A new method in estimation of total hexavalent chromium in Portland pozzolan cement

R. Sharma, D. K. Sharma*

School of Physical Science, Lovely Professional University, (Phagwara, India)

*devsarita@gmail.com

 

ABSTRACT

Variamine blue was used first time for the detection of hexavalent chromium from cement samples. In present method, cement was treated sequentially with water, sulphate and carbonate buffer to extract soluble, sparingly soluble and insoluble hexavalent chromium respectively. Extracted Cr (VI) was determined using variamine blue as chromogenic reagent. The determination is based on the reaction of hexavalent chromium with potassium iodide in an acid medium to liberate iodine. This oxidizes variamine blue to form a violet coloured species having an absorption to maximum at 556 nm. Energy-dispersive X-ray spectroscopy (EDX) and Infrared Spectroscopy (IR) confirmed the complete extraction of hexavalent chromium by sequential extraction process. SRM 2701 (Reference material from NIST, USA) was used for revalidating the results. The percentage of recovery for proposed and reference method (diphelycarbazide method) varied from 98.5 to 101 and 97.5 to 100.5. Whereas, their relative error percentage varied from -1.5 to 0.33 and -2.5 to 0.5.

 

RESUMEN

Un nuevo método en la determinación del cromo hexavalente total en cemento Portland puzolánico. El azul de variamina se utilizó por primera vez para la detección de cromo hexavalente en muestras de cemento. En el presente método, el cemento se trató secuencialmente con agua, y tampones sulfato y carbonato para extraer el cromo hexavalente soluble, poco soluble e insoluble, respectivamente. El Cr (VI) extraído se determinó utilizando azul de variamina como reactivo cromógeno, por reacción del cromo hexavalente con yoduro de potasio en un medio ácido para liberar yodo. Esto oxida al azul de variamina para formar una especie de color violeta con una absorción máxima a 556 nm. El análisis por energía dispersiva de rayos X (EDX) y la espectroscopía de infrarrojos (IR) confirmaron la extracción completa de cromo hexavalente mediante el proceso de extracción secuencial. Se utilizó SRM 2701 (material de referencia de NIST, EE.UU.) para validar los resultados. El porcentaje de recuperación para el método propuesto frente al de referencia (método de la difenilcarbazida) varió de 98.5 a 101 y 97.5 a 100.5, mientras que su porcentaje de error relativo varió de -1.5 a 0.33 y -2.5 a 0.5.

 

Received 19 February 2016; Accepted 17 October 2016; Available on line 6 April 2017

Citation/Citar como: Sharma, R.; Sharma, D.K. (2017) A new method in estimation of total hexavalent chromium in Portland pozzolan cement. Mater. Construcc. 67 [326], e116. http://dx.doi.org/10.3989/mc.2017.03116.

Keywords: Portland cement; Estimation; Chromium; Variamine blue; UV-visible; IR; EDX

Palabras claves: Cemento Portland; Estimación; Cromo; Azul de variamina; UV-visible; IR; EDX

ORCID ID: R. Sharma (http://orcid.org/0000-0002-5808-0045); D. K. Sharma (http://orcid.org/0000-0002-5113-8531)

Copyright: © 2017 CSIC. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) Spain 3.0.


 

CONTENT

1. INTRODUCTIONTOP

Chromium, in its non toxic +3 oxidation state, is present in raw materials (clay, lime or ore) used for cement production. But at high temperature of cement rotary kiln, this trivalent chromium oxidises into reactive hexavalent form (1). Health of workers in cement industries is at risk due to exposure to this hexavalent chromium. Airborne hexavalent chromium has been proved to be a human respiratory carcinogen and some of its compounds were found to be contact allergens (2, 3, 4). Leaching of Cr (VI) from concrete structures, like water reservoirs and pipes used for water transport can also lead to the contamination of water (5). Thus, the detection of hexavalent chromium in cement sample becomes necessary. Although, the use of cement containing more than 2.0 mg/kg soluble Cr (VI) in hydrated cement, was restricted by European Directives 2003/53/EC (6), yet there is no check on concentration of hexavalent chromium in cement samples used in India. The most common detecting reagent used for Cr (VI) is diphenylcarbazide (DPC) (7, 8). However, the major problem associated with this reagent is interference caused by other metal ions (FeIII, MoIV, CuII, HgII etc.) present in the cement (9). Apart from DPC, many organic molecules were also used for the same. However, the main limitation of these reagent is either their carcinogenic behaviour or less selectivy (10).

