KINETICS OF ION EXCHANGE IN SOIL ORGANIC MATTER

The rate of ion exchange reactions of humic substances (humic acid and peat) with lead ions was investigated at 25 °C. All experiments were carried out under finite solution-volume conditions. To the well-stirred aqueous suspensions of the above samples lead ions were added in such small amounts that, after attainment of the equilibrium, the initial equivalent fraction of lead in the humic substance had increased only by a small amount. The kinetics of such differentially small ion exchange processes were studied as a function of the initial lead content of the samples. The experiments showed that the absolute rates for the uptake of small amounts of lead ions decrease, if the initial lead content of the samples increase. The relative rates of ion exchange, i.e. the rates with respect to the extent of each reaction, showed a more complicated behaviour, as they can exhibit a minimum at a certain initial lead content of the sample. The rate-determining step of the above ion exchange experiments was shown, by interruption tests, to be diffusion of the ions through the Nernst-film surrounding the ion exchanger particles. Theoretical calculations of the rates of film-diffusion controlled ion exchange processes were carried out and the results revealed that the rates of any differential ion exchange reaction should decrease with increasing initial content of the ion adsorbed, their exact magnitude depending, among other quantities, strongly on the separation factor. The relative rates for the attainment of the equilibrium, on the other hand, depend much less on the initial ionic composition of the ion exchanger. A comparison of the experimental and the calculated results yields satisfactory agreement if the experimentally determined dependence of the separation factor on the lead content of the samples is taken into account.     

THE MEASUREMENT AND MECHANISM OF ION DIFFUSION IN SOILS

The diffusive flux of soil cations and anions to an absorbing surface may be measured by a simple, quick, and accurate method using ion exchange resin paper as a sink maintaining, initially, zero concentration of the diffusate at the soil surface. The quantity of ions diffusing from the soil of semi-infinite thickness is directly proportional to the square root of the diffusion time until about half the counter-ions originally on the resin have been exchanged. Values of average effective diffusion coefficients are calculated using this proportionality constant and the concentration of total exchangeable ions. The effect of the counter-ions of the resin on soil-ion flux is small but measurable.     

腐殖酸的数均分子量测定

腐殖酸是广泛存在于土壤、污泥、泥炭、煤和水域中的天然大分子电解质,它对土壤肥力、植物生长、环境中元素的迁移和富集有重要作用,近廿年来日益为人们所重视,但其化学结构和分子量的确切范围目前尚不甚清楚。迄今为止,尚没有一种测定各种腐殖酸分子量的良好方法^[11]。     

Application of Humic Substances Results in Consistent Increases in Crop Yield and Nutrient Uptake

The effect of humic substances on yield and nutrient uptake of grass, maize, potato and spinach was investigated in six field experiments and two pot experiments in a high input cropping system. The humic substances originating from leonardite formations in Canada were applied as liquid solution to the soil (Humifirst liquid) or as a solid incorporated in mineral fertilizers (Humifirst incorporated). Formal meta-analysis of the results of all executed experiments showed that the application of humic substances had an overall positive effect on dry matter yield of the crops and this effect was statistically significant for Humifirst incorporated. In the case of permanent grassland, humic substances promoted mainly the production of the first grass cut, which has the highest grass quality among all cuts during the growing season. Tuber production on the potato field trial showed a high response on the application of humic substances. Total potato yield increased with 13 and 17% for Humifirst liquid and Humifirst incorporated, respectively. The effect of humic substances on maize yield was limited, probably due to the rather high nutrient status of both soils. Finally, the formal meta-analysis showed a consequent increase in nitrogen and phosphorus uptake of all studied crops as well. The effect on potassium and magnesium uptake was also mainly positive, while sodium and calcium uptake were not affected in most of the experiments.     

