1. Introduction
Lignin is an alkylphenol unit that represents an enormous reservoir of renewable aromatic carbons[
1]. It is usually reclaimed from waste streams of the biorefining and papermaking processes. It has some advantages such as the presence of useful functional groups including aldehyde, methoxy, hydroxyl, carbonyl, phenolic and carboxyl groups, low cost, high specific surface area and stability, biodegradability and accessibility[
2]. So lignin or modified lignin is a potential green source of various safe and efficient applications such as adhesive, reinforcing agent, sustained-release agent, petroleum industry, supporting medium[
3], etc. In the field of catalysis, lignin has been applied as a green support for the preparation of efficient heterogeneous catalyst including lignin-SO3Sc(OTf)2, lignin-sO3Cu(OTf), and lignin-IL@NH2. The synthesized LS-IL@NH2 showed a good catalytic prower[
4,
5]. The high added-value utilization of lignin needs further research.
Cobalt and nickel are very low in nature but play an important role in the industry. Usually, they often coexist in ores, copper converter slag[
6,
7,
8,
9,
10], nickel slag, deep-sea manganese nodules[
11,
12], special alloy materials[
13,
14], battery materials[
15,
16,
17,
18], catalysts, sludge and waste water[
19]. With the economic growth, the consumption of cobalt and nickel increases rapidly, and the resources become increasingly exhausted, which caused high price, especially the cobalt has the highest value among them[
20]. Meanwhile, a huge amount of batteries containing cobalt or nickel has been generated worldwide and widely used in modern electronic devices. The spent battery can release toxic organic compound and then result in serious environmental problems and public health while disposed improperly[
21,
22]. Therefore, the recovery and utilization of cobalt is becoming more and more important and urgent to eliminate possible pollutions, as well as to make sustainable production of it [
15,
17,
23,
24,
25].
Due to the different sources, complex components, similar physical and chemical properties, the effective separation and purification of cobalt from multi metals in aqueous phase is extremely difficult. A wide variety of separation strategies has been developed to recover valuable metals and eliminate pollution by chemical precipitation[
26,
27,
28], solvent extraction[
29,
30,
31,
32], resin exchange[
33], membrane separation[
34], biorecovery[
35]. The earliest option for cobalt and nickel separation was chemical precipitation which seldom used alone in modern process because of similar solubility and coprecipitation behavior of Co(OH)
2 and Ni(OH)
2.
Solvent extraction is a separation process which is based on the different distribution component to be separated from two immiscible liquid phases usually water and an organic solvent[
36]. It is efficient and flexible compared with other processes and the main method of nickel and cobalt separation in industry.
Organophosphorus (phosphine) acids extraction agent is mostly suitable for the separation of cobalt and nickel sulfate solution to produce high purity cobalt salts and cobalt metal. Di(2-ethylhexyl)phosphoric acid (
D2EHPA or
P204)[
37,
38], hexyl phosphonic acid mono-2-ethyl-hexyl ester (
P507 or
PC88A)[
39], Bis(2,4,4-trimethylpentyl)phosphinic acid (Cyanex272)[
40] were most widely used, the three extraction solvent acid decreased gradually enhanced the ability of separation of cobalt and nickel in turn. D2EHPA can extract many metallic cations from various Carboxylates[
41], Citrate[
42,
43,
44], chloride[
37,
38,
45], nitric[
46,
47], and sulfate [
48,
49,
50]solutions. After the first commercial process using D2EHPA, P507 was developed and widely used in base-metal solvent extraction[
39]. The single extraction tests show that separation factor (Co/Ni) can reach over 350 and the stripping efficiency of Co can reach 99.5% under the optimal extraction condition[
51,
52]. It was proved that P507 (PC-88A) has good chemical stability, reutilization and low solubility in aqueous and can be applicable to the effective recovery of cobalt from various sulfate and chloride solutions with wider range of Ni/Co ratios. Cyanex 272 can effectively separate Co from Ni at low Co/Ni ratios[
40,
53], and it could extract Co selectively from spent lithium ion batteries (LIBs)containing Co, Li, Ni, and Metal ion impurities such as copper, iron, and aluminum [
54,
55].
