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УДК 504.06 ББК 30.16
SOIL HEAVY METAL ACCUMULATION POTENTIAL OF WILD AND CULTIVATED PLANTS: Review
Merhawi Kidane Tsegay
PhD student in Environmental biotechnology, Russia,Astrakhan State University,
merapg 12@gmail.com
Sukenko Ludimila Timofevna.2
Doctor of biological science, Russian Astrakhan,
The advent of the industrial revolution and anthropogenic impacts has resulted in the release of an increasing number of hazardous heavy metals into the environment. Heavy metals are recognized for their non-biodegradability in the environment, and they have the potential to infiltrate the food chain via edible plants, where they could eventually accumulate in the human body through bio-magnification and pose a grave risk to human health and the environment. As a result, a feasible, cost-effective, and environmentally friendly phytoremediation technology that uses plants to remove toxic heavy metals and other pollutants from soil is the best option. The goal of this review is to look at some potential plants for heavy metal removal and to describe the various mechanisms of soil phytoremediation and plant strategies to overcome the toxic effects of heavy metals.
Key words: Phytoremediation, Hyper-accumulators, Heavy metals, Soil, pollution, pollutant, Environment, ecofriendly, accumulation, hazard.
ПОЧВЕННЫЙ ПОТЕНЦИАЛ АККУМУЛЯЦИИ ТЯЖЕЛЫХ МЕТАЛЛОВ ДИКИХ И КУЛЬТУРНЫХ РАСТЕНИЙ:ОБЗОР
Мерхави Кидане Тсегай
Аспирант, экологической биотехнологии, Россия, Астраханский
государственный университет,
merapg 12@gmail.com
Сукенко Людмила Тимофеевна
доктор биологических наук/, Русская Астрахань
Абстрактный. Промышленная революция и антропогенное воздействие привели к выбросу в окружающую среду все большего количества опасных тяжелых металлов. Тяжелые металлы не поддаются биологическому разложению в окружающей среде, и они могут проникать в пищевую цепочку через съедобные растения, где они могут в конечном итоге накапливаться в организме человека в результате биомагнификации и представлять серьезную опасность для здоровья человека и окружающей среды. В результате наилучший вариант — работающая, экономически эффективная и экологически чистая технология фиторемедиации, в которой используются растения для удаления токсичных тяжелых металлов и других загрязнителей из почвы. Цель этого обзора — рассмотреть некоторые потенциальные растения для удаления тяжелых металлов и описать различные механизмы фиторемедиации почвы и стратегии растений для преодоления токсического воздействия тяжелых металлов.
Ключевые слова: Фиторемедиация, Гипераккумуляторы, Тяжелые металлы, Почва, загрязнение, поллютант, Окружающая среда, экологичность, накопление, опасность.
Heavy metals are not degraded by biological or physical processes, and they persist in the earth for a long period, posing a long-term environmental risk [34] ,disrupting a number of physiological and biochemical processes in crop plants, resulting in poorer soil agricultural
output. Based on biological function, they are categorized as essential or non-essential. Elements like Pb, Cd, As, and Hg are non-essential, poisonous, and have no identified usage in plants [36], whereas Cu, Fe, Mn, Ni, and Zn are required for physiological and biochemical
processes throughout the plant's life cycle [41], though they can be harmful when present in excess.
Topsoil is an ever-changing extremely important component of the natural environment. It influences plant distribution and provides a habitat for a variety of organisms. It regulates the flow of water and chemical substances between the atmosphere and the earth, and it serves as both a source and a storage facility for gases in the atmosphere [30]. The uppermost earth, along with the plant and animal life it supports, the rock on which it develops, its location in the landscape, and the climate it experiences, all combine to form an incredibly sophisticated natural system that is more powerful and intricate than any other machine man has created. Industrial revolution, population increase, and anthropogenic activities, are major sources of hazardous pollutants[18, 19].
