Investigation of the effect of microorganisms and their metabolites on the mobility of metals in a gold tailing dump in South Africa

Abstract

This study investigates the mobility of heavy metals and light metals in abandoned mine tailings dumps from the Krugersdorp. Most areas in South Africa which have mines are suffering from waste contamination. Tailings dumps deposited in the environment by mining companies contain residual heavy metals which are likely to be released and contaminate the environment. In South Africa tailings from the gold mines are of great concern, as such mines are regarded as the largest single source of pollution. Mine tailings are of great danger when not rehabilitated, as this facilitates erosion and washing away during rainfall. Oxygenated rainwater is able to combine with mine tailings and the pyrite minerals released to form acid mine drainage (AMD). There is a need to implement biological methods which are environmentally friendly and inexpensive, in order to solve the problems posed by mine tailings. Tailings samples from abandoned mine tailings dumps in the Krugersdorp mining area were considered, as human health seems to be of serious concern there. The samples were taken using an auger drill. From the top to the bottom of tailings dumps 14 samples were taken. The chemical and mineralogical structure was analysed withX-Ray fluorescence (XRF) and X-Ray diffraction (XRD), respectively. The heavy metals were further analysed by inductively coupled plasmaoptical emission spectrometry (ICP-OES). The results of the XRD have illustrated that quartz was the most abundant mineral in the tailings. Other minerals found were iron catena-silicate, ferrosilite, pyrophylite, hautrurite, andulusite, brown millerite and calcium iron (III) oxide, dialuminium silicate oxide, kyanite, dicalcium silicate and sillimanite. The XRF results showed that major elements were mostly in the form of SiO2, Fe2O3 and Al2O3. Sequential leaching methods were employed to evaluate the availability of metals in the tailings and assess the potential risk of pollution. It was found that metals in the most labile fractions were more likely to be released than those which were in the residual fractions. FTIR analysis was performed to determine the functional groups likely to bind metals and prevent their mobility. The DNA sequencing outcome showed that heterotrophic microorganisms were represented byBacillus sp and Pseudomonassp and autotrophic microorganisms were represented by Leptospirillumsp and Sulfobacillus sp. Batch leaching tests were implemented in order to determine the impact of organic acids, which are released by microorganisms, on the mobility of metals. The findings showed that at high temperatures and concentrations of organic acids the mobility of metals increased. The column leaching method was used in order to simulate the exact field conditions, as the freshly collected samples from the field were used without crushing and drying. Compost was used to assess the role of organic material with a view of revegetating metal-contaminated areas. It was established that the addition of compost affects the metal mobility and the oxidation of mineral sulphides. High concentrations of sulphate was found, the mobility was also restricted by the addition of compost. Metal speciation results determined by the PHREEQC model have shown the presence of free metal ions in the leachate which are more mobile compared to metal occurring in a complex form, which makes distribution of metals on the surface more likely to cause contamination.