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At least 451 records · Page 25Linked to original sources
USGS2021

Cyprosulfamide: Analysis of the herbicide safener and two of its degradates in surface water and groundwater from the Midwestern United States

Herbicide safeners are commonly included in herbicide formulations to selectively protect crops from herbicide toxicity but are poorly understood in terms of their environmental occurrence and fate. This study established an analytical method for a newer safener, cyprosulfamide, and two of its degradates, cyprosulfamide desmethyl and N -cyclopropyl-4-sulfamoylbenzamide, in water via solid-phase extraction and liquid chromatography with tandem mass spectroscopy. To evaluate the potential for off-field transport and transformation of cyprosulfamide, the method was used to analyze groundwater and surface water samples collected near cornfields in the midwestern United States where cyprosulfamide had been applied. All three compounds were detected in surface water samples ( N = 34); N -cyclopropyl-4-sulfamoylbenzamide was most frequently detected (56%), followed by cyprosulfamide (25%) and cyprosulfamide desmethyl (19%). Maximum concentrations ranged from 22.0 to 5185.9 ng/L, with the highest concentrations and detection rates during the growing season. None of our target analytes were detected in groundwater.

ACS Agricultural Science and Technology↗
USGS2020

The effects of phosphatization on the mineral associations and speciation of Pb in ferromanganese crusts

The older layers of thick ferromanganese (FeMn) crusts from the central Pacific Ocean have undergone diagenetic phosphatization, during which carbonate fluorapatite (CFA) filled fractures and pore space and replaced carbonates. The effects of phosphatization on individual trace metal concentrations, speciation, and phase associations in FeMn crusts remain poorly understood yet may be important to metal enrichment mechanisms, paleoceanography, and extractive metallurgy. This study examines the concentrations, speciation, and mineral phase associations of Pb in phosphatized and nonphosphatized layers within three Pacific Ocean crusts using standard chemical and mineralogical techniques, in addition to bulk X-ray absorption spectroscopy (XAS) and microfocused X-ray fluorescence (μ-XRF) mapping. Our findings challenge the conclusions of previous works, which reported that most Pb in phosphatized crusts was associated with the CFA-dominated residual phase after sequential leaching experiments. These results were interpreted as an indication of Pb transfer from the oxide phases to diagenetic CFA during phosphatization. However, our results reveal an inverse correlation of Pb with Ca and P in the μ-XRF mapping of in situ FeMn crust sections and bulk chemistry. Furthermore, XAS measurements reveal that Pb speciation in bulk phosphatized FeMn crust layers is very similar to nonphosphatized layers, indicating only minor, if any, change in speciation during diagenetic phosphatization. Small differences in the EXAFS spectra for one highly phosphatized layer indicate that minor amounts of Pb may have been altered during phosphatization, but the new Pb-bearing phase was not unequivocally identified. Taken together, our results demonstrate that Pb speciation is not significantly affected by phosphatization.

Earth and Space Chemistry↗
USGS2021

Isotope fractionation from In Vivo methylmercury detoxification in waterbirds

The robust application of stable mercury (Hg) isotopes for mercury source apportionment and risk assessment necessitates the understanding of mass-dependent fractionation (MDF) due to internal transformations within organisms. Here, we used high energy-resolution XANES spectroscopy and isotope ratios of total mercury (δ202THg) and methylmercury (δ202MeHg) to elucidate the chemical speciation of Hg and the resultant MDF due to internal MeHg demethylation in waterbirds. In three waterbirds (Clark’s grebe, Forster’s tern, south polar skua), between 17-86% of the MeHg was demethylated to inorganic mercury (iHg) species primarily in the liver and kidneys as Hg-tetraselenolate (Hg(Sec)4) and minor Hg-dithiolate (Hg(SR)2) complexes. Tissular differences between δ202THg and δ202MeHg correlated linearly with %iHg (Hg(Sec)4 + Hg(SR)2), and were interpreted to reflect a kinetic isotope effect during in vivo MeHg demethylation. The product-reactant isotopic enrichment factor (εp/r) for the demethylation of MeHg  Hg(Sec)4 was −2.2 ± 0.1‰. δ202MeHg values were unvarying within each bird regardless of Hg(Sec)4 abundance, indicating fast internal cycling or replenishment of MeHg relative to demethylation. Our findings document a universal selenium-dependent demethylation reaction in birds, provide new insights on the internal transformations and cycling of MeHg and Hg(Sec)4, and allow for mathematical correction of δ202THg values due to the MeHg  Hg(Sec)4 reaction.

