ADSORPTION OF INDIGO CARMINE FROM SOLUTION BY NANOSCALE TITANIUM DIOXIDE

Abstract

In this paper, we looked at the physical and chemical properties of titanium dioxide as an adsorbent for indigo carmine dye from water solutions. The starting sample was characterised using SEM and XRD spectroscopy.It was found that the size of titanium dioxide particles is less than 150 nm, the particles are prone to aggregate formation, and the average particle size of the adsorbent according to X-ray diffraction data is 44,20 nm. It was shown that during the first 5 minutes from the start of the adsorbate-adsorbent contact, the adsorption capacity is 0,3 mg/g, and the maximum value of 1,25 mg/g is reached within 90 minutes. The nature of the kinetic curve indicates that adsorption equilibrium occurs within the first 30–40 minutes from the start of contact between the dye molecules and the adsorbent. A further increase in the contact time of the solution with the adsorbent does not contribute to an increase in the adsorption value. It has been established that the probable mechanism of adsorption is intermolecular interaction at the adsorbate-adsorbent interface due to Van der Waals forces and hydrogen bonds between indigo carmine molecules and titanium dioxide surface groups. This mechanism of interaction at the interface is a direct consequence of the pseudo-second-order kinetic model with an initial adsorption rate of 0,082 mg/g·min. It has been shown that the adsorption capacity of the titanium dioxide surface is 1,25 mg/g. The nature of the curve resembles the Langmuir isotherm (type L5), which has a maximum according to the Hils classification.This type of isotherm indicates monomolecular adsorption at the phase boundary, where dye molecules undergo association processes in solution. It has been determined that the adsorption isotherm of indigo carmine is satisfactorily described by the Langmuir model, compared to other models, as can be seen from the correlation coefficient (R2 = 0,623), i.e., the adsorption of dye molecules occurs at homogeneous (uniform) centres of the titanium dioxide surface, where all active centres are energetically homogeneous and only a monomolecular layer of adsorbate can form on the surface. The calculated adsorption energy according to the Dubinin – Radushkevich equation for the adsorbent surface does not exceed 2 kJ/mol, which indicates the physical adsorption of indigo carmine molecules on the surface of titanium dioxide, and the calculation of the Gibbs free energy of the adsorption process allows us to conclude that adsorption at this temperature is a non-spontaneous, non-equilibrium process.