• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 197
  • 91
  • 63
  • 9
  • 8
  • 6
  • 5
  • 4
  • 4
  • 3
  • 2
  • 2
  • 1
  • 1
  • 1
  • Tagged with
  • 444
  • 201
  • 144
  • 108
  • 95
  • 90
  • 76
  • 70
  • 58
  • 52
  • 49
  • 47
  • 45
  • 38
  • 37
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
21

Elementary analysis and methoxyl content of plant lignins and soil humic acid and dioxane extracts II. Liquid phase adsorption of herbicide (CIPC) on clays isolated from a humic gley soil /

Behmer, Dale Edward, January 1966 (has links)
Thesis (Ph. D.)--University of Wisconsin, 1966. / Vita. Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.
22

Influência das substâncias húmicas aquáticas na determinação de atrazina por imunoensaio (Elisa)

Toscano, Ilda Antonieta Salata [UNESP] 02 1900 (has links) (PDF)
Made available in DSpace on 2014-06-11T19:35:05Z (GMT). No. of bitstreams: 0 Previous issue date: 1999-02Bitstream added on 2014-06-13T18:46:14Z : No. of bitstreams: 1 toscano_ias_dr_araiq.pdf: 387630 bytes, checksum: 64cc6245511279744bb2dd4a814e3afb (MD5) / Substâncias húmicas aquáticas (SHA) foram obtidas por processo de adsorção em resinas macroporosas não-iônicas, XAD 7 e XAD 2, dispostas em série. Após eluição com solução de NaOH, o extrato alcalino de SHA foi acidificado a pH 1,0 para separação em ácidos húmico (AH) que precipita, e ácido fúlvico (AF) o qual permanece em solução. Para caracterização físicoquímica do material húmico (AH e AF), foram feitas análise elementar, determinação do teor de substâncias húmicas e acidez total. Os resultados obtidos por UV-VIS e FTIR indicaram que AH apresenta maior número de grupos aromáticos em relação a AF, que em geral possui mais cadeias alifáticas. A aplicação da técnica imunoquímica, enzyme-linked immunosorbent assay (ELISA), para a determinação do herbicida atrazina em águas foi avaliada em amostra de água contendo alto teor de matéria orgânica (~35 mg L-1) e baixo valor de pH (3,8). O efeito matriz devido a presença de SHA pode ser notado pela perda de sensibilidade da técnica, ou seja, os valores de IC50 variaram de 60 ng L-1, na ausência de SHA, para 112 ng L-1 em concentrações acima de 10,0 mg L-1 de material húmico e para 137 ng L-1 em pH < 5,0. Além disto, pode-se inferir que a luz solar aumentou a velocidade de degradação da atrazina na presença de SHA formando produtos, com partes de suas estruturas, semelhantes ao produto original levando a resultados falso-positivos. A quantidade de material húmico presente na amostra de água foi a principal fonte de erro na análise de atrazina, levando à interações não-específicas entre as SHA e os reagentes enzimáticos. O procedimento ELISA, aplicado neste estudo, pode ser utilizado para determinação de atrazina desde que se faça diluição da amostra até cerca de 2,5 mg L-1 de húmicos e em pH alcalino (7,0 – 9,0). / Aquatic humic substances (AHS) were isolated from water samples using Amberlite XAD 7 and XAD 2. After elution with NaOH solution, the XAD concentrated AHS was fractioned at pH 1.0 resulting in fulvic acid (FA - supernatant) and humic acid (HA - slurry). All humic materials were characterized with respect to elemental analysis, amount of AHS and total acidity. UV and FTIR spectra showed HA aromatic character greater than FA. Enzyme-linked immunosorbent assay (ELISA) was evaluated by analyzing atrazine in rich-humic matter water sample (~35 mg L-1) and acid water (pH 3.8). From all the conditions studied the low pH (pH < 5.0) and high humic substances concentrations (>10 mg L-1) showed the greatest influence. The IC50 values to control sample (no humic) decreased from 60 ng L-1 to 112 ng L-1 to humic solution at >10 mg L-1 and to 137 ng L-1 at pH < 5.0. The presence of AHS alters the photochemical behaviour of atrazine by accelerating its degradation forming metabolites which can be recognized by the antibodies. The assay performance showed a strong dependence on the pH values and amount of humic matter. However, analysis could be carried out directly in samples containing HA or FA that had been adjusted the pH in the range between 7.0 and 9.0, and humic concentration at 2.5 mg L-1.
23

