Kinetic, isotherm, and thermodynamic studies for removal of direct red 12b using nanostructured biosilica incorporated into calcium alginate matrix

This article presents the immobilization of nanostructured biosilica within calcium alginate biopolymer and its potential for the adsorption of Direct Red 12B (DR12B) as an azo dye in batch flow mode reactors. The immobilization of biosilica was performed well according to the images of scanning electron microscopy. Moreover, the results of Fourier transform infra-red analysis confirmed strong binding of dye molecules by surficial functional groups. The adsorption process was pH-dependent and increasing initial pH from 3 to 11 led to the decrease in adsorbed dye from 16.02 to 11.88 mg/g, respectively. Increasing adsorbent dosage from 0.5 to 4 g/L led to decreasing the adsorption of DR12B from 75.13 to 12.10 mg/g, respectively. Among various isotherm models, Langmuir isotherm had the highest correlation coefficient (R² = 0.9924) with a maximum adsorption capacity of 33.78 mg/g. The value of mean free energy (E = 15.81 KJ/mol) related to Dubinin-Radukevisch (D-R) model demonstrated chemical nature of the adsorption process. The results of kinetic study revealed the fitness of experimental data to pseudo-second order model. The negative values of Gibbs free energy (ΔG) and enthalpy change (ΔH) suggested a simultaneous and exothermic adsorption process, respectively.