Acid Erosion of Carbonate Fractures and Accessibility of Arsenic-Bearing Minerals: In Operando Synchrotron-Based Microfluidic Experiment.

Underground flows of acidic fluids through fractured rock can create new porosity and increase accessibility to hazardous trace elements such as arsenic. In this study, we developed a custom microfluidic cell for an in operando synchrotron experiment using X-ray attenuation. The experiment mimics reactive fracture flow by passing an acidic fluid over a surface of mineralogically heterogeneous rock from the Eagle Ford shale. Over 48 h, calcite was preferentially dissolved, forming an altered layer 200-500 µm thick with a porosity of 63-68% and surface area >10× higher than that in the unreacted shale as shown by xCT analyses. Calcite dissolution rate quantified from the attenuation data was 3 × 10-4 mol/m2s and decreased to 3 × 10-5 mol/m2s after 24 h because of increasing diffusion limitations. Erosion of the fracture surface increased access to iron-rich minerals, thereby increasing access to toxic metals such as arsenic. Quantification using XRF and XANES microspectroscopy indicated up to 0.5 wt % of As(-I) in arsenopyrite and 1.2 wt % of As(V) associated with ferrihydrite. This study provides valuable contributions for understanding and predicting fracture alteration and changes to the mobilization potential of hazardous metals and metalloids.

Importance of Superemitter Natural Gas Well Pads in the Marcellus Shale.

A large-scale study of methane emissions from well pads was conducted in the Marcellus shale (Pennsylvania), the largest producing natural gas shale play in the United States, to better identify the prevalence and characteristics of superemitters. Roughly 2100 measurements were taken from 673 unique unconventional well pads corresponding to ∼18% of the total population of active sites and ∼32% of the total statewide unconventional natural gas production. A log-normal distribution with a geometric mean of 2.0 kg h-1 and arithmetic mean of 5.5 kg h-1 was observed, which agrees with other independent observations in this region. The geometric standard deviation (4.4 kg h-1) compared well to other studies in the region, but the top 10% of emitters observed in this study contributed 77% of the total emissions, indicating an extremely skewed distribution. The integrated proportional loss of this representative sample was equal to 0.53% with a 95% confidence interval of 0.45-0.64% of the total production of the sites, which is greater than the U.S. Environmental Protection Agency inventory estimate (0.29%), but in the lower range of other mobile observations (0.09-3.3%). These results emphasize the need for a sufficiently large sample size when characterizing emissions distributions that contain superemitters.

Nanospectroscopy Captures Nanoscale Compositional Zonation in Barite Solid Solutions.

Scientists have long suspected that compositionally zoned particles can form under far-from equilibrium precipitation conditions, but their inferences have been based on bulk solid and solution measurements. We are the first to directly observe nanoscale trace element compositional zonation in <10 µm-sized particles using X-ray fluorescence nanospectroscopy at the Hard X-ray Nanoprobe (HXN) Beamline at National Synchrotron Light Source II (NSLS-II). Through high-resolution images, compositional zonation was observed in barite (BaSO4) particles precipitated from aqueous solution, in which Sr2+ cations as well as HAsO42- anions were co-precipitated into (Ba,Sr)SO4 or Ba(SO4,HAsO4) solid solutions. Under high salinity conditions (NaCl ≥ 1.0 M), bands contained ~3.5 to ~5 times more trace element compared to the center of the particle formed in early stages of particle growth. Quantitative analysis of Sr and As fractional substitution allowed us to determine that different crystallographic growth directions incorporated trace elements to different extents. These findings provide supporting evidence that barite solid solutions have great potential for trace element incorporation; this has significant implications for environmental and engineered systems that remove hazardous substances from water.

Biological Recovery of Platinum Complexes from Diluted Aqueous Streams by Axenic Cultures.

