# Mobility of Iron-Cyanide Complexes in a Humic Topsoil under Varying Redox Conditions

Authors: Thilo Rennert; Tim Mansfeldt
Journal: Applied and Environmental Soil Science
 (2009)

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## Abstract

The potentially toxic Fe-CN complexes ferricyanide,[FeIII(CN)6]3−, and ferrocyanide, [FeII(CN)6]4−, undergo a variety of redox processes in soil, which affect their mobility. We carried out microcosm experiments with suspensions of a humic topsoil
(pH 5.3; Corg 107 g kg-1) to which we added ferricyanide (20 mg l-1). We varied the redox potential
(EH) from −280 to 580 mV by using O2, N2 and glucose. The decrease of EH led to decreasing concentrations of Fe-CN complexes and partial reductive dissolution of (hydrous) Fe and Mn oxides. The dynamics of aqueous Fe-CN concentrations was characterized by decreasing concentrations when
the pH rose and the EH dropped. We attribute these dependencies to adsorption on organic surfaces,
for which such a pH/EH behavior has been shown previously. Adsorption was reversible, because when the pH and EH changed into the opposite direction, desorption occurred. This study demonstrates
the possible impact of soil organic matter on the fate of Fe-CN complexes in soil.


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## Body

## 1. Introduction

As a consequence  of anthropogenic inputs, the Fe-CN
complexes ferrocyanide,[FeII(CN)6]4−, and ferricyanide, [FeIII(CN)6]3−,
may be present in soil. Their occurrence is mainly due to the deposition of
wastes originating from industrial processes such as coal gasification, pig Fe
production, or paper recycling [1]. In these wastes, Fe-CN complexes are mainly
present as sparingly soluble compounds, for example, Fe4[FeII(CN)6]3,
KFe[FeII(CN)6], or K2Zn3[FeII(CN)6]2 [2]. A further source is road salt, to which Na4[FeII(CN)6]
or Fe4[FeII(CN)6]3 are added as an
anticaking agent [3]. The solubility of Fe4[FeII(CN)6]3,
KFe[FeII(CN)6], and K2Zn3[FeII(CN)6]2 increases under neutral and alkaline conditions resulting in a release of Fe-CN
complexes into the soil solution [4]. The presence of dissolved Fe-CN complexes
is an environmental concern because they tend to decompose to extremely toxic-free
CN (HCNg,aq and CNaq−)  when irradiated by
daylight [5].Fe-CN complexes undergo a variety of processes in soil
that affect their speciation and mobility. First, they form a redox couple [6](1)[FeII(CN)6]4−⟷[FeIII(CN)6]3−+e−,EH=356mV, where EH is the redox potential referred to
the standard H2 electrode. Birnessite, δ-MnO2, rapidly oxidizes [FeII(CN)6]4− to [FeIII(CN)6]3− [7], whereas the oxidation
by O2 is slow [8]. Second, dissolution and precipitation of
compounds such as Fe4[FeII(CN)6]3 and Mn-containing hexacyanoferrates govern the concentrations of aqueous Fe-CN
complexes, especially under reducing conditions [4, 9–11]. Third, Fe-CN
complexes adsorb on soil minerals such as goethite [8]  and amorphous Al
hydroxides [12]. However, the mechanism, by which Fe-CN complexes are sorbed,
differs between the type of the complex and that of the mineral. Ferrocyanide
forms an Fe4[FeII(CN)6]3-like
layer on the goethite surface, whereas [FeIII(CN)6]3− forms monodentate inner-sphere complexes on it  [8, 13]. Both complexes weakly
adsorb on γ-AI2O3 as outer-sphere complexes
[12]. The pH affects adsorption and precipitation because both increase at
decreasing pH.In neutral and humic soils, additional processes may
occur.  Adsorption of both complexes in Fluvisols is enhanced by soil organic
matter (SOM) because the soils did not adsorb the complexes after destruction
of SOM [14]. Reduction of [FeIII(CN)6]3− by
SOM and subsequent precipitation of Fe4[FeII(CN)6]3 occurred during the transport in a humic topsoil [15]. Ferricyanide is reduced
by humic acids, and afterwards [FeII(CN)6]4− is
associated with them [16]. Additionally, [FeIII(CN)6]3− oxidizes simple organic compounds such as phenols, alcohols, aldehydes, and
ketones [17]. In neutral and
slightly acidic soils, precipitation of Mn hexacyanoferrates dominates the
mobility of Fe-CN complexes [9, 11]. Small amounts of Fe-CN complexes
are adsorbed by SOM in the form of reduced [FeII(CN)6]4−; pyrogenic
organic matter such as charred straw also adsorbs [FeII(CN)6]4− [11].These studies suggest an important influence of SOM on
the mobility and speciation of Fe-CN complexes. Apparently, redox reactions are
involved in interactions between the soil mineral phase, SOM, and Fe-CN
complexes. However, these interactions have not been studied yet under varying
redox conditions. Therefore, we investigated the fate of Fe-CN complexes in
suspensions of an uncontaminated humic topsoil to which[FeIII(CN)6]3− was added and varied the redox potential. Information on previously unknown
processes occurring in humic topsoils under varying redox conditions improves
the overall knowledge on the fate of Fe-CN complexes in soil.


