Browsing by Subject "arsenic"
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Item Adsorption of As(V), As(III) and methyl arsenic by calcite and the impact of some groundwater species(2009-05-15) Jones, Robert GarretThe objective of this research was to investigate the retention of arsenate (iAsV), arsenite (iAsIII), monomethyl arsenate (MMAsV) and dimethyl arsenate (DMAsV) by calcite and assess the impact of dissolved Ca2+, Mg2+, phosphate and sulfate on arsenic solubility, adsorption and precipitation phenomena. Adsorption kinetics of iAsV, evaluated at a low and high concentration, was a relatively rapid process, with a fast initial reaction rate within the first few minutes and a subsequent slower reaction rate as equilibrium was approached. The relative adsorption of arsenicals decreased in the following order: iAsV > iAsIII > DMAV > MMAV. In no case was a clear adsorption maximum observed with increasing dissolved arsenic concentration. Dissolved 0.01 M Ca2+ resulted in an increase in iAsV adsorption; however, in the presence of 0.1 M Ca2+ adsorption of iAsV was decreased. The presence of Mg2+ as 0.01 M Mg(NO3)2 resulted in decreased iAsV adsorption probably the result of a lower iAsV affinity for adsorbed Mg2+ as compared to Ca2+. Phosphate and sulfate were highly competitive with iAsV in adsorption to calcite and both resulted in decreased iAsV adsorption. The total prevention of iAsV adsorption at initial equimolar arsenic/phosphate concentrations > 88 ?M each could be from the consumption of available calcite surface sites by the specific adsorption of phosphate. Equilibrium modeling, using the geochemical and mineral speciation of equilibrium model (MINTEQA2), indicated that at low concentrations of arsenate or phosphate solid-phase precipitation was not likely and adsorption processes likely controlled solubility. At high concentrations of arsenate Ca3(AsO4)2 ? 3 2/3 H2O and Ca3(AsO4)2 ? 4 1/4 H2O solid phases could be controlling arsenate solubility. This study indicates that arsenic adsorption response by calcite was different than that of phosphate suggesting that arsenic may not be specifically adsorbed to calcium at the calcite surface. Reduction and biomethylation of arsenic decreased adsorption, suggesting that processes which could affect the speciation of arsenic in the environment, could increase arsenic mobility in environmental systems where calcite and dissolved aqueous calcium play a predominant role in controlling arsenic solubility. Dissolved aqueous concentrations of magnesium, phosphate and sulfate generally reduced the ability of arsenic to be adsorbed to calcite.Item Assessment of arsenate bioavailability in iron-rich environments: development of a high-pressure liquid chromatography method of quanitification for arsenate sorbed by Fe3+-substituted chelating resins in arsenic-bearing ferrihydrite suspensions(Texas A&M University, 2005-08-29) Roberts, Melissa DelaneGiven that the mobility, bioavailability, and toxicity of arsenate in natural systems is often controlled by the strong binding capacity of iron oxyhydroxides, the objective of this study was to document the interactions of Dowex M4195 Fe3+-substituted chelating resins (a potential fieldbased tool for the quantification of potential arsenate bioavailability) and arsenic-bearing ferrihydrite (AFH) as a function of suspension pH, suspension concentration, and background electrolyte concentration. In 0.5 g AFH/L (0.001 M NaNO3) suspensions, arsenate sorption to the resins was proportional to the degree of acidification of the AFH suspensions by the resins. H+-enhanced dissolution of ferrihydrite artificially increased the arsenate in solution, causing a consistent overestimation of potential arsenate bioavailability. Resin-induced acidification was decreased with increasing suspension concentration. Arsenate sorption to the resins in 0.5 g/L suspensions at pH 8 decreased with increasing NaNO3 concentrations, reflecting the decreasing activity of arsenate under these conditions. The results of this study indicate that the high buffer capacity of natural soils would prevent acidification as a result of resin introduction. Thus, Dowex M4195 Fe3+-substituted chelating resins should provide a reasonable assessment of potential arsenate bioavailability from poorly-crystalline iron oxide minerals. Possibly more importantly, Dowex M4195 Fe3+-substituted chelating resins appear to be a new choice of passive equilibrium sampling device that should work well for the determination of bioavailable arsenate concentrations in the field.Item Characterization and stabilization of arsenic in water treatment residuals(Texas A&M University, 2004-11-15) Wee, Hun YoungThe characterization of water treatment residuals containing arsenic was investigated in the first study. Arsenic desorption and leachability from the residuals were the focus of this study. Arsenic leaching from water treatment residuals was found to be underestimated by the toxicity characteristic leaching test (TCLP) due to the pH of the leachates being favorable for As(V) adsorption. Competitive desorption of arsenic