Using Environmental Protection Agency occurrence and concentration data, it is estimated that about 6 million pounds of arsenic-bearing residuals (ABSR) will be generated annually in the United States when full compliance with the new standard for arsenic in drinking water (10 μg/L) is realized. Effective management of disposal of ABSR requires both a full characterization of the materials and an understanding of the environment in which the disposal will occur. Currently, there are different testing methods to evaluate the stability of ABSR, the principal of which is the EPA Toxicity Characteristics Leaching Procedure (TCLP). These tests indicate that common ABSRs may be disposed in mixed-solid waste landfills. However, this and previous work shows that these testing methods may significantly underestimate the degree and mechanism of arsenic mobilization from the residuals, because critical physical and chemical dissimilarities exist between the tests and landfill conditions. In addition, no current testing methods simulate the mineralogic aging in those ABSR, which exhibit further complexity. Landfill disposal involves liquid and solid residence times on the order of months and decades, respectively, whereas leaching tests are completed in two days or less. Consequently, time dependent re-mineralization of residuals that would be routinely expected in landfill time scales is not addressed by standard leaching tests. Treating arsenic brines by co-precipitation with iron oxyhydroxides is an established and effective remediation method for small quantities of highly concentrated liquid arsenic waste, such as brines derived from mine tailings, ion exchange resin regeneration, and reverse osmosis treatment of drinking water. However, amorphous ferric hydroxide (AFH) is expected to exhibit mineralogical aging analogous to the observed natural evolution of ferrihydrite to goethite and hematite. The aim of this research is to develop methods for characterization of AFH sludges precipitated from concentrated arsenic brines which exhibit mineralogical aging and to evaluate the impact of such aging on arsenic leachability. Overall, aging the sludge resulted in consistently higher arsenic release.

Three iron oxide materials have been studied for uptake of three anions (arsenate, phosphate and fluoride) and a cation (cadmium) from aqueous solutions. Two of the materials were produced using original procedures developed at Loughborough University. The former material was conditioned by a controlled freeze-thaw procedure to enhance granularity and the latter was air-dried at room temperature. Their capacities were compared with a commercially available material supplied by GEH Wasserchemle, Germany. Pore size distributions and specific surface area values were determined by N2 analysis at 77 K. All samples possessed a reasonable specific surface area, in the range 200-300 m2/g and were mesoporous. Samples produced at Loughborough University also contained some macropores, evidence of a more amorphous structure or lack of pH control during production. X-ray diffraction indicated that all samples had some b-FeOOH present and that the chloride content and production pH affected the material crystallinity. Crystallinity increased with increasing chloride content and a higher production pH resulted in the presence of more than one phase. Chemical characterisation was also completed on all three samples. The point of zero net proton charge and isoelectric point for each material was obtained by potentiometric batch titrations and zeta potential measurements respectively. The difference in these values increased with a higher chloride content and all samples studied possessed a positive surface at low pH and negative surface at high pH. These parameters were not greatly affected by the background electrolyte concentration, implying that the background electrolyte is not specifically adsorbed. However, arsenate and phosphate appeared to be specifically adsorbed as the isoelectric point decreased. The uptake capacities for arsenate, phosphate. fluoride and cadmium of all three samples were obtained by measuring batch isotherms at 25 degrees C. The pH range was 4-9, using various initial concentrations up to a maximum of approximately 30 uM. For all anionic species studied, the capacity decreased with increasing pH, and the reverse trend was noted for cadmium. The Langmuir model provided a good fit for the anionic isotherms and the Freundlich model for the cationic isotherms. The materials studied possessed a markedly higher capacity for fluoride than arsenate and phosphate, with an intermediate capacity for cadmium. This indicates that fluoride is attached to the surface via monodentate (single) bonds, whilst both arsenate and phosphate are primarily attached to the surface via bidentate (two) bonds. Cadmium is probably bound by both these mechanisms. The effect of competing anions on arsenic uptake capacity was determined using mini-column experiments of binary (arsenate-fluoride, arsenate-Phosphate and phosphate-fluoride) and ternary (arsenate-fluoride-phosphate) mixtures. Arsenate removal was strongly affected by the presence of phosphate, but was only slightly lower in the presence of fluoride. (Continues...).

