I am working on adsorption of organic molecules on titanium dioxide nanoparticles, using cerium sulfate to oxidase the molecules. I noticed that increasing the concentration the qe values goes to negative numbers. What is the meaning? Are the nanoparticles being dissolved?
TiO2 is hardy soluble. That is not the reason.
The process you describe is not an adsorption process, if you do an oxidation process you have a decompostion of the molecules.
I think you should rethink you experiment, and your measuring approach for the concentration of the organic compounds.
Btw. qe ist not the capacity, qe ist the loading in equilibrium state, the best way to obtain the capacity is by a Langmuir isotherm calculation, but be sure the you are also in the saturation range of the adsorbent.
Dear Thomas Mayer-Gall many thanks for your answer!
Let me try to give more details, I am studying the adsorption of few bisphosphonates on TiO2, I left the molecules interacting with TiO2, then I centrifuge and remove an aliquot. This aliquot I allow to react with cerium sulfate, then I measure the absorbance at 320 nm. I already did the calibration curve, my problem occurs when I try to measure how much bisphosphonate adsorbed on TiO2. Initial concentration - final concentration in few cases is lower than 0.
If the final concentraion goes to zero or below, then you would have no change to the the adsorbance?
Following qe=((Ci-Ce)*v)/m I am obtaining values of qe lower than zero, since the only value which can change is Ce, it is looks like that after adsorption I have a concentration higher than at the beginning of the experiment. How could be it possible?
If Ci - Ce is 0 then you got no adsorption, if Ce is bigger then Ci this result in in the measurment error.
Leonardo Dias, In addition to specifying the adsorbent, you should also specify the adsorbate. It is also important to know whether the oxidation takes place before or during adsorption. Moreover, only one Ce value corresponds to initial concentration of Ci, and not several. Ce is the equilibrium concentration of adsorbate, not the current concentration.