Adsorption mechanism of toxic metal ions by attapulgite

A series of adsorption tests were conducted to analyze the sorption capacity of attapulgite. Ions such as cobalt, nickel, lead and cadmium were adsorbed from waste solutions onto attapulgite surface. Adsorption depletion tests were performed as a function of solid to liquid ratio, conditioning time, and heavy metal ion concentration to identify the mode and extent of interactions in the system. Conditioning time data confirm that ion exchange is nearly complete after 30 seconds indicating the fast kinetics of the ion exchange process. Solid to liquid ratio data suggest that optimum ratio is 50g/l. The adsorption isotherms constructed as a function of heavy metal concentration and pH reveal that adsorption of metal ions increases in the order of Co < Ni < Cd < Pb. The attapulgite was found to be rather receptive to the adsorption of heavy metal ions, and fairly high amounts of calcium, potassium, and magnesium ions were desorbed from the attapulgite into the solution. A quantitative analysis of the adsorption results indicates that a one-to-one ion exchange mechanism is responsible for the incorporation of cations into the structure of attapulgite. The results show that attapulgite, similar to sepiolite, is a potential clay mineral for the removal of toxic metal ions from wastewater streams.