**4. Conclusions**

456 Mass Transfer - Advanced Aspects

reduction speed, the diffusion process takes place and it is possible to calculate the overall

0

The overall mass transfer coefficient depends on the diffusion rate as well as on the chemical

The variation of Ln [Ag+]t/[Ag+]0 as a function of t\*Q/V, is a straight line with a slope equal to –K. Figure 17 shows the results obtained in the case of a system containing a feed solution of Ag(I) 100 mg L-1 with different HNO3 concentrations and 1 mol L-1 of HA in the stripping

Fig. 17. Variation of de Ln[Ag+]t/[Ag+]0 as function of t\*Q/V in the feed compartment for different nitric acid's concentrations. (S) Without HNO3, pH = 5.4; () [HNO3] = 0.1 mol L-1; () [HNO3] = 0.5 mol L-1; (z) [HNO3] = 1 mol L-1. Stripping solution: [HA] = 1 mol L-1

The value of K (slope) obtained for the system in absence of HNO3 is 0.1779 cm s-1,with a R2 value of 0.94. For a HNO3 concentration of 0.1 mol L-1, in the feed solution, the value of K (0.1175 cm s-1) is very close to the former. In these two cases the K values are high for a system with a diffusion control. It is important to note that under these conditions, the formation of silver nanoparticles not only occurs in the feed phase-membrane interface but also in the bulk of the feed solution, then the process is controlled by the redox reaction. On the other hand, when the concentration of HNO3 is 0.5 mol L-1, the value of the overall mass transfer coefficient is 0.0483 cm s-1. When the HNO3's concentration is 1 mol L-1, there are

<sup>+</sup> = − (1)

[ ] [ ] *Ag <sup>t</sup> <sup>Q</sup> Ln K t Ag V*

+

mass transfer coefficient of the Ag+ using the equation (1).

K = overall mass transfer coefficient

Q = effective area of the membrane (11.34 cm2)

reaction between silver ions and ascorbic acid.

Where:

solution.

t = time (sec)

V = volume (250 cm3)

We have developed a methodology for the recovery of silver (I) from aqueous solutions on a microfiltration membrane using ascorbic acid as a reducing agent. Under certain conditions, it is possible to recover about 99% of the silver contained in the aqueous solutions. The silver particles are deposited in nanometric and submicron sizes. The shape of these particles depends on the hydrodynamic and chemical conditions of the system. Silver particles can be obtained as dendrites, decahedra and hexagonal plates. We have analyzed the mass transfer process of the species involved in the system in order to explain the observed phenomena and to correlate the morphology of the particles obtained, with the mass transfer process. We can conclude that the reaction between silver and ascorbic acid occurs at the interface membrane-feed solution. The permeation rate of ascorbic acid into the membrane is linked to the Ag+ mass transfer process. Finally, the global coefficient of mass transfer is related to the morphology of the particles obtained. At high K values, silver dendrites nanoparticles are obtained; whereas if the value of K decreases the deposit of silver particles corresponds to a slow crystallization process. The methodology proposed allows the efficient recovery of Ag (I) ions and allows the obtaining of microfiltration membranes modified by Ag particles, which can be used as filters for the removal of microorganisms contained in water.

Silver Recovery from Acidic Solutions by Formation of

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The authors gratefully acknowledge the financial support of the Universidad de Guanajuato, Mexico and Spanish Ministry through the project MAT2009-14741-C02-02. Oswaldo Gonzalez would like to thank CONACYT for financial support.

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**20** 

*Spain* 

**Particles Formation Using Supercritical Fluids** 

The particle precipitation into micro and nanoparticles has been an active research field for decades (Chattopadhyay & Gupta, 2001; Kalogiannis et al., 2005; Rehman et al., 2001; Reverchon, 1999; Velaga et al., 2002; Yeo&Lee, 2004). The greatest requirement in the application of nanomaterials is its size and morphology control which determine the potential application of the nanoparticles, as their properties vary significantly with size. Micro and nanoparticles can be obtained by different techniques. Conventional techniques (spray drying, solute recrystallization, coacervation, freeze-drying, interfacial polymerization) present drawbacks such as excessive use of solvent, thermal and chemical solute degradation, structural changes, high residual solvent concentration, and mainly, difficulty of controlling the particle size (PS) and particle size distribution (PSD) during processing

However, the application of supercritical fluids (SCFs) is an attractive alternative for this particle formation because remove these drawbacks. These supercritical fluids have larger diffusivities than those of typical liquids, resulting in higher mass-transfer rates. Moreover

There are two main ways of precipitating micro and nanoparticles using supercritical fluid as solvent, the RESS technique (Rapid Expansion of Supercritical Solutions); or using it as antisolvent, the SAS technique (Supercritical AntiSolvent); the choice between one or another depends on the active substance high or low solubility in the supercritical fluid. The RESS process consists of solubilising the active ingredient of interest in the supercritical fluid and then rapidly depressurising this solution through a nozzle, thus causing the precipitation, extremely fast, of this compound. In other words, the process is based on the transition of active compound from soluble to insoluble state when the carbon dioxide passes from the supercritical to the gaseous phase. This technique has been applied on the particle precipitation and co-precipitation of many active ingredients/polymers (Kongsombut et al., 2009; Sane & Limtrakul, 2009; Turk et al., 2006; Vemavarapu et al., 2009;

The SAS technique, in all its variants, generally consists of spraying a solution of the solute to be precipitated into the supercritical fluid. The mass transfer behavior of the droplets is thought to be a key factor affecting particle morphology (Werling & Debenedetti, 1999). The volumetric expansion of the solvent reduces the solvation capacity of the solvent, causing the supersaturation of the liquid phase and the consequent generation of the particles. The SAS process has been carried out for many particles precipitation and polymeric encapsulation of particles of active ingredients (Ai-Zheng et al., 2009; Chong et al., 2009a;

(He et al., 2004), so these techniques for particle formation may not be advisable.

its solvent power and selectivity can be tuned altering the experimental conditions.

**1. Introduction** 

Wen et al., 2010).

A. Montes, M. D. Gordillo, C. Pereyra and E. J. Martinez de la Ossa *Department of Chemical Engineering and Food Technology, Faculty of Science, UCA* 

