SOLIDIFICATION OF HEAVY METALS IN...

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SOLIDIFICATION OF HEAVY METALS IN ETTRINGITE AND ITS STABILITY RESEARCH

Bei Wu(1), Xiangguo Li(2), Baoguo Ma(2), Minglei Zhang(3)

(1) Microlab, Faculty of Civil Engineering and Geosciences, Delft University of Technology, the Netherlands

(2) State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, P. R. China

(3) Hainan Landao Environmental Protection Industry Co., Ltd, P. R. China

Abstract

Solidification threshold and stability of heavy metals in Ettringite-dominated curing system was researched in the paper. Three criteria, hydration or damage degree tested by X-ray diffraction (XRD) together with scanning electron microscopy (SEM) and leaching toxicity tested by toxicity characteristic leaching procedure(TCLP) according to Chinese national standard(HJ 557-2009) of samples doped with heavy metal ions such as Zn2+, Cu2+ and Pb2+, were proposed in the solidification threshold and stability studies. Considering test results from hydration degree and leaching toxicity of samples, solidification thresholds of Zn2+, Cu2+ and Pb2+ in Ettringite were around 1.32%, 2.2% and 6% (in mass), respectively. Heavy metal ions leaching concentration of samples after accelerated carbonation, acid and alkali corrosions or heating treatment respectively, were all far below the limits. It meant that the curing system had highly resistance to these simulated environments and was very stable.

Keywords: Ettringite; heavy metal; solidification; threshold; stability

1. INTRODUCTION

Ettringite, approximately 3CaO·Al2O3·3CaSO4·32H2O, has been firstly written in paper by Moore and Taylor in 1970[1]. It can be produced by reactions among aluminate, lime and gypsum in the Portland cement hydration[2], and is the main product of sulphoaluminate cement. It is also formed from hydration of calcium sulphoaluminate (3CaO·3Al2O3·CaSO4,) in presence of calcium hydroxide[3].

Crystal structure of Ettringite was researched by many scientists[1,4,5,6].In 1973, Taylor reviewed the crystal chemistry of the Ettringite-group minerals, the typical structure of which consists of two parts: columns and channels(Fig. 1)[7]. Composition of the columns is {Ca6[Al(OH)6]2·24H2O}6+ while that of channels is {(SO4)3·2H2O}6- per half unit cell in each case. Each[Al(OH)6]

3-octahedron is linked to its neighbours through three Ca2+ ions, which are 8-co-ordinated by four OH- groups from the [Al(OH)6]

3- octahedron above and four H2O molecules below, and each shares two edges with different [Al(OH)6]

3-octahedrons. Hydrogen from H2O molecules form the surface of columns. In channels between the columns per unit

Second International Conference on Microstructural-related Durability of Cementitious Composites, 11-13 April 2012, Amsterdam, The Netherlands

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of {Ca6[Al(OH)6]2·24H2O}6+, there are four potential anion sites, three of which are occupied by SO4

2- and the rest one is occupied by two water molecules. Researches on other natural minerals of Ettringite group[8-12], such as Sturmanite, Jouravskite and Bentorite, show that Ca2+, Al3+ or SO4

2- in Ettringite structure all can be partially replaced by other foreigner, especially some heavy metal ions or ion group. It means Ettringite has the potential to solidify heavy metals in its structure during crystallization.

Fig. 1: (left) Broad features of the Ettringite structure, projected along the prism axis. C represent the columns [Ca3 Al(OH)6·12H2O]3+, while S represent the SO4

2- ions and H2O molecules groups in the channels. (right) Ettringite: part of a single column projected on

(1120). Unmarked circles represent H2O molecules.

Actually, many studies have been done on the incorporation of heavy metals in synthesized Ettringite or Ettringite-based solidification systems, such as calcium sulfoaluminate cement. Results show that Al3+ can be replaced by some trivalent ions like Fe3+, Cr3+ and Mn3+, Ca2+

can be substituted by bivalent ions as Zn2+, Mn2+, Co2+ or Ni2+. Moreover, SO42- can be

replaced by SeO42-, CrO4

2- etc. [13-20]. However, solution pH value, temperature and carbonation all can influence the formation

and existence of Ettringite. It is generally believed that the most stable pH range for Ettringite is between 10.5 and 13[21, 22]. For Ettringite incorporated with heavy metals, it is a little different. Chromium Ettringite can only appear when the solution pH is around 11[23, 34], and selenium Ettringite is not stable when the pH is above 12.0[25]. In CaO–Al2O3–CaSO4–H2O system, Ettringite is the main product when the temperature is below 50 . However, this temperature can increase to 85 when the concentration of sulphate is very high[21]. Decomposition products of Ettringite by heating is amorphous, a kind of monophase containing 10~13 water molecules. Moreover, when exposed to carbon dioxide and moisture, Ettringite is decomposed by carbonation. Higher moisture can slow down the carbonation reaction [26, 27].

