1. IntroductionIn aqueous media the aluminium ion Al3+ exists in a variety of soluble and insoluble species. High concentrations of aluminium may affect the buffer action and transport of elements in aquatic ecosystems, this element being toxic for aquatic organisms. Although understanding the chemistry and effects of aluminium in aqueous environments has substantially increased in latest years, some supplementary research is required to identify the sources of aluminium and mechanisms of its transport from the soil solution to ground surface waters in order to assess the effects of aluminium on trace metals and the organic carbon cycle, phosphorus compounds, as well as the action of different aluminium-containing species on aquatic organisms. Since many plant species are sensitive to micromolar concentrations of Al, the potential for soils to be Al toxic is considerable. Speciation of

aluminium is a critical issue when assessing the effects of Al in soil solutions because not all its chemical species are equally toxic. The soluble aluminium in soil solutions exists as the Al3+ ion along with chemical combinations of OH-, F-, SO4

-, PO43+ and organic compounds. Though the

recent investigations have often ignored the presence of aluminium non - hydroxylic complexes, it may be assumed that the complex organic species and fluorides are the predominant forms of aluminium in dilute acidic surface waters (with a low ionic strength). The concentration of inorganic aluminium increases exponentially with decreasing solution pH. This phenomenon is analogous to the theoretical dependence of the solubility of mineral phases on pH values. At the same time, the concentration of organic aluminium species correlates better with the variations in the organic carbon concentration from surface waters than with the environmental pH. Fortunately, most of the Al is bound by ligands or occurs

Central European Journal of Chemistry

The role of hydroxy aluminium sulfate minerals in controlling Al3+ concentration and speciation in acidic soils+

* E-mail: ipovar@yahoo.ca+ The article has been presented at the 18th Romanian International Conference on Chemistry and Chemical Engineering - RICCCE18 - held in New Montana, Sinaia, Romania on 4-7 September, 2013.

Received 30 September 2013; Accepted 20 December 2013

Abstract:

© Versita Sp. z o.o.

Keywords: Aluminium heterogeneous speciation • Chemical heterogeneous equilibria • Complex formation • Gibbs energy • Hydroxyl aluminium sulfate thermodynamic stability

Institute of Chemistry, Academy of Sciences, Chisinau, MD 2028, Republic of Moldova

Igor Povar*, Oxana Spinu

RICCCE 18

A thermodynamic approach for the complex chemical equilibria investigation of two-phase systems containing hydroxy aluminium sulfate (HAS) minerals in soils has been developed. This approach utilizes thermodynamic relationships combined with original mass balance constraints, where the HAS mineral phases are explicitly expressed. The factors influencing the distribution and concentrations of various soluble aluminium species have been taken into account. The new type of diagrams, based on thermodynamic, graphical and computerized methods, which quantitatively describe the distribution of soluble and insoluble inorganic, and organic, monomeric and polymeric aluminium species in acidic soil solutions in a large range of pH values has been used. The thermodynamics of equilibria of different natural HAS in soils, the conditions under which solids involving common ions can coexist at equilibrium, the acid-base and mineral equilibria and complex formation have been examined. It has been proved that the presence of sulfate ion dramatically alters the HAS mineral solubility under acidic conditions. The obtained data regarding the factors influencing Al speciation, based on the constructed diagrams of heterogeneous chemical equilibria, are in good agreement with the current experimental and theoretical evidence. The proposed approach is intended to determine the dominant processes that are responsible for the Al3+ concentration levels and its speciation in acidic soils.

Cent. Eur. J. Chem. • 12(8) • 2014 • 877-885DOI: 10.2478/s11532-014-0540-4

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The role of hydroxy aluminium sulfate minerals in controlling Al3+ concentration and speciation in acidic soils

in other non-toxic forms such as aluminosilicates and other Al-containing minerals. However, the solubilization of Al is enhanced by low pH and the Al toxicity is a major factor restraining plant growth on acidic soils; the toxic effect being dependent on the plant species and the concentrations of individual Al species in soil solution.

