Phytoremediation of heavy metal contaminated soil using Senna hirsuta (L.), Panicum maximum (Jacq.) and Helianthus annuus (L.)

Phytoremediation is a new and emerging technology that utilizes the ability of some plants to clean contaminated soil. The technology is well known and established in the developed countries but its use in the tropics is limited. The phytoremediation potential of three local plant species, Senna hirsuta (L.), Panicum maximum (Jacq.) and Helianthus annuus (L.) was evaluated in potted experiments using heavy metal contaminated soil from the Sansu Tailings Dam of AngloGold Ashanti, Obuasi Mine, Ghana. Six different soil treatments were used; raw tailings material, uncontaminated topsoil, mixtures of tailings and topsoil at three different ratios (1:1; 1:2, 1:3) and tailings+NPK fertilizer (TF). The experiment was laid out in a completely randomised design with three replicates at the Rehabilitation Nursery of AngloGold Ashanti, Obuasi. Samples of plants were harvested at 30 days (1st harvest), 60 days (2nd harvest) and 90 days (3rd harvest). The concentrations of seven heavy metals (As, Fe, Zn, Cu, Pb, Cd and Au) were analysed in samples of the soils and plant organs (roots and shoots) before transplanting and after harvest using the Atomic Absorption Spectrophotometer. Results obtained showed that S. hirsuta was the best accumulator for all heavy metals (As, Fe, Zn, Cu, Pb, Cd, and Au) among the plants used for the experiment in all three harvest. Generally more metals were accumulated in the shoots than in the roots of plants. The highest metal accumulation ratios were recorded for Au in the roots (248.8) and shoots (582.0) of S. hirsuta in treated soil having equal tailings+top soil (1:1). S. hirsuta, P. maximum and H. annuus recorded a bioaccumulation ratio greater than 1 for As only in the topsoil (0:1) and less than 1 for Fe in all the treated soils. S. hirsuta recorded bioaccumulation ratio greater than 1 for Zn in all the treated soils at the end of the 3rd harvest with the exception of raw tailings. P. maximum recorded the highest Zn bioaccumulation ratio of 39 at the end of the 3rd harvest whilst H. annuus recorded the highest of 24 during the second harvest. Bioaccumulation ratio recorded by S. hirsuta, P. maximum and H. annuus for Cu were less than 1 in raw tailings but greater than 1 in tailing+NPK fertilizer (TF). The application of NPK fertilizer had a positive influence in the bioaccumulation of Cu in the raw tailings (1:0). The highest bioaccumulation ratio (12.53) for Cu among the plants cultivated in tailing+NPK fertilizer (TF) was recorded by S. hirsuta. The highest Pb bioaccumulation ratio among the plants was recorded by S. hirsuta. H. annuus recorded the highest bioaccumulation ratio (15.61) for Cd. All the plants had bioaccumulation ratio greater than 1 for Au. The highest bioaccumulation ratio (27.84) recorded for Au was by Senna hirsuta cultivated in topsoil (0:1). The species accumulation factors and bioaccumulation ratios gives an indication of the plants’ affinity for specific heavy metals and their potential for optimal metal accumulation during the period of cultivation. P. maximum cultivated in treated soil 1:3 recorded the highest reduction (63.8%) of As. The application of the NPK fertilizer did not have any positive influence in the reduction of Fe in treated soil tailings amended with fertilizer by the plants. S. hirsuta cultivated in top soil recorded the highest percentage reduction of Fe (65.2%). Tailings amended with NPK fertilizer planted with S. hirsuta and P. maximum enhanced the reduction of Zn. The highest percentage reduction of Zn (86.1%), Cu (26.3%), Cd (40.0%) and Au (64.9%) were also recorded in treatment soils planted with S. hirsuta. The highest percentage reduction of As (63.8%) and Pb (39.3%) was recorded in tailings+top soil (1:3) planted with P. maximum. The result indicates that Senna hirsuta has great potential for phytomining of Cu, Cd and Au and P. maximum phytomining potential for As, Zn and Pb. The capability of these plants species to tolerate high levels of heavy metals thus provides useful information for their selective exploitation as phytoremediants in phytoremediation of contaminated mine sites.

Source: KNUST