Contamination of soil by toxic chromium (Cr) is a rising health issue due to over-exploitation and industrial production. Toxicity can be decreased by bioremediation because some microorganisms are ...able to convert highly toxic hexavalent chromium Cr(VI) into less toxic trivalent chromium Cr(III) by secreting chromate reductase. Moreover, microorganisms are able to remove Cr by adsorption on microbial cell walls. Plants can also be used for phytoremediation by uptaking Cr from soil into plant organs. Here, we review the speciation of Cr in soil, remediation methods to remove Cr, bioremediation challenges, and remaining ecological impacts after bioremediation. We present the mechanisms of microbial remediation, phytoremediation and plant–microbial combined remediation and applications.
Due to ecologically unsustainable mining strategies, there remain large areas of phosphate mining wasteland contaminated with accumulated lead (Pb). In this study, a Pb-resistant ...phosphate-solubilizing strain of Pseudomonas sp., LA, isolated from phosphate mining wasteland, was coupled with two species of native plants, ryegrass (Lolium perenne L.) and sonchus (Sonchus oleraceus L.), for use in enhancing the reduction of bioavailable Pb in soil from a phosphate mining wasteland. The effect of PbCO3 solubilization by Pseudomonas sp. strain LA was evaluated in solution culture. It was found that strain LA could attain the best solubilization effect on insoluble Pb when the PbCO3 concentration was 1% (w/v). Pot experiments were carried out to investigate the potential of remediation by ryegrass and sonchus in phosphate mining wastelands with phosphate rock application and phosphate-solubilizing bacteria inoculation. Compared to the control group without strain LA inoculation, the biomass and length of ryegrass and sonchus were markedly increased, available P and Pb in roots increased by 22.2%–325% and 23.3%–368%, respectively, and available P and Pb in above-ground parts increased by 4.44%–388% and 1.67%–303%, respectively, whereas available Pb in soil decreased by 14.1%–27.3%. These results suggest that the combination of strain LA and plants is a bioremediation strategy with considerable potential and could help solve the Pb-contamination problem in phosphate mining wastelands.
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•Phosphate solubilizing bacteria-plants jointly reduced lead bioavailability in soils.•Native strain could effectively enhance the growth and lead accumulation of plants.•Native plants ryegrass and sonchus could effectively accumulate lead in soils.•Native strain had effective phosphate solubilizing and lead resistance capacities.
Native strain Pseudomonas sp. LA could enhance the reduction of Pb bioavailability by native ryegrass and sonchus in phosphate mining wastelands.
Phosphate ore is an important raw material for manufacturing fertilizers and phosphorous chemical products. While most of the phosphate resources cannot be directly treated as feed stock due to the ...low grade of P2O5 and high content of impurities. In order to obtain a qualified phosphate concentrate, the beneficiation of the low-grade phosphate ore is, hence, of great necessity. Many beneficiation techniques can be employed to upgrade the P2O5 grade of phosphate ores based on their characteristics in chemical composition and texture. The flotation process is most widely applied to balance the P2O5 recovery ratio and cost. In this review, the dominant techniques for the beneficiation of phosphate ores are introduced. Moreover, the factors that affect the flotation of phosphate ore, including the properties of mineralogy, flotation reagents (depressants and collectors) and flotation medium, were systematically analyzed.
In recent years, with industrial pollution and the application of agricultural fertilizers with high cadmium (Cd) content, soil Cd pollution has become increasingly serious. A large amount of Cd is ...discharged into the environment, greatly endangering the stability of the ecological environment and human health. The use of microorganisms to induce Cd precipitation and mineralization is an important bioremediation method. Itis highly efficient, has a low cost, enables environmental protection, and convenient to operate. This article summarizes the pollution status, pollution source, biological toxicity and existing forms of Cd, as well as the biomineralization mechanism of microbial induced Cd(II) precipitation, mainly including microbial-induced carbonate precipitation, microbial-induced phosphate precipitation and microbial-induced sulfide precipitation. Factors affecting the bioremediation of Cd, such as pH, coexisting ions, and temperature, are introduced. Finally, the key points and difficulties of future microbe-induced Cd(II) biomineralization research are highlighted, providing a scientific basis and theoretical guidance for the application of microbe-induced Cd(II) immobilization in soil.
