Heterogeneous microchemistry between CdSO4 and CaCO3 particles under humidity and liquid water

Carbonate rocks as natural buffers: Exploring their environmental impact on heavy metals in sulfide deposits

Carbonate rocks are closely related to the genesis and spatial distribution of polymetallic sulfide deposits. The natural buffering of carbonate rocks can reduce the ecological impact of heavy metals produced by mining and smelting. Ignoring the buffering effect of carbonate rocks on the heavy metals in the mine environment leads to inaccurate ecological risk assessment, wasting land resources and funds. This study investigates Cd, Zn, and Pb distribution and speciation in the water and soil-rice system in the polymetallic sulfide deposit at Daxin, Guangxi. The study aims to reveal the effects of the natural buffering of carbonate rocks on the migration and transformation of heavy metals. The results show that the water Zn and Cd concentrations decreased from 1857.0 to 0.9 mg L-1 to 0.16 and 0.001 mg L-1, respectively, from the mining area to 4 km downstream. The natural buffering of carbonate increases the water pH from 2.80 to 7.64, resulting in a tendency for Cd, Zn, and Pb to separate from the aqueous phase and enrich the sediments. Soil Cd content in the mining area reached 110.0 mg kg-1 (mean value 55.88 mg kg-1), and rice Cd seriously exceeded the maximum limit. However, the weathering of carbonate reduces the migration ability and bioavailability of Cd. Soil Cd is mainly in the Fe-Mn bound and carbonate-bound fractions, resulting in lower Cd content in downstream soils (mean value 2.73 mg kg-1). Soil CaO, tFe2O3, and Mn hindered the uptake of soil Cd by rice rendering a lower exceedance of Cd in downstream rice. Therefore, this study recommends a farmland management plan under the premise of rice Cd content and integrated soil Cd content, which ensures food safety and fully utilizes farmland resources. This result provides a scientific basis for ecological risk assessment, mine environmental protection, and management in the carbonatite sulfide mine environment.

Enhancing phase identification in waste-to-energy fly ashes: Role of Raman spectroscopy, background fluorescence, and photobleaching

Waste-to-Energy (WTE) facilities incinerate ∼11% (∼ 222 Mt) of global solid waste, generating bottom and fly ashes. Landfilling these ashes is costly, and risks releasing contaminants into the environment. Instead, using WTE ashes in secondary industrial applications can circumvent such environmental risks. However, their secondary use is restricted by their inconsistent mineralogy, which may vary due to fluctuating waste composition and combustion conditions. Therefore, there is a need for rapid and reliable monitoring of WTE fly ash mineralogy. Here, we evaluate the employment of Raman spectroscopy for that purpose. Our initial investigation of 12 unique WTE fly ashes resulted in excessive fluorescence, rendering key Raman peaks obscure. To address this issue, we report that a mere 2 min of photobleaching can significantly reduce this fluorescence, facilitating the detection of calcite, calcium sulfate, zincite, and carbon - phases previously undetectable in original spectra. These results show the potential of Raman spectroscopy for rapid monitoring of WTE fly ash mineralogy, which could be beneficial in diverting these ashes from landfill.

Degradation of CdS Yellow and Orange Pigments: A Preventive Characterization of the Process through Pump-Probe, Reflectance, X-ray Diffraction, and Raman Spectroscopy

Cadmium yellow degradation afflicts numerous paintings realized between the XIXth and XXth centuries. The degradation process and its kinetics is not completely understood. It consists of chalking, lightening, flaking, spalling, and, in its most deteriorated cases, the formation of a crust over the original yellow paint. In order to improve the comprehension of the process, mock-up samples of CdS in yellow and orange tonalities were studied by means of structural analysis and optical characterization, with the principal techniques used in the field of cultural heritage. Mock ups were artificially degraded with heat treatment and UV exposure. Relevant colorimetric variation appears in CIE Lab coordinates from reflectance spectra. XRD, SEM-EDS, and Raman spectroscopy revealed the formation of cadmium sulfate, whilst time-resolved photoluminescence and pump-probe transient absorption spectroscopy suggest the formation of a defective phase, compatible with Cd vacancies and the formation of both CdO and CdSO₄ superficial clusters.

