phytoremediation of iron cyanide complexes in soil and groundwater

PURDUE UNIVERSITY
GRADUATE SCHOOL
Thesis Acceptance
This is to certify that the thesis prepared
By
Entitled
Complies with University regulations and meets the standards of the Graduate School for originality
and quality
For the degree of
Final examining committee members
, Chair
Approved by Major Professor(s):
Approved by Head of Graduate Program:
Date of Graduate Program Head's Approval:
Dong-Hee Kang
Phytoremediation of Iron Cyanide Complexes in Soil and Groundwater.
Doctor of Philosophy
M. K. Banks
C. Johnston
R.S. Govindaraju
P. Schwab
31 July 2006
M. K. Banks
Darcy Bullock
UMI Number: 3239788
3239788
2007
UMI Microform
Copyright
All rights reserved. This microform edition is protected against
unauthorized copying under Title 17, United States Code.

ProQuest Information and Learning Company
300 North Zeeb Road
P.O. Box 1346
Ann Arbor, MI 48106-1346
by ProQuest Information and Learning Company.

i
PHYTOREMEDIATION OF IRON CYANIDE COMPLEXES IN SOIL AND
GROUNDWATER
A Dissertation
Submitted to the Faculty
of
Purdue University
by
Dong-Hee Kang
In Partial Fulfillment of the
Requirements for the Degree
of
Doctor of Philosophy
August 2006
Purdue University
West Lafayette, Indiana
ii
ACKNOWLEDGMENTS
There are many people who I would like to thank for their contribution to this
dissertation. First and foremost, I would like to thank my advisor, Professor Katherine
Banks of the School of Civil Engineering for her continuous support of my Ph.D
program. Professor Banks always gave me clarity when I had a question about my
research or writing. She consistently allowed this dissertation to be my own work and
guided me in the right direction. She showed me different ways to approach a research

problem and the need to be persistent to accomplish any goal. She is most responsible for
assisting me with the writing of this dissertation as well as directing me towards a
challenging research project. Without her encouragement and constant guidance, I could
not have finished this dissertation. I sincerely want to thank Professor Paul Schwab of the
Department of Agronomy for his assistance with the statistical analyses and analytical
methods development. He always made me comfortable and provided truthful advice
about my future when I stopped by his office. Thanks also to Professor Rao S.
Govindaraju of the School of Civil Engineering for asking insightful questions to help me
think through my research direction. I wish to thank Professor Cliff Johnston of the
Department of Agronomy who offered guidance and research direction on the adsorption
experimental design. In addition, I would like to thank Professor James Alleman, Chair of
iii
the Department of Civil, Construction, and Environmental Engineering at Iowa State
University for instruction on the toxicity assays.
I would like to acknowledge my colleagues from the Banks research group for
their assistance and support during my doctoral work. I would especially like to thank
James Hunter, who gave me confidence when I doubted myself and helped me develop
innovative research ideas. (More importantly, he taught me to how to play hard and
control stress!) I also would like to thank my fantastic coworker, Lee-Yen Hong, for her
friendship, encouragement, and research discussions. Also, many thanks to my other
friends at Purdue University for their support: Yong Sang Kim, Sybil Sharvelle, Agnes
Szlezak, Eric McLamore, and Jason Hickey. I also appreciate the help provided by Dr.
Changhe Xiao with his patient explanations during instrument repair.
Special recognition goes to Dr. Mi-Youn Ahn, who provided insightful
comments and reviewed my work on very short notice. Also, thanks to Dr. Andrew R.
Zimmerman (Assistant Professor of Geological Sciences at the University of Florida),
who gave me useful information about oxidation and enzyme activity. I want to thank
Professor Won Chul Cho of the Department of Civil Engineering at Chung-Ang
University (Seoul, Korea) who was my MS advisor for his continuous support. Also, I
appreciate the assistance of Dr. David Tsao (BP Corporation, IL) and Dr. Wang-Cahill

