Biogenic Calcium Carbonate with Hierarchical Organic–Inorganic Composite Structure Enhancing the Removal of Pb(II) from Wastewater

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.