The potential for constructed wetland mechanisms to treat alkaline bauxite residue leachate: carbonation and precipitate characterisation

Metal-organic framework functionalized with deep eutectic solvent for solid-phase extraction of Rhodamine 6G in water and cosmetic products

An NH2 -MIL-53(Al)-DES(ChCl-Urea) nanocomposite was synthesized for extraction and determination of Rhodamine (Rh) 6G from environmental and cosmetic samples. The deep eutectic solvent (DES) was prepared by mixing choline chloride and urea in a mole ratio of 1:2. NH2 -MIL-53(Al)-DES(ChCl-Urea) nanocomposite was synthesized using the impregnation method at a ratio of 60:40 (w/w). The optimum conditions were determined after NH2 -MIL-53(Al)-DES(ChCl-Urea) characterization was performed. The optimum conditions were determined as pH 8, adsorbent amount of 15 mg, total adsorption-desorption time of 6 min, and enrichment factor of 20. The recovery values of the solid-phase extraction method for water and cosmetic samples under optimum conditions were between 95% and 106%. NH2 -MIL-53(Al)-DES(ChCl-Urea) nanocomposite was an economically advantageous adsorbent because of its reusability of 15 times. All analyses were performed using the ultraviolet-visible spectrophotometer. The linear range, limit of detection, and limit of quantification of the method were 100-1000, 9.80, and 32.68 μg/L, respectively. The obtained results showed that the synthesized nanocomposite is a suitable adsorbent for the determination of Rh 6G in water and cosmetic samples. The real sample applications were verified with the high-performance liquid chromatography system.

Influence of sediment quality and microbial community on the functioning capacity of a constructed wetland treating alkaline leachate after 5.5 years in operation

Constructed wetlands (CWs) have been demonstrated as a cost-effective alternative to chemical treatment systems for mine waters, with the microbial communities attributed to promoting carbonation and aiding pH neutralization. However, few data are available for the long-term use of CWs treating alkaline leachates nor the activity of microbes within them. To investigate the feasibility of CW to buffer alkaline pH, a pilot-scale wetland was implemented in 2015 to treat alkaline bauxite residue leachate. After 5.5 years, samples of supernatant water and sediment were taken at 0.5 m increments along the 11 m long wetland. Waters were analysed for pH, EC and metal(loid) content, while sediment was subjected to physico-chemical assessment and element fractionation. Microbial biomass and community were assessed by phospholipid fatty acid analysis (PLFA) and functionality by the Rapid Automated Bacterial Impedance Technique (RABIT). Evidence presented demonstrates that the CW operating for 66 months effectively treats bauxite residue leachate, with reduced influent pH from 11.5 to 7.8. Trace element analysis revealed effective reduction in Al (94.9 %), As (86.7 %) and V (57.6 %) with substrate analysis revealing a frontloading of elevated pH and trace element content in the first 5 m of the wetland. Sediment Al, As and V were present mostly (>94 % of total) in recalcitrant forms. Sediment Na was mostly soluble (48-62 %), but soils were not sodic (ESP < 15 %). Investigations into the microbial community revealed greatest biomass was in the first 5 m of the wetland, where pH, EC and metal contents were greatest. Microbial respiration using endemic Phragmites australis as a substrate demonstrates an ability to cycle recalcitrant carbon sources within a CW system. These novel microbial findings highlight the need for further investigation into the microbial communities in alkaline CWs.

Neutralization of bauxite residue with high calcium content in abating pH rebound by using ferrous sulfate