Fan for guidance on the design of greenhouse study and financial support of the project.
Finally, I would like to thank my lovely wife and best friend, Hye Jeong Lee, for
her patience and for keeping my life in proper perspective and balance, and my parents
for their endless encouragement and constant support.
iv
TABLE OF CONTENTS
Page
LIST OF TABLES vii
LIST OF FIGURES ix
LIST OF ABBREVIATIONS xi
ABSTRACT xii
CHAPTER 1. INTRODUCTION 1
CHAPTER 2. LITERATURE REVIEW 4
2.1. Manufactured Gas Plants Sites 4
2.2. Classification of Cyanide Compounds 6
2.3. Cyanide Toxicity 7
2.4. Solubility of Iron Cyanide Complexes 8
2.5. Fate and Transport of Cyanide 9
2.6. Microbial Degradation of Cyanide 11
2.7. Phytoremediation of Cyanide Contaminants 13
2.8. Modeling of Phytoremediation Processes 17
2.9. References 19
CHAPTER 3. DISSERTATION OBJECTIVES AND HYPOTHESES 33
CHAPTER 4. SELECTION OF PLANT VARIETIES FOR
PHYTOREMEDIATION OF IRON CYANIDE COMPLEXES 35
4.1. Introduction 36
4.2. Materials and Methods 39
4.2.1. Soil Preparation 39
4.2.2. Plant Species 39
4.2.3. Germination Assay 40

4.2.4. Root Characteristics 41
4.2.5. Statistical Analysis 41
4.3. Results and Discussion 42
v
Page
4.3.1. Germination. 42
4.3.2. Root Characteristics 44
4.4. Conclusions 45
4.5. References 47
CHAPTER 5. PHYTOREMEDIATION OF IRON CYANIDE COMPLEXES
USING CYANOGENIC AND NON-CYANOGENIC PLANT SPECIES 56
5.1. Introduction 57
5.2. Materials and Methods 59
5.2.1. Soil Preparation 59
5.2.2. Plant Selection 60
5.2.3. Greenhouse Methods 60
5.2.4. Analysis of Cyanide 61
5.2.5. Toxicity Assay 62
5.2.6. Statistical Methods 63
5.3. Results and Discussion 64
5.4. Conclusions 68
5.5. References 70
CHAPTER 6. SORPTION OF IRON CYANIDE COMPLEXES ONTO CLAY
MINERALS, MANGANESE OXIDES, AND SOIL 80
6.1. Introduction 81
6.2. Materials and Methods 83
6.2.1. Soil Preparation 83
6.2.2. Clay Mineral Preparation 83
6.2.3. Manganese Oxide Synthesis 84
6.2.4. Adsorption 84

6.2.5. Cyanide Analysis 85
6.2.6. Acid Extraction 85
6.2.7. CEC and AEC 86
6.3. Results and Discussion 87
6.4. Conclusions 90
6.5. References 92
CHAPTER 7. THE ROLE OF TRAMETES VILLOSA LACCASE
ON OXIDATION AND ADSORPTION OF FERROCYANIDE 100
7.1. Introduction 101
7.2. Materials and Methods 103
7.2.1. Materials 103
7.2.2. Enzyme Reactions 104
7.2.3. Adsorption Assessment 104
vi
Page
7.2.4. Cyanide Analysis 105
7.2.5. Laccase Analysis 106
7.3. Results and Discussion 106
7.4. Conclusions 110
7.5. References 111
CHAPTER 8. EFFECT OF PLANTS ON LANDFILL LEACHATE
CONTAINING CYANIDE AND FLUORIDE 120
8.1. Introduction 121
8.2. Description of Field Site 125
8.3. Materials and Methods 128
8.3.1. Plant Selection 128
8.3.2. Soil and Leachate Analysis 128
8.3.3. Fluoride and Cyanide Adsorption 129
8.3.4. Greenhouse Study 130
8.3.5. Cyanide Analysis for Soil and Plant Biomass 130

8.3.6. Fluoride Analysis for Soil and Plant Biomass 131
8.3.7. Assessment of Root Characteristics 132
8.3.8. Statistical Analysis 133
8.4. Results and Discussion 133
8.4.1. Leachate Toxicity 133
8.4.2. Root Characteristic 137
8.4.3. Fluoride and Cyanide Adsorption 138
8.4.4. Soil pH 138
8.4.5. Fluoride Concentration 140
8.4.6. Cyanide Concentration 141
8.5. Conclusions 142
8.6. References 144
CHAPTER 9. CONCLUSIONS AND FUTURE RESEARCH 165
9.1. Conclusions 165
9.2. Future Research 167
APPENDIX 169
VITA 196
vii
LIST OF TABLES
Table Page
Table 2.1 Total Cyanide Concentrations in Contaminated
Soil and Groundwater 25
Table 2.2 Environmental Cyanide Compounds 26
Table 2.3 Potential Risks from Daily Exposure to Cyanide Compounds 27
Table 2.4 Equilibrium Reactions and Constants (log K
o
) 28
Table 2.5 Adsorption of Iron Cyanide Complexes 29
Table 2.6 Plants Used in Phytoremediation Applications 30
Table 2.7 Phytoremediation of Cyanide 31