with phosphate was significant because phosphate tends to compete with As(V) on the surface of the metal hydroxide for adsorption sites. However, arsenic desorption by the competition of sulfate and chloride was found to be negligible. The pH in the leachate was a critical variable in controlling arsenic stability in the residuals. The release of arsenic from the residuals was elevated at low and high pH due to the increase dissolution of the adsorbents such as Fe and Al hydroxides. In the second phase of the study, the stabilization techniques for arsenic contained residuals and were examined to develop methods to suitably stabilize arsenic to eliminate and/or minimize leaching. A decrease of arsenic leaching was achieved by the addition of lime to the residuals and believed to be due to the formation of less soluble and stable calcium-arsenic compounds. However, it is suggested that the ordinary Portland cement (OPC) should be added with the lime for the long term stabilization because lime can be slowly consumed when directly exposed to atmospheric CO2. The solidification and stabilization (S/S) technique with lime and OPC was shown to be successfully applied by the immobilization of a wide variety of arsenic tainted water treatment residuals.Item Factors influencing the efficiency of arsenic extraction by phosphate(Texas A&M University, 2005-11-01) Yean, Su JinExtraction with sodium phosphate has been used as a method of accessing arsenic in soils. Arsenic extraction efficiency by phosphate from rice-paddy soils of Bangladesh usually has been low and highly variable between soils. The major objectives of this study were to examine the relationships between phosphate-extractable arsenic and soil iron-oxide composition and to investigate the experimental factors which might influence arsenic-extraction efficiency from rice-paddy soils of Bangladesh by phosphate. Statistical analysis of approximately 500 surface soils from Bangladesh indicated that phosphate-extractable arsenic was well correlated with total soil arsenic (r2 = 0.832) and oxalate-extractable arsenic (r2 = 0.825), though extraction efficiency varied widely (5 - 54 % of the total soil arsenic). The thanas with the lowest arsenic contents generally also had the soils with the lowest arsenic-extraction efficiencies. Quantity of phosphate-extractable arsenic was weakly correlated with the soil iron-oxide content, but extraction efficiency (i.e., the proportion of phosphate-extractable arsenic to total soil arsenic) was not correlated with any iron-oxide parameter. Arsenic extraction was strongly influenced by reaction variables such as sample grinding, phosphate concentration, principal counterion, reaction pH, and reaction time. The extraction efficiency was impacted by the influence of these individual factors on reaction kinetics and accessibility of arsenic adsorption sites for ligand exchange by phosphate. A portion of the arsenic was readily exchanged during the first few hours of extraction, followed by a much slower subsequent extraction. These results indicate that some of the arsenic is easily exchanged, but for a substantial portion of the arsenic, either the reaction kinetics is very slow or the sites are not accessible for reaction with phosphate. Extraction by phosphate is a useful procedure for the assessment of readily ligand-exchanged arsenic.Item Investigations into arsenate-induced neural tube defects in a mouse model(2009-05-15) Hill, Denise SuzanneNeural tube defects (NTDs) are malformations affecting about 2.6/1000 births worldwide, and 1/1000 in the United States. Their etiology remains unknown, and is likely due to interaction of genetic susceptibility factors with environmental exposure. Of the many environmental agents considered to potentially contribute to NTD risk, arsenic is one that is surrounded in controversy. We have developed a model system utilizing maternal intraperitoneal (I.P.) exposure on E7.5 and E8.5 to As 9.6 mg/kg (as sodium arsenate) in a normal inbred mouse strain, LM/Bc/Fnn, that is sensitive to arsenate-induced exencephaly. We investigated arsenate induced gene expression changes using DNA microarrays of embryonic anterior neural tube tissue, as well as monitoring of metabolic function in conjunction with the administration of select compounds to rescue the normal phenotype. Finally, to address questions concerning the importance of route of administration and potential maternal toxicity, a teratology study was performed using three arsenate doses administered orally. Regarding the gene expression study, we identified several candidate genes and ontology groups that may be responsible for arsenate?s teratogenicity. Genes include: engrailed 1 (En-1), platelet derived growth factor receptor alpha (Pdgfr?) and ephrinA7 (EphA7). Gene ontology groups identified include oxidative phosphorylation, redox response, and regulation of I-kappaB kinase/NF-kappaB cascade. Acute arsenate exposure induced disruption of mitochondrial function and dependent glucose homeostasis: subsequent hyperglycemia was teratogenic. Maternal treatment with insulin or n-acetyl