عنوان مقاله [English]
The issue of soil contamination by waste chemicals has raised concerns about the environment. Among them, heavy metals are also known to be important in low concentrations due to their non-degradability and physiological effects on living organisms. In addition to natural processes, heavy metals can be used by human resources such as mining activities, landfilling of various wastes and wastes, use of sewage sludge in agricultural lands, waste leachate, excessive use of chemical fertilizers, fossil fuels and the like cause soil pollution. Heavy metals do not decompose in the environment, so they need to be removed from the environment. On the other hand, the exorbitant costs of physical and chemical methods have led to efforts to achieve cheaper methods. The use of conventional physical and chemical treatment methods is not economical and cost-effective in cases where the concentration of heavy metals is low, and may even lead to the production of secondary effluents, which in turn will lead to more treatment problems. Therefore, researchers have developed a new method using the potential of plants to eliminate soil pollution and preserve this national capital. Some plants, by developing special mechanisms, can accumulate heavy metals in high concentrations, which are called hyperaccumulation plants. The most characteristic disadvantages of this method are limited to sites with medium and low pollution, low biomass production, shallow roots, low biological access of metals in the soil, especially calcareous and young soils in arid and semi-arid regions, time consuming. These unavoidable constraints are forcing researchers to refine traditional approaches and apply new phytoremediation techniques to minimize the constraints and ensure widespread use of phytoremediation.
Selecting suitable species for phytoremediation of heavy metals is one of the critical stages of phytoremediation. Another solution to overcome the problems mentioned in phytoremediation is to use heavy metal-resistant bacteria and plant growth stimulants to strengthen the root system, produce more biomass and increase the bioavailability of metals by plants. Therefore, the use of microbial inoculation will help plant species to effectively filter heavy metals from the soil.The application of phytoremediation technique has been less in our country so far and only in laboratories and experimentally and due to the ignorance and negligence of officials has never been used as an effective method in refining heavy metals in contaminated soils. Therefore, it is necessary to introduce and replace plants and apply new strategies based on the study of cooperation between plants - microbial communities, to be able to expand and localize modern phytoremediation, to reduce soil health risk and increase food security effectively and economically.
To achieve the above goals, the following suggestions can be considered:
1- Introducing suitable crops to replace cultivation in soils contaminated with heavy metals: Contamination of agricultural lands with heavy elements is a serious risk for the production of healthy crops in Iran and the world. Therefore, it is necessary to reduce the contaminants in the soil by accumulating them in the roots of plants or depositing them in the root zone and transferring them to the food chain by using the bio-stabilization technique. Unlike other phytoremediation techniques, plant stabilization does not remove metal from the soil, but by stabilizing them by adsorption or deposition in the root zone, it reduces their risks to human health and the environment. Plants with high bio-concentration factor (BCF, the ratio of metal concentration in the root of the plant to its concentration in the soil) and low transfer factor (TF ratio of metal concentration in the stem to its concentration in the root) are suitable for plant stabilization. The absence of high concentrations of metals in the stem eliminates the need to discard harvested plants. In this project, sorghum, millet and flax species are proposed for cultivation in agricultural lands to reduce the burden of contamination in crops and food health for study.
2- Introduction of herbaceous plants (forage) for stabilization of heavy metals in contaminated soils
Consumption of infected forage plants by livestock and domestic animals can be examined from two perspectives. First, the use of contaminated forage causes disturbances in livestock and domestic animals, such as reduced milk production and growth rate, reduced resistance to binaries and infections, and impaired reproduction. Second, by consuming animal products, pollutants will enter the human food cycle. Also, due to the multiplicity of pollution industries and ways of its transmission, currently the best way to reduce these risks is to take seriously how to feed and control the diet of livestock. Therefore, the use of specific plant species to immobilize contaminants in the soil, through surface adsorption and accumulation by the root, intra-root adsorption or deposition in the root zone and physical stabilization of soils or in other words plant stabilization is also necessary. Plant stabilization reduces the mobility of pollutants and prevents their transfer to groundwater and the atmosphere. Therefore, in this study, several types of forage plants that have economic aspects and good yield, including sainfoin (Onobrychis aucheri Boiss), Timothy (Phleum pratense), Lolium perenne, Lemon Grass Tall fescue, Dactylis glomerata, Agropyron desertorum, Agropyron desertorum in order to produce healthy forage and restore vegetation in pastures and animal health are recommended for research.
