Printer Friendly

Dissolution and solubility of the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution in aqueous solution at 25[degrees]C and pH 2.

1. Introduction

Arsenic in natural waters is a worldwide problem. Arsenic has been known from antiquity to be highly toxic for animals and the majority of plants [1, 2]. Although arsenic has been classified at the top of the priority list of the most hazardous substances [3, 4], the crystal structures and the solubility as well as the thermodynamic properties of numerous arsenates remain poorly determined. This is an important handicap because an in-depth study of the arsenate behaviour in soils, sediments, and natural waters that have been subjected to pollution requires a precise knowledge of the possible precipitating phases, their crystal chemistry, and their thermodynamic properties. Furthermore, in multicomponent aqueous systems, the precipitation of solid solutions is always a possibility such that the study of arsenate solid solutions involving substitution between atoms of similar size and character is worthwhile [5, 6]. Previous studies indicate that, at high concentrations, compounds in this series could be a limiting phase for arsenic in natural aqueous environments [7, 8]. In solution, Ba tends to associate with As at pH 7.47-7.66, forming BaHAs[O.sub.4] x [H.sub.2]O and [Ba.sub.3][(As[O.sub.4]).sub.2] [9]. This chemical interaction is used to remove As from aqueous solutions, but its efficiency depends on the solubility of both elements, which is altered by the physicochemical conditions of water [10, 11]. Tiruta-Barna et al. [12] also found that the leaching behaviour of arsenic from a compacted coal fly ash was controlled by the weak soluble phase BaHAs[O.sub.4] x [H.sub.2]O. The only available data on this series correspond to the strontium endmember, whose space group (Pbca), cell parameters, and atomic positions of Sr and As were determined by Binas and Boll-Dornberger [13]. From powder diffraction data, Martin et al. [14] determine the same space group for the barium endmember and suggest that SrHAs[O.sub.4] x [H.sub.2]O and BaHAs[O.sub.4] x [H.sub.2]O are isomorphous and therefore good candidates to form solid solutions. The previous data indicate that SrHAs[O.sub.4] x [H.sub.2]O and BaHAs[O.sub.4] x [H.sub.2]O form a complete solid solution where [Sr.sup.2+] ions substitute for [Ba.sup.2+]. Both endmembers crystallize in the same orthorhombic Pbca space group, with lattice parameters that vary significantly with composition [4].

The determination of the equilibrium behaviour in SS-AS systems requires knowledge of both the endmember solubility products and the degree of nonideality of the solid solution [5]. Unfortunately, the lack of thermodynamic data for arsenates is even greater than the lack of crystal-chemical data. The [K.sub.sp] value for BaHAs[O.sub.4] x [H.sub.2]O was determined to be [10.sup.-4.70] by Robins [10], [10.sup.-24.64] by Essington [15], [10.sup.-5.31] by Itoh and Tozawa [16], [10.sup.-0.8] by Orellana et al. [17], [10.sup.-5.51] by Davis [18], and [10.sup.-5.60] by Zhu et al. [9], which showed a great inconsistency of the currently accepted solubility data. Moreover, the solubility of the strontium endmember as well as the thermodynamic properties of the solid solution is unknown [4]. The fact that there is a significant preferential partitioning of barium toward the solid phase seems to indicate a lesser solubility ([approximately equal to] one order of magnitude) for the strontium endmember. A thermodynamic character of these compounds needs to be confirmed 4].

In the present study, a series of the ([Ba.sub.x][Sr.sub.1-x])Has[O.sub.4] x [H.sub.2]O solid solution with different Ba/(Ba + Sr) atomic ratios was prepared by a precipitation method. The resulting solid solution particles were characterized by various techniques. This paper reports the results of a study that monitors the dissolution and release of constituent elements from synthetic ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solutions using batch dissolution experiments. The solid solution aqueous solution reaction paths are also discussed using the Lippmann diagram to evaluate the potential impact of such solid solutions on the mobility of arsenic in the environment.

2. Experimental Methods

2.1. Solid Preparation and Characterization. The experimental details for the preparation of the samples by precipitation were based on the following equation: [X.sup.2+] + HA[O.sub.4.sup.2-] + [H.sub.2]O = XHAs[O.sub.4] x [H.sub.2]O, where X = [Ba.sup.2+] or [Sr.sup.2+]. The ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solutions were synthesized by controlled mixing of a solution of 100 mL 0.5 M [Na.sub.3]As[O.sub.4] and a solution of 100 mL 0.5 M Ba[(Cl[O.sub.4]).sub.2] and Sr[(Cl[O.sub.4]).sub.2], so that the (Ba + Sr)/As molar ratio in the mixed solution was 1.00. The amounts of Ba[(Cl[O.sub.4]).sub.2] and Sr[(Cl[O.sub.4]).sub.2] were varied in individual syntheses to obtain synthetic solids with different mole fractions of Ba/(Ba + Sr) (Table 1). Reagent grade chemicals and ultrapure water were used for the synthesis and all experiments. The initial solutions were slowly mixed in a covered beaker in a course of 10 minutes at room temperature (25 [+ or -] 1[degrees]C). The resulting solutions were kept at 70[degrees]C and stirred at a moderate rate (100 rpm) using a stir bar. After one week to crystallize, the precipitates were allowed to settle. The resultant precipitates were then washed thoroughly with ultrapure water and dried for 24 h at <110[degrees]C to avoid decomposition of the solid samples obtained.

The composition of the solid sample was determined. 50 mg of sample was digested in 10 mL of 2 M HN[O.sub.3] solution and then diluted to 50 mL with 2% HN[O.sub.3] solution. It was analysed for Ba, Sr, and As using an inductively coupled plasma atomic emission spectrometer (ICP-OES, Perkin Elmer Optima 7000DV). The synthetic solids were also characterised by powder X-ray diffraction (XRD) with an X'Pert PRO diffractometer using Cu Ka radiation (40 kV and 40 mA). Crystallographic identification of the synthesised solids was accomplished by comparing the experimental XRD pattern to standard compiled by the International Centre for Diffraction Data (ICDD), which were card 00-023-0823 for BaHAs[O.sub.4] x [H.sub.2]O and card 01-074-1622 for SrHAs[O.sub.4] x [H.sub.2]O. The morphology was analysed by scanning electron microscopy (SEM, Joel JSM-6380LV). Infrared transmission spectra (KBr) were recorded over the range of 4000-400 [cm.sup.-1] using a Fourier transformed infrared spectrophotometer (FT-IR, Nicolet Nexus 470 FT-IR).