Ecologically significant concentrations of aluminium are identified in surface waters due to acidic deposition, which constitutes the main way by which the atmospheric pollutants affect the soil complex equilibria. Mineral acids derived from rainfalls mobilize aluminium from minerals, which is transported to the adjoining surface waters. Thus, the release of toxic Al3+ is one of the most serious consequences of soil acidification. Introduction of acidic rainwater into the soils results in a considerable change in soil solution composition, especially in the so −2

4SO concentration. In the soil solution, so −24SO is one

of the major anions, generally ranging from 1.5×10-4 to 1.25×10-3 mol L-1 in different natural waters. The sulfate activity of surface waters and ground waters is rather constant and is set at 2.5×10-3 mol L-1. In some soils the concentrations of inorganic monomeric aluminium are typically between 1×10-4 and 2×10-4 mol L-1 [1]. Under typical soil conditions, the Al activity is usually controlled by the solubility of gibbsite (s)A(OH)3

or

kaolinite [2]. Nevertheless, in areas of sulfide oxidation, high concentrations of sulfate modify the aqueous geochemistry of Al. The acidic sulfate soil waters generally contain high concentrations of dissolved aluminium. In these acidic soils, the Al solubility appears to be controlled by a variety of basic Al sulfate phases (as alunogen, alunite, jurbanite and basaluminite [3,4]).

The identification of mineral species that control the Al solubility in acidic soil solutions, reached in sulfate ion, is a difficult procedure. Firstly, as it was proved by the X-ray diffraction, the solid phases could be amorphous. Secondly, the differentiation of adsorbed sulfate from that structural is a supplementary difficulty. In numerous studies theoretical calculations have been carried out in order to determine the Al mineral species that are formed in acidic sulfate soils [4]. Authors [5] affirm that the retention of sulfate by the acidic soils is a consequence of the solubility of basaluminite and/or alunite. The basaluminite solubility and its control of Al concentration in aqueous sulfate acidic systems have been recently examined for a series of natural acidic waters [4,6]. Using the Al, S and O, K-XANES spectroscopy in combination with the elemental analysis of groundwater precipitates, it has been demonstrated that for values of pH 4.0-5.2 the Al concentration is controlled by a poor crystalline mineral - hydrobasaluminite (with the empirical formula ) that under

conditions of relatively low humidity is very unstable and tends to dehydrate with formation of basaluminite. The same authors have demonstrated that the concentration of soluble Al in the pH range of 3.5-7 in acidic sulfate soils is controlled by a mixture of Al amorphous hydroxide and basaluminite. Furthermore, at low pH values and high so −2

4SO activity, alunite can be the most stable hydroxy sulfate mineral [7].

On the other hand, the majority of soil water samples studied by authors [4] with pH below 5 were oversaturated with respect to jurbanite. Their best predictions were obtained by assuming a jurbanite type equilibrium for pH<5 and a gibbsite type equilibrium for higher pH values, but to what extent this really reflects the real processes was uncertain.

Additionally, Appelo and Postma [8] considered that in groundwater the gibbsite model was reasonably accurate with pH>4.5 and a jurbanite model better for pH<4.5. Karathanasis and others [9] surmised that the change occurring between pH 4.5–5 is the boundary for control between basic aluminium sulfates and aluminosilicates. The activity in solutions with pH<5 seems to be controlled by jurbanite, alunite, and gibbsite, depending on the so −2

4SO activity in the soil

solution. In solutions with pH>5 an amorphous (s)A(OH)3 controls the Al activity [9].High concentrations of aluminium in dilute acidic

waters are of interest from several points of view: aluminium is an important pH buffer; aluminium can influence the cycle of important elements, such as phosphorus, organic carbon and metal ions; aluminium is potentially toxic to aquatic organisms.