The most commonly used amine extractants are tertiary amine and quaternary ammonium salts. The extraction mechanism of metal ions belongs to anion exchange or ion association [
56,
57]. Cobalt could be extracted (93.6%) from chloride leach liquors of spent nickel-metal hydride (Ni-MH) batteries in two stages with Alamine 336[
58]. 97.5 wt.% of Co(II) could be extracted using Alamine 336 from leaching spent NieCd batteries with hydrochloric acid[
59].
Oxime extraction agent was well known for its widespread application in copper extraction[
60], but it can also be used in nickel extraction for its high selectivity. Employing LIX 84-I as the extractant, nickel was separated to the extent of >99% from solutions containing ammonium sulphate in a single stage at phase ratio of 1.0[
61].
The solvent extraction is efficient to a variation in the metal contents in the wastes or ores compared with other processes. Currently, it is one of the most popular methods used for the removal or separation of metallic species from mixed metal aqueous phase[
20], it has also been successfully applied for recovery of cobalt or nickel from various resources[
62]. One or more methods form a complete flowsheet for the cobalt, nickel and other metal ions separation because of the poor efficiency when using a single way. In addition, new materials and new technologies are also the focus of future development.
In this study, lignin was at first modified to a cationic form by the incorporation of quaternary ammonium groups, which could interact with P507 resulting in new ILs. Then QPILs and P507 were employed for testing the extraction of Co(II) and Ni(II) from weak chloride media.
2. Materials and Methods
2.1. Materials and Reagents
The operation in the experimental process was divided into two parts: synthesis and extraction. Lignin was supplied by Shanghai C-reagent Biotechnology Co. Ltd.. Chemicals used for synthesis of ionic liquids, such as N,N-Dimethyltetradecylamine (DT), epichlorohydrin(EH), sodium ethanol, 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (P507) were purchased from Sinopharm Chemical Reagent Co., Ltd.. All the inorganic compounds were analytical regents. Cobalt chloride and nickel chloride were produced by Kelong Chemical Co., Ltd.. NaOH and HCl were used to adjust the aqueous acidity, which were purchased from TCI Chemical Co., Ltd. The organic diluent kerosene was obtained from Aladdin Biochemical Technology Co., Ltd.. All chemicals were used without any purification.
2.1. Synthesis of QP-ILs
Two stages were needed to synthesize the QP-ILs and the scheme was shown as
Scheme 1.
2.1.1. Synthesis of Quaternary Ammonium Lignin (QAL)
The mixture of N,N-Dimethyl tetradecyl amine (DT) and epichlorohydrin(EH)with a molar ratio of 10:7 was transferred to a tri-neck flask installed with a condenser and a stirrer in an ice−salt bath (NaCl/ice = 1:3 by weight) for 1 h. After that, the reactants were left overnight for complete reaction. Epoxypropyl Dimethyl tetradecyl amine Chloride(EDTC) can be detected by silver nitrate with the appearance of white precipitate and is ready for preparation of quaternary ammonium lignin.
0.4 g of lignin was reacted with 50 mL of a sodium hydroxide solution in a warm bath at 80 °C for 20 min. Then, EDTC was added into the mixture, and the reaction was conducted under constant magnetic stirring for 5 h at 50±5 °C until a brown emulsion was obtained. The mixture was centrifuged to obtain the QAL. The chemical reaction is demonstrated in
Figure 1 (a) and (b).
2.1.2. Synthesis of Quaternary Ammonium Lignin (QAL)
[QAL][OH]: 10 g (0.278 mol) [QAL] was dissolved in 100 mL of ethanol, and added dropwise into the ethanol solution containing sodium alkoxide. The solutions were stirred for 4 h at 50℃. The mixture was centrifuged at 8000 r/min for 10 min to remove the white precipitate of sodium chloride. Then, the filtrates were shaken with equal volume of DI water for half an hour to get [QAL][OH] by the hydrolysis of [QAL][OR].
QP-ILs: QP-ILs was prepared by combining [QAL][OH] (0.12 mol/l) and P507 (mole ratio is 1.1:1). The mixture was vigorously agitated for 12 h at 50℃ under reflux and left to settle. After a while, an aqueous layer formed at the bottom. The upper phase was poured into a vacuum rotatory evaporator (353 K, 20 mbar, 60 min) to remove residual water and ethanol. The chemical reaction is demonstrated in
Figure 1 (c).
