1
Elemental metals for environmental remediation: lessons from hydrometallurgy 1 
R.A. Crane(a), C. Noubactep(b,c) 2 
 (a) Interface Analysis Centre, University of Bristol, 121 St. Michael’s Hill, Bristol, BS2 8BS, UK. 3 
(b) Angewandte Geologie, Universität Göttingen, Goldschmidtstraße 3, D - 37077 Göttingen, Germany. 4 
(c) Kultur und Nachhaltige Entwicklung CDD e.V., Postfach 1502, D - 37005 Göttingen, Germany. 5 
Correspond author; e-mail: cnoubac@gwdg.de; Tel. +49 551 39 3191, Fax. +49 551 399379 6 
Abstract 7 
In the mining industry, the separation of economically valuable metals from gangue materials 8 
is a well established process. As part of this field, hydrometallurgy uses chemical fluids 9 
(leachates) of acidic or basic pH to dissolve the target metal(s) for subsequent concentration, 10 
purification and recovery. The type and concentration of the leach solution is typically 11 
controlled to allow selective dissolution of the target metal(s), and other parameters such as 12 
oxidation potential, temperature and the presence of complexing/chelating agents. In the 13 
remediation industry the use of elemental metals (M0) for the removal of aqueous 14 
contaminant species is also a well established process. Removal is achieved by the oxidative 15 
corrosion of the M0 and associated pH and/or redox potential change. Whilst the two 16 
processes are directly opposed and mutually exclusive they both stem from the same 17 
theoretical background: metal dissolution/precipitation reactions. In the mining industry, with 18 
each prospective ore deposit physically and chemically unique, a robust series of tests are 19 
performed at each mine site to determine optimal hydrometallurgical fluid composition and 20 
treatment conditions (e.g. fluid temperature, flow rate) for target metal dissolution/yield. In 21 
comparison, within the remediation industry not all such variables are typically considered. In 22 
the present communication a comparison of the processes adopted in both industries are 23 
presented. The consequent need for a more robust empirical framework within the 24 
remediation industry is outlined. 25 
Keywords: Environmental remediation, Extractive metallurgy, Intrinsic reactivity, Metal 26 
dissolution, Zerovalent Metal. 27 
 2
1 Introduction 28 
The primary mechanisms of contaminant removal by elemental metals (M0) are considered to 29 
be adsorption, chemical reduction, complexation, co-precipitation, incorporation and size-30 
exclusion [1-6]. Chemical reduction at the M0 surface has often been cited as a key 31 
mechanism of contaminant removal. However, it should be recognised that although chemical 32 
reduction can have a profound effect on contaminant aqueous stability (e.g. solubility, 33 
degradation), it does not explicitly imply contaminant removal [7]. Chemically reduced 34 
contaminants can be removed via surface mediated accumulation/precipitation, but unless 35 
structurally entrapped/incorporated within the M0 corrosion production, the pollutant specie is 36 
always available for re-release/re-dissolution. It can therefore be stated that when determining 37 
the mechanisms which govern the efficacy of a M0 material for aqueous contaminant 38 
remediation, key focus must be applied to: (i) the mechanism of aqueous M0 corrosion; and 39 
(ii) the type, concentration and distribution of corrosion products formed.  40 
When testing M0 (typically Fe0 and Zn0) for environmental remediation, the influence of 41 
several operational parameters on the materials solubility can be investigated including 42 
solution pH, particle size, and the type and intensity groundwater flow [8-10]. A similar 43 
approach is applied in hydrometallurgical investigations. It has been shown that metal 44 
dissolution increases with (i) increasing stirring speed, (ii) acidic or basic pH value, and, (iii) 45 
decreasing particle size [11]. When testing M0 for remedial applications, the extent of 46 
contaminant removal is nominally defined by measuring the aqueous concentration of the 47 
pollutant specie at any given time. The dissolution of the M0 is generally also measured; 48 
however little comparative emphasis is typically placed on this variable [8]. With the 49 
solubility of M0 a key factor with regard to the materials performance for contaminant 50 
removal, this seems counterintuitive. For example, at pH < 4.5 the significantly high 51 
solubility of both FeII and FeIII dictates that minimal contaminant removal will be achieved 52 
[12]. 53 
 3
The objective of the present communication is to highlight the fundamental importance of 54 
tailoring the physical and chemical composition of M0, and mechanism of 55 
