**1. Introduction**

Industrial wastes can generally be classified as wastes rich in organic matter on one hand and wastes rich in inorganic matter on the other. Cyanide (CN- ) and heavy metals (viz. copper, nickel, iron, zinc, cadmium, chromium, silver, gold, etc.) form a significant part of the latter type of wastes. Free cyanide (CN- ) is industrially important chemical because of its some unique properties of binding various transition metals to form metal-cyanides (MxCN) complexes of variable stability and toxicity (Sharpe 1976). Therefore, cyanide finds enormous applications in variety of industrial processes. Industries like gold and silver mining, electro‐ plating, printed circuit board manufacturing and jewellery units emanate large-volume lowtenor effluents containing anionic MxCN complexes like gold-cyanide i.e. [Au(CN)2] and silver-cyanide i.e. [Ag(CN)2] - (Vapur et al 2005). The total cyanide, gold (Au) and silver (Ag) content in these effluents ranges from 5-25, 1-2 and 5-10 mg/L, respectively (Patil 1999). The discharge limits for total cyanide is 0.2 mg/L, while for Au and Ag the standards are yet to be set and currently not available. Apart from Au and Ag many other heavy metals normally occur in the effluents in low quantity and concentrations. If inappropriately managed, cyanide and metals or their complexes can be mobilised and carried into the food web as a result of leaching from waste dumps, contaminated soils and waters. At each level of food chains, the concentration of metals increases which results into a phenomenon called biomagnification.

© 2013 Itankar et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Since cyanide is toxic and Au and Ag being precious metals, non-renewable and finite resource; their complete removal from effluents is the key.

1995). The advantages of passive bioremediation (i.e. biosorption) are: (i) non-living biomass is not subject to toxicity limitations; (ii) costly nutrients and aseptic conditions not required; (iii) the process is very rapid; (iv) waste biomass from fermentation and many other natural and industrial sources could be a cheap source of biomass; (v) biosorbent could be operated at wider range of pH, temperature and metal concentration; (vi) established theories, conven‐ tions and formulae could be applied to the adsorbent and many others (Paknikar et al. 2003). Although biosorption of metal cations have been studied extensively, however, very little is known about the removal and recovery of anionic metal species (viz. MxCN) from aqueous

Resource Recovery from Industrial Effluents Containing Precious Metal Species Using Low-Cost Biomaterials…

http://dx.doi.org/10.5772/56965

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With the above background in mind, investigations were aimed at screening diverse range of low-cost biomass obtained from different sources for the removal and recovery of precious anionic metal species like gold- and silver-cyanide from aqueous wastes with the emphasis on

The low-cost biosorbents in the present study were collected from diverse sources (as given below). Some of these biosorbents are reported for the removal of diverse metal species from acqueous solutions (Mohan & Pittman, 2006), while some of them have been employed for the

**i. Agricultural wastes/by-products:** Coconut fibres, Cow dung cakes, Groundnut

**ii. Industrial wastes/by-products:** Dairy waste sludge, Saw dust, Sugarcane bagasse

**iii. Municipal solid waste components:** Nirmalya (waste flowers), Compost and

**iv. Fungal and Bacterial (waste) biomass:***Ganoderma lucidum*, Yeast waste biomass,

*Mucor heimalis*, *Penicillium* sp., *Streptomyces* waste biomass, *Streptoverticillium* waste biomass, Wood rotting fungal waste and Bacterial consortium (capable of degrading cyanide and thiocyanate). Biomass of the fungal cultures viz. *Mucor heimalis* and *Penicillium* sp*.* available in our laboratory were obtained by cultivating them asepti‐ cally in Sabouraud's medium (Composition: Glucose 20 g; Peptone 10 g; Distilled water 1000 ml; pH 4.5-5.0) for 4-5 days at 30°C on rotary shaker incubator (150 rpm). Biomass was harvested after growth by filtering through muslin cloth and washed with distilled water 3-4 times in order to remove the organic traces. After washing, the biomass was subjected to drying in oven at 50-60°C for 2-3 days till the constant weight was obtained. Biomass was then ground using electric mixer in order to obtain particle size of ≤ 500 μm (0.5 mm). Later the biomass were stored in glass bottles with

the development of a laboratory / pilot scale technology.

shells, Rice husk and Rice straw

suitable air tight caps for further use.

and Tea powder waste.

