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A major environmental problem with aluminum is its toxicity to plants growing in highly weathered acid soils. Aluminum is one of the more abundant elements in soils, making up approximately 7% of the solid matter in an average soil. The solubility of aluminum in soils is initially controlled by minerals that are present that have the highest solubility. Pedogenic processes slowly remove the more soluble minerals, so that the more stable ones supporting the lowest activity of aluminum ultimately control its solubility. The widespread presence of aluminosilicates in soils is accompanied by a reciprocal solubility relationship between aluminum, silica. The presence of hydroxy-interlayered vermiculite in a number of these soils suggests that an amorphous aluminum hydroxide phase forming in the interlayer could also be an active component in these systems, since some of the data fall only slightly below this solubility line.

Reports on the solubility of Fe during fluctuating redox conditions in soils and sediments are often difficult to interpret. This study was undertaken to examine the solubility relationships of Fe in suspensions of amorphous Fe(OH) 3 , Fe 2 O 3 (hematite), and soils subjected to changing redox conditions. A decrease in pe + pH was accomplished by the addition of organic matter and H 2 (g) in soils, and by H 2 (g) with a Pt catalyst in Fe(III) oxide suspensions. Addition of H 2 (g) and a Pt catalyst to Fe(III) oxide suspensions resulted in their dissolution and subsequent precipitation of two distinct Fe 3 O 4 products. With small additions of O 2 (0.2 mL of air), equilibrium was maintained with Fe 3 O 4 (magnetite). X-ray diffraction confirmed its presence. When large amounts of atmospheric O 2 (g) were added (15 mL of air), the precipitate designated Fe 3 O 4 (amorp) was formed with a mean log of the equilibrium constant based on activities (K°) of 40.65 ± 0.11. X-ray diffraction showed this predpitate to be amorphous. This precipitate controlled Fe solubility within the pe + pH range of 4.0 to 12.7. In soil suspensions subject to changing redox, the Fe 3 O 4 (amorp) precipitate and Fe(OH) 3 (amorp) appear to control Fe solubility. Crystalline Fe(III) oxides including Fe 2 O 3 (hematite) and FeOOH(goethite) can only control Fe solubility under more highly stable, oxidizing conditions. Only with prolonged and intensive reduction is Fe 3 O 4 (magnetite) expected to form and control Fe solubility.

Many soils used for tree fruit production before 1968 contain elevated Pb concentrations resulting from use of lead arsenate pesticides. The chemical form of Pb in these soils may influence its bioavailability and the need for and type of remediation actions. The competitive chelation method was used to determine the single-ion activity of Pb 2+ in aqueous suspensions of 11 lead arsenate contaminated soils and, for comparison, three Pb-enriched roadside soils. The activity of Pb 2+ ranged from 10 -9.37 to 10 -7.21 , consistent with values published elsewhere, and was inversely related to pH. The soils appeared to fall into two solubility groups, one where Pb 2+ activity was consistent with control by a Pb-phosphate mineral, possibly Pb 5 (PO 4 ) 3 OH or Pb 3 (PO 4 ) 2 , and a second group where Pb 2+ activity was consistent with control by Pb(OH) 2 . Both groups contained calcareous soils. Adding monocalcium phosphate to two of the soils from the higher Pb solubility group shifted one soil to the lower solubility group but had no effect on the second soil. The experimental results suggest that Pb solubility in lead arsenate contaminated soils is not appreciably higher than in soils contaminated by Pb from gasoline and that phosphate amendment may not always reduce soil Pb solubility.

The solubility of Fe in soils is largely controlled by Fe oxides; ferrihydrite, amorphous ferric hydroxide, and soil-Fe are generally believed to exert the major control. Fe(III) hydrolysis species constitute the major Fe species in solution. Other inorganic Fe complexes are present, but their concentrations are much less than the hydrolysis species. Organic complexes of Fe including those of organic acids like citrate, oxalate, and malate contribute slightly to increased Fe solubility in acid soils, but not in alkaline soils.

