THERMODYNAMICS OF K–Ca ION EXCHANGE IN SOILS1

Cation exchange characteristics of the K:Ca saturated forms of five soils were measured at 25°C and 50°C. The rates of isotopic exchange of ⁴²K and ⁴⁵Ca were too fast to be measured except that of ⁴²K in the K:Ca Harwell soil at 25°C. The slower isotopic exchange of K in this soil was attributed to the presence of a zeolite, clinoptilolite. The intra-particle diffusion coefficient, D_i, of K in this soil increased with K-saturation to a maximum at about 40 per cent K, probably because of the ‘blocking’ action of the larger hydrated Ca ions at small K-saturations in clinoptilolite. The CEC, measured by isotopic exchange along the K:Ca adsorption isotherm, decreased with increasing temperature probably because some interlayer spaces collapsed. The standard free energy, enthalpy, and entropy changes were negative for the reaction Ca-soil+2K⁺? 2K-soil+Ca⁺⁺. These results seem to show that K is more strongly bound than Ca by the soil and that the Ca-preference shown by the isotherm at small external electrolyte concentration is caused by entropy changes in solution. Calculated activity coefficients of the exchangeable ions changed with K-saturation similarly at both temperatures but values at 50°C were smaller than at 25°C.     

Timing,magnitude, and impact of acidic deposition on sensitive forest sites

Adverse effects of acidic deposition on forest health are most likely to occur in forests which develop a thick raw or “mor” humus layer in which the effective cation exchange capacity is highly sensitive to acid input. A study of the trend of exchangeable Ca and Mg ions in sensitive humus layers over the past six decades indicated that a downward shift in equilibrium has occurred that is consistent with theories of ion mobilization and coincident in time with increasing acidic deposition in the mid-1900's. Independent records of a base cation mobilization in wood supports the view of a change in the root zone in sensitive forest sites and in lake water chemistry. Induced nutrient deficiency in declining forest stands was indicated by a high Al∶Ca ratio in fine root tip tissue as a marker of altered uptake efficiency which leads to increased vulnerability to biotic factors and greater sensitivity to abiotic stresses. Evidence indicates that sensitive sites were damaged by acidic deposition 20 to 40 yr ago, long before the problem of “acid rain” was recognized.     

THERMODYNAMICS OF CATION EXCHANGE ON CLAY: Ca-K-MONTMORILLONITE

Radioactive elution and batch equilibration techniques have been used to obtain isotherms at 30° C, 50°C, and 70° C for the exchange of Ca for K on K-montmorillonite. From these the changes in enthalpy and entropy were + 3.87 kcal/mole and +6.7 e.u./mole of Ca respectively. The activity coefficient for K decreased from approximately 1.5 to 1 and for Ca from 1 to approximately 0.5 as the fraction of K on the clay increased from 0 to 1. The results are discussed especially in relation to hydration of the ions and adsorption of water on the clay. The entropy of adsorbed water is inferred, from the results, to be less than that for liquid water.     

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。     

THERMODYNAMICS OF SINGLY CHARGED ION EXCHANGES IN THE TRACE REGIONS ON CAMP BERTEAU MONTMORILLONITE

Ion exchange equilibria for seven pairs of singly charged ions between aqueous chloride solutions and a montmorillonite clay have been studied at 25 °C for low values of the total normality of the equilibrium solution. Only the trace region for the preferred ion was investigated. Since the analogue of Henry's law was found to be valid, the exchanges being almost ideal, standard free energies were calculated from experimental data in the region where the analogue of Henry's law is obeyed by extrapolating to half-loading with one of the ions. The results satisfy the requirement of additivity and are in good agreement with known values.     

