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Modeling the Distribution of Aluminum Speciation in Soil Water Equilibria with the Mineral Phase Jurbanite 1,2
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C. Y. Wang , S. P. Bi , W. Tang , N. Gan , R . Xu , and L. X. Wen 1
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Department of Chemistry, State Key Laboratory of Pollution Control and Resource Reuse of China, Nanjing University, 210093, Peoples Republic of China Department of Chemistry, Xuzhou Normal University, 221009, Peoples Republic of China Corresponding author: email:
[email protected] 2
This paper presents the use of an equilibrium-based computer model to investigate the speciation of Al in soil solutions assumed to be in equilibrium with mineral phase jurbanite. The model predicts the distribution of various inorganic Al species, Al-organic matter complexes, and polymeric-Al species in solution as a function of pH. Using data from several published sources, the model demonstrates how change in soil solution composition impacts Al solution chemistry in equilbrium with several Al solid phases: jurbanite, basaluminite, alunite and gibbsite. Emphasis is placed on jurbanite due to its presence in soils impacted by acidic deposition. In the presence of jurbanite, the model predicts that SO will have a substantial influence on the distribution of Al species and concentrations of soluble Al, while concentrations of organically complexed and fluoride complexed Al are minimal in the p H range studied. The model also predicts Al speciation for published soil solution data, assuming soil solutions were in equilibrium with jurbanite. Predicted concentrations of total dissolved Al, inorganic Al and Al-organic complexes agreed within an order of total dissolved Al, inorganic Al and Al-organic complexes agreed within an order of magnitude, however, the model consistently over predicts concentrations using the current set of constants. Nevertheless, the model results imply the presence and dissolution of jurbanite in soils impacted by acidic deposition will markedly influence soil solution Al chemistry. 2-
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© 2004 American Chemical Society In Environmental Impact of Fertilizer on Soil and Water; Hall, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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Introduction
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The release of toxic A l h a s become one of the most serious consequences of anthropogenic soil acidification (7). Speciation of aluminum (Al) is a critical issue when assessing the effects of A l in soil solutions because not all chemical forms of A l are equally toxic. In order to better understand the effects of acid precipitation on soil and predict toxic concentration of A l in soil solutions, it is necessary to have a means to predict how the various forms of A l will respond to changes in the composition of soil solutions (2). Because of the widespread presence of solid phase gibbsite Al(OH) in soils, control of A l solubility via gibbsite dissolution has been widely used in the modeling of soil solution chemistry (5-5). However, the A l concentration and the forms of A l species in soil solutions are related to the type of soil solid present, composition of the soil solution and soil p H (6,7). Significant changes in soil solution composition, such as a change in the dominant anion species, will impact A l solution chemistry and possibly the solid phase A l species present. Acidic deposition represents one mechanism whereby atmospheric pollutants can influence soil chemistry, especially as H S 0 is one of the most important components of acidic rainfall and acidic surface water (8,9). Introduction of acidic rainwater into the soil results in a significant change in soil solution composition, especially in the concentration of S 0 \ Similar changes occur for soil in contact with pyrite or amended with sulfur, as oxidation reactions result in the release of sulfate to the soil solution. With an increased concentration of sulfate in the soil solution, the activity of A l is greatly modified (10-12). The presence of sulfate may also change the relative stability of Al-containing minerals in the soil. In acid sulfate surface waters, aluminum oxysulfate minerals are more stable than gibbsite (73). A t low p H values and higher S0 * activities, jurbanite Al(S0 )(OH)-5H 0 appears to be the most stable phase of oxysulfate that will form in soils (14-21). The formation and presence of jurbanite has been reported in the B-horizon of soils( 13,15). Models that predict A l speciation in soil solutions for soils impacted by acidic deposition, therefore, should take into account the possible presence of aluminum oxysulfate minerals such as jurbanite. Direct measurement of the various potential A l species that may be present in soil solution is time-consuming and often not complete. Most often only a measure of the total dissolved A l concentration is possible. Computer-based chemical speciation models which assume chemical equilibrium in soil solutions are a simple and convenient way to predict the individual concentration of A l specie that may be present. In this paper we explore the use of a computer model to determination the distribution of A l species in soil solutions in equilibrium with the presence of solid phase jurbanite. The objectives of this effort are: to characterize the distribution of A l species in soil solutions; to evaluate the effects 3
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In Environmental Impact of Fertilizer on Soil and Water; Hall, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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that changes (pH, concentrations of S0 ", and solubility of jurbanite, etc.) in soil solution composition will have on A l speciation, and to compare predicted versus actual concentrations of total dissolved A l , inorganic A l and Al-organic complexes for published soil solution data. 4
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Theory Similar to our previous work (3,22-24), the model was constructed with the following assumptions: (1) The concentration of A l jurbanite:
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in soil water is controlled by the solubility of
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Al(S0 )(OH)-5H 0+H „ 4
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» A1 +S0 +6H 0 4
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log K =-3.52 10
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This reaction exists in a finite range of pH, when p H and C * fit the condition that pH>log ( K ^ K ^ Q o / ) (Κ,,, C * see référencerai)). Setting Qo4 1 lO^mol-L' (S0 ~ concentration in natural waters range from 5.0* 10" to 2.5 *10" mol-L" based on our monitoring for fifty soil solution samples), jurbanite will be in its stable phase and doesn't dissolve at p H below 3.5. (2) For the sake of simplicity, the studied soil solution is assumed as a dilute solution system with low ionic strength, so the effect of ionic strength need not be taken into account (3,25). (3) Natural occuring organic acid in soil waters is depicted as a trinary acid proposed by Schecher and Driscoll (25). Two kinds of organically complexed A l (AlOrg, AlHOrg*) are taken into consideration. In this paper we use 0.43 mol Orgmol* D O C (dissolved organic carbon) (3,25,26). (4) Polynuclear A l is assumed to only exist in the form of dimer A l ( O H ) and trimer A l ( O H ) in acidic soil solutions. Al-phosphate complexes are expressed as AlH P0 (2
