Divalent Cd and Pb uptake on calcite {1014} cleavage faces: an XPS and AFM study

The Capture of Cadmium from Solution during the Replacement of Calcite by Apatite

The capture of cadmium (Cd) from phosphate-containing solutions during the replacement of CaCO3 by phosphate phases such as hydroxylapatite (HAP) and tricalcium phosphate (TCP) has been studied under high and low temperature and pressure conditions using atomic force microscopy, scanning electron microscopy equipped with an X-ray spectrometer and a backscattered electron detector, Raman spectroscopy, and microprobe analysis. Starting with cubes of Carrara Marble (polycrystalline calcite) and single crystals of calcite, a new solid phosphate phase was observed to incorporate Cd from solution, formed under different pressure and temperature conditions tested. Results showed that Cd precipitated in a new phase on the surface of all samples tested. In Carrara Marble, pseudomorphic replacement of CaCO3 is restricted possibly due to kinetic limitation caused by the adsorption of Cd complexes formed in solution at reactive surface sites and the variation of fluid composition inside the sample. However, on the sample surface, this kinetic limitation is less influential, so the new phase could incorporate higher amounts of Cd faster. Furthermore, this reaction at room temperature was found to have similar and/or better Cd-uptake efficiency as HAP and CaCO3 in pure Cd solution through the precipitation of Cd-containing phosphate crystals on the sample surface. Both reactions were able to capture Cd in the precipitating phase structure and could provide a mechanism for simultaneous Cd and phosphate removal from solutions contaminated with both, in industrial or natural settings.

Bimetal-Organic Frameworks Incorporating Both Hard and Soft Base Active Sites for Heavy Metal Ion Capture

The design and synthesis of stable porous materials capable of removing both hard and soft metal ions pose a significant challenge. In this study, a novel metal-organic framework (MOF) adsorbent named CdK-m-COTTTB was developed. This MOF material was constructed using sulfur-rich m-cyclooctatetrathiophene-tetrabenzoate (m-H4COTTTB) as the organic ligand and oxygen-rich bimetallic clusters as the inorganic nodes. The incorporation of both soft and hard base units within the MOF structure enables effective removal of various heavy metal ions, including both soft and hard acid species. In single-component experiments, the adsorption capacity of CdK-m-COTTTB for Pb2+, Tb3+, and Zr4+ ions reached levels of 636.94, 432.90, and 357.14 mg·g-1, respectively, which is comparable to specific MOF absorbents. The rapid adsorption process was found to be chemisorption. Furthermore, CdK-m-COTTTB exhibited the capability to remove at least 12 different metal ions in both separate and multicomponent solutions. The material demonstrated excellent acid-base stability and renewability, which are advantageous for practical applications. CdK-m-COTTTB represents the first reported pristine MOF material for the removal of both hard and soft acid metal ions. This work serves as inspiration for the design and synthesis of porous crystalline materials that can efficiently remove diverse heavy metal pollutants.

Pb Removal Efficiency by Calcium Carbonates: Biogenic versus Abiogenic Materials

The sorption of heavy metals on mineral surfaces plays a key role in controlling the fate and bioavailability of harmful elements through dissolution-precipitation reactions. Here, we investigate the efficiency of Pb removal from highly contaminated waters by two calcium carbonate hard tissues, scallop shells (up to 99.9 mol %; -biocalcite) and cuttlefish bones (up to 90.0 mol %; bioaragonite), which template the precipitation of the highly insoluble mineral cerussite (PbCO3). The experiments show that both biomaterials are about five times more effective Pb scavengers (5 mmol of cerussite precipitated/g sample) than their inorganic counterparts (∼1 mmol/g). We relate this enhanced Pb scavenging capacity of biocarbonates to their composite organic-inorganic nature, which modulates their specific nano- and microstructural features and defines their larger surface areas, solubility, and reactivity compared to those of their inorganic counterparts. The oriented growth of cerussite progressively passivates the bioaragonite surface, reducing its long-term Pb scavenging capacity. In contrast, the randomly oriented growth of cerussite crystals on biocalcite prevents surface passivation and explains why biocalcite outperforms bioaragonite as a long-term Pb scavenger. The use of biocarbonates could be a key for designing more efficient decontamination strategies for heavy metal-polluted waters.

Microbial functionalities and immobilization of environmental lead: Biogeochemical and molecular mechanisms and implications for bioremediation

The increasing environmental and human health concerns about lead in the environment have stimulated scientists to search for microbial processes as innovative bioremediation strategies for a suite of different contaminated media. In this paper, we provide a compressive synthesis of existing research on microbial mediated biogeochemical processes that transform lead into recalcitrant precipitates of phosphate, sulfide, and carbonate, in a genetic, metabolic, and systematics context as they relate to application in both laboratory and field immobilization of environmental lead. Specifically, we focus on microbial functionalities of phosphate solubilization, sulfate reduction, and carbonate synthesis related to their respective mechanisms that immobilize lead through biomineralization and biosorption. The contributions of specific microbes, both single isolates or consortia, to actual or potential applications in environmental remediation are discussed. While many of the approaches are successful under carefully controlled laboratory conditions, field application requires optimization for a host of variables, including microbial competitiveness, soil physical and chemical parameters, metal concentrations, and co-contaminants. This review challenges the reader to consider bioremediation approaches that maximize microbial competitiveness, metabolism, and the associated molecular mechanisms for future engineering applications. Ultimately, we outline important research directions to bridge future scientific research activities with practical applications for bioremediation of lead and other toxic metals in environmental systems.