The widespread use of platinum in high-tech and catalytic applications has led to the production of diverse Pt loaded wastewaters. Effective recovery strategies are needed for the treatment of low concentrated waste streams to prevent pollution and to stimulate recovery of this precious resource. The biological recovery of five common environmental Pt-complexes was studied under acidic conditions; the chloro-complexes PtCl42- and PtCl62-, the amine-complex Pt(NH3)4Cl2 and the pharmaceutical complexes cisplatin and carboplatin. Five bacterial species were screened on their platinum recovery potential; the Gram-negative species Shewanella oneidensis MR-1, Cupriavidus metallidurans CH34, Geobacter metallireducens, and Pseudomonas stutzeri, and the Gram-positive species Bacillus toyonensis. Overall, PtCl42- and PtCl62- were completely recovered by all bacterial species while only S. oneidensis and C. metallidurans were able to recover cisplatin quantitatively (99%), all in the presence of H2 as electron donor at pH 2. Carboplatin was only partly recovered (max. 25% at pH 7), whereas no recovery was observed in the case of the Pt-tetraamine complex. Transmission electron microscopy (TEM) revealed the presence of both intra- and extracellular platinum particles. Flow cytometry based microbial viability assessment demonstrated the decrease in number of intact bacterial cells during platinum reduction and indicated C. metallidurans to be the most resistant species. This study showed the effective and complete biological recovery of three common Pt-complexes, and estimated the fate and transport of the Pt-complexes in wastewater treatment plants and the natural environment.

The Leakage Risk Monetization Model for Geologic CO2 Storage.

We developed the Leakage Risk Monetization Model (LRiMM) which integrates simulation of CO2 leakage from geologic CO2 storage reservoirs with estimation of monetized leakage risk (MLR). Using geospatial data, LRiMM quantifies financial responsibility if leaked CO2 or brine interferes with subsurface resources, and estimates the MLR reduction achievable by remediating leaks. We demonstrate LRiMM with simulations of 30 years of injection into the Mt. Simon sandstone at two locations that differ primarily in their proximity to existing wells that could be leakage pathways. The peak MLR for the site nearest the leakage pathways ($7.5/tCO2) was 190x larger than for the farther injection site, illustrating how careful siting would minimize MLR in heavily used sedimentary basins. Our MLR projections are at least an order of magnitude below overall CO2 storage costs at well-sited locations, but some stakeholders may incur substantial costs. Reliable methods to detect and remediate leaks could further minimize MLR. For both sites, the risk of CO2 migrating to potable aquifers or reaching the atmosphere was negligible due to secondary trapping, whereby multiple impervious sedimentary layers trap CO2 that has leaked through the primary seal of the storage formation.

Platinum Recovery from Synthetic Extreme Environments by Halophilic Bacteria.

Metal recycling based on urban mining needs to be established to tackle the increasing supply risk of critical metals such as platinum. Presently, efficient strategies are missing for the recovery of platinum from diluted industrial process streams, often characterized by extremely low pHs and high salt concentrations. In this research, halophilic mixed cultures were employed for the biological recovery of platinum (Pt). Halophilic bacteria were enriched from Artemia cysts, living in salt lakes, in different salt matrices (sea salt mixture and NH4Cl; 20-210 g L(-1) salts) and at low to neutral pH (pH 3-7). The main taxonomic families present in the halophilic cultures were Halomonadaceae, Bacillaceae, and Idiomarinaceae. The halophilic cultures were able to recover >98% Pt(II) and >97% Pt(IV) at pH 2 within 3-21 h (4-453 mg Ptrecovered h(-1) g(-1) biomass). X-ray absorption spectroscopy confirmed the reduction to Pt(0) and transmission electron microscopy revealed both intra- and extracellular Pt precipitates, with median diameters of 9-30 nm and 11-13 nm, for Pt(II) and Pt(IV), respectively. Flow cytometric membrane integrity staining demonstrated the preservation of cell viability during platinum recovery. This study demonstrates the Pt recovery potential of halophilic mixed cultures in acidic saline conditions.

