Bioaccumulation and Determination of Chromium at a Peat Moss-Modified

Carbon Paste Electrode

Bridges to the Future Summer Program 1999

EPCC/UTEP

Dr. Jorge Gardea-Torresdey

Dr. Jawad Mahmoud

Michelle Johnson

Abstract

 

A new technique for electrode modification with peat moss for bioaccumulation and subsequent voltrammetric determination of Chromium (III) is described. Peat moss is used as the surface modifier and the resulting electrode exhibits preferential uptake on Chromium (III) from dilute solutions. The preconcentration step is performed under an open-circuit and the voltrammetric response is evaluated with respect to time spent in the preconcentration step, pH, Chromium (III) concentrations and the electrode composition. The best conditions were a Sodium Acetate buffer of pH 3 and a 1 molar ionic strength. The modified electrode contained 20% peat moss. The electrode surface can be regenerated by immersion in a 0.1 M solution of Hydrochloric acid (HCl) for a time of 5 minutes. Convenient and rapid acid renewal allows use of a single modified electrode in multiple analytical determinations of the period of several weeks.

Introduction

 

Voltrammetric determinations may have their sensitivity and selectivity increased by the use of chemically modified electrodes. There are several ways that the modifiers may be incorporated into the carbon-paste electrodes. Some of the most popular ways for the modifier to be incorporated are absorption (1), covalent bonding (2), ion-exchange polymers (3), complexation (4), and direct incorporation of the modifier by mixing it with the graphite used to make the carbon-paste electrode (5).

Under controlled conditions, it has been shown that certain microorganisms have an affinity towards certain heavy metals. Green algae have been shown to accumulate Gold (III) and Copper (II) (6). Different species of mosses have shown an affinity for a variety of heavy metals under different conditions (7). Some the heavy metals that lichens attract are Copper (II) and Lead (II) (8). In our laboratory, the binding of Chromium (III) to peat moss has been explored to remove Cadmium contaminants from water.

Experimental Section

 

Chemically modified carbon paste electrodes (20% peat moss by mass) were prepared by hand-mixing 0.48 grams of graphite powder with 0.20 grams of peat moss and .32 grams of mineral oil. The peat moss had been previously dried, ground and sieved to particles of around 150 micrometers in diameter. The mineral oil was added to form a paste which is the basis of the modified electrode and which was then packed into the end of a glass tube (4 mm inner diameter) and its end was connected to a copper wire. Three 10-milliliter cells were used. The preconcentration cell contained the Chromium (III) solution. The measurement cell contained the supporting electrolyte. The cleaning cell contained 0.1 M HCl. The Ag/AgCl reference electrode and the platinum auxiliary electrode were placed in the measurement cell through the hole in the cell’s cover. Differential pulse voltrammograms were recorded with EG & G Princeton Applied Research A-264 Voltrammetric Analyzer. The data was recorded on an Allen Datagragh 900 series X-Y recorder. The experimental settings that were used were scan of 10 millivolts per second and 50 millivolts amplitude.

Measuring Procedure

 

The 20% peat moss electrode was rinsed with deionized water and placed in the preconcentration cell containing the 5 PPM Chromium (III) solution. The electrode was left in the preconcentration cell for variable lengths of time but with a constant stirring and using an open circuit. The modified electrode was the removed form the preconcentration cell, rinsed with deionized water and then placed in the measurement cell. This cell contained an acetate buffer solution with a specific pH. A constant negative potential was applied to reduce the Cadmium accumulated on the modified electrode’s surface. The potential was –1.00 V and was applied for 60 seconds. An oxidation scan was then performed by differential pulse votrammetry between –1.00 V and +.30 V. The voltammogram was recorded on an X-Y recorder and the modified electrode was then removed from the measurement cell. The modified electrode was rinsed with deionized water and placed in the cleaning cell, which contained 0.1 M HCl for 5 minutes. The time in the cleaning cell, eliminates any Chromium that many still be remaining on the modified electrodes surface. To verify that all the Chromium was removed, a second oxidation scan was performed in the measurement cell. The same modified electrode was used throughout this experiment.

Results and Conclusion

 

The intent of the following experiments were a modified surface that preconcentrates the Chromium ion. As a result, the peat moss was chosen to demonstrate the new preconcentration/voltammetric strategy. The preconcentration step is performed under open circuit conditions and the Chromium uptake response is evaluated with respect to pH, electrode composition, voltammetric waveform, Chromium concentrations and preconcentration times.

Experiments were performed to determine the pH dependence of Chromium (III) binding to the peat moss. The Chromium response increases rapidly upon increasing the pH from 2.0 to 3.0. The maximum Chromium response was reached at pH 3.0, which was used in all subsequent work. At pH values higher than 4.0, the Chromium response diminishes. These findings are illustrated in Figure 1.

Figure 2 illustrates the effect of the paste composition on the preconcentration/voltammetric response for Chromium (III). Carbon paste electrodes containing 5%, 10%, 15%, 20% and 25% peat moss were prepared (together with a mixture of graphite powder and mineral oil). The Chromium response increases up to 20% by weight of peat moss in the carbon paste matrix. This was expected due to the increased amount of binding capacity of the electrode. In percentages above 20%, the Chromium response was diminished due to the reduction of the electrode’s conductivity. The 20% peat moss modified electrode was used in all subsequent work.

