M. Schnitzer

Macromolecular structures of humic substances.

We investigated humic substances by surface pressure and viscosity measurements at different pHs and neutral salt concentrations to elucidate their macromolecular configurations. We observed that such configurations were not unique; they varied with the changes in the medium. The controlling parameters were sample concentration, pH of the system, and the ionic strength of the medium. Model macromolecular structures are proposed on the basis of our investigation and of data published in the literature. With the aid of these models, we were able to resolve the contradictions between the so-called rigid “spherocolloid” and flexible “linear molecule” concepts. Our work shows that humic and fulvic acids behave like rigid spherocolloids at high sample concentrations, low pH, or in the presence of sufficient amounts of neutral electrolytes, but they are flexible linear colloids at low sample concentrations, provided that the pH is not too low or that the ionic strength is relatively low, conditions that normally prevail in soils.

Soil organic matter

Soil organic matter includes plant and animal residues at various stages of decomposition, ranging from fresh undecomposed materials through partially decomposed and short-lived products of decomposition to well-decomposed humus. Plant and animal residues contain sugars, celluloses, hemicelluloses, proteins, lignins, waxes, and lipids. When these materials are incorporated into soils, sugars, celluloses, and hemicelluloses, most proteins undergo rapid decomposition. With decomposition, the proportion of resistant compounds such as lignin gradually increases. Resistant decomposition products and newly synthesized substances accumulate in soil and form complexes with its mineral components. Humus is formed in the latter stage as a result of such continued decomposition, resynthesis, and complexation. Humus is a fairly stable substance and requires very long time, for example, centuries to millennia, to be completely decomposed. Most mineral soils contain less than 5 % by weight of soil organic matter (SOM), but this little quantity of SOM has tremendous biological significance. It provides food for microorganisms, stores nutrients, retains water, acts as mulch, and performs as a soil conditioner and aggregating agent. It makes the soil friable and fertile. Soil organic matter contributes significantly to the sequestration of carbon.

Sorption of metals on humic acid

The sorption on humic acid (HA) of metals from an aqueous solution containing Hg(II). Fe(III), Pb, Cu, Al, Ni, Cr(III), Cd, Zn, Co and Mn, was investigated with special emphasis on effects of pH, metal concentration and HA concentration. The sorption efficiency tended to increase with rise in pH, decrease in metal concentration and increase in HA concentration of the equilibrating solution. At pH 2.4. the order of sorption was: Hg⪢ Fe⪢ Pb⪢ CuAl ⪢ Ni ⪢ CrZnCdCoMn. At pH 3.7. the order was: Hg and Fe were always most readily removed, while Co and Mn were sorbed least readily. There were indications of competition for active sites (CO2H and phenolic OH groups) on the HA between the different metals. We were unable to find correlations between the affinities of the eleven metals to sorb on HA and their atomic weights, atomic numbers, valencies, and crystal and hydrated ionic radii. The sorption of the eleven metals on the HA could be described by the equation Y = 100[1 + exp − (A + BX)], where Y = % metal removed by HA; X = mgHA; and A and B are empirical constants.

Soil organic matter: the next 75 years

Part I of this paper presents an account of what we know at this time about SOM (soil organic matter). It deals with the distribution and functions in soils of SOM, descriptions of methods of extraction and fractionation, and chemistry of the major components. These include carbohydrates, nitrogenous compounds, organic P and S compounds, and last but not least, humic substances. The latter are discussed in greater detail in terms of analytical characteristics, chemical structure, free radical content, colloid-chemical characteristics, physiological effects, uses, and interactions with metals, clay minerals, and organics (especially pesticides). The second part of this paper focuses on the laboratory of the future and the agenda for the next 75 years. There will be major changes in laboratory procedures from wet chemical to sophisticated instrumental methods, using solid-state samples. Gaps in our knowledge of major SOM components and their interactions with metals, clays, and organics will be filled. Because SOM serves as soil conditioner, nutrient source, substrate for microbial activity, preserver of the environment, and major determinant for sustaining or increasing agricultural productivity, SOM chemists are bound to play an important role in the soil science of the future.