The purpose of this study was to evaluate the solidification threshold of different kinds of heavy metal in Ettringite and the stability of the Ettringite-based solidification system under certain conditions, such as accelerate carbonation, acid or alkali corrosions and thermal.

2. EXPERIMENTAL

2.1 Preparation of sample

Ettingite has been synthesized by the hydration of a mixture containing calcium


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sulpoaluminate 4CaO·3Al2O3·SO3 and analytical grade CaSO4·2H2O, with a mole rate of 1:2. The total solid mass was 30g with a water/solid (w/s) of 0.6 according to the following formula [28].

3CaO·3Al2O3·CaSO4+2CaSO4·2H2O+38H2O6CaO·Al2O3·3SO3·32H2O+4Al (OH) 3 (1)

Calcium sulphoaluminate was produced by calcining the stoichiometric mixture of analytical grade CaCO3, Al(OH)3 and CaSO4·2H2O at 1300 for one hour according to the following formula, a little difference from a previous study[29].

3CaCO3+6Al (OH) 3+CaSO4·2H2O→4CaO·3Al2O3·SO3+3CO2+11H2O (2)

In the research on solidification threshold, different types of heavy metals with different dosage were added into the mixture in nitrate form during the synthesis of Ettringite. Samples were hydrated for 30min, 2h and then stopped by grinding with acetone to wash off the water, with ethanol final washing for facilitate drying. The dosage of heavy metals in the mixture was shown in Table 1, which was the content of ions.

Table 1: Contents of heavy metals (ω/ %)

Heavy metal types Contents Zn2+ 0 0.22 0.88 1.32 2.2 Cu2+ 0 0.22 0.88 1.32 2.2 Pb2+ 0 0.88 1.32 2.2 6.0

Matrix in the solidification stability study was a mixture of 4CaO·3Al2O3·SO3, analytical grade CaSO4·2H2O and water with a mass of 360g, 180g and 361.8g respectively. The dosage of Zn2+, Cu2+ and Pb2+ was all 1.32%.Samples were stopped hydrating by acetone washing off water after 28d, then ethanol final washing for drying.

2.2 Analysis

Minerals and morphology of Ettringite were studied by means of X-ray diffraction (XRD, Cu Kα, 30kV, 40mA) and scanning electron microscopy (SEM), respectively. The enthalpy variations of the hydration at various temperatures were measured through isothermal calorimetry. Leaching toxicity of the matrix was tested by toxicity characteristic leaching procedure (TCLP) according to Chinese standard (HJ 557-2009) [30]. Concentrations of heavy metal ions in the solution from TCLP were analysed by inductively coupled plasma atomic emission spectrometry (ICP-AES), which were compared with the limits in Chinese standard (GB5085.3-2007) [31].

3. RESULTS AND DISCUSSION

3.1 Solidification threshold of heavy metals in Ettringite 1. XRD test restults Hydrations of samples doped with Zn, Cu, Pb respectively were stopped by the method

mentioned above, then grinded for XRD test, results of which were shown in Fig. 2. When the content of Zn was 1.32% or lower, Ettringite formation was promoted in early

hydration stage (0.5, 2h). However, when its content was increased to 2.2%, the formation of Ettringite was severely impeded (Fig. 2). During the early hydration , if dosage of Cu is below 2.2%, Ettringite formation were promoted. Even when dosage was around 2.2%, Ettringite


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formation was not affected. This promotion effects of Pb on Ettringite formation was the same as Cu, except from the content of Pb could be increased to 6% or even higher.

2. SEM test results Fresh sections of blank and samples doped with 1.32% Zn, 2.2% Cu, 6% Pb respectively

were picked for SEM tests(Fig. 3). Ettringite crystals in blank were short rod-like with smaller aspect ratio comparing with those in samples mixed with 1.32% of Zn or 6% of Pb, which were mostly fibrous . It meant Zn and Pb could promote the nucleation of Ettringite and the

effect of Pb

was more

obvious. However, crystals in the sample mixed with Cu were almost the same as blank which meant Ettringite nucleation and growth was unaffected by Cu.

Fig. 2: Effects of Zn, Cu, Pb(from left to right) on the Ettringite formation

1.32%Zn


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Fig. 3: Effect of Zn, Cu, Pb on the Ettringite formation(hydration for 2h)

3. Leaching toxicity test Leaching concentrations of samples doped with Zn, Cu, or Pb were described in Fig. 4.

Leaching concentration of Zn, Cu, and Pb were all far below the limits. Considering both the effects of heavy metals on hydration degree and leaching toxicity of samples, solidification threshold of Zn, Cu, Pb in Ettringite was 1.32%, 2.2% and 6% or higher, respectively.

