. The open symbols denote data from 23 randomly chosen particles: 11 from the first group (circles), 6 from the second group (squares), and 6 from the third group (triangles). The solid circles are the compositions for locations A and B in Figure 4, while the solid triangles are spectra for the acicular crystals in the group marked D.

TABLE 2. Comparison of d Spacings Reported for Ettringite and the Chromate Analog of Ettringite with the d Spacings of the Acicular Crystals in the Concrete d-spacings (Å)

particle no. 7 1, 4 4 7 1 3, 4, 5 1, 3, 4 7 3, 7 1, 2, 4, 5, 6 6 4 3 2, 7 5 3 7 6 2, 7 6, 7 7 2 1 1 2 7

Ca6[Al(OH)6]2ettringite (CrO4)3•26H2O present estimated McMurdie Perkins and study uncertainty et al. (32) Palmer (31) 7.70 6.00 5.78 4.66 3.76 3.55 3.47 3.27 3.22 3.04 2.87 2.83 2.78 2.74 2.69 2.64 2.57 2.54 2.43 2.39 2.33 2.24 2.20 2.12 2.01 1.97

0.248 0.152 0.141 0.092 0.060 0.054 0.051 0.046 0.044 0.039 0.035 0.034 0.033 0.032 0.032 0.030 0.028 0.028 0.025 0.024 0.023 0.022 0.021 0.019 0.017 0.017

5.758 4.703 3.604 3.477 3.268 3.241 3.018 2.807 2.774 2.697 2.615 2.563 2.523 2.420 2.409 2.350 2.230 2.203 2.122 2.009 1.9732

5.73 4.73 3.74 3.53 3.29 3.04 2.87 2.80 2.80 2.74 2.68 2.65 2.58 2.43 2.36 2.23 2.23 2.11 2.00 1.98

h,k,l

1,0,3 2,0,0 2,1,0 2,0,4 2,1,2 2,1,3 3,0,0 1,1,6 2,2,0 2,2,0 1,0,4 3,1,0 3,1,0 3,1,2 2,1,6 3,1,3 1,1,8 3,1,4 2,0,8 3,2,0 2,2,6 4,1,0 4,0,6 4,1,4

ZAF analyses suggest that their compositions (atom % Ca ) 50.5, Al ) 32.7, Cr ) 16.7; atom % Ca ) 55.5, Al ) 25.9, Cr ) 18.6; for A and B, respectively) are close to a sulfate hydrocalumite (3CaO‚Al2O3‚CaSO4‚nH2O) and its chromate analogue (atom % Ca ) 57.1, Al ) 28.6, S+Cr ) 14.3) (Figure 2). Further, the hexagonal form of the plate labeled A is consistent with the morphology reported for hydrocalumites (15, 35). S was not detected in the EDX spectrum and only 4188

