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.

Accelerated alkalinity regulation and long-term dry-wet aging durability for bauxite residue remediated with biomass pyrolysis.

Biomass fermentation provides a potential route toward the ecological disposal for the bauxite residue (BR) with high alkalinity issues. However, how to accelerate the remediation of the alkaline problem with a long-term durability is still a big challenge. Herein, we investigated the acceleration of the decomposition of straw toward organic acid species via a pyrolysis strategy as well as the pH stability during long-term dry-wet aging for the treated BR. The pH of pyrolytic BR at 300 °C is stabilized at around 8.90 after 70 days' dry-wet aging. During the aging, the main Ca-contained alkaline minerals of calcite and cancrinite are dissolved and the content of exchangeable Na+ is reduced. This pyrolysis process can decompose straw quickly and produce more organic matters that are easily degraded to fulvic and humic acid as evidenced by 3D fluorescence spectrum analysis. Compared to the fermentation with straw under natural conditions, the alkalinity regulation of BR after pyrolysis is featured with shorter period and lower pH as well as long-term pH stability. Therefore, the synergistic pyrolysis of BR with straw provides an alternative method to address the alkaline issues, which is conducive to promoting the soil formation of BR.

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.

Selective Electrocatalytic Water Oxidation to Produce H2O2 Using a C,N Codoped TiO2 Electrode in an Acidic Electrolyte.

Production of hydrogen peroxide (H2O2) via in situ electrochemical water oxidation possesses great potential applications in the energy and environment fields. In this work, for the first time, we reported a C,N codoped TiO2 electrode for selective electrocatalytic water oxidation to produce H2O2 in an acidic electrolyte. An electrochemical anodic oxidation method combined with postcalcination in the presence of urea was applied to fabricate such a C,N codoped TiO2 electrode, which was evidenced by detail structural characterizations. The calcination temperature and urea atmosphere were found to play key roles in its catalytic performances; the optimized 600N sample exhibited an onset potential of 2.66 V (vs Ag/AgCl) and a Tafel slope of 51 mV dec-1 at pH 3. Under the optimal applied potential, the cumulative H2O2 concentration for this sample reached 0.29 µmol L-1 cm-2 h-1. More importantly, a simple recalcination strategy was developed to recover the deactivation electrode. This study proposed an efficient C,N codoped TiO2 electrode toward water oxidation to selectively produce H2O2 in the acidic electrolyte, which could be further used to in situ generate H2O2 for the energy- and environment-related fields with water as the precursor.

Effect of amendments on the leaching behavior of alkaline anions and metal ions in bauxite residue.

A column leaching experiment was used to investigate the efficacy of amendments on their ability to remove alkaline anions and metal ions from bauxite residue leachates. Treatments included, simulated acid rain (AR), phosphogypsum + vermicompost (PVC), phosphogypsum + vermicompost + simulated acid rain (PVA), and biosolids + microorganisms (BSM) together with controls (CK). Results indicated that amendment could effectively reduce the leachate pH and EC values, neutralize OH-, CO32-, HCO3-, and water soluble alkali, and suppress arsenic (As) content. Correlation analysis revealed significant linear correlations with pH and concentrations of OH-, CO32-, HCO3-, water-soluble alkali, and metal ions. BSM treatment showed optimum results with neutralizing anions (OH-, CO32-, and HCO3-), water soluble alkali, and removal of metal ions (Al, As, B, Mo, V, and Na), which was attributed to neutralization from the generation of small molecular organic acids and organic matter during microbial metabolism. BSM treatment reduced alkaline anions and metal ions based on neutralization reactions in bauxite residue leachate, which reduced the potential pollution effects from leachates on the soil surrounding bauxite residue disposal areas.