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Organic Ion-associate Phase Extraction / Back-microextraction for the Pre-concentration and Determination of Lithium Using 2,2,6,6-Tetramethyl-3,5-heptanedione by Liquid Electrode Plasma Atomic Emission Spectrometry and GF-AAS in Environmental Water

We developed an ion-associate phase (IAP)-extraction / acid back-extraction system for the pre-concentration and atomic spectrometric determination of lithium trace amounts in water. The chelating reagent for lithium works also as a constituent of the extraction phase. The lithium in a 10 mL sample solution was converted through a chelate complex reaction with 2,2,6,6-tetramethyl-3,5-heptanedione (HDPM). Addition of a benzyldimethyltetradecylammonium ion caused the formation of an IAP suspension in the solution. Centrifugation of the solution led to the isolation of a liquid organic phase and the lithium complex was extracted as the upper phase from the centrifuge tube. After the aqueous phase was removed, lithium was back-extracted with a 400 µL nitric acid solution from the IAP. The acid phase was measured using liquid-electrode-plasma atomic-emission-spectrometry (LEP-AES) or graphite-furnace atomic-absorption spectroscopy (GF-AAS). The detection limits were 0.02 mg/L for LEP-AES and 0.02  g/L for GF-AAS. This system was applied to the determination of environmental waters. The HDPM in the organic phase was reusable.


Introduction
Lithium is used in a wide range of fields, including lithium-ion batteries, glass ceramics, and metal alloys. In recent years, the use of lithium rechargeable batteries has been increasing. 1 Over 90% of lithium worldwide originates from Australia, Chile, Argentina, and China. 2 Lithium is unevenly distributed in the environment, and trace amounts are present in environmental water. 3 Liquid-electrode-plasma atomic-emission-spectrometry (LEP-AES) is an easy method for quantifying multiple elements simultaneously, since it requires only 40 L of sample. [4][5][6] With this method, lithium is preferably measured in an acid solution. [4][5][6] On the other hand, graphite-furnace atomic-absorption spectroscopy (GF-AAS) is a highly sensitive method and Seawater was filtered using a mixed cellulose ester membrane filter (0.45 μm pore size) from Advantec Toyo (Japan). For the entire sample preparation process, a 50 mL plastic centrifuge tube (As One, Japan) was used.

Optimization of Extraction
In order to extract lithium from water, crown ethers 1,17 and porphyrins 18 have been used as chelating reagents in the past, apart from -diketones 1,19 such as 2,2,6,6-tetramethyl-3,5-heptanedione (dipivaloylmethane, HDPM). [20][21][22] In this study, HDPM was selected because it is relatively cheap and acts as an organic anion. We investigated the optimum amount of the chelating reagent HDPM to be used for the recovery of lithium ( Figure   1-A). Recovery was obtained by double extraction. However, it was almost the same as the recommended procedure, but to the second extraction KOH was not added, and 0.4 mL of HDPM was added instead of 0.6 mL. Recovery (R) is R, % = (I 1 -I 2 ) / I 1× 100 (1) where I 1 is the intensity of first extraction and I 2 is the intensity of the second extraction.
Different amounts of HDPM were added from 200 µL to 1000 µL. Recovery was almost maximal and constant above 600 µL, this is 2,000 times the molar ratio of lithium in this experiment. When the amount of HDPM is small, the ion associate phase will float, making it difficult to separate from the aqueous phase. The addition 600 µL HDPM was recommended because ease of handling and economic reasons.
The HDPM acid dissociation constant pKa is 11.75. 23 Since the chelating reagent HDPM forms a complex with lithium in a strongly basic solution, the pH was adjusted here with a KOH

Analytical Sciences
Advance Publication by J-STAGE Received January 6, 2020; Accepted March 9, 2020; Published online on March 20, 2020 DOI: 10.2116/analsci.20SBP13 solution. The results are shown in Figure 1-B. The reaction between lithium and DPM in strong alkaline solution follows Equations (2) and (3).
As the KOH concentration increases, the dissociated DPMincreases and reacts with more organic cations. Lithium can be recovered more quantitatively by increasing the volume of the organic phase (IAP). However, when the KOH concentration exceeds 1.0 mol/L, the emission intensity of lithium decreases.
The standing time, that is the time from the addition of the KOH solution until the addition of organic cation, was investigated. The results are shown in Figure 1-C. When the cation was added immediately after the KOH solution, the LEP-AES intensity was slightly lower, but almost the same result was obtained for 10 -60 min time difference between the two.
From this result, the formation of the lithium complex was considered to reach equilibrium within 10 min after the addition of the KOH solution.
In this IAP extraction method, the DPMserves as the constituent ion (organic anion) of the IAP. For this reason, the IAP can be formed only by adding organic cations. Therefore, the types of reagents added can be reduced. Benzyldimethyltetradecylammonium ion (C14BzDMA + ), benzyldimethyldodecylammonium ion (C12BzDMA + ), and benzethonium ion (Ben + ) were investigated as the cations potentially constituting the IAP. C14BzDMA + , the most hydrophobic of the three cations, showed the highest recovery of lithium. Therefore, C14BzDMA + was recommended as the organic cation for this study.

