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Na3VO4·12H2O and Na2SO3 were products of Aladdin and Kelong reagent companies, respectively. D2EHPA was purchased from Energy Chemical Trading Co., Ltd. (Shanghai); Tributyl phosphate (TBP) was acquired from Tianjin Kemiou Chemical Reagent Co., Ltd., and other reagents such as n-hexane, carbon tetrachloride, benzene and trichloromethane were bought from the companies of China. All reagents were of analytical reagent grade and used without purification.
Two natural brine samples named as S1 (discharged water of a saline Potash Fertilizer Plant) and S2 (saline field brine) were collected from different places of Qinghai Province, China.
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Two steps were included in the vanadium pre-purification process: extracting vanadium with D2EHPA and stripping it with H2SO4 solution. In present work, both steps were optimized to ensure the maximum recovery of vanadium ions. The complicacy of brine samples makes them impossible to be directly used in optimizing the process, pure vanadium solutions with concentration of 5 mg/L (dissolved in 0.5 mol/L H2SO4) were adopted. The concentrations of vanadium in the solutions were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES) (Agilent 5100, Agilent technologies, USA).
Before extraction, sufficient amount of Na2SO3 was added to reduce vanadium (V) to vanadium (IV), since D2EHPA had better extraction property toward vanadium (IV) [23].
In the extraction step, organic phase, containing D2EHPA (extraction reagent), TBP (interface modifier) and diluent, with total volume of 3 mL was contacted with the same volume of vanadium solution at an oscillation speed of 300 rpm. To optimize the extraction conditions, the effects of D2EHPA and TBP concentrations, diluents, pH of vanadium solution and time on extraction efficiency (E) were investigated. After sufficient contacting, the mixed solution was further centrifuged at 5 000 rpm for 5 min to ensure the complete separation of inorganic and organic phase. The extraction efficiency (E) was calculated by
where
$ {C_{\rm{t}}} $ and$ {C_{\rm{I}}} $ are the vanadium concentrations in inorganic phase before and after extraction, respectively.$ {C_{\rm{t}}} - {C_{{\rm{I}}} } $ is the vanadium concentration in organic phase.In the stripping step, 2 mL organic phase loaded with vanadium was stripped by the same volume of H2SO4 on an oscillator shaking at 300 rpm for 10 min. To optimize the stripping conditions, H2SO4 with different concentrations were applied as the stripping reagents. After centrifuging the mixed solution at 5 000 rpm for 5 min, vanadium concentration in the stripped inorganic phase was determined. The stripping ratio (Eb) was calculated by
where
$ {C_{{{\rm{bI}}} }} $ is the vanadium concentration in the stripped inorganic phase.The parameters which can acquire the highest vanadium extraction and stripping efficiencies were selected as the optimum conditions for pre-purifying vanadium.
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Since trace level of vanadium existed in brine water, ICP-MS (iCAP-QC+ICS5000+, Thermo Fisher Scientific, USA) was used to get the vanadium concentration in these samples. To ensure the optimum vanadium purification conditions determined by ICP-AES are reliable in trace vanadium concentration measurement by ICP-MS, experiments were carried out with standard vanadium solutions of 20 μg/L, but concentrations were acquired by ICP-MS. The operation parameters of ICP-MS were listed in Table 1.
Parameter Value Plasma power/W 1 550 Cool flow/(L·min−1) 14 Nebulizer flow/(L·min−1) 0.97 Auxilliary flow/(L·min−1) 0.80 Sample depth/mm 5.00 Torch horizontal position/mm −0.48 Torch vertical position/mm 1.07 Spray chamber temperature/℃ 2.7 Extraction lens 2/V −90.67 CCT focus lens −0.96 Table 1. The operational parameters of ICP-MS.
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Brine samples S1 and S2 are saturated salt solutions with obvious crystal structures. They were two times diluted so as to make the pH adjustment easier.
