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.     

Thermodynamics of K-Ni and Ca-Ni exchange reactions on kaolinite clay

Exchange equilibria involving nickel ions on potassium and calcium kaolinites were studied using exchange isotherms and derived thermodynamic parameters. The equilibrium was attained within 3 h and maximum adsorption occurred at pH 5.5. The exchange at pH 5.5 and an equilibrium time of 3 h yielded isotherms, equilibrium constants and standard fne energy values indicating a higher preference for nickel ions on the kaolinite surface. A stronger binding of nickel ions on the kaolinite surface was suggested by the enthalpy changes and the entropy changes indicate an increase in order of the system which may be due to changes in hydration states of nickel, potassium and calcium ions. The surface-phase activity coefficients of the cations showed that with increasing nickel saturation, in the Ca-Ni system f_Ni continuously increased while f_Ca decreased; in the K-Ni system f_K first increased and then decreased while f_Ni first decreased and then increasad, which may be attributed to differences in the distribution of ions in the Gouy-Stem layer. The excess thermodynamic functions showed that the exchange reactions were non-ideal: that is the mixture of cations oh the kaolinite surface was more stable and more tightly bound than the homo-ionic forms.     

Thermodynamics of exchange of zinc with Mg-bentonite and Mg-illite

The ion-exchange equilibria of Zn with Mg-bentonite and Mg-illite have been studied with the use of exchange isotherms and thermodynamic parameters. The exchange isotherms and the equilibrium constants point to a lower preference for Zn than Mg ions by both clays. Preference differed with temperature. Free energy changes were in agreement with this conclusion. Positive enthalpy and entropy changes occurred during Zn exchange with bentonite, whereas the reverse was the case with illite. These changes indicate diversity in the strength of binding and order of distribution of Zn ions on surfaces of the two clays.     

不同阴离子溶液中怀俄明州蒙脱石的Na-Zn交换选择性

The effect of background anion on cation exchange reactions, such as Na-Ca and Na-Cu exchange reactions, on montmorillonites has been studied, but the results are not always clear and discrepancies exist in the literature. In this study, the exchange of zinc (Zn²⁺) for sodium (Na⁺) on Wyoming montmorillonite was investigated at 298°K using Cl^-, ClO₄^-, NO₃^-, OAc^-, and SO₄^2- solution media at a constant total metal charge concentration of 0.0200 molc L^-1. Results indicated that the clay CEC values were essential similar for Cl^-, ClO₄^-, NO₃^- and SO₄^2- solution media with an average CEC of 0.856 ± 0.008 molc kg^-1; in an OAc⁺ solution the clay CEC was much higher than that in other anion media. The specific adsorption of Zn (SAZn), as defined by the extraction of Zn using 0.05mol L^-1 Na₂-EDTA, was different in the various background solutions. The highest value for SAZn was 0.359 ± 0.0350molc kg^-1, which occurred in OAc-solution. There was essentially no difference in the total apparent adsorbed metals (the sum of adsorbed equivalents of Na and Zn per kilogram of clay, Q) among the various background solutions. The average Q for all anion media was 0.807 ± 0.011 mol_c kg^-1 and was independent of exchanger composition. Experimental results indicated that there were no significant monovalent cation complexes such as ZnCl⁺ or ZnNO₃⁺ that were adsorbed by montmorillonite. The Na-Zn exchange isotherms indicated that there was an adsorption preference for Zn over Na on Wyoming montmorillonite.     

THERMODYNAMICS OF THE EXCHANGE OF NICOTINE ON ALUMINIUM-MONTMORILLONITE

The ion exchange equilibria involving the interaction of nicotine with Almontmorillonite was studied thermodynamically. The exchange isotherms at 30° and 60°C indicated a preference for Al ions by montmorillonite as compared with nicotinium ions. A separation factor supported the conclusion. The enthalpy gain indicated tighter binding of Al. The entropy gain indicated a diffused and disordered arrangement of nicotinium ions in the Goüy layer with Al forming a more ordered arrangement in the Stern layer.     

