Simultaneous Preconcentration and Determination of Cu(II), Ni(II), and Co(II) in Food and Environmental Samples by the Application of Chelate Adsorption on Amberlite XAD-1180

A simultaneous preconcentration and determination procedure for solid phase extraction on AXAD-1180 as 2,6-dimethylmorpholinedithiocarbamate (DMMDTC) chelates and spectrophotometric determinations of Cu (II), Ni (II), and Co (II) in food and environmental samples is proposed in the present work. The effect of some SPE parameters, such as reagent amount, sample pH, eluent type, concentration, and volume, sample and eluent flow rate, and sample volume, on trace metal ion recovery (R%) for the method developed in the standard model solution medium was investigated. Cu(II), Ni(II), and Co(II) retained as DMMDTC complexes on Amberlite XAD-1180 were eluted with 10 mL of 1 M HNO3 (in acetone). Foreign ions were also studied individually on the recovery of trace metal ions using the developed method. Cu(II), Ni(II), and Co(II) ions were preconcentrated and separated from the sample using the developed SPE method, and their concentrations were simultaneously determined using the UV-Vis spectrophotometric method. The spectrophotometric determination was made by measuring the absorbance of colored chelates of metal ions complexed with DMMDTC in a surfactant medium (1% Triton X-100) at wavelengths of 460, 328, and 342 nm for Cu(II), Ni(II), and Co(II), respectively. To test the method’s accuracy, certified reference materials (CRM 1204 waste water and TMDA-70.2 Ontario lake water) were analyzed using the proposed method, and metal recoveries were calculated to be between 97.1 and 100.7%. The proposed method worked well with the wheat flour sample. Wheat flour has Cu(II) and Ni(II) contents of 2.16 μgmL−1 and 0.56 μgmL−1, respectively.


Introduction
Cereals and their by-products are the important part of daily diet (Cheli et al. 2010). The major cereals are wheat, rice, oats, barley, rye, corn, and millet. Among them, wheat is one of the most consumed cereals. They are grown on nearly 60% of the cultivated area in the world (Koehler et al. 2014). Because of their high average consumption, cereal products are considered among the primary dietary sources of heavy metals (Conti et al. 2000). It is well known that plants grown in contaminated soil by heavy metals are a major route of entry for heavy metals in to the food chain. Due to their not biodegradability, metals accumulate in tissues, and their amounts increase gradually along the food chain (Wang et al. 2011;Cheli et al. 2010); Farooq et al. 2010).
Heavy metal ions including cobalt, nickel, and cobalt ions are significant environmental contaminants (Mitra et al. 2022;Masindi and Muedi 2018), because they can gradually accumulate in a person's body through the food chain and endanger human health (Pinar Gumus and Soylak 2021). The main applications for copper are in the production of paints and pigments, tanneries, fertilizers, and baths for cleaning and plating. A higher copper content consumed over time may result in lung cancer, liver damage, renal damage, sleeplessness, Wilson disease, vomiting, and diarrhea. The US EPA has set a 1.3 mg/L copper maximum allowable level (Mi et al. 2015;Ge et al. 2015;Aksu and Işoǧlu 2005). Vasodilation, flushing, and cardiomyopathy are some of the toxicological consequences that cobalt can cause (Linna et al. 2020). A frequently utilized heavy metal ion and hazardous substance is nickel. Chemical industries, electroplating, mining, refining, and paint and ink formulation units are some of the sources of its toxicity (Genchi et al. 2020;Ma et al. 2019). Thus, it is essential to create a technique that is quick, simple, inexpensive, and sensitive to track the extensive diffusion of trace metal pollutants in the environment.
For the accurate and precise determination of toxic metal ions in food, modern instrumental techniques such as inductively coupled plasma-mass spectrometry (ICP-MS) (Nardi et al. 2009), flame/graphite furnace atomic absorption spectrometry (FAAS/GFAAS) (Abbasi Tarighat 2016;Sánchez-Moreno et al. 2010), and electroanalytical techniques (Jin Mei and Ainliah Alang Ahmad 2021) are important. Because of their low concentration and strong ion impact interference from numerous food additives, proteins, bases, salts, and acids, determining the presence of heavy metal contamination in food samples is significantly more challenging .
In studies on solid-phase extraction, a functional chelating group is added to the sorbent. The development of new sorbents containing functional groups, chemically bonding functional groups on existing sorbents (immobilized or functionalized sorbents), and physically bonding the groups on the sorbent by impregnating the solid sorbent with a solution containing the chelating ligand are the three distinct methods that can be used for this purpose (coated, impregnated, or loaded sorbents). Alternatively, in solid-phase extraction investigations where the production of a chelate compound is used, a chelating agent may be directly added to the sample when the metal-chelate is retained on a suitable sorbent (Laskar et al. 2016;Moulay 2018).
Dithiocarbamates were added to an aqueous phase containing a variety of metals. Ammonium pyrrolidinedithiocarbamate (APDC), hexamethylenedithiocarbamate (HMDC), and sodium diethyldithiocarbamate (NaDDTC) reagent were among the dithiocarbamates used. DTCs can generate colorful chelates that are stable during SPE separation procedures when heavy metal ions such as Mn(II), Co(II), Cu(II), and Ni(II) are used. The formed metal dithiocarbamate complexes were then extracted with a solid phase, such as silica gel, polyurethane foam, activated carbon, or Amberlite XAD resins (Abu-El-Halawa and Zabin 2017; Zhen et al. 2012;Beyramabadi et al. 2012). DTCs also don't interact with alkali or alkaline earth metals to create complexes (Cesur and Aksu 2006). By using an appropriate and efficient ligand for the measurement of trace metals without immobilization and loading the reaction onto the solid phase in actual samples, including alkali and alkaline earth metals, a selective and efficient solid phase extraction method may be developed.
Modern instrumental techniques are not as selective, straightforward, quick, adaptable, or economical as UV-Vis spectrophotometry based on complex-forming chemicals. Because metal DTC complexes have a high UV absorbance after solvent extraction using nonpolar organic solvents, UV-Vis spectrophotometry is often used to analyze them (Balakrishnan et al. 2019).
In the presented study, a new solid-phase extraction method was developed for the preconcentration and separation of Cu(II), Ni(II), and Co(II) ions. In an earlier study, a UV-Vis spectrophotometric method using dimethylmorpholinedithiocarbamate (DMMDTC) was used to identify metal ions at the submilligram per ml level in a surfactant medium (Triton X-100).

