Effects of NO - American Chemical Society

Article 3-

42-

Effects of NO and SO ions on the coupled reaction -extraction-crystallization process of MgCl and CO 2

2

Guilan Chen, Xingfu Song, Yanxia Xu, and Jianguo Yu Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 23 May 2017 Downloaded from http://pubs.acs.org on May 31, 2017

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Industrial & Engineering Chemistry Research

Effects of NO−3 and SO2− ions on the coupled reaction−extraction−crystallization 4 process of MgCl2 and CO2 Guilan Chen, Xingfu Song,* Yanxia Xu, Jianguo Yu* National Engineering Research Center for Integrated Utilization of Salt Lake Resources, East China University of Science and Technology, Shanghai 200237, China ABSTRACT：The effects of NO−3 and SO2− ions on the H+ concentration in organic 4 phase and crystal morphology of solid product nesquehonite (MgCO3·3H2O) in the coupled reaction−extraction−crystallization process were investigated. During the coupled process, the H+ concentration in organic phase increased with the increase in concentration of NO−3 ions and decreased with the increase in concentration of SO2− 4 ions, indicating that NO−3 ions promoted the carbonization of MgCl2 while SO2− ions 4 inhibited the same. The rod-like MgCO3·3H2O crystals became short and thick in the presence of both NO−3 and SO2− ions. Energy-dispersive X-ray spectroscopy (EDS), 4 inductively coupled plasma atomic emission spectroscopy (ICP-AES) and molecular dynamic simulation were used to clarify the mechanism by which impurity ions modified the structure. The results indicated that the NO−3 and SO2− anions could be 4 selectively adsorbed on the top facet ( 01 1 ) of MgCO3·3H2O, preventing the growth of crystal surfaces, which results in the decrease in length and increase in diameter of MgCO3·3H2O product. The morphology modification by NO−3 ion was mainly due to the selective adsorption of NO−3 ions. Both selective adsorption of ions and the incorporation of ions accounted for the morphology modification by SO2− ions. 4

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1. INTRODUCTION Chemical reaction, coupled with extraction and crystallization, can improve the yields of product and overcome the limitations of reaction equilibrium and inhibition of product formation. An important application of the coupled process is to prepare carbonates such as Na2CO31, CaCO32-3, SrCO34 and Li2CO35. The chemical equations can be described as follows, MCl 2(aq) + CO 2(g) + H 2O(aq) → MCO 3 ↓ (s) + 2HCl(aq)

(1)

HCl(aq) + R 3 N(o) → R 3 N ⋅ HCl(o)

(2)

where g, aq, o, and s denote gaseous phase, aqueous phase, organic phase and solid phase, respectively; M and R3N refer to metal element and tertiary amine, respectively. In the coupled process, tertiary amines are added to extract the produced HCl out of the aqueous solution, so that the reaction is pushed in the forward direction. In our previous

work,6

MgCO3·3H2O

was

prepared

using

a

coupled

reaction-extraction-crystallization process. As seen from Eqs 1-2, the coupled process is a multi-phase and multi-reaction complex system, including extraction of HCl and the crystallization of carbonate. From the perspectives of environmental protection and economy, it is very promising to prepare MgCO3·3H2O using industrial effluents, concentrated seawater or brine. However, impurity ions such as Na+, K+, Ca2+, SO2− 4 and NO−3 ions exist in these solutions. The presence of impurity ions has influences on both extraction of acid and crystal growth of solid products in industrial processes. Effects of inorganic salts (Na2SO4 and NaCl) on the extraction of different carboxylic acids (acetic acid, propionic acid, butyric acid, valeric acid, and caproic


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acid) using tributylphosphate were studied by Ingale.7 It was found that the distribution coefficient of acid increased in the presence of salts. Keshav8 found that the KD of propionic acid increased first and then decreased as the concentration of NaCl, NaSO4 or K2HPO4 increased. San-Martin9 showed that lactic acid extraction was not influenced by presence of lactose in the aqueous phase, whereas the Cl− ions negatively affected the extraction of lactic acid. Schunk10-13 investigated the effects of impurities (e.g., sodium nitrate, sodium chloride, sodium sulfate and sodium citrate) on the extraction of citric acid and acetic acid by tri-n-octylamine. Their results indicated that even very small amounts of impurities could considerably reduce the amount of carboxylic acid extracted from the aqueous phase into the organic phase. Many industrial crystallization processes are also influenced by impurity elements. Impurity ions can affect the nucleation and crystal growth of solid product, and consequently change the crystal morphology and even the crystal phase14-18. Two typical examples of such industrial crystallization processes are the crystallization of 17 CaSO4 and CaCO3. Effects of K+,19 Mg2+,16, 19-20 Cu2+,19, 21 F−,22 and PO3− ions on 4