Vegetable waste.

**2. Materials and methods**

**2.1. Low-cost biosorbents**

wastes.

first time.

The conventional methods adopted for the treatment of MxCN contaminated effluents is alkaline chlorination oxidation process (Ganczarczyk et al 1985). Although this method of treatment can be very efficient in detoxifying free cyanide bearing wastes, it is not effective when challenged with anionic metal species such as MxCN (Eckenfelder 1989). Other methods, such as copper catalyzed hydrogen peroxide oxidation, ozonation, electrolytic decomposition, etc. requires large inputs of energy, cost intensive materials and are rarely used for treatment of MxCN containing wastes. Furthermore, at low concentration, metal-cyanide recovery by conventional means is either not possible and/or very expensive. Thus, there is a big techno‐ logical breach, which needs to be bridged immediately.

Biological treatment systems (i.e. bioremediation) for the detoxification of toxic and hazardous wastes has immense potential of becoming and effective alternative because of their several advantages over conventional methods; and therefore being explored by the researchers all over the world (Patil et al., 2012; Patil and Paknikar, 1999b). However, biological methods like biodegradation / biodetoxification using live microorganisms are subject to toxicity of cyanide and metals. Therefore, removal of precious metal-cyanides species from wastes requires immediate attention of scientists and technologists. The challenge is not limited only to their removal, but also extends to finding competent and inexpensive ways of possible recovery and recycling. It was assumed that if a competent process for removal/recovery could be established, MxCN could be conserved, which in the authors opinion would be an innovative strategy of resource recovery. Since MxCN are anionic chemical species, therefore in principle, a well established physico-chemical methods can be used for removal and recovery of precious metal species. A few physico-chemical methods have been tried for adsorption of Au- and Agcyanide (Niu and Volesky 2001) and MxCN using activated carbon or inorganic chemically active adsorbents (Lee et al 1998). However, the practical utility and cost-effectiveness of these processes are not yet established. Biosorption of metal "cations" have been studied extensively (Paknikar et al 2003). However, very few attempts have been made to adopt this technology for possible removal and recovery of "anions" such as MxCN (Patil 2012; Patil and Paknikar 1999); especially Au- and Ag-cyanide (Niu & Volesky 2001). Literature clearly shows the paucity of references on the removal/recovery of precious Au- and Ag-cyanide using low-cost waste biomass.

It is known that biomass like bacteria, fungi, algae, plants, agricultural biomass and different agro-based industrial waste and byproducts have the ability to bind metals, in some cases selectively, from aqueous solutions (Paknikar *et al.*, 2003). This phenomenon is named as 'metal biosorption' and the biomass responsible for the process are known as 'biosorbents'. Biosorp‐ tion is combination of the processes such as electrostatic interactions, ion exchange, complex‐ ation, formation of ionic bonds, precipitation, nucleation, etc. Biomass surfaces are usually charged. The functional groups like phosphoryl, carboxyl, sulphahydryl and hydroxyl of membrane proteins, lipids and of other cell wall components are responsible for adsorption of metal (both cationic and anionic species). The overall interfaces are a result of complexity of biomass surfaces and chemical/ physical properties of metal ions (Modak and Natarajan, 1995). The advantages of passive bioremediation (i.e. biosorption) are: (i) non-living biomass is not subject to toxicity limitations; (ii) costly nutrients and aseptic conditions not required; (iii) the process is very rapid; (iv) waste biomass from fermentation and many other natural and industrial sources could be a cheap source of biomass; (v) biosorbent could be operated at wider range of pH, temperature and metal concentration; (vi) established theories, conven‐ tions and formulae could be applied to the adsorbent and many others (Paknikar et al. 2003). Although biosorption of metal cations have been studied extensively, however, very little is known about the removal and recovery of anionic metal species (viz. MxCN) from aqueous wastes.

With the above background in mind, investigations were aimed at screening diverse range of low-cost biomass obtained from different sources for the removal and recovery of precious anionic metal species like gold- and silver-cyanide from aqueous wastes with the emphasis on the development of a laboratory / pilot scale technology.
