Contribution of toxic elements: Hexavalent chromium in materials used in the manufacture of cement

Cement and Concrete Research, Vol. 24, No. 3, pp. 533-541, 1994 Copyright © 1994 Elsevier Science Ltd Printed in the USA. All rights reserved (XX)8-8846/94 $6.00 + .00

Pergamon

CONTRIBUTION OF TOXIC ELEMENTS: HEXAVALENT CHROMIUM IN MATERIALS USED IN THE MANUFACTURE OF CEMENT

M. Frias, M.I. S~lnchez de Rojas, N. Garcia, M.P. Lux~tn Instituto Eduardo Torroja (CSIC) Apartado 19002. 28080 - Madrid (Spain) (Refereed) (ReceivedMarch 22; in f'malform Sept. 27.1993) ABSTRACT One of the problems currently affecting certain industries, and more specifically cement companies, is the need to determine and control the content of water soluble hexavalent chromium because, due to its toxic effects, it may soon be regulated. The study covers the determination of hexavalent chromium in materials related to the construction sector, namely raw materials, clinker and additions. Spectrophotometric and inductively coupled plasma emission spectrometry (ICP) techniques are compared. Also included is the setting up of the most favourable test conditions for chromate extraction and a study of possible interferences in the colorimetric quantification.

Introduction The cement manufacturing sector has been a pioneer in carrying out work related to the environment, especially the avoidance of emissions of gases and dust during the manufacture of cements (1,2), because of their harmful effect on plants and animals, not to mention human beings. However very little work has been done on the measurement and control of harmful trace elements present in cements. Currently this subject is being debated in international forums; therefore the European Standards Committees will have to acknowledge and evaluate the importance and repercussion that this represents for the cement industry in the future. The toxic effect on hexavalent chromium on health are well known. Various sources add chromium to industrial portland cements, with raw materials and additions being the principal sources. Other sources include wear of mortaring elements used in grinding. Oxidizing conditions at certain stages of clinker manufacture may transform trivalent chromium into hexavalent chromium (Cr (VI)), which is much more soluble in water and easily penetrates the skin. This work, which begins a series of investigations of construction materials and the environment, is a study carded out in order to quantify the contents of hexavalent chromium in cements, and in the added materials, including industrial by-products so widely used today in the production of mixed cements and covered by international standards (3). A test methodology for determining the content of Cr (VI) in cement is proposed. Possible interferences with the test are discussed. 533

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Exnerimental Materials For this study different materials related to the manufacture of portland cement have been chosen, namely, Raw materials: a. Limestone (samples 1-3); b. Clay products (samples 4-6); c. Slate (sample 7); d. Gypsum (samples 8-10). Clinkers: e. (samples 11-15). Additions : f. Pozzolan (samples 19-20); g. Fly ash (samples 21-25); h. Electrofilter dust (samples 26-30); i. Slag (samples 31-34); j. Opaline rock (samples 35-37). Methods Fineness A way to express fineness is through the specific surface area of the materials. A Sympatec Helos 12KA spectrometer was used for the determination of the specific surface area by laser diffraction (4). This set up offers the possibility of directly determining the specific surface area of materials aRer previously determining the value of the real density. -

- Extraction The method applied in the research corresponds to that described in the Finnish Standard SFS 5183 (5), which covers the methodology used to determine water soluble chromium (VI) in cement samples. This test consists mainly of the solubilisation of the hexavalent chromium by stirring of the sample in water for a predetermined time. Procedure: 25 g of cement is suspended in 25 mL of water, stirred vigorously 15 min with magnetic stirrer and filtered, then the analysis continues as referred in the Standard SFS 5183 (5). - Colorimetric Quantitative determinations for water soluble hexavalent chromium have been carded out using the colorimetric method using a Model 330 Perkin-Elmer UV-VIS spectrophotometer. This colorimetric determination is based on producing a chemical reaction between the hexavalent chromium and a diphenylcarbazide in an acid medium, giving rise to a coloured complex (red-purple) that, according the Lambert-Beer law, shows a linear relationship between the concentration of the element and the absorbance of the coloured complex at a wavelength (540 nm). This method is similar to specific other standard methods (6). Emission (ICP) In parallel with the colorimetric determination, quantitative analyses have been carried out using the ICP emission spectrometry, Model 5500 of Perkin-Elmer, as an alternative method since for this type of analytical determinations it offers greater advantages over the traditional method (7), as will be seen in this work. -

