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Effects of anions on calcium arsenate crystalline structure and arsenic stability
Accepted Manuscript Effects of anions on calcium arsenate crystalline structure and arsenic stability
Jie Lei, Bing Peng, Yan-Jie Liang, Xiao-Bo Min, Li-Yuan Chai, Yong Ke, Yang You PII: DOI: Reference:
S0304-386X(17)30753-3 doi:10.1016/j.hydromet.2018.03.007 HYDROM 4771
To appear in:
Hydrometallurgy
Received date: Revised date: Accepted date:
13 September 2017 31 January 2018 6 March 2018
Please cite this article as: Jie Lei, Bing Peng, Yan-Jie Liang, Xiao-Bo Min, Li-Yuan Chai, Yong Ke, Yang You , Effects of anions on calcium arsenate crystalline structure and arsenic stability. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Hydrom(2017), doi:10.1016/ j.hydromet.2018.03.007
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ACCEPTED MANUSCRIPT Effects of anions on calcium arsenate crystalline structure and arsenic stability Jie Leia, Bing Penga, b, Yan-Jie Lianga,b,c,*, Xiao-Bo Mina,b,*, Li-Yuan Chaia,b, Yong Kea,b, Yang Youa
Institute of Environmental Science and Engineering, School of Metallurgy and
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Chinese National Engineering Research Center for Control & Treatment of Heavy
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Environment, Central South University, Changsha, Hunan 410083, China
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Metal Pollution, Changsha, Hunan 410083, China
Xiang Guang Copper Co., Ltd., Yang gu, Shandong, 252327, China
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*Corresponding authors at: Institute of Environmental Science & Engineering, School of
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Metallurgy and Environment, Central South University, Changsha, Hunan 410083, China.
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E-mail addresses: [email protected] (Y.-J. Liang), [email protected] (X.-B. Min).
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Abstract Based on the crystallization reaction between calcium and arsenate, three anions, namely, chloride (Cl-), sulfate (SO42-), and carbonate (CO32-), were introduced during the formation of calcium arsenate in this study. The effects of these anions on arsenic
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stability and calcium arsenate crystalline properties were characterized using TCLP,
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XRF, XRD, SEM-EDS, TEM and particle size distribution (PSD). The results show
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that these anions significantly affect the calcium arsenate crystalline structure and
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arsenic-leachate concentration. The effect of chloride on arsenic stabilization was mainly a result of the formation and growth of calcium arsenate apatite
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(Ca5(AsO4)3OH and Ca5(AsO4)3Cl) crystal. Sulfate is propitious to improve the formation of Ca4(OH)2(AsO4)2·4H2O crystals and enhance their stability effectively
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under corresponding sulfate concentration treatments. A small amount of carbonate is beneficial to the formation and growth of Ca4(OH)2(AsO4)2·4H2O crystals. However,
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when the CO32- concentration was higher than 0.5 mol/L, Ca-As compounds were not precipitated. Instead, CaCO3 and Na3Ca(CO3)2·5H2O were generated successively,
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inhibiting calcium-arsenate crystallization and thus releasing more arsenic into aqueous solution. The trends of grain size are opposite to that of arsenic-leachate concentration, indicating that larger calcium arsenate crystals are more stable in the TCLP test. The internal structure of different Ca-As compounds can be changed and transformed by adding the three anions within a certain range. In summary, the results
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of this paper provide some meaningful information for the treatment of arsenic-containing wastewater or arsenic-bearing sludges using lime precipitation. Keywords: High arsenic-bearing sludge; Calcium arsenate crystallization; Anions,
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Arsenic-leachate concentration; Grain size; Lime precipitation.
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1. Introduction Arsenic is one of the most toxic metalloid found in industrial wastewater which has caused increasing public concern (Wang et al. 2011; Wu et al. 2016). Lime precipitation is used widely in arsenic-bearing wastewater treatment because of its
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low cost and simple process (Chen et al. 2015; Li et al., 2011). However, this process
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generates a large amount of arsenic sludge, which is classified as hazardous waste due
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to its high arsenic content (Cui et al., 2014; Fei et al., 2017; Min et al., 2017; Morales
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et al., 2010). Various types of technologies, such as cementation (Choi et al., 2009; Li et al., 2017; Randall, 2012), polymeric encapsulation (Shaw et al., 2008),
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hydrothermal precipitation (Vinals et al., 2010; Peng et al., 2017), mineralization (Chai et al., 2017), co-treatment solidification (Li et al., 2016; Liu et al., 2017),
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mechanical milling (Liang et al., 2017) and vitrification (Zhao et al., 2017), have been developed to reduce the mobility of arsenic and other heavy metals in the sludge.
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Calcium arsenate crystallization is one of the most common methods for reducing arsenic-leachate concentration from the arsenic-bearing precipitates (Donahue and
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Hendry, 2003). Arsenic-leaching behavior is mainly affected by the solubility and phase-transfer of the arsenic compounds (Ke et al., 2017; Ozverdl and Erdem, 2010; Xie et al., 2013). Many researchers have shown that lime addition reduces As mobility in contaminated slurries due to the formation of low-solubility Ca-As precipitates such as Ca4(OH)2(AsO4)2·4H2O and Ca5(AsO4)3OH (Bothe and Brown, 1999; Camacho et al., 2009; Lei et al., 2017).
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It is well known that arsenic-containing wastewater often contains multiple anions such as Cl-, SO42-, and CO32-. It has been proven that these anions have a significant influence on arsenic precipitation (Meng et al., 2002; Rivas et al., 2012; Sun et al., 2017; Welham et al., 2000). There are studies that show chloride can enhance arsenic
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removal through the nano-zero-valent iron process (Tanboonchuy et al., 2012). Other
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studies have found that sulfate has different effects on arsenate removal under
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different concentrations (Sun et al., 2006, Zhang et al., 2008). Calcium arsenate
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precipitate formed as a result of lime addition is not stable, because it can decompose to calcium carbonate and release arsenic under an attack of atmospheric CO2 which is
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easily converted to carbonate (Luo et al., 2010). However, CO2 combined with ferrous is used to evaluate arsenic immobilization in red mud, and the aqueous arsenic
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concentration effectively decreased (Li et al, 2012). Consequently, these anions may play an important role in the removal and immobilization of arsenic and other heavy
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However, the influence of these anions on arsenic immobilization has not been
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systematically studied. In fact, the properties and stability of crystals are susceptible to anions during the process of crystallization (Crundwell, 2016). Previous studies have demonstrated that sulfate ions can distort calcite structure when incorporated into the calcite crystal lattice (Kontrec et al., 2004). Merely changing the anion types and dosages can also affect the rate of formation and crystalline structure of the synthetic compounds (Petrik et al., 2009). Hence, the objective of this study was to
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arsenic-containing wastewater based on lime precipitation.
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2. Materials and methods
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process of arsenic sludge treatment based on calcium arsenate crystallization or high
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2.1. Materials
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Calcium hydroxide (Ca(OH)2), nitric acid (HNO3), sulfuric acid (H2SO4), glacial acetic acid (CH3COOH), sodium arsenate (Na3AsO4·12H2O) over 99%, sodium
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chloride (NaCl), sodium sulfate (Na2SO4) and sodium carbonate (Na2CO3) were purchased from the Sinopharm Chemical Reagent Company Limited in China. All the
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reagents were of analytical grade, and all the dilutions were made using high-purity de-ionized water (18 MΩ cm-1 resistivity) obtained from a mole analytical ultrapure
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water system (Molecular, Molresearch 1010A, China). The bottles and glassware used for storing samples were washed in 10% (v/v) HNO3 for 24 h and rinsed with
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ultrapure water to avoid the effects of impurities. 2.2. Experimental procedure A schematic diagram of the experimental set up is shown in Fig. 1. The crystallization experiments were performed in 500-mL flasks containing a mixture of Ca(OH)2 and Na3AsO4·12H2O in a 2.0:1 Ca/As molar ratio and 4.0:1 liquid/solid mass ratio under stirring with a glass rod for 2 min. The initial pH value of the
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mixture was detected using a pH meter (Mettler-Toledo, FE20). The mixture was reacted on a magnetic stirring apparatus at 25°C for 5 h. The precipitates were separated using a vacuum filter, washed with de-ionized water three times, oven-dried
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at 60°C for 24 h, and then subjected to characterization and a leaching toxicity test.
