I have prepared activated carbons with two different physical treatment methods, i.e. CO2 activation (surface area: 269 m2/g, porosity: 0.07 cc/g) and Steam+CO2 activation (surface area: 442 m2/g, porosity: 0.078 cc/g). Is it possible that the first sample can show better adsorption properties (to some extent) than the latter one? Specifically for heavy metals adsorption? Because I have got results like this.
The adsorption experiments were conducted in same conditions (pH, agitation, room temp.).
Furthermore, does the working volume of adsorbate solution affect the adsorption efficiency though the adsorbent dose remains the same?
Thank you.
Dear Sirajus, the surface ares is not the only factor that affect the adsorption capacity of an adsorbent. Yes, it is very important, but, in many times the PZC (which change with pH), the surface chemistry and the type of pores are factors driving the adsorption in large extension. Sure that the pH, agitation and temperature are equal when you do the adsorption experiments. Compare the PZC value an make conclusions.
Thank you Carlos. But yes, the adsorption experiments were conducted in same conditions (pH, agitation, room temp.). Do I still need to compare the PZC values?
Yes, PZC is one of the most important parameter for Physisorption process. You have to optimise the PZC of your material to have a better adsorption capacity. At first, you can find a procedure to determine the PZC of your materials (on google), it quite simple. Then, you can have a better understanding of your material not only base on the surface area
BR,
Determine the PZC values and make Diffuse reflectance Infrared. As said by Le Phuc, that is very easy, the mass titration technique can be useful for you. Good Luck!!!
Hello, concerning your last question about the importance of adsorbate solution volume for the same amont of adsorbent, the anwser is yes, since you are changing the number of moles of methal by changing the solution volume. If you are doing kinetic assays V, concentration, weight of dried carbon, pH, temperature, stirring must not change.
About the lower adsorption capacity of the Carbon with lower BET area it is not uncommon since it depends on:
- surface chemistry, that we usually acess by determining the pH at the point of zero charge (PZC) (my steam activated carbons attain PZC values as high as 11).
Since your carbons may have distinct PZC values, altougth your starting adsorbate solution has the same pH the pH during the adsorption process may be diferent due to the carbons.
- pore sizes must also be considered (i.e. Volume of micropores and mesopores). In our works we verify that micropore size distributions allow to have a deeper understanding of the adsorption of pharmaceutical compounds onto activated carbons.
Good luck
AM
Thank you Dr. Ana for your answer. I am learning a lot from all of your contributions.
Hello. It is imperative that you get to know the chemical makeup of the carbon surface. Chemical functionality is more relevant to metal adsorption than surface area. Also, it is not only important to look at the activating agent but also the conditions in which the carbon is prepared. Were the carbonization/activation temperatures the same in both cases? Were the carbons prepared in a single carbonization/activation step or in two distinct steps, carbonization followed by activation? Temperature will be paramount in dictating the chemical makeup of the activated carbon surface. Oxigenated acidic groups (such as carboxylic and lactonic) are more labile than oxigenated or nitrogenated basic groups and, thus, are eliminated from the surface at lower temperatures. Usually, for carbonization/activation temperatures above 600 oC, the resulting activated carbons are of basic nature, meaning they will present high pHPZC. In adsorbate solutions with pH lower than the pHPZC of the carbon, the net charge of the carbon surface is
predominantly positive, thus favoring the sorption of anions. Therefore, such carbon will be only appropriate for metals which will present themselves as anions in the solution at that pH. If you are dealing with divalent cations, e.g., Cu2+, Ni2+, Zn2+ and Cd2+, you need to understand the speciation of the metal in aqueous solution to define the temperature at which the activated carbon will be produced such that you control the acidic/basic nature of the surface in order to favor the adsorption of specific species. Copper, for example, will precipitate at pHs higher the 4.5 and thus need to be adsorbed at pHs lower than that. For this example, the surface of the carbon need to be predominantly negatively charged at that pH, meaning it should have a very low pHPZC and that can be achieved with high concentration of acidic oxygenated groups at the surface such as carboxylic or lactonic. The negatively charged groups will favor electrostatic attraction of the cations to the surface. The adsorption mechanism itself can be ion exchange, complexation and others.
