Humic Acid Aggregates Research Articles

Destabilization and aggregation of aqueous humic acids solution by metal ions

The aggregation of soil humic acid (HA) after addition of cations was investigated by nephelometric measurements of HA solution turbidity. Here we show that aggregation is promoted by the presence of positive ions in the solution and that the order of increasing effectiveness in turbidity follows Schulze-Hardy rule. Also, the obtained results indicate that the cation with the largest ionic radius within a common valence is the most effective coagulant. Influence of unions follows the sequence: chloride < nitrate < sulphate, but it is insignificant comparing the influence of cation. This confirms that HA molecules are of negative charge.

Characterization of Soil Amended with the By‐Product of Corn Stover Fermentation

Corn (Zea mays L.) stover is a potential biofuel; however, removing this stover from the land may increase the risk of erosion and reduce soil organic matter (SOM). Land application of corn stover fermentation by‐product, which is about 70% lignin, may reduce the environmental risk from biofuel harvest by helping to stabilize soil structure. A column study, with soil collected from a toeslope (noneroded, Svea; fine loamy, mixed, superactive, frigid Pachic Hapludoll) and a shoulder slope (severely eroded, Langhei; fine loamy, mixed, superactive, frigid Typic Eutrudepts) was conducted to evaluate the effect of fermentation by‐product on soil properties. Soil was either not amended (control) or amended with corn stover or by‐product at 0.75, 3.0, and 6.1 g kg−1 Soils were incubated for 123 d at ambient temperature in a laboratory, with an initial water‐filled pore space (WFPS) of 0.6 m3 m−3 and drying cycles to 0.35 m3 m−3 WFPS. Compared with the control, amending soil with 6.1 g by‐product kg−1 increased CO2 flux by 68% and increased soluble C and microbial biomass C by about 20%. In the severely eroded soil, humic acid concentration (r2 = 0.97, p = 0.009) and aggregate stability (r2 = 0.98, p = 0.005) increased linearly with increased by‐product concentration. Water‐holding capacity, bulk density, and aggregate distribution were not changed by soil amendments. Careful management of stover removal (avoiding eroded or erosion prone areas) and selective placement and rates of the by‐product will contribute to a sustainable use of corn stover for ethanol production.

Effects of ionic strength on the binding of phenanthrene and pyrene to humic substances: three-stage variation model

This study compared the effects of ionic strength on the binding constants ( K doc) of selected polycyclic aromatic hydrocarbons (PAHs) (phenanthrene and pyrene) and a terrestrial humic acid (Leonardite Humic Acid) in different electrolyte solutions (KCl, KBr, MgCl 2 and MgSO 4). Distinct trends were found in K doc variation depending upon the range of ionic strength resulting from added electrolytes. These trends demonstrated similar shapes for all the systems studied, while degree of variation increased with hydrophobicity of the PAHs. Furthermore, different types of electrolytes had different effects on the interactions between humic acid (HA) and the PAHs. These differences were primarily caused by types of cation, not anion. To describe the complicated effects of ionic strength on K doc, we developed a three-stage variation model that encompasses increasing and decreasing trends and plateaus in K doc associated with ionic strength, as well as the mechanisms behind these trends, including the variation of HA structure configuration, HA aggregation and the salting-out effect. This model illustrated the importance of sufficient experimental data when interpreting the influence of ionic strength on the trends in K doc variation.

Soft X-ray spectro-microscopy investigation of the formation and ageing of Eu(III)-induced humic acid aggregates

Aggregation of Aldrich humic acid at pH 4.3 and after loading with Eu(III)-cations at pH 5.3 is investigated by scanning transmission X-ray microscopy (STXM). C ls-NEXAFS spectra extracted from STXM image stacks for the low pH aggregates show no significant variation, whereas Eu(III)-humate formation is observed to be accompanied by segregation into zones with different optical densities. These zones exhibit distinctly different NEXAFS features, indicative of the presence of a separation of organic species or components. Increasing humate aggregate densification over a period of several months is observed. The spectrum of the unloaded HA cannot be reproduced by a superposition of spectra extracted from the segregated humate zones; there is a distinct metal bonding effect on the molecular states of the functional groups involved in complexation.

Trichloroethylene adsorption by activated carbon preloaded with humic substances: effects of solution chemistry

Trichloroethylene (TCE) adsorption by activated carbon previously loaded (“preloaded”) with humic substances was found to decrease with increasing concentrations of monovalent ions (NaCl), calcium (until solubility was exceeded), or dissolved oxygen in the preloading solution. For a given percentage of organic carbon removal during humic acid loading, greater reductions in TCE adsorption occurred with increasing monovalent ion concentration and calcium concentration at constant ionic strength. However, this effect was related primarily to the amount of humic material adsorbed—the reduction in TCE adsorption was independent of the ionic composition of the preloading solution when compared at similar humic acid loading. Experiments were performed which showed that calcium ions can associate with humic material after the humic has been adsorbed, which subsequently reduces TCE uptake, but this effect does not dominate when calcium is present during humic loading. At sufficiently high calcium concentrations (approaching solubility), aggregation or co-precipitation of humic acid mitigated the effects of preloading. In contrast to the effects of ionic composition, the presence of dissolved oxygen did fundamentally change the mechanism by which organic macromolecules compete with TCE. TCE uptake was lower when preloading by poly(maleic acid) (PMA) occurred in the presence of dissolved oxygen, even when the amount loaded was the same. One explanation invokes a coupling mechanism promoted by the carbon surface, which results in either additional blockage of TCE sorption sites, additional site competition, or both. In all experiments, the effects of preloading were consistent with those reported previously, which have been interpreted as a loss of high-energy sites available to TCE, causing a significant reduction in the site-energy heterogeneity, and reduced uptake in the low concentration region.

