应用Multi-Langmuir模型评价土壤的表面电荷特性

用返滴定技术测定土壤的可变电荷量(Qv),应用Muti-Langmuir模型评价土壤的表面电荷特性。结果表明,黄壤和黄棕壤可变电荷量的变化范围在0～45cmolkg-1,红壤为0～21cmolkg-1,砖红壤和赤红壤为0～14cmolkg-1。可变电荷量(Qv)依赖土壤悬液的pH,3点位模型能很好地描述6种土壤的可变电荷量随pH的变化关系;黄壤、黄棕壤、湖南红壤、江西红壤、赤红壤和砖红壤的pK1分别为4.45、4.46、4.76、4.62、4.66和4.74,可变电荷量Qv1分别为11.7、9.64、9.31、7.14、4.86和5.95cmolkg-1。黄壤和黄棕壤pK1较红壤、砖红壤和赤红壤约低0.3。pK1与ZPC呈现极显著的线性关系,可变电荷量(Qv(i))与有机质含量呈极显著的正相关。     

铁氧化物与土壤表面电荷性质的关系

The relationship between iron oxides and surface charge characteristics in variable charge soils (latosol and red earth) was studied in following three ways.(1)Remove free iron oxides (Fed) and amorphous iron oxides (Feo) from the soils with sodium dithionite and acid ammonium oxalate solution respectively.(2) Add 2% glucose (on the basis of air-dry soil weight) to soils and incubate under submerged condition to activate iron oxides,and then the mixtures are dehydrated and air-dried to age iron oxides.(3) Precipitate various crystalline forms of iron oxides onto kaolinite.The results showed that free iron oxides (Fed) were the chief carrier of variable positive charges.Of which crystalline iron oxides (Fed-Feo) presented mainly as discrete particles in the soils and could only play a role of the carrier of positive charges,and did little influence on negative charges.Whereas the amorphous iron oxides (Feo),which presented mainly fas a coating with a large specific surface area,not only had positive charges,but also blocked the negative charge sites in soils.Submerged incubation activated iron oxides in the soils,and increased the amount of amorphous iron oxides and the degree of activation of iron oxide,which resulted in the increase of positive and negative charges of soils.Dehydration and air-dry aged iron oxides in soils and decreased the amount of amorphous iron oxides and the degree of activation of iron oxide,and also led to the decrease of positive and negative charges.Both the submerged incubation and the dehydration and air-dry had no significant influence on net charges.Precipitation of iron oxides onto kaolinite markedly increased positive charges and decreased negative charges.Amorphous iron oxide having a larger surface area contributed more positive charge sites and blocked more negative charge sites in kaolinite than crystalline goethite.     

A review of the problems associated with applying the terms surface charge and zero point of charge to soils

Abstract

Many soil properties are determined by the surface charge on the soil particles. A range of methods for determining the surface charge properties is available from the literature. However, the interpretation of the results obtained and the theoretical basis of some of the methods is confusing with different terms having different meanings for different authors. Thus, the study of surface charge properties and the theoretical basis is particularly difficult for a newcomer to the field. This paper presents a review of the major concepts as used in the current literature and highlights the problems encountered when applying these concepts to the determination of the surface charge properties.     

Ionic strength effects on surface charge and adsorption of phosphate and sulphate by soils

Two surface soils (Patua and Tokomaru) of contrasting mineralogy were incubated with several levels of either CaCO₃ or HC1. The effects of ionic strength on pH, on surface charge, and on the adsorption of phosphate and sulphate were measured in three concentrations of NaCl. The pH at which the net surface charge was zero (point of net zero charge—PZC) was 1.8 for the Tokomaru soil and 4.6 for the Patua soil: differences that can be related to mineralogical composition. There was an analogous point of zero salt effect (PZSE) that occurred at pH 2.8 for the Tokomaru soil and at 4.6 for the Patua soil. The presence of permanent negative charge in the Tokomaru soil resulted in an increase in PZSE over PZC. The effect of ionic strength on adsorption varied greatly between phosphate and sulphate. For phosphate, there was a characteristic pH above which increasing ionic strength increased adsorption and below which the reverse occurred. This pH (PZSE for adsorption) was higher than the PZC of the soil and was 4.1 for the Tokomaru soil and 5.3 for the Patua soil. In contrast, increasing ionic strength always decreased sulphate adsorption and the adsorption curves obtained in solutions of different ionic strengths converged above pH 7.0. If increasing ionic strength decreases adsorption, the potential in the plane of adsorption must be positive. Also, if increasing ionic strength increases adsorption, the potential must be negative. This suggests that, depending upon pH, phosphate is adsorbed when the potential in the plane of adsorption is either positive or negative, whereas sulphate is absorbed only when the potential is positive.     

