Phytoremediation of Heavy Metal Contaminated Soil Using Thelypteris accuminata (Houtt.) C.V. Morton and Nephrolepis exaltata (L.) Schott

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Phytoremediation provides an alternative remediation method for clean-up of heavy metal contaminated soils. The application is especially important in tropical developing countries due to its cost effective and aesthetically pleasing solution. In this study two ferns Thelypteris accuminata and Nephrolepis exaltata were evaluated in potted experiment to examine their phytoremediation potential using contaminated soil from the Sansu Talings Dam of AngloGold Ashanti, Obuasi Mine, Ghana. Four different soil treatments were used; raw tailings material, uncontaminated topsoil, mixtures of tailings and topsoil at two different ratios (1:0, 0:1, 1:1 and 1:3). The experiment was laid out in a completely randomised design with three replicates at the Plant House Nursery of the Department of Theoretical and Applied Biology, KNUST. Samples of plants were harvested at 30 days (1st harvest), 60 days (2nd harvest) and 90 days (3rd harvest). The concentrations of three heavy metals (As, Pb and Cd) were analysed in samples of the soils and plant organs (rhizoids and fronds) before transplanting and after harvest using the Atomic Absorption Spectrophotometer. The mean differences in concentration of the metals in the rhizoids and fronds were separated using Tukey’s B multiple comparison test (p<0.05). The bioaccumulation potential of heavy metals in the plants was determined from the bioaccumulation and translocation factors. The total mean content of heavy metals (As, Pb and Cd) varied between plants and treatments with As being most accumulated (41.30 mg/kg) by Nephrolepis exaltata cultivated in tailings only. Arsenic levels in the tailings only (1:0) and topsoil + tailings (1:1) exceeded the WHO recommend standard for As in agricultural soils; while Pb and Cd levels were below the standards. The concentrations of all metals were higher and significantly different in the rhizoids than in the fronds. The translocation factor showed that Thelypteris accuminata is a good phytotranslocator for Cd (8.06) while Nephrolepis exaltata is best for As (5.17). The highest bioaccumulation factor was recorded for Cd in the topsoil with Thelypteris accuminata having 44.29 whilst Nephrolepis exaltata had a ratio of 60.77. The percentage reduction of heavy metals in the soil among the plants was significantly different during the three harvest periods. In the treatment soil, topsoil + tailings (1:3) 95.23% reduction was recorded for Cd by Thelypteris accuminata as against 98.21% reduction of Cd by Nephrolepis exaltata. The microbial counts in the soil samples after each harvest were significantly different for the treatments. The different levels of heavy metals accumulation by the two species of ferns, indicate their tolerance to high levels of heavy metals, preferably with Nephrolepis exaltata. The capacities of these ferns to accumulate and translocate heavy metals provide useful information for their metal exploitation as phytoremediating species for the remediation of contaminated mine sites.
A thesis submitted to the Department of Theoretical and Applied Biology, Kwame Nkrumah University of Science and Technology in partial fulfillment of the requirements Master of Science degree in Environmental Science, 2016