We selected thermodynamic data for 45 fluorine minerals and solution species that may play important roles in the fluorine chemistry of soils. Equilibrium reactions and stability constants for these minerals and species were calculated and used to develop stability diagrams in soils.Aluminum fluorid

Thermodynamic data were used to develop equilibrium reactions and constants for 83 selenium (Se) minerals and solution species that relate to soils. The solubilities of the metal-selenate minerals are very high in well-aerated soils and are not expected to persist in these systems. The solubilities

A modification of the EDTA method of determining ion activities in soil developed by Norvell was used to measure the solubility of various ferric hydroxides. The solubility products of jarosite (KFe₃(SO₄)₂(OH)₆) and amorphous Fe(OH)₃ were also determined on a natural jarosite nodule taken from an acid sulphate soil. For amorphous ferric hydroxide and lepidocrocite, the method gave pK° values of 39.02 ± .35 and 42.48 ± .23, respectively. Hematite and goethite were too inert to yield reproducible solubilities. For a mixture of jarosite and ferric hydroxide, the respective pK° values were 98.56 and 39.93. These values were used to develop an Eh-pH stability diagram that helps to explain the occurrence of jarosite under acid oxidizing conditions in soils.

Summary Soil samples obtained from genetic horizons of an upland and acid sulphate soil of Sierra Leone, were equilibrated with 0.01 M CaCI using a 1:2 soil to solution ratio. Al3+ activity was estimated from total Al measured in the equilibrium extract, by accounting for hydrolysis and the formation of other complex species. The Al3+ activity was pH-dependent, but the Al(OH)3 ion product was I'airly constant throughout the upland soil profile; Al3+ activity was near that expected for equilibrium with kaolinite and quartz. In the acid sulphate soil alunite appeared to control the activity of Al3+.

Four divalent Mn phosphate compounds were prepared and identified. The compound, Mn3(PO4)2 . 3H2O slowly transformed to Mn5H2(PO4)4 . 4H2O, when the solid was left in contact with the mother liquor. Dissociation constants were determined for the following reactions: where log K° is the log of the equilibrium constant, (K°). The mineral Mn(H2PO4)2 . 2H2O (manganese [II] di-[dihydrogen phosphate] dihydrate) was identified, but its solubility constant was not determined because of high ionic strength corrections. X-ray diffraction patterns, densities, and refractive indices were determined for the four minerals. Additional Index Words: Chemical formation, manganese precipitates, manganese(II) hydrogen phosphate, trimanganese diphosphate, pentamanganese dihydrogen tetraphosphate, manganese(II) di-(dihydrogen phosphate). View complete article To view this complete article, insert Disc 4 then click button8

Corn (Zea mays L.) ‘Pioneer-3932A’ was grown in the greenhouse on a loamy sand amended with cadmium, cadmium-spiked sewage sludge, and sewage sludge. Soil pH was adjusted to approximately 5.7, 6.7, and 7.8, and phosphorus was applied at 0, 200, and 400 µg/g soil. Soil treatments were applied 0, 8, and 16 weeks prior to planting, and the plants were grown for 5 weeks. Increasing the pH of the soil amended with inorganic cadmium (CdSO₄) decreased the Cd concentration of corn seedlings by approximately 67%. The Cd concentration was reduced approximately 47% for a similar increase in pH on soil amended with Cd-spiked sludge. The Cd concentration of corn seedlings was significantly reduced by the addition of P. Increasing incubation time decreased the availability of added inorganic Cd, but increased the availability of Cd applied with sewage sludge. Increasing the amount of Cd added to the soils increased the Cd concentration of the corn seedlings regardless of the form of Cd used. Cadmium concentrations in the corn seedlings were significantly correlated with DTPA-extractable soil Cd. Solubility measurements of Cd in the acidified soils showed undersaturation with respect to all known Cd minerals, whereas in soils above pH 7.25 the Cd solubility was limited by CdCO₃ at a CO₂ level of 0.003 atm or higher and showed the expected hundredfold decrease in solubility for each unit increase in pH. The minerals Cd₃(PO₄)₂ and Cd(OH)₂ are much too soluble to account for the precipitation of Cd in these soils.

