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.     

土壤中水合氧化物型表面的化学区分Ⅰ.表面电荷

本文研究了氧化物对土壤、粘土和合成无定形铝硅酸盐的表面电荷的贡献.结果表明,水合氧化物型表面可进一步区分为电荷性质完全不同的两类亚表面.在所研究的样本中,氧化铁和氧化铝是带可变正电荷的亚表面,而氧化硅是带可变负电荷的亚表面.     

IRON AND ALUMINIUM AS CEMENTING SUBSTANCES OF SOIL AGGREGATES

A method is described for extracting fractions of soil iron and aluminium without removing organic matter. The extraction is by acetylacetone in a non–polar solvent and is virtually complete in zoo h. Similar amounts of iron and aluminium are extracted by acetylacetone in either benzene or water, but organic matter is extracted in detectable amounts only by acetylacetone in water. Extraction in aqueous solution after acetylacetone in benzene removes only organic matter, with no appreciable release of iron and aluminium. Metals extracted by acetylacetone in benzene are therefore considered to be bonded to that part of the organic matter which is extracted by acetylacetone. Water alone does not extract significant amounts of organic matter after removal of iron and aluminium by acetylacetone in benzene. The refolding of the hydrophilic surface units of the organic matter, caused by non–polar solvents, may hinder the extraction by water.     

Temporal Variations of Aluminium Fractions in Streams in the Delsbo Area, Central Sweden

Stream waters were sampled weekly during spring and monthly during summer and autumn in 1998. The streams are more or less acidified, and some have been treated with lime. The aluminium fractions (total monomeric Al, organic monomeric Al, inorganic monomeric Al) were determined colourimetrically with pyrocatechol violet combined with cation exchange using Continuous Flow Analysis (Autoanalyzer I). The levels of inorganic monomeric aluminium varied substantially, between <3 to 271 µg/l. The levels were higher in untreated than in limed waters and twice as high in the most humic waters as in less humic waters. The importance of aluminium mobilisation from the catchments was obvious, with higher aluminium concentrations in surface runoff (unbalanced stream waters) compared to lake outlets (balanced and precipitated lake water). The highest mean levels were measured at spring, whereas the highest single peaks occurred during summer. Inorganic monomeric and total monomeric aluminium was best correlated to ion ratio and pH whereas acid soluble aluminium and organic monomeric aluminium was best correlated to TOC, water colour and iron.     

EFFECT OF ORGANIC CONSTITUENTS AND COMPLEXED METAL IONS ON AGGREGATE STABILITY OF SOME EAST ANGLIAN SOILS

Paired soils of high and low organic matter content from the East Anglian silts were used to determine the role of different organic constituents and complexed metal ions in relation to aggregate stability. Although leaching with periodate and borate caused some loss of stability for most of the soils, the changes were much less than those due to extraction of the soils with pyrophosphate or acetylacetone which were more effective in removing organic materials complexed with iron and aluminium. Amounts of iron and aluminium extracted were not well correlated with changes in stability. The results indicate that in these soils polysaccharides are less important to aggregate stability than organic matter bonded to the clay particles through association with aluminium or iron.     

Laboratory studies of crust development in Irish and Iraqi soils II. Effects of some physico-chemical constituents on crust strength and seedling emergence

The influence of soil constituents including sand, silt, clay, organic substances, aluminium and iron on crust development and seedling emergence was investigated in a selection of 30 soils. After treatment with simulated rain, soil crust strength as measured by penetration resistance to an upward-moving probe tended to decrease with increasing total organic carbon content, but the relationship was not a simple one. Consideration of other factors showed that in some soils (soils of 1.5–2% total organic carbon), alkali-exractable carbon and clay contents had the greatest influence on penetration resistance values whereas in other soils (2.4–3% total organic carbon), coarse sand, fine sand, EDTA-extractable aluminium and humic acid carbon were highly correlated with penetration resistance. In a group of soils with around 4% total organic carbon, oxalate-extractable aluminium appeared to exert a dominant influence. There was a good correlation between humic acid carbon and penetration resistance after rain treatment in soils containing less than 40% sand. A similar relationship was also noted with EDTA-extractable iron.Combination of all factors in a multiple regression analysis accounted for a considerable proportion of the variation in penetration resistance of soils with and without simulated rain application and in seedling emergence of barley. Organic carbon, and the humic acid fraction in particular, were most important in determining crust strength for all soils as a group.A significant correlation between plastic limit moisture and the logarithm of penetration resistance is explained partially, at least, by the relationships that exist between organic carbon alone, clay alone and a combination of both, with variation in moisture.     