Environmental and human health risks of potentially harmful elements in mining-impacted soils: A case study of the Angouran Zn-Pb Mine, Iran

The soil pollution status, levels of exposure, and potential ecological and health risks to workers and residents by potentially harmful elements (PHEs) in the Angouran mining area (Iran), the biggest Zn-Pb mine in the Middle East, were studied. To this aim, topsoil (0-5 cm) samples (n = 63) from different land-uses were analyzed for their total PHEs concentrations. Mine worker's blood analysis and in vitro digestion extractions were applied in conjunction with human health risk assessment (HHRA) to assess the potential health impacts by exposure to PHEs. The maximum PHEs total concentrations were found in the soils near the waste rock dumps. HHRA indicated that ingestion of soils may induce a non-carcinogenic risk due to As and Pb (for both age groups of children and adults), while dermal contact for children may induce the same type of risk due to Cd, and Pb. The carcinogenic risks (CR_s) of As, Cd, Cr, and Ni through ingestion route were above the acceptable value of 1 × 10^-4, and children may face greater health risks. The average blood Zn, Pb, and Cd levels in the mine workers largely exceeded the safe concentration for adults, while 30% of the workers were tested positive for As in blood. In vitro digestion extractions indicated that the highest bioaccessible contents of As, Cd, Pb, Ni and Zn were found for the industrial-residential and mine soils in the area, while those of Cr and Cu were observed in the agricultural use soils. This study illustrates that a combination of techniques, including geochemical analysis, in vitro bioaccessibility extractions, HHRA, and blood analysis, is a workable integrated approach for evaluating pollution and health risks in mining districts.

Photoelectrochemical Immunosensor Based on a 1D Fe2O3/3D Cd-ZnIn2.2Sy Heterostructure as a Sensing Platform for Ultrasensitive Detection of Neuron-Specific Enolase

Lung cancer is a high-mortality cancer related to the concentration of neuron-specific enolase (NSE). In this work, a sandwich-type photoelectrochemical (PEC) immunosensor was constructed for ultrasensitive detection of NSE, which is based on iron trioxide/indium zinc cadmium sulfide (Fe₂O₃/Cd-ZnIn_2.2S_y) as a sensing platform and Ag-modified polyaniline (Ag@PANI) as a signal amplification label. The 1D Fe₂O₃ porous nanorods with a large specific surface area were synthesized by calcination of Fe-MIL-88A and etching of NaOH. To improve the photocurrent response, the 3D architecture Cd-ZnIn_2.2S_y was combined with the 1D Fe₂O₃ porous nanorods to form a 1D Fe₂O₃/3D Cd-ZnIn_2.2S_y heterostructure. Specifically, the Fe₂O₃/Cd-ZnIn_2.2S_y heterostructure with a good energy level matching (the two can form a stepped energy level matching, which accelerates the transfer rate of electrons) can improve the separation efficiency of electron-hole pairs (e^-/h⁺) under visible light irradiation, which enhances the photocurrent response. Ag@PANI has a strong electron transport capability and can be used as a secondary antibody marker for the signal amplification of the immunosensor. The sensor exhibits a good linear detection range of 100 fg/mL to 100 ng/mL with a low detection limit of 33.5 fg/mL. Moreover, the constructed sandwich-type PEC immunosensor shows good performance and possesses excellent specificity, selectivity, and stability over a period of 4 weeks for NSE detection. With these excellent properties, the immunosensor can be extended to analyze and diagnose other disease biomarkers.

Mechanisms of Ni removal from contaminated groundwater by calcite using X-ray absorption spectroscopy and Ni isotope measurements

A flow-through cell (FTC) experiment was conducted to identify mechanisms of Ni removal by calcite through study of changes in Ni speciation and Ni isotope signature during the treatment of simulated Ni-contaminated groundwater. Synthetic Ni-contaminated groundwater was pumped through a FTC packed with crushed natural calcite. Effluent samples were collected to determine concentrations of anions, cations, and for Ni isotope-ratio measurement. X-ray absorption spectroscopy (XAS) was performed on chosen spots of the solid phase along the FTC length. Isotope data indicated multiple mechanisms affected Ni removal in the FTC system. Ni adsorption to and coprecipitation with calcite dominated the early part of the experiment yielding a fractionation factor of ε = -0.5 ‰. Subsequently, Ni precipitation as a Ni-hydroxide phase became the major process controlling Ni removal, resulting in a fractionation factor ε = -0.4 ‰. XAS analysis confirmed the presence of both Ni(OH)₂ and (Ni, Ca)CO₃ types of Ni local structural environments. Results from this study highlight the potential of Ni isotopes as auxiliary tools to determine the processes involved in Ni attenuation from the environment. The characterization of mechanisms involved in Ni removal from solution is necessary to evaluate potential impacts to the environment and to develop effective remediation strategies.

