Speciation of Cr and V within BOF steel slag reused in road constructions

Journal of Geochemical Exploration 88 (2006) 10 – 14 www.elsevier.com/locate/jgeoexp

Speciation of Cr and V within BOF steel slag reused in road constructions Perrine Chaurand a,*, Je´roˆme Rose a, Je´re´mie Domas b, Jean-Yves Bottero a a

CEREGE, UMR 6635 CNRS-Univ. Paul Ce´zanne, IFR PMSE, Europole de l’Arbois, BP 80, 13545 Aix en Provence Cedex 04, France b INERIS, Domaine du petit Arbois, Baˆtiment Lae¨nnec, BP 33, 13545 Aix en Provence Cedex 04, France Received 30 March 2005; accepted 19 August 2005 Available online 11 November 2005

Abstract Basic Oxygen Furnace (BOF) steel slag is a residue from the basic oxygen converter in steel-making operations, which is partially reused as an aggregate for road constructions. It is essentially composed of calcium, silicon and iron but also contains potential toxic elements present as traces, like chromium (Cr, 2600 mg kg 1) and vanadium (V, 690 mg kg 1), which can be released. The linked results of chemical analysis, XRD and SEM-EDX enabled to identify the main mineral phases composing BOF slag and EDX micro-analyses indicated that V and Cr were associated to dicalciumferrite. A 47-days static leaching test at a laboratory scale with a controlled pH of 5 (pHstat leaching test) showed that Cr was little released, while V was significantly released. Finally, X-ray absorption near-edge structure (XANES) spectra of 3 BOF slag samples were recorded (brawQ, leached 47 days at pH 5 and aged 2 years in a lysimeter). XANES spectra showed that Cr is present at octahedral coordination in the trivalent form, the less mobile and less toxic one, and that its speciation does not evolve during natural ageing and leaching at pH 5. They also indicated that V is predominantly present in the + 4 oxidation state and seems to become oxidized to the pentavalent form (the most toxic form) during natural ageing. D 2005 Elsevier B.V. All rights reserved. Keywords: Chromium (Cr); Vanadium (V); X-ray absorption spectroscopy (XANES); Leaching behaviour; Dicalciumferrite

1. Introduction and materials and methods Reuse of waste materials has become very important within the past decade because of the reinforcement of environmental legislations that require minimizing waste disposal. Steel making operations are specifically concerned by this problem because of generation of huge quantity of by-products. Basic Oxygen Furnace (BOF) steel slag is a by-product in steel-making operations, with an estimated 12 million tons generated

* Corresponding author. Tel.: +33 442971543; fax: +33 442971559. E-mail address: [email protected] (P. Chaurand). 0375-6742/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2005.08.006

annually in Europe. Precisely, BOF steel slag is a residue from the basic oxygen converter, where the pig iron is converted into steel by injecting pure oxygen. Some of the slag is recycled to the blast furnace while a significant portion is used in road construction (e.g. asphaltic or unbound layer) due to its very high stability and superior skid and wear resistance. But even if BOF slag is attractive as a building material, its environmental impacts must be known. Indeed, even if BOF Steel slag is composed essentially of calcium, silicon and iron, potential toxic elements are also present as traces like chromium (Cr, 2600 mg kg 1) and vanadium (V, 690 mg kg 1), and can be released. The element analysis and the mineral composition are es-

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Table 1 Range of chemical composition of raw BOF steel slag (average of 5 replicates from 80 Am to N2 mm) (ICP-AES analysis after mineralization) Major component (g kg

BOF-slag

1

)

Traces (g kg

1

)

Ca

Fe

Si

Mn

Mg

Al

Ti

P

Cr

V

296 F 6

218 F 2

59.4 F 0.6

47.3 F 0.9

26.4 F 0.3

13.1 F 0.4

5.0 F 0.1

4.9 F 0.3

2.4 F 0.1

0.69 F 0.05

sential but not sufficient for environmental impact assessment, since it remains unknown whether the individual components will ever be released. It is essential to predict the long-term behaviour (and to assess the associated environmental and human health risks) of BOF steel slag by determining the rate and the mechanisms of metals release. In the present study interest was focused on the different mineral phases composing BOF steel slag and on the evolution of Cr and V speciation during leaching and ageing because their mobility and toxicity strongly depend on their atomic environment and oxidation state. Chromium is a redox active metal that persists as either Cr(III) or Cr(VI) in the environment. These two oxidation states have opposing toxicity and mobility: trivalent Cr is an essential nutriment at low amounts and a little-toxic element at higher content and is mostly insoluble in water, while hexavalent Cr is very toxic and readily transported (Fendorf, 1995; Rai et al., 1989). Vanadium is a metal which exists in oxidation states ranging from 0 to +5 and the most common valence states are +3, + 4 and + 5. The multiple oxidation states and the facility, with which it changes coordination environments and oxidation states, confer a level of complexity to the chemistry of V and the characteristics of which have just begun to emerge. Vanadium at trace amounts represents an essential element for normal cell growth, but can be toxic when present at higher concentration (Pyrzynska and Wierzbicki, 2004). The vanadium compounds have different nutritional and toxic properties: their toxicity usually

