Characterizing the reactivity of metallic iron upon methylene blue 
discoloration in Fe
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0/MnO2/H2O systems 
Noubactep C. 
Angewandte Geologie, Universität Göttingen, Goldschmidtstraße 3, D - 37077 Göttingen, Germany. 
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e-mail: cnoubac@gwdg.de; Tel. +49 551 39 3191, Fax: +49 551 399379 
 
Abstract 
A simple method is proposed for testing the reactivity of elemental iron materials (Fe0 
materials) using methylene blue (MB) as reagent. The method is based on the oxidative 
reactivity of FeII for reductive dissolution of MnO2. FeII is produced in-situ by the oxidation 
of a Fe0 material. The in-situ formed FeII reacted with MnO2 delaying the bulk precipitation of 
iron corrosion products and thus MB co-precipitation (MB discoloration). For a given MnO2, 
the extent of MB discoloration delay is a characteristic of individual Fe0 materials under given 
experimental conditions. The MB discoloration method for testing the reactivity of Fe0 
materials is facile, cost-effective and does not involve any stringent reaction conditions. 
Keywords: Adsorption, Co-precipitation, Methylene blue, Manganese oxides, Reactivity, 
Zerovalent iron.
Introduction 
Since the introduction of permeable reactive barriers of metallic iron for groundwater 
remediation (Fe0 PRB technology), various Fe0 types were tested and mostly successfully 
used for environmental remediation. Despite the reported successes, little progress has been 
made toward characterizing the variability in reactivity among Fe0 samples from different 
sources [1,2]. Available works attempted to relate corrosion rates to: (i) the rate of hydrogen 
evolution [3,4] or (ii) to the extend of contaminant removal by used Fe0 materials [1,5]. 
Among the reactivity parameters, the composition of the aqueous solution, the Fe0 elemental 
composition (alloying elements) and the surface properties (specific surface area, oxidation 
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state) have been largely discussed [3,6,7]. However, none of these parameters is independent 
and Fe
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0 aqueous reactivity further depends on method of manufacture [8,9]. According to Van 
Orden [8], whether a metal is cast, forged, wrought or welded are as important as the 
environment in the corrosion process. One major limitation of the current material testing 
procedures is that each material is used for remediation tests and the reactivity is ascertained 
at the end of the possibly cost-intensive and time consuming experiments. An alternative 
approach consisting in testing the reactivity of Fe0 materials in dilute EDTA 
(ethylenediaminetetraacetic acid) was recently presented [2]. However, EDTA is a strong 
chelating agent known to delay the process of iron hydroxide precipitation [10] at near neutral 
pH which is characteristic for most natural systems. 
There is a need to develop simple, reliable and cost-effective methods to test the reactivity of 
Fe0 materials. Ideally such methods should (i) be performed under conditions where iron 
solubility is not enhanced, and (ii) be applicable for material screening in preliminary works 
to avoid analytical cost. This study proposed a simple approach to compare the reactivity of 
Fe0 toward methylene blue discoloration. Four materials of known relative reactivity [2] were 
tested to ascertain the efficiency of the new method. The selected materials are representative 
for the large array of powdered and granular Fe0 materials used in environmental remediation. 
Background of the experimental methodology 
Methylene blue (MB) discoloration in the presence of Fe0 results mostly from MB co-
precipitation with in situ generated iron corrosion products [11,12]. Therefore, in a Fe0/H2O 
system, the extend of MB discoloration should increase steadily from the start of the 
experiment to the time of total discoloration. If the process of corrosion product precipitation 
is delayed, the resulting MB discoloration is also delayed. 
The differentiation of the reactivity of Fe0 materials in Fe0/MB/H2O systems is based on the 
reaction of FeII with MnO2 [13,14]. FeII is generated in-situ from Fe0 oxidation. Because 
liberated FeII species are used for the reductive dissolution of MnO2 (at the MnO2 surface), 
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they are not available for quantitative MB co-precipitation. Thus, the blue intensity of MB 
solution will not decreased as rapidly as in the system without MnO
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2. Accordingly, for 
parallel experiments with a given MnO2 and various Fe0 samples, the extent of MB 
discoloration will be a reflection of the chemical reactivity of individual Fe0 samples. 
Therefore, if a Fe0 sample is low reactive no or little delay of MB discoloration will be 
observed (assumption 1). For reactive Fe0 samples the more reactive the sample the more 
extensive the discoloration delay (assumption 2). 