Table 4.1 Soil Characteristics 50
Table 4.2 Plant Varieties 51
Table 4.3 Germination Assays 52
Table 4.4 Root Characteristics of Surface Area, Average Diameter, and Tips 53
Table 5.1 EC
50
(%) and Cyanide Concentration of Leachate 74
Table 5.2 EC
50
(%) of Soil Samples 75
Table 5.3 Overall Mass Balance of Cyanide after 4 Months (%) 76
Table 6.1 Acid Extractable Aluminum, Calcium, Iron,
Magnesium, and Manganese 95
Table 6.2 Freudlich Isotherm Parameters as a Function of Sorbents 96
Table 7.1 Freundlich Adsorption Isotherm Parameters 114
viii
Table Page
Table 8.1 Chemical Characteristics for Soils Surrounding the Landfill Site 151
Table 8.2 Sebree Landfill Leachate Composition 152
Table 8.3 Cyanide and Fluoride Concentrations in Plants 153
Table 8.4 Initial Cyanide and Fluoride Concentrations in Plant Samples
Before Initiation of Greenhouse Study 154
Table 8.5 Cyanide and Fluoride Concentrations in Leachate and Soil for
Greenhouse Study 155
Table 8.6 Plant Height as Affected by Landfill Leachate 156
ix
LIST OF FIGURES
Figure Page
Figure 2.1 Enzymatic Reactions Converting Free Cyanide into Asparagines 32
Figure 4.1 Germination of Sorghum, Switchgrass, and Flax after 7 Days

Exposure to Control, 500 mg/kg, and 1000 mg /kg Total Cyanide 54
Figure 4.2 Root Growth of Sorghum, Switchgrass, and Flax after 4 Weeks
Exposure to Control and 1000 mg /kg Total Cyanide 55
Figure 5.1 Cyanide Concentrations in Plant Biomass and Soil Samples
after 4 Months Exposure to 1000 mg /kg Total Cyanide A) Cyanide
Concentrations in Roots B) Cyanide Concentrations in Plants C)
Cyanide Concentrations in Soil Samples 77
Figure 5.2 Water Soluble Cyanide Concentrations in Soil after 4 Months
of Exposure to 1000 mg/kg Total Cyanide A) Water Soluble Cyanide
Concentrations in Soil Samples B) Average Fraction of Water Soluble Cyanide 78
Figure 5.3 Cyanide Concentrations in Soil after 4 Months of Exposure
to 1000 mg/kg A) Weak Acid Dissociable Cyanide Concentrations in Soil
B) Average Fraction of Weak Acid Dissociable Cyanide 79
Figure 6.1 Freundlich Isotherms for Clay Minerals and ACE and CEC Curve 97
Figure 6.2 Freundlich Isotherms for Prussian Blue on Manganese Oxides
as Affected by pH 98
Figure 6.3 Freundlich Isotherms for Prussian Blue on Drummer Soil
as Affected by pH 99
Figure 7.1 A) Disappearance of Ferrocyanide in the Presence of T. Versicolor
Laccase as Influenced by Reaction Time B) pH Effect on Laccase Mediated
Disappearance of Ferrocyanide C) Effect of T. Versicolor Laccase Activity
and Initial Concentration of Ferrocyanide on Oxidation 115
x
Figure Page
Figure 7.2 Adsorption Isotherms of Iron Cyanide on Aluminum
Hydroxide at pH 3.7, 6.2 and 8.4 as Affected by Laccase 116
Figure 7.3 Adsorption Isotherms of Iron Cyanide on Montmorillonite
at pH 3.7, 6.2 and 8.4 as Affected by Laccase 117
Figure 7.4 Adsorption Isotherms of Ferricyanide and Ferrocyanide on
A) Aluminum Hydroxide or B) Montmorillonite at pH 3.7, 6.2

