The state and adsorption behaviour of traces of cadmium in aqueous solutions

The state of trace cadmium in aqueous nitrate solutions has been analysed by dialysis, centrifugation and electromigration. It has been found that Cd2+ ions prevail in solutions of pH < 5, whereas colloidal forms are present at higher pH, together with the ionic or molecular products of cadmium hydrolysis. The nature and abundance of the colloids depends on the concentration of cadmium. Mostly pseudocolloids are formed in greater than or equal to 5 x 10-7 M solutions due to adsorption of Cd(0H)2 and Cd0H+ on foreign colloidal impurities. At 5 x 10-6 M polymerisation of the hydrolytic products probably takes place with formation of colloidal cadmium hydroxide, or basic salts. Adsorption of traces of cadmium has been studied on glass and plexiglass as a function of pH, age and composition of the solution and concentration of cadmium. Probable mechanisms of adsorption have been proposed. (author)