Robert G. Ford

Speciation of arsenic in sulfidic waters

Formation constants for thioarsenite species have been determined in dilute solutions at 25°C, ΣH2S from 10-7.5 to 10-3.0 M, ΣAs from 10-5.6 to 10-4.8 M, and pH 7 and 10. The principal inorganic arsenic species in anoxic aquatic systems are arsenite, As(OH)30, and a mononuclear thioarsenite with an S/As ratio of 3:1. Thioarsenic species with S/As ratios of 1 : 1,2 : 1, and 4 : 1 are lesser components in sulfidic solutions that might be encountered in natural aquatic environments. Thioarsenites dominate arsenic speciation at sulfide concentrations > 10-4.3 M at neutral pH. Conversion from neutral As(OH)30 to anionic thioarsenite species may regulate the transport and fate of arsenic in sulfate-reducing environments by governing sorption and mineral precipitation reactions.

Kinetics and mechanisms of Zn complexation on metal oxides using EXAFS spectroscopy

Zn(II) sorption onto Al and Si oxides was studied as a function of pH (5.1-7.52), sorption density, and ionic strength. This study was carried out to determine the role of the various reaction conditions and sorbent phases in Zn complexation at oxide surfaces. Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to probe the Zn atomic environment at the metal oxide/aqueous interface. For both amorphous silica and high-surface-area gibbsite, Zn sorption kinetics were rapid and reached completion within 24 h. In contrast, Zn sorption on low-surface-area-gibbsite was much slower, taking nearly 800 h for a sorption plateau to be reached. In the case of silica, EXAFS revealed that Zn was in octahedral coordination with first-shell oxygen atoms up to a surface loading of approximately 1 micro molm(-2), changing to tetrahedral coordination as surface loading and pH increased. For the high-surface-area gibbsite system, the Znz.sbnd;O first-shell distance was intermediate between values for tetrahedral and octahedral coordination over all loading levels. Zn formed inner-sphere adsorption complexes on both silica and high-surface-area gibbsite over all reaction conditions. For Zn sorption on low-surface-area gibbsite, formation of Znz.sbnd;Al layered double hydroxide (LDH) occurred and was the cause for the observed slow Zn sorption kinetics. The highest pH sample (7.51) in the Zn-amorphous silica system resulted in the formation of an amorphous Zn(OH)(2) precipitate with tetrahedral coordination between Zn and O. Aging the reaction samples did not alter the Zn complex in any of the systems. The results of this study indicate the variability of Zn complexation at surfaces prevalent in soil and aquatic systems and the importance of combining macroscopic observations with methods capable of determining metal complex formation mechanisms.

THE ROLE OF AL IN THE FORMATION OF SECONDARY NI PRECIPITATES ON PYROPHYLLITE, GIBBSITE, TALC, AND AMORPHOUS SILICA : A DRS STUDY

