Sorption capacity of ornamental herbaceous plants under conditions of soil contamination with lead

. The sorption capacity of some types of ornamental herbaceous plants was studied under conditions of controlled soil contamination with lead ions in order to obtain species-hyperaccumulators of heavy metals that can be recommended for phytoremediation of soils of technogenic environment. The concept of using plants to restore a polluted environment is not new. More than 300 years ago, plants were proposed to be used in wastewater treatment. The use of plants to restore the environment has been called "green reclamation", "botanical bioremediation", etc. In essence, phytoremediation includes a person's ability to enhance the natural inactivation or restoration of contaminated sites and, as a result, is a process that occupies an intermediate position between technical and natural restoration. Studies have shown that the greatest sorption capacity was noted in seedlings of Brassica napus L. and Ricinus commúnis L., the lowest sorption capacity was shown by seedlings of Phacelia tanacetifolia Benth. The lowest ability to accumulate heavy metal ions was shown by seedlings of Phacelia tanacetifolia Benth., whose metal transfer factor is 0.78, so we cannot recommend this type for the technology of phytoremediation of soils of the technogenic region. In Brassica napus L. and Ricinus commúnis L. the metal transfer factor for lead is almost equal to 1, which makes it possible to attribute this plant species to heavy metal hyperaccumulators and recommend it for use in phytoremediation technology of soils contaminated with lead ions.


Introduction
Currently, the contribution of motor transport to the pollution of the biosphere of the Donetsk region is considered dominant and amounts to 72.7% of the total volume of pollutants entering the urban environment [1][2].The rapid growth of the fleet of cars observed in recent years has caused a sharp deterioration of the environmental situation in the Donetsk region.The level of lead pollution in the urban environment and especially in soils with the ability to sorption and deposit it has increased.The accumulation of lead in soils is also favored by the weak mobility of most of its compounds at high pH [3][4][5][6]9].In addition, biogeochemical anomalies of lead are more static and durable than others, since soils are able to accumulate it throughout the entire period of anthropogenic impact.Lead belongs to the group of heavy metals of the 1st class of environmental hazard, capable of causing various toxicoses and carcinogenic hereditary mutations in humans and animals.Therefore, phytoremediation of soils contaminated with lead ions is one of the leading technologies for urban soil restoration [5,.
Phytoremediation is a method of using plants to remove or neutralize pollutants, it is a promising green strategy for cleaning polluted soils of the urban environment [3,6].Currently, research on the profitability of bioremediation of contaminated soils is becoming more and more relevant.Phytoremediation not only helps to remove pollutants from the soil, but also prevents soil leaching, thereby maintaining or improving soil structure and increasing fertility.This method can be used in combination with other technologies, especially at the last stage of soil remediation.

Materials and Methods
Decorative herbaceous plants were used as objects of research during the experiment: Ricinus communis L., Brassica napus L., Phacelia tanacetifolia Benth.
Studies of the sorption capacity of seedlings were carried out according to the scheme of a complete one-factor five-level experiment (Table 1).Lead nitrate was used as a contaminant by stoichiometric ratio.Lead concentrations were 0, 0.5 MPC, 1 MPC, 1.5 MPC, 2 MPC.
Plant seeds were germinated according to their biological characteristics.Cultivation was carried out for thirty days, daylight duration of 14 hours, temperature of 20-22°C and soil humidity of about 70% of total humidity.Each vessel was filled with 350 g of soil sifted through a soil sieve with a hole diameter of 3 mm, into which lead nitrate was previously introduced according to the experimental scheme.
The lead content in the plant material was determined by the method of atomic absorption spectroscopy according to V. Price on the atomic absorption spectrophotometer "Saturn-3".The method is based on acidic opening of plant raw materials, spraying the obtained solutions into an acetylene-air flame or introducing the resulting solution into a graphite furnace with a spectrophotometer followed by electrothermal atomization [9].The obtained data were processed statistically using specially developed programs.