## 2. Materials and Methods

### 2.1. Soils and Soil Characterization

The soil sample under study is the A horizon of an
arable Stagnic Fluvisol located in the floodplain of the river Oder in Germany
(52°43′N,14°13′E). The sample
was dried at 333 K, sieved to <2 mm, and
homogenized. We determined the soil texture by sieving and the pipette method [18].
Soil pH was measured potentiometrically in
0.01 M CaCl2 with a soil:solution ratio of 1:2.5. The
contents of crystalline Fe oxides as well as of amorphous Fe and Mn oxides were
determined by extraction with dithionite-citrate and oxalate solutions,
respectively [19, 20]. We analyzed total C by oxidation at
1223 K and heat
conductivity detection with a Vario EL elemental analyzer (Elementar Analysensysteme,
Hanau, Germany). We checked the presence of carbonate, but none was detected.
Black carbon (BC) is one of the major aromatic compounds present in soil and is
a product of incomplete combustion of biomass [21]. The BC contents were
determined as described in detail by Brodowski et al. [22]. Briefly, BC is
oxidized to benzene polycarboxylic acids (BPCAs) by concentrated HNO3, cleaned,
derivatized to trimethylsilyl derivatives, and subsequently analyzed by gas
chromatography (Hewlett Packard 6890; Hewlett Packard, Waldbronn, Germany) with
flame-ionization detection. The BPCA carbon yields were multiplied with a
correction factor of 2.27 to obtain BC contents [23]. This factor provides a
conservative minimum estimate of BC contents in soil [22].


### 2.2. Microcosm Experiments

The microcosms (Figure1) were made of glass with the
dimensions 28 cm inner diameter at the top and 21 cm inner height. In two
experiments (no. 1 and no. 2), we suspended 1.2 kg soil in
6 L demineralized H2O.
The suspensions were stirred to prevent sedimentation. The microcosms were
wrapped with Al foil to exclude daylight. Six days after suspending the soil (no. 1)
and one day after suspending the soil (no. 2), a K3[FeIII(CN)6]
solution was added to obtain an initial concentration of 20 mg [FeIII(CN)6]3−l−1. Redox potentials and pH were continuously measured in the
suspensions with Pt electrodes and an Ag/AgCl reference system (EMC 33,
Meinsberg, Ziegra-Knobelsdorf, Germany) and EGA 153 pH electrodes (Meinsberg, Ziegra-Knobelsdorf, Germany),
respectively. The measured redox potentials were related to the potential of
the standard H2 electrode, for example, at
293 K, 207 mV were added.
Temperature was continuously recorded. In the following, redox potentials are
given as the potential of the standard H2 electrode, EH.
We aimed to decrease the redox potential in the experiments by flushing the
suspensions with N2 at a flow rate of 80 ml min−1 and
additions of 2 (no. 1) and 5 g (no. 2) glucose to stimulate microbial activity and
to induce reducing conditions. The two experiments also differed in the numbers
and the moments of the glucose additions. At the end of the experiments, O2 was flushed through the suspensions to reaerate. In intervals of 24 or 48 hours, about 20 ml were taken from the suspensions with a syringe and a tube through the
sampling port. These samples were filtered
(0.45 μm cellulose-nitrate filter)
and analyzed for Fe, Mn, and total CN. We did not add acid or base to vary the
pH. The experiments lasted 56 (no. 2) and 103 (no. 1) days. In experiment (no. 2),
we accelerated the establishment of reducing conditions, whereas we retarded it
with (no. 1). After the microcosm experiments, aliquots of the suspensions were
filtered, the filters were air dried and total CN was extracted from soil.Figure 1
Scheme of a microcosm and control device.