Potassium additions on ionic equilibria,selectivity and diffusion of cations in soils

Abstract

In a soil system variation in the concentration of any one ion as induced by external addition might bring changes in the ionic‐equilibria, diffusion rate and strength of adsorption of all the ions involved. In four Indiana soils the changes in ionic equilibria, selectivity coefficient and rate of diffusion coefficient for K, Na, Ca and Mg were investigated at 5 levels of added K. The experiments were carried out under controlled laboratory conditions by incubating soils for 3 weeks at 25C. All soils had a greater fraction of Ca and Mg on the exchange phase than in solution, whereas with K and Na the reverse occurred. Potassium adsorption isotherms for all the soils differed indicating the difference in the nature of soil materials involved. Chalmers soil with high clay content with high exchange capacity had high differential buffer value for K. In all the soils, K was adsorbed preferentially to Na at all the levels of K addition, Calcium was adsorbed preferentially to Mg on the Zanesville and Toronto soils. However, in Chalmers and Raub soils, reverse was observed when the level of K addition was exceeded 1.0 and 0.5 me K/100g soil, respectively. This difference in Mg for Ca is attributed to smaller proportion of Mg saturation on the exchange surface. Divalent cations were preferentially adsorbed over monvalent ions. Increasing levels of K addition increased the diffusion rates of all the ions under consideration. The rate of diffusion for K and Ca were governed by concentrations of these ions on the exchange and solution phase.     

Geochemical Fractionation and Adsorption Characteristics of Zinc in Thai Major Calcareous Soils

Zinc (Zn) is a vital plant nutrient that is widely deficient in Thai cultivated calcareous soils. The chemical fractionation and adsorption of Zn are among the most important solid- and liquid-phase interactions that determine the retention of Zn in the soils. This study aimed to investigate the fractionation and adsorption isotherms of Zn in cultivated Thai calcareous soils. The results of sequential extractions showed that Zn is mainly distributed in residual fractions followed by organic-bound, iron and manganese oxides-bound, carbonate-bound, and exchangeable Zn, respectively. Zinc adsorption was well fitted by the Langmuir and Freundlich isotherms. Thai calcareous soils had high Zn adsorption capacity. Soil pH, organic carbon, calcium carbonate, cation exchange capacity, and extractable calcium were the major soil properties that affected the Zn adsorption isotherms in these soils. Zinc hydroxide was the solid precipitate and the Zn hydroxide ion (ZnOH⁺) was the dominant Zn ion in alkaline equilibrium solution.     

A new model for cation exchange equilibrium considering the electrostatic field of charged particles

Purpose

The aim of this study was to establish a new equilibrium model for cation exchange that quantitatively describe the three important effects: (1) the effect of electrostatic field around soil particles on exchange adsorption; (2) the effect of ionic interaction energy on Boltzmann distribution of cations; and (3) the effect of hydration radius of cation species on cationic distribution between adsorption phase and solution phase.

Materials and methods

In this paper, the Li/Na, Li/K and K/Na exchange were studied theoretically and experimentally. Purified montmorillonite was used as the experimental material and it was H-saturated in advance. In experimental study, approximately 2.5?g of the H-saturated sample was weighed in each experiment, then LiOH/NaOH, LiOH/KOH or KOH/NaOH mixture solution was added. In each experiment, the suspension was allowed to equilibrate for 48?h with continuous shaking at 298?K in an incubator shaker, and then 1?mol/l HCl was used to adjust pH to pH?=?7 when equilibrium was reached. Finally, the suspension was centrifuged and the concentration and of Li⁺, Na⁺ or K⁺ in supernatant was determined.