3- Introducing ornamental overgrowth plants for refining soils contaminated with heavy metals
In recent years, plant-based environmental treatment technology, or phytoremediation, has been widely pursued as an in-situ cost-effective strategy to clean metals from contaminated sites. In this project, plant extraction means using polluting plants to remove metals or organic pollutants from the soil by accumulating them in harvestable areas. Plant extraction involves planting the plant several times in contaminated soil to bring the metal concentration to an acceptable level. The following species are recommended for review: Parsley (Tagetes patula / erecta), Margaret (Chrysantemum maximum) Marigold (Calendula officinalis / alata), Chrysanthemum (Chrysanthemum indicum), Gladiolus grandiflorus, Pelargonium graveos ).
4- Investigation of bacteria resistant to heavy metals with plant growth stimulant properties with the aim of increasing the growth and production of plant biomass under conditions of metal stress
Using bacteria can be a simple and effective strategy to increase growth as well as bioremediation activity. Bacteria have different mechanisms for decontamination of contaminated environments through biosorption, bioaccumulation, sales, bio-mineralization and biotransformation. On the other hand, plant growth-promoting bacteria typically improve plant growth under stress. Some rhizosphere bacteria that directly and indirectly have beneficial effects on plants are called growth-promoting rhizosphere bacteria. These beneficial bacteria (PGPR) are also called product-enhancing bacteria. Growth-promoting rhizosphere bacteria can play a direct role in increasing plant growth and yield by using various mechanisms. Increased dissolution of insoluble nutrients such as phosphorus, production of ACC-deaminase, production of plant growth hormones such as auxin, nitrogen fixation and production of siderophore (from the point of view of increasing iron absorption) are the most important direct mechanisms. Indirectly, growth-promoting rhizosphere bacteria neutralize or modify the harmful effects of plant pathogens by using various antagonistic mechanisms, thereby increasing plant growth. Competition to absorb substances and occupy suitable sites for the activity of pathogens, production of antibiotics, production of siderophores (from the perspective of removing elements from the pathogens), lytic enzymes and production of hydrogen cyanide are the most important mechanisms used in this method.
5- Study of bacteria capable of producing chelating compounds with the aim of increasing the efficiency of plant extraction by increasing the dissolution and absorption of metals
The efficiency of plant extraction of metals from soils depends on the availability of metals to plants. The magnitude distribution of metals and the intensity of their bonding in the soil are influenced by metal species, age and soil properties. If the availability of metals for plant uptake is low, microorganisms can add chelating or acidifying agents to increase the availability of metals to the soil.
6- Feasibility study of using bacteria to reduce the accumulation of heavy metals in crops grown in soils contaminated with heavy metals with the aim of reducing health risk and increasing food quality
Rhizospheric and endophytic bacteria (with greater potency) in addition to modulating the harmful effects of biological and non-biological stresses (due to heavy metals, salinity, etc.) to the plant can reduce the absorption and accumulation of metals through various mechanisms They are also heavy in the plant (or in the aerial parts). In addition to reducing the destructive effects of heavy metal stresses on plants, heavy metal-resistant bacteria can reduce the availability of heavy metals to plants through various mechanisms in the soil. One of these mechanisms is the adsorption or bioaccumulation of heavy metals inside bacterial cells. At present, heavy metal bioabsorption techniques are recommended for the removal of heavy metals because they are more efficient, less costly, and more environmentally friendly. Root endophytic bacteria prevent their transfer to plant shoots (organs that can be harvested by humans or animals) by adsorption of heavy metals (adsorption of heavy metals on the bacterial cell wall), accumulation, complex formation, reduction and oxidation, etc. Finally, with this method in agricultural lands, the risks of these elements for human and animal health can be reduced.