Synthesis, Structure, and Stability of Gallium Arsenate Dihydrate

Ind. Eng. Chem. Res. 2007, 46, 7875-7882

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Synthesis, Structure, and Stability of Gallium Arsenate Dihydrate, Indium Arsenate Dihydrate, and Lanthanum Arsenate Jean-Francois Le Berre, Raynald Gauvin, and George P. Demopoulos* Department of Mining and Materials Engineering, McGill UniVersity, M. H. Wong Building, 3610 UniVersity Street, Montreal, Quebec, Canada H3A 2B2

This paper reports on the hydrothermal synthesis, structural characterization, and chemical stability-leachability of three metal arsenates, namely gallium arsenate dihydrate (GaAsO4‚2H2O), indium arsenate dihydrate (InAsO4‚2H2O), and lanthanum arsenate (LaAsO4). The new standard synthesis method involves hydrothermal precipitation at 433 K (160 °C) from equimolar (0.3 M) M(III)-As(V) nitrate solutions over a period of 24 h. The produced materials were found to be essentially stoichiometric and to exhibit very good crystallinity. The two dihydrates were found further to be made up of uniformly grown crystallites either aggregated (GaAsO4‚2H2O) or nonaggregated (InAsO4‚2H2O), reflecting their common orthorhombic crystal habit, while LaAsO4 consisted of large aggregated particles with monoclinic habit features. In terms of stability, InAsO4‚ 2H2O and LaAsO4 were found to release less than 1 mg/L arsenic when subjected to a TCLP-like leachability test (24 h contact at pH 5) while GaAsO4‚2H2O released 2.4 mg/L arsenic. An extended leachability study over a period of several weeks resulted in higher concentrations of arsenic released via an incongruent dissolution mechanism. Of the three compounds, LaAsO4 was determined to be the most stable with arsenic equilibrium solubility equal to 4 and 13 mg/L, respectively, at pH 5 and 7 at 22 °C. Introduction The processing of arseniferous ores, concentrates, or secondary industrial materials such as metallurgical dusts and semiconductor wastes imposes the risk of release into the environment of arsenic, a highly toxic element.1 The problem of arsenic control is particularly crucial in the metallurgical industry.2 Smelter flue dusts containing arsenic trioxide and aqueous process effluents containing soluble arsenic (as either arsenite (+III) or arsenate (+V)) need to be treated for the purpose of removing and ultimately immobilizing arsenic. Similar challenges with the generation of arsenic wastes and their control are faced by the GaAs semiconductor industry.3 Depending on the nature of arsenic waste, different techniques can be applied for its removal and immobilization.2,4,5 The key issue with the techniques is to convert arsenic into stable forms that could prevent its release to the environment upon disposal over the long term. In this regard, the arsenic environmental problem is linked to the solubility of its compounds.6 Hence the study of a number of trivalent metal arsenates, considered as candidate materials for the fixation of arsenic, becomes highly important. By far, scorodite (FeAsO4‚2H2O) has received the most attention of all trivalent metal arsenates because of its occurrence as a mineral as well as its formation and reporting to tailings generated by the metallurgical industry.7 Crystalline scorodite has been reported to precipitate during hydrothermal processing of ores8 or via supersaturation-controlled crystallization at 95 °C.9,10 On the other hand, a scorodite precursor phase (amorphous ferric arsenate) has been identified to form and control the solubility of arsenic upon ambient temperature neutralization of uranium mill effluents.11 In addition to the study of its preparation, scorodite has been the subject of several investigations in terms of crystal structure determination12,13 and solubility-stability testing.11,14-16 * To whom correspondence should be addressed. E-mail: [email protected].