Hope that will help you understand better your situation.
Excellent explanation Dr. Leandro. I thank you for your conceptual representation of the experimental system.
Dear Sirajus Salehin, I have attached to this message two book chapters I wrote with Prof. Franca: one about activated carbons produced from agri-food wastes; and other about low-cost adsorbents prepared from agricultural wastes. I hope they will help you in better understanding the matter of the relevance of the chemical makeup of the carbon surface to the adsorption of metals.
Also, please find in attachment another chapter we wrote on column adsorption.
Adsorption capacity of activated carbons is determined not only by their surface area and internal porous structure but also with the functional groups at the carbon pore surface. Activated carbons are known to contain oxygen, hydrogen, chlorine and sulphur atoms. These atoms are derived from the starting material and become a part of the chemical structure as a result of carbonization or chemically bonded to the carbon during activation or during oxidation. These atoms are bound to the edges of the carbon layers and form surface groups that greatly affect the adsorption behavior of the activated carbon. Carbons having equal surface area but having been prepared by different methods or different activation treatments have different functional groups and hence different adsorption characteristics.
This is due to the formation of different types and concentrations of surface oxides on the carbon surfaces, (Mattson and Mark, 1971; Dubinin 1987)
1.The carbons may have low pore volumes. Pore volumes of 0.56 cm3/g and above are good.
2.Carbons may have high pore size which is undesirable. A pore size of 22 Å is good
3.Aim for low density of about (0.3 g/cm3) and the ash content of (18 %).
High ash content leads to higher conductivity,
4. Check for low pH values as a value of 4.5 affects adsorption. Aim for
Ph 7
Could be differences in surface chemistry, or differences in textural properties (porosity).
You can easily check differences in porosity by comparing pore size distribution of both carbons with a method suitable to your isotherm (I would expect mostly micropores and some mesopores present in both cases).
If this does not shed any light on interpreting your results, maybe you have similar porosoty but different surface chemistry. How to check for this? I would heat the sample in inert atmosphere (pyrolysis, carbonization) up to some 900 oC to desorb surface groups and then do your heavy metal adsorption.
Should be A LOT of literature about this (comparison of CO2 vs H2O activation methods (e.g. Rodriguez Reinoso Carbon 1995 THE USE OF STEAM AND CO2 AS ACTIVATING AGENTS IN THE PREPARATION OF ACTIVATED CARBONS or the classical paper of Tomkow in Fuel 1977).
The removal is likely driven by surface chemistry of the activated carbon upon the treatment. You may check the occurrence through FTIR and boehm titration. There would be few possible mechanisms here: i. ion exchange/anchor if acidic functional groups are presence/abundance, ii. C-pi cation interaction if the surface is bold, or iii. combination of (i) and (ii). Higher removal due to functional groups/surface chemistry does not necessarily means higher adsorption rate. Rich pore texture and high surface area may enhance the rate.
Very high surface are of any sorbent indicates presence of narrow pores. These may be inaccessible for larger molecules of sorbate. Our nanoporous carbon fibers have surface areas up to 2,000 m2/g and their effective pore diameter is about 0.6 nm. After modification, they exhibit high efficiency in removal of heavy metals, As, Hg, Cr, Se, Sb... We are looking for a producer.
YES it is possible. It depends on several factors, characteristic for both AC and adsorbate. Jacek
Dear Sirajus,
A good indication of the functional groups present on the surface of your activated carbons is the determination of the Point of Zero Charge. Depending on both, the working pH in the solution and the PZC, the functional groups would be available to interact with the metal ions or not.
The second point to take care is porosity, the surface can be higher due to micropores but all of these micropores are not going to be useful to remove ions in water phase.
BR, V
Dear Sirajus, the participants of discussion are fully right. However, carbon is generally not very effcient sorbent for metals. Which metals are you interested in? Maybe removal of As, Sb, Cr, Ni, Pb... from water. If so, we have developed a very efficient sorbent for these metals, much better than the commercial materials. Patents are pending and we are looking for producer. We can only send sample after the non-diclosue-agreement is signed. Regards, Dusan