2.2. Dissolution Experiments. 1.5 g of the synthetic ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid was placed in 250 mL polypropylene bottle. 150 mL of 0.01 M HN[O.sub.3] (initial pH 2) was added to each bottle. The bottles were capped and placed in a temperature-controlled water bath (25 [+ or -] 1[degrees]C). Water samples (3mL) were taken from each bottle on 18 occasions (1h, 3h, 6h, 12h, 1d, 2d, 3d, 5d, 10d, 20d, 30d, 40d, 50d, 60d, 75d, 90d, 120d, and 180d). After each sampling, the sample volume was replaced with an equivalent amount of ultrapure water. The samples were filtered using 0.20 [micro]m pore diameter membrane filters and stabilised with 0.2% HN[O.sub.3] in 25 mL volumetric flask. Ba, Sr, and As were analysed by using an ICP-OES. After 180 d of dissolution, the solid samples were taken from each bottle, washed, dried, and characterised using XRD, FT-IR, and SEM in the same manner as described above.

2.3. Thermodynamic Calculations. Associated with each dissolution is an assemblage of solid phases, a solution phase containing dissolved calcium, phosphate, arsenate, and a pH value. Assuming equilibrium has been reached, the thermodynamic data can be calculated using established theoretical principles. In this study, the simulations were performed using PHREEQC (Version 3.1.1) together with the most complete literature database minteq.v4.dat, which bases on the ion dissociation theory. The input is free format and uses order-independent keyword data blocks that facilitate the building of models that can simulate a wide variety of aqueous-based scenarios [19]. The aqueous species considered in the calculations were [Ba.sup.2+], BaO[H.sup.+], BaAs[O.sub.4.sup.-], BaHAs[O.sub.4], Ba[H.sub.2]As[O.sub.4.sup.+], [Sr.sup.2+], SrO[H.sup.+], SrAs[O.sub.4.sup.-], SrHAs[O.sub.4], Sr[H.sub.2]As[O.sub.4.sup.+], [H.sub.3]As[O.sub.4], [H.sub.2]As[O.sub.4.sup.-], Has[O.sub.4.sup.2-], and As[O.sub.4.sup.3-]. The activities of [Ba.sup.2+](aq), [Sr.sup.2+](aq), and Has[O.sub.4.sup.2-](aq) were firstly calculated by using PHREEQC, and then the ion activity products (IAPs) for ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O, BaHAs[O.sub.4] x [H.sub.2]O, and SrHAs[O.sub.4] x [H.sub.2]O were determined according to the mass-action expressions by using Microsoft Excel.

3. Results and Discussion

3.1. Solid Characterizations. The composition of the synthetic solid depends on the initial Ba: Sr: As mole ratio in the starting solution. To ensure that the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution was formed, the precipitation was conducted by mixing barium solution, strontium solution, and arsenate solution at low rate. Results suggest that the crystal was the intended composition of ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O. The atomic (Ba + Sr)/As ratios were 1.00 which is a stoichiometric ratio of ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O. The atomic Ba/(Ba + Sr) ratios ([X.sub.Ba] = 0.00, 0.21, 0.42, 0.61, 0.82, 1.00) were almost the same as those of the precursor solutions (x = 0.00, 0.20, 0.40, 0.60, 0.80, 1.00). No [Na.sup.+] and N[O.sub.3.sup.-] were detected in the prepared solid (Table 1).

XRD, FT-IR, and SEM analyses were performed on the solid samples of ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O before and after dissolution (Figures 1, 2, and 3). As illustrated in the figures, the results of the analyses on materials before the dissolution were almost indistinguishable from the following reaction. No evidence of secondary mineral precipitation was observed in the dissolution experiment.

The XRD patterns of the obtained solids indicated the formation of the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution, which has the same type structure as BaHAs[O.sub.4] x [H.sub.2]O (ICDD PDF 00-023-0823) and SrHAs[O.sub.4] x [H.sub.2]O (ICDD PDF 01-074-1622) (Figure 1). The patterns correspond exactly with the database patterns and no impurities are observed. The solid solution is complete, with the space group Pbca (orthorhombic) being retained throughout. BaHAs[O.sub.4] x [H.sub.2]O and SrHAs[O.sub.4] x [H.sub.2]O are the two endmembers of a structural family series. When subjected to XRD, they produce the same reflections; but the reflections exist at different two-theta values; that is, the reflective planes are the same but "d" spacings are different. All the compounds have indicated the formation of a solid phase differing only in reflection location, reflection width, and absolute intensity of the diffraction patterns. The reflection peaks of BaHAs[O.sub.4] x [H.sub.2]O and SrHAs[O.sub.4] x [H.sub.2]O were slightly different from each other. The reflections of the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solutions shifted gradually to a lower-angle direction when the mole fraction [X.sub.Ba] of the solids increased (Figure 1).

The FT-IR spectra of the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solutions are shown in Figure 2. The spectra may be divided into three sections: (a) hydroxyl-stretching region, (b) water HOH bending region, and (c) arsenate As-O stretching and OAsO bending region [9]. The free arsenate ion, As[O.sub.4.sup.3-], belongs to the point group [T.sub.d]. The normal modes of the tetrahedral arsenate ion are [v.sub.1], symmetric As-O stretching; [v.sub.2], OAsO bending; [v.sub.3], As-O stretching; and [v.sub.4], OAsO bending. In the undistorted state, only the absorptions corresponding to [v.sub.3] and [v.sub.4] vibrations are observed. The two remaining fundamentals [v.sub.1] and [v.sub.2] become infrared active when the configuration of the As[O.sub.4.sup.3-] ions is reduced to some lower symmetry [9]. The degenerate modes are split by distortion of the arsenate groups through lack of symmetry in the lattice sites. As shown in Figure 2, the bands of As[O.sub.4.sup.3-] appeared around 815.74, 814.92, and 693.82 [cm.sup.-1] ([v.sub.3]) and 1747.66, 1662.27, 1620.03, and 1442.49 [cm.sup.-1] ([v.sub.1]) for the endmember BaHAs[O.sub.4] x [H.sub.2]O and around 854.23, 814.92, 704.10 [cm.sup.-1] ([v.sub.3]), and 1465.20 [cm.sup.-1] ([v.sub.1]) for the endmember SrHAs[O.sub.4] x [H.sub.2]O. For the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solutions, the [v.sub.1] and [v.sub.3] bands shifted slightly to a smaller wavenumber when the mole fraction [X.sub.Ba] of the solids increased.

