## 3 Heavy Metal Removal Approaches
### Bioaccumulation  
Bioaccumulation is an intricate biological phenomenon that involves the accumulation of heavy metal ions within microbial cells. It is a process that occurs at a slower rate in comparison to biosorption, as it entails the active participation of several metabolic pathways [61]. In contrast to the phenomenon of biosorption, characterized by the adsorption of metal ions onto the exterior surface of microbial cells, the process of bioaccumulation entails the uptake and subsequent retention of said metal ions by living microbial cells suspended within a medium [96]. Furthermore, the bioaccumulation process necessitates the persistent monitoring and observation of said cells. Owing to the multifaceted nature of this process, a thorough understanding of the underlying mechanisms is crucial to comprehend the impact of bioaccumulation on the environment and its potential applications in bioremediation [96]. The process of bioaccumulation presents an advantage in reducing various stages of biosorbent preparation, such as biomass collection, drying, preparation through washing and crushing, and storage. However, it is subject to significant influence by experimental conditions, as reported in [75]. The presence of pollutants in wastewater poses a substantial challenge to microbial growth and results in competition with heavy metals selected for bioaccumulation. Furthermore, these pollutants can bind to the bacterial outer surface and disrupt cell heavy metal accumulation, leading to further challenges in the bioaccumulation process [97]. It is noteworthy that numerous species of fungi, bacteria, algae, and plants have been identified as playing an essential role in the elimination of heavy metals, thus reinforcing the importance of considering biotic factors when assessing the effectiveness of bioaccumulation [98,99]. \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Biosorbent** & **Heavy Metal Ion** & **Heavy Metal Biosorption Capacity (mg/g)** & **References** \\ \hline _Pluytolaca americana_ L. biomass & Pb (II) & 10.83 & [85] \\ \hline _Pseudomonas aeruginosa_ PU21 (Rip64) & Cu (II) & 23 & [86] \\ \hline _Pseudomonas aeruginosa_ PU21 (Rip64) & Pb (II) & 70 & [86] \\ \hline _Pseudomonas aeruginosa_ PU21 (Rip64) & Cd (II) & 58 & [86] \\ \hline _Halomonas_ BVR 1 & Cd (II) & 12.023 & [87] \\ \hline Mix bacterial biomass of _Enterobacter ludwigii,_ & Cu (II) & 6.52 & [88] \\ _Zogoloa ramigerais,_ and _Comamonas testosteroni_ & _Staphylococcus xylosus_ biomass & As (III) & 54.35 & [89] \\ \hline _Staphylococcus xylosus_ biomass & As (V) & 61.34 & [89] \\ \hline _Bacillus strain_ MRS-2 (ATCC 55674) & Pb (II) & 206.5 & [90] \\ \hline _Bacillus subtilis_ & Pb (II) & 57 & [91] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The heavy metals removal by microbial-based biomasses and heavy metal biosorption capacity. Among the various living systems that exist, it has been observed that microbes exhibit a noteworthy capacity for both effectively removing heavy metals as well as demonstrating resistance towards them [63; 100]. Bacteria, which are single-celled microorganisms, have been observed to exhibit the extraordinary ability to selectively assimilate heavy metal ions through the utilization of cell surface receptors, which are specialized proteins that are embedded within the outer membrane of the cell. Following the initial binding of these heavy metals to the receptors, the bacteria have been observed to efficiently accumulate and store them within their cellular structures, thereby enabling them to maintain their physiological functions under adverse environmental conditions [101]. Notably, certain bacterial cells, such as those belonging to the Pseudomonas, Klebsiella, and Mycobacterium species, have exhibited a remarkable ability to resist heavy metal ions that have been extracted from diverse contaminated sites. It has been observed that bacteria that have been isolated from areas that have been contaminated with heavy metals have generally developed a resistance to these ions [102].