This study investigated the cycling of Mn and Fe as oxic surface water entered shallow sediments and was reduced, and as reduced groundwater became exposed to oxygen and was oxidized. A Mn-oxide doped gel probe sampler was developed to study in situ rates of reductive dissolution and was validated with laboratory studies with ascorbic acid and Shewanella oneidensis MR-1 as model reductants. The sampler was deployed in the bank sediments of Lake Tegel, Berlin, Germany. Modeling of the diffusion-controlled reaction converted the mass loss from the gels in the sampler to a profile of pseudo-first-order rate constants as a function of depth. The rate constants were highest at depths with high dissolved Fe and low operationally defined fractions of reducible oxides of Fe and Mn in the sediments.
\r\n\r\nA laboratory column experiment showed that 1.3-m water table fluctuations, as observed in bank filtration sites around Berlin, were able to provide sufficient dissolved oxygen delivery for Pseudomonas putida GB-1, an obligate aerobe, to oxidize Mn(II) in situ. Accumulation of Mn on the quartz sand in the column at the end of the experiment was limited to the top 60 cm, as measured with X-ray fluorescence (XRF), and X-ray absorption spectroscopy (XAS) analysis confirmed that the solid formed was a Mn(IV) oxide characteristic of biogenic origin. After a period of \u201cfilter ripening\u201d in the column, rates of in situ oxidation were still lower than rates in engineered aerated sand filters.
\r\n\r\nAdjacent to a production well with a water table that fluctuates up to 7 m annually at Lake Tegel, however, sediments collected from a borehole did not show any significant accumulation of Mn or Fe with depth, as measured by XRF; analysis of the speciation of Mn and Fe in the solid phase by XAS suggested a slight increase in the proportions of total Mn as Mn(II) and of total Fe as Fe(II) with depth. At this location, vertical zonation of groundwater may preclude the co-occurrence of reduced Mn and Fe with dissolved oxygen entrapped by water table fluctuations. Whether groundwater changes from oxidizing to reducing conditions or vice versa, the behavior of Mn and Fe reflects a complex interaction between sediments, solutes, microbial activity, and hydrology.
", "doi": "10.7907/TB3A-Z569", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:1864", "collection": "thesis", "collection_id": "1864", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05182009-162828", "primary_object_url": { "basename": "wildmanthesisallchapters.pdf", "content": "final", "filesize": 2236242, "license": "other", "mime_type": "", "url": "/1864/9/wildmanthesisallchapters.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "Biogeochemical Implications of Changing Groundwater and Surface Water Hydrology at Lake Powell, Utah and Arizona, and the Merced River, California, USA", "author": [ { "family_name": "Wildman", "given_name": "Richard Alan", "clpid": "Wildman-Richard-Alan" } ], "thesis_advisor": [ { "family_name": "Hering", "given_name": "Janet G.", "clpid": "Hering-J-G" } ], "thesis_committee": [ { "family_name": "Sessions", "given_name": "Alex L.", "clpid": "Sessions-A-L" }, { "family_name": "Rossman", "given_name": "George Robert", "clpid": "Rossman-G-R" }, { "family_name": "Hering", "given_name": "Janet G.", "clpid": "Hering-J-G" }, { "family_name": "Adkins", "given_name": "Jess F.", "clpid": "Adkins-J-F" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis examines some effects of surface water and groundwater hydrology on the mobility of trace elements and phosphorus in natural environments. Three separate field sites are studied: 1) the shoreline of Lake Powell, a large reservoir on the Colorado River in Utah and Arizona where the surface elevation fluctuates on yearly and multi-yearly timescales, 2) the Colorado River inflow region to Lake Powell, where the sediment delta has been exposed due to low water levels, and 3) the lower Merced River, which is located in the San Joaquin Valley, California, amidst extensive agricultural development.
\r\n\r\nOn the shoreline of Lake Powell, depth profiles of manganese and uranium were used to estimate the redox state of sediment porewater. Samples were collected before and after a fluctuation in reservoir level exposed two sampling locations to air and then resubmerged them. Results indicate that reducing conditions are re-established at different rates in two nearby shoreline locations, and that manganese reduction occurs more rapidly than uranium reduction upon resubmergence.
\r\n\r\nIn the Colorado River inflow region of Lake Powell, sediment samples were collected from the lakebed and shoreline. Measurements indicate that particle size anticorrelates with the concentrations of most elements and clay minerals and explains much, but not all, of the variation in trace elements. Spatial trends of particle size imply that low reservoir levels may induce resuspension of fine sediment, a process that may lead to increased primary productivity observed in monitoring data. Sequential extractions performed on these sediment samples suggest that phosphorus, the limiting nutrient in Lake Powell, is primarily associated with calcite and biogenic apatite. Sorption experiments indicate that fine particles sorb much more phosphorus than coarse particles, and that only a small amount of the sediment-associated phosphorus is desorbed during sediment resuspension. When reservoir levels are low, measurements of dissolved phosphorus suggest that sediment resuspended by the Colorado River may supply phosphorus to the photic zone under specific hydrologic conditions.
\r\n\r\nSamples of groundwater collected from beneath the Merced River were analyzed for a suite of trace elements. Statistical analyses suggest that hydrologic processes generally influence the transport of trace solutes more than redox chemistry, and results vary between strontium, barium, uranium, and phosphorus.