cysteine, an antioxidant and precursor of glutathione synthesis, proved highly successful in rescuing both the normal phenotype, and to differing degree, the maternal hyperglycemia. Maternal oral arsenate administration also resulted in exencephaly, with exposed embryos exhibiting a positive linear trend with arsenate dosage. There were also linear trends in the relationships between arsenate dose and anomalies involving several components of the axial skeleton: the vertebrae and calvarium. There was no evidence of maternal toxicity as shown by lack of differences in maternal body weight gain, liver, and kidney weights. In conclusion, maternal arsenate exposure (regardless of exposure route) was teratogenic in our model, primarily causing NTDs. Responsible mechanisms may involve disruption of redox and glucose homeostasis as well as expression of established NTD candidate genes.Item Macroscopic and spectroscopic investigation of interactions of arsenic with synthesized pyrite(2009-05-15) Kim, Eun JungSulfide minerals have been suggested to play an important role in regulating dissolved metal concentrations in anoxic environments. Pyrite is the most common sulfide mineral and it has shown an affinity for arsenic, but little is known about the arsenic retention mechanisms of pyrite. In this study, interactions of arsenic with pyrite were investigated in an anoxic environment to understand geochemical cycling of arsenic better and to predict arsenic fate and transport in the environment better. A procedure using microwaves was studied to develop a fast and reliable method for synthesizing pyrite. Arsenic-pyrite interactions were investigated using macroscopic (solution phase experiments) and microscopic (X-ray photoelectron spectroscopic investigation) approaches. Pyrite was successfully synthesized within a few minutes via reaction of ferric iron and hydrogen sulfide under the influence of irradiation by a conventional microwave oven. The SEM-EDX study revealed that the nucleation and growth of pyrite occurred on the surface of elemental sulfur, where polysulfides are available. Compared to conventional heating, microwave energy results in rapid (< 1 minute) formation of smaller particulates of pyrite. Higher levels of microwave power can form pyrite even faster, but faster reaction can lead to the formation of pyrite with defects. Arsenic removal by pyrite was strongly dependent on pH and arsenic species. Both arsenite (As(III)) and arsenate (As(V)) had a strong affinity for the pyrite surface under acidic conditions, but As(III) was removed more effectively than As(V). Under acidic conditions, arsenic removal continued to occur almost linearly with time until complete removal was achieved. However, under neutral to alkaline conditions, fast removal was followed by slow removal and complete removal was not achieved in our experimental conditions. A BET isotherm equation provided the best fit to arsenic removal data, suggesting that surface precipitation occurred at high arsenic/pyrite ratio. The addition of competing ions did not substantially affect the ultimate distribution of arsenic between the pyrite surface and the solution, but changing pH affected arsenic stability on pyrite. X-ray photoelectron spectroscopy revealed that under acidic conditions, arsenic was removed and formed solid phases similar to As2S3 and As4S4 by reaction with pyrite. However, under neutral to alkaline conditions, arsenic was removed and formed As(III)-O and As(V)-O surface complexes, as well as As2S3/As4S4-like precipitates. As pH increases, the amount of arsenic that formed As2S3/As4S4-like precipitates decreased, while the amount that formed As(III)-O and As(V)-O surface complexes increased. Under alkaline conditions, a FeAsS-like phase was also detected.Item Methyl arsenic adsorption and desorption behavior on iron oxides(Texas A&M University, 2005-08-29) Lafferty, Brandon JamesArsenic is a toxic element that is widely distributed throughout the earth??s crust as a result of both natural geologic processes and anthropogenic activities. In virtually all environments, methylated forms of arsenic can be found. Because of the widespread distribution and toxicity of arsenic and methyl-arsenic, their adsorption behavior on soil minerals is of great interest. Although considerable attention has been given to the behavior of inorganic arsenic on mineral surfaces, little research has been conducted regarding interactions of the methyl-arsenic forms. The objective of this study was to compare the adsorption and desorption behavior of methylarsonate (MMAsV), methylarsonous acid (MMAsIII), dimethylarsinate (DMAsV), dimethylarsinous acid (DMAsIII), arsenate (iAsV), and arsenite (iAsIII) on iron oxide minerals (goethite and ferrihydrite) by means of adsorption isotherms and adsorption envelopes. Additionally, desorption envelopes were obtained using sulfate and phosphate as competitive ligands. Arsenic was measured by FI-HG-AAS. MMAsV and iAsV were adsorbed in higher amounts than DMAsV on goethite and ferrihydrite at all pH values studied. Although MMAsV and iAsV were adsorbed quantitatively