the crystallization of CaSO4 and effects of Mg2+,23-24 Mn2+,25 Zn2+,25 and SO2− ions14, 4 26

on morphology and phase of CaCO3 have been widely investigated. Hasson27 found

that Al3+ or Fe3+ ions changed the crystal morphology of gypsum from thin plates to thick dihydrate crystals. Tracy14, 26 showed that SO2− 4 ions had a controlling influence on both the phase and morphological development of CaCO3. The morphology of calcite crystal was less perfect as the sulfate concentration increased, but the morphologies of aragonite and vaterite were less modified by sulfate. Mg-carbonate



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has various crystal phases (such as lansfordite, artinite, nesquehonite and hydromagnesite)28 and morphologies (such as rod-like, sheet-like, needle-like, plate-block shaped and fan shaped)29. The effects of cations Na+, K+ and Ca2+ on the coupled process have been reported in our previous work.30 The results indicated that the morphology of nesquehonite changed from rod-like to needle-like when Na+ or K+ ions were added to the solution and some spherical amorphous nano Mg/Ca-carbonate were formed in the presence of Ca2+ ion. The overview presented above shows the importance of understanding the effects of impurities in industrial production. Recent studies are focused on investigating the absorption mechanism of CO2,31 extraction mechanism of acid,32 crystallization mechanism5 and the effects of technical parameters33-34 in the coupled process. However, reports on the effects of impurity ions on the coupled process are scarce. In this context, the effects of NO −3 and SO 2− anions on the reaction and crystal 4 morphology of solid product in the coupled process were investigated. Furthermore, EDS, ICP-AES and molecular dynamic simulation were used to clarify the morphology modification mechanism of NO−3 and SO2− ions. 4

2. EXPERIMENTAL 2.1. Materials MgCl2·6H2O (≥99.0 wt%), MgNO3·6H2O (≥99.0 wt%), MgSO4·7H2O (≥99.0 wt%), NaOH (≥99.8 wt%) and isoamylol (≥99.5 wt%) were purchased from Shanghai Lingfeng Chemical Reactant Co. Ltd., China. Ethanol (≥99.7 wt%) was purchased from Sinopharm Chemical Reagent Co. Ltd., China. Commercial extractant N235,


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which is a mixture of tertiary amines R3N (R: ~C8 – C10) with an effective amine concentration of 2.04 mol·L−1, was purchased from Shanghai Rare-earth Chemical Co. Ltd., China. CO2 with a mole fraction of 0.995 was supplied by Shanghai Hukang Industrial Gas Co. Ltd., China. All reagents were used as received without further purification. Deionized water was used in all experiments.

2.2. The Coupled Reaction−Extraction−Crystallization Experiment The experimental setup used in this work was the same as that used in our previous work.6 25 mL MgCl2 (2 mol·L−1) solution with or without impurities, 50 mL ethanol, 50 mL N235 and 25 mL isoamylol were added in the reactor. The amount of impurity added was expressed as the molarity of the impurity in aqueous phase after the reagent addition and the concentration range of NO−3 or SO2− ions was 0 to 75 4 mmol·L−1 (mM). A water bath was used to maintain the reaction temperature at 25 °C. The solution was stirred with a stirring rate of 300 r·min−1 for about 20 min to ensure thorough mixing of the organic and aqueous phases. Then, CO2 was bubbled into the reactor at a rate of 30 mL·min−1 for 120 min. After completion of the reaction, the suspension was filtered. The leachate was separated in a separating funnel after standing for 12 h. The concentration of H+ ion in the organic phase (mol·kg−1) was determined by dissolving the organic phase in ethanol and titrating using a calibrated NaOH solution (0.1 mol·L−1) with phenolphthalein as the indicator. The solid was washed with anhydrous ethanol and deionized water until the impurity ions could not be detected in the filtrate, and then dried at 60 °C for 12 h. The phase composition of the solid was identified by



