No abstract

This work presents kinetic and thermodynamic analysis of the adsorption of three dyes: alizarin, eriochrome blue black R, and fluorescein onto goethite, Co-goethite, and magnetite of medium Brunauer-Emmett-Teller surface area. The adsorption of the dyes on the oxides is fast. Fluorescein exhibits the lowest level adsorption for all adsorbents with goethite being the most efficient adsorbent when milligrams of dye per gram of oxide are considered. The Langmuir isotherm is the most appropriate for describing the adsorption data for the following systems: alizarin-magnetite and eriochrome and fluorescein with the three adsorbents, while alizarin-goethite and alizarin-Co-goethite show better fits with Freundlich isotherm. Molecular modeling using MM2 was used to analyze the steric effects on adsorption.

The adsorption of fully protonated tetracycline (TC) using Na-montmorillonite (Na-MMT) as an adsorbent is studied by DFT calculations. Geometric, electronic and magnetic properties are analyzed. To the best of our knowledge, this is the first theoretical DFT research about Na-MMT as an adsorbent of TC cationic specie. Two Na-MMT models are considered: dry and hydrated Na-MMT. The incorporation of four water molecules in the interlayer space results in the expansion of the d 001 parameter from 10.13 Å (dry) to 12.85 Å (hydrated). Finally, when TC molecule is adsorbed in the basal space of MMT, the increment of the d 001 parameter is about 10 Å respect to hydrated model. Dry and hydrated Na-MMT present a non-magnetic (µ = 0 µ B ) and semiconductor behavior. Nevertheless, the TC-MMT system becomes conductor, and presents a very important magnetic moment (µ = 10.60 µ B ). After TC adsorption in MMT a potential valley with a slight depth in the MMT interlayer is generated. For this reason, it is proposed that TC adsorption would provide active sites for the adsorption of new species (like Na ions and water molecules).