The use of chemical potentials to express nutrient availabilities

The absorption of an ion is balanced, electrically, by either an uptake of an equivalent amount of counter ions or an exchange for ions of the same charge. An ion such as phosphate can be removed from a many-component solution in different ways, e.g., together with potassium, calcium, or magnesium, or in exchange for hydroxyl ions. In each individual case the status of the nutrient in the soil is expressed by a different chemical potential, and without further knowledge it is not possible to say which potential best expresses the availability. Schofield's phosphate potential implies phosphate uptake associated with calcium. This potential, like Woodruff's potassium potential, has been chosen arbitrarily and does not apply to the general process of phosphate uptake.A physiologically meaningful nutrient potential must be selected relative to the uptake process. Thus, if nutrient A is taken up with counter ion B, then AB's chemical potential is of interest. The fact that uptake processes are not sufficiently understood and proposed mechanisms are not generally accepted puts serious limitations on the usefulness of the potential concept. A few possibilities for selecting nutrient potentials are discussed.Since the nutrient capacity is the differential between nutrient content and potential, the above problem of selection is also inherent in the capacity.     

Selectivity of Ion Exchange as a Sign of Soil Quality

A hypothesis is tested that allows study of not only the quantity but also the quality of ion exchange in soils, composts, peats, and its mixtures, which is common in natural and synthetic ion exchangers, on the basis of the ion-exchange process and its selectivity. For practical application of ion exchange in the soil not only the quantity of base cations and hydroxon ions bound by the solid soil phase is significant but also their readiness to ion exchange or ion exchange elasticity. Selectivity of ion exchange, expressed by selectivity coefficient, reflects the readiness of ion exchange while ion-exchange thermodynamics show the degree of spontaneity of the ion-exchange process. The results document that soil humus is the highest-quality ion exchanger in soil and that its quality may be assessed by the proposed method. The effort to replace humus by any synthetic ion exchangers cannot be successful: Although they have a high value of cation exchange capacity, they also show high selectivity for ions of higher valence and low ion-exchange elasticity, and their desorption of calcium and magnesium is poor.     

Kinetic Speciation of Ni(II) in Model Solutions and Freshwaters: Competition of Al(III) and Fe(III)

The competing ligand exchange method was used to investigate the competitive binding of Ni(II) by Al(III) and Fe(III) in model aqueous solutions and freshwaters. Graphite furnace atomic absorption spectrometry and adsorptive cathodic stripping voltammetry were used to monitor the rate of uptake of the Ni by Chelex 100 chelating resin and dimethylglyoxime as the competing ligands, respectively. The results have revealed that Ni(II)–humate complexes were more labile in presence of the mixture of Al(III) and Fe(III), compared to the lability of the Ni(II)–humate complexes when only one of the two, Al(III) or Fe(III), was present. The environmental significance of this work is that in model solutions simulating freshwater containing humic substances and the target trace metal Ni(II) and cations, Al(III) and Fe(III), the competitive binding of Ni(II), Al(III) and Fe(III) by humic substances makes Ni(II)–humate complexes labile, releasing free Ni²⁺–aqua complex, which reported to be toxic.     

作物秸秆碳在土壤中分解和转化规律的研究

采用14C标记秸秆,在大田和实验室的研究结果表明,秸秆的分解速率主要取决于C/N比。施入土壤后,土壤微生物迅速增加,尤其是细菌。秸秆降解首先形成非结构物质,其中大部分转化为富里酸,进而转化为胡敏酸。分解产物对土壤腐殖质的更新,从腐殖质表面官能团或分子断片开始,逐步进行。非结构物质可与腐殖酸的单个分子产生交联作用,在一定条件下,交联的复合分子可进入腐殖质分子核心的成分中。腐殖酸单个组分在土壤中的转化和重新分配,仅仅与腐殖质表面官能团的反交换过程密切相关。粘土矿物选择性吸附胡敏酸,而且优先吸附胡敏酸中低分子成分。     

Mechanism of reduction of exchangeable aluminum by gypsum application in acid andosols

