Extraction of Cu(II) ions from aqueous solutions by free fatty acid-rich oils as green extractants

Journal of Water Process Engineering 33 (2020) 100997

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Extraction of Cu(II) ions from aqueous solutions by free fatty acid-rich oils as green extractants

T

Siti Fatimah Abdul Halima, Siu Hua Changa,*, Norhashimah Moradb a b

Faculty of Chemical Engineering, Universiti Teknologi MARA, Cawangan Pulau Pinang, 13500 Permatang Pauh, Penang, Malaysia School of Industrial Technology, Universiti Sains Malaysia, 11800 Penang, Malaysia

A R T I C LE I N FO

A B S T R A C T

Keywords: Extraction Cu(II) ions Jatropha oil Palm fatty acid distillate Palm kernel fatty acid distillate Green extractants

Three types of free fatty acid (FFA)-rich oil, i.e. Jatropha oil (JO), palm fatty acid distillate (PFAD) and palm kernel fatty acid distillate (PKFAD), were investigated into their potentiality as green extractants for Cu(II) ion extraction from aqueous solutions. The physicochemical properties of FFA-rich oils and their progressive phase separations with Cu(II)-containing aqueous solutions were studied. The effect of initial Cu(II) ion concentration on the extraction efficiency by different FFA-rich oils along with their pH-extraction isotherms and back-extraction (stripping) were also explored. The results revealed that PKFAD showed the most favorable physicochemical properties and phase separation behavior while achieving a high extraction efficiency (> 95%) among all the FFA-rich oils studied. Therefore, PKFAD is a prospective green exractant for Cu(II) ions from aqueous solutions.

1. Introduction In recent decades, heavy metals in industrial effluents have drawn considerable attention of both researchers and practitioners from all around the globe owing to their lethal toxicity at low concentrations (≥ 1 ppm of LD50 (median lethal dose) [1]), non-biodegradability, bioaccumulation, and inherent persistency [2]. Among the various heavy metals, copper is one of the toxic priority pollutants listed by the U.S. Environmental Protection Agency [3]. In particular, cupric (Cu(II)) ions, which are found ubiquitously in effluents produced by a wide variety of industries such as electroplating, mining, printed circuit board manufacturing, and metal finishing [4], have exacerbated various deleterious degradations towards the living organisms such as fish and shellfish [5] when effluents laden with Cu(II) ions are discharged directly into the waterways. The conventional effluent treatment techniques for Cu(II) ions include chemical precipitation [6], coagulation-flocculation [7], membrane filtration [8], ion exchange [9], and adsorption [10]. However, each of these techniques suffers from its own intrinsic shortcomings and none of them is universal in treating all kinds of industrial effluents. Established since the 1940s [11], solvent extraction is one of the prevalent effluent treatment techniques for metal ions due to its high efficiency, high selectivity, ease of operation, and low operating and maintenance costs [12]. Its working principle is based on the relative solubility of metal ions between an aqueous solution and an organic ⁎

solvent which are immiscible with each other [13]. Typically, the organic solvent is made up of an extractant and a diluent where the former binds and converts metal ions into extractable species while the latter governs the overall solvent conditions [14]. Nevertheless, the conventional extractants (di-2-ethylhexyl phosphoric acid (D2EHPA), Cyanex 272, LIX 84, Aliquat 336 [14]) and diluents (kerosene, nhexane, chloroform, toluene [14]) used in solvent extraction are mostly derived from petroleum resources which are detrimental to living organisms when they are released to the environment and could be inordinately expensive due to their finite resources. Of late, there is increasing research in the application of renewable and environmentally benign organic solvents in extracting metal ions from aqueous solutions. For instance, Chang et al. [15], Wahab et al. [16], and Bhatluri et al. [17] successfully utilized D2EHPA as an extractant and fresh or waste vegetable oil (soybean oil [15], palm oil [16], or coconut oil [17]) as a diluent to extract various metal ions (Cu(II) [15,16], Cd(II) [17], or Pb(II) [17] ions) from aqueous solutions. However, these organic solvents were not entirely green since one of the components of the solvents, i.e. D2EHPA (the extractant), was a hazardous petroleum product. In another study, Ong et al. [18] hydrolyzed the waste vegetable oil in a Cu(II)-containing aqueous solution to generate free fatty acids (FFAs) (green extractants) which concomitantly extracted Cu(II) ions by a cation-exchange mechanism. This simultaneous oil hydrolysis and solvent extraction process is a prospective effluent treatment technique in view of the totally green organic solvent used. However,

Corresponding author. E-mail address: [email protected] (S.H. Chang).

https://doi.org/10.1016/j.jwpe.2019.100997 Received 25 July 2019; Received in revised form 22 September 2019; Accepted 11 October 2019 2214-7144/ © 2019 Elsevier Ltd. All rights reserved.