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During the chemical analysis using ICP emission, only the first part of the previously mentioned standard was applied, referring to the placing in solution of the soluble chromium, since it is unnecessary to form the complex with diphenylcarbazide. Once the liquid that has been in contact with the sample has been filtered, 5 mL of the solution are taken and diluted with distilled water into volume flask of 25 mL capacity, in which the determination is later carried out. Results and Discussion The study considered two different aspects of the determination of hexavalent chromium: optimising methods, and quantification of Cr (VI) in different materials. I. STUDIES TO OPTIMIZE METHODS Influence of Cr extraction variables on the solubility of hexavalent chromium. Although a methodology exists, because it can be affected by interferences, it was decided to first optimize some variables in the work that could have an important effect on analytical results. Studies have centred on three Cr extraction variables: Stirring time, stirring rate and fineness. These variables are closely related to the solid/liquid contact time and the mobility of the sample in the aqueous medium. The test material used was an industrial clinker since it is the main product for commercial cements. Three physical values were fixed for each variable, as shown in Table I, in order to explore their possible relationship to the quantification of hexavalent chromium. TABLE I.- TEST CONDITIONS

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a) Influence of time and rate of stirring. Figure 1 shows the absorbances obtained with the Colorimetric method for extractions of hexavalent chromium from the clinker (0.4% absorbance = 4.8 mg/kg of Cr VI) at a constant fineness (2510 cm2/g), but with varied times and stirring rates. The analytical results show that since similar absorbance results were obtained, these parameters did not affect the test. For this reason moderate working conditions were established, 15 minutes and 400-500 revolutions per minute, as the test time and rate of stirring respectively; at all times extreme values were avoided that could cause sedimentation of the particles and/or splashing the walls of the tank. In some cases stirring rate was increased to 700 rpm when working with slates and electrofllter dust.

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STIRRING TIME (min)

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0,41

0,43 0,42 ABSORBANCES 1%)

0,44

CLINKER (2.510 cm2/g) Fig. 1. -TIME AND STIRRING RATE INFLUENCE b) Influence of fineness To study this variable the same clinker was used and underwent three different grindings until specific surfaces of 1490 cm2/g, 2510 cm~/g and 4100 cm2/g were obtained, determined with the use of granulometric laser equipment (4). Figure 2 shows the analytical absorbances for the three fineness levels selected. From these it can be deduced that fineness is a physical parameter that can play an important part in the quantitative determination of soluble chromium, provided that samples are used that have large particle sizes. Therefore the materials fineness recommended for chrome VI extraction tests must be a minimum of 2500 cm2/g of specific area. Interferences in the colorimetric determination All colorimetric methods are based on the formation o f a coloured complex whose colour intensity can be related to actual content of that element. In reality, this complex will be influenced by various interferences that can later affect the chemical determinations. A bibliography review on colorimetric method (6,8) clearly indicates that there are numerous elements and substances that can give rise to interferences that might modify the formation of the complex, in this specific case that formed between hexavalent chromium and diphenylcarbazide. There are two main mechanisms for action: 1. Action on the Chromium VI, 2. Action on the diphenylcarbazide. In the first case, two different types of interference may occur: a) A decrease in the content of chromium (VI) in the solution due to the formation of a compound between the cation and an interfering element. A typical example is the presence of chlorides in the medium to form chromyl chloride. To test this,

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ABSORBANCES (%)

0,46 0,45 0,44 0,43 0,42 0,41 0,4 0,39 0,38 0,37 0,36

2510

1480

4020

CLINKER: SPECIFIC SURFACE (cm2/g) Fig. 2.- FINENESS INFLUENCE various solutions were prepared of a chromium standard (30 parts per million, ppm) to which different percentages of KCI were added, based on the chloride contents present in Spanish cements (0.01%, 0.05% and 0.1%). The analytic results (Figure 3) show that these concentrations of chloride not affect absorbance values. b) Decrease in chromium (VI) due to the presence of agents that reduce the chromium (VI) to a lower (Cr IV).

ABSORBANCES (%)

0,51 0,505 0,5 0,495 0,49 P

P+0.01

P+0.05

P+0.1

P (CHROMIUN STANDARD, 30 ppm) + KCI (%) Fig. 3.- CHLORIDES INFLUENCE

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From the variety of reducing agents (nitrites, sulphides, etc.) sodium sulphite was selected to carry out the study in question.

