Bauxite residue is a strong alkaline solid waste generated during alumina extraction, which poses high pollution risk to the surrounding environment in the storage process (Jiang et al. 2022; Wu et al. 2020). Currently, the concern for the safe disposal of bauxite residue has received more attention. The establishment of a sustainable vegetation cover (revegetation) is widely considered to be one of the most important steps in the management of closed bauxite residue disposal areas (Ke et al. 2021). Successful bauxite residue remediation requires not only the establishment of the most suitable vegetation species, but more importantly, the development of the most suitable growing medium (Menzies and Kopittke 2021). In many cases, if suitable quality soil (topsoil or subsoil) is not available, it may be necessary to amend the residues with various amendments and fertilizers. Successful remediation of bauxite residue is a combination of improvement of the growing medium and selection of plant species best suited to the environmental conditions, coupled with the regulation of the chemical, physical and microbiological properties of the residues (Bray et al. 2018).
At present, it is generally recognized that the most effective and implementable ecological rehabilitation method for bauxite residue on a large scale is the conversion of residues into a soil-like substrates, which supports and accelerates the revegetation process of bauxite residue disposal areas (Rezaei Rashti et al. 2019a, b). Therefore, the alkalinity regulation is only the first step in the process of bauxite residue remediation, the addition of organic matter must be integrated to improve the physical and chemical properties of bauxite residue and increase its fertility for soil formation in bauxite residue (Guo et al. 2022). The most effective treatment for remediated bauxite residue is usually considered to be a combination of gypsum and organic waste, which has been confirmed in laboratory and field studies (Jones et al. 2012; Li et al. 2018, 2019; Courtney et al. 2014). It not only lowers the pH but also improves the physicochemical properties, thus promoting the bauxite residue as a substrate to support plant growth. Compared to gypsum used solely, combining gypsum and organic amendments into the surface layer has a greater impact on subsurface remediation. For example, Li et al. (2017) found that the addition of gypsum and biosolids or cattle manure in the surface layer significantly contributed to lower pH and exchangeable sodium content as well as increased exchangeable calcium content and plant growth in bauxite residue compared to adding gypsum only. A recent study, with pH modulating the humic-mineral interactions in bauxite residue, revealed that the organic molecules derived from decomposition of the nitrohumic acid results in the formation of organo-mineral complexes and microagglomerates by binding to cations (Ren et al. 2021). As a result, the organic matter shifted over time from the granular form of < 200 mm diameter to the < 53 mm diameter class, which represents the binding of humus carbon to the mineral component (Courtney et al. 2013).
Newly stockpiled bauxite residue usually contains small amounts of organic matter (OM) that is generally measured by organic carbon content (Xue et al. 2021). The OM has an important role in the physicochemical properties and biological functions of the soil, including soil physical structure, water retention, and nutrient supply and cycling (Li and Huang 2015). In the process of bauxite residue remediation, the organic molecules obtained from the decomposition of the added exogenous OM can interact with minerals as organic cements, resulting in the formation of aggregation. Therefore, a rapid accumulation of a stable organic matter pool in bauxite residue is required to accelerate its soil formation. Furthermore, the increase in soil organic carbon from a single addition of an amendment will not persist unless the organic carbon is converted to a more stable or physically protected organic carbon pool (Banning et al. 2014). The stability of mineralogical, chemical and physical properties are important factors in improving the stability of organic matter and nutrients in bauxite residue. For example, changes in mineralogy during weathering process will affect the total amount and type of OM in mineral-organic assemblages, as well as the properties of these complexes (Mikutta et al. 2009). Considering the rapid changes in mineralogy in newly stockpiled bauxite residue, the organic carbon pool is unstable in the early stages of soil formation trajectory. Consequently, if a long-term stable increase of organic matter in bauxite residue is desired, the addition of organic carbon through exogenous amendments should be well considered with respect to the effects of physical, chemical and mineralogical changes in the bauxite residue. Also rapidly variable chemical, physical and mineralogical conditions can affect the contribution of microorganisms to organic matter cycling and stabilization, depending on whether they convert more organic carbon to CO2 through respiration or assimilate this carbon into the organism (Banning et al. 2008).
The stability of organic matter, especially dissolved organic matter, needs to be highlighted due to the lack of organic matter in bauxite residue, which usually requires additional supplementation during the actual restoration process. The rapid accumulation of organic matter in bauxite residue has a positive impact on vegetation growth, and further affects the soil formation process of bauxite residue. The stabilization mechanism of organic matter is strongly influenced by the complicated interaction between minerals-OM-microorganisms (Wu et al. 2019). Understanding the interaction of dissolved organic matter with sodalite and cancrinite, a typical alkaline mineral in bauxite residue, is important for the sequestration of organic carbon in eco-engineered rehabilitation for bauxite residue. Furthermore, the action mechanism of calcium ions that are commonly applied to reduce alkalinity on the stabilization of organic carbon from exogenous added organic amendments is still unclear, which will govern the bauxite residue remediation efficiency. The objective of this study is to investigate the effects of calcium ions on the adsorption and immobilization of organic matter (fulvic and humic acid) by sodalite and cancrinite and their interaction mechanisms by using various microspectroscopic methodologies including excitation-emission matrix spectra (EEMS), fourier transformed infrared spectroscopy (FTIR), thermogravimetric analysis (TG-DSC) and atomic force microscope (AFM). This work provides us with well understanding of the dynamic changes of organic carbon during the ecological remediation of bauxite residue disposal areas with calcium-based materials.