Patrick MacCarthy

Limitations in the use of commercial humic acids in water and soil research

Seven samples of commercial humic acids, purchased from five different suppliers, were studied, and their characteristics were compared with humic and fulvic acids isolated from streams, soils, peat, leonardite, and a dopplerite sample. Cross-polarization and magic-angle spinning /sup 13/C NMR spectroscopy clearly shows pronounced differences between the commercial materials and all other samples. Elemental and infrared spectroscopic data do not show such clear-cut differences but can be used as supportive evidence, with the /sup 13/C NMR data, to substantiate the above distinctions. As a result of these differences and due to the general lack of information relating to the source, method of isolation, or other pretreatment of the commercial materials, these commercial products are not considered to be appropriate for use as analogues of true soil and water humic substances, in experiments designed to evaluate the nature and reactivity of humic substances in natural waters and soils.

The Principles Of Humic Substances

Two principles are presented that define the molecular nature and ecological role of humic substances (HS). The First Principle (i) accounts for and organizes an extensive body of apparently disparate data relating to the inability to purify and establish a molecular structure for HS; (ii) offers a conceptual framework for dealing with HS and for evaluating the applicability and limitations of various experimental methods; and (iii) identifies molecular heterogeneity, in combination with pronounced chemical reactivity, as constituting the essence of HS. Five corollaries to the First Principle spell out its consequences in more specific detail. New definitions of HS that offer greater insight into the molecular nature of these materials arise from the First Principle. The inapplicability of the molecular structure concept to HS is explained. The concept of hypothetical pseudostructures is introduced to help visualize the chemical reactions and interactions of HS without the unjustified assignment of specific structures to the material as a whole. Constraints in the design of experiments and in the interpretation of experimental data caused by the heterogeneous nature of HS are discussed. The Second Principle makes a connection between the molecularly heterogeneous and chemically reactive nature of HS and the ecological need for a reactive and persistent medium for plant growth. Concepts presented herein have broad implications in many fields, including chemistry, geochemistry, environmental and soil sciences, and ecology.

Quantitative evaluation of XAD-8 and XAD-4 resins used in tandem for removing organic solutes from water

Abstract The combined XAD-8 and XAD-4 resin procedure for the isolation of dissolved organic solutes from water was found to isolate 85% or more of the organic solutes from Lake Skjervatjern in Norway. Approximately 65% of the dissolved organic carbon (DOC) was first removed on XAD-8 resin, and then an additional 20% of the DOC was removed on XAD-4 resin. Approximately 15% of the DOC solutes (primarily hydrophilic neutrals) were not sorbed or concentrated by the procedure. Of the 65% of the solutes removed on XAD-8 resin, 40% were fulvic acids, 16% were humic acids, and 9% were hydrophobic neutrals. Approximately 20% of the hydrophilic solutes that pass through the XAD-8 resin were sorbed solutes on the second resin, XAD-4 (i.e., they were hydrophobic relative to the XAD-4 resin). The fraction sorbed on XAD-4 resin was called XAD-4 acids because it represented approximately 85–90% of the hydrophilic XAD-8 acid fraction according to the original XAD-8 fractionation procedure. The recovery of hydrophobic acids (fulvic acids and humic acids) and the hydrophobic neutral fraction from XAD-8 resin was essentially quantitative at 96%, 98%, and 86%, respectively. The recovery of XAD-4 acids from the XAD-4 resin was only about 50%. The exact reason for this moderately low recovery is unknown, but could result from π-π bonding between these organic solutes and the aromatic matrix of XAD-4. The hydrophobic/hydrophilic solute separation on XAD-8 resin for water from background Side A and Side B of the lake was almost identical at 65 and 67%, respectively. This result suggested that both sides of the lake are similar in organic chemical composition even though the DOC variation from side to side is 20%.

Statistical evaluation of the elemental composition of humic substances

Elemental data (C, H, O, N, S, atomic H/C and O/C ratios) for humic acids (410 samples), fulvic acids (214 samples) and humin (26 samples), isolated from environments all over the world, were compiled from the literature. These data were analyzed statistically using the mean, median, mode, range, standard deviation and t-test. The large data base allows statistically significant differences between various humic substances to be established. Prior to some of the statistical evaluations, the humic substances were grouped according to source—soil, freshwater, marine or peat. The average elemental compositions for humic acid, fulvic acid and humin, in toto, and also when segregated by source, are presented. Standard deviations for carbon contents are remarkably small suggesting that perhaps an optimum composition exists for humic substances in nature. The evaluation shows that humic acids in toto are significantly (P < 0.0005) different from fulvic acids in toto with respect to C, N, and O contents and atomic H/C ratios. Fulvic acid in toto is significantly (P < 0.0005) different from humin in toto in terms of C and O contents. When segregated by source, some significant (P < 0.0005) differences between humic acids isolated from freshwater, marine, and soil environments are evident; similarly, significant differences are found between fulvic acids from freshwater and soil sources. Three van Krevelen diagrams based on: (1) the 650 samples of humic acid, fulvic acid and humin; (2) humic acids segregated by source, and; (3) fulvic acids segregated by source, are discussed.

