Simulation study for the formation of alkaline efflorescence on bauxite residue disposal areas following the phosphogypsum addition

Potential of artificial soil preparation for vegetation restoration using red mud and phosphogypsum

Red mud and phosphogypsum have long been a focus and challenge in global industrial waste management, and their low-cost and large-scale utilization technology has always been an urgent need. This study is based on the strong acid-base neutralization reaction between red mud and phosphogypsum, which contain an elemental composition similar to that of natural soil, red mud itself has characteristic of clay minerals, and other auxiliary materials (i.e. rice husk powder, bentonite, fly ash, polyacrylamide flocculant and microbial suspension) were added, so as to explore the potential of synergistically prepared artificial soil for vegetation restoration. The results showed that the artificial soils exhibited physicochemical characteristics (e.g., pH, moisture content, cation exchange capacity) similar to those of natural soil, along with abundant organic matter, nitrogen, phosphorus, and potassium contents, meeting the growth requirements of plants. The artificial soils were able to support favorable growth of suitable plants (e.g., sunflower, wheat, rye grass), accumulating high levels of diverse enzymatic activities, comparable to those in natural soils (e.g., catalase, urease, phosphatase), or even surpassing natural soils (e.g., sucrase), and rich microorganism communities, such as Cyanobacteria, Proteobacteria, Actinobacteria in the bacteria domain, and Ascomycota in the fungi domain, were initially developed. It's suggested that preparing 1 ton of artificial soil entails synergistic consumption of 613.7 kg of red mud and 244.6 kg of phosphogypsum, accounting for mass proportions of 61.4 % and 24.5 %, respectively. In future, more evaluations on the leaching loss of nutrients and alkalinity and the environmental risks of heavy metals should be conducted to more references for the artificial soil application. In summary, the preparation of artificial soil is a very simple, efficient, scalable and low-cost collaborative resource utilization scheme of red mud and phosphogypsum, which has great potential for vegetation restoration in some places such as tailings field and soil-deficient depression.

Study on the mechanism and reaction characteristics of metal-supported phosphogypsum as oxygen carrier in a chemical looping gasification application

This study aimed to explore the chemical looping gasification (CLG) reaction characteristics of the metal-supported composite phosphogypsum (PG) oxygen carriers (OCs) and the thermodynamic mechanism. The FactSage 7.1 thermodynamic simulation was used to explore the oxygen release and H2S removal mechanisms. The experimental results showed that the syngas yield of CLG with PG-CuFe2O4 was more than that with PG-Fe2O320/CuO40 or PG-Fe2O330/CuO30 OC at 1023 K when the water vapor content was 0.3. Furthermore, the maximum syngas yield of the CO selectivity was 70.3% and of the CO2 selectivity was 23.8%. The H2/CO value was 0.78, and the highest carbon conversion efficiency was 91.9% in PG-CuFe2O4 at the gasification temperature of 1073 K. The metal-supported PG composite oxygen carrier was proved not only as an oxygen carrier to participate in the preparation of syngas but also as a catalyst to catalyze coal gasification reactions. Furthermore, both the experimental results and FactSage 7.1 thermodynamic analysis revealed that the trapping mechanism of H2S by composite OCs was as follows: CuO first lost lattice oxygen as an oxygen carrier to generate Cu2O, which, in turn, reacted with H2S to generate Cu2S. This study provided efficient guidance and reference for OC design in CLG.

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.

Physico-Mechanical Evaluation of Geopolymer Concrete Activated by Sodium Hydroxide and Silica Fume-Synthesised Sodium Silicate Solution

Commercial sodium hydroxide (NaOH) and sodium silicate (SS) have remained two of the leading alkaline activators widely used in producing geopolymer concrete, despite some identified negatives regarding their availability and additional CO₂ emissions relating to the overall manufacturing process. This study reports the viability of developing geopolymer concrete using a laboratory-synthesised silica fume (SF)-derived SS solution in combination with NaOH at a molarity of 10M as an alternative binary alkali-alkaline activator to Ground Granulated Blast Furnace slag (GGBS). The use of SF in the development of geoolymer activators will pave the way for the quality usage of other high-silica content by-products from nature, industry, and agriculture. In the currently reported proof of concept, four geopolymer concrete batches were produced using different alkaline activator/precursor-A/P ratios (0.5 and 0.9) and SS to NaOH-SS/SH volume ratios (0.8/1.2 and 1.2/0.8), to establish the impact on the engineering performance. Two controls were adopted for ordinary and geopolymer concrete mixes. The engineering performance was assessed using slump and compaction index (CI) tests, while the Unconfined Compressive Strength (UCS) and tensile splitting (TS) tests were measured at different curing ages in accordance with their appropriate standards. The results indicated a reduction in slump values as the A/P ratio decreased, while the CI values showed a reversal of the identified trend in slump. Consequently, mix GC2 attained the highest UCS strength gain (62.6 MPa), displaying the superiority of the alkali activation and polymerisation process over the CSH gel. Furthermore, the impact of A/P variation on the UCS was more pronounced than SS/SH due to its vital contribution to the overall geopolymerisation process.

