Microbial Treatment of Heavy Metal Leachates

Abstract: Ore-mining, metallurgy and other industrial activities represent the source of heavy metal and radionuclide contamination in terrestrial and aquatic environments. Physico-chemical processes are employed for heavy metal removal from industrial wastewater. However, limitations due to the cost-effectiveness and the use of contaminating reagents make these processes not environmentally friendly. Biotechnological approaches can overcome the limitations of physico-chemical processes. Biological alternatives for heavy metal removal are based on the physical, chemical and metabolic properties present in microorganisms. Amongst the different methods, metal precipitation by biogenic sulphide and biosorption are the most relevant. Biogenic sulphide is produced by bacteria in an anaerobic process dependent on the use of sulphur moieties as electron acceptors. Due to the combined removal of acidity, metals and sulphate, sulphate-reduction is the most promising process for the treatment of acid mine drainage. On the other hand, biosorption is based on the microbial uptake of organic and inorganic metal species by the physico-chemical mechanisms described as adsorption. Due to the number and geographical distribution of mines, the Bolivian Andean region is seriously affected by mining contamination. Moreover, scarce technological resources make it difficult to manage the metal pollution. For these reasons, the development of a robust system for production of biogenic sulphide, which will be operated with a minimum of surveillance, is needed. In this context, two important issues were addressed: 1) dependency on the concentrations of electron donor and acceptor, and/or 2) an increase in the available SRB biomass growth-surface. The use of agricultural and solid wastes, widely available, proved to be a good source of electron donors for sulphide production. Bacteria able to produce sulphide can either utilise macromolecules or fermentation products of organic matter as electron donors. The use of cellulosic/hemicellulosic raw materials and volatile fatty acids resultant from degradation of solid organic matter were demonstrated to be suitable electron donors alternatively to those traditionally employed (e.g. lactic acid, ethanol). Sulphide production of 5 mM and 16 mM were achieved when wheat straw and total volatile fatty acids were used, respectively. The augmentation of cell density through the establishment of microbial growth in biofilm-packed bed reactors allows enhancement of sulphide production. Concentrations of 15 mM and 9 mM were achieved using Poraver® and pumice stone respectively, with lactic acid as electron donor. While using total volatile fatty acids, a production of 16 mM was achieved. The utilisation of molecular tools such as 16S rRNA gene amplification, nested PCR and FISH allowed the identification of sulphidogenic bacteria in the different inocula employed in the present study. Aditionally, two new bacterial species, Clostridium boliviense and Clostridium algarum, which are able to produce sulphide using xylan as electron donor and sulphite and thiosulphate as electron acceptors, were described. Heavy metal precipitation and biosorption by algal-bacterial biomass have the potential to be combined with waste and/or wastewater treatment processes such as anaerobic digestion, BOD and nutrient removal contributing further to achieve a sustainable process. Since most of the mines are located in areas where accessibility is limited, a combination of anaerobic digestion of biomass available in the region and biological sulphide production may be advantageous. On the other hand, the advantage of using the algal-bacterial biomass is that selective removal of a particular heavy metal can be achieved. In addition, the algal-bacterial biomass used as adsorbent agent can be regenerated and reused.