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an X-ray diffractometer (XRD; D/max 2550, Rigaku, Japan) using Cu Kα radiation (λ = 1.54178 Å), with a scanning rate of 2° min−1 and a scanning 2θ range of 10 to 80°. The morphology and elemental composition of the solid were characterized by a scanning

electron

microscope

(SEM;

Quanta

250,

FEI

Co.,

USA)

and

energy-dispersive X-ray spectrometer (EDS; TEAM, EDAX Co., USA) after sputter coating a thin layer of Pt. The average diameter and length of the solid product were estimated by direct measurements of about 100 crystals from the SEM images. The contents of NO −3

and SO 2− 4

ions in the solid product were analyzed by ion

chromatography (IC; DX-600, Dionex Co., USA) and inductively coupled plasma atomic emission spectrometry (ICP-AES; ARCOS FHS12, SPECTRO Co., Germany), respectively.

2.3. The Direct Extraction Experiment In the direct extraction experiment, 35 mL of 2 mol·L−1 HCl solution (to ensure that the value of n(HCl):n(N235) equals that in the impurity free solution in coupled experiment) with or without impurities, 50 mL N235 and 25 mL isoamylol were added to a glass flask and mixed in a shaker with a frequency of 150 rpm at 25 °C for 4 h. Preliminary experiments showed that the extraction reached equilibrium within 4 h. After equilibration, the coexisting phases were separated by separating funnel. The concentrations of NO−3 and SO2− ions in the aqueous phase and H+ concentration in 4 the organic phase were determined by the above mentioned methods. The Cl− concentration in the aqueous phase was determined by titration using a calibrated AgNO3 solution with K2CrO4 as indicator. The concentrations of Cl−, NO−3 and SO2− 4


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ions in the organic phase were calculated by applying a material balance. The pH of the organic phase was measured by a Mettler-Toledo pH meter.

3. RESULTS AND DISCUSSION 3.1. Effects of Impurities on the H+ Concentration in Organic Phase In general, NO−3 and SO2− ions are the main anionic impurities present in industrial 4 effluents, concentrated seawater or brine. In this study, the studied concentration range of NO−3 and SO2− ions was selected as 0 to 75 mmol·L−1 to cover the possible 4 concentration range of the impurities. Figure 1 shows the variation of H+ concentration in organic phase with the impurity ions concentration in coupled process. The reaction between MgCl2 and CO2 in the coupled process can be expressed by the following equation. MgCl 2 + CO 2 + 4H 2 O + 2R 3 N → MgCO 3 ⋅ 3H 2 O ↓ +2R 3 N ⋅ HCl

(3)

The H+ concentration in organic phase was 0.742 mol·kg─1 in the impurity-free system. When NO−3 ions were added to the solution, the H+ concentration in organic phase increased to 0.792 mol·kg─1 with the increase in NO−3 ions concentration from 0 to 75 mmol·L−1. The added impurity Mg(NO3)2 may react with CO2 to produce MgCO3·3H2O and HNO3 in the presence of N235. If the entire amount of the impurity Mg(NO3)2 is assumed to be involved in the reaction, the maximum H+ concentration in the organic phase can be up to 0.761 mol·kg─1. However, the experimental results showed that the H+ concentration in organic phase was higher than 0.761 mol·kg─1, which meant that NO−3 ions promoted the carbonization of MgCl2.



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Figure 1. Effects of NO−3 and SO2− ions on the H+ concentration in organic phase 4 An opposite trend was observed in the presence of SO2− ions. The increase in 4 concentration of SO2− ions from 0 to 75 mmol·L−1 led to a drop in H+ concentration 4 in the organic phase from 0.742 mol·kg─1 to 0.666 mol·kg─1, which indicated that SO 2− 4

ions had a negative effect on the coupled process and should be removed during

the pretreatment of raw materials. Since crystallization and extraction coexist in the coupled process, direct extraction experiments are carried out to determine the extraction equilibrium and study how impurity ions affect the reaction. Table 1 shows the effects of NO−3 and SO2− ions on 4 the liquid–liquid equilibrium in the direct extraction process. It was found that both NO−3 and SO2− ions were extracted into the organic phase. Moreover, with the 4 increase in the ion concentration, the H+ concentration in the organic phase was almost unchanged, while the Cl− concentration decreased and the corresponding impurity ion concentration increased. In our system, HNO3/H2SO4 acids may be formed by NO−3 /SO2− ions and H+ of HCl. Schunk et al35 reported that the highest 4 value for the partition coefficient of HNO3 and H2SO4 in tri-n-octylamine was about 500 and 400, respectively, whereas the partition coefficient was about 200 for HCl.