and 8.4 as Affected by Laccase 118
Figure 7.5 Laccase Adsorption on Aluminum Hydroxide or Montmorillonite
at pH 3.7, 6.2 and 8.4 119
Figure 8.1 Sebree Landfill Site and Tree Establishment Areas 157
Figure 8.2 Cyanide Concentrations as a Function of Fluoride Concentration
in Solution After Exposure to Soil at Two pH Levels 158
Figure 8.3 Fluoride Concentrations as a Function of Cyanide Concentration
in Solution After Exposure to Soil at Two pH Levels 159
Figure 8.4 Daily Irrigation Rate per Species 160
Figure 8.5 Root Characteristics after Exposure to Landfill Leachate 161
Figure 8.6 Soil pH vs. Leachate Concentration 162
Figure 8.7 Fluoride Concentrations in Soil and Plant Biomass A) Soluble
Fluoride Concentration in Soil B) Fluoride Concentration in Roots
C) Fluoride Concentration in Stems D) Fluoride Concentration in Leaves 163
Figure 8.8 Cyanide Concentrations in Soil and Plant Biomass A) Cyanide
Concentration in Soil B) Cyanide Concentration in Roots C) Cyanide
Concentration in Stems D) Cyanide Concentration in Leaves 164
xi
LIST OF ABBREVIATIONS
ANOVA: analysis of variance
AOAC: Association of Official Analytical Chemists
APHA: American Public Health Association
ATP: adenosine triphosphate
AWWA: American Water Works Association
BET: surface analysis method (Brunauer, Emmett and Teller, 1938)
EC
50
: concentration of a compound when mortality is observed
CDTA: cyclohexylene diamine tetraacetic acid
MGP: manufactured gas plant

NRT: normalized relative transpiration
PZC: Isoelectric point (pI) is the pH at which a molecule carries no net electrical charge
(Point Zero Charge)
RCRA: Resource Conservation Recovery Act
RSG: relative seed germination
SPL: spent potliner material
SPT: microtox solid phase test
STD: standard deviation
TISAB: total ionic strength adjustment buffer
WAD: weak acid dissociable cyanide
xii
ABSTRACT
Dong-Hee Kang, Ph.D. Purdue University, August, 2006. Phytoremediation of Iron
Cyanide Complexes in Soil and Groundwater. Major Professor: M. Katherine Banks.
High concentrations of cyanide in soil can result from contamination by road
salt, electroplating waste, and residuals from manufactured gas plant sites. The most
toxic species is “free” cyanide (CN
_
, HCN), but this form is generally rare in
contaminated soil and groundwater. Iron cyanides are often predominant in
environmental samples and have low toxicity. Unfortunately, free cyanides are the
thermodynamically favorable species in solution, and degradation of iron cyanide
compounds to the free cyanides can be accelerated by sunlight and microorganisms.
There were two objectives of this research project. The first objective was to
investigate the potential for phytoremediation of cyanide contaminated soils using
cyanogenic plants. The second objective was to assess the fate and transport of cyanide
compounds in vegetated soil. The results indicate that germination and root growth for
cyanogenic plants were higher than for the non-cyanogenic plant in the presence of
cyanide. In addition, root biomass had higher cyanide concentrations than plant shoots.
After 4 months of plant growth, soil cyanide concentration was reduced approximately

17~32%. The mineral sorption capacity for cyanide was greatest for clay at low pH.
xiii
Acid extractable elements also enhanced the adsorption capacity of the clays.
Manganese oxide and laccase enhanced oxidation of ferrocyanide to ferricyanide,
resulting in a more mobile contaminant. In addition, the use of phytoremediation to
reduce landfill leachate volume, and cyanide and fluoride concentrations in groundwater
was assessed. Cyanide was degraded by the plants while fluoride accumulated in plant
biomass. The results reported in this dissertation can be used in the design of
phytoremediation projects for cyanide impacted soil and groundwater.
1
CHAPTER 1.
INTRODUCTION
Cyanide is a deadly poison that can result in human respiratory failure. The
most toxic chemical species of cyanide are HCN
(g)
and CN
-
,
or the “free cyanides”.
Cyanide generally refers to all compounds containing the –CN group which has a triple
bond between carbon and nitrogen. This compound tends to react readily with many
chemical elements, subsequently producing a wide variety of cyanide complexes. Iron
cyanide complexes are common cyanide contaminants.
Cyanide pollutants may be harmful to humans as a result of surface runoff and
movement through soil into potable groundwater. Soil contaminated with cyanide
compounds may result from a variety of industrial and municipal activities. Sodium
ferrocyanide [Na
4
Fe
II