Reagents and Apparatus
Stock solutions of Cu(II), Ni(II), and Co(II) ions were prepared from 1000 mgL −1 high purity compounds (in 0.5 molL − 1 HNO3), supplied by (Merck). Other used chemicals were of analytical grade of Merck (Germany).
The pH values were adjusted with HCl, NaOH, Sorenson's buffer, and ammonium buffer to pH s below 3.73, in the range 4-6 and in the range 7-10, respectively.
NIST, TMDA-70.2 (Environmental Matrix Reference Material Canada) and CRM 1204 wastewater (Tubitak -UME, Turkey) were used as certified reference materials.
Ultra-pure water (18 MΩ cm -1 ) was produced using a water treatment system (Merck, Darmstadt, Germany). UV-Vis spectroscopy was used to measure the analyte ions in a 1-cm quartz cuvette (Spectroquant Pharo 300, Merck, Darmstadt, Germany) (190-1100 nm). An ISOLab pH meter was used to measure pH. A lead fluid peristaltic pump with a flow-rate-adjustable BT/101S model was used to pump the fluid into the resin column that was positioned vertically. An analytical balance (Radwag, Radom, Poland) with a sensitivity of 0.00001 was used to determine the solid weights.
The morphologies of the raw AXAD-1180 (without metal ions) and the loaded with metal ions was observed with EDX (Shimadzu, Carl Zeiss AG -EVO 50). Perkin Elmer-SCIEX ELAN DRC-e ICP-MS was used for the parallel analyses of the samples to be compared.

Preparation of the SPE Column
The effective length and inner diameter of the glass preconcentration column are 100 mm and 10 mm, respectively. Two hundred milligram of AXAD-1180 resin was soaked in 25 mL of ultra-deionized water to create a slurry. Glass cotton was positioned at the bottom and top of the column, and the column was packed with resin to ensure appropriate settling of the AXAD-1180 during sample loading.
De-ionized water was used to clean the column, and then 1 M nitric acid and 1 M hydrochloric acid were used to condition it. After each experiment, the resin in the column was washed thoroughly with 1 M nitric acid, deionized water, and with appropriated buffer solution.

Pre-investigation of Metal Ions Recovery in AXAD-1180/DMMDTC Solid Phase Extraction
Preliminary research was conducted on the adsorption and recovery yields of Pb(II), Zn(II), Cd(II), Hg(II), Cu(II), Ni(II), Co(II), and Mn(II) metal ions for the AXAD-1180/DMMDTC SPE method (Table 1). According to the results, the highest recovery was achieved for Cu(II), Ni(II), and Co(II) ions. Co(II) recovery was lower than Cu(II) and Ni(II) recovery. However, by using XAD-1180N-DMMDTC column optimization, this value was achieved at a rate of more than 90%.