Effect of application mode (capping and amendment) on the control of cadmium release from sediment by apatite/calcite mixture and its phosphorus release risk

In this research, the influence of application mode (capping and amendment) on the control of cadmium (Cd) liberation from sediment by apatite/calcite mixture and its phosphorus release risk were investigated. The results showed that calcite addition had a limited effect on the speciation of Cd in sediment, but apatite addition had a significant impact on the fractionation of Cd in sediment. Apatite amendment could effectively immobilize the most readily mobilized Cd by transferring the acid-soluble fraction to the reducible and residual fractions. Apatite addition also could effectively reduce the concentration of toxicity characteristic leaching procedure (TCLP)-leachable Cd in sediment, and apatite had a much higher reduction efficiency of TCLP-leachable Cd than calcite. Apatite/calcite mixture capping could reduce the risk of Cd liberation from sediment into the overlying water, and the controlling efficiency of apatite/calcite mixture capping was higher than that of apatite/calcite mixture amendment. The effect of apatite/calcite mixture addition on the concentration of reactive soluble phosphorus (SRP) in the overlying water was limited. The introduction of calcite into the apatite capping layer could lower the risk of phosphorus release from apatite to the overlying water as compared to single apatite capping. However, the apatite/calcite mixture capping layer still had a relatively high risk of phosphorus liberation into the overlying water. Results of this work suggest that apatite/calcite mixture has a high potential to be used as a capping material to control Cd release from sediment from the perspective of controlling efficiency and application convenience.

Calcium carbonate as sorbent for lead removal from wastewaters

Low-cost and largely available industrial by-products such as calcite (CaCO₃) have been considered as sorbents to remediate wastewaters from toxic elements, such as lead, in compliance with the European circular economy strategy. To date few articles are reporting results on lead sorption at the calcite-water solution interface by X-ray photoelectron spectroscopy (XPS) and this investigation aims to clarifying the mechanism of the interaction of Pb²⁺ model solutions over a wide concentration range, from 0.1 μM to 80 mM, with commercial calcite. X-ray powder diffraction (XRPD), scanning electron microscopy (SEM, EDX) and XPS analysis indicate that when CaCO₃ particles are soaked in Pb²⁺ 0.1 mM and 1 mM solutions, hexagonal platelets of hydrocerussite [(PbCO₃)₂ Pb(OH)₂] precipitate on its surface. When the concentration of Pb²⁺ is equal or higher than 40 mM, prismatic acicula of cerussite [PbCO₃] precipitate. Solution analysis by atomic emission spectroscopy (ICP-AES) and ICP-mass spectrometry (ICP-MS) indicate that Pb²⁺ removal efficiency is nearly 100%; when the initial Pb²⁺ concentration was equal to 0.1 μM it was below the limit of detection (LOD) and the efficiency could not be determined. The sorption capacity (q_e) increases linearly with increasing initial Pb²⁺ concentration up to a value of 1680 (20) mg/g when the initial Pb²⁺concentration is 80 mM. These findings suggest that heterogeneous nucleation and surface co-precipitation occur and calcite can be well considered a very promising sorbent for Pb²⁺ removal from wastewaters within a wide initial concentration range.

Insights into the effect of cations on cathodic behavior and microstructure in cadmium electrochemical recovery process

Secondary resources provide an essential source for cadmium recovery, but also bring severe environmental problems. Due to the short process and high product purity, electrodeposition is suitable for realizing the reduction, reuse, and recycling of cadmium, while the complex composition of the resources contributes to a complicated electrochemical system. In this work, the effect of common cations (Cu²⁺, Ni²⁺, Fe²⁺, and Zn²⁺) on cadmium electrochemical recovery was investigated from the perspective of electrochemical behavior and microstructure. The results indicated that Cu affected the electrochemical process most prominently, which was deposited on the cathode and formed microcell with Cd, not only impeding the recovery of Cd, but also influencing the purity severely. Comparatively, Ni showed a relatively minor effect, which made the formal potential more negative and alleviated cathodic polarization to some degree. Besides, Fe²⁺ was oxidized by the oxygen released from the anode, and followed by the reaction with Cd, resulting in the redissolution of Cd. With respect to Zn, a low concentration of Zn (≤1 g L^-1) had little influence on electrochemical behavior of Cd, while it was deposited simultaneously with Cd on the cathode at a high concentration (5 g L^-1). Based on the microstructural characterization, the lithops-like cathode cadmium grew up in the presence of Cu, while dendritic Cd was formed affected by Zn, Fe²⁺, and Ni, especially Fe²⁺.

An efficient strategy to screen an effective catalyst for NOx-SCR by deducing surface species using DRIFTS

HYPOTHESIS

Transition metal supported TiO₂ is one of the hottest catalysts in the field of selective catalytic reduction (SCR) of nitrogen oxides. Various formulas have been put forward for an enhanced activity. However, seldom work emphasizes on easy and fast screening of an effective catalyst.

EXPERIMENTS

In this work, Diffuse Reflection Fourier Transform Infrared (DRIFTS) screened catalyst by analyzing intermediates during SCR.

FINDINGS

TiO₂ provided main adsorption sites for NH₃ and the "Eley-Rideal" mechanism dominated the catalysis. The transition metals served as the bridge of electron transport. Moreover, the area reduction rate of adsorbed NH₃ and NH₄⁺ species in DRIFTS represented the electron-transfer rate as well as catalytic activity. In other words, a faster area reduction indicated a better SCR activity. Therefore, this work supplied a fast strategy to screen the most effective catalyst among different materials even without using a nitrogen oxides detector. At the same time, less ammonia and nitrogen oxides were used or discharged.

