Vaterite calcium carbonate for the adsorption of Congo red from aqueous solutions

• Vaterite calcium carbonate as adsorbent for removal of Congo red.
• Ethylene glycol (EG) is a stabilizer to produce spherical vaterite CaCO3.
• Vaterite CaCO3 has a positive surface charge at pH 5–9.
• Vaterite CaCO3 adsorbed up to 16.6 mg/g of Congo red from aqueous solution.

The present study describes the evaluation of vaterite calcium carbonate (CaCO3) for the adsorption of Congo red (CR) from aqueous solution. Vaterite CaCO3 is produced via a precipitation method assisted by ethylene glycol (EG), which acts as stabilizer to prevent transformation of vaterite to others polymorphs of CaCO3. The crystal structure of vaterite CaCO3 formed was proven by SEM, XRD and FTIR. The highest zeta potential of the vaterite CaCO3 was 15.2 mV, which was obtained at pH 5. Batch adsorption studies illustrated that the adsorption of CR to the vaterite CaCO3 was dependent on different parameters, such as pH, initial dye concentration and temperature. The adsorption isotherm data were highly consistent with Langmuir isotherm model, while the adsorption kinetics data fitted better to the pseudo-second-order model. The maximum adsorption capacity of the CaCO3, 32.60 mg/g, was obtained from the adsorption kinetic experiment under adsorption conditions, pH 5, adsorbent dose 0.2 g, adsorbate concentration 100 mg/L and temperature 25 °C. The adsorption of CR on the CaCO3 was proven to be endothermic and non-spontaneous in nature.