Journal of Water Process Engineering 33 (2020) 100997

S.F.A. Halim, et al.

based on the ASTM D445 test method [27], whereas the water solubility was determined by measuring the equilibrium water content in an FFA-rich oils by means of the Karl Fischer titration method [28] after mixing the solvent with an aqueous solution containing 100 mg/L of Cu (II) ions and 200 mM of Na2SO4 (inert salt) under the following conditions: equilibrium pH (pHeq) of 4.0 [19,29], organic to aqueous phase ratio of 1.5:1, and temperature of 50 °C. While the FA compositions were analyzed in accordance with the standard AOAC 996.06 test method [30], the FFA contents were determined through a titration method as described by Zenevicz et al. [31]. The latter began with mixing 2 g of an FFA-rich oil with 30 mL of an equivolume solution of anhydrous ethanol and diethyl ether. The mixture was then titrated against a 0.1 M KOH solution with phenolphthalein as an indicator. Lastly, the FFA content of the FFA-rich oil was calculated by [31]:

this process is more complex and incurs a higher operating cost than the traditional solvent extraction due to the more severe operating conditions (225 °C vs. 25 °C of operating temperature, 480 min vs. 6 min of operating time, pre-treatment of product for glycerol removal vs. no pre-treatment of product, sensitive extraction condition that requires an absence of oxygen vs. non-sensitive extraction condition [18,19]) involved. To reap the benefits of FFAs as green extractants and to avoid the additional hydrolysis process during solvent extraction as reported by Ong et al. [18], three types of naturally FFA-rich oils, namely, Jatropha oil (JO) (with 14 wt% FFAs [20,21]), palm fatty acid distillate (PFAD) (with 90 wt% FFAs [22]), and palm kernel fatty acid distillate (PKFAD) (with 80-90 wt% FFAs [23]) were studied as the potential renewable and green extractants (without any diluents) for Cu(II) ion extraction from aqueous solutions in this work. In general, JO is non-edible vegetable oil which is mainly converted into biodiesel for use in diesel engines [24]. PFAD and PKFAD, on the other hand, are by-products from the physical refining of crude palm oil [22] and crude palm kernel oil [23], respectively, both of which are widely utilized as fatty acid (FA) sources for oleo-chemical, soap and animal feed industries [25,26]. As much as we know, these FFA-rich oils have not been used to extract metal ions from aqueous solutions. To this end, the physicochemical properties of JO, PFAD, and PKFAD were first determined and their progressive phase separations with Cu(II)-containing aqueous solutions were investigated. Next, the effect of initial Cu(II) ion concentration on the extraction efficiency was studied and the pH-extraction isotherms of Cu(II) ions with JO, PFAD, and PKFAD were explored. The roles of JO, PFAD, and PKFAD as extractants for Cu(II) ions were also discussed and verified. Lastly, back extraction or stripping of Cu(II) ions from Cu(II)-loaded PKFAD was investigated with different stripping agents.

FFA content (wt%) =

VKOH × MKOH × MMFFA 10 × Ms

(1)

where VKOH is the volume of KOH solution (ml) used in the titration, MKOH is the molarity of KOH solution (mol/L), MMFFA is the average molar mass of FFAs, and MS is the sample mass (g). Other properties of FFA-rich oils such as melting point and density were obtained from the literature [32–34].

2.4. Preparation of aqueous and organic phases In extraction experiments, the aqueous phases containing specific initial concentrations (100, 500, and 1000 mg/L) of Cu(II) ions were prepared by dissolving appropriate amounts of CuSO4·5H2O in distilled water loaded with 200 mM Na2SO4, whereas the organic phases were prepared with different FFA-rich oils, i.e. JO, PFAD, and PKFAD, without any pre-treatment or addition of chemicals. In stripping experiments, the organic phase were Cu(II)-loaded PKFAD obtained from the extraction experiments, whereas the aqueous phases were prepared from various types of stripping agents (H2SO4, HNO3 and HCl) at different concentrations from 0.5 to 2.0 M.

2. Experimental study 2.1. Materials The FFA-rich oils were supplied by local vendors, i.e. JO by Bionas Sdn. Bhd., PFAD and PKFAD by Sime Darby Sdn. Bhd., all of which was used directly without further purification. Copper sulphate pentahydrate (CuSO4.5H2O) (Merck, ≥ 99.6% purity), diethyl ether (J.T.Baker, ≥ 99% purity), anhydrous ethanol (HmbG Chemicals, ≥ 99.85% purity), sulfuric acid (H2SO4) (Merck, ≥ 98% purity), hydrochloric acid (HCl) (Fisher Scientific, ≥ 37% purity), nitric acid (HNO3) (Fisher Scientific, ≥ 65% purity), sodium hydroxide (NaOH) (Qrec, ≥ 99% purity), potassium hydroxide (KOH) (Qrec, ≥ 99% purity), sodium sulfate (Na2SO4) (Qrec, ≥ 99% purity), and phenolphthalein (Merck, 1% in ethanol) were used as received.

