COMPARATIVE EFFECTS OF POLLUTED SOIL-ENVIRONMENT ON THE UPTAKE AND BIOACCUMULATION OF HEAVY METALS IN MAIZE (ZEA MAYS L.) AND ROSELLE (HIBISCUS SABDARIFFA L.)

Abstract

Crop production is limited by adverse environmental factors which include soil pollution with heavy metals. Soil is the major sink for heavy metals released into the environment. Plants grown on polluted soil are a potential risk to man due to food-chain contamination. Therefore, the comparative effects of polluted soil in the uptake and bioaccumulation of maize (Zea mays L.) and roselle (Hibiscus sabdariffa L) were investigated in 2017 and 2018. The aim of the research was to determine levels of heavy metals in tissues of maize and roselle cultivated on selected polluted and unpolluted soils in South West Nigeria with a view to discouraging the practice of cultivating on polluted lands and the consumption of such crops. The studies were conducted in the screenhouse of Crop, Soil and Pest Management Department, the Federal University of Technology, Akure. Plant (edible and non-edible), soil and water samples were collected from polluted and unpolluted sites concurrently between April and May 2017 from 4 States namely: Ekiti, Ondo, Osun and Lagos. The sampled areas from each State were: (i) Mechanic workshops in Ado Ekiti (AM-waste), (ii) Ondo State Waste Management Authority (M-waste), (iii) The iron and steel recycling factory, Ile- Ife (MS-waste) and (iv) Alaba international market, Ojoo, Lagos (E-waste). Polluted soil samples (4) at a depth of 0 cm to 30 cm from 10 different points on the dumpsites at 50 m apart from each location were collected. Unpolluted soil samples (4), which served as the control substrates, were collected at a distance of 2000 m away from the dumpsites, giving a total of 8 soil samples in all. Soil samples were also collected from each location and wrapped properly in a foil paper for microbial count and identification. Early varieties of maize (TZE pop STR c4) seeds were obtained from IITA, Ibadan, while roselle (Hibiscus sabdariffa L.) seeds were obtained from the Federal College of Forestry, Jos, Plateau State. The experiments were conducted at 3 and 5 months for maize and roselle, respectively. Soil physical and chemical properties were determined before planting. Standard solution for 8 elements (Cd, Cu, Cr, Fe, Mn, Ni, Pb and Zn) in all samples was also determined. The experimental design was a Completely Randomised Design (CRD). Maize and roselle seeds were sown in 8 soil samples (4 polluted and 4 unpolluted) and replicated three (3) times giving a total of 96 experimental units. A series of growth parameters (plant height, number of leaves, number of branches, stem diameter and leaf area), yield (number of cobs, calyces per plant) and yield components (fresh and dry weights of plants). Also determined were components of chlorophyll. Data generated from the experiments were subjected to analysis of variance (ANOVA) using SPSS version 20 and means were separated using Tukey’s test at p < 0.05 level of probability. Assessment of heavy metal in soil was determined using enrichment factor (EF), Geoaccumualtion Index (Igeo), Contamination Factor (CF) and Pollution load index (PLI). Bioaccumulation tools used were; Accumulation Coefficient (AC), Bio-accumulation factor (BAF), Mobility ratio (MR) and Translocation factor (TF). Elemental concentration was determined using X-Ray Fluorescence (XRF). Results obtained show that the study locations had varying concentrations of heavy metals higher than acceptable limits. E-waste and MS-waste had the highest mean concentration of Cu (12712.73 mg/kg) and Fe (2015.48 mg/kg), respectively. Cd was least concentrated among all metals. The control had concentrations lower than acceptable limits while Cd and Pb were not detected. Furthermore, plant samples collected from the study areas accumulated varying levels of heavy metals that were above acceptable limits set by WHO and NAFDAC. Similarly, results showed that water samples had significant concentrations of heavy metals that were higher than allowable limits set by NESREA and USEPA. It was also observed that maize and roselle plants sown in polluted soil accumulated heavy metals higher than acceptable limits. This was responsible for the significant effects on growth parameters, reduction in yield components and contents of chorophyll when compared to the control. Bioaccumulation indices revealed that maize and roselle were accumulators of metals. Therefore, the plants should not be cultivated on polluted soil and not to be consumed for health risk. However, maize and roselle did not accumulate more than 1000 mg/kg. This shows that they are not fit for remediation purposes.