Is it necessary to conduct adsorption isotherm with optimum parameter or we can conduct it with constant pH, time and dosages with varying concentration?
It is Ok, to the best of my knowledge, to determine adsorption isotherms at a controlled pH (and an ionic strength, if important), a given dosage and with varied concentrations of a sorbate. Obviously, a selected dosage has to guarantee that the meaningful part of a sorbate is removed from solution during adsorption, and the remaining concentration is measurable, e.g., a 20-80% removal range. However, there may be also "a dosage effect" representing either interactions between sorbent particles/surfaces or "a sorbent dissolution" (a release of some material) such that the measured adsorption isotherms become unpleasantly "dosage-dependent". I would add more: if (and it is always so), the final use of the adsorption isotherm data will be applying the equilibrium physico-chemical models and the interpretation of model parameters [there are almost no papers without that], the adsorption kinetics have to be examined. Moreover, adsorption kinetics can be dependent on a specific batch parameters such that the tests are desirable at different solute concentrations. And, finally, even adsorption kinetics do not necessarily show that a full equilibrium is reached. The desorption tests if show the lack of hysteresis will convincingly prove that the equilibrium models are applicable. Unfortunately, a vast majority of solution adsorption studies deals with the data for which no real proofs for attainment of equilibrium are provided or, which is worse, a hysteresis is clearly demonstrated, and the equilibrium models are used without a shadow of doubts. Which suggests future problems in understanding and model interpretations.
Generally, with dosage at particular concentration you can optimize the dosage amount. Then, you can vary solution concentrations, pH, temperature of the solution, at the optimize dosage to obtain adsorption isotherms. Even, with varying dosage concentration at particular concentration you can draw adsorption isotherms also with changing other parameters. Once, you have all the parametric studies, you can run one final experiment at optimum parametric values to specify the uptake capacity.
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