Investigation of mineral commodity residues based on alkalinity, solubility and other physicochemical aspects aiming the management of amazonian acidic soils

Abstract

The large amounts of mineral residues generated by the bauxite-alumina industries in the Amazon — with a production scale in the same order of magnitude as their source commodities — have been seen as new sources of raw materials (secondary mines) and/or an inseparable part of a sustainable production system, i.e., a source for the generation of co-products within a circular economy system. In the present study, two alkaline residues from this mining-metallurgical industry were assessed for their potential to amend productive acidic Amazonian soils, namely, (1) insoluble solid residue from the Bayer process (bauxite residue, BR) and (2) ash from energy production from coal (coal combustion residues, CCRs: fly ash, FA, and bottom ash, BA). A physicochemical investigation was performed to evaluate the possible benefits that these residues can offer to the soil‒plant system. The alkalinity of the residues was adjusted “to a value of pH 8–10” by leaching with H3PO4 using a central composite experimental design. The chemical analyses indicated high levels (total and soluble) of essential elements such as Ca and S in the CCRs. All residues showed a high cation exchange capacity (CEC). Regarding the water holding capacity (WHC), FA showed a higher value than the other residues (68.6%). After pH adjustment, available P increased significantly for all residues, and the Ca and S contents remained high for the CCRs, while in BR, there was a decrease in available Na, and aluminum (Al3+) was not available because the potential acidity (H + Al) < 0.6. Finally, complementary analyses showed that with respect to mineralogy, BR is composed mainly of iron oxyhydroxides and aluminosilicate phases, while carbonate, sulfide and silicate phases dominate the CCRs. The neutralizing character, the presence of nutrients in the CCRs and the unavailability of Al3+ in BR are positive physicochemical characteristics for the management of Amazonian acid soils; the use of this residues would add to the circular economy and sustainability of the Amazon.