**4.1. Thermal treatment**

Among the plants that can be used in phytoremediation of mine tailings, studies have focused on *Eriophorum angustifolium*, a plant resistant to substrates with a wide range of pH from 10.9 to 2.7. Other species of plants that can be grown in a low pH environment are *Carex rostrata*, *Eriophorum scheuchzeri*, *Phragmites australis*, *Typha angustifolia*, *Typha latifolia*, which grows to a

The studies made since 1977 by the American biologist Dr. Robert Brooks [16], have shown that metals can be extracted from plants (e.g. Ni, Zn, Pb and Au), but the facility of this process depends on the density and solubility of the elements. From the first experiments was obtained 0.01 g of Ni from few kilograms of plant biomass and, more recently, 10 g of Au were obtained from a two hectares of rape culture, established in the vicinity of abandoned mines in Cali‐

So far there have been numerous studies on the phytoremediation process, having examined the species of plants that have a greater ability to accumulate heavy metals, factors affecting the results of phytoremediation and areas that should be remediated with plants. In terms of treatment, storage or heavy metal recovery from the biomass resulted from the phytoreme‐

pH value of 2.1, 4.4, 2.1, 3.0 and 2.5 respectively [14].

**Figure 2.** Phytoremediation of heavy metals polluted soil [2]

314 Environmental Risk Assessment of Soil Contamination

**4. Methods for heavy metal recovery**

diation process the reference studies are scant.

fornia.

During the thermal treatment (incineration or pyrolysis), the organic matter from biomass is destroyed and metals remain in ash as oxides [25] which can be recovered by hydrometallur‐ gical processes [22,23,26] and bio-hydrometallurgical processes [27], ion exchange [28,29,30,31,32], flotation [19,33], magnetic field [34,35] or electrolysis [3,18,21] and bioelectrochemical procedures [36].

Due to the high content of oxygen, plant biomass can be easily incinerated, requiring only small volume of air during the combustion. In addition, relatively low sulfur content of the biomass is an advantage because there are no harmful gases released in the atmosphere during the combustion [20]. A negative aspect of incineration at very high temperatures (over 600 °C) is the fact that some heavy metals, including those with the greatest risk of toxicity, are volatile: Pb, Zn, Cd, Se [37]. Volatilization of metals can be exploited as an advantage in the recovery of metals, because these elements condense in the cooler areas of the incineration oven and are adsorbed on fine particles of ash retained in the cyclone or on filters [37,38]. The metal recovery is more efficient due to higher metal concentrations by mass of ash [20,37]. The mechanism which determines the behavior of metals during incineration process is characterized by three aspects: (a) evaporation in the combustion areas and condensation of metals in the lower temperature zones of the furnace, (b) physical adsorption on specific surface area of ash particles, and (c) chemo-absorption [38]. The surface area of ash particle is determined primarily by unburned carbon and assessed by electron microscopy (SEM) and is a determin‐ ing factor for the adsorption of volatilized metals [18,38].

To minimize the risk of volatile metals reaching the atmosphere, the incineration plant shall be fitted with ash particle filtering and retrieval (filters and cyclone). Mercury, selenium and arsenic are metals with the highest percentage of vaporization into incineration system. Mercury is highly volatile and can be delivered almost entirely as vapor in the form of HgO and HgCl2. Oxidized form of mercury is easily collected from air pollution control system [37].

Keller et al. [3] demonstrated by their study that, better results of the thermal treatment of the biomass are obtained by pyrolysis, under reducing conditions, compared to incineration. The researchers aimed only the recovery of volatile metals (especially copper and zinc), from shoots of Salix use in phytoremediation. These statements are subject to the heavy metal content of ash, which have to lie under the maximum permissible level if they are used as amendment.

#### **4.2. Hydrometallurgical processes**

Extraction of heavy metals by ash leaching is a complex chemical process which offers the possibility of obtaining quantitative precipitated metal. The solutions used for leaching of ash must be environmentally friendly, efficient, cheap and with a high capacity for regeneration.

