Interactions between organic matter and alkaline minerals in bauxite residue: implication for soil restoration

Ecological restoration of bauxite residue has received extensive attention, and organic matter plays a crucial role in the soil formation process of bauxite residue. However, the interaction between organic matter and alkaline minerals in bauxite residue is not well understood. In this work, molecular spectroscopic techniques combined with isothermal titration calorimetry (ITC) were employed to investigate the interactions between humic acid (HA) and four representative alkaline minerals in bauxite residue (calcite, garnet, sodalite, and cancrinite). The results show that the adsorption processes of HA onto calcite and garnet were primarily governed by monolayer surface adsorption and controlled by surface reactions, which were different for sodalite and cancrinite. Both garnet and cancrinite had strong binding affinities with fluorescent HA, while cancrinite only bound with a small fraction of HA. In contrast, the bindings of calcite and sodalite with fluorescent HA were weak. The ITC results indicate distinct thermodynamic properties of different alkaline minerals in the interaction with HA. The molar enthalpy of calcite was - 45.88 kJ/mol, which was much higher than those of garnet, sodalite, and cancrinite, suggesting that calcite exhibited a relatively uniform interaction mechanism with HA dominated by enthalpy change, while the others showed heterogeneous entropy-driven mechanisms. The findings contribute to a better understanding on the microscale connections between organic matter and alkaline minerals in bauxite residue.