Vanadium concentrations in these brines samples were measured by the standard recovery technique. Furthermore, to reduce the matrix effect of the huge coexisting ions, 30 mL diluted brine water was extracted by 6 mL organic phase, but the phase ratio of the stripping step is still 1:1. The vanadium in diluted brine and standard spiked diluted brine samples were purified and then their concentrations were measured by ICP-MS. The standard addition recovery (
$ {E}_{\rm r} $ ) of each sample was acquired by[24-25]where,
$ {C}_{\mathrm{m}\mathrm{B}} $ and$ {C}_{\mathrm{a}\mathrm{B}} $ are the measured vanadium concentrations of the diluted brine and standard spiked diluted brine samples, respectively.$ {C}_{\mathrm{a}} $ is the spiked standard vanadium concentration.Hence, the actual vanadium concentration of the brine samples (
$ {C}_{\mathrm{B}} $ ) were:because the brine samples were two times diluted.
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The effect of D2EHPA concentration [varied from 10% to 30% (v/v)] on vanadium extraction is shown in Fig.1(a). Although vanadium extraction efficiency increases slowly with increasing D2EHPA concentrations, it achieves 76% when D2EHPA concentration is only 10%. The highest vanadium extraction efficiency is 87% when D2EHPA concentration is 30%. Hence, D2EHPA concentration of 30% is sufficient to achieve relative high vanadium extraction efficiency.
To avoid the third phase formation in the extraction process, TBP is used as the interface modifier. The volume concentration of TBP was changed from 5% to 25% (v/v). As shown in Fig.1(b), the extraction efficiencies of V(IV) are independent of TBP concentrations and keep nearly constant. This is different from the results of Zhang et al.[23] where V(IV) extraction was enhanced by TBP, but consistent with the finding of Cherafhi et al.[26]. Although TBP has negligible effect on vanadium extraction from pure solutions, small floccule appeared in the saline lake brine samples at TBP concentrations less than 10% (data not shown for concise). Hence, TBP concentration was chosen to be 20%.
In addition, n-hexane, methylbenzene, benzene and trichloromethane were respectively employed as diluent to evaluate their effects on vanadium extraction, the acquired results are illustrated in Fig.1(c). The highest and lowest extraction efficiencies are obtained with n-hexane and trichloromethane as the diluents, respectively. While the extraction efficiencies are close to each other when methylbenzene and benzene are the diluents. This phenomenon is correlated with the stability of D2EHPA-vanadium ion complex and influenced by diluents. More stable D2EHPA-vanadium ion complex can be formed when D2EHPA is polymerized[26], and it is much easier for D2EHPA to form dipolymer in the diluent with lower polarity. The polarity of n-hexane, methylbenzene, benzene and trichloromethane are 0, 2.4, 2.7 and 4.1, respectively. Therefore, the most effective extraction of vanadium is observed when n-hexane was selected as the diluent.
The forms of vanadium(IV) in water solutions are very complicated and vary with solution pH[27]. Hence, solution pH is an important factor that affects the extraction ratio. At pH lower than 4.5, vanadium(IV) is mainly existed as VO2+ and convert to HV2O5− at higher pH. To avoid the influences caused by vanadium(IV) form, vanadium solutions with pH of 0.75 to 4.5 were selected. As shown in Fig.1(d), vanadium extraction efficiencies rise linearly within solution pH of 0.75 to 3.00, then keep almost constant with further increase of pH, which is in line with the references[22, 28]. D2EHPA is an acid extractant, the effective extraction of vanadium ions is caused by exchanging H+ ions in D2EHPA with VO2+ ions through the reaction VO2++2H2A2=VOA2·2HA+2H+[29]. Here, D2EHPA was represented by HA. Therefore, increasing the solution pH will facilitate the extraction process. Moreover, the forms of vanadium ions are also influenced by vanadium concentration, higher vanadium concentration leads to a lower pH range for VO2+ to be stable[16]. The investigated vanadium concentration here was only 5 mg/L, two orders of magnitude lower than the vanadium concentrations in references (500 mg/L[26] or 1 810 mg/L[28], optimum extraction pH of ~1.8). Therefore, the maximum vanadium extraction efficiency is obtained at pH of 3.00 and remains constant in pH from 3.00 to 4.50. To ensure high extraction efficiency, solution pH of 3.0 was selected.
