The efficiency of the red alga Mastocarpus stellatus for remediation of cadmium pollution

This work reports the results of the study for cadmium binding by the dead red macroalga Mastocarpus stellatus. Kinetics sorption experiments demonstrated the high rate of metal biosorption: the system attained over 50% of the total biomass cadmium uptake within 2 min of contact and over 90% in the first 9 min. The kinetic data were successfully described by a pseudo-second order model with rate constants ranging from 1.06 to 10 g mmol−1 min−1, as a function of initial metal concentration and temperature. The equilibrium binding was accurately represented in terms of Langmuir and Langmuir–Freundlich models. The sorption isotherms at constant pH showed uptake values as 0.49 mmol g−1 (at pH 2.4), 0.56 mmol g−1 (at pH 4) and 0.59 mmol g−1 (at pH 6), while the affinity constant values were between 0.6 and 5 mmol−1 L (Langmuir fit). The acid–base properties of the alga were also studied, obtaining the total number of acid groups, 2.5 mmol g−1, and their apparent pK value, 1.56, using the Katchalsky model. Desorption studies were conducted employing different HNO3 concentrations and desorption times.