### 2.3. Analyses and Interpretation

Fe and Mn concentrations in the filtrates as well as
in the dithionite-citrate and oxalate extracts were determined by induced
coupled plasma atomic emission spectrometry using a CIROSCCD instrument (Spectro Analytical Instruments, Kleve, Germany). Additionally, the
concentrations of other major cations in the filtrates were measured with the
same instrument and concentrations of major inorganic anions by ion
chromatography with conductivity detection (Dionex DX 500, Idstein, Germany). Total
CN in the filtrates was analyzed by acid distillation (Micro-Distiller,
Eppendorf-Netheler-Hinz, Hamburg, Germany) and subsequent spectrophotometrical
detection at 600 nm (detection limit 1 μg l−1; Lambda 2, Perkin
Elmer, Überlingen, Germany) [24]. We checked the presence of free CN, but none
was detected. Total CN present in soil after the experiments was extracted using
1 M NaOH in duplicate [24] and
subsequently determined as in the filtrate in duplicate.Partially, we interpreted the data with the
geochemical speciation program ECOSAT 4.7 [25] using EH, pH and the concentrations
of all cations and anions analyzed to check possible precipitation of Fe4[FeII(CN)6]3,
Mn2[FeII(CN)6], and Mn3[FeIII(CN)6].
The solubility constants of the compounds are as follows: lg Ksol=−84.5 (Fe4[FeII(CN)6]3;
[4]), lg Ksol=−25 (Mn2[FeII(CN)6]),
and lg Ksol=−27 (Mn3[FeIII(CN)6]2;
[9]).


## 2.1. Soils and Soil Characterization

The soil sample under study is the A horizon of an
arable Stagnic Fluvisol located in the floodplain of the river Oder in Germany
(52°43′N,14°13′E). The sample
was dried at 333 K, sieved to <2 mm, and
homogenized. We determined the soil texture by sieving and the pipette method [18].
Soil pH was measured potentiometrically in
0.01 M CaCl2 with a soil:solution ratio of 1:2.5. The
contents of crystalline Fe oxides as well as of amorphous Fe and Mn oxides were
determined by extraction with dithionite-citrate and oxalate solutions,
respectively [19, 20]. We analyzed total C by oxidation at
1223 K and heat
conductivity detection with a Vario EL elemental analyzer (Elementar Analysensysteme,
Hanau, Germany). We checked the presence of carbonate, but none was detected.
Black carbon (BC) is one of the major aromatic compounds present in soil and is
a product of incomplete combustion of biomass [21]. The BC contents were
determined as described in detail by Brodowski et al. [22]. Briefly, BC is
oxidized to benzene polycarboxylic acids (BPCAs) by concentrated HNO3, cleaned,
derivatized to trimethylsilyl derivatives, and subsequently analyzed by gas
chromatography (Hewlett Packard 6890; Hewlett Packard, Waldbronn, Germany) with
flame-ionization detection. The BPCA carbon yields were multiplied with a
correction factor of 2.27 to obtain BC contents [23]. This factor provides a
conservative minimum estimate of BC contents in soil [22].


## 2.2. Microcosm Experiments

The microcosms (Figure1) were made of glass with the
dimensions 28 cm inner diameter at the top and 21 cm inner height. In two
experiments (no. 1 and no. 2), we suspended 1.2 kg soil in
6 L demineralized H2O.
The suspensions were stirred to prevent sedimentation. The microcosms were
wrapped with Al foil to exclude daylight. Six days after suspending the soil (no. 1)
and one day after suspending the soil (no. 2), a K3[FeIII(CN)6]
solution was added to obtain an initial concentration of 20 mg [FeIII(CN)6]3−l−1. Redox potentials and pH were continuously measured in the
suspensions with Pt electrodes and an Ag/AgCl reference system (EMC 33,
Meinsberg, Ziegra-Knobelsdorf, Germany) and EGA 153 pH electrodes (Meinsberg, Ziegra-Knobelsdorf, Germany),
respectively. The measured redox potentials were related to the potential of
the standard H2 electrode, for example, at
293 K, 207 mV were added.
Temperature was continuously recorded. In the following, redox potentials are
given as the potential of the standard H2 electrode, EH.
We aimed to decrease the redox potential in the experiments by flushing the
suspensions with N2 at a flow rate of 80 ml min−1 and
additions of 2 (no. 1) and 5 g (no. 2) glucose to stimulate microbial activity and
to induce reducing conditions. The two experiments also differed in the numbers
and the moments of the glucose additions. At the end of the experiments, O2 was flushed through the suspensions to reaerate. In intervals of 24 or 48 hours, about 20 ml were taken from the suspensions with a syringe and a tube through the
sampling port. These samples were filtered
(0.45 μm cellulose-nitrate filter)
and analyzed for Fe, Mn, and total CN. We did not add acid or base to vary the
pH. The experiments lasted 56 (no. 2) and 103 (no. 1) days. In experiment (no. 2),
we accelerated the establishment of reducing conditions, whereas we retarded it
with (no. 1). After the microcosm experiments, aliquots of the suspensions were
filtered, the filters were air dried and total CN was extracted from soil.Figure 1
Scheme of a microcosm and control device.