Alterations of Fractures in Carbonate Rocks by CO2-Acidified Brines.

Fractures in geological formations may enable migration of environmentally relevant fluids, as in leakage of CO2 through caprocks in geologic carbon sequestration. We investigated geochemically induced alterations of fracture geometry in Indiana Limestone specimens. Experiments were the first of their kind, with periodic high-resolution imaging using X-ray computed tomography (xCT) scanning while maintaining high pore pressure (100 bar). We studied two CO2-acidified brines having the same pH (3.3) and comparable thermodynamic disequilibrium but different equilibrated pressures of CO2 (PCO2 values of 12 and 77 bar). High-PCO2 brine has a faster calcite dissolution kinetic rate because of the accelerating effect of carbonic acid. Contrary to expectations, dissolution extents were comparable in the two experiments. However, progressive xCT images revealed extensive channelization for high PCO2, explained by strong positive feedback between ongoing flow and reaction. The pronounced channel increasingly directed flow to a small region of the fracture, which explains why the overall dissolution was lower than expected. Despite this, flow simulations revealed large increases in permeability in the high-PCO2 experiment. This study shows that the permeability evolution of dissolving fractures will be larger for faster-reacting fluids. The overall mechanism is not because more rock dissolves, as would be commonly assumed, but because of accelerated fracture channelization.

Recovery of critical metals using biometallurgy.

The increased development of green low-carbon energy technologies that require platinum group metals (PGMs) and rare earth elements (REEs), together with the geopolitical challenges to sourcing these metals, has spawned major governmental and industrial efforts to rectify current supply insecurities. As a result of the increasing critical importance of PGMs and REEs, environmentally sustainable approaches to recover these metals from primary ores and secondary streams are needed. In this review, we define the sources and waste streams from which PGMs and REEs can potentially be sustainably recovered using microorganisms, and discuss the metal-microbe interactions most likely to form the basis of different environmentally friendly recovery processes. Finally, we highlight the research needed to address challenges to applying the necessary microbiology for metal recovery given the physical and chemical complexities of specific streams.

Dissolution-Driven Permeability Reduction of a Fractured Carbonate Caprock.

Geochemical reactions may alter the permeability of leakage pathways in caprocks, which serve a critical role in confining CO2 in geologic carbon sequestration. A caprock specimen from a carbonate formation in the Michigan sedimentary Basin was fractured and studied in a high-pressure core flow experiment. Inflowing brine was saturated with CO2 at 40°C and 10 MPa, resulting in an initial pH of 4.6, and had a calcite saturation index of -0.8. Fracture permeability decreased during the experiment, but subsequent analyses did not reveal calcite precipitation. Instead, experimental observations indicate that calcite dissolution along the fracture pathway led to mobilization of less soluble mineral particles that clogged the flow path. Analyses of core sections via electron microscopy, synchrotron-based X-ray diffraction imaging, and the first application of microbeam Ca K-edge X-ray absorption near edge structure, provided evidence that these occlusions were fragments from the host rock rather than secondary precipitates. X-ray computed tomography showed a significant loss of rock mass within preferential flow paths, suggesting that dissolution also removed critical asperities and caused mechanical closure of the fracture. The decrease in fracture permeability despite a net removal of material along the fracture pathway demonstrates a nonintuitive, inverse relationship between dissolution and permeability evolution in a fractured carbonate caprock.

Chromium(III) oxidation by three poorly crystalline manganese(IV) oxides. 2. Solid phase analyses.

