KINETICS OF ION EXCHANGE IN SOIL ORGANIC MATTER

The rate of ion exchange of humic substances (humic acid and peat in the hydrogen form) with lead ions was investigated. As in processes occurring in nature, the lead solution was added in all our experiments slowly and continuously at a constant feed rate to the well-stirred water-suspended humic substances. Control experiments were also performed with a well-defined synthetic ion- exchange resin (carboxylic acid type). The uptake of divalent lead ions by the humic substance and by the ion-exchange resin, which is coupled with the release of an equivalent amount of hydrogen ions, proceeds as a function of time according to a characteristic sigmoidal curve. During the initial phase, the rate of ion exchange increases continuously with time, even though the humic substances become more saturated with lead ions. Theoretical calculations, based on a film diffusion process as the rate-determining step, show that this behaviour should indeed be observed if any solution, containing exchangeable counter ions, is added continuously to an ion exchanger suspended in a solvent. The theory predicts further, in agreement with the experimental results, that in the initial phase of the ion-exchange reaction, the uptake of the counter ions is proportional to the square of the time. If experiments are performed with different rates of addition of the counter ions, the time required to exchange a certain amount of counter ions should be proportional to α^-1/2, where α is the rate at which the ions are added. This prediction is also in accordance with the experimental results.     

Competitive sorption of Pb by bentonite and sphagnum peat

In mixtures of known amounts of Ca-saturated Sphagnum peat and bentonite, the competitive sorption of lead ions by these components from a solution was investigated as a function of their ratio in the mixture and the solution concentration. By means of separation of the components after attainment of equilibrium and using Pb-212 as a tracer, the amounts of lead ions taken up by each sorbent were determined independently. The experiments show that at any concentration used, peat will sorb more lead ions than bentonite relatively to its amount (in me) present in the mixture at any ratio of the components. Starting from pure peat and increasing in the mixture the fraction of bentonite causes the peat to sorb-relatively to its percentage amount in the mixture-more lead ions. Starting with pure bentonite and increasing in the mixture the fraction of peat causes the clay mineral to sorb-relatively to its percentage amount in the mixture-fewer lead ions. Theoretical calculations are in quantitative agreement with experimental results.     

Effects of humic and fulvic acids on growth of tobacco. 2. Tobacco growth and ion uptake 1

Experiments were conducted in growth chambers of the Phytotron to evaluate the effects of humic and fulvic acids on growth and ion uptake by tobacco plants grown to flowering.

When small amounts of humic or fulvic acids were added to media used to grow seedlings, the dry weight of roots and tops and the height of the plants subsequently grown in a nutrient solution with small amounts of humic or fulvic acids increased, while at high rates of humic acids they decreased. Generally, there was a trend for low rates of humic acids to increase concentration of Ca and Mg in roots and leaves and to decrease Fe concentration in roots.

The total content of N, K, P, Ca, Mg and Fe in the stalk and leaves increased in the presence of small amounts of humic or fulvic acids in the nutrient solution. Low rates of humic or fulvic acids in the growth medium facilitated the adsorption of Ca, Mg and Fe and their translocation within the plant. For high rates of humic acids there was a trend to decrease the total content of N, K, P, Ca and Fe in the roots and leaves of the plant.     

镉在胡敏酸上的吸附动力学和热力学研究

采用镉离子选择电极研究了镉在胡敏酸上的吸附特征。实验结果表明 ,胡敏酸对镉的等温吸附规律与介质pH有关。当pH为 3 0 0和 3 50时符合Freundlich方程 ,当pH在 4 0 0～ 6 50之间更好地符合Langmuir方程 ;在pH为 3 0 0～ 6 50之间 ,吸附量与pH成显著正相关。温度升高吸附量降低 ,吸附热与反应介质的pH有关 ,pH升高 ,吸附反应放热增加 ;胡敏酸对镉的吸附作用力随介质pH改变发生较大变化 ,当pH为6 50时主要为配位基交换作用。胡敏酸对镉的吸附含有部分不释放氢的静电吸附 ,其吸附反应动力学用Elovich方程拟合效果较好     

The effect of ion exchange on the solubility of fluoride compounds

The influence of ion exchange processes on the apparent solubility of fluoride compounds was examined by observing changes in free F^? and total F levels when suspensions of sparingly soluble fluoride species (e.g., CaF₂, AlF₃) and aluminium smelter wastes were equilibrated with a range of materials having different cation exchange capacities. The exchanger materials used included synthetic resins, clay minerals, a humic acid and Mn(IV) oxide. The amount of fluoride ion released from the fluoride salts and F^? rich wastes was found to increase in the presence of solids capable of exchanging cations, and the magnitude of the effect tended to be determined by the number of exchange sites available and the affinity of the fluoride compound cation for the exchange material. In some instances (e.g., with illite and alumina wastes) the released fluoride ion attacked the substrate and formed soluble complex ions.     