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Therefore, HNO3 and H2SO4 would be preferentially extracted by N235, resulting in a drop in the extraction of HCl.

Table 1. Effects of NO−3 and SO2− ions on the liquid–liquid equilibrium in the 4 direct extraction process Ions equilibrium concentration/(mol·kg─1)

Concentration of impurity ions ─1

SO2-4

3

Organic phase -

NO-3

SO2-4

pH

1.104

1.105

0

0

5.75

0.00013

1.095

1.087

0.0080

5.95

0.071

0.00022

1.096

1.081

0.0134

6.02

37.5

0.096

0.00039

1.095

1.074

0.0201

6.05

50

0.121

0.00065

1.097

1.059

0.0267

6.11

75

0.172

0.00135

1.096

1.040

0.0400

6.13

15

0.067

0.00033

1.099

1.084

0.0080

5.60

25

0.088

0.00056

1.099

1.082

0.0133

5.49

37.5

0.107

0.00250

1.095

1.056

0.0193

5.35

50

0.138

0.00519

1.098

1.045

0.0245

5.33

75

0.161

0.01325

1.095

1.026

0.0341

5.13

Cl

NO

SO

0

0.021

0

0

15

0.053

25

24

+

Cl

/(m mol·L )

NO-3

Aqueous phase −

H

Additionally, it was also observed that the addition of NO−3 and SO2− ions affected 4 the pH of organic phase. The organic phase pH changed from 5.75 to 6.13 as the concentration of NO−3 ions increased from 0 to 75 mmol·L−1, but it decreased from 5.75 to 5.13 when the concentration of SO2− ions increased from 0 to 75 mmol·L−1. 4 As shown in Table 1, the H+ equilibrium concentration in the organic phase was approximately the same in all cases. Hence, the difference in organic phase pH was certainly caused by NO−3 and SO2− ions. In other words, the organic phase pH 4 increased in the presence of NO−3 ions, while it decreased in the presence of SO2− 4 ions.



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Based on the results of the direct extraction experiments, it can be speculated that a portion of NO−3 and SO2− ions will be preferentially extracted by N235 in the coupled 4 process. The presence of NO−3 ions would lead to an increase in the organic phase pH while the effect of SO2− ions would be the opposite, which has been confirmed by the 4 experimental data (shown in Figure 2). Wang et al.36 reported that the stronger the basicity of organic amine, the easier the precipitation of Mg2+, and the higher the H+ concentration in organic phase. Therefore, the changes in the H+ concentration in organic phase induced by the presence of NO−3 and SO2− ions in the coupled process 4 may be interpreted as follows. When NO−3 ions were added to the solution, a portion of the NO−3 ions were extracted into the organic phase. Then, the presence of these NO−3 ions raised the organic phase pH, which promoted the carbonization of MgCl2, and therefore, the H+ concentration in organic phase increased. In the case of SO2− 4 ions, the organic phase pH was lowered in the presence of SO2− ions, which inhibited 4 the carbonization of MgCl2, and resulted in the decrease in H+ concentration in organic phase.

Figure 2. Effects of NO−3 and SO2− ions on the organic phase pH in coupled process 4


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3.2. Effects of Impurities on Solid Product Figure 3 shows the XRD patterns of the products prepared with different concentrations of NO −3 and SO2− ions. All the diffraction peaks (Figure 3a, c) 4 matched with the values in the standard JCPDS card 20-0669 of MgCO3·3H2O and no impurity peaks were detected, indicating that the presence of NO−3 and SO2− ions 4 within the studied concentration range did not induce the phase transformation of MgCO3·3H2O. Although the diffraction peaks were consistent with the standard peaks, the 2θ values of the two highly intense peaks shifted slightly in the presence of NO−3 and SO2− ions. As displayed in Figure 3b, NO−3 ions led to the 2θ shift of (101) peak 4 from 13.689 to 13.530°, and the 2θ shift of (002) peak from 23.144 to 23.001°, corresponding to the increase in lattice spacing from 6.463 to 5.531 Å for (101) and from 3.840 to 3.864 Å for (002), respectively. In the case of SO2− ions (Figure 3d), 2θ 4 of (101) shifted from 13.689 to 13.527°, and 2θ of (002) shifted from 23.144 to 22.998°, corresponding to the increase in lattice spacing from 6.463 to 5.534 Å for (101) and from 3.840 to 3.869 Å for (002), respectively. These peak shifts may have been caused by the incorporation of impurity ions. The radius of SO2− is larger than 4 is that of NO−3 , thus the interplanar spacing of (101) and (002) in the presence of SO2− 4 larger than that in the presence of NO−3 .



