M.H.B. Hayes

Humic substances: Considerations of compositions, aspects of structure, and environmental influences

Humic substances (HS) are the major components of the mixture of materials that comprise soil organic matter, and these substances, which are by far the most abundant organic materials in the environment, are themselves complicated mixtures of biologically transformed organic debris. However, it is likely that many of the solubilization properties of the mixtures arise from the presence of nonhumic components that are intimately associated with the HS and that cannot be separated effectively from these components. Separation and fractionation techniques are improving, and most of the instruments needed to advance awareness of the composition and aspects of structures are now in place. There is a need, however, to review the operational definitions that now apply and to put in place a classification system that will take into account origins and some compositional characteristics. The broad based definition of humin is especially unsatisfactory. This umbrella term covers a mixture of materials that are insoluble in aqueous systems and that contain nonhumic components such as long chain hydrocarbons, esters, acids, and even relatively polar structures of microbial origin, such as polysaccharides and glomalin, that can be associated with the nonpolar moieties and with soil minerals, as well as plant components that are highly resistant to decomposition. Advances in the humic sciences in recent times have been impressive, and the questioning of ‘ingrained’ theories and concepts is opening vistas through which we are seeing new concepts of size, shape, and association; the remarkable developments in nuclear magnetic resonance are also allowing better interpretations of compositions and of aspects of structures. From these advances will emerge a more fundamental understanding of HS functions in such important roles as the stabilization of soil aggregates, the binding of anthropogenic organic chemicals, and the sequestration of C from atmospheric CO2. The latter function is especially important at this time because we need to know why some soils sequester more C than others and why the qualities (or composition and structure) of HS in some soils are different from those in others.

Interactions at the soil colloid-soil solution interface

About 20 years ago the emphasis in soil chemistry research switched from studies of problems related to scarcities of plant nutrients to those arising from soil pollutants. The new problems have come about because of the excessive uses of fertilizers, the inputs from farm and industrial wastes, the widespread applications of anthropogenie xenobiotic chemicals, and the deterioration of soil structure resulting from certain modern agriculture practises. The International Society of Soil Science (ISSS) recognized these problems and challenges. A provisional Working Group was set up in 1978 to focus attention on soil colloids with a view to understanding better the interactions wh ich take place at their surfaces. It was recognized that these interactions are fundamental to problems of soil fertility, as weIl as to those of soil pollution. After the group had received the official support of ISSS at its 12th International Congress in New Delhi in 1982 it set as its priority the assembling and evaluation of information, relevant to the soil and environmental sciences, concerning the composition and structure of soil colloids. Prior to that aseries of Position Papers were published in the Bulletin of the International Society of Soil Science (Vol. 61, 1981) outlining the state of knowledge about the composition and properties of soil colloids.

Soil colloids and their associations in aggregates

I: Soil Inorganic Colloids.- 1 Structure, Crystal Chemistry, and Origin of the Phyllosilicate Minerals Common in Soil Clays.- 2 Some Properties of Clays and of other Soil Colloids and their Influences on Soils.- 3 Some Properties of Soil and Synthetic Iron Oxides.- 4 Some Observations on the Formation and Transformation of Iron Oxides.- 5 Interaction of Stable and Metastable Monomeric Iron(III) Species with a Kaolinitic Soil Clay.- 6 The Use of Mossbauer Spectroscopy in the Study of Soil Colloidal Materials.- 7 Soil Manganese Oxides.- 8 Structures and Genesis of Allophanes and Imogolite and their Distribution in Non-Volcanic Soils.- 9 Organo-alumino Polymer Associations and their Significance in Soil and Environmental Sciences.- II: Soil Organic Colloids.- 10 Genesis, Isolation, Composition and Structures of Soil Humic Substances.- 11 Composition, Origins, Structures, and Reactivities of Soil Polysaccharides.- 12 Microorganisms, Enzymes and Soil Colloid Surfaces.- III: Water and Clays.- 13 Structure and Dynamics of Water at Clay Surfaces. Inferences from Neutron Scattering Studies.- 14 Behaviour and Microstructure of Clay Minerals.- IV: Soil Aggregates.- 15 Characterisation of the Sand, Silt, and Clay Fractions of some Mollisols.- 16 Interparticle Forces in Relation to the Stability of Soil Aggregates.- 17 Associations of Colloids in Soil Aggregates.- 18 Soil Aggregates - Formation and Stability.- V: Soil Conditioners and Soil Aggregates.- 19 Applications of Polymeric Substances as Physical Soil Conditioners.- 20 Applications of Soil Conditioners for Agriculture and Engineering.- 21 Erosion Control in the Tropics.- Author Index.

