Calcium ions improve organic carbon stability in bauxite residue through alkaline minerals

Abstract

Abstract Purpose The interaction of minerals and organic matter significantly affects the stability of organic carbon, which is key to the rehabilitation of bauxite residue. However, due to the microscopic structure of minerals and the chemical composition of dissolved organic matter (DOM), the interaction mechanism between alkaline minerals and DOM in the presence of calcium ions that are widely used as amendments is still poorly understood. Methods In this work, the adsorption mechanism of organic matter by sodalite and cancrinite in the presence of calcium ions was investigated by fourier transform infrared spectroscopy (FTIR), excitation-emission matrix fluorescence (EEMs), thermogravimetric analysis (TG-DSC) and atomic force microscope (AFM). Results The calcium ions can act as bridges to connect the negative sites on the mineral surface with the polysaccharide functional groups of organic matter, thus allowing the DOM to enter the pore channels of mineral structure to sequester organic carbon via ligand-exchange and/or electrostatic interactions. Furthermore, DOM located in the pore structure within cancrinite may be better protected due to physical isolation. The 3D view of the interacted minerals-organic matter assemblages provided visual evidence that the adsorption sites distributed on the surface of cancrinite increased by the action of calcium ions, with different adsorption capacity of individual sites for DOM. Conclusions The calcium ions enhance the ability of alkaline minerals to trap dissolved organic carbon, and the work highlighted the significance of organo-mineral associations in the microscopic mineral structure for the organic carbon sequestration in the revegetation process of bauxite residue disposal areas.