Arsenic contamination in groundwater is a worldwide problem. The existing portable field test kits can not provide accurate results when the arsenic concentration is around 10 μg L-1 or lower. This research first was focused on the development and validation of methods in which portable instrumentation, such as electrochemistry instruments or quartz crystal microbalances, could be used to accurately determine arsenic concentrations in water even when the concentration is below 10 μg L-1. A modified anodic stripping voltammetry (ASV) and cathodic stripping voltammetry (CSV) method with measurement at a microarray electrode manufactured by TraceDetect Inc. was developed. When the ASV method with a gold electrode was applied for real water analysis, the detection limit of arsenite was 2.2 μg L-1, and for arsenate was 0.13 μg L-1. In the CSV method the more commonly used hanging mercury drop electrode was replaced with a mercury film array electrode. Under the optimum condition, this method had a detection limit for arsenite of 0.58 μg L-1 and for arsenate of 2.7 μg L-1. A method for the determination of arsenic using a quartz crystal microbalance was developed in which the crystal surface was modified in situ by dithiolthreitol, an arsenite-selective ingand. The method was applied to real water sample analysis with a limit of 0.6 μg L-1. The second was concerned with an investigation of the kinetics of the reactions that are the basis of several currently available field test kits (as exemplified by the Hach Kit) using inductively coupled plasma mass spectrometry (ICP-MS) with the goal of improving the performance of the test kit. The time for arsine gas reaches to the maximum concentration in the headspace of the vessel was about 60 min without continuous stirring and only 20% of arsenic was absorbed on the test strip. To speed up the arsine generation, continuous stirring condition can be applied. It also made more arsine absorbed on the test strip. The SEM study proves the structure of the darker colored compound. For the lighter colored compounds, the information is not enough to make a conclusion.

<p> A continuation of studies of main group compounds carried out in this laboratory has led to the investigation of the structural and oxidative chemistry of several compounds and mixtures of the elements phosphorus, arsenic, sulfur and selenium. A number of questions and a lack of data regarding known compounds of these elements lead to an examination of the series P4-X^AsX^S3-Y^SeY, X = 0-4, Y = 0-3. Raman spectra of several members of the series have been recorded. The 31P and natural abundance 77Se nmr spectra of these compounds were also recorded, including a reinvestigation and complete assignment of the 31P nmr spectrum of P4Se3 . Several trends in the coupling constants and chemical shifts were noted and an empirical equation was devised for the 31P nmr spectral assignments for quaternary members of the series. The compound formerly believed to be P2As2S3 was shown to be PAs3S3. The crystal structure of an occupationally disordered crystal of stoichiometry P2As2S3 is also reported in this thesis.</p> <p> The compound As4S4 and 1:1 As:Se fused mixtures were separately oxidized with the Lewis acids AsF5 and SbF5 to produce the new cations As3S4+ and As3Se4+. The cations were characterized as the salts As3S4 (AsF6), As3S4 (SbF6), As3Se4 (AsF6 ) and As3Se4 (SbF6) by Raman spectroscopy and infrared spectroscopy. The determination of the crystal structures of the first three salts is also reported.</p> <p> Reactions of As4S4 with PF5, PCl5, BCl3, SO3, NbF5 , TaF5 and WF6 were shown to not proceed or, in the case of PF5, to not produce a stable adduct. The action of AsF5 on compounds and mixtures of heavy main group elements (e.g., Sb2Te3) of groups V and VI produced no new compounds that were identified and generally just gave known homo-polyatomic cations of the chalcogens.</p> <p> Oxidation of As4S4 with the halogens (X) chlorine and bromine produced AsX3 and S2X2 while the reaction of SbCl5 with As4S4 or 1:1 As:Se fused mixtures produced SbCl3, AsCl3 and the salts SCl3 (SbCl6) and SeCl3 (SbCl6),respectively. The crystal structure of SCl3 (SbCl6) is reported here along with unit cell data for SeCl3 (SbCl6) and SBr1.2Cl1.8 (SbCl6) and the Raman data for all three compounds. Only AsSI was produced by the oxidation of As4S4 by I2 in SO2 while a reaction between molten P4Se3 and I2 gave the new compound αP4Se3I2. Raman spectra for both AsSI and αP4Se3I2 were recorded as well as the 31P nmr spectrum for the latter.</p> / Thesis / Doctor of Philosophy (PhD)

