I'm studying heavy metal adsorption with some absorbents.
But At the adsorption isotherm experiment,
The pH decreased by increased concentration in the solution...
Is it normal Common phenomenon?
I couldn't find a paper that considers this phenomenon...
Usually whenever I do adsorption experiment the pH effect would be studied by controlling the working solution pH. Because, depending on the adsorbate that you're working on (heavy metal) the pH might not be exactly same for all different concentration due to the ionic movement of the adsorbate on to the solvent. For example, if you working anionic adsorbate like Cr(IV) its pH is ranging b/n 6-14, so the concentration d/c will result change in solution pH. Therefore, its recommend to regulate the pH prior attempting adsorption experiment. you can use HCl and NaOH to alter the pH then study the isotherm at a fixed pH to have reliable data.
It's very important for pH batch studies, for the pH of each solution to be adjusted to desired value with dropwise addition of analar grade acid and base. However for concentration and Biosorption isotherm (langmuir and freundlich ). Initial concentration increase favours the removal of metals at optimum pH. So one has to control the pH to optimal value. Several journals I have seen, use pH 6
Sorry to make you misunderstand.
I already adjusted the pH of all solutions that were used in the adsorption experiment.
I mean, even I did adsorption experiments with the solution adjusted the same pH, the pH of the solution after experiments were decreased by the concentration
(For example, the initial pH of the solutions were 5
And concentrations of the solutions were 5, 10, 20, 50, 100 mg/L .....
But the pH after adsorption experiments were 4.7, 4.6, 4.5, 4.4....
I don't know why the pH of the solutions was changed by the concentration after adsorption experiments.)
Thank you for helping me again.
But Could I get more information????
In my opinion, this is normal, during the adsorption process there is a change in the absorbent and absorbate species. This change causes a change in the pH of the solution. Some heavy metals form charged and acidic species. To control the pH of the solution, use a strong buffer solution.
Dear Jeon Han Gyeol this is indeed a very interesting technical question. Personally I never worked in this field as we are synthetic chemists. I appears that virtually all studies about heavy metal adsorption deal with the effect of changes in pH value on the adsorption process. As a typical example please have a look at the following RG link:
The Influence of pH and Adsorbent Concentration on Adsorption of Lead and Zinc on a Natural Goethite
Article The Influence of pH and Adsorbent Concentration on Adsorptio...
There are numerous other papers like this in the literature. However, they don't give a direct answer to your question.
One possible explanation is that the metals are perhaps adsorbed in the form of oxides, hydroxides, or carbonates. When these basic species are removed from the solution, there is a possibility that the solution becomes more acidic, i.e. the pH decreases.
In this context please have a look at the following potentially useful research article:
An assessment of pH-dependent release and mobility of heavy metals from metallurgical slag
Article An assessment of pH-dependent release and mobility of heavy ...
Perhaps it would also be helpful to know more about the nature of your adsorbent and see if it contains acidic groups..
I hope this helps. Good luck with your work and best wishes, Frank Edelmann
Dear Jeon Han Gyeol,
I have also seen this case when our group use the citric acid-modified lignocellulosic materials to remove some heavy metal cations such as Fe (III), Pb (II), Ni(II), and so on. The results could be explained to be due to the ion-exchange process happened together with the adsorption process, it often is regarding to the -COOH and -OH functional groups on the surface of absorbents. For example: one Fe3+ ions can exchange from one H+ ions till to three H+ ions on the surface of absorbents; therefore, the number of H+ ions in the adsorbent solution will increase after occurring the adsorption process --> The pH value of adsorbent solution will reduce. Hope that the response will help you to understand more clearly. Beside that, you could refer my some articles regarding to the same phenomena as the following link:
Article A Theoretical and Experimental Study on Esterification of Ci...
Best regards.
Alemayehu Kidanemariam Farouq Busari Noer Komari Farouq Busari Dang T. Nguyen
Thank you very much for your helps!!
Changing concentration may change pH. However, in the case of adsorption, the concentrations of the adsorbate are very low and, consequently, the changes in the concentration of the adsorbate during the process are very small. A buffer may be used if necessary.
I am facing the same challenge, the adsorbent increased pH of the working solution.