\r\n", "doi": "10.7907/2BQF-KR29", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis:1044", "collection": "thesis", "collection_id": "1044", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-03212007-235559", "primary_object_url": { "basename": "WangThesis.pdf", "content": "final", "filesize": 3801652, "license": "other", "mime_type": "application/pdf", "url": "/1044/1/WangThesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Studies on Environmental Relevance of Quorum Sensing Signal Decay", "author": [ { "family_name": "Wang", "given_name": "Ya-Juan (Megan)", "clpid": "Wang-Ya-Juan-Megan" } ], "thesis_advisor": [ { "family_name": "Leadbetter", "given_name": "Jared R.", "orcid": "0000-0002-7033-0844", "clpid": "Leadbetter-J-R" } ], "thesis_committee": [ { "family_name": "Leadbetter", "given_name": "Jared R.", "orcid": "0000-0002-7033-0844", "clpid": "Leadbetter-J-R" }, { "family_name": "Newman", "given_name": "Dianne K.", "orcid": "0000-0003-1647-1918", "clpid": "Newman-D-K" }, { "family_name": "Hering", "given_name": "Janet G.", "orcid": "0000-0002-0865-1946", "clpid": "Hering-J-G" }, { "family_name": "Hoffmann", "given_name": "Michael R.", "orcid": "0000-0001-6495-1946", "clpid": "Hoffmann-M-R" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Signal degradation impacts all communications. Although acyl-homoserine lactone (acyl-HSL) quorum sensing signals are known to be degraded by defined laboratory cultures, little is known about their stability in nature. Here, we show for the first time that acyl-HSLs are biodegraded in soils sampled from diverse US sites. When amended to soil samples at physiologically relevant concentrations, \u00b9\u2074C-labeled acyl-HSLs were mineralized to \u00b9\u2074CO\u2082 rapidly and, at most sites examined, without lag. A lag-free turf soil activity was characterized in further detail. Heating or irradiation of the soil prior to the addition of radiolabel abolished mineralization, whereas protein synthesis inhibitors did not. Mineralization exhibited an apparent Km of 1.5 \u00b5M acyl-HSL, ca. 1000-fold lower than that reported for a purified acyl-HSL lactonase. Under optimal conditions, acyl-HSL degradation proceeded at a rate of 13.4 nmol . h\u207b\u00b9 . g of fresh weight soil\u207b\u00b9. An MPN of 4.6 \u00d7 105 cells . g of turf soil\u207b\u00b9 degraded physiologically relevant amounts of hexanoyl-[1-\u00b9\u2074C]HSL to \u00b9\u2074CO\u2082. The results implicate a real-world challenge for acyl-HSL-producing bacteria to outpace biological acyl-HSL degradation and thus to successfully engage in cell-cell communications in soils and other environments. Furthermore, high-affinity acyl-HSL degraders were enriched in oligotrophic biofilm reactors that were inoculated with the turf soil. When supplied at physiological concentration, acyl-HSL was observed to be >95% consumed through the reactors. Six bacterial strains were isolated belonging to Variovorax, Mesorhizobium, Bradyrhizobium and Labrys genera. Unambiguous growth of the six isolates on physiological amount of acyl-HSLs under oligotrophic condition was demonstrated. A Variovorax strain SOD31 exhibited acyl-HSL-limiting growth kinetics with a half-saturation constant of 1.7 \u00b5M, which is in high agreement with what we observed from the turf soil. The results suggest that strain SOD31 may be an active contributor to the soil\u2019s activity of degrading acyl-HSLs. This research reinforces the importance of signal decay as intrinsic to bacterial cell-cell communication, as well as providing bases for further studies towards how signal decay may influence community function and structure in naturally occurring microbial communities.", "doi": "10.7907/C00R-DG30", "publication_date": "2007", "thesis_type": "phd", "thesis_year": "2007" }, { "id": "thesis:5043", "collection": "thesis", "collection_id": "5043", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-12182006-102819", "primary_object_url": { "basename": "Campbell_Thesis_Complete.pdf", "content": "final", "filesize": 5706066, "license": "other", "mime_type": "application/pdf", "url": "/5043/13/Campbell_Thesis_Complete.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Biogeochemical Mechanisms of Arsenic Mobilization in Haiwee Reservoir Sediments", "author": [ { "family_name": "Campbell", "given_name": "Kate Marie", "orcid": "0000-0002-8715-5544", "clpid": "Campbell-Kate-Marie" } ], "thesis_advisor": [ { "family_name": "Hering", "given_name": "Janet G.", "orcid": "0000-0002-0865-1946", "clpid": "Hering-J-G" } ], "thesis_committee": [ { "family_name": "Hering", "given_name": "Janet G.", "orcid": "0000-0002-0865-1946", "clpid": "Hering-J-G" }, { "family_name": "Newman", "given_name": "Dianne K.", "orcid": "0000-0003-1647-1918", "clpid": "Newman-D-K" }, { "family_name": "Rossman", "given_name": "George Robert", "orcid": "0000-0002-4571-6884", "clpid": "Rossman-G-R" }, { "family_name": "Hoffmann", "given_name": "Michael R.", "orcid": "0000-0001-6495-1946", "clpid": "Hoffmann-M-R" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Naturally-occurring arsenic (As) in the Los Angeles Aqueduct, a major drinking water source for the City of Los Angeles, is removed by precipitating an amorphous iron (Fe) oxyhydroxide floc in the aqueduct waters. The floc is removed via sedimentation at Haiwee Reservoir, where the Fe- and As-rich sediments provide a unique field site for studying the mechanisms of As mobilization to sediment porewater.
\r\n\r\nA gel probe equilibrium sampler was developed to measure the porewater concentrations and As sorption behavior in Haiwee Reservoir sediments. The gels consisted of a polyacrylamide polymer matrix and were 92% water. Undoped gels (clear gels) were used to determine porewater composition, and hydrous ferric oxide (HFO)-doped gels were used to measure in situ As adsorption chemistry. Gels were placed in a plastic holder, covered with a membrane filter, and allowed to equilibrate with the sediment porewaters. This study combined data from the gel probe samplers, gravity cores, and laboratory studies, to elucidate the biogeochemical processes governing As partitioning between the solid and aqueous phases. The gel probe device allowed for in situ observation of the effect of porewater chemistry on As adsorption.
\r\n\r\nArsenic was reduced from As(V) to As(III) in the upper layers of the sediment, but the change in redox state did not cause As to be mobilized into the porewaters. Arsenic mobilization occurred during reductive dissolution of Fe(III) oxides. Arsenate and Fe(III) reduction were probably microbially mediated. Arsenic sorption onto the HFO-doped gels was inhibited at intermediate depths, probably due to dissolved carbonate produced from organic carbon mineralization. The partitioning of As onto the sediment in this region may be primarily controlled by porewater chemistry, rather than sorption site availability. Deeper in the sediment column, the Fe(III) phase was partially transformed to carbonate green rust, possibly sequestering dissolved carbonate. In this region, As adsorption onto HFO-doped gels was controlled by dissolved phosphate. The accumulation of As in the porewater in this region may be due to lack of available surface adsorption sites on the sediment. Arsenic partitioning between solid and aqueous phases depends on microbially driven diagenetic processes, as well as porewater composition.
