Adsorption of growth suppressor chromium (III) on potassium sulfate crystals in aqueous solution

Adsorption isotherms of chromium (III) a crystal growth suppressor, on the surface of potassium sulfate crystal powder suspended in saturated solutions were measured at 45 degrees C at different constant pH (= 2.5, 3.4 and 5). The higher the pH, the larger amount of chromium (III) was adsorbed. The pH effect on the adsorption was discussed and reasonably explained by a modified Langmuir adsorption model, in which two hydrolysis products of [Cr(H2O)(6)](3+), i.e., [Cr(H2O)(5)(OH)](2+), and [Cr(H2O)(4)(OH)](+) are assumed to be both adsorption active and to be adsorbed independently to each group of active sites on the surface of potassium sulfate crystals. The fraction of the first hydrolysis product, [Cr(H2O)(5)(OH)](2+), present in the solution is numerically shown to decrease as the pH is increased in the pH range of pH greater than or equal to 3 and the surface coverage is also shown to decrease accordingly in the corresponding pH range. Weaker growth-suppression at higher pH is explained by assuming only the first hydrolysis product to be growth-suppression active. The adsorbed amount of the second hydrolysis product (not growth-suppression active) is also shown numerically to increase as the pH is increased. This increase leads to an increase of the total adsorbed chromium (III) in the pH range up to 5. (C) 1998 Elsevier Science B.V. All rights reserved.