2.5. Extraction procedure Extraction of Cu(II) ions was performed based on the shake-out test as delineated in our previous work [35]. Firstly, a volume of 22.5 mL FFA-rich oil (JO, PFAD or PKFAD) was mixed with a prepared Cu(II)containing aqueous solution at 1.5:1 organic to aqueous volume ratio in a glass-stoppered conical flask. The flask was then agitated on an incubator shaker at 150 rpm and 50 °C for 8 min. The mixture was left to settle for 5 min for phase separation. Next, about 10 ml sample was taken from the aqueous phase with a syringe and the pH of the sample was measured with a pH meter. If the desired pHeq was not obtained, the pH of the aqueous phase was adjusted with 1 M H2SO4 or 1 M NaOH and the mixture was mixed for another 8 min. This step was repeated until the desired pHeq was obtained. Finally, the mixture was transferred into a separating funnel for overnight phase disengagement and about 15 ml sample was withdrawn from the aqueous phase for chemical analysis with a FAAS. The percentage extraction (%E) of Cu(II) ions was calculated according to:

2.2. Equipment An incubator shaker (Lab Companion, SI-300) was used to mix the aqueous and organic phases and a pH meter (Hanna Instruments, HI11310) was used to measure the pH of aqueous phase before and after extraction. The concentrations of Cu(II) ions in aqueous phases after extraction were determined by a flame atomic absorption spectrophotometer (FAAS) (Shimadzu, AA7000), whereas the viscosities and functional groups of organic phases were analyzed by a digital viscometer (Graigar, NDJ-8S) and an attenuated total reflectanceFourier transform infrared (ATR-FTIR) spectroscopy (Shimadzu, IRPrestige-21), respectively.

%E=

[Cu]i,aq − [Cu]f,aq [Cu]i,aq

X 100%

(2)

where [Cu]i,aq and [Cu]f,aq are the initial and final Cu(II) ion concentrations, respectively, in the aqueous feed phase. All extraction experiments were carried out in duplicate or triplicate at 50 °C and the relative standard deviation between replicate samples within an experiment range was less than 3%.

2.3. Characterization of FFA-rich oils The FFA-rich oils, i.e. JO, PFAD, and PKFAD, used in this work were characterized for their viscosities, water solubility, FA compositions, and FFA contents. The viscosity was measured with a digital viscometer 2

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Among these factors, carbon chain length appears to be the most fitting in explaining the viscosity order of FFA-rich oils obtained in this work. In this regard, JO, which was rich in 18-carbon oleic acid (42.0 wt%), was the most viscous, followed by PFAD that was abundant in 16-carbon palmitic acid (49.7 wt%) and PKFAD that was loaded with 12-carbon lauric acid (49.1 wt%) as depicted in Table 1. Similar findings on the increasing viscosity of fatty acids with their carbon chain length had also been reported by several researchers [40,41]. Nonetheless, all these FFA-rich oils were substantially more viscous than the conventional petroleum-based organic solvents used in SX, for instance kerosene (1.64 mPa.s @ 25 °C [14]), n-hexane (0.297 mPa.s @ 25 °C [14]), and chloroform (0.53 mPa.s @ 25 °C [14]). This is an undesirable trait since the viscous FFA-rich oils would reduce their mobility and hinder the transport of solutes across them during the SX process. Providentially, the high viscosities of FFA-rich oils could be moderated by heating which would reduce the intermolecular cohesive forces in the oils [42], mixing or stirring that would minimize the boundary layer resistance (concentration polarization) in the oils [14], adding suitable viscosity reducers to the oils [43], and so forth. In spite of the greater viscosities of FFA-rich oils than those of the conventional organic solvents, it is noteworthy that JO was as viscous as vegetable oil (44–54 mPa.s @ 25 °C [14]), while PFAD and PKFAD were about a factor of two less viscous than vegetable oil. This implies that FFA-rich oils, particularly PFAD and PKAD, were more favorable than vegetable oil to be used as green organic solvents for SX, at least from the lower viscosity point of view. With respect to water solubility, PKFAD was found to dissolve the most amount of water (0.983 wt% @ 50 °C), followed by JO (0.511 wt% @ 50 °C) and PFAD (0.389 wt% @ 50 °C) as shown in Table 1. This suggests the order of decreasing polarity of PKFAD > JO > PFAD which may be explained by the variations in their FFA contents and FA chain length. On the whole, FFAs are FAs that occur as free (unesterified) molecules in nature as opposed to FAs which are often esterified to lipid molecules [44]. Being free molecules, FFAs carry polar heads of free eOH groups of the carboxylic acid moieties which interact favorably with water compared to their lipid bound-FA counterparts that have no free eOH [44]. As a result, the higher the FFA content of an oil, the more water soluble it is. In light of the highest FFA content of PKFAD (92.2 wt%), along with the shortest carbon chain length of its bulk 12-carbon lauric acid, PKFAD exhibited the greatest water solubility (0.983 wt% @ 50 °C) among the FFA-rich oils (Table 1). Nevertheless, the larger FFA content of PFAD (85.0 wt%) and the shorter carbon chain length of its bulk 16-carbon palmitic acid than those of JO (FFA content of 16.9 wt% and bulk 18-carbon oleic acid) did not result in the larger water solubility of PFAD. This may be attributed to the smaller degree of unsaturation of PFAD than that of JO (unsaturated to saturated FA ratio of PFAD of 1:1 vs. that of JO of 3:1 (Table 1)) which had rendered it less polar and, thus, dissolved less water than JO (0.389 wt% @ 50 °C vs. 0.511 wt% @ 50 °C (Table 1)). In spite of their abilities in dissolving trace amounts of water (< 1 wt%), all FFA-rich oils are practically non-polar due to their extremely low dielectric constants, i.e. < 4 [45,46], and, therefore, could be used as the potential organic solvents for SX. In a nutshell, PKFAD exhibited the most favorable physicochemical properties for solvent extraction among all FFA-rich oils studied on account of its low melting point of 22 °C, low density of 0.89 g/cm3 @ 70 °C, moderate viscosity of 23.0 mPa.s @ 50 °C, high FFA content of 92.2 wt%, and low water solubility of 0.983 wt% @ 50 °C (Table 1). PFAD and JO were less favorable due to the high melting point of 45 °C for the former and the high viscosity of 50.2 mPa.s @ 50 °C for the latter.

