**1.1.1 Synthesis of silver nanomaterials**

440 Mass Transfer - Advanced Aspects

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

1. Metal Replacement. It is one of the most popular and economical methods. It consists of a cartridge containing iron, wool and wooden chips or spirals. A solution circulates with a constant flux through the cartridge. As the silver is removed, iron is depleted, producing sediment. Finally the sediment is refined to recover the silver. One cartridge recovers 90% of silver, two in a series, 95%. Although the cost of implementation is low,

2. Electrolytic recovery. This technology was introduced in the year 1930. It uses a cell with two electrodes immersed in a solution, to which a constant current is applied. The silver is reduced to pure metal on the cathode (usually stainless steel). There are two basic types of this technic: one where the cathode rotates in a solution and another where the solution flows around the cathode. The recovery is around 96% (20 to 60

3. Precipitation. It was the first practical method for silver recovery. It has been used for over 50 years, so it is highly developed. It also precipitates copper, cadmium, mercury, lead, nickel and tin, amongst other metals. It uses a precipitant together with a flocculating agent to increase the size of the particles. Silver is recovered by filtration and then refined, with a yield of 99%. However, the equipment and the precipitant are

4. Distillation. It is normally used together with the external management of effluents. It reduces the amount of liquid to be transported:80 to 100% of water could be removed, leaving thick or solid silver. With this method 99% of silver could be recovered. The

cost is high and it is recommended almost exclusively for industrial laboratories. 5. Ion Exchange. This technology can be used in solutions that have low percentages of silver, like stabilizers or wash water. In this process the metallic silver is obtained through a reversible process in which ions are exchanged between a solid (resin) and water with ionized salts. With a single column more than 90% of the silver could be

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

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

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

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

structural arrangement of the molecules (Tong-Xiang et al., 2009).

recovered. With two columns in a series, about 99% could be recovered.

the cost of refining is higher than the value of the recovered silver.

grams/hour of high purity) and it is easy to operate.

five are:

expensive.

recovery processes.

**1.1 Nanoparticles** 

two decades.

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 Ag particles is still a great challenge.

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

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

Silver Recovery from Acidic Solutions by Formation of

measuring range of 0° to 180°, with an uncertainty of ± 0.1°.

stirring speed of 600 rpm at both compartments.

obtained with the optical microscope.

**3.1 Silver nanoparticles formation on the microfiltration membranes** 

variation with respect to its initial weight.

**3. Results and discussion** 

Nanoparticles and Submicroparticles of Ag on Microfiltration Membranes 443

After every experiment, the membrane was removed from the system and the water was eliminated by evaporation. Subsequently, the membrane was weighed to determine the

The characterization of the metallic silver particles was carried out with a SEM (Scanning Electronic Microscope) Hitachi S-3000N coupled to an EDAX InCAx-sight analyzer. Contact angle of the microporous membrane with ascorbic acid solutions and Ag (I) in HNO3 solutions was measured using the CAM 200 from KSV Instruments Ltd. This unit has a

The recovery of Ag (I) was carried out using Ag+ 100 mg L-1 and HNO3 0.25 mol L-1 as the feed solution, and a solution of ascorbic acid (HA) 1 mol L-1 as the stripping solution. The contact time of the membrane with the feed and stripping solutions was 30 minutes, with a

After 30 minutes, the membrane was removed from the cell. The side of the membrane in contact with the feed solution showed a deposit, while any deposits were observed on the face in contact with the stripping solution. Figure 2a shows a part of the microfiltration membrane that was in contact with the feed solution. Figure 2b corresponds to an image

Fig. 2. Images of (a) microfiltration membrane after contact with the feed (Ag (I)) and stripping (HA) solutions; and (b) small part of the membrane through an optical microscope This deposit may have been caused by the formation of metallic silver by the reduction of Ag (I) with ascorbic acid. The deposit seems uniform both to the naked eye and through the optical microscope. In addition, the formation of silver on the membrane is consistent with the decrease concentration of Ag+ ions in the feed solution (see Figure 3). No concentration

(a) (b)

of Ag+ was detected in the stripping solution.

nanoparticles depends on different parameters such as, silver, nitric and ascorbic acids concentrations and the stirring rate of the solutions. Considering the extensive applications of nano and submicrometer Ag particles as catalysts, conductive adhesives, display devices, passive components, inkjet printing, photon emission, and higher order multiples resonances substrates (Dai et al., 2011; Hu et al., 2010; Xu et al. 2008; Sung et al., 2010; Gloskowskii et al., 2008), this methodology could be used in a wide range of industrial applications.
