Pore Engineering and Competitive Kinetics for Selective Dye Adsorption on γ‑Fe₂O₃ Nanoparticles

Adsorption process can be utilized to selectively separate the dyes from the medium based on the nature of charge and size of dye molecules. The separation of dyes having similar charges and sizes is still an intricate problem. Here, we present the competitive adsorption studies of cationic and similarly sized methylene blue (MB) and safranin O (SO) systems on porous γ-Fe₂O₃ nanoparticles. To demonstrate the relation between pore size and selectivity, γ-Fe₂O₃ nanoparticles were lixiviated using a citrate buffer and the lixiviated samples were subjected to further pretreatment methods before dye adsorption studies. The pretreated samples exhibit enhanced pore size, which results in higher adsorption capacity for both MB and SO dyes. The selectivities for MB exhibited by pretreated samples with average pore sizes of 3.4 and 2.2 nm are 10% and 22%, respectively. On the other hand, MB, despite being less preferred than SO from a thermodynamic point of view and possessing a relatively larger size, has a higher pseudo-second-order adsorption kinetic constant. Provided an accommodating pore size in γ-Fe₂O₃ nanoparticles, by virtue of greater mass transfer of MB to accessible sites in the pore region, the selectivity of MB is enhanced. While a larger pore is equitable to the dye molecules resulting in the loss of selectivity, we hypothesize that combined factors of pore size requisite in adsorbent, thermodynamics and kinetics of adsorption, and dye molecular size need to be harnessed for a better selectivity in dye adsorption.