Manufactured soil: A new use for bauxite residue

• Published in: The Science of the total environment 2024-12, Vol.955, p.177105, Article 177105
• Main Authors: Weng, X., Haynes, R.J., Zhou, Y.F.
• Format: Article
• Language: English
• Subjects: aluminum oxide; bauxite; Bauxite residue; cation exchange capacity; composts; Dune sand; electrical conductivity; Engineered soil; environment; exchangeable sodium; green waste; Manufactured soil; microbial biomass; mineralization; Optimized bauxite residue; soil microorganisms; toxicity
• ISSN: 0048-9697; 1879-1026; 1879-1026
• DOI: 10.1016/j.scitotenv.2024.177105

Bauxite processing residue produced at an Alumina refinery was used to produce manufactured soil. The residue was first acidified and then leached to remove excess salts. Green waste compost was added at rates of 5 and 10 % w/w, with or without the addition of 20 % w/w dune sand. The products were dried, crushed and sieved and the chemical, physical and microbial properties of the materials characterized. Products had an electrical conductivity and pH of 1.0–1.1 dS m−1 and 8.0–8.1 respectively in 1:5 soil:water extracts and corresponding values in saturation paste extracts were 5.4–5.7 dS m−1 and 7.4–7.5. The effective cation exchange capacity was 202–351 mmolc kg−1, exchangeable Na percentage was 41–46 %, Colwell-extractable P was 108–119 mg kg−1 and the Toxicity Characteristic Leaching Procedure showed that extractable metal concentrations were an order of magnitude below regulatory limits so the material can be considered as non-hazardous. Diethylenetriaminepentaacetic acid-extractable Cu, Zn and Mn were below reported critical levels. Addition of compost lowered concentrations of extractable Cr, As and V and drying the material further reduced extractable levels. When the material was dried it shrunk and solidified and the aggregates produced had a low macroporosity and high microporosity and water holding capacity compared with dune sand. Aggregates were very stable as measured by wet sieving. Compost addition increased organic C content, and the size (microbial biomass C) and activity (basal respiration and the activity of enzymes involved in C, N, S and P mineralization) of the soil microbial community. It was concluded that the materials produced had soil-like chemical, physical and microbial properties and could potentially be used as a soil substitute. [Display omitted] •Bauxite residue was acidified and leached to produce optimized bauxite residue.•A manufactured soil product was made by adding 10 % compost to the liquid residue and then drying it.•Compost incorporation significantly lowered concentrations of extractable Cr, As and V.•The product formed had soil-like chemical, physical and microbial properties.