2.6. Stripping procedure The stripping experiments were conducted by using the same procedures as the extraction experiments, but without the pHeq adjustment after mixing of aqueous and organic phases. An organic to aqueous volume ratio of 1:1 and a temperature of 25 °C were applied throughout all the stripping experiments. The percentage of stripping (%S) of Cu(II) ions was given by:

%S =

[Cu] f , strip [Cu]i, aq − [Cu] f , aq

X 100%

(3)

where [Cu]f,strip is the final concentration of Cu(II) ions in the aqueous stripping phase. All stripping experiments were carried out in duplicate or triplicate and the relative standard deviation between replicate samples within an experiment range was less than 4%. 3. Results and discussion 3.1. Physicochemical properties of FFA-rich oils The physicochemical properties of an organic solvent govern the extraction behavior of the organic solvent and determine its suitability for use in SX [13]. Table 1 shows the physicochemical properties (melting point, density, viscosity, FA composition, FFA content, and water solubility) of different FFA-rich oils (JO, PFAD, and PKFAD) studied in this work. It can be seen that the melting points of JO and PKFAD were well below room temperature (25 °C), i.e. 4.5 °C and 22 °C, respectively, whereas that of PFAD was above room temperature, i.e. 45 °C. As a result, JO and PKFAD were literally liquids at ambient conditions and could be employed as the potential organic solvents for ambient SX, whereas PFAD was semi-solid and could only be used for high-temperature SX. Nevertheless, all these FFA-rich oils were found to have analogous densities of less than 1 g/cm3, i.e. 0.86-0.89 g/cm3 (Table 1), suggesting that all of them float on water. This provides a key insight concerning the configuration of aqueous and organic phases when they are applied in an SX system. Table 1 also reveals that JO was significantly more viscous (50.2 mPa.s @ 50 °C) than PFAD (24.3 mPa.s @ 50 °C) and PKFAD (23.0 mPa.s @ 50 °C) and these findings are in good agreement with the previous works [36–39]. Being lipid materials, FFA-rich oils owe their viscosities to three key features of FA composition: carbon chain length, degree of unsaturation, and double-bond configuration [40]. In general, the viscosity of a lipid material increases with the carbon chain length, but decreases with the degree of unsaturation and the cis double-bond configuration, of its bulk FAs [41]. Table 1 Physicochemical properties of JO, PFAD and PKFAD. Physicochemical properties

JO

PFAD

PKFAD

Melting point (oC) Density @ 70 °C (g/cm3) Viscosity @ 50 °C (mPa.s) FA (C:Dd) composition (wt%) Lauric acid (12:0) Myristic acid (14:0) Palmitic acid (16:0) Stearic acid (18:0) Oleic acid (18:1 cis) Linoleic acid (18:2 cis) Water solubility @ 50 °C (wt%) FFA (wt%) Unsaturated to saturated FA ratio

4.5a 0.89a 50.2

45b 0.86b 24.3

22c 0.89c 23.0

0.0 0.1 14.9 7.9 42.0 33.4 0.511 16.9 3:1

0.2 1.2 49.7 4.4 34.4 8.1 0.389 85.0 1:1

49.1 17.0 10.0 2.3 11.0 1.6 0.983 92.2 1:7

a b c d

3.2. Progressive phase separations of FFA-rich oils with Cu(II)-containing aqueous solutions

[32]. [33]. [34]. C:D = number of carbon atoms: number of double bonds.

Rapid separation of the organic and aqueous phases without 3

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Fig. 1. Progressive phase separations of (a) JO, (b) PFAD, and (c) PKFAD from Cu(II)-containing aqueous solutions (i) without and (ii) with pH adjustments.