Therefore, there is a need of simple and sensitive reagent for estimation of concentration of Cr (VI). Recently, variamine blue had been used for detection of hexavalent chromium in steel, industrial effluents, natural water samples and soil samples (11). Simple reaction conditions, no extraction or heating and high colour stability has made variamine blue a better reagent than others (11). In this paper, variamine blue is used for detection of Cr (VI) in cement samples. Sequential extraction and detection of Cr (VI) (water soluble, sparingly soluble and insoluble chromium (VI)) has been carried out using this reagent.

2. EXPERIMENTALTOP

2.1. InstrumentationTOP

A HACH Spectrophotometer (Made in USA) with 1 cm quartz cell was used for the absorbance measurements. Residues left after extraction of water soluble Cr (VI) (extraction 1) and extraction of sparingly soluble Cr (VI) (extraction 2) were dried and ground. Infrared spectra of these powders were then recorded from KBr pellets in the range 4000-200 cm−1 on a SHIMADZU FTIR 8400S spectrophotometer. EDX was performed with a JEOL Model JED – 2300. The cement sample (S1) and its residue left after extraction of insoluble Cr (VI) (extraction 3) was dried and ground. The fine powder, thus obtained, was used to take EDX spectrum.

2.2. Reagents and solutionTOP

All the chemicals used were of analytical / GR grade. Variamine blue dye solution (0.05%) was prepared by dissolving 0.05 g in 25 mL absolute alcohol and then the volume was made 100 mL using distilled water. A solution of potassium iodide (2.0%) was prepared by dissolving 2.0 g potassium iodide in water and diluting it up to 100 mL. A solution of sodium acetate (2M) was prepared by dissolving 16.407 g sodium acetate in distilled water and the volume made up to 100 mL in a volumetric flask. Sulfuric acid (1M) was prepared by diluting 6.95 mL of stock H2SO4 to the mark in a 250 mL volumetric flask with distilled water. Stock solution of Cr (VI) was prepared by dissolving 0.2829 g, K2Cr2O7 in distilled water to make the volume 1L. Standard solutions of 0.05, 0.1, 0.2, 0.04, 0.08, 1 ppm were made by dilution of stock solution.

2.3. MaterialsTOP

Cement samples used are fly ash based Portland Pozzolan Cement of different brands and the %age of pozzolanic material is not less than 10% and not more than 25% (Table 1). The study of chemical composition of PPC samples (Table 2) has been carried out at Shankar Laboratory, New Delhi. Due to the unavailability of hexavalent chromium certified reference cement, SRM 2701 Hexavalent Chromium in contaminated Soil from NIST (National Institute of Standards and Technology, USA) was used for revalidating the method accuracy. The certified concentration of Cr (VI) in SRM 2701 is 551.12 ± 34.5 mg/Kg.

Table 1. Sample details
Brand name ACC Jaypee Birla Prism Mysem Reliance
Sample ID S1 S2 S3 S4 S5 S6
Weak/year of Manufacturing 23/2015 22/2015 21/2015 22/2015 21/2015 21/2015
Table 2. Chemical composition of Portland Pozzolan Cement
Sample ID Constituents %
LOI SiO2 Fe2O3 Al2O3 CaO MgO SO3 IR Na2O K2O Cl
S1 2.23 31.7 3.3 9.3 41.7 1.9 1.8 8.51 0.28 0.2 0.29
S2 2.41 32 3.4 8.8 40.2 1.4 2.16 10.6 0.21 0.18 0.22
S3 2.42 31.2 3.4 9.5 41 1.7 2.16 9.12 0.37 0.25 0.18
S4 2.33 31 3.21 8.1 42.2 1.5 2.1 8.65 0.45 0.31 0.19
S5 2.11 32 3.4 9.8 40 1.9 2.16 9.65 0.18 0.18 0.21
S6 2.63 32 3.4 10.8 40.1 1.3 1.8 8.3 0.21 0.11 0.19