Oxidation-induced leaching of sulphate and cations from acid sulphate soils

Leaching of sulphate and cations from horizon samples of two acid sulphate soils (0.9 to 1.6% S in subsoil) was studied in the laboratory. Samples were incubated and eluted with water at 20 °C and 5 °C until apparent exhaustion of leachable S resources. The leachates were analyzed for pH, SO₄-S, Fe, Al, Mn, K, Ca, Mg, and Na. Oxidation of sulphide was retarded at the lower temperature. From all the originally water-logged samples the sulphate formed was initially washed out with base cations (mainly with Mg), but the proportion of acid counter ions (predominantly Al) increased with proceeding oxidation and acid formation. In the most acid leachates, pH was 2.6 to 2.8. In the transition layer between reduced and oxidized horizons, sulphide oxidation had been going on for some time, and acid cations were the main counter ions for sulphate already at the beginning of the experiment. In the totally oxidized surface horizons, sulphate was leached only in moderate quantities, and the sum of cation equivalents (mainly base species) exceeded that of sulphate, suggesting some removal of other anions. Leaching losses in the laboratory experiment, corresponding to drainage-induced loading of waters in field experiments during the course of many decades, point out the environmental danger associated with deep drainage of potentially acid sulphate soils.     

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.     

Studies on the thermodynamics of zinc exchange in montmorillonite

The ion exchange equilibria and mechanism of Zn exchange with Na montmorillonite has been studied with the help of adsorption isotherms and thermodynamic parameters. The exchange isotherms, K and △G° values indicated a spontaneity of reaction and a higher preference of Zn for the montmorillonite surface. A stronger binding of Zn and changes in the hydration rates of Zn and Na with increase in order were suggested by enthalpy an entropy effects respectively. The surface phase activity coefficients and the excess thermodynamic functions were indicative of a nonideal heterogeneous exchange in which the mixture of ions on the montmorillonite surface was more stable and more tightly bound with significant differences in the hydration rates of the ions in the mixture with respect to the homo-ionic forms.     

Speciation and mobilization of aluminium and cadmium in podzols and cambisols of S. Sweden

Processes governing the mobilization of Al and Cd in podzols and cambisols of S. Sweden having different tree layer vegetation (Picea abies, Fagus sylvatica, or Betula pendula) were investigated. Speciation of Al and Cd in soil solutions were performed by a column cation exchange procedure (cf. Driscoll, 1984) in combination with thermodynamic calculations. Podzols in spruce and beech stands were characterized by a high release of organic compounds from the O/Ah horizons, resulting in a high organic complexation of Al (c. 93%) in the soil solution from the E horizon (15 cm lysimeters). Organic complexes were mainly adsorbed/precipitated in the upper Bh horizon and the overall transport of Al at 50 cm depth was governed by a pH dependent dissolution of a solid-phase Al pool. In the cambisols, inorganic Al forms were predominant at both 15 and 50 cm depth, and Al solubility was closely related to solution pH. Secondary minerals like synthetic gibbsite, jurbanite, kaolinite or imogolite could generally not explain measured solution Al³⁺ activities. Results instead indicated that the relatively large organically bound solid-phase Al pools present in both soil types could do so. The column fractionation procedure could be used only qualitatively for Cd, but results strongly indicated that Cd-organo complexes contributed significantly to the overall mobilization of Cd in the podzol E horizons. In all other soil solutions, Cd²⁺ was the predominant species. Both solid-phase and solution chemistry suggests that ion exchange processes controlled the Cd²⁺ activities in these solutions. All reactive solidphase Cd was extractable by NH₄Cl and Cd²⁺ activities could in most cases effectively be modeled by the use of ion exchange equations. Solubilized Al³⁺ efficiently competed for exchange sites and played an important role for the Cd mobilization in these soils.     

Changes in aluminum pools of andisols due to soil acidification

Abstract

Aluminum concentrations in soil solutions are not only controlled by inorganic clay minerals but also by organically bound aluminum. The objective of this study was to determine which pools contribute to Al dissolution. Soil samples were taken at various distances from tree trunks and at various depths at the Rolling Land Laboratory (RLL), Hachioji, Tokyo. Selective dissolution techniques were used to analyze the changes in pools of solid-phase aluminum. Soil pH values around Hinoki cypresses were in the aluminum buffer range. Exchangeable aluminum contents in soils under Hinoki cypresses were 104 mmol_c kg-^?1 on the average. This value was similar to that of the cation exchange capacity (CEC) of Andisols at RLL at a soil pH of 4. The relationship between the soil pH and exchangeable, organically bound, and amorphous aluminum pools showed that dissolved aluminum ions in the soil solution were primarily derived from the amorphous Al pool. Dissolved aluminum ions were substituted with base cations of soils, resulting in the increase of the content of exchangeable Al and/or the formation of complexes with organic matter which increased the proportion of organically bound Al pools. Increase in the proportion of organically bound Al pools indicated the importance of complexation with soil organic matter for controlling the aluminum concentration in the soil solution.     