The sodium hypochlorite oxidation of humic acids and prepared lignins

Herman, William Allan January 1977 (has links)
In order to investigate the relative merits of a selective oxidant for the degradation of natural polymers, humic acid extracts from three Alberta soil sites and three British Columbia soil sites, and two Kraft's prepared wood lignins were oxidized with 1.6 N NaOCl at room temperature (23°C) for 5 days. The oxidation products included CO₂, highly volatile acids and organic solvent soluble (OSS) products with the relative size of each fraction determined by carbon content. OSS products were characterized by Infrared (IR), Thin Layer Chromatography (TLC) and Nuclear Magnetic Resonance (NMR) techniques and identified after methylation and separation by Gas Liquid Chromatography (GLC) methods involving co-injecting authentic compounds and matching elution time and temperature of some of the components with that of the authentic compounds. Proceeding from the known chemistry of the NaOCl reaction, the CO₂ and highly volatile acid products could only be derived from the aliphatic chain or saturated ring components of humic acids or lignin and represented 66 to 82% of the products assuming no destruction of aromatic structure. Benzene carboxylic acid products were derived from the aromatic component of the starting materials. Estimates of the degree of aromaticity of the starting materials, using GLC and potentiometric titration data, were substantially less than those calculated from proposed model humic acid and lignin structures in the soils literature. An unidentified oily component was found in the oxidation products of two of the humic acid preparations. The results of this study indicated humic acid and lignin are composed of a mixed aliphatic-aromatic compound system. The relatively more mature humic acid preparations were found to be of greater aromaticity than the less mature samples. It was found 2 that NaOCl was not totally selective in differentiating Sp² from Sp³ carbon hybrids; as a result the total discrimination between aliphatic and aromatic structures was not a safe assumption. It is postulated that aromatic ring opening may occur at sites of hydroxyl group substitution on the ring structures resulting in an apparent less aromaticity and the generation / Land and Food Systems, Faculty of / Graduate
24

Fluorescence huminových kyselin 3D excitačně emisní metodou. / Fluorescence spectroscopy of humic acids by excitation-emission matrix.

Mlčoch, Tomáš January 2008 (has links)
The aim of the diploma thesis was measurement and characterization of EEM (excitation emisssion spectra) of IHSS humic acids standards and soil, lignite humic and fulvic acids and humates prepared from different sources. The aim was to follow the differences in their fluorescence spectra and to determine precise location of fluorescence peaks. Target of this work was to find the optimal way for exporting of scan data to the 3D EEM spectrofluorographs. The dependence of relative intensity of fluorescence on pH value was studied further.
25

Influence of Humic Acids on the Migration Behavior of Radioactive and Non-Radioactive Substances Under Conditions Close to Nature -Synthesis, Radiometric Determination of Functional Groups, Complexation-

Nitsche, Heino, Heise, Karl-Heinz, Bernhard, Gert, Schmeide, Katja, Pompe, Susanne, Bubner, Marianne 31 March 2010 (has links) (PDF)
The interaction behavior of humic acids with uranium(VI) and the influence of humic substances on the migration behavior of uranium was investigated. A main focus of this work was the synthesis of four different humic acid model substances and their characterization and comparison to the natural humic acid from Aldrich. A radiometric method for the determination of humic acid functional groups was applied in addition to conventional methods for the determination of the functionality of humic acids. The humic acid model substances show functional and structural properties comparable to natural humic acids. Modified humic acids with blocked phenolic OH were synthesized to determine the influence of phenolic OH groups on the complexation behavior of humic acids. A synthesis method for 14C-labeled humic acids with high specific activity was developed. The complexation behavior of synthetic and natural humic acids with uranium(VI) was investigated by X-ray absorption spectroscopy, laser-induced fluorescence spectroscopy and FTIR spectroscopy. The synthetic model substances show an interaction behavior with uranium(VI) that is comparable to natural humic acids. This points to the fact that the synthetic humic acids simulate the functionality of their natural analogues very well. For the first time the influence of phenolic OH groups on the complexation behavior of humic acids was investigated by applying a modified humic acid with blocked phenolic OH groups. The formation of a uranyl hydroxy humate complex was identified by laserspectroscopic investigations of the complexation of Aldrich humic acid with uranium(VI)at pH 7. The migration behavior of uranium in a sandy aquifer system rich in humic substances was investigated in column experiments. A part of uranium migrates non-retarded through the sediment, bound to humic colloids. The uranium migration behavior is strongly influenced by the kinetically controlled interaction processes of uranium with the humic colloids. The influence of humic acids on the sorption of uranium(VI) onto phyllite was investigated in batch experiments using two different humic acids. The uranium(VI) sorption onto the rock phyllite is influenced by the pH-dependent sorption behavior of the humic acids.
26

