Effect of Competing Metals and Humic Substances on Uranium Mobilization from Noncrystalline U(IV) Induced by Anthropogenic and Biogenic Ligands.

Anthropogenic and biogenic ligands may mobilize uranium (U) from tetravalent U (U(IV)) phases in the subsurface, especially from labile noncrystalline U(IV). The rate and extent of U(IV) mobilization are affected by geochemical processes. Competing metals and humic substances may play a decisive role in U mobilization by anthropogenic and biogenic ligands. A structurally diverse set of anthropogenic and biogenic ligands was selected for assessing the effect of the aforementioned processes on U mobilization from noncrystalline U(IV), including 2,6-pyridinedicarboxylic acid (DPA), citrate, N,N'-di(2-hydroxybenzyl)ethylene-diamine-N,N'-diacetic acid (HBED), and desferrioxamine B (DFOB). All experiments were performed under anoxic conditions at pH 7.0. The effect of competing metals (Ca, Fe(III), and Zn) on ligand-induced U mobilization depended on the particular metal-ligand combination ranging from nearly complete U mobilization inhibition (e.g., Ca-citrate) to no apparent inhibitory effects or acceleration of U mobilization (e.g., Fe(III)-citrate). Humic substances (Suwannee River humic acid and fulvic acid) were tested across a range of concentrations either separately or combined with the aforementioned ligands. Humic substances alone mobilized appreciable U and also enhanced U mobilization in the presence of anthropogenic or biogenic ligands. These findings illustrate the complex influence of competing metals and humic substances on U mobilization by anthropogenic and biogenic ligands in the environment.

Redox-Driven Recrystallization of PbO2.

Lead(IV) oxide (PbO2) is one of the lead corrosion products that forms on the inner surface of lead pipes used for drinking water supply. It can maintain low dissolved Pb(II) concentrations when free chlorine is present. When free chlorine is depleted, PbO2 and soluble Pb(II) will co-occur in these systems. This study used a stable lead isotope (207Pb) as a tracer to examine the interaction between aqueous Pb(II) and solid PbO2 at conditions with no net change in dissolved Pb concentration. While the dissolved Pb(II) concentration remained unchanged, significant isotope exchange occurred that indicated that substantial amounts (24.3-35.0% based on the homogeneous recrystallization model) of the Pb atoms in the PbO2 solids had been exchanged with those in solution over 264 h. Neither α-PbO2 nor ß-PbO2 displayed a change in mineralogy, particle size, or oxidation state after reaction with aqueous Pb(II). The combined isotope exchange and solid characterization results indicate that redox-driven recrystallization of PbO2 had occurred. Such redox-driven recrystallization is likely to occur in water that stagnates in lead pipes that contain PbO2, and this recrystallization may alter the reactivity of PbO2 with respect to its stability and susceptibility to reductive dissolution.

Reduction of U(VI) on Chemically Reduced Montmorillonite and Surface Complexation Modeling of Adsorbed U(IV).

Adsorption and subsequent reduction of U(VI) on Fe(II)-bearing clay minerals can control the mobility of uranium in subsurface environments. Clays such as montmorillonite provide substantial amounts of the reactive surface area in many subsurface environments, and montmorillonite-containing materials are used in the storage of spent nuclear fuel. We investigated the extent of reduction of U(VI) by Fe(II)-bearing montmorillonite at different pH values and sodium concentrations using X-ray absorption spectroscopy and chemical extractions. Nearly complete reduction of U(VI) to U(IV) occurred at a low sodium concentration at both pH 3 and 6. At pH 6 and a high sodium concentration, which inhibits U(VI) binding at cation-exchange sites, the extent of U(VI) reduction was only 70%. Surface-bound U(VI) on unreduced montmorillonite was more easily extracted into solution with bicarbonate than surface-bound U(IV) generated by reduction of U(VI) on Fe(II)-bearing montmorillonite. We developed a nonelectrostatic surface complexation model to interpret the equilibrium adsorption of U(IV) on Fe(II)-bearing montmorillonite as a function of pH and sodium concentration. These findings establish the potential importance of structural Fe(II) in low iron content smectites in controlling uranium mobility in subsurface environments.