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Fig. 1. Schematic diagram of the experimental procedure.
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2.3. Characterization A leaching toxicity test was conducted to evaluate the arsenic-leaching behavior of calcium-arsenic compounds according to the toxicity characteristic leaching procedure (TCLP, USEPA SW-846, Method 1311) (USEPA, 1992), which is the
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current USEPA protocol for determining whether waste is hazardous. The chemical
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composition of precipitates was detected using an X-ray fluorescence spectrometer
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(XRF, Shimadzu-1800, operated at 40 kV, 95 mA). The phase and crystallinity of all
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samples were characterized using X-ray diffraction (XRD, Rigaku D/MAX 2550 VPC diffractometer, 40 kV, 20 mA). The average grain size of the crystals was calculated
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using the Scherrer equation as follows (Min et al., 2013): (1)
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where d is the average grain size; k is the Scherrer constant, taken to be 0.89; λ is the wavelength of the X-ray (0.15418 nm); β is the corrected half-width of the diffraction
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peak; and θ is the diffraction peak angle. In addition, scanning electronic microscope coupled by energy dispersive
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spectroscopy (SEM-EDS, Nova Nano SEM 230), laser particle size analyser (OMEC LS-POP VI) and a transmission electron microscope (TEM, JEM-2100F) were used to observe the morphological features and surface characteristics of the Ca-As precipitates, respectively.
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3. Results and discussion 3.1 Characterization of the precipitate from the control experiment The major elements of the precipitate obtained at a Ca/As molar ratio of 2.0:1, a liquid/solid mass ratio 4.0:1 and a temperature of 25°C for 5 h are shown in Table 1:
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Ca 37.9%, O 32.5%, As 23.7 and Na 5.75. The characterization of the control sample
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is presented in Fig. 2. As shown in Fig. 2a and 2b, the general particles are in irregular
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shapes, and no obvious crystal structure is presented. The amorphous particles
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agglomerate into large particles. EDS results show the control sample is mainly composed of Ca and As, indicating that the amorphous particles are Ca-As
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compounds. Fig. 2c shows a wide range of particle size distribution from 0.2 μm to 70 μm. The size of the particles is distributed in three concentrated areas approximately
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0.54 μm, 4.66 μm and 19.3 μm. The median particle size (D50) of 15.5 μm reflects indirectly that most small particles are agglomerated into large particles. Fig. 2d
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shows that the characteristic diffraction peaks of Ca(OH)2 (portlandite), CaHAsO4·2H2O (pharmacolite) and Ca5(AsO4)3OH (johnbaumite) are observed in the
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control sample. A leaching test was performed using the TCLP method, and the results indicated that the arsenic-leachate concentration was 28.26 mg/L, which is greater than the regulation limit of 5 mg/L for As.
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Table 1. Chemical compositions of control sample obtained using XRF (mass fraction, %) Element Ca O As Na Percentage 37.9 32.5 23.7 5.75
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Fig. 2. Characterization of the control sample: (a) SEM, (b) EDS, (c) Particle size distribution, and (d) X-ray diffraction pattern.
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3.2. Chemical composition of precipitates The elemental composition of precipitates, obtained by crystallization reaction with different anions, was determined using XRF, and the results are shown in Fig. 3. It shows that there are no obvious changes in chemical composition in the Cl- and SO42-
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samples, while CO32- caused a significant change. As shown in Fig. 3a and Fig. 3b,
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the percentages of arsenic presented in the residuals are approximately 24% and 23%,
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respectively. With an increase in anion concentration, the content of Cl and S
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increased slightly, indicating that a portion of Cl- and SO42- entered into precipitates during the calcium-arsenate crystallization process. However, as shown in Fig. 3 c,
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with the increasing CO32- concentration from 0.1 mol/L to 1.0 mol/L, the content of As in the precipitates decreased from 23.7% to 11.7%, and the content of Ca
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decreased from 34.6% to 22.2%. At the same time, the content of Na and O increased relatively. The results indicate that Cl- and SO42- can maintain As in residual and show
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an insignificant effect on the content of Ca-As compounds in precipitates. However, the increase in CO32- hinders the formation of Ca-As compounds and leads to arsenic
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release into solution.
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Fig. 3. Elemental composition of precipitates obtained by crystallization reaction with different anions: (a) chloride, (b) sulfate and (c) carbonate.
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3.3. Effect of anions on Ca-As compound crystalline structure and arsenic stability 3.3.1. Effects of Cl- concentration To investigate the influence of anions on the phase composition and crystal
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structure of the precipitates, a crystallization reaction was performed at 25°C for 5 h
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with a Ca/As molar ratio of 2.0, maintaining the anion concentration within the range
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of 0.1–1.0 mol/L. The XRD patterns and grain sizes of the samples synthesized at
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different NaCl concentrations are presented in Fig. 4. Fig. 4a shows that the characteristic diffraction peaks of Ca(OH)2, CaHAsO4·2H2O and Ca5(AsO4)3OH are
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observed when the Cl- concentration is 0.1 mol/L. This is similar to that of the control sample. The main reactions are shown in from Eq. 2 to Eq. 4. However, when the Cl-
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concentration increases from 0.3 mol/L to 1.0 mol/L, CaHAsO4·2H2O diffraction peaks disappear, while Ca5(AsO4)3Cl (turneaureite) diffraction peaks become stronger,
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validating the formation and growth of stable calcium arsenate apatite crystals (including johnbaumite and turneaureite). The grain size was calculated using
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Scherrer’s equation and the results are shown in Fig. 4b. It can be seen that the grain size increased gradually with the increase in Cl- concentration. The results indicate that a low concentration of chloride shows no obvious influence on the crystallization reaction, but a high concentration of chloride can accelerate the formation and growth of calcium arsenate apatite in the form of Ca5(AsO4)3OH and Ca5(AsO4)3Cl. 𝐶𝑎(𝑂𝐻)2 + 𝑁𝑎3 𝐴𝑠𝑂4 ∙ 12𝐻2𝑂
CaH𝐴𝑠𝑂4 ∙ 2𝐻2 𝑂 + 3𝑁𝑎𝑂𝐻 + 9𝐻2 𝑂
(2)
ACCEPTED MANUSCRIPT 5𝐶𝑎(𝑂𝐻)2 + 3𝑁𝑎3 𝐴𝑠𝑂4 ∙ 12𝐻2𝑂
𝐶𝑎5 (𝐴𝑠𝑂4 )3 𝑂𝐻 + 9𝑁𝑎𝑂𝐻 + 12𝐻2 𝑂
5𝐶𝑎(𝑂𝐻)2 + 3𝑁𝑎3 𝐴𝑠𝑂4 ∙ 12𝐻2𝑂 + 𝑁𝑎𝐶𝑙
𝐶𝑎5(𝐴𝑠𝑂4 )3 𝐶𝑙 + 10𝑁𝑎𝑂𝐻 + 12𝐻2 𝑂
(3) (4)
Fig. 4b shows the effect of Cl- concentration on arsenic-leachate concentration using the TCLP test. It can be seen that the arsenic-leachate concentration decreased
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from 28.26 mg/L to 0.08 mg/L gradually as the Cl- concentration increased from 0
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mol/L to 1.0 mol/L. When Cl- concentration increased to 0.8 mol/L, the
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arsenic-leachate concentration reached the regulatory level (less than 5 mg/L).
