Giovanni Battista De Giudici

Surface control vs. diffusion control during calcite dissolution: Dependence of step-edge velocity upon solution pH

The reaction of the (101–4) calcite surface during dissolution was studied as a function of pH (HCl) by liquid-cell Atomic Force Microscopy (AFM) under ambient conditions of both temperature (22 ∞C) and partial pressure of carbon dioxide (PCO2). A flow through AFM liquid reaction cell was used, and solutions were renewed at a controlled flow rate and sampled for chemical analysis. In the pH range 7.5–4.3, surface dissolution proceeds via the formation of etch pits delimited by steps of height 3.3 ± 0.3 Å or a multiple of this elementary distance. Rhombohedral pits form and grow at the surface by anisotropic step retreat, at a velocity of 4.2 ± 0.2 nm/s for the steps oriented along the two equivalent directions [4 – 41]+ and [481 – ]+, and 1.1 ± 0.2 nm/s for the steps oriented along the other two equivalent directions [441]– and [481 ]–. At the pH value of 2.7, the slower rate slightly increases and is equal to 1.6 ± 0.2 nm/s. At the pH value of 1.7, microtopography measurements indicate that micrometric steps have a meandering shape, and move as a family at a velocity higher than 10 nm/s. These meandering steps are made of nanometric segments oriented along [441]+, [481]+, [4 41]–, and [481 ]–, and they terminate their run against sloped rough etch pits with sides made of narrow terraces delimited by steps oriented along the [481 – ]+. Finally, the present study indicates that reactive area (i.e., the total amount of reactive atomic sites) of a calcite surface increases in response to higher solution acidity firstly by an increase in the step density, and secondly by production of additional roughness at step edges. The change in step edge-velocity observed below pH = 2.7 was interpreted as the response of the dissolving surface to the loss of buffering capacity at the Stern layer. DE GIUDICI: STEP EDGES BEHAVIOR AT CALCITE SURFACE 1280 (Fenter et al. 2000). These measurements confirm the general scheme of surface complexation models. However, X-ray reflectivity studies mainly represent the conditions on the terrace surfaces, whereas calcite surface reactions proceed mainly at step-edge kink sites. X-ray photoelectron spectroscopy (XPS) on freshly cleaved and water-exposed calcite surfaces provided direct evidence for the formation of two distinct surface sites, >CaOH∞ and >CO3H∞ (Stipp and Hochella 1991). In the attempt to define the electric field at the calcite surface for a given aqueous solution, the point of zero charge was measured to fall in the pH range 8–9.5 (10 < I < 10; Somasundaran and Agar 1967; Mishra 1978; Vdović 2001). Van Cappellen et al. (1993) proposed a model of the interface between calcite and water by applying surface complexation theory based on a general oxide surface model with distinct surface hydration sites at the calcite surface as determined by XPS. Many previous AFM investigations have illustrated calcite surface behavior under surface-controlled conditions, and have provided accurate measurements of step-retreat velocity. The aim of the present work is to investigate the behavior of the calcite surface as a function of pH by using in situ AFM techniques. AFM-molecular-scale measurements allow one to determine how matter is removed from calcite surface. Overall dissolution rates are determined by measuring solute concentration. In particular, the questions we would like to answer are: (1) what kind of step morphology appears in AFM images when dissolution is governed by diffusional processes at the interface (transport control), and (2) Do step-retreat velocities remains constant when calcite dissolves at high dissolution rates? EXPERIMENTAL METHODS At the microscopic scale, the calcite surface dissolves via the formation and growth of etch pits and the retreat of isolated steps, as previously shown (e.g., Liang et al. 1996; Jordan and Rammensee 1998). Pits appearing on the cleavage surface of calcite are delimited by elementary steps oriented along the [4 – 41] or [481–] crystallographic directions. For the (101 –4) cleavage surface, these directions are equivalent crystallographically. However, there are two possible non-equivalent steps along each direction. The surface pits orient in such a way that equivalent steps are adjacent, whereas non-equivalent steps are parallel to each other (Fig.1). For one type of step, the step-edge atoms on the upper terrace overhang, and an acute angle of 78∞ is formed, whereas at the opposite side of the pit, the step-edge atoms do not overhang, and an obtuse angle of 102∞ is formed. According to previous