Phytoremediation
Definition
Historical
Technical
Potentially toxic metals first interact with plants at the roots, where they are taken up by mass flow and diffusion. These metal pollutants are made bioavailable for plants through root secretion of metal-chelating molecules into the surrounding rhizosphere [30], metal reductase in the plasma membrane, and proton extrusion from roots [26]. Several mechanisms of phytoremediation exist [2,9,31,32]. In phytoextraction, soil contaminants are taken up through the roots and accumulate in the shoots [33,34]. In general, higher concentrations of metal in the growth environment result in higher accumulations in plant tissue [35–40]. Then, the contaminated shoot tissues are processed using a variety of disposal methods, such as heat and extraction treatments [41]. For instance, the tissues may be harvested and incinerated as hazardous waste, with the ash being discarded in landfills [42,43], or utilized for the re-extraction of trace elements [44,45]. The harvested biomass can alternatively be used as feedstock for biofuel production or pyrolyzed to form biochar [46–49]. Phytostabilization is a process in which metallic contaminants are immobilized through root adsorption and metal precipitation and stabilized through complex formation or reduction [50]. The immobilization and stabilization of metals to a nontoxic form within the plant prevents interference with cellular metabolism [51]. Phytovolatilization converts potentially toxic metals to more-volatile forms that are removed to the atmosphere through transpiration [44].[1]
Remedial Media
Hemp grows quickly and has deep, wide roots [15]. It can adapt to different soil conditions and grows in a variety of climates [36]. Many studies showed that hemp has a high tolerance to metals [52]. Industrial hemp can often take up metals and store them in different parts of the plant, with no detriment to the plant itself (Table 1) [36,53,54]. When employed for phytoremediation purposes, toxins can accumulate in the roots, leaves, and stalks [35]. Therefore, the leaves are not harvested for food or used for personal care; however, the stalks can be utilized for building materials, paper, cloth, and biofuel [55]. Since 1998, hemp has been successfully used to remove soil contaminants from agricultural lands that were heavily contaminated by the 1986 Chernobyl nuclear disaster [56]. In 2008, in an Italian farming region contaminated by a nearby steel plant, hemp was grown to leach pollutants, such as dioxin, from the soil [57]. Dioxins are considered toxic as they cause cancer, affect reproduction and development, damage the immune system, and interfere with hormones. Once remediation is complete, plant material containing dioxins can be used to produce energy. Beyond cleaning soil, research is being conducted on using hemp fibers to create absorption material capable of filtering out metals from contaminated water [58].[1]
- ↑ 1.0 1.1 Placido, D.F.; Lee, C.C. "Potential of Industrial Hemp for Phytoremediation of Heavy Metals." Plants 2022, 11, 595. https://doi.org/10.3390/plants11050595