Blending bauxite residues with multiple byproducts improves capping materials for tailings storage facilities.

Amelioration and management of large volumes of tailings resulting from alumina refining is a major challenge owing to the high alkalinity and salinity of residues. Blended byproduct caps are a potential new and more cost-effective approach to tailings management, where tailings are blended with other local byproducts in order to reduce pH, salinity and toxic elements. Here, alkaline bauxite residue was blended with four byproducts (waste acid, sewage water, fly ash and eucalypt mulch) to create a range of potential capping materials. We leached and weathered materials in the glasshouse with deionized water over nine weeks to investigate if byproducts on their own or in combination improved cap conditions. Combining all four byproducts (10 wt % waste acid, 5 wt % sewage water, 20 wt % fly ash and 10 wt % eucalypt mulch) achieved lower pH (9.60) compared to any byproduct applied individually, or un-remediated bauxite residue (pH 10.7). Leaching decreased EC by dissolving and exporting salts and minerals from the bauxite residue. Fly ash addition increased organic carbon (likely from non-combusted organic material) and nitrogen, while eucalypt mulch increased inorganic phosphorus. Addition of byproducts also decreased the concentration of potentially toxic elements (e.g., Al, Na, Mo and V) and enhanced pH neutralisation. Initial pH with single byproduct treatments was 10.4-10.5, which decreased to between 9.9-10.0. Further lowering of pH and salinity as well as increased nutrient concentrations may be possible through higher addition rates of byproducts, incorporation of other materials such as gypsum, and increasing leaching/weathering time of tailings in situ.

Coupling microbial and abiotic amendments accelerates in situ remediation of bauxite residue at field scale.

Bauxite residue is a highly saline-sodic tailings material formed as a by-product of the Bayer process for alumina production. In situ remediation of bauxite residue has the potential to provide an effective means for accelerated rehabilitation of residue storage areas. However, previous work has predominantly only used chemical and physical amendments to date, limiting rates of pH neutralisation and extent of remediation. Combining these abiotic amendments with recently developed microbial biotechnology for pH neutralisation may transform bauxite residue into a productive soil material in a shorter timeframe. Here we investigated the effects of microbial and abiotic amendments (compost plus tillage), both in isolation and combined, on remediation of key bauxite residue properties in field scale trials (10 × 15 m × 2 m deep field plots). Triplicate residue samples were collected to 30 cm depth from each plot in quarterly field sampling campaigns. Changes in chemical and physical properties were monitored to assess remediation performance under different amendments. After one year, field plots amended with a microbial treatment had significantly (p < 0.05) lower average pH (8.99-9.46) in the upper 20 cm than the control (10.3). The combined microbial-abiotic treatment also had improved physical structure, higher organic C and lower electrical conductivity than the microbial treatment alone. The strong performance of the microbial-abiotic treatment is attributed to the combined benefits of bioneutralisation from microbial fermentation products, enhanced leaching of alkaline pore water and salts due to tillage and compost, and addition of highly stable C and N in compost. Combining novel microbial biotechnology with common abiotic amendments is therefore suggested for accelerating in situ remediation progress towards a material amenable for plant growth.