Aggregation Of Humic Substances Research Articles

Humic Substances: From Supramolecular Aggregation to Fractal Conformation—Is There Time for a New Paradigm?

Natural organic matter, including humic substances (HS), comprises complex secondary structures with no defined covalent chemical bonds and stabilized by inter- and intra-molecular interactions, such as hydrogen bonding, Van der Waal’s forces, and pi-pi interactions. The latest view describes HS aggregates as a hydrogel-like structure comprised by a hydrophobic core of aromatic residues surrounded by polar and amphiphilic molecules akin a self-assembled soft material. A different view is based on the classification of this material as either mass or surface fractals. The former is intended as made by the clustering of macromolecules generating dendritic networks, while the latter have been modelled in terms of a solvent-impenetrable core surrounded by a layer of lyophilic material. This study reviews the evolution of the increasingly refined models that appeared in the literature, all capable to describing the physicochemical properties of HS. All the models are critically examined and revisited in terms of their ability to provide key information on the structural organization of HS. Understanding how the molecular association pathway influences aggregation of HS also provides a key acknowledgment of their role in the environment.

Revisit soil organic matter

<p indent="0mm">The accurate and comprehensive understanding of the molecular composition, source, and stabilization mechanism of soil organic matter (SOM) is vital for soil management and the preservation of organic carbon. Currently, a series of key scientific issues concerning the formation and stabilization of SOM is highly contentious. This review summarized the advances in the sources and molecular structure of SOM, and presented a systematic analysis of the different views, conflicts, and problems concerning soil humic substances in the past decades. The traditional view of soil science believes that SOMs are mainly composed of humic substances which are macromolecular compounds and are derived from plant and animal residues through a series of mineralization and humification processes. Soil enzymes and minerals, including phenol oxidase, laccase, peroxidase, iron oxide, manganese oxide, promote the polymerization of organic molecules through additive nucleophilic reactions and free radical catalyzation. ¹³C-NMR and FT-ICR-MS analyses supported the existence of humic substances in soils based on the structure and the molecular composition of SOM. However, with the application of biomarker analysis, high-resolution mass spectrometry, spatial resolution spectroscopy, and other advanced techniques, there is an emergence of new perspectives on the source and the structure of SOM. HPLC-SEC showed the average molecular weight of Aldrich humic acid was significantly smaller than previously thought and the aggregation of humic substances was affected by acetic acid, pH and ionic strength. These evidences suggest that humic substances are aggregates of small organic compounds with a molecular weight of hundreds to thousands of Daltons, rather than macromolecules of tens to hundreds of thousands of Daltons. Furthermore, <italic>in situ</italic> observation of soil particles by STXM revealed significant heterogeneity of SOM at <sc>50 nm</sc> spatial resolution. In terms of the origin of SOM, model calculations and biomarker analysis have indicated that microbial necromass is the major constituent of SOM which accounts for ~33%–62% of SOM in different ecosystems. Collectively, microbial residues may contribute predominantly to the SOM pool and traditional concept “humification” may not occur in the soil. These new findings are contrary to the traditional theory of soil chemistry and soil biochemistry and subvert our understanding of SOM. Therefore, it is imperative to perform further in-depth studies to fill the knowledge gaps regarding the sources, formation processes, and molecular structures of humus. However, further intensive investigations on SOM are restricted by the lack of efficient and reliable extraction procedures. In addition, the conversion coefficients from biomarker to necromass are not accurate and mass spectrometry could not provide a comprehensive analysis on the whole organic molecules in the soil. The studies on enzymatic reactions and microbial synthesis were mainly performed in simple suspension systems, which could not simulate the complex soil matrix. Consequently, future studies are needed to improve the efficiency and accuracy of the extraction and analysis of SOM and to develop a real-time study platform for the interfacial reactions among minerals, organic matters and microorganisms. Meanwhile, <italic>in situ</italic> characterization techniques and modeling should be employed and combined to clarify the sources and the underlying mechanisms of SOM stabilization at molecular level.

