Alkalinity neutralization of bauxite residue by nitrohumic acid: Mineral transformation and subsequent formation of organo-mineral complexes

Organic materials are receiving increased attention in reducing alkalinity of bauxite residue to achieve effective pedogenesis and reclamation. However, the dominant chemical reactions underlying this process remain unclear. Therefore, in this study, a comprehensive investigation of the decrease in the alkalinity of bauxite residue and the transformation of its constituent minerals owing to the action of nitrohumic acid (NA) was conducted. The results indicated that NA can be used to control the efficacy of the mineral dissolution-triggered neutralization of bauxite residue. Specifically, addition of NA at < 3% induced the neutralization of soluble Al(OH)4−, Na2CO3, and NaOH, while NA addition >3% further induced the dissolution of cancrinite, calcite, and andradite. This resulted in the contents of amorphous iron oxides and organic-matter complexed Fe increased from 0.11 and 0.27 g/kg to 2.75 and 6.97 g/kg, respectively, and the binding energy of Fe 2p shifted to higher values, which may attribute to the formation of organo-Fe associations. Further, NA broke the binding of K with Mg, Fe, and Ti containing minerals in bauxite residue, and resulting in the increase of free potassium. This study provides insight regarding the mineral transformation mechanism associated with NA-induced alkalinity reduction.