恒电荷土壤与可变电荷土壤对Cl~-和NO_3~-吸附的差异及其机理

对 3种可变电荷土壤和 4种恒电荷土壤在不同 pH、不同浓度、不同相伴阴阳离子下混合体系中Cl-和NO3-的吸附进行了测定。结果表明 ,在Cl-和NO3- 共存体系中 ,Cl-比例增大使可变电荷土壤Na+吸附量及OH-释放量增加 ,而对恒电荷土壤影响不大。Cl-和NO3-吸附量随平衡Cl-和NO3-浓度增加而增大 ,随pH升高而减少。但恒电荷土壤在上述各种条件下对Cl-和NO3-吸附均相同 ,而可变电荷土壤对Cl-吸附量大于NO3-的吸附量 ;NO3-、Cl-的选择系数为 0.51～0.78,Cl-和NO3-的相对吸附量分别为56.9%和 43.1%。在不同相伴阳离子下 ,可变电荷土壤平衡溶液Cl-/NO3-比值均小于 1,且为Na+K+Ca2+Mg2+Fe3+;而恒电荷土壤Cl-/NO3-比值为 1左右 ,且不受阳离子类型的影响。由此认为 ,Cl-和NO3-在两类土壤中均以电性吸附为主 ,恒电荷土壤对Cl-和NO3-的亲合力及吸附机理相同 ;而可变电荷土壤对Cl-的亲合力 NO3- ,Cl-存在着专性吸附     

Ionic-strength and pH effects on the sorption of cadmium and the surface charge of soils

Two Oxisols (Mena and Malanda), a Xeralf and a Xerert from Australia and an Andept (Patua) and a Fragiaqualf (Tokomaru) from New Zealand were used to examine the effect of pH and ionic strength on the surface charge of soil and sorption of cadmium. Adsorption of Cd was measured using water, 0.01 mol dmp^?3 Ca(NO₃)₂, and various concentrations of NaNO₃ (0.01–1.5 mol dm^?3) as background solutions at a range of pH values (3–8). In all soils, the net surface charge decreased with an increase in pH. The pH at which the net surface charge was zero (point of net zero charge, PZC) differed between the soils. The PZC was higher for soils dominated by variable-charge components (Oxisols and Andept) than soils dominated by permanent charge (Xeralf, Xerert and Fragiaqualf). For all soils, the adsorption of Cd increased with an increase in pH and most of the variation in adsorption with pH was explained by the variation in negative surface charge. The effect of ionic strength on Cd adsorption varied between the soils and with the pH. In Oxisols, which are dominated by variable-charge components, there was a characteristic pH below which increasing ionic strength of NaNO₃ increased Cd adsorption and above which the reverse occurred. In all the soils in the normal pH range (i.e. pH>PZC), the adsorption of Cd always decreased with an increase in ionic strength irrespective of pH. If increasing ionic strength decreases cation adsorption, then the potential in the plane of adsorption is negative. Also, if increasing ionic strength increases adsorption below the PZC, then the potential in the plane of adsorption must be positive. These observations suggest that, depending upon the pH and PZC, Cd is adsorbed when potential in the plane of adsorption is either positive or negative providing evidence for both specific and non-specific adsorption of Cd. Adsorption of Cd was approximately doubled when Na rather than Ca was used as the index cation.     