Formation of secondary Ni precipitates is an important mechanism of Ni retention in neutral and alkaline clay/water systems. However, the structure and composition of these secondary phases, and their stability is still disputable. Using existing structure refinement data and new ab-initio FEFF 7 calculations we show that Ni-edge X-ray absorption fine structure spectroscopy alone may not be able to unequivocally discriminate four possible candidate compounds: α-Ni(OH)2, the isostructural but Al-substituted layered double hydroxide (Ni-Al LDH), and 1:1 and 2:1 Ni-containing phyllosilicates. Hence, we investigated the potential of diffuse reflectance spectroscopy (DRS) in determining in situ the Ni phase forming in the presence of four sorbents, pyrophyllite, talc, gibbsite, and amorphous silica. The 3A2g → 3T1g(F) band (ν2) of octahedrally coordinated Ni2+ could be reliably extracted from the reflectance spectra of wet pastes. In the presence of the Al-free talc and amorphous silica, the ν2 band was at ≈14,900 cm−1, but shifted to 15,300 cm−1 in the presence of Al-containing pyrophyllite and gibbsite. This shift suggests that Al is dissolved from the sorbent and substitutes for Ni in brucite-like hydroxide layers of the newly forming precipitate phase, causing a decrease of the Ni-O distances and, in turn, an increase of the crystal-field splitting energy. Comparison with Ni model compounds showed that the band at 14,900 cm−1 is a unique fingerprint of α-Ni(OH)2, and the band at 15,300 cm−1 of Ni-Al LDH. Although the complete transformation of α-Ni(OH)2 into a Ni phyllosilicate causes a significant contraction of the Ni hydroxide sheet as indicated by band positions intermediate to those of α-Ni(OH)2 and Ni-Al LDH, incipient states of silication do not influence Ni-O distances and cannot be detected by DRS. The first evidence for the formation of a precipitate was obtained after 5 min (pyrophyllite), 7 hr (talc), 24 hr (gibbsite), and 3 days (amorphous silica). For both pyrophyllite and talc, where sufficiently long time series were available, the ν2 energy slightly increased as long as the Ni uptake from solution continued (3 days for pyrophyllite, 30 days for talc). This may be explained by a relative decrease of relaxed surface sites due to the growth of crystallites. Our study shows that the formation of both α-Ni(OH)2 and Ni-Al LDH may effectively decrease aqueous Ni concentrations in soils and sediments. However, Ni-Al LDH seems to be thermodynamically favored when Al is available.

Stability of layered Ni hydroxide surface precipitates—a dissolution kinetics study

In recent years, studies have shown that sorption of metals onto natural materials results in the formation of new mineral-like precipitate phases. However, the stability of the precipitates and the potential long-term release of the metal back into the soil solution are poorly understood. Therefore, we investigated the influence of residence time and dissolution agent on the release of nickel from three sorbents, pyrophyllite, talc, and gibbsite, complementing the macroscopic observations with X-ray absorption fine structure (XAFS) and diffuse reflectance spectroscopies (DRS), and high-resolution thermogravimetric analysis (HRTGA). Dissolution of the surface precipitates was compared to dissolution of reference Ni compounds. In the sorption experiments conducted at pH 7.5, Ni-Al layered double hydroxide (LDH) formed in the presence of pyrophyllite and gibbsite, and a-Ni hydroxide formed with talc, in line with former studies. The stability of the phases decreased from Ni-Al LDH on pyrophyllite to a-Ni hydroxide on talc to Ni-Al LDH on gibbsite. This sequence could be explained by the greater stability of precipitates with Al-for-Ni substituted hydroxide layers compared to pure Ni hydroxide layers, and by the greater stability of precipitates with silicate-for-nitrate exchanged interlayer. With increasing residence time, all precipitate phases drastically increased in stability, as was documented by decreasing amounts of Ni released by nitric acid (HNO3) and ethylenediaminetetraacetic acid (EDTA) treatments. This aging effect may be partly explained by the silicate-for-nitrate exchange during the first days of reaction, and subsequently by silicate polymerization and partial grafting onto the hydroxide layers (Ford et al., 1999). However, even Si-free, Ni-reacted gibbsite showed a substantial aging effect, suggesting that factors other than interlayer silication may be equally important. Such a factor may be crystal growth due to Ostwald ripening. The Ni precipitates which remained at the end of the dissolution experiments were structurally similar to the precipitates at the beginning of the dissolution, indicating that no preferential dissolution of a less stable phase occurred. Therefore, the precipitate phase within each sorbent system was apparently homogeneous in structure. Copyright

Frontiers in metal sorption/precipitation mechanisms on soil mineral surfaces

Spectroscopic studies provide evidence that inorganic contaminants may be incorporated into precipitates at the surface of soil and sediment minerals.Surface precipitates may form via several mechanisms that are dependent on the unique characteristics of the interfacial region between solid and solution.In general, surface complexation models (SCMs)capture most of the salient features of the interfacial region.However,current SCMs fail to capture the dynamics of mineral surfaces,thus limitingtheir ability to predict the composition and structure of potential surface precipitates.This review outlines the current implementation of surface precipitation models,spectroscopic studies that highlight the need to develop more comprehensive SCMs,and future research directions that will help .ll existing knowledge gaps.Successful modeling approaches to describe surface precipitation phenomena are a necessary component for the evaluation of long-term inorganic contaminant transport in soil and sediment systems.

