An optimum pH of 5.0 for the adsorption of Cr 6ϩ by chitosan was determined by using a stirred-batch reactor method at constant pH. When a column containing chitosan was used to bind Cr 6ϩ in a situation where pH could not be held constant because of pH changes caused by the chitosan itself, significant binding occurred only at solution pH 1 and 2. When chitosan was pretreated with sulfuric acid in a range of 7-70 mol % sulfuric acid : moles glucosamine residue, maximum binding occurred at pH 6.0. Under these conditions, a column containing 0.500 g acidtreated chitosan (35% mole ratio) reduced the concentration of Cr 6ϩ in 713 bed volumes of 25 ppm Cr 6ϩ solution to Յ5 ppm in the effluent. A similar column of pretreated chitosan reduced Cr 6ϩ concentration in 1042 bed volumes of industrial chromium plating rinse water initially containing 18 ppm Cr 6ϩ to Յ5 ppm. Capacity experiment results indicated 60 mg chromium bound per gram of treated chitosan at pH 6.0. Commercial resin IRA-67 was also investigated as a Cr 6ϩ binding agent.
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