Shown in Figure 3 are differential pulse voltammograms for solutions of increasing Chromium (III) concentrations from 2 PPM to 8 PPM after three minutes of preconcentration time. Observed between -.75 V and -.35 V is a well-defined Chromium peak when the potential was scanned between -1.00 V and +.30 V. The higher the concentration of Chromium (III), the more of the ion was accumulated on the surface of the peat moss modified electrode, thus causing the peak to be larger (compare Peak A to Peak D).

The relationship of the Chromium response at the peat moss modified electrode to the concentration of Chromium (III) used is illustrated in Figure 4. When using a preconcentration time of three minutes, the response is linear up to 8 PPM. Deviations from linearity are observed at higher concentrations due to the nature of preconcentration process (saturation of binding sites). Using shorter preconcentration times or unstirred solutions may extend the linear range.

The relationship between the Chromium peak and the preconcentration time is shown in Figure 5. Using a concentration of 5 PPM Chromium (III) solution, the time the peat moss modified electrode was left in the preconcentration cell was varied from two and a half minutes to fifteen minutes. As the time increases, the Chromium response rises rapidly at first and then begins to slow down. As the electrode becomes saturated at a time of 10 minutes, in the preconcentration cell, the Chromium response levels out and results in a line with no significant slope.

A precision trial was performed to determine the precision of the equipment and the methods used in this research. Figure 6 illustrates the results. A 4 PPM Chromium (III) solution was used with a 3 minute preconcentration time and a voltammogram was then run between –1.0 V and +.30 V. This was repeated 10 times. The resultant mean and standard deviation of the results was 75.8 +/-4.50.

The use of peat moss modified carbon paste electrodes offer an alternative for preconcentration/voltammetric measurements of Chromium (III). This approach possesses an advantage of high sensitivity and selectivity. There is research in the laboratory continuing with the goal of extending this method into different directions.

 

References

  1. W. J. Albery, M. J. Eddowes, H. A. O. Hill and A. B. Hillman, J. Am. Chem. Soc., 103 (1981) 3904.
  2. K. J. Stutts and R. M. Wightman, Anal. Chem., 54 (1983) 1576.
  3. G. Nagy, G. A. Gerhardt, A. K. Oke, M. E. Rice and R. N. Adams, J. Electroanal. Chem., 188 (1985) 1074
  4. S. V. Prabhu, R. P. Baldwin and L. Kryger, L. Anal. Chem., 59 (1986) 1790
  5. R. P. Baldwin, J. K. Christensen and L. Kryger Anal. Chem., 58 (1986) 1790
  6. J. Gardea-Torresdey, D. Darnall and J. Wang, Anal. Chem., 60 (1988) 72
  7. K. J. Williams and T. G. Thompson, Int. Rev. Gesamten Hydrobiol., 33 (1936) 271
  8. M. Connor, E. Dempsey, M. R. Smith and D. H. S. Richardson, Electroanalysis, 3 (1991) 331

Figure 1: Dependence of the Chromium (III) peak on the pH of the Sodium Acetate buffer solution.

Conditions : Preconcentration time of 7 minutes
15% Peat Moss (w/w) modified electrode
5 PPM Chromium (III) solution
Scan rate of 10 mV/sec
Initial Potential of –1.0V to final potential of +.30V

Figure 2: Effect of electrode paste composition on the Chromium (III) peak current.

Conditions : Preconcentration time of 7 minutes
Sodium Acetate buffer of pH 3
5 PPM Chromium (III) solution
Scan rate of 10 mV/sec
Initial Potential of –1.0V to final potential of +.30V
 

Figure 3: Voltammograms obtained for increasing Chromium (III) concentrations. (A) 2 PPM (B) 4 PPM (C) 6 PPM (D) 8 PPM

Conditions : Preconcentration time of 3 minutes
Sodium Acetate buffer of pH 3
20% Peat Moss (w/w) modified electrode
Scan rate of 10 mV/sec
Initial Potential of –1.0V to final potential of +.30V


Peak A = 2 PPM Chromium (III) solution
Peak B = 4 PPM Chromium (III) solution
Peak C = 6 PPM Chromium (III) solution
Peak D = 8 PPM Chromium (III) solution

Figure 4: Dependence of the peak current on increasing concentrations of Chromium (III) solutions.

Conditions : Preconcentration time of 3 minutes
Sodium Acetate buffer of pH 3
20% Peat Moss (w/w) modified electrode
Scan rate of 10 mV/sec
Initial Potential of –1.0V to final potential of +.30V<

Figure 5: Effect of preconcentration time on the Chromium (III) peak current.

Conditions : Sodium Acetate buffer of pH 3
20% Peat Moss (w/w) modified electrode
5 PPM Chromium (III) solution
Scan rate of 10 mV/sec
Initial Potential of –1.0V to final potential of +.30V

Figure 6: Determination of precision.

Conditions : Preconcentration time of 3 minutes
Sodium Acetate buffer of pH 3
20% Peat Moss (w/w) modified electrode
4 PPM Chromium (III) solution
Scan rate of 10 mV/sec
Initial Potential of –1.0V to final potential of +.30V
?

State Reports | UT System | Customer Service Statement | Site Feedback | Required Links
500 West University Avenue | El Paso, Texas 79968 | Dean's Office: (915) 747-5536 | Advising Office: (915) 747-8027