Adsorbent dosage has significant effect on adsorption capacity.

its only in few Experimental Design where the model shows that the dosage is not significant...but actually, dosage is a decisive factor

Maximum sorption capacity (determined by using different concentrations of solutions) should not be dependent on sorbent/ solution ratio. However, I have never seen two isotherms at the same graph, which are obtained using different sorbent/ solution ratio. Actually, I am planning to do that in the near future.

But mystery for me is the following - why sorption capacity increases, when higher sorbent/ solution ratio is used?

Artis, thanks to your comment. It is interesting to see sorption capacity increases with increasing sorbent/solution ratio. As I observed from literatures, higher solid-solution ratio usually results in lower sorption capacity. Of course solution pH is the most significant factor controlling the sorption capacity, which may be closely related to SSR. Have your considered pH in your study on adsorptions?

Of course, sorption efficiency (expressed in %) will be higher, if more sorbent (for example, 1.0 g) is used than less (for example, 0.2 g). BUT, sorption capacity (expressed as mg/g ) will be lower (as I presume, it should be true only in cases, when full saturation is not reached).

Yes, I always study the effect of pH in my sorption experiments. Actually, sorbent/ solution ratio in this case plays significant role:

1) there are materials with high buffering capacity, so if a lot of sorbent is used, it will significantly influence the pH of the solution. Even if there is no buffering capacity, it will change the pH of the solution. Of course, you can adjust pH during the sorption, but it will take more efforts (time, acid or base used) compared to situations if less sorbent is used.

2) If "to much" sorbent is used (e.g. low sorbent/ solution ratio), it will results in complete removal of substance from the solution. BUT, I strongly suggest that, it should never happen in pH studies, because it will not correctly show the influence of pH. For example, there is 100 % removal of copper ions at pH 4. Than the removal at >4 also will be 100 %. But how can it show the effect of pH? So, we need the use less sorbent in our experiments. :)

Actually, I have seen in some publications that the line in graph showing the effect of pH is almost horizontal, and wrong conclusion is made - that there is no effect of the pH.

One of the reasons why capacity decreases is the aggregation caused by high dosage as this leads to decrease the total surface area, in addition, the adsorption "density" is reduced due to the unsaturation of the adsorption sites

Thank you Yehya! Do you have any references, where this explenation is shown?

Daniel has a good point, but it seems difficult to take surface area into consideration because it's hard to obtain such parameters for most of the lab scale adsorption experiment with application into low cost sorbemts.

2012 Asia Pacific Conference on Environmental Science and Technology

Advances in Biomedical Engineering, Vol.6

Journal of Engineering Science and Technology

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To sumup, it is believed that experimental error, particularly the role of dissolvable organics acting as the ligands for trace metals is introducing errors into the test results. Other factors like the flocculation of fine particles that would reduce specific surface area etc. could impact the sorption capacity by weakening the amount of sorbed metals, but this effect only exists in short term, rather than in the longrun, e.g. the equilibrium sorption capacity might not be influenced. However, the latter argument is still obscure in mechanism, since no experiments are available to the writer at present.

Thank you for all the replies. It all helps!

You can refer to the following reference. Liao, X.-P.; Shi, B. Adsorption of fluoride on zirconium (IV)-impregnated collagen fiber. Environ. Sci. Technol. 2005, 39 (12), 4628-4632. I hope this helps.

According to the surface site heterogeneity model, the surface is composed of sites with a spectrum of binding energies. At low adsorbent dose, all types of sites are entirely exposed and the adsorption on the surface is saturated faster, showing

a higher qe value. But at higher adsorbent dose, the availability of higher energy sites decreases with a larger fraction of lower energy sites occupied, resulting in a lower qe value.