Metal contamination has emerged as one of the most prominent issues in today's environmental crises. The so-called pollutants can be found as free metal ions, soluble metal complexes, exchangeable metal ions, organically bound metals, precipitated or insoluble compounds such as oxides, carbonates, and hydroxides, or as a component of silicate materials [24]. The introduction of such metals is caused by both natural geological processes and human activities. Though there are many elements that cause environmental hazards; Lead (Pb), cadmium (Cd), zinc (Zn), nickel (Ni), copper (Cu), mercury (Hg), and arsenic (As) are the primary threats [3, 4]Examples of natural inputs include weathering of mineral-rich rocks, from groundwater or subsurface layers of soil, atmospheric deposition of volcanic activity, and transport of continental dust. Humans release heavy metals into the soil through the disposal of industrial effluents, the application of sewage sludge, military maneuvers, mining, metal smelting, electroplating, landfill operations, the discarding of industrial solid and liquid waste, the use of agricultural chemicals, gas exhausts, and the production of energy and fuel [23, 24].
So far, the most commonly used methods of dealing with heavy metal soil pollution are the conventional methods; such as physical, chemical, excavation, soil- washing, soil-capping, thermal therapy, electro-kinetics, or chemical (oxidation or reduction) means . Such treatment acts have been blamed for high energy consumption, site destruction, and logistical demands. Recently, a new alternative technology, known as bioremediation, which uses bacteria, fungi, or plants to deal with such environmental problem, has emerged due to its low cost, simplicity, and efficiency [11]. This paper examines the potential of plant-based cleaning to be a great alternative to so-called conventional methods.
Phytoremediation
Phytoremediation is a new bioremediation technology that has a lot of potential for cleaning up and recovering damaged sites [31]. In both industrialized and developing countries, there is a constant need for contaminated site cleanup; phytoremediation should be given attentive, serious, and immediate consideration as a cost-effective, promising, and pioneering environmental technical solution.
The discovery of floras capable of absorbing hazardous metals in 50-500 times greater quantities than regular plants has revolutionized this bioremediation. The potential of these hyper-accumulators to accumulate toxic metals in their shoots is substantially greater than that of non-accumulators [36]. Several plants that flourish in hazardous metal-contaminated soil collect considerable amounts of heavy metals in their roots, branches, and shoots. Despite the fact that there are over 400 varieties of hyper-accumulator plants[28], little research has been done to determine whether natural hyper-accumulators or other compatible plants can perform phytoextraction in the field. Plants must generate enough biomass while accumulating large levels of metals, and metal-accumulating plants must be amenable to agricultural operations so that they may be planted and harvested repeatedly[15].Identifying or creating a suitable plant, optimizing soil and crop management practices, and developing biomass processing and metal extraction procedures are all critical components of phytoremediation's success[29, 40]. The absorption capacity of high biomass crop plants such as Indian mustard ("Brassica juncea"), sunflower ("Helianthus annuus"), and maize ("Zea mays") has been studied extensively[6, 13, 20].
Heavy metals' effects on soil
Growth and metabolism of organisms require elements including Cu, Co, Ni, Zn, Mo, Fe, and, Mn but their concentrations can easily exceed acceptable limits, causing harm to them. Increased level of Pb in soil, for example, could reduce soil fertility, whereas minimum Pb levels may inhibit key plant processes like "mitosis, photosynthesis, and absorption of water, resulting in toxic symptoms like dark green leaves, wilting of older leaves, stunted foliage, brown short leaves, and roots"[2]. Others, including Cd, Pb, As, and Hg, are believed to be with no biological role, and even very low amounts might be hazardous, such chemicals also have a negative impact on soil microorganisms, resulting in changes in the variability, population size, and overall activity of microbial communities[27]. High concentrations of plant metal uptake from soils and the aquatic environment may pose a significant health risk due to food-chain implications. Ingesting such plants can deplete some vital nutrients in the body, resulting in "lowered immune defenses, intrauterine development retardation,
malnutrition-related impairments, and a high prevalence of upper gastrointestinal cancer" [17].
Adaptations of plants to heavy metals in contaminated soils
Accumulation and transport: Hyper-accumulator plant roots in the rhizosphere release protons to acidify the soil, which mobilizes metal ions and improves metal bioavailability [9]. The lipophilic cellular barrier can prevent metal ions from entering cells due to their charge; however, three types of secretion can facilitate the crossing process[36]:
(I) Proteins that function as transporters: A variety of molecules, including metal ion transporters and complexing agents, mediate heavy metal absorption and translocation in plants. Specialized transporters (channel proteins) or H+-coupled carrier proteins are found in the plasma membrane of root cells and are required for heavy metal ion uptake from the soil. They can transport specific metals across cellular membranes and mediate metal transfer from roots to shoots[39]. These proteins have a specific binding domain that attaches to and transports metal ions from the extracellular space into cells, such as the hyperaccumulator "Thlaspi caerulescens," which has a higher Zn2+ capacity than its cousin "T. arvense" [16, 22], implies that carrier proteins are important.