The need of making arsenic contaminated ground water potable is urgent in parts of Burkina Faso. An implementation of a treatment design using Granular Ferric Hydroxide (GFH) is under development. Water from a tube-well in Lilgomdé, Yatenga province, Burkina Faso, has been treated with the adsorbent GFH through column experiments. The water had an arsenic concentration varying between 99 and 215 μg/L and an average pH of 7.9. The study has shown that arsenic, predominantly in the form of arsenate, can be adsorbed to the material in significant amounts despite a high natural pH and the presence of ions competing with arsenic for adsorption sites on the GFH. When run through the column, the pH of the effluent water drastically decreased in the beginning. However, the low pH was soon followed by a slower readjustment towards the pH of the influent water. The adsorption of phosphates and fluorides was also studied. Both competitors exist in higher molar quantities than arsenic in the ground water. Even though arsenic displays a higher affinity for the GFH, an average 44 % of total phosphate and 64 % of the fluoride were adsorbed, making them a factor affecting the results of the study. Hydrogen carbonate is also believed to affect the adsorption process but this could not be confirmed. The empty bed contact time (EBCT), describing the average time of contact between the adsorbent and the water, has shown to be of importance. Increasing the EBCT resulted in notably more arsenic being adsorbed per volume GFH. When increasing the contact time, the study showed that reducing the speed of the flow was more effective than increasing the volume of the adsorbent. The GFH was also found to have a self-regenerating ability to a certain extent. When interrupting the experiment and leaving the column material in the aqueous solution for several days, the arsenic adsorption capacity after the break was shown to be higher than just before it. A 13 % increased capacity was shown in one experiment. Conclusively, the results of this study suggest no hindrances towards developing larger scale columns and prototypes to be applied at tube-well pump stations. Further investigations on the treatment method with GFH, on arsenic contaminated water, are recommended.

Phosphonate antiscalants are commonly used in nanofiltration and reverse osmosis water treatment to prevent membrane fouling by mineral scale. In many circumstances it is desirable to remove these phosphonate compounds before concentrate disposal or further treatment. This research investigated the removal of phosphonate compounds from simulated membrane concentrate solutions using ferric hydroxide adsorbents. Two phosphonate antiscalants were investigated, Permatreat 191® (PT191) and nitrilotrimethylphosphonic acid (NTMP). Batch adsorption isotherms and column breakthrough and regeneration experiments were performed on two commercial adsorbents and a ferric hydroxide loaded polyacrylonitrile fiber adsorbent prepared in our laboratory. The best performing adsorbent was Granular Ferric Hydroxide® (GFH) obtained from GEH Wasserchemie. Adsorption isotherms measured after 24-hour equilibration periods showed initial concentration effects, whereby the isotherms were dependent on the initial adsorbate concentration in solution. Significant differences in adsorption behavior were observed between the PT191 and the NTMP adsorbates. Differences in adsorption behavior between NTMP and PT191 are all consistent with the PT191 containing fewer phosphonate functional groups per molecule than NTMP. Desorption rates were bimodal, with 40-50% of the adsorbed phosphonate being released on a time scale of 10-24 hours, while the remaining fraction was released approximately one order of magnitude more slowly. The slow desorbing fraction primarily resulted from equilibrium effects resulting from significant phosphonate adsorption, even in 1.0 mol/L NaOH solutions. Complete regeneration could not be achieved, even after eluting the adsorbent columns with more than 300 bed volumes of 1.0 mol/L NaOH. However, the incomplete regeneration had only a minor effect on phosphonate uptake in subsequent column breakthrough experiments.

Lowering the Maximum Contaminant Level (MCL) for arsenic in drinking water in the U.S., has caused a significant increase in the volume of Arsenic Bearing Solid Residuals (ABSRs) generated by drinking water utilities. Most of the affected utilities are smaller water treatment facilities, especially in the arid Southwest, and are expected to use adsorption onto solid sorbents for arsenic removal. Because of their high adsorption capacity and low cost, iron sorbents are used treatment technology and, when the sorbent's capacity is spent, these ABSRs are disposed in municipal solid waste (MSW) landfills and as a consequence arsenic is likely being released into leachate. However, a mature landfill is a biotic, reducing environment, which causes arsenic reduction and mobilization from the ABSRs. It is well documented that iron and sulfur redox cycles largely control arsenic cycling and, because iron and sulfur are ubiquitous in MSW, it is suspected that they play key roles in arsenic disposition in the landfill microcosm. The purpose of this study is to investigate the degree to which sulfate can prevent arsenic from leaching into landfill through biomineralization and to study ABSRs biogeochemical weathering effect on arsenic sequestration. The primary routes of iron and sulfate reduction in landfills are microbially mediated and biomineralization is a common by-product. In this case, biomineralization is the transformation of ferric (hydr) oxides into ferrous iron phase and sulfate into sulfide minerals such as: siderite (FeCO₃), vivianite (Fe₃(PO₄)₂), iron sulfide (FeS), goethite (α-FeOOH), and realgar (AsS). In this work, long-term microbial reduction and biomineralization of iron, sulfur, and arsenic species are evaluated as processes that both cause arsenic release from landfilled ABSRs and may possibly provide a means to re-sequester As in a recalcitrant solid state. The work uses long-term, continuous flow-through laboratory-scale columns in which controlled conditions similar to those found in a mature landfill prevail. In these simulated landfill column experiments, formation of biominerals, same as those that would naturally occur in typical non-hazardous MSW landfills, will be investigated. The feed contains lactate as the carbon source and primary electron donor, and ferric iron, arsenate, and a range of sulfate concentrations as primary electron acceptors. Our results suggest that biomineralization changes the stability of arsenic through a number of different processes including (i) release of arsenic through reductive dissolution of iron-based ABSRs; and (ii) readsorption/incorporation of released arsenic to secondary biominerals. The influence of biominerals, which have less surface area and adsorption capacity than original AFH, on the retention of arsenic is also investigated in this study. Our results show that the concentration of sulfate fed to the system affects the biomineral formation, and that the relative amounts and sequence of precipitation of biominerals affect the free arsenic concentration that can seemingly be engineered by the concentration of sulfate fed to the system. Comparison between the columns with different sulfate concentrations indicate that inflow sulfate concentration higher than 2.08 mM decreases As mobilization to <50%.