Except for the above conditions, the effect of time on extraction efficiency was also investigated and data are illustrated in Fig.1(e). The extraction efficiency can reach 65% within 2 minutes and 10 minutes is enough to achieve extraction equilibrium. In fact, the relatively low initial vanadium concentration (5 mg/L) may be another reason for the fast extraction kinetics. To guarantee the stable vanadium extraction efficiency, extraction time of 30 min was employed.
The extracted vanadium must be stripped and further used for vanadium concentration determination. H2SO4 with concentrations of 0.5 to 4 mol/L were applied as the stripping reagents. From Fig.1(f), it can be found that the stripping efficiencies display positive correlation with the increased H2SO4 concentrations and (88.7±2.5)% of the loaded vanadium can be stripped by H2SO4
$\geqslant $ 3.0 mol/L. On the contrary to the extraction procedure, more acid environment will benefit the stripping efficiency. Hence, 3.0 mol/L H2SO4 was used as the stripping solution.Based on the above investigations, the vanadium pre-purification process can be determined (Fig.2). After adjusting the valence of vanadium and solution pH, 3 mL vanadium solution is extracted by the organic phase composed of 30% D2EHPA (v/v), 20% TBP (v/v) in n-hexane for 30 min at pH 3.0; and then vanadium in the organic phase is stripped by 3 mol/L H2SO4 of 2 mL for 10 min. The phase ratio of each step was 1:1.
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The calibration curve of 51V measure by ICP-MS within 0 to 20 μg/L is: A = 53 171 CV (μg/L) – 4 168.9 (R2=0.99). Here, A is the measured intensity of 51V and CV is the vanadium concentration (µg/L). Thus, the sensitivity of ICP-MS for vanadium is 53 171 cps/(μg/L). The limits of detection (LOD) and quantification (LOQ) can be further estimated to be 1.88 ng/L (3 Sb/s) and 6.27 ng/L (10 Sb/s), respectively. Where Sb is the standard deviation of the blank and s is the slope of the analytical curve analytical curve [9].
To ensure whether the vanadium pre-purification process is reliable in trace vanadium measurement by ICP-MS, three standard vanadium solution with concentration of 20 μg/L are undergone the vanadium pre-purification process (Fig.2). Vanadium concentrations in the inorganic phases of each step were quantified by ICP-MS. The extraction and stripping efficiencies as well as their corresponding relative standard deviations (RSDs) are calculated and listed in Table 2. All the extraction and stripping efficiencies are higher than 93% with small RSDs. Hence, the vanadium pre-purification process is reliable to be used in trace vanadium concentration measurement by ICP-MS.
Measured number Extraction efficiency/% Stripping efficiency/% Calculated value Average RSD/% Calculated value Average RSD/% 1 94.50 93.84 0.37 95.14 96.34 1.41 2 93.68 97.82 3 93.32 96.05 Table 2. The extraction and stripping efficiencies of the standard samples.
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S1 and S2 are natural brine samples collected from different places of Qinhai Province, China. The concentrations of the main co-existing ions in these two samples were listed in Table 3. It is clear that the concentrations of Na+, K+, Ca2+, Mg 2+ and Cl− are extremely high, so their matrix effect to vanadium concentration determination must be reduced.