\r\n", "doi": "10.7907/T3WC-DA42", "publication_date": "2007", "thesis_type": "phd", "thesis_year": "2007" }, { "id": "thesis:1097", "collection": "thesis", "collection_id": "1097", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-03242006-105717", "primary_object_url": { "basename": "01-TitleAndAcknowledgements.pdf", "content": "final", "filesize": 91515, "license": "other", "mime_type": "application/pdf", "url": "/1097/1/01-TitleAndAcknowledgements.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "TiO\u2082-Photocatalyzed As(III) Oxidation and its Applicability to Water Treatment", "author": [ { "family_name": "Ferguson", "given_name": "Megan Alameda", "clpid": "Ferguson-Megan-Alameda" } ], "thesis_advisor": [ { "family_name": "Hering", "given_name": "Janet G.", "clpid": "Hering-J-G" } ], "thesis_committee": [ { "family_name": "Hering", "given_name": "Janet G.", "clpid": "Hering-J-G" }, { "family_name": "Rossman", "given_name": "George Robert", "clpid": "Rossman-G-R" }, { "family_name": "Hoffmann", "given_name": "Michael R.", "clpid": "Hoffmann-M-R" }, { "family_name": "Lewis", "given_name": "Nathan Saul", "clpid": "Lewis-N-S" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Ingestion of arsenic (As) in drinking water can lead to several chronic illnesses, including bladder, lung, and skin cancers. Due to natural weathering reactions, As is present in many drinking water sources throughout the United States and other countries. The U.S. drinking water standard has recently been revised from 50 micrograms L-1 to 10 micrograms L-1, thus requiring thousands of water distribution facilities to implement new As removal procedures. However, most As removal technologies treat As(V) much more effectively than As(III), so a pre-oxidation step is recommended for source waters containing As(III) at significant concentrations. The photocatalyzed oxidation of As(III) on titanium dioxide (TiO\u2082) has been critically evaluated as a potential technology to achieve pre-oxidation.
\r\n\r\nBatch slurry studies showed that sorption of both As(III) and As(V) to TiO\u2082 occurs rapidly. Changes in As(III) concentration adsorbed to the TiO\u2082 surface, whether resulting from varying dissolved As(III) concentration or from the presence of competitive adsorbates, affected the rate of photooxidation. A transition from first-order to zero-order reaction kinetics was observed as the TiO\u2082 surface became saturated with As(III). Experiments targeting the reaction mechanism (which used hydroxyl radical quenchers, an alternate electron acceptor, and superoxide dismutase) suggested that superoxide plays a major role as an oxidizing agent in this system.
\r\n\r\nAs(III) was rapidly photooxidized in a fixed-bed, flow-through reactor. Catalyst poisoning or severe mass transport limitations were not observed for the conditions studied, although complete As(III) oxidation could not be achieved due to constraints of reactor geometry. With synthetic groundwater solutions, the reaction appeared to be inhibited by the presence of competitive adsorbates. Use of natural sunlight resulted in more rapid As(III) oxidation than that observed with irradiation from the 365 nm lamp used for other experiments.
\r\n\r\nAs arsenic detection methods improve, reporting levels and perhaps regulatory standards will be lowered, requiring more distribution systems to consider technologies for As treatment. In contrast to methods of pre-oxidation currently in use, TiO\u2082 photocatalysis involves no hazardous chemicals and generates no toxic reaction byproducts. Based on these findings, TiO\u2082-photocatalyzed As(III) oxidation could be a viable pre-oxidation technology for certain small water distribution facilities.
", "doi": "10.7907/093T-4643", "publication_date": "2006", "thesis_type": "phd", "thesis_year": "2006" }, { "id": "thesis:1680", "collection": "thesis", "collection_id": "1680", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05082006-143046", "primary_object_url": { "basename": "Cornu_c_2002.pdf", "content": "final", "filesize": 5807149, "license": "other", "mime_type": "application/pdf", "url": "/1680/1/Cornu_c_2002.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Photocatalysis Under Periodic Illumination", "author": [ { "family_name": "Cornu", "given_name": "Catherine J. G.", "clpid": "Cornu-Catherine-J-G" } ], "thesis_advisor": [ { "family_name": "Readhead", "given_name": "Anthony C. S.", "orcid": "0000-0001-9152-961X", "clpid": "Readhead-A-C-S" }, { "family_name": "Hoffmann", "given_name": "Michael R.", "orcid": "0000-0001-6495-1946", "clpid": "Hoffmann-M-R" } ], "thesis_committee": [ { "family_name": "Hoffmann", "given_name": "Michael R.", "orcid": "0000-0001-6495-1946", "clpid": "Hoffmann-M-R" }, { "family_name": "Blake", "given_name": "Geoffrey A.", "orcid": "0000-0003-0787-1610", "clpid": "Blake-G-A" }, { "family_name": "Hering", "given_name": "Janet G.", "clpid": "Hering-J-G" }, { "family_name": "Lewis", "given_name": "Nathan Saul", "orcid": "0000-0001-5245-0538", "clpid": "Lewis-N-S" }, { "family_name": "Readhead", "given_name": "Anthony C. S.", "orcid": "0000-0001-9152-961X", "clpid": "Readhead-A-C-S" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "In the first part of this work, the use of periodic illumination is investigated as a means to improve the quantum yield of aqueous phase photocatalysis. We find that quantum yield enhancements are obtained under periodic illumination compared to those under continuous illumination with the same maximum photon absorption rate. This result is in agreement with previous literature reports. However, no enhancements are obtained when compared to those under continuous illumination with the same average photon absorption rate. Ways in which the use of periodic illumination in remediation systems might nevertheless be more cost-effective than continuous operation are discussed. In the second part of this work, periodic illumination is applied as a tool to investigate the kinetic behavior of the intermediates involved in photocatalysis. Two transitions are observed in the quantum yield as a function of the active light period, corresponding to separate lifetimes of two intermediates. These lifetimes are found to be relatively insensitive to the physical properties of the TiO2 photocatalyst particles, the compound being photocatalytically oxidized, or the oxygen concentration. However, a strong correlation with the pH of the solution is observed. For example, one intermediate's lifetime increases exponentially with pH, while the other one's decreases exponentially. Principles from electrochemistry, namely Nernst's law and the Butler-Volmer equation, allow us to conclude that the intermediate with a decreasing lifetime with pH is a reducing species, while the other one is an oxidizing species. The intermediates' lifetimes are equal at a pH ~ 8, concurrent with a minimum in quantum yield. Below pH 8 the reducing species has a longer lifetime, indicating that interfacial electron transfer to oxygen is the slowest step under these conditions.
\r\n\r\nThe observed lifetimes are compatible with those previously measured for superoxide radicals (reducing species), and hydroxyl radicals, surface trapped holes, or surface bound hydroxyl radicals (oxidizing species).