(II) Natural chelators: Plants can produce natural chelators that are far less toxic and biodegradable than EDTA (Ethylene-Diamine-Tetra Acetic acid), which binds to metal ions and renders them uncharged. An uncharged ion has a high mobility, making it much easier to pass through a biological membrane. Plants contain two natural chelators known as "phyto-chelatin (PC) and metallothionein (MT)" [33, 35].
(III) Organic acid synthesis: Some organic acids, such as "malic acid" and "citrate," have been identified as positive bio-reagents for accelerating heavy metal absorption by the root. This mechanism is much more visible in root-shoot transportation.
Detoxification: Heavy metals typically cause cell damage by attaching to essential proteins, interfering with cellular functions, and suppressing cell control. Plants, thankfully, have evolved systems to protect themselves from the harmful effects of heavy metals; for example;
(1) Chelation: Not only does the detoxification phase but also play an important role in the accumulation and passage of heavy metals. Chelators typically have ligands (the most common of which are "histidine" and "citrate") that bind to metal ions, and chelated metal ions, which appear uncharged and inert to react with other substances, significantly reduce harm to cells.
(2) Vascular compartmentalization: The "vacuole" is commonly viewed as the primary storage location for wastes in plant cells, and vascular compartmentalization is very effective
in managing metal ion distribution and concentration. To compartmentalize a vacuole is to "arrest and imprison" harmful metal ions, confining them to a small area so that they do not reach other areas of the cell[13].
(3) Volatilization: Some varieties of plants avoid lasting toxic metal damage by transforming metallic ions into a volatile state. A good example is the bio-process of mercury, a worldwide volatile pollutant that can store in human bodies. However, not all plants have this ability, and even among those that do, the small amount of accumulation and spatial distribution has hampered their widespread cultivation [29].
Kinds of phytoremediation
Rhizofiltration: It is a method that uses plants (both terrestrial and aquatic) to captivate, concentrate, and precipitate toxins from polluted areas using their roots. Manufacturing discharge, farming runoff, and mine drainage can all be partially treated via rhizofiltration [10]. It is a relatively new technique that has been generally recognized because of its environmental friendliness as a more sustainable answer for the dealing of aqueous contaminated sites [38].Dominant species suitable for; include "Water hyacinth (Eich-hornia sp.), water lettuce (Pistia sp.), duckweeds (Lemna sp., Spirodella sp.), Thlaspi caerulescens, Sedum Alfred, Arabidopsis Halleri, Brassica campestris, and Pistia stratiotes and little water ferns (Azolla, sp.)". Rhizofiltration predominantly retains "lead, cadmium, copper, nickel, zinc, and chromium" inside the roots [12].Indian mustard, sunflower, tobacco, spinach, corn, and rye" are often employed for the removal of lead (Pb) from water or soil. This method has the advantage of being able to handle a wide spectrum of heavy metals while also producing a recyclable metal-rich plant waste with minimum environmental impact [1, 40].
Phytoextraction (phyto-accumulation)
The absorption and carriage of chemicals from the earth to the shoot parts of a plant through roots is termed as phytoextraction. Hazardous metals from polluted soils are absorbed, concentrated, and precipitated into aboveground biomass (shoots), which is gathered and burned after treatment[26]. Alternatively, the high-biomass plant can be paired with chelates or a soil amendment to increase the ability to drain metals from the environment. Frequently available phytoextractors comprise "Agrostis tenuis (Pd hyper-accumulators), Streptanthus polygaloides (Ni hyper-accumulators), Aerollanthus subacaulis (Cu hyper-accumulators), Haumaniastrum robertii (Co hyper-accumulators), Maytenus bureaviana (Mn hyper-accumulators), Thlaspi tatrense, and Thlaspi caerulescens (Zn hyper-accumulators), Lecythis ollaria (Se hyper-accumulator), and Pteris vittata (As hyper-accumulators)" [32]. Typically, a plant is considered as a hyper-accumulator, if the metal concentration in the
shoot is greater than 0.1%, the ratio of the shoot to root concentration is also regularly greater than 1, which describes the ability to transfer metals (metal uptake) from root to shoot and the formation hyper-tolerance capacity.