The high alkalinity of bauxite residue and its sustained release impose major limitation on its reuse and ecological disposal. It has been confirmed from sustained rehabilitation that gypsum can effectively reduce the alkalinity of bauxite residue by continuously releasing Ca2+ to react with carbonate and hydroxide. However, the combined bauxite residue with high calcium content exhibits stubborn alkalinity for most alkaline reduction methods employing cations to consume carbonate. In this study, we have aimed to address this knowledge gap by investigating the dose-response relationship in the alkaline reduction induced by ferrous sulfate (FS) neutralization. The pH, exchangeable sodium percentage (ESP), and CO32-/HCO3- of bauxite residue decreased from 10.6, 44.1%, and 42.7/24.5 mg/kg to 8.1, 27.7%, and 0.7/18.0 mg/kg, respectively. Approximately 20-55 days were required for the neutralization reaction to reach equilibrium. The FS induced an increase in free iron oxide (Fed) and amorphous iron oxide (Feo), and partial dissolution of alkaline minerals including calcite, cancrinite, and kaolinite in bauxite residue. Further, addition of FS also affected the kinetic dissolution process of bauxite residue; the acid neutralization capacity of bauxite residue to pH 7 decreased from 0.21 mol H+/kg solid to 0.02 mol H+/kg solid. The results showed FS to be a potential candidate for improving the characteristics of the combined bauxite residue, and guide the FS application for the disposal of the combined bauxite residue.

The Microbiology of Metal Mine Waste: Bioremediation Applications and Implications for Planetary Health

Mine wastes pollute the environment with metals and metalloids in toxic concentrations, causing problems for humans and wildlife. Microorganisms colonize and inhabit mine wastes, and can influence the environmental mobility of metals through metabolic activity, biogeochemical cycling and detoxification mechanisms. In this article we review the microbiology of the metals and metalloids most commonly associated with mine wastes: arsenic, cadmium, chromium, copper, lead, mercury, nickel and zinc. We discuss the molecular mechanisms by which bacteria, archaea, and fungi interact with contaminant metals and the consequences for metal fate in the environment, focusing on long-term field studies of metal-impacted mine wastes where possible. Metal contamination can decrease the efficiency of soil functioning and essential element cycling due to the need for microbes to expend energy to maintain and repair cells. However, microbial communities are able to tolerate and adapt to metal contamination, particularly when the contaminant metals are essential elements that are subject to homeostasis or have a close biochemical analog. Stimulating the development of microbially reducing conditions, for example in constructed wetlands, is beneficial for remediating many metals associated with mine wastes. It has been shown to be effective at low pH, circumneutral and high pH conditions in the laboratory and at pilot field-scale. Further demonstration of this technology at full field-scale is required, as is more research to optimize bioremediation and to investigate combined remediation strategies. Microbial activity has the potential to mitigate the impacts of metal mine wastes, and therefore lessen the impact of this pollution on planetary health.

Modelling Hydrological Performance of a Bauxite Residue Profile for Deposition Management of a Storage Facility

Accurate scheduling of bauxite residue (red mud) deposition time is required in order to prevent the risk of storage facility failure. This study was conducted to precisely determine the hydraulic parameters of bauxite residue and investigate the capability of HYDRUS to accurately estimate the residue moisture profile and the timing for its deposition. The hydraulic properties of the bauxite residue profile were determined by solving an inverse problem. A one-dimensional hydrological model (HYDRUS-1D) was validated using a 300 mm long column filled with bauxite residue and exposed to a dynamic lower boundary condition. After numerical validation, the model was used to simulate the moisture profile of bauxite residue under the climatic conditions of an alumina refinery site in Queensland, Australia, as well as other scenarios (i.e., high (300 mm) and small (1.7 mm) rainfall events of the site). This study showed that the HYDRUS model can be used as a predictive tool to precisely estimate the moisture profile of the bauxite residue and that the timing for the re-deposition of the bauxite residue can be estimated by understanding the moisture profile and desired shear strength of the residue. This study revealed that the examined bauxite residue approaches field capacity (water potential −10 kPa) after three days from a low rainfall event (<1.7 mm) and after eight days from an intense rainfall event (300 mm) at the time of disposal. This suggests that the bauxite residue can be deposited every four days after low rainfall events (as low as 1.7 mm) and every nine days after high rainfall events (as high as 300 mm) at the time of deposition, if bauxite residue experiences an initial drying period following deposition.

