Kinetics and equilibrium studies from the methylene blue adsorption on diatomite treated with sodium hydroxide

• The purified diatomite presented high surface area and well porous structure.
• The kinetics data fitted well to the pseudo-second order model.
• The equilibrium data followed Langmuir isotherm models better.
• Thermodynamic data indicated adsorption process was endothermic and spontaneous.
• Diatomite purified by NaOH was an effective adsorbent to remove basic dyes.

Diatomite was treated with sodium hydroxide to remove impurity in order to improve its performance as an adsorbent. The raw diatomite and purified diatomite were characterized by scanning electron microscopy, energy dispersive X-ray analysis and Brunauer–Emmett–Teller adsorption. It was found that the surface area was in order of 15.87 m2 g− 1 for raw diatomite and 31.35 m2 g− 1 for purified diatomite. Scanning electron microscopy images showed the well-developed porous structure of purified diatomite. Purified diatomite improved a more than tenfold increase in adsorption amount from 1.72 mg g− 1 to 18.15 mg g− 1 and removal efficiency from 8.60% to 90.75% for methyelen blue initial concentration 100 ppm respectively. The kinetics studies showed that experiment data followed pseudo-second-order model better. The equilibrium data was fitted to Langmuir and Freundlich adsorption isotherms and was found that Langmuir model presented the best fit, showing maximum monolayer adsorption capacity of 27.86 mg g− 1. The thermodynamic parameters such as the standard enthalpy, standard entropy and standard free energy were evaluated. The obtained results indicated the adsorption of methylene blue onto diatomite treated with sodium hydroxide is endothermic and spontaneous process and confirmed the applicability of this purified inorganic material as an efficient adsorbent for basic dyes.