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Meanwhile, the grain size of the calcium arsenate crystal increases gradually along with the Cl- concentration, which is opposite to the trend of the arsenic-leachate
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concentration. The results indicate that a larger-size calcium arsenate crystal particle is more stable than a smaller one, and chloride ion can promote an increase in the size
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of the calcium arsenate crystals. When a chloride ion exists, the calcium arsenate crystallization reaction will move in the direction of Eq. 3 and generate Ca5(AsO4)3Cl
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under certain conditions. At the same time, the reaction of Eq. 2 will also be promoted, because Ca5(AsO4)3OH has similar structure and properties as compared to
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Ca5(AsO4)3Cl (Biagioni et al., 2017). The chloride ion potentially acts as a precursor in the formation of Ca5(AsO4)3OH and Ca5(AsO4)3Cl.
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Fig. 4. The XRD pattern (a), arsenic-leachate concentration and grain size (b) of Ca-As compounds obtained by crystallization reaction with different Clconcentrations.
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The diffraction peak intensity of Ca4(OH)2(AsO4)2·4H2O initially decreased and then
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increased when the SO42- concentration increased form 0.1 mol/L to 1.0 mol/L. In
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particular, the Ca4(OH)2(AsO4)2·4H2O diffraction peaks disappeared and were
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replaced by Ca5(AsO4)3OH and NaCaAsO4·3H2O (sodium calcium arsenate hydrate) under 0.5 mol/L SO42- concentration. The results indicate that sulfate addition mainly
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resulted in the formation of Ca4(OH)2(AsO4)2·4H2O (Eq. 5). However, Ca5(AsO4)3OH and NaCaAsO4·3H2O were generated when the SO42- concentration was near 0.5
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mol/L (Eqs. 3 and 6). Actually, the sulfate ion will react with calcium and produce CaSO4 (Eq. 7), which will also participate in the calcium-arsenate crystallization
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reaction and promote the formation of Ca4(OH)2(AsO4)2·4H2O (Eq. 8). Although calcium sulfate was produced and then take part in another reaction immediately, this
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process and behavior might be vital to the formation of Ca4(OH)2(AsO4)2·4H2O. 4𝐶𝑎(𝑂𝐻)2 + 2𝑁𝑎3 𝐴𝑠𝑂4 ∙ 12𝐻2𝑂 𝐶𝑎(𝑂𝐻)2 + 𝑁𝑎3 𝐴𝑠𝑂4 ∙ 12𝐻2𝑂 Ca( H)2 + 𝑁𝑎2 𝑂4
𝐶𝑎4 (𝑂𝐻)2 (𝐴𝑠𝑂4 )2 ∙ 4𝐻2 𝑂 + 6𝑁𝑎𝑂𝐻 + 20𝐻2 𝑂 NaCa𝐴𝑠𝑂4 ∙ 3𝐻2 𝑂 + 2𝑁𝑎𝑂𝐻 + 9𝐻2 𝑂
𝐶𝑎 𝑂4 + 2𝑁𝑎 H
3Ca( H)2 + 2𝑁𝑎3 𝐴𝑠𝑂4 ∙ 12𝐻2 𝑂 + 𝐶𝑎 𝑂4 𝑁𝑎2 𝑂4 + 20𝐻2 𝑂
(5) (6) (7)
𝐶𝑎4(𝑂𝐻)2 (𝐴𝑠𝑂4 )2 ∙ 4𝐻2 𝑂 + 4𝑁𝑎 H + (8)
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The effect of sulfate on grain size and arsenic stability is also shown in Fig. 5b. When the SO42- concentration is 0.1 mol/L, the arsenic-leachate concentration is 2.75 mg/L, which is lower than that of the control sample (28.26 mg/L) and meets the standard level (less than 5 mg/L). Thus, sulfate is beneficial in reducing the
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arsenic-leachate concentration from Ca-As precipitates. Along with an SO42-
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concentration increase from 0.1 mol/L to 1.0 mol/L, arsenic-leachate concentration
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initially increased to 10.17 mg/L and then decreased to 3.75 mg/L. Combined with
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XRD results, the generation of stable Ca4(OH)2(AsO4)2·4H2O crystals should be the main reason for the reduction in the arsenic-leachate concentration. The trend of grain
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size was opposite to that of arsenic-leachate concentration, further illustrating that larger Ca4(OH)2(AsO4)2·4H2O crystals are more stable. However, Ca5(AsO4)3OH and
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NaCaAsO4·3H2O were generated when the SO42- concentration was near 0.5 mol/L. Perhaps NaCaAsO4·3H2O is not very stable compared to Ca5(AsO4)3OH and
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Ca4(OH)2(AsO4)2·4H2O and that is the reason the arsenic-leachate concentration
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Fig. 5. The XRD pattern (a), arsenic-leachate concentration and grain size (b) of Ca-As compounds obtained by crystallization reaction with different SO42concentrations.
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were generated when the carbonate concentration is 0.1 mol/L. When CO32-
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concentration increased from 0.3 to 1.0 mol/L, the phase composition of the
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precipitates is different from one another. There are no calcium-arsenic compounds
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formed except Ca4(OH)2(AsO4)2·4H2O under a CO32- concentration less than 0.3 mol/L. With increasing carbonate concentrations, CaCO3 (calcite), Na3AsO4 (sodium
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arsenate), Na3AsO4·12H2O (sodium arsenate hydrate), and Na3Ca(CO3)2·5H2O (gaylussite) were generated successively. The results indicate that a low carbonate
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concentration would not be disadvantageous for arsenic-leachate concentrations but would promote the crystallization of Ca4(OH)2(AsO4)2·4H2O. However, a higher
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carbonate concentration might inhibit the reaction between AsO43- and Ca2+ and produce calcium-carbonate compounds.
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The effect of carbonate on grain size and arsenic stability under a CO32concentration from 0 mol/L to 1.0 mol/L is presented in Fig. 6b. It can be seen that the arsenic-leachate concentration initially decreased from 28.26 mg/L to 18.66 mg/L and then increased dramatically to 1412.50 mg/L after the CO32- concentration exceeded 0.5 mol/L. However, the lowest arsenic-leachate concentration of precipitates under the function of carbonate was still higher than the threshold value of the TCLP
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standard level. Because there are no Ca-As compound crystals generated under a high CO32- concentration, the grain size of precipitates is not valuable for arsenic-leachate concentration. Under a low concentration of carbonate, calcium arsenates can normally be generated mainly because of the excessive calcium, and the carbonate
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reacts with the excessive calcium ion first. Therefore, a low concentration of
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carbonate shows no obvious influence on the crystallization of Ca-As compounds.
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calcium ions and prevent the formation of calcium arsenates. The diffraction peak of
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Fig. 6. The XRD pattern (a), arsenic-leachate concentration and grain size (b) of Ca-As compounds obtained by crystallization reactions with different CO32concentrations.
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3.4. Effect of anions on morphological features Morphological features play an important role in the structure and stability of Ca-As compounds (Zhu et al., 2006). The morphology of the precipitates identified by XRD was examined systematically using SEM-EDS and the results are shown in Fig.
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7 and Fig. 8. The particle size distribution of these samples is provided in Fig. 9. As
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observed in Fig. 7, the anions have a significant influence on the crystal structure of
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the precipitates. Fig. 7a–d is the SEM micrograph of the Ca-As compounds under a
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different Cl- concentration treatment. It can be seen from the pictures that the particles of the Ca-As compounds become more compact and smooth with increasing Cl-
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concentration. The particle size distribution curve in Fig. 9a shows that these precipitates share nearly the same particle size distribution and median diameter (D50).
