Solidification floating organic drop for dispersive liquid–liquid microextraction estimation of copper in different water samples

Safwan Mohammad Fraihat

doi:10.1515/gps-2025-0067

Article Open Access

Solidification floating organic drop for dispersive liquid–liquid microextraction estimation of copper in different water samples

Safwan Mohammad Fraihat

Published/Copyright: August 1, 2025

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From the journal Green Processing and Synthesis Volume 14 Issue 1

Abstract

A selective, sensitive, and eco-friendly microextraction method followed by the solidified floating organic drop-dispersive liquid–liquid microextraction was investigated to determine copper(ii) ions by spectrophotometry. The developed method depends on forming a colored complex between copper ion and 2-aminophenol with a maximum absorbance at a maximum absorption wavelength of 440 nm. This enables the estimation of copper ions in different water samples by direct spectrophotometry. The key factors that influence the stability of the colored association complex and the performance of the extractive method were studied and optimized. A quantitative range of 10–75 µg·L⁻¹ of copper was detected. The calibration equation was determined and found to have a slope of 0.0141 and an intercept of 0.0382. The limit of detection was 3.0 µg·L⁻¹, while the limit of quantification was 9.3 µg·L⁻¹. The relative standard deviation (n = 3) was evaluated to be 0.62%. Common ion interference effects were investigated, and the method was fairly selective. The proposed method was applied practically for the estimation of copper levels in common water samples. The reliability and accuracy of the developed method were proved by the analysis of different types of water samples.

Keywords: copper; dispersive liquid–liquid microextraction; spectrophotometry

1 Introduction

Copper is a significant trace element to countless living species to control the metabolic activities of glucose and lipid; in addition, copper is important to maintain heart and blood health. On the other hand, copper becomes hazardous at greater doses because of possible cell membrane interaction with copper, limiting transport across the cell wall [1]. It is found that a shortage of Cu is associated with anemia and other health complications, while excessive Cu has the potential to be detrimental and related to Parkinson’s and Alzheimer’s diseases [2].

Copper ions are released into environmental water from several industrial activities, for instance, dyeing, petrochemical, and metal plating, which discharge huge amounts of Cu into the environmental sites. Moreover, the widespread of copper in agricultural activities such as pesticides and fertilizers made the problem of well water contamination with copper a more serious problem for water plants and fish, which increase the levels of copper in the environment of fresh water, plants, and water-living organisms. Consequently, bioaccumulation of copper and other heavy metals in living organisms arises and merges into the food chain [3].

Many health organizations have set regulations for permissible levels of copper in different water samples. In drinking water, the acceptable permissible limit of copper is 2.0 mg·L⁻¹, according to World Health Organization [4], while the permitted level for the U.S. Environmental Protection Agency is 1.3 mg·L⁻¹ [5].

Numerous analytical techniques for identifying copper and other heavy metals rely on advanced quantitative techniques such as electrochemical methods, atomic absorption spectrometry (AAS), and inductively coupled plasma (ICP) [6,7,8,9]. The primary disadvantages of these methods, however, are the high energy consumption and instrumentation cost, as well as the possibility of spectrum interferences that could skew the results. Flame atomic absorption spectrometry is less sensitive, but it requires less equipment and upkeep. Other methods such as electrochemical [10], ultraviolet-visible spectrophotometry [11], and high performance liquid chromatography [12] have been reported for the determination of Cu in various samples.

The determination of heavy metals usually requires a sensitive and selective extractive method since these metals are usually present in very low levels in water and food samples, as well as for monitoring of water quality [13,14]. In most cases, these extractive methods are based on using selective chelating agents that react selectively with the requested metal ions.

Several extractive methods were developed for the estimation of metal ions, which are commonly used, such as solid-phase extraction [15] and liquid–liquid extraction techniques [16]. These extractive methods are complex and cost-effective. They are also time-consuming, labor-intensive, and usually use great amounts of hazardous organic solvents and reagents.

