Elsevier

Journal of Hazardous Materials

Volume 181, Issues 1–3, 15 September 2010, Pages 1170-1174
Journal of Hazardous Materials

Short communication
Elemental metals for environmental remediation: Learning from cementation process

https://doi.org/10.1016/j.jhazmat.2010.05.085Get rights and content

Abstract

The further development of Fe0-based remediation technology depends on the profound understanding of the mechanisms involved in the process of aqueous contaminant removal. The view that adsorption and co-precipitation are the fundamental contaminant removal mechanisms is currently facing a harsh scepticism. Results from electrochemical cementation are used to bring new insights in the process of contaminant removal in Fe0/H2O systems. The common feature of hydrometallurgical cementation and metal-based remediation is the heterogeneous nature of the processes which inevitably occurs in the presence of a surface scale. The major difference between both processes is that the surface of remediation metals is covered by layers of own oxide(s) while the surface of the reducing metal in covered by porous layers of the cemented metal. The porous cemented metal is necessarily electronic conductive and favours further dissolution of the reducing metal. For the remediation metal, neither a porous layer nor a conductive layer could be warrant. Therefore, the continuation of the remediation process depends on the long-term porosity of oxide scales on the metal surfaces. These considerations rationalized the superiority of Fe0 as remediation agent compared to thermodynamically more favourable Al0 and Zn0. The validity of the adsorption/co-precipitation concept is corroborated.

Introduction

The use of metallic iron (Fe0) for environmental remediation is now well established [1], [2], [3], [4]. However, the exact mechanism of aqueous contaminant removal in the presence of Fe0 is not fully understood. It is univocally accepted that contaminant removal is due to the process of iron oxidative dissolution (iron corrosion). However, a net discrepancy exists on the role of the oxide scale on Fe0 in the process of contaminant removal. Oxide scale formation on Fe0 at pH > 4.5 is a fundamental characteristic of aqueous iron corrosion [5], [6], [7], [8]. The universal oxide scale on Fe0 is either regarded as beneficial (blessing) or inhibitory (curse) for aqueous contaminant removal in the presence of Fe0.

The prevailing concept was introduced in the early phase of investigations regarding the mechanism of aqueous contaminant removal by Fe0 [9], [10]. This concept considers that contaminant is removed mainly by a heterogeneous chemical reduction, ideally at the surface of Fe0. Accordingly, the oxide scale on Fe0 is a curse as its represents a diffusion barrier slowing down the kinetics of contaminant removal [11], [12]. The initial model assuming the local existence of oxide-free Fe0 in the aqueous solution was proven unrealistic by Bonin et al. [13]. A new conceptual model for the reductive transformation was proposed [8], [13], [14]. The conceptual model of Bonin et al. [13] indicated that the reductive transformation is controlled by electron transfer through the surface film. Accordingly the film must be electronic conductive. However, no such conductive film is expected in nature [6], [15], [16]. Moreover, the concept regarding oxide scale as curse is built on the premise that Fe0 is a strong reducing agent. The concept is strictly applicable only to reducible contaminants.

It is important to notice that the reductive transformation concept has never been univocally accepted [17], [18]. For example, Warren et al. [17] wrote that “a convincing mechanism for the reductive dehalogenation of haloorganics by zero-valence metals has not yet been proposed. Matheson and Tratnyek [9] maintained that dehalogenation was not mediated by H2(g) or FeII in the bulk aqueous-phase solution, suggesting that observed reactions take place at the metal surface.” Three years later, O’Hannesin and Gillham [1] acknowledged that “there is a broad consensus that the process is an abiotic redox reaction involving reduction of the organic compound and oxidation of the metal”. Despite this “broad consensus”, the reductive transformation concept has felt to explain many experimental observations [19], [20], [21].

An alternative concept regards the oxide scale on Fe0 as beneficial (a blessing) for the process of aqueous contaminant removal [22], [23], [24], [25]. Independent researchers could traceably demonstrate that quantitative contaminant removal is only observed when iron corrosion products are allowed to precipitate in the system [26], [27], [28], [29], [30], [31]. Their results suggest that adsorption and co-precipitation are the fundamental (not the dominant or the major) contaminant removal mechanisms. Accordingly, relevant contaminants could be further (quantitatively) chemically transformed (reduced or oxidized). The first merit of this concept it that its explains why a contaminant like zinc which is non-reducible by Fe0 (Table 1) could be quantitatively removed in the presence of Fe0 [32].

The present communication is motivated by recent publications speaking disparagingly about the concept of adsorption/co-precipitation as fundamental mechanisms of aqueous contaminant removal in the presence of Fe0 [33], [34]. The similarities between aqueous contaminant removal by Fe0 and metal iron cementation on elemental metals (mostly Al0, Fe0, Zn0) will be discussed with the aim to present results from the hydrometallurgical process of cementation which could help to understand and further develop the process of aqueous contaminant removal by Fe0. Both processes are heterogeneous and the metal surface is covered by a scale acting as diffusion barrier. For the sake of clarity the diffusion barrier in the Fe0 remediation will first be presented.

Section snippets

Aqueous contaminant removal by metallic iron

Aqueous iron corrosion on which remediation with metallic iron is based is a heterogeneous electrochemical process. A simplistic mechanism for iron oxidative dissolution involves four major steps: (i) diffusion of the oxidizing agent (H+, O2, contaminant) to the Fe0 surface, (ii) adsorption of the oxidizing agent onto the iron surface, (iii) the reduction of the oxidizing agent, and (iv) diffusion of reaction products (including FeII species) away from the reactive site on Fe0. Because aqueous

Cementation and its use in the hydrometallurgy

Cementation is an electrochemical process by which a more noble metal ion (Mn+ – Eq. (1)) is precipitated from solution and replaced by a metal higher in the electromotive series (M1m+ – Eq. (2)) [39], [40], [41], [42], [43], [44], [45]. Cementation, also known as contact reduction or metal displacement, is necessarily a spontaneous heterogeneous reaction (ΔG0 < 0) that takes place through the galvanic cell M10/M1m+/Mn+/M (Eq. (3)):mMn++mnemM0E0(V)nM10nM1m++mneE10(V)nM10+mMn+nM1n++mM0ΔE

Cementation using Al, Fe and Zn

The control of the pH value is a key task for the cementation process for a variety of reasons including: (i) corrosion damage of reactors, (ii) excess dissolution of the reducing metal (Al, Fe and Zn), and (iii) hydroxide precipitation. Accordingly, the determination of the optimal pH value is an important economical issue for any cementation plant. The impact of pH on the performance of Al, Fe and Zn as reducing metal will be discussed on the basis of the results from Hg2+ cementation by Al0,

Diffusion layers on remediation elemental metals

Diffusion is a spontaneous process involving mobility of species due to the existence of a concentration gradient in a system. The extend of diffusion depend on (i) the properties of the diffusing species (including their size) and (ii) the structure of the diffusion layer (connectivity, morphology, porosity, pore site distribution or tortuosity). Here the diffusion layer is a precipitated scale (oxide scale).

Oxide layers on remediation metals are formed at pH > 5.0 which is the pH of natural

Concluding remarks

The formation of surface scale on immersed elemental metals is a common feature for remediation with metallic elements and electrochemical cementation (Table 2). In both cases the surface scale primarily inhibits the metal dissolution and thus the kinetics of the concerned process. The formation of an oxide film on the cementation agent can be prevented (or limited) by a rational selection of the operational conditions (e.g. pH value, amount of cementation agent, and mixing operations).

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