Results and discussion

In theoretical study, firstly, considering the ionic interaction energy in bulk solution, a modified Poisson?CBoltzmann equation was obtained; secondly, considering the electrostatic field around soil particles, the relationship between the selectivity coefficient and surface potential of particles was established; and thirdly, the modification factors were introduced to modify the effective charge of two cation species that involved in exchange because of the difference in hydration radius. Finally, the new models for describing cation exchange equilibrium were developed. Both theoretical and experimental results showed that the electrostatic field, the ionic interaction energy and the difference in hydration radius of two cation species strongly influenced cation distribution or cation exchange equilibrium. The results indicated that the effective charge could be obtained through either experimental determination or theoretical calculation, and the theoretically predicted values met the experimental results well. Therefore, the ion exchange selectivity series of different cation species with the same valence could be evaluated quantitatively.

Conclusions

New equilibrium models for describing cation exchange were established, for which the three important effects was quantitatively taken into account: the electrostatic field on exchange adsorption, the ionic interaction energy on Boltzmann distribution and the hydration radius of cation species. Both theoretical analyses and experimental results demonstrated that the cation hydration radius and the ionic interaction energy strongly influenced the exchange equilibrium considering the electrostatic field of charged particles.     

Interactions between montmorillonite and fulvic acid

The isotherms at 20°C for the adsorption of ¹⁴C-labelled fulvic acid from aqueous solutions by montmorillonite containing different exchangeable cations, have been determined. Below a concentration of 0.45 mg/ml and at near neutral pH, all samples give linear isotherms, the slope of which increases in the order Ba²⁺ < Ca²⁺ < Zn²⁺ < La³⁺ < Al³⁺ <Cu²⁺ < Fe³⁺. In the absence of interlayer expansion, the shape of the isotherms is interpreted in terms of solute entry into intercrystalline pores within a clay domain. The affinity of fulvic acid for the clay surface is related to the ionic potential or polarising power of the exchangeable cation. It is inferred that fulvic acid adsorbs by hydrogen bonding between an anionic group of the acid and a water molecule in the primary hydration shell of the saturating cation. Comparison of the data with those for the humic acid extracted from the same organic matter source, indicates that secondary interactions between adsorbed molecules or directly with the montmorillonite surface, contribute to the overall affinity. With samples saturated with Al³⁺, Fe³⁺, and Cu²⁺ ions, part of the adsorbed fulvic acid may also be attached to the clay by a complexation reaction involving the metals as such, or when they exist as polyhydroxy compounds at the mineral surface.     

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.     

Surface loading effect on Cd and Zn sorption by kaolinite and montmorillonite from low concentration solutions

Studies of Cd and Zn sorption using Na-saturated kaolinite and montmorillonite, and low metal solution concentrations similar to those found in the environment, showed that metal sorption affinity (measured by K _d values) decreased markedly with increasing surface metal loading for both layer silicates. For equilibrium solution concentrations <0.1 μmol L^?1 for Cd, and < 1 μmol L^?1 for Zn, both metals were sorbed with greater affinity by kaolinite than montmorillonite. These results were probably due to the higher proportion of weakly acidic edge sites present on kaolinite surfaces. In the case of Zn there was an affinity reversal for equilibrium solution concentrations > 1 μmol L^?1, which was attributed to the permanent charge sites of montmorillonite. Cadmium ions were sorbed, by kaolinite, with greater affinity than Zn for equilibrium solution concentrations between 0.3 to 1.5 μmol L^?1. This result was attributed to retention of these metal ions through electrostatic attraction by permanent charge sites present on the kaolinite used in this work. According of these results it seems that metal sorption by these layer silicates involves predominantly edge weak acid sites at lower surface coverages (higher affinity sites), and permanent charge sites at higher metal coverages (lower affinity sites). It was concluded that Cd and Zn sorption by those two layer silicates is greatly influenced by surface metal coverage, and results cannot be extrapolated from low to high surface coverages, and viceversa.     