Authors [10] suggested that the formation of basic aluminium sulfate minerals

, also named hydroxyl - aluminium sulfate (HAS), may contribute to sulfate preservation in soils. The identification of the mineral species controlling the solubility of Al in acidic waters rich in sulfate has presented researchers with several challenges. It is noticeable that there are inconsistencies in the literature on the process regulating aqueous aluminium concentration in dilute acidic waters. If one intends to improve the models that have been developed to evaluate the degree and effects of soil acidification, it is essential to develop a better understanding of the mechanisms of aluminium mobilization.

In order to better understand the effects of acid precipitation on soil and predict the toxic concentration of Al in soil solutions, it is essential to have a tool to predict how the various species of Al will respond to changes in the composition of soil solutions containing the HAS minerals. Chemical modeling represents

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I. Povar, O. Spinu

a unique method for the prediction of long-time aluminium behavior in soil solutions [11]. On the other hand, chemical modeling of aqueous solutions is restricted by availability of correct thermodynamic data, underlying assumptions of chemical equilibrium that may or may not apply, accessibility of full equilibrium data for the modeled system, along with quality of these data. Discrepancies between observed and predicted solid phases reflect kinetic factors, disequilibria processes in natural systems, and the uncertainties of thermodynamic data. The main objective of this paper consists of the thermodynamic analysis and graphical representation of the distribution of various species of Al, both in saturated aqueous solutions and HAS solid phases. Diagrams of heterogeneous complex equilibria, based on graphical and numerical computerized methods [11], which quantitatively depict the repartition of various soluble and insoluble, inorganic and organic, monomeric and polymeric aluminium species in heterogeneous multicomponent systems “HAS mineral – saturated aqueous solution”, have been used in order to determine the dominant processes that are responsible for the aluminium ion concentration levels in acidic soils.

2. Computational details In this paper, as the aluminium insoluble species, a series of minerals with general composition

has been investigated. Their solubility can be controlled by the following dissolution-precipitation equilibrium:

(1)

where SK is the solubility product of aluminium mineral. The ionic charges and influence of ionic strength are omitted for simplicity. The concentration can be calculated from the SK Eq. 1:

(2)

In this paper the following minerals have been analyzed: 1) jurbanite , 2) basaluminite

3) alunite

4) alunogel and 5) gibbsite . Alongside the Eq. 1 a large set of possible

equilibria in the system „mineral – soil solution” has been considered. In the soil solution Al is found to

be in different soluble forms, including and its hydroxo, fluoride, sulfate and organic complexes. There are also phosphate complexes and polymeric species of aluminium. Organic solutes in natural waters are presented as a triprotic acid OrgH 3 proposed by Driscoll and Schecher [3], while two species of Al-organic solutes are depicted. Their thermodynamic constants were calibrated using field observations, and a good fit of the data was obtained (r2 = 0.86 and n = 31). As polynuclear species of aluminium, the dimmer and trimmer

have been

taken into account. The polymeric species with the greater molecular masses , where i = 6, 8 and 13 are not examined since they are not practically formed in the soil solution due to the low polymerization kinetics. At the same time, the organic acids in soil solution inhibit their formation [11]. The phosphate complexes of aluminium can be presented as . The silicate complexes of aluminium are ignored because of their weak complex formation properties. The combination of the mass balance (MB) conditions formulated within the method of residual concentrations (RC) [12-14], the law of mass action (LMA) equations in the considered systems (see Supplemental Table 1 in the Supplementary Information) and Eq. 2 gives the following set of equations:

(3)

(4)

(5)

(6)

,

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The role of hydroxy aluminium sulfate minerals in controlling Al3+ concentration and speciation in acidic soils

(7)

(8)

(9)

The quantities 0

kC and rkC (mol L-1) in Eqs. 3-8 represent

respectively the analytical and residual concentrations of component “k” in the heterogeneous system, while

kC∆ symbolizes its quantity in the precipitate in one litre of solution [15]. The final set of nonlinear algebraic equations obtained Eqs. 3-8 involves only the free ionic concentrations and

kC∆

terms, as well as the stability

and solubility product constants. Within the RC method, the relations and are valid. In the Eq. 9 0

HC denotes the excess of +H ions versus hydroxyl ions in biphasic mixtures,

[16].

