Pore diffusion vs Surface diffusion

[lpy2000]

Hello everyone,
How can i know if the biomaterial that i use to adsorp heavy metal from aqueous solution is releted to the pore diffusion or Surface diffusion or both?
How important it is to learn about the pore diffusion and surface diffusion?
If we know which diffusion is the main one and then how can we apply it to another step?

I am looking forward to hearing from you all.

Best regards

 

[Fizzhead]

lpy2000:

In either situation, you have surface diffusion occurring. I believe your question is which is the dominant mechanism, absorption/adsorption at the materials external surface, or on the internal surfaces within the pore network. With biomaterials, the mechanisms can be difficult to identify because you may have enzyme enhanced, or retarded, surface diffusion at cell membranes, etc. To answer your question accurately you need to know what type of absorption/adsorption occurs with the biomaterial you are using. First guess, I would say both mechanisms probably play a role, only question is which one dominates. By utilizing the internal surface area of the pore network, you could have much more surface area to saturate, thus increasing the loading capabilities considerably, depending upon the porosity of the material. For a first cut at answering this question, apply a variable pressure gradient and measure the changes in flow and absorption/adsorption. Both overall and kinetically. Pore diffusion will be more greatly affected by the pressure gradient variability than surface diffusion because it is actually a combination of pore flow rate and surface absorption/adsorption inside the pore network. Note that just because you see an increase in flow does not, by itself, mean you have answered the question. Assuming you have a significant pore network available, the change in the pressure gradient should produce increased flow regardless of what happens with respect to absorption/adsorption. You should collect your data, pressure gradient, flow variance and absorption/adsorption variance and then analyze the data against various mass transfer models to determine which the data best fits. This answer then gives you a direction for further investigation.

 

[lpy2000]

Hi Fizzhead,
Thank you very much for your kind suggestion. I now have some overviews about it.
Regarding your questions, i could not understand some points you mentioned as follow:
1) The mechanisms can be difficult to identify cause of Enzyme enhanced..? What kind of enzyme enhanced? Enzyme that produced by biomass? and how much it influence the adsorption. I never heard about this before and it would be nice if you could share me some idea.
2) I guess that to do a pressure gradient, we need to experiment it in a column right? How about if i just use the Continuous Stirring Tank Reactor (CSTR), how can i check the pressure gradient. Now i consider only to check the influence of turbine speed mixing and the influence of temperature. would these help to explain the surface diffusion and pore diffusion?
3) If my friends ask me how important is it to know whether the adsorption is mainly occupy by pore diffusion or surface diffusion, how should i reply?
Sorry for my weak english, and i am very new to this field. All comments are welcomed.

Best regards,
LPY2000

 

[Fizzhead]

LPY2000:

In bioabsorption/adsorption, the potential for enzyme effect depends upon the process you are using, the particular biomaterial and the metal you are absorbing/adsorbing. Especially if the biomaterial is still living, the absorption/adsorption onto the membrane surface, or mass transport across, membranes can be dramatically affected by the active assistance, or resistance, provided by enzymes. The problem is that if you are measuring the absorption/adsorption kinetics in the presence of this assistance or resistance, it must be somehow accounted for in the mathematical model you use to correlate the results. Otherwise, your results are potentially skewed. Unless of course, you just want to develop a reasonably accurate process control model which you can do utilizing the bulk mass transfer coefficient, knowing that any enzyme effects are accounted for in the general parameter.

Your question/answer about use of the CSTR is 50/50. If you measure the effects of differing mixer speeds you might see some effect. But, it would be the effect of the higher mixer speed thinning the boundary layer at the surface of the biomaterial. Any enhanced kinetics measured this way would be a chance to correlate the bulk mass transfer coefficient with mixer speed, but would not answer the original question. In my first response I was thinking of something like a bed of the biomaterial through which you could force flow at varying rates and measure substrate uptake rates. However, if your process normally employs a CSTR, then the measurement you really want for equipment sizing, or process improvement is the bulk mass transfer coefficient correlated to mixer speed.

The temperature effect would be measurable, assuming you vary the range sufficiently. However, the main cause of any observable effect is bound to be the change in viscosity of the liquid medium with temperature. Lower viscosity, keeping everything else constant, results in faster diffusion of the substrate through the boundary layer. The mass transfer coefficient is normally a weak function of temperature and therefore, temperature may not be economically viable as a process control variable. IF you measure the effect AND can differentiate between the viscosity effects and the mass transfer coefficient variation with temperature, you would still not be able to distinguish between surface absorption/adsorption and pore absorption/adsorption.

As mentioned above, for a controllable process variable in a CSTR, I would measure the bulk mass transfer coefficient and correlate it to mixer speed and stop. With this information, you can determine the best operating regime for your CSTR process. The mathematical modeling to further differentiate between the two mechanisms is fairly complex. How badly do you want to know it?

The surface diffusion mechanism is the diffusion of the substrate (the metal you are absorbing/adsorbing) across the fluid boundary layer surrounding the biomaterial. It is, generally speaking, a function of the substrate's chemical properties in solution, the concentration gradient across the boundary layer (the difference between the "bulk concentration and the concentration at the actual material surface) and the kinetics of the absorption/adsorption process at the material surface. Where a pore exists, both the concentration gradient and the boundary layer shape can be distorted because of fluid flow into the pore (depending on the pore diameter and flow characteristics). Be aware that the fluid flow rate into the pore is a function of the pore diamater, the tortuosity (relative straightness and variability in diameter over the pore length) and the pressure gradient across the pore. So, all sorts of questions arise about the characteristics of the pore network. Whats the average pore diameter? What is the distribution of pore diameters? Pore tortuosities? Below a certain diameter, the pores initially take up fluid via capillary action, but once full, all mass transfer within the pore depends upon diffusion from the mouth of the pore into the pore and then to the walls. Etc. Etc.

As you can see, the question can become quite complex very fast. In general though, if you induce a greater pressure drop across a porous solid you will induce greater flow. For surface diffusion, the greater flow thins the boundary layer resulting in a shorter diffusion path and this effect can be measured and correlated with many standard diffusion models. If greater absorption/adsorption effects are noted, then most likely these are due to increased pore diffusion, which can be modeled from the same test results and a representative porosity and 'average pore diameter' postulated for the material. So, having written a small treatise on mass transfer variability in porous solids, the answer to your original question is that for the bulk kinetic data you probably want for process modeling purposes, you probably do not need to give a hoot what the distribution is between surface difusion and pore diffusion.

 

[lpy2000]

Hi Fizzhead,

I got a lot of idea from you. Thank you so much for you kind help.