studies, pit growth is controlled by anisotropy in velocities; for low CO 3 concentration in solution, the obtuse step (noted as [4 – 41]+ ) has a higher velocity than the acute step (noted as [441]–). Thus, the shape of a rhombohedral pit depends upon the differences between fast and slow step-retreat velocities. Based on these geometric properties of the calcite surface, we were able in this study to identify the crystallographic directions controlling the calcite surface features. A large specimen of optically pure calcite (Island spar) was gently and carefully cleaved in the laboratory to obtain a fresh (101–4) cleavage surface. AFM measurements were performed during dissolution of calcite in distilled water (MQ) at different concentrations of HCl and under ambient atmospheric conditions. The first images were collected during dissolution in MQ water, and then the initial solution reservoir was changed to HCl solutions having lower pH values (according to the sequence of initial pH values 3.3, 3, 2.3, and 1.3). Measurements were repeated on several calcite grains, and good reproducibility of surface features and kinetic data was obtained for each set of experimental conditions. The AFM investigation was performed in Contact Mode using Molecular Imaging AFM equipped with Digital Instruments software and a 7.5 ¥ 7.5 mm scanner. Acoustical noise and chemical contamination were avoided by protecting the liquid cell with an environmental chamber. We chose Si tips with a nominal force constant of 0.2 N/m. The tip-surface force in the liquid was always lower than 1 nN. Images were collected by displaying them in both height and deflection mode. Both the solution reservoir and the liquid cell were open to the atmosphere and the experiment was run at the room temperature of 22 ∞C. The increase of temperature in the liquid cell due to the incident laser light was compensated by the continuous renewal of the solution. A peristaltic pump equipped with two lines (input and output) provided continuous renewal at a controlled flow rate, Q, of 60 mL/min. In this way, the solution droplet (100 mL) between the surface and the tip is completely renewed in about 90 s. As observed by AFM, for Q > 40 mL/ min step-retreat velocities do not depend upon flow rate. Total FIGURE 1. Highly simplified geometry of rhomboedral etch pits at (101 – 4) cleavage surface of calcite. DE GIUDICI: STEP EDGES BEHAVIOR AT CALCITE SURFACE 1281 surface area interacting with the solution, A, was 1 ¥ 10 m. A was measured as the size of the surface (macroscopically flat) exposed to solution in the liquid cell, and does not account for atomic roughness and surface area of microscopic faces that are not on the horizontal plane. The error in A measurement was estimated by AFM and SEM images to be <10%, and depends on the amount of mineral surface that is not on the horizontal plane. Step velocities were estimated by measuring the change in distances between step edges delimiting etch pits and a fixed point, using a slightly modified standard method (e.g., Teng et al. 1999). The center of impurity grains was assumed as a reference point, for two reasons: (1) impurity grains dissolve at a velocity much slower than the calcite surface and (2) after dissolution of a terrace area where a grain is anchored, the grain appears in AFM images in the same horizontal position on the newly formed terrace. The two points above can be probed easily by measuring the size of grains in the sequential images presented in this work, and the positions of the reference grains on the dissolving calcite surface precisely located. Such anchoring is probably due to attractive forces between grain surfaces and the calcite surface. Distances between a step edge and grains of impurities were measured on height profiles by using Digital Instrument software. Retreat velocities of step edges were obtained by calculating the difference between the measured distances between edges and impurity grains and computing these for the time elapsed between the images. The observed drift of the scanner does not imply significant errors for two sequential images (<1.5%). Thus, the error in step-edge retreat corresponds to the error in calibration of piezoelectrics, which is theoretically as low as 1% of the measured distance, but was estimated realistically (with slight excess) to be higher and equal to 5–10%. For the condition of pH = 1.7, step-edge retreat velocities were estimated on the basis that surface features change completely from one image to the next (see section results). Output solutions were collected for chemical analysis. The dissolution rate, R, (mol/m·s) of the reaction was measured as:

