**1. Introduction**

438 Mass Transfer - Advanced Aspects

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Vartanyan T.A., Leonov N.B. & Przhibel'skii S.G. (2010). Application of localized surface

Vartanyan T.A., Khromov V.V., Leonov N.B. & Przhibel'skii S.G. (2011). Shaping of surface

Warmack R.J. & Humphrey S.L. (1986). Observation of two surface plasmon modes on gold particles. *Phys. Rev*. B Vol.34, No.4, (August 1986). pp. 2246 -2252, ISSN 1098-0121 Wenzel T., Boshbach J., Steitz F., & Trager F. (1999) In situ determination of the shape of

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1642, ISSN 0021-9606

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Amsterdam

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Despite environmental regulations, wastewaters generated by some industries are in some cases discharged into lakes, rivers or reservoirs after inefficient treatments or without any pretreatment for the elimination or reduction of certain pollutants. Effluents may sometimes contain valuable elements with significant commercial value such as precious metals. Recovery of these metals is important because they could be harmful to aquatic life in lakes and rivers and because of its economic value.

There are legal provisions regarding the composition of an effluent: in the case of liquids containing silver, it is a maximum of 5 ppm. It is known, however, that the silver ion could create a complex especially with thiosulfate, which has little effect on health.

Even if silver would not affect health and there were no restrictions to its discharge, there is an important reason to recover it: its value and scarcity. Annual global demand for silver is currently of 24,500 metric tons, used in a vast array of industrial and consumer products. For example, silver is widely used in industrial electroplating as a protective coating or as adornment. Silver reflects light very well, so it is used in car headlights and mirrors.

A laboratory that uses silver in its production could discharge monthly, a value of 150 to 1,800 dollars in silver.

Worldwide, approximately 57% of the silver present in discarded products is recovered. It has the highest rates of recovery among the most commonly used metals, but much of it is still lost in the various emissions to the environment.

#### **Silver recovery**

Various methodologies have been reported for the recovery of this metal ion, with efficiencies that vary depending on the experimental conditions.

Among the most common methods for silver recovery are: 1) Metal Replacement; 2) Electrolytic Recovery; 3) Precipitation; 4) Distillation; 5) Ion Exchange and 6) The use of

Silver Recovery from Acidic Solutions by Formation of

**1.1.1 Synthesis of silver nanomaterials** 

Ag particles is still a great challenge.

Nanoparticles and Submicroparticles of Ag on Microfiltration Membranes 441

(Granqvist, 2007; Bavykin & Walsh, 2010), electricity carriers (Conte et al., 2004), fire resistant materials (Hamdani et al., 2010), computing and data storage (Jimenez & Jana, 2007), sensors (Yun et al., 2008; Rivas et al. 2009), water treatment (Thavasi et al., 2008), catalysis (Cheng et al., 2010) and early identification of cancer cells (Nanomedicine, 2007). Although many of these applications still remain untested, the investment over the last few years has been large: the USA allocated more than a billion dollars in 2005 (Pedreño A., 2005), and more recently Japan and the European Union invested 770 and 1400 million euros, respectively in scientific programs involving nanomaterials (EU Official Website, 2010). Among the metallic nanomaterials, silver has been intensively studied because of its wide applications including catalysis, electronics, photonics, and photography (Maillard et al., 2010). Furthermore, low-dimensional silver materials may be utilized as interconnectors or

as active components in the manufacture of micro/nanodevices (Sun et al., 2002).