Isotherm, kinetics, and adsorption mechanism studies of diethylenetriaminepentaacetic acid-modified banana/pomegranate peels as efficient adsorbents for removing Cd(II) and Ni(II) from aqueous solution

Two novel absorbents were synthesized for the first time by banana and pomegranate peels using diethylenetriaminepentaacetic acid (DTPA) modification to eliminate Cd(II) and Ni(II) of sewage. The DTPA-modified peels performed significantly higher adsorption capacity than unmodified materials. The maximum adsorption capacities of DTPA-modified banana/pomegranate peel were 46.729/46.296 mg/g for Cd(II), and 29.240/16.611 mg/g for Ni(II). Adsorption isotherm and kinetics models were simulated to determine their removal efficiency and potential for recovery of these two heavy metals. As the results, the adsorption reached equilibrium within 5 min and was well described by the pseudo-second order model and Langmuir isotherm. The surface morphology analysis of the synthetic materials by Scanning Electron Microscopy-Energy Dispersive X-ray spectroscopy, Fourier Transform Infrared spectroscopy, and X-ray Photoelectron Spectroscopy, implied that ion exchange, complexation, and physical adsorption may together contribute to Cd(II) and Ni(II) loading on DTPA-modified peels. This study demonstrates the feasibility of waste peels as cost-efficient bio-absorbents to remove Cd(II) and Ni(II) in sewage systems, and discovers potential adsorption mechanism of efficiency improvements after DTPA modification.

Heavy metal distribution in particle size fractions of floodplain soils from Dongchuan, Yunnan Province, Southwest China

The heavy metal pollution level in soils is heavily affected by the soil particle size distribution. However, the heavy metal loss during particle size extraction and the effect of calcite on the heavy metals removal in terms of the particle size are unclear. In this study, the distribution of heavy metals (Cu, Cd, Cr, Co, Ni, Zn, Pb, U, and V) was determined in five particle fractions (> 2, 2-0.25, 0.25-0.02, 0.02-0.002, and < 0.002 mm) of two soil and one sediment samples collected from the floodplain of Dongchuan, Yunnan Province, Southwest China. The sampled floodplain soils were mainly composed of gravel and sand fractions (> 97%). The concentrations of all nine heavy metals in the sampled soils and sediment increase significantly with decreasing particle sizes. The maximal loss rate of Cd and Cu reaches 54% and 8.6%, respectively, which should be considered in the process of particle size fraction extraction in soils. The removal amount and removal rate of heavy metals in solution by pure calcite ranks in the order of Pb²⁺ > Cu²⁺ > Cr⁶⁺, while the removal rate of Pb (93.13%) is much higher than that of Cu (24.56%) and Cr (10.71%), which increase with the calcite particle size decreasing. The stabilization of carbonate minerals in soils is crucial for heavy metal pollution control in floodplain soils with high carbonate concentrations in Dongchuan, China.

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.

Aerobic and anaerobic removal of lead and mercury via calcium carbonate precipitation mediated by statistically optimized nitrate reductases

The nonbiodegradability nature of heavy metals renders them resident in food chain and subsequently, destructing the entire ecosystem. Therefore, this study aimed to employ nitrate reduction-driven calcium carbonate precipitation in remediation of lead and mercury aerobically and anaerobically by Proteus mirabilis 10B, for the first time. Initially, Plackett-Burman design was employed to screen of 16 independent variables for their significances on periplasmic (NAP) and membrane-bound (NAR) nitrate reductases. The levels for five significant variables and their interaction effects were further optimized by central composite design. The maximum activities of NAP and NAR recorded 2450 and 3050 U/mL by 2-fold enhancement, comparing with non-optimized medium. Under aerobic and anaerobic optimized remediation conditions, the changes in media chemistry revealed positive correlation among bacterial growth, nitrate reductase activity, pH, NO₃^- and NO₂^- consumption and removal of Ca²⁺, Pb²⁺ and Hg²⁺. Subsequently, the remediated precipitates were subjected to mineralogical analysis; energy dispersive X-ray patterns exhibited characteristic peaks of C, O and Ca in addition to Pb and Hg. Scanning electron microscope depicted the presence of bacterial imprints and protrusions on rough and smooth surface bioliths. However, X-ray diffraction indicated entrapment of PbCO₃, Pb₂O, CaPbO₃, Hg and Hg₂O in calcite lattice. Interestingly, such approach is feasible, efficient, cost-effective and ecofriendly for heavy metals remediation.

High and fast adsorption of Cd(II) and Pb(II) ions from aqueous solutions by a waste biomass based hydrogel

A waste biomass based hydrogel soybean residue-poly(acrylic acid) (SR–PAA) was prepared through a fast one-step reaction by UV radiation technology. SR–PAA was used to remove Cd(II) and Pb(II) ions from aqueous solutions. Effect of pH value, temperature, initial concentration, contact time, competitive ions in the solutions on metal ions adsorption and desorption/regeneration capacity of SR–PAA was discussed in detailed. It was found that the adsorption equilibrium was achieved within 20 min, and maximum adsorption for Cd(II) and Pb(II) ions were 1.43 and 2.04 mmol g⁻¹, respectively. Besides, adsorption thermodynamic analysis indicates that the process of Cd(II) and Pb(II) ions adsorption was spontaneous, feasible and exothermic in nature. And experimental data fitted the pseudo-second-order and Freundlich isotherm model well. Moreover, XPS spectra analysis proves that the metal ions were adsorbed on SR–PAA due to the interaction of carboxyl, hydroxyl and amine with these ions as ionic bond, coordination bond and electrostatic interaction.