From the stoichiometric composition of minerals of type the useful relations follow:

(10)

The equilibrium concentration of ion is determined by the Eq. 2. From the MB Eqs. 3-8 for a certain pH value, taking into account the Eq. 10, one can obtain a system of 5 nonlinear equations with 5 unknown variables, respectively and (or ). In the case of alunite we get a system of 6 equations with a sixth additional unknown variable ][ +K . The obtained systems of nonlinear equations can be easy solved by obvious computerized numerical methods [15-18]. Then, the residual concentrations of the “k” component are calculated r

kC [15-20], knowing the initial composition of heterogeneous mixture, e.g. all the quantities 0

kC . Using the LMA and MB combined equations, it is possible to calculate the diagrams of distribution of aluminium ion both in solid and liquid phases (species in aqueous solution), i.e. the diagrams of heterogeneous chemical equilibrium (DHCE) [11]. In the heterogeneous system, where the examined reactions proceed concomitantly, the following equations for calculation of the solute mole fractions can be written:

(11)

The subscript index “tot” symbolizes the sum of mole fractions of all the soluble species. From the Eq. 3 and 11 the following identity results:

The DHCE are externally similar to the diagrams of species distribution in the homogeneous systems. The lasts usually are fitted in the coordinates

, as in the absence of polynuclear complexes the solute mole fractions of species in aqueous solution (more frequently, written as f or a) are functions only of the solution рН and do not depend on initial concentrations of components. In the case of heterogeneous equilibria, the solute mole fractions of species also depend on the initial composition of mixture and, for a given pH, are functions of five variables

. In this case it is reasonably to construct diagrams in coordinates while values of other quantities are kept constant.

Within the thermodynamic approach, previously developed [11,15,20] and extended in this paper for the solid phases with a more complex composition, it has been demonstrated that the values of the total Gibbs energy totSG ,∆ for the investigated complex processes, where a large set of simultaneous reactions take place, can be calculated by the following equation:

(12)

In order to compare the totSG ,∆ values for different minerals, the Gibbs energy referred to one mole of aluminium, totSG ,′∆ , has been calculated:

(13)

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I. Povar, O. Spinu

The solid phase is stable if 0, >∆ totSG . The condition 0, =∆ totSG corresponds to the beginning of mineral dissolution/precipitation.

The procedure of the DHCE construction includes the following essential steps [11]:

i) Thermodynamic determination of the mineral stability area. For that reason, from Eqs. 12 or 13, the values of the Gibbs energy for the examined process are calculated;

ii) The solute mole fractions of all the species ,

containing the aluminium ion, for the thermodynamic stability area of mineral established in the previous step, are computed using Eqs. 11;

iii) In order to get a whole diagram, in the case of aqueous homogeneous solution, the solute mole fractions will be calculated from the equations used for construction of the usual diagrams of species distribution. In this case, under the conditions of the mononuclear species formation, the values do not depend on the initial concentrations of solution components.

3. Results and discussion

3.1. Thermodynamic stability of hydroxy aluminium sulfate mineralsThe diagrams constructed here totSG ,∆

(pH) allow easy

determination of the stability area of minerals in function on the initial composition of multicomponent systems. Figs. 1 and 2 show the variation of the Gibbs energy versus pH in the “aqueous saturated solution - mineral” systems for different concentrations of individual components. These dependencies prove that the aluminium residual concentration can vary within the large limits for the same pH value. Therefore, the pH value is not the exclusive factor controlling the soluble aluminium concentration. The analysis of obtained data proves that the thermodynamic stability areas of studied soil minerals become considerably larger at increasing total (analytical) concentrations. Calculations also demonstrate that significant amounts of inorganic ligand

−F , as well as organic ligand −3Org , affect significantly the solubility of minerals. Under the examined conditions alunogel is thermodynamically unstable. For high concentrations of the aluminium and sulfate ions

