Removal characteristics of Pb(II) and Cd(II) from aqueous single metal solutions by vermicompost combined with nano-CaO

Vermicompost (CV) and nanomaterials have been identified as potential amendments to adsorb environmentally toxic heavy metals. This study analyzes multi-aspect properties of CV and its nanocomposite (CV-nCa) with nano calcium oxide (nCaO), which include pH, electrical conductivity (EC), cation exchange capacity (CEC), ash content and surface characteristics, aiming to compare their potential for removing aqueous Pb(II) and Cd(II). The results demonstrated that CV-nCa had a higher pH, EC, CEC, ash content, pore parameters and specific surface area than CV. Adsorption isotherm studies showed that CV-nCa had a higher adsorption capacities for Pb(II) and Cd(II), with the maximum adsorption capacities being 255.1 mg/g and 137.7 mg/g. The adsorption rates (ARs) by CV-nCa were both above 99.72% within the initial concentration of Pb(II) being 200-800 mg/L and Cd(II) being 100-200 mg/L. In addition, CV-nCa can effectively remove Pb(II) and Cd(II) within a wider pH range (3-6) and a lower mass concentration (MC) of 4 g/L. Kinetic studies showed that the Pb(II) and Cd(II) adsorption by CV-nCa were both best fitted with the pseudo-second-order model. FTIR analysis demonstrated that the interaction of CV and CV-nCa with Pb(II) and Cd(II) mainly depended on thearomatic/aliphatic acids, some primary/secondary alcohols and some silicate and carbonates. XRD analysis showed that the addition of nCaO into CV mainly increased CaCO3 content and affected the adsorption characteristics. Thus, combining nCaO with vermicompost may broaden heavy metal remediation and vermicompost utilization in the environment.