Among newly developed preconcentration methods is dispersive liquid–liquid microextraction (DLLME), which has overcome many complications associated with conventional liquid extraction and solid-phase extraction, such as long extraction time and low preconcentration factor. The method is based on using a mixture of micro-volumes of a mixture of extracting and dispersing solvents.

DLLME yields a greater analyte enrichment factor (EF) and uses a lower quantity of organic extracting solvent than more traditional classical procedures [17,18]. It uses a water-miscible, polar disperser solvent such as ethanol or acetonitrile and extracting organic solvents. The ease of use and quick extraction are two benefits of this method. The enhancement of the efficiency of the extraction is implemented by homogenizing both aqueous phase and organic phase containing the analyte, which can be employed in different ways such as heating, shaking by vortex, or ultrasound radiation. The organic phase is separated by the aid of centrifugation, and finally, the separated drop is diluted and analyzed with the appropriate technique [19,20,21,22].

One of the recently developed (DLLME) methods is based on using a low-density eco-friendly solvent that can extract the desired analyte as a solidified floating organic solvent (SFO-DLLME) [23,24,25]. Those methods are considered simple preconcentration ways that improve the detection limits (LODs) of analytes. The basis of the SFO-DLLME methodology depends on holding the analyte in the aqueous mixture with a tiny drop of the extraction solvent, which has a lower density than the aqueous phase with a low melting point. As a result, a floating solidified analyte-rich phase will be separated and removed. SFO-DLLME methods were developed to enhance the greenness and the extraction efficiency using some cofactors, which include ultrasonic-assisted extraction or vortexing-assisted extraction and, respectively [26]. The surface area between the solutes and the solidified floating organic droplets is increased by these factors, which speeds up the mass transfer rate of the solutes into the organic drops.

Recently, deep eutectic solvents were implemented as extracting media; these solvents are formed by the interaction between hydrogen bond acceptor materials and hydrogen bond donor materials under continuous stirring and heating until the homogenous solution is obtained. The deep eutectic solvents can be made from combinations of non-toxic and eco-friendly materials. Some researchers have investigated deep eutectic solvent applications for the extraction of chromium from environmental samples with improved results [27,28,29,30,31].

However, the studies related to the preparation of deep eutectic solvents with different starting materials and their utilization for microextraction are still limited.

Figure 1: Major steps of the proposed procedure.

Figure 1

Schematic diagram of SFO-DLLMS general steps: (a) Water sample plus buffer and salt, (b) Triton added, (c) vortex shaking, (d) centrifugation and then cooling, and (e) measurement of absorbance.

This study aimed to develop a sensitive and selective preconcentration liquid-microextraction method based on a solidified floating organic drop DLLME (SFO-DLLME) coupled with spectrophotometric analysis. In this method, 2-Amino phenol (2AMP) was used as a coloring reagent for the selective analysis of copper ions in aqueous solutions. The method used ethanol as a dispersive solvent and dodecanol as an extracting solvent; the significant factors that influence the extraction efficiency include extracting and dispersing solvents, vortex time, pH, and the extracting solvent, which were examined and optimized. The method was successfully applied for real water samples.

During the design of novel analytical methods, the toxicity and hazard of the reagents used are just as significant as any other analytical feature [32]. According to the 12 tenets of green chemistry, it is therefore imperative to create techniques that are less hazardous to both people and the environment. The developed preconcentration method could be used in conjunction with other analytical techniques, such as AAS, inductively coupled plasma-optical emission spectrometry, and inductively coupled plasma-mass spectrometry. If instrumentation such as this is used, then a much lower detection limit could be obtained.

2 Materials and methods

2.1 Instrument

The spectrophotometer used was a double-beam (Shimadzu 1900-Japan) with a wavelength range of 200–900 nm, a bandwidth of 2.0 nm, and a scanning speed of 200 nm·min⁻¹. It is accurate within 0.2 nm of wavelength. Microcells measuring 10.0 mm in diameter were used to measure the absorbance of solutions. The pH meter (model 8521, Hanna, NY, USA) was used in this study to record the pH measurements, vortex shaker (model heidolph), Germany, and centrifuge LW scientific.