Exchange of Adsorbed Pb(II) at the Rutile Surface: Rates and Mechanisms.

The dynamics of Pb(II) at mineral surfaces affect its mobility in the environment. Pb(II) forms inner- and outer-sphere complexes on mineral surfaces, and this adsorbed pool often represents a large portion of the bioaccessible Pb in contaminated soils. To assess the lability of this potentially reactive adsorbed Pb(II) pool at metal oxide surfaces, we performed Pb(II) isotope exchange measurements between dissolved Pb(II) enriched in 207Pb and natural isotopic abundance Pb(II) adsorbed to rutile at pH 5, 6, and 7. We find that ∼95% of the adsorbed lead is exchangeable. An initially fast exchange (<1 h) is followed by a slower exchange that occurs on a time scale of hours to days. Pb LIII-edge extended X-ray absorption fine structure spectra indicate that similar binding mechanisms are present at all pH values and Pb(II) loadings, implying that differences in exchange rates across the pH range examined are not attributable to changes in the coordination environment. The slower exchange at pH 5 may be associated with interparticle and intraparticle diffusion resulting from particle aggregation. These findings demonstrate that the dissolved Pb(II) pool can be rapidly replenished by adsorbed Pb(II) if this pool is drawn down incrementally by biological uptake or a shift in chemical conditions.

Metal-Catalyzed Hydrolysis of RNA in Aqueous Environments.

The stability of RNA in aqueous systems is critical for multiple environmental applications including evaluating the environmental fate of RNA interference pesticides and interpreting viral genetic marker abundance for wastewater-based epidemiology. In addition to biological processes, abiotic reactions may also contribute to RNA loss. In particular, some metals are known to dramatically accelerate rates of RNA hydrolysis under certain conditions (i.e., 37 °C or higher temperatures, 0.15-100 mM metal concentrations). In this study, we investigated the extent to which metals catalyze RNA hydrolysis under environmentally relevant conditions. At ambient temperature, neutral pH, and ∼10 µM metal concentrations, we determined that metals that are stronger Lewis acids (i.e., lead, copper) catalyzed single-stranded (ss)RNA, whereas metals that are weaker Lewis acids (i.e., zinc, nickel) did not. In contrast, double-stranded (ds)RNA resisted hydrolysis by all metals. While lead and copper catalyzed ssRNA hydrolysis at ambient temperature and neutral pH values, other factors such as lowering the solution pH and including inorganic and organic ligands reduced the rates of these reactions. Considering these factors along with sub-micromolar metal concentrations typical of environmental systems, we determined that both ssRNA and dsRNA are unlikely to undergo significant metal-catalyzed hydrolysis in most environmental aqueous systems.

Ligand-Induced U Mobilization from Chemogenic Uraninite and Biogenic Noncrystalline U(IV) under Anoxic Conditions.

Microbial reduction of soluble hexavalent uranium (U(VI)) to sparingly soluble tetravalent uranium (U(IV)) has been explored as an in situ strategy to immobilize U. Organic ligands might pose a potential hindrance to the success of such remediation efforts. In the current study, a set of structurally diverse organic ligands were shown to enhance the dissolution of crystalline uraninite (UO2) for a wide range of ligand concentrations under anoxic conditions at pH 7.0. Comparisons were made to ligand-induced U mobilization from noncrystalline U(IV). For both U phases, aqueous U concentrations remained low in the absence of organic ligands (<25 nM for UO2; 300 nM for noncrystalline U(IV)). The tested organic ligands (2,6-pyridinedicarboxylic acid (DPA), desferrioxamine B (DFOB), N,N'-di(2-hydroxybenzyl)ethylene-diamine-N,N'-diacetic acid (HBED), and citrate) enhanced U mobilization to varying extents. Over 45 days, the ligands mobilized only up to 0.3% of the 370 µM UO2, while a much larger extent of the 300 µM of biomass-bound noncrystalline U(IV) was mobilized (up to 57%) within only 2 days (>500 times more U mobilization). This work shows the potential of numerous organic ligands present in the environment to mobilize both recalcitrant and labile U forms under anoxic conditions to hazardous levels and, in doing so, undermine the stability of immobilized U(IV) sources.

