Environ. Sci. Technol. 1999, 33, 3806-3811
Arsenic Immobilization by Calcium Arsenate Formation JAMES V. BOTHE, JR. AND PAUL W. BROWN* Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802
Lime additions to arsenic-containing wastes have been proven to be beneficial in reducing the mobility of dissolved arsenic, presumably through the formation of lowsolubility calcium arsenates. However, the role of calcium arsenate formation in reducing the concentrations of dissolved arsenic has not been well established. Therefore, slurries with varying Ca/As ratios were equilibrated, and the compounds that formed at elevated pH values were established. In contrast to the literature, Ca3(AsO4)2 was not observed, rather Ca4(OH)2(AsO4)2‚4H2O, Ca5(AsO4)3OH (arsenate apatite), and Ca3(AsO4)2‚32/3H2O had formed. The equilibrium concentrations of arsenic were found to be the lowest at high pH. Minimum arsenic concentrations in equilibrium with Ca4(OH)2(AsO4)2‚4H2O and Ca5(AsO4)3OH were 0.01 and 0.5 mg/L, respectively. Because arsenate apatite is stable to near-neutral pH values, the extent of its solid solubility with Ca5(PO4)3OH was determined. This was done to assess the effects of phosphate ion on the possible release of arsenate ion. Although equilibrium arsenate ion concentrations increased with decreasing pH, solid solution formation did not occur under ambient conditions. Rather, the arsenate apatite formed at the expense of Ca5(PO4)3OH.
Introduction Arsenic is of environmental concern due to its toxic properties. It occurs naturally in about 245 minerals, which when subjected to weathering can release soluble arsenic into natural waters. Arsenic can also be found in the waste streams from a variety of industrial processes. For example, arsenic waste can be generated from petroleum refining, glass melting, and smelting of ores that are mined for their lead, copper, zinc, gold, and silver. Arsenic is also released into the environment by the dispersion of arsenic-containing fertilizers, pesticides, and wood preservatives (1-3). A common method for removing arsenic from aqueous waste streams is through precipitation. Typical precipitates are arsenic sulfides, calcium arsenates, or ferric arsenates. Each of these precipitates have limited pH ranges within which they exhibit solubility minima. For example, those calcium arsenates that exhibit the lowest equilibrium concentrations of arsenate ion are stable at high pH, whereas ferric arsenates (i.e., scorodite) are stable only at low pH (4). The use of lime (CaO) is by far the most common method of treating industrial waste. Such waste includes incinerator ash, petroleum sludge, phosphoric acid residue, steel pickle liquor, hydrocarbon waste, and flue gas cleaning sludges from * Corresponding author phone: (814)865-5352; fax: (814)863-7040; fax:
[email protected]. 3806
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 21, 1999
fossil fuel-burning power plants (5). However, the literature indicates a lack of knowledge as to the precipitates that form when lime is used to remove soluble arsenic. An evaluation of arsenic encapsulation by cement indicated that the addition of lime to the cement binder minimized the concentration of soluble arsenic in the leachate (6). This study also concluded that Ca3(AsO4)2 formation resulted in the reduction. A further reduction of arsenic concentrations when lime was added to a suspension of pyrite fines has been attributed to Ca3(AsO4)2 formation (7). Studies modeling the solubility behavior of arsenates using thermodynamic data have used Ca3(AsO4)2 as the prototype mineral responsible for arsenic uptake in the presence of calcium (8-10). Although there are several calcium arsenates that can precipitate from an aqueous solution over a wide range of pH, anhydrous Ca3(AsO4)2 is not one of them. Comprehensive studies on calcium arsenates by Guerin (11) and Pierrot (12) indicate that only the hydrated forms, such as Ca3(AsO4)2‚xH2O, are thermodynamically stable in aqueous solutions. Bothe and Brown (13) determined the solubility product constants and free energies of formation of a variety of calcium arsenates. Nishimura and Robins (14) recently reevaluated the solubility and stability regions of various calcium arsenate hydrates but were not able to synthesize the arsenate apatite, Ca5(AsO4)3OH (johnbaumite), for which little information exists in the literature. Apatites are a class of minerals that are compositionally varied but share the same crystal structure and have been investigated as host materials for long-term immobilization of a number of environmentally hazardous elements including lead, uranium, cadmium, iodine, and bromine (15-18). For example, one method used for the removal of dissolved arsenic involves the crystallization of mimetite or lead chloroarsenate apatite, Pb5(AsO4)3Cl, which was shown to reduce the aqueous arsenic concentration to less than 0.20 ppb (19). Apatites are attractive hosts because they tend to be stable over broad ranges of pH. The present study is part of a larger effort directed toward establishing the stabilities of solids precipitated at ambient temperature and which are capable of sequestering toxic and hazardous species (20, 21). The objectives of the presently described study are to identify those calcium arsenates, including the apatite Ca5(AsO4)3OH, that precipitate under alkaline conditions; to establish the conditions under which they are stable; and to establish the processes responsible for the immobilization of arsenic in the presence of lime.