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As shown in Fig. 9d, the median diameters are 16.8 μm, 18.7 μm and 18.8 μm, which are close to that of the control sample (15.5 μm). Compared to the SEM image of the
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control sample in a large agglomerated particle (Fig. 2a), Fig. 7a is mainly for capturing the small amorphous particles. The EDS results from Fig. 8a and Fig. 2b
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show that the chemical composition of particle A as shown in Fig. 7a is different from that of the control sample. The content of Ca and As shown in Fig. 7a is 29.0% and 37.2%, while that shown in Fig. 2a is 33.8% and 21.0%, respectively. The Ca/As ratio of particle A is much lower than that of the control sample. Combined with the results of XRD, the amorphous particles in Fig. 7a might be CaHAsO4·2H2O, while the agglomerated particle in Fig. 2a might be Ca5(AsO4)3OH. Fig. 7d shows rod-shaped
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chloride promotes the formation and growth of calcium arsenate apatite crystals, the
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structure of which is more stable than the amorphous calcium arsenates (Biagioni et
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SEM microphotographs of the samples affected by different SO42- concentrations
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greater than 0.8 mol/L. The EDS spectra (Fig. 8d and f) show that the micro-area elements at point D and point F in the strip crystals are Ca, Na, O and As. Combined
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with the XRD results, these crystals are Ca4(OH)2(AsO4)2·4H2O. However, the microstructure is not in an obvious crystal-shape at an SO42- concentration of 0.5
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mol/L (Fig. 7f). Thus, the calcium arsenate crystals are rarely formed in the crystallization system with a 0.5-mol/L sulfate addition. Combined with the XRD pattern (Fig. 5a) and EDS diagram (Fig. 8e), the poorly crystallized particles are probably NaCaAsO4·3H2O, thus leading to more arsenic leaching out during the TCLP test, which corresponds with the results of other studies (Yoon et al., 2010). The particle size distribution shown in Fig. 9b also demonstrates that the main
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distribution areas of particles under the 0.5-mol/L sulfate treatment are smaller than the particles under the 0.1-mol/L or 1.0-mol/L sulfate treatments. The results indicate that low or high sulfate will lead to calcium arsenate crystal form transformation and densification, which will increase its crystallinity and stability.
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The SEM micrographs of samples of different CO32- concentration treatments are
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presented in Fig. 7i–m. Fig. 7i shows that a longer and larger strip crystal with a
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length of from 10–15 μm was observed when the CO32- concentration was 0.1 mol/L.
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The EDS spectra (Fig. 8g) combined with the XRD patterns (Fig. 6a) illustrate that the strip crystal is Ca4(OH)2(AsO4)2·4H2O. Fig. 9c shows that the particle size
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distribution of this Ca-As compound is relatively larger than that of the others, having a larger median diameter of 26.5 μm. However, the strip crystal became incomplete
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and released amorphous particles gradually along with the increasing CO32concentration (Fig. 7j and 7k), causing the size of the particles to decrease. This may
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be a result of the Ca2+ and Na+ reacting with carbonate and generating CaCO3 or Na3Ca(CO3)2·5H2O, which would lead to arsenic emission to aqueous solution (Zhu
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et al., 2006). When CO32- the concentration is 0.8 mol/L, the SEM image shows particles tend to aggregate in a flocculent structure (Fig. 7k). The EDS spectra (Fig. 8h) shows the microregion elemental distribution contains Ca, O, Na and As. Combined with XRD patterns, this finding indicates that the flocculent structure is Na3AsO4·12H2O within in a matrix of calcium hydroxide. A shown in Fig. 7l, bulk crystals were observed when the CO32- concentration reached 1.0 mol/L. The EDS
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spectra (Fig. 8i) showed the existence of Ca, Na and C, illustrating that the bulk crystal
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Na3Ca(CO3)2·5H2O.
Therefore,
during
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treatment
of
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arsenic-containing wastes with high carbonate ion concentration, such as high alkaline arsenic-bearing sludges in antimony smelting, the formation of calcium arsenate
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crystals may be affected. Excessive carbonate should be avoided as much as possible
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in the treatment of arsenic-containing wastewater or arsenic-bearing sludges based on
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calcium arsenate crystallization.
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Fig. 7. SEM images of Ca-As compounds with different anion concentrations.
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Fig. 8. EDS plots of Ca-As compounds with different anion concentrations.
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Fig. 9. Particle size distribution of precipitates obtained by crystallization reaction with different anions: (a) chloride, (b) sulfate and (c) carbonate, and the variation in the cumulative size distribution: (d) chloride, (e) sulfate and (f) carbonate.
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Typical bright-field TEM and high-resolution transmission electron microscopy (HRTEM) were employed to investigate the internal structure of the as-prepared Ca-As compounds. As shown in Fig. 10a and 10d, it can be seen that the particle size is at a nanometre scale, and the crystals are randomly distributed under a 1.0-mol/L
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Cl- concentration influence. Combined with the XRD and SEM results, these crystals
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might be Ca5(AsO4)3OH and Ca5(AsO4)3Cl. The TEM micrographs and the HRTEM
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images of Ca4(OH)2(AsO4)2·4H2O affected by 1.0 mol/L SO42- and 0.1 mol/L CO32-
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are shown in Fig. 10b and 10e and Fig. 10c and 10f, respectively. It can be seen that the μm-scale particle is Ca4(OH)2(AsO4)2·4H2O, corresponding with the strip crystal
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shown in the previously mentioned SEM micrograph (Fig. 7h and 7i). The HRTEM images show that the diffraction fringes are consistent with lattice fringes, illustrating
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the crystalline grains are nanoparticles. The results also indicate that the internal structure of the Ca4(OH)2(AsO4)2·4H2O crystals is affected by different anions, and
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the structure is quite distinguishable.
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Fig. 10. TEM and HR-TEM images of Ca-As compounds. (a)TEM and (d) HR-TEM images of calcium arsenate apatite with 1.0-mol/L Cl- addition, (b)TEM and (e) HR-TEM images of Ca4(OH)2(AsO4)2·4H2O with 1.0-mol/L SO42- addition, (c)TEM and (f) HR-TEM images of Ca4(OH)2(AsO4)2·4H2O with 0.1-mol/L CO32- addition.
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4. Conclusion The calcium arsenate crystallization reaction can be affected by common anions, such as chloride, sulfate and carbonate. Chloride and sulfate show an insignificant effect on the content of Ca-As compounds in precipitates, while carbonate results in a
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decrease in Ca-As compounds. The anions have an effect on the arsenic-leachate
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concentration which is related to the grain size of the calcium arsenate crystals.
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Chloride can contribute to the crystallization reaction and promote the formation and
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growth of arsenic apatite, which can effectively reduce the arsenic-leachate concentration of the precipitates. When the Cl- concentration increased from 0 mol/L
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to 1.0 mol/L, the arsenic-leachate concentration decreased from 28.26 mg/L to 0.08 mg/L. The chloride ion potentially acts as a precursor in the formation and growth of
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Ca5(AsO4)3OH and Ca5(AsO4)3Cl crystals, resulting in the arsenic-leachate concentration gradually decreasing. Sulfate is beneficial in reducing the
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arsenic-leachate concentration from Ca-As precipitates, because it is propitious to improve the formation and growth of Ca4(OH)2(AsO4)2·4H2O. Low or high sulfate
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will lead to calcium arsenate crystal form transformation and densification, which will increase its crystallinity and stability. A low carbonate concentration does not have a detrimental effect on the arsenic-leachate concentration in the short term. In contrast, a small amount of carbonate is beneficial for the formation and growth of Ca4(OH)2(AsO4)2·4H2O. However, more carbonate would capture Ca2+ and Na+ to generate CaCO3 and Na3Ca(CO3)2·5H2O instead of Ca-As compounds, which would
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prevent the calcium arsenate crystallization reaction. The trend of grain size being in opposition to that of the arsenic-leachate concentration illustrates that the larger crystal particles of Ca-As compounds are more stable than the smaller ones. Thus, promoting grain size growth of stable calcium
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arsenates by controlling the anion concentration to reduce the arsenic-leachate
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concentration is a proposed method for industrial applications.
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Acknowledgements
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This work was supported by the Special Program on Environmental Protection for Public Welfare (201509050), the National Natural Science Foundation of China
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(51704337, 51474247, 51634010), the National Key R&D Program of China (2017YFC0210402), and the Co-Innovation Center for Clean and Efficient Utilization
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of Strategic Metal Mineral Resources for financial support.
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Highlights
The anions, including chloride, sulfate and carbonate, have an effect on the calcium arsenate crystallization reaction.
