Abstract:
Heavy Metal (HM) pollution of soil is of global concern in view of the great risk HMs pose to human and animal health through the food chain. Remediation by plants has become an attractive means of HM removal because of its eco-friendliness. However, most plants often used are crops and vegetable needed as foods. Tithonia diversifolia (Td) and Chromolaena odorata (Cho) are fast growing ubiquitous weeds whose phytoremediation capabilities are yet to be fully understood. Therefore, the aim of this study was to determine the physiology and HM phytoremediation capabilities of Td and Cho.
Three dumpsites (Irese, New stadium area and Onyearugbulem) and a control site (Ijare), all in Ondo State were purposively selected for this study. Three soil samples on which Td and Cho grew were collected from each dumpsite. Thirty-six Uncontaminated Soil Samples (USS) were collected from Ijare. A screen house experiment was conducted using soils from dumpsites and control. Ten grammes (g) of CdCl2, ZnCl2, FeCl2, CuCl2 and Pb (NO3)2 were introduced to 6.5 kg of six USS (3 for Td and 3 for Cho) to simulate contaminated soils. Six USS were used as control. Soil analysis was done before planting and after harvest. Viability test (Emergent %) was carried out on both plants. Growth parameters (shoot-length, root-length and stem-girth), Relative growth rate (RGR) and plant biomass were also determined. Chlorophyll content (photosynthetic rate), HMs concentration in roots and shoots were analysed using atomic absorption spectrophotometry. Data obtained were subjected to descriptive analysis and ANOVA with Duncan Multiple Range Test at α0.05.
The HMs concentrations (mg/kg) in dumpsite soils were: Cd, 0.01-0.08; Zn,0.48-1.92; Fe,0.99-353.87; Cu,0.10-3.04 and Pb,0.20-1.40, while in simulated-polluted-soil, the HMs were; Cd, 942.88; Zn,630.40; Fe,919.45; Cu,612.60; Pb,962.33. Mean HMs concentrations in soil planted with Td after harvest were: Cd, 0.01-0.18±0.008; Zn,0.24-1.92±0.04; Fe,4.97-12.05±0.263; Cu,0.10-3.04±0.038; Pb,0.30-1.00±0.079 , while in Cho were:Cd, Not-detectable-0.20±0.009; Zn, 0.12-1.91±0.014; Fe, 4.99-9.10±0.37; Cu,0.07-3.04±0.014 and Pb,0.20-0.60±0.07. The HMs concentration in simulated-polluted soils indicated a reduction in HM. The seed emergent for Td was 93.33% and Cho, 86.77%. This indicated that both passed the viability test. Shoot-length (185cm), root-length (45.40cm) and stem-diameter (6.90cm) values for Td were more than that of Cho (90.00, 26.11 and 2.61 cm). The RGR of Td ranging from 0.06 to 0.16 showed more tolerance to pollution load than Cho (0.01 to 0.09). Fresh weight (mg) of shoots (5.60-110.61) and roots (1.02-47.98) for Td produced more biomass than shoot (2.41-53.11) and roots (0.39-7.18) of Cho. Likewise, the dry weight (mg) of Td shoots (0.89-20.66) and roots (0.22-16.58) was higher than the corresponding shoot (0.10-4.57) and root (0.05-0.36) of Cho. Generally, total chlorophyll content (mg/) was higher in Td (3.20-29.59) than Cho (0.92-17.39). Uptake of HMs concentration in shoots and roots of Td were more than in Cho. Data analysed showed that Td values were significantly higher than Cho.
The phytoremediation abilities possessed by Tithonia diversifolia and Chromolaena odorata enabled photosynthetic activities and heavy metal uptake. The plants reduced heavy metals in the polluted soil and can be used for phytoremediation.