KINETICS OF ION EXCHANGE IN SOIL ORGANIC MATTER. IV. ADSORPTION AND DESORPTION OF Pb2+, Cu2+, Cd2+, Zn2+ AND Ca2+ BY PEAT

The equilibria as well as the rates of adsorption and desorption of the ions Pb²⁺, Cu²⁺, Cd²⁺, Zn²⁺, and Ca²⁺ by soil organic matter were determined in batch experiments as a function of the amount of metal ions added to an aqueous suspension of HCl-washed peat. Simultaneous determination of the metal ions and hydrogen ions in the solution by atomic absorption spectrophotometry and pH-measurements showed that the adsorption of one divalent metal ion by peat was coupled with the release of two hydrogen ions. Since this equivalent ion-exchange process causes a corresponding increase of the electric conductivity of the solution, the rates of the adsorption and desorption processes were determined by an immersed conductivity electrode. The distribution coefficients show that the selective order for the metal adsorption by peat is Pb²⁺ > Cu²⁺ > Cd²⁺≌ Zn²⁺ > Ca²⁺ in the pH range of 3·5 to 4·5. The slope of -2, as observed in a double logarithmic plot of the distribution coefficients versus the total solution concentration confirms the equivalence of the ion-exchange process of divalent metal ions for monovalent H₃O⁺ -ions in peat. The absolute rates of adsorption, as well as the rates for the fractional attainment of the equilibrium, increase with increasing amounts of metal ions added. This behaviour is also observed for the subsequent desorption of the metal ions by H₃O⁺-ions. At a given amount of metal ions added, the absolute rates of adsorption decrease in the order Pb²⁺ > Cu²⁺ > Cd²⁺ > Zn²⁺ > Ca²⁺, while the rates for the fractional attainment of the equilibrium decrease in the order Ca²⁺ > Zn²⁺≌ Cd²⁺ > Pb²⁺ > Cu²⁺. The half times for adsorption and desorption were in the range of 5 to 15 sec.     

The influence of ion‐exchange substrates on grass growth in sandy soils

The effect of 2% (v/v) addition of ion exchange substrate to a sandy soil on the growth of Dactylis glomerata L., used as a test species, was studied. Four different exchange additives having pH values in the range 4.45–7.14 were evaluated in pot experiments. These additives were prepared as mixtures of monoionic forms of a strong acid cation exchanger and a weak base anion exchanger, loaded with nutrient ions. The additions of the ion exchange substrates to the sand affected plant growth advantageously, increasing the total dry biomass 3.8– 8.5 times. The most efficient additive appeared to be the substrate with the lowest pH because it contained the largest stock of nutrient elements. The excessive acidity of the additive was spontaneously neutralized in the mixtures with sand due to reaction of the acid excess with carbonates present in the natural sandy soil.     

Use of a coupled equilibrium model to describe the buffering of protons and hydroxyl ions in some acid soils

The buffering of protons and hydroxyl ions in acid soils was studied by the addition of small amounts of HCl, H₂SO₄, and NaOH in consecutive batch experiments using surface soils and subsoils from two Cambisols and one Podzol. A chemical equilibrium model was used to study the main buffer processes. The model included inorganic complexation and multiple cation exchange, and also the solubility of jurbanite and Al(OH)₃ for the subsoils. Buffering of protons was predicted quite well by the model for the surface soil of the Spodi-Dystric and Spodic Cambisols, suggesting that multiple cation exchange was the main buffer process. For the Podzol surface soil, however, the model overestimated proton buffering by cation exchange considerably. Hydroxyl buffering in acid surface soils could be described well by the model for the Podzol soil only. For the Cambisols, hydroxyl buffer reactions included not only cation exchange, but also solubilization of large amounts of organic matter and presumably deprotonation of dissolved organic carbon (DOC). Modelling proton and hydroxyl buffering in subsoils suggested that equilibrium with AJ(OH)₃ was not maintained for the Podzol and spodic Cambisol. Sulphate sorption had to be considered to describe titration experiments in all three soils. The assumption of jurbanite being in equilibrium with soil extracts was useful only for the Spodi-Dystric Cambisol.     

