The accumulation of metals, in particular of metals known as heavy in the plants poses problem. However, so with the state of traces, these metals are essential to the life, they can with stronger concentrations appear toxic . So to limit the risks, we have to study the effects of these pollutants on the living organisms . Among the techniques of phytorehabilitation, we find the phytoextraction. So, we are interested in the phytoextraction in the barley (Hordium vulgare) of a soil contaminated artificially by zinc and cadmium and the influence of these metals presence on the barley growth. The results show that the barley is tolerant in the zinc and the cadmium; it presents no sign of stress after 4 weeks of culture in soil contaminated by these metals. The accumulated zinc arrests at the level of roots and it is not transferred towards the air parts. On the other hand, the barley accumulates more cadmium compared to zinc .
Most sites contaminated with heavy metals, often have a diverse flora tolerant apparently well high levels of metals. The phytorehabilitation is based on the use of plants for the remediation of contaminated sites. It is defined as the use of plants to remove or render less mobile and less toxic environmental contaminants. These techniques are not very aggressive to the environment and relatively inexpensive. They can be applied to organic or inorganic pollutants.
Among these techniques, the phytoextraction relates to mainly the metals absorption of the ground with the plants. The ideal plant for the phytoextraction must be able to accumulate and tolerate strong contents of metals: they are plants known as accumulator or hyper accumulator. The phytoextraction term relates to mainly the absorption of the ground metals with the plants. The plants can accumulate metals, such as: Fe, Mn, Zn, Cu, Mg, Mo, necessary to their good development, but are also able to accumulate metals, such as: Cd, Cr, Pb, Co, Ag, Hg. The tolerance and this bioaccumulation are made possible by an adaptation of the plant, with the introduction of new cellular level of physiological capacities [
The soil chosen as model of study is a soil which comes from the National Institute of Agronomy of El Harrach (Algiers). The samples of ground were taken in a 5 - 10 cm depth. The site of taking away is in a zone distant from any roads and all industrial activities having an agricultural activity. The sample of the studied ground was treated according to the standard ISO 11464. It is dried with the free air (20˚C ± 2˚C) is crushed and filtered on a sieve of mesh of 2 mm, then preserved in a dry place.
The measures of pHwater and pHKCl are taken according to the standard ISO 10390, 10 g of soil are put in suspension in 25 mL of distilled water (or potassium chloride, KCl in 1 mol∙L−1), agitated during 1 hour then left with the rest during 2 hours. The pH is then measured using a pH-meter. The pHwater measures the real acidity and takes into account the ions H3O+ free in the solution of the ground. The pHKCl measures the potential acidity which takes into account the ions H3O+ free in the solution of the soil and those moved by the KCl. The granulometry was determined by following the NF standard X 31 - 107 (French National Organization for Standardization 2004) and at the level of the soil analysis laboratory of the INRA.
It represents the fraction of water stored in micropores, capacity of retention. In water is measured according to the method by pressure the principle of which bases on the determination of the water quantity of a sample of ground retained under a strength of retention not exceeding 1000 g∙cm−2 [
The cation exchange capacity CEC represents the total quantity of exchangeable cations that the soil can adsorb. The CEC was determined by the method cobaltihexamine chloride (Co(NH3) 6Cl3) according to standard NF X 31 - 130 (AFNOR, 2004). The principle of this method is that the ions cobaltihexamine (
The content in organic carbon was determined according to the standard ISO 14235. It is about an oxidation of the organic carbon of the soil by the bichromate of potassium (K2Cr2O7) in excess, in acid medium (H2SO4), and hot. The chromium VI is reduced by the organic carbon to chromium III. The remaining Cr VI is then measured. For that purpose, 150 mg of soil are mixed in 5 mL of a solution of potassium bichromate to 0.27 mol∙L−1 and in 7.5 mL of concentrated sulphuric acid. After centrifugation and filtration (0.45 µm), the Cr VI is measured in 580 nm by means of an UV-VISIBLE spectrophotometer. The dosage remits in a curve of calibration established from glucose, which undergoes the same protocol as the ground.
Volumetric determination of the carbon dioxide (CO2) released under the effect of an acid very at room tempe- rature (of the laboratory) by the sample of ground prepared for essay (French National Organization for Standardization, on 1987).