Intercomparison and Refinement of Surface Complexation Models for U(VI) Adsorption onto Goethite Based on a Metadata Analysis.

Adsorption of uranium onto goethite is an important partitioning process that controls uranium mobility in subsurface environments, for which many different surface complexation models (SCMs) have been developed. While individual models can fit the data for which they are parameterized, many perform poorly when compared with experimental data covering a broader range of conditions. There is an imperative need to quantitatively evaluate the variations in the models and to develop a more robust model that can be used with more confidence across the wide range of conditions. We conducted an intercomparison and refinement of the SCMs based on a metadata analysis. By seeking the globally best fit to a composite dataset with wide ranges of pH, solid/sorbate ratios, and carbonate concentrations, we developed a series of models with different levels of complexity following a systematic roadmap. The goethite-uranyl-carbonate ternary surface complexes were required in every model. For the spectroscopically informed models, a triple-plane model was found to provide the best fit, but the performance of the double-layer model with bidentate goethite-uranyl and goethite-uranyl-carbonate complexes was also comparable. Nevertheless, the models that ignore the bidentate feature of uranyl surface complexation consistently performed poorly. The goodness of fitting for the models that ignore adsorption of carbonate and the charge distributions was not significantly compromised compared with that of their counterparts that considered those. This approach of model development for a large and varied dataset improved our understanding of U(VI)-goethite surface reactions and can lead to a path for generating a single set of reactions and equilibrium constants for including U(VI) adsorption onto goethite in reactive transport models.

Effects of Cu(II) and Zn(II) on PbO2 Reductive Dissolution under Drinking Water Conditions: Short-term Inhibition and Long-term Enhancement.

Lead oxide (PbO2) has the lowest solubility with free chlorine among Pb corrosion products, but depletion of free chlorine or a switch from free chlorine to monochloramine can cause its reductive dissolution. We previously reported that Cu(II) and Zn(II) inhibited PbO2 reductive dissolution within 12 h. Here, we expanded on this work by performing longer duration experiments and further exploring the underlying mechanisms. Between 12 and 48 h, Cu(II) and Zn(II) had no discernible effect on PbO2 reductive dissolution. From 48 to 192 h, Cu(II) and Zn(II) enhanced PbO2 reductive dissolution. Dissolved oxygen (DO) concentrations followed the same trends as PbO2 reductive dissolution, indicating that the DO was produced by PbO2 reductive dissolution. On the basis of extended X-ray absorption fine structure spectra, we hypothesize that the inhibitory effect of Cu(II) and Zn(II) on PbO2 reductive dissolution (<12 h) is caused by decreasing abundance of protonated sites on the PbO2 surface. The enhanced dissolution (>48 h) may be caused by competitive adsorption of Cu(II) and Zn(II) with Pb(II), which could limit the adsorption of Pb(II) onto PbO2 that could otherwise inhibit reductive dissolution. This study indicates that stagnation time plays a vital role in determining cations' effects on the stability of Pb corrosion products.

Effect of Aluminum on Lead Release to Drinking Water from Scales of Corrosion Products.

The occurrence of aluminum in scales on lead pipes is common. This study aimed to identify factors that influence Al accumulation on oxidized lead surfaces and to determine whether the presence of Al impacts Pb release from corrosion products to water. Al accumulation and Pb release were monitored both with and without the addition of phosphate as a corrosion inhibitor. Pb coupons with corrosion scales were exposed to chlorinated water for up to 198 days to investigate Al accumulation and Pb release. Al accumulation was facilitated by Pb corrosion products, but its accumulation was inhibited by phosphate addition. During the study period, the formation of Al deposits did not affect Pb release when phosphate was absent. In an Al-free system, the addition of 1.0 mg/L phosphate (as P) lowered the dissolved Pb concentration below 1.0 µg/L. In a system containing 200 µg/L Al, the emergence of phosphate's effect on Pb control was delayed, and the dissolved Pb concentration decreased but stabilized at a higher value (10-12 µg/L) than in the Al-free system. Phosphohedyphane (Ca2Pb3(PO4)3Cl) was formed in all phosphate-containing systems, and PbO2 was formed independent of phosphate addition. The effect of Al on Pb release was probably related to its influence on the composition and morphology of Pb-containing minerals on coupon surfaces. The laboratory study has unavoidable limitations in its ability to simulate all conditions in real lead service lines, but this study still highlights the importance of considering the influence of Al when designing Pb corrosion control strategies.

