Preparation and characterization of calcium alginate-based composite adsorbents for the removal of Cd, Hg, and Pb ions from aqueous solution

Composite adsorbent materials containing calcium alginate, clinoptilolite, and coal-derived humic acid were prepared. Humic acid (HA), clinoptilolite (CL), alginate (AL), alginate-entrapped humic acid (AL/HA), clinoptilolite (AL/CL), and humic acid/clinoptilolite (AL/HA/CL) samples were characterized. The effectiveness of the samples as adsorbents for the removal of cadmium (Cd), mercury (Hg), and lead (Pb) were studied in a series of batch-adsorption experiments. For the AL, AL/HA, AL/CL, and AL/HA/CL adsorbents, uptake versus time data were evaluated using two kinetic models, a linear and a non-linear pseudo-first-order and a pseudo-second-order model. The data for each metal ion on all adsorbents showed good correspondence with the pseudo-second-order kinetic model. The equilibrium data were fitted to Langmuir, Freundlich, Dubinin-Radushkevich, and Temkin isotherm models. The results show that a non-linear method seems more appropriate for obtaining isotherm parameters. The non-linear Freundlich and Langmuir models for Pb and Hg produced a best fit with high R-2 value (0.99). For HA adsorbent, the equilibrium data for Cd removal better fit to the non-linear Dubinin-Radushkevich isotherm.