ACS Earth and Space Chemistry↗
USGS2011

A novel photosynthetic strategy for adaptation to low-iron aquatic environments

Iron (Fe) availability is a major limiting factor for primary production in aquatic environments. Cyanobacteria respond to Fe deficiency by derepressing the isiAB operon, which encodes the antenna protein IsiA and flavodoxin. At nanomolar Fe concentrations, a PSI−IsiA supercomplex forms, comprising a PSI trimer encircled by two complete IsiA rings. This PSI−IsiA supercomplex is the largest photosynthetic membrane protein complex yet isolated. This study presents a detailed characterization of this complex using transmission electron microscopy and ultrafast fluorescence spectroscopy. Excitation trapping and electron transfer are highly efficient, allowing cyanobacteria to avoid oxidative stress. This mechanism may be a major factor used by cyanobacteria to successfully adapt to modern low-Fe environments.

Biochemistry↗
USGS2009

Comprehensive isolation of natural organic matter from water for spectral characterizations and reactivity testing

A variety of approaches were tested to comprehensively isolate natural organic matter (NOM) from water . For waters with high NOM concentrations such as the Suwannee River, Georgia, approaches that used combinations of membrane concentrations, evaporative concentrations, and adsorption on nonionic XAD resins, ion exchange resins and iron oxide coated sand isolated over 90% of the NOM. However, for waters with low NOM concentrations, losses of half of the NOM were common and desalting of NOM isolates was a problem. A new comprehensive approach was devised and tested on the Seine River, France in which 100 L of filtered water was sodium softened by ion exchange and vacuum evaporated to 100 mL. Colloids (32% of the NOM) were isolated using a 3,500 Dalton membrane by dialysis against 0.1 M HCl and 0.2 M HF to remove salts and silica. On the membrane permeate, hydrophobic NOM (42%) was isolated using XAD-8 resin and hydrophilic NOM (26%) was isolated using a variety of selective desalting precipitations. The colloid fraction was characterized by IR and NMR spectroscopy as N-acetylamino sugars.

Georgia↗
USGS2011

Redox chemistry and natural organic matter (NOM): Geochemists' dream, analytical chemists' nightmare

Natural organic matter (NOM) is an inherently complex mixture of polyfunctional organic molecules. Because of their universality and chemical reversibility, oxidation/reductions (redox) reactions of NOM have an especially interesting and important role in geochemistry. Variabilities in NOM composition and chemistry make studies of its redox chemistry particularly challenging, and details of NOM-mediated redox reactions are only partially understood. This is in large part due to the analytical difficulties associated with NOM characterization and the wide range of reagents and experimental systems used to study NOM redox reactions. This chapter provides a summary of the ongoing efforts to provide a coherent comprehension of aqueous redox chemistry involving NOM and of techniques for chemical characterization of NOM. It also describes some attempts to confirm the roles of different structural moieties in redox reactions. In addition, we discuss some of the operational parameters used to describe NOM redox capacities and redox states, and describe nomenclature of NOM redox chemistry. Several relatively facile experimental methods applicable to predictions of the NOM redox activity and redox states of NOM samples are discussed, with special attention to the proposed use of fluorescence spectroscopy to predict relevant redox characteristics of NOM samples.

