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On the Operating Mode of Bimetallic Systems for Environmental Remediation

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Chicgoua Noubactep at Georg-August-Universität Göttingen
Chicgoua Noubactep
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Abstract

This letter challenges the concept that Fe(0)/Me(0) bimetallic systems enhance contaminant reduction on Me(0) surfaces. It is shown on a pure thermodynamic perspective that any enhancement of contaminant reduction by Fe(0) in the presence of a second more electropositive elemental metal (Me(0)) is the result of an indirect process resulting from iron corrosion. This demonstration validates the concept that aqueous contaminant removal in the presence of Fe(0) mostly occurs within an in situ generated oxide film on Fe(0).
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On the Operating Mode of Bimetallic Systems for Environmental 1
Remediation 2
Noubactep C. 3
Angewandte Geologie, Universität Göttingen, Goldschmidtstraße 3, D - 37077 Göttingen, Germany. 4
e-mail: cnoubac@gwdg.de; Tel. +49 551 39 3191, Fax: +49 551 399379 5
6
Abstract 7
This letter challenges the concept that Fe0/Me0 bimetallic systems enhance contaminant 8
reduction on Me0 surfaces. It is shown on a pure thermodynamic perspective that any 9
enhancement of contaminant reduction by Fe0 in the presence of a second more 10
electropositive elemental metal (Me0) is the result of an indirect process resulting from iron 11
corrosion. This demonstration validates the concept that aqueous contaminant removal in the 12
presence of Fe0 mostly occurs within an in-situ generated oxide film on Fe0. 13
14
Keywords: Adsorption; Bimetallic system; Co-precipitation, Zerovalent iron; Reduction. 15
16
A metallic surface can be involved in chemical reactions in various ways: a metallic material 17
can serve as a redox agent or catalyst, facilitating a reaction, or it can release metal species 18
into the system [1,2]. Elemental iron (Fe0) and Fe0/Me0 bimetallic systems used in water 19
remediation (Fe0-H2O systems) are typical systems were all these three reaction paths might 20
be involved: (i) Fe0 might serve as reducing agent (direct reduction), (ii) Fe0 surface might 21
serve a catalyst for instance for the reduction through molecular (H2) or atomic hydrogen (H) 22
and (iii) Fe0 might release FeII and H/H2 into the system. A Fe0/Me0 system is a system where 23
the metallic surface should serve as a catalyst for contaminant reduction through hydrogen 24
(H/H2). 25
A survey of the voluminous literature on environmental remediation with Fe0 shows that all 26
factors increasing Fe0 oxidation enhance contaminant removal. These factors include (i) the 27
presence of molecular oxygen [3], (ii) the addition of a second more electropositive metal 28
(e.g. Ag0, Co0, Cu0, Ni0, Pd0, Pt0, Ru0) yielding bimetallic systems [4,5], and (iii) increasing 29
the surface area of iron by reducing its particle size [4]. Increasing Fe0 oxidation is directly 30
correlated with increased generation of iron corrosion products (e.g. iron oxyhydroxides) 31
which are well-known for their adsorptive capacity for both organic and inorganic compounds 32
[6]. Iron corrosion products are formed as an oxide film at the Fe0 surface. To reach the 33
underlying Fe0 surface a contaminant molecule should migrate across the film. 34
In discussing aqueous contaminant removal in the presence of Fe0, reduction at the Fe0 35
surface and adsorption onto iron corrosion products have traditionally been evaluated as 36
separate, independent processes that occur simultaneously or sequentially. Thereby the 37
dynamic nature of the formation of the oxide film on Fe0 [8] has been almost overseen. 38
However, during their formation and transformation iron corrosion products likely sequestrate 39
foreign species, including contaminants [9]. Therefore, the author of ref. [9] has revisited the 40
concept of reductive transformations [3,10] and introduced a new concept considering 41
adsorption and co-precipitation of contaminants with iron corrosion products as primordial 42
removal mechanism. The present letter shows that the conception that bimetallic systems 43
enhance reductive transformation by Fe0 is incompatible with the premise that Fe0 is the 44
reducing agent in Fe0-H2O systems (statement 1). It has been reported that the presence of 45