(24.3 mPa.s @ 50 °C (Table 1)) and lowest water solubility (0.389 wt% @ 50 °C (Table 1)) among the FFA-rich oils. The low viscosity of PFAD enhanced the creaming rate of PFAD droplets from emulsion to organic layer according to Stoke’s Law [47], while the lowest water solubility in PFAD indicates its lowest polarity among the FFA-rich oils which prompted its separation from the polar aqueous solution based on the ‘like dissolves like’ aphorism [19]. Between PKFAD- and JO-aqueous systems without pHeq adjustment, the former was faster in breaking up the emulsion layer due to its much lower viscosity (23.0 mPa.s @ 50 °C vs. 50.2 mPa.s @ 50 °C (Table 1)), albeit its higher water solubility (0.983 wt% @ 50 °C vs. 0.511 wt% @ 50 °C (Table 1)), than the latter. With pHeq adjustment, however, soap was formed in both the PFADand PKFAD-aqueous systems following the addition of 1 M NaOH during pHeq adjustment, which in turn increased the emulsion breaking time, i.e. from 1.67 min without pHeq adjustment (Fig. 1b(i)) to > 300 min with pHeq adjustment (Fig. 1b(ii)) for PFAD-aqueous system and from 3 min without pHeq adjustment (Fig. 1c(i)) to 5 min with pHeq adjustment (Fig. 1c(ii)) for PKFAD-aqueous system. The substantially larger increment in the emulsion breaking time encountered by the PFAD-aqueous system than that by the PKFAD-aqueous system when pHeq was adjusted from 3.4 to 4.0 may be attributed to the greater amount of soap (surfactant) formed in the PFAD-aqueous system (Fig. 1b(ii)) as a result of its higher proportion of unsaturated FAs (unsaturated to saturated FA ratio of 1:1 vs. 1:7 (Table 1)) which were more reactive than their saturated counterparts due to their double bonds [49]. This finding is consistent with that of Kamba et al. [48] who reported the increasing emulsion stability with surfactant concentration. Meanwhile, no soap was formed in JO-aqueous system when pHeq was adjusted from 5.4 to 4.0 due to the addition of 1 M H2SO4 instead of 1 M NaOH into the system. Consequently, JO-aqueous system did not undergo a significant change in the emulsion breaking time when pHeq was adjusted, i.e. from 5 min without pHeq adjustment (Fig. 1a(i)) to 5.5 min with pHeq adjustment (Fig. 1a(ii)). On top of the variations observed in the emulsion breaking time among the organic-aqueous systems and between those without and with pHeq adjustment within an organic-aqueous system, the color

emulsion or crud formation at the organic-aqueous interface is a desirable trait in SX. In this work, the progressive phase separations of different FFA-rich oils, i.e. JO, PFAD, and PKFAD, from Cu(II)-containing aqueous solutions were investigated without and with pHeq adjustments (Fig. 1). The initial Cu(II) ion concentration in the aqueous solution was maintained at 100 mg/L throughout the investigation. Without pHeq adjustment, the pHeq of the JO-, PFAD-, and PKFADaqueous systems were attained at 5.4, 3.4, and 3.4, respectively. To adjust and maintain the same pHeq of 4.0 across different organicaqueous systems, a few drops of 1 M H2SO4 was added to the JO-aqueous system, while a trace amount of 1 M NaOH was added to both the PFAD- and PKFAD-aqueous systems. As can be seen in Fig. 1, a thick and bulky emulsion layer that almost filled up the entire separating funnel was formed between the organic and aqueous layers at the outset of phase separation, i.e. t = 0, in all the organic-aqueous systems, either without or with pHeq adjustment. This unstable emulsion layer then broke up with time due to creaming, flocculation, and coalescence [47] until complete phase separation occurred (Fig. 1). The emulsion breaking time, i.e. the time required to break up the emulsion layer, however, varied from one organic-aqueous system to another and between those without and with pHeq adjustments within an organic-aqueous system. Without pHeq adjustment, the increasing order of the emulsion breaking time was PFAD-aqueous system (1.67 min (Fig. 1b(i))) < PKFAD-aqueous system (3 min (Fig. 1c(i))) < JO-aqueous system (5 min (Fig. 1a(i))), whereas with pHeq adjustment, the order was PKFAD-aqueous system (5 min (Fig. 1c(ii))) ≈ JO-aqueous system (5.5 min (Fig. 1a(ii))) < < PFADaqueous system (300 min (Fig. 1b(ii))). The former order could be deduced from the variations in the viscosity and water solubility among the FFA-rich oils, while the latter was influenced by the presence of soap, i.e. sodium salts of FAs due to the reaction of FAs with NaOH, in the system which could act as a surfactant to stabilize the emulsion layers by reducing the liquid-liquid interfacial tension and prolong the emulsion breaking time [48]. Without pHeq adjustment, PFAD-aqueous system took the shortest time to break up the emulsion layer due to its relatively low viscosity 4

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Fig. 2. Effect of initial Cu(II) ion concentration on %E of Cu(II) ions by JO (○), PFAD (□), and PKFAD (Δ). Fig. 3. pHeq-extraction isotherms of Cu(II) ions with JO (), PFAD (), and PKFAD ().

intensity of the organic layer within an organic-aqueous system also experienced notable changes when pHeq was adjusted, particularly for PFAD- (Fig. 1b(i, ii)) and PKFAD-aqueous (Fig. 1c(i, ii)) systems where the color intensity of the organic layers changed from somewhat lighter to much darker. This suggests that there may have been a steep rise in the concentration of Cu(II) ions in the organic layers of PFAD- and PFKAD-aqueous systems when pHeq was adjusted from 3.4 to 4.0. For JO-aqueous system, on the other hand, a subtle change in the color intensity of the organic layer from darker to lighter implies a decline in the Cu(II) ion concentrations in the organic layer of JO-aqueous system when pHeq was adjusted from 5.4 to 4.0 (Fig. 1a(i, ii)).