Extraction of metal ions sorbed on Mn(IV) oxides and CaCO3 using ion exchange resins

The ability of ion exchange resins to promote release of metal ions (Cu, Ph, Cd or Zn) presorbed on different structural forms of Mn(IV) oxide or CaCO₃ has been investigated using exchangers possessing different types of functional groups and charged in either the H⁺ or Na⁺ form. The exchanger uptake reflected the degree of soluble salt or labile complex displaced from the substrate surface, and its magnitude varied with metal ion involved, exchanger type and initial counter ion (i.e. H⁺ forms retrieved more than Na⁺ forms). With Mn(IV) oxides substrate structure was important, with CaCO₃ a major factor was dissolution of the matrix. The amount of calcite dissolved was controlled by the number of exchange sites introduced, and exchanger type. The `labile metal' levels determined by the exchanger technique did not directly correlate with bonding fraction category values obtained using a series of chemical extractant solutions.     

Modelling cations in three acid soils with differing acid input in Germany

Prediction of concentration changes of cations in soil solutions is complex, and chemical models are necessary for the purpose. The objective of this study was to determine whether the reactions considered in a coupled equilibrium model were appropriate to predict cation concentrations when the initial equilibrium was disturbed by adding small amounts of electrolytes. Multi-ion sorption in three acid soils (two Cambisols and a Podzol) was studied by sequentially adding small amounts of electrolytes to samples of the soils in batch experiments. A chemical equilibrium model that included inorganic complexation and multiple cation exchange was used to interpret the results. For the subsoils, the solubility of jurbanite was also included in the model. Model results for the two Cambisol surface soils agreed well or satisfactorily with the measured pH and sorption values of Na, K, Mg, Ca, Mn, Al and Fe, with a few exceptions. Linear correlation coefficients were generally between 0.97 and 1, and the regressin coefficients for cations (modelled against measured) lay between 0.6 and 1.3. For the subsoils sorption of sulphate was described satisfactorily for the spodic dystric Cambisol and to some extent for the spodic Cambisol. Correlation coefficients for subsoils lay between 0.63 and 1, and the regression coefficients (modelled against measured) were between 0.9 and 1.6 for the Cambisols. The model did not predict pH and sorption data in surface and subsurface soils with very small amounts of exchange capacities, pointing to the significance of cation buffering resulting from exchange sites. This study showed the usefulness and limitations of equilibrium models to predict the composition of the soil solution.     

红壤对铝、锰离子的吸附特征 Ⅰ.铝、锰离子的非等当量交换

对铝、锰离子在红壤中的吸附特征进行了研究,结果表明:离子交换能（或吸附能）的次序为铝>锰>钙>钾,但同一离子在不同固相中的交换吸附能力有异。离子的代入量和代出量随其加入量而递增,而代入量占所加数量的百分数则递减,遵守一般的离子交换规律。非等当量性为铝或锰离子参与交换所出现的一种特殊现象,是由于其水解产物的非电性吸附叠加于电性吸附所致,尤以在砖红壤体系中者为甚;非电性吸附量与平衡溶液浓度的关系符合Freundlish方程。     

Applications of the Continuous Flow Stirred Cell (CFSC) technique to adsorption of zinc, cadmium and mercury on humic acids

We discuss the sorption of Zn (II), Cd (II) and Hg (II) by H-saturated humic substances using the continuous flow stirred cell technique. Sorption isotherms were determined at two different flow rates and for each of two different humic preparations using radioisotopes of the above metals.

Sorption of mercury using two different anions (NO₃⁻ or Cl⁻) revealed very different behaviour indicating the importance of the anionic species in such studies. In general the adsorption isotherms determined for these metal ion-humic interactions were similar to those predicted from the Langmuir equation.

The data indicate two distinct mechanisms for the adsorption of the metal ions by the humic preparations. At the low metal concentrations used in all the CFSC measurements, sorption occurs without the release of a significant quantity of H⁺ ions, presumably by chelate formation. At higher metal concentrations interactions also involve the release of H⁺ ions from carboxyl-type exchange sites and another model may be required.     