The content in total nitrogen is determined by the method Kjeldahl, this standard has for principle to mineralize the organic nitrogen by the sulphuric acid in the presence of an activator of mineralization, distillation of the ammoniacal formed nitrogen and titration in a solution of boric acid (French National Organization for Standardization, on 1987).
The quantification of metals in solution is performed by atomic absorption spectroscopy (AAS). In AAS, the analyses are realized using a spectrophotometer (correction of background noise by deuterium lamp). The produced flame is a flame air/acetylene. In the case of Cr, it is reducing and in other cases, it is oxidizing.
The quantification of the metal content “total” requires dissolution of the soil. So we chose the protocol using aqua regia, 0.5 g of soil were introduced into a glass reactor with 9 ml HNO3 (69%) and 3 ml HCl (37%). Mineralization is realized using a digester. The digests are then brought to a final volume of 50 ml with distilled water. The metals were analyzed by AAS.
We work with a soil having been already of use to an agricultural activity, this soil is poor in nitrogen, phosphor and potassium; we thus proceeded to fertilization. The quantities are estimated on the basis of the recommendations of T I.B.C (Technical Institute of the Big Cultures), N/P/K: 70/50/50 unity by hectare for the Hordium vulgare. The used products are: the urea as the source of nitrogen; KH2PO4 as source of phosphor, K2SO4 as source of potassium. Products are dissolved in some demineralized water.
The ground is filled in 30 pots with 15 pots are contaminated with zinc. For that, solutions were prepared from the zinc sulfate and were added to the soil so that it is substantially in the soil after addition: 500, 1000, 2000, 3000 and 4000 mg of zinc per kg of soil (3 pots for each concentration). The contamination was realized by applying the solution of zinc corresponding to 90% of water saturation on the surface, and it is maintained for 24 h at room temperature until complete absorption of the solution.
The remaining 12 pots are contaminated by cadmium. For this, solutions were prepared from the cadmium sulfate were added to the soil so that it is added in the soil after 25, 50, 100 and 200 mg of cadmium per kg of soil (three pots for each concentration) by following the same procedure applied to the soil contaminated with zinc. The remaining three pots are not contaminated and therefore serve as a witness.
The Hordeum vulgare (barley) was planted on October 19th, 2009 and cultivated on November 19th, 2009 under natural light in an ambient temperature between 14˚C - 23˚C, the seeds used come from TIBC. During this phase of growth, the plants are watered to 90% of their capacity in order to maintain retention of the nutrients, and especially to maintain the metals in the soil and prevent leakage. The volume of water added depends on the water retention capacity of the soil and water requirements of the plant. During the culture, all the plants are sprayed with water. The amount of water to add is determined after all of the weighing pot-soil-plant.
Was stopped after one month of culture, we proceed to the separation of the root portion of the aerial part (leaves) by cutting the plant at the root bulb separate leaves. The plant material was dried at 40˚C for 72 hours and then ground in a mortar (porcelain). The digestion is carried out to determine the metal content in the plant. The metal content in the different parts of the plant (root, leaves) was determined after acid digestion. One introduced into the glass reactors 1 g of dry plant material and 10 ml of 69% HNO3. The dissolution is carried out in accordance with a temperature program: 200˚C for 30 min. The digests are then brought to a final volume of 50 ml with distilled water. The metals were analyzed by AAS.
After the culture of the barley, we proceeded to the determination of the growth parameters namely: the length of the leaf, the length of the root, the dry weight biomass (roots and leaves). The dry weight was determined after drying of biomass for 72 hours at 40˚C.
The presence of metals in the various parts of the plant indicates that there is accumulation but also transloca- tion towards the air parts (transport of these elements of the roots towards the air parts). Accumulation and the translocation can be evaluated by two parameters: the bioaccumulation factor and the translocation factor.
The bioaccumulation factor is described as:
CHMP: Concentration of heavy metals in the plant.
CHMS: Concentration of heavy metals in the soil.
The presence of metals in different parts of the plant indicating that there is accumulation but also translocation to the aerial parts, that is to say, the transport of these roots to shoots. The accumulation and translocation can be measured by two parameters: the factor translocation and accumulation factor.