Calcium-sodium exchange in some calcareous soils and a montmorillonite clay as compared with predictions based on double layer theory

A comparative study between Gapon's equation and the standard diffuse double-layer theory using activities of the ions in the equilibrium solution indicate that calcium-sodium exchange equilibria in calcareous soils cannot be exclusively attributed to valence and concentration effects. The large correction factors for surface charge densities necessary to make the experimental data fit the theoretical values stress the fact that some specific adsorption properties must be involved which are found to be partially due to organic matter and calcium carbonate.In the case of clay, the good agreement between the theoretical values and experimental data - with a correction factor of 1.6 for the surface charge density - is observed only above 50% Na saturation of the cation exchange capacity. At high Ca saturation of montmorillonite, specific adsorption energies are found to be dominant presumably due to tactoid formation.     

Response of the sorption behavior of Cu,Cd, and Zn to different soil management

This study investigated the effect of different farming practices over long time periods on the sorption‐desorption behavior of Cu, Cd, and Zn in soils. Various amendments in a long‐term field experiment over 44 y altered the chemical and physical properties of the soil. Adsorption isotherms obtained from batch sorption experiments with Cu, Cd, and Zn were well described by Freundlich equations for adsorption and desorption. The data showed that Cu was adsorbed in high amounts, followed by Zn and Cd. In most treatments, Cd ions were more weakly sorbed than Cu or Zn. Generally, adsorption coefficients K_F increased among the investigated farming practices in the following order: sewage sludge ≤ fallow < inorganic fertilizer without N ≈ green manure < peat < Ca(NO₃)₂ < animal manure ≤ grassland/extensive pasture. The impact of different soil management on the sorption properties of agricultural soils for trace metals was quantified. Results demonstrated that the soil pH was the main factor controlling the behavior of heavy metals in soil altered through management. Furthermore, the constants K_F and n of isotherms obtained from the experiments significantly correlated with the amount of solid and water‐soluble organic carbon (WSOC) in the soils. Higher soil pH and higher contents of soil organic carbon led to higher adsorption. Carboxyl and carbonyl groups as well as WSOC significantly influenced the sorption behavior of heavy metals in soils with similar mineral soil constituents.     

THE INFLUENCE OF MAGNESIUM IN SALINE AND SODIC SOILS: A SPECIFIC EFFECT OR A PROBLEM OF CATION EXCHANGE?

Na-Mg and Na-Ca exchange isotherms were determined at electrolyte concentrations of 500, 50, 5 and 1 meq per litre for illite, vermiculite and montmorillonite and for three soils containing illite and montmorillonite. This enabled comparisons to be made of clay swelling, dispersion and soil hydraulic conductivity changes between the Na-Mg and Na-Ca systems at known SAR, ESP and electrolyte concentration. Na-Mg montmoriUonite and a montmorillonitic soil behaved identically to the Na-Ca systems at the same ESP and electrolyte concentration: there was no specific effect. At the same SAR, the higher ESP in the Na-Mg system caused greater changes than in the Na-Ca systems. The Na-Mg vermiculite, illite, illitic soil and mixed illite-montmorillonitic soil showed greater changes than the Na-Ca systems at the same ESP, and there was a specific effect. At the same SAR, the higher ESP in all the Na-Mg systems apart from vermiculite increased the differences, but for vermiculite with a lower ESP, the differences were reduced. The lack of a specific effect for montmorillonite is probably related to the demixing of cations with the divalent ions concentrated on the non-swelling (internal) surfaces of the crystals.     