The used methodology for testing the reactivity of Fe0 in MnO2/MB/H2O systems consists in 
following the process of MB discoloration in a given MnO2/H2O system as influenced by 
various Fe0 while testing the validity of assumptions 1 and 2. It should be kept in mind that 
MB discoloration and not MB removal is discussed in this study. For the discussion of MB 
removal TOC measurements for instance should have been necessary to account for MB 
reduction to colorless leuco-methylene blue (LMB) [15] which remains in solution. 
Materials and Methods 
Solutions 
Methylene blue (MB) is a traditionally favourite dye of choice for laboratory and technical 
purposes [16,17]. Its molecule has a minimum diameter of approximately 0.9 nm [18] and is 
also used as redox indicator [15]. The used initial concentration was 12 mg/L (~0.037 mM) 
MB and it was prepared by diluting a 1000 mg/L stock solution. All chemicals were analytical 
grade. 
Solid materials 
One scrap iron (ZVI1) and three commercially available iron materials (ZVI2 to ZVI4) have 
been tested in the present study. Table 1 summarizes the main characteristics of these 
materials together with their iron content. Before used ZVI4 (Tab. 1) was crushed and sieved; 
the size fraction 1.0-2.0 mm was used without any further pretreatment. The specific surface 
area of the materials were not available nor determined. This parameter is known as one of the 
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most important reactivity factors [20]. However, it is not the objective of this study to 
investigated the impact of the specific surface area on the reactivity of the material, but rather 
to compare the material in the form in which they could be used in field applications. 
Therefore, all other materials were used as obtained. Crushing and sieving ZVI4 aimed at 
working with materials of particle size relevant for field applications. The materials differ 
regarding their characteristics such as content of metallic iron, additives, grain size and shape. 
No information about the manufacture process (e.g. raw material, heat treatment) was 
available. 
The used MnO2 is commercial sample from Merck (85 - 90% MnO2; synthetic pyrulosite). 
The powdered sample was used without any pre-treatment. 
Discoloration studies 
Batch experiments with a shaking intensity of 50 min-1 were conducted in essay tubes for an 
experimental duration of 6 days. The essay tubes were immobilized on a support frame. A 
rotary shaker HS 501 D from “Janke & Kunkel”, DCM Laborservice was used. The batches 
consisted of 0 to 9.0 g/L of the synthetic pyrulosite (MnO2) and 5 g/L of each Fe0. A parallel 
experiments with the Fe0 materials alone was performed. The extent of MB discoloration in 
the investigated systems was characterized by allowing 0.0 to 0.20 g of MnO2 and 0.11 g of 
each Fe0 to react in sealed sample tubes containing 22.0 mL of a MB solution (12 mg/L) at 
laboratory temperature (about 20 ± 2 ° C). Initial pH was ~7.8. After equilibration, up to 3 mL 
of the supernatant solutions were carefully retrieved (no filtration) for MB measurements (no 
dilution). Each experiment was performed in triplicate and averaged results are presented. 
Analytical methods 
MB concentrations were determined by a Cary 50 UV-Vis spectrophotometer at a wavelength 
of 664.5 nm using cuvettes with 1 cm light path. The pH value was measured by combined 
glass electrodes (WTW Co., Germany). Electrodes were calibrated with five standards 
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following a multi-point calibration protocol in agreement with the current IUPAC 
recommendation [21]. 
Expression of experimental results 
After the determination of the residual MB concentration (C) the corresponding percent MB 
discoloration was calculated according to the following equation (Eq. 1): 
P = [1 - (C/C0)] * 100%      (1) 
where C0 is the initial aqueous MB concentration (about 12 mg/L), while C gives the MB 
concentration after the experiment. The operational initial concentration (C0) for each case 
was acquired from a triplicate control experiment without additive material (so-called blank). 
This procedure was to account for experimental errors during dilution of the stock solution, 
MB adsorption onto the walls of the reaction vessels and all other possible side reaction 
during the experiments. 