Pd0 speeds up the reduction reaction as follows: on the Pd0 surface, molecular hydrogen (H2) 46
from iron corrosion is adsorbed and dissociated into more reducing atomic H; atomic H 47
attacks chlorinated contaminants (R-X) and transforms them to R-H and Cl- [11]. Therefore, 48
the better well-dispersed the Pd0 in the Fe0/Pd0 system, the higher the catalytic effect. But, as 49
recalled above, an universal oxide film shields the bimetallic surface [9,12]. Catalytic 50
hydrodehalogenation is a well-known decontamination process [13]; it differs from the 51
reductive dehalogenation reactions by Fe0 and Fe0/Pd0 systems in that the catalytic surface 52
(Pd0) and the electron donor (H2) are supplied as two separate reagents. 53
To demonstrate the absurdity of statement 1, lets consider the bimetallic system Fe0/Pd0 and 54
a chlorinated hydrocarbon (RCl) to be reduced by the bimetallic. The involved electrode 55
potentials (E0) are: 0.915 V for the couple PdII/Pd0, 0.41 to 0.59 V for the couple RCl/R° [14], 56
and –0.44 V for the couple FeII/Fe0. The higher the E0 value, the stronger the reducing 57
capacity of Fe0 for the oxidant of a couple. Comparing the three E0 values, it is evident that 58
PdII and RCl are concurrent oxidants for Fe0, PdII been the strongest. Therefore, if any RCl 59
removal enhancement is observed in the presence of PdII it is indirectly related to Fe0 60
oxidation. Thus enhanced contaminant reduction by bimetallics [4,5,15] is an argument for 61
indirect reduction (by FeII or H/H2 within the oxide film on Fe0) [12]. Because of the 62
omnipresence of the oxide film, even if the reducing agent is H/H2, the reduction is not likely 63
to occur at the Pd0 surface. On the other hand FeII adsorbed onto the oxide film (FeII(s) or 64
structural FeII) has been shown to be a very strong reducing agent [16]. 65
For illustration, consider an ideal redox indicator for the titration of Fe0 by PdII (Eq. 1) having 66
a standard potential of 0.238 V. The redox half-reaction of the indicator is described by 67
equation 2 [17], where Indox
is the coloured oxidized form of the indicator, Indred is the 68
corresponding colourless reduced form, n is the number of electrons transferred (typically 1 or 69
2), and m is the number of protons transferred (typically 0, 1 or 2) and is dependent on the 70
pH. 71
Fe0 + Pd2+ ⇔ Fe2+ + Pd0 (1) 72
Indox
+ ne- + m H+ ⇔ Indred (2) 73
Indred + Pd2+ ⇔ Indox
+ Pd0 (3) 74
The end of the titration (Fe0 depletion) is detected by the appearance of a colour in the 75
solution. This colouration of the solution corresponds to the oxidation of the reduced form of 76
the indicator to the oxidized form (Eq. 3) by Pd2+ ions. To obtain accurate results, the lowest 77
possible amount of indicator should be used. In the titration context, no one can claim that 78
PdII enhances the reduction of Indox
by Fe0. PdII oxidizes both Indox
and Fe0. In this 79
competition Fe0 is the stronger electron donor. Therefore, if any Indox
reduction enhancement 80
is observed in the presence of PdII, it can only indirectly be related to Fe0 oxidation. It is true 81
that the surface of Pd0 and not dissolved PdII is the catalyst for contaminant reduction by 82
H/H2. The Pd0 surface is however, shielded as a rule, and dissolved PdII will (at least partly) 83
co-precipitate with iron hydroxides and will no more be available for catalytic activity. 84
In conclusion, Pd0 and other bimetallic elements (Co0, Cu0, Ni0, Pt, Ru0…) can not 85
significantly enhance contaminant reduction by elemental iron (electron from Fe0 or from 86
H/H2). Consequently, the reported increased contaminant removal by bimetallic systems is a 87
result of secondary redox processes within the oxide film on Fe0 (electron from FeII or from 88
H/H2). This conclusion is a further negation of the well-established concept of direct reductive 89
transformations as major decontamination process [9, 12]. 90
For the further development of the iron reactive wall technology target experiments should be 91
performed to investigate the influence of hydrodynamic shear stress on the transport, transfer 92
and reaction rates within the oxide film, as well as film detachment under experimental 93
conditions pertinent to natural situations. This means that experiments must be performed 94
under conditions which favour oxide film formation and transformation. Redox processes 95