## 2.3. Analyses and Interpretation

Fe and Mn concentrations in the filtrates as well as
in the dithionite-citrate and oxalate extracts were determined by induced
coupled plasma atomic emission spectrometry using a CIROSCCD instrument (Spectro Analytical Instruments, Kleve, Germany). Additionally, the
concentrations of other major cations in the filtrates were measured with the
same instrument and concentrations of major inorganic anions by ion
chromatography with conductivity detection (Dionex DX 500, Idstein, Germany). Total
CN in the filtrates was analyzed by acid distillation (Micro-Distiller,
Eppendorf-Netheler-Hinz, Hamburg, Germany) and subsequent spectrophotometrical
detection at 600 nm (detection limit 1 μg l−1; Lambda 2, Perkin
Elmer, Überlingen, Germany) [24]. We checked the presence of free CN, but none
was detected. Total CN present in soil after the experiments was extracted using
1 M NaOH in duplicate [24] and
subsequently determined as in the filtrate in duplicate.Partially, we interpreted the data with the
geochemical speciation program ECOSAT 4.7 [25] using EH, pH and the concentrations
of all cations and anions analyzed to check possible precipitation of Fe4[FeII(CN)6]3,
Mn2[FeII(CN)6], and Mn3[FeIII(CN)6].
The solubility constants of the compounds are as follows: lg Ksol=−84.5 (Fe4[FeII(CN)6]3;
[4]), lg Ksol=−25 (Mn2[FeII(CN)6]),
and lg Ksol=−27 (Mn3[FeIII(CN)6]2;
[9]).


## 3. Results and Discussion

### 3.1. Soil Characteristics

The soil  has a sandy-clayey texture (563 g kg−1 sand, 124 g kg−1 silt, 313 g kg−1 clay) and is moderately
acidic (pH 5.3 in CaCl2). It contains 107 g kg−1 organic
C of which 10.3% is BC that has the potential to interact with [FeIII(CN)6]3− [11]. Another important feature of the soil is that it contains Fe and Mn
oxides, which may dissolve under reducing conditions (dithionite-extractable Fe
9870 mg kg−1, oxalate-extractable Fe and Mn 7800 and 1650 mg kg−1,
resp.).