Transport of Cd2+ through saturated porous media: Insight into the effects of low-molecular-weight organic acids

Low-molecular-weight organic acids (LMWOAs) are ubiquitous in the aquatic environment and consequently may affect the heavy metal transport in aquifer systems. In this study, the influences of LMWOAs on the transport of Cd²⁺ under different pH conditions in saturated porous media were evaluated. For this, three LMWOAs such as acetic acid, tartaric acid, and citric acid were employed. A two-site nonequilibrium transport model was applied to simulate the transport data. Under acidic conditions (pH 5.0), the results indicated that LMWOAs inhibited the transport of Cd²⁺ even at the low concentrations of organic acids (i.e., 0.05 and 0.1 mM). The inhibition effects might be attributed to the complexation role of the sand surface-bound organic acids and also electrostatic interaction. Meanwhile, the inhibition effects of LMWOAs on Cd²⁺ transport in the following order of citric acid > tartaric acid > acetic acid, which was also in agreement with the decreasing complex stability constants between Cd²⁺ and LMWOAs. This order may be dependent on their molecular structures (i.e., amount and type of functional groups) and complexing strength. Interestingly, when the LMWOA concentrations 0.5 mM, tartaric acid and citric acid still inhibited Cd²⁺ transport, while acetic acid slightly enhanced the Cd²⁺ mobility due to its weaker complexing strength. However, under neutral conditions (pH 7.0), LMWOAs generally enhanced the transport of Cd²⁺. The transport-enhancement of LMWOAs was ascribed to the formation of stable aqueous non-adsorbing Cd-organic acid complexes. In addition, citric acid could obviously inhibit the transport of Cd²⁺ under competitive transport conditions (i.e., with competing cations), which is mainly due to different complex affinities of citric acid to Pb²⁺ and Cd²⁺. These findings demonstrate that LMWOAs may inhibit or facilitate Cd²⁺ transport under different environmental conditions. Thus, environmental assessment concerning the transport of heavy metals should consider the roles of organic acids.

Heavy metals in handloom-dyeing effluents and their biosorption by agricultural byproducts

The Madhabdi municipality in the Narsingdi district of Bangladesh is a well-known area for textile, handloom weaving, and dyeing industries. These textile industries produce a considerable amount of effluents, sewage sludge, and solid waste materials every day that they directly discharge into surrounding water bodies and agricultural fields. This disposal poses a serious threat to the overall epidemic and socio-economic pattern of the locality. This research entailed the collection of 34 handloom-dyeing effluent samples from different handloom-dyeing industries of Madhabdi, which were then analyzed to determine the contents of the heavy metals iron (Fe), zinc (Zn), copper (Cu), chromium (Cr), manganese (Mn), lead (Pb), and cadmium (Cd). Average concentrations of Fe, Cr, Cu, Pb, Mn, and Zn were 3.81, 1.35, 1.70, 0.17, 0.75, and 0.73 mg L^-1, respectively, whereas Cd content was below the detectable limit of the atomic adsorption spectrophotometer. The concentrations of Fe, Cr, Cu, Pb, and Mn exceed the industrial effluent discharge standards (IEDS) for inland surface water and irrigation water guideline values. A biosorption experiment of the heavy metals (Fe, Cr, Cu, Mn, and Zn) was conducted without controlling for any experimental parameters (e.g., pH, temperature, or other compounds present in the effluent samples) by using four agricultural wastes or byproducts, namely rice husk, sawdust, lemon peel, and eggshell. Twenty grams of each biosorbent was added to 1 L of effluent samples and stored for 7 days. The biosorption capacity of each biosorbent is ranked as follows: eggshell, sawdust, rice husk, and lemon peel. Furthermore, the biosorption affinity of each metal ion was found in the following order: Cu and Cr (both had similar biosorption affinity), Zn, Fe, Mn. The effluents should not be discharged before treatment, and efficient treatment of effluents is possible with eggshell powder or sawdust at a rate of 20 g of biosorbent per liter of effluents.

Removal of cadmium from aqueous solutions using industrial coal fly ash-nZVI

Batch experiments were conducted to test the effects of various solution properties, such as pH, temperature, initial concentration and anoxic and aerobic atmosphere, on Cd removal by nanoscale zerovalent iron (nZVI) supported on industrial coal fly ash. Cd (II) could be removed by adsorption on fly ash-nZVI in a very short time (5 min) with high removal rates (greater than 99.9%) over a wide range of concentration (5-100 mg l^-1). Cd (II) was physically adsorbed on the surface of fly ash-nZVI. The preparation of fly ash-nZVI can incorporate the use of waste media, making the overall adsorbent more removal efficient and low cost.

Toxicity effects on metal sequestration by microbially-induced carbonate precipitation

Biological precipitation of metallic contaminants has been explored as a remedial technology for contaminated groundwater systems. However, metal toxicity and availability limit the activity and remedial potential of bacteria. We report the ability of a bacterium, Sporosarcina pasteurii, to remove metals in aerobic aqueous systems through carbonate formation. Its ability to survive and grow in increasingly concentrated aqueous solutions of zinc, cadmium, lead and copper is explored, with and without a metal precipitation mechanism. In the presence of metal ions alone, bacterial growth was inhibited at a range of concentrations depending on the metal. Microbial activity in a urea-amended medium caused carbonate ion generation and pH elevation, providing conditions suitable for calcium carbonate bioprecipitation, and consequent removal of metal ions. Elevation of pH and calcium precipitation are shown to be strongly linked to removal of zinc and cadmium, but only partially linked to removal of lead and copper. The dependence of these effects on interactions between the respective metal and precipitated calcium carbonate are discussed. Finally, it is shown that the bacterium operates at higher metal concentrations in the presence of the urea-amended medium, suggesting that the metal removal mechanism offers a defence against metal toxicity.