Magnetization of Bauxite Residue to Enhance the Removal Efficiency Towards Heavy Metals

Bauxite residues are a mass of industrial wastes derived from aluminum metallurgy. This work provided a simple pyrolysis method to magnetize the bauxite residue to serve as a magnetic adsorbent towards heavy metals removal. The X-ray diffraction patterns and Mossbauer spectrum results confirmed the partial reduction of iron species with an obvious enhancement in magnetization. The magnetized bauxite residue exhibited excellent removal efficiencies for Cu²⁺, Cd²⁺ and Pb²⁺ with maximum adsorption capacities of 219.0 mg g^-1, 275.4 mg g^-1, and 100.4 mg g^-1, which could be quickly separated through a magnet. The adsorption equilibrium data were fitted to the Langmuir isotherm model, while the adsorption kinetics followed a pseudo-first-order model. According to the characterization results, chemical precipitation and sorption was the major mechanism for the removal of Cu²⁺, Pb²⁺, and Cd²⁺. Thus, the magnetized bauxite residue exhibited promising applications for heavy metals removal in wastewater.

Dynamic Variations of Soil-Formation Indicators in Bauxite Residue Driven by the Integration of Waste Solids and Microorganisms

Soil-formation process is critical to ecological rehabilitation on bauxite residue disposal areas. In this study, a soil column experiment was taken to assess the dynamic variations of soil-formation indicators in bauxite residue driven by the integration of waste solids and microorganisms. Results showed that the combination of waste solids and microorganisms significantly decreased the alkalinity, accumulated organic carbon content, and improved aggregate stability of bauxite residue. Compared with waste solids treatments, the addition of acid-producing microorganisms enhanced the changes of soil-formation indicators. The integration of waste solids and microorganisms increased the content of aliphatic carbon, presenting low thermal stability in the residues. The integration of waste solids and microorganisms provides a potentially effective method for soil formation and ecological remediation on bauxite residue disposal areas.

Integrating column leaching experiments and geochemical modelling to predict the long-term alkaline stability during erosion process for gypsum amended bauxite residue

Gypsum amendment is widely used to resolve alkalinity issues and implement sustainable management for bauxite residue disposal areas (BRDAs). Amended BRDAs under natural conditions suffer from long-term erosion processes. Nevertheless, the effect of erosion on amendment efficacy is rarely assessed. In this study, by integrating the geochemical modelling of PHREEQC and column leaching experiments, the dissolution of alkaline solids in bauxite residue (BR) and gypsum amendment, as well as their environmental behaviors, were determined through a 1-year simulated rainfall leaching experiment. The PHREEQC simulation results demonstrated that Na⁺ ion strength, CO₂ partial pressure and rainfall, all affected the saturation index (SI) of calcite significantly and accelerated its corrosion, leading to the dissolution of gypsum and calcite in a relatively stable state. However, Na⁺ ion strength and rainfall significantly acted on the SI of gypsum, which lead to loss of Ca²⁺ and reduction of alkaline stability. In addition to the effects of Na⁺ and Ca²⁺ on the saturation concentration of gypsum and calcite solution, Na⁺ and Ca²⁺ also exhibited significant effects on the equilibrium of chemical species reactions. The column results confirmed that stability of gypsum and calcite was consistent with the simulation results of PHREEQC in the BRDAs environment. Furthermore, multiple linear regressions revealed differences in combined contributions of rainwater and atmospheric CO₂ on the stability of calcite and gypsum. The PHREEQC simulation provides a new approach to predict long-term alkaline stability of BR as well as to establish sustainable remediation on BRDAs during erosion process.