Well-crystallized solids formed (Figure 3). The cell parameters a and b and volume increased and c decreased in a nonlinear way with [X.sub.Ba]. The flaky appearance increased; XBa that is, the morphology of the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solutions changed from short prismatic or granular crystals ([X.sub.Ba] = 0) to platy or blades crystals ([X.sub.Ba] =1).

3.2. Evolution of Aqueous Composition. The solution pH and element concentrations during the dissolution experiments at 25[degrees]C and initial pH 2 as a function of time are shown in Figure 4 for the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution. The experimental results indicated that the dissolution could be stoichiometrical only at the very beginning of the process, and then dissolution became nonstoichiometrical and the system underwent a dissolution-recrystallization process that affects the ratio of the substituting ions in both the solid and the aqueous solution.

When dissolution progressed at the initial pH 2, the aqueous pHs increased rapidly from 2.00 to 5.51-7.71 within the first hour of the experiment. For the dissolution of the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4]-[H.sub.2]O solid solution with [X.sub.Ba] = 0.00, 0.21, 0.42, and 0.61, the solution pHs increased gradually from 1h to 120 h and after that decreased gradually until they reached the steady state after 2160 h. For the dissolution of the solids with [X.sub.Ba] = 0.82 and 1.00, the aqueous pHs varied only slightly after 1 h and reached the steady state after 2160 h. The aqueous pHs decreased with the increasing [X.sub.Ba] of the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution.

For the dissolution of the solids with [X.sub.Ba] = 0.21, 0.42, and 0.61, the aqueous Ba concentrations increased rapidly and reached the peak values within the first hour and then decreased gradually and reached the lowest peak values in 240 h. After that, they increased gradually and reached the steady state after 2160 h. For the dissolution of the solids with [X.sub.Ba] = 0.82 and 1.00, the aqueous Ba concentrations increased rapidly within the first six hours and then increased gradually in 6-480 h. After that, they decreased gradually and reached the steady state after 2160 h. The aqueous Ba concentrations increased with the increasing [X.sub.Ba] of the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution.

When dissolution progressed at the initial pH 2, the aqueous Sr concentrations increased rapidly and reached the peak values within 120-480 h and then decreased gradually until they reached the steady state after 2880 h. The aqueous Sr concentrations increased with the increasing [X.sub.Ba] of the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution with 0 < [X.sub.Ba] < 0.21 (or 1 < [X.sub.Sr] < 0.79) and decreased with the increasing [X.sub.Ba] of the solids with 0.21 < [X.sub.Ba] < 1 (or 0.79 < [X.sub.Sr] < 0); that is, the aqueous Sr concentrations had the highest value for the dissolution of ([Ba.sub.0.21][Sr.sub.0.79])Has[O.sub.4] x [H.sub.2]O.

The aqueous As(V) concentrations increased rapidly and reached the peak values within the first hour and then decreased gradually and reached the lowest peak values in 48 h. After that, they increased gradually and reached the second peak values in 120-720 h. And then they decreased gradually and reached the steady state after 2880 h. The aqueous As(V) concentrations had the highest value for the dissolution of ([Ba.sub.021][Sr.sub.0.79])HAs[O.sub.4] x [H.sub.2]O.

3.3. Determination of Solubility and Free Energies of Formation. For the stoichiometric dissolution of the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution according to

([Ba.sub.x][Sr.sub.1-x]) Has[O.sub.4] x [H.sub.2]O = [xBa.sup.2+] (aq) + (1 - x) [Sr.sup.2+] (aq) + Has[O.sub.4.sup.-] (aq) (1)

a stoichiometric ion activity product, LAP, can be written as

IAP = [{[Ba.sup.2+]}.sup.x][{[Sr.sup.2+].sup.(1-x)] {Has[O.sub.4.sup.-]}. (2)

The activities of [Ba.sup.2+](aq), [Sr.sup.2+](aq), and Has[O.sub.4.sup.2-](aq) were firstly calculated by using PHREEQC [19], and then the IAP values for ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O, BaHAs[O.sub.4] x [H.sub.2]O, and SrHAs[O.sub.4] x [H.sub.2]O were determined according to the massaction expression (2). The aqueous pH, Ba, Sr, and As concentrations had reached stable values after 2880 h dissolution and the LAP values of 2880 h, 3600 h, and 4320 h were considered as [K.sub.sp] of the solids (Figure 4 and Table 2).

For (1),

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. (3)

Rearranging,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. (4)

The standard free energy of reaction ([DELTA][G.sup.0.sub.r]), in kJ/mol, is related to [K.sub.sp] at standard temperature (298.15 K) and pressure (0.101 MPa) by

[DELTA][G.sup.0.sub.r] = -5.708 log [K.sub.sp]. (5)

The solution chemistry representing equilibrium involving the solution phase and ([Ba.sub.rx][Sr.sub.1.-x])HAs[O.sub.4] x [H.sub.2]O, along with the calculated log [K.sub.sp] using PHREEQC, is shown in Table 2. Based on the obtained literature data, [DELTA][G.sup.0.sub.f][[Ba.sup.2+]] = -560.77kJ/mol, [DELTA][G.sup.0.sub.f] [[Sr.sup.2+]] = -559.84kJ/mol, [DELTA][G.sup.0.sub.f][Has[O.sub.4.sup.2-]] = -714.59kJ/mol, AG[micro][[H.sub.2]O] = -237.141 kJ/mol, and the free energies of formation, [DELTA][G.sup.0.sub.f] [([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O], were also calculated (Table 2).