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 19, 2000

a small amount of Si observed, thus these crystals are nearly pure chromate hydrocalumite (3CaO‚Al2O3‚CaCrO4‚nH2O). However, the S peak may be obscured by the Au peak, and the ZAF program may underestimate the amount of S. Obstruction of the S peak may partially explain the small deviation from the idealized CrO42-/SO42--hydrocalumite composition in Figure 2. The thin crystals further on the left of Figure 3a,b (point C) are different in that they contain a substantial amount of calcium, little Al, and some Si (O ) 67.5, Ca ) 25.1, Si ) 3.6, Cr ) 2.8, Al ) 0.9 atom %) (Figure 4C). The high ratios of Ca to Si, Cr, and Al suggest that the spectrum is most likely a Ca-O phase with some impurities or intermixed with other phases. In cement, the most likely Ca-O phase is portlandite (Ca(OH)2). Assuming that Si, Cr, and Al are present as oxides, the measured O:Ca ratio of 2.18 is close to the expected value of 2.00 for portlandite. Portlandite was also observed in the TEM where electron diffraction patterns on a Ca-rich phase yielded d spacings of 4.91, 3.13, 1.92, 1.18, 1.15 Å which correspond to the reported peaks of 4.90, 3.112, 1.927, 1.1762, and 1.1432 Å, respectively (PDF #040733). An EDX spectrum was obtained for even further to the left of point C. This spectrum had a high O content (82.9 atom %). Ca and Si were present at 12.4 and 3.6 atom %. Al and Cr were present in minor amounts (0.6 and 0.5 atom %). A carbon peak was present, but we were not able to ascertain if the carbon was in the crystal or if the peak was from the underlying carbon base. The Ca:(Si+Al) ratio is 3.0, and the Ca:O ratio is 6.7. One of the few phases that has a Ca:Si ratio of 3 is alite (3CaO‚SiO2), a common constituent of cement clinker. However, there would have to be substantial hydration of this phase to achieve the observed O:Ca ratio. The morphology of the acicular crystals in the aggregate (Figure 3a, particle D) is similar to the ettringite. These crystals are comprised of Ca-Al-Cr-O (Figure 4D) but unlike the ettringite crystals previously discussed, they contain little or no S. However, ZAF-4 analyses suggest the chemical composition (atom % Ca ) 56.45 ( 2.41, Al ) 24.57 ( 2.93, Cr ) 18.98 ( 0.68; average of five spectra) is consistent with that of a chromate hydrocalumite phase (Figure 2). Thus, these crystals are nearly a pure chromate analogue of sulfate hydrocalumite and an apparent pseudomorph after ettringite. There appears to be a thin coating in the area near F in Figure 3a. Closer examination of this area (Figure 3c) shows that it is at least partially comprised of small, thin, acicular crystals (Figure 3a,c E and F). EDX spectra (Figure 4E,F) indicate that the crystallites are comprised of O Ca, Al, Si, and Cr. The two small acicular crystallites at E (Figure 3a,c) have a composition (atom % Ca ) 56.4, Si ) 14.3, Al ) 15.4, Cr ) 13.8). Based on the crystal morphology, the high oxygen content, and the yellow color of the sample, these crystallites are likely part of a solid solution between Cr(VI)-ettringite and a Si-ettringite (Ca6Al2(OH)12(SiO3)3‚25H2O). Si substitution in ettringites is known to occur (7, 36-38), and the chemical composition matches a 51% Cr(VI)-ettringite and 49% Si-ettringite which has a calculated composition of Ca ) 54.5, Si ) 13.4, Al ) 18.2, Cr ) 13.8 atom %. Another spectrum (F) obtained from the less distinct coating near the top of the image is different than E. There is more Si and O and less Ca, Al, and Cr (O ) 85.9, Ca ) 6.96, Si ) 4.82, Al ) 1.54, and Cr ) 0.76 atom %). The material appears to be an aggregate of the thin acicular crystals similar to those observed at E and a thin coating rather than a homogeneous solid. Assuming that chromium is present as Cr(III) and normalizing this composition to (Al,Cr)2O3, the coating has a composition of 6.0CaO‚4.2SiO2‚(Al,Cr)2O3‚nH2O. Written in this manner, the spectrum indicates a Si-ettringite cemented with a SiO2. Acicular Crystals in Matrix. In addition to the crystal aggregate, we observed acicular crystals embedded in a matrix

FIGURE 3. Crystalline aggregate (a) in concrete with greater magnification in the areas labeled A (b) and E (c). The upper-case letters denote areas where EDX spectra (Figure 4) were obtained. (Figure 5a). The matrix (Figure 5b) is comprised of Ca, Si, O with minor amounts of Al. The small amount of C is likely from the underlying carbon stub. The material is a calcium silicate hydrate gel that is often abbreviated as CSH in the concrete literature. The formula for CSH can be represented as xCaO‚SiO2‚xH2O, where x is the Ca:Si ratio in the gel (39). For the EDX spectrum in Figure 5b, the observed Ca:Si ratio of 1.57 is close to the average value of 1.6 for mature cement pastes (39). The observed Ca:Si ratio is within the expected range of 2.2-1.4 for pH between pH 12.5 and 12.0 (40). This relatively high ratio suggests that the pH in the concrete was still fairly high when the silica gel was formed. The measured oxygen content of 79.5 atom % is reasonably close to the expected value of 76.3 for x equal to 1.57. The EDX spectrum for the acicular crystals in Figure 5A yields O ) 88.20, Ca ) 7.56, Al ) 1.93, S ) 1.61, and Cr ) 0.71 atom %. Carbon is not included because it was believed that the signal is derived from the carbon base on which the thin crystal is supported. This spectrum is likely a composite of the spectra from the acicular crystal and the matrix. The (S+Cr):Al ratio of 1.2 is closer to that expected for ettringite (1.5) than for the sulfate hydrocalumite (0.5). The acicular crystals must have formed before the CHS matrix in which they are embedded. These crystals contain Cr; therefore, the gel was not formed at the time the concrete was mixed but was formed after the concrete was contaminated. Conceptual Model. The mineralogical relationships observed in the concrete can be combined with knowledge about sulfate attack on cement (40) to develop a conceptual model of Cr(VI)-concrete interactions. Anhydrous portland cement is typically comprised of alite (3CaO‚SiO2), belite (2CaO‚SiO2), tricalcium aluminate (3CaO‚Al2O3), calcium aluminum ferrite (4CaO‚Al2O3‚Fe2O3), and gypsum (CaSO4‚