Layered, poorly crystalline Mn(IV)O(2) phases are abundant in the environment. These mineral phases may rapidly oxidize Cr(III) to more mobile and toxic Cr(VI) in soils. There is still, however, little knowledge of how Cr(III) oxidation by Mn(IV)O(2) proceeds at the microscopic and molecular levels. Therefore, the sorption mechanisms of Cr(III) and Cr(VI) on Random Stacked Birnessite (RSB), δ-MnO(2), and Acid Birnessite (AB) were determined by Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS). These three synthetic Mn(IV)O(2), which are poorly crystalline phases and have layered structures, were reacted with 50 mM Cr(III) at pH 2.5, 3, and 3.5 before being analyzed by EXAFS. The results indicated that Cr(VI) was loosely sorbed as an outer-sphere complex on Mn(IV)O(2), while Cr(III) was tightly sorbed as an inner-sphere complex. Further research is needed to understand why Cr(III) stopped being significantly oxidized by Mn(IV)O(2) after 30 min. This study, however, demonstrated that the formation of a Cr surface precipitate is not necessarily responsible for the cessation in Cr(III) oxidation. Indeed, no Cr surface precipitate was detected at the microscopic and molecular levels on Mn(IV)O(2) surfaces reacted with Cr(III) for 1 h, although the Cr(III) oxidation ceased before 1 h of reaction at most employed experimental conditions.

Chromium(III) oxidation by three poorly-crystalline manganese(IV) oxides. 1. Chromium(III)-oxidizing capacity.

The Cr(III)-oxidizing capacity of three layered poorly crystalline Mn(IV)O(2) phases, i.e. δ-MnO(2), Random Stacked Birnessite (RSB), and Acid Birnessite (AB), was determined in real-time and in situ, using Quick X-ray Absorption Fine Structure Spectroscopy (Q-XAFS). The results obtained with this technique, which allows the measurement of the total amount of Cr(VI) produced in the system, indicated that the Cr(III) oxidation reaction had ceased between 30 min and 1 h under most experimental conditions. However, this cessation was not observed with a traditional batch technique, which only allows the measurement of Cr(VI) present in solution and thus neglects the amount of Cr(VI) that may be sorbed to Mn(IV)O(2). This study also demonstrated that the Mn(IV)O(2) phase oxidizing the highest amount of Cr(III), which is positively charged in solution, was the mineral featuring the most negatively charged surface. Also, the results indicated that the presence of Mn(II) and/or Mn(III) impurities inside the Mn(IV)O(2) structure could enhance the mineral's capacity to oxidize Cr(III). The information provided in this study will be useful in predicting the capabilities of naturally occurring Mn oxide minerals, which are similar to the three synthetic Mn(IV)O(2) investigated, to oxidize Cr(III) to toxic and mobile Cr(VI) in the soil of contaminated sites.

Arsenic fractionation in mine spoils 10 years after aided phytostabilization.

Aided phytostabilization using a combination of compost, zerovalent iron grit and coal fly ash (CZA) amendments and revegetation effectively promoted the biological recovery of mining spoils generated at a gold mine in Portugal. Selective dissolution of spoil samples in combination with solid phase characterization using microbeam X-ray absorption near edge structure (µXANES) spectroscopy and microbeam X-ray fluorescence (µXRF) mapping were used to assess As associations in spoils ten years after CZA treatment. The results show that As preferentially associates with poorly crystalline Fe-oxyhydroxides as opposed to crystalline Fe-(oxyhydr)oxide phases. The crystalline Fe(III)-phases dominated in the treated spoil and exceeded those of the untreated spoil three-fold, but only 2.6-6.8% of total As was associated with this fraction. Correlation maps of As:Fe reveal that As in the CZA-treated spoils is primarily contained in surface coatings as precipitates and sorbates. Arsenic binding with poorly crystalline Fe-oxyhydroxides did not inhibit As uptake by plants.

Biosupported bimetallic Pd-Au nanocatalysts for dechlorination of environmental contaminants.

