Due to its toxicity, most of the fly ash is landfilled
after detoxification, or recycled as a secondary material . Since some of the elements (e.g. Cu and Zn) are present in high concentration and may permit an economic recovery, fly ash may be considered as an artificial ore . The leached and recovered metals may be recycled for re-use as raw materials . Conventional pyro- or hydro-metallurgical techniques in fly ash detoxification and heavy metal recovery include thermal treatment, chloride evaporation process and chemical leaching. Although these techniques provide a rapid treatment and complete destruction of toxic compounds in fly ash, they are very energy intensive and result in the release of hazardous emissions during treatment. The high cost and the negative environmental impact of conventional methods have led to the investigation selleck products of bioleaching (considered a clean technology) as an alternative in the extraction of heavy metals from fly ash  and .
The main focus in bioleaching was initially the recovery of metals from insoluble metal sulfide minerals in mining ores, based on the ability of microorganisms NVP-BKM120 order to oxidize reduced iron and sulfur compounds, via the production of organic or inorganic acids. There are patents on pilot- or commercial-scale bioleaching plants, with most focused on low-grade ore . Recently, however, there have been interests in the application of bioleaching in industrial wastes as increasingly vast quantities of hazardous industrial wastes (such as spent catalyst, electronic waste, MSW incineration fly ash etc.), are generated  and . Although much has been reported on bioleaching by the chemolithoautotrophic acidophilic microorganisms of the genus Acidithiobacillus, fly ash is not a suitable substrate for bioleaching due to its high pH . Acidithiobacillus sp. grow well under pH 2–3, while fungi are generally able to grow over a wide pH range, from 1.5 to 9.8  and . Fungal bioleaching of Buspirone HCl heavy metals have been reported for solid wastes including
electronic scrap material , spent refinery processing catalyst  and  and incineration fly ash ,  and . In general, bioleaching may be conducted using either one-step or two-step. In the former, the microorganism is incubated together with the metal-bearing waste. In two-step bioleaching, the microorganism is first cultured in the growth media and incubated for a period of time before the metal-bearing waste is added to the culture and the incubation continued. In order to better exploit this intrinsic capability of selected microorganisms for metal leaching and recycling, more efforts are needed to understand the behavior of both the microorganisms and the metal substrate during bioleaching.