**4. Biosorption**

released directly into the environment. Metals exhibit health issues [9] if their concentrations exceed allowable limits. Even when the concentration of metals does not exceed these limits, there is still a potential for bioaccumulation and associated chronic toxicity as heavy metals are known to be accumulative within biological systems [10]. These metals include arsenic, cadmium, chromium, copper, lead, mercury, nickel, and zinc [4, 11]. Industrial effluents are known to contain heavy metals which originate from metal plating, mining activities, smelting, battery manufacture, tanneries, petroleum refining, paint manufacture, pesticides, pig-

ment manufacture, and printing and photographic industries [1, 11–14].

Heavy metals are usually defined as metals having density more than 5 g/cm<sup>3</sup>

**Category of heavy metal Example of heavy metals**

Essential Copper (Cu)

Non-essential Lead (Pb)

**Table 1.** Essential and non-essential heavy metals.

classified as essential and non-essential metals. The metals which are need for normal cellular growth are essential metals e.g. zinc, nickel, copper, etc. Such metals are required in low concentrations (nM), but at higher concentrations (μM to mM) all heavy metals have detrimental effects to organisms [16, 17]. If the metals have no known biological function, they are called as non-essential metals e.g. e.g., lead, cadmium, mercury [18]. Such metals are toxic at any concentration [8]. The list of essential and non-essential heavy metals is given (**Table 1**). There are 90 naturally occurring elements in periodic table, 21 are non-metals, 16 are light metals and the remaining 53 (with As included) are heavy metals [19]. In periodic table, transition elements are mostly heavy metals. They have incompletely filled 'd' orbitals which allow heavy-metal cations to form complex compounds that may or may not be redox-active. In this way, heavy metals play an important role as 'trace elements' (cobalt, copper, nickel, and zinc) in sophisticated biochemical reactions and are important cofactors for metallo-proteins and enzymes [8]. The toxicity of heavy metal ions starts when their concentration becomes higher

> Nickel (Ni) Iron (Fe) Zinc (Zn)

Magnesium (Mg)

Mercury (Hg) Cadmium (Cd)

Tin (Sn) Arsenic (As) [15]. They are

**2. Heavy metals**

22 Biosorption

Biosorption is defined as "ability of biological materials to accumulate heavy metals from wastewater through metabolically mediated (by the use of ATP) or spontaneous physicochemical pathways of uptake (not at the cost of ATP), or as a property of certain types of inactive, non-living microbial biomass which bind and concentrate heavy metals from even very dilute aqueous solutions" [1, 5, 32]. It is a complex process that depends on differentfactors like cell physiology, physicochemical factors such as pH, temperature, contact time, ionic strength, and metal concentration, chemistry of the metal ions, cell wall composition of microorganisms [5, 33, 34]. Biosorption of different heavy metals e.g. cadmium, silver, lead, nickel etc. by using microorganisms like fungi, algae or bacteria was studied by different groups [34–42].
