Diffusion dialysis

Diffusion dialysis is a diffusion process in which ions are transported across an ionic membrane due to a concentration difference and can be described in a similar way as the dialysis process. The flux of an ion i across a membrane is in absence of concentration polarization given by 

The other concept is shown in figure VI - 45b and this process is often referred to as Donnan dialysis. Since there is in fact not a fundamental difference between diffusion dialysis and Donnan dialysis, the former name is preferred and will be used. Also here ion exchange membranes are applied and figure - 45b left shows a cation-exchange membrane. Since the membrane is permeable for cations, will diffuse from the left compartment (phase I) to the right compartment (phase II). 

Diffusion dialysis type of processes are from an engineering point of view rather simple and energy efficient processes in which feed and permeate are pumped counter-currendy, like most dialysis type of processes. No external driving force such as pressure difference or an electrical potential difference are required. A simplified process scheme is shown in figure VI - 46. 

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Many related processes use charged membranes and/or EMF. Electrodialytic water dissociation (water splitting), diffusion dialysis, Donnan dialysis, and electrolysis are related processes. Electrowsis (chlorine-caustic) is a process of enormous importance much of which is processed through very special membranes. 

Dijfusion Dialy The propensity of and OH" to penetrate membranes is useful in diffusion dialysis. An anion-exchange membrane will block the passage of metal cations while passing hydrogen ions. This process uses special ion-exchange membranes, but does not employ an applied electric current. 

Nomura et al. (1987a) attempted to minimize product inhibitory effect on the aspartate kinase step in lysine biosynthesis and enhance L-lysine production from Brevibacterium Jlavum QL-5 using a combined ED-F system. However, lysine production was not statistically different from that obtained in diffusion dialysis fermentation and about 20% greater than that achieved during conventional fermentation, thus making practically ineffective such a use of ED. 

Three other processes using ion exchange membranes (Donnan dialysis, diffusion dialysis and piezodialysis) are covered in Chapter 13. 

Now the major application of dialysis is the artificial kidney and, as described in Chapter 12, more than 100 million of these devices are used annually. Apart from this one important application, dialysis has essentially been abandoned as a separation technique, because it relies on diffusion, which is inherently unselec-tive and slow, to achieve a separation. Thus, most potential dialysis separations are better handled by ultrafiltration or electrodialysis, in both of which an outside force and more selective membranes provide better, faster separations. The only three exceptions—Donnan dialysis, diffusion dialysis and piezodialysis—are described in the following sections. 

Figure 13.4 Schematic of a diffusion dialysis process to separate acids from heavy metal/acid mixtures...

In addition to the processes discussed so far there are two more electromembrane separation processes in which the driving force is not an externally applied electrical potential but a concentration gradient. The processes are referred to as diffusion dialysis and Donnan dialysis. Diffusion dialysis is utilizing anion- or cation-exchange membranes only to separate acids and bases from mixtures with salts. Donnan dialysis can be used to exchange ions between to solutions separated by an ion-exchange membrane. Both processes have so far gained only limited practical relevance [4] and will not be discussed in this chapter. 

To simultaneously recover the metal and sulfuric acid from spent process liquors of nickel electrolysis, Xu and Yang [95] tested diffusion dialysis successfully. The membrane used was surface-cross-linked with aqueous ammonium to decrease waste volume expansion caused by the water osmosis. They could control nickel leakage within 4% and recover about 70% of the add. 

Alternatively to diffusion dialysis, Pierard et al. [96] suggested electrodialysis as a regeneration process. In the case study involving acid pickling before electroplating, they demonstrated the selection of ion-exchange membrane couples as well as the development of tools to promote the use of electrodialysis in industrial applications. 

Xu, T.W. and Yang, W.H. (2004) Tuning the diffusion dialysis performance by surface cross-linking of PPO anion exchange membranes - simultaneous recovery of sulfuric add and nickel from electrolysis spent liquor of relatively low acid concentration. J. Hazardous Mater., 109 (1-3), 157-164. 

Wodzki R and Szczepanski P. Treatment of electroplating rinse solution by continuous membrane extraction and diffusion dialysis. Pol J Environ Stud, 2001 10(2) 101-111. 

Diffusion dialysis is a special class of Donnan dialysis where anion-exchange membranes are generally used. The membranes do not allow cations to pass through, but anions are free to move across the membrane to equilibrate. However,... 

Donnan dialysis and diffusion dialysis techniques are used [30] to deacidify and concentrate actinide bearing acidic solutions. Near complete deacidification could be achieved from 8 M nitric acid solutions with a high decontamination factor for the actinides. With cation-exchange membranes, the deacidification is accomphshed by the build up of neutralized salt in the feed. With an anion-exchange membrane, the feed acidic solution is deacidrfied free of salt buildup. By superimposing direct osmosis with free deacidiflcation, it was simultaneously possible to concentrate the deacidified actinide solution [30]. 

Mathur, J.N. et al., Diffusion dialysis aided electrodialysis process for concentration of radionuclides in acid medium, 7 Radioana/. Nucl. Chem. 232, 237, 1998. 

Influence of Acid Type and Concentration on Diffusion Dialysis Recovery.975... 

The process of diffusion dialysis is also mentioned briefly, as an attempt to recover spent acid from WTR, after alum recovery. The objective of this process is to estimate the efficacy of the process in (a) raising the pH of the treated WTR and (b) lowering the pH of the fresh WTR before undergoing alum recovery in the DMP. 

FIGURE 34.7 Integrated process comprising a DMP unit and a diffusion dialysis unit. 

The dialysis coefficient U for a given component in solution in diffusion dialysis cell is given by the amount of the component that is transported per unit active membrane area, per unit time, and per unit concentration difference of the component. This translates into the following equation ... 

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