Analytical Sciences
The amount of 0.1 M C14BzDMA + solution added as organic cation was investigated ( Figure 1-D). By adding C14BzDMA + , IAP was formed by Equation (4). When the amount of 0.1 M C14BzDMA + solution added was 2.0 mL, the lithium emission intensity was the highest.
In this case, there is about 30 times more HDPM than C14BzDMA + in molar ratio. The addition of 3.0 mL of the C14BzDMA + solution led to solidification of the liquid organic phase when nitric acid was added for back extraction. This was probably due to the precipitation of the ion associate of the nitrate ion and the excess C14BzDMA + solution. Thus, the optimal addition amount of organic cation was 2.0 mL.
higher the number of rotations, the shorter the centrifugation time required to achieve lithium recovery. However, the plastic centrifuge tubes were sometimes deformed due to the multiple use. Therefore, centrifugation for 30 min at 3000 rpm was adopted in this study.

Back-extraction
The IAP extracted the lithium-complex that was dissolved in an acetonitrile 0.36 mL and 2 M nitric acid 0.04 mL, and then measured it with a LEP-AES analyzer. Since the volume of the IAP was about 0.4 mL, the solution dissolved IAP is about 0.8 mL. The sensitivity by IAP extraction/organic solvent dissolution increased 12 times compared to the standard solution without concentration. In the dissolved IAP, the precision of the emission intensities of LEP-AES was poor without hydrogen normalization. 5 This was due to the influence of coexisting organic substances during LEP-AES measurement, namely, the organic solvent used for IAP dissolution and IAP constituent ions.
With the LEP-AES measurement, Nakayama et al. 6 reported that when the IAP was dissolved in methanol and nitric acid, copper and manganese were sensitized but lead and zinc were slightly desensitized.
By adding acid (HX), the lithium extracted into IAP can be back-extracted into acid solution by Equation (5).
{Li/DPM, C14BzDMA + DPM -} + HX HDPM + C14BzDMA + X -+ Li + (5) We tested the use of different acids, such as sulfuric acid and nitric acid, in the back-extraction of lithium. The results are shown in Figure 3. To extract lithium from IAP, a moderately large ion, such as nitrate ion, quantitatively extracts lithium. Large ions such as nitrate ions are considered to extract lithium ions more strongly into the aqueous phase than Nitric acid solution is preferred for GF-AAS measurement. Nitric acid was recommended as the optimal acid for back extraction from IAP.
In order to extract lithium quantitatively, the concentration of nitric acid and sulfuric acid were examined. The results are shown in Figure 4. In consideration of safety, the study was In the LEP-AES measurement, when lithium was back-extracted from IAP with acid, the sensitivity was improved about 2 times compared to the case where IAP was dissolved in organic solvent (acetonitrile 360 L and 2 M nitric acid 40 L).
The absorption spectra showed that the upper phase consisted only of the chelating reagent HDPM and the lower phase consisted of IAP (DPM -C14BzDMA + ). It was presumed that DPM -, which was once ion-associated, did not complex with lithium again.
The upper phase of the recovered organic phase could be reused as a chelating reagent, but the lower phase could not be used as it was. Therefore, to reuse the reagent of the lower phase, as a result of trial and error, the organic cation (C14BzDMA + ) of IAP (C14BzDMA + DPM -) was replaced with potassium by washing with a saturated solution of potassium chloride (3.33 M), and then the acid was added. As a result, it was recovered as the chelating reagent HDPM. Using the recovered HDPM as the chelating reagent, the recovery of lithium was 93%, which was slightly higher than the recovery achieved by using a commercially available chelating reagent.
We compared the performance of nitric acid and hydrochloric acid in washing the lower phase. Washing with hydrochloric acid, contrary to washing with nitric acid, we were able to extract lithium in the same manner as with a commercially available chelating reagent.

Optimized analytical procedure
The procedure of scheme of the ion-associate phase extraction / acid back-microextraction method for atomic spectrometry are illustrated in Figure 5.  For GF-AAS measurement, lithium in fresh water samples was measured. The lithium concentration in Oyabe River water was 0.038 g/L, and Sho River water was 0.428 g/L. For the spring water (Uriwari-syozu) sample, a spike recovery test was performed at concentration of 0.2 g/L (Table 1). Lithium sub g/L level in the spring water could be recovered quantitatively.

Figure 5
Scheme of the ion-associate phase extraction / acid back-microextraction method for atomic spectrometry.