The solubility of iron in soils is controlled by Fe(OH)3(soil)in well‐oxidized soils, by Fe3(OH)s(ferrosic hydroxide) in moderately oxidized soils, and by FeCO3(siderite) in highly reduced soils. The Fe(III) hydrolysis species Fe(OH)2 +, and Fe(OH)3° are the major solution species of inorganic Fe, but they are maintained too low to supply available iron to plants. Iron is absorbed by plants as Fe2+ and must be in the general range >10–7.7 M to avoid iron deficiency. The redox of soil‐root environments must be

The major cause of iron deficiency in plants is the insolubility of Fe (III) oxides in soils. Minimum solubility occurs in the pH range of 7.4 to 8.5 where the dominant solution species Fe(OH)3° is present at approximately 10‐10.4 M. The critical level of soluble iron for plants is approximately 10‐8 M, so the ferric oxides are too insoluble to supply adequate iron to plants unless modifying factors intervene. Major factors that modify iron solubility relationships in soils include oxidation‐reduction processes and the presence of chelating agents. Reduction processes near respiring plant roots enable ferric chelates to dissociate and Fe3+ to reduce to Fe2+. It is necessary for Fe2+ to increase above the critical level to supply adequate iron to plants. Competition of metal ions for chelating agents gives rise to many nutrient interactions. Redox relationships also affects metal solubility relationships in soils. When combined these factors greatly affect iron availability to plants. Important nu...

The concentration of ferric ion and the ion product of ferric hydroxide in the Weld loam (Aridic Paleustoll) were estimated from equilibria of EDTA (ethylenediaminetetraacetic acid) chelates. Additions of ferric chloride to the Weld loam raised the solubility of soil iron at least seven‐fold. The (Fe3+)(OH‐)3 ion product increased from approximately 10‐39.2 to at least 10‐38.4 after ferric chloride was added. The solubility of soil iron decreased slowly with time after fertilization. After one month the (Fe3+)(OH‐)3 ion product had fallen to 10‐39.1, virtually the same level as in the unamended soil. The results are consistent with the hypothesis that iron oxides of high specific surface or poor crystalinity control the solubility of ferric ion in soil solution. 1. This paper was presented, edited, and accepted as part of the International Symposium on Iron Nutrition and Interactions in Plants, which was published in the Journal of Plant Nutrition 5 (4 to 7), 1982. Due to error in editorial handl...

Theoretical calculations were used to develop an electron titration curve for a hypothetical soil to show the characteristic redox poises (pe + pH values) maintained by various manganese and iron minerals. An experimental electron titration of soil was made by adding either oxidant or reductant and measuring the pe + pH after 7 days and after 14 days. The resulting redox poises were compared with theoretically developed poises. The results show that many redox-related mineral transformations occur slowly and that poorly defined, mixed valency solids often result. Nevertheless, the present study demonstrates how redox poises are controlled by mineral transformations that can be identified. Redox solubility diagrams show how Mn 2+ and Fe 2+ solubilities can be predicted from pe + pH measurements.

Barbotage de CO 2 a travers deux schistes distilles. Transformations minerales accompagnant ce barbotage. Solubilite de B, Mo et F avant et apres la recarbonatation

A solubility diagram was developed that shows the activity of various ionic and molecular species of Si in equilibrium with amorphous silica as a function of pH. Below pH 8.0 only uncharged Si(OH)₄ contributes significantly to total soluble Si. The solubility of Si was measured on two soils in which equilibrium was approached from both undersaturation and supersaturation over a 50-day period. In this study, 0.02M CaCl₂ was included in all suspensions during their equilibration to flocculate colloidal Si and preclude its inclusion as soluble Si. The acid Paxton soil supported approximately 19 ppm of Si in solution while the calcareous Ulysses soil supported 25 ppm. These levels are intermediate between the solubility of quartz (2.8 ppm Si) and amorphous silica (51 ppm Si) and lie very near the level predicted for equilibrium between kaolinite and montmorillonite based on Kittrick's thermodynamic calculations.

The pKₛₚ value of a highly purified synthetic variscite, Al(OH)₂H₂PO₄, was found to be 30.5 at 25° C. The equilibrium between variscite and its constituent ions in solution was attained only very slowly. The solubility criteria obtained in this study would indicate that the immediate reaction products of applied phosphate in acid soils are much more soluble than variscite but, upon ageing, these intermediate reaction products are slowly transformed into variscite which may coexist with gibbsite as stable solid phases.