In comparison to scorodite, much less information is available for all other trivalent metal arsenates. As part of a research program that focuses on the development of advanced technologies for the immobilization of arsenic from metallurgical and chemical processing operations, a systematic study of the stability of several trivalent metal arsenates has been undertaken in our laboratory. The first of these lesser known metal arsenates studied was mansfieldite (AlAsO4‚2H2O),17,18 followed by the study of the scorodite-mansfieldite solid solution series.19 The different compounds were hydrothermally synthesized by adapting a method developed earlier for the preparation of scorodite.20 These studies revealed that the increase of the aluminum fraction in the scorodite structure tends to destabilize the material. At this point it was thought that the smaller size of the aluminum metal ion (0.535 Å21) vis-a`-vis the size of the Fe3+ (0.645 Å21) may have been responsible for the increased solubility of mansfieldite. Hence it was decided to examine other trivalent metal arsenate compounds with larger cation size, namely Ga3+ (0.620 Å21), In3+ (0.800 Å21), and La3+ (1.032 Å21). The synthesis, characterization, and stability evaluation of these metal arsenate compounds is reported here. In particular, this paper deals with the dihydrates of gallium and indium and the anhydrite of lanthanum arsenate. GaAsO4‚2H2O and InAsO4‚2H2O are members of the group MIIIXO4‚2H2O, where M can be Al, Fe, Ga, Cr, In, ... and X is As or P. These compounds, considered as variscite (AlPO4‚ 2H2O) isostructural species,22 are characterized by an orthorhombic structure and the space group Pbca (No. 61). They are by extension isostructural with scorodite and mansfieldite. In contrast, no dihydrate for lanthanum arsenate is found to exist but only its anhydride: LaAsO4. The latter has a monoclinic structure with the space group P21/m (No. 11). Of the three metal arsenates, only InAsO4‚2H2O has been found to exist in nature as the mineral yanomamite.23 Previous studies dealing with the synthesis and characterization of these metal arsenates are summarized in the remainder of this section. No information on their chemical stabilities-solubilities has been reported.

10.1021/ie061514v CCC: $37.00 © 2007 American Chemical Society Published on Web 10/31/2007

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Paˆques-Ledent and Tarte24 were the first investigators to work on the synthesis and characterization of GaAsO4‚2H2O. Their synthesis method involved reaction of a 0.3 M solution of gallium nitrate at initial pH 2.4 with Na2HAsO4 at 403 K (130 °C). Later, in 1974, Ronis and d’Yvoire25 reported on the preparation of stoichiometric and nonstoichiometric gallium arsenate dihydrate by hydrolysis of Ga(H3AsO4)3‚5H2O in an H3AsO4 solution with initial molar ratio As/Ga ) 11. The temperature employed varied from 363 to 443 K (from 90 to 170 °C) and the reaction time ranged from 2 h to 3 months. Initially a poorly crystalline nonstoichiometric precipitate formed which with time or temperature elevation was transformed to stoichiometric crystalline product. Finally, in 1997, Dick26 reported on the crystal structural parameters of GaAsO4‚2H2O produced by hydrothermal synthesis. Dick’s synthesis method involved the equilibration of Ga2O3 in 1 M K2HAsO4 solution (molar ratio As/Ga ≈ 13) at pH 2 and 423 K (150 °C) for a period of 5 days. For the synthesis of InAsO4‚2H2O, two studies have been reported. The first one, by Tarte and Paˆques-Ledent,27 involved equilibration of amorphous InAsO4 in 0.25 M H3AsO4 solution at pH 2.4 and 408 K (135 °C) over a period of 3 days. More recently, Tang et al.28 reported on the crystal structure of InAsO4‚2H2O following its hydrothermal synthesis. Their method of synthesis was based on the use of a solution made up of InCl3, As2O5‚xH2O, TMAOH ((CH3)4NOH‚5H2O), CsNO3, Cd(C2H3O2)‚2H2O, and water with the molar ratio 4:1: 2:4:8:776. The mixture was heated to react at 433 K (160 °C) over a period of 12 days. The method produced two phases, namely the desired one InAsO4‚2H2O (around 80% yield), and CsIn(HAsO4)2. In the case of lanthanum arsenate, Botto and Baran29 first and Pradhan and Choudhary30 later were able to synthesize LaAsO4, by thermal treatment at 1173 K (900 °C) and hydrothermal synthesis at 473 K (200 °C), respectively. The latter method involved reaction of a stoichiometric amount of La2O3 (previously dissolved in a hot dilute nitric acid solution) with (NH4)2HAsO4 in the presence of urea ((NH2)2CO) and heating until precipitation occurred at 473 K (200 °C). The removal of arsenic(V) from waste solutions via its precipitation in the form of arsenate solids with Ga(III), In(III), or La(III) has attracted only limited attention. Thus, Tokunaga et al.31 studied the removal of arsenic(V) from aqueous solution by lanthanum compounds, and they found that LaAsO4 formed in the acid pH range. The precipitation of lanthanum arsenate according to a follow-up study by the same authors32 allowed higher removal of arsenic(V) (99% of the initial 0.25 mM) than from ferric arsenate (76% removal). Recently, the removal of arsenic(V) from synthetic and real waste solutions generated by the GaAs semiconductor industry by precipitation of gallium arsenate was reported.33 The above study, however, did not include characterization of the precipitated metal arsenate phases nor did it report on their stability. All the above reviewed studies failed to report stability data for the three metal arsenates namely, GaAsO4‚2H2O, InAsO4‚ 2H2O, and LaAsO4. Furthermore, the synthesis methods described vary from one metal arsenate to another and appear to lead to product contamination with other coprecipitated phases (such as CsIn(HAsO4)228) or nontransformed materials (such as Ga2O326 or amorphous InAsO427) or simply adsorbed arsenic due to the high arsenic to metal molar ratio.25,26 As such they are not suitable for producing pure simple model compounds for unequivocal testing of their stabilities-solubilities. It is the object indeed of the present study to report on a new synthesis