Lime potential of variable charge soils

The lime potentials of several electrodialysed variable charge soils of Australia and China were directly measured with two ion-selective electrodes when titrated with Ca(OH)₂. The lime potential (pH – 0.5 pCa) was higher for the Ferralsol than that for the Acrisol or the Luvisol. The presence of organic matter led to a lower pH – 0.5 pCa value. The pH – 0.5 pCa curve was closely related to the pH curve for a given soil, but was more distinct than the latter. Lime potential ranges of 1.5–3.0, 3.0–5.0 and 5.0–7.0 corresponded to pH ranges of about 4.0–5.2, 5.2–7.0 and 7.0–8.2 respectively.     

Sulphate sorption by variable charge soils

The sorption of sulphate (SO^2?₄) by three variable charge soils from the Canary Islands (Spain) was studied. Sulphate sorption decreased with increasing pH. Only negligible amounts of SO^2?₄ were sorbed above pH 6.5. When the soils were washed with an indifferent electrolyte (0.01 M KCl), more SO^2?₄ was recovered than had been sorbed. This indicated a release of native SO^2?₄ Sulphate replaced hydroxyl ions (OH) and co-ordinated H₂O molecules, as well as very small amounts of silicate (Si). No measurable amount of phosphate (P) was released. On average hydroxyl release accounted for 50% of SO^2?₄ sorbed, the rest being accounted for by the increase in negative charge as measured by K⁺ adsorption. The results presented here are consistent with the sorption of SO^2?₄ through a ligand exchange mechanism, but in a different plane of sorption to that of phosphate.     

Pedogenesis and surface charge of some Andic soils in Washington,U.S.A.

Chemical and surface charge characteristics were determined for three soils that were formed in a mantle of volcanic tephra along a bio-climatic gradient in forest in northcentral Washington state. The soils are Nevine silt loam (Typic Vitrandept), Manley silt loam (Entic Cryandept), and Moses silt loam (Andic Cryochrept). Selective dissolution analyses for Al, Fe, and Si suggests that the weathering products in the ash mantle are poorly crystalline aluminosilicates, hydrous and crystalline Fe-oxides, and Al-humus complexes in decreasing order of abundance. The chemical and physical data indicate that Moses silt loam has undergone a greater degree of weathering than either Nevine silt loam or Manley silt loam, and we attribute this to differences in the weathering environment. The measured points of zero charge parameters are typical for soils that are dominated by amphoteric surfaces: the soils exhibit charge reversal, and charge magnitude is dependent upon pH and electrolyte concentration. The point of zero salt effect ranged from 5.1 to 5.3, and the point of zero net charge ranged from 4.3 in Nevine silt loam to 5.4 in Moses silt loam. Pedogenesis has not caused large enough differentiation among the soils to allow us to use zero charge parameters as indicators of the degree of pedogenesis.     

磷的吸附和表面电荷特征及其与华南地区某些土壤矿物的关系

The phosphate adsorption and surface charge characteristics of the tropical and subtropical soils derived from different parent materials in China were determined, and their relations to soil mineralogy were analysed. The results showed that all soil phosphate adsorption curves were well fitted by Freundlich equation and Langmuir equation. The maximum buffering capacity of P ranged from 66 to 9 880 mg kg^-1, with an increasing order of purple soil, skeletal soil, red soil, lateritic red soil, yellow soil and latosol; and the highest value was 149 times the lowest value, which indicated great differences among these soils in phosphate adsorption and supplying characteristics. The pH₀ (zero point of charge) values obtained by salt titration-potential titration varied from 3.03 to 5.49, and the highest value was found in the latosol derived from basalt whereas the lowest value was found in the purple soil. The correlation analysis indicated that the main minerals responsible for phosphate adsorption in the soils were gibbsite, amorphous iron oxide and kaolinite; and the pH₀ was mainly controlled by kaolinite, gibbsite and oxides.     