ACS Symposium Series↗
USGS2026

Comparing in vivo methylmercury detoxification in hunted duck: Implications for wildlife and human health

Great Salt Lake (GSL) waterfowl have elevated mercury (Hg) concentrations, and in vivo detoxification of neurotoxic and bioaccumulative methylmercury (MeHg) can occur through Hg-selenium (Se) complexation, influencing the suitability of Hg consumption advisories. Here, we present Hg chemical speciation and Hg stable isotope measurements of brain, breast muscle, liver, and kidney samples from two GSL duck species─northern shoveler ( Spatula clypeata ) and cinnamon teal (Spatula cyanoptera). Chemical and stable isotope measurements, along with high energy resolution fluorescence detected X-ray absorption near-edge structure (HERFD-XANES) spectroscopy, indicate the occurrence of in vivo MeHg demethylation in both bird species. The percentage of total Hg (THg) as MeHg (% MeHg) varied with tissue type (brain > breast muscle > kidney > liver), and differences between δ 202 THg and δ 202 MeHg were linearly correlated with % MeHg. Demethylated inorganic Hg was a mixture of Hg-dithiolate (Hg(SR) 2 ) and Hg-tetraselenolate (Hg(Sec) 4 ) in both bird species. Notably, liver THg concentrations were elevated in northern shoveler liver tissues relative to those of cinnamon teal (7.54 ± 4.69 mg/kg versus 2.19 ± 1.27 mg/kg, dry weight, respectively) and % MeHg liver values were significantly lower (40% ± 19% versus 68% ± 14%), indicating taxonomic differences in Hg detoxification and depuration. THg concentrations in waterfowl species from this study were comparable to previously reported levels, indicating that elevated Hg concentrations in northern shoveler and cinnamon teal at GSL have persisted over the past two decades. Due to in vivo demethylation, we show that THg is not an effective proxy for MeHg within duck species, carrying implications for current GSL waterfowl consumption advisories and assessments exposure risk. This study highlights important differences in MeHg detoxification between waterbird taxa and provides further insights into Hg toxicity risk to GSL waterbirds.

Utah↗
USGS2001

In situ spectroscopic and solution analyses of the reductive dissolution of Mn02 by Fe(II)

The reductive dissolution of MnO 2 by Fe(II) under conditions simulating acid mine drainage (pH 3, 100 mM SO 4 2 - ) was investigated by utilizing a flow-through reaction cell and synchrotron X-ray absorption spectroscopy. This configuration allows collection of in situ, real-time X-ray absorption near-edge structure (XANES) spectra and bulk solution samples. Analysis of the solution chemistry suggests that the reaction mechanism changed (decreased reaction rate) as MnO 2 was reduced and Fe(III) precipitated, primarily as ferrihydrite. Simultaneously, we observed an additional phase, with the local structure of jacobsite (MnFe 2 O 4 ), in the Mn XANES spectra of reactants and products. The X-ray absorbance of this intermediate phase increased during the experiment, implying an increase in concentration. The presence of this phase, which probably formed as a surface coating, helps to explain the reduced rate of dissolution of manganese(IV) oxide. In natural environments affected by acid mine drainage, the formation of complex intermediate solid phases on mineral surfaces undergoing reductive dissolution may likewise influence the rate of release of metals to solution.

Environmental Science & Technology↗
USGS2002

Limitations in the use of commercial humic acids in water and soil research

Seven samples of commercial "humic acids", purchased from five different suppliers, were studied, and their characteristics were compared with humic and fulvic acids isolated from streams, soils, peat, leonardite, and a dopplerite sample. Cross-polarization and magic-angle spinning 13C NMR spectroscopy clearly shows pronounced differences between the commercial materials and all other samples. Elemental and infrared spectroscopic data do not show such clear-cut differences but can be used as supportive evidence, with the 13C NMR data, to substantiate the above distinctions. As a result of these differences and due to the general lack of information relating to the source, method of isolation, or other pretreatment of the commercial materials, these commercial products are not considered to be appropriate for use as analogues of true soil and water humic substances, in experiments designed to evaluate the nature and reactivity of humic substances in natural waters and soils.