In the leaching process, the extraction of heavy metals is subject to such factors as the solubility and availability of the metal. The solubility can be influenced by pH, the chemical form of inorganic species, organic matter, and the reducing properties. Most metals from waste indicated a higher solubility in acid solutions [39]. Singer et al. [40] tested the extraction of aluminum with citric acid at different temperatures and the results showed that the extraction of metal was considerably influenced by the concentration of the acid used and by temperature. Based on this study, Machado et al. [23] have studied the recovery of Ni and Zn in a multicomponent solution by precipitation in the form of alkali metal hydroxide. As a result of the experiment the researchers obtained a recovery rate of over 99% and a purity of 92% for Ni and 99.4% for Zn. The main factor which influences the precipitation of metals from NaOH solution was the pH, which defined the precipitation of Ni at pH 14 and the precipitation of Zn at pH 10. Recovery of Cd, Cu and Pb by leaching with NaHCO3 was investigated by Lezcano et al. [22] who obtained different amounts of metal in relation to pH.

To increase the solubility of metals from ores, Hoque & Philip [27] proposed the introduction of a microbial population, to convert insoluble metal sulfides into soluble metal sulfates. For example, for the extraction of copper from copper sulfide, it was oxidized by microorganisms to copper sulfate. The metal ions have been concentrated in the aqueous phase and the solid residues were removed [27]. A similar technology is used for the conversion of solid metal in water soluble form, in the presence of microorganisms. The technology is called biooxidation and is used for the microbiological oxidation of metal-containing minerals to be extracted.

Bio-hydrometallurgical processes are used in copper metallurgy in the presence of the bacterium *Thiobacillus ferooxidans*, carrying bivalent iron to trivalent iron oxidation [41]:

$$\begin{aligned} \text{CuFeS}\_2 &+ 4 \text{ O}\_2 \rightarrow \text{CuSO}\_4 + \text{FeSO}\_4\\ \text{FeSO}\_4 &+ \text{H}\_2\text{SO}\_4 + \text{V}\bullet \text{O}\_2 + \text{bacetia} \rightarrow \text{Fe}\_2\text{(SO}\_4\text{)}\_3 + \text{H}\_2\text{O} \\ 2\text{ Fe}\_2\text{(SO}\_4\text{)}\_3 &+ \text{CuFeS}\_2 + 3\text{ O}\_2 + 2\text{ H}\_2\text{O} \rightarrow \text{CuSO}\_4 + 5\text{ FeSO}\_4 + \text{H}\_2\text{O} \end{aligned}$$

The reaction takes place in aqueous solution, and the last two reactions are cyclic ensuring the continuous development of the leaching process of the chalcopyrite, while copper passes in the solution as sulfate. Another type of bacteria is used to oxidize the sulfur (*Thiobacillus sulfooxidans*). The solutions obtained by leaching, after purification and concentration, can be processed to extract metal ions which we are interested in.

#### **4.3. Electrochemical processes**

Mercury is highly volatile and can be delivered almost entirely as vapor in the form of HgO and HgCl2. Oxidized form of mercury is easily collected from air pollution control system [37]. Keller et al. [3] demonstrated by their study that, better results of the thermal treatment of the biomass are obtained by pyrolysis, under reducing conditions, compared to incineration. The researchers aimed only the recovery of volatile metals (especially copper and zinc), from shoots of Salix use in phytoremediation. These statements are subject to the heavy metal content of ash, which have to lie under the maximum permissible level if they are used as amendment.

Extraction of heavy metals by ash leaching is a complex chemical process which offers the possibility of obtaining quantitative precipitated metal. The solutions used for leaching of ash must be environmentally friendly, efficient, cheap and with a high capacity for regeneration.