increases as the valence increases (Barceloux, 1999). Hence pentavalent compounds are the most toxic. Therefore an accurate determination of oxidation states is very important. An experimental test was performed to assess shortterm leaching behaviour of BOF slag at a fixed pH (pHstat test). BOF steel slag grains (grains larger than 2 mm) were leached in a mechanically mixed tank for a period of 47 days. The test was performed with constant temperature (40 8C), constant liquid to solid ratio (30) and controlled pH (pH 5). Then eluates were filtered through a 0.22 Am filter and released metal contents were analyzed by plasma emission spectrometry ICP-AES. Multi scale structural techniques, used in this study to describe BOF steel slag, include X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) microanalyses and X-ray absorption spectroscopy. Synchrotron-based technique of X-ray absorption near-edge structure (XANES) spectroscopy is the most adapted method permitting in-situ determination of valence and speciation of elements present in solid sample at such low concentrations. This technique is non-destructive, gives element specific information and is not susceptible to matrix effects. Cr and V K-edge XANES measurements were performed on (i) beamline FAME at the European Synchrotron Radiation Facility-ESRF (Grenoble, France) and (ii) beamline BL-11.1 at Elettra (Triestre, Italy), respectively. To record Cr and V K-edge XANES spectra, the Si(111) double crystal monochro-

Fig. 1. SEM backscattered electron image of a BOF steel slag (polished section, grains of 200–500 Am).

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Fig. 2. Release of Cr and V from BOF steel slag during pHstat test (pH = 5, 47 days, L/S = 30, grains N 2 mm).

mator was stepped from about 100 eV below the edge to about 200 eV above the edge (Cr K-edge 6 5989 eV, V K-edge 6 5465 eV) and the intensity of the monochromatic X-ray beam was measured as a function of energy before and after absorption by the sample. Three lots of BOF slag (grains larger than 2 mm) were analyzed: one brawQ which was not subjected to leaching (called bBOF slagQ), one leached during pHstat test (47 days at pH 5) (called bleached BOF slagQ) and one subjected to natural ageing (called baged BOF slagQ). The last lot provided by the bLaboratoire Central des

Ponts et Chausse´esQ (Nantes, France) was weathered in a lysimeter (1 m3) placed outside for a period of 2 years. 2. Results and discussion 2.1. Chemical and mineral composition of BOF steel slag Table 1 shows the range of the chemical composition of brawQ BOF steel slag obtained by ICP-AES and ICP-

Fig. 3. Normalized XANES spectra for BOF steel slag samples (grains N 2 mm) and for crystalline standards, recorded (a) at Cr K-edge and (b) at V K-edge.

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MS measurements (results correspond to the average of 5 replicates from 80 Am to N 2 mm). The linked results of chemical analyses, XRD and EDS micro-analyses enabled to identify and characterize six minerals within BOF steel slag (Fig. 1):

ground water (not used as drinking water) are 200 Ag L 1 for V and 2000 Ag L 1 for Cr3+. So, V within BOF steel slag can present a risk for environment (Fig. 2).

! h-dicalciumsilicate (Ca2SiO4) as large grains, containing a solid solution of Ca3P2O8 (1). ! Dicalciumferrite (Ca2Fe2O5), containing Ti and Al (2). ! Solid solution rich in iron ((Fe, Ca, Mn, Mg)O) (3). XRD spectrum of this phase is close to wustite (FeO) spectrum lines (Chavepeyr et al., 1979). ! Lime containing a solid solution of iron and manganese oxides ((Ca, Fe, Mn)O) (not shown in Fig. 1). XRD spectrum of this phase are closed to CaFeO2 spectrum lines (Chavepeyr et al., 1979). ! Calcium hydroxide (Ca(OH)2) and calcite (CaCO3) formed during ageing (lime hydration) (4). ! Iron metal (Fe) (5).

The Cr(III) and Cr(VI) oxidation states in solids can be readily distinguished in Cr XANES spectra by the intensity of the pre-edge peak. As shown in Fig. 3a, the pre-edge peak is almost as intense as the edge-step for hexavalent Cr, whereas the pre-edge peak is usually very weak for trivalent Cr. This difference in the height of the pre-edge peak has been used for the determination of Cr oxidation states (Peterson et al., 1996). Fig. 3a also shows normalized XANES spectra for samples of BOF steel slag, before and after leaching at pH 5 and natural ageing. The absence of pre-edge peak indicates that Cr is present at octahedral coordination in the trivalent form (the less mobile and less toxic form), and that its speciation does not change during leaching at pH 5 or natural ageing.

