## 3 Heavy Metal Removal Approaches
### Bioleaching  
Bioleaching is an exceedingly multifarious and intricate process that involves the utilization of an extensive range of microbial organisms, with a special emphasis on acidophiles. These remarkable chemolithotrophic microorganisms exhibit an exceptional ability to oxidize Fe (II) to Fe (III) and/or reduce sulfur to sulfuric acid, which endows them with the capacity to thrive in environments with low pH levels typically ranging at or below 2.0 [112; 113]. The solubilization of metal sulfides and oxides from ones, thereby enabling the extraction of metal through the segregation of metals in the solid phase from the more water-soluble phase, is accounted for by the production of ferric ions and protons resulting from sulfuric acid [114]. Bioleaching, which involves the reduction of metal ions by microorganisms, plays an indispensable role in the extraction and recovery of heavy metals [115]. The ability of microorganisms to convert solid chemicals within contaminated soil into a soluble substance that can be removed and recovered significantly affects the efficacy of the recovery process, especially given the finite and non-renewable nature of metal resources [116]. In light of this, scholars have proposed bioremediation as a viable means of recovering raw materials from effluent. For instance, an absorbent based on Annona squamosa was utilized to achieve a substantial recovery rate of up to 98.7% for Cd (II) through 0.1 M HCl [117]. Likewise, it is noteworthy to mention that the application of _Pseudomonas aeruginosa_ biomass, in conjunction with a highly concentrated solution of hydrochloric acid (0.1 M HCl), has been observed to yield a remarkably substantial rate of retrieval for the elusive heavy metal known as cadmium (II), with recovery rates reaching an impressive 82% [118]. Additionally, a noteworthy 100% recovery rate of Cu (II) was reported by utilizing volcanic rock matrix-immobilized _P. putida_ cells that possessed surface-displayed cyanobacterial metallothioneeins at a pH of 2.35. Analogously, 100% recovery of Cu (II) was obtained from activated sludge at a pH of 1.0 [119]. It is a matter of significance to highlight that the incorporation of the autochthonous variant _Enterobacter_ sp. J1 brought about a remarkable increase in the recuperation percentage, exceeding 90% for both copper and lead ions when the pH level was at 2 [120].  
\begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Bacteria** & **Heavy Metals** & \begin{tabular}{c} **Initial Heavy Metal** \\ **Concentration (mg/L)** \\ \end{tabular} & \
\begin{tabular}{c} **Heavy Metal Removal** \\ **Capacity (\%)** \\ \end{tabular} &
\begin{tabular}{c} **References** \\ \end{tabular} \\ \hline _Pseudomonas aeruginosa_ PU21 (Rip64) & Pb (II) & 50 & 98 & [86] \\ _Pseudomonas aeruginosa_ PU21 (Rip64) & Cu (II) & 50 & 98 & [86] \\ _Pseudomonas aeruginosa_ PU21 (Rip64) & Cd (II) & 50 & 98 & [86] \\ _Enterobacter ludivigii, Zoogloa ramigerais, and_ _Comanomas testosterone_ mix culture & Pb (II) & 10 & 72.6 & [88] \\ _Enterobacter ludivigii, Zoogloa ramigerais, and_ _Comanomas testosterone_ mix culture & Cu (II) & 2.5 & 83.9 & [88] \\ _Pseudomonas aeruginosa_ & As (III and V) & 10 & 98 & [110] \\ _Pseudomonas strain As-11_ & As (III) & 130 & 48 & [111] \\ _Pseudomonas strain As-11_ & As (V) & 312 & 78 & [111] \\ _Microbacterium paraoxydans_ VSVM IIT (BHU) & Cr (VI) & 50 & 91.62 & [9] \\ _Microbacterium paraoxydans_ VSVM IIT (BHU) & Cd (II) & 50 & 89.29 & [9] \\ _Microbacterium paraoxydans_ VSVM IIT (BHU) & Pb (II) & 50 & 83.29 & [9] \\ \hline \hline \end{tabular}
\end{table}
Table 2: Heavy metal uptake capacity of bacterial strains.