The mean [K.sub.sp] values were calculated for BaHAs[O.sub.4] x [H.sub.2]O of [10.sup.-5,52] ([10.sup.-5.49] - [10.sup.-5.53]) at 25[degrees]C, for SrHAs[O.sub.4] x [H.sub.2]O of [10.sup.-4.62] at 25[degrees]C. The corresponding free energies of formation ([DELTA][G.sup.0.sub.f]) were determined to be-1543.99 [+ or -] 0.18 kJ/mol and-1537.94 [+ or -] 0.02 kJ/mol. The [K.sub.sp] value of [10.sup.-5.52] for BaHAs[O.sub.4] x [H.sub.2]O is approximately 19.12log units higher than 10-24'64 reported by Essington [15] and approximately 4.72log units lower than [10.sup.-0.8] reported by Orellana et al. [17], but in accordance with those of Robins [10], Davis [18], and Zhu et al. [9] ([10.sup.-4.70], [10.sup.-5.51,] and [10.sup.-5.60], resp.). Essington [15] determined the solubility of BaHAs[O.sub.4] x [H.sub.2]O(c) based not on his own experimental measurement, but on the data of Chukhlantsev [20] for [Ba.sub.3][(As[O.sub.4]).sub.2](c). He took the [Ba.sub.3][(As[O.sub.4]).sub.2] solid used by Chukhlantsev [20] for BaHAs[O.sub.4] x [H.sub.2]O(c) and recalculated the original analytical data. Orellana et al. [17] determined the solubility product only from precipitation experiments. The aqueous solution in his experiment might not reach equilibrium and was still supersaturated with regard to the solid. Based on our experimental results and those available in the literature, the solubility product for BaHAs[O.sub.4] x [H.sub.2]O should be around [10.sup.-5.50].

BaHAs[O.sub.4] x [H.sub.2]O is less soluble than SrHAs[O.sub.4] x [H.sub.2]O. For the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution, the solubility decreased as [X.sub.Ba] increased when 0.00 < [X.sub.Ba] < 0.82 and increased as [X.sub.Ba] increased when 0.82 < [X.sub.Ba] < 1.00. The ([Ba.sub.x][Sr.sub.1-x])Has[O.sub.4] x [H.sub.2]O solid solution had a minimum solubility product of 10-5'61 at [X.sub.Ba] = 0.82. This variation tendency is in accordance with the change of the unit cell parameters [4].

3.4. Saturation Index for BaHAs[O.sub.4] x [H.sub.2]O and SrHAs[O.sub.4] x [H.sub.2]O. The [K.sub.sp] values of [10.sup.-5.52] for BaHAs[O.sub.4] x [H.sub.2]O and [10.sup.-4.62] for SrHAs[O.sub.4] x [H.sub.2]O were used in the calculation using the program PHREEQC in the present study. The calculated saturation indices for BaHAs[O.sub.4] x [H.sub.2]O show a trend of increasing values as the composition of the solid phases approaches that of the pure-phase endmember, BaHAs[O.sub.4] x [H.sub.2]O (Figure 5). At the beginning of the dissolution of the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution, the BaHAs[O.sub.4] x [H.sub.2]O saturated index (SI) values increased with time until the aqueous solution was oversaturated with respect to BaHAs[O.sub.4] x [H.sub.2]O, and then the SI values decreased slowly. At the end of the dissolution experiment (28804320 h), the aqueous solution was saturated with respect to BaHAs[O.sub.4] x [H.sub.2]O for the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution with SI = -0.06~0.10.

The calculated saturation indices for SrHAs[O.sub.4] x [H.sub.2]O show a distinctly different trend than those for BaHAs[O.sub.4] x [H.sub.2]O (Figure 5). For the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution with x > 0.2, the SrHAs[O.sub.4] x [H.sub.2]O saturated index (SI) values decreased as the SrHAs[O.sub.4] x [H.sub.2]O mole fraction decreased or the BaHAs[O.sub.4] x [H.sub.2]O mole fraction increased for the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution with x > 0.2. The SrHAs[O.sub.4] x [H.sub.2]O saturated index (SI) values increased with time for the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution with x > 0.2. For the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution with x [less than or equal to] 0.2, the SrHAs[O.sub.4] x [H.sub.2]O saturated index (SI) values increased as the SrHAs[O.sub.4] x [H.sub.2]O mole fraction increased or the BaHAs[O.sub.4] x [H.sub.2]O mole fraction decreased for the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution with x [less than or equal to] 0.2. At the beginning of the dissolution, the SrHAs[O.sub.4] x [H.sub.2]O saturated index (SI) values increased with time until the aqueous solution was oversaturated with respect to SrHAs[O.sub.4] x [H.sub.2]O, and then the SI values decreased slowly. At the end of the dissolution experiment (4320 h), the aqueous solution was saturated with respect to SrHAs[O.sub.4] x [H.sub.2]O for the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution with x [less than or equal to] 0.2, while the aqueous solution was undersaturated with respect to SrHAs[O.sub.4] x [H.sub.2]O for the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution with x > 0.2.

3.5. Lippmann Diagram

3.5.1. Construction of Lippmann Diagram. Understanding solid solution aqueous solution (SSAS) processes is of fundamental importance. However, in spite of the numerous studies, the availability of thermodynamic data for SSAS systems is still scarce [6]. Lippmann extended the solubility product concept to solid solutions by developing the concept of "total solubility product [summation][[PI].sub.SS]," which is defined as the sum of the partial activity products contributed by the individual endmembers of the solid solution [5, 6]. At thermodynamic equilibrium, the total activity product [summation][[PI].sub.SS], expressed as a function of the solid composition, yields Lippmann's "solidus" relationship. In the same way, the "solutus" relationship expresses [summation][[PI].sub.SS] as a function of the aqueous solution composition. The graphical representation of "solidus" and "solutus" yields a phase diagram, usually known as a Lippmann diagram [5, 6]. A comprehensive methodology for describing reaction paths and equilibrium end points in solid solution aqueous solution systems had been presented and discussed in literatures [5, 6, 21-23].

In the case of the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution, the term "total solubility product [summation][[PI].sub.SS]" is defined as the ({[Ba.sup.2+]} + {[Sr.sup.2+]})|Has[O.sub.4.sup.2-]} at equilibrium and can be expressed by

[summation][[PI].sub.SS] = [K.sub.Ba][X.sub.Ba]cro][[gamma].sub.Ba] + [K.sub.Sr][K.sub.Sr][[gamma].sub.Sr], (6)

where {*} designate aqueous activity. [K.sub.Ba] and [K.sub.Sr], [X.sub.Ba] and [X.sub.Sr], and [[gamma].sub.Ba] and [[gamma].sub.Sr] are the thermodynamic solubility products, the mole fractions (x, 1-x), and the activity coefficients of the BaHAs[O.sub.4] x [H.sub.2]O and SrHAs[O.sub.4] x [H.sub.2]O components in the ([Ba.sub.x][Sr.sub.1-x])Has[O.sub.4] x [H.sub.2]O solid solution, respectively. This relationship, called the solidus, defines all possible thermodynamic saturation states for the two-component solid solution series in terms of the solid phase composition.