Environmental Science & Technology↗
USGS2001

Effects of sulfur impregnation temperature on the properties and mercury adsorption capacities of activated carbon fibers (ACFs)

Laboratory studies were conducted to determine the role of sulfur functional groups and micropore surface area of carbon-based adsorbents on the adsorption of Hg0 from simulated coal combustion flue gases. In this study, raw activated carbon fibers that are microporous (ACF-20) were impregnated with elemental sulfur between 250 and 650 ??C. The resulting samples were saturated with respect to sulfur content. Total sulfur content of the sulfur impregnated ACF samples decreased with increasing impregnation temperatures from 250 and 500 ??C and then remained constant to 650 ??C. Results from sulfur K-edge X-ray absorption near-edge structure (S-XANES) spectroscopy showed that sulfur impregnated on the ACF samples was in both elemental and organic forms. As sulfur impregnation temperature increased, however, the relative amounts of elemental sulfur decreased with a concomitant increase in the amount of organic sulfur. Thermal analyses and mass spectrometry revealed that sulfur functional groups formed at higher impregnation temperatures were more thermally stable. In general, sulfur impregnation decreased surface area and increased equilibrium Hg0 adsorption capacity when compared to the raw ACF sample. The ACF sample treated with sulfur at 400 ??C had a surface area of only 94 m2/g compared to the raw ACF sample's surface area of 1971 m2/g, but at least 86% of this sample's surface area existed as micropores and it had the largest equilibrium Hg0adsorption capacities (2211-11343 ??g/g). Such a result indicates that 400 ??C is potentially an optimal sulfur impregnation temperature for this ACF. Sulfur impregnated on the ACF that was treated at 400 ??C was in both elemental and organic forms. Thermal analyses and CS2extraction tests suggested that elemental sulfur was the main form of sulfur affecting the Hg0 adsorption capacity. These findings indicate that both the presence of elemental sulfur on the adsorbent and a microporous structure are important properties for improving the performance of carbon-based adsorbents for the removal of Hg0 from coal combustion flue gases.

Environmental Science & Technology↗
USGS2003

Binding of mercury(II) to aquatic humic substances: Influence of pH and source of humic substances

Conditional distribution coefficients ( K DOM ‘) for Hg(II) binding to seven dissolved organic matter (DOM) isolates were measured at environmentally relevant ratios of Hg(II) to DOM. The results show that K DOM ‘ values for different types of samples (humic acids, fulvic acids, hydrophobic acids) isolated from diverse aquatic environments were all within 1 order of magnitude (10 22.5 ± 1.0 −10 23.5 ± 1.0 L kg - 1 ), suggesting similar Hg(II) binding environments, presumably involving thiol groups, for the different isolates. K DOM ‘ values decreased at low pHs (4) compared to values at pH 7, indicating proton competition for the strong Hg(II) binding sites. Chemical modeling of Hg(II)−DOM binding at different pH values was consistent with bidentate binding of Hg(II) by one thiol group (p K a = 10.3) and one other group (p K a = 6.3) in the DOM, which is in agreement with recent results on the structure of Hg(II)−DOM bonds obtained by extended X-ray absorption fine structure spectroscopy (EXAFS).

Environmental Science & Technology↗
USGS2004

Alkaline hydrolysis/polymerization of 2,4,6-Trinitrotoluene: Characterization of products by 13C and 15N NMR

Alkaline hydrolysis has been investigated as a nonbiological procedure for the destruction of 2,4,6-trinitrotoluene (TNT) in explosives contaminated soils and munitions scrap. Nucleophilic substitutions of the nitro and methyl groups of TNT by hydroxide ion are the initial steps in the alkaline degradation of TNT. Potential applications of the technique include both in situ surface liming and ex situ alkaline treatment of contaminated soils. A number of laboratory studies have reported the formation of an uncharacterized polymeric material upon prolonged treatment of TNT in base. As part of an overall assessment of alkaline hydrolysis as a remediation technique, and to gain a better understanding of the chemical reactions underlying the hydrolysis/polymerization process, the soluble and precipitate fractions of polymeric material produced from the calcium hydroxide hydrolysis of unlabeled and 15 N-labeled TNT were analyzed by elemental analysis and 13 C and 15 N nuclear magnetic resonance spectroscopy. Spectra indicated that reactions leading to polymerization included nucleophilic displacement of nitro groups by hydroxide ion, formation of ketone, carboxyl, alcohol, ether, and other aliphatic carbons, conversion of methyl groups to diphenyl methylene carbons, and recondensation of aromatic amines and reduced forms of nitrite, including ammonia and possibly hydroxylamine, into the polymer. Compared to the distribution of carbons in TNT as 14% sp 3 - and 86% sp 2 -hybridized, the precipitate fraction from hydrolysis of unlabeled TNT contained 33% sp 3 - and 67% sp 2 -hybridized carbons. The concentration of nitrogen in the precipitate was 64% of that in TNT. The 15 N NMR spectra showed that, in addition to residual nitro groups, forms of nitrogen present in the filtrate and precipitate fractions include aminohydroquinone, primary amide, indole, imine, and azoxy, among others. Unreacted nitrite was recovered in the filtrate fraction. The toxicities and susceptibilities to microbial or chemical degradation of the polymeric materials remain unknown.

