**7. Membrane separation processes**

Membrane separation processes are also an advanced water treatment that could be used for the removal of trace concentrations of metal ions from water and wastewater. Membrane separation processes (**Figure 6**) consist of passing the water flow through semipermeable membranes with certain properties (pore size or electrical

**Figure 6.** *Scheme of the membrane separation processes.*

charge) that allow the passage of water molecules but retain the dissolved metal ions [11, 106–109].

Depending on the forces-actions used for separation and the size of the separated particles, the membranes can be classified into.


Since membrane treatment technologies require operation at high pressures, the membranes must present compressive strength. The properties that characterize membranes are permeability, porosity, hydrophilicity, surface charge, and thermal/ mechanical stability.

Most of the studied membranes are composite materials based on polymer supports [110–112]. The use of these processes presents the following advantages: they constantly ensure good water quality, no chemical reagents are used, they lend themselves to automation, and they are compact. At the same time raise technical issues: insufficient selectivity, relatively weak transmembrane fluxes, exploitation problems, possibility of membrane fouling, requiring pretreatment step and periodic membrane cleaning, and also economic problems: many and different manufacturers with different technologies and prices.

### **8. Electrochemical separation processes**

Electrochemical treatment processes were first used for ores electrorefining in the mineralogical industry. Researchers were reluctant to use them due to the need for energy consumption and initial investment in special equipment, which leads to increased processing costs. After studies that demonstrated their efficiency and the need for low maintenance of the equipment, the use of these methods in the water treatment processes with metal ions content became more and more promising. Electrochemical processes for treating water with metal ions content can be divided into electrocoagulation, electro flocculation, and electrodeposition.

The electrocoagulation process takes place in a simple electrolysis cell, which contains sacrificial electrodes, which could be from the same or different materials (**Figure 7**). In most of cases, the electrodes are formed from Fe3+ or Al3+. The

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

### **Figure 7.** *Scheme of electrocoagulation processes.*

electrocoagulation process consists of the dissolution of the anode, H2 and HO� generation at the cathode, coagulant (Al(OH)3) formation, destabilization and neutralization of metal ions by introducing electric current, aggregation of the destabilized metal ions and flocks/sludge formation [113–118]. The main advantage of this process consists in the fact that no additional reagents are required, and the coagulant is formed *in situ* due to the electrical dissolution of the sacrificial electrodes. The obtained flock/sludge is stable and easy to be removed. The hydrogen formation contributes to the removal of tiny particles. The main disadvantages are represented by the possibility of cathode passivation and the need of the periodical replacement of the sacrificial anode.

In the case of electroflotation, the destabilized heavy metals adhere to the oxygen and hydrogen molecules released by the reactions from the electrodes and float to the surface of the liquid from where they are removed. The formed sludge presents better stability in this case and the process requires a shorter time. Most of the time, a combination of electrocoagulation and electroflotation is used [119–121].

Electrodeposition is an effective method of selective recovery of dissolved metals in order to recycle/reuse them. It is advantageous because it does not require additional reagents and no sludge is formed. Dissolved metals from wastewater are deposited at the cathode according to the reaction (10). In this case, it is preferable for the anode to be insoluble in order not to contaminate the recovered metals. At the anode, the reaction takes place according to the reaction (11). Side reactions such as the formation of hydrogen gas may occur during the process, reaction (12). This method is selective but is sensitive to the composition of water to be treated, and the efficiency is negatively influenced by the side reaction of hydrogen formation [11, 122, 123].

$$\mathbf{\color{red}{Catod \( - \)}} : \qquad \mathbf{M}^{\mathbf{n}^+} + \mathbf{n} \mathbf{e}^- = \mathbf{M} \tag{10}$$

$$\text{Anod} \,(+): \qquad \text{4OH}^- = \text{O}\_2 + 2\text{H}\_2\text{O} + 4\text{e}^- \tag{11}$$

$$\mathbf{H}^+ + \mathbf{e}^- = \mathbf{1}/2\mathbf{H}\_2 \tag{12}$$

### **9. Conclusions, remarks, and future perspectives**

It can be seen from the literature study that each method of removing metal ions from water has both advantages and disadvantages. The choice of one or another is

made following several conditions, such as knowledge in the field, experience in a certain method, the composition of the influent, the desired removal efficiency to be achieved, and the operating conditions. Sometimes two or more methods are combined and used to obtain the desired results. Chemical precipitation is frequently used in the treatment of waters with a high content of heavy metals. It presents low capital costs, simple operation condition, and a high treatment efficiency, it can be easily automated, but in some cases, even if high removal degree is obtained, residual concentrations below the maximum allowed concentrations are still not reached, so it is necessary to be followed by an advanced treatment method. The main disadvantages of chemical precipitation are underlined by the need for chemical reagents addition for pH adjustment and by the fact that great quantities of sludge are obtained, which need further treatment or special disposal. The same disadvantages are encountered in the case of using coagulation-flocculation processes. The efficiency of chemical precipitation or coagulation-flocculation processes also depends on the efficiency of the method used to remove the resulting sludge. Ion exchange processes and adsorption processes are used to treat large volumes of wastewater with a low content of metal ions. In the case of ion exchange processes, the initial investment in ion exchange columns is necessary, the problems raised in this case are given by the costs of ion exchange resins and their selectivity, stability, and reusability. Adsorption processes are the most promising treatment process of heavy metals removal from aqueous solutions due to low costs, easy operation, and the multitude of existing adsorbent materials, especially low-cost adsorbent. Also, in this case, the problem of regeneration and reuse of adsorbent materials arises. Membrane separation processes develop very high removal degrees of metal ions, but in this case, the costs of the technology and the possibility of membrane fouling limit their use at a large scale. Electrochemical processes have the advantage of selective removal of metal ions and the possibility of their recovery and reuse without the need for additional consumption of chemical reagents. At the same time, it presents the disadvantage of high energy consumption. Accordingly, for each case, studies must be carried out starting from the laboratory scale and then updated to the pilot scale to determine the optimal treatment method. Most of the studies were carried out on synthetic water. To clearly specify the efficiency of a certain treatment process or another, for different situations encountered in practice, it is necessary to carry out studies on real waters. Besides the metal of interest, the sample matrix also presents a major influence on the performance of the process treatment. Future studies must be focused on the development of cost-effective materials and methods that involve low treatment costs, high efficiency, and minimal impact on the environment.