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P (CHROMIUM SATNDARD, 30 ppm) + Na2SO3 (g/50mL) Fig. 4.- SODIUM SULPHITE INFLUENCE Figure 4 indicates the absorbances obtained for a chromium standard (30 ppm) with different amounts of sodium sulphite (0.001g/50mL, 0.01g/50mL, 0.1g/50mL and 2g/50mL respectively). The strong influence of these reducing compounds on the quantification of water soluble hexavalent chromium can be appreciated. As for the second case of the action of interference agents on the diphenylcarbazide, this is another possible way that may cause analytical errors. This mechanism is foreseeable with some metallic elements (Fe, Ti, Zn, etc.) that may react with the diphenylcarbazide to give rise to compounds that, in the majority of cases, are purple in colour and very similar to those obtained with chromium. Of the three interferences elements cited above, iron was selected as the most common element, found in the greatest percentage in construction materials. To verify the possible interference of this metallic element, a standard solution was first taken of I mg/L of iron concentrate, from with the corresponding aliquots were taken to obtain contents of llxg/50mL, 21xg/50mL and 201ag/50mL. First of all, the same amount of prepared chromium standard (30 ppm) was placed in each matrix. The results are shown in Figure 5, where it can be seen that Fe, as the interference element in these concentrations, does not influence the quantification of hexavalent chromium. Interference in plasma determinations In order to quantify chromium using ICP, the work can be carded out with different wavelengths. Of these, 267.71 nm was chosen as being the ideal one for these types of samples, since the others show important spectral interferences with iron and aluminium, majority elements in the construction materials selected for this study.

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ABSORBANCES (%)

0.505

0.5

0.495

P

P+ 1

P+2

P+20

P (CHROMIUM STANDARD, 30 ppm) + FE (111)(#g/mL) Fig 5.- FERRIC IRON INFLUENCE

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5

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II. QUANTIFICATION THE CONTENTS OF Cr (VI) Quantitative analyses were carried out on the water soluble chromium using the two techniques mentioned in the previous section: Visible Spectrophotometry as the reference method, and Inductively Coupled Plasma (ICP) as the alternative method. Figure 6 shows the average contents of chromium (VI), expressed in mg/kg, obtained for each of the types of materials considered in this study, both when the determination was made using the Colorimetry method and when carried out using the alternative Plasma method.

mg/kg

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SAMPLES 1-3:Limestone; 4-6:Clay Products; 7:Slate; 8-10:Gypsum; 11-15:Clinker; 19-20:Pozzolan; 21-25:Fly ash; 26-30:Elect.dust; 31-34: Slag; 35-37:Opaline rock Fig. 7.- Cr (VI): COLORIMETRIC AND ICP METHODS These results reveal that the hexavalent chromium content in industrial cements comes from samples that have undergone oxidation processes and high temperatures. The raw materials (a-d) do not show soluble chromium in their composition. When the manufacturing process is carried out, the resulting product, clinker (e), has an average content of hexavalent chromium of about 4 mg/kg, which may even reach 8 mg/kg in some samples.Thus, clinker is the largest supplier of soluble chromium in cements, followed by fly ash (g), with an average amount of approximately 0.5 mg/kg and electrofilter dust (h), which contributes very small amounts (< 0.3 mg/kg). The rest of the samples analyzed (slags (i), natural pozzolanic (f) and opaline rock (j)) did not show soluble chromium. The results obtained with colorimetric techniques to quantify hexavalent chromium in each of the samples studied, gave rise to very similar results when compared with the emission values using ICP (Figure 7) for the same samples. In view of this similarity in the results for the two analytical methods, the ICP technique offers advantages that are very interesting when compared with coiorimetric technique: speed, no need for specific solutions, determination of total soluble chromium, whether or not it is masked.

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With regard to interferences, it has been found that, for the concentrations analyzed, only reducing agents have a noticeable influence on colorimetric determinations for chromium (VI). Nevertheless, if what is desired is a decrease in the concentration of hexavalent chromium because of its toxic effects, these reducing agents could have a very positive influence for reducing Cr (VI) to a lower valence state, implying a decrease or removal of the hazard from soluble chromium in clinkers. References 1.- W. HINZ. Zement Kalk Gips, 42, n.3, 136-141 (1989). 2.- W. KREFTA. Zement Kalk Gips, 41, n.4, 193-201 (1988). 3.- EUROPAN STANDARD (ENV 197-1). Cement: Composition, Specifications and Conformety Criteria. Part 1: Common Cements. ' 4.- M. FRIAS, M.I. SANCHEZ DE ROJAS, M.P. LUXAN, N. GARCIA. Cement and Concrete Research, 21, n.5, 709-717 (1991). 5.- FINNISH STANDARDISATION ORGANISATION, SFS 5183. Determination of watersoluble chromium (VI) in cement. 6.- ASTM D 1687-86. Standard Test Methods for chromium in water. 7.- M.I. SANCHEZ DE ROJAS, M.P. LUXAN, M. FRIAS. Materiales de Construcci6n, 36, n. 202, 31-46 (1986). 8.- J. ROWLAND. Varian Instruments at Work, n. UV-20, 1-4 (1982).