Optimization of the spectrophotometric determination of Aqueous Cyanide: Application on Samira (Niger) Gold Mine Groundwater Analysis

 

Hassane Adamou Hassane, Rabani Adamou*, Maman Maazou Ahmed, Alassane Abdoulaye

Faculté des Sciences et Techniques, Université Abdou Moumouni, Niamey/Niger.

*Corresponding Author E-mail: arabani@refer.ne

Cyanide is very toxic to humans, and its usage in gold mining potentially poses serious environmental threats. In this paper, by revising the well-known colorimetric method based on the chlorination of cyanide with chloramine-T and subsequent reactions with pyridine-barbituric acid reagent, we have developed a robust and sensitive UV-Visible spectrophotometric method for cyanide determination in aqueous media.

 

The optimization of some analytical parameters, like the duration of the colored complex formation (50 min), the amount of chloramine-T (1 mL) and the medium sodium hydroxide concentration (0.05 M), has allowed us to highly improve the stability and exaltation of the colored complex absorption signal. The obtained limits of detection (LOD) and quantification (LOQ) are respectively 0.2 and 0.7 µg L^-1. In addition to this sensitivity, our approach seems to be one of the most reliable methods for free cyanides determination in environmental matrices. The recovery rate in spiked solutions is more than 95%.

 

Application on thirteen (13) groundwater samples (drillings, piezometers and open pit mining waters), adjacent to and around the Samira (Niger) gold mine, has shown that the obtained cyanide rates are less than the World Health Organization (WHO) drinking-water quality guideline value (70 µg L^-1). Nevertheless, the observed values indicate a potential contamination of the ground water resources, which previously not contained this pollutant.

INTRODUCTION:

Cyanides are naturally found in environmental matrices (air, water, plants, microbes and fungi…). Observed compounds are cyanogen, hydrogen cyanide, simple cyanides, complex or metallocyanides and organic cyanides^1-3. The most predominant form is hydrogen cyanide (HCN) at concentrations in the range 1.5 – 1.7 x 10^-1ppb_v in air, with an average life time of 2.5 years³. In addition to natural cyanides, anthropogenic activities result in an important environmental contamination. Each year, approximately 2 million tons of cyanides are produced in the world, significantly for mining, chemical and pharmaceutical industriesneeds⁴. Elsewhere, plastic waste and other nitrogen-containing materials combustion releases a large quantity of hydrogen cyanide in the atmosphere³.

 

Cyanide compounds are highly toxics to humans and other species even in very small doses and many are rapid-acting poisons^3,5-10. Exposure to cyanides harms the brain and heart and can even cause coma and death^3,11. The effect severity depends on the mode of exposure and the form of cyanide. Ingestion or inhalation of a small amount respectively of cyanide salts or hydrogen cyanide can be deadly regardless. Indeed, cyanide produces toxic effects at levels of 500 µg L^-1 of blood, and death have occurred at levels of 3,000 µg L^{-1 12}.Cyanide environmental impacts are horrible. On January 2,000, when the Baia gold mine (Roumania) cyanides spill, Tisza and Danube rivers were polluted. Besides the ecological damage of their ecosystem and fauna (fish, birds and carnivores), the cyanides concentration in some places was 100 times more than the limit value for drinking water¹³.

 

According to the above health effects and environmental impacts, cyanides determination in environmental matrices requires special attention^3,13,14. Therefore, the goal of this work is to develop a robust, sensitive and cost-effective method for cyanides analysis in aqueous media. Different methods are available on the market for cyanides determination in environmental matrices. These methods are based on voltammetry¹⁵, potentiometry^15,16, titrimetry¹⁷, spectrophotometry^2,17-24, colorimetry¹⁵, fluorometry¹⁵ techniques and are often preceded by a time consuming distillation, chromatography or micro-diffusion pretreatment steps^25-29. Most of these methods are characterized by a lack of sensitivity, reproducibility and reagent instability^3,18. The most sensitive methods are time consuming and have a prohibitive price^{2,19-21,25,30}. Thus, in order to allow cyanide traces analysis in groundwater, we have revised the colorimetric method based on the cyanide chlorination by chloramine-T and subsequent colored complex reactions by optimizing some analytical parameters by using the robust and affordable UV-Visible spectrophotometer.