Fig. 4: Leaching toxicity of samples doped with Zn, Cu, Pb(hydration for 24h)

3.1 Solidification stability of heavy metals in Ettringite Solidified matrix was stopped hydrating after28d by acetone washing off water, and with

ethanol final washing for drying. Then, it was divided to small parts(50g) for stability testing. 1. Accelerated carbonation Samples were placed in conditions described in Table 2. Concentration of MgCl2 solution

was 0.5mol/L and CO2 pressure was 1 MPa. After 45d carbonation, samples were taken for XRD and leaching toxicity test, results of which were shown in Fig. 5, 6.

5 μm

Blank 2.2%Cu 6%Pb


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Table 2: Accelerated carbonation conditions

Carbonation parameter L/S

0.25 0.50 0.75 Pure water, CO2 A1 A2 A3

MgCl2 solution, CO2 A4 A5 A6 MgCl2 solution, air A7

Fig. 5 : XRD test for samples after 45d carbonation (left :pure water, right: MgCl2 solution)

Fig. 6 : Leaching toxicity of samples after 45d carbonation

XRD test results showed that no matter mixed with pure water or MgCl2 solution, after 45 d accelerated carbonation, no Ettringite existed in samples, even the liquid/solid (L/S) ratio was 0.25. Only in normal carbonation condition, there was still Ettringite in the sample mixed with MgCl2 solution. However, leaching toxicity test told that the concentrations of Zn, Cu were far below limits and Pb was undetected. Ettringite was decomposed but the solidification matrix was still stable and safe, could not do harms to the surroundings.

2. Acid and alkali corrosions In acid and alkali corrosions test, samples were soaked in three types of buffer solutions, of

which the pH were 4, 6.86 and 9.18 respectively. And the liquid/solid was 2:1. After soaked for 7 days, the samples were dried for XRD and leaching toxicity tests, which

were described in Fig. 7. XRD test results showed Ettringite was almost unchanged. Only in


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solution with pH of 9.18, there was a little Ettringite decomposed. Leaching concentration of Zn or Cu (Pb was undetected) was less than limits and nearly the same as the blank, which meant the solid matrix had high resistance to acid or alkali corrosions in this study.

Fig. 7 : XRD test and leaching toxicity of samples after acid and alkali corrosions for 7 d

3. Thermal stability Matrix samples were heated in stove under 120℃for 24 h, then tested by XRD and TCLP.

XRD results indicated there was no diffraction property in samples, which meant Ettringite decomposed to a monophase containing 10~12 water molecules [32]. Leaching concentration of Zn, Cu and Pb were all below the limits(Table 3). This was to say Ettringite would be decomposed by heating, but the solid was still stable and safe to surroundings.

Table 3 Leaching toxicity of samples after thermal treatment

No. Zn Cu Pb

Leaching concentration/(mg·L-1

) 23.8 13.6 3.7

Limits/(mg·L-1

) 100 100 5

4. CONCLUSIONS

The solidification threshold of Zn in Ettringite was 1.32% or little higher in mass percentage. That of Cu was 2.2%. And for Pb, the threshold was 6% or more. The solidification matrix was stable after carbonation, acid or alkali corrosions and thermal treatment, even the Ettringite was decomposed under certain circumstance. The leaching concentrations of heavy metal ions were still lower than limits.

ACKNOWLEDGEMENTS

The work described in the paper is funded by the National Natural Science Foundation of China (51002110).

References [1] Moore, A.E., Taylor, H.F.W., Crystal structure of Ettringite, Acta Crystallogr.B26(1970)386-393. [2] Runzhang Y, et al, Cementitious materials science, 2nd Edn (Wuhan, 1996). [3] Yanmou W, et al, Sulphoaluminate cement(Beijing, 1999). [4] Bannister, F. A., Hey, M. H. and Bernal, J. D., Ettringite from Scawt Hill, Co. Antrim. Mineral.

Mag., 24(1936), 324-329.


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[5] Courtois, A., Dusausoy, Y. et al. Etude préliminaire de la structure crystalline de I’ Ettringite, C. R. Acad. Sci. D266(1968)1911-1913.