Lessons from the Terra Preta de Índios of the Amazon region for the utilisation of charcoal for soil amendment

Neste trabalho faz-se a divulgacao do potencial de carvoes e residuos orgânicos parcialmente carbonizados visando obter materiais que mimetizam a materia orgânica do solo das Terras Pretas de Indio da Amazonia, e que sirvam como condicionadores de solo e sequestrem carbono de forma recalcitrante e reativa. Pesquisas desenvolvidas por grupos brasileiros e estrangeiros tem contribuido para o entendimento do surgimento e utilizacao das Terras Pretas de Indio da Amazonia. Aqui sao divulgados resultados de estudos quimicos no sentido do desenvolvimento do conhecimento cientifico e tecnologico e de inovacao no aproveitamento de subprodutos orgânicos, principalmente de industrias de biocombustiveis, carvao vegetal metalurgico e outros, buscando imitar a excelente performance da chamada Terras Pretas de Indio da Amazonia.

Humic materials from an organic soil: A comparison of extractants and of properties of extracts

Five extraction procedures and thirteen extracting reagents, which included dipolar aprotic solvents, organic chelating agents, pyridine, ethylenediamine, sodium hydroxide, ion-exchange resins and two salts (sodium pyrophosphate and ethylenediamine hydrochloride), were used to extract humic materials from an organic soil. Extractabilities increased in the general order: salts < organic chelating agents < dipolar aprotic solvents < pyridine < ethylenediamine = sodium hydroxide, and the amounts of the soil organic matter extracted by the reagents in the series ranged from 13 to 63%. Gel chromatography techniques indicated that extracts in dipolar aprotic solvents were predominantly of intermediate and low molecular-weight values, and it is suggested that the more highly oxidised soil humic materials were extracted in these. The more efficient solvents extracted materials with a range (high—low) of molecular-weight values. Data from elemental analysis and from E.S.R. measurements indicated that ethylenediamine altered the chemical nature and the composition of extracts. Dipolar aprotics, by the same criteria, were found not to alter the humic extracts, and can be regarded as mild reagents for the extraction of a less representative (of the total) fraction of soil organic matter. Sodium hydroxide in solution, despite its oxidation effects, was the best of the reagents tested for isolating extracts which were representative of a wide range of soil humic substances.

Dissolved organic carbon losses from grazed grasslands under different management regimes.

Dissolved organic matter (DOM) is fundamental to many biogeochemical processes in soils and natural waters. Despite the large number of studies reporting on DOM losses from forest soils and in surface waters there is little published data on exports from managed grasslands. The objective of our study was to determine the extent of short-term exports of dissolved organic carbon (DOC) from managed grazed grasslands and to evaluate the influence of fertilizer management and drainage regime. DOC discharged from grazed grassland plots, with a range of management strategies. was determined over 2 months. Total export varied from 42 to 118 kgCha(-1), and was greater from some plots than literature estimates for annual losses from all catchment types. There was a significant (P = 0.048) positive correlation between DOC export and rates of nitrogen application for treatments with no artificial drainage. Increased dry matter production arising from increased fertilizer-N inputs is suggested as an important factor in this relationship. DOC export was significantly (P = 0.032) reduced by artificial drainage and adsorption of DOC to soil surfaces and the restriction of decomposition due to waterlogging are suggested as two possible explanations.

Pressurised pyrolysis of Miscanthus using a fixed bed reactor.