2.2 Reagents

Analytical-grade chemicals were used, and highly purified water was used during this study.

Pure CuCl₂ and (2AMP) were purchased from Fluka. Acetic acid–sodium acetate buffer (HA/NaA), NaCl, triton X-100, triton X-114, sodium lauryl sulfate (SLS), ammonium cetyl bromide (CTAB) were purchased from Merck. Undecanol, octanol, dodecanol, HCl, and sodium hydroxide were from Sigma. Double-distilled water was used to prepare aqueous solutions.

A standard stock solution of 100 µg·mL⁻¹ (7.4 × 10⁻³ M) of copper chloride was used to prepare serial dilutions. 2AMP was prepared freshly equivalent to 1.00 × 10⁻² mol·L⁻¹ in absolute ethanol.

About 2% surfactant and 5.0% NaCl solutions were prepared in distilled water; buffer solutions were prepared from 0.10 M (HA/NaA) solutions, and the pH was adjusted using HCl and NaOH solutions.

2.3 Methods

2.3.1 DLLME-SFO procedure

Aqueous solutions containing appropriate amounts of copper were adjusted to pH 5.0 using 1.0 mL of the buffer solution in a 10.0 mL graduated conical centrifuge tube. An aliquot of 2AMP (1.0 mL of 0.10 g/100 mL) prepared in ethanol (disperser solvent), 1.0 mL of NaCl solution, and 1.0 mL of Triton-100 solution were added. Then, the mixture was shaken in a water bath at 60°C for 30 min; then, it was equilibrated to room temperature, and then, 200 µL of dodecanol was added and vortexed for 2 min. The formation of a cloudy solution appeared as a result of the dispersion of the fine droplets of extraction solvent in the sample solution. Then, the solution was centrifuged at 4,000 rpm for 2 min. The dodecanol-containing phase was separated at the top of the sample solution. The conical tube was then kept in an ice bath, and the dodecanol layer was solidified after 5 min and attached to the inner surface of the tube. The aqueous phase was efficiently extracted, while the solidified solvent underwent immediate liquefaction and was then diluted to 1.0 mL with ethanol. The solution was then transferred into the microcuvettes for measuring the absorbance at 440 nm. This procedure was repeated for the calibration standards and the blank solutions.

3 Results and discussion

3.1 Absorption spectra

The developed method is based on the reaction of copper ions in an aqueous medium with 2AMP as a chelating agent, producing a colored association complex with a maximum absorption at about 440 nm (Figure 2). The resultant complex is extracted with a selected organic solvent (dodecanol), with the aid of a dispersing solvent (ethanol) and the surfactant (Triton X-100), which facilitate the extraction performance of copper ions from aqueous to the surface floating organic extracting solvent.

Figure 2

Absorption spectrum of Cu-2AMP complex (orange series) and 2AMP (blue series).

3.2 Stoichiometric ratio of copper and 2AMP (Job’s method)

The method of continuous variation [33] was employed by using equimolar (1.00 × 10⁻⁴ M) solutions of CuCl₂ and 2AMP in water and ethanol, respectively. A series of 1.0 mL portions of Cu and 2AMP were made up comprising different complementary volumes in 10 mL calibrated measuring flasks. Absorbance was plotted against different ligand-to-metal mole ratios, as shown in Figure 3.

Figure 3

Effect of variation of (copper mole ratio) on the efficiency of separation: Experimental conditions: 100 µL of standard 5.00 mg·L⁻¹ of Cu solution, with 1.0 mL of each of the following solutions: 0.10 M HA/NaA buffer (pH = 5.0, 5.0% NaCl, 2.0% of selected surfactant and different using 1.00 × 10⁻⁴ M of each copper and 2AMP solution.

It is clear that the stoichiometry of the complex is (1:2) metal-to-ligand mole ratio.

3.3 Effect of different factors on the extraction efficiency

Several factors that affect the formation and stability of the associated complex and extraction efficiency include pH, chelating agent concentration, shaking duration, type and volume of the extracting solvent, which must be adjusted in order to achieve a high EF.

