This document summarizes an investigation into the effects of palladium nanoparticles (PdNPs), pH, and temperature on the reduction of hexavalent chromium (CrVI) to its less toxic form CrIII. Key findings include: 1) The optimal conditions for the reduction reaction were pH 4 and 45°C temperature. 2) Reaction kinetics followed pseudo-first order behavior, indicating a linear decrease in CrVI over time. 3) PdNPs catalyzed the reaction, decreasing the activation energy by 20±10 kJ/mol and increasing the rate constant over an order of magnitude. 4) Under optimal conditions, a 26% reduction of CrVI was achieved.

1. Investigation of Effects of Palladium Nanoparticles, pH and Temperature on the Reduction Reaction of Hexavalent Chromium
Diana Cheng, Rachelle Manel, Grace Tuazon, Dr. Rajabi*
Department of Chemistry, Chem 141, California State University, Sacramento
Hexavalent Chromium (Cr(VI)) is one of the leading contaminants in many
hazardous waste sites and proven to be carcinogenic. Therefore, it is
necessary to reduce Cr(VI) to its less hazardous oxidation state, Cr(III). It
has been shown that formic acid is a strong reducing agent and can
undergo direct mineralization to CO2 without side reactions. By changing
the environment of the reaction (i.e. temperature and pH), it can affect the
rate of reaction. Furthermore, using Palladium nanoparticles (PdNPs) can
catalyze the reduction of Cr(VI). Investigation in the effects of pH,
temperature, and PdNPs were accomplished by monitoring the reduction of
Cr(VI) at 350 nm over a twenty minute time period on the UV-vis
spectrophotometer. An optimal environment for these reactions was found
to be at a temperature of 45oC and pH 4. By studying the reaction in
presence and absence of PdNPs, activation energies were determined to
be (30 +/- 10) kJ/mol and (50 +/- 20) kJ/mol, respectively. In comparison,
the catalytic efficiency of the overall reaction was estimated to be 140%.
Heavy metals are naturally occurring chemical elements that is known to be
harmful to living organism because they cannot be readily degraded and will
accumulate in the organism over time consequently interfering with biological
pathways. Cr(VI) is highly toxic and its presence in the environment is
primarily due to contamination from industrial processes. It is highly mobile in
water with the potential risk for drinking water contamination being a
significant public health issue. Therefore, the ability to effectively reduce
Cr(VI) to its less toxic oxidation state is of great importance. Cr(VI) can
reduce under acid soil conditions due to humic acid however the rate of
reduction is very slow. Catalyzing PdNPs can reduce Cr(VI) efficiently due to
its high surface to volume ratio for electron transferring.
HOOCH  CO2 + 2H+ + 2e- (1)
Electron Transfer to PdNPs
Electron transfer from PdNPs to Cr(VI)
2 Cr(VI) + 6 e-  2 Cr(III) (2)
-1.4000
-1.2000
-1.0000
-0.8000
-0.6000
-0.4000
-0.2000
0.0000
0 200 400 600 800 1000 1200
ln([Cr(VI)])(mM)
Time (seconds)
pH Study: ln[Cr(VI)] vs time
pH 2 pH 4 pH 5
-0.900
-0.800
-0.700
-0.600
-0.500
-0.400
-0.300
-0.200
-0.100
0.000
0 100 200 300 400 500 600 700 800 900 1000 1100 1200
ln[Cr(VI)](mM)
Time (s)
45 degrees 60 degrees 75 degrees
Temperature Study (with PdNPs): ln[Cr(VI)] vs time
-0.32
-0.31
-0.31
-0.30
-0.30
-0.29
0 200 400 600 800 1000 1200 1400
ln[Cr(VI)](mM)
Time (s)
45 degrees 60 degrees 75 degrees