application/deployment, for environmental applications. In order to aid discussion, the 56 
procedures performed in the hydrometallurgy industry are comparatively examined. 57 
2 Hydrometallurgy 58 
Hydrometallurgy is a metallurgical process by which metals are extracted from an ore body 59 
using chemical reagents. To optimize the extraction efficiency, the process can be operated at 60 
a high temperature and under pressure [13]. Extracted metals are then separated to produce a 61 
concentrate or an intermediary product [13-15]. Another common metallurgical process is 62 
pyrometallurgy. This method, however, is typically considered as significantly more energy 63 
intensive and less versatile than hydrometallurgy [13,14-17]. For example, Zn was produced 64 
for over several hundred years via pyrometallurgy, however, in the 1980’s, a fully 65 
hydrometallurgical process was invented (Sherritt autoclave process) and production plants 66 
are now in abundant operation using this technology. Pyrometallurgy will not be further 67 
considered in this work as the focus in on processes occurring in aqueous solutions (for ore 68 
processing and metal corrosion). 69 
2.1 Dissolving metals in hydrometallurgy 70 
The dissolution of metals (metal leaching) from minerals can occur either through biological, 71 
chemical or electrochemical processes. Due to its low-cost, simple application and high metal 72 
yield, the most common method employed to date in the hydrometallurgy industry has been 73 
the use of chemical leachates. 74 
Leaching involves the use of aqueous solutions containing a lixiviant, the type of which 75 
leachate selected is typically dependent on the type of target metal, however, the type and 76 
concentration of any gangue material is generally also considered [11,18-21]. For example, 77 
chemical reducing agents within the gangue material (iron, organics, sulphur, etc.) can 78 
provide adsorptive sites for any redox-amenable contaminant species, such as uranium, 79 
 4
decreasing the efficiency of in-situ uranium leaching [19]. In such cases, complexing agents 80 
(such as dissolved carbonate), high temperatures and/or oxidising agents can be incorporated 81 
into the lixiviant to significantly enhance the uranium extraction yield [19]. 82 
2.2 Lessons from hydrometallurgy 83 
The myriad different factors to consider when assessing the metal/H2O system is an area of 84 
research that transcends both the hydrometallurgy and remediation industries.  85 
Similar to ore bodies, the reactivity of engineered metals depends primarily on three factors: 86 
(i) the nature and proportion of any associated, impurity or alloying elements (or gangue 87 
materials), (ii) the method of manufacture (or diagenesis), and (iii) the stability of the surface 88 
oxide layer when immersed in an aqueous environment. Until recently, materials have 89 
typically been characterised for the removal extent of selected contaminants without 90 
addressing the intrinsic reactivity which typically correlates with the metal’s tendency to 91 
dissolve [8-10]. 92 
The importance of this issue was recently presented for Fe0 [7,8]. Because Zn0 is currently 93 
investigated as an alternative to Fe0, Table 1 summarises the experimental conditions of 94 
recent work by Salter-Blanc and Tratnyek [6] and selected references therein. Table 1 95 
highlights that, when testing different Zn0 materials for environmental remediation, mass 96 
loadings from 25 to 250 g/L were tested under various mixing intensities and various 97 
experimental durations. The arising question is how to compare the obtained results? The 98 
electrochemical reactivity of Zn0 for contaminant reduction is given by the standard redox 99 
potential of the couple ZnII/Zn0 (-0.763 V), which is considered to be constant for Zn0 100 
regardless from the used particle size. However, numerous other factors must be constant in 101 
order to successfully compare different experiments, including: (i) the M0 mass loading; (ii) 102 
the presence of any associated, impurity or alloying elements; (iii) the solution pH; (iv) the 103 
solution temperature; (v) the particle size (surface area); and (vi) the solution stirring/agitation 104 
speed.  105 
 5
3 The metal/H2O system 106 
The aqueous solubility of metal ions (Mn+) from elemental metal (M0) at pH > 4.5 (natural 107 
waters) is strongly influenced by physical and chemical composition of the initial surface 108 
oxide layer [12,25,26]. Because contaminated natural waters contain target pollutants in trace 109 
amounts, the solvent (H2O) is typically present in large stoichiometric abundance. Therefore, 110 
the metal/H2O system in natural waters should be regarded as the primary domain for aqueous 111 