In the leaching process, the extraction of heavy metals is subject to such factors as the solubility and availability of the metal. The solubility can be influenced by pH, the chemical form of inorganic species, organic matter, and the reducing properties. Most metals from waste indicated a higher solubility in acid solutions [39]. Singer et al. [40] tested the extraction of aluminum with citric acid at different temperatures and the results showed that the extraction of metal was considerably influenced by the concentration of the acid used and by temperature. Based on this study, Machado et al. [23] have studied the recovery of Ni and Zn in a multicomponent solution by precipitation in the form of alkali metal hydroxide. As a result of the experiment the researchers obtained a recovery rate of over 99% and a purity of 92% for Ni and 99.4% for Zn. The main factor which influences the precipitation of metals from NaOH solution was the pH, which defined the precipitation of Ni at pH 14 and the precipitation of Zn at pH 10. Recovery of Cd, Cu and Pb by leaching with NaHCO3 was investigated by Lezcano

To increase the solubility of metals from ores, Hoque & Philip [27] proposed the introduction of a microbial population, to convert insoluble metal sulfides into soluble metal sulfates. For example, for the extraction of copper from copper sulfide, it was oxidized by microorganisms to copper sulfate. The metal ions have been concentrated in the aqueous phase and the solid residues were removed [27]. A similar technology is used for the conversion of solid metal in water soluble form, in the presence of microorganisms. The technology is called biooxidation and is used for the microbiological oxidation of metal-containing minerals to be extracted.

Bio-hydrometallurgical processes are used in copper metallurgy in the presence of the bacterium *Thiobacillus ferooxidans*, carrying bivalent iron to trivalent iron oxidation [41]:

+ H2O

The reaction takes place in aqueous solution, and the last two reactions are cyclic ensuring the continuous development of the leaching process of the chalcopyrite, while copper passes in

et al. [22] who obtained different amounts of metal in relation to pH.

+ CuFeS2+3O2+2H2O →CuSO4+ 5 FeSO4+ H2O

**4.2. Hydrometallurgical processes**

316 Environmental Risk Assessment of Soil Contamination

CuFeS2+4O2→CuSO4+ FeSO4

2 Fe2(SO4)3

FeSO4+ H2SO4+½O2+ bacteria→Fe2(SO4)3

Electrochemical processes for the extraction of heavy metals have the advantage of selective recovery of metals, depending on the metals reduction potential of the metal to be extracted, but in order to obtain a high purity for every metal, tests must be performed to optimize the factors which influence the metal deposition on the electrode (pH and the electrolyte concen‐ tration, the temperature of the electrolytic bath and the metal species). The metal extraction by electrolysis is common use in the metallurgy of zinc, copper, nickel, etc.

Each metal has a specific ion discharge potential, which corresponds to the minimum potential at which an ion electrode begins to discharge continuously and visible (substance discharge). When a substance has more ions, they are discharged successively as they achieve the potential of each download. On this basis is realized the separation and selectively deposition of a number of metal ions from the same solution, if their discharge potentials are differing by at least 0.2 V, otherwise they are deposited at the same time. The ions can be electro-gravimetric separated with a determination error of less than 0.1%.

Fukuta et al. [42] have obtained the selective recovery of Cu, Ni and Zn with sodium sulfide, but in 2011, Machado et al. [43] conclude that, because of the difference in deposition potential of only 0.25 V, the separate recovery of Ni and Zn by electrolysis may be compromised, with the risk of co-deposition of the two metals. For this reason, the research was continued and the electrolyte was used to test the removal of the two metals by precipitation. Ni extraction by electrochemical processes was tested also by Lee [18]. The subject solution of this experiment was a spent electro-less nickel plating solution, the electrode used as the anode was made of platinum, and the cathode was made of stainless steel. Just as in the previous studies, pH played a key role in the extraction of Ni, and, at the end of the process, the metal was obtained in the form of nickel hydroxide and nickel fine particles.

The study conducted by Kirkelund et al. [21] to remove the Cd from the residual plant material by electrochemical methods is based on the principle of electro-migration of ions in solution, in an electric field. The researchers used membranes for the anions and cations exchange to optimize the process. Optimization of results for metal extraction was pursued also by Modin et al. [36] who applied a bio-electrochemical process for recovering Cu, Pb, Cd and Zn in dilute solutions. The anode was inoculated with micro-organisms from the sewage sludge and in the anode chamber a nutrient solution was circulated. The advantage of this method is the less energy consumption for the metal discharge and the selectivity of metal extraction [44].