3.4. pHeq-extraction isotherms A pHeq-extraction isotherm is crucial to predicting the extraction characteristics of an organic solvent with respect to a solute under a variety of conditions. Hence, the pHeq-extraction isotherms of Cu(II) ions with various FFA-rich oils were investigated and the results were shown in Fig. 3. The initial Cu(II) ion concentration in the aqueous solutions was maintained at 100 mg/L throughout the investigation. In general, a sigmoid curve was obtained for all the pHeq-extraction isotherms, where the %E of Cu(II) ions was minimum (< 20%) at pHeq of less than 3.40 for all FFA-rich oils, followed by a dramatic increase from 23 to 88% at pHeq of 4.0 to 4.8, 18 to 93% at pHeq of 3.4 to 4.2, and from 8 to 92% at pHeq of 3.4 to 4.5 for JO, PFAD, and PKFAD, respectively, achieving the maximum (> 95%) at pHeq of 5.4, 4.7, and 4.7 for JO, PFAD, and PKFAD, respectively, and plateauing thereafter for all FFA-rich oils (Fig. 3). Owing to the high pHeq dependency of different FFA-rich oils in Cu (II) ion extraction, it is presumed that the active components that bound with Cu(II) ions during the extraction process were the FFAs contained in FFA-rich oils since they possessed acidic protons (H+) in their free eOH groups of carboxylic acid moieties [55] and thus could function as cationic extractants [56]. According to the extraction reaction of Cu(II) ions with FFAs (Eq. 4), FFA molecules extract Cu(II) ions by releasing and exchanging their H+ reversibly with Cu(II) ions where its equilibrium position shifts to the right and increases the %E when H+ decreases (increasing pHeq), and shifts to the left and reduces the %E when H+ increases (decreasing pHeq). The pH-extraction isotherms (Fig. 3) obtained in this work are consistent with those reported in similar previous studies [19,57–59]. They also justify the earlier conjectures about the plausible drastic increments in the Cu(II) ion concentrations in the organic layers of both PFAD- and PFKAD-aqueous systems when adjusting the pHeq from 3.4 to 4.0 (Fig. 1 b(i,ii) and c (i,ii)) based upon the remarkable rise in the %E of Cu(II) ions from pHeq of 3.4 (18% for PFAD and 8% for PKFAD (Fig. 3)) to 4.0 (85% for PFAD and 75% for PKFAD (Fig. 3)), as well as the possible reduction in the Cu (II) ion concentrations in the organic layers of JO-aqueous system when adjusting the pHeq from 5.4 to 4.0 (Fig. 1 a(i,ii)) due to the decline in the %E of Cu(II) ions from pHeq of 5.4 (96% (Fig. 3)) to 4.0 (23% (Fig. 3)). As can be seen in Fig. 3, the pHeq-extraction isotherm of Cu(II) ions with PFAD exhibited the greatest steepness of the sigmoid curve, followed by those with PKFAD and JO. This implies that PFAD was the strongest extractant, with a pH50 (pHeq at which 50% of Cu(II) ions was extracted) of 3.5, PKFAD was the intermediate extractant, with a pH50 of 3.8, and JO was the weakest extractant, with a pH50 of 4.3 (Fig. 3). This is the characteristic of cationic extractants in which a low pH50 of the extractant for a solute signifies its high strength and propensity in extracting the solute from highly acidic aqueous media, whereas a high pH50 indicates its low strength and inclination in extracting the solute

3.3. Effect of initial Cu(II) ion concentration on extraction efficiency Since the initial solute concentration in aqueous solutions often varies from one source to another, it is vital to study the effect of initial solute concentration on the extraction efficiency of a solvent for the solute. Fig. 2 shows the effect of different initial Cu(II) ion concentrations (100, 500, and 1000 mg/L) on the %E of Cu(II) ions by various FFA-rich oils (JO, PFAD, and PKFAD) at pHeq of 4.0. It reveals that PFAD achieved the highest %E of Cu(II) ions throughout the different initial Cu(II) ion concentrations studied, followed closely by PKFAD and then JO. Regardless of the types of FFA-rich oil studied, %E of Cu(II) ions were the lowest at 100 mg/L of initial Cu(II) ion concentration, i.e. 23% for JO, 85% for PFAD and 75% for PKFAD. They then enhanced steadily with the initial Cu(II) ion concentration from 100 to 500 mg/L, reached or closely approached a small peak at 500 mg/L, i.e. 36% for JO, 95% for PFAD and 91% for PKFAD, and plateaued or declined somewhat thereafter to 1000 mg/L (Fig. 2). The rise in the %E of Cu(II) ions from 100 to 500 mg/L attained by all FFA-rich oils (Fig. 2) was ascribed to the increase in the driving force for Cu(II) ion transfer which caused the equilibrium position of the extraction reaction of Cu(II) ions with FFAs to shift to the right and improved the extraction. The extraction reaction of Cu(II) ions with FFAs is expressed as: 2+ Cuaq +

