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Geology topics

D.J. Pinckney

Publications and source records attributed to D.J. Pinckney.

7 recordsLinked to original sources

USGS1988

Characterization of humic acid fractions by C-13 nuclear magnetic resonance spectroscopy

Soil humic acids from different environments were fractionated by adsorption chromatography on Sephadex and characterized by C‐13 nuclear magnetic resonance (NMR) spectroscopy. The C‐13 NMR spectra of the fractions consist of some sharp, well‐resolved lines and some broad bands in contrast to the spectra of the unfractionated humic acids, where the bands are broader and less well‐resolved. The marked increase in resolution is apparently due to increased homogeneity of the fractions. These spectra are compared to the spectra of model compounds.

Environmental Technology Letters↗

USGS1982

Molecular size of aquatic humic substances

Aquatic humic substances, which account for 30 to 50% of the organic carbon in water, are a principal component of aquatic organic matter. The molecular size of aquatic humic substances, determined by small-angle X-ray scattering, varies from 4.7 to 33 Å in their radius of gyration, corresponding to a molecular weight range of 500 to greater than 10,000. The aquatic fulvic acid fraction contains substances with molecular weights ranging from 500 to 2000 and is monodisperse, whereas the aquatic humic acid fraction contains substances with molecular weights ranging from 1000 to greater than 10,000 and is generally polydisperse.

Arizona, Colorado, Hawaii, Florida, Massachusetts,↗

USGS1977

Chemical structure of humic acids - Part 1, a generalized structural model

A new model is proposed for the structure of humic acids. In this model humic acid is pictured as being made up of a hierarchy of structural elements. At the lowest level in this hierarchy are simple phenolic, quinoid, and benzene carboxylic acid groups. These groups are bonded covalently into small particles. Particles of similar chemical structure are linked together by weak bonds to form "homogeneous" aggregates. Two or more different types of aggregates may be linked together to form mixed aggregates. Complexes of humic acid and clay minerals are also formed.

Journal of Research of the U.S. Geological Survey↗

USGS1977

Chemical structure of humic acids - Part 2, the molecular aggregation of some humic acid fractions in N, N-dimethylformamide

Humic acid fractions form molecular aggregates in solution. In previous studies we have shown by small angle X-ray scattering that the size of these aggregates is a function of pH. In this study we have found that the size of the aggregates of two humic acid fractions in water and buffers and in dimethylformamide solutions can be changed by oxidation with molecular oxygen and air. These results cast new light on the bonding mechanisms that cause aggregation of the humic acid particles in solution. We have interpreted the changes in aggregation sizes as being brought about by changes in intermolecular and intramolecular hydrogen bonding of the humic particles. Solvation of the humic molecules by dimethylformamide interferes with some of the hydrogen bonding reactions between proton donor and acceptor groups on the same humic acid molecules or on different molecules.

Journal of Research of the U.S. Geological Survey↗

USGS1975

Pyrrolidone - a new solvent for the methylation of humic acid

In the past, humic acid has been methylated by suspending it in a solution of diazomethane in diethyl ether, and degrading the partly methylated humic acid to release those parts of the molecule that were methylated. Only small fragments of the molecule have been identified by this technique. In the procedure described here the humic acid is dissolved in 2-pyrrolidone and methylated by the addition of diazomethane in diethyl ether and ethanol to the solution. Because the humic acid is completely dissolved in the reaction medium, disaggregation of the humic acid particles takes place and much more complete methylation is obtained. The methylated products may be fractionated by countercurrent distribution and analyzed by mass spectrometry.

Journal of Research of the U.S. Geological Survey↗

USGS1973

The fractionation of humic acids from natural water systems

Humic acids, the most abundant organic components of natural water systems, are complex mixtures of molecular aggregates of different chemical and physical properties. The first step in the study of such a mixture is the fractionation of the mixture. The most common approach with humic acids is to attempt to obtain a molecular weight fractionation by gel-permeation chromatography. However, since the preponderance of evidence indicates that the components of humic acid do not fulfill the basic criteria of uniform shape and chemical structure necessary for obtaining a molecular fractionation on gel-permeation media, molecular weight distributions in humic acids cannot be evaluated by this method. A fractionation dependent upon chemical structure can be obtained by the manipulation of elution conditions on a gel-permeation column. This procedure provides a beginning in the isolation and identification of discrete components of humic acids.

Journal of Research of the U.S. Geological Survey↗

USGS1973

Determination of the association and dissociation of humic acid fractions by small angle X-ray scattering

A procedure has been devised for the fractionation of humic acid samples from different environments. This procedure involves fractionation of the sample by adsorption chromatography on a Sephadex G-50 column followed by chromatography on either a G-25 or a G-100 column. The fractions of the solutions are then examined by small angle X-ray scattering. Three different types of behavior have been detected among the humic acid fractions: (1) Some fractions show very little change in aggregation at pH values above 3.5. (2) One fraction forms aggregates at pH values above and below pH 7, but at pH 7 it is completely dissociated. (3) In some fractions the degree of aggregation decreases with increasing pH. However, even at pH values as high as 11.5 some large particles are still present. These differences in association behavior are due to the interaction of different attractive and repulsive forces. In many aggregating systems only one type of attractive force is dominant; however in humic acid systems hydrogen bonding, π bonding between planar aromatic moieties, and other coulombic interactions apparently all play a role in the formation of molecular aggregates.

Journal of Research of the U.S. Geological Survey↗