DISSOLUTION KINETICS OF SILICATE MINERALS IN AQUEOUS CATECHOL SOLUTIONS

Fine particle samples of silicate minerals and of gibbsite were shaken with solutions of pyrocatechol, 4-nitro-pyrocatechol or 3,4-dihydroxybenzoic acid at pH 9·3–9·7 and 25°C. In 63 days, from 1·4 per cent (illite) to 17·9 per cent (nepheline) of the silica present was dissolved. Except for kaolinite in solutions of the two substituted catechols, the aluminosilicates dissolved incongruently, leaving residues enriched with aluminium. When accumulated amounts of elements (Si, Al, Na, K) dissolved from nepheline were plotted against time on log-log paper straight lines were obtained. For quartz, kaolinite, and in some cases gibbsite similar plots could be approximated by two straight-line segments. The plots for microcline and oligoclase may be described by a combination of three-line segments. Several possible reaction mechanisms are discussed but a mechanism which explains the similar rate profiles has not yet been formulated.     

Influence of organic matter on phosphate adsorption by aluminium and iron oxides in sandy soils

The phosphate adsorption capacity (P_max) of samples from various horizons of five Danish podzolized soils were investigated before and after organic matter removal. Removal of organic matter had no direct influence on P_max suggesting that organic matter did not compete with phosphate for adsorption sites. In the soils investigated aluminium and iron oxides were the main phosphate adsorbents. Thus, more than 96% of the variation in P_max could be accounted for by poorly crystalline aluminium and iron oxides (extractable by oxalate) and by well-crystallized iron oxides (taken as the difference between dithionite-citrate-bicarbonate-extractable iron and oxalate-extractable iron). Organic matter affected phosphate adsorption indirectly by inhibiting aluminium oxide crystallization. The resulting poorly crystalline oxides had high P_max. In contrast, the influence of organic matter on the crystallinity of the iron oxides, and therefore on their capacity to adsorb phosphate, seemed limited.     

Evidence for microbial decomposition of organic acids during podzolization

Microbial aspects of podzolization were studied by percolating organic acids through sterile and non-sterile soil in columns. Six columns containing sand from a fluvioglacial sediment were percolated (90 mm d^?1) with mor extract and an oxalate-citrate solution. In three of these, sterilized soil solutions were used. Weathering was enhanced by organic acids which formed complexes with aluminium and iron. In the non-sterile soil columns weathering by organic acids was inhibited at 7–8 cm because the acids were degraded by microorganisms. Weathering was evident from colour change, contents of extractable aluminium and iron, and the micromorphology. Enhanced weathering in the sterile columns was also suggested by larger amounts of aluminium, iron, silica and base cations leached from the columns. Comparison of the output of aluminium, iron and silica from the sterile soil with that from the non-sterile soil, suggests that probably an aluminium-iron-silicate phase was formed in the deeper parts of the non-sterile columns.     

Sorption of isotopically exchangeable and non-exchangeable phosphate by some soils of Colombia and Brazil, and comparisons with soils of southern Nigeria

Fourteen soils from Colombia and Brazil provided a wide range of sorption characteristics. Curves of sorbed phosphate that was exchangeable to ³²P were described by Freundlich's equation, and of non-exchangeable phosphate by Temkin's equation. Exchangeable phosphate was associated with aluminium in poorly-crystalline oxides and in organic complexes. Non-exchangeable phosphate was related to aluminium in organic complexes, and especially to the ratio of AI/C in them. In Nigerian soils similar mechanisms controlled sorption of phosphate but oxides and organic complexes of iron were important. The concentration of phosphate in solution when affinities of soil for exchangeable and non-exchangeable phosphate are equal, and the importance of organic matter, are discussed in relation to soil management and to responses of crops to fertilizer phosphate. The results indicate that sorption curves should not be split into sections.     

Forms of Al in soil and soil solution in a long‐term fertilizer application experiment

Under the conditions of a long‐term fertilizer experiment, this study aimed to determine the contents of total and exchangeable aluminium in soil as well as the Al concentration in the soil solution. Additionally, Al speciation was evaluated with the use of the MINTEQA2 software. The results obtained indicated that under the conditions of long‐term application of different mineral fertilizers or farmyard manure, the soil reaction changed to a great extent (pH 3.58–6.78). At the same time, the content of total Al in soil fluctuated from 18.85 to 22.13 g/kg and that of exchangeable Al ranged from 1.42 to 102.66 mg/kg. The concentration of Al in the soil solution was highly differentiated (5.19–124.07 μmol/L) as well as that of free aluminium ions (Al³⁺) (0–16.9 μmol/L). In acidic soils, aluminium complexes with organic matter are the predominant forms of Al in the soil solution. In soils with neutral soil reaction, there were no free aluminium ions. Soil liming and addition of organic amendment were the treatments that restricted the presence of toxic aluminium forms in soil.     