The research of the influence of heavy metal ions on the aggregation of humic substances on the mica by atomic force microscopy

The conformation of molecules of humic substances of peats sorbed on hydrophilic mica in the absence and presence of heavy metal ions was studied by atomic-force microscopy in semi-contact mode in an aqueous medium. A model of the lignin-carbon complex is proposed, which ensures the conformation of the three-dimensional ring structure of humic substances depending on pH and allows accurate description of the surface structure of humic substances. The sizes of single globular molecules of humic substances were determined: 140-320 nm in diameter and 5-25 nm in height. Possible ways of the interaction of humic substances of peats with metal cations (Pb 2 + and Zn 2 +) with account of conformational changes of humic substances have been established. It has been proven that lead cations are sorbed mainly on the surface of humic molecules, and sorption of zinc cations proceeds with reorientation of humic substances around zinc ions.

Abiotic reversible self-assembly of fulvic and humic acid aggregates in low electrolytic conductivity solutions by dynamic light scattering and zeta potential investigation

The aggregation of humic substances and their interaction with filtration media (membranes, soils) has implications for our understanding of membrane fouling during water treatment, the facilitated transport of contaminants, and the transport of organic matter through the microbial loop. To investigate the aggregation of fulvic and humic acids in low electrolytic conductivity solutions, laboratory studies of simulated environmental water samples as well as actual environmental water samples were examined. Intensity-, volume-, and number-based particle size distributions (PSDs) were obtained by dynamic light scattering. Aggregates were categorized into three ranges, i.e., 10–100nm, 100–1000nm, and >1μm. Individual biomacromolecules and the aggregates between 10nm and 1μm were presumed to be precursors for the formation of a large 5-μm-sized-particle. The self-assembly of the large-in-volume, few-in-number, 5-μm-sized particle was observed in real-time and occurred in unfiltered samples and in samples filtered (0.45μm) at a nominal size one order of magnitude smaller. The supramicrometer-sized particle formed, dissipated, and spontaneously re-formed over turbulent/quiescent cycles in the presence of sodium azide indicating reversible abiotic self-assembly. Zeta potential analyses demonstrated that colloidal stability increased as concentration increased. DLS studies of the environmental water samples were comparable to those of the simulated laboratory samples. The operational range of the instrumentation used in these experiments was 0.6nm–6μm; therefore, aggregates larger than 6μm may exist in these solutions.

Molecular-size-distribution-dependent aggregation of humic substances by Na(I), Ag(I), Ca(II), and Eu(III)

Molecular-size-distribution (MSD)-dependent aggregation of Fluka and International Humic Substance Society humic acids (HAs) in the presence of Na(I), Ca(II), Ag(I), and Eu(III) were studied using high-performance size-exclusion chromatography. Larger molecular size fractions (>8000Da) rich in aliphatic carbons were observed to aggregate in the presence of Na(I) and Ca(II), whereas both large and small molecular size fractions concurrently aggregated in the presence of Ag(I) and Eu(III). In view of the non-specific binding of Na(I) and Ca(II) with HAs, the electric-field screening effect is probably the main cause of HA aggregation by Na(I) and Ca(II), preserving the intrinsic hydrophobicity of the HAs. Reduced surface charges and the hydrophobic heterogeneities of different MSD fractions of HAs by direct binding of Ag(I) and Eu(III) are postulated to induce insignificant dependence of aggregation on the different MSD fractions.

Effects of electrolyte and nonelectrolyte additives on adsorption and surface rheological characteristics of humic acid salt solutions

The du Nouy and oscillating droplet shape methods are employed to study the effects of the ionic strength and pH of a medium, as well as the addition of nonelectrolytes (lower alcohols and acetone), on the adsorption and surface rheological characteristics of aqueous solutions of humic acid salts (sodium humates) at the liquid-air interface. When added in concentrations at which the aggregation of humic substances is not yet observed, strong electrolytes (NaCl and HCl) decrease the equilibrium surface tension and increase the dilatational viscoelastic modulus of aqueous sodium humate solutions. The aggregation of humic substances enhances the surface tension and reduces the viscoelastic modulus of surface layers. Nonelectrolyte additives decrease the surface tension and dilatational modulus of aqueous humic acid salt solutions. The equilibrium surface tension of sodium humate-nonelectrolyte mixed solutions is described in terms of two different models, namely, a relatively exact model of polyelectrolyte-nonionic surfactant adsorption and a simple additive model. It is shown that the additive model may be used to predict the equilibrium surface tension for the mixtures of high- and low-molecular-mass surfactants.

