Assessment of As and Hg in mine tailings and indigenous grass : a case study of non-functional New Union Gold Mine, South Africa

Abstract

The present study was carried out to assess the uptake of toxic metals, arsenic (As) and mercury (Hg), by Cynodon dactylon grass species at New Union gold mine tailings, Limpopo Province, South Africa. The samples were collected from New Union Gold Mine tailings and from Ka-Madonsi Village (control), and concentrations of As and Hg in soil and plant material were determined by ICP-MS. The average dry weight concentrations of As and Hg in mine tailing dam A were 2.53 and 1.18 µg/g, respectively, and 2.24 and 0.91 µg/g, respectively, in mine tailings B. The average dry weight of As and Hg in the control soil samples were 0.30 and 0.05 µg/g, respectively. The C. dactylon, on average dry weight, absorbed 5.45 µg/g of As, and 1.72 µg/g of Hg from mine tailings A, and 4.29 µg/g of As; and 1.55 µg/g of Hg from mine tailings B. The control grass, on average dry weight, absorbed 0.53 µg/g and 0.01 µg/g of As and Hg, respectively. In most cases, significant differences were observed between bioaccumulation of Hg and As in plant tissue from mine sites and control sites. The study showed that the bioconcentration factor was less than 1 for the acidic mine tailings, with exception of the root and rhizome system which was greater than 1. This contrasted sharply with the control site where the pH was alkaline, and As and Hg levels were low. The pH values were significantly different (p <0.05) between the soils originating from mine tailings and the control site. The electrical conductivity (EC) of soil at mine tailings A and B were 1847.35 and 1777.5 µS/cm, respectively. The EC for the control site was significantly different (p < 0.05) at 543.3 µS/cm. C. dactylon was found to be capable of bioaccumulation of As and Hg, effective at soil stabilization, and grew well under the acidic conditions. The control grass was healthy and thrived under the alkaline conditions.