Grantee Research Project Results
Final Report: Attenuation of Chromium in Alkaline Environments Chromium Substitution in Ettringites and C4AH12- Monosulfates
EPA Grant Number: R823388Title: Attenuation of Chromium in Alkaline Environments Chromium Substitution in Ettringites and C4AH12- Monosulfates
Investigators: Palmer, Carl D.
Institution: Portland State University
EPA Project Officer: Aja, Hayley
Project Period: October 1, 1995 through September 30, 1998
Project Amount: $176,630
RFA: Exploratory Research - Engineering (1995) RFA Text | Recipients Lists
Research Category: Safer Chemicals , Land and Waste Management
Objective:
Chromium interaction with concrete is an important area of research from several different perspectives. Chromium is a widely used toxic industrial metal and a common contaminant in soil and water. Spills of chromium solutions at industrial sites often occur onto concrete and cement. Interactions between the chromium-laden solutions and the cement may require the removal, treatment, and disposal of concrete as a hazardous material. Chromium-contaminated groundwater entering basements has been observed interacting with the concrete basement walls. There also is an interest in the use of cement mixtures for the stabilization of wastes, including those that contain chromium. Also, there is interest in using chromium additives in cement clinker to produce high-strength cements and refractory materials. The report provides basic information about the occurrence of phases in Cr(VI)-contaminated concrete and thermodynamic properties of some of these phases. The information is applicable not only to the behavior of chromium in cement systems, but also to the behavior of chromium in other highly alkaline systems such as naturally alkaline soils, chromium ore-processing wastes, alkaline waste materials, and fly ash. This information will be useful to environmental scientists and materials scientists who are studying the fate of chromium in industrial processes and in the environment.
Summary/Accomplishments (Outputs/Outcomes):
The results are described in several chapters focusing on particular aspects of the work. The key points of the report are summarized below.
1. The thermodynamics of the Cr(VI)-H2O systems were reviewed and summarized.
a. Using previously published studies, we determined that for the reaction
the log K at 25?C is 6.504 ? 0.014. Fitting reported Log K versus temperature data over the temperature range of 0? to 100?C resulted in a log K of 6.496 ? 0.010 at 25?C, and and of 5.13 ? 31 kJ mol-1, 141.6 ? 1.0 J mol-1 K-1, and 163.2 ? 7.3 J mol-1 K-1, respectively. Partial molal quantities of formation were calculated assuming the formation constants for CrO42- reported by Shock, et al. (1997), are correct.
b. The formation constant for dichromate (Cr2O72-) via the reaction
at 25?C is log K = 14.62 ? 0.08. Fitting reported Log K versus temperature data over the temperature range of 15? to 100?C resulted in a log K of 14.62 ? 0.08 at 25?C, and and of 20.7 ? 3.7 kJ mol-1, 210.3 + 12.1 J mol-1 K-1, and 661 + 125 J mol-1 K-1, respectively. Partial molal quantities of formation were calculated assuming the formation constants for CrO42- reported by Shock, et al. (1997), are correct.
2. A sample of Cr(VI)-contaminated concrete was examined by optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Both SEM and TEM were equipped with energy dispersive x-ray spectroscopy (EDX) units.
a. Several acicular crystals comprised of Ca, Al, S, Cr, and O were observed. The composition of 23 of these crystals had an average composition, normalized to Ca, Al, and S+Cr, very close to an idealized ettringite (Ca6[Al(OH)6]2(SO4)3?26H2O) in which there is substantial substitution of CrO42- for SO42-.
b. Variation in the composition with the O+H content can be attributed to loss of water of hydration from the crystal structure due to heating in the electron beam under high vacuum. The measured variability in Ca, Al, and Cr+S concentrations with O+H concentrations are nearly identical to the theoretical trends for ettringite.
c. Ettringite also was identified by selected area diffraction and convergent beam electron diffraction. Seven different crystals were chosen based on their morphology and general chemistry. Of the 26 d-spacings obtained, 25 could be associated with ettringite.
d. An aggregate of crystals comprised of hexagonal plates, acicular crystals, and a coating partially composed of small acicular crystals was examined. The hexagonal plates have a composition similar to the Cr(VI) analog of monosulfate. Some of these crystals contain C and Si and may be thaumasite (Ca6Si2(OH)12(SO4)2(CO3)2?24H2O). Surprisingly, the acicular crystals also have a composition similar to the Cr(V1) analog of monosulfate. The small acicular crystals in the coating also may be an analog of monosulfate in which Si is substituting for Al. Some of the coating is comprised of Ca, Si, Al, and Cr, with a Ca:Si:Al+Cr ratio of 3:2:1.
e. Acicular crystals containing chromium believed to belong to the ettringite group were embedded in what appears to be a calcium-silica gel. Thus, this gel was formed after contamination of the concrete occurred.
f. A conceptual model for chromate-concrete interactions that is consistent with our observations was developed.
