Marble Waste Sludges as Effective Nanomaterials for Cu (II) Adsorption in Aqueous Media

This study evaluated the waste generated by a Spanish marble-producing company as adsorbent for the removal of copper (Cu [II]) from aqueous media. Six marble waste sludge samples were studied, and the following operational parameters were analyzed in discontinuous regime, including pollutant concentration, pH, temperature, nature of aqueous medium, and ionic strength. The applicability of the adsorbent material was assessed with experiments in both continuous and discontinuous regimes under close-to-real-life conditions. A pseudo-second order model yielded a better fit to the kinetic data. Application of the intraparticle diffusion model revealed two well-differentiated adsorption stages, in which the external material transfer is negligible and intraparticle diffusion is the controlling stage. The equilibrium study was better fitted to a Freundlich-type isotherm, predicting elevated maximum adsorption values (22.7 mg g^-1) at a relatively low initial Cu (II) concentration (25 ppm), yielding a highly favorable chemisorption process (n >> 1). X-ray fluorescence study identified calcite (CaCO₃) as the main component of marble waste sludges. According to X-ray diffraction analysis, Cu (II) ion adsorption occurred by intercalation of the metallic cation between CaCO₃ layers and by the formation of surface complexes such as CaCO₃ and Cu₂(CO₃)(OH)₂. Cu (II) was more effectively removed at medium pH, lower temperature, and lower ionic strength of the aqueous medium. The salinity and dissolved organic matter in surface, ground-, and waste-waters negatively affected the Cu (II) removal process in both continuous and discontinuous regimes by competing for active adsorption sites. These findings demonstrate the applicability and effectiveness of marble-derived waste sludges as low-cost and readily available adsorbents for the treatment of waters polluted by Cu (II) under close-to-real-life conditions.