Miscanthus x giganteus was pyrolysed, in a fixed bed reactor in a constant flow of dinitrogen gas, at a rate of 13°C/min from ambient to 550°C, then held for 25 min at this temperature. The pressures employed ranged from atmospheric to 26 bar. The major compounds identified in the bio-oil were water, phenol, and phenol derivatives. The water contents impact on the usefulness of the bio-oil as a fuel. However, the phenols could provide useful platform chemicals and products. The properties of the char were determined using elemental analyses, surface area measurements using the Brunauer-Emmett-Teller equation, a calorimetric bomb, Scanning Electron Microscopy, and solid state (13)C NMR spectroscopy. The chars were highly carbonised, especially at the higher pressures, and provided thermally stable materials. Pressure impacted greatly on the surface area. Char formed at atmospheric pressure had a surface area of 162 m(2)/g, whereas that from the highest pressure applied was only 0.137 m(2)/g.

Chemistry and potential mutagenicity of humic substances in waters from different watersheds in Britain and Ireland

Humic substances are amorphous organic macromolecules responsible for the hue of natural waters. They are also known to be precursors of mutagens formed on chlorination prior to distribution of drinking water. In this study humic substances from the waters of primary streams, from major rivers, and from reservoirs were isolated and fractionated into humic acids (HA), fulvic acids (FA) and XAD-4 acids using columns of XAD-8 and of XAD-4 resins in tandem, and the fractions from the different sources were chlorinated and assayed for mutagenicity. CPMAS 13C NMR spectroscopy showed marked differences in compositions not only between HA, FA, and XAD-4 acids from the same water samples, but also between the same fractions from water samples from different watersheds. There were found to be strong similarities between the fractions from watersheds which had closely related soil types. Aromaticity was greatest in HAs, and lowest in XAD-4 acids, and carboxyl contents and aliphatic character were greatest in the XAD-4 acids. Carbon content decreased in the order HA > FA > XAD-4 acids, and amino acids and neutral sugars contents decreased in the order HA > XAD-4 > FA. Titration data complemented aspects of the NMR data, demonstrating that carboxyl content decreased in the order XAD-4 acids > FA > HA, and indicated that phenolic character was highest in HAs and lowest in the XAD-4 acids. All samples tested gave rise to bacterial mutagens on chlorination. Although the mutagenicities were of the same order of magnitude for the chlorinated humic samples from the different sources, the samples which showed the greatest number of revertant bacterial colonies were from the Thames and Trent, large rivers with humic materials from diverse environments, and relatively high in amino acid contents.

Kinetics of levulinic acid and furfural production from Miscanthus × giganteus

This study investigated the kinetics of acid hydrolysis of the cellulose and hemicellulose in Miscanthus to produce levulinic acid and furfural under mild temperature and high acid concentration. Experiments were carried out in an 8L-batch reactor with 9%-wt. biomass loading, acid concentrations between 0.10 and 0.53 M H2SO4, and at temperatures between 150 and 200°C. The concentrations of xylose, glucose, furfural, 5-hydroxymethylfurfural and levulinic acid were used in two mechanistic kinetic models for the prediction of the performance of ideal continuous reactors for the optimisation of levulinic acid and the concurrent production of furfural. A two-stage arrangement was found to maximise furfural in the first reactor (PFR - 185°C, 0.5M H2SO4, 27.3%-mol). A second stage leads to levulinic acid yields between 58% and 72%-mol at temperatures between 160 and 200°C.

Characterisation of the products from pyrolysis of residues after acid hydrolysis of Miscanthus

Platform chemicals such as furfural and hydroxymethylfurfural are major products formed during the acid hydrolysis of lignocellulosic biomass in second generation biorefining processes. Solid hydrolysis residues (HR) can amount to 50 wt.% of the starting biomass materials. Pyrolysis of the HRs gives rise to biochar, bio-liquids, and gases. Time and temperature were variables during the pyrolysis of HRs in a fixed bed tubular reactor, and both parameters have major influences on the amounts and properties of the products. Biochar, with potential for carbon sequestration and soil conditioning, composed about half of the HR pyrolysis product. The amounts (11-20 wt.%) and compositions (up to 77% of phenols in organic fraction) of the bio-liquids formed suggest that these have little value as fuels, but could be sources of phenols, and the gas can have application as a fuel.