3. Ettringite (Ca6[Al(OH)6]2(SO4)3?26H2O) is an important hydration product of Portland and super-sulfated cements, has been used as a coating for paper, and occurs naturally in alkaline environments. There has been some uncertainty regarding the solubility of ettringite at 25?C and very little information about its solubility at other temperatures. To clarify these issues with regard to this important phase, we synthesized ettringite and conducted a series of solubility experiments at pH between 10.5 and 13.0 and temperatures from 5? to 75?C. Equilibrium was established in 4 to 6 days, and samples were collected between 10 and 36 days. We obtained the following results:
a. The log KSP for the reaction
Ca6[Al(OH)6]2(SO4)3?26H2O 6Ca2+ + 2Al(OH)-4 + 3SO2-4 + 4OH- + 26H2O
at 25?C calculated for dissolution experiments (-45.0 + 0.2) is not significantly different from the log KSP calculated for precipitation experiments (-44.8 ? 0.3) at the 95 percent confidence level. There is no apparent trend in log KSP with pH, and the mean log KSP,298 is -44.9 ? 0.3.
b. The solubility product decreases linearly with the inverse of temperature, indicating a constant enthalpy of reaction from 5? to 75?C. The enthalpy and entropy of reaction, Hr? and Sr?, were determined from the linear regression to be 204.6 ? 0.6 kJ mol-1and 170 + 38 J mol-1 K-1. The Gr? at 25?C obtained from the log KSP is 256 ? 1.8 kJ mol-1.
c. Using our values for log KSP, Hr?, and Sr?, and published partial molal quantities for the constituent ions, we calculated the free energy for formation, Gf,298?, the enthalpy of formation, H?f,ettringite, and the entropy for formation, S?ettringite, to be -15211 ? 19 kJ mol-1, -17549 ? 7 kJ mol-1, and 1867 ? 59 J mol-1 K-1, respectively. Assuming C?P,r is zero, the heat capacity of ettringite, C?P,ettringite, is 592 + 143 J mol-1 K-1. The H?f,ettringite and S?ettringite values are within two standard deviations of the values of -17539 kJ mol-1and 1861.6 J mol-1 K-1 listed by Wagman et a1. (1982) and Viellard and Rassineux (1992).
4. The substitution of Cr(VI) into ettringite has been proposed as an important reaction in Cr(VI)-cement iterations. Several researchers have synthesized Cr(VI)-containing ettringite, and we have observed such phases in Cr(VI)-contaminated concrete. However, there is very little information about the thermodynamic properties of this phase. To fill this gap in our knowledge, we synthesized Ca6[Al(OH)6]2(CrO4)3?26H2O, the Cr(VI) analog of ettringite, and conducted a series of solubility experiments at pH between 10.5 and 13.0 and temperatures from 5? to 75?C. Equilibrium was established in approximately 2 days, and samples were collected between 8 and 30 days. We obtained the following results:
a. The log KSP varied with pH, unless a CaCrO4 ion-pair was included in the speciation model. The log value of the formation constant for this complex, obtained by minimizing the variance in the ion activity products, is 2.77.
b. The log KSP for the reaction
Ca6[Al(OH)6]2(CrO4)3?26H2O 6Ca2+ + 2Al(OH)-4 + 3CrO2-4 + 4OH- + 26H2O
at 25?C calculated for dissolution experiments (-41.6 ? 0.2) is significantly different from the log KSP calculated for precipitation experiments (-41.3 ? 0.3) at the 95 percent confidence level, but not at the 97.5 percent confidence level. The differences between the mean values for the precipitation and dissolution experiments are very small and less than that expected for a 5-percent analytical error; therefore, we did group these data. There is no apparent trend in log KSP with pH, and the mean log KSP,298 is -41.4 ? 0.3.
c. The log of the solubility product increases with increasing temperature, indicating a positive enthalpy of reaction from 5? to 75?C. The log solubility product (log KSP) of Ca6[Al(OH)6]2(CrO4)3?26H2O at 25?C is -41.4 ? 0.3. The temperature dependence of the log KSP is
Log KSP = 498.88 27497 / T 181.09 log(T)
The values of G?r and H?r for the dissolution reaction are 236.6 ? 3.9 and 77.5 ? 2.4 kJ mol-1. The values of C?P,r and S?r are -1506 ? 140 and -534 ? 83 J mol-1K-l. The G?r at 25?C obtained from the log KSP is 236.6 + 3.9 kJ mol-1.
d. Using these values and published partial molal quantities for constituent ions, we calculated values of G?f,298 = -15131 ? 19 kJ mol-1, H?f = -17330 ? 8.6 kJ mol-1, S? = 2.19 ? 0.10 kJ mol-1 K-1, and C?P = 2.12 ? 0.53 kJ mol-1 K-1.
