The use of color dyes in modern society presents a great challenge to the environment. Thus, extensive studies have been conducted in the last 30 years on the removal of color dyes from aqueous solutions such industrial wastewater. In this study, the removal of alizarin red S (ARS), an anionic dye, from solution by raw calcite (Cal) and heat-treated calcite (HCal) was conducted and compared under different physico-chemical conditions. Based on the isotherm study, the ARS removal capacities increased from 167 to 251 mmol/kg after the Cal was heated to 1000 °C for 3 h. The X-ray diffraction analyses showed no difference in the calcite phase between Cal and HCal after ARS sorption. Fourier-transform infrared results also showed no change in the calcite phase after ARS sorption, except a slightly increase in wavenumber from 713 to 727 cm⁻¹ for the OCO bending of HCal at high ARS sorption levels. SEM observations showed about the same particle size and morphology before and after ARS sorption. The TGA data showed the formation of CaO after Cal was heated, and CaO converted back into calcite after being in contact with water or ARS solution for 24 h and then being air-dried. Thus, the high ARS removal could be due to CaO produced after Cal being heated. The findings from this research proved that there is great potential in the use of calcite, a low-cost and readily available Earth material, after heat treatment for the removal of contaminants from water.

Significant increase in use of color dyes in modern society exerted a great pressure on environmental and water qualities. As such, studies for the removal of color dyes from water have been conducted extensively. In this study, common Earth materials dolomite and calcite were evaluated and contrasted for their removal of toluidine blue (TB), a cationic dye, before and after heat treatment. An increase by a factor of 3 in TB removal capacity from 3.5 to 10 mmol/kg was achieved after dolomite (Dol) was heated to 800 °C (designated as HDol). In contrast, the TB removal capacity increased by more than 100 times from 2 to 220 mmol/kg after calcite (Cal) was heated to 1000 °C (designated as HCal). For Dol and HDol, the TB removal increased as the solution’s pH increased but decreased with increases in the solution’s ionic strength. For Cal and HCal, the influence of the equilibrium solution’s pH and ionic strength on TB removal was negligible. The free energy of TB sorption on Dol, HDol, and Cal were −7 to −15 kJ/mol. The results suggested different removal mechanism for TB by Dol vs. Cal. X-ray diffraction data for Dol showed a slight increase in calcite content after heat treatment. For Cal, CaO was produced after heating, which converted back to calcite after 24 h of mixing with TB solutions. The significant TB removal by HCal could be attributed to its reaction with CaO. Thus, the best solution is to use freshly treated Cal for the removal of TB from solution.