THE REACTION OF FLUORIDE WITH SOILS AND SOIL MINERALS

The reaction of sodium fluoride solution with soils and soil minerals at constant pH has been assessed as a possible single-value characteristic in the classification of soils, particularly those containing significant amounts of poorly ordered inorganic material. A suitable method involves reaction of the soil or clay at 25°C with 0·85 M sodium fluoride at pH 6·8, the amount of OH^? released after 25 min being taken as the single-value characteristic. There was a statistically significant correlation between the amount of OH^? released and the amount of alumina extracted with cold 5 per cent sodium carbonate solution from both freely drained and poorly drained Scottish soils. The multiple correlation coefficients were improved for freely drained soils by including the dithionite-extracted ferric oxide and the silica soluble in 5 per cent sodium carbonate solution. The procedure is not suitable for surface soils, because of interference by organic matter, or for carbonate-containing soils unless carbonate is removed.     

IDENTIFICATION OF PHOSPHATE-REACTIVE SITES OF HYDROUS ALUMINA FROM PROTON CONSUMPTION DURING PHOSPHATE ADSORPTION AT CONSTANT pH VALUES

The amounts of hydroxyl ions released from hydrous alumina during phosphate adsorption were measured at constant pH values and at different phosphate concentrations using an automatic titrator. The hydroxyl ions released were plotted against phosphate adsorbed and the equations that gave best fit to the data were differentiated. The variation in the differentials (dOH/dP) with increasing phosphate adsorption was consistent with the theory, proposed below, on the type of sites responsible for phosphate adsorption, by synthetic hydrous alumina containing bayerite and pseudoboehmite, at different concentrations. At low phosphate adsorption which corresponded to low phosphorus concentration, adsorption took place mainly by displacing aquo groups (Al-H₂O). With increase in phosphorus concentration hydroxo groups (Al-OH) became the predominant sites of adsorption. At still higher concentrations, where phosphate adsorption displacing aquo and hydroxo groups was nearly completed, the hydroxyl bridges linking aluminium atoms (Al-OH-Al, ol groups) were broken. This created new sites which were responsible for additional adsorption of phosphate at these concentrations.     

Aluminium solubility mechanisms in moderately acid Bs horizons of podzolized soils

The processes controlling the retention and release of aluminium in acid forest soils are still subject to controversy, and therefore a universal hypothesis as to what mechanisms are operating has not been firmly established. By studying the Bs horizons of Swedish and Swiss podzolized soils, and by analysing data in the literature, we have found that aluminium hydroxide, and in some cases also poorly ordered imogolite, may control Al solubility in moderately acid (pH > 4.2–4.3) Bs horizons. The strongest evidence in support of the presence of a quickly reacting Al(OH)₃ pool came from the temperature dependence of Al solubility in a Bs horizon, which was consistent with the reaction enthalpy of an Al(OH)₃ phase such as gibbsite, and from the observation that the ion activity product for Al(OH)₃ was the same regardless of whether equilibrium was reached from over‐ or undersaturation. The pool of Al(OH)₃ is commonly small and may be completely dissolved after large additions of acid. This may be explained by the continuing redissolution of reactive Al(OH)₃ to form less soluble imogolite‐type phases. By using the same methods it was found that soil suspensions did not reach equilibrium with poorly ordered imogolite even after 17 days. Thus, imogolite probably does not control Al solubility in the short term in many soils despite the common occurrence of this mineral. This is due to the relatively slow kinetics of imogolite formation and dissolution, especially at low temperatures and at small solution H₄SiO₄ concentrations.     