Modelling changes in cations in the topsoil of an Amazonian Acrisol in response to additions of wood ash

Predictions of changes in soil solution chemistry and exchangeable cations which occur on ash deposition after slash burning are complex and may be facilitated by the use of chemical models. Multi-ion sorption in the topsoil of an Amazonian Acrisol was studied by sequentially adding small amounts of electrolytes to soil and mixtures of soil and ash in batch experiments. A chemical equilibrium model that included inorganic complexation, multiple cation exchange and sparingly soluble salts (aluminium hydroxide and magnesian calcites) was used to interpret the results. The model predicted well the pH and sorption values in all experiments in which there was no addition of ash. The model suggested that cation exchange was the main process determining concentrations of soil solutions in all cases where neutral salt solutions were added, and that proton buffering was achieved by the dissolution of Al(OH)₃ which was followed by Al³⁺ adsorption. Calculation of ion activity products in solutions from various batch experiments in soil + ash mixtures suggested that magnesian calcites of differing solubility may be in equilibrium with the activities of Mg²⁺ and Ca²⁺ in solution. An incongruent dissolution of Mg resulted in less soluble magnesian calcites in the ash. The model estimated satisfactorily the pH and the sorption of ions for all experiments with differing ash additions to the soil. Most of the Ca and significant amounts of Mg added in the ash are expected to remain for a long time in the soil and may determine the Ca and Mg status of the soil solution, primarily controlled by principles of solubility products.     

Abbau und Umwandlung von Pflanzenrückständen und ihren Inhaltsstoffen durch die Mikroflora des Bodens

Degradation and transformation of plant residues and their components by the microflora of the soil This review describes recent results of publications in this area. It condsiders the dynamics of degradation under field and laboratory conditions and indicates some of the problems of simulation models. The preponderant part of transformation processes apparently takes place in a relatively small fraction of soil organic matter which also includes the biomass. Methods for the quantitative measurements of the biomass have been considerably improved. The turnover rates of plant residues are effectively influenced by their chemical composition, for instance by their C/N ratio and their content of lignin or polysaccharides. C/N ratios also seem to influence the socalled priming effect and the transformation of increasing amounts of plant residues added to the soil. Some progress has been also made in the transformation of plant, residues under different climatic conditions. Experiments with polysaccharides and glucose have indicated that a major portion of the residual carbon residues in soil are contained in the biomass or N-containing microbial metabolites. A small part is also present in the phenolic constituents of humic compounds. Root excretions also contribute carbohydrates. Recent experiments indicated the transportation of a considerable amount of photosynthetic products from sprouts through roots into the soil. They cause intensive microbial turnover processes in the root zone. N-containing compounds are stabilized in the soil biomass. Proteins and aminopolysaccharides are furthermore stabilized by sorption on humic compounds or clay particles. They are even more effectively stabilized by linkage into humic compounds. Plant residues contain appreciable amounts of free and polymerized phenols which are degraded by microorganisms or incorporated into humic compounds. Easily oxidisable phenols are more slowly degraded in soil than the more stable ones. This is especially true for low concentrations of phenols added to the soil. The pH-values of the soil and its content of humic compounds influence essentially the degradation rate. Lignins belong to the most important natural biopolymers. Methods for their specific labelling with 14°C have essentially promoted studies about their degradation and transformation in soil and have enlarged the knowledge about lignin degrading organisms. Beside lignins and other plant polyphenols, melanins may play an important role in the formation of humic compounds. These melanins are synthesized form carbohydrates by some microscopic fungi through secondary metabolic reactions. They are formed from phenols, quinones, polyenes, aminoacid and aminosugar derivatives. Similar to humic acids they are relatively stabile against rapid microbial degradation. Analytical methods developed in the last years indicate that both groups of polymers contain similar building blocks.     

淹水还原条件下红壤中葡萄糖及腐殖酸对铁锰形态的影响

通过设置不同的葡萄糖/腐殖酸配比模拟还原性土壤体系,考察长期(约74 d)淹水培养过程中铁锰元素在土壤溶液/矿物相中的分布形态演变。结果表明:在淹水培养初期,葡萄糖的添加可以促进铁锰离子的还原溶出,同时土壤中可交换态和酸可提取态铁、以及可交换态锰的含量也会随之增加;而腐殖酸的添加则会促进土壤中可氧化态铁/锰含量的升高。随着培养时间的增加,铁锰离子浓度及各个土壤提取形态的铁锰含量大多呈现降低趋势,铁锰元素逐渐转化成提取性更低的矿物形态。因此,淹水环境中铁锰还原溶出-分布形态演变受到土壤中有机物质种类和含量的显著影响,呈现出不同的金属移动性和生物有效性。     

Multivariate statistical analysis of mass spectra as a tool for the classification of the main humic substances according to their structural and conformational features

