We used three different methods in the preparation of activated carbon from cadena de amor (Antigonon leptopus Hook & Arn.) stems, namely: pure carbonization (PC), carbonization before activation (CBA), and activation before carbonization (ABC). The samples were carbonized at 500 degrees Celcius for three hours, and were activated using sulfuric acid. As per the CBA and ABC samples, the adsorption capacities were in accordance to the results of the positive control (bamboo activated charcoal), wherein the concentrations decreased from initial concentration of 60 ppm of potassium chromate aqueous solution after adsorption . However, the PC samples resulted in increase of concentration from 60 ppm to around 70-80 ppm after adsorption.
Additionally, SEM micrographs showed that PC had no pores, while the other treatments showed evident and noticeable pores.
a. Could this be used in explaining the yields of adsorption?
b. Also, what could be the possible explanations behind this (increase in concentration after adsorption using PC)?
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Thank you.
There must be some contamination from some sources. If we consider also that PC didn't adsorb chromium ions at all still initial conc. of metal ion soln. can't get enhanced. I wonder whether the original plant from which activated carbon was prepared was itself previously contaminated with chromium. What was the pH at which experiments were conducted ? And which chromium were you experimenting?There is a chance that previously contaminated chromium ions may get desorbed at acidic pH. You probably have have measured total chromium. So enhanced Cr conc may be either a combination of Cr(III) & Cr(VI) or Cr ions in metal soln. plus desorbed chromium ions from sorbent.
Thank you very much. We will re-conduct the set-up for PC following your recommendations.
The law of mass conservation must be fulfilled. Thus, two situations are possible: either less likely, i.e. contamination of the adsorbent surface by the adsorbat (there would have to be a concentration difference and conditions conducive to desorption), or more likely, i.e. adsorption of some water with simultaneous lack of affinity of the adsorbent to the adsorbate.
Please check the porosity by porosimeter (N2 Physisorption). If there is no porosity, then carbonisation has not occured. Secondly try to measure Cr directly in solution using techniques like ICP or AAS & also measure Cr in PC to conclude.
What was the methodology used for the determination of the chromium concentration? Is it selective to chromium?
Depending on the procedure used you could have a false positive from the water-soluble organic compounds (WSOC) present in the surface of the carbon from an incomplete carbonization.
Has the PC passed through any washing step?
SEM images would not show the pores with they are to small (nanometric scale). The ideal is to check the porosity via N2 or CO2 adsorption as stated above.