Providing safe drinking water and wastewater remediation are constant worldwide challenges. Adsorption is an attractive alternative to conventional techniques such as coagulation, precipitation (chemically or electrochemically), hybrid membranes, and ion-exchange for the purification of water. Biochar-based composite sorbents are increasingly popular because a range of surface chemical and physical treatments can impart performance and environmental benefits to the material. This is ideal for rural areas where more costly conventional methods may not be readily available or affordable. This dissertation focused on three different projects involving high surface area (~700 m2/g) Douglas fir biochar based multifunctional engineered adsorbents. Chapter II focuses on arsenic (III) adsorptive removal onto magnetic iron oxide dispersed onto biochar. This chapter highlights the adsorptive and redox properties of biochar composites for pollutant toxicity reduction. Chapter III focuses on pollutant toxicity neutralization after adsorption, simultaneous adsorption, and multi-phase adsorption. A MIL-53-MOF magnetite/magnetic biochar composite model system was used to demonstrate simultaneous chromium (VI) adsorption and organic pollutant rhodamine (RhB) degradation. Chapter IV is focused on tailoring the biochar to change its physical properties (enhance hydrophobicity) to achieve a specific pollutant treatment requirement (buoyancy). Oil spill remediation was used as a model example for this purpose and lauric acid-decorated magnetite biochar composite was introduced. The composites and their pollutant-loaded analogues were extensively characterized using BET, SEM, TEM, EDS, XRD, VSM, PZC, Elemental analysis, TGA, DSC, FT-IR and XPS.

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A kinetic study of the oxidation of arsenic(III) by chromium(VI) was made in acetic acid-acetate buffers at ionic strengths 1.5M and 3.0M. The reaction rate showed a first order dependence on the arsenic(III), total chromium(VI), and acetic acid concentrations. Two possible mechanisms for the reaction have been suggested. One possibility is the formation of an arsenic-chromium adduct in which either the acid chromate or dichromate ion is involved. A second possible mechanism postulates adduct formation involving only the acid chromate ion, followed by a CrO₄= catalyzed decomposition of the adduct. In either suggested mechanism, the acetate and hydroxide ion catalyzes the decomposition of the adduct. The rate dependence on total chromium(VI) concentration is contrary to previous reports which postulate an adduct or ester mechanism. Induced reaction studies show the initial step involves the oxidation of one arsenic(III) species by chromium(VI) with the formation of tetravalent chromium as-a reactive intermediate. The stoichiometry or the overall reaction was found to be 3 As(III) + 2 Cr(VI) = 3 Aa(V) + 2 Cr(III). / M.S.

For this study, I have constructed a logistic regression model, using existing datasets of environmental parameters to predict the probability of As concentrations above 5 parts per billion (ppb) in Virginia groundwater and to evaluate if geologic or other characteristics are linked to elevated As concentrations. Measured As concentrations in groundwater from the Virginia Tech Biological Systems Engineering (BSE) Household Water Quality dataset were used as the dependent variable to train (calibrate) the model. Geologic units, lithology, soil series and texture, land use, and physiographic province were used as regressors in the model. Initial models included all regressors, but during model refinement, attention was focused solely on geologic units. Two geologic units, Triassic-aged sedimentary rocks and Devonian-aged shales/sandstones, were identified as significant in the model; the presence of these units at a spatial location results in a higher probability for As occurrences in groundwater. Measured As concentrations in groundwater from an independent dataset collected by the Virginia Department of Health were used to test (validate) the model. Due to the structure of the As datasets, which included As concentrations mostly (95-99%) = 5 ppb, and thus few (1-5%) data in the range > 5 ppb, the regression model cannot be used reliably to predict As concentrations in other parts of the state. However, our results are useful for identifying areas of Virginia, defined by underlying geology, that are more likely to have elevated As concentrations in groundwater. Results of this work suggest that homeowners with wells installed in these geologic units have their wells tested for As and regulators closely monitor public supply wells in these areas for As. / Master of Science

Arsenic sorbs strongly to the surfaces of Fe(III) (hydr)oxides. Under aerobic conditions, oxygen acts as the terminal electron acceptor in microbial respiration and Fe(III) (hydr)oxides are highly insoluble, thus arsenic remains associated with Fe(III) (hydr)oxide phases. However, under anaerobic conditions Fe(III)-reducing microorganisms can couple the reduction of solid phase Fe(III) (hydr)oxides with the oxidation of organic carbon. When ferric iron is reduced to ferrous iron, arsenic is mobilized into groundwater. Although this process has been documented in a variety of pristine and contaminated environments, minimal information exists on the mechanisms causing this arsenic mobilization. Arsenic mobilization was studied by conducting controlled microcosm experiments containing an arsenic-bearing ferrihydrite and an Fe(III)-reducing microorganism, Geobacter metallireducens. Results show that arsenic mobility is strongly controlled by microbially-mediated disaggregation of arsenic-bearing iron nanoparticles. The most likely controlling mechanism of this disaggregation of iron oxide nanoparticles is a change in mineral phase from ferrihydrite to magnetite, a mixed Fe(III) and Fe(II) mineral, due to the microbially-mediated reduction of Fe(III). Although arsenic remained associated with the iron oxide nanoparticles and was not released as a hydrated oxyanion, the arsenic-bearing nanoparticles could be readily mobilized in aquifers. These results have significant implications for understanding arsenic behavior in aquifers with Fe(III) reducing conditions, and may aid in improving remediation of arsenic-contaminated waters. / Master of Science