可变电荷土壤及矿物的交换性碱

可变电荷土壤及矿物的酸度特征具有明显的两性性质,以往对其酸性研究颇多,而对它同时表现出交换性碱认识甚少。近三、四十年来,随着对阴离子吸附研究的深入,对可变电荷土壤及矿物的交换性碱有了全新的认识。本文从交换性碱的研究方法、释放机理、交换性碱的影响因子及动力学性质等方面进行了较为全面的综述。     

Adsorption of sulphate and fluoride by variable charge soils

The adsorption of sulphate and fluoride by three variable charge soils was studied. Adsorption increased with increase in the amount of sulphate added at constant _pH, and decreased with increase of _pH.
The ratios of the amount of released OH⁻to that of the adsorbed SO^2-₄ at pH 5–0 were 0.12, 0.14 and 0.20 for the three soils, respectively, much lower than the corresponding OH⁻/F⁻ ratios which ranged from 0.3 to 1.0. For a ferric acrisol the OH⁻ released accounted, on average, for only 15% of the SO^2-₄ adsorbed, leaving more than 60% to be explained by the decrease in positive charge and the increase in negative charge carried by the soil.     

Determination of the water hysteresis and specific surface of soils by electronic microhygrometry and psychrometry

A procedure was developed for studying water retention hysteresis using a dew-point hygrometer and a Peltier psychrometer. Sorption curves derived using the former instrument can be applied for assessing the specific surface of soils by the single-point method (relative humidity 20%). It was shown that water hysteresis in soils and similar porous media with developed specific surfaces occurs in the potential range of ?7.0 to ?0.2 kJ/kg, which was previously inaccessible for experimental study. Based on the results obtained, the main hysteresis loop and scanning curves were plotted with the help of existing theories and then used in the SIM-DARS software to simulate results of physical experiments on nonisothermal water exchange in systems with a stepwise initial moisture distribution.     

可变电荷土壤对酸雨敏感度的影响因子

The sensitivity of a large number of variable charge soils to acid rain was evaluated through examining pH-H₂SO₄ input curves. Two derivative parameters, the consumption of hydrogen ions by the soil and the acidtolerant limit as defined as the quantity of sulfuric acid required to bring the soil to pH 3.5 in a 0.001mol L^-1 Ca(NO₃)₂ solution, were used. The sensitivity of variable charge soils was higher than that of constant charge soils, due to the predominance of kaolinite in clay mineralogical composition. Among these soils the sensitivity was generally of the order lateritic red soil > red soil > latosol. For a given type of soil within the same region the sensitivity was affected by parent material, due to differences in clay minerals and texture. The sensitivity of surface soil may be lower or higher than that of subsoil, depending on whether organic matter or texture plays the dominant role in determining the buffering capacity. Paddy soils consumed more acid within lower range of acid input when compared with upland soils, due to the presence of more exchangeable bases, but consumed less acid within higher acid input range, caused by the decrease in clay content.     

Size and charge characteristics of kaolinitic soils in SE Queensland

Some kaolinitic soils derived from basaltic material contain 2:1 minerals as a discrete phase and/or interstratified with kaolin as determined by X-ray diffraction (XRD) and by chemical analysis of DCB-treated clays, while others show little or no evidence of 2:1 minerals. Mineral impurities were not detected by electron diffraction in the latter group. There is good agreement between the interpretation of XRD traces and the SiO₂/Al₂O₃ ratio given by chemical analysis. Some kaolin is seen by electron microscopy to be of very small particle size, and two distinct morphological types have been recognized. High surface area measurements are consistent with fine particular size. These profiles contain kaolin with higher than normal CEC and permanent negative charges which cannot be accounted for by the presence of 2:1 clay minerals alone and which are attributed to a degree of isomorphous substitution. This substitution and/or mineral impurities may have limited kaolin crystal growth during neoformation.     

Specific surface area and surface charges of some argentinian soils

The specific surface areas of nine argentinian soils obtained by adsorption followed the order N₂ < ethylene glycol < H₂O, attributed to the presence of smectites (verified by XRD analysis) and some organic coating. The H₂O₂ treatment of these soils modified the former order of surface area determined by different absorbents. This order was modified because the mineral surface was evidenced and an increase of cation adsorption was produced by organic matter removal. This fact was supported by the decrease of PZC values of soils after H₂O₂ treatment. The specific surface area of initial soils obtained by water and ethylene glycol adsorptions showed a good correlation with carbon content and CEC of untreated soils and with the PZC of protonated soils.     