The effects of these variables are discussed in the following:

3.3.1 Effect of the concentration of 2AMP

The impact of the amount of 2AMP was studied in the range of (0.005–0.05) mol·L⁻¹; it was observed that the extraction efficiency increased up to 1.0 × 10⁻⁴ Mol·L⁻¹ and then remained constant above 2.00 × 10⁻⁴ Mol·L⁻¹, as shown in Figure 4.

Figure 4

Effect of concentration of 2AMP on the efficiency of separation.

Experimental conditions: 100 µL of standards 5.00 mg·L⁻¹ Cu solution, with 1.0 mL of each of the following solutions: 0.10 M HA/NaA buffer (pH = 5.0, 5.0% NaCl, 2.0% of selected surfactant.

3.3.2 Effect of extracting solvent

Extracting solvent must meet certain criteria, including low density, immiscibility with water, low volatility, having a melting point close to room temperature, non-toxic, and the ability to dissolve copper complex efficiently.

Selecting the extracting solvent is very important to achieve the selectivity of the extracted complex. To examine the effect of different extracting solvents on the enhancement of the DLLME into the organic solvent, octanol, decanol, undecanol, and dodecanol were used. The results are shown in Figure 5, which reveals that dodecanol is the preferred extracting solvent due to its maximum ability to extract the associated complex formed between copper ions and 2-aminophenol in solution. The volume of dodecanol extracting solvent was examined in the range of 50–200 µL. The results show that the extraction efficiency of copper ions from an aqueous solution is affected by the volume used. Therefore, 200 µL of dodecanol was selected as a suitable volume of the extracting in subsequent experiments.

Figure 5

Effect of type of extracting solvent.

Experimental conditions: 100 µL of standard 5.00 mg·L⁻¹ of Cu solution with 1.0 mL of each of the following solutions: 1.0 mL 2AMP, 0.10 M HA/NaA buffer (pH = 5.0, 5.0% NaCl, 2.0% of Triton X-100, and 200 µL of different types of extracting solvent.

3.3.3 Effect of pH

One of the limiting factors that influence the formation and stability of the complex between copper ions and 2AMP is the pH of the solution. Moreover, the pH affects the efficiency of the extraction process.

As shown in Figure 6, the maximum recovery was obtained at pH around 5.0; this value was used in further experiments; it was noted that at higher pH, turbidity starts to appear, whereas at lower pH, the recovery decreases.

Figure 6

Effect of pH of the phase on Cu extraction efficiency.

Experimental conditions: 100 µL of 5.00 mg·L⁻¹ Cu solution, with 1.0 mL of each of the following solutions: 1.0 mL of 2AMP, 0.10 M HA/NaA buffer of selected pH, 5.0% NaCl, and Triton-X100.

3.3.4 Effect of nature of surfactant

The selection of a suitable surfactant is essential to a successful extraction procedure. A variety of surfactants, including Triton-X-100, Triton-X-114, CTAB, and SLS, were investigated.

The influence of the nature and volume of these micelles on DLLME experiment showed that the efficiency and speed of the extraction were highly enhanced using (2%) of Triton-X100, as shown in Figure 7; this result indicates that the complex formed between the copper ion and the organic ligand is mostly non-ionic forming miscible phase with the non-ionic surfactant and highly soluble with the extracting organic solvent.

Figure 7

Effect of surfactant type on Cu extraction efficiency.

Experimental conditions: 100 µL of 5.00 mg·L⁻¹ Cu solution, with 1.0 mL of each of the following solutions: 2AMP, 0.10 M HA/NaA buffer (pH = 5.0, 5.0% NaCl, 2.0% of selected surfactant.

3.3.5 Effect of heating time

The influence of heating time was monitored by heating the mixture before extraction steps at 50°C; it was decided to consider 30-min duration of heating at 50°C as an optimum condition, as shown in Figure 8.

Figure 8

Effect of heating time (50°C) on Cu extraction efficiency.