2.3. General Procedure for Co and Ni Extraction
The extraction experiments were carried out by mixing equal volume of aqueous and organic phase in a separatory funnel. The conditions so generated (solution pH, extraction time, temperature and solvent concentration) were applied for separation of cobalt and nickel from aqueous solution containing 0.5 and 2 mg/ml, respectively. The pH (0.5-4.5) of the aqueous solution was adjusted to the desired value by adding dilute HCl or NaOH before equilibration. After phase disengagement, the aqueous phase was separated and its equilibrium pH was measured by the pH meter.
The concentration of nickel and cobalt ions in aqueous phases before and after extraction was determined by ICP-OES. The stock solutions of cobalt chloride(0.5 mg/ml Co.), nickel chloride(0.5, 2 mg/ml Ni.) and the mixture of cobalt chloride and nickel chloride in which concentrations of cobalt and nickel were 0.5 and 2 mg/ml, respectively were prepared from analytical grade reagents. All extraction experiments were carried out from 1 M NaCl solution to maintain constant ionic strength.
The concentration of metal ions in the organic phase was calculated from the difference between the metal ion concentration in the aqueous phase before and after extraction.
The extractants P507, QP-ILs were used for separation of Co and Ni in the extractant concentrations ranging from 2%-40%. The solution pH, extraction time and temperature were also analyzed. The extraction rate E, distribution ratio D and separation factor β were obtained by Eqs.(1), (2) and (3), respectively.
Where, Ca stands for the original metal ion concentration in the aqueous phase before extraction and Ce is the equilibrium metal concentration in the aqueous phase after extraction. Co stands for the equilibrium metal concentration in the organic phase. DCo and DNi denote the distribution ratios of Co and Ni, respectively.
2.4. Characterization
The structures of synthesized compounds were identified by Nuclear Magnetic Resonance Spectroscopy (NMR) and Fourier Transform Spectroscopy (FT-IR). NMR spectra of QP-ILs was recorded using AVANCE IIIHD 600 (Bruker, Switzerland) in d6-DMSO solution. FTIR was carried out on a Nicolt 380 spectrometer (Thermo Fisher Scientific,U.S.A).
4. Conclusions
In this study, a new process for the high value-added utilization of lignin was established: a new ionic liquids with Quaternary Ammonium Lignin which can act as QP-ILs for the extraction of Co and Ni were synthesized. The spectroscopy methods such as 1HNMR and FT-IR were used to identify its structure. Two different extractants P507 and QP-ILs were selected to investigate the effect of extraction time, temperature, pH value and extractant concentration on the extraction of Co and Ni in the mixed solution. The main conclusions are as follows:
With the increase in time, the extraction ratios for Co and Ni were both increased, but the former was much higher than the latter. The maximum extraction ratios of Co was 98.55% and 99.34% in P507 and QP-ILs at time of 10 min, and the best βCo/Ni value was 1142 in QP-ILs at the same time.
The extraction capacity of QP-ILs for Co was larger than that of P507. The Co and Ni extraction processes by P507 and QP-ILs were both endothermic. Increasing temperature was in favor of Co and Ni extraction. The maximum extraction ratio and βCo/Ni value were 92.57% and 329.82 at the temperature of 85℃ in QP-ILs.
The pH value has a significant effect on extraction ratio and separation efficiency. The extraction ratios for Co and Ni were both increased with the increasing pH value. Under the condition of pH = 4.5, the extraction ratio of Co reached 97.67% and 99.12% in P507 and QP-ILs, respectively. While the optimal βCo/Ni value was 480.21 at pH4.0 in QP-ILs because of lower extraction ratio for Ni.
The concentration of extractant is an important factor affecting the extraction ratio and separation factor. They all rise with increasing concentration. If the concentration continues to increase, almost all cobalt ions are extracted into the organic phase. The best separation results occurred in 40% QP-ILs solution.
According to the above experimental results, the selectivity of the QP-ILs for Co can be used as a good extractant for separating them from weak acid solution. However, the separation efficiency is related to many factors and further research is needed.