Sorption and desorption of zinc and cobalt in different soils of West Bengal, soil clays and humic acids

Sorption of Zn and Co was carried out on four West Bengal soils and their clays and humic acids. Adsorption was measured from aqueous solution at pH 6.0 without any background electrolyte. The adsorption data conformed to Langmuir equation enabling computation of adsorption maxima and energy constant. Removal of mobile humic matter by alkali extraction resulted in a decrease of sorption by 34 to 80 per cent. Both humic acids and clays adsorbed the metals in amounts much below their intrinsic CECs. Zinc was adsorbed to a much lesser extent than Co by both clays and humic acids. A considerable percentage of sorbed Zn and Co was desorbed in presence of electrolytes such as NaCl, KCl, MgCl2 and CaCl2. The desorbing efficiency of the electrolytes decreased in the sequence: CaCl>MgCl2>>KCl>NaCl. Much higher desorption by bivalent cations, Ca2+ and Mg2+ suggested that the Sorption was predominantly non-specific i.e. electrostatic in nature.