I am studying the adsorption of lincomycin on activated carbons. I have determined the equilibrium time and now I would like to begin with the isotherms construction. Therefore, I need to specify the initial concentration of adsorbate (lincomycin) to be adsorbed. I have thought about working in the range from 10 to 50 ppm of lincomycin. However, in order to work with concentrations similar to those found in water bodies like rivers (250 ng/L) I would have to work in a smaller range than selected (e.g. 100-500 ng/L). It could be a problem for detecting concentration of adsorbate and measuring adsorbent quantities, influencing the reproducibilty of experiments. How would you determine an adequate initial concentration for experiments? Thank you
Dear Oscar Balladares,
We have faced the same problem when conducting adsorption isotherms for the removal of a variety of pharmaceuticals by ODTMA clay micelles complex and activated carbon.
In most cases we used a concentration of 20-50 ppm in order to conduct the analysis by HPLC. However, for some drugs such as ibuprofen we have conducted two experiments: one using a concentration of 50 ppm and the second using a concentration of 0.1 ppm. In the first experiment we determined the concentration of the pharmaceutical residue using HPLC and in the second experiment we used LC-MS.
When we compared the results from the two experiments we have found that Qmax and k in both experiments were about the same. Therefore, if you use a concentration of 10-50 ppm, the adsorption results will give a good indication on the removal of the compound when exists in a low concentrations such as that in water bodies.
Hoping this will be helpful,
Rafik
Thank you so much Dr. Rafik, If I don't bother you, do you have any paper published where this procedure is mentioned? It seems a very good and logic answer for me and, in theory, adsorption models should predict this behavior. Anyway, I would like to have some documents that support this information. Thank you again for your valuable help. Kind regards
Dear Oscar,
Attached please find the requested paper.
For your consideration,
Rafik
Dear Oscar, I would think that you have really to clearly decide whether the ppm range is relevant for you. Finally, you will want to publish your data, and the first (reasonable) question you will get is why do you work in the ppm range when your contaminants are at ppb. To add, you are apparently working in the ppt range, instead of mg/L at the ng/L?! Practically, as a potential user of such data, I will always consider them relevant for the ppm range, and the use at lower concentrations, although possible, but will be made with caution. In addition, you are working with a microporose material such as activated carbon; its effective surface, active sites and the pores available may be be essentially different, depending on the actual loading and the concentrations. It is in contrast to liquid-like materials and sorbents, including "octanol" model". In activated carbons, you may get yourself in saturation range of some sorption sites, which is totally non-relevant for the concentration range where isotherms are relatively more linear. If you have no strong justification, why the ppm range is sufficient, e.g., in terms of mechanistic understanding (which is also finally for this range), or, in looking for analogy between your sorbent and other non-porose materials, I would suggest examining, foremost, analytical capabilities to go down in concentrations, if it does not work. it is worth strongly weighing the scientific and applicative relevance of further work.
Hello Dr. Mikhail
Thank you for your valuable comment. I would like to add some infomation about my study. I am using activated carbons, natural zeolites and synthetic zeolites in order to remove lincomycin, so the primary objective of this research is to compare the efficiency of these materials in the adsorption of this adsorbate. Therefore, my objective is to find a cheap, useful and efficient material that shows promissory behavior for completing this goal. I would be grateful for any other feedback you could give me. Kind regards
Oscar
Dear Oscar, I guess the problem you are dealing, i.e., "a most effective, cheap adsorbent", is very general, and the sorbents you are examining, are typically characterized by non-linear sorption isotherms. If it is indeed an applicative issue, I would go foremost for practically relevant concentrations, and check whether lincomycin distribution coefficients are concentration-dependent (if not, you might go to higher solute concentrations). And, besides multiple "classic" questions (kinetics, dosage effect, pH/ionic strength, reversibility), I would take into account also that in almost any relevant water to be treated there are competitors, e.g., other contaminants as well as dissolved organic matter (DOM). DOM may have a blockage effect on sorption of organic compounds by multiple porose materials; perhaps, this blockage effect could be reduced on negatively charged surfaces, due to rejecting multiple negatively charged components of DOM. Anyway, there are some studies on sorption of your compound (or similar ones) by regular clays, which might be really cheap and effective adsorbents for positively charged organic molecules. The efficacy of clays to adsorb organic cations may be strongly enhacned by using potassium-exchanged clays; you might want to look for some papers, by Stephen Boyd, Brian Teppen and Hui Li (typically in EST).
Thank you so much for your valuable comments Dr. Mikhail. I will take them all in account. Kind regards