Mechanochemically incorporating magnesium sulfate into antigorite to provide active nucleation sites for efficient precipitation of cadmium ions from weak acidic solution

Utilization of natural clay minerals for the treatment of heavy metal cadmium contamination is appealing as the affordable and readily accessible raw materials. However, the low reactivity of natural serpentine limits its practical application for Cd removal. In the present study, mechanochemical activation of antigorite-type serpentine (SP) as example was introduced to enhance its removal capacity for heavy metal of cadmium high enough for practical use. It was found ball-milling at 600 rpm for 60 min for antigorite resulted in the increased release of hydroxyl group to facilitate the precipitation of Cd²⁺, giving a capacity of 27.4 mg/g for the treatment of 100 mg/L Cd²⁺ for 120 min at room temperature, which was 10 times higher than that of the pristine antigorite (2.5 mg/g). More significantly, magnesium sulfate (MgSO₄, MS) was introduced for the first time to process antigorite, thus to form MgSO₄-incorporated antigorite. As a result, the removal capacity for Cd²⁺ was dramatically increased to 239.7 mg/g with the equal antigorite dosage (the molar ratio of SP/MS = 1:0.5), which is also much higher than the other reported clay minerals. Results showed that, MgSO₄ incorporation promoted the reactivity of antigorite and provided numerous SO₄^2- active sites, which allowed the heterogeneous nucleation of basic cadmium sulfate (CdSO₄·3.5 Cd(OH)₂·xH₂O) precipitate on antigorite, therefore not requiring high alkalinity support as the conventional formation of cadmium hydroxide does. Correspondingly, under the new mechanism, the Cd precipitation could take place in a wide pH range, even from pH 3.0, which was a rarely reported phenomenon happening on natural minerals. Based on these findings, this study demonstrated the effectiveness of mechanochemical incorporation of sulfate for enhancing the Cd²⁺ removal capacity of serpentine, as well as the high efficiency of new pathway for Cd²⁺ precipitation. Moreover, the potential of low-cost serpentine as alternative stabilizers for the highly-effective remediation of heavy metal contamination may be expected.

Selective recovery of heavy metals from wastewater by mechanically activated calcium carbonate: Inspiration from nature

In nature, the calcium carbonate shows different interactions with different metal ions. Inspiration from this natural phenomenon, in this work, the selective recovery of heavy metals from wastewater by mechanically activated calcium carbonate was investigated. The changes in Ca²⁺ concentration, pH value and metals uptake ratio of solution showed that M²⁺ (M = Cu, Mn, Zn and Ni) were endowed with different migration rules, resulting in the various interaction with the calcium carbonate in metal-bearing solution. The combination of XRD, SEM, and stereomicroscope affirmed that the adsorbed M²⁺ rarely change the lattice structure of calcium carbonate, while the adsorbed Cu²⁺ and Zn²⁺ could convert the mineral phase from calcium carbonate to posnjakite and hydrozincite, respectively. As a result of phase transition, 15% Cu²⁺ and 6% Zn²⁺ were uptaken with initial concentration of 1 mM for 100 min, however, the unsatisfactory recovery prevented the efficient recycling of metal. The mechanically activated calcium carbonate had a superior solubility at the solid/liquid interface, promoting mineral phase transformation on the premise of weak displacement adsorption. Hence, the uptake ratio of Cu²⁺ and Zn²⁺ were significantly increased to 99% and 53% at the same condition. Finally, Cu²⁺ was recovered from polymetallic systems from complex environment with high precision. The concept of selective recycling in this research guides the development of innovative processes from natural information.

Influence of collecting substrate on the Raman imaging of micron-sized particles

The influence of six common substrates on the Raman imaging of micron-sized inorganic aerosol particles was examined. Laboratory-generated single-component particles of calcite (CaCO₃) and mixed particles of calcite (CaCO₃), nitratine (NaNO₃), hematite (Fe₂O₃) and anglesite (PbSO₄) were deposited by cascade impaction on Ag, In, Si, SiO₂, microscope slide and TEM-grid substrates. The spectral contribution of substrates to Raman images of the deposited particles was evaluated by Multivariate Curve Resolution. The shape and intensity of the substrate spectra affect the effectiveness capability of the spectral deconvolution. The substrates were characterized and compared with respect to their effect on the reconstruction of Raman images of aerosol particles. The TEM-grid substrate yielded spatially stable sample measurements with a homogeneous spectral contribution, satisfactory Raman map reconstruction and the potential for application in other techniques (e.g., SEM-EDX).

