**14. Biotechnological intervention: genetically engineered microorganisms (GEM)**

**12.2. Fluidized bed and air lift bioreactors**

**Table 11.** Use of different bioreactors for biosorption of metal ions.

Indicates the dry weight of the biosorbent; <sup>b</sup>

experiments at laboratory scale; and #

**Type of reactor**

92 Biosorption

a

reactors are associated with the low mass transfer [38].

**12.3. Stirred tank bioreactors**

These two reactors almost work on the same principle of separation and can be operated in the batch mode. The reactor contains liquid, gaseous, and solid phases. The solid phase is a biosorbent on solid particles used for the retention of metals. The reactor operates with the idea that the gas allows the liquid containing the metal species to be removed to rise. The liquid then flows upward through the middle of the reactor and comes back down through the edges resembling a fountain [191]. In this the liquid is in continuous movement and moves the entire volume of the column. The metal species then adhere to the biosorbent. Once the biosorbent is harvested, the target molecule is separated. Since the particles are in continuous movement, it is preferred and also reduces the clogging effect of the biosorbent. Fluidized

Indicates continuous biosorption experiments.

**Biosorbent Metal species Biosorption efficiency Reference**

Sand grains Cu (II), Pb (II), Ni (II) 96%, 93%, 98%a\* [193]

*Scenedesmus incrassatulus* Cr (VI) 43.5%a# [195]

*Trichoderma viridae* Cr (VI) 60%b\* [194]

*Ulva reticulate* Cu (II), Co (II), Ni (II) 56.3%, 46.1%, 46.5%a# [198] Sewage sludge Cr (VI), Ni (II) 90%a# [199] *Microcystis aeruginosa* Pb (II), Cd, (II), Hg (II) 80%, 90%, 90%a# [200]

Indicates the wet weight of the biosorbent; \*

Indicates batch biosorption

FBRs *Pseudomonas aeruginosa* Cd (II), Cr (VI) 67.17%, 49.25%a\* [191]

ALRs *Trichoderma viridae* Cr (VI) 94.3%b\* [194]

STRs *Rhizopus arrhizus* Cr (VI) 70.5%a# [196]

PBCs *Aspergillus niger* Cu (II) 83.96%a# [197]

Liquid phase can be separated from the solid phase by a membrane system. Though the pro-

The efficiency in the removal of metal ions largely depends on the type of bioreactor, type of biosorbent, and operating conditions. Recent studies evaluated the efficiency of different biosorbents in the removal of metal ions by using various types of bioreactors (**Table 11**).

Many researchers have attempted pilot-scale studies to make the technology of biosorption available at the industrial scale. A small pilot plant with a three-zone contact settling was developed in a single vessel using anaerobically digested sludge as the biosorbent for the removal of Cu (II) ions. The efficient metal removal (similar to the batch experiments) of 90 mg/g of the

cess is simple, the cost of operation is high due to high energy requirements [192].

**13. Application of the biosorption process at pilot scale**

Most biosorbents sequester metal ions by using cell-surface moieties. However, they lack the property of specificity and affinity for metals. By using the available genetic engineering technologies specific tailoring can be done to the microbial biosorbents with required selectivity and affinity for metal ions [204]. Genetic engineering technology involves altering the genetic material of the organism in order to develop an efficient strain for the removal of metal ions against the wide range of contaminants present in the wastewater [205]. One such emerging strategy which has received increased attention in recent times is the use of metal-binding proteins such as metallothioneins and phytochelatins. For example, *E. coli* was modified to express phytochelatin 20 on its surface enhancing the accumulation of Hg by 25 times over that by wild-type strains [204]. The technology also offers the advantage of developing microbial strains that can withstand complex environmental conditions and stressful situations. A major obstacle associated with the molecular approach is that it has been applied to only limited bacterial strains like *Escherichia coli.* Hence, other microorganisms need to be explored using this molecular intervention. **Table 12** shows the list of selected genetically engineered bacteria used for the removal of metal ions.


**Table 12.** Use of genetically engineered microorganisms for biosorption of metal ions.
