Characteristics of adsorption interactions of cadmium(II) onto humin from peat soil in freshwater and seawater media.

The present study examined Cd solubility in pH 2-12 fresh and seawater media with and without humin to determine Cd species composition. The study, based on the Langmuir-Hinshelwood kinetics model, was conducted to determine the kinetic parameters of Cd(II) adsorption onto humin. We employed the Langmuir and Freundlich models to derive thermodynamic parameters. Aquo (Cd(H2O) 6 (2+) ) and chloro- (CdCl(+) and CdCl2) complexes were responsible for Cd(II) adsorption onto humin. Results showed Cd as Cd(II) and Cd(H2O) 6 (+2) was water soluble at 2 < pH < 7; with a portion of the soluble Cd precipitating as Cd(OH)2. The Cd(II) rate constant (k) in freshwater was 0.5 × 10(-3) (min(-1)), occurring in a single phase, while in seawater fast and slow phase values for k were 31.88 × 10(-3) and 6.2 × 10(-3) (min(-1)), respectively. The adsorption curves showed a better fit with the Langmuir than the Freundlich model.