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Cadmium removal by bioclastic granules (Lithothamnium calcareum): batch and fixed-bed column systems sorption studies

ABSTRACT The potential of Bioclastic Granules – BG (calcium-carbonate-based material) using the algae Lithothamnium calcareum as sorbent for the removal of Cd(II) from aqueous solutions by sorption was evaluated through batch and continuous systems tests using a fixed-bed column. Sorption process variables, in particular pH (2–7), particle size (<38–300 μm), initial BG concentration (0.1–1.0 g L−1), initial Cd(II) concentrations (5–400 mg L−1) and contact time (5–240 min), were evaluated. Adsorption isotherm profiles of Cd(II) per BG were similar to an L-type, or Langmuir type, with the adsorption forming a monolayer of approximately 0.61 μm, with a qmax of 188.74 mg g−1 and kL of 0.710 L mg−1. Thomas’s model considers that sorption is not limited to a chemical reaction but is controlled by mass transfer at the interface. In the present study, the obtained value of kTh was 0.895 mL h−1 mg−1, reaching a sorption capacity qo of 124.4 mg g−1. For the Yoon–Nelson model, it was possible to obtain two important parameters to describe the behavior of the column, the rate constant (kYN), obtaining a value of 0.09 h−1 and an τ of 82.12 h corresponding to the time required for sorption to occur of 50% of the solute in the rupture curve. X-ray diffraction and scanning electron microscopy analyses coupled to the X-ray dispersive energy system (SEM/EDS) of the BG after the Cd(II) ion sorption tests evidenced the formation of crystals with the prevalence of a new mineral phase (otavite).