 

The obtained optimum conditions were used to establish a calibration curve which allowed cyanides contents determination of thirteen (13) groundwater samples of Samira (Niger) gold Mine, after ten (10) years of activities. In order to assess the reproducibility of the developed method, the recovery rates of different cyanides spiked solutions were measured.

 

EXPERIMENTATION:

Analytical reagents

Potassium cyanide, KCN was used as a standard. The stock solution (100 mg L^-1) was prepared by dissolving potassium cyanide in a sodium hydroxide solution (0.1 mol L^-1) prepared in distilled water. Working solutions were prepared by diluting the stock solution by the same sodium hydroxide solution. For samples cyanide chlorination (formation of CNCl), a solution of chloramine-T (10 g L^-1) is prepared in distilled water using chloramine-T trihydrate (CH₃C₆H₄SO₂NClNa. 3H₂O, 98%). The coloration solution was prepared by dissolving sodium hydroxide (1.75 g), barbutiric acid (4.2 g), pyridine-4-carboxylic acid (3.4 g) in 250 mL of distilled water. The medium pH is adjusted to 5.2 with a 1M sodium hydroxide solution. The obtained mixture was vigorously stirring for 1hour at 30 °C with a Velp Scientific (Italy) heating and magnetic stirrer. After that, it was filtered through a Whatman paper before usage. In order to follow the coloration reaction, an indicator solution of phenolphthalein (0.1%) was prepared in ethanol. Glacial acetic acid (20%) prepared in distilled water was used to decolorize the medium.

 

 

The different solution were covered with aluminum foil and conserved in a refrigerator at 4°C. All agents and chemicals were obtained from Merck and are at least analytical grade.

 

Samples collection preservation and storage

Groundwater samples were collected in polyethylene containers of 500 mL, covered in aluminum foil in order to avoid cyanides compounds photodecomposition. They were preserved by using a sodium hydroxide (pH>12) and conserved in a refrigerator at 4°C. Samples spiked with potassium cyanide were prepared at the Samira gold mine site. Spiked solutions were prepared, preserved and storage in the same conditions as the real environmental samples. However, neither the nature of the spiked solutions nor their water cyanides contents are notified to the researcher until the end of the laboratory work.

 

Real samples and spiked solution were collected, preserved and storage by the Samira gold mine groundwater sampling equips which is assisted by the gold mining group environmental supervisor and one responsible of our laboratory. The samples collection, preservation and storage procedures were performed according to the Canadian environment assessment protocols manual for water quality sampling³¹.

 

Apparatus

All the absorbance measurements were performed at room temperature with a double beam Thermo Evolution 300 spectrophotometer. A standard Hellma (Mulheim Germany) quartz cuvette (path-length ℓ= 1 cm) and micropipette Pipetman (5-50 µm or 10 – 100 µm, Gilson –France) were used. The analytical medium pH adjustment was done by a BASIC pH meter 20 Crison.

 

Principle of the method and analytical measurements

Principle of the method

The most important step of the method is the chlorination of cyanide by chloramine-T to form cyanogen chloride (CNCl), a relatively more stable cyanide compound. After that, formed CNCl will react with pyridine-barbituric acid to forma colored complex with a maximum absorbance at 598 nm. The obtained complex concentration is proportional to the medium cyanides content. The two main steps of the colored complex formation are described in figure 1^{32, 33}.

 

Figure 1-a : Cyanide chlorination with chloramine-T

 

Figure 1-b: Formation of the colored complex with pyridine-barbituric acid

 

 

Analytical measurements

To 20mLof a known standard working solution of cyanide into a volumetric flask of 50 mL, 20 mL of 0.1 M NaOH solution were added. After that, two drops of phenolphthalein (0.1%) were introduced in the medium. A pink coloration, characteristic of phenolphthalein color in alkaline medium, is observed. Under a smoothly magnetic stirring, 2 mL of the glacial acetic acid solution (20%) were required to decolorize the medium in order to obtain an appropriate pH for the chlorination reaction. Thereafter, in maintaining the magnetic stirring, 2mL of chloramine-T trihydrate were added in the decolorized solution. After only 2 minutes, 5 mL of the coloration reagent were added. Finally, the 50 mL flask is completed with distilled water. At the beginning, a red staining color was observed which progressed and stabilized finally to violet coloration.