The Ability of Phosphate To Prevent Lead Release from Pipe Scale When Switching from Free Chlorine to Monochloramine.

For lead pipes that contain PbO2(s) as a major component of their scales, a change in the residual disinfectant from free chlorine to monochloramine can destabilize the PbO2(s) and result in dramatic increases in aqueous lead concentrations. Such a scenario occurred in Washington, D.C., in late 2000. That problem was ultimately addressed by the addition of phosphate as a corrosion inhibitor, but it took several months for lead levels to drop below regulatory values. This study sought to determine whether adding phosphate prior to switching the disinfectant could mitigate lead release. Using synthetic tap water and new lead pipes, we developed a set of lead pipes with scales rich in PbO2(s) and then studied their response to a change from free chlorine to monochloramine. Total lead concentrations remained below 10 µg/L for pipes that received phosphate prior to and during the switch. In contrast, total lead concentrations increased from less than 5 µg/L to more than 150 µg/L as a result of the disinfectant switch when phosphate was not present. Characterization of the pipe scales demonstrated that plattnerite (ß-PbO2(s)) was the dominant component of the scale prior to the switch, and that the scale gradually transformed into one containing a lead phosphate solid chemically similar to phosphohedyphane (Ca2Pb3(PO4)3(Cl,F,OH)(s)) when phosphate was present.

Formation and Transport of Cr(III)-NOM-Fe Colloids upon Reaction of Cr(VI) with NOM-Fe(II) Colloids at Anoxic-Oxic Interfaces.

Natural organic matter-iron (NOM-Fe) colloids are ubiquitous at anoxic-oxic interfaces of subsurface environments. Fe(II) or NOM can chemically reduce Cr(VI) to Cr(III), and the formation of Cr(III)-NOM-Fe colloids can control the fate and transport of Cr. We explored the formation and transport of Cr(III)-humic acid (HA)-Fe colloids upon reaction of Cr(VI) with HA-Fe(II) colloids over a range of environmentally relevant conditions. Cr(VI) was completely reduced by HA-Fe(II) complexes under anoxic conditions, and the formation of Cr(III)-HA-Fe colloids depended on HA concentration (or molar C/Fe ratio) and redox conditions. No colloids formed at HA concentrations below 3.5 mg C/L (C/Fe ratio below 1.6), but Cr(III)-HA-Fe colloids formed at higher HA concentrations. In column experiments, Cr(III)-HA-Fe(III) colloids formed under oxic conditions were readily transported through sand-packed porous media. Colloidal stability measurements further suggest that Cr(III)-HA-Fe colloids are highly stable and persist for at least 20 days without substantial change in particle size. This stability is attributed to the enrichment of free HA adsorbed on the Cr(III)-HA-Fe colloid surfaces, intensifying the electrostatic and/or steric repulsion interactions between particles. The new insights provided here are important for evaluating the long-term fate and transport of Cr in organic-rich redox transition zones.

Effect of Cu(II) on Mn(II) Oxidation by Free Chlorine To Form Mn Oxides at Drinking Water Conditions.

The chemical oxidation of dissolved Mn(II) to Mn(III/IV) oxides (MnOx) can lead to the accumulation of Mn deposits in drinking water distribution systems. However, Mn(II) oxidation by free chlorine is quite slow under mild conditions (e.g., pH 7.7 and 1.0 mg/L Cl2). This study found a significant role for Cu(II) in Mn(II) oxidation under conditions relevant to the supply of chlorinated drinking water. At pH 7.7, dissolved Cu(II) accelerated Mn(II) oxidation more than 10 times with a dose of 20 µg/L. Solid characterization revealed that during Mn(II) oxidation, Cu(II) adsorbed to freshly formed MnOx and produced Mn-Cu mixtures (denoted as MnOx-Cu(II)). An autocatalytic model for the reaction kinetics suggested that the freshly formed MnOx-Cu(II) had a much higher catalytic activity than that of pure MnOx. Solid CuO also catalyzed Mn(II) oxidation, and kinetic modeling indicated that after an initial oxidation of Mn(II) facilitated by the CuO surface, the freshly formed MnOx-Cu(II) on CuO surface played the dominant role in accelerating further Mn(II) oxidation. This study indicates a high potential for the formation of Mn oxides at locations in a drinking water distribution system or in premise plumbing where both Mn(II) and Cu(II) are available. It provides insights into the co-occurrence of other metals with Mn deposits that is frequently observed in distribution systems.