Effect of Al hydroxide polymers on cation exchange of montmorillonite

Al hydroxide polymers (AlHO) can significantly influence the cation exchange behaviour of clays. We have determined the effect of synthesized AlHO on Ca–Na, Zn–Na and Pb–Na exchange for a series of exchanger compositions and two Al loadings at pH 6.0 and an ionic strength of 0.01  m . The preference for Ca on the siloxane surface of the clay–AlHO system (CAlHO) was greater than for the pure clay, and the average K _V (Vanselow selectivity coefficient) was determined to be 2.16 and 1.24, respectively. The selectivity coefficients for the exchange reactions Zn–Na and Pb–Na were not directly determined in CAlHO systems, because heavy-metal ions bind as well to the clay surface as to the AlHO over a wide range of pH. We have estimated the effect of the presence of AlHO on the selectivity coefficients of Zn–Na and Pb–Na exchange by extrapolation of the experimental results of Ca–Na, Zn–Na and Pb–Na exchange for pure clay and Ca–Na exchange for CAlHO. The average K _V was increased by the presence of the AlHO from 1.23 to 2.16 for Zn–Na exchange and from 1.59 to 2.77 for Pb–Na exchange. The increase in the preference for the divalent cations is probably caused by parallel alignment of clay platelets by sorption of AlHO. Increasing the amount of AlHO did not change the selectivity for Ca–Na exchange, and probably the structure of the system or the arrangement of the clay platelets and AlHO particles was not substantially changed. This was supported by the linear reduction of the cation exchange capacity with amount of AlHO present at pH 6.6. It seems likely that the selectivity coefficients for Ca–Na, Zn–Na and Pb–Na exchange that we found apply in naturally occurring montmorillonite–AlHO systems.     

Release of Al by hydroxy-interlayered vermiculite and hydroxy-interlayered smectite during determination of cation exchange capacity in fine earth and rock fragments fractions

The fine earth (<2 mm) and rock fragments (>2 mm) fractions of two soils derived from Oligocene sandstone have been examined to assess the origin of the discrepancies between cation exchange capacity (CEC) and effective CEC (ECEC). The soils differ in terms of acidity: soil A is more acid than soil B. When the A samples are treated with BaCl₂, the solution became sufficiently acid (pH < 4·5) to dissolve and to maintain Al in solution. From these samples more Al is released than base cations. Aluminium was continuously replenished even after 192 h, so that the ECEC was always larger than the CEC. Samples from soil B contain less H and Al ions, and the BaCl₂ solution could not lower the pH below 5·0. In these samples little Al is released, and the base cations dominate the exchangeable pool of ions. This Al can be considered to be exchangeable, and a good agreement exists between the ECEC and the CEC. The source of non-exchangeable Al in the A samples is the OH-Al polymers of the hydroxy-interlayered vermiculite (HIV) and hydroxy-interlayered smectite (HIS) that tend to dissolve during the BaCl₂ treatments. In the less acid B samples the Al polymers are not affected by BaCl₂ treatment. Different results were obtained when the clays, extracted from an Na-dispersed suspension, were treated with BaCl₂ solution. Because the clays are no longer acid, no H⁺ is released, and the OH-Al polymers are not dissolved. Therefore, the saturating ions play an important role in the dissolution of the OH-Al polymers and cause differences between the CEC and ECEC. We discount organic matter and specifically Al-organo complexes as a source of non-exchangeable Al. Both A and B soils contain very similar pyrophosphate-extractable Al, but show substantial differences in the amount of exchangeable Al.     