### 3.2. Microcosm Experiments

At the  beginning of the experiments, the redox conditions
were oxidizing (Figures2(a) and 2(b)), and flushing the soil suspensions with
N2 led to a decrease of EH. Under the N2 atmosphere, an almost constant EH in the range of 340 to 370 mV was
achieved in both experiments. The first addition of glucose induced a strong
decrease of EH to approximately 100 mV (no. 1) and even negative
potentials (no. 2). The difference between the experiments is caused by the
varying amount of glucose added. Negative redox potentials with (no. 1) were
achieved later, after a further addition of glucose. The decrease in EH is connected with an increase in pH revealing their well-known reciprocal
relation when the EH varies. The initial pH measured in both experiments is higher
than the soil pH determined in CaCl2. This is caused by the stronger
release of Al3+ from soil in the presence of CaCl2 compared
to the release from soil suspended in water as in the microcosm experiments.
The establishment of reducing conditions was accompanied with the disappearance
of NO3− (c < 0.1 mg l−1) in both
experiments after the EH had dropped to <350 mV. Previously, 40
± 5 mg NO3−l−1 were present in each
experiment. The main inorganic cations were
Ca2+(150–270 mg l−1)
and Mg2+ (4–10 mg l−1), the main anion was SO42− (100–140 mg l−1). The concentrations of dissolved Mn accompanied
with additions of glucose that induced a drop in EH (Figures 3(a), 3(b))
leading to reductive dissolution of (hydrous) Mn oxides in soil. Reductive
dissolution of (hydrous) Fe oxides occurred at very low redox potentials only (after
1860 hours with no. 1) as shown by the increase of the aqueous Fe concentration
to 3.5 mg l−1. The release of Fe and Mn ions induced by dissolution
of oxides governed the mobility and solubility of Fe-cyanide complexes in a
subsoil low in organic C by precipitation of sparingly soluble phases such as
Fe4[FeII(CN)6]3 and Mn2[FeII(CN)6]
[10]. Similarly, precipitation of Mn2[FeII(CN)6]
was the main process of [FeIII(CN)6]3− added
to humic and neutral soils [11].Redox potentials and pH during the microcosm experiments (a)
no. 1 and (b) no. 2. “+[Fe(CN)6]3−”
denotes addition of ferricyanide, “+ G” denotes additions of glucose, “+ N2”
and “+ O2” denote the gas purged through the suspensions.
(a)(b)Fe and
Mn concentrations during the microcosm experiments (a) no. 1 and (b) no. 2. “+ G”
denotes additions of glucose, “+N2” and “+ O2”
denote the gas purged through the suspensions.
(a)(b)After decreasingEH due to the N2 atmosphere and subsequent additions of glucose, the concentrations of Fe-CN
complexes decreased in both experiments (Figures 4(a), 4(b)). The decrease in
experiment no. 1 was retarded compared to (no. 2), similar to the less pronounced
decrease in EH. Qualitatively, the [FeIII(CN)6]3− added is reduced to [FeII(CN)6]4− during the
establishment of reducing conditions as shown by direct measurements and
geochemical calculations [10]. The [FeII(CN)6]4− formed is completely dissociated because all pKa values are <4.19 [26], which is at least 2 pH units lower than the pH during the
experiments (Figures 2(a),
2(b)).Total Fe-CN complex concentrations during the microcosm
experiments (a) no. 1 and (b) no. 2. “+ G”
denotes additions of glucose, “+N2” and “+ O2”
denote the gas purged through the suspensions.
(a)(b)After 1200 hours, the Fe-CN concentration in
experiment (no. 1)  increased from 14.7 to 16.7 mgl−1, while the pH
slightly decreased and the EH increased (Figures 2(a), 4(a)).
Subsequently, under more reduced conditions, the Fe-CN concentrations
continuously decreased to approximately 2.5 mg l−1. The course of
Fe-CN concentrations in experiment (no. 2) revealed similar features: after a
steep decrease to 1.5±0.5 mg l−1, the Fe-CN concentration increased
to 5.2 mg l−1. Simultaneously, theEH increased to 80 mV
and the pH decreased (Figure 2(b)). After reaeration (EH>400mV), the  level of the initial concentration was almost equalled with 18.1 (no. 1)
and 17 mg l−1 (no. 2) while the pH dropped to 7.5 (no. 1) and 6.6 (no. 2).
Conversely, decreasing concentrations were linked  with increasing pH during the
experiments. In both experiments, the sum of the amounts of CN extracted from
soil after the experiments and of the aqueous species equalled the amount
initially added. This indicates that microbial degradation of Fe-CN complexes