1×10-3 mol L-1 (Fig. 1) and 1×10-2 mol L-1 (Fig. 2) at certain pH values, the condition

0, >∆ totSG occurs for all the minerals, except for alunogel. One can identify the pH areas in which the aluminium minerals are thermodynamically stable, for

example in Fig. 1: alunite (3.2 ÷ 8.6), basaluminite (4.0 ÷ 9.0), gibbsite (4.2 ÷ 10.6) and jurbanite (3.0 ÷ 6.8). Furthermore, the maximal thermodynamic stability of minerals varies as a function of the pH of heterogeneous mixtures. In the pH area of 3.0 ÷ 5.0 the most stable mineral is jurbanite, then for the pH values of 5.2 ÷ 5.8 is basaluminite and for the pH values greater than 6.0 is gibbsite.

The analysis of data illustrated in Figs. 1 and 2 reveals that the thermodynamic stability area for the Al hydroxyl-sulfate minerals becomes considerably larger by increasing the total (analytical) concentration of aluminium ion. The presence of sulfate also alters dramatically the Al solubility under acidic conditions, where other, less soluble minerals than gibbsite, can control the aqueous geochemistry of aluminium. Our analysis shows that a certain influence as well as the mineral solubility causes the presence of

2 4 6 8 10

-12

-8

-4

0

4

5

4

3

2

1

∆G` S,

tot/2

.3RT

pH

1. Alunite 2. Basaluminite 3. Gibbsite 4. Alunogel 5. Jurbanite

Figure 1.

2 4 6 8 10

-12

-8

-4

0

4

5

4

3

21

∆G` S,

tot/2

.3RT

pH

1. Alunite 2. Basaluminite 3. Gibbsite 4. Alunogel 5. Jurbanite

Figure 2.

Total Gibbs energy versus рН in the system “saturated aqueous solution – mineral”. Concentrations (mol L-1):

0OrgC , = 1×10-4, 0

FC = 5×10-6, = = 1×10-3.

Total Gibbs energy versus рН in the system “saturated aqueous solution – mineral”. Concentrations (mol L-1):

0OrgC , = 1×10-4, 0

FC = 5×10-6, = = 1×10-2.

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The role of hydroxy aluminium sulfate minerals in controlling Al3+ concentration and speciation in acidic soils

large amounts of inorganic ligand −F and organic ligand −3Org . These minerals include alunite

, jurbanite and basaluminite .

Our data confirmed the early finding [21] that jurbanite is stable up to the pH range of 3–5 while alunite is stable at higher pH values than jurbanite, up to 4–7, depending on the sulfate activity (Figs. 1 and 2). Above these pH ranges gibbsite is the most stable phase.

The obtained results, based on thermodynamic calculations, explain the following experimental confirmation: (a) the increase in sulfate concentration in soil solutions leads to a considerable decrease of the aluminium residual concentration and (b) high concentrations of so −2

4SO play an important detoxification role for aquatic organisms [22].

3.2. Diagrams of heterogeneous chemical equilibria (DHCE) in the systems „mineral – multicomponent saturated aqueous solutions”

3.2.1. The system „jurbanite – aqueous solution”DHCE for the investigated system are represented in Figs. 3 and 4. The diagram reflects clearly the distribution of various aluminium chemical forms both in the solid phase and saturated solutions. Solute mole fractions of soluble aluminium forms are changing strongly at the pH value of dissolution – formation of the solid phase. For a given total concentration of aluminium in acidic and neutral solutions the equilibrium concentrations of mononuclear and polynuclear aluminium hydroxo - complexes are negligible.

The Al3+ activity is strongly correlated with the soil solution pH and it increases as pH decreased. The presence of an inorganic ligand, fluoride ion −F , does not practically affect the solubility of jurbanite, while the organic ligand −3Org has a little influence. The solubility of jurbanite increases under alkaline conditions. The anionic hydroxo – complex in alkaline media becomes the prevailing species. From Figs. 3 and 4 one can clearly see that the thermodynamic area of jurbanite stability increases with the increase of the analytical concentrations of aluminium and sulfate ions in the heterogeneous mixture. In the case of 1×10-2 M in the strong acidic solutions in the pH region up to 2.0 the predominant species is +

4AlSO .

