Heavy Metals Accumulation by Indigenous Plants Growing in Contaminated Soil in a Gold Mining Area in Ghana

Abstract

Phytoremediation is an environmentally friendly, low cost biotechnological process that is fast gaining prominence in the cleaning of contaminated soils in the tropics. The accumulative potential of indigenous or native plants for heavy metals in mine tailings at the Storage Facility of the Chirano Gold Mine Limited, Ghana was investigated. Plant species were sampled in five plots, each 32 m². Samples of plants were harvested and separated into roots and shoots and the corresponding rhizospheric soil samples collected. They were analysed for the total concentrations of heavy metals; Arsenic (As), Cadmium (Cd) and Zinc (Zn) using Atomic Absorption Spectrophotometer. Bioavailable fractions of studied metals in the soil samples were determined with the formation of metal complex with Ethylenediaminetetraacetic acid and ammonium acetate reagents. The hyper-accumulation potential and mobility of heavy metals within the plants were determined from the bioaccumulation and translocation factors. Differences in mean concentrations of heavy metals in the plants (shoot, root and whole) were separated using Tukey B Analysis of Variance, SPSS version 20, at significant level of p < 0.05. The mean concentrations of total and bioavailable As, Cd, and Zn in the soil samples varied with Zinc being most predominant (13.20 mg/kg). However, As was the most available with 39% bioavailability. The soil elemental concentrations of As (3.0 mg/kg), Cd (0.29 mg/kg) and Zn (13.20 mg/kg) were below the WHO recommended standards of 12 mg/kg, 1.4 mg/kg and 200 mg/kg for As, Cd and Zn respectively. The concentrations of all metals in the plant organs varied between species. In all plant species Zinc was the most accumulated heavy metal, recording the highest level of accumulation (135.76 mg/kg) in the root of Euphorbia heterophylla. Bioaccumulation factor as expressed by total and bioavailable metal concentrations in soil indicate that all the plant species demonstrate good hyperaccumulation and phytostabilisation potential for Zn and Cd whilst 13 and 8 plant species demonstrate good phytoextraction potential for As. The translocation factor indicate that 8 plant species are good phytostabilisers for Zn, 7 plant species for Cd and 10 plant species for As. The accumulative and phytostabilisation potential of these plant species provide useful information about their selective exploitation for effective phytoremediation of the tailings dam at Chirano Gold Mine.

Keywords: phytoremediation, heavy metal, hyperaccumulation, phytostabilisation, indigenous plants, bioavailable.