Environmental Science & Technology↗
USGS2003

Arsenic speciation and reactivity in poultry litter

Recent U.S. government action to lower the maximum concentration levels (MCL) of total arsenic (As) (10 ppb) in drinking water has raised serious concerns about the agricultural use of As-containing biosolids such as poultry litter (PL). In this study, solid-state chemical speciation, desorbability, and total levels of As in PL and long-term amended soils were investigated using novel synchrotron-based probing techniques (microfocused (μ) synchrotron X-ray fluorescence (SXRF) and μ-X-ray absorption near-edge structure (XANES) spectroscopies) coupled with chemical digestion and batch experiments. The total As levels in the PL were as high as ≈50 mg kg - 1 , and As(II/III and V) was always concentrated in abundant needle-shaped microscopic particles (≈20 μm × 850 μm) associated with Ca, Cu, and Fe and to a lesser extent with S, Cl, and Zn. Post-edge XANES features of litter particles are dissimilar to those of the organo-As(V) compound in poultry feed (i.e., roxarsone), suggesting possible degradation/transformation of roxarsone in the litter and/or in poultry digestive tracts. The extent of As desorption from the litter increased with increasing time and pH from 4.5 to 7, but at most 15% of the total As was released after 5 d at pH 7, indicating the presence of insoluble phases and/or strongly retained soluble compounds. No significant As accumulation (<15 mg kg - 1 ) was found in long-term PL-amended agricultural surface soils. This suggests that As in the PL may have undergone surface and subsurface transport processes. Our research results raise concerns about long-term PL amendment effects on As contamination in surrounding soil−water environments.

Environmental Science & Technology↗
USGS2003

Molecular-scale characterization of uranium sorption by bone apatite materials for a permeable reactive barrier demonstration

Uranium binding to bone charcoal and bone meal apatite materials was investigated using U L III -edge EXAFS spectroscopy and synchrotron source XRD measurements of laboratory batch preparations in the absence and presence of dissolved carbonate. Pelletized bone char apatite recovered from a permeable reactive barrier (PRB) at Fry Canyon, UT, was also studied. EXAFS analyses indicate that U(VI) sorption in the absence of dissolved carbonate occurred by surface complexation of U(VI) for sorbed concentrations ≤ 5500 μg U(VI)/g for all materials with the exception of crushed bone char pellets. Either a split or a disordered equatorial oxygen shell was observed, consistent with complexation of uranyl by the apatite surface. A second shell of atoms at a distance of 2.9 Å was required to fit the spectra of samples prepared in the presence of dissolved carbonate (4.8 mM total) and is interpreted as formation of ternary carbonate complexes with sorbed U(VI). A U−P distance at 3.5−3.6 Å was found for most samples under conditions where uranyl phosphate phases did not form, which is consistent with monodentate coordination of uranyl by phosphate groups in the apatite surface. At sorbed concentrations ≥ 5500 μg U(VI)/g in the absence of dissolved carbonate, formation of the uranyl phosphate solid phase, chernikovite, was observed. The presence of dissolved carbonate (4.8 mM total) suppressed the formation of chernikovite, which was not detected even with sorbed U(VI) up to 12 300 μg U(VI)/g in batch samples of bone meal, bone charcoal, and reagent-grade hydroxyapatite. EXAFS spectra of bone char samples recovered from the Fry Canyon PRB were comparable to laboratory samples in the presence of dissolved carbonate where U(VI) sorption occurred by surface complexation. Our findings demonstrate that uranium uptake by bone apatite will probably occur by surface complexation instead of precipitation of uranyl phosphate phases under the groundwater conditions found at many U-contaminated sites.

