Browsing by Subject "phosphate"
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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 Ambient pH signaling influences phosphate transport in Neurospora crassa(Texas A&M University, 2007-04-25) Kennedy, Patrick WadeIn the course of our efforts to resolve Pi transport mechanisms in the model fungus Neurospora crassa we identified a null mutant, par-1, that displays enhanced Pi transport activity specifically under alkaline growth conditions. The PAR-1 protein is related to PalF of Aspergillus nidulans, which is one component of an ambient pH signaling pathway that is conserved among fungi. A deletion mutant for the PacC homolog, another component of the same pathway, phenocopies par-1, demonstrating that a defect in pH signaling is responsible for the altered Pi transport activity. Our results indicate that pH signaling in N. crassa plays an important role in coordinating high and low affinity Pi transport in response to ambient pH, but through different mechanisms. Sulfate acquisition also is influenced by pH signaling, suggesting that this regulatory system has a broad role in nutrient uptake and homeostasis.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 Novel mechanisms for regulating polyphosphate metabolism in Saccharomyces cerevisiae(Texas A&M University, 2005-11-01) Neef, Daniel WilhelmTo ensure a continuous supply of phosphate, living organisms have devised complex mechanisms to regulate the uptake and subsequent utilization of this essential nutrient. An important aspect of phosphate metabolism is the storage of excess phosphate as the polymer, polyphosphate. Despite the importance of this polymer to all living organisms, much needs to be learned about its synthesis, storage, or utilization. Furthermore, little is known about the regulatory mechanisms that determine when polyphosphate synthesis or degradation is appropriate. Our work has shown that polyphosphate is a dynamic molecule whose levels fluctuate during the cell cycle. Polyphosphate levels are high in G1, and subsequently drop as the cell uses free phosphate during cell division. Mitotic induction of phosphate regulatory genes, including the acid phosphatase gene PHO5, replenishes polyphosphate levels late in the mitosis. Furthermore, we have shown that Mcm1 and Fkh1, two cell cycle dependent transcriptional activators, contribute to mitotic activation of PHO5. In addition, we have elucidated the importance of regulating polyphosphate synthesis. Strains lacking the cyclin Pho80 have increased expression of the polyphosphate synthase genes, PHM1-4, and thus have highly elevated polyphosphate levels. Hyperaccumulation of polyphosphate results in severe growth defects on medium containing high levels of sorbitol, presumably through the polyphosphate-dependent overacidification of the vacuole.