Visualising the molecular alteration of the calcite (104) - water interface by sodium nitrate

The reactivity of calcite, one of the most abundant minerals in the earth’s crust, is determined by the molecular details of its interface with the contacting solution. Recently, it has been found that trace concentrations of NaNO₃ severely affect calcite’s (104) surface and its reactivity. Here we combine molecular dynamics (MD) simulations, X-ray reflectivity (XR) and in situ atomic force microscopy (AFM) to probe the calcite (104) – water interface in the presence of NaNO₃. Simulations reveal density profiles of different ions near calcite’s surface, with NO₃⁻ able to reach closer to the surface than CO₃²⁻ and in higher concentrations. Reflectivity measurements show a structural destabilisation of the (104) surfaces’ topmost atomic layers in NaNO₃ bearing solution, with distorted rotation angles of the carbonate groups and substantial displacement of the lattice ions. Nanoscale AFM results confirm the alteration of crystallographic characteristics, and the ability of dissolved NaNO₃ to modify the structure of interfacial water was observed by AFM force spectroscopy. Our experiments and simulations consistently evidence a dramatic deterioration of the crystals’ surface, with potentially important implications for geological and industrial processes.

Incorporation of Pb at the calcite (104)-water interface

Lead (Pb) is a common environmental pollutant, and its transport in surface waters and groundwater is controlled in part by sorption and precipitation reactions at mineral surfaces. Using in situ specular and resonant anomalous X-ray reflectivity measurements, we investigated the interaction of the calcite (104) surface with a dilute Pb- and EDTA-bearing solution that is slightly undersaturated with respect to calcite. The X-ray results reveal Pb coherently substituting for Ca in the near-surface layers of strained calcite with Pb/(Pb + Ca) atom fractions as high as 0.28 in the outermost layer. The larger ionic radius of Pb(2+) relative to Ca(2+) is accommodated in calcite by vertical displacements of Pb relative to the Ca site. In situ atomic force microscopy images obtained during the reaction suggest that Pb incorporation below the surface occurs after initial dissolution followed by regrowth of a strained epitaxial Pb-rich calcite solid-solution at the calcite (104)-water interface. This process could produce a widespread host phase for Pb in groundwater aquifers and soil pore fluids.

Strontium, nickel, cadmium, and lead substitution into calcite, studied by density functional theory

We have used density functional theory to predict the ion exchange energies for divalent cations Ni(2+), Sr(2+), Cd(2+), and Pb(2+) into a calcite {10.4} surface in equilibrium with water. Exchange energies were calculated for substitution into the topmost surface layer, at the mineral-fluid interface, and into the second layer of the solid. This information can be used as an indicator for cation substitution in the bulk phase, such as for the uptake of toxic metals from the environment and the growth of secondary phases. In both the surface and in the second layer, Ni(2+), Cd(2+), and Pb(2+) substitute exothermically and Sr(2+) substitutes endothermically. Our results agree with published experimental data that demonstrate trace metal coprecipitation with calcite as a sink for Ni(2+), Cd(2+), and Pb(2+), whereas Sr(2+) has a distribution constant significantly smaller than 1. Ni(2+) substitution is favored at the mineral-fluid interface compared with bulk substitution, which also agrees with experimental data. Our results predict that Ni(2+), Cd(2+), and Pb(2+) form a stable solid solution with calcite. Successful prediction of the experimental results gives us confidence in our ability to predict the divalent cation preference for surfaces rather than for sites within the bulk crystal structure, which cannot be directly derived from experiment.

Metal release from sandstones under experimentally and numerically simulated CO2 leakage conditions

Leakage of CO2 from a deep storage formation into an overlying potable aquifer may adversely impact water quality and human health. Understanding CO2-water-rock interactions is therefore an important step toward the safe implementation of geologic carbon sequestration. This study targeted the geochemical response of siliclastic rock, specifically three sandstones of the Mesaverde Group in northwestern Colorado. To test the hypothesis that carbonate minerals, even when present in very low levels, would be the primary source of metals released into a CO2-impacted aquifer, two batch experiments were conducted. Samples were reacted for 27 days with water and CO2 at partial pressures of 0.01 and 1 bar, representing natural background levels and levels expected in an aquifer impacted by a small leakage, respectively. Concentrations of major (e.g., Ca, Mg) and trace (e.g., As, Ba, Cd, Fe, Mn, Pb, Sr, U) elements increased rapidly after CO2 was introduced into the system, but did not exceed primary Maximum Contaminant Levels set by the U.S. Environmental Protection Agency. Results of sequential extraction suggest that carbonate minerals, although volumetrically insignificant in the sandstone samples, are the dominant source of mobile metals. This interpretation is supported by a simple geochemical model, which could simulate observed changes in fluid composition through CO2-induced calcite and dolomite dissolution.

Coupled dissolution and precipitation at the cerussite-phosphate solution interface: implications for immobilization of lead in soils

In situ atomic force microscopy (AFM) has been used to study the interaction of phosphate-bearing solutions with cerussite, PbCO3, (010) surfaces. During the dissolution of cerussite we observed simultaneous growth of needle-shaped or spherical pyromorphite phases. This occurred at two different pH values and ionic strengths relevant to soil solution conditions. The initial dissolution processes occurring at the cerussite solid-phosphate solution interface were clearly distinguished, and heterogeneous nucleation and growth rates of pyromorphites at phosphate concentrations ranging from 0.1 μM to 10 mM were quantitatively defined. Enhanced cerussite dissolution in the presence of high salt (NaCl or NaF) concentrations leads to an increase in pyromorphite nucleation and growth rates. The newly formed pyromorphites were found to be stable upon contact with water or citrate-bearing solutions under acidic or alkaline conditions in the pH range 4-8. These in situ observations may improve the mechanistic understanding of processes resulting in lead immobilization in diverse soil systems as well as to enhance the effectiveness of phosphate-based treatments for remediation of lead-polluted soils.