Mechanism of reduction of exchangeable aluminum in acid Andosols treated with gypsum was studied by using cation exchange resin methods to determine the amount of polymerized aluminum. Two types of acid Andosols were used as test soils: Kitakami light colored Andosol (fine, mixed, mesic, Andic Dystrochrept) and Kawatabi thick high humic Andosol (medial, mesic mixed Alic Pachic Melanudand). Polymerization of aluminum in the soil solution of both Kitakami and Kawatabi Andosols treated with gypsum was suggested based on an analysis using cation exchange resin methods, whereas that in monomer aluminum solution was not detected. Accumulation of polymerized aluminum in both Kitakami and Kawatabi Andosols was determined by using cation exchange resin, and the amounts of polymer aluminum trapped by the resin and the ratio of polymer aluminum to monomer aluminum were increased with the incubation time. The values of CEC which decreased in the Kitakami Andosol after gypsum treatment were almost equivalent to the amounts of cation exchange sites occupied by polymer aluminum ions which were calculated based on the decrease of the values of Y _l. We conclude that the mechanism of reduction of exchangeable aluminum in strongly acid Andosols treated with gypsum is as follows: firstly, exchangeable aluminum adsorbed on the cation exchange sites of soils may be released into the soil solution due to the increase in the ion strength caused by gypsum application, and then monomer aluminum in soil solution may be polymerized in the presence of soil colloidal materials. Consequently, the polymer aluminum formed in the soil solution may be selectively and irreversibly fixed on the cation exchange sites of 2 : 1 clay minerals.     

A method for chemical fractionation of chloride from complex effluents

A method was developed to fractionate chloride ions from complex effluents using an ion exchange column. The procedure utilizes a strongly basic anion exchange resin with hydroxide ions as the exchange ion. The chloride within the effluent is exchanged for hydroxide which is less toxic to Ceriodaphnia dubia. The addition of hydroxide to the solution requires a pH adjustment to a physiologically acceptable pH for Ceriodaphnia with an acid. The fractions are added to Ceriodaphnia and the ET ₅₀ (median lethal time) determined. The column was applied to two effluents with suspected chloride toxicity. The column successfully removed virtually all of the chloride from the effluents. A significant reduction in toxicity was found following the process for one of the effluents. In addition, a control that consisted of the original effluent at the ionic strength of the column fractions revealed that chloride was a causative toxicant in both of the effluents.     

EPR spectra of humic acids and their metal complexes

On the basis of ESR spectra, humic substances are believed to be hydroquinone type polymer radicals with considerable cation exchange and redox capacity. All 3d-transition metal humates appear to be ionic high spin complexes. Manganese (II) ions doped in raw peats and peat humic acids are coordinated octahedrally with six oxygen-containing functional groups, e.g., carboxylate, phenolic, hydroxyl, carbonyl ligands, whereas copper (II) ions are in square planar arrangement with two carboxylate and two aliphatic nitrogen ligands. Doping with vanadyl(II) ions results in a square pyramidal arrangement with four oxygen-containing ligands. Diamagnetic manganese(VII), chromium(VI), molybdenum (VI) and vanadium(V) oxoanions in acidic solution are reduced by humic acids into paramagnetic manganese(II), chromium(III), molybdenum(V) and vanadium(IV) ions. Diamagnetic copper(I) ions, on the other hand, are oxidized by humic acids into paramagnetic copper(II) ions.In contrast to polyronic acid gels with outer-sphere structure, 3d-transition metal ions are generally bound to humic acids as inner-sphere chelate complexes.     