Silicon,aluminum, and oxalic acid interactions in two California forest soils

Abstract

The release of solid‐phase soil aluminum (Al) from two soils was studied under acidic conditions and also in the presence of monosilicic acid. The soils support mixed‐conifer forests in the mid‐elevation Western Sierra Nevada in northern California, but differ in their state of development and mineralogy as shown by Al, iron (Fe), and silicon (Si) concentrations. The pyrophosphate‐extractable Al (Al_p) pool, which was a main source of released Al, decreased after a two‐month leaching with nitric (HNO₃) or oxalic (HO₂C‐CO₂H) acids. Addition of monosilicic acid (SiO₂.XH₂O) to the acid extractants resulted in a further decrease of Al. Solution monosilicic acid was removed from solution by sorption on Fe oxides/hydroxides in the soil with the higher dithionite‐extractable Fe pool. In the less developed soil with lower pedogenic Fe, the formation of short‐range‐ordered aluminosilicates, even in the presence of a strong Al chelator, was responsible for the removal of a portion of the monosilicic acid from solution. Pedogenic Fe inhibited the formation of short‐range‐ordered aluminosilicates more than the presence of a strong Al chelator. Both the solution phase and surface reactions are important in the pedogenic formation of alumino‐silicate minerals.     

FORMATION, COMPOSITION AND STRUCTURE OF HYDROXY-ALUMINOSILICATE IONS

Formation of hydroxy-Al (HA) and hydroxy-aluminosilicate (HAS) ions was studied in solutions of A1C1₃, orthosilicic acid and NaOH. Evidence for incorporation of Si into HA is that HA retarded the development of silicomolybdate colour and Si was retained by a cation-exchanger. The reaction is believed to be mainly a condensation reaction between the hydroxyl groups in Al–OH–Al and Si–OH groups. The HAS with Si/Al ratio 0.3–0.6 showed infrared spectra similar to allophane-like constituents in Ando soils.     

低分子量有机酸对高岭石中铝释放的影响

选择了几种土壤中可能存在的低分子量脂肪羧酸 ,研究了它们对高岭石中铝释放的影响。结果表明 ,有机酸可以通过络合作用促进高岭石中铝的释放。几种有机酸对体系中可溶性铝影响的大小顺序为 :草酸 >柠檬酸 >丙二酸 >苹果酸 >乳酸。草酸、柠檬酸和乳酸对可溶性铝释放的促进作用随体系pH的升高而减小 ,其中草酸体系中可溶性铝随pH的变化幅度最大。在苹果酸体系中 ,可溶性铝随pH的升高而稍有增加。体系中的可溶性铝随有机酸浓度的增加而增加 ,而交换性铝随柠檬酸浓度的增加而减少 ,先随苹果酸浓度增加而增加 ,然后又逐渐减少。与对照相比 ,柠檬酸和草酸使交换性铝的量减小 ,苹果酸和乳酸在低pH下使交换性铝明显增加 ,而苹果酸在较高pH下使交换性铝减少。有机酸影响释放出的铝在固液相间的分配比 ,苹果酸在低pH下使体系中释放出的大部分铝以交换性形态存在 ,而在较高pH下 ,大部分铝以可溶形态存在。在草酸体系中 ,释放出铝的大部分都以可溶形态存在。不同有机酸的不同表现与体系中铝的溶解平衡、铝的吸附 -解吸平衡、有机酸的吸附 -解吸平衡、有机酸的离解平衡和铝与有机酸的络合 -离解平衡有关。     

THE MECHANISM OF PHOSPHATE ADSORPTION BY KAOLINITE, GIBBSITE, AND PSEUDOBOEHMITE

The adsorption isotherms (20°C) of phosphate on two potassium kaolinites and two aluminium oxides have been determined at pH values from 3 to 10 and at concentrations ranging from 10^?4 to 10^?2M. The reversibility of the adsorption with respect to pH and concentration has also been examined. The isotherms result from at least three types of adsorption site (regions I, II, and III) of widely different reactivities. The number of adsorption sites increases to a limit with decrease in pH for regions I and II. The behaviour of region III is more complex. The different adsorbents behave in essentially the same manner and differ only in the number of adsorption sites. It is tentatively suggested that regions I and II are located on an edge –Al(OH)₂ of the adsorbents, while region III results from penetration into some amorphous region of the crystal surface.     