Results and Discussion 
Effect of MnO2 on the process MB discoloration in Fe0/H2O systems 
Figure 1 compares the extent of MB discoloration in the three investigated systems: (i) 0 to 
9.0 g/L MnO2 (System I), (ii) 0 to 9.0 g/L Fe0 (System II) and (iii) 5 g/L Fe0 and 0 to 9.0 g/L 
MnO2 (System III). From Figure 1a it can be seen that System II is more efficient at 
discolouring MB at all tested material loadings. This observation is consistent with the 
intrinsic nature of both materials. MnO2 acts mostly as an absorbent with limited adsorption 
capacity. Fe0 and its oxidation products can adsorb MB. Oxidation products of Fe0 include 
Fe(OH)2, Fe(OH)3, FeOOH, Fe2O3, Fe3O4, and green rust [22-24]. Their role in the 
mechanisms of aqueous contaminant removal is yet to be properly characterized [24,25]. It is 
certain that, during their precipitation corrosion products sequestrate MB in their structure 
[11]. Therefore, in a Fe0/MB/H2O system, MB discoloration may occur via adsorption and co-
precipitation. Reduction to colourless LMB is also likely to occur. Whatever the actual MB 
discoloration mechanisms in Fe0/H2O systems are, the addition of various amounts of MnO2 
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to 5 g/L Fe0 should have increased the extend of MB discoloration relative to the experiment 
with 5 g/L Fe
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0 alone as represented in Fig. 1a. Accordingly, the discoloration extend of 52 % 
with 5 g/L Fe0 in the absence of MnO2 should increase with increasing MnO2 loading to reach 
95 % discoloration at 9.0 g/L MnO2. This theoretical prediction (Fig. 1a) could not be verified 
experimentally as shown in Fig. 1b. Instead of increasing with increasing MnO2 loading, the 
discoloration extent decreased from 52 % at 0 g/L MnO2 to 40 % for MnO2 loadings > 2 g/L. 
This observation is compatible with MB co-precipitation as main discoloration mechanism. 
The observed discoloration extent of 40 % for MnO2 loadings > 2 g/L corresponds to the 
discoloration in: (i) system I (MnO2 alone) for MnO2 loadings > 6.5 g/L or (ii) system II (Fe0 
alone) for Fe0 loadings > 2.0 g/L. This observation suggests that mixing 5 g/L Fe0 with 0 to 
9.0 g/L MnO2 results in a system less efficient at discolouring MB than a system with only 
2.5 g/L Fe0. Note that if MB was acting as redox indicator, the discoloration extend should 
have steadily increased with increasing MnO2 loading. This observation corroborates the 
prediction that iron hydroxides sequestrate MB while precipitating. 
Effect of various Fe0 on the process of MB discoloration 
Figure 2 compares the extent of MB discoloration by tested Fe0 materials in the presence and 
in the absence of MnO2. From Fig. 2a the following order of efficiency can be observed: 
ZVI3 < ZVI1 ≅ ZVI4 < ZVI2. This order of efficiency was the same than that obtained by 
Noubactep et al. [2] while testing the same materials for UVI removal and for Fe dissolution in 
0.02 M EDTA ( see also Tab. 2). These results suggest that MB discoloration in Fe0/H2O 
systems is a simple method to differentiate the reactivity of materials. However, due to the 
diversity of possible discoloration mechanisms, it was important to make sure that Fe0 
oxidative dissolution is evidenced. For this purpose MnO2 is added to the systems to evidence 
the effects of in-situ generated FeII. Fig. 2b summarises the results. The same order of 
reactivity was found (ZVI3 < ZVI1 ≅ ZVI4 < ZVI2). A close consideration of Fig. 2b reveals 
that ZVI3 is low reactive (assumption 1: no delay of MB discoloration) because unlike for the 
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three other materials no initial discoloration decrease was observed upon MnO2 addition. All 
other materials are more reactive than ZVI3 (assumption 2: the more extensive the 
discoloration delay the more reactive the sample). Powdered ZVI2 was the most reactive 
material. The increased MB discoloration efficiency with ZVI2 is due to the increased Fe
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surface area resulting in a better solid/H20 contact. Again granular ZVI1 and ZVI4 showed 
very close reactivity, suggesting that the MB discoloration method is as powerful as the two 
other tests but essentially simpler. The reactivity of used materials is also reflected by the 
final pH value (Eq. 2). According to Eq. 2 (H+ production), the more reactive a material, the 
lower the final pH value (Tab. 2). 
2 Fe
2+
 + MnO2 + 2 H2O  ⇒  2 FeOOH + Mn
2+
 + 2 H+   (2) 
The verification of assumptions 1 and 2 validates the efficiency of MB discoloration for 
testing the reactivity of Fe0 materials in Fe0/MnO2/H2O systems. 
Conclusions 
Methylene blue has been used for the characterization of the reactivity of Fe0 materials. The 
proposed method is simple and does not involve any stringent reaction conditions. This 
method is a good alternative for reported costly instrumental procedures [2]. The proposed 
method has been successfully applied to differentiate the reactivity of four Fe0 materials of 
known relative reactivity. This method can be used for material screening prior to cost-
intensive investigations. Moreover, the test can be further developed to yield a characteristic 
parameter for Fe0 materials similar to iodine number or methylene blue number for the 
characterization of activated carbons [16,18]. 