Chloride and sulfate contribute to the crystallization reaction while carbonate would prevent the formation of calcium arsenate. The arsenic leachate concentration of Ca-As compounds is related to the grain
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size of calcium arsenate crystals.
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The larger crystal particles of calcium arsenate are more stable than the
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smaller one.
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Jie Lei, Bing Peng, Yan-Jie Liang, Xiao-Bo Min, Li-Yuan Chai, Yong Ke, Yang You PII: DOI: Reference:
S0304-386X(17)30753-3 doi:10.1016/j.hydromet.2018.03.007 HYDROM 4771
To appear in:
Hydrometallurgy
Received date: Revised date: Accepted date:
13 September 2017 31 January 2018 6 March 2018
Please cite this article as: Jie Lei, Bing Peng, Yan-Jie Liang, Xiao-Bo Min, Li-Yuan Chai, Yong Ke, Yang You , Effects of anions on calcium arsenate crystalline structure and arsenic stability. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Hydrom(2017), doi:10.1016/ j.hydromet.2018.03.007
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ACCEPTED MANUSCRIPT Effects of anions on calcium arsenate crystalline structure and arsenic stability Jie Leia, Bing Penga, b, Yan-Jie Lianga,b,c,*, Xiao-Bo Mina,b,*, Li-Yuan Chaia,b, Yong Kea,b, Yang Youa
Institute of Environmental Science and Engineering, School of Metallurgy and
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Chinese National Engineering Research Center for Control & Treatment of Heavy
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b
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Environment, Central South University, Changsha, Hunan 410083, China
c
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Metal Pollution, Changsha, Hunan 410083, China
Xiang Guang Copper Co., Ltd., Yang gu, Shandong, 252327, China
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*Corresponding authors at: Institute of Environmental Science & Engineering, School of
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Metallurgy and Environment, Central South University, Changsha, Hunan 410083, China.
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E-mail addresses: [email protected] (Y.-J. Liang), [email protected] (X.-B. Min).
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Abstract Based on the crystallization reaction between calcium and arsenate, three anions, namely, chloride (Cl-), sulfate (SO42-), and carbonate (CO32-), were introduced during the formation of calcium arsenate in this study. The effects of these anions on arsenic
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stability and calcium arsenate crystalline properties were characterized using TCLP,
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XRF, XRD, SEM-EDS, TEM and particle size distribution (PSD). The results show
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that these anions significantly affect the calcium arsenate crystalline structure and
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arsenic-leachate concentration. The effect of chloride on arsenic stabilization was mainly a result of the formation and growth of calcium arsenate apatite
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(Ca5(AsO4)3OH and Ca5(AsO4)3Cl) crystal. Sulfate is propitious to improve the formation of Ca4(OH)2(AsO4)2·4H2O crystals and enhance their stability effectively
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under corresponding sulfate concentration treatments. A small amount of carbonate is beneficial to the formation and growth of Ca4(OH)2(AsO4)2·4H2O crystals. However,
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when the CO32- concentration was higher than 0.5 mol/L, Ca-As compounds were not precipitated. Instead, CaCO3 and Na3Ca(CO3)2·5H2O were generated successively,
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inhibiting calcium-arsenate crystallization and thus releasing more arsenic into aqueous solution. The trends of grain size are opposite to that of arsenic-leachate concentration, indicating that larger calcium arsenate crystals are more stable in the TCLP test. The internal structure of different Ca-As compounds can be changed and transformed by adding the three anions within a certain range. In summary, the results
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of this paper provide some meaningful information for the treatment of arsenic-containing wastewater or arsenic-bearing sludges using lime precipitation. Keywords: High arsenic-bearing sludge; Calcium arsenate crystallization; Anions,
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Arsenic-leachate concentration; Grain size; Lime precipitation.
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1. Introduction Arsenic is one of the most toxic metalloid found in industrial wastewater which has caused increasing public concern (Wang et al. 2011; Wu et al. 2016). Lime precipitation is used widely in arsenic-bearing wastewater treatment because of its
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low cost and simple process (Chen et al. 2015; Li et al., 2011). However, this process
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generates a large amount of arsenic sludge, which is classified as hazardous waste due
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to its high arsenic content (Cui et al., 2014; Fei et al., 2017; Min et al., 2017; Morales
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et al., 2010). Various types of technologies, such as cementation (Choi et al., 2009; Li et al., 2017; Randall, 2012), polymeric encapsulation (Shaw et al., 2008),
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hydrothermal precipitation (Vinals et al., 2010; Peng et al., 2017), mineralization (Chai et al., 2017), co-treatment solidification (Li et al., 2016; Liu et al., 2017),
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mechanical milling (Liang et al., 2017) and vitrification (Zhao et al., 2017), have been developed to reduce the mobility of arsenic and other heavy metals in the sludge.
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Calcium arsenate crystallization is one of the most common methods for reducing arsenic-leachate concentration from the arsenic-bearing precipitates (Donahue and
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Hendry, 2003). Arsenic-leaching behavior is mainly affected by the solubility and phase-transfer of the arsenic compounds (Ke et al., 2017; Ozverdl and Erdem, 2010; Xie et al., 2013). Many researchers have shown that lime addition reduces As mobility in contaminated slurries due to the formation of low-solubility Ca-As precipitates such as Ca4(OH)2(AsO4)2·4H2O and Ca5(AsO4)3OH (Bothe and Brown, 1999; Camacho et al., 2009; Lei et al., 2017).
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It is well known that arsenic-containing wastewater often contains multiple anions such as Cl-, SO42-, and CO32-. It has been proven that these anions have a significant influence on arsenic precipitation (Meng et al., 2002; Rivas et al., 2012; Sun et al., 2017; Welham et al., 2000). There are studies that show chloride can enhance arsenic
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removal through the nano-zero-valent iron process (Tanboonchuy et al., 2012). Other
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studies have found that sulfate has different effects on arsenate removal under
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different concentrations (Sun et al., 2006, Zhang et al., 2008). Calcium arsenate
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precipitate formed as a result of lime addition is not stable, because it can decompose to calcium carbonate and release arsenic under an attack of atmospheric CO2 which is
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easily converted to carbonate (Luo et al., 2010). However, CO2 combined with ferrous is used to evaluate arsenic immobilization in red mud, and the aqueous arsenic
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concentration effectively decreased (Li et al, 2012). Consequently, these anions may play an important role in the removal and immobilization of arsenic and other heavy
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metals.
However, the influence of these anions on arsenic immobilization has not been
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systematically studied. In fact, the properties and stability of crystals are susceptible to anions during the process of crystallization (Crundwell, 2016). Previous studies have demonstrated that sulfate ions can distort calcite structure when incorporated into the calcite crystal lattice (Kontrec et al., 2004). Merely changing the anion types and dosages can also affect the rate of formation and crystalline structure of the synthetic compounds (Petrik et al., 2009). Hence, the objective of this study was to
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arsenic-containing wastewater based on lime precipitation.
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2. Materials and methods
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process of arsenic sludge treatment based on calcium arsenate crystallization or high
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2.1. Materials
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Calcium hydroxide (Ca(OH)2), nitric acid (HNO3), sulfuric acid (H2SO4), glacial acetic acid (CH3COOH), sodium arsenate (Na3AsO4·12H2O) over 99%, sodium
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chloride (NaCl), sodium sulfate (Na2SO4) and sodium carbonate (Na2CO3) were purchased from the Sinopharm Chemical Reagent Company Limited in China. All the
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reagents were of analytical grade, and all the dilutions were made using high-purity de-ionized water (18 MΩ cm-1 resistivity) obtained from a mole analytical ultrapure
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water system (Molecular, Molresearch 1010A, China). The bottles and glassware used for storing samples were washed in 10% (v/v) HNO3 for 24 h and rinsed with
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ultrapure water to avoid the effects of impurities. 2.2. Experimental procedure A schematic diagram of the experimental set up is shown in Fig. 1. The crystallization experiments were performed in 500-mL flasks containing a mixture of Ca(OH)2 and Na3AsO4·12H2O in a 2.0:1 Ca/As molar ratio and 4.0:1 liquid/solid mass ratio under stirring with a glass rod for 2 min. The initial pH value of the
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mixture was detected using a pH meter (Mettler-Toledo, FE20). The mixture was reacted on a magnetic stirring apparatus at 25°C for 5 h. The precipitates were separated using a vacuum filter, washed with de-ionized water three times, oven-dried
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at 60°C for 24 h, and then subjected to characterization and a leaching toxicity test.