Temperature Study (no PdNPs): ln[Cr(VI)] vs time
Time
(sec)
pH 2 pH 4 pH 5
A350 [Cr(VI)]
(mM)
A350 [Cr(VI)]
(mM)
A350 [Cr(VI)]
(mM)
0 1.031 0.665 0.870 0.561 1.102 0.711
300 0.999 0.645 0.763 0.492 1.092 0.705
600 0.937 0.605 0.653 0.421 1.055 0.681
900 0.871 0.562 0.474 0.306 1.008 0.65
1200 0.807 0.521 0.413 0.266 0.959 0.619
Time
(sec)
45⁰C 60⁰C 75⁰C
A350 [Cr(VI)]
(mM)
A350 [Cr(VI)]
(mM)
A350 [Cr(VI)]
(mM)
0 1.071
±0.100
0.691
±0.002
1.003
±0.197
0.647
±0.003
0.938
±0.29
6
0.605
±0.005
300 1.046
±0.129
0.675
±0.002
0.982
±0.212
0.634
±0.003
0.907
±0.32
9
0.585
±0.005
600 1.028
±0.146
0.663
±0.003
0.903
±0.314
0.583
±0.005
0.841
±0.40
4
0.542
±0.006
900 0.976
±0.203
0.629
±0.003
0.874
±0.326
0.564
±0.005
0.763
±0.48
9
0.492
±0.006
1200
0.924
±0.258
0.596
±0.004
0.788
±0.419
0.508
±0.006
0.688
±0.57
4
0.444
±0.007
Time
(sec)
45⁰C 60⁰C 75⁰C
A350 [Cr(VI)]
(mM)
A350 [Cr(VI)]
(mM)
A350 [Cr(VI)]
(mM)
0 1.157
±0.011
0.746
±0.015
1.147
±0.002
0.740
±0.003
1.146
±0.003
0.739
±0.004
300 1.153
±0.013
0.744
±0.017
1.145
±0.001
0.739
±0.001
1.145
±0.003
0.739
±0.004
600 1.152
±0.015
0.743
±0.020
1.142
±0.003
0.737
±0.004
1.143
±0.003
0.737
±0.004
900 1.148
±0.001
0.740
±0.001
1.142
±0.003
0.737
±0.004
1.140
±0.002
0.735
±0.003
1200 1.146
±0.002
0.739
±0.003
1.137
±0.009
0.734
±0.012
1.135
±0.005
0.733
±0.006
ResultsAbstract
Introduction
Experimental
Figure 2: Plot of ln(Cr(VI)) vs time of pH of 2, 4, and 5 study at temp. of 600C
Table 2: Absorbance values and [Cr(VI)] of pH of 2, 4, and 5
study at temperature of 600C
Figure 3: Plot of ln(Cr(VI)) vs time of temp. of 45, 60, and 750C study at pH = 4 with
PdNPs
Table 3: Absorbance values and [Cr(VI)] of temperature of 45,
60, and 75oC study at pH = 4 with PdNPs
Table 4: Absorbance values and [Cr(VI)] of temp. of 45, 60, and
75oC at pH = 4 without PdNPs
Figure 4: Plot of ln(Cr(VI)) vs time of temp. of 45, 60, and 750C study at pH = 4 with
PdNPs
Study Reaction
0.5 mM Cr(VI)
(mL)
0.5 mM Acetate
Buffer (mL)
56.12 mM Formic
Acid (mL)
0.2 mM PdNPs
(mL)
Temp. (°C)
Temp.
Study
1 30 7.3 of pH 4 1.8 0.9 45
2 30 7.3 of pH 4 1.8 0.9 60
3 30 7.3 of pH 4 1.8 0.9 75
pH
Study
1 30 7.3 of pH 2 1.8 0.9 60
2 30 7.3 of pH 4 1.8 0.9 60
3 30 7.3 of pH 5 1.8 0.9 60
PdNPs
Study
1 30 8.2 of pH 4 2.7 0.0 45
2 30 8.2 of pH 4 2.7 0.0 60
3 30 8.2 of pH 4 2.7 0.0 75
Results & Discussion
= (50 +/- 20) kJ/mol
= (30 +/- 10) kJ/mol
References
Acknowledgements
pH 5:
y = -0.0001x - 0.3257
R² = 0.9512
k = 1.2 ±0.2 x 10-4 mM*s-1
pH 2:
y = -0.0002x - 0.3905
R² = 0.9805
k = 2.1 ±0.2 x 10-4 mM*s-1
pH 4:
y = -0.0007x - 0.5384
R² = 0.9749
k = 6.6 ±0.5 x 10-4 mM*s-1
45⁰C:
y = -0.0005x - 0.097
R² = 0.933
k = 4.6 ±0.7 x 10-4 mM*s-1
60⁰C:
y = -0.0002x - 0.4168
R² = 0.9543
k = 2.0 ±0.3 x 10-4 mM*s-1
75⁰C:
y = -0.0003x - 0.4759
R² = 0.97
k = 2.6 ±0.3 x 10-4 mM*s-1
45⁰C:
y = -3.E-06x - 0.2933
R² = 0.9704
k = 7.6 ±0.8 x 10-6 mM*s-1
60⁰C:
y = -7E-06x - 0.3007
R² = 0.9445
k = 7.0 ±1.0 x 10-6 mM*s-1
75⁰C:
y = -8E-06x - 0.3011
R² = 0.9403
k = 7.6±0.1 x 10-6 mM*s-1
PdNPs Synthesis:
In a round-bottomed flask, 0.0300 g of palladium acetate was dissolved in 10 mL of
DMF and reacted with excess NaBH4; mixture was stirred at 25oC for 40 minutes to
generate PdNPs and then dispersed into colloids with H2SO4. The solution was
allowed to settle. The PdNPs were rinsed with H2O and filtered. The PdNPs were
rinsed with DMF and H2O to remove any traces of NaBH4. PdNPs were dried and
dissolved into acetate buffer (2mg/ml).