Cr(VI) Adsorption on Engineered Iron Oxide Nanoparticles: Exploring Complexation Processes and Water Chemistry.

Surface-functionalized magnetic nanoparticles are promising adsorbents due to their large surface areas and ease of separation after contaminant removal. In this work, the affinity of Cr(VI) adsorption to 8 nm surface-functionalized superparamagnetic magnetite nanoparticles was determined for surface coatings with amine (trimethyloctadecylammonium bromide, CTAB) and carboxyl (stearic acid, SA) functional groups. Cr(VI) adsorbed more strongly to the CTAB-coated nanoparticles than to the SA-coated materials due to electrostatic interactions between positively charged CTAB and anionic Cr(VI) species. The adsorption of Cr(VI) by CTAB- and SA-coated nanoparticles increased with decreasing pH (4.5-10), which could be simulated by a surface complexation model. Cr(VI) removal performance by the nanocomposite was evaluated for two realistic drinking water compositions. The co-occurrence of divalent cations (Ca2+ and Mg2+) and Cr(VI) resulted in decreased Cr(VI) adsorption as particles were destabilized, leading to the aggregation and lower effective surface area, confirming the importance of the overall water composition on the performance of novel engineered nanomaterials for water treatment applications.

Geochemical Stability of Dissolved Mn(III) in the Presence of Pyrophosphate as a Model Ligand: Complexation and Disproportionation.

Dissolved Mn(III) species have recently been recognized as a significant form of Mn in redox transition zones, but their speciation, stability, and reactivity are poorly understood. Besides acting as the intermediate for Mn redox chemistry, Mn(III) can undergo disproportionation producing insoluble Mn oxides and aqueous Mn(II). Using pyrophosphate(PP) as a model ligand, we evaluated the thermodynamic and kinetic stability of Mn(III) complexes. They were stable at circumneutral pH and were prone to (partial) disproportionation at acidic or basic pH. With an initial lag phase, the kinetics of Mn(III)-PP disproportionation was autocatalytic with the produced Mn oxides promoting the disproportionation. X-ray diffraction and the average Mn oxidation state indicated that the solid products were not pure Mn(IV) oxides but a mixture of triclinic birnessite and δ-MnO2. Addition of synthetic analogs of the precipitates eliminated the lag phase, confirming their catalytic roles. Thermodynamic calculations adequately predicted the stability regime of Mn(III)-PP. The present results refined the constant for Mn(PP)25- formation, which allows a consistent and quantitative prediction of equilibrium speciation of Mn(III)-Mn(II)-birnessite with PP. A simple and robust model, which incorporated the thermodynamic constraints, autocatalytic rate law, and verified reaction stoichiometry, successfully simulated all kinetic data.

Permanent CO2 Trapping through Localized and Chemical Gradient-Driven Basalt Carbonation.

Recent laboratory and field studies have demonstrated that basalt formations may present one of the most secure repositories for anthropogenic CO2 emissions through carbon mineralization. In this work, a series of high-temperature, high-pressure core flooding experiments was conducted to investigate how transport limitations, reservoir temperature, and brine chemistry impact carbonation reactions following injection of CO2-rich aqueous fluids into fractured basalts. At 100 °C and 6.3 mM [NaHCO3], representative of typical reservoir conditions, carbonate precipitates were highly localized on reactive mineral grains contributing key divalent cations. Geochemical gradients promoted localized reaction fronts of secondary precipitates that were consistent with 2D reactive transport model predictions. Increasing [NaHCO3] to 640 mM dramatically enhanced carbonation in diffusion-limited zones, but an associated increase in clays filling advection-controlled flow paths could ultimately obstruct flow and limit sequestration capacity under such conditions. Carbonate and clay precipitation were further enhanced at 150 °C, reducing the pre-reaction fracture volume by 48% compared to 35% at 100 °C. Higher temperature also produced more carbonate-driven fracture bridging, which generally increased with diffusion distance into dead-end fractures. In combination, the results are consistent with field tests indicating that mineralization will predominate in buffered diffusion-limited zones adjacent to bulk flow paths and that alkaline reservoirs with strong geothermal gradients will enhance the extent of carbon trapping.