has not occurred.The course of Fe-CN concentrations with an increase
under subsequent oxidizing conditions, the low Fe concentrations as well as
geochemical calculations clearly indicated that precipitation ofFe4[FeII(CN)6]3 cannot have occurred. Precipitation of Mn hexacyanoferrates as the main process
removing Fe-CN complexes from solution is also very questionable because (i)
the presence of Mn ions is necessary for precipitation, but their concentrations
are rather low (Figures 3(a),
3(b)), (ii) Mn hexacyanoferrates once
precipitated are stable under subsequent oxidizing conditions [9, 10], which
contradicts the mobilization of Fe-CN complexes observed after reaeration, (iii)
the pH dependency of the dissolution of Mn2[FeII(CN)6]
is weak in the range at the end of the experiments (6.9 to 7.8) as checked by
geochemical calculations, and (iv) the pH during the experiments (up to 7.8)
generally prevents precipitation.Furthermore, Fe-CN complexes possibly adsorbed on
mineral surfaces were not desorbed when the pH decreased from 7.8 to 7.5 and from
7.3 to 6.6 as found here after reaeration because desorption rises with growing
pH. Therefore, we found the rather surprising situation that decreasing pH and
increasingEH enhanced the mobility of Fe-CN complexes in soil.
Theoretically, a positive effect of increasing pH on adsorption on mineral
surfaces is possible when the pH increases to the pKa of the
conjugate acid of an anion adsorbing on a mineral surface [27]. However, as
pointed out before, the pKa values of ferrocyanic acid are much
lower than the pH during the experiments so that adsorption on the mineral soil
phase cannot have determined the dynamics of Fe-CN complexes.The previous discussion points up that any process
that includes inorganic soil minerals cannot explain the dynamics of the Fe-CN
complex concentrations observed. Therefore, interactions with organic surfaces
must be considered for a comprehensive explanation. Adsorption of[FeIII(CN)6]3− on organic surfaces occurred by two different pathways [11]. First, it weakly
adsorbed on oxidized functional groups such as carboxyls, carbonyls, and
alcohols under neutral conditions by hydrogen bonds. Then, [FeIII(CN)6]3− underwent partial reduction to [FeII(CN)6]4− as inferred from Fourier transform infrared (FTIR) spectroscopy and by
voltammetry [11, 16]. However, this type of adsorption should be unimportant here
because under oxidizing and neutral conditions at the beginning of experiment (no. 2),
the Fe-CN concentrations did not decrease. Second, [FeIII(CN)6]3− was adsorbed by phenols and was reduced to [FeII(CN)6]4− [11]. Phenolic species are present in SOM, especially in BC. Ferrocyanide
formed inner-sphere complexes with the organic matrix as again inferred from
FTIR spectroscopy. Alkaline and reducing conditions enhanced reduction and
adsorption [11]. Thus, [FeII(CN)6]4− previously
adsorbed on phenolic species such as quinones is released into solution when
the EH rises or the pH drops, which accords with the dynamics of
Fe-CN complexes found here in the experiments. At the beginning of experiment (no. 2),
oxidizing conditions prevailed and the pH decreased. Consequently, the initial
Fe-CN concentration remained almost unaffected. The same geochemical milieu
applied to the end of both experiments, EH increased, and the pH
dropped. This was followed by remobilization of adsorbed Fe-CN complexes (Figures
4(a),
4(b)). The mobility of aqueous Fe-CN complexes depended on pH and EH as we would expect when they interact with quinone species. Therefore, we
assume that these aromatic species are the most important reactants in the soil
matrix with regard to Fe-CN complexes and that the observed dynamics of Fe-CN
complexes in this soil was most likely and to a large proportion governed by
interactions with SOM. The soil contains large amounts of Fe oxides, but the
effect of pH on the mobility of Fe-CN complexes in the microcosm experiments
documents that adsorption on their surfaces seems to be marginal. The
prevailing alkaline pH in this experiment limits adsorption on mineral surfaces
and excludes precipitation. However, Fe-CN complexes are not entirely mobile
under alkaline conditions as long as organic adsorbents are present, and reducing
conditions are necessary to induce these adsorptive
processes.