Method
A Sorenson's buffer solution was passed through the AXAD-1180 column to precondition it (pH 5). Fifty milliliter of a solution containing 10 g of Cu(II), Ni(II), and Co(II) (from each) were prepared. One milliliter of 0.1% (w/v) DMMDTC chelating agent was added to the solution after the pH was adjusted to the desired value (in the range 2-10). The solution was passed through the column filled with AXAD-1180 at a flow rate of 8.5 mL min −1 after waiting for the production of metal-DMMDTC chelates for 10 min. At a flow rate of 5.5 mL min −1 , 10 mL of 1 mol L −1 HNO3 in acetone were used to elute the metal ions that had been retained on the column. By using a UV-Vis spectrophotometric method that was carried out in accordance with prior descriptions in our investigations, the concentration of the metal ions was determined.

Determination of Metal Ions
Cu(II), Ni(II), and Co(II) concentrations can be determined by using a UV-Vis spectrophotometric method previously described in micellar media containing TX-100 at pH 3.0-7.0 with DMMDTC as a chelating ligand (Topuz et al. 2017). The selectivity of the proposed method for the spectrophotometric determination of the metal ions is increased due to the DMMDTC ligand and the formation of stable complexes with the metal ions in the specific pH range.

Microwave Digestion Procedure Was Applied for Wheat Flour Sample
In a microwave digestion system, 0.4 g of wheat flour samples was digested with 5 mL of HNO 3 (65%) and 1 mL of H 2 O 2 (30%) and diluted to 25 mL with deionized water. In the same way, a blank digest was performed. Every sample solution was clear. The microwave system digestion conditions were 5 min for 170 °C, 30 bar, 15 min for 190 °C, 35 bar, and 10 min for 50 °C, 25 bar, respectively.

Characterization of AXAD-1180 Resin and the Metal Ion Adsorption
When compared to the image of AXAD-1180, SEM images of AXAD-1180 resin loaded with the investigated metal ions show surface modifications. This modification might be explained by the metal ions bonding to the surface of the AXAD-1180 resin (Figs. 1, 2, 3, and 4).

Influence of the Sample pH
The most important characteristic among the evaluated variables for the adsorption of metals on the AXAD-1180 was found to be pH. The pH of a 50 mL sample solution containing 1 mg L −1 of Cu(II), Ni(II), and Co(II) ions was adjusted to be in the range of 0.62-9.97 in order to assess the impact of pH on the retention efficiency. As can be seen in Fig. 5, The quantitative retention of the metal ions on the AXAD-1180 was obtained after pH 4.3. Due to hydrolysis, pH above 10.0 was not tested. Finally, a pH of 5 was chosen for further studies.

Influence of the Sample Flow Rate
To optimize the sample flow rate, 50 mL solutions of 1 mg L −1 Cu(II), Ni(II), and Co(II) ions were pH-adjusted to 5.0 before being pumped through the column at rates ranging from 1,76 to 11.76 mL min −1 . The results (Fig. 6) showed that the retention of the heavy metals on AXAD-1180 was unaffected by the sample flow rate fluctuation in this range. However, a 3 mL min −1 sample flow rate was chosen since a slow sample flow rate lengthens the analytical process.

Influence of the Volume of DMMDTC
The chelating compound DTC is effective in interacting with a variety of metal ions to create stable complexes. The effectiveness of these metal chelates' enrichment depends on the amount of DTC present. In this experiment, the volume of 0.1 KDMMDTC was examined at between 50 and 2000 L. The experimental findings displayed in Fig. 7 demonstrated that as the volume of DMMDTC increased in this range, the extraction efficiencies of all metal chelates increased. In order to save reagent, the following experiments used DMMDTC of 1 mL.
The remaining experiments, then, employed quantities of 10 mL of eluent for desorption of Cu(II), Ni(II), and Co(II). It was also investigated how the eluent flow rate affected the recovery of metals. Quantitative recoveries for Cu(II), Ni(II) and Co(II) were obtained at flow rates ranging from 1.42 to 7.59 mL min −1 with 1 molL −1 HNO 3 in acetone. There was no significant difference between the results (Fig. 10). A low elution flow rate should be used to allow enough time for the solid phase and eluent to reach equilibrium. For the following experiments, an eluent flow rate of 2 mL min −1 was selected.