The standard addition technique was used for vanadium concentration determination in natural brine samples. The measured vanadium concentrations in diluted brine and standard spiked diluted brine samples are listed in Table 4. The average standard addition recoveries (Er) are found to be (104.4±0.3)% for brine sample S1 and (101.2±0.5)% for brine sample S2 (Table 4). The admitted standard addition recovery ranges are 80%~120%, 90%~110% and 95%~105%, if the concentrations are
$\leqslant $ 1 mg/L, 1~100 mg/L and$\geqslant $ 100 mg/L, respectively[30]. The actual vanadium concentrations (CB) of S1 and S2 are (6.58±0.08) µg/L and (6.96±0.15) µg/L, respectively (Table 4). These values are much lower than 1 mg/L, so the obtained standard addition recovery of sample S1 and S2 are acceptable. Moreover, the RSD values for both brine samples are less than 2.5% (Table 4). With the high recovery and small RSDs, the method obtained in present work can be used for determining the concentration of trace vanadium in natural complicated waters, such as seawater and saline lake brines.Sample name Na+ K+ Mg2+ Ca2+ Cl− SO42− S1
S25 595
66 22124 702
35 228101 413
15 82819 215
18 610207 616
105 85651 376
48 725Table 3. The concentrations(mg/L) of the main co-existing ions in saline sample S1 and S2.
Sample Measured number $ {C}_{\mathrm{m}\mathrm{B}} $/(μg·L−1) $ {C}_{\mathrm{a}} $/(μg·L−1) $ {C}_{\mathrm{a}\mathrm{B}} $/(μg·L−1) Er/% Average/% RSD/% $ {C}_{\mathrm{B}} $/(μg·L−1) Average/(μg·L−1) RDS/% S1 1 3.47 10.67 14.45 104.1 104.4 0.30 6.66 6.58 1.3 2 3.44 10.67 14.67 104.4 6.59 3 3.40 10.67 14.62 104.8 6.49 S2 1 3.58 10.67 13.97 100.7 101.2 0.54 7.11 6.96 2.2 2 3.46 10.67 14.28 101.7 6.80 3 3.52 10.67 14.71 101.2 6.96 Table 4. Measured vanadium concentration in the diluted and standard spiked diluted brine samples and the actual vanadium concentration of brine samples.
Determining the Concentration of Trace Vanadium in Natural Saline Lake Brines
doi: 10.11804/NuclPhysRev.39.2021054
- Received Date: 2021-07-02
- Rev Recd Date: 2021-08-21
- Available Online: 2022-06-29
- Publish Date: 2022-06-29
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Key words:
- vanadium /
- determination /
- extraction /
- saline lake brine /
- purification
Abstract: To determine the concentration of trace vanadium in saline lake brines, a vanadium pre-purification process is established to reduce the matrix effect of the huge coexisting ions. Two steps, extraction and stripping, are included in the process. The factors affecting the vanadium purification efficiencies are investigated in detail and the optimum conditions are determined to be: vanadium in the solutions was extracted by the organic phase containing 30% D2EHPA (v/v), 20% TBP (v/v) in n-hexane for 30 min at pH 3.0, and then stripped with 3 mol/L H2SO4 for 10 min. Trace vanadium in two natural brine samples are pre-purified using this process and their concentrations are determined by inductively coupled plasma-mass spectrometry (ICP-MS) with sensitivity and limit of detection (LOD) for 51V are 53 171 cps/(µg/L) and 1.88 ng/L, respectively. The standard addition recoveries of the brine samples are ~100% but with small relative standard deviations (RSD<0.6%), indicating that the method can be used to measure the concentration of trace vanadium in natural complicated waters, such as seawater and saline lake brines.
Citation: | Jian CHU, Qinggang HUANG, Ruiqin GAO, Weiping WANG, Xiaojie YIN, Xiaolei WU, Wei TIAN, Sha LI, Zhi QIN, Jing BAI. Determining the Concentration of Trace Vanadium in Natural Saline Lake Brines[J]. Nuclear Physics Review, 2022, 39(2): 238-244. doi: 10.11804/NuclPhysRev.39.2021054 |