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]
The feasibility of in-situ contaminants’ removal is mainly credited due to its porous and hydrophilic surface structure, as well as the strong recalcitrance on levels of toxicity. For example, hemp fbers were chosen as the remediator of heavy metal ions (i.e., lead (II), zinc (II) and cadmium (II)). The metal removal efciencies of hemp that were persuasive ranging from 17.5 to 39% in single/ ternary ion metal(s) solutions. Campbell et al. (2002) observed large reductions of benzo(a)pyrene (~ 33.5%), but inconsistent results on chrysene from -50% to 64% in the contaminated soil. It was also estimated that contaminant accumulation was highly selective on hemp parts. For example, the accumulation of nickel, lead and cadmium in hemp leaves were 4–12 times larger than the metal in other parts like fibers, seeds and herbs (Linger et al. 2002). It makes the potential remanufacture/reuse of less contaminated hemp parts possible, which aligns with the circular economy.[2]
Intoxicant Matters
- Chromium
In hemp, chromium (Cr) is absorbed passively with other essential metals [61]. Chromium accumulates significantly in the root system (for both seed and fiber varieties of hemp) and less so in the stems, leaves, and seeds [36,38,59,60]. As observed in other plants, hemp’s ability to immobilize Cr in the vacuoles of root cells may explain its high accumulation in the roots of the plant [61,62]. A higher amount of Cr in the roots of hemp from contaminated sites leads to significantly higher proline accumulation and an increase in the phenolics content. Both proline and phenolics accumulation prevent oxidative injury in plants. At the same time, when Cr accumulates in root systems, chlorophyll, carotenoids, and dry biomass decrease significantly [60].
- Zinc
Hemp can tolerate high concentrations of zinc (Zn), and most of the Zn absorbed by hemp was retained in the roots [39]. Zn shoot restriction means minimal damage to photosynthetic activity and healthy plant growth. Low concentrations of Zn are also found in seeds, making them suitable for alimentary use [52,63]. In contrast, Malik et al. found that Zn accumulates at higher levels in shoots than in roots [59]. Angelova et al. reported Zn concentrations in the following order: flower > seeds > roots > stems > leaves > fiber.
- Copper
In hemp, copper (Cu) accumulates in the leaves but not in the fibers [64]. Glutathionedisulfide reductase (GSR) and phospholipase D-α (PLDα) are major antioxidant enzymes that protect plant cells against oxidative damage caused by reactive oxygen species (ROS) produced under metal-stress conditions. The expressions of GSR and PLDα were found to be induced in hemp that have accumulated high concentrations of Cu [52]. Other studies have reported an increase in aldo-keto reductase, an NAD(P)H-dependent enzyme, in hemp grown under Cu stress [65,66]. This reductase is involved in the detoxification process by improving the scavenging capacity of the cell [67]. Authors proposed that this protein reduces ions, making them available for interaction with other proteins, such as phytochelatins, that can transport them to the vacuole [65]. Although photosynthesis was not affected in that study, Cu treatment resulted in a significant reduction in the aerial parts of the plant as well as the root-system architecture. Angelova et al. reported Cu concentrations in the following order: flower > seeds > roots > stems > leaves > fiber.
- Selenium
Hemp can grow in selenium (Se)-laden soil and accumulates selenomethionine and methylselenocysteine in seed embryos [40]. This means that the seeds can be used as a Se supplement for humans as well as livestock. Se is also present in other above-ground parts. Se found in the flower (where CBD and terpenes are produced and extracted) and stems (where fiber is produced) has no negative effect on the yield of such metabolites or on fiber quality. Se in the leaves can be applied as fertilizer for crops growing in low-Se soil.
- Cadmium / Nickel / Lead
Hemp can take up high concentrations of cadmium, nickel, and lead from metalcontaminated environments, such as soil from abandoned mines [37]. For each of these metals, their concentration was highest in the leaves. In another study with the same metal contamination, the quality of fibers and hurds was not affected, which allowed them to be used in products like composite materials [53]. Growing in soil with high concentrations of Cd has no negative effect on hemp germination, but as the plants mature, they accumulate Cd in aerial parts, negatively influencing photosynthesis and inhibiting plant growth [35]. Surprisingly, Cd accumulation was highest in the roots, but root growth was not inhibited. Ahmad et al. also demonstrated that Pb accumulated mainly in the leaves [52], although Angelova et al. reported Pb concentrations in the following order: flower > roots > stems > leaves > seeds > fiber.
- ↑ 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
- ↑ Wu, Y., Trejo, H.X., Chen, G. et al. Phytoremediation of contaminants of emerging concern from soil with industrial hemp (Cannabis sativa L.): a review. Environ Dev Sustain 23, 14405–14435 (2021). https://doi.org/10.1007/s10668-021-01289-0