Influence of Humic Acids on the Migration Behavior of Radioactive and Non-Radioactive Substances Under Conditions Close to Nature -Synthesis, Radiometric Determination of Functional Groups, Complexation-: Influence of Humic Acids on the Migration Behavior of Radioactive and Non-Radioactive Substances Under Conditions Close to Nature -Synthesis, Radiometric Determination of Functional Groups, Complexation-

Nitsche, Heino, Heise, Karl-Heinz, Bernhard, Gert, Schmeide, Katja, Pompe, Susanne, Bubner, Marianne January 2000 (has links)
The interaction behavior of humic acids with uranium(VI) and the influence of humic substances on the migration behavior of uranium was investigated. A main focus of this work was the synthesis of four different humic acid model substances and their characterization and comparison to the natural humic acid from Aldrich. A radiometric method for the determination of humic acid functional groups was applied in addition to conventional methods for the determination of the functionality of humic acids. The humic acid model substances show functional and structural properties comparable to natural humic acids. Modified humic acids with blocked phenolic OH were synthesized to determine the influence of phenolic OH groups on the complexation behavior of humic acids. A synthesis method for 14C-labeled humic acids with high specific activity was developed. The complexation behavior of synthetic and natural humic acids with uranium(VI) was investigated by X-ray absorption spectroscopy, laser-induced fluorescence spectroscopy and FTIR spectroscopy. The synthetic model substances show an interaction behavior with uranium(VI) that is comparable to natural humic acids. This points to the fact that the synthetic humic acids simulate the functionality of their natural analogues very well. For the first time the influence of phenolic OH groups on the complexation behavior of humic acids was investigated by applying a modified humic acid with blocked phenolic OH groups. The formation of a uranyl hydroxy humate complex was identified by laserspectroscopic investigations of the complexation of Aldrich humic acid with uranium(VI)at pH 7. The migration behavior of uranium in a sandy aquifer system rich in humic substances was investigated in column experiments. A part of uranium migrates non-retarded through the sediment, bound to humic colloids. The uranium migration behavior is strongly influenced by the kinetically controlled interaction processes of uranium with the humic colloids. The influence of humic acids on the sorption of uranium(VI) onto phyllite was investigated in batch experiments using two different humic acids. The uranium(VI) sorption onto the rock phyllite is influenced by the pH-dependent sorption behavior of the humic acids.
27

Πετρολογική-ορυκτολογική μελέτη του λιγνίτη Κρυανής (Ν. Φθιώτιδας) και δυνατότητες εξωενεργειακής εφαρμογής του

