Fig. 6. Kinetics of Cr oxidation by the coarse clay-size Mn oxides at pH 4. The oxidation tests were conducted at (a) 200 µM and at (b) 400 µM of initial Cr(III) concentration. Data were plotted according to the first-order rate equation (Sparks, 1989). Concentration: K_t = Cr(III) at time t; K= Cr(III) at equilibrium.