## 3.1. Soil Characteristics

The soil  has a sandy-clayey texture (563 g kg−1 sand, 124 g kg−1 silt, 313 g kg−1 clay) and is moderately
acidic (pH 5.3 in CaCl2). It contains 107 g kg−1 organic
C of which 10.3% is BC that has the potential to interact with [FeIII(CN)6]3− [11]. Another important feature of the soil is that it contains Fe and Mn
oxides, which may dissolve under reducing conditions (dithionite-extractable Fe
9870 mg kg−1, oxalate-extractable Fe and Mn 7800 and 1650 mg kg−1,
resp.).


## 3.2. Microcosm Experiments

At the  beginning of the experiments, the redox conditions
were oxidizing (Figures2(a) and 2(b)), and flushing the soil suspensions with
N2 led to a decrease of EH. Under the N2 atmosphere, an almost constant EH in the range of 340 to 370 mV was
achieved in both experiments. The first addition of glucose induced a strong
decrease of EH to approximately 100 mV (no. 1) and even negative
potentials (no. 2). The difference between the experiments is caused by the
varying amount of glucose added. Negative redox potentials with (no. 1) were
achieved later, after a further addition of glucose. The decrease in EH is connected with an increase in pH revealing their well-known reciprocal
relation when the EH varies. The initial pH measured in both experiments is higher
than the soil pH determined in CaCl2. This is caused by the stronger
release of Al3+ from soil in the presence of CaCl2 compared
to the release from soil suspended in water as in the microcosm experiments.
The establishment of reducing conditions was accompanied with the disappearance
of NO3− (c < 0.1 mg l−1) in both
experiments after the EH had dropped to <350 mV. Previously, 40
± 5 mg NO3−l−1 were present in each
experiment. The main inorganic cations were
Ca2+(150–270 mg l−1)
and Mg2+ (4–10 mg l−1), the main anion was SO42− (100–140 mg l−1). The concentrations of dissolved Mn accompanied
with additions of glucose that induced a drop in EH (Figures 3(a), 3(b))
leading to reductive dissolution of (hydrous) Mn oxides in soil. Reductive
dissolution of (hydrous) Fe oxides occurred at very low redox potentials only (after
1860 hours with no. 1) as shown by the increase of the aqueous Fe concentration
to 3.5 mg l−1. The release of Fe and Mn ions induced by dissolution
of oxides governed the mobility and solubility of Fe-cyanide complexes in a
subsoil low in organic C by precipitation of sparingly soluble phases such as
Fe4[FeII(CN)6]3 and Mn2[FeII(CN)6]
[10]. Similarly, precipitation of Mn2[FeII(CN)6]
was the main process of [FeIII(CN)6]3− added
to humic and neutral soils [11].Redox potentials and pH during the microcosm experiments (a)
no. 1 and (b) no. 2. “+[Fe(CN)6]3−”
denotes addition of ferricyanide, “+ G” denotes additions of glucose, “+ N2”
and “+ O2” denote the gas purged through the suspensions.
(a)(b)Fe and
Mn concentrations during the microcosm experiments (a) no. 1 and (b) no. 2. “+ G”
denotes additions of glucose, “+N2” and “+ O2”
denote the gas purged through the suspensions.
(a)(b)After decreasingEH due to the N2 atmosphere and subsequent additions of glucose, the concentrations of Fe-CN
complexes decreased in both experiments (Figures 4(a), 4(b)). The decrease in
experiment no. 1 was retarded compared to (no. 2), similar to the less pronounced
decrease in EH. Qualitatively, the [FeIII(CN)6]3− added is reduced to [FeII(CN)6]4− during the
establishment of reducing conditions as shown by direct measurements and
geochemical calculations [10]. The [FeII(CN)6]4− formed is completely dissociated because all pKa values are <4.19 [26], which is at least 2 pH units lower than the pH during the
experiments (Figures 2(a),
2(b)).Total Fe-CN complex concentrations during the microcosm
experiments (a) no. 1 and (b) no. 2. “+ G”
denotes additions of glucose, “+N2” and “+ O2”
denote the gas purged through the suspensions.
(a)(b)After 1200 hours, the Fe-CN concentration in
experiment (no. 1)  increased from 14.7 to 16.7 mgl−1, while the pH
slightly decreased and the EH increased (Figures 2(a), 4(a)).
Subsequently, under more reduced conditions, the Fe-CN concentrations
continuously decreased to approximately 2.5 mg l−1. The course of
Fe-CN concentrations in experiment (no. 2) revealed similar features: after a
steep decrease to 1.5±0.5 mg l−1, the Fe-CN concentration increased
to 5.2 mg l−1. Simultaneously, theEH increased to 80 mV
and the pH decreased (Figure 2(b)). After reaeration (EH>400mV), the  level of the initial concentration was almost equalled with 18.1 (no. 1)
and 17 mg l−1 (no. 2) while the pH dropped to 7.5 (no. 1) and 6.6 (no. 2).
Conversely, decreasing concentrations were linked  with increasing pH during the
experiments. In both experiments, the sum of the amounts of CN extracted from
soil after the experiments and of the aqueous species equalled the amount
initially added. This indicates that microbial degradation of Fe-CN complexes
has not occurred.The course of Fe-CN concentrations with an increase
under subsequent oxidizing conditions, the low Fe concentrations as well as
