BRAZILIAN RED LATOSOL A TYPIC SOIL AS AN EXCHANGER - A THERMODYNAMIC STUDY INVOLVING CU, ZN, CD, HG, PB, CA AND NA

The thermodynamic cationic exchange process involving divalent (Cu, Zn , Cd, Hg, Pb and Ca) and monovalent (Na) cations in Brazilian red Lato sol soil was studied. Using a batchwise method, the exchange was monit ored as a function of the added cation concentration and the aqueous s uspension of the soil at different temperatures. The isotherm series o btained were adjusted to a modified Langmuir equation, whose results w ere compared with the proposed Rawat method. The cationic exchange equ ilibria constants (In K) vary from 1.97 to 9.80 for the Langmuir equat ion and 7.06 to 13.50 for the Rawat method. The variation in enthalpie s obtained by applying the van't Hoff equation gave, for Langmuir and Rawat procedures, exothermic values for Cu (65.5 and 97.3), Cd (36.9 a nd 45.6) and Pb (43.0 and 50.7) Id mol(-1), and endothermic values for Zn (40.8 and 30.5), Hg (15.0 and 11.3), Ca (30.4 and 40.0) and Na (32 .7 and 42.3) kJ mol(-1). The exchanges proceed spontaneously as indica ted by the free energy values: Cu (14.2 and 27.2), Zn (21.6 and 32.0), Cd (16.1 and 23.2), kg (13.8 and 22.9), Pb (22.6 and 28.3), Ca (17.0 and 25.9) and Na (9.9 and 19.3) id mol(-1) at 323 K. These results sug gest that the interaction occurs by complex formation between the orga nic matter of the soil matrix and the cations dispersed in aqueous sol ution.