(CN)
6
)] and potassium ferrocyanide [K
4
Fe
II
(CN)
6
)] are used in road
salts as anti-clumping agents. Alkaline cyanide solutions are used in heap leaching of
gold mining ores. Soils contaminated with cyanide complexes are often located near
former manufactured gas plant (MGP) sites.
Manufactured gas was produced during the period between 1830 and 1950. The
production facilities, or manufactured gas plants (MGPs), were used to provide gas for
municipal lighting, heating, and residential use. The last operational plant was closed in
the 1950s. At former gas works facilities, the process of degassing coal to produce
2
natural gas produced H
2
S and HCN at concentrations ranging from 500 to 1000 mL/m
3
(Kjeldsen, 1999). These contaminants were scrubbed from the gas using a purification
box containing bog iron ore. The spent ore with a pH of 2 to 5 and 1 to 2% cyanide
(Young and Theis, 1991) was used as fill material in the surrounding area. The form of
cyanide found at these sites is predominantly iron cyanides, such as Prussian blue
[Fe
III
4
(Fe
II

(CN)
6
)
3
].
Considerable attention recently has focused on cyanide near manufactured gas
sites because of high concentrations in the soil and detection in groundwater (Kiikerich
and Avin, 1993; Meeussen et al., 1994; Ghohs et al., 1999). Thermodynamic calculations
indicate that free cyanide should predominate the chemical equilibrium in soil and that
Prussian blue should govern solubility (Meeussen et al., 1995). However, complexed
cyanide is often found in groundwater, indicating that the speciation of cyanide is
determined not by chemical equilibrium but by decomposition kinetics (Meeussen et al.,
1992). Iron cyanide complexes have low toxicity even at high levels of exposure. The
rate of conversation of ferrocyanide and ferricyanide complexes proceeds slowly in the
dark but is greatly enhanced in the presence of low levels of light. Temperature and, to
a lesser degree, pH also can impact degradation rates. Therefore, iron cyanide
complexes cannot be regarded as completely inert (Meeussen et al., 1994).
Phytoremediation is an innovative technology and a cost effective remediation
method that utilizes plants to remove contaminants from soil and water. The extraction
and accumulation of contaminants in harvestable plant biomass, degradation of complex
organic molecules to simple molecules and the incorporation of these molecules into
3
plant tissue, and stimulation of microbial and fungal degradation by release of
exudates/enzymes into the root zone are all proven mechanisms of dissipation.
Production of cyanide is not limited to anthropogenic activities; a number of
plant species can produce this toxin as well. This is of particular problem for food or
forage crops that, under storage conditions, produce acutely toxic concentrations of free
cyanide as a result of the degradation of cyanogenic glucosides. These species of plants
have the capability to produce cyanide, and they also possess the capacity to detoxify it; a
detoxification capacity that is interesting from the perspective of phytoremediation.

Although phytoremediation is a popular remediation approach, there are limitations such
as extensive time compared to other remediation methods, difficulty in establishing
vegetation, phytotoxicity due to hazardous wastes, and limited remediation depth. A
major concern about phytoremediation is the lack of understanding of the mechanisms of
plant-chemical interactions.
There are two objectives of this dissertation research project. The first
objective is to investigate the potential for phytoremediation of cyanide contaminated
soils using cyanogenic plants. Cyanogenic plants are those species that synthesize
cyanogenic glucosides, compounds that readily decompose to cyanide when plant tissue
is injured. Because cyanide is a natural component of these plants, they may possess
enhanced capacities for degrading cyanide. The second objective is to evaluate the fate
and transport of cyanide compounds in planted soil.
4
CHAPTER 2.
LITERATURE REVIEW
2.1. Manufactured Gas Plant Sites
Manufactured gas plants were constructed to provide cost-effective sources of
energy for homes and industries from the mid-19th Century until the 1950s. The
manufacture of coal gas produced many toxic by-products. Some of these wastes could
be sold after further processing, others were recycled on-site, and the remaining residual
typically disposed of either on-site or at the local trash dump (ERL, 1987). Therefore,
significant contamination of soils and groundwater has been reported at gasworks and
disposal sites. The production of gas from coal was based on the work of Robert Boyle
(1691) and William Murdoch (1796). The first gasworks company, the Gas Light and
Coke Company, was established in 1812, and supplied gas for light and heat for a large
city (Turczynowicz, 1993). Gas usage was quickly adopted globally. After 1812, over
1037 separate gas companies operated in the United Kingdom, each with at least one
operational site resulting in approximately 3000 to 4000 sites within the UK alone (ERL,
1987; Turczynowicz, 1993).
The first use of manufactured gas in the United States was in Newport, Rhode