Pd(CH3CO2)2 + 2NaBH4 + 6H2O → Pd + 7H2 + 2B(OH)3 + 2CH3COONa (4)
Kinetics Protocol:
Each reaction consisted of 30 mL of 0.5 mM Cr(VI) solution, 1.8 mL of stock 56.12 mM
formic acid, and 7.3 mL of 0.5 mM acetate buffer. The Cr(VI) solution was made with
K2Cr2O7 and ultrapure H2O (0.5mM). The acetate buffer was prepared with NaO2C2H3
and ultrapure H2O (0.5mM); pH was achieved with glacial acetic acid. Water baths
were used to establish the varying temperature conditions.
For each study, three reactions were run according to the volumes in Table 1.Aliquots
were drawn every five minutes for 20 minutes. Reaction was stopped by immersing
solution in an ice bath. Shizmadu UV-2401 PC UV-vis spectrophotometer was used to
take absorbance values at 350 nm. All absorbance values were converted to [Cr(VI)]
by using Beer’s Law, A = εbc. The molar absorptivity, ε, was 1550 M-1cm-1.
Table 1: Outline of Procedure for all studies Done
• Using the 45⁰C and 60⁰C trials, the activation energy (Ea) was calculated
for the reaction with PdNPs catalyst (50 ± 20 kJ/mol) and without PdNPs
(30 ± 10 kJ/mol).
• The overall difference being 20 ± 10 kJ/mol .
• The activation energy was determined by equation 3
• The efficiency of this reaction was 140 % using PdNPs as catalyst.
Conclusion
We would like to thank CSU Sacramento for providing chemicals and
instrumentation for this project.
A special thanks to Dr. J. Houston for providing equipment needed to carry
out the synthesis of PdNPs.
(1) Lenntech B.V. “Heavy Metals”. The Netherlands. 2014.
http://www.lenntech.com/processes/heavy/heavy-metals/heavy-metals.htm
Agency for Toxic Substances and Disease Registry. “Enviromental Health and
Medical Education: Chromium.” 2008.
(2) US Environmental Protection Agency. “Chromium Compounds”. 2013.
http://www.epa.gov/ttn/atw/hlthef/chromium.html
http://www.atsdr.cdc.gov/csem/csem.asp?csem=10&po=4
(3) Omole et al. “Palladium nanoparticles for catalytic reduction of Cr(VI) using
formic acid.” Applied Catalysis B: Environmental. 2007. 76, 158-167.
Accessed on ScienceDirect.
Miyake, H. et. al., Formic Acid electrooxidation on Pd in acidic solutions
studied by surface enhanced infrared absorption spectroscopy, Phys. Chem.
Chem. Phys., 2008, 10, 3662-3669
• The optimal environment for the reduction of Cr(VI) using formic acid
was pH of 4 and temperature of 45oC.
• The order of reaction for Cr(VI) was pseudo-first order indicating a linear
decrease when plotted ln(Cr(VI)) as a function of time.
• Experimental results was in agreement with literature values for Cr(VI)
being first order.
• PdNPs played a large role in the catalysis the reaction of PdNPs by
increasing the rate constant by greater than a order of magnitude.
• The effects of PdNPs catalyzed decreased the activation energy by (20
+/- 10) kJ/mol.
• A 26 % reduction was achieved with optimal environments.
Figure 1: Electron transfer of
Formic acid to PdNPS
Figure 5: Reaction coordinate diagram for the activation energy of PdNPs
The activation energy can be determined
by using equation 3.
Ea = 𝑅 ∗
ln(
𝑘1
𝑘2
)
1
𝑇2
−
1
𝑇1
(3)