**1. Introduction**

Metals present numerous benefits for everyday life. They have contributed to the development of civilization, to the modernization and development of industries. In some cases, they are essential for maintaining the metabolism of the human or animal body, or they are indispensable in the growth of plants as microelements [1, 2]. However, if metals exceed certain concentration levels (even trace amounts), they can contribute to environmental pollution and can lead to devastating effects on living organisms [3, 4]. **Figure 1** shows the main sources of metal pollution and their transport through an environment. Water is the most responsible environmental factor, with the help of which the heavy metals present at improper concentrations can reach living organisms. Furthermore, heavy metals have the property of bioaccumulation since they could not be biodegradable, leading to critical health issues [5–7]. Therefore, water pollution with heavy metals represents a global concern and the World Health Organization (WHO) established the maximum admitted level of heavy metal concentrations in drinking water [8].

**Figure 1.** *Sources and transport of heavy metals through an environment.*

The removal of heavy metals from water sources or wastewater before discharge is an important problem that must be carried out to minimize pollution, reduce their interference with beneficial uses, and eliminate their negative effects on the environment. In some cases, in addition to eliminating the toxic effect of the metals present in waters upon the environment, these water treatment methods also have the purpose of recovering and revaluing the metals, especially if we are talking about rare and precious metals (such as gold, silver, platinum, palladium, rhodium, ruthenium, iridium, and osmium). The present chapter summarizes the treatment methods employed for heavy metals removal, describing new advanced developments, and highlighting the advantages and disadvantages of each in terms of efficiency, accuracy, feasibility, and kinetic.

### **2. Chemical precipitation**

Chemical precipitation (**Figure 2**) is the process of transforming soluble metal ions into insoluble metal compounds using various precipitating agents, such as hydroxides, carbonates, sulfides, sulfates, phosphates, chlorides, and sodium borohydride, usually followed by a separation step (sedimentation, filtration, settling, and centrifugation) [9–11].

The most frequently used method in industry is precipitation under hydroxide form. The method is based on the low solubility of metal hydroxides (reaction 1) at alkaline values of the mass reaction pH [1, 3].

$$\rm{M^{n+}\\_aq} + n(\rm{OH})^{-}\,\_{\rm{aq}} = \rm{M(OH)}\_{\rm{n\,\,pp}} \tag{1}$$

The solubility of hydroxides is dependent on pH. For each metal, there are solubility diagrams that represent the graphical representation of the solubility function of *Heavy Metals Removal from Water and Wastewater DOI: http://dx.doi.org/10.5772/intechopen.110228*

**Figure 2.** *Scheme of the precipitation system.*

pH, which present importance in the establishment of the pH value at which the desired metal ions present the lowest solubility [12]. Different metals have different values of the optimal precipitation pH, so maximizing the removal efficiency of a certain metal can lead to a significant decrease in the removal degree of another metal.

Sometimes for this reason it is necessary to use two stages of treatment to obtain a high removal degree of all the metals present in the wastewater. On the other hand, the fact that metals present in water precipitate at different pH values can lead to a selective separation of them. The main advantages of the method are: (1) easy of operation, could be applied without requiring a pretreatment; (2) could remove several parameters at once; (3) could be performed at ambient temperature; (4) could be automated; and (5) presents low costs if lime is used as a precipitating agent. At the same time, the method presents also some disadvantages, such as requires a high amount of precipitating agent, consequently generating a large amount of sludge (especially if lime is used as precipitating agent); the pH must be strictly controlled; does not remove complex metals; the efficiency of the separation step influence the treatment performance [13]. Among the metals that can be removed using this method, could be mentioned: iron, copper, zinc, cadmium, beryllium, cobalt, mercury, manganese, and aluminum. Chromium can only be removed if it is found in the trivalent form.

Besides pH, other parameters and factors must be considered (such as temperature, ionic strength, formation of complexes, formation of other solid phases, alkalinity, and formation of buffer solutions) because they affect the solubility of metal hydroxides and consequently the efficiency of metal ions removal through precipitation. The metal ions'solubility increases with the temperature increasing, with ionic strength increasing, or with the formation of complexes. Around the optimum pH value of metal ions precipitation under hydroxide form takes place in the formation of another solid phase, such as carbonates form. Certain anions present in wastewater from metal processing (chlorines, sulfates) can also precipitate the metals present. The carbonates and sulfates present in the water, as well as some organic anions can precipitate with the calcium ions added in the form of lime to adjust the pH. The metal ions'solubility decreases as the formatted precipitate is subject to aging and is transformed into ideal crystals [12].

Instead of hydroxide precipitation or together with it is used precipitation under carbonates forms (reaction 2).

$$\rm{M^{n+}\,\_{aq}} + n(CO\_3)^{2-}\,\_{aq} = M\_2(CO\_3)\_{n\text{ pp}}\tag{2}$$

In contrast to metal hydroxides, metal carbonates are formed at lower values of the reaction mass pH, and present higher density, which leads to the improvement of the removal process performance. Lower metal solubilities can be obtained by increasing the carbonate dose at pH values lower than the optimal pH for the precipitation of hydroxides. The main disadvantages of this method are represented by its reduced kinetic and by the possible foaming of the reaction mass due to CO2 release [14]. This method is frequently used for lead removal due to the high solubility of lead hydroxide [15, 16].