", "doi": "10.7907/ZDAH-V967", "publication_date": "2002", "thesis_type": "phd", "thesis_year": "2002" }, { "id": "thesis:1829", "collection": "thesis", "collection_id": "1829", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05162002-135325", "primary_object_url": { "basename": "Full_Thesis.pdf", "content": "final", "filesize": 3390258, "license": "other", "mime_type": "application/pdf", "url": "/1829/1/Full_Thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Response of Soil Mineral Weathering to Elevated Carbon Dioxide", "author": [ { "family_name": "Stephens", "given_name": "Jennie Catherine", "clpid": "Stephens-Jennie-Catherine" } ], "thesis_advisor": [ { "family_name": "Hering", "given_name": "Janet G.", "clpid": "Hering-J-G" } ], "thesis_committee": [ { "family_name": "Hering", "given_name": "Janet G.", "clpid": "Hering-J-G" }, { "family_name": "Morgan", "given_name": "James J.", "clpid": "Morgan-J-J" }, { "family_name": "Randerson", "given_name": "James T.", "clpid": "Randerson-J-T" }, { "family_name": "Hoffmann", "given_name": "Michael R.", "orcid": "0000-0001-6495-1946", "clpid": "Hoffmann-M-R" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Understanding the rates of weathering of soil minerals and the factors that may either enhance or inhibit these rates is a crucial part of understanding many processes from the watershed to the global scale. One potentially important factor in mineral weathering that is not yet well understood is the effect of elevated CO2 concentrations on weathering rates. Here, the direct and indirect effects of elevated soil CO2 are examined in field and laboratory-based studies, and the incorporation of the relationship between CO2 and mineral weathering in soil chemistry models is critically evaluated. \r\nAt Mammoth Mountain, California, volcanic ash soil is exposed to naturally occurring high levels of CO2 from a magmatic source. Comparative analyses of chemical and mineralogical characteristics of exposed and control soils suggest that decade-long exposure to elevated CO2 concentrations has altered soil dissolution rates. Indirect effects of elevated soil CO2 at this site, including vegetation mortality and a decrease in pH, have significant potential to alter weathering rates. Laboratory dissolution studies on whole soils under varying conditions of pH, PCO2, and concentrations of oxalate (chosen as a proxy for low-molecular-weight organic acids associated with vegetation in soils) were designed to assess both the direct and indirect effects of CO2. The results of these experiments provide confirming evidence that CO2 does not directly influence soil dissolution rates under acidic conditions. However, soil dissolution rates are sensitive to indirect effects of elevated CO2, including changes in pH and organic acid concentration. The inclusion of a direct CO2 dependence in a widely used soil chemistry model, PROFILE, may be perpetuating confusion on this issue. Erroneous conclusions in future model applications could result if this relationship is not removed from the PROFILE model. \r\nA significant and striking decrease in the specific surface area of the soil material was observed during all soil dissolution experiments. These observations call into question the informal convention of normalizing reported dissolution rates to the initial surface area. For effective comparison of weathering rates and identification of the factors influencing them, changes in surface area must be accounted for in reporting dissolution rates.", "doi": "10.7907/9ET7-QX09", "publication_date": "2002", "thesis_type": "phd", "thesis_year": "2002" }, { "id": "thesis:1903", "collection": "thesis", "collection_id": "1903", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05212002-125705", "primary_object_url": { "basename": "Giammar-Thesis.pdf", "content": "final", "filesize": 6713351, "license": "other", "mime_type": "application/pdf", "url": "/1903/1/Giammar-Thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Geochemistry of uranium at mineral-water interfaces: rates of sorption-desorption and dissolution-precipitation reactions", "author": [ { "family_name": "Giammar", "given_name": "Daniel", "clpid": "Giammar-D" } ], "thesis_advisor": [ { "family_name": "Hering", "given_name": "Janet G.", "clpid": "Hering-J-G" } ], "thesis_committee": [ { "family_name": "Hering", "given_name": "Janet G.", "clpid": "Hering-J-G" }, { "family_name": "Rossman", "given_name": "George Robert", "clpid": "Rossman-G-R" }, { "family_name": "Morgan", "given_name": "James J.", "clpid": "Morgan-J-J" }, { "family_name": "Hoffmann", "given_name": "Michael R.", "clpid": "Hoffmann-M-R" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The extraction and processing of uranium for use in the nuclear weapons program and in commercial nuclear energy has led to extensive contamination of the environment. Migration of uranium is also a concern for the proposed long-term nuclear waste disposal in geologic repositories. Reactions occurring at mineral surfaces significantly affect the mobility of uranium in the environment. Both the equilibrium and kinetics of reactions at mineral surfaces must be understood in order to predict the extent of reactions on time scales pertinent to human exposure. Such information is needed to establish input parameters for reactive transport models and to design remediation technologies.\n\nRates of uranium sorption on mineral surfaces and the dissolution of uranium-containing minerals have been investigated. Rates of sorption onto and desorption from goethite, an important environmental sorbent, were determined by measuring the responses of goethite suspensions (pre-equilibrated with or without uranium) to perturbations of the solution chemistry. Dissolution rates were measured for a set of laboratory-synthesized minerals: the uranyl oxide hydrate schoepite, the uranyl silicate soddyite, and a uranyl phosphate phase. These minerals have been observed in contaminated environments and are produced during the corrosion of spent nuclear fuel. Mineral dissolution and transformation were monitored in batch reactors, while dissolution rates were quantified in flow-through reactors. In both sorption and dissolution-precipitation studies, measurements of bulk solution chemistry were integrated with solid phase characterization.\n\nWhile sorption processes were rapid, dissolution and surface-precipitation reactions occurred more slowly. Adsorption and desorption reactions of uranium onto or from goethite reached greater than 50% completion within minutes and completion on a time-scale of hours. In some uranium-goethite suspensions, a meta-stable sorption state persisted for as long as three weeks before a schoepite-like phase precipitated. Dissolution reactions proceeded at time-scales of hours for schoepite and days to weeks for soddyite and the uranyl phosphate. Common groundwater cations affected dissolution rates and, in several cases, resulted in the precipitation of uranium in secondary phases. In several schoepite and soddyite batch dissolution experiments, uranium ultimately reprecipitated in sodium or cesium uranyl oxide hydrate phases which subsequently controlled the dissolved uranium concentration.", "doi": "10.7907/EJ9E-XW58", "publication_date": "2001", "thesis_type": "phd", "thesis_year": "2001" }, { "id": "thesis:3006", "collection": "thesis", "collection_id": "3006", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-08042006-091727", "primary_object_url": { "basename": "Kneebone_pe_2000.pdf", "content": "final", "filesize": 11953906, "license": "other", "mime_type": "application/pdf", "url": "/3006/1/Kneebone_pe_2000.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Arsenic geochemistry in a geothermally impacted system : the Los Angeles Aqueduct", "author": [ { "family_name": "Kneebone", "given_name": "Penelope E.", "clpid": "Kneebone-P-E" } ], "thesis_advisor": [ { "family_name": "Hering", "given_name": "Janet G.", "clpid": "Hering-J-G" } ], "thesis_committee": [ { "family_name": "Hering", "given_name": "Janet G.", "clpid": "Hering-J-G" }, { "family_name": "List", "given_name": "John", "clpid": "List-J" }, { "family_name": "Morgan", "given_name": "James J.", "clpid": "Morgan-J-J" }, { "family_name": "Bercaw", "given_name": "John E.", "clpid": "Bercaw-J-E" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Elevated arsenic concentrations in the Los Angeles Aqueduct (LAA) derive from geothermal inputs in Hot Creek. This study examines arsenic geochemistry in three sections of the LAA system: Hot Creek, Crowley Lake and Haiwee Reservoir.