SOIL ALKALINITY. I. EQUILIBRIA AND ALKALINITY DEVELOPMENT

Theoretical relationships between pH, CO₂ partial pressure and alkalinity (bicarbonate + carbonate concentrations) have been shown to apply to solutions and calcite and soil suspensions. The exchange of Na onto three Ca clays shows that Ca is preferred but with negative free energies of exchange. With decreasing total electrolyte concentration, the preference for Ca increases, so that only when the concentration is above 10^-3M will significant amounts of exchangeable Na be found in soils. The preference for Ca is illite > montmorillonite > vermiculite. Dilution of the mixed Na-Ca clay suspensions causes exchange, desorption of Na and. in some cases Ca. This desorbed or ‘alkaline’ Na (and Ca) is replaced by H some of which attacks the clays. The apparent hydrolysis coefficient, KG = H adsorbed (Na)/Na adsorbed (H), varies between 5 and 9 × 10⁵, increases with increase in electrolyte concentration and varies in the order vermiculite > illite > montmorillonite. The concentration of alkaline Na + Ca increases with increasing ESP, and with clay type in the same order as the KG values. The pH calculated from the alkaline Na + Ca, assumed equal to the alkalinity was equal to measured values except for montmorillonite where the calculated values were low. Magnesium release by acid attack of the montmorillonite may explain the differences. Vermiculite rich soils will be most likely to accept exchangeable Na and to hydrolyse and develop alkalinity.     

Adsorption and binding of amino acids on homoionic montmorillonite and kaolinite

The adsorption and binding of amino acids (aspartic acid, cysteine, glycine, proline and arginine), ranging in molecular weight from 115 to 174 and in isoelectric point from pH 2.8 to 10.8, to montmorillonite [M] and kaolinite [K] homoionic to H, Na, Ca, Zn, La or Al were studied in unbuffered suspensions. Aspartic acid was adsorbed and bound on M homoionic to Ca or Zn but only adsorbed to K homoionic to Ca or Zn. Cysteine was only adsorbed on M homoionic to Al and adsorbed and bound on M homoionic to Zn and on K homoionic to H or Zn. Proline was adsorbed and bound on M homoionic to H, Ca or Zn and on K homoionic to H; it was adsorbed but not bound on both M and K homoionic to Al. Arginine was adsorbed and bound on M homoionic to H or Al and only adsorbed on K homoionic to Ca or Zn. Glycine was not bound on any of the clays. The amounts of amino acid adsorbed and bound (measured by the loss of amino acid from solution) and the class (Giles et al., 1974a and b) of both the adsorption and binding isotherms (retention against ultimate washing with water) were dependent on the type of amino acid, the type of clay, the type of cation predominant on the clays and the basicity or the additional function moiety (e.g. carboxyl, thiol, guanido) of the amino acid. The relative values of the non-standard free energy of clay-amino acid complexes that had isotherms of the Giles C-class were dependent on the type and molecular weight of the amino acid and on the exchangeable cation on the clay.     

Comparison of Five Adsorption Isotherms for Prediction of Zinc Retention in Calcareous Soils and the Relationship of their Coefficients with Soil Characteristics

Abstract

Zinc (Zn) deficiency is believed to be a consequence of reactions taking place between soluble Zn and the soil solid phase. This study was carried out to obtain quantitative relationships between Zn in equilibrium solution and that retained by the soil solids in calcareous soils. Twenty calcareous soils (saturated paste pH 6.9–7.9; calcium carbonate equivalent 4.64–22.80%) from Tehran province, Iran, were equilibrated with varying solution concentrations of Zn, and the amounts removed from the solution were used to check the fit to five adsorption isotherms, namely, Freundlich, Langmuir, Temkin, Gunary, and two‐surface Langmuir. Adsorption data of all soils showed statistically significant fit to the first four adsorption isotherms, but only 7 of the 20 soils tested showed fit to the two‐surface Langmuir. Coefficients of the adsorption isotherms showed statistically significant relationship with soil characteristics. Clay percentage, calcium carbonate equivalent percentage, and cation exchange capacity appeared to be the most influential soil characteristics with regard to Zn adsorption, whereas soil organic matter seemed to be of no importance under the conditions of this study.     