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Fig. 1. Schematic diagram of the experimental procedure.
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2.3. Characterization A leaching toxicity test was conducted to evaluate the arsenic-leaching behavior of calcium-arsenic compounds according to the toxicity characteristic leaching procedure (TCLP, USEPA SW-846, Method 1311) (USEPA, 1992), which is the
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current USEPA protocol for determining whether waste is hazardous. The chemical
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composition of precipitates was detected using an X-ray fluorescence spectrometer
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(XRF, Shimadzu-1800, operated at 40 kV, 95 mA). The phase and crystallinity of all
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samples were characterized using X-ray diffraction (XRD, Rigaku D/MAX 2550 VPC diffractometer, 40 kV, 20 mA). The average grain size of the crystals was calculated
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using the Scherrer equation as follows (Min et al., 2013): (1)
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peak; and θ is the diffraction peak angle. In addition, scanning electronic microscope coupled by energy dispersive
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3. Results and discussion 3.1 Characterization of the precipitate from the control experiment The major elements of the precipitate obtained at a Ca/As molar ratio of 2.0:1, a liquid/solid mass ratio 4.0:1 and a temperature of 25°C for 5 h are shown in Table 1:
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Ca 37.9%, O 32.5%, As 23.7 and Na 5.75. The characterization of the control sample
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is presented in Fig. 2. As shown in Fig. 2a and 2b, the general particles are in irregular
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shapes, and no obvious crystal structure is presented. The amorphous particles
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agglomerate into large particles. EDS results show the control sample is mainly composed of Ca and As, indicating that the amorphous particles are Ca-As
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compounds. Fig. 2c shows a wide range of particle size distribution from 0.2 μm to 70 μm. The size of the particles is distributed in three concentrated areas approximately
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0.54 μm, 4.66 μm and 19.3 μm. The median particle size (D50) of 15.5 μm reflects indirectly that most small particles are agglomerated into large particles. Fig. 2d
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shows that the characteristic diffraction peaks of Ca(OH)2 (portlandite), CaHAsO4·2H2O (pharmacolite) and Ca5(AsO4)3OH (johnbaumite) are observed in the
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control sample. A leaching test was performed using the TCLP method, and the results indicated that the arsenic-leachate concentration was 28.26 mg/L, which is greater than the regulation limit of 5 mg/L for As.
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Table 1. Chemical compositions of control sample obtained using XRF (mass fraction, %) Element Ca O As Na Percentage 37.9 32.5 23.7 5.75
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Fig. 2. Characterization of the control sample: (a) SEM, (b) EDS, (c) Particle size distribution, and (d) X-ray diffraction pattern.
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3.2. Chemical composition of precipitates The elemental composition of precipitates, obtained by crystallization reaction with different anions, was determined using XRF, and the results are shown in Fig. 3. It shows that there are no obvious changes in chemical composition in the Cl- and SO42-
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samples, while CO32- caused a significant change. As shown in Fig. 3a and Fig. 3b,
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the percentages of arsenic presented in the residuals are approximately 24% and 23%,
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respectively. With an increase in anion concentration, the content of Cl and S
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increased slightly, indicating that a portion of Cl- and SO42- entered into precipitates during the calcium-arsenate crystallization process. However, as shown in Fig. 3 c,
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with the increasing CO32- concentration from 0.1 mol/L to 1.0 mol/L, the content of As in the precipitates decreased from 23.7% to 11.7%, and the content of Ca
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decreased from 34.6% to 22.2%. At the same time, the content of Na and O increased relatively. The results indicate that Cl- and SO42- can maintain As in residual and show
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an insignificant effect on the content of Ca-As compounds in precipitates. However, the increase in CO32- hinders the formation of Ca-As compounds and leads to arsenic
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release into solution.
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Fig. 3. Elemental composition of precipitates obtained by crystallization reaction with different anions: (a) chloride, (b) sulfate and (c) carbonate.
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3.3. Effect of anions on Ca-As compound crystalline structure and arsenic stability 3.3.1. Effects of Cl- concentration To investigate the influence of anions on the phase composition and crystal
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structure of the precipitates, a crystallization reaction was performed at 25°C for 5 h
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with a Ca/As molar ratio of 2.0, maintaining the anion concentration within the range
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of 0.1–1.0 mol/L. The XRD patterns and grain sizes of the samples synthesized at
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different NaCl concentrations are presented in Fig. 4. Fig. 4a shows that the characteristic diffraction peaks of Ca(OH)2, CaHAsO4·2H2O and Ca5(AsO4)3OH are
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observed when the Cl- concentration is 0.1 mol/L. This is similar to that of the control sample. The main reactions are shown in from Eq. 2 to Eq. 4. However, when the Cl-
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concentration increases from 0.3 mol/L to 1.0 mol/L, CaHAsO4·2H2O diffraction peaks disappear, while Ca5(AsO4)3Cl (turneaureite) diffraction peaks become stronger,
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validating the formation and growth of stable calcium arsenate apatite crystals (including johnbaumite and turneaureite). The grain size was calculated using
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Scherrer’s equation and the results are shown in Fig. 4b. It can be seen that the grain size increased gradually with the increase in Cl- concentration. The results indicate that a low concentration of chloride shows no obvious influence on the crystallization reaction, but a high concentration of chloride can accelerate the formation and growth of calcium arsenate apatite in the form of Ca5(AsO4)3OH and Ca5(AsO4)3Cl. 𝐶𝑎(𝑂𝐻)2 + 𝑁𝑎3 𝐴𝑠𝑂4 ∙ 12𝐻2𝑂
CaH𝐴𝑠𝑂4 ∙ 2𝐻2 𝑂 + 3𝑁𝑎𝑂𝐻 + 9𝐻2 𝑂
(2)
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𝐶𝑎5 (𝐴𝑠𝑂4 )3 𝑂𝐻 + 9𝑁𝑎𝑂𝐻 + 12𝐻2 𝑂
5𝐶𝑎(𝑂𝐻)2 + 3𝑁𝑎3 𝐴𝑠𝑂4 ∙ 12𝐻2𝑂 + 𝑁𝑎𝐶𝑙
𝐶𝑎5(𝐴𝑠𝑂4 )3 𝐶𝑙 + 10𝑁𝑎𝑂𝐻 + 12𝐻2 𝑂
(3) (4)
Fig. 4b shows the effect of Cl- concentration on arsenic-leachate concentration using the TCLP test. It can be seen that the arsenic-leachate concentration decreased
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from 28.26 mg/L to 0.08 mg/L gradually as the Cl- concentration increased from 0
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mol/L to 1.0 mol/L. When Cl- concentration increased to 0.8 mol/L, the
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arsenic-leachate concentration reached the regulatory level (less than 5 mg/L).
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Meanwhile, the grain size of the calcium arsenate crystal increases gradually along with the Cl- concentration, which is opposite to the trend of the arsenic-leachate
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concentration. The results indicate that a larger-size calcium arsenate crystal particle is more stable than a smaller one, and chloride ion can promote an increase in the size
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of the calcium arsenate crystals. When a chloride ion exists, the calcium arsenate crystallization reaction will move in the direction of Eq. 3 and generate Ca5(AsO4)3Cl
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under certain conditions. At the same time, the reaction of Eq. 2 will also be promoted, because Ca5(AsO4)3OH has similar structure and properties as compared to
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Ca5(AsO4)3Cl (Biagioni et al., 2017). The chloride ion potentially acts as a precursor in the formation of Ca5(AsO4)3OH and Ca5(AsO4)3Cl.