Metal sulfides are compounds that also present a much lower solubility than metal hydroxides. Thus, metal ions can be removed from aqueous solutions by precipitating them in the form of sulfides according to reactions 3–6 [1, 11, 17, 18].

$$\text{H}\_2\text{S}=\text{HS}^- + \text{H}^+\tag{3}$$

$$\mathbf{H}\mathbf{S}^- = \mathbf{S}^{2-} + \mathbf{H}^+ \tag{4}$$

$$\mathbf{M}^{2+} + \mathbf{S}^{2-} = \mathbf{M} \mathbf{S}\_{\text{PP}} \tag{5}$$

$$\text{M}^{2+} + \text{HS}^{-} = \text{MS}\_{\text{pp}} + \text{H}^{+} \tag{6}$$

For the precipitation of metals in the form of sulfides, soluble sulfides can be used, for example, sodium sulfide (Na2S), calcium polysulfide (CaS), or sodium hydrosulfide (NaHS), or insoluble sulfides, such as ferrous sulfide (FeS). The partially soluble ferrous sulfide is added as a suspension. Most metals have lower solubility than FeS, so heavy metals precipitate as sulfides, while FeS is solubilized (reaction 7). The reaction takes place around pH = 8, and Fe precipitates in the form of hydroxides [1, 11, 17].

$$\mathbf{M}^{2+} + \mathbf{FeS} = \mathbf{MS} + \mathbf{Fe}^{2+} \tag{7}$$

Due to its reducing character, an important advantage of this method is the fact that FeS can be used directly in the removal process of Cr(VI) from water (reaction 8). The sulfur is oxidized to its basic state, and chromium precipitates in the form of hydroxide [19–21].

$$\text{H}\_2\text{CrO}\_4 + \text{FeS} + 2\text{H}\_2\text{O} = \text{Cr(OH)}\_3 + \text{Fe(OH)}\_3 + \text{S} \tag{8}$$

Another advantage of the precipitation under sulfide ions is the ability to direct the precipitation of complex metals. In this case, it does not require any pretreatment steps or multiple stages to remove different metals. Unfortunately, the sulfide precipitates are in the form of small particles that present poor sedimentation properties, sometimes requiring the use of coagulation agents. The main disadvantage of this method consists of the strict conditions for operation and handling of the resulting sludge due to the possibility of toxic hydrogen sulfide formation. The advantages and disadvantages of the main precipitation agents used for the removal of metal ions from water and wastewater are summarized in **Table 1**.

*Heavy Metals Removal from Water and Wastewater DOI: http://dx.doi.org/10.5772/intechopen.110228*


**Table 1.**

*The advantages and disadvantages of the main precipitation agents used for the removal of metal ions from water and wastewater.*

Precipitation in the form of sulfates is generally applied to remove barium from aqueous solutions [22–24]. Precipitation in the form of phosphates is generally applied for the elimination of trivalent metals (iron, aluminum, and chromium). The precipitation in the form of chlorides is particularly applied in combination with the oxidation of cyanides for the removal and recovery of silver [25]. Sodium borohydride is an effective reducing agent and is used to remove Pb, Hg, Ni, Cd, Co, Cu, and some precious metals [26, 27].

All these mentioned factors and variables that influence the precipitation raise difficulties in the evaluation of the treatment process. So that predicting the efficiency of the process requires the evaluation of a theoretical and experimental study for each individual case. For example, Serrano and his coworkers studied the removal of Fe (III), Cd(II), and Zn(II) using hydroxides precipitation combined with flotation. The precipitation efficiency was determined function of pH, metal ions initial concentration, treatment time, and dosage of the precipitating agent. The simultaneous removal of 99% for all three studied metal ions was obtained using a pH value of 10.3, an initial concentration of metal ions between 1 mol/L to 15 mol/L, and a treatment time of 15 minutes [9]. Zhang and Duan studied the removal of heavy metals by precipitation using magnesium hydroxy carbonate as a precipitating agent. They obtained a residual concentration under the maximum admitted value using a dosage of the precipitation agent of 0.3 g/50 mL of residual solution at a pH value of 7.1 [12]. Sadeghi et al. studied the removal of lead through precipitation using sodium sulfide and sodium carbonate as precipitation agents. In each case was obtained a removal efficiency

>95% at pH = 11 [15]. The lead ions removal was also studied by precipitation with sodium carbonate by Hu et al. In this case, the researchers used a ball milling process to increase the reaction between lead salts and the precipitation agent, achieving a removal degree of 99% [16]. Regarding precipitation in the form of sulfides, studies have been focused on minimizing the production of toxic hydrogen sulfide. In this regard, Phol studied the metal ions precipitation using other sulfur-containing precipitation agents, such as potassium/sodium thiocarbonate (STC), 2,4,6 trimercaptotiazine (TMT), sodium dimethyldithiocarbamate (SDTC), 1,3 benzenediamidoethanethiol (BDETH 2), 2,6-pyridinediamidoethanethiol (PyDET), or pyridine-based thiol ligand (DTPY) [17]. In this way, the metal ions are binding by the precipitation agent and form metal complexes and are avoided the H2S formation but to obtain an efficient removal of metal ions is necessary to use a higher dose of precipitation agents. Prokkola et al. studied the metal ions precipitation from acidic mine drainage (AMD) by using HS, resulting from sulfate reduction reaction. The resulted H2S gas and ionic HS� during anaerobic treatment were recycled in the precipitation process. The optimum pH value of the precipitation process was 5.5, when is achieved a residual concentration of metal ions <30 μg/L [18]. It is observed that regardless of the precipitation agent used, the degree of metal ions removal from aqueous solutions is influenced by the pH value, treatment time, precipitation agent dose, and initial concentration of metal ions in the solution. By adjusting these influencing factors, the removal capacity of metal ions from wastewater can be significantly improved, which provides the theoretical basis for the practical application of these technologies from the laboratory scale, updated to pilot scale, and further to the industrial level.