The translocation factor (FT) is used to assess the capacity of phytoextraction plants, more particularly, it indicates the ability of the plant to transfer the metal from the root to the leaves, is calculated as follows:
TAP: metal content in the aerial parts (mg∙kg−1).
TRP: metal content in the root portions (mg∙kg−1).
The physicochemical characterization of the soil was realized by following the standardized protocols. All the results are presented at
Parameters | Units | Soil |
---|---|---|
Clay (<2 µm) | g∙kg−1 | 33.27 |
Fine silt (2/20 µm) | 25.31 | |
Coarse silt (20/50 µm) | 18.62 | |
Fine sand (50/200 µm) | 15.56 | |
Coarse sand (200/2000 µm) | 7.79 | |
pHwater | 8.12 | |
pHKCl | 7.8 | |
CEC | meq/100g | 20.55 |
Organic carbon (CO) | 1.25 | |
N total | 0.13 | |
Ca2+ | 15.6 | |
Mg2+ | 2.35 | |
Na+ | 0.18 | |
K+ | 1.2 | |
CaCO3 | 5.25 | |
P2O5 | 21.06 | |
Zn | mg∙kg−1 | 72 |
Cd | 0 |
Following a contamination by cadmium,
The evolution length of the roots and leaves according to the concentration of cadmium are represented by
The evolution of the dry weight of the roots and the leaves is represented by
Following a contamination by zinc,
The evolution length of the roots and leaves according to the concentration of zinc are represented by
The evolution of the dry weight of the roots and the leaves is represented by
After the digestion of the roots and leaves of the barley while following the protocol quoted previously, we performed the analysis of cadmium in roots and leaves of the harvested plant, the results are represented in
Contamination By Cd (mg/kg of soil) | Length of the roots (cm) | Length of the leaves (cm) | Dry weight of the roots (g) | Dry weight of the leaves (g) | Total biomass (g) |
---|---|---|---|---|---|
0 (Control) | 20.95 ± 1.30 | 22.00 ± 1.40 | 0.1234 ± 0.0021 | 0.4641 ± 0.0031 | 0.5875 ± 0.0026 |
25 | 18.50 ± 1.20 | 21.16 ± 1.30 | 0.1204 ± 0.0035 | 0.4488 ± 0.0028 | 0.5808 ± 0.0032 |
50 | 18.24 ± 1.50 | 20.67 ± 1.30 | 0.1320 ± 0.0028 | 0.4245 ± 0.0038 | 0.5715 ± 0.0033 |
100 | 17.45 ± 1.20 | 19.87 ± 1.35 | 0.1271 ± 0.0017 | 0.4443 ± 0.0042 | 0.5449 ± 0.0034 |
200 | 16.03 ± 1.30 | 19.18 ± 1.35 | 0.1163 ± 0.0019 | 0.4191 ± 0.0049 | 0.5354 ± 0.0034 |
Contamination By Zn (mg/kg of soil) | Length of the roots (cm) | Length of the leaves (cm) | Dry weight of the roots (g) | Dry weight of the leaves (g) | Total biomass (g) |
---|---|---|---|---|---|
0 (Control) | 20.95 ±1.30 | 22.00 ± 1.40 | 0.1444 ± 0.0015 | 0.5202 ± 0.0047 | 0.6646 ± 0.0031 |
500 | 20.40 ± 1.20 | 20.83 ± 1.30 | 0.1293 ± 0.0009 | 0.5234 ± 0.0038 | 0.7164 ± 0.0024 |
1000 | 19.98 ± 1.30 | 20.66 ± 1.10 | 0.1425 ± 0.0012 | 0.5497 ± 0.0042 | 0.6922 ± 0.0027 |
2000 | 19.60 ± 1.10 | 19.77 ± 0.90 | 0.1618 ± 0.0018 | 0.6396 ± 0.0034 | 0.6679 ± 0.0026 |
3000 | 19.48 ± 0.90 | 19.73 ± 1.40 | 0.1445 ± 0.0026 | 0.5546 ± 0.0031 | 0.6689 ± 0.0029 |
4000 | 19.14 ± 1.10 | 19.65 ± 1.20 | 0.1295 ± 0.0023 | 0.4876 ± 0.0051 | 0.6171 ± 0.0037 |
contaminated soil, the results are also shown in
After the digestion of the roots and leaves of the barley following the procedure mentioned above, we performed the analysis of zinc in roots and leaves of the harvested plant, the results are shown in
These results allowed us to determine the bioaccumulation factor and factors translocution for each zinc content in the contaminated soil, the results are also shown in
The effect of heavy metals on plant growth, especially root growth has been reported by several authors [