\r\n\r\nAt Hot Creek, the accumulation of arsenic in the sediments is strongly influenced by hot springs and plants. Solid-phase arsenic is present as As(III) throughout the sediment column with an identifiable organic phase in the surface sediment and a sulfide phase in the deep sediment. Sediments carry only a small fraction of the arsenic load in Hot Creek. Concentration profiles obtained with a novel, high resolution pore water sampler reveal a net flux of arsenic out of the sediments. Sediment-water exchange of arsenic in Hot Creek depends on initial arsenic concentrations and the presence of other ions in solution; this latter effect may be partly due to colloid aggregation and peptization.
\r\n\r\nIn Crowley Lake, water column profiles obtained during stratified and well-mixed conditions provide no evidence for sediment-water exchange of arsenic. Algal uptake and transformation of arsenic is not sufficient to perturb the distribution of arsenic species in the water column or to accumulate substantial arsenic concentrations in the sediment. Arsenic is associated, and immobilized, with sulfide in the sediments, which are permanently anoxic. Sediment arsenic concentrations are modestly elevated; release of arsenic from the sediments driven by decreased water column arsenic concentrations would be gradual and would not threaten the water quality of the LAA.
\r\n\r\nIn Haiwee Reservoir, arsenic is deposited in the sediments as a result of the Los Angeles Department of Water and Power's interim arsenic management plan. This solid-phase arsenic is unstable with respect to reductive dissolution of the iron oxyhydroxide with which it is associated. A strong redox gradient exists below the sediment water interface and solid-phase arsenic speciation evolves rapidly with depth from an As(V) to an As(III) phase. The potential for release of this arsenic to the overlying water poses a threat to the LAA water supply.
\r\n", "doi": "10.7907/83BV-0B62", "publication_date": "2000", "thesis_type": "phd", "thesis_year": "2000" }, { "id": "thesis:566", "collection": "thesis", "collection_id": "566", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-02082008-163932", "primary_object_url": { "basename": "Lloyd_t_1999.pdf", "content": "final", "filesize": 10469466, "license": "other", "mime_type": "application/pdf", "url": "/566/1/Lloyd_t_1999.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Dissolution of Fe(III)- and Mn(III,IV)- (hydr)oxides by deferrioxamine B", "author": [ { "family_name": "Lloyd", "given_name": "Thomas", "clpid": "Lloyd-T" } ], "thesis_advisor": [ { "family_name": "Morgan", "given_name": "James J.", "clpid": "Morgan-J-J" } ], "thesis_committee": [ { "family_name": "Morgan", "given_name": "James J.", "clpid": "Morgan-J-J" }, { "family_name": "Hering", "given_name": "Janet G.", "clpid": "Hering-J-G" }, { "family_name": "Bercaw", "given_name": "John E.", "clpid": "Bercaw-J-E" }, { "family_name": "Hoffmann", "given_name": "Michael R.", "clpid": "Hoffmann-M-R" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\n\nMicrobially produced siderophores are thought to have evolved to increase the availability of iron in oxic aqueous systems. Historically, it has been assumed that siderophores interact with iron found in Fe(III)-(hydr)oxides, removing iron from the solid to form ferrated aqueous species of the siderophore available for uptake by the microorganism. However, this proposed interaction of siderophore with the solid phase has not been examined in detail.\n\nThe work presented in this thesis examines the interaction of a model trihydroxamic acid siderophore (deferrioxamine B) with synthetic Fe(III)-, Mn(III), Mn(IV)- (hydr)oxide particles. The experiments were designed to characterize the mineral dissolution process mediated by deferrioxamine B (DFA). DFA was found to dissolve all the solid particles used in the study. The Fe(III)- (hydr)oxides dissolved exclusively by a nonreductive mechanism. The dissolution is a surface-mediated process. Dissolution of manganite was found to be analogous to the Fe(III)-(hydr)oxide dissolution process, while the dissolution of pyrolusite was found to follow a complicated mechanism that involves reductive processes. The interactions between DFA and the solid surface are examined experimentally and theoretically.\n\nUnder the conditions in which (i) the concentration of DFA in solution remains essentially constant over the course of the reaction, i.e., the concentration of the ferrated form of DFA (FA) is much less than [DFA][...] over the whole time of the reaction, and (ii) the concentration of available surface sites remains constant, the dissolution rate can be modeled as a pseudo-zero [...] order reaction, where\n\n \t\tR = k'.\n\nUnder the same conditions and where the concentration of adsorbed DFA is measured, the dissolution rate can be modeled as a pseudo-first order process, where\n\n\t\tR = k [[...]Fe \u2014 DFA],\n\nwhere [[...]Fe \u2014 DFA] is defined as the concentration of adsorbed DFA. Conditions (i) and (ii) imply that a necessary condition for observation of a pseudo-zero [...] order process is that [[...]Fe \u2014 DFA] remains constant over the course of the reaction. Where these conditions fail, the dissolution process becomes more complicated, and the experiments presented here do not address these cases in detail.\n", "doi": "10.7907/dxzk-z221", "publication_date": "1999", "thesis_type": "phd", "thesis_year": "1999" }, { "id": "thesis:787", "collection": "thesis", "collection_id": "787", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-02272008-111006", "primary_object_url": { "basename": "Johansen_amj_1999.pdf", "content": "final", "filesize": 15355652, "license": "other", "mime_type": "application/pdf", "url": "/787/1/Johansen_amj_1999.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Aerosol chemistry over remote oceanic regions", "author": [ { "family_name": "Johansen", "given_name": "Anne Marie Jorunn", "clpid": "Johansen-A-J" } ], "thesis_advisor": [ { "family_name": "Hoffmann", "given_name": "Michael R.", "clpid": "Hoffmann-M-R" } ], "thesis_committee": [ { "family_name": "Hoffmann", "given_name": "Michael R.", "clpid": "Hoffmann-M-R" }, { "family_name": "Cass", "given_name": "Glen Rowan", "clpid": "Cass-G-R" }, { "family_name": "Morgan", "given_name": "James J.", "clpid": "Morgan-J-J" }, { "family_name": "Hering", "given_name": "Janet G.", "clpid": "Hering-J-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nAtmospheric sampling of aerosol was carried out from ships during four separate month-long research cruises over remote oceanic regions. Three cruises took place in the Arabian Sea, during the inter-, SW- and NE-monsoons. An additional cruise was focused on the tropical North Atlantic Ocean. Identical sampling techniques were employed for all cruises. One high volume dichotomous virtual impactor and two low volume collectors were used to collect aerosol particles for the quantification of trace metals, anions and major cations. Ferrous iron concentrations were determined immediately after sample collection on board the ship using a ferrozine extraction technique. Data were analyzed by principal component, enrichment factor and weighted multiple linear regression analyses.