2.4. Extraction of water soluble Cr (VI) species (Extraction 1)TOP

The Cr (VI) from cement samples was extracted using five day extraction process (12, 13). In this method, 100 mL distilled water was added to 1.0 g cement sample. The contents were mixed thoroughly with a glass rod. This mixing was repeated for five days to ensure complete leaching of hexavalent chromium. After five days, filtration was done using Whatman 42 filter paper. Two washings were required. Insoluble part was kept for estimation of sparingly soluble and insoluble chromium and filtrate was used for detection of water soluble hexavalent chromium.

2.5. Extraction of sparingly soluble Cr (VI) species (Extraction 2)TOP

Sparingly soluble Cr (VI) is present in cement in the form of strontium chromate (14). For its detection, residue left after extraction 1 was transferred to a 250 mL glass beaker. To this was added, 40 mL of ammonium sulfate buffer (0.05 M ammonium sulfate and 0.05 M ammonium hydroxide) and contents were stirred for 30 min. Filtrate obtained after stirring was used to estimate sparingly soluble chromium and residue was kept for estimation of insoluble chromium (14).

2.6. Extraction of insoluble Cr (VI) species (Extraction 3)TOP

To the residue obtained after extraction 2, was added 40 mL of carbonate buffer (3% (w/v) sodium carbonate and 2% (w/v) sodium hydroxide) in 250 mL glass beaker. The reaction mixture was stirred for 30 minutes and filtered (14). Filtrate was used to estimate the concentration of insoluble chromium.

2.7. Determination methodTOP

To the filtrate of extraction 1, 1 mL of 2% potassium iodide was added. Addition of sulphuric acid (1.0 mol/L, 1 mL, the oxidation of iodide to iodine was effective in the pH range 1.0 to 1.5) to the reaction mixture gave appearance of yellow colour indicating the liberation of iodine. To this was added variamine blue (0.05%, 0.5 mL) and 2 mL of 2 mol/L sodium acetate solution (Buffer solution). The resulting solution was kept for 10–15 minutes to ensure the completion of reaction (appearance of violet colour). The absorbance at 556 nm, of the coloured solutions had a linear relationship to the Cr (VI) concentration. The similar procedure was used for filtrates obtained after extraction 2 and 3. All the solutions were also analyzed for chromium concentration by standard DPC method (15). The concentration of Cr (VI) can be calculated using the following formula: C(Cr (VI)) = C V1/M V2.

Where C(Cr (VI)) is the concentration of hexavalent chromium (ppm), C is the concentration of Cr (VI) in μg/ml, V1 is the volume of water in which the original sample is suspended, V2 is the volume of filtrate which is transferred to the volumetric flask, cm3. M is the mass of the cement sample.

3. RESULTS AND DISCUSSIONTOP

Cr (III) is slightly soluble in alkaline medium in the form of Cr (OH)6-3 and therefore, generally does not interfere in detection of Cr (VI). But if it is in excess, Cr (OH)6-3 can bind with Cr (III)-DPC complex to give positive interference in detection of Cr (VI) by standered DPC method (11). To overcome this interference by Cr (III), variamine blue is used as chromogenic reagent in detection of Cr (VI) because the mechanism of action in this case is different. In this method iodide ion gets oxidized into iodine on reaction with chromate in an acidic medium. The liberated iodine oxidized variamine blue, in the presence of sodium acetate, (buffer solution, pH 4) is to form a violet-coloured solution (Figure 1). This coloured solution gave maximum absorbance at 556 nm. The reagent blank had a negligible absorbance at this wavelength and was set at zero.

Figure 1. Oxidation of Variamine Blue (VB) by liberated Iodine (I2).

 

3.1. Analytical ParametersTOP

The calibration graph is linear and can be explained by equation: y = 0.062x – 0.001, where y is integrated absorbance and x is analyte concentration (Figure 2). The regression value obtained for the calibration plot is 0.998. The detection limit (DL= 3.3σ/S) and quantification limit (QL= 10 σ/S) of Cr (VI) were found to be 0.05 and 0.16 μg/ml, where σ is the standard deviation of blank samples (n=5) and S is slope of calibration curve. The experimental data to get calibration curve is given in supplementary material.