Alkalinity neutralization and structure upgrade of bauxite residue waste via synergistic pyrolysis with biomass

Bauxite residues, a large volume solid waste, are in urgent need of effective disposal and management. Especially, strategies to alleviate the high alkalinity of bauxite residue remain a big challenge. Here, we developed a synergistic pyrolysis to neutralize the alkalinity of bauxite residue and upgrade the structure of biomass simultaneously. By cooperating the catalytic feature from bauxite residue, rice straw, a cellulose-enriched biomass, could prefer to produce acidic components under a hypothermal pyrolysis temperature (below 250 °C) and partial oxygen-contained atmosphere as evidenced by the synchronous TGA-FTIR analysis. In return, these in-situ produced acidic components neutralized the bauxite residue profoundly (pH decreased from 11.5 to 7.2) to obtain a neutral product with long-term water leaching stability. Also, a higher pyrolysis temperature led to neutral biochar-based products with well-defined carbonization characteristics. Thus, the biomass-driven pyrolysis strategy provides a potential to dispose the alkalinity issue of bauxite residue and further opportunities for the sustainable reuse and continuing management of bauxite residue.

Effect of phosphogypsum and poultry manure on aggregate-associated alkaline characteristics in bauxite residue

Bauxite residue is a highly alkaline solid waste with poor physical structure which ultimately limits plant growth. Ecological reconstruction is an effective strategy to improve its environmental management, although soil formation process still requires further investigation. Here, an incubation experiment was used to investigate the effects of phosphogypsum and poultry manure, on aggregate size distribution and aggregate-associated exchangeable bases of bauxite residue. Phosphogypsum and poultry manure additions significantly increased the proportion of 2-1 mm residue aggregates and enhanced mean weight diameter (MWD) of residues in the 0-20 cm and 20-40 cm layers, although little effect was evident in the 40-60 cm layer. Phosphogypsum addition reduced pH and EC values to approximately 8.5 and 200 mS/cm in different size aggregates at 0-20 cm. Exchangeable Ca²⁺ concentration was improved, especially in 0.25-0.05 mm and <0.05 mm aggregates, following amendment additions. The relative contents of katoite and cancrinite in >0.25 mm aggregate fractions were relatively higher, which was consistent with changes in pH. Phosphogypsum and poultry manure changed the microstructure and surrounding pores of residue aggregates, whilst the concentration of Ca on microaggregate surfaces was higher than that on macroaggregates. These findings reveal that application of phosphogypsum and poultry manure directly alter the distribution of exchangeable bases and alkaline indicators within residue aggregates, resulting in aggregate size distribution and microstructure variations.

Removal of Al, Ga, As, V and Mo from alkaline wastewater using pilot-scale constructed wetlands

The study was initiated to evaluate constructed wetland technology as a method for treating alkaline (pH 8.0-8.6) drainage high in Al, Mo, V, As and Ga originating from bauxite residue storage areas. Pilot-scale horizontal flow constructed wetlands were operated over a 40-week period using three filter materials (granitic gravel, bauxite and alum water treatment sludge), and half of the wetlands were planted with Phragmites australis and the other half left unplanted. Gravel was the least effective medium for removing the target elements, while of the two active media, water treatment sludge was more effective than bauxite. Plants removed only small amounts of elements into their above- and below-ground dry matter (0.4-4.9% of that added). Nonetheless, the presence of plants greatly increased the effectiveness of all three media since their presence decreased effluent pH values by 0.5-1.3 pH units and that of the filter media by 0.4 pH units. Removal of elements followed the order Al > Ga > V > As > Mo. For planted wetlands, total elemental removal ranged from 18 to 98% for gravel, 80 to 99% for bauxite, and 93 to 99% for water treatment sludge. The lowest removal was for Mo (ranging from 18% for gravel to 93% for water treatment sludge) and the highest for Al (ranging from 98% in gravel to 99% in water treatment sludge). A sequential fractionation scheme for As, V and Mo on filter material at the end of the experiment showed that for bauxite and water treatment sludge, V and As were concentrated in the NaOH extractable fraction while Mo was concentrated in the less strongly adsorbed NaHCO₃ extractable fraction. It was concluded that a constructed wetland with water treatment sludge as an active filter material is an effective technology for removal of the target elements from the alkali drainage.