method for gallium arsenate dihydrate (GaAsO4‚2H2O), indium arsenate dihydrate (InAsO4‚2H2O), and lanthanum arsenate (LaAsO4), on the detailed characterization of these compounds, and on the evaluation of their chemical stabilities with reference to scorodite. Experimental Section Hydrothermal Synthesis. For the synthesis of gallium arsenate dihydrate, indium arsenate dihydrate, and lanthanum arsenate, analytical-reagent grade As2O5‚xH2O, Ga(NO3)3‚xH2O, In(NO3)3‚5H2O, and La(NO3)3‚6H2O were dissolved in 250 mL of deionized water in equimolar concentrations (0.3 M As(V) and 0.3 M Ga(III) or In(III) or La(III)). The resulting solution was placed in a 2-L Parr Ti-made autoclave equipped with a glass liner and heated to 433 K (160 °C) while being stirred at 400 rpm for 24 h under 89 psi (∼610 kPa) pressure. The final reaction slurry was filtered after cooling using a pressure filter (P ) 40 psi or ∼270 kPa) and a 0.1 µm filter paper. The solid material was washed four times with 250 mL of hot water. The wet cake was finally dried at 313 K (40 °C) to constant weight with the purpose of removing the moisture. No higher drying temperature was employed as a precaution against undesirable secondary phase crystallization or transformation effects. Stability Testing. The stabilities of the synthesized metal arsenates were evaluated using the same procedure as the one used previously by the authors to study the stability of mansfieldite18 or the stability of the scorodite-mansfieldite solid solution materials.19 Basically, it involved five stages (of 24 h each) of contacting the solids with water at a defined pH before they were subjected to extended (up to 4 months) stability measurements. The initial five-stage treatment was applied for removing any poorly crystalline or adsorbed species, to ensure that only the stability of the specific crystalline arsenate was determined. All measurements were done at 295 ( 1 K (22 ( 1 °C). Following the five-stage leaching pretreatment, the metal arsenates were equilibrated at pH 5 and 7 in magnetically stirred slurries of 1 g of solids/40 mL of solution for at least 6 weeks to determine their stability potentials. The pH was kept constant over this leachability testing period with the manual addition (when necessary) of 0.01 M NaOH. Experiments at pH 7 were duplicated for experimental reproducibility. Over the extended leachability experiment, the flasks were stoppered, while at regular intervals 1-mL samples were collected, filtered (using 0.025 µm filter membrane), and analyzed for arsenic and occasionally for gallium, indium, or lanthanum. At the end of the experiments, the solids were pressure filtered, washed, and dried to constant weight. Characterization. X-ray diffraction (XRD) and thermogravimetric analysis (TGA) techniques were used to identify the nature of the synthesized materials and their respective crystallization water content. XRD analysis was performed on a Rigaku Rotaflex D-Max diffractometer equipped with a rotation anode, a copper target (Cu KR1 λ ) 1.5406 Å), a monochromator composed of a graphite crystal, and a scintillator detector. The diffractometer used 40 kV and 150 mA. Scanning took place between 10° and 100° 2θ with a 0.04° or 0.1° step and an acquisition time of 3 s by step. TGA analysis was done with a Perkin-Elmer thermogravimetric analyzer (model TGA-7). The acquisition was between 323 K (50 °C) and 673 K (400 °C) with a heating rate of 10 K (10 °C) per minute. The purge gas was argon. For the morphological characterization of the different metal arsenates produced, a Hitachi S-4700 field emission gun