Drinking water contaminated with arsenic is a worldwide problem, especially in developing nations. The research presented in this dissertation describes two major goals: development of hybrid homopolymer polyacrylonitrile (PAN)-based sorbents for arsenate removal from drinking water and understanding regeneration of arsenate from ferric hydroxide-based adsorbents. The homopolymer PAN fiber was chemically modified to introduce functional groups using NaOH and hydrazine hydrate (HH) separately, or in combination of both. The modified fibers were characterized using Fourier transform infrared spectroscopy (FTIR) and ion exchange measurements. The ferric hydroxides were impregnated onto functionalized fibers using two iron loading procedures. The best arsenate removal performance was obtained using the simplest pretreatment procedure of soaking in 10% NaOH at 95 °C for ninety min, followed by precipitation coating of ferric hydroxide. This suggests that adsorbents based on a low-cost PAN fabric may be produced in developing areas of the world where commercial products may not be available. A density functional theory (DFT) molecular modeling was used to compare free energies of reactions and activation barriers in the formation of arsenate-ferric hydroxide complexes. Slow kinetics associated with arsenate adsorption and desorption attributed to the high activation barriers in forming and breaking bonds with the ferric hydroxides. Another aspect of regeneration study focused on the effects of underlying properties of the ferric hydroxides-loaded adsorbents on arsenate recovery. The arsenate loaded ferric hydroxide adsorbent containing no or weak base functionalities can be regenerated using NaOH, while addition of NaCl to NaOH solution is required for same recovery of arsenate from the adsorbents containing strong base anion exchange functionalities. Moreover, the irreversible fraction of arsenate on the adsorbent can be reduced by increasing the concentration of NaOH. Thus, understanding arsenate desorption kinetics and effects of support properties of ferric hydroxide-based adsorbents are important for environmental fate of arsenate and in designing adsorption systems for removing arsenate from potable water.

TDO utilizes b-type heme as a cofactor to activate dioxygen and insert two oxygen atoms into free L-tryptophan. We revealed two unidentified enzymatic activities of ferric TDO from Ralstonia metallidurans, which are peroxide driven oxygenation and catalase-like activity. The stoichiometric titration suggests that two moles of H2O2 were required for the production of one mole of N-formylkynurenine. We have also observed monooxygenated-L-tryptophan. Three enzyme-based intermediates were sequentially detected in the peroxide oxidation of ferric TDO in the absence of L-Trp including compound I-type and compound ES-type Fe-oxo species. The Fe(IV) intermediates had an unusually large quadrupole splitting parameter of 1.76(2) mm/s at pH 7.4. Density functional theory calculations suggest that it results from the hydrogen bonding to the oxo group. We have also demonstrated that the oxidized TDO was activated via a homolytic cleavage of the O-O bond of ferric hydroperoxide intermediate via a substrate dependent process to generate a ferrous TDO. We proposed a peroxide activation mechanism of the oxidized TDO. The TDO has a relatively high redox potential, the protonated state of the proximal histidine upon substrate binding as well as a common feature of the formation of ferric hydroxide species upon substrate or substrate analogues binding. Putting these together, we have proposed a substrate-based activation mechanism of the oxidized TDO. Our work also probed the role of histidine 72 as an acid-base catalyst in the active site. In H72S and H72N mutants, one water molecule plays a similar role as that of His72 in wild type TDO. MauG is a c-type di-heme enzyme which catalyze the biosynthesis of the protein-derived cofactor tryptophan tryptophylquinone. Its natural substrate is a monohydroxylated tryptophan residue present in a 119-kDa precursor protein of methylamine dehydrogenase (MADH). We have trapped a novel bis-Fe(IV) intermediate from MauG, which is remarkably stable. A tryptophanyl radical intermediate of MADH has been trapped after the reaction of the substrate with the bis-Fe(IV) intermediate. Analysis by high-resolution size-exclusion chromatography shows that MauG can tightly bind to the biosynthetic precursor and form a stable complex, but the mature protein substrate does not.