Lability, solubility and speciation of Cd, Pb and Zn in alluvial soils of the River Trent catchment UK

Alluvial soils can store a wide range of metal contaminants originating from point and diffuse sources. The biological health of these soils is important as they act as an interface between terrestrial and aquatic environments, therefore playing an important role in maintaining the quality of surface waters. The aim of this work was to examine the lability, solubility and bioavailability of Pb, Zn and Cd in the top (0-15 cm) and sub soil (35-50 cm) of metal contaminated alluvial soils from the Trent catchment, U.K. Samples (n = 46) were collected from within 10 m of the river bank. Sources of contamination include historical mining, industry, sewage treatment works and energy production. Enrichment factors based on total metal concentrations showed that contamination in soils declined with distance from the mining areas before rising again as a result of general urbanisation and identified point sources (e.g. river dredging activities). Pore waters were extracted and isotopic dilution and single extraction assays were undertaken on the soils to assess the lability and solubility of the metals. Multi-element isotopic dilution assays were used to determine the labile pool or E-value of these metals in the soil. E-value concentrations were found to range between 0.5 and 14 mg kg(-1), 11-350 mg kg(-1) and 25-594 mg kg(-1) for Cd, Pb and Zn, respectively. Comparison of the E-value assay with the EU standard extraction assay for trace element availability (0.05 M EDTA) showed that EDTA extractions generally over-estimated the E-value for Zn and Pb, with the difference being greater as contamination levels increased. Bioavailability of the metals was assessed by speciating the pore waters [MSol] using WHAM 7 to obtain estimates of free ion activities (M(2+)). Values of (M(2+)) were compared to published 'median critical limits' for soils that estimate levels of protection for 95% of biological species. For each of the three metals, (M(2+)) was found to exceed these critical limits at some sites. Solubility of the metals are reported using Kd values expressed using both the total and E-value as the solid phase. Finally we examine the use of different metal pools (total, E-value, EDTA-extractable) and different measures of Fe oxide pools (total, free total, free amorphous), in predicting [MSol] concentrations and (M(2+)) using WHAM 7 in assemblage modelling mode. Overall best simultaneous model predictions for the three metals were obtained using the E-values. Larger over-estimates of [MSol] and (M(2+)) were produced using the EDTA and total metal pools whereas a better fitting in the prediction was obtained when models used either the total or the free total FeOx pools.

Phytoremediation of heavy metals by calcifying macro-algae (Nitella pseudoflabellata): implications of redox insensitive end products

To evaluate the phytoremediation of heavy metals in water and understand the biochemistry of end products of calcifying macro algae (charophytes), an 84-wk laboratory experiment was conducted. Eighteen microcosms were maintained with and without plants. These were given different heavy metal treatments: no heavy metals, 0.2mgL(-1) Cr(6+) and 0.01mgL(-1) Cd. Accumulation observed to be 0.06% Cr by dry weight and for Cd it was 0.02%. The bioconcentration factors were 3000 and 25000 for Cr and Cd, respectively. Ratios of heavy metal accumulation in alkaline (i.e., calcified areas) to acidic areas of plants were 6 to 4 (for Cr) and 1 to 1 (for Cd). This elucidated an association between heavy metal accumulation and calcification. This was validated by sequential extraction of sediments. It was shown that in microcosms with plants, the heavy metals were mainly in redox insensitive and less bioavailable carbonate bound form (39-47%). This was followed by organic-bound form (23-34%). Carbonate bound end products will ensure long term storage of heavy metals and after plant senescence these will not re-enter the water column.

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

Laboratory experiments using in situ Raman imaging combined with ex situ TOF-S-SIMS demonstrate the behavior of CdSO4·8/3H2O microparticles in contact with 101¯4 CaCO3 (calcite) surface under three different experimental conditions representative of unpolluted atmosphere. The contact of CdSO4·8/3H2O particles with CaCO3 surface in humid air (RH∼40-80%) does not induce any chemical reaction. In contrast, the condensation of a water drop on CdSO4·8/3H2O/CaCO3 interface causes the free dissolution of CdSO4·8/3H2O particle in the drop. A CdSO4·8/3H2O microcrystal is reformed after gentle drying with a CdSO4·H2O coating of the CaCO3 surface. The TOF-S-SIMS image of the CaCO3 surface provides evidence of a thin layer corresponding probably to insoluble coating of CdCO3 (otavite) or CdxCa1-xCO3 solid solution at the liquid-solid interface. This layer armours the CaCO3 from further dissolution and stops the reaction. The deposition of CdSO4·8/3H2O particle in water drop previously in contact with CaCO3 for a long time generates CdCO3 small rhombohedral crystals while gentle drying provokes the crystallization of bar shape crystals of CaSO4·2H2O (gypsum). These laboratory results provide valuable chemical prediction for a possible fate of cadmium rich particles emitted in the atmosphere and thus, can contribute to realistic assessment of human exposure to Cd hazard.