Ind. Eng. Chem. Res., Vol. 46, No. 24, 2007 7877 Table 1. Percent Arsenic Precipitated and M/As Molar Ratio of Metal Arsenate Compounds compound

% As precipitated

molar ratio M/As

gallium arsenate (M ) Ga) indium arsenate (M ) In) lanthanum arsenate (M ) La)

92% ( 3% 87% ( 3% 88% ( 3%

0.98 ( 0.04 0.96 ( 0.04 0.98 ( 0.04

scanning electron microscope (FEG-SEM) was used. Prior to microscopic examination, the particles were deposited on a carbon film and coated with a thin layer of AuPd. In addition to FEG-SEM observation, the powder particle size distribution was determined using a HORIBA LA-920 particle size analyzer. Isopropyl alcohol was used as the medium. Solutions were subjected to inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis with a Thermo Jarrel Ash Trace Scan Machine. Standards of 50 and 100 mg/L for each element and a blank (2 vol % HNO3) were used for the calibration. All standards were prepared from ICP grade standards of 1000 mg/L. The standard deviation was evaluated at 5%, by analyzing the same sample more than 10 times. This technique was used to determine gallium, indium, lanthanum, and arsenic concentrations in the solution samples as well as for the determination of the solid product composition after their digestion in an HCl (8.6 vol %)-HNO3 (4.3 vol %) solution (180 mg of solids in 115 mL of solution) heated at 343 K (70 °C).

Figure 1. TGA analysis of GaAsO4‚2H2O, InAsO4‚2H2O, and LaAsO4.

Results and Discussion Synthesis and Characterization. The synthesis method used in this work is the same as the one used by Le Berre et al. for the production of mansfieldite18 and scorodite-mansfieldite solid solution series.19 This method is based on the use of an initially acidic solution (pH ∼1 at 295 K (22 °C)) consisting of 0.3 M Ga(NO3)3 (or In(NO3)3 or La(NO3)3) and 0.3 M H3AsO4 heated at 433 K (160 °C) and allowed to react for 24 h, according to the following reaction stoichiometries:

Ga(NO3)3 (aq) + H3AsO4 (aq) + 2H2O f GaAsO4‚2H2O(s) + 3HNO3 (aq) (1) In(NO3)3 (aq) + H3AsO4 (aq) + 2H2O f InAsO4‚2H2O(s) + 3HNO3 (aq) (2) La(NO3)3 (aq) + H3AsO4 (aq) f LaAsO4 (s) + 3HNO3 (aq) (3) All three compounds synthesized were in the form of white powders. The yanomanite (InAsO4‚2H2O) mineral is also known to be white.23 Analysis of the final solution and the solids following digestion allowed for the determination of the percentage of precipitated arsenic for each of the studied metal arsenate systems and the metal to arsenic molar ratio (Table 1). The near-1 metal to arsenic molar ratio confirms the stoichiometric composition of the precipitated compounds (refer to reactions 1-3). The reaction yield was high with more than 85% of the initial arsenic reporting to the product. In comparison, the corresponding percentages of precipitated arsenic under the same conditions for scorodite and mansfieldite were respectively 98% and 61%.18 Assuming that the equilibrium was reached at the end of each precipitation reaction, the above percentages of arsenic precipitation data may be interpreted as indications of the relative stabilities of the various metal arsenates synthesized here vis-a`-vis scorodite and mansfieldite.

Figure 2. XRD patterns of GaAsO4‚2H2O, InAsO4‚2H2O, and LaAsO4.

This, however, can only be verified upon performance of stability testing as done later in this paper. The presence and number of structural waters was determined by TGA analysis (Figure 1). The weight loss percentage found for gallium arsenate dihydrate was 14.8%, which is in agreement with the theoretical one (14.7%) corresponding to two waters. Gallium arsenate dihydrate lost its structural water at a temperature less than 673 K (400 °C). In contrast, indium arsenate dihydrate lost its waters in sequence at higher temperatures. At 673 K (400 °C), the maximum temperature that our TGA equipment could reach, there was still some water associated with the indium arsenate solids. It was possible to determine the first weight loss step to be 6.4%, which upon extrapolation gives 12.8% of total weight loss. This value is in agreement with the theoretical one (12.4%) expected from the two waters of the indium arsenate dihydrate. The TGA results also confirmed the absence of structural water in the lanthanum arsenate precipitate. The production of crystalline GaAsO4‚2H2O, InAsO4‚2H2O, and LaAsO4 was confirmed by XRD analysis (Figure 2). The produced materials exhibited good crystallinity and matched closely the JCPDS (Joint Committee on Powder Diffraction Standards) files for GaAsO4‚2H2O (No. 26-0667), for InAsO4‚ 2H2O (No. 47-1780), and for LaAsO4 (No. 15-0756). The produced materials were characterized further in terms of particle size and morphology. The three materials exhibited distinctly different particle size distributions (Figure 3). LaAsO4 appeared to be a mixture of two populations of particles: one around 2.5 µm and one around 25 µm; the mode particle sizes for GaAsO4‚2H2O and InAsO4‚2H2O were respectively 8 µm and 2 µm. These apparent particle differences were explored