Τσιμικλής, Δημήτριος 11 October 2013 (has links)
Η εργασία έχει ως στόχο τη μελέτη των χαρακτηριστικών του λιγνίτη Κρυανής (Νομού Φθιώτιδας), προκειμένου να προσδιοριστούν οι συνθήκες απόθεσης του λιγνίτη στον παλαιοτυφώνα, αλλά και να εξεταστεί η καταλληλότητά του ως εδαφοβελτιωτικό και οργανοχουμικό λίπασμα. Η Νεογενής λεκάνη Ζελίου-Αγνάντης αποτελεί τμήμα της Υποπελαγονικής ζώνης. Τα περιθώρια και το υπόβαθρο της λεκάνης αποτελούνται από Ανω-Τριαδικούς δολομίτες και Ανω-Τριαδικούς και Ιουρασικούς ασβεστόλιθους, οι οποίοι υπόκεινται πυριτόλιθων και σχιστολίθων με εναλλαγές οφιολίθων. Τα πετρώματα του Παλαιογενούς απουσιάζουν, πιθανότατα λόγω διάβρωσης. Το λιγνιτικό κοίτασμα φιλοξενείται σε ιζήματα Μειοκαίνου έως κατωτέρου Πλειστοκαίνου και διακρίνεται σε τρεις σχηματισμούς: τον ανώτερο λιγνίτη, τον ενδιάμεσο και τον υποκείμενο. Ο υποκείμενος και ο ανώτερος λιγνιτικός σχηματισμός περιλαμβάνουν κυρίως χονδρόκοκκα κλαστικά υλικά, ενώ ο ενδιάμεσος ορίζοντας αποτελείται από στρώματα λιγνίτη που εναλλάσσονται με λεπτόκοκκα κλαστικά υλικά και ασβεστολιθικά πετρώματα. Στην περιοχή δειγματοληψίας παρατηρήθηκαν εκτός από το λιγνίτη, χουμώδεις άργιλοι, ιλυόλιθοι, άμμος και ψαμμίτες. Στη λεκάνη δραστηριοποιούνται μια σειρά από μικρά επιφανειακά ορυχεία, όπως αυτά της Κρυανής, Αγνάντης και Γολεμίου. Από τον ανώτερο ορίζοντα του επιφανειακού ορυχείου Κρυανής συλλέχθησαν συνολικά 20 δείγματα λιγνιτών μέσω δειγματοληψίας ανοικτής αύλακας. Η δειγματοληψία καλύπτει συνολικό πάχος στρωμάτων 20,93 m με πάχος λιγνίτη 11,54 m. Πραγματοποιήθηκαν προσεγγιστική και στοιχειακή ανάλυση, ανθρακοπετρογραφική εξέταση και ορυκτολογική ανάλυση. Επιπλέον προσδιορίστηκε η περιεκτικότητα σε χουμικά συστατικά, ώστε να διαπιστωθεί η καταλληλότητα του λιγνίτη Κρυανής ως εδαφόβελτιωτικού ή/και οργανοχουμικού λιπάσματος. Ο επικρατέστερος λιθότυπος είναι ο matrix, ενώ ακολουθούν ο ορυκτομιγής και σπανιότερα ο ξυλιτικός. Τα στρώματα που παρεμβάλλονται μεταξύ των λιγνιτών είναι κυρίως ιλυόλιθοι, άργιλοι, καθώς και φακοί ψαμμιτών. Η υγρασία, η τέφρα, η περιεκτικότητα σε πτητικά συστατικά είναι 25,37%, 34,37% και 48,69% (επί ξηρού) κατά μέσο όρο αντίστοιχα. Ο μόνιμος άνθρακας ανέρχεται κατά μέσο όρο σε 23,53%, ενώ η θερμαντική ικανότητα είναι κατά μέσο όρο 13,98 MJ/kg. Η ορυκτολογική ανάλυση έδειξε ότι τα κύρια ορυκτά είναι ο χαλαζίας, ο χλωρίτης και τα αργιλικά (ιλλίτης). Η περιεκτικότητα σε χουμικά συστατικά είναι σε αρκετά υψηλά επίπεδα: τα χουμικά οξέα κυμαίνονται από 47,0% έως 56,6% σε δείγματα επί ξηρού και τα φουλβικά συστατικά από 1,8% έως 10,8%. Mε βάση αυτά τα στοιχεία και σε σύγκριση με αποτελέσματα προηγούμενης μελέτης που πραγματοποιήθηκε σε αρκετές λεκάνες του Ελλαδικού χώρου ο λιγνίτης Κρυανής θεωρείται κατάλληλος για χρήση ως εδαφοβελτιωτικό και οργανοχουμικό λίπασμα. / This study aims to determine the features of the Kryani lignite deposit (Prefecture of Fthiotida, Central Greece) in order to assess the palaeoenvironmental conditions during peat deposition and to test the potential of certain lignite beds for soil improvement media. The Neogene Zeli-Agnanti Basin is part of the Sub-Pelagonian Ζone. The margins and the basement of the basin consist of Upper Triassic dolomites and Upper Triassic and Jurassic limestones, overlain by shale and chert strata alternating with ophiolites. Palaeogene rocks are lacking presumably due to erosion. The lignite deposit is hosted in sediments deposited from Miocene to Early Pleistocene and distinguished into lignite seams: the lower, the intermediate and the upper ones. The lower and the upper formations consist mainly of clastic, relatively coarse-grained rocks, whereas the intermediate one formation includes lignite beds alternating with fine-grained clastic and calcareous rocks. At the sampling site, except of lignite layers, Neogene sediments such as humic clay, siltstone, sandstone and sand were encountered. The basin includes several small deposits, namely these of Kryani, Agnanti, Golemi. Applying channel sampling, twenty lignite samples were collected from the upper seam exposed at the Kryani open pit mine. Inorganic intercalations were additionally sampled. The total thickness of the sampled profile is 20.93 m and the cumulative lignite thickness is 11.54 m. The matrix lithotype is dominant with the mineral-rich lithotype being less common. Xylite- and charcoal-rich lithotypes seldom. The intercalations consist mainly of siltstone, claystone, as well as of sandstone lenses or layers, sometimes including pebbles. Proximate and ultimate analyses, along with maceral and mineralogical analyses provide evidence for the depositional environment of the Kryani lignite deposit. Additionally, the extraction and the separation of humic substances (humic and fulvic acids, humins) constitute criteria for the adequacy of certain layers as soil-improvement media. The predominant lithotype is the matrix one, followed by the mineral-rich lithotype and rarely the xylite-rich lithotype. The intercalates between the main lignite is siltstone, claystone and sandstones lenses. The moisture, ash and the content of volatile components are 23.9%, 45.06% and 30.98% respectively. The carbon fix averages at 23.53% and the calorific value 13.98 MJ/kg. The mineralogical analysis proved that the main minerals are quartz, chlorite and the clay (illite). The humic substances content is quite high: the humic acids display values from 47.0% to 56.6% (on dry basis) and fulvic acids 1.8% to 10.8%. The results compared with these of previous studies conducted in several areas of Greece, prove that Kryanis lignite is suitable for application as soil-improvement media and organic fertilizer.
28