Chemical Model Structures for Soil Organic Matter and Soils

A hypothesis for the three-dimensional structure of soil organic matter (SOM) and whole soil, based on an improved humic acid (HA) model that resulted from comprehensive investigations combining geochemical, colloid-chemical, electron-microscopic, IR, 13C-NMR, and X-ray spectroscopic as well as agri

Analysis of carboxyl groups in soil humic acids by a wet chemical method, Fourier-Transform Infrared spectrophotometry, and solution-state carbon-13 nuclear magnetic resonance. A comparative study

Carboxyls are important functional groups that affect polarity and reactivity in humic acids (HAs). Carboxyls were analyzed in eight soil HAs by three methods based on widely differing principles: (i) wet chemical analysis, (ii) Fourier-Transform Infrared spectrophotometry (FT-IR), and (iii) liquid-state 13 C Nuclear Magnetic Resonance (13C NMR). The objective was to uncover the suitability of each of these methods for the analysis of COOH groups in HAs and the extent to which the three methods agreed with each other in quantitative measurements of COOH groups in HAs. In regard to reaction mechanisms, the chemical Ca-acetate method is based on ion-exchange of H of COOH for Ca of Ca-acetate. From FT-IR spectra of HAs, COOH groups were determined by totaling absorbances at 1720-1710 cm−1 (COOH) and 1620-1600 cm−1 (COO−), whereas from 13C NMR spectra of HAs, COOH groups were computed by integration of 175-185 ppm area. Good correlations were found between the three methods although the COOH values computed by 13C NMR were higher, as a result of the inclusion of small amounts of esters, amides, and lactones, than those obtained by the other two methods. Thus, depending on the equipment and facilities available, soil scientists have a choice of methods that can be used for determining COOH groups in HAs.

The retention of hydrophobic organic compounds by humic acid

Abstract A humic acid, extracted from the Ah horizon of a Black Chernozem soil was found to fix or strongly retain significant amounts of hydrophobic organic compounds. Only approximately 8 per cent of the total alkanes and fatty acids occurring in the humic acid could be removed by exhaustive extraction with organic solvents. Another 19 and 11 per cent of the two classes of compounds could be removed by organic solvents after methylation of the humic acid, but extraction of the bulk of these compounds required extensive chromatographic separations of the methylated humic acid. Alkanes and fatty acids were estimated to account for up to 0.34 and 0.47 per cent of the weight of the original humic acid, respectively. Other hydrophobic organic compounds extracted were dialkyl phthalates, some of which are toxic, and di-sec-butyl adipate, which were estimated to account for about 1 per cent of the initial weight of the humic acid. The organic esters were shown not to be artifacts but are considered to be of biosynthetic origin. The data reported herein show that 100 g of humic acid can firmly retain up to 2 g, and possibly more, of hydrophobic organic compounds by a mechanism that most likely involves adsorption on external surfaces and in internal voids of a molecular sieve-type structural arrangement. The adsorption behaviour of humic substances for organic compounds, some of which may be toxic pollutants, should be of interest to those concerned with environmental and geochemical problems.

STRUCTURAL STUDIES ON SOIL HUMIC ACIDS BY CURIE-POINT PYROLYSIS-GAS CHROMATOGRAPHY/MASS SPECTROMETRY

Humic acids (HAs) extracted from the Ap horizon of a Haplaquoll (Bainsville) and the Bh horizon of a Haplaquod (Armadale) were investigated by Curie-point pyrolysis (Cp Py)-gas chromatography (GC)/mass spectrometry (MS). More than 300 pyrolysis products, tentatively identified by conventional electron impact (EI) and field ionization (FI) MS, were common to both samples, although very different in their intensity distributions. Whereas the Armadale HA produced predominantly alkanes and olefins, the Bainsville HA yielded large quantities of phenol derivatives. Alkanes and olefins formed homologous series up to C30 and n-alkylbenzenes up to C18. Of particular interest were the intense signals originating from aromatics and alkyl-substituted aromatics, especially phenol derivatives and methyl-substituted naphthalenes. In addition, heterocyclic compounds such as furans, pyrroles, and pyridines with and without alkyl substituents were also detected. Our data indicate the presence of aryl-alkyl structures in the HA network and contribute to a better understanding of the chemical composition and structure of soil organic matter (SOM).

The nitrogen distribution in soils formed under widely differing climatic conditions

Abstract Amino acids, amino sugars and ammonia were determined in 6N HCl hydrolyzates of soils formed under arctic, cool temperate, subtropical and tropical climates. Soils from warmer climates yielded relatively more amino acids and amino sugars but less NH3 than did those from colder regions. The amino acid composition of all soils, however, was remarkably similar, resembled that of lake sediments and was somewhat similar to that of bacteria, suggesting the importance of microorganisms in their formation. Climate appears to have little effect on the amino acid composition of soils.

Alkanes and fatty acids in humic substances

Abstract Straight-chain fatty acids are shown to occur in natural humic acids and fulvic acids to a substantially greater extent than has previously been estimated. The amounts chemically bonded to the substrate usually exceed those physically adsorbed. Quantities of individual fatty acids and phenolic acids in typical samples have been estimated.