Release of glucose from dissolved and mineral-bound organic matter by enzymatic hydrolysis

Sorption of dissolved organic matter (DOM) by poorly crystalline minerals during their formation may protect large amounts of carbon in soils from mineralization. We investigated the bioavailability of carbohydrates in DOM and after co-precipitation with short-range ordered aluminosilicates. Carbohydrates originated from soil solutions collected in situ at two depths of a Dystric Cambisol, and from litter extracts. Quantification of substrate-specific degradability was achieved by the addition of β-glucosidase at an optimal concentration and subsequent determination of glucose release. Depending on DOM composition, 0.6–41.4 mg g⁻¹ C⁻¹ of glucose was enzymatically released from dissolved carbohydrates. Co-precipitated carbohydrates were partially accessible, resulting in a glucose release of 0.7–5.2 mg g⁻¹ C⁻¹. Restricted enzymatic depolymerization due to co-precipitation may contribute to accumulation of easily degradable substrates in 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.     

ADSORPTION OF SELENITE, PHOSPHATE AND SULPHATE ON HYDROUS ALUMINA

Selenite was adsorbed on a positively charged hydrous alumina at a solution pH of 5.0, and the OH ^? released and changes in the surface charge were measured. The adsorption isotherm levelled off at high concentrations suggesting a definite adsorption maximum. The OH^? released yielded a curvilinear relationship of increasing slope with the selenite adsorbed. The positive charge on the alumina surface was neutralized by the adsorbed selenite and the net charge became close to zero as the adsorption of selenite was near maximum. The molar ratio of the ligands (OH^?+ OH₂) displaced over selenite adsorbed was more than one at surface saturations of <0.5, but it was approaching one with increasing adsorption of selenite. The results are explained in terms of preferential adsorption of divalent (SeO²₃^?) and monovalent (HSeO^?₃ selenite on a highly positively and less positively charged surface respectively, in agreement with the VSC^? VSP model of Bowden et al. (1973; 1977). The supply of SeO²₃^? for adsorption is considered to be favoured by an expected lower H activity in solution adjacent to the positively charged surface than that in the bulk solution. A general discussion is given on the ligand exchange adsorption of selenite, phosphate and sulphate on hydrous alumina.     

Reaktionen von Ca-Montmorillonit und Ca-Vermiculit mit Kohlensäure

Reaction of Ca-Montmorillonite and Ca-Vermiculite with CO₂ The treatment of Ca-montmorillonite and Ca-vermiculite with carbonic acid (Pco₂ = 2 atm) has shown the following results: In the pH range of ± 3.9–6.0 Ca is exchanged by H directly only on pH dependent (variable) charge sites (Cav), whereas, Ca on permanent charge sites (Ca_p) is exchanged only by lattice cations mainly Al released by proton attack. Therefore, with regard to proton uptake from carbonic acid the two Ca-clays behave as extremely weak acids. The exchange of Ca_v proceeds rapidly that of Ca_p much more slowly, the first one is essentially reversible the latter at least not within a short period of time. The proportion of Ca_v is found from a graphical extrapolation of the Ca saturation-pH-function (fig. 3). Al released during the initial stage of the reaction is non exchangeable, prevents the expandable minerals partly from collapsing on K tratement (verm. much more than mont.) and has an OH/Al ratio of about 2. On treating the same clay sample several times or at low suspension concentrations exchangeable Al with OH/Al = O occurs. Exchangeable hydrogen was never detected. Vermiculite is less resistent against carbonic acid than montmorillonite. The results were discussed in respect to the mechanism of release of lattice constituents and in respect to Al release under the influence of CO₂-concentrations of soils.     

Effects of pH and aluminosilicate composition on KMg exchange selectivity of amorphous aluminosilicates

The KMg cation exchange selectivity of some allophanic soil clays, imogolite and a range of synthetic hydrous oxides and amorphous aluminosilicates was studied using solutions 0.05N with respect to KCl and MgCl₂. The preference of the aluminosilicates for Mg²⁺ increased with pH, which is in qualitative agreement with the predictions based on double-layer theory. Large quantitative discrepancies indicated, however, specific interaction of potassium with aluminosilicates of low and intermediate Al/(Al + Si) molar ratio. The potassium selectivity of these materials probably arises from swelling-pressure effects and steric restriction on the entry of the larger hydrated Mg²⁺ into channels in the aluminosilicate aggregates. Hydrous oxides of iron and aluminium were found to specifically adsorb magnesium at alkaline pH values and evidence indicates that specific adsorption by these materials also occurred in soil clays and in Fe(NO₃)₃-treated synthetic aluminosilicates.     

