Optimization, characterization and adsorption properties of natural calcite for toxic As(III) removal from aqueous solutions

The potential of natural (N) and magnetized (M) forms of different rocks (calcite and dolomite) and clays (bentonite, kaolinite, and hematite) were evaluated for the removal of As(III) from aqueous solutions, in order to optimize and scrutinize the most suitable adsorbent. The order observed for efficiency of As(III) removal was N-calcite > N-dolomite > M-calcite > M-dolomite > N-bentonite > M-bentonite > M-kaolinite > N-kaolinite > M-hematite > N-hematite. On the basis of this analysis, natural calcite was further selected for in-depth analysis of various adsorption parameters such as time, pH, temperature, dosage, and concentration of As(III) ions. An excellent adsorption capacity of 19.05 mg‧g−1 was displayed by calcite for As(III), which was significantly higher than that previously reported for other studies. The characterization of adsorbent calcite was performed by various analytical techniques:XRD, XRF, FTIR, SEM-EDS, TG/DTA and PZC. The kinetic investigation revealed that the adsorbent successfully removed over 98% of As(III) from aqueous solution after 160 minutes of equilibration time. The adsorption data was well fitted to the Freundlich isotherm model and fairly described by the pseudo-second-order mechanism. The mean energy of adsorption (E) determined from the Dubinin-Radushkevich (D-R) model was less than 8 kJ‧mol-1 , which indicates a physical adsorption process. The calculated thermodynamic parameters revealed that the adsorption of As(III) by calcite is endothermic, favourable and spontaneous.