High crystallinity Si‐ferrihydrite: An insight into its Néel temperature and size dependence of magnetic properties

(1) Ferrihydrite, an antiferromagnetic iron oxyhydroxide with resulting magnetization due to uncompensated spins, is of great importance for the cycling of many trace metals in the environment. Four ferrihydrite samples prepared with 1.3 to 3.5 wt% of Si at different synthesis temperatures (7.5C, 22C, 50C, and 75C) were studied by temperature- dependent hysteresis loops, ZFC/FC susceptibility curves, ac susceptibility and Mossbauer spectroscopy. The incorporation of Si into the ferrihydrite during synthesis changed the properties of this mineral. Interestingly, seven sharp lines were observed in the X-ray diffraction pattern of the ferrihydrite samples prepared at 50C and 75C. In general, both XRD and magnetism demonstrate that particle size decreased (from 23 nm to 2 nm) and particle size distribution narrowed as the synthesis temperature was lowered. Those samples prepared between 7.5C and 50C showed the expected superparamagnetic behavior of ferrihydrite below 300 K. The ferrihydrite prepared at 75C was unusually coarse-grained and had a blocking temperature above 300 K. Extrapolation of induced magnetization from the largest particles with the highest crystallinity allowed an estimate of a ferrihydrite Neel temperature of around 422 K. We also present XRD and magnetic data from large natural Si-ferrihydrite collected from a marine shallow-water hydrothermal area that formed at a temperature of approximately 88C.

Nest construction by a ground-nesting bird represents a potential trade-off between egg crypticity and thermoregulation

Predation selects against conspicuous colors in bird eggs and nests, while thermoregulatory constraints select for nest-building behavior that regulates incubation temperatures. We present results that suggest a trade-off between nest crypticity and thermoregulation of eggs based on selection of nest materials by piping plovers (Charadrius melodus), a ground-nesting bird that constructs simple, pebble-lined nests highly vulnerable to predators and exposed to temperature extremes. Piping plovers selected pebbles that were whiter and appeared closer in color to eggs than randomly available pebbles, suggesting a crypsis function. However, nests that were more contrasting in color to surrounding substrates were at greater risk of predation, suggesting an alternate strategy driving selection of white rocks. Near-infrared reflectance of nest pebbles was higher than randomly available pebbles, indicating a direct physical mechanism for heat control through pebble selection. Artificial nests constructed of randomly available pebbles heated more quickly and conferred heat to model eggs, causing eggs to heat more rapidly than in nests constructed from piping plover nest pebbles. Thermal models and field data indicated that temperatures inside nests may remain up to 2–6°C cooler than surrounding substrates. Thermal models indicated that nests heat especially rapidly if not incubated, suggesting that nest construction behavior may serve to keep eggs cooler during the unattended laying period. Thus, pebble selection suggests a potential trade-off between maximizing heat reflectance to improve egg microclimate and minimizing conspicuous contrast of nests with the surrounding substrate to conceal eggs from predators. Nest construction behavior that employs light-colored, thermally reflective materials may represent an evolutionary response by birds and other egg-laying organisms to egg predation and heat stress.

Characterization of salt cake from secondary aluminum production.

