ECOLOGICAL ENGINEERING OF SOIL-PLANT SYSTEMS TO REHABILITATE BAUXITE RESIDUES: CURRENT PROGRESS, BARRIERS AND INNOVATIONS

To date about 4 Gt of bauxite residues (red mud (RM) and seawater neutralized (SN) RM) have been accumulated at alumina refineries worldwide. These RM dams across different climatic zones are posing grave financial and ecological challenges to refineries and local communities, as their rehabilitation into sustainable ecosystems is very slow and expensive when conventional capping methods are used. As a result, developing new rehabilitation technologies which are scientifically robust, site-feasible and costeffective has become even more important. The emerging technology, ecological engineering of in situ “soil formation” from bauxite residues (or red mud) has shown promise, with expected outcome to reduce the volume of soil materials required for reconstructing root zones and establishing a sustainable vegetation cover. This technology integrates engineering inputs (biomass organic matter inputs, tolerant microbial consortia, water) with soil biogeochemical and ecological processes and purposely accelerates alkali-mineral dissolution, removal of undesirable factors and development of physicochemical and biogeochemical properties in the residues in situ. The resultant growth media (technosols) are capable of supporting sustainable growth of target plant communities under local climatic conditions.

Technosols are a group of soil developed from artefacts and technic hard parent material of organic and mineral nature, of which origin can be either natural or technogenic (Schad, 2018). Reviewing historic and current literature has demonstrated a clear conceptual shift from “soil remediation” to integrated ecoengineering of soil formation in the RM for its rehabilitation. Recent reports on long-term field trials have demonstrated the possibility of the formation of functional technosol from aged RM amended with organic and inorganic treatments, with successful vegetation cover. The eco-engineering technology for RM-soil formation is proposed to consist of three major, coupled and dynamically inter-dependent phases: bioneutralisation, hydrogeochemical stabilization and physical structure development, and evolution and rehabilitation of biogeochemical structure and functions. For field-deployment, it needs to be further developed with well defined transition phases, indicators and their criteria for target vegetation types, in order to monitor the progress of soil formation and evaluate properties of resultant technosol required for target plant communities under local climatic conditions. This paper aims to review research progress achieved so far, identify key barriers and limitations to be resolved by research, and propose critical phases of in situ soil formation. Further field-scale trials are urgently needed to clearly define operational risks and associated field operational requirements, under local operational and climatic conditions.