**1. Introduction**

Nowadays, the development of various industries (such as mining stone refining, battery making, and the creation of pesticides) and the increase of industrial wastewater from factories are considered serious problems for the environment and humans. The lack of usable water resources on the planet and water pollution with various contaminants such as metals, semi-metals, pesticides, drugs, and other persistent organic elements has become a major concern worldwide. Among the various types of water pollutants, heavy metals are among the most important due to their toxicity, stability, resistance to environmental degradation, and long-term accumulation in the food chain. The most dangerous ions for human health and other living organisms are Cr, Fe, Se, V, Cu, Co, Ni, Cd, Hg, As, Pb, and Zn [1–3].

Despite the negative environmental effects, heavy metals are still used as main and important materials in various industries such as mining, coating, smelting, plastic, fabric, painting, etc. Therefore, it is necessary to remove heavy metals from industrial wastewater to reduce their impact on the environment. Also, the separation of heavy metals from wastewater can be economically important due to the high price of these metals. All kinds of physical and chemical methods have been investigated and used to treat industrial wastewater. Methods such as oxidation/reduction, sedimentation, ion exchange, reverse osmosis, membrane filtration, coagulation and flotation and chemical precipitation, electrochemical techniques, and adsorption on activated carbon and plant residues [2–7]. Choosing a specific purification method depends on various factors such as the type and concentration of heavy metals, whether the wastewater is homogeneous or heterogeneous, the required removal percentage, and the cost of the treatment process. But limitations such as cost, time, and efficiency have limited the use of these methods. Activated carbon and Nano adsorbents have a very high efficiency in this regard, but the cost of activation and synthesis of nanoparticles is high [8].

In recent years, biological adsorption has become a promising alternative method for wastewater treatment. Especially in research, it has been determined that surface adsorption is the most effective method for separating heavy ions and dyes from wastewater, and biological adsorbents are very promising to achieve this goal. The most important advantages of this method include high absorption capacity, low cost and economic productivity (especially when the bio absorbent can be recycled and heavy metals can be reused), high efficiency and productivity, minimal consumption of chemicals and creating sludge, the possibility of recovering metals, the renewable nature of biological adsorbents, the ability to be used in a wide range of changes in operating conditions and environmental compatibility pointed out [9].

In this chapter, researches carried out in the field of biological absorption and the use of algae to remove heavy metals from wastewater since 1977 have been studied. The main characteristics of algae that cause the high capacity to absorb metal ions heavy in them are investigated and the mechanisms of biological absorption of metals in algae have been reviewed. Also, the effect of operating parameters on the absorption efficiency of heavy metals by algae and the process conditions reported for optimal absorption of heavy metal ions have been discussed and investigated.

## **2. Biological removal of heavy metals (mechanism of biosorption)**

The process of biological absorption includes two phases: the solid phase and the liquid phase (including particles that will be absorbed). Due to the high affinity of the adsorbent with metal ions, complex processes with mechanisms such as; there are chemical absorption, surface absorption, ion exchange, absorption by physical forces, entrapment inside the fibrillar capillaries and the space between the polysaccharide network in passing through the cell wall and membrane. The amount of absorption is obtained from the following formula, where C0 is the initial concentration and C is the final concentration after contact with the adsorbent, and in the following, the absorption power of all types of adsorbents will be expressed with the help of this formula [9–11]:

$$\%adsorbed = \left(\frac{C\_o - C}{C\_o}\right) \times 100\tag{1}$$

*Biological Treatment of Heavy Metals with Algae DOI: http://dx.doi.org/10.5772/intechopen.110301*

**Figure 1.** *A plausible mechanism of biosorption.*

Accumulation of heavy metals in microorganisms mainly takes place in two phases; the first phase occurs at the cell surface. Absorption is fast and passive and is completely dependent on cell metabolism, the second phase of active absorption of ions into the algal cell cytoplasm, which is dependent on cell metabolism, and its other name is called intracellular absorption (**Figure 1**) [12].