Structural properties of biologically controlled hydrozincite: An HRTEM and NMR spectroscopic study

Abstract The microscopic properties of biomineral hydrozincite [Zn5(CO3)2(OH)6] from Naracauli Creek (SW Sardinia) were investigated by using X-ray diffraction (XRD), nuclear magnetic resonance spectroscopy (NMR), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). Because the biomineral hydrozincite turned out to significantly deviate from the ideal structure of hydrozincite, synthetic and geologic hydrozincite samples were also investigated for comparison. SEM imaging shows that biomineral hydrozincite is made of small platelet-shaped crystallites having a 20-50 nm long side at the shortest and other sides measuring hundreds of nanometers long. These are interlaced to form sheaths several micrometers long. HRTEM analysis of the biomineral samples shows an imperfectly oriented aggregation of the nanocrystals that is discussed in terms of mesocrystals. Transmission electron microscopy (TEM) and XRD analysis indicate a progressive decrease in the size of the particles in the biomineral compared to the synthetic and geologic hydrozincite samples, with coherent diffraction domains in the biomineral hydrozincite that are smaller by 30-50% than in the other samples investigated in this study. 13C magic angle spinning (MAS) and cross polarization magic angle spinning (CPMAS) NMR spectra show more than one peak for all the investigated samples, despite the fact that carbon atoms have a unique crystallographic position in the hydrozincite structure. The additional peaks can reflect the presence of lattice defects typical of nanocrystals as indicated by the HRTEM images, where high concentration of lattice defects, such as grain boundaries and stacking modes, can be observed both in the biomineral and in the synthetic samples. Further additional peaks in the NMR spectra of biomineral samples are attributed to organic molecules, relicts of the biomineralization process, in agreement with the filaments observed in SEM images of biomineral samples

Assessment of the applicability of a “toolbox” designed for microbially assisted phytoremediation: the case study at Ingurtosu mining site (Italy)

The paper describes the fieldwork at the Italian test site of the abandoned mine of sphalerite and galena in Ingurtosu (Sardinia), with the aim to assess the applicability of a “toolbox” to establish the optimized techniques for remediation of soils contaminated by mining activities. A preliminary characterization—including (hydro)geochemistry, heavy metal concentration and their mobility in soil, bioprospecting for microbiology and botany—provided a data set for the development of a toolbox to deliver a microbially assisted phytoremediation process. Euphorbia pithyusa was selected as an endemic pioneer plant to be associated with a bacterial consortium, established with ten selected native strains, including metal-tolerant bacteria and producers of plant growth factors. The toolbox was firstly assessed in a greenhouse pot experiment. A positive effect of bacterial inoculum on E. pithyusa germination and total plant survival was observed. E. pithyusa showed to be a well-performing metallophyte species, and only inoculated soil retained a microbial activity with a high functional diversity, expanding metabolic affinity also towards root exudates. These results supported the decision to proceed with a field trial, investigating different treatments used singly or in combination: bioaugmentation with bacterial consortia, mycorrhizal fungi and a commercial mineral amendment. Microbial activity in soil, plant physiological parameters and heavy metal content in plants and in soil were monitored. Five months after the beginning, an early assessment of the toolbox under field conditions was carried out. Despite the cold season (October–March), results suggested the following: (1) the field setup as well as the experimental design proved to be effective; (2) plant survival was satisfactory; (3) soil quality was increased and bioaugmentation improved microbial activity, expanding the metabolic competences towards plant interaction (root exudates); and (4) multivariate analysis supported the data provided that the proposed toolbox can be established and the field trial can be carried forward.

Microscopic surface processes observed during the oxidative dissolution of sphalerite

Sphalerite cleavage surface dissolution in acidic (HC1) and oxygen-saturated solutions was investigated by liquid-cell Atomic Force Microscopy (AFM). The sphalerite surface was cleaved in the laboratory and then mounted in the liquid cell at 298 K with a fast continuous renewal of the interacting solution. AFM data indicate that unreacted (110) surfaces are characterised by flat surface terraces delimited by step edges aligned along [110] crystallographic directions. AFM imaging allowed us to investigate removal of matter only at pH = 0 (HC1). Under these conditions, etch pits develop that are delimited by 1–3-nm-high step edges. However, surface terraces are covered by nanometric protrusions, while the step edges are microrough. Ex-situ solution chemistry measurements performed in flow-through-reactor indicates strong undersaturation with respect to both zinc sulphide and zinc sulphate. The reactivity of the dissolving (110) surface decreases significantly during the 24 hours of run time. Such a decrease suggests a change in the mechanism governing the overall dissolution process. We interpret nanometric protrusions as due to oxidative reactions at the interface that result in a reorganisation of the surface at the nanometric scale. The mechanism limiting the rate of sphalerite dissolution would be the process of protrusion formation and dissolution. A similar phenomenon was observed in an AFM study of the galena surface. Finally, we propose that the process of protrusion formation could be general in the oxidative dissolution of metal sulphides.

