Biomining of copper. Copper was the first metal extracted by biomining. During the period 1950-1980, as compared to conventional metallurgical techniques, biomining appeared as economically viable and potential technology to recover Cu

Fig. 14.15 Metal specific chelating resin

from low grade ore, like copper sulfide. It has been reported that the Lo Aguirre mine in Chile processed about 16,000 t ore per day between 1980 and 1996 using biomining [27].

Fungal leaching of manganese ore. Recovery of Mn from low grade ore of Mn by using pyrometallurgical and hydrometallurgical methods is expensive because of high energy and capital inputs. Besides, it also contributes a lot to environ­mental pollution. On the other hand biomining of Mn from manganiferous ores using microbial leaching is cost effective as well as environment friendly. It has
been reported that a fungus Penicillium citrium can solubilize or extract 64.6% of Mn from the low grade ore [28].

Biomining of gold. Using cyanide method, it is very much difficult to extract gold, when gold is covered with insoluble metal sulfides. Biomining of these sulfide films is the best option to achieve satisfactory gold recovery. Gold extraction plants of Sao Benzo in Brazil, Ashanti in Ghana, Tamboraque in Peru are known to have such biomining facilities. A series of demonstration plants was also commissioned during 2002 in the Hutti Gold Mines in Karnataka [27].

Recovery of chromium from tannery sludge. About 40% of total Cr used in tanning industry end up in the sludge. Cr is non-biodegradable and can easily accumulate in food chain causing serious health effects to human beings. Use of microfungi due to their biochemistry and relatively high immunity to hostile conditions such as pH, temperature etc. provide a better alternative to commercial leaching processes. It has been demonstrated that chromium from tannery sludge can be bioleached up to 99.7% using indigenous acidophilic fungi, A. thiooxidans [29]. Another Cr recovery option from tannery waste is to grow potential Cr accumulating fungi in tannery waste and subsequent extraction of Cr from the harvested biomass. In an extensive study on Cr accumulation by fungal biomass, the author identified a fungal strain, Paeciomyces lilacinus which can accumulate Cr up to 18.9% of their dry biomass [30].

Bioleaching of economical metals from electronic and galvanic waste. These contain various valuable metals. Microbial process involving both bacteria and fungi, which produce inorganic and inorganic acids, can mobilize these metals from the waste. Metals such as Al, Ni, Pb, and Zn have been reported to be extracted by this process. Microbial leaching has also been found effective to recover Ni and Cd from spent batteries [31].

Phytoextraction of metal. Phytoextraction of metals from low or moderately contaminated soil or waste material is recommended but not an option for highly contaminated soil. In later case, it may take decades or even centuries to reduce the contaminant concentration to an acceptable limit. Instead of using low biomass hyperaccumulator plants, high yielding plants along with addition of chelating agent proved to be better method to phytoextract metal from soil. Uses of different plants in chelant-induced phytoextractiopn are summarized in Table 14.3.

However, often application of chelants can result in residual toxicity in soil on which it is applied. Thus, natural accumulation of metals would be the best option provided application of mycorrhizal fungi, plant growth promoting rhizobacteria and other beneficial microbes in soil that can enhance the efficiency of extraction processes [32]. It has also been reported that plants colonized by the AM fungi not only enhance growth, but also significantly increase Pb uptake in root and higher translocation to the shoot at all given treatments [33]. It has also been seen that three mycorriza inoculated plant glomus species namely G. lamellosum, G. intraradices, G. proliferum and their consortia greatly enhance accumulation of Cr from tannery waste to plants.