3.2.2. The system „basaluminite – aqueous solution”Diagrams DHCE for various concentrations of sulfate are presented in Figs. 5-8. The solute mole fractions of soluble aluminium species change to a large extent at the pH value of precipitation - dissolution of the solid phase. One can observe that with the increase of the so −2

4SO the nature species and pH range essentially change. For the studied mixture compositions in acidic and neutral solutions, the equilibrium concentrations of mononuclear hydroxo-complexes are small, while the polynuclear complexes almost do not form within the examined range of the variation of pH and total sulfate concentration.

The concentration of the toxic aluminium ion in acidic media decreases significantly with increasing

, and for 1×10 mol L-1 becomes negligible. The predominant species in alkaline solutions is the anionic hydroxo-complex . With the increase of the concentration of sulfate ions a substantial modification of the stability fields of other soluble species occurs. In the case 2.5 ×10-5

mol L-1 and

1×10-4 mol L-1, within the range of pH 4.0 – 5.0

2 4 6 8 100,0

0,2

0,4

0,6

0,8

1,0

8 7 6

5

4

3

2

1

γ ij

pH

1. AlOHSO4

2. Al3+

3. Al(OH)2+

4. Al(OH)4-

5. AlSO4+

6. AlOrg7. AlHOrg+

8. AlH2PO42+

Figure 3.

2 4 6 8 100,0

0,2

0,4

0,6

0,8

1,0

4

3

2

1

γ ij

pH

1. AlOHSO4

2. Al3+

3. Al(OH)4-

4. AlSO4+

Figure 4.

Diagrams of heterogeneous chemical equilibrium in the system “saturated aqueous solution – jurbanite”. Concentrations (mol L-1): 0

OrgC , = 1×10-4, 0FC =

5×10-6, = = 1×10-3.

Diagrams of heterogeneous chemical equilibrium in the system “saturated aqueous solution – jurbanite”. Concentrations (mol L-1): 0

OrgC , = 1×10-4, 0FC =

5×10-6, = = 1×10-2.

882

I. Povar, O. Spinu

the complexes AlOrg and +AlHOrg prevail, and for 1×10-3 mol L-1 this range of pH narrows, while for 1×10-2 mol L-1 the formation of these complexes is

blocked by the predominant complexes of aluminium with sulfate ion, +

4AlSO and (see Fig. 8). In Fig. 9 is illustrated the variation of the solute mole fraction of the solid phase - basaluminite, in function of pH and . One can observe the areas of the mineral existence, e.g. the areas of the heterogeneous system, coincide with the areas of the thermodynamic stability of this mineral (Fig. 1), at the same time the values grow up with the increase of .

3.2.3. The system „alunite – aqueous solution”DHCE for the investigated system are represented in Fig. 10. For the given value of in acidic and neutral

2 4 6 8 100,0

0,2

0,4

0,6

0,8

1,0

8

76

5 3

42 1

γ ij

pH

1. Al4(OH)10SO4

2. Al3+

3. AlOH2+

4. Al(OH)4-

5. AlF2+

6. AlOrg 7. AlHOrg+

8. AlH2PO42+

Figure 5. Distribution curves of the soluble species and solid phase in the system „basaluminite – saturated aqueous solution” for 2.5.×10-5 mol L-1, , 0

OrgC , = 1×10-4, 0

FC =5×10-6 mol L-1.

2 4 6 8 100,0

0,2

0,4

0,6

0,8

1,0

9

8

7

65

4

3

2

1

γ ij

pH

1. Al4(OH)10SO4

2. Al3+

3. AlOH2+

4. Al(OH)4-

5. AlF2+

6. AlSO4+

7. AlOrg 8. AlHOrg+

9. AlH2PO42+

Figure 6.