 

After a time (t), the solution absorbance measurement is performed using the spectrophotometer Evolution 300. Absorption spectra were recorded over the range 450 to 750 nm against a blank. The colored complex spectra presented two peaks located at 525 and 598 nm. The last wavelength (598 nm) corresponding to the height peak of the intense and broad absorption band between 550 – 650 nm, was selected for the analytical signal investigations (Figure 2). The absorbance values for performing the calibration graph were acquired by measuring the absorption signal of a series of standard working solutions.

 

All the analytical measurements were carried out under the same conditions. Measurements were performed under incandescent light and working solutions of different concentrations were protected from solar ultraviolet radiations with aluminum foil in order to avoid their photodecomposition. Curves representation and statistical processing of data were carried out with the software ISIS Draw 2.4 and Microcal Origin 6.00.

 

Figure2: Absorption spectrum of the colored complex

 

RESULTS AND DISCUSSIONS:

Optimization of operating conditions

Optimization of each analytical parameter was performed independently. The initially used reagent rates were obtained from the official colorimetric reference method of environmental matrices cyanides determination with chloramines-T and subsequent pyridine-barbituric acid reactions². In parallel, alternating variable search during the optimization process was performed to found the appropriate concentration value for each reagent.

 

Optimization of the colored complex formation time

Optimization of the colored complex formation time (t) was performed with a standard concentration of cyanides 100 µg L^-1. Absorbance values were measured during 0 to 90 minutes period. The obtained absorption spectra were presented in figure 3.

 

Figure 3: Colored complex absorption spectra change in time

 

At the beginning (from 0 - 15 min), one observed, on the colored complex formation spectra (Figure 3), the presence of two broad absorption bands with their maximum peaks respectively at λ₁ = 525 nm and λ₂ = 598 nm. Here, the most important absorption is observed between 450 to 550 nm with a maximum peak at 525 nm. Over time, this peak is weakened and one assisted to a bathochromic effect accompanied with an important increase of the absorption intensity around 598 nm (hyperchromic effect). The observed changes were probably due to the presence of two complexes in the medium. The firstly formed complex has an extinction coefficient e₁ =2.418 10⁴ L mol^-1cm^-1with an absorption maximum located around 525 nm. This complex is responsible of the firstly red coloration observed in the medium. The second complex has an extinction coefficient e₂ =5.53810⁴ L mol^-1 cm^-1 and its absorption band extend from 550 to 650 nm with a maximum absorption peak at 598 nm. This complex is responsible of the observed violet coloration. It’s relatively more stable than the first formed complex which coloration disappeared along the time. Indeed, during the used period 0–90 min, the medium coloration progressed from red and stabilized to violet. The above results validated the choice of the wavelength λ₂ = 598 nm for the colored complex study after the optimized reaction time.

 

The representation of the complex absorption intensity versus time will give us more information on the optimized duration of the colored complex formation (Figure 4).

 

igure 4:Absorption of the colored complex in time at λ₂ = 598 nm

 

 

An increase in the absorbance intensity of the colored complex is observed in the used period 0 -90 minutes (Figure4). The absorption signal increased and reached a plateau after 50 minutes. Then, it remained almost constant until 90 minutes. Thus, approximately fifty (50) minutes was needed for the complete development of the violet coloration.

