COMPETITIVE SORPTION OF HEAVY-METAL BY SOILS - ISOTHERMS AND FRACTIONAL FACTORIAL-EXPERIMENTS

zCompeting ions strongly affect heavy metal sorption onto the solid su rfaces of soil. This study evaluated competitive sorption of Cd, Cu, N i, Pb and Zn on three soils: Calcixerollic Xerochrept, Paralithic Xero rthent and Lithic Haplumbrept. Monometal and competitive sorption isot herms were obtained at 25 degrees C. The individual effect of ions on retention of the others was ascertained by a fractional factorial anal ysis design. Most of the sorption isotherms belonged to type L subtype 2 in the classification of Giles. In competitive sorption the initial linear part was shorter and the knee sharper when compared with monom etal sorption isotherms. Parameters related to sorptive capacity, such as Point B, Langmuir monolayer and Freundlich distribution coefficien t, were higher in monometal than in competitive sorption, and in basic soils than in acidic soil. Calcium desorbed at different points of th e sorption isotherms indicated that cationic exchange with Ca was the main retention mechanism in calcareous soils. For Pb, the ratio Ca des orbed/Pb sorbed was close to one; for Cu, Ni and Zn the ratio ranged f rom 1.20 to 1.37, probably due to partial dissolution of calcium carbo nates by hydrolytic processes during retention. On the other hand, Cd had a ratio around 0.6 reflecting another additional retention mechani sm, probably surface complexation. Fractional factorial design confirm ed that the presence of the cations investigated reduced the amount of the five metals retained, but the presence of Cu and Pb in the system depressed Ni, Cd and Zn sorption more than the inverse. Cation mobili ty was enhanced when equilibrium concentration increased and the effec t was higher in Ca-saturated soils. (C) 1998 Elsevier Science Ltd. All rights reserved.