The specific adsorption of organic and inorganic phosphates by variable‐charge oxides

We measured the reaction of inorganic phosphate and of four organic phosphates with three aluminium hydroxides or oxides. We compared the fit of the Langmuir equation with that of an equation designed to allow for the feedback effect of adsorption on the electric potential. We also fitted a four‐plane model to describe the effects of pH on adsorption and ζ potential. For inorganic phosphate, the Langmuir equation described adsorption curves poorly, indicating that there was a large feedback effect of adsorption on the electric potential of the adsorption plane. For the organic phosphates, the deviations from the Langmuir equation were not as marked, indicating that there was little feedback effect. Nevertheless, there was a large effect on the ζ potential of the phosphated aluminium (hydr)oxides. We suggest that when a large organic phosphate molecule is adsorbed, the charge conveyed to the surface is repelled electrostatically to the outside of the new surface. There is therefore a large effect on the ζ potential and a small effect on the potential of the adsorption plane. This suggestion was supported by the fit of the four‐plane model, for which, with increasing molecular weight, the mean position of the charge conveyed to the surface by adsorption was moved further from the plane carrying the potential determining ions (H⁺ and OH^?).     

ACCUMULATION OF CADMIUM AND ZINC FROM DIFFUSE IMMISSION ON ACID SANDY SOILS,AS A FUNCTION OF SOIL COMPOSITION

Sandy soils, in the border area of Belgium and the Netherlands (the Kempen region), are heavily contaminated by atmospheric deposition of cadmium and zinc from nearby smelters. Groundwater contamination by leaching from these low retention soils is subject of study. There are reports of high cadmium and zinc concentrations in groundwater in the area, but in most cases the direct sources are unknown. In an attempt to predict present or future risk of groundwater contamination by soil leaching, metal binding processes (retardation) were studied that are specific for these soil types under the existing acidifying conditions. From four fields nine contaminated profiles were sampled and analyzed for cadmium and zinc. Average concentrations of 131 μg g^-1 zinc and 1.6 μg g^-1 cadmium with maximum values of 2989 μg g^-1 respectively 16.3 μg g^-1 were found. In addition pH and contents of organic matter, aluminium, iron, and manganese were determined. The relative importance of these soil parameters for metal retardation is derived from the profiles. The data show that organic matter is the most important soil component for binding cadmium and zinc. Adsorption of cadmium and zinc on aluminium, iron and manganese (hydr) oxides appears to be of minor importance at low pH (<5.5).     

Inositol phosphate esters in some surface soils of Bangladesh

Concentrations of inositol mono-, di- and tri-, tetra-, penta- and hexaphosphate ranged from 5.0 to 12.5, 10.0 to 27.6, 20.0 to 45.0, 22.5 to 64.6, and 20.0 to 275.4 ppm, respectively, in samples of soils that were poorly drained and mostly derived from alluvium. Multiple correlation suggested that the amounts of esters of inositol phosphate other than inositol monophosphate were significantly correlated with organic phosphorus, total phosphorus, organic carbon, total nitrogen, inositol phospate, clay, exchangeable iron and aluminium, and pH. Individually, inositol hexa-, penta-, and di- and triphosphates were significantly correlated with organic phosphorus, total phosphorus, organic carbon, total nitrogen, inositol phosphate, and exchangeable iron and aluminium; inositol tetraphosphate with organic carbon, total nitrogen, inositol phosphate, and exchangeable iron and aluminium; inositol monophosphate with none.     

Effect of peroxidation on soil electrochemical properties

Treatment with hydrogen peroxide to destroy the organic matter shifts the surface charge towards positive values. This is in agreement with the suggestion that per oxidation produces artefacts in the soil residues, since metals released from organic matter precipitate as hydroxide coating and produce positively charged surfaces. In the presence of 0.1 M sodium pyrophosphate, treatment with hydrogen peroxide always shifts the surface charge towards negative values, notwithstanding the removal of the electronegative organic components. It is suggested that the bulk of electropositive iron and aluminium oxides in soil are associated with the organic matter and removed by the treatment, so that permanent negative charges of clay minerals dominate in soil residue.     

