Analysis of the variable charge of two organic soils by means of the NICA-Donnan model

We have tested to see if the generic set of NICA‐Donnan model parameters, used to describe isolated humic substances, can also describe soil humic substances in situ. A potentiometric back‐titration technique was used to determine the variable surface charge of two organic peat soils at three different ionic strengths. The non‐ideal, competitive‐adsorption NICA‐Donnan model was used to simulate the surface charge, by assuming a bimodal distribution of H⁺ affinity on the soil solid phase. The model provided an excellent fit to the experimental data. The Donnan volume, V_D, varied slightly with ionic strength, although the variation was less than for humic substances in solution. The values obtained for the parameters that define the affinity distributions, the intrinsic proton binding constant (log K_i^int) and the heterogeneity of the site (m_i), were similar to those observed for isolated soil humic acids. The abundance of carboxylic groups in the whole soil represented 30% of the typical value for isolated soil humic acids. The composition of the organic matter of the whole soils, obtained by ¹³C CPMAS NMR, was comparable to the characteristic composition of soil humic acids.     

Measurement of the potentially available charge and the dissociation behaviour of humic acid from cobaltihexammine adsorption

The use of a new method for characterizing humic acids based on the measurement of the cobaltihexammine cation exchange capacity (CEC) vs. pH is examined. The method was first verified on a macroporous weak-acid cation exchange resin: charge vs. pH curves and pK values from cobaltihexammine adsorption identify with results from discontinuous acid-base titrations at high ionic strength (5 M). Similar agreement was obtained for humic acids from a Podzol soil: the cobaltihexammine-CEC identifies with the macromolecular charge (from continuous acid-base titrations) at ionic strength of 3 and 5 M, but slightly underestimates (5%) the humic acid charge at 1 M ionic strength. The cobaltihexammine method is suitable for determining the potentially available charge in the pH range 2 to 12 and the intrinsic pK values of humic acids.     

Effect of soil depth on acid properties of humic substances extracted from an ombrotrophic peat bog in northwest Spain

The most southerly ombrotrophic peat bogs in Europe are in Galicia (northwest Spain). The humic matter in them originates from chemical processes in anaerobic conditions. We studied the acid properties of fulvic acids and humic acids isolated from two peat horizons of an ombrotrophic peat bog by potentiometric titration. Solutions containing 25, 50 and 100 mg l⁻¹ of each humic substance were titrated at ionic strengths 0.005 m , 0.01 m and 0.1 m (with KNO₃ as the inert electrolyte). Charge curves were analysed with a Donnan model to determine the intrinsic proton binding parameters. The concentration of the humic substance affected the charge curves more significantly at pH exceeding 6, and tended to disappear at greater concentrations. The proton binding conditional constants decreased with increasing ionic strength, this effect being more significant in the carboxylic groups with less affinity for protons. The proton binding constant of the carboxyl groups in a fulvic acid was one order of magnitude less than the value for the corresponding humic acid, whereas for the phenolic groups the values for both fractions were similar. The total content of acid groups was approximately 2 mol kg⁻¹ greater in the fulvic fraction than in the humic fraction. Both humic fractions from the lower horizon contained more acid groups than those from the upper horizon, mainly because the content of carboxyl groups increases with soil depth. Therefore, the humic substances in the lower horizon of the peat will be more negatively charged, which will affect their solubility and the binding of metal ions.     

A model of solid-solution interactions in acid organic soils, based on the complexation properties of humic substances

CHAOS (Complexation by Humic Acids in Organic Soils) is a quantitative chemical model of organic soils that incorporates complexation by the functional groups of humic substances and non-specific ion-exchange reactions. The two types of interaction are linked by the net humic charge, Z, which depends on the extents of proton and metal complexation, and which in turn determines ionic concentrations in the diffuse part of the electrical double layer, by a Donnan equilibrium. CHAOS was found to account satisfactorily for the results of acid-base titration experiments (pH range 3–5) with soil samples, giving reasonable simultaneous predictions of solution pH and concentration of A1³⁺. Predictive calculations with CHAOS suggest that organic soils acidified by acid rain would respond on a time-scale of years-to-decades to reductions in rain acidity. An associated effect might be an increase in the concentration of dissolved organic matter in the soil solution.     