Island in 1806. The city of Baltimore developed the first gas compamy in 1817. Across
5
the United States of America, from 1100 to 3000 gaswork sites operated between 1806 to
the mid 1970s (Shifrin et al., 1996).
The coal gasification process generated a number of by-products. Highly
contaminated solid waste was produced from the final stage of gas purification, where the
gas was filtered to remove cyanide and hydrogen sulfide. Two possible purifier
reactions resulting in the formation of ferrous cyanide [Fe(CN)
2
] were:
Fe(OH)
2
+ 2HCN Fe(CN)
2
+ 2H
2
O (2.1)
FeS + 2HCN Fe(CN)
2
+ H
2
S (2.2)
Cyanides in the raw gas stream also may have been present as ammonium cyanide,
NH
4
CN. Ferrous and ammonium cyanides can react to form complex ammonium
ferrocyanide compounds. Ammonium ferrocyanide is soluble in water and readily
dissociates to yield ferrocyanide ions, [Fe(CN)
4+
6

]. In the presence of excess iron,
ferrocyanide ions can react to form thermodynamically stable ferric ferrocyanide,
Fe
4
[Fe(CN)
6
]
3
, also known as Prussian blue (Shifrin et al., 1996):
4Fe
3
+
+ 3Fe(CN)
4+
6
Fe
4
[Fe(CN)
6
]
3
(2.3)
Prussian blue is commonly used as a blue pigment in dyes. The extensive
underground infrastructure of a gas manufacturing plant, comprising of tanks, pipes and
foundations for gasholders, also can be considered solid waste. Elevated cyanide
6
concentrations near manufactured gas sites have been detected (Table 2.1). With a
drinking water standard of 0.2 mg/L for free cyanide, nearly all of the reported values
shown in Table 2.1 would violate the US EPA drinking water regulations if identified in
the U.S. Another important observation from the data in Table 2.1 is that there is very

little relationship between soil and groundwater concentrations. This strongly indicates
that cyanide concentrations in water are controlled by dissolution/precipitation of solid
phases.
2.2. Classification of Cyanide Compounds
All compounds contained in the cyanide group CN
-
are subdivided into weak acid
dissociable and strong complexes, as shown in Table 2.2. This classification includes
free and strong metal complexed cyanides. Free cyanides, hydrogen cyanide (HCN
(g,
aq)
) and cyanide ion (CN
-
(aq)
), are defined as the forms of molecular and ionic cyanide
released into solution by the dissolution and dissociation of cyanide compounds and
complexes (Smith et al., 1991). In addition to WAD cyanide, other toxicologically
important forms of cyanide are free cyanide, sodium cyanide (NaCN), potassium cyanide
(KCN), and moderately and weakly complexed metal-cyanides.
Weak complexes include tetracyanozincate [Zn (CN)
4
2-
] and tetracyanocadminate
[Cd(CN)
4
2-
], while the strong complexes contain iron cyanide complexes, which are
ferro- and ferricyanide complexes ([Fe(CN)
6
4-

