214

14 Recovery of Precious Metals from Electronic and Other Secondary Solid Waste

27 belonged to Amycolatopsis and Streptomyces genera. When these heavy metals

are subjected to chemical precipitation using hydrogen sulfide, they showed strong

accumulation of lead [23]. To measure the availability of silicone dioxide, Al, Ca,

Mg, Pb, Cu, and Zn, hibiscus was grown on waste foundry sand (WFS). Actinomyces

sp. isolated from WFS was used to bioleach the treated sand. Hibiscus plant was

grown on both treated and untreated sand for determining the presence of met-

als by plant and was calculated by atomic emission spectroscopy technique. Results

showed that there was a reduction of metal level in WFS. This observation proves

that bioleaching by Actinomyces sp. in WFS was adequate in the Hibiscus rosasinensis

growth [24].

Extraction of gold and silver from gold-containing minerals using alkaline

condition is known as cyanidation process. Water-soluble complex is formed by

cyanide with gold which is responsible for gold extraction. Eco-friendly way for

the gold extraction processes can be achieved by using biogenic cyanide referred

to as alkaline bioleaching. Precious metals such as gold can be obtained from

Electronic wastes such as PCBs. Gold from the metallic particles of crushed waste

PCBs can be dissolved by Chromobacterium violaceum, a mesophilic gram-negative

bacterium, and by a facultative anaerobe C. violaceum. It was found that combining

C. violaceum with chemical methods or with other mechanism such as iodide,

Pseudomonas aeruginosa, and Pseudomonas fluorescens can improve gold leaching

efficiency by reinforcement of the cyanide generation. The efficiency was found

to be 70% for gold leaching using C. violaceum [25]. Bio-recovery from electronic

waste material was done using Bacillus megaterium where gold was obtained as

gold cyanide complex. These bacteria were subjected to pre-treatment, mutation

and allowed to grow at different pHs. The results show that mutated alkaline

bacteria (B. megaterium) were found to be more effective than that grown in normal

physiologic pH in gold biorecovery [26].

14.5

Application of Bioleaching for Recovery

of Individual Metals

14.5.1

Gold

In gold mining process, metal residues are often thrown back which causes ground

water pollution. This is due to the fact that the leachate contains harmful metals like

cadmium and lead. Electronic scrap contains more gold content compared to that of

the natural gold ores. Therefore, electronic scraps are considered to be a cheap and

alternative gold source. Studies show that significant gold recovery can be obtained

from bioleaching using cyanogenic bacteria. Pure cultures like P. fluorescens and

C. violaceum and cyanogenic bacteria play a vital role in degrading cyanide. In

cyanogenic bioleaching, adapted strains are observed to be more effective than

unadapted ones. For instance, at pH 9.0, 9.5, and 10, the adapted cyanogen C.

violaceum bioleached 18%, 22.5%, and 19% Au, respectively, compared unadapted

strain which could extract only 11% Au at pH 7.0 [27, 28].