2+m + (HR)2, org ⇄ (CuR2 (HR)m )org + 2Haq 2

(4)

where the subscripts aq and org correspond to the aqueous and organic phases, respectively, (HR)2 represents the dimeric form of FFAs, and m is the number of monomeric FFAs engaged in the Cu(II)-organic complex. Meanwhile, the peaks achieved at 500 mg/L (Fig. 2), as well as the plateaus or drops obtained subsequently (Fig. 2), denote the increasing mass transfer resistance of Cu(II) ions from aqueous to organic phases which aggravated the extraction. Nevertheless, it should be noted that the absolute amounts of Cu(II) ion extracted increased with the initial Cu(II) ion concentration for all FAA-rich oils. These findings are comparable with other similar previous work [50,51]. Considering the typical range of Cu (II) ion concentration in real electroplating wastewater of 1.1–283 mg/L [52–54], the initial Cu(II) ion concentration of 100 mg/L was selected for further study. 5

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from lowly acidic aqueous media [55]. The ascending order of Cu(II) ion extraction strength of JO < PKFAD < PFAD obtained could be elucidated from the major FA contained in each of the oils. JO, which contained predominantly oleic acid (18:1 cis) (Table 1), was the weakest extractant due to the steric hindrance imposed by its long chain length of 18 carbons and cis configuration that inhibited the access to the complexing sites [49]. This finding agrees well with the decline in the extraction strength of different carboxylic acids for Ni(II) ions with their steric bulkiness as reported by Preston & Preez [60]. Nevertheless, between PFAD and PKFAD which were mainly composed of palmitic acid (16:0) and lauric acid (12:0) (Table 1), respectively, the greater steric hindrance imposed on palmitic acid by its longer chain length of 16 carbons did not seem to reduce its extraction strength for Cu(II) ions. This may be ascribed to the relatively lower polarity of PFAD than PKFAD as reflected by its lower water solubility (0.389 wt% @ 50 °C vs. 0.983 wt% @ 50 °C (Table 1)). Since Cu(II)-organic complexes were practically nonpolar due to their long nonpolar hydrocarbon tails, more Cu(II)-organic complexes could dissolve in PFAD rather than in PKFAD, once again, based on the principle of ‘like dissolves like’. In spite of the deviations in the extraction strength for Cu(II) ions among the FFA-rich oils, all of them were capable to extract more than 95% of Cu(II) ions from aqueous solutions at distinct pHeq, i.e. 4.7 for PFAD and PKFAD, and 5.4 for JO (Fig. 2), which were as good as, if not better than, the classical extractants for Cu(II) ions such as D2EHPA [61], Cyanex 272 [62] and LIX 84-I [63] diluted in various kinds of diluents.

3.5. Verification of the roles of FFA-rich oils as extractants To verify the roles of FFA-rich oils as extractants, with FFAs being the active components in Cu(II) ion extraction as proposed in Sec. 3.4, the pHeq of the aqueous phases in JO-, PFAD-, and PKFAD-aqueous systems were measured before and after extraction without pHeq adjustment (Fig. 4). It was discovered that the aqueous phases experienced considerable pHeq drops before and after extraction, i.e. from 5.7 to 5.4 for JO-aqueous system and from 5.7 to 3.4 for both PFAD- and PKFAD-aqueous systems as shown in Fig. 4. These pHeq drops denote the release of acidic H+ from FFA-rich oils into aqueous phases which were presumed to be derived from the FFAs contained in FFA-rich oils as shown in Eq. 4. This presumption was supported by the FTIR analysis of FFA-rich oils loaded with Cu(II) ions where the amplitudes of the strong and broad C]O stretching bands of carboxylic acids, which spanned over a range from 1650 to 1790 cm−1 [64], decreased with the Cu(II) ion concentration loaded in the oils (Fig. 5). This may be attributed to the increasing cleavage of OeH bonds in the carboxylic acid moieties at one end of the FFAs to liberate more acidic H+ for cation exchange with Cu(II) ions, which in turn diminished the intensity of C]O stretching bands of carboxylic acids with Cu(II) ions loaded in the oils. In fact, similar findings have been reported in our previous works [19,65].

Fig. 5. C]O stretching bands of carboxylic acids for (a) JO, (b) PFAD, and (c) PKFAD loaded with Cu(II) ions (···) and without Cu(II) ions (− − −).

