Cation exchange prediction model for copper binding onto raw brown marine macro-algae Ascophyllum nodosum: Batch and fixed-bed studies

Abstract

The cation exchanger properties of brown marine macro-algae Ascophyllum nodosum for copper separation from aqueous solutions were studied in batch and continuous mode. The total amount of light metals present on the surface of raw biomass was 2.4 mEq/g. The raw macro-algae were converted in different ionic forms achieving a similar binding capacity, indicating that the conversion of seaweeds to different ionic forms consists in an ion exchange process. Carboxylic (approximate to 1.3 mEq/g) and sulphonic (approximate to 1.1 mEq/g) groups were identified as the main functional groups responsible for cations binding. Equilibrium and kinetic experiments were conducted at different pH values using different algae forms. Cation exchange equilibrium model was formulated using a mass action law being able to determine the selectivity coefficients between all ionic species for carboxylic and sulphonic groups. In the fixed-bed column, for four cycles of saturation/elution/regeneration, the operating capacity varied between 0.6 and 0.8 mEq Cu2+/g, treating 27-33 L of influent until the breakthrough point of 0.02 mEq Cu2+/L, corresponding to a service capacity of 301-367 BV. Higher elution efficiency was observed for 3.0% HCl in counter-flow mode and no biomass damage was observed after four elution cycles. A mass transfer model, considering equilibrium given by the mass action law, and a linear driving force model for intraparticle diffusion, was able to predict well the ion exchange process during the saturation and elution steps for all chemical species in the liquid and solid phase. The regeneration step was successfully performed with CaCl2 0.1 M at pH = 8.0 making possible the reuse of the biomass in multiple cycles.