 

In many methods used worldwide, including the reference method for cyanides determination in environmental matrices², the optimal duration observed for the colored complex formation is eight (8) minutes. The present optimization study has shown us that this duration is insufficient for the complete formation of the colored complex. This is probably one of cyanides rate under estimation in environmental matrices. Indeed, a concentration of cyanide measured after 8 minutes represented only the 3/5 of the exact cyanides concentration present in the medium. Moreover, in figure 4, the colored complex absorbance signal at just eight (8) minutes is instable, that may partly explain many observed errors in different laboratories for a same cyanides sample. Hence, a small gap in the signal measurement time will result in a different absorbance value and a different cyanides rate of the sample. Therefore, additionally to the gain of sensitivity, the relative stability of the colored complex absorbance signal after 50 minutes, will improve the reliability of cyanides determination method. Hence, in this study duration of 50 minutes is observed for the colored complex formation before performing any absorbance signal measurement.

 

Optimization of medium chloramine-T content

Chloramine-T allows the formation of cyanogen chloride (CNCl), an important step in the medium cyanide ions mobilization for the formation of the colored complex. In the reference colorimetric method, 2 mL of chloramine-T were used for the chlorination step. In this study, in order to optimize the medium chloramine-T content, different volumes of chloramine-T trihydrate (V_Cl-T) ranged from 0 to 5 mL were used for a fixed concentration of cyanide [CN^-] =200 µg L^-1.

 

Otherwise, in order to reduce the complete analysis duration, all the samples series needed for the medium chloramine-T content optimization were prepared together. For each sample, the beginning time is noticed in order to firmly respect the 50 minutes needed for the complete formation of the colored complex. Before the first sample absorbance measurement, the base line is performed with the reference. The reference contained also the used cyanide rate (200 µg L^-1) and its preparation respects all the colored complex formation steps except absence of chloramine-T in the medium. The absorption spectra of the different absorbance measurements were shown in figure 5.

 

Figure 5: Absorption spectra of the medium chloramine-T content optimization

 

 

The absence of noticeable absorption at 598 nm in the reference sample spectrum (Figure 5, spectrum a) indicates the great importance of chloramine-T in the process of the colored complex formation. The remaining absorption spectra indicate that the addition of chloramine-T in the reaction medium results in the formation of the colored complex. However, the medium chloramine-T contents didn’t correlate with the absorbance signal increases. Indeed, the curve representing the measured absorption intensity versus added chloramine-T content shows an important fluctuation in the studied range (Figure 6).

 

Figure 6: Curve of chloramine-T volume optimization

 

 

In figure 6, the general aspect of the obtained curve seems to show an increase of the absorbance signal followed by a relative slight decrease respectively for the ranges 0 to 1 mL and 1 to 5 mL. However, a focus between 0.1 to 2 mL shows significant fluctuations of the colored complex absorbance signal with the medium chloramine-T content (Figure 6inset). In the used range, the important absorption intensities were obtained for 0.1, 0.25, 1 mL of chloramine-T trihydrate. Here, the absorption signal highly fluctuates for 0.1 and 0.25 mL of  chloramine-T while it’s relatively stable around 1 mL.

 

This investigation shows that a dosage of cyanides with chloramine-T content of 0.1 or 0.25 mL, despite the relative gain in sensitivity, will be a major source of analytical errors.  Indeed, a small uncertainty on the used volume will give different values for the medium cyanide concentrations. Therefore, according to the relative high concentration of the reagent and the potential uncertainty on the medium chloramine-T content, the volume 1 mL seems to be the more appropriate for the method precision improvement.

 

Traditionally, the used amount of chloramine-T in cyanides analysis is 2mL. At this concentration, the measured absorption intensity for the colored complex is weak and less stable compared to results observed for 1 mL. Thus, the use of 1 mL of chloramine-T in the reaction medium will give more accurate measurements and will improve the developed method sensitivity. Moreover, the chosen content (1 mL) will help to save 50% of used reagent at each measure.

 

Optimization of NaOH content in sample preservation

Real water samples collected for cyanides analysis were preserved in high alkaline medium, pH greater than 12. In practice, in order to avoid samples contamination three (3) or four (4) NaOH pellets were generally added. This practice would be a potential source of errors in the case that the medium NaOH content will interfere in the colored complex formation. For this reason, we have performed the colored complex formation with a standard solution of cyanide (100 µg L^-1) prepared in different concentrations of sodium hydroxide. The pH concerned in this study was ranged between 12 and 14. Obtained absorption spectra were shown in Figure 7.