The influences of temperature on aqueous aluminium chemistry

Temperature affects the solubility of Al(OH)₃(s), the solubility product formed, the hydrolysis and molecular weight distribution of aqueous Al species as well as the pH of the solutions. In the present work, identical solutions of inorganic Al (400, 600, and 800 μg Al L^?1) were stored for 1 mo at either 2 or 25 °C. In the solutions stored at 25 °C pH varied from 4.83 to 5.07, while in the corresponding solutions stored at 2 °C pH varied from 5.64 to 5.78. In spite of the relatively low pH at 25 °C, significant amounts of high molecular weight Al species were precipitated from the solution and the solubility product (log^* K _s ) of (Al(OH)₃) (s) was low (9.0). Substantial amounts of high molecular weight Al species were also formed at 2 °C, but the majority was present as colloids in the solution. The solubility product (converted from 2 into 25 °C) was 10.2, reflecting a solubility product of an amorphous (Al(OH)₃)(s) phase. The different physico-chemical forms of Al present at 2 and 25 °C should have relevance for water/soil chemistry modeling.     

Chemistry of soil aluminum

Abstract

Better understanding of soil aluminum has had dramatic effects on the interpretation of many aspects of soil chemistry. Aluminum is a Group III element, metallic in nature, and exhibits both ionic and cuvaient bonding. It is the most plentiful of all metallic cations of the earth's crust. It is released from octahedral coordination with oxygen in minerals by weathering processes. Once released, the trivalent Al ion assumes octahedral coordination with six OH₂ groups each of which dissociates a H ion in sequence as pH increases. The resulting hydroxy‐Al ions are absorbed to the cation exchange capacity of the soil. Here they polymerize on charged surfaces and in the interlayers of the clay minerals obstructing both the contraction of the clay lattice and the exchange of cations. Soluble Al is toxic to most plants, and reacts readily with soluble phosphates converting them to relatively insoluble and plant‐unavailable forms. Adsorbed and polymerized aluminum affects actual lime requirements of soils by its acidic nature and indicated lime requirements by its effect on the buffers of the lime requirement test. The level of exchangeable Al has been suggested as an Index of lime requirement of acid soils, but this may be an adequate Index for liming only on highly weathered soils.     

EFFECT OF SOIL pH AND ORGANIC MATTER ON LABILE ALUMINIUM IN SOILS UNDER PERMANENT GRASS

The rates of extraction of Na, K, Mg, Ca, and Al with 1M NH₄ NO₃ from the mineral-and organic-rich layers of some Park Grass (Rothamsted) soils were measured at the pH of the soil. Below pH 3.7 exchangeable Al, derived from the kinetics curve, increases with decreasing soil pH and is less in the organic-rich layer. The sum of the basic exchangeable cations, ∑(Na + K + Mg + Ca), increases with increasing soil pH and is more in the organic-rich layer. The extraction of exchangeable Al obeys first order kinetics, the rate constant being similar for all the soils (mean value 36 ± 7 × 10^?6|s^?1), which implies that exchangeable Al is released from surfaces with similar properties for the adsorption of Al, and that the rate is not affected by soil pH and organic matter. The rate of extraction of non-exchangeable Al is the same in the mineral-and organic-rich layers of each soil, and is maximal at about pH 3.7, decreasing sharply at more and less acid pH values.     

SOLUBLE ALUMINIUM AND CALCIUM-ALUMINIUM EXCHANGE IN RELATION TO THE pH OF DILUTE CALCIUM CHLORIDE SUSPENSIONS OF ACID SOILS

Measurements of pH and A1 concentration were made on 10^-2 M CaCl₂, suspensions of a number of acid soils that had been limed to give 3 range of pH values, and exchangeable A1 and Ca+Mg were determined in 1.0 M NH₄Cl extracts. The variation of pH with A1 concentration did not support the theory that pH is controlled by the solubility of Al(OH)₃. For some of the soils, proton release on hydrolysis of A1₃₊ions in solution accounted for the pH values, and explained quantitatively the variation of pH with the Ca:Al balance of the exchange complex, taking account of the selectivity coefficient for exchange, K_ca→A1 Although K_ca→A1 was smaller for soils containing more humus, their pH values were also less than those predicted by the hydrolysis of A1³⁺ in solution, indicating that they contained other sources of protons, presumably the carboxyl groups in humus.     

