**4.1. Significance**

Bioremediation offer different advantages such as low operating cost, minimum ratio of disposable sludge volume, high efficiency in detoxifying very dilute effluents and even *in situ* remediation [30, 43, 44]. Bacteria detoxify heavy metals in a variety of different ways [45]. Although various types of tolerance mechanisms have been reported in bacteria for heavy metal stress, Cd detoxification has only been restricted to efflux pumps. The plasmid encoded *cad* systems in (*Staphylococcus aureus*) and the *czc* system (*Alcaligenes eutrophus*) are best characterized. These systems actively maintain Cd ions outside the intracellular environment, hence avoiding its toxic effects. Consistently, many researchers reported that sensitive bacteria can accumulate 3–15 times more Cd than resistant strains [46]. The most important aspect of Cd ions is that they covalently bind to sulfhydryl groups. Although this is partially the cause for its high toxicity, this feature is also used by several organisms to render the metal harmless to the cell, through sequestration with metal-detoxifying ligands, the metal becomes less bioavailable.

and it involves the formation or presence of two or more separate coordinate bonds between a polydentate ligand and a single central atom, physical adsorption; adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It is a surface phenomenon. Generally the adsorption process is classified as physisorption, characteristic of weak van der Waals forces, or chemisorption, characteristic of covalent bonding. It may also occur due to electrostatic attraction, precipitation; it is the creation of a solid in a solution or inside another solid during a chemical reaction or by diffusion in a solid. When this reaction occurs in a liquid solution, the solid formed is called the "precipitate" and the chemical that causes the solid to form is called the "precipitant") or complexation (it consists of a central atom or ion, which is usually metallic and is called the coordination centre, and a surrounding array of bound molecules or ions, that are known as ligands or complexing agents. Many metal-containing compounds, especially those of transition metals, are coordination complexes). There may involve a single process or combination of these processes [50, 51]. If the metal binding to cell wall is metabolism dependent then it involves energy from ATP. The ligands present on the cell wall of biological material such as phosphoryl, carboxyl, carbonyl, sulfhydryl and hydroxyl groups immobilizes the metal ion [32] and then uptake occurs [5]. Other factors that effect the metal uptake by living biomass includes nature of heavy metals ions, conditions of the medium, cell wall composition, etc. [5]. The uptake process by living biomass involves adsorption to cell wall and enter-

Biosorption of Heavy Metals

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http://dx.doi.org/10.5772/intechopen.72099

The metabolism independent process mostly occurs in biomass consisting of dead cells [54]. The adsorption process is the main key point behind such physicochemical biosorption mechanism. The adsorption process can be ionic interactions or physiochemical adsorption. Presence of anionic ligands on bacterial cell wall (carboxyl, amine, hydroxyl, phosphate, and sulfhydryl groups) also plays an important role in metal biosorption. Living biological mass is preferred over dead mass, because living cells have ability for continuous metal uptake, and self-replenishment [27, 29, 31]. Previously it is reported that adsorption is a rapid process while accumulation is slow and energy dependent [29, 31 52–53]. The fate of metal inside cell can be accumulation, detoxification and/or efflux depending on the nature of bacteria [31, 55, 56]. In past few decades, many groups worked on heavy metal resistant bacteria that can be used for bioremediation [27, 29, 31, 56–58]. Many workers reported that cells of bacteria of genera

In order to have the physiological effect on the growth of cells, heavy metals must enter the cell [19, 59, 60]. Metal uptake system in bacteria is grouped in two types; one is fast and unspecific, constitutively expressed and does not require ATP. They are usually driven only by the chemiosmotic gradient across the cytoplasmic membrane of bacteria. The second type of uptake system is highly specific, slow, inducible and dependent on ATP, in addition to the chemiosmotic gradient. They are only induced in times of need, starvation or a special meta-

*Alcaligenes* and *Pseudomonas* can be used for bioremediation purpose [45].

ing into the cytoplasm [29, 31, 52, 53].

**5.3. Metal accumulation**

bolic situation [61].

**5.2. Metabolism independent biosorption**