Modelling pH buffering and aluminium solubility in European forest soils

The pH buffering and aluminium solubility characteristics of acid soil are important in determining the soil's response to changes in precipitation acidity. The chemistry of soil organic matter (humic substances) plays a key role in both processes, yet is complex and still poorly understood. Nevertheless, models of humic substance chemistry have been developed, one of which is WHAM–S, which contains a model (Model V) of proton and metal binding at discrete sites on humic substances and considers electrostatic effects on the binding strength. Here we have tested the ability of WHAM–S to model solution pH and Al using batch titration studies on organic and mineral soil horizons from forested sites in Norway, Germany and Spain, with ambient pH values from 3.73 to 5.73. We optimized the model predictions by adjusting the amounts of soil aluminium and humic substances within defined limits, taking the contents of copper chloride‐extractable Al and the base‐extractable organic matter as starting values. The model simulated both pH and dissolved Al well with optimized amounts of aluminium and humic substances within the defined limits (root mean squared error for pH from 0.01 to 0.22, for p[Al]_aq (total dissolved Al) from 0.03 to 0.49, five data points). Control of dissolved Al by dissolved organic matter was important particularly at above‐ambient pH. In two mineral horizons we improved the fits by assuming that Al could precipitate as Al(OH)₃. The optimized model also gave reasonable predictions of pH and dissolved Al in supernatants obtained by repeated leaching of the soil horizons. The results show that humic substances dominate the control of pH and dissolved Al in most of the horizons studied. Control by Al(OH)₃ occurs but is the exception.     

恒pH自动滴定系统及其在土壤表面化学研究中的应用

利用MIA 3型自动分析系统的硬件 ,根据研究的需要开发了一个专用软件 ,建成适用于恒pH自动滴定系统。并对一个含氧化铁很高的砖红壤的表面羟基的释放过程进行了测试应用。结果显示 ,在Na2 SO4 溶液中 ,羟基释放量明显受pH和SO2 - 4加入量的影响 ,羟基释放量的动态变化很好地符合Elovich方程。     

The solubility characteristics of organic Al precipitates in allophanic Bs horizons could mimic those of Al hydroxides, and indicate the solubility of associated allophanes

It is shown that Al-humate and fulvate precipitates in Bs horizons of pH > 4.6 can be the source of the soluble aluminium which is rapidly released in equilibrium studies to give log₁₀{Al³⁺} + 3pH values near 9.4 at 8°C, so that it is not necessary to postulate an anomalously reactive but sparingly soluble Al(OH)₃ phase. These Al-organic precipitates will have reached equilibrium in the natural soil environment with the more slowly reacting hydroxy-aluminium precipitates present, including proto-imogolite allophane, but can release Al³⁺ much more rapidly than the inorganic precipitates in laboratory equilibrations and soil leaching episodes that yield lysimeter waters. Equilibrium concentrations of Al reported in a range of Bs horizons indicate that the allophanes present are less soluble than proto-imogolite sols prepared in the laboratory and matured for up to 2 years.     

