Elemental sulphur reduction under acidic and thermal conditions for selective metals recovery

Mining, mineral and metallurgical industries have shown tremendous growth all over the world for the provision of metals and mineral commodities. Extraction of metals applying different technologies generates a variety of undesirable materials. The environmental pollution caused by this kind of wastewaters, containing high concentrations of dissolved heavy metals negatively impacts the living organisms, including humans. Metals of particular interest in acid mine drainage and industrial wastewaters include copper, zinc, cadmium, arsenic, manganese, aluminium, lead, nickel, silver, mercury, chromium and iron, in a concentration that can range from 10-6 to 102 g/L.

Physicochemical methods to treat these waters are widely applied. Despite effective treatment, these methods are expensive and generate considerable amounts of non-reusable sludge. The immobilization of heavy metals in the form of metals sulphide through microbial mediated sulphur compounds reduction and precipitation is one of the biological approaches widely studied as an alternative over physico-chemical methods. This process is based on the oxidation of simple organic compounds by microorganisms under anaerobic conditions, utilizing sulphur compounds as electron acceptors and generating hydrogen sulphide. The biologically produced hydrogen sulphide reacts with dissolved heavy metals to form insoluble metal sulphides precipitates. Sulphate reduction is the biological process most used for the treatment of these streams with metals sulphides as products.

Normally, wastewater from mining or metallurgic industries contains low amounts of organic matter. In order to completely reduce the sulphate into sulphide, carbon sources or electron donors need to be added to the medium. Stoichiometrically, elemental sulphur is energetically more attractive as electron acceptor than sulphate to produce sulphide, since only two electrons (eq. 1) are needed in the process, instead of eight (eq. 2), with the consequent reduction of the electron donor and, therefore, cost savings in the process.

(1) S + H2 à S2- + 2H+                                 

(2) SO42- + 4H2 à S2- + 4H2O                                       

Elemental sulphur can be reduced with several electron donors and the hydrogen sulphide produced can be used to precipitate metals in the same manner as after the sulphate reduction process.

The conditions in which sulphur reduction occurs influence the efficiency and rate of the process and the efficiency of metals recovery as well. Some important conditions for sulphur reduction and selective precipitation of heavy metals are pH and temperature.

Although the optimum pH for sulphur-reducing bacteria is usually stated around neutral values, the sulphur reduction process at lower pH values can be interesting and useful, since in mining and metallurgical industries large amounts of waste and process streams are produced at a low pH. Moreover, considering that metal precipitation is pH-dependent, variation of pH values in a bioreactor can be applicable to make the selective metals recovery viable.

Furthermore, each sulphur-reducing microorganism has an optimum temperature and temperature range, but generally growth and conversion rates are higher at elevated temperatures. Taking into account that the energy needed to cool or heat bioreactors contributes to the costs of the process, it is reasonable to operate them at a temperature close to the temperature of the sulphur containing waste or process stream.

This project will be mainly focused in the bio-reduction process with elemental sulphur as electron acceptor in acidic and thermal conditions, as well as in a comprehensive study of microbial communities responsible for the sulphur reduction in a bioreactor.