ION EXCHANGE IN SOILS FROM THE ION PAIRS K–Ca,K–Rb,AND K–Na1

Isotopic exchange of ⁴³K, ²⁴Na, ⁸⁶Rb, and ⁴⁵Ca was used to measure the equilibria between some arable soils and mixed chloride solutions of the ion pairs, K–Ca, K–Rb, and K–Na; the results were interpreted by a thermodynamic treatment of the exchange isotherm (Gaines and Thomas, 1953). The cation exchange capacities of the soils were not constant for the three ion pairs and decreased appreciably with K-saturation in the K–Ca systems because Ca ions were trapped inside the interlayer spaces. That soils preferred K to Na in the K–Na systems and Rb to K in the K–Rb systems, was explained in terms of the standard free-energy changes of the exchange reactions. The activity coefficients of the adsorbed ions, f, changed with K-saturation differently for the three ion pairs: fNa and fCa decreased continuously with increasing K-saturation whereas fRb remained almost constant. However, with increasing K-saturation, fK increased in the K–Na systems, remained unchanged in the K–Rb systems, and first increased and then decreased in the K–Ca systems. These changes are interpreted in terms of the effect of the various ions on the interlayer space of 2 : 1 type clay minerals and the possible distribution of adsorbed ions between the Gouy and Stern layers.     

EFFECTS OF ADSORBED CATIONS ON PHYSICAL PROPERTIES OF TROPICAL RED EARTHS AND TROPICAL BLACK EARTHS

Similar relative effects were obtained for plastic limits, percentage stable aggregates, and hydraulic conductivities when a tropical red earth, consisting predominantly of kaolin and iron oxide, and a tropical black earth consisting predominantly of montmorillonite, were saturated with Ca, Mg, K, or Na ions or combinations of these. Plastic limits were independent of the nature of the saturation cations, but were significantly dependent upon clay type. Percentage stable aggregates of artificially prepared aggregates and hydraulic conductivity were affected by the four cations in the order Ca = Mg > K Na and were also significantly affected by clay type. Combinations of the four cations generally gave intermediate effects with Ca-saturated soils more susceptible to deterioration by K or Na than Mg-saturated soils. The results confirm the generally deleterious effect of Na ions and show that K ions have effects similar to Na on the properties investigated. They suggest that heavy potash fertilization without liming may lead to structural deterioration of these soils.     

2︰1和1︰1型黏土矿物胶体凝聚中Hofmeister效应的比较研究

土壤胶体是土壤具备肥力与生态功能的物质基础,土壤胶体凝聚与分散影响着土壤中一系列微观过程和宏观现象。采用动态光散射技术比较研究三种碱金属阳离子(Li+、Na+、K+)引发不同类型黏土矿物(2︰1型蒙脱石和1︰1型高岭石)胶体凝聚中的Hofmeister效应。研究发现,Li+、Na+、K+作用下蒙脱石、高岭石胶体的凝聚速率、临界聚沉浓度及活化能都存在明显差异,表现出强烈的Hofmeister效应。当电解质浓度为20 mmol·L–1时,K+引发蒙脱石胶体凝聚的速率为66.61 nm·min–1,远高于Na+、Li+引发蒙脱石胶体凝聚速率(5.93、4.41 nm·min–1);而与之对应的临界聚沉浓度则呈现K+(蒙脱石21.8 mmol·L–1、高岭石34.6 mmol·L–1)低于Na+(蒙脱石57.6 mmol·L–1、高岭石85.8 mmol·L–1)低于Li+(蒙脱石81.8 mmol·L–1、高岭石113.9 mmol·L–1)规律,胶体凝聚中活化能可合理解释此现象。电解质浓度为25 mmol·L–1时,Li+、Na+、K+引发蒙脱石、高岭石胶体凝聚的活化能分别为1.97 kT、1.43 kT、0 kT和2.94 kT、1.71 kT、0.49 kT,说明蒙脱石、高岭石胶体凝聚过程中Hofmeister效应序列均为Li+相似文献