عنوان مقاله [English]
Heavy metals are one of the most important causes of environmental pollution in the world due to the industrialization of societies. Wastewater production is one of the problems of waste accumulation in waste collection sites. Urban wastewater contains almost all known chemical elements and persistent pollutants in the environment such as heavy metals that are not biodegradable. Due to the ability of heavy metals to accumulate and cause toxicity in living organisms, this type of pollution is considered a serious and fundamental problem. Some heavy metals such as lead, cobalt and cadmium can be distinguished from other pollutants, because not only are they not biodegradable, but they can accumulate in the structure of living organisms and cause various diseases, even in high concentrations. be very low The use of plants as an effective and cost-effective technology to remove metal pollutants from contaminated soils and waters has been recommended as a new environmentally friendly method instead of other costly measures.
The purpose of this research was to investigate the absorption of some heavy metals in urban wastewater by vetiver and canna plants and its effect on some physiological and anatomical characteristics, as well as to evaluate the resistance of vetiver and oleander plants against heavy metals in urban wastewater. For this purpose, an experiment was conducted in the form of a randomized complete design with three replications using vetiver and canna plants. The plants at the same size and the same weight including vetiver plant and the rhizomes of canna plants were obtained from their cultivation places in Guilan province and placed in the pots at the end of the winter season that is, in the month of March, containing 10 kg of sifted soil. The control pots were irrigated with 500 ml of urban water and the treated pots with 500 ml of urban wastewater twice a week, and this process of watering the control pots and the treatment continued for 6 months. In addition, 3 pots containing 10 kg of sifted soil without plants were also irrigated with urban wastewater during 6 months, like the treated samples.
After 6 months, the amount of heavy metals in different organs of vetiver and canna plants was evaluated and the anatomical structure of the roots of vetiver and canna plants treated with urban wastewater was compared to the control plants. Also, the amount of lipid peroxidation of the cell membrane, the amount of proline and soluble sugars in the leaves of the plants treated with urban wastewater were measured in comparison with the control plants.
The results of investigating the effect of 6 months of irrigation with urban wastewater and urban water on the amount of zinc metal accumulation in the roots, stems and leaves of vetiver and canna plants showed that the amount of zinc was more than the standard level (200 mg/kg). The transfer factor (TF) for zinc element (Zn) was calculated as 0.72, which indicated the accumulation of zinc element in vetiver roots under treatment. Despite the high absorption of about 150 mg/kg zinc by treated plants, a significant amount of zinc element was observed in the final soil, which indicated the high contamination of urban wastewater with zinc element. The bioaccumulation of zinc element in the present research was 2.45 and this shows that vetiver plant is a good plant accumulator for zinc element in the root. In addition, the highest concentration of zinc element was found in the root of canna plant.
The results of investigating the effect of 6 months of irrigation with urban wastewater and city water on the amount of chromium metal accumulation in the roots, stems and leaves of vetiver and canna plants showed that the bioaccumulation rate of chromium in the treated canna plant was 4.43 and can indicate this the issue is that canna is a good absorber for chromium. The transfer factor (TF), which is the ratio of chromium metal concentration in the dry weight of shoot to root, was 1.64 for canna plant. This amount shows that the transfer of chromium to the aerial organs happened more, although the chromium element was also significantly accumulated in the roots.
The results of investigating the effect of 6 months of irrigation with urban wastewater on the root anatomy of canna and vetiver plants showed that the diameter of the root and the diameter of the central cylinder in the treated plants were reduced compared to the control plants, as well as the number of vascular vessels in the treated plants compared to the control. Heavy metals have different effects on the anatomy of plant roots, and the absorption and accumulation of metals in high concentrations has led to structural changes in the roots of various plants.
According to the investigation, the stress of cadmium and copper caused a decrease in the thickness of the mentioned parenchyma. According to the acidification of the soil after six months of irrigation with urban wastewater and the standard limit set for zinc metal, it was observed that zinc metal in the pure soil (soil without plants that was irrigated with urban wastewater with each irrigation period) was higher than the standard level. The effect of 6 months of irrigation with urban wastewater on the amount of malondialdehyde, proline and soluble sugar in leaves of vetiver and canna showed that the plants that were treated with urban wastewater had higher levels of malondialdehyde in their leaves than the control plants, so that high concentrations of zinc and cadmium led to an increase in lipid peroxidation in treated plants.
Based on the results of this research, 6 months of watering vetiver plant with urban wastewater containing heavy elements of zinc and chromium, a high accumulation of zinc was observed in vetiver plant, which shows that vetiver plant is a good absorber for zinc. In the canna plants treated with urban wastewater contaminated with heavy elements of zinc and chromium, high accumulation of zinc was also observed in the canna plants, which shows that the canna plant is also a good absorber for zinc, but since the large amount of these elements for these plants are stress-producing, the evidence of physiological and anatomical stress was observed in the treated plants. The highest level of urban wastewater pollution used in this research was attributed to the zinc element, and according to the bioaccumulation factor and the results of this research, canna and the vetiver plants are good absorbers for zinc and chromium elements. In such a way that both elements zinc and chromium were mostly accumulated in the roots of canna, but the investigation of the transfer factor showed that zinc and chromium elements were more transferred to the aerial parts of the canna plant and so the canna plant has the ability to absorb these two metals through plant extraction, and the vetiver plant, which did not show significant transfer in the aerial parts of the plant, showed the ability to absorb these two metals through plant stabilization. Therefore, it is recommended to plant the studied plants in soil polluted by urban water to refine the soil.