2H2O). When this cement clinker is mixed with water, several hydration products are formed including calcium silicate hydrate gel (CSH), portlandite (Ca(OH)2), hydroxy-hydrocalumite (4CaO‚Al2O3‚13H2O or C4AH13 in the cement notation), and ettringite. As [SO42-] decreases in the cement porewater, ettringite converts to a SO4-hydrocalumite (3CaO‚ Al2O3‚CaSO4‚nH2O) that is often referred to as calcium monoaluminosulfate or simply monosulfate. Hydrogarnet (3CaO‚Al2O3‚6H2O) is more stable than C4AH13, consequently, it is found in mature cements. However, oxyanion substitution in C4AH13 can make it more stable (15). The concrete pad from which the sample was taken was located behind the plating shop. It was subjected to periodic infiltration by precipitation as well as intermittent exposure to Cr(VI)-laden solutions. Precipitation that infiltrates the concrete equilibrates with the portlandite, C4AH13, and CSH. This initial pore water should be a Ca-OH water with minor amounts of SO42-, Al, and Si (40). As chromic acid, represented here as H2CrO4, is spilled, it enters the concrete and dissolves Ca(OH)2 and C4AH13. [Ca2+] and [Al(OH)4-] increase until saturation with respect to chromate ettringite is attained and it precipitates:

2Ca(OH)2(s) + 3H2CrO4(aq) + 4CaO‚Al2O3‚13H2O(s) + 14H2O / 3CaO‚Al2O3‚3CaCrO4‚32H2O(s) (1) The solution is then at an invariant point, and the pH and the solution composition remain constant until Ca(OH)2 or C4AH13 are consumed. The amount of chromate-ettringite formed will be less than the amount of sulfate-ettringite precipitated for the same amounts of H2CrO4 or H2SO4 added because the solubility product (KSP) for chromate-ettringite VOL. 34, NO. 19, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

4189

FIGURE 4. EDX spectra for the points labeled in Figure 3. is (log KSP ) -44.8) more than 3 orders of magnitude lower than the KSP of chromate-ettringite (log KSP ) -41.46) (31, 33). If sulfate ettringite is present in the concrete, the chromate-ettringite can also form by an exchange reaction:

3CaO‚Al2O3‚3CaSO4‚32H2O(s) + XCrO42- / 3CaO‚Al2O3‚3Ca(S1-XCrX)O4‚32H2O(s) + XSO42- (2)

Without the addition of more H2CrO4, [CrO42-] decreases as it precipitates in the chromate ettringite or as the pore water mixes with infiltrating precipitation. When [CrO42-] is sufficiently low, chromate-hydrocalumite (3CaO‚Al2O3‚ CaCrO4‚12H2O) becomes the stable phase. The hexagonal forms such as those in Figure 3a,b (particle A and B) developed, and the previously formed chromate ettringite transforms to chromate-hydrocalumite:

3CaO‚Al2O3‚3CaCrO4‚32H2O(s) / For sulfuric acid attack on concrete, continued addition of H2SO4 results in the precipitation of gypsum for pH < 11.6, the precipitation of gibbsite and gypsum as sulfate ettringite dissolves for pH < 10.6, and ultimately the formation of amorphous silica as CHS dissolves (40). Under H2CrO4 attack, the formation of CaCrO4(s) requires the addition of more acid than that required to form CaSO4‚2H2O because the solubility product of CaCrO4 is about 400 times greater than that of gypsum. 4190

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 19, 2000

3CaO‚Al2O3‚CaCrO4‚12H2O(s) + 2Ca2+ + 2CrO42- + 20H2O (3) The transformation of sulfate-ettringite to sulfate-hydrocalumite is well discussed in the literature (15, 41-46), and the transformation of chromate-ettringite to chromatehydrocalumite is expected based on the analogy with the sulfate system. Further, powder X-ray diffraction data of

This conceptual model for Cr(VI)-concrete reactions is consistent with the observations of the precipitates identified in the Cr(VI)-contaminated concrete and models for sulfate attack of concrete. It is likely that the phases we have identified are present at other sites where concrete has been contaminated by hexavalent chromium. Quantitative models require more information about the thermodynamic properties of these phases as well as solid-solution/aqueous solution reactions between the various components.