Microscopic processes ruling the bioavailability of Zn to roots of euphorbia pithyusa L. Pioneer plant

Euphorbia pithyusa L. was used in a plant growth-promoting assisted field trial experiment. To unravel the microscopic processes at the interface, thin slices of E. pithyusa roots were investigated by micro-X-ray fluorescence mapping. Roots and rhizosphere materials were examined by X-ray absorption spectroscopy at the Zn K-edge, X-ray diffraction, and scanning electron microscopy. Results indicate some features common to all the investigated samples. (i) In the rhizosphere of E. pithyusa, Zn was found to exist in different phases. (ii) Si and Al are mainly concentrated in a rim at the epidermis of the roots. (iii) Zn is mostly stored in root epidermis and does not appear to be coordinated to organic molecules but mainly occurs in mineral phases such as Zn silicates. We interpreted that roots of E. pithyusa significantly promote mineral evolution in the rhizosphere. Concomitantly, the plant uses Si and Al extracted by soil minerals to build a biomineralization rim, which can capture Zn. This Zn silicate biomineralization has relevant implications for phytoremediation techniques and for further biotechnology development, which can be better designed and developed after specific knowledge of molecular processes ruling mineral evolution and biomineralization processes has been gained.

Coupled pot and lysimeter experiments assessing plant performance in microbially assisted phytoremediation.

We performed an experiment at pot scale to assess the effect of plant growth-promoting bacteria (PGPB) on the development of five plant species grown on a tailing dam substrate. None of the species even germinated on inoculated unamended tailing material, prompting use of compost amendment. The effect of inoculation on the amended material was to increase soil respiration, and promote elements immobilisation at plant root surface. This was associated with a decrease in the concentrations of elements in the leaching water and an increase of plant biomass, statistically significant in the case of two species: Agrostis capillaris and Festuca rubra. The experiment was repeated at lysimeter scale with the species showing the best development at pot scale, A. capillaris, and the significant total biomass increase as a result of inoculation was confirmed. The patterns of element distribution in plants also changed (the concentrations of metals in the roots of A. capillaris and F. rubra significantly decreased in inoculated treatments, while phosphorus concentration significantly increased in roots of A. capillaris in inoculated treatment at lysimeter scale). Measured variables for plant oxidative stress did not change after inoculations. There were differences of A. capillaris plant–soil system response between experimental scales as a result of different substrate column structure and plant age at the sampling moment. Soil respiration was significantly larger at lysimeter scale than at pot scale. Leachate concentrations of As, Mn and Ni had significantly larger concentrations at lysimeter scale than at pot scale, while Zn concentrations were significantly smaller. Concentrations of several metals were significantly smaller in A. capillaris at lysimeter scale than at pot scale. From an applied perspective, a system A. capillaris—compost—PGPB selected from the rhizosphere of the tailing dam native plants can be an option for the phytostabilisation of tailing dams. Results should be confirmed by investigation at field plot scale.

Uptake of Cd in hydrozincite, Zn5(CO3)2(OH)6: evidence from X-ray absorption spectroscopy and anomalous X-ray diffraction

The activity of a biological photosynthetic community promotes the seasonal precipitation of hydrozincite, Zn 5 (CO 3 ) 2 (OH) 6 , from heavy-metal contaminated waters of the Rio Naracauli stream, Sardinia. The precipitation removes from waters not only zinc, but also other heavy metals, such as Cd, Cu, Pb. The phenomenon has remarkable environmental implications, and may have remediation applications. In this study, we investigate the nature of Cd binding to hydrozincite by release tests in deionized water, backed by X-ray absorption spectroscopy (XAS) spectra collected at the Cd K -edge, and by synchrotron-based anomalous X-ray diffraction (AXRD) spectra. Release tests indicate that Cd is weakly bound to hydrozincite, being released to a significantly higher rate than Zn. The absence of a residual corresponding to a crystalline phase in the anomalous diffraction pattern indicates that, up to bulk concentration of 2 wt%, Cd occurs in hydrozincite in an essentially disordered environment. The analysis of the weak signal of the second shell in the extended X-ray absorption fine structure (EXAFS) spectra suggests a local environment similar to cadmium carbonate, but distinct from otavite. We conclude that Cd is bound to hydrozincite as a disordered amorphous surface precipitate. The loose nature of the binding suggests a limited potential of hydrozincite both as a control on the mobility of cadmium in natural waters, and as a remediation tool for contaminated effluents.

