Continuous Ultrasound-Assisted Leaching

DISCRETE VERSUS CONTINUOUS ULTRASOUND-ASSISTED LEACHING 

Ultrasound-assisted leaching involves irradiating a sample-leachant system in order to facilitate the transfer of the target analytes to the liquid phase. 

The general procedure for developing discrete US-assisted leaching (DUSAL) is simpler than that of its dynamic counterpart (continuous US-assisted leaching or CUSAL), as the former requires no specific knowledge to design and construct the continuous manifold and optimize interrelated flow variables as in CUSAL. 

For CUSAL, the sample is weighed and placed in a leaching chamber both ends of which are tightly connected to the flow manifold in order to avoid leakage of the liquid phase and allow passage of the leachant. Two types of sample chamber have so far been used for this purpose, namely  

Before the next run, the system is flushed by circulating the leachant for a preset time in order to avoid cross-contamination. 

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DISCRETE VERSUS CONTINUOUS ULTRASOUND-ASSISTED LEACHING... 

Figure 4.2. Sample cells for continuous ultrasound-assisted leaching. (A) One-piece cell and (B) DIsmountable cell. (Reproduced with permission of Elsevier, Ref [2].)...

Figure 4.5. Experimental set-up for continuous ultrasound-assisted leaching in an open (A) and closed manifold (B). C — coil, CR — collection reservoir, L — leachant, LC — leaching chamber, PC — personal computer, PL — propagating liquid, PP — peristaltic pump, S / — switching valve, UP — ultrasonic probe, 1/1/ — waste and 1/1/6 — water bath.

The variables to be optimized in a continuous ultrasound-assisted leaching approach can be classified into two groups, namely those concerning the relative position of the probe and sample cell and their surroundings, and those related to the sonicator settings. 

Fig. 3.7. Approaches to implementing continuous ultrasound-assisted leaching. (A) With direct monitoring of the process. (B) and (C) With a derivatization step before detection. (Bl) With leaching in the recirculation mode. (Cl) With leaching in the go forward and back mode. (B2) and (C2) Derivatization-detection steps. P peristaltic pump, UP ultrasound probe, SS solid sample, DB de-bubbler, D detector, SV switching valve, LC leaching carrier, R reagent, W waste. (Reproduced with permission of Elsevier.)...

The principal advantages of continuous ultrasound-assisted leaching are modest consumption of sample and reagents, the need for few or none of the chemicals required for dissolution in the manual method and the expeditiousness of analyses (about 25 soil samples per hour can be processed in the determination of boron). 

A. Caballo-Lopez, M.D. Luque de Castro, Continuous ultrasound-assisted leaching of phenoxyacid herbicides in soil and sediment with in-situ sample treatment, Chro-matographia 58 (2003) 257. 

FIGURE 2.34 Experimental setup used for the two modes of continuous ultrasound-assisted leaching and the possibilities of their coupling to other steps of the anal)dical process. One, two or three steps can he used in a single method. Abbreviations used are LC, leaching carrier PP, peristaltic pump UP, ultrasonic probe EC, extraction chamber WB, water bath W, waste SV, selection valve EX, extract E, eluent IV, injection valve SPC, solid-phase column DR, derivatization reagent RC, reaction coil and D, detector. (Reprinted from Luque-Garcia, J. L. and Luque de Castro, M. D., Trends Anal. Chem., 22, 41-47, 2003. With permission from Elsevier.)... 

A continuous ultrasound-assisted device was studied by an experimental design in order to determine the leaching parameters... 

In many situations, ultrasound-assisted leaching is an expeditious, inexpensive, efficient alternative to conventional extraction techniques and, in some cases, even to supercritical fluid and microwave-assisted extraction. A number of applications to both organic and inorganic analytes in a wide variety of samples exist. Most are conducted by hand. As with automatic extractions, applications involving continuous systems are still very scant and hence one possible target for future research. 

There are three different dynamic approaches to ultrasound-assisted leaching in continuous-flow systems, namely ... 

The experimental conditions influencing an ultrasound-assisted continuous leaching process were studied using an experimental design... 

The use of ultrasound-assisted slurry is a simple, efficient alternative to circumvent problems associated to digestion of samples with complex matrices derived from the required hazardous conditions, but also to leaching when efficiencies are not quantitative. Applications involving slurries prepared by ultrasonic assistance are continuously proposed with different detection systems, which demonstrate the versatility of slurries for metallic elements determination. 

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