PRECIPITATION OF AMORPHOUS ALUMINOSILICATES FROM SOLUTIONS CONTAINING MONOMERIC SILICA AND ALUMINIUM IONS

Monomeric silica solutions were mixed with solutions containing AlCl₃, or Al(OH)_2-55Cl_0-45 to give a SiO₂/Al₂O₃, ratio varying from 8 to 8.0 and were kept at pH 6–0, 7.0, or 8.0 over a period of about I year. All precipitates which formed from solutions with SiO₂/Al₂O₃ ratios of 1–0 or higher were X-ray amorphous. The SiO₂/Al₂O₂, ratio of the precipitate varied in the range from 1.0 to 3.0. It increased with the Si concentration and the SiO₂/Al₂O₃ ratio of the parent solution and with reaction time. The pH did not affect the SiO₂/Al₂O₃ ratio of the precipitate but did affect its dispersion and flocculation. Al³⁺ and Al(OH)_2.55^+0.45 reacted differently with silica and yielded different reaction products. These differences were interpreted in terms of the stability of hydroxy-Al polymer units in the reaction. The amount of NaOH per Al atom which was required to reach and maintain the same pH, increased with the increasing SiO₂/A1₂O ratio of the precipitate, but the value was generally lower than that predicted from ‘one in four’ substitution of Si by Al in four-fold co-ordination. A ‘neutralization’ of the positive charge on the hydroxy-Al cations by silica was pointed out also to be a cause of the extra acidity. Implications of these observations on synthetic amorphous aluminosilicates to the genesis, structure, and properties of their natural counterparts were discussed.     

Phosphate sorption in allophanic soils and release of sulphate, silicate and hydroxyl

Phosphate (P) sorption and the concomitant release of sulphate (SO,), silicate (Si) and hydroxyl ion (OH) were determined on three allophanic soils from Spain, at different P concentrations. P effectively replaced SO₄, Si and OH. However, at every stage of P sorption, the molar ratios of the total amounts of anions released (SO₄+ Si + OH) to that of the sorbed P were low. The amount of added P affected the relative proportions of SO₄, Si and OH exchanged. At low concentrations of P, phosphate sorption was accompanied mostly by release of the adsorbed SO₄ with some Si. As more P was sorbed an increasing displacement of OH was also observed.     

SORPTION OF INORGANIC PHOSPHATE BY IRON- AND ALUMINIUM- CONTAINING COMPONENTS

The amounts of inorganic P sorbed by a range of Fe- and Al- containing components varied appreciably and decreased in the order allophane > fresh Al gel > Fe gel pseudoboehmite > aged Al gel > dried Fe gel > Fe-coated kaolinite > haematite > goethite > akaganeite > gibbsite = ground kaolinite > dispersed kaolinite. Al gel sorbed 30 to 70 times more P than gibbsite, and Fe gel sorbed approximately 10 times more P than its crystalline analogues (haematite, goethite. and akaganeite). Despite large differences in the extent of P sorption, the form of the isotherm was essentially the same for each sorbent. The ability of freshly-prepared Al gel suspensions to sorb P decreased with ageing, a property not shown by Fe gel. Drying of Fe gel at 80°C, however, caused an approximately 4-fold decrease in P sorption. Precipitation of Fe gel (2% Fe) on the surface of kaolinite increased P sorption by a factor of 10. The occurrence of Fe gel as a coating apparently presents more sorption sites to solution per unit weight of Fe gel than Fe gel alone. A linear relationship (r= 0.98) was obtained between the amount OH^? sorbed per unit increase in pH value (‘hydroxyl buffering’) and the overall P sorption maximum for each sorbent. Hydroxyl buffering provided a better index of P sorption potential than specific surface area. Except for the crystalline Fe sorbents, isotherms obtained by plotting fractional sorption saturation against final solution P concentration for the sorbents were essentially coincident with those for several contrasting soils. For crystalline Fe components a lower relative amount of weaker sorption, as opposed to chemisorption, of the overall sorption maximum was obtained. Differences in the extent of P sorption. however, appear to be primarily related to the number of functional M-OH groups presented at the solid-solution interface.     

THE REACTIONS OF SOLUBLE ALUMINIUM WITH MONTMORILLONITE

The amount of aluminium sorbed by calcium bentonite from basic aluminium chloride solutions depends on the ratio of OH to A1 in the original solution. Except when OH/A1 = 0, only part of the sorbed A1 is exchanged by dilute solutions of barium or calcium chlorides and the exchange capacity of the clay decreases in proportion to the amount of aluminium retained by the clay. Al-bentonite appears to contain a mixture of A1³⁺ and a basic cation in which OH/Al is 2.5. Neutralizing Al-bentonite with calcium acetate of pH 7.2 restores only part of the exchange capacity and some charged interlayer aluminium is thought to remain in the clay at this pH. It seems possible, therefore, that charged interlayer aluminium may be found even in soils with a neutral reaction.