Formation and Aggregation of Lead Phosphate Particles: Implications for Lead Immobilization in Water Supply Systems.

Phosphate is commonly added to drinking water to inhibit lead release from lead service lines and lead-containing materials in premise plumbing. Phosphate addition promotes the formation of lead phosphate particles, and their aggregation behaviors may affect their transport in pipes. Here, lead phosphate formation and aggregation were studied under varied aqueous conditions typical of water supply systems. Under high aqueous PO4/Pb molar ratios (>1), phosphate adsorption made the particles more negatively charged. Therefore, enhanced stability of lead phosphate particles was observed, suggesting that although addition of excess phosphate can lower the dissolved lead concentrations in tap water, it may increase concentrations of particulate lead. Adsorption of divalent cations (Ca2+ and Mg2+) onto lead phosphate particles neutralized their negative surface charges and promoted their aggregation at pH 7, indicating that phosphate addition for lead immobilization may be more efficient in harder waters. The presence of natural organic matter (NOM, ≥ 0.05 mg C/L humic acid and ≥ 0.5 mg C/L fulvic acid) retarded particle aggregation at pH 7. Consequently, removal of organic carbon during water treatment to lower the formation of disinfection-byproducts (DBPs) may have the additional benefit of minimizing the mobility of lead-containing particles. This study provided insight into fundamental mechanisms controlling lead phosphate aggregation. Such understanding is helpful to understand the observed trends of total lead in water after phosphate addition in both field and pilot-scale lead pipe studies. Also, it can help optimize lead immobilization by better controlling the water chemistry during phosphate addition.

Water metal contaminants in a potentially mineral-deficient population of Haiti.

This study aimed to characterize metal contaminant concentrations and assess temporal and spatial variability in the main drinking water sources of Cap-Haïtien, Haiti. Water sources from five communities were sampled in two seasons, June (2014) and October (2014), and analysed for a suite of metals. A geographic information system was used to examine the spatial distribution of sampling points. Metal concentrations were below the US Environmental Protection Agency (USEPA) primary drinking water standards. Mean manganese concentrations were comparatively higher in wells (254.5 µg/L), exceeding the USEPA secondary drinking water standard (50 µg/L). Higher mean Mg/Ca and Ba/Ca ratios (range 2.3-3.4) may indicate different interactions between seawater and groundwater throughout the year. Although metal concentrations were within the limits of the USEPA drinking water standards, emerging contaminants, such as manganese, showed concentrations in excess of recommended limits. These metals may interact with background nutritional status with potential implications for growth and development.

Roles of Transport Limitations and Mineral Heterogeneity in Carbonation of Fractured Basalts.

Basalt formations could enable secure long-term carbon storage by trapping injected CO2 as stable carbonates. Here, a predictive modeling framework was designed to evaluate the roles of transport limitations and mineral spatial distributions on mineral dissolution and carbonation reactions in fractured basalts exposed to CO2-acidified fluids. Reactive transport models were developed in CrunchTope based on data from high-temperature, high-pressure flow-through experiments. Models isolating the effect of transport compared nine flow conditions under the same mineralogy. Heterogeneities were incorporated by segmenting an actual reacted basalt sample, and these results were compared to equivalent flow conditions through randomly generated mineral distributions with the same bulk composition. While pure advective flow with shorter retention times promotes rapid initial carbonation, pure diffusion sustains mineral reactions for longer time frames and generates greater net carbonate volumes. For the same transport conditions and bulk composition, exact mineral spatial distributions do not impact the amount of carbonation but could determine the location by controlling local solution saturation with respect to secondary carbonates. In combination, the results indicate that bulk mineralogy will be more significant than small-scale heterogeneities in controlling the rate and extent of CO2 mineralization, which will likely occur in diffusive zones adjacent to flow paths or in dead-end fractures.