Study of the acid‐base properties of a peat soil and its humin and humic acid fractions

For accurate interpretation of cation binding to natural organic matter, the proton binding behaviour of both solid and dissolved natural organic matter must first be established. In the present study, potentiometric titrations of samples of humin and humic acid extracted from a peat soil were performed at different ionic strengths. Humic acid (HA) samples in solution (dissolved humic acid, DHA) and in suspension (aggregated humic acid, AHA) were titrated. The corresponding charge curves were analysed with the NICA‐Donnan model and the results were compared with those previously obtained for the peat soil. Good reproduction of the DHA charge curves was obtained, and in the case of the AHA, the exact forms of the charge curves were not reproduced at pH < 6 because of the conformational changes and dissolution of the sample that took place throughout the titration. The peat and humin displayed similar proton binding behaviour, which was different to that of the humic acid. With the Donnan approach, the value of parameter b, that relates the Donnan volume to the ionic strength, was the same for peat and humin and less than that of the humic acid, for both the dissolved samples and those in suspension, indicating that the effect of ionic strength on the charge is greater in the peat and humin than in the HA. The ratio between the contents of phenolic groups and of carboxylic groups was greater in peat and humin than in humic acid. The model used revealed that the acid‐base behaviour of the peat is closer to that of the humin than to that of the humic acid.     

Proton interactions with soil organic matter: the importance of aggregation and the weak acids of humin

Samples of three organic‐rich soils (ombrotrophic peat, podzol H‐horizon, humic ranker) were extensively washed with dilute nitric acid, dialysed against deionised water, and then subjected to acid‐base titrations over the pH range 3–10, in 0.3–300 mm NaNO₃, and with soil concentrations in the range 2–150 g l^?1. The results for the three soils were quantitatively similar. Comparison of the titration data with previously published results for humic acids isolated from the same soils showed the soil organic matter to have a greater ionic strength dependency of proton binding and to possess relatively greater buffering capacity at high pH, attributable to weak acid groups (c. 2–5 mmol g^–1) in the humin fraction of the soils. To describe the soil titration data quantitatively, we modified Humic Ion‐Binding Model VI‐FD, which utilizes a fixed Donnan volume to describe counterion accumulation, by increasing the content of weak acid groups. When artefacts in pH measurement caused by the suspension effect were taken into account, the resulting Model VI‐FD2 provided good or fair simulations of all the titration data. The results suggest that soil structure, specifically aggregation, plays a significant role in cation binding by organic soils in situ. The lack of dependence of the titration results on soil suspension concentration suggests that the findings can be applied to soils in situ.     

Negative charge — pH dependence of organic matter of forest soil under influence of liming as determined from titration curves

The dependences between negative charge and pH for organic matter of limed and unlimed profiles of sandy acidic forest soils were determined on the base of ion exchange and titration curves measurements. Subtracting the titration curves of the supernatant from the titration curves of the respective suspensions the quantities of base consumed by solid phases were determined. They were interpreted in terms of negative charge after corrections with the quantities of initial exchangeable basic cations and exchangeable hydrogen. For investigated organic material the charge increased slowly in acidic pH region and much faster in alkaline pH region. The zones of the fast increase of charge occured at higher pH's for deeper horizons, enriched with fulvic acids. The observed changes of organic matter charge due to liming were related to the increase of fulvic to humic acids ratio. The negative charges of organic matter in limed and unlimed profiles estimated for high pH were better correlated with fulvic to humic acids ratio than when estimated for lower pH levels.     

An Experimental and Modelling Study of Cu2+ Binding on Humic Acids at Various Solution Conditions. Application of the NICA-Donnan Model

Humic substances are characterized by a strong binding capacity for both metals and organic pollutants, affecting their mobility and bioavailability. The understanding of the mechanisms of proton and metal binding to humic substances is of fundamental importance in geochemical modelling and prediction of cation speciation in the environment. This work reports results on copper binding on humic acids obtained through a thorough experimental and modelling approach. Two humic acids, a reference purified peat humic acid isolated by the International Humic Substances Society (IHSS) and a humic acid from a Greek soil, were experimentally studied at various pH values (4, 6 and 8), humic acid concentrations (ranging from 20 to 200 mg?L^?1) and ionic strength (0.1 and 0.01 M NaNO₃). The binding of copper to humic acids was determined over wide ranges of copper ion concentrations using a copper ion selective electrode. The copper binding isotherms obtained at different conditions have shown that copper binding is dependent on the pH and ionic strength of the solution and on the concentration of both humic acids. Copper binding experimental data were fitted to non-ideal competitive adsorption NICA-Donnan model and the model parameter values were calculated. Both Cu²⁺ and CuOH⁺ species binding to humic acid with different binding affinities were considered. Two sets of the NICA-Donnan parameters have been calculated: one for humic acid concentrations of ??100 mg?L^?1and one for humic acid concentration of 20 mg?L^?1. The meaning of the parameters values for each concentration level is also discussed.     