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Fig. 4. The XRD pattern (a), arsenic-leachate concentration and grain size (b) of Ca-As compounds obtained by crystallization reaction with different Clconcentrations.
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The diffraction peak intensity of Ca4(OH)2(AsO4)2·4H2O initially decreased and then
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increased when the SO42- concentration increased form 0.1 mol/L to 1.0 mol/L. In
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particular, the Ca4(OH)2(AsO4)2·4H2O diffraction peaks disappeared and were
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replaced by Ca5(AsO4)3OH and NaCaAsO4·3H2O (sodium calcium arsenate hydrate) under 0.5 mol/L SO42- concentration. The results indicate that sulfate addition mainly
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resulted in the formation of Ca4(OH)2(AsO4)2·4H2O (Eq. 5). However, Ca5(AsO4)3OH and NaCaAsO4·3H2O were generated when the SO42- concentration was near 0.5
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mol/L (Eqs. 3 and 6). Actually, the sulfate ion will react with calcium and produce CaSO4 (Eq. 7), which will also participate in the calcium-arsenate crystallization
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reaction and promote the formation of Ca4(OH)2(AsO4)2·4H2O (Eq. 8). Although calcium sulfate was produced and then take part in another reaction immediately, this
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process and behavior might be vital to the formation of Ca4(OH)2(AsO4)2·4H2O. 4𝐶𝑎(𝑂𝐻)2 + 2𝑁𝑎3 𝐴𝑠𝑂4 ∙ 12𝐻2𝑂 𝐶𝑎(𝑂𝐻)2 + 𝑁𝑎3 𝐴𝑠𝑂4 ∙ 12𝐻2𝑂 Ca( H)2 + 𝑁𝑎2 𝑂4
𝐶𝑎4 (𝑂𝐻)2 (𝐴𝑠𝑂4 )2 ∙ 4𝐻2 𝑂 + 6𝑁𝑎𝑂𝐻 + 20𝐻2 𝑂 NaCa𝐴𝑠𝑂4 ∙ 3𝐻2 𝑂 + 2𝑁𝑎𝑂𝐻 + 9𝐻2 𝑂
𝐶𝑎 𝑂4 + 2𝑁𝑎 H
3Ca( H)2 + 2𝑁𝑎3 𝐴𝑠𝑂4 ∙ 12𝐻2 𝑂 + 𝐶𝑎 𝑂4 𝑁𝑎2 𝑂4 + 20𝐻2 𝑂
(5) (6) (7)
𝐶𝑎4(𝑂𝐻)2 (𝐴𝑠𝑂4 )2 ∙ 4𝐻2 𝑂 + 4𝑁𝑎 H + (8)
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The effect of sulfate on grain size and arsenic stability is also shown in Fig. 5b. When the SO42- concentration is 0.1 mol/L, the arsenic-leachate concentration is 2.75 mg/L, which is lower than that of the control sample (28.26 mg/L) and meets the standard level (less than 5 mg/L). Thus, sulfate is beneficial in reducing the
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arsenic-leachate concentration from Ca-As precipitates. Along with an SO42-
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concentration increase from 0.1 mol/L to 1.0 mol/L, arsenic-leachate concentration
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initially increased to 10.17 mg/L and then decreased to 3.75 mg/L. Combined with
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XRD results, the generation of stable Ca4(OH)2(AsO4)2·4H2O crystals should be the main reason for the reduction in the arsenic-leachate concentration. The trend of grain
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size was opposite to that of arsenic-leachate concentration, further illustrating that larger Ca4(OH)2(AsO4)2·4H2O crystals are more stable. However, Ca5(AsO4)3OH and
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NaCaAsO4·3H2O were generated when the SO42- concentration was near 0.5 mol/L. Perhaps NaCaAsO4·3H2O is not very stable compared to Ca5(AsO4)3OH and
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Ca4(OH)2(AsO4)2·4H2O and that is the reason the arsenic-leachate concentration
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Fig. 5. The XRD pattern (a), arsenic-leachate concentration and grain size (b) of Ca-As compounds obtained by crystallization reaction with different SO42concentrations.
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were generated when the carbonate concentration is 0.1 mol/L. When CO32-
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concentration increased from 0.3 to 1.0 mol/L, the phase composition of the
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precipitates is different from one another. There are no calcium-arsenic compounds
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formed except Ca4(OH)2(AsO4)2·4H2O under a CO32- concentration less than 0.3 mol/L. With increasing carbonate concentrations, CaCO3 (calcite), Na3AsO4 (sodium
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arsenate), Na3AsO4·12H2O (sodium arsenate hydrate), and Na3Ca(CO3)2·5H2O (gaylussite) were generated successively. The results indicate that a low carbonate
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concentration would not be disadvantageous for arsenic-leachate concentrations but would promote the crystallization of Ca4(OH)2(AsO4)2·4H2O. However, a higher
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carbonate concentration might inhibit the reaction between AsO43- and Ca2+ and produce calcium-carbonate compounds.
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The effect of carbonate on grain size and arsenic stability under a CO32concentration from 0 mol/L to 1.0 mol/L is presented in Fig. 6b. It can be seen that the arsenic-leachate concentration initially decreased from 28.26 mg/L to 18.66 mg/L and then increased dramatically to 1412.50 mg/L after the CO32- concentration exceeded 0.5 mol/L. However, the lowest arsenic-leachate concentration of precipitates under the function of carbonate was still higher than the threshold value of the TCLP
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standard level. Because there are no Ca-As compound crystals generated under a high CO32- concentration, the grain size of precipitates is not valuable for arsenic-leachate concentration. Under a low concentration of carbonate, calcium arsenates can normally be generated mainly because of the excessive calcium, and the carbonate
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reacts with the excessive calcium ion first. Therefore, a low concentration of
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carbonate shows no obvious influence on the crystallization of Ca-As compounds.
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However, when the carbonate concentration continues to increase, it will compete for
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calcium ions and prevent the formation of calcium arsenates. The diffraction peak of
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Fig. 6. The XRD pattern (a), arsenic-leachate concentration and grain size (b) of Ca-As compounds obtained by crystallization reactions with different CO32concentrations.
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3.4. Effect of anions on morphological features Morphological features play an important role in the structure and stability of Ca-As compounds (Zhu et al., 2006). The morphology of the precipitates identified by XRD was examined systematically using SEM-EDS and the results are shown in Fig.
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7 and Fig. 8. The particle size distribution of these samples is provided in Fig. 9. As
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observed in Fig. 7, the anions have a significant influence on the crystal structure of
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the precipitates. Fig. 7a–d is the SEM micrograph of the Ca-As compounds under a
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different Cl- concentration treatment. It can be seen from the pictures that the particles of the Ca-As compounds become more compact and smooth with increasing Cl-
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concentration. The particle size distribution curve in Fig. 9a shows that these precipitates share nearly the same particle size distribution and median diameter (D50).