Acknowledgments The author wishes to express his appreciation of Dr. J. McCarthy for his suggestions and assistance with the electron microscopy, Dr. Georg Grathoff and R.B. Perkins for their helpful comments, and Dr. Eric Reardon for his suggestions for greatly improving the manuscript. The initial stages of this work were funded from a grant from RSKERR Environmental Research Laboratory, Ada, OK. The bulk of the analyses was supported under a STAR grant from the National Center for Environmental Research and Quality Assurance, Office of Research and Development, U.S. Environmental Protection Agency. The work has not been reviewed by U.S. EPA and does not necessarily reflect the views of the Agency.

Literature Cited

FIGURE 5. Acicular crystals likely from the ettringite group (EDX spectrum B) embedded in a matrix of calcium silicate hydrate gel (EDX spectrum A). residuals in our chromate ettringite solubility study (31) indicate the presence of a chromate hydrocalumite (25). This type of transformation may have resulted in pseudomorphs of the original ettringite crystals as seen in Figure 3a (particle D). Both the CrO42--ettringite and the CrO42--hydrocalumite are covered by the Si-ettringite and SiO2. There appears to be a transition from a pure Si-ettringite (mixed with SiO2), through a solid solution of Si- and CrO4-ettringite, to what was once a nearly pure CrO42--ettringite. A likely source of the silica is the CSH. As the pH decreases with the continued addition of H2CrO4, there is an increase in solubility of CSH (39, 40) and hence an increase in the aqueous silica concentration. The combined effect of the acid attack on CSH and on the calcium aluminates in the cement is the formation of the Si-ettringite. When the addition of H2CrO4 ceases and there is no addition of water by infiltration of precipitation, the pore water evaporates. A consequence of this evaporation is that the ions in solution are concentrated and pH increases. With increasing pH, the solubility product of CSH decreases and it should precipitate. Such a mechanism could explain the formation of the CSH around the chromium enriched ettringite crystals in Figure 3.

(1) Nriagu, J. O. Production and uses of chromium; Nriagu, J. O., Nieboer, E., Ed.; John Wiley & Sons: New York, 1988; Vol. 20, pp 81-104. (2) Palmer, C. D.; Wittbrodt, P. R. Environ. Health Perspectives 1991, 92, 25-40. (3) Burke, T.; Fagliano, J.; Goldoft, M.; Hazen, E.; Iglewicz, R.; McKee, T. Environ. Health Perspectives 1991, 92, 131-137. (4) Gougar, M. L. D.; Scheetz, B. E.; Roy, D. M. Waste Management 1996, 16, 295-303. (5) Lin, T.; Lin, C.; Wei, W. J.; Lo, S. Environ. Sci. Technol. 1993, 27, 1312-1318. (6) Kindness, A.; Macias, A.; Glasser, F. P. Waste Management 1994, 14, 3-11. (7) Omotoso, O. E.; Ivey, D. G.; Mikula, R. J. Mater. Sci. 1998, 33, 507-513. (8) Omotoso, O. E.; Ivey, D. G.; Mikula, R. J. Mater. Sci. 1998, 33, 515-522. (9) Zamorani, E.; Sheikh, A. A.; Serrini, G. Nuclear Chem. Waste Management 1988, 8, 239-245. (10) Teramoto, H. Am. Ceramics Soc. Bull. 1972, 51, 625-629. (11) Teramato, H.; Koie, S. J. Am. Ceramic Soc. 1976, 59, 522-525. (12) Chan, C.-F.; Ko, Y.-C. J. Am. Ceramic Soc. 1992, 75, 2857-2861. (13) Gancy, A. B.; Wamser, C. A. Suppression of water pollution caused by solid wastes containing chromium compounds; U.S. Patent No. 3,981,965; Allied Chemical Corporation: Morris Township, NJ, 1976. (14) Myneni, S.; Traina, S. J.; Logan, T. J. Retention of arsenate and chromate by ettringite in alkaline waste environments; American Chemical Society: San Diego, CA, 1994; pp 525-527. (15) Zhang, M., Ph.D. Thesis. University of Waterloo: Waterloo, Ontario, Canada, 2000, pp 171 p. (16) Ecology and Environment, I. Final Remedial Investigation Report; United Chrome Products: Corvallis, OR, 1985; Vol. 1 and 2. (17) McKinley, W. S.; Pratt, R. C.; McPhillips, L. C. Civil Eng. 1992, 62, 69-71. (18) Palmer, C. D.; Wittbrodt, P. R. Geochemical characterization of the United Chrome Products Site, Final Report. In Deep Aquifer Drilling Technical Report; CH2M-Hill: 1990. (19) Stanley, J. T.; Palmer, C. D.; Downham, D. A.; Johansen, A. M.; McCarthy, J. Identification of mixed crystalline solids; Extraction and Processing Division Congress of the Three M Society: Denver, CO, 1993; pp 433-445. (20) Goldstein, J. I.; Newbury, D. E.; Pechliln; Joy, D. C.; Romig, A. D.; Lyman, C. E.; Fiori, C.; Lifshin, E. Scanning electron microscopy and X-ray microanalysis, 2nd ed.; Plenum Press: New York, 1992. (21) Link Analytical, L. ZAF-4/FLS Operators Manual; Oxford Instruments (UK) Ltd., Analytical Systems Division, Microanalysis Group: 1991. (22) Hauff, P. L.; Foord, E. E.; Rosenblum, S. Am. Mineralogist 1983, 68, 1223-1225. VOL. 34, NO. 19, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