\r\n\r\nThe mineralogical composition of aerosol collected over the Arabian Sea reflected that of an average of the earth's crust, while over the Atlantic Ocean, shale appeared to best represent the sampled mineral dust. With exception of the SW-monsoon samples, which were characterized by weak continental influences, the aerosol samples contained excess water-soluble non-sea-salt calcium that appeared to be of crustal origin, in the form of CaCO[...] or CaSO[...].\r\n\r\nFerrous iron concentrations accounted for 0.3, 1.3, and 0.5% of the total observed Fe during the inter- and NE-monsoons over the Arabian Sea, and over the Atlantic Ocean, respectively. Absolute Fe(II) concentrations were determined at 5.2, 9.8, and 3.1 [...]g [...] for the corresponding seasons, with most of the Fe(II) (> 80%) present in the fine fraction. Fe(II) did not exhibit a simple relationship with any of the sources indicating the complexity of iron redox reactions is this system.\r\n\r\nNon-sea-salt sulfate (NSS-SO[...]) sources were identified and quantified with the use of multiple linear regression analyses. Biogenically-derived [...] contributed significant amounts of NSS-SO42-, especially during the SW-monsoon, when anthropogenic sources were small. Biogenic SO[...]/methansulfonic acid (MSA) weight ratios were determined and found to be in agreement with reported literature values from pristine oceanic regions, varying from 6.8 at 24\u00b0C to 17.7 at 28.9\u00b0C.", "doi": "10.7907/P5HP-F516", "publication_date": "1999", "thesis_type": "phd", "thesis_year": "1999" }, { "id": "thesis:506", "collection": "thesis", "collection_id": "506", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-02052008-100328", "primary_object_url": { "basename": "Weavers_l_1998.pdf", "content": "final", "filesize": 6046414, "license": "other", "mime_type": "application/pdf", "url": "/506/1/Weavers_l_1998.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Enhancement of ultrasonic and ultraviolet irradiation with chemical oxidants", "author": [ { "family_name": "Weavers", "given_name": "Linda K.", "clpid": "Weavers-L-K" } ], "thesis_advisor": [ { "family_name": "Hoffmann", "given_name": "Michael R.", "clpid": "Hoffmann-M-R" } ], "thesis_committee": [ { "family_name": "Hoffmann", "given_name": "Michael R.", "clpid": "Hoffmann-M-R" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Morgan", "given_name": "James J.", "clpid": "Morgan-J-J" }, { "family_name": "Hering", "given_name": "Janet G.", "clpid": "Hering-J-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nThe combination of ultrasound and ozone was used to study the degradation of nitrobenzene, 4-nitrophenol, 4-chlorophenol, cyclohexene, and pentachlorophenol in aqueous solutions. Using ultrasonic frequencies of 20 and 500 kHz revealed apparent enhancement at 20 kHz and antagonism at 500 kHz for the combined system although the first-order degradation rate constants in the absence of 03 (ozone) was typically a factor of 10 larger at 500 kHz. A comparison of the first-order degradation rate constants for nitrobenzene, 4-nitrophenol, and 4-chlorophenol by sonication, ozonation, and the combination of sonication and ozonation revealed that the observed enhancement upon the combination of ultrasound and ozone is mainly the result of thermolytic decomposition of ozone in a collapsing cavitation bubble. A continuous flow reactor closed to the atmosphere, open to the atmosphere, and open to the atmosphere with gas bubbling was used to probe the effects of ultrasound on [...] mass transfer. Enhanced mass transfer of [...] appeared to be the result of rapid decomposition of [...] in a cavitation bubble. The degradation of pentachlorophenol and observation of intermediates showed OH attack of the aromatic ring. A chemical kinetic model was developed to gain insight into the formation of radicals in various bubbles at 20 and 500 kHz with and without [...] present. The model revealed [...] pyrolysis slightly before the end of collapse followed rapid free- radical formation in the last nanoseconds due to [...] and [...] decomposition.\r\n\r\nIn addition, a new advanced oxidation process, photoactivated periodate, was developed to investigate the decomposition of triethanolamine, its associated chemical oxygen demand, and the oxidation of an industrial wastewater. The optimal [...] for COD degradation was determined to be pH 7.6 due to the combined effects of pH on the speciation of TEA and [...]. Increasing the [...] ratio increased the degradation rate up to an apparent saturation value. Irradiation with a 1000 W Hg(Xe) lamp increased the pseudo first-order degradation rate constant of COD by a factor of 5.5 for synthetic TEA solutions and 2.3 for the industrial wastewater as compared to irradiation with a 1000 W Xe lamp.\r\n", "doi": "10.7907/WPCP-PA50", "publication_date": "1998", "thesis_type": "phd", "thesis_year": "1998" }, { "id": "thesis:194", "collection": "thesis", "collection_id": "194", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01152008-142257", "primary_object_url": { "basename": "Packman_ai_1997.pdf", "content": "final", "filesize": 41426789, "license": "other", "mime_type": "application/pdf", "url": "/194/1/Packman_ai_1997.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Exchange of colloidal kaolinite between stream and sand bed in a laboratory flume", "author": [ { "family_name": "Packman", "given_name": "Aaron Ian", "clpid": "Packman-A-I" } ], "thesis_advisor": [ { "family_name": "Brooks", "given_name": "Norman H.", "clpid": "Brooks-N-H" }, { "family_name": "Morgan", "given_name": "James J.", "clpid": "Morgan-J-J" } ], "thesis_committee": [ { "family_name": "List", "given_name": "E. John", "clpid": "List-E-J" }, { "family_name": "Hering", "given_name": "Janet G.", "clpid": "Hering-J-G" }, { "family_name": "Blake", "given_name": "Geoffrey A.", "clpid": "Blake-G-A" }, { "family_name": "Murray", "given_name": "Bruce C.", "clpid": "Murray-B-C" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Experiments were conducted in a recirculating flume to study the exchange of colloids between a stream and sand bed. Observations of the net transport of kaolinite clay from the stream to the bed were used to validate models for colloid exchange which include the hydraulics of water exchange along with the particle-specific effects of filtration and settling.\r\n\r\nThe flume allowed extensive control of the stream conditions (slope, depth, velocity) and bed parameters (bed depth, bedform height, wavelength, and velocity). All experiments had steady, uniform stream flow and well-developed bedforms. To define the chemistry of the system, the composition of the flume water was controlled, the clay was reproducibly prepared, and the sand was cleaned prior to every experiment.\r\n\r\nExperiments involved observing the exchange of both a conservative lithium chloride tracer and the kaolinite tracer. Net exchange was determined by measuring the change of tracer concentration in the stream water. Vertical profiles of the tracer distribution in the bed were also measured.\r\n\r\nColumn experiments were performed to measure kaolinite filtration by the bed sediment. The pH and ionic strength of both the flume and column water were controlled to vary the extent of filtration.