Sorption of cadmium by complexes of kaolinite with humic acid

Abstract

Levels of cadmium (Cd) in New Zealand pastoral soils have increased due to Cd impurities in applied fertilisers. As there is little information on the interaction of Cd with soil mineral‐organic matter complexes, the sorption of Cd by complexes of kaolinite with humic acid has been investigated. Sorption was measured at pH and ionic strength values typically found for solutions of pastoral soils in New Zealand. Sorption increased with the content of humic acid in the complex, and as the pH of the medium was raised from 4.2 to 6.3. Sorption was also influenced by the ionic strength of the ambient solution, notably by the nature of the cation in the added electrolyte. The experimental data were interpreted in terms of the effect of solution pH and ionic composition on the charge characteristics of kaolinite and humic acid. These factors, in turn, influence clay particle association as well as the clay‐humic and metal‐humic interaction.     

Measurement of electrostatic and site-specific associations of alkali metal cations with humic acid

A discontinuous acidimetric titration method incorporating ultrafiltration was developed to measure the association of a soil humic acid with Li⁺ t, Na⁺ and K⁺ (pH 3 to 8). In addition, possible site-specific binding of these alkali metal cations was investigated using desorption experiments at pH 1. Li, Na and K cations behaved equivalently in the titrations and the amounts of these cations associated with the humic acid was measurable at all pH values between 3 and 8. Up to 90% of the total alkali metal cation was humate-associated at pH 8. The absolute amount of humic-associated cation did not depend on the alkali metal cation concentration, but rather on the solution alkalinity. In addition, the net charge of the humate polyanion made a negligible contribution to the electroneutrality of the bulk solution under all conditions. These results are consistent with a diffuse layer model of hydrated humic acid in which the alkali metal cations neutralize the humic charge. The association of Na+ andK+ with humic acid at pH 1 was successfully described by a Langmuir adsorption model. The number of sites per g of humic acid was very small, and greater for K+ than for Nat. Lithium cations exhibited no detectable humic association at pH 1. These differences suggest that humic acids may have a small number of specific binding sites for which the size of the hydrated cation is important.     

Proton and copper binding by humic acid: application of a discrete-site/electrostatic ion-binding model

A discrete-site/electrostatic model of ion binding by humic substances has been applied to proton-and copper-binding data for a soil humic acid. The proton data cover the _pH range 3–9 and the ionic strength range 0.001 m -0.1 m , while those for copper refer to the _pH range 4–5, ionic strengths between 0.005 m and 0.1 m , and _p[Cu] values between 3 and 8. The model is able to explain the major observed trends, including the dependence of proton and copper binding on ionic strength and the binding of copper as a function of _pH. However, the calculated ionic strength dependence of copper binding is slightly less than that observed. In addition, the model has been used to predict ratios of protons released to copper bound under different conditions, on the basis of the separately estimated parameters for proton and copper binding. The model correctly predicts the ratios to be between 1 and 2, and to decrease with increasing bound copper and with _pH.     

Partitioning of organic carbon, aluminium and cadmium between solid and solution in soils: application of a mineral–humic particle additivity model

The partitioning of chemical elements between the solid and solution phases in soil is fundamental in understanding processes such as leaching and bioavailability. Here I present a model in which the partitioning of Cd, Al and carbon in both mineral and organic soils can be simulated in the pH range 2–8. A two‐phase additivity approach simulates ion adsorption by the soils using a hydrous ferric oxide and humic type surface. A model for the partitioning of soil humic matter has also been developed in which the NICA–Donnan model calculates humic surface charge. Other key processes represented include mineral solubilization and solution speciation. Methods for deriving model input parameters either from analytical data or by parameter optimization were used. Acid ammonium‐oxalate‐extractable Fe was used to estimate the amount of hydrous ferric oxide, and reactive humic substances were estimated by a scaled down version of the International Humic Substances Society method for the extraction of humic and fulvic acid. For initial calculation the 0.1 m HCl‐extractable Al was used to estimate reactive Al. Optimization of reactive Al improved the fit of both the total dissolved Al data and the adsorbed Cd. The model for the solid–solution partitioning of humic substances could simulate reasonably well the release of carbon in the pH range 4–8 for both the organic and mineral soils.     

