## 3 Heavy Metal Removal Approaches
### Some Other Advanced Microbial-Based Approaches for Heavy Metal Removal  
#### 3.6.1 Rhizoremediation  
The process of rhizoremediation, which is a method employed to eliminate heavy metals from the soil, involves the utilization of microorganisms that are found in the rhizosphere of plants. Rhizoremediation is a technique that combines phytoremediation and bioaugmentation, which are two techniques used to cleanse contaminated substrates [125]. Rhizoremediation is a highly innovative and cutting-edge approach that has been developed to address the pressing issue of organic pollutants and heavy metal contaminants that are present in our environment [126]. This method is based on the use of plants and rhizospheric microorganisms, which have proven to be highly effective in the process of restoring polluted environments. Indeed, the success of rhizoremediation cannot be overstated, as it is an eco-friendly, cost-effective, and highly efficient means of dealing with the problem of pollution [126]. However, it is important to note that the success of rhizoremediation at any given site is dependent on a wide range of factors, including the level of soil contamination and the quantity of metal contaminant present in the soil, as well as the ability of plants to aggressively take up metals from the soil. Therefore, these factors must be carefully considered and taken into account when implementing rhizoremediation to ensure that the process is as effective as possible [127]. Ultimately, the use of rhizoremediation represents a crucial step forward in the fight against pollution, and we must continue to explore and develop this innovative approach to safeguard our environment for future generations [68].  
Plants that are employed in the process of rhizoremediation can be categorized into two distinct groups, namely the hyperaccumulators and non-hyperaccumulators. The former group comprises plants that possess an exceedingly high ability to accumulate heavy metals, albeit at the expense of biomass efficiency [128]. Conversely, the latter group has a comparatively lower capacity for extraction of heavy metals when compared to hyperaccumulators, but their total biomass yield is considerably higher. Moreover, non-hyperaccumulators exhibit rapid growth characteristics [129]. In the context of removing heavy metals from contaminated soils, certain plants utilize a variety of processes, which can be observed in Figure 2.  
Phytostabilization is a process which involves the immobilization of heavy metals in soil by the adsorption or precipitation of heavy metals. The term phytoextraction is related to the translocation of heavy metal ions from the soil and their accumulation in plants (root, stem, and leaves). In phytovolatilization, heavy metals are absorbed by plant roots and pushed up through the xylem stream. The heavy metal ions transfer to the upperparts of the plant, such as the leaves and stem, then convert into a volatile form and are released into the atmosphere [130,131]. This scientific approach has been integrated with several other techniques to create a comprehensive method for soil renewal. Currently, the application of plants and plant-growth-promoting bacteria (PGPB) is being evaluated as an effective and environmentally acceptable method for soil renewal [132]. PGPB in the soil that are close to the roots of plants play a vital role in facilitating the absorption or adsorption of heavy metals by plants. In addition to enhancing remediation abilities, PGPB act as a plant growth stimulant by secreting hormones and metabolites. This stimulates growth, solubilizes minerals, fixes nitrogen, and protects against pathogens. Other biotic and abiotic stresses that plants face can also be relieved by PGPB. It is important to note that the process of rhizoremediation is not only environmentally friendly but also enhances soil quality and promotes the growth of healthy plants [133,134].