within a film on iron is well documented in the context of microbiologically influenced 96
corrosion [18]. 97
Acknowledgments 98
Thoughtful comments provided by Angelika Schöner (Martin-Luther-University of Halle, 99
Germany) on the draft manuscript are gratefully acknowledged. 100
References 101
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... On the other hand, the transformation process is slowed down as the natural oxide scale develops at the Fe 0 surface. It is in this context that bimetallic systems were introduced, wherein a second metal is combined with Fe 0 [127,128]. The second metal primarily has three functions: (i) Acts as a hydrogenating catalyst, (ii) prevents the formation of the oxide film on the Fe 0 surface, and (iii) induces Fe 0 to release electrons due to the difference in reduction potentials [127,129,130]. ...
... The second metal certainly disturbs the formation of the oxide scale [63][64][65], but cannot really prevent it in the long term. The property of the second metal to induce electron release from Fe 0 is a fundamental aspect and likely the most accurate (Section 3) [79,128,131]. ...
... As demonstrated by Noubactep [128] in the pH range of natural waters, the second metal primarily induces Fe 0 oxidative dissolution and accelerates-or at least sustains-the corrosion process, which in turn induces contaminant removal. Remember that Fe 0 is a generator of contaminant scavengers. ...
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... Having neglected this key evidence, researchers have been hopelessly seeking for "common underlying mechanisms" for reactions in Fe 0 /H 2 O systems "that provide a confidence for design that is non-site-specific" [67]. However, used treatability experiments were ill-designed (shaken or stirred) and are interpreted with wrong descriptors like the k SA concept [22][23][24][25]. ...
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Catalytic hydrodehalogenation of chlorinated ethylenes using palladium and hydrogen for the treatment of contaminated water
Article
  • Sep 1995
A kinetic model is presented for the catalytic hydrodehalogenation of chlorinated ethylenes using Pd and H2 under water treatment conditions. All five chlorinated ethylenes, including tetrachloroethylene (PCE) and vinyl chloride, were completely removed from tap water within 10 minutes at room temperature by 0.5 g of 0.5% Pd on alumina and 0.1 atm H2. Ethane accounted for 55–85% of the mass balance in these systems. Ethene was a reactive intermediate whose maximum concentration accounted for less than about 5% of the initial substrate. Palladium on granular carbon was also an effective catalyst, although ethane yield for PCE was somewhat lower than with Pd-alumina (55% versus 85%). The transformation of PCE was first order with respect to both substrate and amount of metal, with a half-life of t12 = 9 min for 0.055 μmole Pd (583 μg of 1% Pd on powdered activated carbon). Addition of ∼10 mg/L of nitrite to the water decreased the rate constant by about 50%. The nitrite concentration decreased by about 25% over the course of the reaction. Addition of nitrate or sulfate had smaller effect on the rate of PCE transformation; chloride had no effect. The presence of oxygen greatly reduced the amount of ethane produced regardless of the catalyst support. Bisulf1de poisoned the catalyst.
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From Bacon to Barriers: A Review on the Passivity of Metals and Alloys
Article
  • Mar 2002
Passivity of metals and alloys is a phenomenon of utmost technical importance as it prevents many construction materials from rapid deterioration. For the majority of metals, passivity is based on the spontaneous formation of a thin oxide layer (the passive film), in a specific environment. This film can slow corrosion (dissolution) reactions by many orders of magnitude. The present review tries to give an overview of several important aspects and factors that influence film formation, stability and breakdown. Emphasis is on chemical stability and its connection to thermodynamic, kinetic and electronic aspects of the metal/oxide/environment system. Although some metals (e.g. Fe) and alloys (e.g. Fe/Cr) are treated with greater depth, the focal point is the description of general mechanistic approaches – for specific systems the reader is referred to secondary literature.