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Figure 3. Particle size distribution (volume-based) of GaAsO4‚2H2O, InAsO4‚2H2O, and LaAsO4 materials.

further by FEG-SEM observation (Figure 4). The gallium arsenate dihydrate particles appeared to be in the form of aggregates of individual crystallites, whereas the indium arsenate dihydrate was made largely of nonaggregated uniformly grown crystallites. The lanthanum arsenate particles, on the other hand, had a completely different appearance, apparently reflecting the different crystal system (monoclinic) for this arsenate as opposed to orthorhombic for gallium and indium arsenates. Crystal Structure. A more detailed analysis of the XRD data was undertaken for the characterization of the crystal structures of the produced metal arsenates. Tables 2-4 list the detailed XRD data. These three tables compare the peak positions in d values and the intensity of the experimental pattern with those of reference materials. The d values were determined using Bragg’s law:

2d sin θ ) nλ

(4)

where n is the order of reflection (n ) 1 in this case), θ is the Bragg angle (rad), λ is the incident wavelength of the monochromatic X-rays (Cu KR1 λ ) 1.5406 Å), and d represents the distance between atomic layers in the crystal. The experimental patterns of GaAsO4‚2H2O and LaAsO4 matched very well with the theoretical ones. In the case of InAsO4‚2H2O, a systematic deviation (up to 0.1 Å) was observed between the experimental pattern and the JCPDS one. The deviation can be due to an experimental source (e.g., type of XRD equipment used) or to a minor crystal imperfection. The XRD analysis was extended to the determination of the lattice parameters of each compound produced. GaAsO4‚2H2O and InAsO4‚2H2O present an orthorhombic structure (isostructural with variscite), whereas LaAsO4 presents a monoclinic structure. Lattice parameters and planes are related by

1 h2 k2 l2 ) + + d2 a2 b2 c2

(5)

Figure 4. FEG-SEM images of (a) GaAsO4‚2H2O, (b) InAsO4‚2H2O, and (c) LaAsO4.

1 1 h2 k2 sin2 β l2 2hl sin β ) 2 + + 22 ac d sin β a2 b2 c monoclinic case (6)

this work are presented in Table 5 and compared to data reported in the literature. It can be seen in general that the crystal lattice parameters of the synthesized phases to match well those of the reference materials. Stability. Classification of a waste material as hazardous is done mainly on the basis of United States Environmental Protection Agency (EPA)’s Toxicity Characteristic Leaching Procedure (TCLP). This leachability test involves contact of the solids with extraction water at pH 5.0 ( 0.2 over a period of 24 h at 293 K (20 °C).35 According to EPA’s Toxicity

(

orthorhombic case

)

where d is the distance between atomic layers in a crystal calculated with Bragg’s law (eq 4), h, k, and l are the Miller indices, and a, b, c, and β are the lattice parameters. The lattice parameters were calculated by considering both planes at high or low angles and by obtaining the average of the different values. The values obtained for each compound produced in