Effect of soil invertebrates on the formation of humic substances under laboratory conditions

The complete polymerization of phenols and proteins (one of the processes involved in the formation of humic substances) was explained. It was shown that fly (Bibio marci) larvae and earthworms (Aporrectodea caliginosa) participate in the complete polymerization of phenols and proteins. In a laboratory experiment, invertebrates participated in the degradation of organic matter and the synthesis of humic substances, which was proved in experiments with ¹⁴C-labeled phenols and proteins. The same organic substances (phenols and proteins) without the impact of invertebrates were used as the control substances. The distributions of the ¹⁴C isotope in alkaline extracts separated by solubility in acids (humic and fulvic acids) was compared to those of the control substances. The portion of the ¹⁴C isotope in the humic acids in the excrements of Bibio marci was higher than that in the control substances. The content of ¹⁴C-labeled humic substances in the excrements of the earthworm Aporrectodea caliginosa exceeded the control values only in the experiment with proteins. When clay material was added to the organic substances, the portion of the ¹⁴C isotope in the humic acids increased in both experiments with phenols and proteins. When these substrates passed through the digestive tracts of the invertebrates, the polymerization of organic substances and the inclusion of proteins and phenols into humic acids occurred.     

Effects of mineral and monocarboxylic acids on the molecular association of dissolved humic substances

The conformational structure of dissolved humic substances is an important property that controls the reactivity of humus in the soil solution. High performance size-exclusion chromatography was used here to study the changes in molecular size of different humic substances brought about by addition of mineral (HCl) and monocarboxylic (formic, acetic, propionic, and butyric) acids. The CPMAS-NMR spectra showed that humic substances had varying chemical composition and that the ratio of hydrophilic to hydrophobic carbon (HI/HB) was greater for a humic acid from soil than for ones from oxidized coal and lignite. All humic substances showed a decrease in UV absorbance of chromatographic peaks when treated with either HCl or monocarboxylic acids. This was due to the hypochromic effect by which the absorptivity of associated molecules is decreased when they are separated. We attributed the molecular separation upon acid treatment to the formation of intermolecular hydrogen bonding that alters the original conformation stabilized mainly by weaker hydrophobic interactions. Addition of organic acids not only further decreased peak absorbances of humic acids but also caused their shift to larger elution volumes, indicating a larger conformational disruption than with HCl. The extent of the molecular size changes showed a relation to the number of carbons of monocarboxylic acids and to the HI/HB ratios of humic materials. The larger the carbon content of organic acids and the smaller the HI/HB ratio of humic materials, the larger was the decrease of the average molecular size of humic acids. These results suggest that dissolved humic substances associate predominantly by hydrophobic forces and that the apolar components of humic substances largely control their aggregation and reactivity in the environment.     

RATE PROCESSES IN THE DESORPTION OF PHOSPHATE FROM SOILS BY ION-EXCHANGE RESINS

Phosphate extracted by ion-exchange resins in the chloride and sodium forms from a deep river-gravel soil under widely varying conditions is always from the isotopically exchangeable or‘labile’ pool. At any reaction time, a constant fraction of this pool is desorbed by the chloride form of the anion-exchange resin alone, irrespective of the pH and phosphate manuring of the soil. If, however, a sodium: cation exchange resin is included, increasing fractions of the‘labile pool’ are desorbed with decreasing soil acidity. Phosphate desorption by the anion-exchange resin alone and with the cation exchange resin is shown to be‘particle-diffusion’ controlled in the anion exchange resin and neither a‘chemical reaction’ nor a‘film-diffusion’ mechanism. Over the pH range 4·5–8·5, values between 4·8 and 0·9 × 10^-9 cm² sec^-1 were calculated for the interdiffusion coefficient of the phosphate: chloride exchange process in the resin. The isotopically exchangeable phosphate in the soils seems to behave like sparingly soluble or weakly dissociating compounds towards ion-exchange resins and its rate of desorption depends on the nature and composition of the resins.     