Acknowledgments 
Josee Vouffo (student research assistant) is acknowledged for technical support. The work 
was granted by the Deutsche Forschungsgemeinschaft (DFG-No 626/2-2). 
References 
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Table 1: Main characteristics and iron content of the four tested Fe0 materials. The material code 
(“code”) are from the author, the given form is as supplied; d (μm) is the diameter of the supplied 
material and the Fe content is given in % mass. 
 Supplier(a) Supplier denotation  code  form d 
(μm) 
Fe 
(%) 
MAZ, mbH Sorte 69(b) ZVI1  fillings - 93(c)
G. Maier GmbH FG 0000/0080 ZVI2  powder ≤ 80 92(d)
Würth Hartgussstrahlmittel ZVI3  spherical 1200 n.d.(e)
ISPAT GmbH Schwammeisen ZVI4 spherical 9000 n.d. 
(a) List of suppliers: MAZ (Metallaufbereitung Zwickau, Co) in Freiberg (Germany); Gotthart Maier 
Metallpulver GmbH (Rheinfelden, Germany), ISPAT GmbH, Hamburg (Germany), Connelly GPM Inc. 
(USA), 
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(b)Scrap iron material; (c) Mbudi et al. [19]; (d) average values from material supplier, (e)not 
determined. 
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Table 2: Extent of MB discoloration (PMB) at a Fe0 mass loading of 9.0 g/L, decrease of the 
percent MB discoloration (ΔP
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MB), and final pH value in the presence of 5 g/L Fe0 as the 
MnO2 loading varied from 0 to 1.1 g/L. For ZVI3 no decrease (ΔPMB = 3.5 %) was observed 
under the experimental conditions (50 min-1 for 6 days). Data for the rate of iron dissolution 
in a 2 mM EDTA solution (aEDTA) and the percent uranium removal (PU) are presented for 
comparison. 
Material PMB ΔPMB pHfinal aEDTA(a) PU(a)
 (%) (%)  (μM/h) (%) 
ZVI1 48.8 -4.8 7.8 1.95 81.0 
ZVI2 100.0 -38.4 7.4 2.65 100.0 
ZVI3 31.9 3.5 8.6 1.54 63.1 
ZVI4 49.1 -8.7 7.9 1.86 87.0 
(a) Values according to Noubactep et al. [2]. 261 
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Figure 1 262 
0 2 4 6 8
20
40
60
80
100 (a)
theoretical 5 g/L Fe0 + MnO2
 MnO2
 Fe0
 Fe0 + MnO2
M
B
 re
m
ov
al
 / 
[%
]
material / [g/L]
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264  
0 2 4 6 8
0
20
40
60
80
100 (b)
experiment
t = 6 days
N = 50 min-1
Fe0 = ZVI1
 Fe0
 Fe0 + MnO2
 MnO2
M
B
 re
m
ov
al
 / 
[%
]
material / [g/L]
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267 
 
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268 
Figure 2 
 
0 2 4 6 8 10
0
20
40
60
80
100
(a)
 ZVI3
 ZVI4
 ZVI1
 ZVI2
M
B
 re
m
ov
al
 / 
[%
]
Fe0 / [g/L]
 269 
270  
0 2 4 6 8 10
20
40
60
80
100
(b)
 ZVI1
 ZVI2
 ZVI3
 ZVI4
M
B
 re
m
ov
al
 / 
[%
]
MnO2 / [g/L]
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Figure Captions 278 
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Figure 1: Methylene blue discoloration by MnO2, Fe0, and “Fe0 + MnO2” for 6 days as a 
function of the material loading (Fe0, MnO2). ZVI1 is the used Fe0 source. In Fig 
1a the theoretical discoloration extent assuming additive removal of Fe0 (5 g/L) 
and varying amount of MnO2 is represented. Fig. 1b represents experimental 
results for the three systems together with the line corresponding to MB 
discoloration by 5 g/L Fe0 (no MnO2 addition). All lines are not fitting functions, 
they simply connect points to facilitate visualization. 
Figure 2: Methylene blue discoloration by the individual Fe0 materials (a) and 5 g/L each 
material and varying amounts of MnO2 (b). The experiments were performed for 6 
days while shaking at 50 min-1. The lines are not fitting functions, they simply 
connect points to facilitate visualization. 
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