Bicomponent adsorption equilibrium of tetracycline and oxytetracycline on rice husk ash: Mathematical estimation of the process

Adsorption is nowadays an efficient technology for the removal of pollutants such as tetracycline (TC) and oxytetracycline (OTC) in water. Using mathematical tools such as modeling allows for a more in-depth understanding of the process to improve it to the point where it can be used in industry. In this work, the behavior of the bicomponent adsorption equilibrium of TC and OTC on rice husk ash and its mathematical estimation was studied. Two groups of data were used in this research: in group 1, the initial concentration of OTC was fixed, and the initial concentration of TC varied, and in group 2, TC was fixed, and OTC varied. Adsorption conditions were C0 = 20-160 mg L-1; pH = original solution; mads = 1.6 g; Vsol = 100 mL; T = 298K; agitation speed v = 300 rpm. The estimation model was of the additive type, which considered one term for each component and the Langmuir and Freundlich adsorption isotherms. The experimental data were processed in R Open software and the parameter used as the goodness of fit was the residual standard error (RSE). The four models obtained from the combination of the two components and the isotherms presented an RSE of less than 0.3, which indicates that the estimation of the adsorption equilibrium in the mixture had high precision. For group 1, the models that best fitted the experimental data were those corresponding to Langmuir-Langmuir and Langmuir-Freundlich interactions, whereas, for group 2 data they were Langmuir-Freundlich and Freundlich-Freundlich. The adsorption behavior in the mixture for TC was of Langmuir type and OTC had a behavior closer to the Freundlich isotherm. The interaction coefficients allowed us to determine that the mixing effect between the components was synergistic. The adsorption equilibrium on a bicomponent mixture of TC and OTC on rice husk ash was estimated with minimal error through a hybrid model.