ReviewBiological treatment of precious metal refinery wastewater: A review
Section snippets
Water usage in South Africa
Water is considered to be a vital and limited resource; population growth, industrial developments and other pressures faced by developing countries have lead to structured measures to ensure sustainable management of this important resource. In a country that only receives an annual rainfall of approximately 480 mm – half that of the world average of 860 mm, South Africa experiences a large divide with regard to water quantity and quality that is accessible to rural and urban communities (
Xenobiotics in the environment
Wastewater streams from metal refineries contain complex mixtures of xenobiotic compounds, many of which are solvents commonly used during the extraction of metals. Xenobiotic compounds including amines and amides are used in extraction of many metals from their ores. Intentional or accidental release of these compounds could be hazardous not only to the environment but also to humans, exhibiting acute or chronic toxicity (Rieger et al., 2002). These compounds are generally poorly degraded in
Extraction of platinum
Platinum group metals (PGMs) consist of ruthenium (Ru), rhodium (Rh), palladium (Pa), osmium (Os), iridium (Ir) and platinum (Pt) and are extremely scarce compared to other precious metals due to their low abundance and the complex processes that are required to extract these metals from ores (Bernardis et al., 2005). In the last few years there has been a dramatic increase in the production of these technologically important metals. These metals are separated using the unique chemistry that
Wastewater characteristics
The solvents and extractants commonly used are generally aromatic in nature. Some of these include β-hydroxyxime, amine, and amide in the extraction of Pt and Pa and methyl isobutyl ketone (MIBK) for the extraction of gold (Stoffels et al., 1998). Commonly utilised diluents are composed of a mixture of aromatic monocyclic hydrocarbons. In the extraction of gold (Au), using MIBK, diluents are not commonly used (Hudson, 1982). The majority of the compounds making up an example of a diluent,
Treatment options
The definition of wastewater is a combination of liquids or wastes carried in water from various sources including residential areas, industry and commercial centres. High concentrations of oxygen demands, organic materials and inorganic materials are typically associated with wastewaters (Sonune and Ghate, 2004). Water is considered essential in mining, from dust suppression and drilling to refining of ore. Thus management of the release of the wastewater is crucial as mining operations
Using activated sludge
Aerobic degradation in the presence of oxygen is considered to be a relatively simple, inexpensive and environmentally sound way to degrade wastes (Yamada and Kawase, 2006). It has been shown to be successful in many processes and in the treatment of a variety of wastes. Activated sludge systems are very useful in the treatment of metal refinery wastewaters, results that were achieved by Nemukula (2005) demonstrating their potential. Factors that are critical in the optimal degradation of the
Reactors
Chemical degradation of aromatic and aliphatic xenobiotics has proved to be a costly operation in the past and researchers have been forced to look for alternative ways in which to breakdown these compounds. Biodegradation has been shown to be an effective solution to this problem, as it results in the complete mineralisation of the compounds. The treatment of toxic xenobiotics using aerobic processes is limited by their intrinsic properties. They are slow to degrade, and due to their toxicity,
Downstream metal recovery using biosorption
Metal refinery wastewaters contain trace amounts of PGMs. The ability to recover these trace amounts could have large economic benefits when viewed in the context of the volume of wastewater that is produced. Biosorbents are normally derived from suitable biomass and can be useful in the effective removal and recovery of metals from dilute wastewater streams. Their major advantages are that they are usually very low cost, they decrease metal concentrations considerably and they are especially
Concluding remarks
Although several physicochemical options are widely utilised in the successful treatment of industrial wastewaters containing heavy metals or organic solvents, biological wastewater treatment options are rapidly gaining support. These options are being shown to be technologically and economically feasible, even for the treatment of PGM wastewaters. The two most likely reactor configurations are MBRs and two-phase bioreactors which both offer the advantage of optimising cell/substrate contact
Acknowledgements
This research was funded by Anglo Platinum Ltd. and supported by the Technology and Human Resources for Industry Programme of the South African National Research Foundation and Department of Trade and Industry.
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