Biologically produced monometallic palladium nanoparticles (bio-Pd) have been shown to catalyze the dehalogenation of environmental contaminants, but fail to efficiently catalyze the degradation of other important recalcitrant halogenated compounds. This study represents the first report of biologically produced bimetallic Pd/Au nanoparticle catalysts. The obtained catalysts were tested for the dechlorination of diclofenac and trichlorethylene. When aqueous bivalent Pd(II) and trivalent Au(III) ions were both added to concentrations of 50 mg L(-1) and reduced simultaneously by Shewanella oneidensis in the presence of H(2), the resulting cell-associated bimetallic nanoparticles (bio-Pd/Au) were able to dehalogenate 78% of the initially added diclofenac after 24 h; in comparison, no dehalogenation was observed using monometallic bio-Pd or bio-Au. Other catalyst-synthesis strategies did not show improved dehalogenation of TCE and diclofenac compared with bio-Pd. Synchrotron-based X-ray diffraction, (scanning) transmission electron microscopy and energy dispersive X-ray spectroscopy indicated that the simultaneous reduction of Pd and Au supported on cells of S. oneidensis resulted in the formation of a unique bimetallic crystalline structure. This study demonstrates that the catalytic activity and functionality of possibly environmentally more benign biosupported Pd-catalysts can be improved by coprecipitation with Au.

Palladium nanoparticles produced by fermentatively cultivated bacteria as catalyst for diatrizoate removal with biogenic hydrogen.

A new biological inspired method to produce nanopalladium is the precipitation of Pd on a bacterium, i.e., bio-Pd. This bio-Pd can be applied as catalyst in dehalogenation reactions. However, large amounts of hydrogen are required as electron donor in these reactions resulting in considerable costs. This study demonstrates how bacteria, cultivated under fermentative conditions, can be used to reductively precipitate bio-Pd catalysts and generate the electron donor hydrogen. In this way, one could avoid the costs coupled to hydrogen supply. The catalytic activities of Pd(0) nanoparticles produced by different strains of bacteria (bio-Pd) cultivated under fermentative conditions were compared in terms of their ability to dehalogenate the recalcitrant aqueous pollutants diatrizoate and trichloroethylene. While all of the fermentative bio-Pd preparations followed first order kinetics in the dehalogenation of diatrizoate, the catalytic activity differed systematically according to hydrogen production and starting Pd(II) concentration in solution. Batch reactors with nanoparticles formed by Citrobacter braakii showed the highest diatrizoate dehalogenation activity with first order constants of 0.45 ± 0.02 h⁻¹ and 5.58 ± 0.6 h⁻¹ in batches with initial concentrations of 10 and 50 mg L⁻¹ Pd, respectively. Nanoparticles on C. braakii, used in a membrane bioreactor treating influent containing 20 mg L⁻¹ diatrizoate, were capable of dehalogenating 22 mg diatrizoate mg⁻¹ Pd over a period of 19 days before bio-Pd catalytic activity was exhausted. This study demonstrates the possibility to use the combination of Pd(II), a carbon source and bacteria under fermentative conditions for the abatement of environmental halogenated contaminants.

Assessment of aided phytostabilization of copper-contaminated soil by X-ray absorption spectroscopy and chemical extractions.

Field plots were established at a timber treatment site to evaluate remediation of Cu contaminated topsoils with aided phytostabilization. Soil containing 2600 mg kg⁻¹ Cu was amended with a combination of 5 wt% compost and 2 wt% iron grit, and vegetated. Sequential extraction was combined with extended X-ray absorption fine structure (EXAFS) spectroscopy to correlate changes in Cu distribution across five fractions with changes in the predominant Cu compounds two years after treatment in parallel treated and untreated field plots. Exchangeable Cu dominated untreated soil, most likely as Cu(II) species non-specifically bound to natural organic matter. The EXAFS spectroscopic results are consistent with the sequential extraction results, which show a major shift in Cu distribution as a result of soil treatment to the fraction bound to poorly crystalline Fe oxyhydroxides forming binuclear inner-sphere complexes.

Virus disinfection in water by biogenic silver immobilized in polyvinylidene fluoride membranes.