The term "total solubility product [summation][[PI].sub.SS]" can also be expressed by

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], (7)

where [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] are the activity fractions of the aqueous Ba and Sr components. This relationship, called the solutus, defines all possible thermodynamic saturation states for the two-component solid solution series in terms of the aqueous phase composition.

For a solid solution with fixed composition [X.sub.Ba] = 1 - [X.sub.Sr], a series of minimum stoichiometric saturation scenarios as a function of the aqueous activity fraction [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] of the substituting ions in the aqueous solution can be described by

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. (8)

For the endmembers BaHAs[O.sub.4] x [H.sub.2]O and BaHAs[O.sub.4] x [H.sub.2]O and [X.sub.Ba] = 1 and [X.sub.Ba] = 0, the endmember saturation equations can be written as

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. (9)

A Lippmann phase diagram for the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution is a plot of the solidus and solutus as log [summation][[PI].sub.SS] on the ordinate versus two superimposed aqueous and solid phase mole fraction scales on the abscissa.

3.5.2. Lippmann Diagram for the Ideal Solid Solution. In the ([Ba.sub.x][Sr.sub.1-x])Has[O.sub.4] x [H.sub.2]O system, the endmember solubility products are -5.52 for BaHAs[O.sub.4] x [H.sub.2]O and -4.62 for SrHAs[O.sub.4] - [H.sub.2]O; that is, they differ by about 0.9 log units. The ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solutions are assumed ideal. In this case, the pure-phase solubility curves for BaHAs[O.sub.4] x [H.sub.2]O and SrHAs[O.sub.4] x [H.sub.2]O are quite distinct from the solutus, and the solution compositions along the solutus are clearly undersaturated with respect to both pure BaHAs[O.sub.4] x [H.sub.2]O and SrHAs[O.sub.4] x [H.sub.2]O solids. The dotted horizontal tie lines indicate the relation between the solid mole fractions and the aqueous activity fractions at thermodynamic equilibrium (points T2 and T1) or at primary saturation (points P2 and P1) with respect to a ([Ba.sub.0.61][Sr.sub.0.39])Has[O.sub.4] - [H.sub.2]O solid. The dashed curve gives the series of possible [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] aqueous compositions that satisfy the condition of stoichiometric saturation with respect to a ([Ba.sub.0.61][Sr.sub.0.39])HAs[O.sub.4] x [H.sub.2]O solid. Point Ml is the "minimum stoichiometric saturation" point for a ([Ba.sub.0.61][Sr.sub.0.39])HAs[O.sub.4] x [H.sub.2]O solid.

As with all the "minimum stoichiometric saturation" curves, the "pure endmember saturation" curves plot above the solutus for all aqueous activity fractions, concurring with the solutus only for [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] equal to one. This can be seen in Figure 6, in which the curves [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] have been represented for the same solid solution as in Figure 6. Note that the two curves intersect at the point "S." This point represents an aqueous solution at simultaneous stoichiometric saturation with respect to both endmembers.

The hypothetical reaction path is also shown in Figure 6, in relation to Lippmann solutus and solidus curves for the ([Ba.sub.0.61][Sr.sub.0.39])HAs[O.sub.4] x [H.sub.2]O solid solution. The reaction path of a stoichiometrically dissolving solid solution moves vertically from the abscissa of a Lippmann diagram, originating at the mole fraction corresponding to the initial solid solution composition. The pathway shows initial stoichiometric dissolution up to the solutus curve, followed by nonstoichiometric dissolution along the solutus, towards the more soluble endmember.

The difference between the solubility products of the endmembers involves a strong preferential partitioning of the less soluble endmember towards the solid phase. 0ur dissolution data indicate a final enrichment in the BaHAs[O.sub.4] x [H.sub.2]O component in the solid phase and a persistent enrichment in the [Sr.sup.2+] component in the aqueous phase. The possibility of formation of a phase close in composition to pure BaHAs[O.sub.4] x [H.sub.2]O seems unavoidable, given the lower solubility of BaHAs[O.sub.4] x [H.sub.2]O and the oversaturation with respect to BaHAs[O.sub.4] x [H.sub.2]O and the relatively high solubility of SrHAs[O.sub.4] x [H.sub.2]O and the undersaturation with respect to SrHAs[O.sub.4] x [H.sub.2]O (Figure 5). From the point of view of the crystallization behavior, all data seem to indicate that there is a preferential partitioning of barium into the solid phase [4].

3.5.3. Lippmann Diagram for the Nonideal Solid Solution. Complementary powder XRD measurements indicated that the cell parameters increased in a nonlinear way with [X.sub.Ba] indicating the solid solution is complete but could be nonideal [4]. The solid phase activity coefficients of BaHAs[O.sub.4] x [H.sub.2]O ([[gamma].sub.Ba]) and SrHAs[O.sub.4] x [H.sub.2]O ([[gamma].sub.Sr]) as components of the solid solution can be calculated as a function of composition using the Redlich and Kister equations [5, 6], expressed in the form

ln [[gamma].sub.Ba] = [(1 - x).sup.2] [[a.sub.0] - [a.sub.1] (3x (1 - x)) + ...] , ln [[gamma].sub.Sr] = [x.sup.2] [[a.sub.0] - [a.sub.1] (3(1 - x) - x) + ...], (10)

where x and 1 - x are the mole fractions ([X.sub.Ba] and [X.sub.Sr]) of the BaHAs[O.sub.4] x [H.sub.2]O and SrHAs[O.sub.4] x [H.sub.2]O components in the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution; the Guggenheim coefficients [a.sub.0] and [a.sub.1] can be determined from an expansion of the excess Gibbs free energy of mixing [G.sup.E] [5, 6].