Ind. Eng. Chem. Res., Vol. 46, No. 24, 2007 7879 Table 2. XRD Data of GaAsO4‚2H2O JCPDS files (26-0667) from Ronis and d’Yvoire25

this work 2θ (deg)

d (Å)

intensity

2θ (deg)

d (Å)

intensity

16.00 ( 0.04 17.52 ( 0.04 17.88 ( 0.04

5.53 ( 0.02 5.06 ( 0.01 4.96 ( 0.01

67 13 22

20.16 ( 0.04 22.00 ( 0.04 23.68 ( 0.04

4.40 ( 0.01 4.04 ( 0.01 3.75 ( 0.01

100 11 18

27.08 ( 0.04 28.40 ( 0.04 29.68 ( 0.04 30.12 ( 0.04 32.88 ( 0.04

3.29 ( 0.01 3.14 ( 0.01 3.01 ( 0.01 2.96 ( 0.01 2.72 ( 0.01

14 53 59 26 4

33.84 ( 0.04 34.96 ( 0.04 35.16 ( 0.04 35.36 ( 0.04 36.16 ( 0.04 36.32 ( 0.04 39.44 ( 0.04 39.82 ( 0.04 41.96 ( 0.04 45.76 ( 0.04 50.32 ( 0.04 55.84 ( 0.04 62.12 ( 0.04

2.65 ( 0.01 2.56 ( 0.01 2.55 ( 0.01 2.54 ( 0.01 2.48 ( 0.01 2.47 ( 0.01 2.28 ( 0.01 2.26 ( 0.01 2.15 ( 0.01 1.98 ( 0.01 1.81 ( 0.01 1.65 ( 0.01 1.49 ( 0.01

18 27 31 47 15 25 14 6 12 11 11 15 10

15.985 17.443 17.832 20.073 20.165 22.319 23.707 26.749 27.080 28.401 29.655 30.095 32.852 33.783 33.849 34.938 35.164 35.350 36.130 36.342 39.383 39.855

5.54 5.08 4.97 4.42 4.40 3.98 3.75 3.33 3.29 3.14 3.01 2.97 2.72 2.65 2.65 2.57 2.55 2.54 2.48 2.47 2.29 2.26

70 20 30 80 100 40 60 30 20 70 70 50 30 50 30 60 20 50 30 50 10 10

Characteristic Rule,36 the regulatory limit for TCLP constituents (in this case arsenic) is 100 times the Drinking Water Standards Maximum Contaminant Level (MCL). The drinking water standard for arsenic (MCL) was until recently 50 µg/L but was reduced in 2006 by the EPA to 10 µg/L. Hence the regulatory limit, specific to the United States, for arsenic was reduced from 5 to 1 mg/L. Testing of the three metal arsenates synthesized in this work with a modified TCLP method (for details, refer to Le Berre et al.18) yielded the leachability results reported in Table 6. According to these data, InAsO4‚2H2O and LaAsO4 meet the new regulatory limit of 1 mg/L while GaAsO4‚ 2H2O is over this value but lower than the previous limit of 5 mg/L. In order to obtain further information on the long-term stability potentials of the produced metal arsenates, extended leachability tests (minimum 48 days) at pH 5 and 7 were performed as well. The obtained results, in terms of arsenic release with time, are shown in Figures 5 and 6 for pH 5 and 7, respectively. As can be seen, the arsenic concentration increased with time, reaching equilibrium values in the case of

Figure 5. Influence of time on arsenic concentration released in solution from GaAsO4‚2H2O, InAsO4‚2H2O, and LaAsO4 at 295 K (22 °C) and pH 5.

(hkl) (111) (200) (020) (002) (201) (121) (112) (202) (221) (122) (311) (131) (113) (032) (231) (132) (203) (400) (040) (213) (331) (420)

LaAsO4 of 4 (pH 5) and 13 mg/L (pH 7). No equilibrium was reached in the cases of GaAsO4‚2H2O and InAsO4‚2H2O. In the latter cases, the arsenic concentration, reached after 48 days at pH 7, was respectively 90 and 15 mg/L. This means that the actual solubility of these compounds is higher than these values. The good degree of reproducibility of the reported data can be appreciated with the duplicate test data obtained at pH 7 for LaAsO4 (Figure 6). In Figure 6, a comparison is made in terms of their leachability results among GaAsO4‚2H2O, InAsO4‚2H2O, LaAsO4, and two variscite isostructural species, namely scorodite (FeAsO4‚ 2H2O) and mansfieldite (AlAsO4‚2H2O). It appears that the stability of GaAsO4‚2H2O is similar to that for mansfieldite. Of all trivalent metal arsenates, these two are the least stable whereas scorodite is the most stable. In Table 7, the arsenic leachability data from the various trivalent metal arsenates is presented as a function of cation size. No systematic tendency is observed. However, it can be noticed that cations of smaller size (i.e., Al and Ga) than that of Fe(III) render the respective

Figure 6. Comparison between GaAsO4‚2H2O, InAsO4‚2H2O, LaAsO4, scorodite (FeAsO4‚2H2O), and mansfieldite (AlAsO4‚2H2O) in terms of arsenic concentration released in solution with time at pH 7 and 295 K (22 °C).