Modelling pH buffering and aluminium solubility in European forest soils

The pH buffering and aluminium solubility characteristics of acid soil are important in determining the soil's response to changes in precipitation acidity. The chemistry of soil organic matter (humic substances) plays a key role in both processes, yet is complex and still poorly understood. Nevertheless, models of humic substance chemistry have been developed, one of which is WHAM–S, which contains a model (Model V) of proton and metal binding at discrete sites on humic substances and considers electrostatic effects on the binding strength. Here we have tested the ability of WHAM–S to model solution pH and Al using batch titration studies on organic and mineral soil horizons from forested sites in Norway, Germany and Spain, with ambient pH values from 3.73 to 5.73. We optimized the model predictions by adjusting the amounts of soil aluminium and humic substances within defined limits, taking the contents of copper chloride‐extractable Al and the base‐extractable organic matter as starting values. The model simulated both pH and dissolved Al well with optimized amounts of aluminium and humic substances within the defined limits (root mean squared error for pH from 0.01 to 0.22, for p[Al]_aq (total dissolved Al) from 0.03 to 0.49, five data points). Control of dissolved Al by dissolved organic matter was important particularly at above‐ambient pH. In two mineral horizons we improved the fits by assuming that Al could precipitate as Al(OH)₃. The optimized model also gave reasonable predictions of pH and dissolved Al in supernatants obtained by repeated leaching of the soil horizons. The results show that humic substances dominate the control of pH and dissolved Al in most of the horizons studied. Control by Al(OH)₃ occurs but is the exception.     

Sephadex-Gelfiltration von Huminsäuren und deren Gehalt an Aminosäuren

Gel filtration on Sephadex of humic acids and their content of amino acids

• 1 Brown humic substances from the A_h of an iron-humus podzol were separated on Sephadex G 25 into three fractions. The first fraction (Ve/Vo = 1.0–1.5) has the lowest value of Q 4/6 (5.0–8.0), the second the highest Q 4/6 (10.0–12.0) and the third a middle one (8.0–10.5). The adsorption effect may take place during separation.
• 2 The asymmetric form of the integral elution curves of the first fraction indicated that the humic substances are a set of particles with different size. The separation is influenced by diffusion into the pores of gel matrix.
• 3 In the first fraction were determined 16, in the second 13 and in the third 10 amino acids.
• 4 The titration curves of the first fraction (after separation on G 25) indicated that the rate of pH change with alkali added becomes its maximum at pH 6.5–7.5 at different ionic strengths.
• 5 Dialysed humic acids (H⁺ form) were fractionated on G 75. The alkali consumption up to pH 7.5 in the eluate shows that small amounts of mineral acids increase the conductivity of the solution. Their V_e is lower than that of the second fraction of humic acids. Therefore it is impossible to determine the alkali consumption by the second fraction and by the humic acids as a whole without their total desalting.

     

Adsorption and desorption of Cd on humic acid fraction of soils

The adsorption of ionic Cd has been investigated on three humic acids isolated from podzol, rendzina and brown Mediterranean soils of Tuscany. The adsorption isotherms have been determined at 5 and 25°C. Cadmium adsorption was described by the Langmuir adsorption equation. Langmuir parameters were related to the functional groups content of humic acids and decreased in the following order: rendzina>brown Mediterranean soil>podzol. Adsorption was independent on temperature and increased with pH. Desorption experiments with 0.1 N NH₄OAc and 0.25 M Cu (OAc)₂ proved that Cd is adsorbed on humic acid about 50% in an exchangeable form and 50% in coordination complexes.     

ACIDIC PROPERTIES OF PLANT LIGNINS AND HUMIC MATERIALS OF SOILS

The acidic properties of lignins from the native vegetation of a virgin Ella loamy sand and the humic acids, the 1, 4-dioxane extractable humic fraction and residual humic acids from the virgin and cultivated soils were investigated by determining solid-state cation exchange capacities for Cu and Ca and by non-aqueous potentiometric titration, using potassium methoxide in a benzene-methanol mixture as titrant. The lignins and soil dioxane extracts were titrated in pyridine; the humic acids in dimethylformamide. Platinized Pt and CH₃OH modified calomel electrodes formed the electrode combination. The CEC of the humic acids were much higher than the CEC of lignins with both cations but the Cu: Ca absorption ratios were less for humic acids than for lignins. Titration curves of lignins showed two potential breaks with total acidities of 960 to 1,190 me/100 g. Titration curves of the humic acids showed three to six potential breaks while curves of the soil dioxane extracts showed three and four. The total acidities of the two types of humic isolates were 1,100-1,200 me/100 g. The equivalent weights of most lignins and humic materials were very similar although the acid groups might be different with respect to types and acid strength. Lignins from other plant materials and humic isolates from other soils were also investigated by non-aqueous titration to determine the general applicability of the results obtained with isolates from the Ella soils.     

