I have some literature reports which suggest that the functional groups present on the surface of the activated carbon get depleted.
Domingues RR, Trugilho PF, Silva CA, et al. Properties of biochar derived from wood and high-nutrient biomasses with the aim of agronomic and environmental benefits. PLOS ONE. 2017;12:e0176884.
Further, the FTIR reports also suggest that there is apparently no functional group present on the surface of activated carbon due to the absence of peaks in the FTIR spectra.
Please explain.
1. The surface functional groups on activated carbon were determined using Boehm's titration , you can find out what kind of functional groups (acidic group like Carboxyl, Lactone, Phenol and basic group ) and how quantitative it is in your active carbon.
2. The Cr (VI) removal at various pH levels can be attributed to the fact that various forms of Cr (VI) such as H2CrO4, HCrO-4, Cr2O-7, and CrO-4 are present in the environment depending on the solution pH. At pH less than 1.0, H2CrO4 is predominant than the other forms of Cr (VI). In pH range of 1–6, HCrO-4 and Cr2O-7 coexist with HCrO-4 predominance in the solution. at low pH, the surface of activated carbon was positively charged due to excess of H+ ions and then electrostatic attraction bond was developed between the adsorbate (HCrO-4) and adsorbent molecule.
for more detail, you can see this article
Article Characterization and application of biomass gasifier waste m...
Abhishek Ajmani ,
Adsorption at the surface of a substrate can be both physical and chemical. Therefore, the adsorption at the surface of activated carbon can be influenced by the ions that are possibly adsorbed onto the activated carbon through hydrogen bonding. Based on the nature and strength of the ions adhering to the activated carbon, the degree of adsorption of chromium will vary.
Anuj Prajapati I have earlier mentioned that the FTIR spectra suggested lack of functional groups owing to absence of peaks. I have attached FTIR spectra for your reference.
Abhishek Ajmani After seeing your FTIR results, I conclude that your result was good before and after chromium treatment. Raw SSSP shows various peak due to the presence of several functional groups and after treatment peak position or intensity was shifted or changes due to chromium treatment. Due to the activation process, many functional groups that were previously found in raw SSSP were lost. So that's reason lack of peak present in your SSAC but main groups functional groups that are directly responsible for chromium treatment were present (like 3357cm−1 may OH group, two peaks at 2925, 2855 cm−1 may be attributed to symmetric, asymmetric –CH2 stretching and so on ). After chromium treatment, a new peak was found in your (SSAC+Cr) activated carbon at 497 which shows Cr-O bond. that was confirmed that adsorption of chromium has occurred.
Anuj Prajapati Thank you for analyzing the results. Do you have any reference literature for the results, you have suggested? Please provide the literature in support of your analysis. Thanks in advance.
Dear Abhishek,
pH affects the speciation of Cr in solution which you can model using software such as PHREEQC (https://www.ndsu.edu/webphreeq/) and Visual Minteq (https://vminteq.lwr.kth.se/). I did not inspect the FTIR spectrum myself but based on Abhishek Ajmani his observation of the OH group, it is likely to belong to an aromatic compound (e.g. phenol), which does not dissociate practically speaking. The C-H2 is also not part of a functional group. For further reading I recommend (Article Fourier Transform Infrared (FTIR) Spectroscopy of Biological Tissues
;Chapter Interpretation of Infrared Spectra, A Practical Approach
). Since suitable functional groups, if present at all, were below the detection limit of FTIR, this suggests that adsorption was mainly governed by physisorption as mentioned by Dominique Frances Hoover, although chemisorption cannot be ruled out if the peak at 497 cm-1 belongs to C-Cr. It may also be conceivable that a chelating agent was present in the solution used for the adsorption experiment. If the chelating agent contains dipoles indirect adsorption of Cr via physisorption of the chelating agent is also a possibility.Kind regards, Rob.
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