The excess Gibbs free energy of mixing [G.sup.E] has not been measured for the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution. Assuming that a stoichiometric saturation state was attained in the dissolution experiments, the Guggenheim coefficients [a.sub.0] and [a.sub.1] can be estimated from

ln [K.sub.sp] = x(1 - x)[a.sub.0] + x(1 - x)(x - (1 - x)) [a.sub.1] + (1 - x) ln [(1 - x)[K.sub.Ba]] + x ln [x[K.sub.Sr]]. (11)

Log IAP (log [K.sub.sp]) values of the samples taken after 2880 h dissolution are shown as a function of [X.sub.Ba] in Figure 7. A plot of these log [K.sub.sp] values versus solid mole fraction of BaHAs[O.sub.4] x [H.sub.2]O shows that the log [K.sub.sp] values are close to and slightly lower than what would be expected for an ideal solid solution. Fitting the [K.sub.sp] values as a function of solid composition to (11) yields a best fit with a two-parameter Guggenheim model of [a.sub.0] = 1.55 and [a.sub.1] = -4.35 ([R.sup.2] > 0.95) (Figure 7).

The diagram of Figure 8 is a typical Lippmann diagram for a solid solution with a negative enthalpy of mixing. The negative excess free energy of mixing produces a fall of the solutus curve with respect to the position of an equivalent ideal solutus. This means that the solubility of intermediate compositions is significantly smaller than that of an equivalent ideal solid solution. Indeed, for a certain range of solid compositions, the solutus values plot below the solubility product of the less soluble endmember. The system shows an "alyotropic" minimum at [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] where the solutus and solidus curves meet. Such a point represents a thermodynamic equilibrium state in which the mole fraction of the substituting ions in the solid phase equals its activity fraction in the aqueous solution. Obviously, at the alyotropic point, the equilibrium distribution coefficient is equal to unity [5, 6].

3.5.4. Solid Solution Aqueous Solution Reaction Paths. A Lippmann diagram for the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution for the ideal case when [a.sub.0] = 0.0 is shown in Figure 9. In addition to the solutus and the solidus, the diagrams contain the total solubility product curve at stoichiometric saturation for the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution at x = 0.21, 0.42, 0.61, and 0.82. The saturation curves for pure endmembers BaHAs[O.sub.4] x [H.sub.2]O (x = 1.00) and SrHAs[O.sub.4] x [H.sub.2]O (x = 0.00) have also been plotted in the chart. Also included are the data from our study, plotted as ({[Ba.sup.2+]} + {[Sr.sup.2+]}){HAs[O.sub.4.sup.2-]} versus [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII].

In general, the location of data points on a Lippmann diagram will depend on the aqueous speciation, degree to which secondary phases are formed, and the relative rates of dissolution and precipitation. As ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O dissolves in solution, aqueous [Ba.sup.2+] is converted into Ba, BaO[H.sup.+], BaAs[O.sub.4.sup.-], BaHAs[O.sub.4], and Ba[H.sub.2]As[O.sub.4.sup.+], and aqueous [Sr.sup.2+] is converted into [Sr.sup.2+], SrO[H.sup.+], SrAs[O.sub.4.sup.-], SrHAs[O.sub.4], and Sr[H.sub.2]As[O.sub.4.sup.+]; aqueous HAs[O.sub.4.sup.2-] is converted primarily into As[O.sub.4.sup.3-], [H.sub.2]As[O.sub.4.sup.-], and [H.sub.3]As[O.sub.4] and only a fraction remains as [Ba.sup.2+], [Sr.sup.2+,] and HAs[O.sub.4.sup.2-]. The smaller values of the activity fractions are a consequence of the [Ba.sup.2+] and [Sr.sup.2+] speciation. For the plot of the experimental data on the Lippmann diagram, the effect of the aqueous [Ba.sup.2+] and [Sr.sup.2+] speciation was considered by calculating the activity of [Ba.sup.2+] and [Sr.sup.2+] with the program PHREEQC.

The experimental data plotted on Lippmann phase diagrams show that the ([Ba.sub.0.61][Sr.sub.0.39])HAs[O.sub.4] x [H.sub.2]O precipitate dissolved stoichiometrically at the beginning and approached the Lippmann solutus curve and then overshot the Lippmann solutus curve, the saturation curves for pure endmembers BaHAs[O.sub.4] x [H.sub.2]O, and the stoichiometric saturation curve for x = 0.61. After about 1h dissolution, the aqueous solution was oversaturated with respect to BaHAs[O.sub.4] x [H.sub.2]O and the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution. And then, the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] decreased from 0.61 to 0.21 in 240 h with no obvious change in the log[summation][[PI].sub.SS] value, indicating the dissolution path for this precipitate may involve stoichiometric dissolution to the Lippmann solutus curve and overshot the Lippmann solutus curve followed by a possible exchange and recrystallization reaction. From 240 to 4320 h, the log[summation][[PI].sub.SS] value decreased further, but the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] increased from 0.21 to about 0.43, and the dissolution followed the saturation curve for pure endmembers BaHAs[O.sub.4] x [H.sub.2]O and approached the intersection with the minimum stoichiometric saturation curve for x = 0.61.

4. Conclusions

With the increasing [X.sub.Ba], the morphology of the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solutions changed from short prismatic or granular crystals ([X.sub.Ba] = 0) to platy or blades crystals ([X.sub.Ba] = 1). The synthetic solids used in the experiments were found to have no obvious variation after dissolution. During the dissolution of the solid solution, the aqueous pH values and component concentrations increased rapidly at the beginning and then varied slowly with time and finally exhibited stable state after 2880 h dissolution. With the increasing [X.sub.Ba], the aqueous pH value decreased and the aqueous Ba concentration increased. At [X.sub.Ba] = 0.21, the aqueous Sr and As concentrations had the highest values. The solubility products ([K.sub.sp]) for BaHAs[O.sub.4] x [H.sub.2]O and SrHAs[O.sub.4] x [H.sub.2]O were calculated to be [10.sup.-5.52] and [10.sup.-4.62], respectively. The corresponding free energies of formation ([DELTA][G.sup.0.sub.f]) were -1543.99 [+ or -] 0.18 kJ/mol and -1537.94 [+ or -] 0.02 kJ/mol. BaHAs[O.sub.4] x [H.sub.2]O is less soluble than SrHAs[O.sub.4] x [H.sub.2]O. For the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution, the solubility decreased as [X.sub.Ba] increased when 0.00 < [X.sub.Ba] < 0.82 and increased as [X.sub.Ba] increased when 0.82 < [X.sub.Ba] < 1.00. The([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution had a minimum solubility product of [10.sup.-5.61] at [X.sub.Ba] = 0.82. The Guggenheim coefficients were determined to be [a.sub.0] = 1.55 and [a.sub.1] = -4.35 for the ([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O nonideal solid solution. The constructed Lippmann diagram was a typical Lippmann diagram for a nonideal solid solution with a negative enthalpy of mixing, which produced a fall of the solutus curve with respect to the position of an equivalent ideal solutus, which indicated that the solubility of intermediate compositions is significantly smaller than that of an equivalent ideal solid solution. The system shows an "alyotropic" minimum at [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. At the end of the To make (someone or something) go in a direction experiment, the dissolution followed the saturation curve for the pure endmember BaHAs[O.sub.4] x [H.sub.2]O and approached the intersection with the minimum stoichiometric saturation curve on the Lippmann diagram.

