I conducted an adsorption experiment of arsenic on soil in the presence of different doses of silicon as competing ions to see the effect of silicon on arsenic adsorption and desorption. I took 5 concentrations of silicon, namely 0, 1, 10, 100 and 1000 ppm. Although adsorption behaviour was correct and in line with what I expected to be in the presence of 0 to 100 ppm of silicon, I was shocked to see that in the presence of VERY high concentration of silicon i.e. 1000 ppm, the adsorption isotherms are showing opposite trends, the Langmuir bonding energy coefficient (Kl) shows the impossible negative value and the Freundlich exponential coefficient too showing values (more than 1). Why is this happening in the presence of a very high concentration of silicon as competing ion?
As soon as you start forming multilayers, neither of these isotherms is suitable for it. Freundlich is a purely empirical low coverage isotherm and Langmuir can handle exactly one monolayer and that's it. For more, BET would be the default choice.
Regarding the backgrounds of this, see the derivation of Langmuir and BET by statistical thermodynamics which you can e.g. find in "Concepts in surface physics" by Desjonqueres/Spanjaard.
Check out for
1.competitive adsorption
2.surface complexion
3.precipitation
4.electrostatic interactions
5.solibilty effects
Hello, very interesting question. What does "the Langmuir bonding energy coefficient (Kl) shows the impossible negative value" mean? Foremost, I would check how this "negative" coefficient was obtained. I hope it was not done by the data linearization (and a respective Langmuir model linearization) but through the direct non-linear data fit. It appears that with the rise of As (which one?) sorption isotherms go up (like in BET? as mentioned above by Jürgen Weippert?). It is not necessarily multilayer adsorption; such BET-like isotherms or the Freundlich models with exponent exceeding 1 may reflect lateral interactions in 'a monolayer" which enhance sorption with rise of As concentration or cooperative changes in a sorbent matrix (like in protein molecules where several molecules or ions interact stronger than each one taken separately). Both mechanisms lead to the Freundlich exponent exceeding 1. However, I understood that the major issue is why it happened at in particular high concentration of the competing anion (silicate?). There might be very many mechanisms difficult to address without knowing details but foremost it is important to know what was pH at high concentrations of "a competitor'? was it constant compared to other systems? pH may affect As speciation, soil organic matter dissolution, concentration of dissolved organic matter in solution phase, ionization of surface groups and precipitation of some components (capable of involving other components due to entrapment). I hope pH was always controlled and the same in the experiments which could help in the identification of mechanisms
- Badges
- Science method
- Similar topics
- Methodology
- Crystallography
- X-ray Diffraction