Interactions between acid solutions and Venezuelan tropical soils

This study addresses the impact of exogenous acid on tropical topsoil. The main objectives are to ascertain the physico-chemical parameters and processes that determine the buffer capacity (β) and to evaluate the effects of anions and pH on the leaching of Na, K, Ca, Mg, Fe, Si and Al. The topsoils were classified into two groups. First the carbonate group (n = 6), comprising samples collected in a tectonically-active area with β values of between 75 and 3600 meq/kg and CaCO3 values from 0.8 to 19 wt. %. Second the non-carbonate group (n = 15), comprising mainly highly weathered soils from the plains and the Guiana Shield, which yield the lowest β values (1–30 meq/kg), as well as the lowest cation exchange capacity (CEC), base saturation (BS) and pH. β values display a linear correlation with % CaCO3 and CEC for the former group, and with organic carbon, CEC and BS for the latter. Fifty milliliters of water (initial pH = 5.6) added to 1 g dry-soil produced a release of cations ranging between 4.3 and 68 meq/soil kg. This is explained by a desorption-dissolution process. Strong acid solutions, e.g. HNO3, H2SO4 and HCl (initial pH = 3.3), in relation to water, induces increased cation release. This process produces consumption ≡ 60–70% of H+ added. Protons consumed were predominantly compensated by Ca2+ + Mg2+ released into solution. The amounts and relative proportions of cations desorbed are independent of the initial anionic composition of the inorganic acid solution. However, a weak acid solution, e.g. CH3COOH solution (initial pH = 3.3) produced remarkable increases of the relative proportion of both Na+ and K+. It may be concluded that ionic exchange, controlled by organic matter and carbonate dissolution, is the main proton consuming mechanism.