Bioaccumulation of Radionuclide Metals in Plants: A Case Study of Cesium

Abstract

Phytoremediation is a new and fast-developing technology with nondestructive properties and ideal for removal of heavy metals and radionuclides from industrial effluents. Many plant species have high efficiency for adsorption and uptake of cations including heavy metals and radionuclides from contaminated soil and water. Plant screening and selection for remediation purposes is considered an important and a major step for implication of this technique. Taking into account geological characteristics and climatic conditions of a particular geographic area, most suitable plant species must be selected and implemented to achieve high rates of remediation. In this study, three plant species (Amaranthus chlorostachys var. Chlorostachys, Calendula alata Rech. F., Fl. Iranica, and Chenopodium album) were studied with the objectives to evaluate their potential for uptake of cesium from wastewater containing cesium salt. At the first step, plant seeds were selected and grew in hydroponic system using “Hoagland” solution. “Hoagland” solution is a standard medium made from distilled water enriched with specified nutrients for plant growth studies. After 2 months, plants were incubated in solutions with three different concentrations of stable cesium. The first set of experiments were carried out using four containers including a control solution containing distilled water free of Cs, and three other distilled water solutions containing 0.5, 2, and 5 mg l⁻¹ of CsCl, respectively. In the second set of experiments, standard Hoagland solution was used. A. chlorostachys and C. album plants were placed in containers filled with Hoagland medium mixed with equal volume of cesium solution made from dissolving CsCl in distilled water, so that the final concentration of CsCl was 1 mg l⁻¹ in first container and 2.5 mg l⁻¹ in the second. For each set of experiments, several similar containers were used to ensure validity of results. Experiments were arranged in randomized block design for a period of 15 days. Then, C. album that showed to be an effective accumulator of cesium was selected for anatomical studies. After separation, aerial organs (stem, leaf) of C. album plant were laid in a solution of alcohol and glycerin for fixation and anatomical investigations. The concentration of cesium in the growth solution, (Hoagland diluted with distilled water containing cesium), and in plants (dried and digested in acid) was measured by atomic absorption spectrophotometry (Varian Spectra AA-55B). The results of the present study showed that A. chlorostachys remediated 65 ± 4.11 % of cesium from simulated wastewater. Efficiency of C. alata in phytoremediation of cesium chloride (5 mg l⁻¹) and Hoagland medium was 89.35 ± 0.25 %. Comparison of plants remediation potentials showed higher efficiency of A. chlorostachys while in bioaccumulation potential comparison, A. chlorostachys showed higher efficiency. Anatomical changes studies of C. album plants showed that important change was increase in crystals (entering cesium to crystalline structure) quantity in stem parenchyma and their color embrace, due to cesium uptake. This tolerant plant converted cesium ions to crystals molecules in shoots. Plants grew healthy in contaminated environments and the remediation efficiency of cesium reached close to 90 % in C. alata when cesium salt concentration was 2.5 mg l⁻¹ and Hoagland solution was used. It was concluded that in a proper culturing condition, all these plants are tolerant to radionuclides and could be suitable candidates for remediation of radionuclide wastes. Consequently, it was proved that phytoremediation could be a complimentary treatment technique for removal of radionuclides from waste discharges at nuclear sites.