metal corrosion and precipitation reactions. The corrosion process may also be significantly 112 
influenced by the presence of the contaminants and other ubiquitous water species (including 113 
dissolved CO2 and O2). It is therefore not unexpected, that pure M0 oxide/hydroxides are 114 
typically the most abundant aqueous corrosion products identified [27-29]. 115 
4 Hydrometallurgy versus contaminant removal in metal/H2O systems 116 
In the hydrometallurgy industry, numerous different physico-chemical parameters are 117 
typically modified in order to improve the technique, including: (i) leachate composition (pH, 118 
presence of complexing/chelating agents, etc.); (ii) leachate temperature; and (iii) subsurface 119 
permeability. Depending on the specific chemistry of the target metal, the following 120 
respective alterations can be applied: (i) the leachate pH is buffered to either strongly acidic or 121 
basic and/or complexing/chelating agents are used; (ii) the leachate temperature is raised; and 122 
(iii) secondary subsurface fracturing techniques are employed to significantly enhance the 123 
subsurface permeability. In the present section the potential use of such processes in the 124 
remediation industry is comparatively discussed. The influence of M0 (or ore) particle 125 
size/surface area and mixing intensity/agitation is also included for discussion. 126 
4.1 Effect of solution pH  127 
The aqueous concentration of H+ (pH = -log[H+]) is arguably the important parameter for both 128 
the hydrometallurgy and remediation industries. Indeed, both metal solubility and metal 129 
speciation are strongly pH dependent (Figure 1). In aqueous systems, hydroxides are the most 130 
soluble phase of metals. Fig.1 compares the solubility of AlIII, FeII, FeIII and ZnII hydroxides 131 
 6
and shows that in the pH range of natural waters (pH 5.0 to 9.5), AlIII and FeIII hydroxides are 132 
relatively low soluble while FeII and ZnII hydroxides are relatively soluble [30]. 133 
In the hydrometallurgy industry, the pH can be shifted to more acidic or more basic ranges to 134 
optimize the extraction yield. For example, in-situ uranium leaching can be achieved by 135 
carbonate solutions (pH > 6.0) or by sulphuric acidic solutions (pH < 3.0) [19]. In the 136 
remediation industry, the solution pH must be maintained at a value which is favourable for 137 
metal oxide precipitation (e.g. pH > 4.5 for Fe0) [7,12]. This corresponds to the pH range of 138 
natural waters wherein the metal surface is considered bound by a ubiquitous oxide layer 139 
[12,25,29,31]. The initial oxide layer is considered porous and non protective, with 140 
subsequent transformations dependent on the reactivity of metal with the surrounding 141 
environment. 142 
4.2 The effect of temperature 143 
In general, increasing the solution temperature significantly accelerates the dissolution rate in 144 
both processes. In hydrometallurgy, temperatures can be discretionary elevated to optimize 145 
the extraction yield [11]. In remediation metal/H2O systems, ambient (room) temperatures 146 
(20.0 to 24.0 °C) are typically too high to accurately represent the subsurface environment, 147 
which is generally between 10.0 to 15.0 °C. 148 
4.3  The effect of subsurface permeability 149 
In the hydrometallurgy industry the permeability of the subsurface can be significantly 150 
improved by the use of secondary subsurface fracturing techniques, such as hydraulic, 151 
pneumatic or explosive processes. Such processes can be employed in the remediation 152 
industry to: (i) improve the flow rate of a contaminant plume which is passing through a 153 
permeable reaction barrier; or (ii) to facilitate the movement of nanoscale M0 into soil pores 154 
for in-situ aqueous pollutant treatment. Logistical factors such as workforce safety and the 155 
potential for accidental aqueous contaminant or nanoscale M0 release into the local 156 
groundwater systems however must be considered. 157 
 7
4.4 The effect of particle size 158 
Assuming the composition and shape of a M0 material remains constant; its tendency and rate 159 
of dissolution typically increases as particle size decreases. The specific surface area (SSA) 160 
and the particle diameter (d) are inversely proportional (SSA = 6/πd) [32]. Therefore, surface 161 
area exposure can be regarded as a useful indicator of: (i) the solubility of an ore body in the 162 
hydrometallurgy industry; or (ii) the aqueous reactivity and associated contaminant removal 163 
efficacy of the M0 in the remediation industry. However, this respectively assumes that: (i) the 164 
ore body is soluble in the leachate solution; and (ii) the M0 is a stronger chemical reducing 165 