\r\n\r\nTwo mechanisms are responsible for exchange between the stream and stream bed--an advective pore-water flow driven by bedform-induced pressure variations at the surface of the bed (pumping), and the burial and release of water due to bedform motion (turnover). Pumping causes kaolinite to be carried deep in the bed where it is trapped due to filtration and settling. Turnover causes continuous mixing of the uppermost portion of the bed which hinders penetration of clay to the deeper bed.\r\n\r\nModels incorporating the relevant physical and chemical processes controlling colloid transport were developed to predict the net exchange of kaolinite between the stream and sand bed. Model parameters were nondimensionalized so the models can be applied to problems of any scale. Models for conservative solutes were developed to predict the exchange with a finite bed and exchange due to fast-moving bedforms. Models for colloid transport were developed which are based on the solute transport models, but include the impact of settling on particle flow paths and filtration along path lines. When input data were taken from laboratory experiments, the models generally predicted the flume results well with no calibration. In all flume experiments, filtration and settling of colloids in the bed were sufficiently high so that it could be assumed that all colloid which entered the bed was irreversibly trapped.\r\n\r\nAdditional model simulations were performed to demonstrate the effect of major input variables on exchange. These simulations cover cases not examined in experiments and provide a sensitivity analysis for the model inputs.\r\n", "doi": "10.7907/FYMP-BR13", "publication_date": "1997", "thesis_type": "phd", "thesis_year": "1997" }, { "id": "thesis:5047", "collection": "thesis", "collection_id": "5047", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-12182007-084901", "primary_object_url": { "basename": "Hua_i_1996.pdf", "content": "final", "filesize": 7486810, "license": "other", "mime_type": "application/pdf", "url": "/5047/1/Hua_i_1996.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "The sonochemistry of aqueous solutions", "author": [ { "family_name": "Hua", "given_name": "Inez", "clpid": "Hua-I" } ], "thesis_advisor": [ { "family_name": "Hoffmann", "given_name": "Michael R.", "clpid": "Hoffmann-M-R" } ], "thesis_committee": [ { "family_name": "Hoffmann", "given_name": "Michael R.", "clpid": "Hoffmann-M-R" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Gray", "given_name": "Harry B.", "clpid": "Gray-H-B" }, { "family_name": "Morgan", "given_name": "James J.", "clpid": "Morgan-J-J" }, { "family_name": "Hering", "given_name": "Janet G.", "clpid": "Hering-J-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nThe ultrasonic irradiation of organic compounds in aqueous solution is studied under various physical conditions. The observed sonochemical rate constants can be analyzed to yield information about the nature of the bubbles which exist during acoustic cavitation and to determine which parameters are significant for enhancing cavitation chemistry. Aqueous solutions of p-nitrophenylacetate (PNPA), p-nitrophenol (PNP), and carbon tetrachloride (CCl4) were sonicated in various reactor configurations and with different saturating gases. The production of hydroxyl radical ([...]) and hydrogen peroxide (H2O2) was quantified at four different ultrasonic frequencies.\r\n\r\nThe most significant features of the sonolytic hydrolysis of PNPA at 20 kHz are that the observed first order rate constants are independent of pH in the range of 3 to 8, but significantly influenced by the saturating gas (Kr, Ar, or He). Furthermore, the apparent activation parameters for hydrolysis of PNPA ([...], [...], [...]) are significantly different in sonicated systems than in solutions under ambient conditions. These changes are attributed to changes in the physical nature of the solvent when a layer of transient supercritical water is formed around the cavitation bubbles.\r\n\r\nThe psuedo-first order degradation rate constants for the transformation of PNP in a near-field acoustical processor (NAP) increases with increasing power-to-volume ratio over the range of 0.98-7.27 W/[...]. An increase in the power-to-area ratio results in an increasingly fast degradation rate up to 1.2 W/[...] and a slight decrease thereafter. The nature of the background gas influences the degradation rate constant and product distribution. The fastest rate is observed when a mixture of Ar and O2 is used to saturate the solution.\r\n\r\nThe sonolytic degradation of CCl4 is investigated in Ar- and O3-saturated aqueous solutions. The chlorine mass balance is typically >70%, mostly in the form of chloride ion (Cl-). The reactive intermediate, dichlorocarbene, is identified and quantified by means of trapping with 2,3-dimethyl-2-butene. Low concentrations (0.01-0.1 [...]) of hexachloroethane and tetrachloroethylene are detected. Ultrasonic irradiation of a mixture of PNP and CCl4 results in the acceleration of the sonochemical degradation rate of p-NP.\r\n\r\nBoth the ultrasonic frequency and the saturating gas influence the production of [...] and H2O2 during sonolysis of buffered, aqueous solutions. Ultrasonic frequencies of 20,40,80 and 500 kHz are used, and the background gas in solution are Kr, Ar, He or O2. Both species are found at the highest rates during sonolysis of a Kr-saturated solution at 500 kHz, whereas the lowest rates of production are observed during sonolysis of a He-saturated solution at 20 kHz. A range of rate constants for the production of [...] and H2O2 are observed at each frequency because each of the four saturating gases has different physical properties (e.g., thermal conductivity.) However, the range narrows at the higher ultrasonic frequencies (80 and 500 kHz) compared to that observed at the lower frequencies (20 and 40 kHz).\r\n", "doi": "10.7907/YWAK-2730", "publication_date": "1996", "thesis_type": "phd", "thesis_year": "1996" }, { "id": "thesis:5074", "collection": "thesis", "collection_id": "5074", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-12192007-155028", "type": "thesis", "title": "The Kinetics of Redox Reactions of Mn(II) and Mn(III) in Aqueous Systems: Homogenous Autoxidation of Mn(II) and the Formation and Disappearance of Mn(III) Complexes", "author": [ { "family_name": "Klewicki", "given_name": "J. Kenneth", "clpid": "Klewicki-J-Kenneth" } ], "thesis_advisor": [ { "family_name": "Morgan", "given_name": "James J.", "clpid": "Morgan-J-J" } ], "thesis_committee": [ { "family_name": "Morgan", "given_name": "James J.", "clpid": "Morgan-J-J" }, { "family_name": "Blake", "given_name": "Geoffrey A.", "clpid": "Blake-G-A" }, { "family_name": "Cass", "given_name": "Glen Rowan", "clpid": "Cass-G-R" }, { "family_name": "Hering", "given_name": "Janet G.", "clpid": "Hering-J-G" }, { "family_name": "Hoffmann", "given_name": "Michael R.", "clpid": "Hoffmann-M-R" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The kinetics of manganese redox reactions are important for understanding redox cycles in natural waters. This study examined the kinetics of the homogenous oxidation of Mn(II) and formation and disappearance of Mn(III) complexes.
