An optimum pH of 5.0 for the adsorption of Cr6+ by chitosan was determined by using a stirred-batch reactor method at constant pH. When a column containing chitosan was used to bind Cr6+ 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 Cr6+ in 713 bed volumes of 25 ppm Cr6+ solution to :55 ppm in the effluent. A similar column of pretreated chitosan reduced Cr6+ concentration in 1042 bed volumes of industrial chromium plating rinse water initially containing 18 ppm Cr6+ to less than or equal to 5 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 Cr6+ binding agent. (C) 2004 Wiley Periodicals, Inc.