 

Figure 7: Optimization of the medium NaOH concentration

 

 

In figure 7, it was observed that the preservation NaOH content greatly influenced the colored complex absorption intensity. Indeed, one assisted to an hyperchromic effect, when the NaOH content of the medium increases from 0.01 to 0.05 M (pH = 12 to 13). Beyond this value, a general hypochromic aspect is observed for pH = 13 to 14 (Figure 7 in set).

 

Therefore, it’s very important to know the used NaOH content for cyanide samples preservation. If note, a pH correction of the medium have to be conducted in the laboratory prior to the colored complex formation and analysis in order to exclude observed potential errors. Here, NaOH 0.05 M (pH=12.7) gives the relatively higher absorption intensity. This concentration was chosen in order to improve the sensitivity of our method.

 

Calibration curve and analysis of real samples

Calibration curve

The calibration curve is performed under the following optimal conditions obtained during the analytical parameters optimization studies:

·         The maximum absorption signal is measured at 598 nm,

·         The observed duration for the colored complex is 50 minutes,

·         The volume of chloramine-T used in the reaction medium is equal to 1mL,

• The sodium hydroxide concentration used in the reaction medium is 0.05M.

The working standard diluted solutions were used to perform the calibration curve. Serial dilutions between 0 to 250 µg L^-1 were prepared. The working solutions were scanned between 450 to 750 nm. All absorbance signal measurements were carried out in triplicate and the results were expressed as a mean values. The curves displaying the absorbance spectra versus cyanides concentration is shown in figure 8.

 

Figure 8: Spectra displaying the variation of the colored complex absorption signal versus cyanides concentration

 

In figure 8, it was found that, more the concentration of cyanide increased the colored complex absorbance signal was improved. The representation of the curve displaying the measured absorbance versus solutions cyanide concentration which represented the calibration curve A = f (C) is plotted in figure 9. In the studied cyanide concentration range (0-250 µg L^-1), the treatment of the data by linear regression analysis gives a satisfactory correlation (R²=0.99963).

 

Figure 9: Calibration curve

Analytical figures of merit of our method

This curve is used to determine the limits of detection (LOD) and quantification (LOQ), which are respectively given by the following formulas: LOD= 3 s and LOQ= 10 s, with S the standard deviation of 10 replica of a relatively low solution concentration of 1 µg L^-1 cyanides³⁷. After that, the absolute limit of detection (ALOD), calculating using 2.5 mL is determined. To assess the relevance of these two values, we have calculated the ratio of conformity (R') of the method which is expressed by the formula, , where  is the average calculate concentration value³⁴. The obtained ratio of conformity R' is 6. A value of R’ between 4 and 10 means that the used concentration is adequate for the LOD and LOQ determination.

 

The performances of our method are listed in Table1.

 

Table 1: Merit of our method

Parameters	Results
Absorption maximum (nm)a	598
Linearity range (µg L-1)b	0–250
Correlation coefficient (R2)c	0.99963
Recovery rate (%)d	>95
Limit of detection (µg L-1)e	0.2
Limit of quantification (µg L-1)f	0.7
ALOD (µg)g	0.5
Ratio of conformityh	6
Precisioni	99,6% (intra-day), 98,2% inter-day

 

^aWavelength chosen of the analysis, ^bRange linear dynamic concentration, ^cCorrelation coefficient, ^dRecovery rate, ^eThe limit of detection of the method, ^fThe limit of quantification of the method, ^gAbsolute limite of detection, ^hRatio of conformity, ⁱprecision of intra-day analysis (n=6) and inter-days (one week).

 

The obtained method with a recovery rate superior to 95% and a very weak relative standard deviation (% RSD = ±0.5 %, n = 6) has a very significant accuracy. The intra-day and inter-days analysis results comparison have shown that the method precision is excellent (> 98%). The observed limits of detection (LOD) and quantification  (LOQ) which are respectively 0.2 and 0.7 µg L^-1 indicate the very high sensitivity of the method, more than others colorimetric methods^19,20. Therefore, the revised UV-Visible spectrophotometric method will be suitable for cyanides traces analysis in aqueous media.

