Adsorption of methylene blue on comminuted raw avocado seeds: Interpretation of the effect of salts via physical monolayer model

In this work, the effect of salts on the adsorption of a textile pollutant (Methylene blue: MB) on comminuted raw avocado seeds (CRAS) was interpreted via a physical monolayer model (PMM). To this aim, ten MB adsorption isotherms were determined at 25 °C and pH 10 in the presence of the next salts: NaCl, Na2CO3, Na2SO3, NaHCO3, C6H5Na3O7, NaHSO3, Na2B4O7·10H2O, Na2S2O3, NaNO2 and Na2HPO4. Under the same experimental conditions, a further adsorption isotherm was also determined without any salt addition and taken as reference data. In order to achieve a theoretical interpretation of the experimental data and to address the effect of salts on adsorption mechanism, a physical monolayer model (PMM) was applied to all the retrieved data. The model showed that Na2CO3, NaHCO3, Na2B4O7, Na2S2O3, and NaNO2 salts have no effect on the adsorption of MB dye, while the presence of NaCl, Na2SO3, C6H5Na3O7, NaHSO3 and Na2HPO4 salts in the solution caused a slight decrement of MB adsorption capacity due to a competitive effect between MB dye and salts on CRAS adsorption site. The modeling analysis showed that MB was adsorbed with a mixed parallel and non-parallel orientation on CRAS adsorbent surface. Moreover, the adsorption energies varied from 19.44 to 22.36 kJ/mol in tested systems. Adsorption energy values revealed that the process evolved via a physical adsorption, regardless the salt present in solution. All the investigated salts did not functionalize the CRAS adsorbent surface and also did not change the interaction energy between MB molecules and the adsorbent receptor sites. The interpretation of all the model parameters contributed to explain the adsorption mechanism of MB dye on CRAS. Specifically, the effect of salt on the reduction of MB adsorption capacity seems to be likely due to both steric hindrance effect and to MB molecule aggregation hindrance exerted by the salts, which prevented the formation of subsequent MB adsorbed layers, thus limiting its overall adsorption capacity.