Secondary ion mass spectrometry: The application in the analysis of atmospheric particulate matter

Currently, considerable attention has been paid to atmospheric particulate matter (PM) investigation due to its importance in human health and global climate change. Surface characterization, single particle analysis and depth profiling of PM is important for a better understanding of its formation processes and predicting its impact on the environment and human being. Secondary ion mass spectrometry (SIMS) is a surface technique with high surface sensitivity, high spatial resolution chemical imaging and unique depth profiling capabilities. Recent research shows that SIMS has great potential in analyzing both surface and bulk chemical information of PM. In this review, we give a brief introduction of SIMS working principle and survey recent applications of SIMS in PM characterization. Particularly, analyses from different types of PM sources by various SIMS techniques were discussed concerning their advantages and limitations. The future development and needs of SIMS in atmospheric aerosol measurement are proposed with a perspective in broader environmental sciences.

Influence of stearic acid coating of the NaCl surface on the reactivity with NO2 under humidity

In the atmosphere, sea salt aerosols, containing mainly NaCl, can accumulate fatty acids on surface and undergo spatially heterogeneous chemistry with atmospheric nitrogen oxides.

Lead and cadmium phytoavailability and human bioaccessibility for vegetables exposed to soil or atmospheric pollution by process ultrafine particles

When plants are exposed to airborne particles, they can accumulate metals in their edible portions through root or foliar transfer. There is a lack of knowledge on the influence of plant exposure conditions on human bioaccessibility of metals, which is of particular concern with the increase in urban gardening activities. Lettuce, radish, and parsley were exposed to metal-rich ultrafine particles from a recycling factory via field atmospheric fallouts or polluted soil. Total lead (Pb) and cadmium (Cd) concentrations in of the edible plant parts and their human bioaccessibility were measured, and Pb translocation through the plants was studied using Pb isotopic analysis. The Pb and Cd bioaccessibility measured for consumed parts of the different polluted plants was significantly higher for root exposure (70% for Pb and 89% for Cd in lettuce) in comparison to foliar exposure (40% for Pb and 69% for Cd in lettuce). The difference in metal bioaccessibility could be linked to the metal compartmentalization and speciation changes in relation to exposure conditions. Metal nature strongly influences the measured bioaccessibility: Cd presents higher bioaccessibility in comparison to Pb. In the case of foliar exposure, a significant translocation of Pb from leaves toward the roots was observed. To conclude, the type of pollutant and the method of exposure significantly influences the phytoavailability and human bioaccessibility of metals, especially in relation to the contrasting phenomena involved in the rhizosphere and phyllosphere. The conditions of plant exposure must therefore be taken into account for environmental and health risk assessment.

Foliar or root exposures to smelter particles: consequences for lead compartmentalization and speciation in plant leaves

In urban areas with high fallout of airborne particles, metal uptake by plants mainly occurs by foliar pathways and can strongly impact crop quality. However, there is a lack of knowledge on metal localization and speciation in plants after pollution exposure, especially in the case of foliar uptake. In this study, two contrasting crops, lettuce (Lactuca sativa L.) and rye-grass (Lolium perenne L.), were exposed to Pb-rich particles emitted by a Pb-recycling factory via either atmospheric or soil application. Pb accumulation in plant leaves was observed for both ways of exposure. The mechanisms involved in Pb uptake were investigated using a combination of microscopic and spectroscopic techniques (electron microscopy, laser ablation, Raman microspectroscopy, and X-ray absorption spectroscopy). The results show that Pb localization and speciation are strongly influenced by the type of exposure (root or shoot pathway) and the plant species. Foliar exposure is the main pathway of uptake, involving the highest concentrations in plant tissues. Under atmospheric fallouts, Pb-rich particles were strongly adsorbed on the leaf surface of both plant species. In lettuce, stomata contained Pb-rich particles in their apertures, with some deformations of guard cells. In addition to PbO and PbSO4, chemical forms that were also observed in pristine particles, new species were identified: organic compounds (minimum 20%) and hexagonal platy crystals of PbCO3. In rye-grass, the changes in Pb speciation were even more egregious: Pb-cell wall and Pb-organic acid complexes were the major species observed. For root exposure, identified here as a minor pathway of Pb transfer compared to foliar uptake, another secondary species, pyromorphite, was identified in rye-grass leaves. Finally, combining bulk and spatially resolved spectroscopic techniques permitted both the overall speciation and the minor but possibly highly reactive lead species to be determined in order to better assess the health risks involved.