Modeling humic acids transport in a bauxite profile: Los Pijiguaos, Venezuela

Los Pijiguaos bauxite deposit is located in Southern Venezuela, in a tropical rain forest environment. Among the various impurities during the bauxite processing (Bayer process), organic matter compounds present in Bayer liquor can slow the rate of alumina precipitation, and reduce product yield and quality. Soil samples were taken from a 37-m-deep core and the distribution of organic carbon (OC) was investigated as well as its relationship with major oxides Al₂O₃, Fe₂O₃, SiO₂ and with the mineral phases gibbsite, hematite, kaolinite and quartz. Each sample was subjected to an adsorption process of humic acids by interaction with solutions of known concentrations. Maximum adsorption capacity was also studied in each mineral separately, which allowed calculation of the theoretical adsorption capacity of each soil sample. Even if concentrations of OC were higher in the first 5 m of the profile (0.2–0.5%), samples exhibited a low adsorption capacity (0.02%), while at depth between 10 and 37 m, samples showed lower OC concentrations (0.03–0.05%), and a larger OC adsorption capacity (0.08%). The model for the adsorption capacity only works between 10 and 30 m of depth, since in the first 10 m the bauxitic profile seems to be saturated in OC, because the concentration of OC is higher than the maximum predicted by adsorption experiments.