we found that the pseudo-first-order model can be described the adsorption process of methylene blue.
What does Pseudo-first-order kinetics model indicate in liquid-solid adsorption system?
That word Pseudo can mean many things. It probably means it is modeled well by 1st order kinetics -but it isn't actually 1st order kinetics,
As Rick Manner said: it is well described by a first order kinetic model, however it is not. In your case I would say it's because only the concentration of methylene blue is important to describe the adsorption and the "concentration" of active adsorption sites is approximately constant.
as you know, to adsorb sth, firstly mass transfer through diffusion occurs, then substrates arrives to the surface of adsorbent, theb Knudsen diffusion takes place and finally adsorption carries out. this cycle will continue in desorption and in order to find a rate equation, all mass transfer rates of inter/intra particle diffusion and adsorption should be considred simultaneously. then you should find determining step in this cycle and propose a observed/global rate equation through fitting experiments with model.
How many more people will ask this question, and how many more people will be infected with the "pseudo" absurdities of Ho and McKay. PFO is in fact an exponential equation (explicit form) or logarithmic equation (implicit form) with two lumped parameters. Good agreement of these equations with single kinetic curve means nothing. Regards,
I see Miroslaw has expressed his feelings about that word "pseudo". I will take it a step further. Most "kinetic" models are demonstrably wrong. This is very easy to prove because frequently studied chemical reactions typically have 10 or more different articles with different proposed chemical reaction kinetics and at most one of them can be correct. But the academic community likes to see correlations in this form so researches keep producing them. There is nothing wrong with that as long as you understand that this type of "models" are not revelations about the underlying nature of the chemical reaction - they are just correlations that fit the data over the range of conditions where they were developed that sometimes can be cautiously extrapolated a small distance outside of that range.
Rick, It is not first time I "expressed my feelings" about the term "pseudo". Currently, PFO, PSO etc. it is a standard in the description of batch adsorption kinetic data, without which a given paper will not receive a positive assessment. Unfortunately, many use these equations completely without thinking, because others do. Regards,
In short, the concordance of the experimental results with the pseudo-first order equation indicates that the adsorption process requires a single binding point.
A good agreement of the experimental results with the pseudo-first order equation (in fact exponential equation) it is not related to the adsorption mechanism. Regards,
Hello Miroslaw in order for the academic community to learn can you please inform us about what should be rightly represented or refer to journals or how best this model is used or can be use.
Thank you
Dear Dr Ndifreke Etuk Williams, I have repeatedly written about the exponential equation on RG. There are situations where the compatibility of this equation with experimental points allows to state that the process is of the first-order. There are also situations where this equation can only be regarded as empirical. These, unfortunately, prevail. Regards,
For some reason the academic community likes to pretend they understand the fundamental kinetic relationships that describes any given reaction. Clearly they do not because most widely studied reactions all have had at least 5 different "kinetic" models proposed that describe some portions of the reaction vs time curve.
The engineering community understand that even though we are using a "kinetic" form of an equation the relationship developed is a "correlation" that only applies within a limited range of conditions. As a result they are quite cautious if/when they extrapolate beyond this well understood range of conditions.
Rick, it's hard to disagree with you. However, it depends on the goal you set. Sometimes we want to approximate a few points with the chosen type of function, and then we try to pretend that it is a great science. Another time, we perform a wide kineic study (various initial concentrations, temperatures, pressures, flows etc.) and as a result we also have one equation. Only that the latter can be called a kinetic equation, and the first one can not. Regards,
Miroslaw, It is still just a curve fit or at best a correlation unless it still gives the correct answer when you change operating condition to a point significantly outside of the original data range and it still works well.
Rick, Every equation should be verified in industrial conditions. However, this is not always possible, at least for cost reasons. Research may also concern a new catalyst. Regards,
Utilising experimental design for choosing combinations of initial concentrations, adsorbent dosages, particle sizes for a range of adsorption times like a Central Composite Design Matrix or Box Behnken Design while enable the consideration of a wide range of factors required for proper kinetic study
Simulations studies utilizing ASPEN Adsorption with the optimal conditions for adsorbent dosage (bed height), flowrates, influent concentration determined empirically can give the upper limit of adsorbent bed performance on industrial scale.
You need to survey on Adsorption mechanism. Adsorption can be physical or chemisorption and psuedo first or second order reaction has its meaningful concept.
Why I write many times on RG that the description of adsorption kinetic data using the so-called PFO an PSO, which is actually using two empirical functions, is worthless from a scientific point of view. Maybe it's worth ending up disseminating this Ho and McKay nonsense.
Dear Dr. Grzesik
Is it possible for you to prove your notes " the description of adsorption kinetic data using the so-called PFO an PSO, which is actually using two empirical functions, is worthless from a scientific point of view."?
I appreciate your notice
Yadollah Tavan, The answer is in my post. Both functions are empirical and therefore their constants k1 (PFO) and k2 (PSO) are lumped parameters with no physical meaning. You will also find the answer in many of my comments on the publications posted on RG.
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