4191

(23) Rai, D.; Zachara, J. M.; Eary, L. E.; Aisworth, L. E.; Amonette, C. C.; Cowan, J. E.; Szelmeczka, C. E.; Resch, R. W.; Schmidt, C. T.; Smith, S. C.; Girvin, D. C. Chromium reactions in geologic materials; Electric Power Research Institute: 1988. (24) Baron, D.; Palmer, C. D. Solid solutions between jarosite (KFe3(SO4)2(OH)6) and KFe3(CrO4)2(OH)6: Dissolution behavior and implications for the mobility of Cr(VI); Proceedings of the 1998 Fall Meeting of the American Geophysical Union; San Francisco, CA, 1998. (25) Perkins, R. Ph.D. Thesis, Portland State University: Portland, OR, 2000; p 199. (26) Myneni, S. C. B. Ph.D. Thesis, The Ohio State University, 1995; p 249. (27) Myneni, S. C. B.; Traina, S. J.; Logan, T. L. Chem. Geology 1998, 148, 1-19. (28) Kumarathasan, P.; McCarthy, G. J.; Hassett, D. J.; PflughoeftHassett, D. F. Oxyanion substituted ettringites: synthesis and characterization; and their potential role in immobilization of As, B, Cr, Se and V; Materials Research Society: 1990; Vol. 178, pp 83-104. (29) Poellman, H.; Auer, S.; Kuzel, H. J.; Wenda, R. Cement Concrete Res. 1993, 23, 422-430. (30) Poellmann, H.; Kuzel, H. J.; Wena, R. Cement Concrete Res. 1990, 20, 941-947. (31) Perkins, R. B.; Palmer, C. D. Appl. Geochem. 2000, 15, 12031218. (32) McMurdie, H. F.; Morris, M. C.; Evans, E. H.; Paretzkin, B.; Wongng, W.; Chang, Y. Powder Diffraction 1986, 4, 337. (33) Perkins, R. B.; Palmer, C. D. Geochim. Cosmochim. Acta 1999, 63, 1969-1980.

4192

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 19, 2000

(34) Hampson, C. J.; Bailey, J. E. J. Mater. Sci. 1982, 17, 3341-3346. (35) Lea, F. M. The Chemistry of Cement and Concrete, 3rd ed.; Edward Arnold Ltd: Glasgow, 1970. (36) Flint, E. P.; Wells, L. S. J. Res. Natl. Bureau Standards 1944, 33, 471-477. (37) McCarthy, G. J.; Solem-Tishmack, J. K. Hydration mineralogy of cementitious coal combustion byproducts; Grutzeck, M. W., Sarkar, S. L., Eds.; American Society of Civil Engineers: 1994; pp 103-121. (38) Regourd, M.; Hornain, H.; Mortureux, B. Cement Concrete Res. 1976, 6, 733-740. (39) Reardon, E. J. Waste Management 1992, 12, 221-239. (40) Reardon, E. J. Cement Conrete Res. 1990, 20, 175-192. (41) Taylor, H. F. W. Cement Chemistry; Academic Press: 1990. (42) Gabrisova´, A.; Havlica, J.; Sahu, S. Cement Concrete Res. 1991, 21, 1023-1027. (43) Ho¨glund, L. O. Cement Concrete Res. 1992, 22, 217-228. (44) Odler, I.; Abdul-Maula, S. Cement Concrete Res. 1984, 14, 133141. (45) Shi, C. Cement Concrete Res. 1996, 26, 1351-1359. (46) Singh, N. B.; Singh, A. K.; Singh, S. P. J. Am. Ceramic Soc. 1990, 10, 3063-3068.

Received for review September 21, 1999. Revised manuscript received June 19, 2000. Accepted June 26, 2000. ES991089F