2 4 6 8 100,0

0,2

0,4

0,6

0,8

1,0

9

8

7

6

5

4

3

2

1

γ ij

pH

1. Al4(OH)10SO4

2. Al3+

3. Al(OH)4-

4. AlF2+

5. AlSO4+

6. Al(SO4)2-

7. AlOrg 8. AlHOrg+

9. AlH2PO42+

Figure 7.

2 4 6 8 100,0

0,2

0,4

0,6

0,8

1,0

8 7

6

5

4

3

2

1

γ ij

pH

1. Al4(OH)10SO4

2. Al3+

3. Al(OH)4-

4. AlF2+

5. AlSO4+

6. Al(SO4)2-

7. AlOrg 8. AlHOrg+

Figure 8.

4 5 6 7 8 90,0

0,2

0,4

0,6

0,8

1,0

4 3 2 1

γ s

pH

1. C(SO42-) = 2.5 x 10-5M

2. C(SO42-) = 10-4M

3. C(SO42-) = 10-3M

4. C(SO42-) = 10-2M

Figure 9. Distribution curves of the soluble species and solid phase in the system „basaluminite – saturated aqueous solution” for 1×10-4 mol L-1, , 0

OrgC , = 1×10-4, 0

FC = 5×10-6, mol L-1.

Distribution curves of the soluble species and solid phase in the system „basaluminite – saturated aqueous solution” for 1×10-3 mol L-1, , 0

OrgC , = 1×10-4, 0

FC = 5×10-6, mol L-1.

Distribution curves of the soluble species and solid phase in the system „basaluminite – saturated aqueous solution” for 1×10-2 mol L-1, , 0

OrgC , = 1×10-4, 0

FC =5×10-6, mol L-1.

Solute mole fraction (or the degree of precipitation) of basaluminite versus pH for different

values, ,

0OrgC , = 1×10-4, 0

FC = 5×10-6, mol L-1.

883

The role of hydroxy aluminium sulfate minerals in controlling Al3+ concentration and speciation in acidic soils

solutions, the equilibrium concentrations of aluminium mononuclear and polynuclear hydroxocomplexes are extremely small. The fraction of the uncomplexed aluminium ion increases with the growth of in a narrowed area of the рН values of saturated solutions. With the increase of the analytical concentration of the aluminium ion the area of stability of the anionic hydroxocomplex narrows; this becomes the predominant species in the slightly alkaline and alkaline solutions. The analysis of obtained results shows that for the concentration 0

FC = 5×10-4 mol L-1 the complexes+

2AlF and 3AlF bring a sizeable contribution. One can note that the diagrams obtained here significantly differ from the diagrams [22], since the authors of cited paper considered the distribution of inorganic and organic species only in the homogeneous aqueous solution. Our calculations confirmed that the complexes of aluminium with phosphate-ion are formed in insignificant quantities for investigated compositions. The aluminium sulfate complexes prevail in very acidic and acidic solutions. This conclusion correlates with that communicated by authors [22].

4. ConclusionsThe areas of thermodynamic stability for different aluminium-containing minerals with the general composition have been determined for a large area of pH variation and different values of aluminium and sulfate concentrations. Using data from several published sources, the developed thermodynamic approach demonstrates how the changes in soil solution composition affect the Al chemistry in equilibrium with HAS minerals.

1. It has been proved that the so −24SO concentration

has a substantial influence on the aluminium speciation in acidic soils, while the concentrations of organically and fluoride complexed Al are minimal in the studied pH range.

2. Our thermodynamic analysis for a series of different soil solution compositions shows that in decreasing the so −2

4SO

concentration, the amounts of toxic forms of and greatly increase while the concentrations of diminish. When the solution pH is raised above 4.0, Al 3+ forms mononuclear hydroxo-species and complexes

with inorganic ligands such as sulfate, fluoride and also organic species. The obtained data regarding the factors influencing Al speciation, based on the constructed DHCE diagrams, are in good agreement with the previous results obtained by authors [1,2,8-10,21], but they are more specific, taking explicitly into account the heterogeneous character of soil solutions.

