**5.2. Metabolism independent biosorption**

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

Following are given the advantages of biosorption over conventional metal removal methods

**5.** No need for chemical additions as highly selective for uptake and removal of specific

**6.** Functional over wide range of conditions including temperature, pH, presence of other

The process of heavy metal ion binding to bacterial cell wall (peptidoglycan) can be metabo-

Metabolism dependent biosorption is exhibited by living biological material. It involves various mechanisms like chelation; a specific way in which ions and molecules bind to metal ions

becomes less bioavailable.

[47, 48].

24 Biosorption

metals

metal ions, etc.

**4.3. Disadvantages of biosorption**

**5. Biosorption mechanisms**

lism dependent or independent [1].

**5.1. Metabolism dependent biosorption**

**4.2. Advantages of biosorption**

**1.** Cheaper production of biomass (bacteria or fungi)

**7.** Easy and cheaper desorption of metals attached to biomass

**8.** Reduced volume of waste or toxic materials production

The disadvantages of biosorption are stated below [49].

**1.** Saturation of active sites of metal binding ligands

**2.** Reversible sorption of metals on biomass

**2.** Use of biomass for removal of heavy metals

**3.** Multiple heavy metals uptake at a time

**4.** Treatment of large volumes of wastewater

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