Utilization of desilication products as efficient adsorbents for the removal of basic fuchsine

Desilication products (DSPs) are one of the main components of bauxite residue, which is currently discharged without further usage. The present study reports on the use of DSPs as adsorbents for basic fuchsine dye. Using artificial spent liquor, we synthesized not only neat DSP but also solids in the presence of various organics, to account for their likely occurrence during the Bayer process. The physico-chemical properties of all DSPs are similar, except for the specific surface area (SSA), which decreases as the organic content in the final product increases. Further, we compared the adsorption characteristics of DSP to those of Y-type faujasite (FAUY). Strikingly, both time-dependent adsorption measurements and adsorption isotherms showed that DSP, despite its 28-fold smaller SSA, binds 4-5 times more dye molecules. Computational modelling for sodalite (as a model for DSP) and FAUY indicates not only more favourable adsorbent-adsorbate interactions, but also more available free Si-OH sites for binding of fuchsine in the case of sodalite. Finally, we find that organic impurities present in the Bayer liquor do not alter the adsorption capacity of neat DSP to any significant degree; therefore, this adsorbent tolerates numerous organic contaminants without decreasing its affinity to the binding of fuchsine.