3. The results presented here suggest that the formation of HAS minerals plays a major role in sulfate retention in S-impacted soils. Based on our thermodynamic analysis of the soil solution composition, alunite, basaluminite and jurbanite are predicted to prevail or coexist in acidic soils within different pH ranges, controlling the aluminium ion concentration level for various soil environments.

4. It is confirmed that the mobility and toxicity of aluminium in soils are enhanced by the acid deposition. As the soil pH decreases, aluminium is solubilized and the proportion of phytotoxic aluminium ions increases in the soil solution. The thermodynamic approach developed and used in this paper contributes to elucidate the mechanism of controlling aluminium concentration in dilute acidic soil solutions. This approach is intended to determine the dominant processes that are responsible for the aluminium ion concentration level and its speciation controlled in acidic soils by the HAS minerals.

2 4 6 8 10 120,0

0,2

0,4

0,6

0,8

1,0

1. KAl3(SO4)2(OH)6

2. Al3+

3. Al(OH)4-

4. AlF2+

5. AlSO4+

6. Al(SO4)2-

4

3

6

5

1

2

γ ij

pHFigure 10. Diagrams of heterogeneous chemical equilibria in

function of pH in the system “saturated aqueous solution – alunite”. Concentrations (mol L-1): 0

OrgC , , =1×10-4,

0FC =5×10-6, =

5×10-3.

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I. Povar, O. Spinu

T. Larssen et al., Water, Air, Soil Pollut. 101, 137 (1998)W. Stumm, J.J. Morgan, Aquatic chemistry: An introduction emphasizing chemical equilbria in natural waters (Wiley, New York, 1981)C.T. Driscoll, W.D. Schecher, Environ. Biochem. Health 12, 28 (1990)A. M. Jones, R. N. Collins, T. D. Waite, Geochim. Cosmochim. Acta 75, 965 (2011)F. Adams, Y. Rawajfih, Soil Sci. Soc. Am. J. 41, 686 (1977)J. Sánchez-España, I. Yusta, M. Diez-Ercilla, Appl. Geochem. 26, 1752 (2011)R.E. Stoffregen, G.L. Cydan, Amer. Min. 75, 209 (1990)C.A.J. Appelo, D. Postma, Geochemistry, Groundwater and Pollution (A. A. Balkema, Rotterdam, 1993)A.D. Karathanasis, V.P. Evangelou, Y.L. Thompson, J. Environ. Quality 17, 534 (1988)T. Delfosse, F. Elsass, B. Delvaux, Eur. J. Soil Sci. 56, 281 (2005)

I. Povar, V. Rusu, Can. J. Chem. 90, 326 (2012)I. Povar, J. Anal. Chem. 53, 1113 (1998)I. Povar, Russ. J. Gen. Chem. 70, 501 (2000)I. Povar, Can. J. Chem. 79, 1166 (2001)I. Povar, Russ. J. Inorg. Chem. 42, 607 (1997) (Engl. Transl.)I. Fishtik, I. Povar, I. Vataman, Zh. Obshch. Khim. 56, 739 (1986) (In Russian) I. Fishtik, I. Povar, Zh. Obshch. Khim. 57, 31 (1987) (In Russian)I. Fishtik, I. Povar, I. Vataman, Zh. Obshch. Khim. 57, 736 (1987) (In Russian) I. Povar, I. Fishtik, I. Vataman, Izv. Akad. Nauk Mold. SSR, Ser. Biol. Khim. Nauk 5, 57 (1989) (In Russian)I. Povar, Ukr. Khim. Zh. 60, 371 (1994) (In Russian)D.K. Nordstrom, Geochim. Cosmochim. Acta 46, 681 (1982)S. Bi et al., Environ. Geol. 41, 25 (2001)

References

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11][12][13][14][15]

[16]

[17]

[18]

[19]

[20][21]

[22]

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