Figure 2. Calibration graph.

 

3.2. Validation of proposed methodTOP

“SRM 2701 Hexavalent Cr in contaminated Soil” from NIST (National Institute of Standards and Technology, USA) was used for revalidating the results obtained by the proposed method (Table 3).

Table 3. Determination of Cr (VI) in standard sample (SRM 2701 from NIST)
  Hexavalent chromium in SRM 2701 (in ppm) Recovery (%) RSD (%) T -test F- test
Water soluble Sparingly soluble Insoluble Total
Certified value - - - 551.12±34.5 - - - -
Proposed method 125±2.74 115.2±8.70 309.2±14.45 549.4±25.89 99.69 4.7 0.93 0.97
Reference method 126±3.67 114.6±2.97 309.2±14.45 550.8±29.58 99.94 5.3

3.3. Leaching procedure and verification for Cr (VI) extractionTOP

For complete leaching of water soluble hexavalent chromium, five days extraction procedure was followed. Long stirring in water ensured the extraction of some hexavalent chromium that was present in solid phases of cement. Sparingly soluble Cr (VI) was extracted by treatment of residue of extraction 1 with ammonium sulfate buffer (0.05 M (NH4)2SO4–0.05 M NH4OH). However, the insoluble chromium was extracted using carbonate buffer (3% (w/v) sodium carbonate and 2% (w/v) sodium hydroxide).

3.4. IR spectraTOP

In the IR spectrum (16), bands for CO32- ion appeared at 1425, 1497, 875 and 732 cm−1 and for chromate ion in the range 850-950 cm−1 (specially for ZnCrO4 and CaCrO4) (8, 17, 18). IR spectrum of cement sample S1 show band at 877 and 1430 cm−1 (Figure 3). The intensity of these bands decreased after the extraction of water soluble and spirangly soluble Cr (VI) (residue of extraction 1 and 2 respectively). Decrease in intensity of band at 877 cm−1 can either be due to leaching of hexavalent chromium in extraction process or reduction in carbonate concentration (due to formation of carboxyaluminates (19, 20). or replacement of carbonate by sulphate ion in Aft (alumina ferric oxide tri-sulfate) and AFm (alumina ferric oxide mono-sulfate) phases) (21, 22).

Figure 3. Overlay of spectrum of cement sample S1 and its residue after extraction 1 and 2.

 

Since vibrational bands for carbonate and chromate band appeared in the range, 870–880 cm−1, it is difficult to get a distinct information regarding decrease in concentration of chromate ion. Nevertheless, it can be used as supporting data to other techniques like EDX and UV-Vis spectrophotometer.

3.5. EDX spectaTOP

Extraction of hexavalent chromium was also confirmed by EDX spectra of sample S1 and the residues left after extraction 3. Generally chromium gave a peak in range of 5.0 to 6.0 KeV in EDX spectrum (23, 24, 25, 26). In sample S1, this peak appeared at 5.2 KeV in its EDX spectrum (Figure 4), however this appeared as very weak peak after extraction 3. (Figure 5)

Figure 4. EDS spectrum of cement paste before sequential extraction.

 

Figure 5. EDS spectrum of cement paste after sequantial extraction.

 

3.6. Determination of hexavalent chromium in Cement samplesTOP

Variamine Blue method was applied to filtrates of extraction 1, 2 and 3 of six different samples of Portland pozzolan cement for estimation of water soluble, sparingly soluble and insoluble hexavalent chromium. To check the accuracy of proposed method, all the six cement samples were also tested for same using standard DPC methods.

Tables 46 gave a comparison of proposed method with standard method in terms of parametric tests (T and F test), percentage of relative standard deviation (RSD) and percentage of recovery. The percentage recoveries in six cement samples (from S1 to S6) were spiked with known concentration of hexavalent chromium. A null hypothesis indicating that both Cr (VI) determination methods adopted in this research relied on each other was evaluated by comparing both the means with parametric test (T and F test). These tests for the comparison of means were calculated. The calculated values were compared with the tabulated value at a proper degree of freedom (27) and results indicate reliability of these methods from 2 to 12 ppm concentration (28). At a 95% confidence level, the calculated T- and F-values do not exceed the theoretical values (Table 36) indicating no significant difference between the proposed and the reference method. Consequently, the developed method is as accurate and precise as that of reference method (Figure 6).