The aim of this work is to explore the suitability of the complementary use of mass spectra and the corresponding statistical analysis (principal components-Pareto analysis (PCA) and discriminant analysis (DA)) of these spectra to differentiate diverse humic samples as a function of their structural and conformational features. To this end, the mass spectra of humic samples belonging to the main humic fraction types (gray humic acid, brown humic acid, and fulvic acid) were obtained by electrospray ionization mass spectrometry (ESI-MS). The results obtained showed that the application of PCA yielded a clear separation between blanks and humic samples. However, a clear differentiation among the humic fraction types was not achieved. The DA of PCA data, however, yielded a clear separation among the humic substances (HS) samples belonging to each HS fraction type considered: gray humic acids, brown humic acids, and fulvic acids. These results showed that the mass spectra of each humic sample include characteristic mass/charge (m/z) distribution values that can be considered as a "fingerprint" representative of its specific structural features. Our results also indicate that, although the m/z values principally corresponded to single-charged ions, we cannot identify these molecular weight distributions with those of humic samples, since sample molecular fragmentation, as well as partial molecular ionization, cannot be ruled out under our experimental and instrumental conditions.     

Effect of cultivation on chemical characteristics and respiratory activity of crusting soil from Mazowe (Zimbabwe)

Abstract

Clearing and cultivation in crusting soils from Mazowe (Zimbabwe) has lead to severe changes in most physico‐chemical characteristics related to the concentration and distribution patterns of plant nutrients and to the total amount of soil organic matter. Nevertheless, the concentration of the different humus fractions showed lower intensity changes, as did the mineralization rates of the organic matter. The most significant effects of cultivation on the soil chemical characteristics coincided with those considered to favor clay dispersion and crusting phenomena, including generalized desaturation of the exchange complex and losses of divalent ions with a potential bridging effect between soil particles. Concerning the soil organic matter, the humic acid tended to concentrate in the cultivated soils as a probable consequence of selective biodegradation of the other humic fractions. The composition and activity of soil humus suggest low‐performance organo‐mineral interactions: in these soils the active turnover of the plant wastes is not regulated by intense physico‐chemical interactions with the soil mineral fractions, or by physical encapsulation of organic particles. In consequence, the mineralization rates were relatively constant in the soils studied and unrelated to soil organic matter concentration. The results suggests that there is a possibility to revert the early degradation stages of these soils through a rational management of suitable amounts of crop wastes.     

Comparison of thermal and chemical stability of Cu–humic complexes

Purpose

The purpose of this study was to investigate relationships between chemical and thermal stabilities of Cu–humic complexes. The study of the chemical stability was based on pedological methods used for the determination of the bond strength of metal ions in soils by chemical leaching agents. The samples with various contents of the Cu(II) ions and their bond strength were put to the thermal analysis in order to correlate their thermo-oxidative behavior with their stability determined by leaching.

Materials and methods

The humic acid was extracted from the South-Moravian lignite by standard alkaline extraction. The humic sample was used in two different forms: as the solid powder and as the hydrogel prepared by the acidic precipitation of humate. Six various concentrations of copper(II) solutions were used for the complexation of the humic powder and the hydrogel, in order to study the influence of their initial concentration on both the determined stabilities of the prepared complexes. Their chemical stability was assessed in terms of the Cu(II) ions release from the humic acid structure into two different extraction agents (MgCl₂ and HCl solutions). Their thermo-oxidative behavior was investigated employing the thermogravimetry.

Results and discussion

The complexation capacity of the humic hydrogel was higher in comparison with the humic powder. The amounts extractable from the Cu–humic complexes by the used leaching agents are higher for the humic powder, which shows on the lower chemical stability. The thermal degradation of the prepared complexes proceeds in several steps and this character remains also after the removal of the mobile and the ion-exchangeable fractions by the MgCl₂. The elimination of these fractions as well as the extraction of the strongly bound Cu(II) ions shift the thermal degradation to higher temperatures. The incombustible residue increases with the Cu(II) content in the complexes except for the samples extracted by the HCl.

Conclusions

The form of humic sample used for the preparation of the Cu–humic complexes influences both the chemical stability and the thermal one. The main reason is probably a better accessibility of the functional groups in the humic gel, which enables forming stronger binding copper(II) ions. The results showed that the thermal and chemical stabilities are closely related, which corresponds with the shift of the thermal degradation to higher temperatures after removing the less stable fractions from the humic complexes.