The development of innovative water disinfection strategies is of utmost importance to prevent outbreaks of waterborne diseases related to poor treatment of (drinking) water. Recently, the association of silver nanoparticles with the bacterial cell surface of Lactobacillus fermentum (referred to as biogenic silver or bio-Ag(0)) has been reported to exhibit antiviral properties. The microscale bacterial carrier matrix serves as a scaffold for Ag(0) particles, preventing aggregation during encapsulation. In this study, bio-Ag(0) was immobilized in different microporous PVDF membranes using two different pre-treatments of bio-Ag(0) and the immersion-precipitation method. Inactivation of UZ1 bacteriophages using these membranes was successfully demonstrated and was most probably related to the slow release of Ag(+) from the membranes. At least a 3.4 log decrease of viruses was achieved by application of a membrane containing 2500 mg bio-Ag(0)(powder) m(-2) in a submerged plate membrane reactor operated at a flux of 3.1 L m(-2) h(-1). Upon startup, the silver concentration in the effluent initially increased to 271 µg L(-1) but after filtration of 31 L m(-2), the concentration approached the drinking water limit ( = 100 µg L(-1)). A virus decline of more than 3 log was achieved at a membrane flux of 75 L m(-2) h(-1), showing the potential of this membrane technology for water disinfection on small scale.

Concomitant microbial generation of palladium nanoparticles and hydrogen to immobilize chromate.

The catalytic properties of various metal nanoparticles have led to their use in environmental remediation. Our aim is to develop and apply an efficient bioremediation method based on in situ biosynthesis of bio-Pd nanoparticles and hydrogen. C. pasteurianum BC1 was used to reduce Pd(II) ions to form Pd nanoparticles (bio-Pd) that primarily precipitated on the cell wall and in the cytoplasm. C. pasteurianum BC1 cells, loaded with bio-Pd nanoparticle in the presence of glucose, were subsequently used to fermentatively produce hydrogen and to effectively catalyze the removal of soluble Cr(VI) via reductive transformation to insoluble Cr(III) species. Batch and aquifer microcosm experiments using C. pasteurianum BC1 cells loaded with bio-Pd showed efficient reductive Cr(VI) removal, while in control experiments with killed or viable but Pd-free bacterial cultures no reductive Cr(VI) removal was observed. Our results suggest a novel process where the in situ microbial production of hydrogen is directly coupled to the catalytic bio-Pd mediated reduction of chromate. This process offers significant advantages over the current groundwater treatment technologies that rely on introducing preformed catalytic nanoparticles into groundwater treatment zones and the costly addition of molecular hydrogen to above ground pump and treat systems.

Virus removal by biogenic cerium.

The rare earth element cerium has been known to exert antifungal and antibacterial properties in the oxidation states +III and +IV. This study reports on an innovative strategy for virus removal in drinking water by the combination of Ce(III) on a bacterial carrier matrix. The biogenic cerium (bio-Ce) was produced by addition of aqueous Ce(III) to actively growing cultures of either freshwater manganese-oxidizing bacteria (MOB) Leptothrix discophora or Pseudomonas putida MnB29. X-ray absorption spectroscopy results indicated that Ce remained in its trivalent state on the bacterial surface. The spectra were consistent with Ce(III) ions associated with the phosphoryl groups of the bacterial cell wall. In disinfection assays using a bacteriophage as model, it was demonstrated that bio-Ce exhibited antiviral properties. A 4.4 log decrease of the phage was observed after 2 h of contact with 50 mg L(-1) bio-Ce. Given the fact that virus removal with 50 mg L(-1) Ce(III) as CeNO(3) was lower, the presence of the bacterial carrier matrix in bio-Ce significantly enhanced virus removal.

Microbial community dynamics in uranium contaminated subsurface sediments under biostimulated conditions with high nitrate and nickel pressure.