Modelling metal competition for adsorption sites on humic acid

Jeong, Chang-Yoon January 1998 (has links)
No description available.
29

Studies on metal-humic interactions

Hall, Anthony January 1993 (has links)
The favoured option for disposal of low and intermediate level radioactive waste is burial deep underground. In the safety assessment of deep disposal facilities a possibility which has to be considered is the eventual penetration of the engineered and natural barriers by ground water. Unfortunately humic and fulvic acids, present in most natural waters, can combine with radionuclides to produce metal complexes which are potentially more mobile in the environment than the original radionuclides from which they are formed. Migration of such complexes via groundwater and/or subsequently surface water routes could result in accelerated release of radionuclides to the biosphere. Quantitative knowledge of the extent of possible metal humate and fulvate interactions is therefore of considerable importance. In this study the complexation of Ni, Eu and Ce by humic and fulvic acids present in both ground and surface waters has been investigated. In most studies, to simplify the chemistry involved, humics and fulvics are separated from the water in which they occur before determination of their complexation properties. However, the severe conditions employed in the extraction process could conceivably alter the very properties to be measured. Accordingly the complexation properties of humics and fulvics present in a surface water have been studied both before and after extraction. The major part of this study was undertaken using a High Performance Size Exclusion Chromatographic technique especially developed to facilitate determinations on nonextracted materials. A fluorescence technique was also investigated as a more rapid alternative but was found to be of limited applicability. Complexation parameters were derived using a strong and weak site model. The effects of side reactions, ionic strength, pH, ligand type and nature of the cation were taken into account. With respect to measured stability constants, no major differences between ground and surface waters or extracted and non extracted materials were found. However, measured capacities of humics and fulvics for metals showed some variation according to the nature of the material and the system.
30

Toxicity of Inorganic Aluminium in Humic Streams

Andrén, Cecilia M. January 2012 (has links)
Aluminium (Al) has been recognised as a main toxic factor alongside pH in acidified water ecosystems. The toxic effect of Al has been attributed to inorganic Al (Ali), though there are few in situ studies in ambient humic waters which are the focus of this thesis. The aim was to estimate Ali toxicity and thus also Ali concentrations in Swedish humic streams. Subsequently it is necessary to analyse Ali correctly, which was studied by modelling and method intercalibrations. The hypothesis was that the effect of Ali could be followed via physiological effects and Al accumulation, as well as by mortality. Toxicity was studied by in stream exposures of brown trout (Salmo trutta L.) and two salmonid prey organisms (Gammarus pulex and Baetis rhodani) during spring flood. The modelling of the Ali fraction was performed using monitoring data covering all of Sweden with satisfactory results. The essential variables for Ali modelling were determined; Al, DOC, pH and F, while Fe, Ca and Mg had less effect. The automated analytical procedure for Ali (with cation exchange followed by complexation with pyrocatechol violet) was modified and validated and showed to be the preferred method for laboratory analyses. To avoid detrimental effects for brown trout Ali should be &lt;20 µg/L and pH &gt;5.0; mortality was high when the Ali was above 50 µg/L. The invertebrates were more sensitive, as mortalities occurred at pH &lt;6.0 and Ali &gt;15 µg/L for G. pulex, and at pH &lt;5.7 and Ali &gt;20 µg/L for B. rhodani. It is prudent to use a wide view and let the most sensitive species set the tolerance limits; a pH above 5.7-6.0 and Ali below 15-20 µg/L allows the stream ecosystems to thrive. Today, as waters are recovering from acidification, the aim of mitigating liming is to carefully adjust dosage to avoid suboptimal water quality. The thresholds found in this thesis can be used to efficiently but carefully decrease liming, as both Ali and pH levels have to be balanced to sustain the recovering aquatic biota.

Page generated in 0.0163 seconds