Determination of some redox properties of humic acid by alkaline ferricyanide titration

Abstract A method for the direct potentiometric oxidation titration of aqueous phase humic substances with potassium ferricyanide (K3Fe(CN)6) has been developed. Oxidative titrations of humic acid were carried out at varying analyte concentrations, and at values of pH ranging from 5.0 to 11.0. Model synthetic mixtures of phenols and hydroquinones were examined in the same pH range. Ferricyanide titrations of humic acid at pH 9.0 yielded single discrete endpoints, and generally stable potential measurements in the pre-equivalence point region of the titration curve. Cyclic voltammetry of alkaline solutions of humic acid provides evidence for direct interaction between phenolic components in the humic acid and the electrode surface, and that for titrations conducted at pH 9.0 and above, humic acid can contribute to the poising of the potential measurements in the region of the half-titration point.

Isolation of humin by liquid-liquid partitioning

Abstract A method is described for isolating humin from diverse samples by partitioning the organic components between methyl isobutyl ketone (MIBK) and an aqueous phase as a function of pH. The humin isolated by the MIBK method is similar to humin isolated from the same sample by more traditional alkali-extraction methods. The MIBK method may be extended to separate the organic components of humin from its inorganic or non-humic organic constituents. It is shown that the organic components of humin consist of lipids and a humic acid-like material. In this study the MIBK method was applied to a soil, a peat and a stream sediment sample.

Cu(II) binding by a pH-fractionated fulvic acid

Abstract The relationship between acidity, Cu(II) binding and sorption to XAD resin was examined using Suwannee River fulvic acid (SRFA). The work was based on the hypothesis that fractions of SRFA eluted from an XAD column at various pH’s from 1.0 to 12.0 would show systematic variations in acidity and possibly aromaticity which in turn would lead to different Cu(II) binding properties. We measured equilibrium Cu(II) binding to these fractions using Cu 2+ ion-selective electrode (ISE) potentiometry at pH 6.0. Several model ligands were also examined, including cyclopentane-1,2,3,4-tetracarboxylic acid (CP-TCA) and tetrahydrofuran-2,3,4,5-tetracarboxylic acid (THF-TCA), the latter binding Cu(II) much more strongly as a consequence of the ether linkage. The SRFA Cu(II) binding properties agreed with previous work at high ionic strength, and binding was enhanced substantially at lower ionic strength, in agreement with Poisson–Boltzmann predictions for small spheres. Determining Cu binding constants ( K i ) by non-linear regression with total ligand concentrations ( L T i ) taken from previous work, the fractions eluted at varying pH had K i similar to the unfractionated SRFA, with a maximum enhancement of 0.50 log units. We conclude that variable-pH elution from XAD does not isolate significantly strong (or weak) Cu(II)-binding components from the SRFA mixture.

Pyrolysis-mass spectrometry/pattern recognition on a well-characterized suite of humic samples

Abstract A suite of well-characterized humic and fulvic acids of freshwater, soil and plant origin was subjected to pyrolysis-mass spectrometry and the resulting data were analyzed by pattern recognition and factor analysis. A factor analysis plot of the data shows that the humic acids and fulvic acids can be segregated into two distinct classes. Carbohydrate and phenolic components are more pronounced in the pyrolysis products of the fulvic acids, and saturated and unsaturated hydrocarbons contribute more to the humic acid pyrolysis products. A second factor analysis plot shows a separation which appears to be based primarily on whether the samples are of aquatic or soil origin.

Comments on the literature of the humin fraction of humus

Abstract An extensive and critical review of the literature on the humin fraction of humus has demonstrated two patterns of usage of the term which are in conflict with its generally accepted definition. In the first instance, the word humin was widely used to refer to the alkali-soluble products of the hydrolysis of amino acids and/or sugars. In the second instance, what are apparently incorrect translations of German words such as Huminsubstanzen have caused papers describing the humic materials considered collectively to be indexed under the keyword humin in early volumes of Chemical Abstracts. Consequently, 66 of the 201 papers indexed under the keyword humin in Chemical Abstracts, from volume 1 through volume 77 (1972) do not refer to humin as it is usually defined.

Disaggregation and characterization of humin

Abstract Humin was extracted from a stream sediment and a peat using the methyl isobutyl ketone (MIBK) partitioning method. The humin samples were then disaggregated into their component ‘building blocks’ by an extension of the MIBK procedure that avoids degradative loss of any of the constituents of humin. This procedure allows all of the constituents of humin to be investigated separately, in contrast to the more common degradative methods for the study of humin that decompose one or more of the humin components during its investigation. Humin was separated into four fractions: (i) a bitumen fraction, (ii) a bound humic acid fraction; (iii) a bound-lipid fraction, and (iv) an insoluble residue. This paper presents a comparison of the bound humic acids from humins with the humic acids and fulvic acids isolated from the same sources by the methyl isobutyl ketone (MIBK) method. Elemental analyses and measurements of acidity indicate that the bound humic acid has characteristics that are generally intermediate between those of the corresponding humic and fulvic acids.