Soil humic acid aggregation by dynamic light scattering and laser Doppler electrophoresis

Humic acids (HAs), similar to other fractions of humic substances (HSs), have a large number of reactive functional groups enabling them to aggregate in solutions. Regardless of the origin of humic acid (aqueous or soil), this aggregation process is dependent on environmental conditions and strongly influences the mobility of soluble ionic and molecular pollutants. The aim of this work was to monitor the aggregation process of two humic acids isolated from different mineral soils (IHSS Elliot soil HA standard and Rendzic Leptosol HA) in the 2–11 pH range. Changes in aggregate size in HA sols were followed up using dynamic light scattering (DLS), while zeta potential (ZP) measurements in the same pH range were performed applying laser Doppler electrophoresis (LDE) technique. The effect of HA sol concentration and soil source on aggregation was examined as well. Besides, HA samples were characterized using Fourier transform infrared (FT‐IR) spectroscopy. By inspecting HA‐particle‐size dependence on pH, it can be concluded that both HAs in corresponding sols behave as molecular aggregates or supramolecular structures, formed from small individual moieties (sizes < 10 nm) at higher pH values. The ZP vs. pH curve for both HAs revealed the ZP minimum in the 5–7 pH range, caused most likely by dissociation of acidic functional groups prevailing at lower pH values and deaggregation predominating over dissociation at higher pH values.     

Zinc sorption–desorption by two Andepts: effect of pH and support medium

Laboratory experiments were carried out to evaluate the effect of pH, ionic strength and electrolyte composition on zinc sorption–desorption by two Andepts from the Canary Islands (Spain). At the natural soil pH, the soils exhibited little net negative surface charge and small Zn sorption capacities. More than 75% of the sorbed Zn was apparently strongly bonded. The pH greatly influenced the sorption–desorption reactions. Sorption increased with increasing pH, and retention increased abruptly at pH > 6.0. Sorption also occurred at pH values below the point of zero charge (PZC) of the soils, when most of the surface sites are positively charged. Desorption decreased continuously with rising pH and became a trace at pH > 6.0. An increase in the ionic strength of the background electrolyte decreased Zn sorption and enhanced the amount of sorbed metal that could be subsequently released. In the two soils, Zn sorption diminished somewhat in the K and Ca electrolytes as compared with the Na electrolyte. However, this did not happen at small Zn loadings. Desorption was not affected by the type of electrolyte and cation used. The results are consistent with chemisorption being responsible for most of the sorption. The results also suggested a strong affinity sorption or even precipitation at high pHs.     

Determination of DTNB-reactive sulphhydryl groups in soil humic acids: their enrichment in humic acids from volcanic acid soils

Thirteen acid soils were collected from typical volcanic regions in Japan (S content: 0.9–4.1, mean = 2.2g kg^?1; pH (H₂O): 2.81–3.93, mean = 3.33), as well as nine reference soils (S: 0.6–1.7, mean= 1.1 gkg^?1; pH(H₂O): 4.10–4.74, mean = 4.47). Humic acids were extracted from the soils separately with 0.1 M NaOH and precipitated by acidification (pH = 2, HCl). After purification, the humic acids were subjected to colorimetric analysis using a DTNB reagent [5,5′-dithiobis(2–nitrobenzoic acid] for the active -SH group. Since humic acids have significant absorption at 4I2 nm, the coloured compound (5–mercapto-2–nitrobenzoic acid) was separated from the humic acids by ultrafiltration or solvent extraction prior to the colorimetric measurement. Humic acids also caused discoloration of the coloured compound when they coexisted in the reaction solutions. Thus, the reproducible determination of -SH was accomplished by employing a standard addition technique (-SH standard: cysteine). Although -SH contents obtained by the ultrafiltration method were considerably higher than those by the solvent-extraction method, probably due to the denaturation of humic acids by the higher buffer concentration used in the ultrafiltration method, they correlated well with each other. The humic acids from acid soils contained apparently higher concentrations of -SH (120–510, mean = 270mg S kg^?1 by the ultrafiltration method; 8–110, mean = 38mg S kg^?1 by the solvent-extraction method) compared to those from reference soils [20–260, mean = 90mg S kg^?1 by the former; not detectable (<5)-34, mean = 11 mg S kg^?1 by the latter]. This -SH enrichment in the humic acids from acid soils may result from the degradation and subsequent humification of S-rich debris of plants and micro-organisms and/or direct incorporation of volcanic acid gas (H₂S) into the humic acids.