Quantification of the bioccumulation potential of various chemical elements from coal fly ash using Brassia juncea and Spinacia oleracea L and its implication for phytoremediation of coal fly ash dumps

MENVSC

Department of Ecology and Resource Management

South Africa is highly dependent on coal for production of electricity. The combustion of coal for electricity generation produces waste by-products which include fine ash (fly ash) and coarse ash. Fly ash (FA) is produced in large amounts while its utilization is low due to its classification as a hazardous material. Sasol produces about 7 million tons of FA, while Eskom produces about 28 million tons of coal FA annually. FA is a fine by-product from pulverized coal with detrimental effects on plants, soil or land, animals and human beings, and pollutes the air. The present investigation focuses on the examination of the chemical elements bioaccumulation and phytoremediation potential of Indian mustard (Brassica juncea) and Spinach (Spinacia Oleracea L). X-ray fluorescence (XRF) was used to analyze the elemental composition of FA and soil, Scanning Electron Microscope (SEM) to examine the morphology, while X-ray diffraction (XRD) analysis was used to evaluate the mineralogy of FA. The concentrations of metal and non-metal species that are released from FA on contact with water at different conditions were quantified using ICP-MS. Pot culture experiment was conducted to grow Brassica juncea and Spinacia Oleracea L. using FA and soil as growth medium. Leachates from the pots were collected and analysed using ICP-MS. Plant parts from harvested Brassica juncea and Spinacia Oleracea L. were cut separately and analysed using ICP-MS for the concentrations of different metal and non-metal species in plant parts. Plant parts were also used to estimate biomass and chlorophyll content (leaves). To prepare these plant samples for analysis, the powdered plant sample (0.5 g) was digested through aqua regia (HCL:HNO3 = 3:1 (v/v)) to near dryness using hotplate and filled to 100 mL of MilliQ water. The samples were filtered and directly used to determine the chemical elements concentrations. Blanks and internal standards were used for quality assurance during analysis. Chemical elements that are present in FA and sometimes in higher concentrations are associated with detrimental effects in plants, animals and human beings, hence phytoremediation is vital. Bioconcentration factor (BCF) was used to estimate the metal species accumulation ability of the plants from the FA, FA/soil mixtures, while translocation factor (TF) was used to assess the plant species potential for phytoremediation of coal fly ash dumps. Analysis of Varience (ANOVA) was used to statistically test data using Graphpad software package. Relationship between chemical elements in soil, FA and FA+soil growth media and also different plant parts (root, stem and leaf) of B. juncea and S. Oleracea L were calculated using the t - test, ANOVA-Bartlett test, Mann-Whitney Test and Kruskal-Wallis Statistic (KW) depending on each data set. The physicochemical characterization of coal fly ash showed that FA from Grootvlei power station can be classified as class F with an alkaline pH level of 10.62. It showed that particle morphology of this FA had a lower degree of sphericity with irregular agglomerations of many particles while there were dominant spherical particles and smaller sharp needle like particles. It is also an alumino-silicate material as confirmed by the high SiO2 and Al2O3 content, while soil leachates had an average/neutral pH of 7.22 with very high amounts of Si. In both soil and FA, elements that were expected to be readily available to plants included Ca, Si, K, Ba, Mo, Na, Al, Mg, Sr and non-essential elements (Si, Ba, Na, Al, Sr), which, if uptaken by plants’ roots can have negative impacts in plants. Physicochemical analysis of soil, FA and FA+soil leachates showed that the alkalinity of the FA changed over time and there was also a decrease in the EC due to dissolution iv of soluble major oxides, which was promoted by continuous water irrigation. The soil and FA+soil growth media showed similar results. Chemical elements like B, Mn, Fe and Ba were occurring at higher concentrations in leachates for most weeks in the pot culture experiments. However, it was observed that in the eleventh week of leachate collection all these chemical elements decreased to very low concentrations. This suggest that these chemical elements can be reduced over time as plants are being irrigated which is either due to uptake by plants or washed off with water. Statistically, there was a significant difference for different chemical elements of leachates from different growth media for each plant species (B. juncea and S. Olearcea L.). The overall growth rate shows that S. Oleracea L was better than that of B. juncea especially in the FA media over time, while the biomass of the two plant species showed similar results. After all, even though S. Oleracea L had carotenoid content below detection limits, it had higher chlorophyll b than B. juncea for all growth media in general. But, statistically there was no significant difference between the two plant species in terms of growth rate and biomass; even between the plant parts denoting similar growth performance for the two plant species under study. The bioaccumulation potential of the two plant species showed that chemical elements such as Fe, Mn, Ba, Zn and B were highly accumulated by the different parts of the plants. However, the chemical elements such as of Mo, Ni, Cu, and Cr showed the least concentrations. This trend was similar for all growth media and both plant species but this changes over time for different growth media and plant species as increasing and decreasing trends can be noticed. This led to no significant difference between plant species and also growth media, statistically. High BCF values of Fe, Mn, Ba, Zn and B were observed in the different parts of the plants for both plant species. However, Mo, Ni, Cu, and Cr had BCF values less than 1 for most growth media over time. BCF values in plant parts differed with time, growth media and plant species. Translocation of chemical elements shows that the B. juncea plant proved to be an effective phytoremediation plant species since it is effective in translocation of many chemical elements for different growth media to shoots while S. Oleracea L failed to translocate most chemical elements from stem to leaves although it translocated some from root to shoots. Hence, it can be concluded from the study that both species can be used in phytoremediation of coal fly ash dumps but with B. juncea being the most effective accumulator and translocator of many chemical elements. However, it can be recommended that chelating agents like ethylene diamine tetra acetic acid (EDTA) be introduced to solubilize chemical elements from growth media matrix into growth media solution to facilitate the quick transport of chemical elements into xylem, and increase