Mecanismos de transporte de metais pesados em misturas de solos utilizados como camada de base em aterro sanitário.

Abstract

Leachate is a complex effluent composed of several potentially toxic components and one of the main biodegradation products in urban solid wastes. Because of this complexity, the transport of metals in compacted soils and the influence of competitive interaction among metals in the retention process in the base layer should be analyzed. This study aimed to determine the competitive adsorption and diffusion of heavy metals (copper, nickel, and chromium) in soil mixtures used as base layer in a sanitary landfill. Samples of local natural soil and bentonitic soil were collected at the Sanitary Landfill of Campina Grande-PB and stored at the Environmental Geotechnics Laboratory at Federal University of Campina Grade. Subsequently, the local natural soil, bentonitic soil, and mixtures with 5, 15, 20, and 25% of bentonitic soil were prepared to undergo geotechnical, chemical, and mineralogical characterization. Batch tests were conducted for all samples to verify the adsorption of metallic ions, and sorption isotherms were also derived. An experimental planning was also conducted to analyze the factors that influence the diffusion mechanism, considering as predicting variables: content of bentonitic soil, degree of compaction, and contaminant solution concentration. The statistical indicators used to compare the models with the experimental data were: coefficient of determination, residual sum of squares, square root, and mean squared error. In monospecies system, the analyzed soil exhibited a higher retention of Cr+3 compared to the other elements evaluated. Considering the results obtained from the adjustment of data to Langmuir, Freundlich, and Redlich–Peterson models, the adsorption sequence was: Cr+3 > Cu+2 > Ni+2. The extended Langmuir model provided a better adjustment of experimental data in a multispecies solution, as indicated by the statistical indicators analyzed. In the ternary system, a synergic interaction was observed for all elements in mixtures with up to 20% of bentonitic soil. Based on the diffusion test results, among the variables analyzed, the initial concentration in the reservoir was the factor that affected the most the diffusive transport; higher molar concentrations resulted in a faster mass transport. Moreover, in general, the effective diffusion coefficients determined were lower than the diffusion coefficients in free solution. The pH was a determining factor in the adsorption and diffusion of heavy metals in the soil mixtures, directly acting on the solubility of metallic cations and, consequently, on the retention of these contaminants. This factor was more relevant when compared to some properties of the elements, such as ionic radius and electronegativity. Therefore, these mixtures can be used as base layers in sanitary landfills, helping in the immobilization of heavy metals, as long as less than 25% of bentonitic soil is used.