Soils play a vital role in determining the availability of trace elements to plants. The uptake of trace elements by plants constitutes an important link in the food chain of man. There are two important parameters in soils that influence the availability of an element to plants. One of these is the intensity factor or the concentration of an element in the soil solution. The other is the capacity factor or the ability of the solid phases of soils to replenish an element as it is depleted from solution. In soils, trace elements are present in several forms including ions in solution, adsorbed ions, solid phase minerals, and soil organic matter. Recently an extractant was developed for the simultaneous determination of available Zn, Fe, Mn, and Cu in soils. 10 references, 3 figures, 1 table.

The Fe, Zn, Cu, and Mn complexes and the Na salt of EDTA (ethylenediaminetetraacetic acid) were reacted with suspensions of acid, neutral, and calcareous soils. The initial concentration of chelate was 1 × 10⁻⁴M or on a soil weight basis, 11.2, 13.1, 12.7, and 11.0 ppm of Fe, Zn, Cu, and Mn, respectively. Suspensions were aerated and shaken continuously for periods of 2 hours to 30 days. At the termination of each reaction period the pH of the suspensions was measured; following centrifugation, the supernatant solutions were analyzed for Fe, Zn, Cu, Mn, Ca, and Mg by atomic absorption spectrophotometry. The concentration of ¹⁴C-labeled EDTA was determined by liquid scintillation. FeEDTA was stable in soil suspensions of pH 5.7 and 6.1, moderately stable at pH 6.75, and unstable at pH 7.3 and 7.85. This behavior was satisfactorily predicted using formation constants to describe equilibria of EDTA with Fe³⁺, Ca²⁺, and H⁺, with the assumption that the Fe³⁺ concentration was controlled by the solubility of amorphous iron(III) oxides. ZnEDTA and CuEDTA were most stable in suspensions near neutrality. In acid soils Cu and Zn were increasingly displaced by Fe as pH decreased while in calcareous soils these metals were displaced by Ca as pH increased. The loss of Mn from MnEDTA was very rapid in all soils and was essentially complete in less than one day from suspensions of pH 6.1 to 7.85.

The solubility constant for ZnSiO₃ reported in the literature is based on invalid thermodynamic data. The agreement between Zn²⁺ concentrations in some soils and the invalid solubility constant for ZnSiO₃ is coincidental. The presence of this compound in soils is very unlikely.

The dissolution of monocalcium phosphate monohydrate (MCP) in water was followed over a 17-day period; and the resulting solution was compared with solubility isotherms of the system CaO-P₂O₅-H₂O at 25°C. Shaking excess MCP with water at room temperature resulted in rapid dissolution during the first few minutes. Within an hour and continuing for more than 24 hours, the solution was in metastable equilibrium with newly formed dicalcium phosphate (DCP) dihydrate and undissolved MCP. The composition of the solution later changed as anhydrous DCP slowly precipitated and the first-formed DCP dihydrate dissolved. The final solution was in equilibrium with anhydrous DCP and undissolved MCP. Band placement of MCP in soil was found to initiate a series of reactions. The solution phase from the reaction zone was sampled by means of filter papers separating several 5-mm. layers of Hartsells fine sandy loam. Reduced water vapor pressure at the fertilizer band caused water to move inward from the surrounding soil. This transport of moisture resulted from both liquid and vapor movement. A wetted zone formed in the immediate vicinity of the band. The solution sampled at the fertilizer band approximated very closely the composition of the metastable triple-point solution (MTPS) in which P is 3.98M, Ca 1.44M, and pH 1.48. This highly concentrated solution moved slowly away from the fertilizer band into the partially dried soil, dissolving Fe, Al, Mn, and other constituents from the soil. In this complex chemical matrix many different phosphate compounds of Fe, Al, Ca, and Mn are expected to precipitate as the solution reacts with more soil and pH rises gradually to that of the surrounding soil. Water continued to move into the fertilizer zone even after MCP had dissolved. This caused dilution of the first-formed solution and enabled it to move farther into the surrounding soil.