Determination of free DNA in soils

The concentration of free DNA in soils has to be determined in order to understand the fate and the transport of extracellular DNA. A protocol for the extraction and determination of free DNA was developed. The procedure uses separation steps, i.e. centrifugation and ultra‐filtration. The free dsDNA was stained with PicoGreen® and determined fluorimetrically. Samples from different soils, different soil horizons, soil waters and under different land use systems were analyzed. It was found that in nearly all samples free DNA was detectable. Free DNA concentrations of up to 1950 ng (g dried sample)^—1 could be detected depending on depth, soil type and system of land use.     

Solubility control of KCl extractable aluminum in soils with variable charge

Abstract

Solubility and kinetic data indicated that concentrations of aluminum (Al) extracted with 1 M KCl are determined by the solubility of a precipitated A1(OH)₃ phase in soils dominated by variable charge minerals. Kinetic studies examining the release of Al on non‐treated and KCl treated residues indicated the precipitation of an acid‐labile Al phase during the extraction procedure. The log ion activity products estimated for the KCl extracts ranged between 8.1–8.6 for the reaction Al(OH)₃ + 3H⁺ < = > Al³⁺+ 3H₂O, which was similar to the solubility product of several Al(OH)₃phases. The mechanism proposed for Al precipitation indicated that Al released by exchange with added K⁺ hydrolyzed and released H⁺ that was readily adsorbed on surfaces of variable charge minerals. The increased ionic strength of the extracting solution further increased the amount of H⁺adsorbed to the variable charge surface and reduced the H⁺ concentration in the aqueous phase. Consumption of H⁺ induced further hydrolysis of Al, resulting in supersaturation of the extracting solution and formation of polynuclear hydroxy Al species. It was concluded that the 1 M KCl extraction does not quantitatively extract salt exchangeable Al from variable‐charge soils.     

Adsorption of chloride and nitrate by variable charge soils in relation to the electric charge of the soil

A cell consisting of a chloride-selective electrode and a nitrate-selective electrode was directly put in the soil suspension to determine the concentration ratio NO₃^?/Cl^? for studying the adsorption of these two ions by three soil samples from variable charge soils. It was found that such factors as the iron oxide content of the soil, the pH of the suspension, the concentration of the respective anion, the kind of accompanying cations, and the dielectric constant of solvent etc. can all affect the amounts and the ratio of the two anions adsorbed. The adsorption was chiefly caused by coulombic force, but another mechanism, presumably a covalent force between the anion and the metal atom on the surface of soil particles, may also be involved, at least for chloride ions.     

Influences of aluminum ions on the determination of zpc (zero point of charge) of variable charge soils

Influence of Al dissolution on soil ZPC (zero point of charge) measured by a potentio-metric titration (PT) method and a modified salt titration (STPT) method was examined using two strongly weathered soils from Thailand and two volcanic ash soils from Japan. The amount of dissolved Al ions increased with the increase in the concentration of a supporting electrolyte for the strongly weathered soils, while the increase was negligible for the volcanic ash soils.

ZPC value of the strongly weathered soils determined by the PT method was lower than that by the STPT method, due to the greater Al dissolution associated with the higher electrolyte concentration used in the PT method. Al ions adsorbed onto the soil surface would shift the ZPC to a higher pH value not as a result of the formation of hydroxy Al polymers, but due to the blocking of permanent negative charge sites, which could otherwise lower the ZPC. The σp value, as a measure of permanent charge or the amount of 11 or O11 adsorbed by a soil required to attain the ZPC, could be used to describe this phenomenon.

In the STPT method, the salt concentration was not high enough to causc a significant Al dissolution at the ZPC, which is considered to be a more suitable condition than in the PT method because the ZPC value can be evaluated at a low salt concentration as in the ease of field conditions for crop production. Thus, the STPT method is rccommendcd for the determination of the ZPC.