5. A secondary precipitate was consistently observed in a series of dissolution experiments conducted on synthesized Ca6[Al(OH)6]2(CrO4)3?26H2O. The secondary phase was identified as 3CaAl2O3?CaCrO4?15H2O based on the similarities of the x-ray diffraction (XRD) pattern with published data from 3CaAl2O3?CaSO4?15H2O (a heavily hydrated monosulfate). Steady-state ion concentrations indicated that the experimental aqueous solutions were in apparent equilibrium with both the Cr(V1) analog of ettringite and a monochromate phase. We interpreted our experiments in light of this discovery and were able to make the following conclusions:
a. Over the pH range 10.5 to 12.5, the calculated ion activity products for the dissolution reaction
3CaAl2O3?CaCrO4?15H2O 4Ca2+ + 2Al(OH)-4 + CrO2-4 + 4OH- + 15H2O
are not significantly different from the values for the dissolution reaction. The log of the solubility product (log KSP) for the dissolution of 3CaAl2O3?CaCrO4?15H2O at 25?C is -30.33 + 0.28.
b. The temperature dependence of the log KSP obtained from six additional temperatures from 5? to 75?C is
Gr? and Hr? for the dissolution reaction are 173.1 ? 3.7 and 39.1 ? 3.2 kJ mol-1, and Sr? is -450 ? 10 J mol-1 K-1.
c. Using these values and published partial molal quantities for constituent ions, we calculate G?f,298 = -9905 ? 15.7 kJ mol-1,H?f = -11303 ? 8.3 kJ mol-1, and S? =1439 ? 89 J mol-1 K-1 for 3CaAl2O3?CaCrO4?15H2O.
6. Cr(III) substitution for Al in the ettringite structure has been observed. Bentorite (Ca6[Cr(OH)6]2(SO4)3?26H2O) was synthesized and characterized. A series of dissolution and precipitation experiments was conducted. To the best of our knowledge, the results of these experiments are the first reported thermodynamic properties for this phase.
a. The synthesized material was characterized by powder x-ray diffraction, digests, SEM/EDX, and thermogravimetry. Results indicate that the synthesized material is bentorite. One of the synthesized batches may contain amorphous Cr(OH)3, and trace amounts of Si were detected with the EDX.
b. The time scales to achieve equilibrium were much longer than observed for ettringite and its Cr(VI) analog. Consequently, many of the experimental batches had not achieved equilibrium before they were sampled.
c. The log KSP,298 was estimated by fitting an empirical rate equation to the log IAP versus time data and extrapolating to infinite time. This procedure resulted in a log KSP,298 of -50.22 ? 0.40.
d. Experiments conducted at 5? and 45?C had been conducted longer than the minimum time to achieve equilibrium. The log KSP obtained from these two experiments combined with the results from the time series experiments were used to estimate the enthalpy and entropy of reaction. The Gr?, Hr?, and Sr? obtained are 315.2 ? 2.3 kJ mol-1, 366.0 ? 16.4 kJ mol-1, and 260.9 ? 52.4 J mol-1K-l, respectively.
e. Gf?, Hf?, and S? could then be calculated using the reaction data and published values of formation parameters. The obtained values are -14638 ? 2 kJ mol-1, -17089 ? 17 kJ mol-1, and 1704 ? 53 J mol-1 K-1, respectively. Assuming the heat capacity of the reaction is zero, Cp,f? = 541 ? 65 J mol-1 K-1. These results are similar to results obtained for ettringite (Perkins and Palmer, 1999b) and its chromate analog (Perkins and Palmer, 1999a).
Supplemental Keywords:
chromium, ettringite, concrete, alkaline wastes, environmental fate., RFA, Scientific Discipline, Toxics, Waste, Ecosystem Protection/Environmental Exposure & Risk, Environmental Chemistry, Fate & Transport, Hazardous Waste, 33/50, Hazardous, Environmental Engineering, fate and transport, chromium & chromium compounds, bentorite, teratogenic, waste stabilization, alkaline environments, fly ash, ettringites, monosulfates, carcinogen, hazardous chemicals, attenuationProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.