A novel direct Z-scheme Bi2O3/Bi2MoO6 nanocomposite was successfully obtained via simple alkali treatment of Bi2MoO6 succeeded by air calcination approach, which exhibited excellent ...visible-light-driven photocatalytic degradation of phenol and hydrogen evolution reaction due to the direct Z-scheme charge transfer mechanism. This Z-scheme system not only greatly promoting the efficient separation of photogenerated electron-hole pairs, but also preserves strong and continuous redox ability, resulting in the very high photocatalytic performance.
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•Novel direct Z-scheme Bi2O3/Bi2MoO6 composites was successfully obtained.•The composites exhibited excellent photocatalytic degradation of phenol and hydrogen evolution reaction.•Direct Z-scheme photocatalytic mechanism used to explain activity enhancement.•O2−, h+ and OH as the major reactive species in the forming Z-scheme system.•Oxygen vacancy serves as electron mediator to promote the separation of photogenerated electron-hole pairs.
Semiconductor based photocatalytic technology has attracted substantial research attention based on its potential to overcome environmental and energy crisis. The conventional photocatalysts with their issues such as rapid charge recombination are considered unfavorable candidates for practical applications. Herein, a novel direct Z-scheme based photocatalyst composed of Bi2O3/Bi2MoO6 hetrojunction is proposed for efficient photo-degradation of phenol and hydrogen (H2) production reaction. The hetrojunction comprises of ultra-thin (2D) Bi2O3 nanosheets, in-situ grown over 3D Bi2MoO6 microspheres via simple alkali treatment of Bi2MoO6 succeeded by air calcination step. The relative mass ratio of Bi2O3 and Bi2MoO6 could be fine-tuned by controlling the alkali dosage (i.e. NaOH or KOH). Unlike the conventional Bi2O3/Bi2MoO6 hetrojunction, the proposed catalyst follow a direct Z-scheme charge transfer mechanism which permits superior photocatalytic activity with 96.4% phenol degradation efficiency and high hydrogen evolution rate of 52 μmol·g−1 under visible light irradiation. The exuberant performance is accredited to the spatially separated redox charge carriers, excellent light harvesting capability and fast-charge transportation characteristics of the proposed photocatalyst. The present work serves as a primary pathway to design and develop efficient Bi2MoO6-based direct Z-scheme photocatalysts with promising applications in environmental remediation and solar fuel production.
Lead (Pb) pollution is getting more and more serious in phosphate mining wastelands recently. However, seldom studies focused on the bioremediation of Pb pollution in phosphate mining wastelands by ...phosphate-solubilizing bacterium (PSB). In this study, a PSB named LA with high Pb tolerance was isolated from a phosphate mining wasteland. Based on its cell morphology, physiology, and phylogenetic analysis, it was identified as Pseudomonas sp. Its capabilities to solubilize mid-low-grade phosphate rock (PR) and immobilize Pb were assessed in this study. It was found that LA could effectively solubilize PR on PKO culture medium and release soluble phosphate in the culture medium. PR solubilization and Pb immobilization were investigated at different initial Pb concentrations and pH levels. The results showed that soluble phosphate was highly effective in immobilizing Pb and that when the initial concentration of Pb
2+
was 100 mg/L, the immobilization rate of Pb was enhanced. Further, the mechanisms underlying solubilization of PR and biomineralization of Pb ions in LA were evaluated by Fourier transform infrared spectroscopy and X-ray diffraction. The results showed that some functional groups on the PR surface and LA were altered, and LA could form hydroxyapatite and pyrophosphate with Pb ions.