The effect of ozonation on aggregation of humic substances on mica studied by atomic force microscopy

Humic substances influence adsorption and aggregation of colloids and ions in natural aquatic environment and thereby affect drinking water treatment by ozonation. In this paper the effect of ozonation on the ability of humic substances to form supramolecular structures was investigated by atomic force microscopy (AFM) surface topography. Ozonated and unozonated solutions of Suwannee River humic acid and fulvic acid were deposited onto molecularly smooth mica surfaces, left to dry and then imaged by AFM in non-contact mode. Two solutions with pH 4 and pH 7 were investigated. At each pH, significant differences in the aggregation behavior between these humic substances before and after ozonation were observed. In particular, ozonation increased the aggregate size of fulvic acid at the neutral pH. Size exclusion chromatography also revealed similar molecular weight distributions for the humic substances after ozonation. It was argued that difference in the molecular hydrophobicity or factors other than molecular weight could be responsible for the observed aggregation behavior. The outcome of this study is important for a better understanding of supramolecular structures of humic substances in ozonation, particularly those in drinking water treatment.

Aggregation of humic substances by metal ions measured by ultracentrifugation

UV scanning analytical ultracentrifugation has been used to measure the aggregation of dissolved humic substances by metal ions. An adaptation of the Archibald approach to equilibrium technique has been used to monitor the change in average molecular weight, both with time, and also with mass of sample. The results have been analysed to give molecular weight distributions in the aggregated systems. For each metal studied, it was found that there was a threshold concentration below which no aggregation was observed, the magnitude of that concentration depending upon the metal. The range of concentrations from the onset of aggregation to the precipitation of the humic also depends upon the metal. It was found that the metal ions promoted aggregation in the order: La 3+>Cu 2+>Cd 2+>Ag +. Tl 4+, however, were found not to induce aggregation. In addition, the effects of ageing time, solution pH and ionic strength have been studied. It was also found that many of the aggregated humic populations were stable in solution for periods of weeks, and that the aggregation threshold concentration was independent of solution ionic strength. It was found that the aggregation was not the result of simple double layer compression as predicted by the Schulze-Hardy rule. Rather, the metal ions were reducing the intermolecular repulsion by reducing the humic charge.

Aggregation of humic substances in aqueous media as determined by light‐scattering methods

SUMMARYSamples of peat humic acid (PHA) and surface water humic (WBHA) and fulvic acids (WBFA) extracted from Whitray Beck in North Yorkshire, UK and previously studied by analytical (UV scanning) ultracentrifugation at low concentrations (10–20 mg dm−3) (Reid et al., 1990) have been investigated by dynamic and static light‐scattering methods at concentrations up to approximately 1700 mg dm−3. Dynamic light‐scattering gave very low diffusion coefficients for the humic acids (PHA and WBHA) suggesting the presence of aggregates. Static light‐scattering from the fulvic acid (WBFA) showed that the second virial coefficients of this material in various media were negative, which is characteristic of reversible aggregation. The aggregates, which consisted of approximately 20 smaller molecules, could not be dissociated completely by the detergents sodium n‐dodecylsulphate or Triton X‐100, although limited dissociation by the latter may occur. Sedimentation velocity measurements in the same concentration range as the light scattering gave sedimentation coefficients which increased with concentration also consistent with reversible aggregation.The studies illustrate the problems inherent in the characterization of humic substances from natural sources and demonstrate that, at least for lower molecular weight fulvic acids, static light‐scattering from solutions of relatively high concentration in combination with sedimentation equilibrium measurements at low concentrations distinguishes a system having a broad molecular weight distribution from one in which aggregation occurs.