Experimental Methods Sixty-four suspensions were prepared in seven sets made over 4 years. These were made by mixing Ca(OH)2 powder with o-arsenic acid and deionized water at a liquid to solids weight ratio of approximately 10 to attain molar Ca/As ratios varying from 0.80 to 4.0. The ingredients were combined in 125 mL of HDPE Nalgene bottles with zirconia milling media to facilitate mixing of the liquid and solid reactants. The bottles were then tightly sealed with their necks wrapped in electrical tape to minimize intrusion of atmospheric CO2, stored at room temperature (23 ( 1 °C), and periodically agitated. The Ca(OH)2 was made by calcining CaCO3 at 1000 °C for 2 h. The resulting CaO was then hydrated in boiling water, and the product was filtered in open air and dried overnight in a vacuum oven. X-ray diffraction was used to ensure that the Ca(OH)2 did not contain any residual CaCO3. The CaCO3 used was obtained from two sources. The first five sets used reagent-grade CaCO3 obtained from Fisher Scientific; this 10.1021/es980998m CCC: $18.00
1999 American Chemical Society Published on Web 09/16/1999
TABLE 1. Calcium Arsenate Hydrates Precipitated from Aqueous Suspensions with Molar Ca/As Ratios Ranging from 1.50 to 2.50 and Their Associated Solution Chemistry solid-phase assemblage
Ca/As
sample set (days reacted)
Ca4(OH)2(AsO4)2‚4H2O
2.50 2.237 2.25 2.20 2.20
III (411) III (823) II (672) III (411) III (823)
760 690 780 310 320
0.08 0.01 0.01 0.40 0.12
12.54 12.64 12.52 12.14 12.23
Ca5(AsO4)3OH (arsenate-hydroxyapatite)
1.90 1.90 1.67 1.67 1.67
VI (408) VI (668) VI (414) VI (562) VI (674)
780 850 26 18 19
0.14 0.17 10.5 19.5 19.5
12.63 12.72 9.87 9.54 9.77
Ca3(AsO4)2‚32/3H2O Ca3(AsO4)2‚41/4H2O
1.67 1.50 1.50 1.50 1.50
I (74) III (411) III (823) III (137) VI (408)
32 350 320 410 400
Ca (mg/L)
As (mg/L)
3 590 490 820 710
pH
FIGURE 1. X-ray diffraction pattern of the hydrate Ca4(OH)2(AsO4)2‚ 4H2O.
11.18 7.52 7.55 7.36 7.32
contained approximately 0.5 wt % magnesium oxide. The sixth and seventh sets used ultrapure CaCO3 provided by courtesy of Osram-Sylvania (Towanda, PA). The arsenic source was a stock solution of 80.5 wt % H3AsO4 purchased from Riedel-deHae¨n AG (D-3016 Seeize 1, Germany). Aliquots were periodically characterized to monitor the extent of reaction by extracting two 5-mL samples from each suspension. The pH of the first sample was measured in open air, whereas all the separation steps on the second sample were carried out within a nitrogen-filled tent. The second sample was first centrifuged, and then the liquid fraction was filtered through a 0.22-µm syringe filter into a scintillation vial that was tightly sealed with electrical tape. The concentrations of calcium and arsenic were analyzed by DC plasma emission spectroscopy. The solid fractions were extracted via filtration, air-dried, and then characterized using a Scintag fully automated powder X-ray diffractometer; the powdered samples were pressed into a recessed glass slide. Some samples were characterized using TGA (TA Instruments model 2050). The heating rate used during TGA analysis was 10 °C/min in air. Uncoated representative samples were characterized for their morphologies using an ISI-DS130 duelstage scanning electron microscope (SEM) with tungsten filament. Eleven additional suspensions having varying ratios of phosphate to arsenate were prepared to investigate the extent of formation of the solid solution Ca10(AsO4)x(PO4)6-x(OH)2 under ambient conditions. In preparing these suspensions, Ca4O(PO4)2, CaHPO4 (Aldrich), Ca(OH)2 (made from Fisher CaCO3), and H3AsO4 were combined with deionized water at a liquid to solids weight ratio of approximately 10 to produce suspensions having a constant molar Ca/(As + P) ratio of 1.67. The suspensions were allowed to equilibrate for approximately 490 days. Ca4O(PO4)2 was synthesized by firing a mixture of reagent-grade CaCO3 (Fisher) and CaHPO4 (Aldrich) at 1300 °C for 2 h. The following compositions were prepared in accordance with the formula Ca10(AsO4)yo(PO4)6-yo(OH)2:yo ) 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, and 5.5 where yo represents the stoichiometric amount of H3AsO4 added to each suspension.
Results Calcium Arsenate Stability. The calcium arsenate hydrates that precipitated for Ca/As ratios between 1.50 and 2.50 are listed in Table 1. Phase-pure Ca4(OH)2(AsO4)2‚4H2O (Figure 1) was observed to consistently form at Ca/As ratios between 2.00 and 2.50. However, in some suspensions this hydrate
FIGURE 2. SEM micrograph of the hydrate Ca4(OH)2(AsO4)2‚4H2O. precipitated in conjunction with minor amounts of the apatite Ca5(AsO4)3OH within the same range of Ca/As ratios. The micrograph in Figure 2 shows the hydrate Ca4(OH)2(AsO4)2‚ 4H2O to be well crystallized with the crystallites ranging in size from approximately 0.5 to 4 µm. On the basis of the solution chemistry data presented in Table 1, arsenic concentrations in equilibrium with Ca4(OH)2(AsO4)2‚4H2O remain low (