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As shown in Fig. 9d, the median diameters are 16.8 μm, 18.7 μm and 18.8 μm, which are close to that of the control sample (15.5 μm). Compared to the SEM image of the
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control sample in a large agglomerated particle (Fig. 2a), Fig. 7a is mainly for capturing the small amorphous particles. The EDS results from Fig. 8a and Fig. 2b
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show that the chemical composition of particle A as shown in Fig. 7a is different from that of the control sample. The content of Ca and As shown in Fig. 7a is 29.0% and 37.2%, while that shown in Fig. 2a is 33.8% and 21.0%, respectively. The Ca/As ratio of particle A is much lower than that of the control sample. Combined with the results of XRD, the amorphous particles in Fig. 7a might be CaHAsO4·2H2O, while the agglomerated particle in Fig. 2a might be Ca5(AsO4)3OH. Fig. 7d shows rod-shaped
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chloride promotes the formation and growth of calcium arsenate apatite crystals, the
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structure of which is more stable than the amorphous calcium arsenates (Biagioni et
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SEM microphotographs of the samples affected by different SO42- concentrations
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are shown in Fig. 7e–h. Based on these microphotographs, it can be seen that strip crystals 3–5 μm in length were observed under the SO42- at less than 0.3 mol/L or
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greater than 0.8 mol/L. The EDS spectra (Fig. 8d and f) show that the micro-area elements at point D and point F in the strip crystals are Ca, Na, O and As. Combined
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with the XRD results, these crystals are Ca4(OH)2(AsO4)2·4H2O. However, the microstructure is not in an obvious crystal-shape at an SO42- concentration of 0.5
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mol/L (Fig. 7f). Thus, the calcium arsenate crystals are rarely formed in the crystallization system with a 0.5-mol/L sulfate addition. Combined with the XRD pattern (Fig. 5a) and EDS diagram (Fig. 8e), the poorly crystallized particles are probably NaCaAsO4·3H2O, thus leading to more arsenic leaching out during the TCLP test, which corresponds with the results of other studies (Yoon et al., 2010). The particle size distribution shown in Fig. 9b also demonstrates that the main
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distribution areas of particles under the 0.5-mol/L sulfate treatment are smaller than the particles under the 0.1-mol/L or 1.0-mol/L sulfate treatments. The results indicate that low or high sulfate will lead to calcium arsenate crystal form transformation and densification, which will increase its crystallinity and stability.
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The SEM micrographs of samples of different CO32- concentration treatments are
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presented in Fig. 7i–m. Fig. 7i shows that a longer and larger strip crystal with a
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length of from 10–15 μm was observed when the CO32- concentration was 0.1 mol/L.
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The EDS spectra (Fig. 8g) combined with the XRD patterns (Fig. 6a) illustrate that the strip crystal is Ca4(OH)2(AsO4)2·4H2O. Fig. 9c shows that the particle size
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distribution of this Ca-As compound is relatively larger than that of the others, having a larger median diameter of 26.5 μm. However, the strip crystal became incomplete
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and released amorphous particles gradually along with the increasing CO32concentration (Fig. 7j and 7k), causing the size of the particles to decrease. This may
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be a result of the Ca2+ and Na+ reacting with carbonate and generating CaCO3 or Na3Ca(CO3)2·5H2O, which would lead to arsenic emission to aqueous solution (Zhu
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et al., 2006). When CO32- the concentration is 0.8 mol/L, the SEM image shows particles tend to aggregate in a flocculent structure (Fig. 7k). The EDS spectra (Fig. 8h) shows the microregion elemental distribution contains Ca, O, Na and As. Combined with XRD patterns, this finding indicates that the flocculent structure is Na3AsO4·12H2O within in a matrix of calcium hydroxide. A shown in Fig. 7l, bulk crystals were observed when the CO32- concentration reached 1.0 mol/L. The EDS
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spectra (Fig. 8i) showed the existence of Ca, Na and C, illustrating that the bulk crystal
is
Na3Ca(CO3)2·5H2O.
Therefore,
during
the
treatment
of
some
arsenic-containing wastes with high carbonate ion concentration, such as high alkaline arsenic-bearing sludges in antimony smelting, the formation of calcium arsenate
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crystals may be affected. Excessive carbonate should be avoided as much as possible
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in the treatment of arsenic-containing wastewater or arsenic-bearing sludges based on
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calcium arsenate crystallization.
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Fig. 7. SEM images of Ca-As compounds with different anion concentrations.
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Fig. 8. EDS plots of Ca-As compounds with different anion concentrations.
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Fig. 9. Particle size distribution of precipitates obtained by crystallization reaction with different anions: (a) chloride, (b) sulfate and (c) carbonate, and the variation in the cumulative size distribution: (d) chloride, (e) sulfate and (f) carbonate.
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Typical bright-field TEM and high-resolution transmission electron microscopy (HRTEM) were employed to investigate the internal structure of the as-prepared Ca-As compounds. As shown in Fig. 10a and 10d, it can be seen that the particle size is at a nanometre scale, and the crystals are randomly distributed under a 1.0-mol/L
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Cl- concentration influence. Combined with the XRD and SEM results, these crystals
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might be Ca5(AsO4)3OH and Ca5(AsO4)3Cl. The TEM micrographs and the HRTEM
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images of Ca4(OH)2(AsO4)2·4H2O affected by 1.0 mol/L SO42- and 0.1 mol/L CO32-
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are shown in Fig. 10b and 10e and Fig. 10c and 10f, respectively. It can be seen that the μm-scale particle is Ca4(OH)2(AsO4)2·4H2O, corresponding with the strip crystal
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shown in the previously mentioned SEM micrograph (Fig. 7h and 7i). The HRTEM images show that the diffraction fringes are consistent with lattice fringes, illustrating
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the crystalline grains are nanoparticles. The results also indicate that the internal structure of the Ca4(OH)2(AsO4)2·4H2O crystals is affected by different anions, and
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Fig. 10. TEM and HR-TEM images of Ca-As compounds. (a)TEM and (d) HR-TEM images of calcium arsenate apatite with 1.0-mol/L Cl- addition, (b)TEM and (e) HR-TEM images of Ca4(OH)2(AsO4)2·4H2O with 1.0-mol/L SO42- addition, (c)TEM and (f) HR-TEM images of Ca4(OH)2(AsO4)2·4H2O with 0.1-mol/L CO32- addition.
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4. Conclusion The calcium arsenate crystallization reaction can be affected by common anions, such as chloride, sulfate and carbonate. Chloride and sulfate show an insignificant effect on the content of Ca-As compounds in precipitates, while carbonate results in a
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decrease in Ca-As compounds. The anions have an effect on the arsenic-leachate
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concentration which is related to the grain size of the calcium arsenate crystals.
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Chloride can contribute to the crystallization reaction and promote the formation and
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growth of arsenic apatite, which can effectively reduce the arsenic-leachate concentration of the precipitates. When the Cl- concentration increased from 0 mol/L
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to 1.0 mol/L, the arsenic-leachate concentration decreased from 28.26 mg/L to 0.08 mg/L. The chloride ion potentially acts as a precursor in the formation and growth of
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Ca5(AsO4)3OH and Ca5(AsO4)3Cl crystals, resulting in the arsenic-leachate concentration gradually decreasing. Sulfate is beneficial in reducing the
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arsenic-leachate concentration from Ca-As precipitates, because it is propitious to improve the formation and growth of Ca4(OH)2(AsO4)2·4H2O. Low or high sulfate
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will lead to calcium arsenate crystal form transformation and densification, which will increase its crystallinity and stability. A low carbonate concentration does not have a detrimental effect on the arsenic-leachate concentration in the short term. In contrast, a small amount of carbonate is beneficial for the formation and growth of Ca4(OH)2(AsO4)2·4H2O. However, more carbonate would capture Ca2+ and Na+ to generate CaCO3 and Na3Ca(CO3)2·5H2O instead of Ca-As compounds, which would
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prevent the calcium arsenate crystallization reaction. The trend of grain size being in opposition to that of the arsenic-leachate concentration illustrates that the larger crystal particles of Ca-As compounds are more stable than the smaller ones. Thus, promoting grain size growth of stable calcium
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arsenates by controlling the anion concentration to reduce the arsenic-leachate
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concentration is a proposed method for industrial applications.
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Acknowledgements
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This work was supported by the Special Program on Environmental Protection for Public Welfare (201509050), the National Natural Science Foundation of China
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(51704337, 51474247, 51634010), the National Key R&D Program of China (2017YFC0210402), and the Co-Innovation Center for Clean and Efficient Utilization
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of Strategic Metal Mineral Resources for financial support.
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References
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Highlights
The anions, including chloride, sulfate and carbonate, have an effect on the calcium arsenate crystallization reaction.
Chloride and sulfate contribute to the crystallization reaction while carbonate would prevent the formation of calcium arsenate. The arsenic leachate concentration of Ca-As compounds is related to the grain
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size of calcium arsenate crystals.
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The larger crystal particles of calcium arsenate are more stable than the
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smaller one.
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