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Figure 3. XRD patterns of the products prepared with different impurity ion concentrations: (a, b) NO−3 ion; (c, d) SO2− ion. 4 MgCO3·3H2O belongs to the monoclinic system. Structural research on its crystal structure shows that Mg atoms and O atoms in the crystal form MgO6 octahedrons, which then form infinitely long-chains29, 37. As a result, the nesquehonite crystal grows in a single direction and shapes in needle-like or rod-like morphology. The effects of NO−3 and SO2− ions on the morphology of nesquehonite are illustrated in 4 Figures 4-5, and the average diameter and length of the corresponding products are shown in Figure 6. Rod-like MgCO3·3H2O crystals with an average diameter of 2.3 µm and length of 14.0 µm were formed in impurity free solution. When NO−3 ions were added, the MgCO3·3H2O crystals became short and thick, and their average diameter increased to 2.8 µm while their average length decreased to 7.6 µm.


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Moreover, some radial polycrystals were observed when the concentration of NO−3 ions was above 25 mmol·L−1.

Figure 4. SEM photos of the products prepared with different concentrations of NO−3 ions: (a) 0 mmol·L−1; (b) 15 mmol·L−1; (c) 25 mmol·L−1; (d) 37.5 mmol·L−1; (e) 50 mmol·L−1; (f) 75 mmol·L−1.

Figure 5. SEM photos of the products prepared with different concentrations of SO2− 4 ions: (a) 0 mmol·L−1; (b) 15 mmol·L−1; (c) 25 mmol·L−1; (d) 37.5 mmol·L−1; (e) 50



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mmol·L−1; (f) 75 mmol·L−1.

Figure 6. Average diameter and length of the products prepared with different impurity concentrations (a) NO−3 ion; (b) SO2− ion. 4 Similar but more obvious phenomena were observed in the presence of SO2− ions. 4 As shown in Figure 6b, within the studied concentration range, the average diameter increased to 3.5 µm and the average length decreased to 7.3 µm as the concentration of SO 2− ions increased to 75 mmol·L−1. Furthermore, the nesquehonite existed 4 entirely as radial polycrystals when the concentration of SO2− ions was raised up to 4 50 mmol·L−1 (Figure 5e). Valency of the ions is one of the factors that affects the crystal morphology.17 It has been reported that di- and trivalent ions lead to stronger morphology modification of crystals compared to univalent ions.21 Thus, the divalent ions have a greater effect on morphology modification of nesquehonite. SO2− 4 Selective adsorption of impurity ions on the crystal surface and incorporation of impurity ions into the crystal play important roles in crystal morphology modification.19-20, 38 The morphology modification of the MgCO3·3H2O crystal may be attributed to surface adsorption or incorporation of the impurity ions, or both these effects together. To clarify the modification mechanism of NO−3 and SO2− ions, the 4


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elemental compositions of the products were analyzed qualitatively and quantitatively with EDS, IC, and ICP-AES. Based on the IC and ICP-AES results, an uptake ratio (U)17 given by Eq. 4 was used to quantify the amount of impurity ions taken up by nesquehonite after filtration and washing.

U =

nimpurity

(4)

nsolid

where nimpurity is the mole number of impurity ion and nsolid is the mole number of nesquehonite. Figure 7 displays the EDS analysis of solid product in the presence of NO−3 and SO 2− 4

ions. All the crystallization products obtained in the presence of NO−3 or SO2− ions 4

were composed of C, O, and Mg elements, and no N or S elements were detected. This may be attributed to the very small amounts of impurity ions incorporated into the product crystals. IC and ICP-AES analyses were further used to quantify the uptake of impurity ions by the solid product. As shown in Figure 8, NO−3 and SO2− 4 ions were indeed incorporated into the crystals. The uptake ratio of NO−3 ions in the crystals increased with increasing NO−3 concentration up to 37.5 mmol·L−1. Upon further increase in the NO−3 ion concentration, the uptake amount decreased. Overall, the uptake ratio of NO−3 was very small (

Effects of NO - American Chemical Society

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