## 3 Heavy Metal Removal Approaches  
The notion of utilizing biological means to remove heavy metals is exceedingly appealing when juxtaposed with other conventional techniques because biological procedures are characterized by their cost-effectiveness, eco-friendly nature, and their exceptional efficiency when it comes to dealing with low concentrations of heavy metal ions in wastewater [52]. A wide range of heterogeneous biological entities, including various substances such as plant biomass, agricultural detritus, microbial biomass, nanoparticles synthesized through green methods, residual fruit matter, and biopolymers, have been effectively employed in the removal of heavy metals [53]. These aforementioned agents have demonstrated their efficacy in this role, and their use has been widely documented in the literature [54]. It is significant to acknowledge that the application of these biological agents for heavy metal elimination holds immense importance within the realm of environmental science. This is because it presents a sustainable and environmentally conscious approach towards tackling the problem of heavy metal pollution [52,54]. Thus, further research into the application of these agents in heavy metal removal is warranted and is likely to yield valuable insights and contributions to the field. Living organisms, which are vital constituents of the biosphere, including bacteria, algae, and fungi, have been identified as playing significant and emerging roles in the process of heavy metal removal from the environment [55]. Recent advances in biotechnology have enhanced the understanding of the mechanisms through which these organisms mediate the detoxification of heavy metals. These mechanisms include biosorption, biomineralization, bioaccumulation, and bioremediation and are crucial in the removal of heavy metals from ecosystems [10,56]. Therefore, it is evident that the utilization of living organisms in the removal of heavy metals is a promising approach towards achieving sustainable environmental management. Various types of microorganisms, including fungi, bacteria, and algae, can absorb heavy metals from their surrounding environment and transport them into their cells [57]. It has been documented and reported in numerous studies that a diverse range of microbial species possesses the unique capability to convert the extremely hazardous Cr (VI) to a less toxic form of Cr (III). Microbial cells possess the ability to effortlessly internalize Cr (III) owing to their limited solubility in aqueous solutions and negligible toxicological characteristics [58,59]. Numerous techniques for heavy metal bioremediation have been proposed in the past, including biosorption, phytoremediation, bioreduction, and bioaccumulation [58]. Biosorption involves the use of dead biomass, while phytoremediation necessitates the removal of heavy metals through plant mediation. On the contrary, it is imperative to note that bioreduction is a highly intricate process that encompasses the transformation of the oxidation states of heavy metal ions, which is a highly significant phenomenon in the field of environmental chemistry [60]. Conversely, bioaccumulation is a process that is characterized by the absorption and uptake of heavy metal ions into the intracellular space, which is a crucial mechanism that is fundamental to the understanding of the behavior of heavy metals in biological systems. These techniques have been extensively researched and have shown promise in the bioremediation of heavy metals [61]. The process of remediating heavy metals can be achieved through the utilization of two distinct mechanisms: metabolically independent mechanisms, which entail the use of inorganic, lifeless materials, and metabolically dependent mechanisms, which involve the use of living cells of bacteria,fungi, and algae that are capable of carrying out the necessary metabolic processes required to transform the heavy metal contaminants into less harmful forms [4; 62].