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Reduction of Aqueous Transition Metal Species on the Surface of Fe(II)-Containing Oxides
Article
  • Oct 1996
  • GEOCHIM COSMOCHIM AC
Experimental studies demonstrate that structural Fe(II) in magnetite and ilmenite heterogeneously reduce aqueous ferric, cupric, vanadate, and chromate ions at the oxide surfaces over a pH range of 1–7 at 25°C. For an aqueous transition metal m, such reactions are and where z is the valance state and n is the charge transfer number. The half cell potential range for solid state oxidation [Fe(II)] → [Fe(III)] is −0.34 to −0.65 V, making structural Fe(II) a stronger reducing agent than aqueous Fe2+ (−0.77 V). Reduction rates for aqueous metal species are linear with time (up to 36 h), decrease with pH, and have rate constants between 0.1 and 3.3 × 10−10 mol m−2 s−1. Iron is released to solution both from the above reactions and from dissolution of the oxide surface. In the presence of chromate, Fe2+ is oxidized homogeneously in solution to Fe3+.X-ray photoelectron spectroscopy (XPS) denotes a Fe(III) oxide surface containing reduced Cr(III) and V(IV) species. Magnetite and ilmenite electrode potentials are insensitive to increases in divalent transition metals including Zn(II), Co(II), Mn(II), and Ni(II) and reduced V(IV) and Cr(III) but exhibit a log-linear concentration-potential response to Fe(III) and Cu(II). Complex positive electrode responses occur with increasing Cr(VI) and V(V) concentrations. Potential dynamic scans indicate that the high oxidation potential of dichromate is capable of suppressing the cathodic reductive dissolution of magnetite. Oxide electrode potentials are determined by the Fe(II)/Fe(III) composition of the oxide surface and respond to aqueous ion potentials which accelerate this oxidation process.Natural magnetite sands weathered under anoxic conditions are electrochemically reactive as demonstrated by rapid chromate reduction and the release of aqueous Fe(III) to experimental solution. In contrast, magnetite weathered under oxidizing vadose conditions show minimum reactivity toward chromate ions. The ability of Fe(II) oxides to reduce transition metals in soils and groundwaters will be strongly dependent on the redox environment.
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An Overview of Microbiologically Influenced Corrosion
Article
  • Sep 1992
  • ELECTROCHIM ACTA
Biofilms on metal surfaces produce an environment at the biofilm/metal interface that is radically different from that of the bulk medium in terms of pH, dissolved oxygen, organic and inorganic species. The term microbiologically influenced corrosion (MIC) is used to designate corrosion due to the presence and activities of microorganisms within biofilms. In this review we have correlated electrochemical reactions to the activities of microorganisms to show that microorganisms can accelerate rates of partial reactions in corrosion processes or alter the corrosion mechanism.
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A method for the rapid dechlorination of low molecular weight chlorinated hydrocarbons in water
Article
  • Oct 1995
  • WATER RES
1,1,2-Trichloroethylene (TCE), 1,1-dichloroethylene, cis and trans-1,2-dichloroethylene and tetrachloroethylene (PCE), at concentrations of 20 ppm in aqueous solutions were rapidly hydrodechlorinated to ethane (in a few minutes), on the surface of palladized iron in batch experiments that were performed in closed vials. No intermediate reaction products such as 1,1-dichloroethylene, 1,2-dichloroethylenes and vinyl chloride were detected at concentrations > 1 ppm either in the headspace or in solution. The chloromethanes, CCl4, CHCl3 and CH2Cl2 were also dechlorinated to methane on palladized iron; the CCl4 was dechlorinated in a few minutes, the CHCl3, in less than an hour and the CH2Cl2, in 4–5 h. These results indicate that an above-ground treatment method can be designed for the treatment of groundwater contaminated with low molecular weight chlorinated hydrocarbons.
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