geochemical calculations clearly indicated that precipitation ofFe4[FeII(CN)6]3 cannot have occurred. Precipitation of Mn hexacyanoferrates as the main process
removing Fe-CN complexes from solution is also very questionable because (i)
the presence of Mn ions is necessary for precipitation, but their concentrations
are rather low (Figures 3(a),
3(b)), (ii) Mn hexacyanoferrates once
precipitated are stable under subsequent oxidizing conditions [9, 10], which
contradicts the mobilization of Fe-CN complexes observed after reaeration, (iii)
the pH dependency of the dissolution of Mn2[FeII(CN)6]
is weak in the range at the end of the experiments (6.9 to 7.8) as checked by
geochemical calculations, and (iv) the pH during the experiments (up to 7.8)
generally prevents precipitation.Furthermore, Fe-CN complexes possibly adsorbed on
mineral surfaces were not desorbed when the pH decreased from 7.8 to 7.5 and from
7.3 to 6.6 as found here after reaeration because desorption rises with growing
pH. Therefore, we found the rather surprising situation that decreasing pH and
increasingEH enhanced the mobility of Fe-CN complexes in soil.
Theoretically, a positive effect of increasing pH on adsorption on mineral
surfaces is possible when the pH increases to the pKa of the
conjugate acid of an anion adsorbing on a mineral surface [27]. However, as
pointed out before, the pKa values of ferrocyanic acid are much
lower than the pH during the experiments so that adsorption on the mineral soil
phase cannot have determined the dynamics of Fe-CN complexes.The previous discussion points up that any process
that includes inorganic soil minerals cannot explain the dynamics of the Fe-CN
complex concentrations observed. Therefore, interactions with organic surfaces
must be considered for a comprehensive explanation. Adsorption of[FeIII(CN)6]3− on organic surfaces occurred by two different pathways [11]. First, it weakly
adsorbed on oxidized functional groups such as carboxyls, carbonyls, and
alcohols under neutral conditions by hydrogen bonds. Then, [FeIII(CN)6]3− underwent partial reduction to [FeII(CN)6]4− as inferred from Fourier transform infrared (FTIR) spectroscopy and by
voltammetry [11, 16]. However, this type of adsorption should be unimportant here
because under oxidizing and neutral conditions at the beginning of experiment (no. 2),
the Fe-CN concentrations did not decrease. Second, [FeIII(CN)6]3− was adsorbed by phenols and was reduced to [FeII(CN)6]4− [11]. Phenolic species are present in SOM, especially in BC. Ferrocyanide
formed inner-sphere complexes with the organic matrix as again inferred from
FTIR spectroscopy. Alkaline and reducing conditions enhanced reduction and
adsorption [11]. Thus, [FeII(CN)6]4− previously
adsorbed on phenolic species such as quinones is released into solution when
the EH rises or the pH drops, which accords with the dynamics of
Fe-CN complexes found here in the experiments. At the beginning of experiment (no. 2),
oxidizing conditions prevailed and the pH decreased. Consequently, the initial
Fe-CN concentration remained almost unaffected. The same geochemical milieu
applied to the end of both experiments, EH increased, and the pH
dropped. This was followed by remobilization of adsorbed Fe-CN complexes (Figures
4(a),
4(b)). The mobility of aqueous Fe-CN complexes depended on pH and EH as we would expect when they interact with quinone species. Therefore, we
assume that these aromatic species are the most important reactants in the soil
matrix with regard to Fe-CN complexes and that the observed dynamics of Fe-CN
complexes in this soil was most likely and to a large proportion governed by
interactions with SOM. The soil contains large amounts of Fe oxides, but the
effect of pH on the mobility of Fe-CN complexes in the microcosm experiments
documents that adsorption on their surfaces seems to be marginal. The
prevailing alkaline pH in this experiment limits adsorption on mineral surfaces
and excludes precipitation. However, Fe-CN complexes are not entirely mobile
under alkaline conditions as long as organic adsorbents are present, and reducing
conditions are necessary to induce these adsorptive
processes.


## 4. Conclusions and Outlook

The mobility of Fe-CN complexes may be decreased in
soil even under alkaline conditions that minimize sorption processes such as
precipitation of sparingly soluble Fe-CN compounds and adsorption on soil
minerals. In this study, we attribute the reduced mobility to adsorption on
organic surfaces such as quinone groups notably present in BC. However, Fe-CN adsorption
on organic surfaces is rather weak and completely reversible. Further research
is essential to study and to qualify further possible organic adsorbents and to
quantify their bonding strength in order to implement interactions with organic
matter in a comprehensive geochemical model on the fate of Fe-CN complexes in
soil.



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*Source: 857640-2009-03-04.xml*