Analysis of humic acid fouling during microfiltration using a pore blockage–cake filtration model

Fouling by natural organic matter, such as humic substances, is a major factor limiting the use of microfiltration for water purification. The objective of this study was to develop a fundamental understanding of the underlying mechanisms governing humic acid fouling during microfiltration using a combined pore blockage–cake filtration model. Data were obtained over a range of humic acid concentrations, transmembrane pressures, and stirring speeds. The initial flux decline was due to pore blockage caused by the deposition of large humic acid aggregates on the membrane surface, with a humic acid deposit developing over those regions of the membrane that have first been blocked by an aggregate. The rate of cake growth approaches zero at a finite filtrate flux, similar to the critical flux concept developed for colloidal filtration. The data were in good agreement with model calculations, with the parameter values providing important insights into the mechanisms governing humic acid fouling during microfiltration. In addition, the basic approach provides a framework that can be used to analyze humic acid fouling under different conditions.

Ultrafiltration of natural organic matter

Abstract Increasingly stringent regulations for drinking water quality have stimulated the application of ultrafiltration to water treatment. In addition to removing particulate materials from water (including microorganisms, bacteria and viruses), the use of membrane treatment also meets purification requirements. However, irreversible fouling curtails the economic viability of such a process. Experiments in stirred-cells were conducted to evaluate the effects of surface water composition on rejection and fouling of two ultrafiltration membranes with different molecular weight cut-offs (10 and 100 kDa). Experimental solutions consisted of natural organic matter or humic substances in a background electrolyte. The effect of calcium concentration decreased rejection of humic acid under certain circumstances. This is believed due to reduced molecular size with an initial increase in calcium concentration. However, at about 2.5 mM CaCl2, IHSS humic acid aggregates. This aggregation increased rejection, and also caused irreversible fouling of the 100 kDa membrane, presumably as a result of pore size reduction due to internal deposition of aggregates. This was confirmed by blocking law analysis. The variation of transmembrane pressure indicated the importance of a ‘critical flux’ effect. The organics and their various fractions showed differences both in rejection and flux decline. The larger and more UV-absorbing fraction of humic acid was shown to be responsible for irreversible pore adsorption and plugging. The fulvic acid and the hydrophilic fraction showed a smaller and mostly reversible flux decline.

Soft X-Ray Spectromicroscopy Investigation of the Interaction of Aquatic Humic Acid and Clay Colloids

Soft X-ray spectromicroscopy investigations of the interaction of aquatic humic acid (HA) and montmorillonite colloids have been performed in situ at the NSLS X1-A STXM endstation. Images have been recorded of montmorillonite particles, HA aggregates, and mixed suspensions of both montmorillonite + HA and montmorillonite + carboxyl polystyrene microspheres, as reference organic colloids. Special emphasis has been placed on the sample preparation technique in order to keep the colloid particles hydrated during all measurements. C 1s near edge absorption fine structure extracted from STXM image stacks reveals electronic transitions corresponding to functional -COOH and -C6H5 groups present in HA and polystyrene. XANES peak intensities reflect the relative amounts of these two carbon functional groups in the organic particles. For example, the greater amount of carboxyl groups in HA compared to the latex reference particles affects a larger 1s→π* transition intensity. A specific form of montmorillonite–HA particle agglomeration at near-neutral pH has been observed. Under these conditions, we found no separate clustering of HA. Instead, STXM images show the HA to coat the clay mineral surface, leading to nearly a fractal manner of aggregation.

Humic Acid Fouling during Ultrafiltration

Recent studies have shown that natural organic matter (e.g., humic and fulvic acids) is a major foulant during ultrafiltration of surface water. The objective of this study was to develop a more complete understanding of the mechanisms governing humic acid fouling, including the effects of humic acid adsorption, concentration polarization, and aggregate deposition on the rate and extent of fouling. Data were obtained with Aldrich and Suwannee River humic acids using ultrafiltration membranes with a broad range of molecular weight cutoffs. Fouled membranes were also examined using streaming potential and contact angle measurements. The extent of flux decline was greatest for the largest molecular weight cutoff membranes due to the greater relative hydraulic resistance of the humic acid deposit formed on the surface of these membranes. This humic acid deposit reduced the apparent zeta potential and increased the membrane contact angle. Simple static adsorption and concentration polarization caused relatively little flux decline. Humic acid aggregates had a significant effect on fouling only for the larger molecular weight cutoff membranes. The rate and extent of humic acid fouling increased at low pH, high ionic strength, and in the presence of calcium due to changes in intermolecular electrostatic interactions. These results provide important insights into the mechanisms of humic acid fouling during ultrafiltration.

