Calcium carbonate from geological sources (geo-CaCO3, e.g., calcite, aragonite) is used extensively in removing heavy metals from wastewater through replacement reaction. However, geo-CaCO3 has an intrinsically compact crystalline structure that results in low efficiency in pollutant removal and thus its use may produce enormous sludge. In this work, biogenic calcium carbonate (bio-CaCO3) derived from oyster shells was used to remove Pb(II) from wastewater and found to significantly outperform geo-CaCO3 (calcite). The thermodynamics study revealed that the maximum adsorption capacity of bio-CaCO3 for Pb(II) was three times that of geo-CaCO3, reaching up to 1667 mg/g. The kinetics study disclosed that the dissolution kinetics and the rate of intraparticle diffusion of bio-CaCO3 were faster than those of geo-CaCO3. Extensive mechanism research through X-ray powder diffraction (XRD), scanning electron microscopy (SEM), N2 adsorption/desorption test and mercury intrusion porosimetry showed that the hierarchical porous organic-inorganic hybrid structure of bio-CaCO3 expedited the dissolution of CaCO3 to provide abundant CO32- active sites and facilitated the permeation and diffusion of Pb(II) into the bulk solid phases. In addition, Fourier transform infrared spectroscopy (FTIR) study, X-ray photoelectron spectroscopy (XPS) analysis, and the examination of Pb(II) removal ability of bio-CaCO3 after calcination indicated that the organic functional groups of bio-CaCO3 also facilitated the immobilization of Pb(II) into CaCO3 particles, although the major contribution was from the hierarchical porous structure of bio-CaCO3.
Keywords: CaCO3; functional groups; lead; microstructure; porous structure; wastewater treatment.