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.     

A 13C-NMR study of the interactions of soil organic matter with aluminium and allophane in podzols

The stabilization of organic matter in soil by interaction with aluminium (Al) or allophane is important in maintaining soil quality, and in retarding the decomposition of soil organic matter. Complexation of Al by soil organic matter may also ameliorate Al toxicity. Here we use ¹³C-NMR spectroscopy to assess the interaction of soil organic matter with both Al and allophane in two poorly drained podzols containing only trace amounts of iron. The ¹³C-NMR spectrum of the subsoil of the allophane-rich One Tree Point podzol shows an intense peak at 179 p.p.m., assigned to carbon in carboxylic acids. This peak shifts to 177 p.p.m. after removal of allophane (11% of the soil mass) by treatment with HF. We infer that the carboxyl groups in the organic matter are bonded to structural Al on the surface of allophane spherules. In the non-allophanic Te Kopuru podzol, on the other hand, the organic matter apparently interacts with Al ions in the soil solution. This soil also has more aromatic carbon and fewer carbons in carboxyl and carbohydrate structures than the allophanic sample. There is an indication that allophane stabilizes carbohydrate groups as well as carboxyl groups.     

IMOGOLITE AND PROTO-IMOGOLITE ALLOPHANE IN SPODIC HORIZONS: EVIDENCE FOR A MOBILE ALUMINIUM SILICATE COMPLEX IN PODZOL FORMATION

Examination by infrared spectroscopy and electron microscopy of the fine clays (<0.5 μm) dispersed at pH 3.5 from H₂O₂-treated soil indicates that imogolite and proto-imogolite allophanes are concentrated in podzolic B₂ and B₃ horizons, and make up at least 6 percent of one B₂ horizon soil, which contains virtually no layer silicate clays. It is argued here that imogolite-type components are the principal source of extractable aluminium and silicon in such horizons, that they may act as cementing agents in indurated horizons, and that proto-imogolite, a soluble aluminium-silicate complex, is the predominant mobile form in which aluminium is transported to B₂ and lower horizons of podzols. Comparison of the amounts of aluminium extracted by acetic acid with those extracted by EDTA indicates that extractable aluminium in Bhg, Bh, and organic-rich A₂ horizons is present principally in organic complexes. It is proposed that the aluminium fulvates concentrated in these horizons are formed in situ.     

SULPHUR, CARBON, AND NITROGEN CONTENTS OF ORGANIC FRACTIONS FROM ACETYLACETONE EXTRACTS OF SOILS

Ultrasonic dispersion of acid-pretreated soils in aqueous acetylacetone at pH 8, and at a solvent: soil ratio of about 45: I, extracted from 61 to 97 per cent of the organic sulphur from five Scottish soils. Narrowing the ratio to about 9:1 reduced the proportion extracted but was more convenient for the subsequent isolation of the organic components. Gel permeation chromatography of the extracts yielded materials containing from 14 to 60 per cent of the soil organic sulphur. Most of the soils gave four distinct fractions which contained from 21 to 38 per cent carbon, 0.2 to 2.1 per cent nitrogen, and 0.1 to 0.8 per cent sulphur. There was no consistent similarity between corresponding fractions of the various soils. Considerable amounts of iron and aluminium were extracted, presumably incorporated in organic complexes. Since they were obtained under very mild conditions, the fractions should provide useful starting materials for qualitative investigations on the nature of soil organic sulphur.     

Lanthanides in soils of the Cherepovets steel mill impact zone

Contents of different lanthanide forms in soddy-calcareous soils at different distances from the Cherepovets steel mill (Vologda oblast) have been studied. Increased contents of Pr and Tb are found in soils near the pollution source. Less manifested increases in the contents of other lanthanides (from La to Gd) are also observed. Along with the increase in total content, technogenic pollution increases the content of acid-soluble lanthanides and affects their degree of extraction. The residual fraction strongly bound to aluminosilicates contains 80 to 95% of lanthanides. Soil processes result in the partial binding of lanthanides with organic matter (5–18% of their total content) and Fe and Mn (hydr)oxides (0.1–5% of the total content). The individual properties of lanthanides are clearly manifested in their interaction with these soil components. The highest share of the fraction bound to organic matter contains medium lanthanides, and the highest share of the fraction bound to Fe and Mn (hydr)oxides contains heavy lanthanides.