Table 4. Determination of water soluble Cr (VI) in various Cement samples
Sample ID Soluble Cr (VI) Added* (ppm) Proposed method DPC method T-test F-test
Soluble Cr (VI) Found RSD (%) Recovery (%) Soluble Cr (VI) Found RSD (%) Recovery (%)
S1 - 23±1.58 6.9 - 24±2.12 8.8 - 0.42 0.58
  2 24.7±1.07 4.4 98.64 25.4±1.90 7.1 98.23 0.38 0.33
S2 - 20±1.64 8.3 - 20±1.92 9.5 - 0.84 0.77
  4 23.58±1.57 6.7 98.25 24.3±0.97 4 101.25 0.41 0.38
S3 - 23±2.49 10.8 - 26±1.52 5.9 - 0.07 0.36
  6 28.7±1.11 3.9 99.1 32.2±1.68 5.2 100.63 0.01 0.44
S4 - 26±2.63 10.21 - 29±3.02 10.38 - 0.1 0.79
  8 33.9±2.10 6.2 99.71 36.4±2.42 6.7 98.32 0.12 0.79
S5 - 17±1.92 11.3 - 18±2.17 11.91 - 0.49 0.82
  10 27.1±1.05 3.86 100.44 27.6±1.19 4.33 98.57 0.52 0.81
S6 - 19±1.49 7.83 - 20±2.35 7.22 - 0.47 0.40
  12 31.1±2.25 11.73 100.32 31.8±1.35 4.25 99.38 0.57 0.35
*Added as sodium chromate (Na2CrO4), a Mean ± standard deviation (n = 5). a Tabulatedt-value for 8 degrees of freedom at 5% level of significance is 2.306.b Tabulated F-value for (4,4) degrees of freedom at P (0.95) is 6.39.
Table 5. Determination of Sparingly soluble Cr (VI) in various Cement samples
Sample ID Sparingly soluble Cr (VI) Added* (ppm) Proposed method DPC method T-test F-test
Springle soluble Cr (VI) Found RSD (%) Recovery (%) Springle soluble Cr (VI) Found RSD (%) Recovery (%)
S1 - 10±1.48 14.5 - 11±1.58 14.37 - 0.43 0.20
  2 24.7±1.07 10.3 99 25.4±1.90 11.31 100.77 0.19 0.72
S2 - 15±1.14 7.81 - 15±2.07 14.2 - 1.00 0.27
  4 18.9±0.93 4.89 99.58 18.8±0.84 4.45 98.95 0.84 0.85
S3 - 13±1.19 8.91 - 14±1.92 13.55 - 0.46 0.38
  6 18.8±0.96 5.09 98.73 20.2±1.19 5.92 100.8 0.08 0.68
S4 - 15±1.33 9.01 - 16±1.30 8.05 - 0.12 0.97
  8 22.8±1.04 4.55 99.13 23.7±1.65 6.99 98.7 0.35 0.39
S5 - 11±0.23 2.04 - 11±1.62 14.73 - 0.72 0.00
  10 20.9±0.91 4.37 99.52 21.2±2.59 12.21 100.95 0.82 0.07
S6 - 14±1.92 13.94 - 13±1.48 11.59 - 0.39 0.63
  12 26.1±1.87 7.18 100.31 25±1.87 7.5 99.84 0.37 1.00
*Added as Calcium chromate.
Table 6. Determination of Insoluble Cr (VI) in various Cement samples
Sample ID Insoluble Cr (VI) added (ppm) Proposed method DPC method T-test F-test
Insoluble Cr (VI) Found RSD (%) Recovery (%) Insoluble Cr (VI) Found RSD (%) Recovery (%)
S1 - ND - - ND - - - -
  2 1.94±0.36 18.8 97 2.08±0.26 12.44 104 0.5 0.52
S2 - 11±1.36 12.49 - 10.5±1.6 14.2 - 0.64 0.74
  4 14.7±1.32 9 98 14.8±1.97 13.3 98.8 0.9 0.45
S3 - ND - - ND - - - -
  6 5.94±0.99 16.7 99 6.18±0.81 13.17 103 0.69 0.71
S4 - ND - - ND - - - -
  8 8.02±0.93 11.64 100.25 8.1±0.82 10.11 101.25 0.89 0.8
S5 - 8±0.78 9.48 - 10.30±1.10 10.64 - 0.01 0.53
  10 18.4±0.81 4.43 102.4 21.2±2.58 12.2 106 0.07 0.05
S6 - 10±0.74 7.49 - 11.1±1.43 12.9 - 0.15 0.23
  12 21.94.1±1.9 8.5 99.72 23.4±2.12 9.1 101.8 0.28 0.81
*Added as lead chromate (PbCrO4).