Typically, there are four main features of hyper-accumulating plants:
a. Accumulation capacity: "The minimum concentration of Ni, As Pb, Co, and Cu in the shoots of a hyper-accumulator should be greater than 1000 mg/kg dry mass, 10,000 mg/kg Zn and Mn, 100 mg/kg Cd, and 1 mg/kg Au".
b. Translocation capacity: the concentration of heavy metals in the shoots of a plant should be higher than that in the roots [5].
c. Tolerance capacity: A hyper-accumulator should have a high tolerance to toxic contaminants; in particular, the shoot biomass of plants grown in contaminated soils should not decrease significantly.
d. The enrichment factor (EF) index (concentration ratio in the plant to the soil): the EF value should be at least higher than 1 when the content of heavy metal in soils reaches its critical concentration in a hyper-accumulator [14].
Phytovolatilization
In this regard, soil pollutants are absorbed by the plant's body, but a volatile version of the pollutant or a volatile breakdown product is subsequently exhaled through the leaves as water vapor. Bio-methylation of toxic metals like selenium, arsenic, and mercury can result in volatile chemicals that can be discharged into the atmosphere [21]. Although bacteria have long been recognized to play an important part in the volatilization of Se from soils, plants have only lately been identified to do the same. The diffusion of pollutants from stems or other plant components to the leaves is referred to as Phytovolatilization [25]. Often genetically modified plants are employed to absorb contaminants, particularly Hg, but "Brassica juncea and Brassica napus" have been utilized to phytovolatilize Se from the soil naturally [7].
Phytostabilization (phytoimmobilization or photo restoration)
With this respect, hydraulic control is provided, which decreases the vertical drive of contaminants into groundwater; pollutants are physically and chemically immobilized through root sorption and chemical fixation with various soil adjustments[43]. Phytostabilization is effective on fine-textured soils with high organic-matter content, but it can be used in a variety of locations with extensive surface contamination. Plants that stabilize soils should have specific characteristics such as drought tolerance, rapid growth to provide adequate ground coverage, a thick root system, ease of establishment and care in the field, and self-production. For the use of soil adjustments in phytostabilization reduces
biological activity and controls plant metal uptake, heavy metals cannot be completely removed from the environment and must be monitored on a regular basis[8].
The benefits and drawbacks of phytoremediation
Early research suggests that phytoremediation is a promising cleanup solution for a wide range of pollutants. It can treat a wide range of organic and inorganic compounds[25]. Phytoremediation is well-known for its low environmental impact, environmental improvement, public acceptance, and potential application to a relatively large pollution area and does not necessitate expensive equipment or highly trained personnel, it is relatively simple to implement[37]. The primary benefit of phytoremediation is its low cost when compared to traditional clean-up technologies. For example, "the cost of cleaning up one acre of sandy loam soil with a contamination depth of 50 cm with plants was estimated to be $60,000-$100,000, versus $400,000 for the conventional excavation and disposal method"[42]. However, there are several issues and limitations to phytoremediation. Treatment is typically limited to soils within centimeters of the surface; climatic or hydrologic conditions may limit the growth rate of plants in use.
Conclusion
Phytoremediation is with promising prospective for cleansing soil pollutants without affecting the soil's qualities. Furthermore, it is a less expensive, environmentally beneficial, viable, long-term, and visually beautiful technology. However, it is still in the early stages of development, and research efforts must be redirected toward commercial applications. Several studies have suggested that plants and bacteria work together to remove metal pollutants from the soil. Plant growth-promoting bacteria and metal-tolerant microorganisms can be integrated to boost the phytoremediation process radically. Short-term improvements in phytoremediation can be made by selecting more efficient plant varieties and soil amendments, as well as optimizing agronomic practices for plant cultivation. Long-term improvements can be made by isolating genes from various plant, bacterial, and animal sources that can enhance metal accumulation or organic degradation. The highly integrated nature of phytoremediation necessitates collaboration with a wide range of different disciplines. It is clear that harnessing the remarkable ability of green plants and their rhizosphere microbes to accumulate elements and compounds from the environment and perform biochemical transformations are becoming a new frontier of science. Most contaminated sites have hardy, tolerant, weedy, and microbial species, and bioremediation using these and other non-edible species can prevent the contaminant from entering the food web.
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