I have tested Pb adsorption on dry aquatic plants and I found that I have a 90% adsorption efficiency at equilibrium, however adsorption capacity at equilibrium is very low (15.8 mg/g). This seems to be very common, but how can you explain a high efficency having a low adsorption capacity?
"adsorption capacity" is the thermodynamic value (the equilibrium), while "adsorption efficiency" is the kinetic value (the rates)
The number of adsorption sites and available specific surface area determines adsorption capacity. Besides solid/liquid ratio efficiency is also determined by tendency of adsorbate to adsorb compared to stay in solution.
Thank you all for answering!.
Actually I have tested various initial Pb concentrations for the isotherm from 50 ppm to 400 ppm, that's how I obtained the adsorption capacity of 15.8 mg/g, this value corresponds to a 90% of adsorption efficiency. However, I have only tested 3 adsorbent concentrations 2, 4 and 6 g/L, and I chose to use 6 g/L as it seemed to have higher adsorption (in percentage terms).
Compared to other adsorbents, 15.8 mg/g is very low. But I get to adsorb 90% of Pb. Why is this bad?
Dear Paola,
in practical terms you have to evaluate
what would be typical effluent concentration (range)
what residual concentration would be tolerable
how long would it take to loose ability to ensure this concentration.
What would be the cost of adsorbent
cost of alternative adsorbent or removal by ion exchange
.....
First of all, the adsorption capacity that you have obtained is not very low as you stated. Remember that you are using agricultural wastes not AC. Your results are very good as matter of fact compared with the results with other researchers (I am attaching just one research article). I recommend that you make a literature review and include the results that are obtained by other researchers in your publication as a reference.
Second, it is also normal to found that the % removal efficiency is high with the adsorption capacity that you have obtained. My colleagues have stated the reasons in their answers. The initial concentration plays an important role, The active sits are also a key player.
To conclude, you must be happy with your results. I wish you the best of luck in presenting these results in a good way.
Kafia M. Shareef Surchi, Agricultural Wastes as Low Cost Adsorbents for Pb Removal: Kinetics, Equilibrium and Thermodynamics, International Journal of Chemistry Vol. 3, No. 3; August 2011
Factor that would influence adsorptions capacity include
Volume of waste solution and mass of adsorbent. increase the biomass and see how your results would be affected. Best of luck in your research
Increasing adsorption density of a solute on any adsorbent reduces the further tendency of adsorbent to adsorb owing to increasing competition among solute ions or molecules and a decrease in the number of vacant sites available for further adsorption. So your observations are correct. For the removal of heavy metal by any adsorbent, it is important with what tenacity the solute is binding with the adsorbent. I mean, whether the retention is through simple physical or chemical bonding?
Thank you all for your answers.
It doesn´t seem obvious at first, but as Hossam Altaher said, it is not necessarily bad, I could see that adsorption capacity is not that high, but if less adsorbent concentration is tested it migh change as Ioannis Anastopoulos said. I will leave that as a recommendation for future works.
Thank you again!
I thanked the followings:
1-The student that ask this important question.
2-All my seniors who gave the answers to the best of their knowledge.
I am facing similar problem as the one Ms. Paola presented here.
I found this discussions useful.
Thank you very much.
Mr. Abubakar Hamisu Mijinyawa
PhD Scholar (Chemistry)
Sharda University
G. Noida, India