BACKGROUND, AIM, AND SCOPE: The subsurface at the Oak Ridge Field Research Center represents an extreme and diverse geochemical environment that places different stresses on the endogenous microbial communities, including low pH, elevated nitrate concentrations, and the occurrence of heavy metals and radionuclides, including hexavalent uranium [U(VI)]. The in situ immobilization of U(VI) in the aquifer can be achieved through microbial reduction to relatively insoluble U(IV). However, a high redox potential due to the presence of nitrate and the toxicity of heavy metals will impede this process. Our aim is to test biostimulation of the endogenous microbial communities to improve nitrate reduction and subsequent U(VI) reduction under conditions of elevated heavy metals. MATERIALS AND METHODS: Column experiments were used to test the possibility of using biostimulation via the addition of ethanol as a carbon source to improve nitrate reduction in the presence of elevated aqueous nickel. We subsequently analyzed the composition of the microbial communities that became established and their potential for U(VI) reduction and its in situ immobilization. RESULTS: Phylogenetic analysis revealed that the microbial population changed from heavy metal sensitive members of the actinobacteria, alpha- and gamma-proteobacteria to a community dominated by heavy metal resistant (nickel, cadmium, zinc, and cobalt resistant), nitrate reducing beta- and gamma-proteobacteria, and sulfate reducing Clostridiaceae. Coincidentally, synchrotron X-ray absorption spectroscopy analyses indicated that the resulting redox conditions favored U(VI) reduction transformation to insoluble U(IV) species associated with soil minerals and biomass. DISCUSSION: This study shows that the necessary genetic information to adapt to the implemented nickel stress resides in the endogenous microbial population present at the Oak Ridge FRC site, which changed from a community generally found under oligotrophic conditions to a community able to withstand the stress imposed by heavy metals, while efficiently reducing nitrate as electron donor. Once nitrate was reduced efficient reduction and in situ immobilization of uranium was observed. CONCLUSIONS: This study provides evidence that stimulating the metabolism of the endogenous bacterial population at the Oak Ridge FRC site by adding ethanol, a suitable carbon source, results in efficient nitrate reduction under conditions of elevated nickel, and a decrease of the redox potential such that sulfate and iron reducing bacteria are able to thrive and create conditions favorable for the reduction and in situ immobilization of uranium. Since we have found that the remediation potential resides within the endogenous microbial community, we believe it will be feasible to conduct field tests. RECOMMENDATIONS AND PERSPECTIVES: Biostimulation of endogenous bacteria provides an efficient tool for the successful in situ remediation of mixed-waste sites, particularly those co-contaminated with heavy metals, nitrate and radionuclides, as found in the United States and other countries as environmental legacies of the nuclear age.

Electrostatic surface charge at aqueous/alpha-Al2O3 single-crystal interfaces as probed by optical second-harmonic generation.

Second harmonic generation (SHG) spectroscopy was used to characterize the pH-dependent electrostatic charging behavior of (0001) and (102) crystallographic surfaces of corundum (alpha-Al2O3) single-crystal substrates. The pH value of the point of zero charge (pH(pzc)) for each surface was determined by monitoring the SH response during three consecutive pH titrations conducted with 1, 10, and 100 mM NaNO3 carbonate-free aqueous solutions. The crossing point of the three titration curves, which corresponds to the pH(pzc), occurs at pH 4.1 +/- 0.4 for the (0001) surface and pH 5.2 +/- 0.4 for the (102) surface. SHG measurements that were recorded as a function of NaNO3 concentration at fixed pH values were found to corroborate the pH(pzc) values identified in the pH titrations. A comparison of the SHG results with surface protonation constants calculated using a simple electrostatic model suggests that surface relaxation and bonding changes resulting from surface hydration do not account for differences between experimental observations and model predictions. The measured pH(pzc) values for the alpha-Al2O3 single-crystal surfaces are significantly more acidic than published values for Al-(hydr)oxide particles which typically range from pH 8 to 10. This discrepancy suggests that the charging behavior of Al-(hydr)oxide particles is determined by surface sites associated with defects assuming that differences in surface acidity reflect differences in the coordination environment and local structure of the potential-determining surface groups.