Bauxite Residue Disposal and the Environmental Benefit of Açaí, Soil and Gypsum Amendments

Production of alumina from bauxite results in a large amount of bauxite residue. The accumulation of this hazardous material has unfavorable properties for plant growth and the closure of disposal areas is challenging. The recovery of bauxite residue disposal sites (BRDA) requires the addition of suitable amendments to achieve environmental improvement. One of the possible approaches is the application of açaí, soil, or gypsum amendments. This study focusses on a case study site in Pará—a state located in northeastern Brazil. This thesis analyzes the leaching behavior of dissolved inorganic elements of four types of samples: bauxite residue (red mud) without amendments, and with three different amendments—açaí, soil, and gypsum in a proportion of 10%. The leaching pattern of major and minor elements, pH, electrical conductivity (EC), and acid buffering capacity were analyzed in the four types of samples in a sequential batch leaching test comprised of five steps that correspond to a cumulative liquid to solid ratio of 50:1. (i.e., 3.9 g bauxite residue, 0.03 g of amendment for 44 ml of water solution). The analytical data obtained in the batch leaching test was modelled using the PHREEQC geochemical modelling software to assess mineral equilibria and the influence of carbon dioxide on the pH and formation of secondary minerals. The Electrical Conductivity (EC), Exchangeable Sodium Percentage (ESP), and Sodium Absorption Ratio (SAR) measurements suggest that these parameters will decrease (EC: 3.290 to 577, ESP: 19.29 to 1.52, and SAR: 17.06 to 1.90) after the 5-step sequential batch test and differ with each type of amendment. The initial and final batch leaching test results show that each of the four types of samples generally decrease the pH in a range from 11.98 to 10.92. These results imply that the high alkalinity and pH will continue under field conditions if no further actions are taken. The dissolved fraction of Al present in this study demonstrates that in BRDA, highly mobile Al-hydroxide species are available (20 mg/l) and should be addressed for successful remediation. This is important to consider in the implementation of sustainable alternatives of bauxite residue amelioration. The pH does not need to be reduced to neutral pH, values below 9 are acceptable since the concentration of negative Al-hydroxides will be considerably reduced. The geochemical modelling shows that the reaction of bauxite residue with carbon dioxide in the atmosphere can reduce the initial pH from 11.5 to 9.1 (these values are the average of both the first and the final batch tests with and without amendments) and with a higher concentration of carbon dioxide, usually found in the soil, pH values near 7.2 can be reached. Thus, when the formation of carbonates is taken into account, the reaction with atmospheric carbon dioxide can lead to a higher precipitation of carbonates such as calcite, dolomite, and hydrozincite when compared with the formation of carbonates caused by the reaction of carbon dioxide in the soil. These can provide a way to improve the poor environmental conditions of bauxite residue and lead to a feasible rehabilitation of the area. Determination of the minor element composition by XRF method reveals that bauxite residue samples form Barcarena, Pará, Brazil, showed a high concentration of chromium and vanadium, 290 mg/kg and 536 mg/kg respectively. These amounts are present in the solid phase with or without amendments, representing a potential detrimental effect on plant growth in BRDAs