Rapid conversion of alkaline bauxite residue through co-pyrolysis with waste biomass and its revegetation potential

The extreme alkalinity of bauxite residue (BR) leads to difficulty with its reuse. Alkaline leachate and dust generation during the stacking process can pollute surrounding soil, air and water. In this work, co-pyrolysis of bauxite residue and sawdust was applied to rapidly produce a soil-like matrix that met the conditions for plant growth as demonstrated by ryegrass pot experiments. The present study aimed to characterize the detailed changes in physicochemical, mineral weathering, and microbial communities of the pyrolyzed BR with different ratios of saw dust after plant colonization for 2 months. With increasing sawdust addition during co-pyrolysis, the pH of BR decreased from 11.21 to 8.16, the fraction of macro-aggregates 0.25-2 mm in the water-stable agglomerates increased by 29.3%, and the organic carbon concentration increased from 12.5 to 320 mg/kg, whilst facilitating the degree of humification, which were all beneficial to its revegetation performance. The backscattered electron-scanning electron microscope-energy-dispersive X-ray spectrometry (BSE-SEM-EDS) results confirmed the occurrence of sodalite and calcite weathering on aggregate surfaces, and X-ray photoelectron spectroscopy (XPS) results of surface Al and Si compounds identified that some weathering products were clay minerals such as kaolinite. Furthermore, bacterial community composition and structure shifted towards typical soil taxonomic groups. These results demonstrate soil development of treated BR at an early stage. The technique is a combination of alkalinity regulation and agglomerate construction, which accelerates soil formation of BR, thus proving highly promising for potential application as an artificial soil substitute.