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Table 3. XRD Data of InAsO4‚2H2O JCPDS files (47-1781) from Botelho et al.23

this work 2θ (deg)

d (Å)

intensity

2θ (deg)

d (Å)

intensity

15.8 ( 0.1

5.62 ( 0.04

69

17.4 ( 0.1 19.9 ( 0.1 21.7 ( 0.1

5.07 ( 0.03 4.47 ( 0.02 4.10 ( 0.02

49 100 50

23.3 ( 0.1

3.82 ( 0.02

26

26.4 ( 0.1 27.8 ( 0.1 29.1 ( 0.1

3.38 ( 0.01 3.21 ( 0.01 3.07 ( 0.01

35 59 66

31.2 ( 0.1

2.87 ( 0.01

4

32.9 ( 0.1

2.72 ( 0.01

31

34.0 ( 0.1

2.63 ( 0.01

55

34.6 ( 0.1 35.0 ( 0.1 35.6 ( 0.1 37.5 ( 0.1 38.3 ( 0.1 38.9 ( 0.1

2.60 ( 0.01 2.56 ( 0.01 2.52 ( 0.01 2.40 ( 0.01 2.35 ( 0.01 2.32 ( 0.01

45 15 42 10 18 12

40.9 ( 0.1

2.20 ( 0.01

35

42.1 ( 0.1

2.14 ( 0.01

17

43.8 ( 0.1

2.07 ( 0.01

13

44.7 ( 0.1

2.03 ( 0.01

19

46.1 ( 0.1 48.2 ( 0.1 49.0 ( 0.1 49.8 ( 0.1 51.4 ( 0.1

2.01 ( 0.01 1.97 ( 0.01 1.89 ( 0.01 1.86 ( 0.01 1.83 ( 0.01 1.78 ( 0.01

10 6 10 7

53.5 ( 0.1

1.71 ( 0.01

13

54.6 ( 0.1

1.68 ( 0.01

30

55.4 ( 0.1

1.66 ( 0.01

16

56.9 ( 0.1 57.5 ( 0.1 60.4 ( 0.1

1.60 ( 0.01 1.59 ( 0.01 1.53 ( 0.01

10 4 25

61.4 ( 0.1 62.3 ( 0.1 63.2 ( 0.1 65.0 ( 0.1 66.8 ( 0.1

1.51 ( 0.01 1.49 ( 0.01 1.47 ( 0.01 1.43 ( 0.01 1.40 ( 0.01

7 13 8 9 12

15.492 16.926 17.131 19.550 21.332 21.493 22.986 25.933 26.103 27.454 28.707 28.967 30.904 31.905 32.588 33.472 33.708 34.149 34.223 34.675 35.271 37.131 37.969 38.523 38.817 40.117 40.539 40.600 40.675 41.042 41.812 43.368 43.460 43.523 44.354 44.568 44.714 45.790 47.897 48.646 49.472 51.112 53.112 53.305 53.423 53.867 54.125 54.396 55.098 56.434 56.532 56.619 57.125 60.017 60.259 61.135 61.831

5.7190 5.2340 5.1720 4.5370 4.1620 4.1310 3.8660 3.4330 3.4110 3.2461 3.1073 3.0800 2.8912 2.8027 2.7455 2.6750 2.6568 2.6235 2.6180 2.5849 2.5426 2.4194 2.3679 2.3351 2.3181 2.2459 2.2235 2.2203 2.2164 2.1974 2.1587 2.0848 2.0806 2.0777 2.0407 2.0314 2.0251 1.9800 1.8977 1.8702 1.8409 1.7856 1.7230 1.7172 1.7137 1.7006 1.6931 1.6853 1.6655 1.6292 1.6266 1.6243 1.6111 1.5402 1.5346 1.5147 1.4993