3.6. Stripping of Cu(II) ions from Cu(II)-loaded PKFAD Stripping of extracted solutes from solute-loaded solvents is vital for solute recovery and solvent regeneration. In this work, stripping of Cu (II) ions from Cu(II)-loaded PKFAD was investigated with H2SO4, HNO3 and HCl at different concentrations (0.5–2.0 M) as the stripping agents and the findings obtained are depicted in Fig. 6. It was found that the % S of Cu(II) ions increased with the stripping agent concentration (0.5–2.0 M) from 90 to 95 and 88 to 93% for HNO3 and HCl, respectively, but decreased from 94 to 83% for H2SO4. The former was due to the higher H+ concentration at a greater molarity of HNO3 and HCl, while the latter was attributed to the substantially higher viscosity of H2SO4 (24 m·Pa·s@ 20 °C [66]) than the viscosities of HNO3 (0.75 m·Pa·s @ 25 °C [67]) and HCl (1.8 m·Pa·s @ 15 °C [68]) which increased with its concentration [69]. A similar finding was reported by Hu et al. [69] who observed the decline in %S of vanadium from vanadium-loaded hexane with H2SO4 concentration. Despite the negative effect of H2SO4 on %S of Cu(II) ions at high concentrations, its %S accomplished at a low concentration of 0.5 M (94%) was compatible with the %S of HNO3 (95%) and HCl (93%) achieved at a high

Fig. 4. pHeq of aqueous phases in different FFA-rich oil-aqueous systems before (x) and after (◊) extraction without pHeq adjustment. 6

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Acknowledgements This work was supported in part by the Ministry of Energy, Science, Technology, Environment and Climate Change (MESTECC), Malaysia under the E-Science Fund Project No: 03-01-05-SF0847. References [1] K.S. Egorova, V.P. Ananikov, Toxicity of metal compounds: knowledge and myths, Organometallics 36 (2017) 4071–4090, https://doi.org/10.1021/acs.organomet. 7b00605. [2] M.A. Barakat, New trends in removing heavy metals from industrial wastewater, Arab. J. Chem. 4 (2011) 361–377, https://doi.org/10.1016/j.arabjc.2010.07.019. [3] Electronic Code of Federal Regulations - 40 Protection of Environment - PART 141National Primary Drinking Water Regulations - Chapter 1, Subchapter D, Subpart I, Control of Lead and Copper, (1991) https://www.ecfr.gov/cgi-bin/retrieveECFR? gp=&SID=ba40130fba372ec72b573ecae3ecae17&mc=true&n=pt40.25.141&r= PART&ty=HTML#se40.25.141_180. [4] M. Bilal, J.A. Shah, T. Ashfaq, S.M.H. Gardazi, A.A. Tahir, A. Pervez, H. Haroon, Q. 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Fig. 6. %S of Cu(II) ions from Cu(II)-loaded PKFAD by various concentrations of H2SO4, HNO3 and HCl as stripping agents.

concentration of 2.0 M. This could be elucidated from the greater number of moles of H+ (2 mol) produced when one mole of H2SO4 dissociated compared to that (1 mol) of HNO3 or HCl when one mole of HNO3 or HCl dissociated. The greater number of moles of H+ liberated by H2SO4 moved the equilibrium position of the extraction equation (Eq.4) to the left and enhanced the %S. Owing to the high efficiency of H2SO4 at low concentrations, coupled with its low volatility (b.p 335 °C [66]) compared to those of HNO3 (b.p 83 °C [67]) and HCl (b.p 57 °C [68]), 0.5 M H2SO4 was the most preferable stripping agent for Cu(II) ion stripping from Cu(II)-loaded PKFAD from safety and environmental perspectives. 4. Conclusions Among all FFA-rich oils studied, palm kernel fatty acid distillate (PKFAD) exhibited the most favorable physicochemical properties for solvent extraction owing to its low melting point of 22 °C, low density of 0.89 g/cm3 @ 70 °C, moderate viscosity of 23.0 mPa.s @ 50 °C, high FFA content of 92.2 wt%, and low water solubility of 0.983 wt% @ 50 °C. Palm fatty acid distillate (PFAD) and Jatropha oil (JO) were less favorable due to the high melting point of 45 °C for the former and the high viscosity of 50.2 mPa.s @ 50 °C for the latter. PKFAD also showed the fastest (5 min) phase separation with Cu(II)-containing aqueous solutions at an equilibrium pH (pHeq) of 4.0, followed closely by JO (5.5 min) and then PFAD (> 300 min). They also shared a similar pattern (increased, reached a peak, and plateaued or declined slightly) of the initial Cu(II) ion concentration effect on Cu(II) ion extraction. A sigmoid curve was obtained for all the pH-extraction isotherms of Cu(II) ions investigated with different FFA-rich oils and their extraction strengths for Cu(II) ions were found to be in the ascending order of JO < PKFAD < PFAD. Despite their dissimilarities in extraction strengths, all FFA-rich oils were capable to extract more than 95% of Cu (II) ions from aqueous solutions at distinct pHeq, i.e. 4.7 for PFAD and PKFAD, and 5.4 for JO. The roles of FFA-rich oils as extractants, with FFAs being the active components in Cu(II) ion extraction, were examined and verified by the decreasing amplitudes of the carboxylic acid C]O stretching bands with the Cu(II) ion concentration loaded in FFArich oils. The most preferable stripping agent was found to be 0.5 M H2SO4 which stripped 94% of Cu(II) ions from Cu(II)-loaded PKFAD. Author contributions All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version. Declaration of Competing Interest None. 7

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