**5. Ion exchange**

Chemical precipitation and coagulation-flocculation are used to treat water with a high content of metal ions. Their efficiency decreases if the metal ions are found in trace concentrations. In this situation, it is necessary to use advanced water treatment methods. Ion exchange is one such method that can be successfully used to remove metal ions found in low concentrations from water. Ion exchange treatment of waters with metal ions content requires the use of ion exchangers (IX), a water-insoluble compound, that release in water some harmless ions, such as H+ , Na<sup>+</sup> , or OH�, retaining in return the target metal ions, which present a higher affinity for the used IX, according to reaction (9). This process is reversible and takes place until the exhaustion of IX. Usually, the treatment is carried out with two columns filled with IX (**Figure 4**), one is used for operation, while the other is regenerated. After

**Figure 4.** *Scheme of the ion exchange process.*


**Table 3.**

*Efficiency of various ion exchangers applied on different wastewater treatment.*

regeneration, smaller volumes of solutions containing the target metal ions in much higher concentrations are obtained so that they can be recovered, capitalized, or further processed using other methods [46–49].

$$\mathbf{R-A} + \mathbf{Me} \qquad \mathbf{R-Me} + \mathbf{A} \tag{9}$$

where:

R: ion exchanger matrix.

A: the mobile ion of the ion exchanger.

Me: the target metal ion from the residual water of the same sign as A.

Depending on their nature, IX can be: (i) inorganic-natural (zeolites, clinoptilolite) or synthetic (layered double hydroxides) [50–56]; (ii) organic-natural (cellulose, alginic acid, chitin), or synthetic (polycondensation ion exchangers; polymerization ion exchangers) [48, 57–63]. Depending on the type of mobile ion they can exchange, IX are classified as cationic, anionic, or amphoteric. The most important property of IX is the exchange capacity. The exchange capacity represents the number of ions exchanged during the process, equal to the number of functional groups capable of exchange. It is expressed in equivalents per dry product unit or wet product volume unit. In practice, two notions are used: total capacity and useful exchange capacity. The exchange capacity is influenced by constructive factors (the ratio between the column height and diameter), functional factors (percolation speed, specific load, and regeneration level), and chemical factors (the chemical composition of the water to be treated). The disadvantage of ion exchange treatment consists in the fact that the method is sensitive to the pH variations of the influent, and a pre-treatment must be carried out both to remove the particles in suspension and to avoid the precipitation of metal ions on the used resin to avoid the clogging of the exchange column. **Table 3** summarize the experimental conditions and the results of various ion exchanger applied on different wastewater treatments and the obtained metal ions removal efficiencies.

Not all ion exchangers could be used for the removal of metal ions, therefore further studies must be carried on regarding the stability and reusability of various ion exchangers [64, 65]. The challenge in the heavy metals removal from aqueous solution is the development of nontoxic, easily available, and low-cost ion exchangers. For a good design and modeling of the ion exchange process is necessary for each case to study the effect of the main parameters, such as pH, time, flow rate, dosage, initial concentration, bed height, and types of resin.

### **6. Adsorption**

Another advanced water treatment that is efficient use in the removal of trace amounts of metal ions from water and wastewater is adsorption. Adsorption is a separation process consisting of the adhesion of metal ions dissolved in aqueous solutions to the surface of a solid, called adsorbent (**Figure 5**). Separation mechanisms of metal ions from water through adsorption are influenced both by their characteristics and by the characteristics of the adsorbent, being determined by the interactions between these two. These interactions can be of a physical nature, exerted by forces with low energies (e.g., van der Waals-type forces). In this case, the metal ions are adsorbed in the pores of the adsorbent without the involvement of electron transfer. In this case, the process is reversible, the metal ion molecules retained on the

**Figure 5.** *Scheme of the adsorption process.*

adsorbent surface can be removed by desorption, regenerating the adsorbent. If the interaction between metal ions and adsorbents involves the transfer of electrons and the formation of chemical bonds, then the process is called chemical adsorption, or chemosorption. In this case, the metal ions are not attracted to the entire surface of the adsorbent materials, but only to the active zones, which contain functional groups that react with the metal, involving higher energies than those of physical adsorption, a fact that explains the greater selectivity of the chemical adsorption [11, 66, 67].

In adsorption processes, the used adsorbent material represents the essential element for obtaining effective separations. Thus, a multitude of adsorbent materials was studied. The most frequently used is activated carbon both in its powder form and in its granular form, due to its high specific surface area [68–71]. The powder-activated carbon (PAC) could be used together with other treatment processes, for example, in the coagulation-flocculation processes, for obtaining a better separation of the heavy metals. This presents an advantage that it is not necessarily an alternative equipment, those the separation costs are reduced. Unfortunately, in this case, it is not possible to recover the activated carbon from the resulted sludge. Another disadvantage is represented by the fact that is required a great amount of PAC for a specific volume of water to be treated. The use of granular activated carbon (GAC) requires a smaller amount for the treatment of a specific volume with heavy metals content, but in this case, it is necessary to use separate devices for water treatment and for the regeneration of the exhausted GAC. Sometimes the adsorbed metal ions could be instantaneously removed from the surface of GAC appearing in the effluent at higher concentrations than they were initially in the influent [11, 68–71]. With the recent development of nanotechnologies, researchers have turned their attention to the uses of carbon nanotubes as adsorbent materials [72, 73]. Numerous studies have been carried out regarding the use of activated carbon obtained from vegetable waste as an adsorbent material, or various natural adsorbent materials [74–82], or on different wastes [83, 84]. They have the advantage of low costs, but also the disadvantage of developing low adsorption capacities and low selectivity. To obtain high selectivity and increased efficiency for certain specific adsorption processes, various materials were synthesized and studied such as ferrites or oxides [85–87]. Adsorbent materials with magnetic properties have also been developed to improve the separation stage [88–92]. Since the structure of the adsorbent greatly influences the adsorption process, compounds with well-ordered structures, such as layered double hydroxides and metal-organic frameworks, have also been intensively studied [93–97]. To obtain materials with high selectivity and improved absorbent properties, functionalized

materials have been developed by impregnating numerous solid supports with compounds containing various functional groups [51, 98–105]. **Table 4** summarizes the experimental conditions and the efficiency of various adsorbent materials used for the removal of various metal ions from different wastewater.




**Table 4.**

*Research results of various adsorbent materials used for the removal of various metal ions from different wastewater.*

Adsorption is a low-cost and easy-to-operate method for the treatment of water with metal ion content. It presents economic feasibility to be scaled up to the industrial application. The existence of various adsorbent materials and low-cost adsorbent materials increases the versatility of this treatment method. Unfortunately, this method is not suitable for automation. Due to the multitude of adsorbent materials with different adsorptive properties (see **Table 4**), intensive studies are continuously carried out, and the selection of one must be based on a preliminary study performed for the specific effluent that is to be treated.
