Bioleaching approach for extraction of metal values from secondary solid wastes: A critical review

Bioleaching (microbial leaching), being an economical and environmental friendly process, is investigated extensively for metal extraction from secondary solid wastes, viz., electronic wastes, spent catalyst, sludge, slag and fly ash. Bioleaching using acidophiles is routed through generation of ferric (Fe III) and sulphuric acid. Other microbes such as the fungi carryout bioleaching by acidolysis,complexolysis (generation of organic acids), redoxolysis and bioaccumulation method. The cyanogenic microbes in particular possess the ability to extract metal(s) by producing hydrogen cyanide. The bioleaching process is achieved by one-step, two-step and spent medium-step in batch mode or by continuous mode reported to be promising on quantitative extraction of various metals (Ni, Co, Mo, V, Fe, Zn, Cu, Cr, Cd, W, Pb and Mn). Enhanced metal extraction can be accomplished by implementing pretreatment methods like, adding a catalyst, prior adaptation of microbes, bioleaching followed by bioleaching or chemical leaching, ultrasound, and also by optimising the process parameters. Additionally, the use of small size waste particles, low pH, low solid concentration and higher operating temperature could also enhance metal leaching to considerable extent. The review compiles extensive studies on treatment of secondary solid wastes employing chemolithotrophs (acidophiles) and organotrophs (fungi and cyanogens) for metal extraction. The mechanism of bioleaching, candidate microbes, metal extraction efficiency, operational strategies and process improvement are extensively reviewed, discussed and reported.