Lysimeter study with a cambic arenosol exposed to artificial acid rain: II. Input-output budgets and soil chemical properties

The effects of artificial precipitation with different pH levels on soil chemical properties and element flux were studied in a lysimeter experiment. Cambic Arenosol (Typic Udipsamment) in monolith lysimeters was treated for 6 1/2 yr with 125 mm yr^?1 artificial rain in addition to natural precipitation. Artificial acid rain was produced from groundwater with H₂SO₄ added. pH levels of 6.1, 4 and 3 were used. ‘Rain’ acidity was buffered, mainly due to cation exchange with Ca²⁺ and Mg²⁺, which were increasingly leached due to the acid input. The H⁺ retention was not accompanied by a similar increase in the output of Al ions, but a slight increase in the leaching of Al ions was observed in the most acidic treatment. The net flux of SO₄ ^2? from the lysimeters increased with increasing input of H₂SO₄, but in the most acidified lysimeters significant sorption of SO₄ ^2? was observed. The sorption was, however, most likely a concentration effect. The ‘long-term’ acidification effects on soil were mainly seen in the upper O and Ah-horizons, where an impoverishment of exchangeable Ca²⁺ and Mg²⁺ was observed. An increased proportion of Al ions on exchange sites in the organic layer was observed in the pH 3-treated soil. By means of budget calculations the annual release of base cations due to weathering was estimated to be between 33 and 77 mmol_c m^?2.     

THERMODYNAMICS OF COBALT(I1)-SODIUM EXCHANGE ON MONTMORILLONITE CLAY

An attempt has been made to predict the mechanism of Co(I1)-Na exchange on montmorillonite with the help of thermodynamic parameters derived from the experimental results. The exchange isotherms indicated a preference of Co ions for the clay phase, a fact in accordance with the separation factor. The reaction was accompanied by a decrease in the free energy, enthalpy, and entropy of the system revealing a tighter and more orderly structure of Co ions on the montmorillonite surface.     

Effects of acid irrigation and liming in a norway spruce stand (Picea abies [L.] KARST.)

Acid irrigation (pH 2.7 to 2.8; mean annual input 4.1 kmol H⁺ ha^?1 as H₂SO₄) has caused significant changes in the chemistry of the soil of a mature Norway spruce stand (Picea abies [L.] KARST.) after 4 years of treatment. In the surface humus layer around 20% of the exchangeable Ca, Mg, K and Mn ions were leached. This was connected with a decrease of pH and cation exchange capacity. In the mineral soil no changes of pH and cation adsorption were observed. However there was a significant increase of Al³⁺ ions in the soil solution, exceeding 20 mg L^?1, mainly caused by dissolution of Al-hydroxides and Al-hydroxosulphates. Also the concentrations of ionic Cu, Zn and Cd were nearly doubled. Manganese concentrations are fluctuating according to periods with and without acid irrigation, showing reduction and oxidation phases. In contrast to microorganisms, certain moss species and Oxalis acetosella, the mature spruce stand was not severely damaged up to now. It is hypothesized that Ca/Al and Mg/Al ratios of single horizons are insufficient for characterizing Al stress in the field. Liming (4 Mg ha^?1) led to a significant increase of dissolved organic C, which is associated with mobilization of metals such as Pb, Cu and Al in organic complexation. Also nitrification increased in the surface humus layer. As a consequence the nitrate concentrations in the seepage water exceeded 250 mg NO₃ L^?1.     

THE SIGNIFICANCE OF TRUNCATION IN THE EVOLUTION OF SLOPE SOILS IN MID-WALES

The pattern of loss and gain of Fe was quantitatively determined for the soils of two hill slope sequences in mid-Wales. Overall, both sequences showed a net gain in Fe, with individual profiles showing gains related to slope. The pattern of gains is discussed in relation to lateral movement, aerial input and movement of Fe out of the hill mass. It is concluded that creep erosion (truncation) offers the only adequate explanation of the data recorded; further, that truncation is concurrent with vertical redistribution of Fe and constitutes a major evolutionary process on sites with slopes greater that l0°