http://dx.doi.org/10.1155/2014/654168

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors thank the financial supports from the National Natural Science Foundation of China (41263009, 40773059) and the Provincial Natural Science Foundation of Guangxi (2012GXNSFDA053022, 2011GXNSFF018003).

References

[1] E. Fulladosa, J. C. Murat, M. Martinez, and I. Villaescusa, "Effect of pH on arsenate and arsenite toxicity to luminescent bacteria (vibrio fischeri)," Archives of Environmental Contamination and Toxicology, vol. 46, no. 2, pp. 176-182, 2004.

[2] D. Mohan and C. U. Pittman Jr., "Arsenic removal from water/wastewater using adsorbents--a critical review," Journal of Hazardous Materials, vol. 142, no. 1-2, pp. 1-53, 2007

[3] P. L. Smedley and D. G. Kinniburgh, "A review of the source, behaviour and distribution of arsenic in natural waters," Applied Geochemistry, vol. 17, no. 5, pp. 517-568, 2002.

[4] A. Jimenez, M. Prieto, Salvado MA, and S. Garcia-Granda, "Structure and crystallization behavior of the (Ba,Sr)HAs[O.sub.4] x [H.sub.2]O solid-solution in aqueous environments," American Mineralogist, vol. 89, pp. 601-609, 2004.

[5] P. D. Glynn and E. J. Reardon, "Solid-solution aqueous-solution equilibria: thermodynamic theory and representation," American Journal of Science, vol. 290, no. 2, pp. 164-201, 1990.

[6] M. Prieto, "Thermodynamics of solid solution aqueous solution systems," Reviews in Mineralogy and Geochemistry, vol. 70, pp. 47-85, 2009.

[7] J. S. Lee and J. O. Nriagu, "Stability constants for metal arsenates," Environmental Chemistry, vol. 4, no. 2, pp. 123-133, 2007.

[8] B. Planer-Friedrich, M. A. Armienta, and B. J. Merkel, "Origin of arsenic in thegroundwater of the Rioverde basin, Mexico," Environmental Geology, vol. 40, no. 10, pp. 1290-1298, 2001.

[9] Y. Zhu, X. Zhang, Q. Xie et al., "Solubility and stability of barium arsenate and barium hydrogen arsenate at 25[degrees]C," Journal of Hazardous Materials, vol. 120, no. 1-3, pp. 37-44, 2005.

[10] R. G. Robins, "The solubility of barium arsenate: sherritt's barium arsenate process," Metallurgical Transactions B, vol. 16, no. 2, pp. 404-406, 1985.

[11] L. Mondez-Rodriguez, T. Zenteno-Savin, B. Acosta-Vargas, J. Wurl, and M. Imaz-Lamadrid, "Differences in arsenic, molybdenum, barium, and other physicochemical relationships in groundwater between sites with and without mining activities," Natural Science, vol. 5, pp. 238-243, 2013.

[12] L. Tiruta-Barna, Z. Rakotoarisoa, and J. Mehu, "Assessment of the multi-scale leaching behaviour of compacted coal fly ash," Journal of Hazardous Materials, vol. 137, no. 3, pp. 1466-1478, 2006.

[13] H. Binas and K. Boll-Dornberger, "The structure of CaHAs[O.sub.4] x [H.sub.2]O (haidingerite) and SrHAs[O.sub.4] x [H.sub.2]O," Chemie der Erde/Geochemistry, vol. 21, p. 450, 1962.

[14] C. Martin, A. Durif, and M. T. Adverbuch-Pouchot, "Sur le groupe spatial de l'haidingerite. Donneoes cristallographiques sur BaHAs[O.sub.4] x [H.sub.2]O," Bulletin Societe Francaise Mineralogie Cristallographie, vol. 93, p. 397,1970.

[15] M. E. Essington, "Solubility of barium arsenate," Soil Science Society of America Journal, vol. 52, no. 6, pp. 1566-1570,1988.

[16] C. T. Itoh and K. Tozawa, "Equilibria of the barium(II)-arsenic(III,V) water system at 25[degrees]C," Tohoku Daigaku Senko Seiren Kenkyusho Iho, vol. 45, p. 105, 1989.

[17] F. Orellana, E. Ahumada, C. Suarez, G. Cote, and H. Lizama, "Thermodynamics studies of parameters involved in the formation of arsenic(V) precipitates with barium(II)," Boletin de la Sociedad Chilena de Quimica, vol. 45, pp. 415-422, 2000.

[18] J. Davis, "Stability of metal-arsenic solids in drinking water systems," Practice Periodical of Hazardous, Toxic, and Radioactive Waste Management, vol. 4, no. 1, pp. 31-35, 2000.

[19] D. L. Parkhurst and C. A. J. Appelo, "Description of input and examples for PHREEQC version 3--a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations," in U.S. Geological Survey Techniques and Methods, Book 6, chapter A43, pp. 1-497, 2013.

[20] V G. Chukhlantsev, "Solubility-products of arsenates," Journal of Inorganic Chemistry, vol. 1, pp. 1975-1982,1956.

[21] P. D. Glynn, E. J. Reardon, L. N. Plummer, and E. Busenberg, "Reaction paths and equilibrium end-points in solid-solution aqueous-solution systems," Geochimica et Cosmochimica Acta, vol. 54, no. 2, pp. 267-282, 1990.