Finally SEM-EDS micro-analyses indicate that V and Cr are always associated within the same phase: the dicalciumferrite containing Ti and Al (Fig. 1). 2.2. Cr and V release The leaching test shows that Cr is little released (less than 0.02% of total content) while V is significantly released (between 0.4% and 0.5% of total content) even during a relatively short leaching period (Fig. 2). Canadian MCL values (Maximum Containment Level) for

2.3. Chromium K-edge XANES spectra

2.4. Vanadium K-edge XANES spectra V K-edge XANES spectra for seven crystalline standards containing V at different valences (from V(III) to V(V)) and BOF steel slag samples were recorded. The edge energy (E i) of each sample was measured half way up the normalized-edge step, or where the absorption is equal to 0.5 (Fig. 3b). The X-ray edge energy (at half way) of V standards displays a positive shift with increase in the oxidation state of V (Fig. 4). Thus the

Fig. 4. Position of the absorption edge of vanadium vs. its oxidation state of the various BOF steel slag samples.

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V K-edge is a clear signature of its oxidation state (Mansour et al., 2002; Rodella and Mastelaro, 2003). The experimental error on the energy measurement is F0.05 eV. An analysis of the energy position of the edge (Fig. 4) reveals that V is predominantly present in the + 4 oxidation state in BOF steel slag (mean oxidation state of 4.1) and that the mean oxidation state of V seems to increase during natural ageing (4.3) and to lower during leaching at pH 5 (3.9). This last result is in agreement with the V phase diagram. 3. Conclusion The results of the multiple scale structural study permitted to describe the main mineral phases composing BOF steel slag and to identify the bearing phase of Cr and V: a dicalciumferrite containing Ti and Al. Then leaching tests showed that Cr is little released. XANES investigations at Cr K-edge were in line with this result because they showed that Cr is present in BOF slag in the trivalent form, the less mobile and less toxic one, and remains in this form during leaching and natural ageing. Unlike Cr, V is significantly released and predominantly present in BOF slag in the + 4 oxidation state (one of the toxic forms). Moreover V seems to become oxidized to the pentavalent form (the most toxic) during natural ageing. However analyses were performed on entire grains (not only on the altered zone) and results obtained are average. This observation remains to be confirmed by local structural analyses of the altered zone (A-fluorescence X and A-XANES mapping). This work allowed us to show that the reuse of BOF steel slag as aggregates in road construction can involve significant release in the natural environment of

V in a toxic form: the tetravalent form and, maybe, the pentavalent form on the long-term. Acknowledgements We thank ADEME (the French Agency for Environment and Energy Management) and INERIS (the French Institute of Industrial Environment and Risks) for supporting this work. We also thank LCPC (Central Laboratory of Bridges and Roads) for providing us with BOF steel slag samples and V. Briois, L. Olivi, J.L. Hazemann and O. Proux for helpful discussions and technical assistance. References Barceloux, D.G., 1999. Vanadium. Journal of Toxicology 37-2, 265 – 278. Chavepeyr, G., Dumortier, C., Gohy, C., Levert, J.M., Riquier, Y., 1979. Mineralogical study of BOF slag. Silicates Industriels 4410, 217 – 233 (in French). Fendorf, S.E., 1995. Surface reactions of chromium in soils and waters. Geoderma 67 (1–2), 55 – 71. Mansour, A.N., Schmidt, P.H., Baker, W.M., Balasubramanian, M., McBreen, J., 2002. In situ XAS investigation of the oxidation state and local structure of vanadium in discharged and charged V2O5 aerogel cathodes. Electrochimica Acta 47-19, 3151 – 3161. Peterson, M.L., Brown, J., Gordon, E., Parks, G.A., 1996. Quantitative determination of chromium valence in environmental samples using XAFS spectroscopy. Proc. Mater. Res. Soc. Spring. Mtg., vol. 432, pp. 75 – 80. Pyrzynska, K., Wierzbicki, T., 2004. Determination of vanadium species in environmental samples. Talanta 64-4, 823 – 829. Rai, D., Eary, E.A., Zachara, J.M., 1989. Environmental chemistry of chromium. Science of the Total Environment 86 (1–2), 15 – 23. Rodella, C.B., Mastelaro, V.R., 2003. Structural characterization of the V2O5/TiO2 system obtained by the sol–gel method. Journal of Physics and Chemistry of Solids 64-5, 833 – 839.