## 3 Heavy Metal Removal Approaches
### Biosorption  
The phenomenon of biosorption, which is frequently defined as the process of adsorption of heavy metal ions by biomass, is considered to be metabolically independent heavy metal adsorption [71]. This captivating process is generally executed by a diverse range of deceased biomass, encompassing but not limited to plant waste, microalgae, agro-waste, microbial biomass, and agriculture residue [72]. In the course of biosorption, it is widely acknowledged that heavy metal ions tend to adhere to the surface of the biomass using a series of functional groups that are present on said surface. It is, however, worth noting that biosorption can also be executed by live cell biomass. Specifically, living microbial cells possess the exceptional capacity to attach heavy metal ions to their cell surface through the process of passive adsorption and complexation [73]. Biosorption, an intriguing phenomenon for researchers in the field, entails a multifaceted process whereby heavy metal ions adhere to surface functional groups, namely amino, amide, imidazole, sulfonate, and carboxyl. It is noteworthy to emphasize that the pKa value of the medium in which biosorption takes place exerts a substantial impact on the adsorption of heavy metal ions and the binding propensity of functional groups. [PERSON] believed that hydroxyl groups have a pKa value between 9.5 and 13.21. As a result, carboxylate groups are mostly responsible for the negative charges on biomass between these Pka values. Aqueous media requires the deprotonation of carboxylic groups to generate negative charges and H+. As a result, different metal ions can be adsorbed [74]. One of the most crucial and indelible aspects of biosorption is the judicious selection of a suitable material to employ. This material should be easily obtainable, non-hazardous, and economically viable, which represents an important consideration for researchers in the field [75]. The efficacy evaluation of a biosorbent is contingent upon the array of surface groups that are present. For a biosorbent to be deemed optimal, the concentration of surface functional groups must be augmented [25]. The surface morphology of an adsorbent represents a pivotal characteristic in the heavy metal biosorption process. Surfaces that are rough and porous provide a larger surface area, thereby conferring an advantage for the binding of heavy metal ions onto the biosorbent surface, as per reference [76]. It is crucial to carefully scrutinize the surface morphology and functional groups of a biosorbent. A myriad of techniques, including Fourier Transformation Infra-Red (FTIR), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Analysis (EDX), Nuclear Magnetic Resonance (NMR), and X-ray Diffraction (XRD) are readily available to characterize a biosorbent, as per reference [77]. As such, it is imperative to employ these aforementioned techniques judiciously to obtain a comprehensive understanding of the intricate details of the biosorbent's surface morphology and functional groups [76; 77]. The biosorption process, which involves the use of biomass to remove metal ions from aqueous solutions, is influenced by various factors that impact its efficacy. These factors encompass the morphology and composition of the biomass, the presence of multiple metal ions in the media, the ambient temperature, and the pH of the medium [78]. It is commonly observed that a decrease in pH results in competition between cationic heavy metal ions. Conversely, an increase in pH leads to the deprotonation of the biosorbent surface, which causes the exposure of surface functional groups [79]. Additionally, desorption, which involves releasing adsorbed heavy metals from the biosorbent, can be employed to regenerate the biosorbent. The recovery of metal ions can be achieved by manipulating the pH of the medium, which is a critical factor in the process [80].