Author:

Keywords:

bioleaching, Bacillus mucilaginosus, Aspergillus niger, heavy metal stress

Abstract:

Bioleaching uses microorganisms to extract valuable metals from minerals. It has risen as a sustainable alternative to conventional metal recovery processes for low grade ores and industrial waste materials such as incineration ashes as it could be more economical and environmentally friendly [1-2]. However, inherently to these materials is the presence of the hazardous heavy metals that can become toxic to the bioleaching microorganisms when released from its solid form, and potentially can affect the bioleaching performance adversely. In this work, the bacterium Bacillus mucilaginosus and the fungus Aspergillus niger were investigated for their response towards various heavy metals including arsenic, cadmium, cobalt, chromium, nickel, lead and zinc. In the first stage, a series of tests with the plate diffusion method [3] was performed for each metal on both microorganisms to determine the toxic effect. Metals which exhibited negative effects on the growth of the microorganisms in this first test were further investigated in liquid batch cultures. Growth curves were prepared using conventional bacterial/fungal growth counting techniques such as plate counting, optical density measurement and dry biomass determination, and were compared with growth curves obtained under normal conditions. The results showed that cadmium, nickel and arsenic had a negative influence on the growth of B. mucilaginosus whereas A. niger was only sensitive for cadmium. These influences were however only characterized by a delayed or reduced growth, and in neither of the batch cultures a total inactivation or dying off of the microorganisms was seen for the concentrations being used (namely 100 ppm metal component). Furthermore, it was shown that growth totally recovered when microorganism grown in the presence of metal were inoculated in metal free medium. It is assumed that the metals initially bound to different cell structures, dissociate in the absence of the different metals. In conclusion, the investigated metals had a bacteriostatic and fungistatic effect on Bacillus mucilaginosus and Aspergillus niger respectively and worked in no way as a bactericide or fungicide. Both microorganisms exhibited strongly adaptation abilities and presented the potentials to be used in the bioleaching of heavy metal loaded materials, and were good indicator organisms for further research concerning bioleaching approaches.