], [Fe(CN)
6
3-
]), and hexacyanocobaltate
[Co(CN)
6
4-
] (Rennert, 2002). Cyanide containing organic compounds is introduced
naturally into the environment by a number of living systems (Fuller, 1985). However,
7
the concentration of naturally occurring free cyanide is usually less than 0.01 mg/L, and
is not persistent in soil (Meehan, 2000). Free cyanides may be degraded by soil
microorganisms and converted to carbonate and ammonia (Fuller, 1985). Iron-cyanide
complexes are not produced by natural sources and their presence in the soil environment
is caused by anthropogenic input (Meeussen et al., 1992).
2.3. Cyanide Toxicity
Cyanide refers to all compounds containing the CN group in which there is a
triple bond between the carbon and nitrogen. The acute toxicity of free cyanide (CN
-
or
HCN) is the result of its ability to bind heme iron in the oxygen-binding site of the
mitochondrial enzyme, cytochrome oxidase, consequently blocking oxidative energy
metabolism. Brain cells and heart tissue are most vulnerable to serious damage from
diminished oxygen utilization caused by cyanide poisoning (Shifrin et al., 1996). Cyanide
does not accumulate in body tissue and therefore, any effects will be acute, chronic or
sub-chronic (US EPA, 1992; Shifrin et al., 1996). Acute cyanide poisoning in humans
may result in convulsions, vomiting, coma and death. Chronic effects by exposure to
lower concentrations of free cyanide include neuropathy, optical atrophy, and pernicious
anemia (Raybuck, 1992). The toxicity of cyanide is dependent upon the species.
Cyanide may be inhaled, ingested, or adsorbed through dermal contact (Shifrin et. al.,

1992).
Combined forms of cyanide are quite common and have low toxicity. Few
studies are published on human and/or animal exposure to iron-complexed cyanides.
8
Published literature strongly suggests that these compounds have low toxicity even at
high levels of exposure due to limited transformation to free cyanide (Shifrin et al., 1996).
The no-observed adverse effect level (NOAEL) of 3200 mg/kg-day was observed for rats
exposed to ferric-ferrocyanide in drinking water for 12 weeks. This is in contrast to fatal
HCN concentrations of 3.5 mg CN/kg with an LD
50
of 1.1-1.5 mg CN/kg. The LD
50
of
ferric ferrocyanide for mice is projected to be more than 5000 mg/kg, which is greater
than for the Fe (Shifrin et al., 1996). Table 2.3 highlights the potential risks from daily
exposure to cyanide compounds. The lower toxicity of some compounds appears to be
the result of compound stability under acidic conditions and low absorption from the gut
(Schmidt-Nielsen, 1990). However, iron cyanide complexes cannot be regarded as
completely inert (Meeussen et al., 1994), because iron cyanide complexes are easily
decomposed to HCN.
2.4. Solubility of Iron Cyanide Complexes
Iron cyanide complexes are the predominant form of cyanide compounds at MGP
sites, specifically Prussian blue. These cyanide complexes are not stable and tend to
decompose to free cyanide. Thermodynamically, free cyanide should be the dominant
species in soil and groundwater. However, iron-cyanide complexes predominate in soil
(Meeussen et al., 1992; Theis et al., 1994) due to the relatively slow decomposition rate
in absence of sunlight. Table 2.4 shows equilibrium reactions and constants for cyanide
species. Iron cyanide complexes are slightly soluble under acidic conditions. Acidic
conditions prevail due to the generation of sulfuric acid at MGP sites. The dissociation
9

constant (K
a
) in water is 10
-36.9
for ferrocyanide [Fe(CN)
6
4-
] and 10
-43.9
for ferricyanide
[Fe(CN)
6
3-
] (Beck, 1987).
2.5. Fate and Transport of Cyanide
Iron cyanide complexes have not been reported to be toxic. Consequently, the
regulation limit in drinking water is rather high (US EPA: 0.2 mg/L CN
-
). However,
these compounds can be rapidly photodegraded to form toxic HCN (Rader et al, 1993;
Meeussen, 1994). The transport of these compounds poses a serious risk of
groundwater contamination. Prediction of fate of these contaminants is often
complicated by their physical characteristics and complex interactions in soil and
groundwater. The sorption and solubility characteristics of cyanide complexes play a
major role in controlling contaminant fate and transport in the subsurface.
The chemistry of cyanide in soils has been used to predict the equilibrium
concentrations of free cyanide, cyanic acid, and complexed forms in both solution and
solid phase (Meeussen et al., 1995; Kjeldsen, 1999) as well as the kinetics of the
transformations (Meeussen et al., 1992). The solubility of Prussian blue is highest at
moderate redox potential and basic pH. Acidic pH favors precipitation of Prussian blue

as does high redox potential (pE>10) or low redox potential (pE<6). Conversion of Fe-
cyanide complexes to free cyanide species is favored by acidic pH and high redox
conditions.
Meeussen et al. (1992) stated the following regarding the stability of non-toxic
iron-cyanide complexes in soil and groundwater. “Iron cyanide complexes are