at lower concentrations on goethite and ferrihydrite, as arsenic concentration was increased MMAsV was adsorbed in slightly lower quantities than iAsV. DMAsV was not quantitatively adsorbed at any concentration on goethite or ferrihydrite. MMAsV and iAsV exhibited high adsorption affinities on both goethite and ferrihydrite at pH values below 9 and showed decreasing adsorption above this point (more rapidly for MMAsV). DMAsV was adsorbed only at pH values below 8 on ferrihydrite and below 7 on goethite. MMAsV, iAsV, and DMAsV each exhibited adsorption characteristics suggesting specific adsorption on both goethite and ferrihydrite. Increased methyl substitution resulted in increased ease of arsenic release from the iron oxide surface. MMAsIII and DMAsIII exhibited no evidence for any type of specific adsorption under the conditions studied. Phosphate was a more effective desorbing ion than sulfate, but neither desorbed all arsenic species quantitatively.Item Mineralogy and Geochemistry of Pb, Zn and Ag Mine Tailings Originating From Carbonate-Rich Deposits(2012-11-29) McClure, Roberta 1981-Mining for silver, lead, zinc, and copper in Zimapan, Hidalgo State, Mexico has been ongoing since 1576. Unsecured tailings heaps and associated acid mine drainage have presented problems related to soil quality, water quality, and dust emission control in the Zimapan area. Objectives of the study of the mine tailings are (1) to determine mineralogy of the tailings in order to identify acid-producing minerals and heavy metals at risk for release in acidic conditions, and (2) to quantify carbonate minerals and (3) to determine heavy metal content that may be released by the products of sulfide mineral weathering. Representative mine tailings have been sampled from a site located north of Zimapan. Mineralogical characterization has been conducted with X-ray diffraction (XRD), and scanning and transmission electron microscopes (SEM and TEM). Total carbonates have been determined the Chittick procedure. X-Ray Fluorescence (XRF) has been utilized to determine total elemental composition. XRD and SEM analyses have confirmed the presence of pyrite and arsenopyrite indicating a potential for acid mine drainage. Calcite has been confirmed to have a significant presence in the unweathered samples by XRD and the Chittick procedure, with some samples containing an average of 19.4% calcite. NAA and XRF have revealed significant concentrations of toxic elements such as As, Pb and Zn in both the oxidized and unoxidized samples.Item Quantification of potential arsenic bioavailability in spatially varying Geologic Environments at the Watershed Scale Using Chelating Resins(Texas A&M University, 2004-09-30) Lake, Graciela EstherPotential arsenic toxicity in different geologic environments is dependent on total arsenic concentration and arsenic bioavailability. It is important to identify the geologic environments that may sequester arsenic because these systems can act as long-term sources for arsenic as well as retard transport and limit toxicity. Bioavailability is defined as the readiness of a compound or element to be taken up by organisms (Gregorich et al., 2001), while potential bioavailability is possible uptake of a compound or element by organisms. The objective of this research is to quantify the potential bioavailability of arsenic in laboratory microcosms and in different geologic environments in the Nueces and San Antonio River Watersheds, Texas, using a chelating resin as an infinite sink. To assess the applicability of chelating resins to estimate potential arsenic bioavailability in the field, iron-loaded DOWEX M4195 resin was used to extract arsenic from solutions and sediments (pond sediment, river sediment, and ephemeral stream sediment). The average percentage of arsenic sorbed from solution was 66% ? 0.16. Competition studies between arsenate, phosphate, and vanadate suggest there is moderate competition, reducing overall arsenic sorption to the resin in the presence of competing ions. Iron-loaded resin was then exposed to sediment samples spiked with increasing amounts of arsenic over 15, 30, 60 and 90 days. Results of the sediment study showed 1) increased arsenic sorption to the resin over time, 2) small variations of potential bioavailable arsenic among geologically different sediments, and 3) evidence of arsenic sequestration. Field devices that housed iron-loaded resin were used to extract potentially bioavailable arsenic from sediment in six different geologic environments (i.e. lake, river, perennial stream, ephemeral stream, pond, and wetland) in the watersheds over a twenty-eight day period. The wetland (15.7 mmol As/g wet resin) and perennial stream sediments (11.0 mmol As/g wet resin) represented the maximal and minimal calculated potential bioavailability, respectively. However, the potentially bioavailable index calculated from mmol As/g wet resin extracted from field environments and mmol As/ g sediment in digested samples showed sequestration would be high in the wetland environment and high bioavailability in the perennial stream and river environments.