Results
Extensive pollution of agricultural land around the world significantly reduces the productivity of arable land in the cultivation of crops.Thus, the global problem of environmental pollution requires an immediate solution aimed at detoxifying toxic and dangerous pollutants in order to restore the disturbed natural environment.
Currently, all measures have been taken to find less polluting and cost-effective environmentally correct technologies.In this regard, phytoremediation technology is considered as an effective, less costly alternative to generally accepted reclamation technologies for restoring the environment polluted with a wide range of toxic and harmful substances.
Modern technologies for the restoration of soils contaminated with heavy metals such as Pb, Cd, Cr, Ni, Co, Mn, Hg and As, etc. use the sorption properties of plants [7].In developing countries, treated and untreated wastewater is commonly used for irrigation of agricultural land, which leads to the accumulation of heavy metals in soils.This negatively affects the quality of infantry lands.Sorption of heavy metals from polluted soil areas using green technologies is an acceptable approach [8,10].
Natural geological and anthropogenic activities are the main sources of soil pollution by heavy metals.Heavy metals are easily absorbed by plants through the roots and transported to aboveground organs.The absorption of heavy metal ions depends on several factors, such as soil pH, temperature, organic component content, the presence of chelating agents, etc.Among the various soil factors, soil pH is the most important one affecting the mobility of heavy metal ions.
Studies have shown that the ability of plants to accumulate lead ions in their organs depends both on the concentration of metal and on the species-specific characteristics of plants.Thus, in the variants of soil contamination with lead in concentration 0.5 MPC, the content of this metal in the root system of Ricinus commúnis L. seedlings increases on 20%, compared to plants grown on unpolluted soil.With a further increase the concentration of the pollutant to 1 MPC, the concentration of lead ions in the roots of these plants also increased twice (Table 2).Under conditions of introduction of lead ions in concentration of 1.5 MPC, the concentration of this toxicant in the root system increased by 165%, and with a further increase lead ions in soil to 2 MPC, its concentration in the roots of seedlings increased almost 4 times.
In the variants of soil contamination with lead in concentration 0.5 MPC, the content of this metal in the aboveground part of Ricinus communis L. seedlings increases on 52% compared to the control.When the concentration of lead in the soil increases to 1 MPC, the content of the toxicant in plant stems increased almost twice.A similar increase in the concentration of lead ions in the aboveground part was observed with an increase in the concentration of the toxicant in the soil by 1.5 and 2 MPC, under which the metal content in the stems increased by 4.6 times in comparison with plants grown on uncontaminated soil.
In the variants of soil contamination with lead in concentration 0.5 MPC, the content of this metal in the root system of Brássica nápus L. seedlings increases on 20%, compared to plants grown on unpolluted soil.When the concentration of lead in the soil increases to 1 MPC, the concentration of lead ions in the roots of these plants also increased by almost 3 times.Under conditions of introduction of lead ions at a concentration of 1.5 MPC, the concentration of the toxicant in the root system increased almost 4 times, and with a further increase in lead to 2 MPC, its concentration in the roots of seedlings increased almost 5 times.In the variants of soil contamination with lead in concentration 0.5 MPC, the content of this metal in the aboveground part of Brassica napus L. seedlings increased on 45% compared to the control.With a further increase in the concentration of lead in the soil to 1 MPC, the content of the toxicant in plant stems increased almost three times.A similar increase in the concentration of lead ions in the aboveground part was observed with an increase in the concentration of the toxicant in the soil by 1.5 and 2 MPC, under which the metal content in the stems increased by 4-6 times in comparison with plants grown on uncontaminated soil.
In the variants of soil contamination with lead in concentration 0.5 MPC, the content of this metal in the root system of Phacelia tanacetifolia Benth seedlings increases on 18.5%, compared to plants grown on unpolluted soil.With a further increase in the concentration of the pollutant to 1 MPC, the concentration of lead ions in the roots of these plants also increased by almost 75%.But under the conditions of the introduction of lead ions at a concentration of 1.5 MPC, the concentration of the toxicant in the root system decreased sharply, and with a further increase in lead to 2 MPC, its concentration in the roots of seedlings also continued to decrease.This fact can be explained by the protective reaction of the plant to the harmful effects of the pollutant.When introducing lead ions into the soil at a concentration of 0.5 MPC, we noted an increase in the concentration of this metal in the aboveground part of Phacelia tanacetifolia Benth seedlings by 21.3% compared to the control.But under the conditions of the introduction of lead ions at a concentration of 1.5 MPC, the concentration of the toxicant in the aboveground part of the seedlings decreased sharply, and with a further increase in lead ions to 2 MPC, its concentration in plant stems also continued to decrease.

Discussion
The sorption of metals depends on the specific species of plants, the chemical characteristics of metals and the characteristics of the soil.Studies have shown that representatives of hyperaccumulating plants belong to various families, such as Asteraceae, Brassicaceae, Linaceae, Amaranthaceae, Caryophyllaceae, Boraginaceae, Lamiaceae, Solanaceae, etc.One of the indicators of the sorption capacity of plants is the Wilkinson transfer factor, which is the ratio of the concentration of metal in plant organs to the concentration of metal in soil subject to phytoremediation (Table 3).
Studies have shown that in Brassica napus L. and Ricinus commúnis L., the metal transfer factor for lead exceeded 1, which indicates the high ability of these plant species to accumulate heavy metal ions in vegetative organs.The seedlings of Phacelia tanacetifolia Benth showed the least ability to accumulate heavy metal ions.the metal transfer factor of which did not exceed 1.0.Phytoremediation technology consists in using a certain specific plant species to restore the soil by deactivating metal ions in the rhizosphere or translocating them into aboveground parts.The new technology of phytosorption has a number of advantages, such as low cost and high aesthetics of the technology [10].Phytoremediation technology has a number of advantages, such as high economic efficiency, environmental friendliness, aesthetics, and is applicable to a wide range of heavy metals.
Phytoremediation has some disadvantages: 1) the presence of a complex of several types of heavy metals and organic pollutants may present difficulties in accumulation; 2) climatic and hydrological conditions may limit the growth of plants used for reclamation; 3) this is a long process that may take several years, and it is applicable only to the upper layers of the soil; 4) the biomass accumulated by plants can turn into into hazardous waste, for which proper disposal will be necessary.
The effectiveness of the chosen technology depends on plant species resistant to abiotic and anthropogenic factors, their ability to absorb higher concentrations of heavy metals.The selection of plants is carried out taking into account high growth rates, the degree of accumulation of biomass and the rate of absorption of heavy metal ions.

Table 1 .
Experimental scheme of sorption capacity of seedlings in conditions of soil contamination with lead ions

Table 2 .
Content of lead ions by vegetative organs of seedlings

Table 3 .
Heavy metal transport factor by plants