\r\n\r\nThe oxidation of Mn(II) was studied to determine the homogenous oxidation rate in the absence of solid surfaces and biological activity. Experiments were conducted at 35, 45, 50, and 60\u00b0C. The pH was 8.0. The reaction solution was prepared so that at no time during the experiment was the solubility product of any solid phase exceeded. Oxidized Mn was measured using leuco crystal violet dye reagent. Measurable rates were observed for the 45, 50, and 60\u00b0C experiments. An Arrhenius expression was fitted to the rates in order to extrapolate to 25\u00b0C. The second order rate constant for the rate expression
\r\n\r\n-d[Mn(II)]/dt = k\u22c5[Mn(II)\u22c5[O2]
\r\n\r\nwas calculated to be 6.9 \u00b1 1.6 x 10-7 M-1s-1.
\r\n\r\nThe kinetics of disappearance of Mn(III) complexes from aqueous solution were studied. Complexes of pyrophosphate (P2O74-), ethylenediaminetetracetate (EDTA), and citrate (CIT) were synthesized from MnO4- and a Mn(II) salt in a 1:4 ratio in the presence of excess ligand. Concentrations of Mn(III) complex were monitored spectrophotometrically. Experiments were conducted in the pH range of 6 to 9 for pyrophosphate and citrate and 3 to 9 for EDTA. The total manganese concentration was varied between 0.5 and 1.0 mM. Ligand concentrations were varied from 0.5mM to 200mM. Experiments were also conducted to examine the effects of oxygen, light, and ionic strength. Oxygen had a significant effect on only the citrate complex; ionic strength affected only the EDTA complex. Light was found to be insignificant in all cases.
\r\n\r\nThe Mn(III)P2O7 complex was found to disappear from solution relatively slowly providing the ligand was in at least ten-fold excess. Disappearance time scales were on the order of 107 s. The Mn(III)EDTA complex reacted rather rapidly with time scales on the order of 104 s. There were at least two Mn(III)EDTA complexes, a protonated one more stable at low pH and an unprotonated one more stable at high pH. The pKa of the complex appeared to be approximately 5.3. The rate of disappearance of the Mn(III)EDTA had a fractional dependence on pH, probably indicative of an unknown pH dependent intermediate in the decomposition of the complex. The rate was found to increase with increased EDTA, indicating that the rate limiting step was an outer sphere electron transfer from Mn(III)EDTA to an excess EDTA. The rate law for the reaction above pH 6 was found to be
\r\n\r\n-d[Mn(III)EDTA]/dt = k\u22c5[H+]0.31\u22c5[EDTA]1.35\u22c5[Mn(III)EDTA]
\r\n\r\nThe Mn(III)CIT complex was found to undergo a redox cycle. The Mn(III)CIT complex was reduced, forming Mn(II). The Mn(II) was then oxidized in the presence of oxygen to re-form the Mn(III) complex. Both pH and ligand concentration were found to have fractional orders in the rate expression, largely due to the competition between the reduction and the oxidation and possibly complicated by radicals formed by the reaction.
\r\n\r\nThe dissolution of MnOOH by pyrophosphate, EDTA, and citrate was studied. A MnOOH solid was synthesized by oxidizing Mn(II) with hydrogen peroxide at elevated temperatures and high pH. The solid was identified by X-ray diffraction to be \u03b2-MnOOH, with some contamination by Mn3O4. Throughout the dissolution process samples were removed by pipette and filtered. The filtrate was analyzed spectrophotometrically for the presence of Mn(III) complexes and total Mn. The solids captured on the filter were analyzed by an iodine titration technique, coupled with formaldoxime measurements to determine the average oxidation state of the solids. The effects of pH and ligand concentration on rates were examined.
\r\n\r\nPyrophosphate was found to dissolve the Mn(III) solids nonreductively, producing the Mn(III) complex in solution. The dissolution reaction rate was dependent on approximately the half power of [H+], possibly indicative of a surface binding ligand binding on the surface. No dependence on the ligand concentration was found down to a ligand:Mn ratio of 10:1, probably indicative of surface site saturation by ligand.
\r\n\r\nEDTA was found to dissolve the solids reductively with no Mn(III) solution species being observed. The dependence on [H+] was approximately one half order, possibly indicative of a surface binding.
\r\n\r\nCitrate dissolved the MnOOH solids in what appeared to be two steps. There seemed to be an initial stage of nonreductive dissolution, followed by a reductive dissolution. The rate and duration of the two different stages depended on pH. The dependence was slightly greater than first order in [H+], possibly indicating the reaction becomes controlled by reactions of the radicals produced by oxidation of the citrate.
\r\n\r\nThis study has shown that Mn(III) complexes can be formed in pH conditions relevant to natural waters. These complexes can be formed either through oxidation of Mn(II) by strong oxidants in the presence of stabilizing ligands or by dissolution of Mn(III)-containing solids by stabilizing ligands. Once formed, the lifetime of these complexes will depend on the nature of the ligand and chemical characteristics of the aquatic environment. If the ligand does not rapidly reduce Mn(III) these complexes can be powerful mobile oxidants which could significantly affect the local redox environment.
\r\n", "doi": "10.7907/40pv-rq92", "publication_date": "1996", "thesis_type": "phd", "thesis_year": "1996" } ]