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Abstract
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)^sub 6^^sup 2+^) and chloro- (CdCl^sup +^ and CdCl^sub 2^) complexes were responsible for Cd(II) adsorption onto humin. Results showed Cd as Cd(II) and Cd(H2O)^sub 6^^sup +2^ was water soluble at 2 < pH < 7; with a portion of the soluble Cd precipitating as Cd(OH)^sub 2^. The Cd(II) rate constant (k) in freshwater was 0.5 × 10^sup -3^ (min^sup -1^), occurring in a single phase, while in seawater fast and slow phase values for k were 31.88 × 10^sup -3^ and 6.2 × 10^sup -3^ (min^sup -1^), respectively. The adsorption curves showed a better fit with the Langmuir than the Freundlich model.[PUBLICATION ABSTRACT]