Utilization of diatomite/chitosan–Fe (III) composite for the removal of anionic azo dyes from wastewater: Equilibrium, kinetics and thermodynamics

A novel diatomite/chitosan–Fe(III) composite (Fe(III)/Cs@Dia) was synthesized successfully to use as an adsorbent for the removal of anionic azo dye. The composite was further characterized by SEM, FTIR, XPS, and C 13 ${}^{13}\mathrm{C}$ / Si 29 ${}^{29}\mathrm{Si}$ NMR, and then the adsorption batch experiments were carried out as a function of initial dye concentrations, pH values, contact time and temperatures to determine the adsorption capacities of Fe(III)/Cs@Dia. Compared with several typical isotherm models, the adsorption isotherms were all best-fitted by the Langmuir model, with a maximal 2GL dye uptake of 1250   mg   g −1 at pH 6 and 298   K. In addition, kinetic studies followed the pseudo-second-order model and thermodynamic studies revealed that the uptake was exothermic in nature. The adsorption mechanisms were proposed with observation that electrostatic interaction and complexation played the key roles in adsorption process. Moreover, the composite also showed favorable adsorption properties for other four anionic azo dyes (direct yellow R, congo red, methyl orange and direct red 23).