Gel filtration chromatography of humic substances in soil solutions using HPLC-determination of the molecular weight distribution

The molecular weight distribution of humic substances in soil solutions from a dystric cambisol and an orthic podzol were determined by high-performance gel filtration chromatography (Toyo Soda TSKgel G2000 SWXL column). The retention volumes of humic substances were compared to those of narrow molecular weight distributed poly-styrenesulphonates with known molecular weight. A change in mobile phase pH or ionic strength usually resulted in an almost identical change in retention volume for the polystyrenesulphonates and the bulk of humic substances. The possibility of using the former to calibrate the column is discussed. The molecular weight at peak maximum was 1120–1190 and 1800 for the cambisol and podzol samples, respectively, using polystyrenesulphonates as molecular weight standards. No pretreatment of samples was used, but ion exchange is recommended for samples with high Al concentrations.     

The effect of pH on the retention of Cu,Pb, Cd,and Zn by clay-fulvic acid mixtures

The addition of fulvic acid to clay suspensions (kaolinite, illite or montmorrillonite) resulted in increased uptake of Cu, Pb, Cd, and Zn ions over the pH range 3 to 6, due to the limited solubility of one of the metal-fulvate species formed. At higher pH values, residual metal ion was retained in solution, instead of precipitating as hydroxy species. The amount of total metal ion found in solution at equilibrium was determined by the quantity and type of clay added; the amount of organic acid present; and to a lesser extent, pH. The behavior of the clay-fulvic acid systems differed from that observed using other organic materials such as gelatine, tannic acid or a humic acid.     

Quantitative differences in evaluating soil humic substances by liquid- and solid-state C-NMR spectroscopy

We compared the quantitative responses of liquid-state (LS) and solid-state (CPMAS) ¹³C-NMR spectroscopy of four different soil humic substances. The intensities of signals for the alkyl carbons (0–40 ppm) were significantly larger in CPMAS than in LS spectra. This difference is in agreement with the pseudo-micellar model of the conformational nature of humic substances. By this view, the hydrophobic interactions holding together the heterogeneous molecules of humic micelles inhibit the molecular motions of the alkyl carbons, thereby enhancing the spin-lattice relaxation times and consequently lowering the sensitivity of liquid-state NMR. Conversely, regardless of their position in the humic conformation, a better estimation of the number of alkyl carbons can be obtained by CPMAS-NMR because of the cross-polarization of hydrogen nuclei in CH₂ and CH₃ groups. The intensity of the 40–110 ppm region is also slightly lower in LS than in CPMAS-NMR spectra, despite the hydrophilicity of the oxidized and peptidic carbons resonating in this chemical shift interval. Their molecular motion may also be reduced by either the formation of intra- and inter-molecular hydrogen bondings due to poorly acidic hydroxyl groups of saccharides, or the degree of conformational rigidity that a pseudo-micellar arrangement confers even to hydrophilic domains. The higher content of aromatic carbons (110–160 ppm) found in the LS spectra was attributed partly to the high degree of substitution of the aromatic ring that slows down cross-polarization in CPMAS experiments and partly to the relative overestimation of this region by LS-NMR due to a lack of signal in the aliphatic interval. The slightly lower content of carboxyl carbons estimated in CPMAS spectra as compared to LS spectra was also attributed to slow cross-polarization. This work shows that the combined use of both NMR techniques is profitable in conformational analysis of humic substances and of dissolved organic matter in general.     

Soil Organic Matter Content and Quality in Polyfactorial Field Experiments

The polyfactorial long-term field experiments founded in 1979 exist in four different sites till the present time. The experimental design and selected variants have been described in our previous paper (Lipavsky et al. 2002). Besides of the organic carbon and nitrogen contents in soil, dry matter of the main and second products, nitrogen uptake by the main and second products presented in our previous paper (Lipavsky et al, 2002), hot water soluble carbon (C hwl ), humic substances, e.g. humic and fulvic acids have been determined in soil samples taken in autumn each year from six selected variants of organic and mineral fertilisation. The results of the hot water soluble carbon content and humic and fulvic acids contents in the air dried soil samples have been evaluated in this paper. It was shown that humic substances are rather conservative and site specific soil properties that are not much affected by the cropping and fertilisation systems. Hot water soluble carbon, on the other hand, seems to be a dynamic part of the soil organic matter.