agent that the aqueous contaminant specie(s). As a consequence, the intrinsic reactivity of the 166 
M0 should be characterised prior to testing different particle sizes and surface area. 167 
This is routinely performed in the hydrometallurgy industry, wherein numerous factors are 168 
typically considered, including: (i) the concentration of the leaching agent, (ii) the ore surface 169 
area, and (iii) the duration of the leaching process. However, in remediation industry such 170 
factors are often overlooked. In addition, a single M0 particle size and a single method of 171 
application is typically selected for contaminant remediation, whilst in the hydrometallurgy 172 
industry, several different application processes and lixiviant compositions are simultaneously 173 
employed to maximise metal dissolution and recovery [15,19]. 174 
4.5 The effect of the stirring speed 175 
The type and intensity of mixing operations is a strong driver for metal 176 
dissolution/precipitation. In the hydrometallurgical industry by increasing the leachate flow-177 
rate greater metal dissolution is typically ensured. In the remediation industry, the method of 178 
M0 application depends on the physical and chemical structure of the contaminated site. Care 179 
must therefore be taken during empirical tests to use mixing devices and mixing intensity that 180 
are relevant for the conditions of the field. In particular, the formation and transformation of a 181 
surface oxide layer in the vicinity of the metal has a strong bearing on the reactivity of the M0. 182 
For slow mixing speeds (e.g. < 50 min-1) oxide formation is not generally physically disturbed 183 
 8
[8]. A large number of studies have however used considerably higher mixing intensities. For 184 
example, Pang et al. [33] used a shaking intensity of 150 min-1 to keep nano-Fe0 suspended, 185 
while Hao et al. [34] used a mixer stirred at 500 min-1 to ease nitrate transport to the surface 186 
of iron filings. 187 
In addition, it is essential to consider contaminant transport to the metal surface as diffusion-188 
limited and discuss the nature and stability of oxide scales under relevant conditions 189 
[25,29,35]. 190 
5 Concluding remarks 191 
In the present work a comparison has been presented for the testing framework implemented 192 
within the hydrometallurgical industry for ore characterisation and acid/alkaline leaching and 193 
testing framework implemented within the remediation industry for contaminated site 194 
characterisation and aqueous pollutant removal. By comparison the considerably more robust 195 
empirical framework exhibited within the former industry presents a strong consequent need 196 
for more stringent protocols within the latter. It has been outlined that with every 197 
contaminated site chemically and physically unique there exists a strong need for empirical 198 
tests that are site specific and environmentally relevant. Only once this has been achieved, can 199 
a remedial material that is specific for the unique conditions of the contaminated site be 200 
effectively selected.  201 
Acknowledgments 202 
Data on metal hydroxide solubility were kindly provided by A.E. Lewis (University of Cape 203 
Town). The manuscript was improved by the insightful comments of anonymous reviewers 204 
from Fresenius Environmental Bulletin. 205 
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297 
 13
Table 1: The variability of operational conditions employed for batch experiments as 297 
illustrated by: specific surface area (SSA), mass loading (ρ) and redox conditions. The 298 
experimental designs also further differ in terms of mixing devices and intensities, and 299 
the type, concentration and speciation of aqueous contaminant species.. 300 
 301 
Mesh size SSA Density V mZn ρ Redox Ref. 
 (m2/g) (g/cm3) (mL) (g) (g/L)   
30 0,035 - 1000 200 200 anoxic [22] 
30 0,035 - 125 5 40 anoxic [23] 
-10 / 50 0,023 - 120 3,0 25 anoxic [24] 
30 0,038 - 120 3,0 25 anoxic [24] 
< 325 0,350 - 120 3,0 25 anoxic [24] 
< 325 0,620 2,60 80 20 250 anoxic [6] 
20 / 60 0,016 2,34 80 20 250 anoxic [6] 
200 / 325 0,160 3,27 80 20 250 anoxic [6] 
 302 
303 
 14
Figure 1: The pH dependence of metal hydroxide solubility for Al(OH)3, Fe(OH)2, Fe(OH)3 303 
and Zn(OH)2 [30]. It can be observed that if Zn0 is used for environmental remediation, care 304 
must be taken to control ZnII species concentration in the effluent. For Fe0, FeII species are 305 
rendered insoluble by oxidation to FeIII species. At neutral pH values, the solubility of AlIII 306 
and FeIII species is less than 10-7 M. 307 
 308 
4 6 8 10 12
-8
-6
-4
-2
0
 Al3+
 Fe2+
 Fe3+
 Zn2+
m
et
al
 io
ns
 / 
[m
ol
/L
]
pH value
 309