Salt cake is a major waste component generated from the recycling of secondary aluminum processing (SAP) waste. Worldwide, the aluminum industry produces nearly 5 million tons of waste annually and the end-of-life management of these wastes is becoming a challenge in the U.S. and elsewhere. In this study, the mineral phases, metal content and metal leachability of 39 SAP waste salt cake samples collected from 10 different facilities across the U.S. were determined. The results showed that aluminum (Al), aluminum oxide, aluminum nitride and its oxides, spinel and elpasolite are the dominant aluminum mineral phases in salt cake. The average total Al content was 14% (w/w). The overall percentage of the total leachable Al in salt cake was 0.6% with approximately 80% of the samples leaching at a level less than 1% of the total aluminum content. The extracted trace metal concentrations in deionized water were relatively low (μgL(-1) level). The toxicity characteristic leaching procedure (TCLP) was employed to further evaluate leachability and the results indicated that the leached concentrations of toxic metals from salt cake were much lower than the EPA toxicity limit set by USEPA.

Delineating landfill leachate discharge to an arsenic contaminated waterway

Discharge of contaminated ground water may serve as a primary and on-going source of contamination to surface water. A field investigation was conducted at a Superfund site in Massachusetts, USA to define the locus of contaminant flux and support source identification for arsenic contamination in a pond abutting a closed landfill. Subsurface hydrology and ground-water chemistry were evaluated in the aquifer between the landfill and the pond during the period 2005-2009 employing a network of wells to delineate the spatial and temporal variability in subsurface conditions. These observations were compared with concurrent measures of ground-water seepage and surface water chemistry within a shallow cove that had a historical visual record of hydrous ferric oxide precipitation along with elevated arsenic concentrations in shallow sediments. Barium, presumably derived from materials disposed in the landfill, served as an indicator of leachate-impacted ground water discharging into the cove. Evaluation of the spatial distributions of seepage flux and the concentrations of barium, calcium, and ammonium-nitrogen indicated that the identified plume primarily discharged into the central portion of the cove. Comparison of the spatial distribution of chemical signatures at depth within the water column demonstrated that direct discharge of leachate-impacted ground water was the source of highest arsenic concentrations observed within the cove. These observations demonstrate that restoration of the impacted surface water body will necessitate control of leachate-impacted ground water that continues to discharge into the cove.

Examining the efficiency of muffle furnace-induced alkaline hydrolysis in determining the titanium content of environmental samples containing engineered titanium dioxide particles

A novel muffle furnace (MF)-based potassium hydroxide (KOH) fusion digestion technique was developed and evaluated for different titanium dioxide materials in various solid matrices. Digestion of different environmental samples containing sediments, clay minerals and humic acid with and without TiO(2) particles was first performed utilizing the MF-based KOH fusion technique and its dissolution efficacy was compared to a Bunsen burner (BB)-based KOH fusion method. The three types of TiO(2) particles (anatase, brookite and rutile) were then digested with the KOH fusion techniques and microwave (MW)-based nitric (HNO3)–hydrofluoric (HF) mixed acid digestion methods. Statistical analysis of the results revealed that Ti recoveries were comparable for the KOH fusion methods (BB and MF). For pure TiO(2) particles, the measured Ti recoveries compared to calculated values were 96%, 85% and 87% for anatase, brookite and rutile TiO(2) materials, respectively, by the MF-based fusion technique. These recoveries were consistent and less variable than the BB-based fusion technique recoveries of 104%, 97% and 72% and MW-based HNO3–HF mixed acids digestion recoveries of 80%, 81% and 14%, respectively, for anatase, brookite and rutile. Ti percent recoveries and measurement precision decreased for both the BB and MF methods when TiO(2) was spiked into sediment, clay minerals, and humic acid. This drop in efficacy was counteracted by more thorough homogenization of the spiked mixtures and by increasing the mass of KOH in the MF fusion process from 1.6 g to 10.0 g. The MF-based fusion technique is consistently superior in digestion efficiency for all three TiO(2) polymorphs. The MF-based fusion technique required 20 minutes for digestion of 25 samples (based on in-house Lindberg MF capacity) compared to 8 hours for the same number of samples using the BB-based fusion technique. Thus, the MF-based fusion technique can be used to dissolve a large number of samples in a shorter time (e.g., 500 samples per 8 hours) while conserving energy and eliminating health and safety risks from methods involving HF.