Environmental Science & Technology↗
USGS2004

Macroscopic and microscopic observations of particle-facilitated mercury transport from New Idria and Sulphur Bank mercury mine tailings

Mercury (Hg) release from inoperative Hg mines in the California Coast Range has been documented, but little is known about the release and transport mechanisms. In this study, tailings from Hg mines located in different geologic settings-New Idria (NI), a Si-carbonate Hg deposit, and Sulphur Bank (SB), a hot-spring Hg deposit-were characterized, and particle release from these wastes was studied in column experiments to (1) investigate the mechanisms of Hg release from NI and SB mine wastes, (2) determine the speciation of particle-bound Hg released from the mine wastes, and (3) determine the effect of calcinations on Hg release processes. The physical and chemical properties of tailings and the colloids released from them were determined using chemical analyses, selective chemical extractions, XRD, SEM, TEM, and X-ray absorption spectroscopy techniques. The total Hg concentration in tailings increased with decreasing particle size in NI and SB calcines (roasted ore), but reached a maximum at an intermediate particle size in the SB waste rock (unroasted ore). Hg in the tailings exists predominantly as low-solubility HgS (cinnabar and metacinnabar), with NI calcines having >50% HgS, SB calcines having >89% HgS, and SB waste rock having ???100% HgS. Leaching experiments with a high-ionic-strength solution (0.1 M NaCl) resulted in a rapid but brief release of soluble and particulate Hg. Lowering the ionic strength of the leach solution (0.005 M NaCl) resulted in the release of colloidal Hg from two of the three mine wastes studied (NI calcines and SB waste rock). Colloid-associated Hg accounts for as much as 95% of the Hg released during episodic particle release. Colloids generated from the NI calcines are produced by a breakup and release mechanism and consist of hematite, jarosite/alunite, and Al-Si gel with particle sizes of 10-200 nm. ATEM and XAFS analyses indicate that the majority (???78%) of the mercury is present in the form of HgS. SB calcines also produced HgS colloids. The colloids generated from the SB waste rock were heterogeneous and varied in composition according to the column influent composition. ATEM and XAFS results indicate that Hg is entirely in the HgS form. Data from this study identify colloidal HgS as the dominant transported form of Hg from these mine waste materials.

Environmental Science & Technology↗
USGS2005

Freshwater mussel shells as environmental chronicles: Geochemical and taphonomic signatures of mercury-related extirpations in the North Fork Holston River, Virginia

This study utilized freshwater mussel shells to assess mercury (Hg) contamination in the North Fork Holston River that extirpated (caused local extinctions of) a diverse mussel fauna. Shells (n = 366) were collected from five sites situated upstream (two sites), just below (one site), and downstream (two sites) of the town of Saltville, Virginia, where Hg was used to produce chlorine and caustic soda from 1950 to 1972. Shell samples were used to test the (1) utility of geochemical signatures of shells for assessing the spatial variation in Hg levels in the river relative to the contamination source and (2) value of taphonomy (postmortem shell alteration) for distinguishing sites that differ in extirpation histories. Geochemical signatures of 40 shells, analyzed using atomic absorption spectroscopy, indicated a strong longitudinal pattern. All shells from the two upstream sites had low Hg concentrations (<5-31 ??g/kg), shells directly below Saltville had variable, but dramatically higher concentrations (23-4637 ??g/kg), and shells from the two downstream sites displayed intermediate Hg levels (<5-115 ??g/kg) that declined with distance from Saltville. Two pre-industrial shells, collected at Saltville in 1917, yielded very low Hg estimates (5-6 ??g/kg). Hg signatures were consistent among mussel species, suggesting that Hg concentrations were invariant to species type; most likely, highly variable Hg levels, both across sites and through time, overwhelmed any interspecific differences in Hg acquisition. Also, a notable postmortem incorporation of Hg in mussel shells seemed unlikely, as the Hg content was not correlated with shell taphonomy (r = 0.18; p = 0.28). The taphonomic analysis (n = 366) showed that the degree of shell alteration reliably distinguished sites with different extirpation histories. At Saltville, where live mussels have been absent for at least 30 years, shells were most heavily altered and fragmented. Conversely, fresh-looking shells abounded upstream, where reproducing mussel populations are still present. In summary, relic shells offered valuable spatiotemporal data on Hg concentrations in a polluted ecosystem, and shell taphonomic signatures discriminated sites with different extirpation histories. The shell-based strategies exemplified here do not require sampling live specimens and may augment more standard strategies applied to environmental monitoring. The approach should prove especially useful in areas with unknown extirpation and pollution histories. ?? 2005 American Chemical Society.