The aftermath of mining weathered crust elution-deposited rare earth ore produces a large amount of residual ammonium leaching solution, which causes ammonia and nitrogen pollution to the mine site. ...Recently, denitrification by heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria has attracted much attention. However, limited studies exist regarding the denitrification process of HN-AD bacteria. In this study, we combined four strains of HN-AD bacteria, Pseudomonas fulva K3, Pseudomonas mosselii K17, Klebsiella oxytoca A12, and Enterobacter hormaechei A16, obtained from rare earth element leaching sites, to select the best microbial consortium for ammonia nitrogen removal. We designed an ammonia removal process applicable to HN-AD bacteria to directly remove ammonia nitrogen from acidic leaching solutions. The experimental results demonstrated that the most efficient microbial consortium for ammonia nitrogen removal to be K3 + K17 + A16, with a removal efficiency of 89.68% for 8 h. In this process, considering the influencing factors of the ammonia removal process, the larger the influent flow rate and influent ammonia nitrogen concentration, the greater the ammonia nitrogen accumulation and pH decrease in the reactor. In consecutive multi-batch experiments, the ammonia removal process was used to remove ammonia nitrogen, at concentrations of 100-600 mg/L, from the simulated leaching solution at pH 4-7, whereby the effluent ammonia nitrogen concentration was lower than 15 mg/L. The results demonstrate that the ammonia removal process is highly feasible and stable. These findings will provide new ideas for the application of HN-AD bacteria and new methods for the removal of ammonia nitrogen from acidic leaching solutions.
Stable and efficient partial nitrification reaction and nitrite accumulation are essential for the operation of partial nitrification-anaerobic ammonia oxidation process. This paper investigated the ...changes in the performance of a partial nitrification bioreactor during the domestication process. After 40 days of domestication, the partial nitrification reaction remained stable, the nitrite accumulation rate (NAR) increased significantly to 85.7%, and the NO
2
-
-N/NH
4
+
-N ratio stabilized at 1.25 ± 0.04. Both diversity indices (Simpson and Shannon) and richness indices (Ace and Chao1) tend to decrease with domestication. Nitrosomonas as a typical ammonia oxidizing bacterium it increased its relative abundance in the reactor from 0.02 ± 0.007% at P1 to 11.6 ± 2.2% at P2, and at P3 stage the denitrifying functional bacteria norank_f_AKYH76, Dokdonella and unclassified_f_Comamonadaceae reached 33.4 ± 12.5%, 18.8 ± 1.8%, and 10.1 ± 1.3%, respectively, and the increase in the relative abundance of these functional species was also consistent with the accumulation of nitrite nitrogen in the reactor.
Rare earth (RE) with specific application value has become the relatively important strategic resource for all countries. Weathered crust elution-deposited rare earth ore (WCED-REO) are rich in ...middle-heavy RE, and its exploitation restrict the global supply of the middle-heavy RE. The leaching technology directly determines the leaching efficiency of RE in WCED-REO. The selection and use of the leaching agents are the most important parts of the leaching technology. The leaching mechanism is at the core of the leaching technology and also affects the effective utilization of the leaching agents. The research and development of leaching technologies, leaching agents, and leaching mechanisms of WCED-REO are discussed in this article. The development processes of exiting leaching technologies are introduced. The advantages and disadvantages of barrel leaching, pool leaching, heap leaching, in-situ leaching, and other leaching technologies are compared. And the new and practical leaching technologies are put forward. The research progresses of ammonium salt leaching agents, composite ammonium salt leaching agents, and novel non-ammonium leaching agents in recent years are reviewed. The leaching effects of different leaching agents are compared comprehensively to provide guidance for industrial application and research direction of the best leaching agent in the later period. The leaching basic theory, as well as the mass transfer process and leaching kinetics of rare earth are introduced. The mass transfer processes in leaching mechanism are explained from the macroscopic and microscopic perspectives. The further research directions of the leaching technologies, leaching agents, and leaching mechanism of WCED-REO are put forward, which provide the theoretical guidance and technical supports for the green and efficient exploitation of WCED-REO.
•Discussion of the development of the leaching technologies.•Description of the current new and practical leaching technologies.•Compilation of leaching agents in literature.•Overview of advantages and disadvantages of various leaching agents.•Explanation of the leaching mechanism in terms of leaching basic theory of rare earth, mass transfer process, and leaching kinetics.
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