[6] Moore, A.E., Taylor, H.F.W., Crystal structure of Ettringite, Nature (1968)1048-1049. [7] Taylor, H.F.W., Crystal structures of some double hydroxide minerals, Min. Mag. 39(1973)377-

389. [8] Edge, R.A., Taylor, H.F.W., Crystal structure of thaumasite[Ca3Si(OH)6.12H2O](SO4)(CO3),

Acta. Cryst. b27(1971) 594-601. [9] Granger, M.M., Protas, J., Détermination et étude de la structure cristalline de la jourvaskite

Ca3MnIV(SO4)(CO3)(OH)6·12H2O, Acta. Cryst. B25(1969)1943-1951. [10] Gross, S., Bentorite, a new mineral from the Hatrurim area, west of the Dead Sea, Israel, J. Earth

Sci. 29(1980)81-84. [11] Dunn, P.J., Peacor, D.R. et al, Charlesite, a new mineral of the Ettringite group, from Franklin,

New Jersey, Am. Mineral. 68(1983)1033-1037. [12] Peacor, D.R., Dunn, P.J. et al., Sturmanite, a ferric iron, boron analogue of Ettringite, Can.

Mineral. 21(1983)705-709. [13] Berardi, R., Cioffo, R., SantoroL., Matrix stability and leaching behaviour in Ettringite-based

stabilization systems doped with heavy metals, Waste Manag. 17 (8) (1997)535-540. [14] Albino, V., Cioffi, R., Marrocoli, M., Santoro, L., Potential application of Ettringite generating

systems for hazardous waste stabilization, Journal of Hazardous Materials, 51(1996)241-252. [15] Pöllmann, H., Kuzel H.J., Wenda R., Solid solutions of Ettringites: Part II: incorporation of

B(OH)4− and CrO4

2− in 3CaO·Al2O3·3CaSO4·32H2O, Cem. Concr .Res. 23(2)(1993) 422-430. [16] Bensted, J., Varma, S.P., Ettringite and its derivatives: II, Chromium Substitutions, Silicates

Industriels 23(1973)315-318. [17] Klem, W., Bhatty, J. I., Fixation of heavy metals as oxyanion-substituted Ettringite, Portland

Cement Association R&D (2002) 2431a. [18] Cody, A.M., Lee, H., Cody, R.D., et al, The effects of chemical environment on the nucleation,

growth, and stability of Ettringite[Ca3Al(OH)6]2(SO4)3·26H2O, Cem.Concr.Res.24(2004)869-881 [19] M. Zhang, Reardon, E.J., Removal of B, Cr, Mo, and Se from waste water by incorporation into

hydrocalumite and Ettringite, Environ. Sci. Technol. 37 (13)( 2003)2947-2952. [20] Chrysochoou, M., Dermatas, D., Evaluation of Ettringite and hydrocalumite formation for heavy

metal immobilization: Literature review and experimental study, J.Hazard.Mater.136(2006)20-33 [21] Damidot, D., Glasser, F.P., Thermodynamic investigation of the CaO–Al2O3–CaSO4–H2O system

at 50 ◦C and 85 ◦C, Cem. Concr. Res. 22(1992)1179-1191. [22] Damidot, D.,Glasser, F.P., Thermodynamic investigation of the CaO–Al2O3–CaSO4–K2O–H2O

system at 25 ◦C, Cem. Conc. Res. 2(1993)1195-1204. [23] Perkins, R.B., Palmer, C.D., Solubility of Ca6 [Al (OH) 6]2(CrO4)3·26H2O, the chromate analog of

Ettringite at 5–75 ◦C, Appl. Geochem. 15(2000)1203-1218. [24] Perkins, R.B., Palmer, C.D., Solubility of chromate hydrocalumite (3CaO·Al2O3·CaCrO4·nH2O)

at 5–75 ◦C, Cem. Concr. Res. 31(2001)983-992. [25] Baur, I., Johnson, A.C., The solubility of selenate-Aft(3CaO·Al2O3·3CaSeO4·37.5H2O) and

selenite - AFm (3CaO·Al2O3·CaSeO4·xH2O), Cem. Concr. Res. 33(2003)1741-1748 [26] Nishikawa, T., Suzuki, K., Ito, S., et al., Decomposition of synthesized Ettringite by carbonation,

Cem. Concr. Res. 22(1992) 6-14. [27] Zhou, Q., Glasser, F.P., Kinetics and mechanism of the carbonation of Ettringite, Adv. Cement

Res. 12(2000)31-36. [28] Odler, I., Cements containing calcium sulfoaluminate, Special Inor-ganic Cements, E&FN Spon,

London(2000)69- 87. [29] Valenti, G. L., Santoro, L., Garofano, R, High-temperature synthesis of calcium sulphoaluminate

from phosphogysum. Thermochimica.Acta. 1987,113, 269-275. [30] National Environmental Protection Standard of People's Republicof China, Solid waste-Extraction

procedure for leaching toxicity-Horizontal vibration method, HJ 557-2009.


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[31] National Standard of People's Republicof China, Identification standards for hazardous wastes-Identification for extraction toxicity, GB5085.3-2007.

[32] Micheal R. H., Steven K. B., et al., The evolution of structural changes in Ettringite during thermal decomposition. Jour. of Solid State Chem. 179(2006)1259-1272.


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