70 3 30 100 40 10 25 4 20 80 40 35 2 1 30 2 50 10 30 5 45 5 8 5 1 1 8 4 5 4 8 2 4 4 9 1 5 3 2 2 2 7 2 2 2 2 9 8 5 4 4 4 5 8 4 3 3

trivalent metal arsenates much more soluble than those cations of larger size (i.e., In and La). Finally, no relationship between the particle size (data in Figure 3) and leachability (data in Figures 5 and 6) of the trivalent metal arsenates studied in this work was observed. As can be seen from the leachability data (Figures 5 and 6), GaAsO4‚2H2O exhibited faster and higher solubility than the other arsenates even though its particle size was significantly larger than, for example, that of InAsO4‚ 2H2O. Analysis of the leachate solution for indium and lanthanum concentrations as a function of time detected less than 1 mg/L

(hkl) (111) (200) (002) (020) (021) (112) (121) (220) (212) (122) (311) (113) (302) (131) (023) (321) (123) (230) (400) (004) (231) (114) (313) (402) (204) (214) (133) (041) (421) (124) (323) (240) (042) (304) (142) (413) (314) (115) (043) (521) (234) (522) (006) (433) (334) (106) (441) (144) (523) (620) (612) (235) (216) (226) (613) (060) (061)

of each metal at both pHs (5 and 7), which implies that their dissolution is incongruent at that pH range:

InAsO4‚2H2O(s) + H2O f In(OH)3 (s) + H3AsO4*(aq) LaAsO4 (s) + 3H2O f La(OH)3 (s) + H3AsO4*(aq)

(7) (8)

XRD analysis failed to detect the presence of In(OH)3 or La(OH)3 due to their (assumed) amorphous nature and their small fractions as such; therefore, their formation is only a postulation

Ind. Eng. Chem. Res., Vol. 46, No. 24, 2007 7881 Table 4. XRD Data of LaAsO4 this work

JCPDS files (15-0756)

2θ (deg)

d (Å)

intensity

2θ (deg)

d (Å)

intensity

(hkl)

16.3 ( 0.1 18.0 ( 0.1 18.3 ( 0.1 20.4 ( 0.1 21.1 ( 0.1

5.43 ( 0.04 4.92 ( 0.03 4.84 ( 0.03 4.35 ( 0.03 4.21 ( 0.02

3 10 7 5 22

24.7 ( 0.1 26.3 ( 0.1 27.4 ( 0.1 28.1 ( 0.1 29.1 ( 0.1 30.0 ( 0.1 32.9 ( 0.1

3.60 ( 0.02 3.39 ( 0.01 3.25 ( 0.01 3.17 ( 0.01 3.07 ( 0.01 2.98 ( 0.01 2.72 ( 0.01

20 46 6 100 18 62 16

35.3 ( 0.1 36.4 ( 0.1 37.0 ( 0.1 38.7 ( 0.1 40.0 ( 0.1 40.1 ( 0.1 40.9 ( 0.1 41.4 ( 0.1 41.9 ( 0.1 43.3 ( 0.1 44.9 ( 0.1 45.7 ( 0.1

2.54 ( 0.01 2.47 ( 0.01 2.43 ( 0.01 2.32 ( 0.01 2.25 ( 0.01 2.25 ( 0.01 2.20 ( 0.01 2.18 ( 0.01 2.15 ( 0.01 2.09 ( 0.01 2.02 ( 0.01 1.98 ( 0.01

16 5 5 3 13 12 6 6 4 3 24 3

46.8 ( 0.1 47.5 ( 0.1 48.7 ( 0.1 49.8 ( 0.1

1.94 ( 0.01 1.91 ( 0.01 1.87 ( 0.01 1.83 ( 0.01

25 13 3 16

50.9 ( 0.1 52.5 ( 0.1

1.79 ( 0.01 1.74 ( 0.01

22 11

54.3 ( 0.1

1.69 ( 0.01

9

55.8 ( 0.1

1.65 ( 0.01

14

16.250 17.904 18.276 20.398 21.059 24.467 24.667 26.258 27.180 27.990 29.080 29.928 32.875 35.220 35.493 36.325 36.960 38.677 39.909 40.001 40.737 41.283 41.803 43.273 44.760 45.519 46.306 46.765 47.408 48.704 49.725 50.431 50.581 50.822 52.452 54.021 54.264 55.619 55.766

5.45 4.95 4.85 4.35 4.215 3.635 3.606 3.391 3.278 3.185 3.068 2.983 2.722 2.546 2.527 2.471 2.430 2.326 2.257 2.252 2.213 2.185 2.159 2.089 2.023 1.991 1.959 1.942 1.916 1.868 1.832 1.808 1.803 1.795 1.743 1.696 1.689 1.651 1.647

4 12 10 8