外文翻譯--砷酸鈣形成固定砷（英文）

1、Arsenic Immobilization by Calcium Arsenate FormationJ A M E S V . B O T H E , J R . A N D P A U L W . B R O W N *Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsy

2、lvania 16802Lime additions to arsenic-containing wastes have been proven to be beneficial in reducing the mobility of dissolved arsenic, presumably through the formation of low- solubility calcium arsenates. However, the

3、 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 ele

4、vated pH values were established. In contrast to the literature, Ca3(AsO4)2 was not observed, rather Ca4(OH)2(AsO4)2?4H2O, Ca5(AsO4)3- OH (arsenate apatite), and Ca3(AsO4)2?32/3H2O had formed. The equilibrium concentrati

5、ons 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)3- OH were 0.01 and 0.5 mg/L, respectively. Because arsenate apatite is stable t

6、o 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 io

7、n 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.IntroductionArsenic is of environmental co

8、ncern due to its toxic proper- ties. It occurs naturally in about 245 minerals, which when subjected to weathering can release soluble arsenic into naturalwaters.Arseniccanalsobefoundinthewastestreams from a variety of i

9、ndustrial 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 t

10、he 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

11、 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 e

12、quilibrium con- centrations 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 w

13、aste. Such waste includes incinerator ash, petroleum sludge, phosphoric acid residue, steel pickle liquor,hydrocarbonwaste,andfluegascleaningsludgesfromfossil fuel-burning power plants (5). However, the literature indica

14、tes 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 th

15、e concentrationofsolublearsenicintheleachate(6).Thisstudy also concluded that Ca3(AsO4)2 formation resulted in the reduction.Afurtherreductionofarsenicconcentrationswhen lime was added to a suspension of pyrite fines has

16、 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 forarsenicuptakeinthepresenceofcalciu

17、m(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 (1

18、1) 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

19、 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,

20、 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

21、 materials for long-term immobilization of a number of environmentally hazardous elements including lead, ura- nium, cadmium, iodine, and bromine (15-18). For example, onemethodusedfortheremovalofdissolvedarsenicinvolves

22、 the crystallization of mimetite or lead chloroarsenate apatite, Pb5(AsO4)3Cl,whichwasshowntoreducetheaqueousarsenic concentrationtolessthan0.20ppb(19).Apatitesareattractive hosts because they tend to be stable over broa

23、d 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 andhazardousspecies(20,2

24、1).Theobjectivesofthepresently 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 sta

25、ble; and to establish the processes responsible for the immobilization of arsenic in the presence of lime.Experimental MethodsSixty-four suspensions were prepared in seven sets made over 4 years. These were made by mixin

26、g 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 Nalge

27、ne 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, st

28、ored 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

29、 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-gra

30、de CaCO3 obtained from Fisher Scientific; this* Correspondingauthorphone: (814)865-5352;fax: (814)863-7040; fax: etx@psu.edu.Environ. Sci. Technol. 1999, 33, 3806-38113806 9 ENVIRONMENTAL SCIENCE all the reaction produc

31、ts are apatitic in nature. There is a discernible increase in intensity of the (111) reflection with increasing arsenate content. However, this alone is insufficient to signify an increasing degree of substitution of ars

32、enate for phos- phate. As the amount of arsenate ion increased from yo ) 0.5 to 2.0, the (211), (112), and (300) reflections of the apatite phase remained distinct as other more diffuse reflections begin to build up arou

33、nd them. With increasing proportions of arsenate ion, these distinct reflections become completely masked,whereasthediffusereflectionsbecomemoredistinct and ultimately produce the diffraction pattern for crystalline Ca5(

34、AsO4)3OH. To determine the nature of the transition from Ca5(PO4)3OH (yo ) 0.5) to Ca5(AsO4)3OH (yo ) 5.5), the X-rayFIGURE 3. X-ray diffraction patterns for (a) Ca3(AsO4)2?32/3H2O made with the calcium source containing

35、 0.5 wt % magnesium, (b) Ca3(AsO4)2?41/4H2O made with the calcium source containing 0.5 wt % magnesium, and (c) Ca3(AsO4)2?41/4H2O made with the ultrapure calcium source.FIGURE 4. TGA profiles for (a) Ca3(AsO4)2?32/3H2O

36、made with the calciumsourcecontaining0.5wt%magnesium,(b)Ca3(AsO4)2?41/4H2O made with the calcium source containing 0.5 wt % magnesium, and (c) Ca3(AsO4)2?41/4H2O made with the ultrapure calcium source.3808 9 ENVIRONMENTA