Petrology and geochemistry of annually laminated stalagmites from an Alpine cave (Obir, Austria): seasonal cave physiology

Seasonality is encoded in palaeoproxies of secondary cave mineral deposits (speleothems) and the code is becoming cracked. The petrology of calcite stalagmites from Obir, an Alpine (1100 m altitude), perennially wet cave, was characterized by optical and electron backscatter diffraction, and their chemistry by bulk ICP-MS analysis, ion microprobe and synchrotron-based micro-X-ray fluorescence. Vadose water penetrates 70 m through Triassic limestones (with some Pb–Zn mineralization) to the chamber Säulenhalle where the stalagmites were collected. Strong seasonal ventilation in the cave leads to low PCO2 in winter associated with falls in speleothem sulphate S and increase in δ13C values. All samples display autumnal event lamination defined by a narrow, optically visible zone with increases in trace element concentrations, within which synchrotron studies have resolved μm-scale enrichments of Pb and Zn. Small-scale (10 µm) lateral trace element variations reflect alternate flat faces and rough crystal edges, influenced by high Zn content. The elemental covariations are consistent with the transport of Pb, Zn, P, F, Br and I adsorbed onto organic colloids in dripwater, but the final deposition may have been from aerosols and we propose this as a new mechanism requiring further investigation. This study represents the most complete demonstration of how chemical variations are powerful expressions of seasonal cave physiology in humid temperate caves, including the contrast between summer and winter conditions, and the preservation of sub-weekly events during the autumn season.

Comparison of dissolution and surface reactions between calcite and aragonite in L-glutamic and L-aspartic acid solutions

We have investigated dissolution and surface reaction of calcite and aragonite in amino acid solutions of L-glutamic (L-glu) and L-aspartic acid (L-asp) at weak acidity of above pH 3. The surface reactions of calcite and aragonite were related with the dissolution. Calcite was dissolved in both solutions but the dissolution was limited by an adsorption of Ca-carboxylate salt. Aragonite was neither dissolved nor reacted in amino acid solutions because the crystal surface consisted of a hard to dissolve structure.

Spatially resolved product formation in the reaction of formic acid with calcium carbonate (1014): the role of step density and adsorbed water-assisted ion mobility

The reaction of calcium carbonate (1014) single-crystal surfaces with formic acid (HCOOH) vapor was investigated using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). AFM images indicate the reaction produces rather well-defined crystallites, preferentially at step edges and at distinct angles to one another and mirroring the rhombohedral structure of the calcite surface, while exposing unreacted carbonate surface. The size and surface density of the crystallites depend upon substrate step density, exposure time, and relative humidity. XPS data confirmed the crystallite composition as the expected calcium formate product. The AFM images show erosion and pit formation of the calcite surface in the vicinity of the product crystallites, clearly providing the spatially resolved characterization of the source of Ca ions. AFM experiments exploring the effects of water vapor on the reacted surface show that the calcium formate crystallites are mobile under conditions of high relative humidity, combining to form larger crystallites and nanometer-sized crystals with an orthorhombohedral habit consistent with the alpha form, as confirmed by X-ray diffraction. The implications for the reactions described here are discussed.

Humidity-induced restructuring of the calcite surface and the effect of divalent heavy metals

The composition and topography of calcite 10114 cleavage surfaces, with and without exposure to divalent metals, have been investigated as a function of relative humidity. Atomic force microscopy (AFM) was used to understand topographical changes on the calcite surface due to the presence of divalent metal and exposure to different humid environments. Ion scattering spectroscopy (ISS) was used to determine the composition of the near and outermost surface of the calcite after exposure to Cd and Pb and before exposure to the varying humidity conditions. In general, the extent of topographical changes observed on the calcite surface increased with the humidity level, though the initial step density of the cleaved calcite surface affects the extent of surface restructuring. Pretreatment of the calcite surface with aqueous divalent Pb prior to humidity exposure did not appear to alter the humidity-induced structural changes that occurred on the calcite surface. In contrast, calcite pretreated with divalent Cd showed little topographical change following exposure to high humidity. The results suggest that while Pb forms surface precipitates on the calcite surface, Cd exhibits a stronger interaction with the step edges of the calcite surface, which inhibits the ability of the calcite surface to restructure when exposed to a high relative humidity environment.

Heterogeneous chemistry between PbSO4 and calcite microparticles using Raman microimaging

Currently, the smelting activities of lead and zinc are the loudest sources of local pollution by emission in the troposphere of dust of micrometer size containing PbSO(4). As the particles evolve in the troposphere, their chemical and physical properties - and hence their characteristics such as toxicity - change by accumulation of atmospheric heterogeneous reactions. Calcite (CaCO(3)) represents a large part of the mineral fraction in tropospheric aerosols with aerodynamic diameters less than 10 microm. The calcite particles are expected to react with PbSO(4) particles. In an effort to model the chemical behaviour of PbSO(4) individual particles in the troposphere, we present the in situ Raman imaging results during the course of the reactions in a water droplet of PbSO(4) particles with a calcite microcrystal surface. The computer-microcontrolled XY scanning and Z focusing of confocal Raman imaging combined with multivariate curve resolution (MCR) of Raman images have resolved the severe spectral overlaps of the Raman spectra which are not resolved by the spatial resolution of the instrument ( approximately 1 microm(3)). The results pointed out the identification and the mapping of Pb(3)(CO(3))(2)(OH)(2), PbCO(3) and CaSO(4).2H(2)O (gypsum) on the calcite surface.