Investigation of the hydrozincite structure by infrared and solid-state NMR spectroscopy

Abstract To better understand lattice disorder in hydrozincite, natural hydrozincite samples and synthetic analogues were investigated by XRD, FTIR, 13C MAS, and 13C CPMAS NMR. The size of coherent diffraction domains ranges between ~10 nm (Synth1) and ~30 nm (Synth2). FTIR peaks from the antisymmetric CO2-3 stretching v3 mode were observed at 1383 and 1515 cm-1 in all samples. Peaks due to OH vibrations were observed for all the samples at 3234, 3303, and 3363 cm-1, and were sharp only for the samples having larger crystal domains. The 13C MAS and CPMAS NMR spectra showed a main carbon signal at 164 ppm in the Synth2 sample, while two main signals were observed at ~164 and ~168 ppm in the Synth1 sample. The intensity ratio of the latter signals were found to be independent of contact time, in the investigated range between 0.2 and 30 ms. In addition, 13C CPMAS dynamics indicates that the Synth1 sample has shorter T1r with respect to Synth2. This indicates a more effective process of spin diffusion of proton magnetization in the former due to different structural properties of Synth1 and Synth2 samples. In addition, chemical shift anisotropy analysis was attributed to a structural change in the carbonate group or hydrogen bonding for Synth1 and Synth2. This was interpreted as a deviation from the ideal structure generated by linear and planar lattice defects and/or grain boundaries

Efficient artificial mineralization route to decontaminate Arsenic(III) polluted water - the Tooeleite Way

Increasing exposure to arsenic (As) contaminated ground water is a great threat to humanity. Suitable technology for As immobilization and removal from water, especially for As(III) than As(V), is not available yet. However, it is known that As(III) is more toxic than As(V) and most groundwater aquifers, particularly the Gangetic basin in India, is alarmingly contaminated with it. In search of a viable solution here, we took a cue from the natural mineralization of Tooeleite, a mineral containing Fe(III) and As(III)ions, grown under acidic condition, in presence of SO42− ions. Complying to this natural process, we could grow and separate Tooeleite-like templates from Fe(III) and As(III) containing water at overall circumneutral pH and in absence of SO42− ions by using highly polar Zn-only ends of wurtzite ZnS nanorods as insoluble nano-acidic-surfaces. The central idea here is to exploit these insoluble nano-acidic-surfaces (called as INAS in the manuscript) as nucleation centres for Tooeleite growth while keeping the overall pH of the aqueous media neutral. Therefore, we propose a novel method of artificial mineralization of As(III) by mimicking a natural process at nanoscale.

Dissolution of the (001) surface of galena : An in situ assessment of surface speciation by fluid-cell micro-Raman spectroscopy

Abstract The chemical evolution of the galena (001) cleavage surface dissolving in oxygen-saturated solutions was investigated by fluid-cell micro-Raman Spectroscopy (μRS) and solution chemistry. In this novel design of μRS apparatus, the solution in the fluid cell is continuously renewed. A fairly thick (several tens to hundreds of nanometers) layer forms at the galena surface in solutions with pH between 1 and 5.8. This surface layer is composed of Pb oxides, sulfates, and metastable species of sulfur. Native sulfur forms at pH 1 and 4.6, but is not a predominant surface species at pH 5.8. Dissolution rates, measured by solution chemistry, decrease with pH and reaction time. The formation of Pb oxides in these experiments at such low pH values contrasts with thermodynamic predictions based on properties at the macroscale (bulk solution). The in situ assessment of surface speciation confirms that sulfur can partially oxidize at the interface, and indicates that this process of sulfur oxidation depends on pH. We propose that sulfur oxidation may take place, at least partially, during the reaction of dissolved molecular oxygen with S atoms at the galena surface, or in the immediate vicinity. After this first step of reaction, oxygen combines with Pb ions to form Pb oxide at the interface.