Many reports have focused on the synthesis of shape controlled Ag nanostructures, including quasi-spheres, decahedrons, cubes, prisms, rods, wires, tubes, branches, sheets or plates, and belts (Yin et al., 2001; Du et al., 2007; Cobley et al., 2009). Generally, sizecontrolled Ag particles can be realized adjusting the reaction parameters. Evanoff and Chumanov synthesized Ag particles with diameters between 15 and 200 nm, through variations in the reaction time (Evanoff & Chumanov, 2004). By varying the concentration of sodium borohydride (NaBH4) employed in the reaction, Metraux and Mirkin have provided a straight forward and rapid route to Ag nanoprisms withover prism thickness control (Metraux & Mirkin, 2005). By adjusting intensity and spectral properties of the irradiating light, Pietrobon and Kitaev synthesized decahedral Ag nanoparticles with controllable regrowth to larger sizes (Pietrobon & Kitaev, 2008). Also, Yin's laboratory has recently demonstrated that the aspect ratio and optical properties of Ag nanoplates can be tuned with precision, over a wide range through a UV-light-induced reconstruction process (Zhang et al., 2009). However, in these examples of qualitative size control, the results can only be roughly speculated before the experiment (e.g., size-decrease or increase, but with no precise measurement). Quantitative size-control, where the product is size-designed by adjusting the reaction conditions to produce the desired particle sizes predictably and accurately, has not yet been established. Actually, it is well-known that the chemical synthesis of metal nanocrystals is influenced by several thermodynamic and kinetic factors, and much difficulty remains in capturing the distinct stages of nucleation and growth of nanocrystals (Burda et al., 2005). Also, it is very hard to establish a quantitative function to describe the relationship between the synthesis conditions and the size of the product. Therefore, carrying out qualitative and especially quantitative synthesis of size-controlled

The synthesis of silver nanoparticles has been studied searching for an easy control over kinetics. The preparation of the conditions for each of the methods mentioned above, play an important role on the composition, structure and size of nanoparticles, and have a direct impact on their properties. The development of a methodology to provide adequate control

This paper presents a novel approach to recovering silver from aqueous solutions in its most valuable form: the metallic and the formation of particles with different size depending on the experimental conditions. This includes the reduction of silver ions with a reducing agent such as ascorbic acid in a microfiltration system. During reduction of the silver ions, the membrane is used as a support for the metallic silver formed. The size and shape of the

of particle size in a simple way is an important contribution to the synthesis.

new compounds to precipitate silver (soil, silica, clays, etc.). The main features of the first five are:


None of these methods gives any importance to the size or shape of the recovered metallic silver particles. Their main interest is on the efficiency of the recovery process. The recovery of the silver in specific shapes and sizes (nano and submicrometric), is an added value of the recovery processes.

### **1.1 Nanoparticles**

Although nanomaterials have always existed in nature, our understanding of their properties and how they influence their environment has been limited. Many of these materials are currently under study and their applications have been developed over the last two decades.

Their manufacturing has gained importance because of their unusual properties compared to bulk materials. Examples include aluminium nanoparticles of 20 to 30 nm which can spontaneously combust while bulk aluminium is stable (Gromov & Vereshchagin, 2004) and calcium carbonate that forms either a fragile chalk or tough abalone shells, depending on the structural arrangement of the molecules (Tong-Xiang et al., 2009).

The applications of this relatively new technology are large and include: conductive plastics (Aravind et al., 2003), anticorrosive coatings (Gangopadhyay & De, 2000), fuel cells and batteries (González-Rodriguez, 2007; Ponce de León, 2006), solar energy generation (Granqvist, 2007; Bavykin & Walsh, 2010), electricity carriers (Conte et al., 2004), fire resistant materials (Hamdani et al., 2010), computing and data storage (Jimenez & Jana, 2007), sensors (Yun et al., 2008; Rivas et al. 2009), water treatment (Thavasi et al., 2008), catalysis (Cheng et al., 2010) and early identification of cancer cells (Nanomedicine, 2007). Although many of these applications still remain untested, the investment over the last few years has been large: the USA allocated more than a billion dollars in 2005 (Pedreño A., 2005), and more recently Japan and the European Union invested 770 and 1400 million euros, respectively in scientific programs involving nanomaterials (EU Official Website, 2010). Among the metallic nanomaterials, silver has been intensively studied because of its wide applications including catalysis, electronics, photonics, and photography (Maillard et al., 2010). Furthermore, low-dimensional silver materials may be utilized as interconnectors or as active components in the manufacture of micro/nanodevices (Sun et al., 2002).