Effects of solution environment on humic acid fouling during microfiltration

Microfiltration is increasingly used in drinking water treatment for the removal of particles, turbidity, and microorganisms to meet more stringent water quality requirements. Fouling by natural organic matter (e.g., humic and fulvic acids) is a critical factor in microfiltration processes. Experiments were performed to evaluate the effects of solution pH, salt concentration, and ionic composition on the extent and mechanisms of humic acid fouling during microfiltration. Fouling was more severe at high salt concentration due to the tighter packing of the charged humic acids within the fouling layer caused by the increased electrostatic shielding. The effects of solution pH were more complex and likely reflect alterations in humic acid charge, hydrophobicity, and extent of aggregation. Divalent cations such as calcium significantly increased humic acid fouling and aggregation through the formation of intra- or intermolecular bridges between the negatively charged humic acid molecules. These results provide important insights into the physical and chemical interactions governing humic acid fouling during microfiltration.

Facilitated Ion-Transfer of a Hydrophobic Tetraalkylammonium Ion in Nitrobenzene to Aqueous Phase by Humic Acid

Abstract Ion transfer at an interface of organic nitrobenzene and aqueous phases was polarographically studied in the presence of humic acid using an ascending aqueous electrode. The transfer of hydrophobic tetraheptylammonium cation as a supporting electrolyte ion in nitrobenzene to the aqueous phase was greatly facilitated by the incorporation into the micelle-like humic acid aggregates formed around CMC of humic acid.

Humic acid fouling during microfiltration

A major factor limiting the use of microfiltration for surface water treatment is membrane fouling by natural organic matter. The extent and mechanisms of humic acid fouling during microfiltration have been examined using stirred cell filtration experiments and scanning electron microscopy. The extent of fouling was strongly dependent on both the source and preparation of the humic acid solutions. The large flux decline observed during constant pressure microfiltration was caused by the formation of a humic acid deposit located on the upper surface of the membrane. Prefiltration of the humic acid solutions dramatically reduced the rate of fouling through the removal of large humic acid aggregates. The initial fouling in this system was determined almost entirely by the convective deposition of these large particles/aggregates on the membrane surface. This initial deposit accelerated the subsequent rate of humic acid fouling, possibly serving as a nucleation site for deposition of macromolecular humic acids.

Applications of light and X-ray scattering to characterize the fractal properties of soil organic matter

Soil organic matter (SOM) is a heterogeneous assemblage of organic molecules that interact in a variety of ways with each other, with soil mineral surfaces, and with soil mineral colloids. Because of SOM's heterogeneity it is very difficult to define its surface, or the surfaces of the composite materials produced by its interaction with soil minerals. Yet it is at these interfaces where chemical reactions that involve SOM take place. Results are presented which describe the fractal characterization of humic materials using static X-ray and light scattering, and dynamic light scattering (DLS) experiments. The applicability of static X-ray scattering to the direct determination of the fractal dimension of humic materials is established using DLS. Over the length scales studied, humic materials are surface fractals in the solid state, and mass fractals in solution. The longer characterization length scales possible in the static light scattering experiments suggest that at longer characterization length scales humic acid is not fractal, at least not under the solution conditions employed in these experiments. Application of fractal analysis to the characterization of the surface morphology of soil and peat humin samples, the calculation of the hydrodynamic radius of humic acid particles, and the study of humic acid aggregation are discussed.

Fractal dimension of humic acids

Small-angle neutron scattering experiments have been made on solutions of humic acid aggregates with an acidity corresponding to pH 5.0 and at 0.1 M ionic strength. We observe power-law decay of the intensity over one decade of the scattering vector, Q, indicating that the aggregates are fractal. We explain the normalized intensity in the entire Q-range by assuming that the humic acid particles can be described by building units of a radial size, ≤ 25 A, aggregated into clusters with an average radius of 400–500 A. For humic acids obtained from two different sources, we determine the fractal dimension, D = 2.3 ± 0.1. For small values of Q, the measured data of one of the samples extend into the Guinier range giving an average radius of gyration of 320 ± 20 A.

Aggregate formation of humic acids from marine sediments

Humic acids were extracted from marine sediments and the aggregate-forming properties were studied by surface tension measurements and the solubilization of hydrocarbons. The surface tension of aqueous humic acid solutions decreases with the humic acid concentration and becomes constant; the relationship between the surface tension and the logarithm of the concentration of humic acid is represented by two straight lines. The break point is considered to be due to aggregate formation. The solubility of benzo(a)pyrene and icosane in humic acid solution increases with humic acid concentration. A marked solubility increase observed at a humic acid concentration near 0.1% indicates that the hydrocarbon is solubilized in humic acid aggregates. The concentration of aggregation can be determined from the point of intersection of two straight lines representing the relationship between solubility of hydrocarbons and log concentration of humic acid. Concentrations of aggregate formation for various humic acids determined by surface tension measurements and by the solubilization of hydrocarbons are in good agreement.