Divalent heavy metals and uranyl cations incorporated in calcite change its dissolution process

Application and mechanism of carbonate material in the treatment of heavy metal pollution: a review

Among the many heavy metal pollution treatment agents, carbonate materials show strong flexibility and versatility by virtue of their high adsorption capacity for heavy metals and the characteristics of multiple and simple modification methods. It shows good potential for development. This review summarizes the application of carbonate materials in the treatment of heavy metal pollution according to the research of other scholars. It mainly relates to the application of surface-modified, activated, and nano-sized carbonate materials in the treatment of heavy metal pollution in water. Natural carbonate minerals and composite carbonate minerals solidify and stabilize heavy metals in soil. Solidification of heavy metals in hazardous waste solids is by MICP. There are four aspects of calcium carbonate oligomers curing heavy metals in fly ash from waste incineration. The mechanism of treating heavy metals by carbonate in different media was discussed. However, in the complex environment where multiple types of pollutants coexist, questions on how to maintain the efficient processing capacity of carbonate materials and how to use MICP to integrate heavy metal fixation and seepage prevention in solid waste base under complex and changeable natural environment deserve our further consideration. In addition, the use of carbonate materials for the purification of trace radioactive wastewater and the safe treatment of trace radioactive solid waste are also worthy of further exploration.

Carbonate Coprecipitation for Cd and Zn Treatment and Evaluation of Heavy Metal Stability Under Acidic Conditions

Mining wastes or combustion ash are materials of high carbon sequestration potential but are also known for their toxicity in terms of heavy metal content. To utilize such waste materials for engineered carbon mineralization purposes, there is a need to investigate the fate and mobility of toxic metals. This is a study of the coprecipitation of metals with calcium carbonate for environmental heavy metal mitigation. The study also examines the stability of precipitated phases under environmentally relevant acid conditions. For a wide range of cadmium (Cd) and zinc (Zn) concentrations (10 to 5000 mg/L), induced coprecipitation led to greater than 99% uptake from water. The calcium carbonate phases were found to contain amounts as high as 9.9 wt % (Cd) and 17 wt % (Zn), as determined by novel synchrotron techniques, including X-ray fluorescence element mapping and three-dimensional (3D) nanotransmission X-ray microscopy (TXM). TXM imaging revealed first-of-a-kind observations of chemical gradients and internal nanoporosity within particles. These observations provided new insights into the mechanisms leading to the retention of coprecipitated heavy metals during the dissolution of calcite in acidic (pH 4) solutions. These observations highlight the feasibility of utilizing carbonate coprecipitation as an engineered approach to the durable sequestration of toxic metals.

Effect of phosphate on cadmium immobilized by microbial-induced carbonate precipitation: Mobilization or immobilization?

Microbial-induced carbonate precipitation (MICP) is a promising technology to immobilize/remediate heavy metals (HMs) like cadmium (Cd). However, the long-term stability of MICP-immobilized HMs is unclear, especially in farmland where chemical fertilization is necessary. Therefore, we performed MICP treatment on soils contaminated with various Cd compounds (CdCO₃, CdS, and CdCl₂) and added diammonium phosphate (DAP) to explore the impact of phosphate on the MICP-immobilized Cd. The results showed that MICP treatment was practical to immobilize the exchangeable Cd but to mobilize the carbonate and Fe/Mn oxide-bound Cd. After applying DAP, soil pH declined due to ammonium nitrification. At high P/Ca molar ratios (1/2 and 1), partial previously immobilized Cd was released due to the carbonate dissolution. Contrarily, exchangeable Cd transformed to less mobilizable Fe/Mn oxide-bound at low P/Ca molar ratios (1/4 and 1/8). Meanwhile, other treatments were also helpful in avoiding the release of immobilized Cd, such as applying non-ammonium phosphate and adding lime material after soil acidification. Our investigation suggested that the long-term stability of HMs in remediated sites should be carefully evaluated, especially in agricultural areas with phosphate and nitrogen fertilizer input.

Immobilization of heavy metals by microbially induced carbonate precipitation using hydrocarbon-degrading ureolytic bacteria

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Heavy metal toxicity to hydrocarbon-degrading ureolytic bacteria is Cd > Ni > Cr > Cu > Zn.

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The ureolytic bacteria can tolerate heavy metals and co-precipitate heavy metals.

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The precipitated minerals shifted between calcite and brushite depending on the heavy metal.

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The immobilization of heavy metals resulted in removal efficiency reaching 100%.

Crude oil contamination introduces multiple threats to human health and the environment, most of which are from toxic heavy metals. Heavy metals cause significant threats because of their persistence, toxicity, and bio-accumulation. Biomineralization, performed through many microbial processes, can lead to the immobilization of heavy metals in formed minerals. The potential of the microbially carbonate-induced precipitation (MICP) in removal by biomineralization of several heavy metals was investigated. A collection of diverse 11 bacterial strains exhibited ureolytic activity and tolerance to heavy metals when growing in Luria-Bertani (LB) and urea medium. Determination of the minimum inhibitory concentrations (MIC) revealed that heavy metal toxicity was arranged as Cd > Ni > Cr > Cu > Zn. Three hydrocarbon-degrading bacterial strains (two of Pseudomonas aeruginosa and one of Providencia rettgeri) exhibited the highest tolerance (MIC > 5 mM) to Cu, Cr, Zn, and Ni, whereas Cd exerted significantly higher toxicity with MIC <1 mM. At all MICP conditions, different proportions of calcium carbonate (calcite) and calcium phosphate (brushite) were formed. Pseudomonas aeruginosa strains (QZ5 and QZ9) exhibited the highest removal efficiency of Cr (100%), whereas Providencia rettgeri strain (QZ2) showed 100% removal of Zn. Heavy metal complexes were found as well. Cd removal was evidenced by the formation of cadmium phosphate induced by Providencia rettgeri bacterial activity. Our study confirmed that hydrocarbon-degrading ureolytic bacteria not only can tolerate heavy metal toxicity but also have the capability to co-precipitate heavy metals. These findings indicate an effective and novel biological approach to bioremediate petroleum hydrocarbons and immobilize multiple heavy metals with mineral formation. This is of high importance for ecological restoration via stabilization of soil and alleviation of heavy metal toxicity.

First Steps towards Understanding the Non-Linear Impact of Mg on Calcite Solubility: A Molecular Dynamics Study

Magnesium (Mg2+) is one of the most common impurities in calcite and is known to have a non-linear impact on the solubility of magnesian calcites. Using molecular dynamics (MD), we observed that Mg2+ impacts overall surface energies, local free energy profiles, interfacial water density, structure and dynamics and, at higher concentrations, it also causes crystal surface deformation. Low Mg concentrations did not alter the overall crystal structure, but stabilised Ca2+ locally and tended to increase the etch pit nucleation energy. As a result, Ca-extraction energies over a wide range of 39 kJ/mol were observed. Calcite surfaces with an island were less stable compared to flat surfaces, and the incorporation of Mg2+ destabilised the island surface further, increasing the surface energy and the calcium extraction energies. In general, Ca2+ is less stable in islands of high Mg2+ concentrations. The local variation in free energies depends on the amount and distance to nearest Mg in addition to local disruption of interfacial water and the flexibility of surface carbonate ions to rotate. The result is a complex interplay of these characteristics that cause variability in local dissolution energies. Taken together, these results illustrate molecular scale processes behind the non-linear impact of Mg2+ concentration on the solubility of magnesium-bearing calcites.