Humic-mineral interactions modulated by pH conditions in bauxite residues – Implications in stable aggregate formation

Abstract

Bauxite residue exhibiting high sodicity, nutrient deficiency, and poor structure, has a very slow effect on natural vegetation. However, it presents some eco-friendly indicators through substrate improvement, which is the basis for bauxite residue to show pedogenesis and vegetation restoration. To test the effectiveness of acid organic materials toward promoting transformation into a soil-like material, bauxite residue was amended with nitrohumic acid to improve the physical and chemical properties. The pH, total alkalinity, and exchangeable sodium percentage of the treated bauxite residues decreased from 10.2, 2.7 cmol/kg, and 20.6% to 8.0, 0.4 cmol/kg, and 5.1%, respectively, whilst organic matter, available N and P increased from 1.3%, 6.7 mg/kg, and 0.8 mg/kg to 7.2%, 69.7 mg/kg, and 2.4 mg/kg, respectively. The results obtained from excitation-emission matrix spectroscopy also indicate that the nitrohumic acid treated residues have a higher fulvic acid content and present a significant amount of humified material and soil maturation. The high alkalinity of bauxite residue promotes the release of hydroxyl and phenolic hydroxyl groups from fulvic acid to combine with cations. Nitrohumic acid neutralization lowered the IEP and PZC to pH 7.83 and 7.79, and made its surface charge properties closer to the soil; the decreased zeta potential of residues resulted in the particles dispersed state tend to aggregate. And as a result, the number of 0.05–0.25 mm and > 0.25 mm aggregates in the treated bauxite residues significantly increased, the sheet-like fine particles adhere to the surface of the large particles resulted in an accumulation and a layer structure formed. Amount of Ca and Fe in the aggregates as an inorganic binder combined with organic to promote the formation of micro-aggregates; whereas the content of Na and Si, which act as a “dispersant” and “filler” decreased.