Experimental conditions: 100 µL of 5.00 mg·L⁻¹ standard Cu, with 1.0 mL of each of the following solutions: 1.0 mL of 2AMP, 0.1 M (pH = 5.0) HA/NaA buffer (pH = 5.0, 5.0% NaCl, 2.0% Triton-X100).

3.3.6 Analytical performance of the method

Under the optimized variables, the suggested method’s analytical performance was assessed for a 10 mL sample solution. The linear concentration range was 5–75 µg·L⁻¹, calibration graphs were created with a correlation coefficient of 0.9976, and the calibration equation was A = 0.014 C + 0.038 with (R ² = 0.997), where A represents the absorbance and C represents the solution’s content of Cu(II). The limits of detection were calculated as LOD = 3 Sb/m, and LOQ = 10 Sb/m, where m is the calibration curve’s slope following preconcentration and Sb is the standard deviation of ten replicate blank signals.

LOD and LOQ were 3.0 and 9.3 µg·L⁻¹, respectively. The preconcentration factor was calculated to be (10) (Table 1).

Table 1

Figures of merit of the proposed method

Parameter	Results
Wavelength (nm)	440
Beer’s law limits (µg·L⁻¹)	5–75
Molar absorptivity
Sandell’s sensitivity
Slope (b)	0.0141
Intercept (a)	0.038
SD of the intercept	0.013
Regression coefficient (R ²)	0.997
LOD (µ·L⁻¹)	3.0
LOQ (µg·L⁻¹)	9.3
Preconcentration factor	10

LOD = 3.3 SDa/b, LOQ = 10 SDa/b.

3.3.7 Interference

In acidic conditions, the chelating agent 2-aminophenol ligand forms complexes with copper and some other transition metals [34]. Using 50.0 µg·L⁻¹ of Cu standard solution in the presence of different concentrations of specific ions, the impact of possible interference found in samples on the recovery using the proposed extracted method was investigated. The greatest concentration of the foreign ion that, when compared, could alter the analytical signal by more than 5% was identified as the tolerance level. The outcomes are displayed in Table 2.

Table 2

Interference effect studied using 50 µg·L⁻¹ of the analyte and different amounts of foreign ions

Interfering ion	Interference/analyte amount
NaCl, KCl, MgCl₂	1,000
AlCl₃, KNO₃, MgSO₄	500
CaCO₃	300
ZnCl₂	200
CoCl₂	100
Fe³⁺

3.3.8 Analysis of water samples

The suggested procedure was tested for the analysis of Cu(ii) ions in tap water (Amman city), river water (Zarka River), mineral water, and well water by spiking a standard amount of copper ions and determining the percent recoveries. The results are shown in Table 2. This result reveals good recoveries, confirming the good accuracy and applicability of the suggested method (Table 3).

Table 3

Analytical results of water and the recovery of spiked analyte

Sample	Added	Found	% Recovery
Tap water	5.0	4.9	98
River water	10.0	10.2	104
Mineral water	2.0	2.2	101
Well water	6.0	6.2	103

4 Conclusion

A novel method for the preconcentration and determination of Cu is SFO-DLLME combined with spectrophotometry. The proposed method utilized a new ligand (2AMP) as a chromogenic reagent that produces a colored complex with copper ion with an absorption wavelength of 440 nm. The separation procedure was optimized by investigating the factors affecting the separation efficiency. The interfering ions results demonstrate that the proposed method can be successfully applied for the determination of copper ions in different water samples. The method is considered as environmentally friendly because of the minimal use of toxic organic solvents, simplicity, rapidity, sensitivity, and reproducibility.

Acknowledgments

The Author would like to thank The department of Chemistry at the University of Jordan for its support.

Funding information: The study was supported by the University of Jordan. Deanship of Scientific Research.
Author contribution: Safwan Mohammad Fraihat: methodology, optimization, analysis, writing, review and editing.
Conflict of interest: The author states no conflict of interest.
Data availability statement: The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

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Received: 2025-03-26

Accepted: 2025-06-25

Published Online: 2025-08-01

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/gps-2025-0067

Keywords for this article

copper; dispersive liquid–liquid microextraction; spectrophotometry

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