Figure 6. Concentration of Cr (VI) found by proposed method and reference method for samples S1-S6.

 

No insoluble Cr (VI) was detected by both the methods for samples, S1, S3 and S4. Results obtained by proposed method were in good agreement with standard method except for sample S3 and S4, where concentration of water soluble Cr (VI) by proposed method was found to be less than that of obtained by standard method. Concentration of water soluble and sparingly soluble Cr (VI) was found to be maximum in sample S4 and minimum in S5 by both the methods. It was observed that in Portland pozzolan cement samples, 54–55% of the total hexavalent chromium was water soluble. The percentage of sparingly soluble and insoluble Cr (VI) was found to be 30% and 10% respectively. Although, the industries and researchers are more concerned about water soluble hexavalent chromium, sparingly and insoluble Cr (VI) are also toxic (29). Thus detection of all type of Cr (VI) in cement sample is necessary for the safety of workers.

4. CONCLUSIONSTOP

From the present study, we can conclude that variamine blue can be used as a reagent for detection of Cr (VI) in cement samples as the results obtained by using variamine blue method are in good agreement with standard DPC method. Concentration of hexavalent chromium by this method is found to be slighly less than standered DPC method. Because this method is free from interference caused by Cr (III). Thus the results are more accurate and reliable than the standered DPC method. Advantage of this method over standard method is the high colour stability of dye formed. Sequential extraction of hexavalent chromium was confirmed by IR and EDX spectroscopy. It has been observed that in Portland pozzolan cement samples (S1-S6), 54–55% of the total hexavalent chromium was water soluble, 30% sparingly soluble and 10% insoluble Cr (VI). Presence of sparingly and insoluble Cr (VI) cannot be ignored, as these forms can also be toxic if inhaled. Thus, the detection of total hexavalent chromium by a suitable method is necessary for the health of workers in the cement industries.

 

ACKNOWLEDGEMENTSTOP

The authors wish to thank Dr. N.K. Katyal, Ex In-charge “Analytical Lab Service” National Council for Cement and Building Materials, Ballabgarh (Haryana), India for providing the facilities for analysis of cement samples.

 

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SUPPLEMENTARY MATERIALTOP

Calibration data of proposed method (Varamine Blue method)

DR/2010 User Experiment Report: Hexavalent chromium

High Limit: 12 mg/L of Cr (VI)

Low Limit: 0.05 mg/L of Cr (VI)

Monochromator Wavelength: 556 nm

Table 7. Calibration data (Proposed method)
S.No Standard Concentration Absorbance (Optical density)
1 0.05 0.003
2 0.1 0.006
3 0.2 0.010
4 0.4 0.023
5 0.8 0.049
6 1 0.062

Figure 7. Calibration curve (proposed method).

 

Calibration data of Reference method (Diphenycarbazide) [given in table 8]

DR/2010 User Experiment Report: Hexavalent chromium

High Limit: 2.0 mg/L of Cr (VI)

Low Limit: 0.001 mg/L of Cr (VI)

Monochromator Wavelength: 540 nm

Table 8. Calibration data (Reference method)
S.No Standard Concentration Absorbance (Optical density)
1 0.05 0.010
2 0.1 0.041
3 0.2 0.090
4 0.4 0.220
5 0.6 0.290
6 0.8 0.422
7 1 0.520

Figure 8. Calibration curve (Reference method).

 



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