[22] H. Gamsjager, E. Konigsberger, and W. Preis, "Lippmann diagrams: theory and application to carbonate systems," Aquatic Geochemistry, vol. 6, pp. 119-132, 2000.

[23] D. Baron and C. D. Palmer, "Solid-solution aqueous-solution reactions between jarosite (K[Fe.sub.3][(S[O.sub.4]).sub.2][(OH).sub.6]) and its chromate analog," Geochimica et Cosmochimica Acta, vol. 66, no. 16, pp. 2841-2853, 2002.

Xuehong Zhang, (1) Yinian Zhu, (1) Caichun Wei, (1,2) Zongqiang Zhu, (1) and Zongning Li (1)

(1) College of Environmental Science and Engineering, Guilin University of Technology, Guilin 541004, China

(2) College of Light Industry and Food Engineering, Guangxi University, Nanning 530004, China

Correspondence should be addressed to Yinian Zhu; zhuyinian@glut.edu.cn

Received 2 October 2013; Accepted 27 January 2014; Published 17 March 2014

Academic Editor: Stefan Tsakovski

TABLE 1: Summary of synthesis and composition of the
([Ba.sub.x][Sr.sub.1-x])Has[O.sub.4] x [H.sub.2]O solid solution.

                             Volumes of the
                             precursors (mL)
Ba[(Cl[O.sub.4]).sub.2]  Sr[(Cl[O.sub.4]).sub.2]  [Na.sub.3]As[O.sub.4]
0.5 mol/L                       0.5 mol/L               0.5 mol/L

0                                  100                     100
20                                 80                      100
40                                 60                      100
60                                 40                      100
80                                 20                      100
100                                 0                      100

                                      Solid solution
Ba[(Cl[O.sub.4]).sub.2]
0.5 mol/L                            Molecular formula

0                               SrHAs[O.sub.4] x [H.sub.2]O
20                             ([Ba.sub.0.21][Sr.sub.0.79])
                                 HAs[O.sub.4] x [H.sub.2]O
40                       ([Ba.sub.0.42][Sr.sub.0.58])HAs[O.sub.4]
                                       x [H.sub.2]O
60                       ([Ba.sub.0.61][Sr.sub.0.39])HAs[O.sub.4]
                                       x [H.sub.2]O
80                       ([Ba.sub.0.82][Sr.sub.0.18])HAs[O.sub.4]
                                       x [H.sub.2]O
100                             BaHAs[O.sub.4] x [H.sub.2]O

Ba[(Cl[O.sub.4]).sub.2]
0.5 mol/L                [X.sub.Ba]

0                           0.00
20                          0.21
40                          0.42
60                          0.61
80                          0.82
100                         1.00

TABLE 2: Analytical data and solubility determination of the
([Ba.sub.x][Sr.sub.1-x])HAs[O.sub.4] x [H.sub.2]O solid solution.

Sample             Dissolution    pH     Concentration (mmol/L)
                    time (h)              Ba       Sr       As

SrHAs[O.sub.4] x      2880       7.96    0.00     9.08    12.88
[H.sub.2]O            3600       7.90    0.00     9.12    12.83
                      4320       7.95    0.00     9.16    12.53

([Ba.sub.0.21]        2880       7.77    0.91    13.34    15.38
[Sr.sub.0.79])        3600       7.77    0.98    13.32    15.54
HAs[O.sub.4]          4320       7.74    0.94    13.32    15.53
x [H.sub.2]O

([Ba.sub.0.42]        2880       6.86    1.63     8.73    11.68
[Sr.sub.0.58])        3600       6.85    1.56     8.82    11.07
HAs[O.sub.4]          4320       6.83    1.54     8.69    10.81
x [H.sub.2]O

([Ba.sub.0.61]        2880       6.18    3.92     5.18    10.15
[Sr.sub.0.39])        3600       6.10    3.91     5.17    10.17
HAs[O.sub.4]          4320       6.06    3.93     5.20    10.24
x [H.sub.2]O

([Ba.sub.0.82]        2880       5.72    9.45     1.55    11.58
[Sr.sub.0.18])        3600       5.67    9.51     1.56    11.16
HAs[O.sub.4]          4320       5.68    9.57     1.57    11.07
x [H.sub.2]O

BaHAs[O.sub.4] x      2880       5.61   10.53     0.00    11.42
[H.sub.2]O            3600       5.56   10.56     0.00    11.25
                      4320       5.57   10.69     0.00    10.94

                                 Average         [DELTA]
Sample                log          log       [G.sup.0.sub.f]
                   [K.sub.sp]   [K.sub.sp]      (kJ/mol)

SrHAs[O.sub.4] x     -4.62
[H.sub.2]O           -4.62        -4.62         -1537.94
                     -4.62

([Ba.sub.0.21]       -4.70
[Sr.sub.0.79])       -4.69        -4.69         -1538.54
HAs[O.sub.4]         -4.69
x [H.sub.2]O

([Ba.sub.0.42]       -5.11
[Sr.sub.0.58])       -5.13        -5.13         -1541.25
HAs[O.sub.4]         -5.15
x [H.sub.2]O

([Ba.sub.0.61]       -5.45
[Sr.sub.0.39])       -5.51        -5.49         -1543.50
HAs[O.sub.4]         -5.53
x [H.sub.2]O

([Ba.sub.0.82]       -5.57
[Sr.sub.0.18])       -5.63        -5.61         -1544.24
HAs[O.sub.4]         -5.62
x [H.sub.2]O

BaHAs[O.sub.4] x     -5.49
[H.sub.2]O           -5.53        -5.52         -1543.99
                     -5.53
COPYRIGHT 2014 Hindawi Limited
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2014 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:Research Article
Author:Zhang, Xuehong; Zhu, Yinian; Wei, Caichun; Zhu, Zongqiang; Li, Zongning
Publication:Journal of Chemistry
Article Type:Report
Date:Jan 1, 2014
Words:8892
Previous Article:Potential antioxidant anthraquinones isolated from Rheum emodi showing nematicidal activity against Meloidogyne incognita.
Next Article:Preparation and its adsorptive property of modified expanded graphite nanomaterials.
Topics:

Terms of use | Privacy policy | Copyright © 2021 Farlex, Inc. | Feedback | For webmasters