 

Application

The developed method was applied to determine cyanides content in groundwater samples of Samira (Niger) gold mine. Investigations were focused on the area around industrial process residues tailings. The groundwater sampling concerned three (D₁, D₂, D₃) drillings which are used as drinking water supply for the mine and surrounding villages, five (P₁, P₂, P₃, P₄, P₅) piezometers developed to control the industrial process residues tailing impacts on the groundwater and five (Pt₁, Pt₂, Pt₃, Pt₄, Pt₅) open pit mining waters issued for the mineral extraction and used as industrial water. All these installations are adjacent to and around the gold mine.

 

In order to assess the accuracy of the developed method, two (2) kind of spiked samples were analyzed. The cyanides concentrations obtained for the control samples, RV= 0 µg L^-1 and RF= 2.101 mg L^-1. The obtained results are almost identical to the used spiked concentrations. Indeed, the two solutions were prepared with a commercial drinking water called Rarhous. The RV sample (Rharous Virgo) not contained cyanides and the second sample RF (Fortified Rharous) contained       2 mg L^-1 of  cyanide issued from potassium cyanide pellets.

 

Obtained absorbance intensities for real samples were used in the calibration curve to determine their cyanides concentrations. The calculated cyanide contents were shown in Table 2.

 

Table 2: Concentrations given in free cyanides samples

Ground water source		Absorbance	Measured cyanide (µg L-1)
Piezometers	P1 (FCD-P1)	0,053	6,8
	P2 (FCD-P2)	0,047	2,6
	P3 (FCD-P3)	0,048	3,3
	P4 (FCD-P4)	0,041	0
	P5 (FCD-P5)	0,043	0
Pits	Pt1 (MLNW)	0,042	0
	Pt2 (MLB)	0,044	0,5
	Pt3 (MBD)	0,044	0,5
	Pt4 (MSM)	0,044	0,5
	Pt5 (LP)	0,047	2,6
Drillings	D1 (FS)	0,04	0
	D2 (Tw02)	0,044	0,5
	D3 (Tw05)	0,042	0

 

 

Before the mining activities Samira region groundwater sources were free of cyanides. Therefore, cyanide traces detected in the analyzed ground water would come from interactions between the gold mining activities and the ground water resources. However, the maximum content cyanide observed (6.8 µg L^-1) is lower than the limit values in drinking water allowed in the matter by the World Health Organization (WHO)  standards             (70 µg L^-1)³⁵ and the European Union (EU) (50 µgL^-1)³⁶.

 

CONCLUSION:

The use of UV-Visible spectrophotometric instrument and the optimization of the main analytical parameters allowed us to overcome many short comings regarding the instability and inhibition of the absorbance signal of the colored complex observed in current methods used for cyanides analysis in environmental matrices. The developed method is robust, cost-effective and covers a large range of concentrations.

The method analytical figures of merit show that it could be a simple and cheaper alternative to gain sensitivity and reliability, which traditionally requires a heavy and expensive instrumentation. The application of our method on spiked solutions and real gold mining groundwater gives satisfactory results.

 

ACKNOWLEDGMENTS:

This research was founded trough a SOPAMIN (Société de Patrimoine des Mines du Niger) research grant.

 

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35.     World Health Organization. Guidelines for drinking-water quality. 3rd ed. 2008.http://www.who.int/water_sanitation_health/dwq/fulltext.pdf

36.     Official Journal of the European Communities.; Council Directive 98/83/EC. 1988, p L330/42

https://www.fsai.ie/uploadedfiles/legislation/food_legisation_links/water/council_directive_98_83_ec.pdf

(accessed 25/06/2015)

 

 

 

 

Received on 07.07.2015         Modified on 20.07.2015

Accepted on 28.07.2015         © AJRC All right reserved

Asian J. Research Chem. 8(7): July- 2015 ; Page 481-492