Environmental Science & Technology↗
USGS2005

Arsenate adsorption mechanisms at the allophane: Water interface

We investigated arsenate (As(V)) reactivity and surface speciation on amorphous aluminosilicate mineral (synthetic allophane) surfaces using batch adsorption experiments, powder X-ray diffraction (XRD), and X-ray absorption spectroscopy (XAS). The adsorption isotherm experiments indicated that As(V) uptake increased with increasing [As(V)] o from 50 to 1000 μM (i.e., Langmuir type adsorption isotherm) and that the total As adsorption slightly decreased with increasing NaCl concentrations from 0.01 to 0.1 M. Arsenate adsorption was initially (0−10 h) rapid followed by a slow continuum uptake, and the adsorption processes reached the steady state after 720 h. X-ray absorption spectroscopic analyses suggest that As(V) predominantly forms bidentate binuclear surface species on aluminum octahedral structures, and these species are stable up to 11 months. Solubility calculations and powder XRD analyses indicate no evidence of crystalline Al−As(V) precipitates in the experimental systems. Overall, macroscopic and spectroscopic evidence suggest that the As(V) adsorption mechanisms at the allophane−water interface are attributable to ligand exchange reactions between As(V) and surface-coordinated water molecules and hydroxyl and silicate ions. The research findings imply that dissolved tetrahedral oxyanions (e.g., H 2 PO 4 2 - and H 2 AsO 4 2 - ) are readily retained on amorphous aluminosilicate minerals in aquifer and soils at near neutral pH. The inner-sphere adsorption mechanisms might be important in controlling dissolved arsenate and phosphate in amorphous aluminosilicate-rich low-temperature geochemical environments.

Environmental Science & Technology↗
USGS2007

A gel probe equilibrium sampler for measuring arsenic porewater profiles and sorption gradients in sediments: I. Laboratory development

A gel probe equilibrium sampler has been developed to study arsenic (As) geochemistry and sorption behavior in sediment porewater. The gels consist of a hydrated polyacrylamide polymer, which has a 92% water content. Two types of gels were used in this study. Undoped (clear) gels were used to measure concentrations of As and other elements in sediment porewater. The polyacrylamide gel was also doped with hydrous ferric oxide (HFO), an amorphous iron (Fe) oxyhydroxide. When deployed in the field, HFO-doped gels introduce a fresh sorbent into the subsurface thus allowing assessment of in situ sorption. In this study, clear and HFO-doped gels were tested under laboratory conditions to constrain the gel behavior prior to field deployment. Both types of gels were allowed to equilibrate with solutions of varying composition and re-equilibrated in acid for analysis. Clear gels accurately measured solution concentrations (??1%), and As was completely recovered from HFO-doped gels (??4%). Arsenic speciation was determined in clear gels through chromatographic separation of the re-equilibrated solution. For comparison to speciation in solution, mixtures of As(III) and As(V) adsorbed on HFO embedded in gel were measured in situ using X-ray absorption spectroscopy (XAS). Sorption densities for As(III) and As(V) on HFO embedded in gel were obtained from sorption isotherms at pH 7.1. When As and phosphate were simultaneously equilibrated (in up to 50-fold excess of As) with HFO-doped gels, phosphate inhibited As sorption by up to 85% and had a stronger inhibitory effect on As(V) than As(III). Natural organic matter (>200 ppm) decreased As adsorption by up to 50%, and had similar effects on As(V) and As(III). The laboratory results provide a basis for interpreting results obtained by deploying the gel probe in the field and elucidating the mechanisms controlling As partitioning between solid and dissolved phases in the environment. ?? 2008 American Chemical Society.

Environmental Science & Technology↗