Some studies were based on the determination of the biomass of the roots and leaves of biomass to evaluate the toxicity of the metal in plants. Some authors have reported that root growth is more sensitive than the leaves and thus are based on root length to study the toxicity addition to the effects on the production of root biomass and length, the roots may also respond to metal stress by changes in the pattern of root growth and morphology in the present study we investigated the impact of metals on the length of roots and leaves and the production of
Contamination by Cd (mg/kg of soil) | Cd cumulated (mg/kg) | BF (%) | FT × 100 | |||
---|---|---|---|---|---|---|
Roots (cm) | Leaves (cm) | Roots | Leaves | |||
0 (Control) | 0 | 0 | ||||
25 | 416.8 | 38.2 | 16.672 | 0.15 | 9.17 | |
50 | 809.0 | 68.3 | 16.180 | 0.14 | 8.44 | |
100 | 1562.6 | 148.7 | 15.626 | 0.15 | 9.52 | |
200 | 2807.0 | 285.5 | 14.035 | 0.14 | 10.17 | |
Contamination by Zn (mg/kg of soil) | Zn cumulated (mg/kg) | BF (%) | FT × 100 | |||||
---|---|---|---|---|---|---|---|---|
Roots (cm) | Leaves (cm) | Roots | Leaves | |||||
0 (Control) | 4900 ± 210 | 600 ± 26 | 12.25 | |||||
500 | 8450 ± 730 | 1200 ± 61 | 16.90 | 2.40 | 14.20 | |||
1000 | 12450 ± 410 | 1450 ± 73 | 12.45 | 1.40 | 11.65 | |||
2000 | 16400 ± 850 | 1550 ± 88 | 8.20 | 0.80 | 9.45 | |||
3000 | 20700 ± 631 | 1650 ± 110 | 6.90 | 0.60 | 7.97 | |||
4000 | 28700 ± 990 | 1900 ± 97 | 7.17 | 0.48 | 6.62 | |||
root and aboveground biomass.
The lengths of the roots and leaves are shown in
toxicity to organisms [
The biomass of the roots and leaves and total biomass are shown in
Previous studies have shown that plants can suffer toxic effects if the soil reaches a high content of metals or 8 mg/kg for cadmium. The results of this study show that barley has no apparent signs of stress after thirty days of culture, it resisted even at a level of 200 mg/kg of soil. The parameter that has detected a stress due to the presence of cadmium is mainly the reduction of root size. We also note that barley is more tolerant to zinc, we observed no evidence of apparent even at 4000 mg/kg soil stress, and there was even increase the total biomass in the presence of strong zinc concentrations.
The accumulation of metals in the roots is larger than that of the leaves (
The bioconcentration factor (BF) has been widely used to estimate the potential of plants for phytoremediation [
These results are in agreement with those reported by other authors [
We cannot thus qualify our species of accumulator plant if it is cultivated in grounds contaminated by cadmium and zinc but its tolerance with these metals causes us to want to increase this accumulation for example using EDTA as chelating soil, knowing that addition of EDTA solution has a favorable effect on the absorption of metals, due to its properties acidification and chelation, it could be an effective amendment to improve the phytoextraction. However, this chelator is known to be persistent in soil due to its low biodegradability, which may pose a risk to the environment from leaching into groundwater. Therefore, studies should be conducted in order to find other natural and synthetic biodegradable chelating agents such as EDTA alternatives agents. And some studies have reported some bacteria able to increase the solubility and mobility of heavy metals [
It can be concluded from this study that a soil with a diverse flora can be contaminated with heavy metals and the species that grow on the ground floor, just being tolerant species to high concentrations of metals, where the interest should be brought to the soil pollution especially as it often is apparent point in our case. It will obviously not make a difference between the control pot and those contaminated with high concentrations of metals. These plants have a good yet growth contaminated with high levels of zinc and cadmium. We must ensure that contaminated sites are not cultured after decontamination to protect human health.