New insights into the surface oxidation role in enhancing Congo red dye uptake by Egyptian ilmenite ore: Experiments and physicochemical interpretations

Abstract

The continuous discharge of organic dyes into the aquatic environment represents a main reason for water contamination. Removal of dye molecules via adsorption is suggested to be an effective, simple, and low-cost technology. Herein, two representative samples of black ilmenite and its oxidized red form were characterized and tested as adsorbents for Congo red (CR) dye. The high concentrations of iron-titanium oxides in the red ilmenite was due to the surface alteration. The red ilmenite displayed high adsorption capacities against CR compared to the black one at varied pH values (i.e. 5.0–9.0). The Freundlich model described well the CR data at all adsorption temperatures (25, 40, and 50 °C). The physicochemical parameters of the multilayer model were employed to recognize the interface mechanisms between CR molecules and the ilmenite active sites. The values of receptor sites (n) ranged from 0.60 to 0.91 (i.e., n < 1.0 at all temperatures) suggesting a horizontal position and multi–docking adsorption mechanism. In the temperature range of 25–50 °C, the increase of the total removed CR layers (the Nt parameter) from 2.9 to 4.4 resulted in enhancing the adsorption capacity at saturation (Qsat) from 158.23 to 267.96 mg/g. The values of adsorption energies varied from 13.26 to 18.356 kJ/mol reflected that CR adsorption onto the RI was endothermic and associated to physical interactions. Overall, the high performance of the red ilmenite was mainly associated with the control of surface alteration. Furthermore, utilizing the multilayer statistical model described the interaction mechanism between CR and ilmenite adsorption sites at molecular level.