Effect of the Volume of Sample Solutions
Large volumes of actual samples were preconcentrated because there weren't many heavy metal concentrations. In order to determine the maximum volume of sample solution, a metal ion solution volume was increased while maintaining a constant ion concentration (0.1 mg of Cu(II), Ni(II), and Co(II)).
The metal ion standard sample solutions in volumes ranging from 50 to 1200 mL were passed through the column. The results (Figs. 11) showed that a sample volume of up to 750 mL could be obtained for the simultaneous quantitative recovery of Cu(II), Ni(II), and Co(II) on AXAD-1180.

Effect of the Interfering Ions
To investigate the effects of various cations and anions found in wheat flour and environmental samples, alkaline, alkaline earth, transition metals, and widespread anions were added to 100 mL of solution containing 0.01 mg of Cu (II), Ni(II), and Co (II). Table 2 shows the level of tolerance for foreign ions. From the tolerance data, it can be seen that the potentially interfering ions have no significant effects on the preconcentration of Cu (II), Ni(II), and Co (II) ions with the proposed method.

Analytical Performances of the AXAD-1180/DMMDTC SPE Method
Under optimal conditions, the analytical performance of the AXAD-1180/DMMDTC SPE method was investigated. Table 3 summarizes the proposed method's analytical performance. The calibration curves' correlation coefficients (R 2 ) were in the 0.9951-0.9972 range. The method's relative standard deviations (RSD) were found to be <4.6% for all metal ions. As a result of the obtained RSD values, it can be concluded that the proposed procedure demonstrated good repeatability. The limit of detection, defined as three times the standard deviation of a reagent blank (N = 10), was determined to be 4.4, 39.6, and 21.5 μgL −1 for Cu(II), Ni(II), and Co(II), respectively.

Adsorption Characteristics
Adsorption capacity of the sorbent is an important factor to consider when evaluating it because it determines how so much AXAD 1180/DMMDTC is required in a given solution. A batch technique was employed to assess the AXAD-1180's ability to adsorb with DMMDTC. The following ingredients were added to a 50-mL aqueous solution: 0.2 g AXAD-1180, 1 mL 0.1% DMMDTC, and 10 mg the metal ions at pH 5.5. After 24 h of shaking, the mixture was filtered. One milliliter of the supernatant solution was diluted to 50 mL, and the metal ions were determined using the UV-Vis spectrophotometric method. This process was repeated individually for each analyte ion. The obtained capacities of AXAD-1180 were found to be 29, 64, 13.45 and 6.49 μgg −4 for Cu (II), Ni(II), and Co (II), respectively. The stirring time is a measure of the contact time between the AXAD-1180 and sample solution. The effect of stirring time on metal recoveries was investigated by varying it from 3 to 150 min. In each case, 0.2 mg of AXAD-1180 was used, along with a 50 mL aqueous sample containing 1 mg of metals. The results (Fig. 12) showed that time variations in the range of 3-150 min had no significant influence on the adsorption of metal chelates on MWCNTs. It was discovered that SPE with AXAD-1180 and DMMDTC as chelating agents promote immediate interaction between metal chelates and AXAD-1180. According to these results, AXAD-1180/DMMDTC exhibited good adsorption characteristics with an stirring time of about 3 min for 90-97% sorption.

Analytical Applications
To validate the preconcentration method, the suggested approach has been successfully used to determine the amounts of Cu(II), Ni(II), and Co(II) ions in TMDA-70.2 Ontario lake water and CRM 1204 waste water certified reference materials (Table 4). It was discovered that the values founded using the novel method and the certified values of reference materials were similar. This demonstrates that the novel method was used to examine analyte ions in actual samples. The proposed method was also used to separate and determine analytes from a wheat flour sample. The same sample was also analyzed by ICP-MS for comparison and almost similar results were observed (Table 5).

Conclusions
Using AXAD-1180 and DMMDTC, a new solid phase extraction method for UV-Vis spectrometric determination of Cu(II), Ni(II), and Co(II) ions at trace levels in wheat flour and water samples was developed. The method has the advantages of being basic, time-saving, sensitive, costeffective, environmentally friendly and capable of determining the underlying metal ions in food samples as well as recognized reference materials. The matrix components had little impact on the nearly quantitative analyte ion recoveries. Cu(II), Ni(II), and Co(II) concentrations can be determined by using a UV-Vis spectrophotometric method previously described in micellar media containing TX-100 at pH 3.0-7.0 with DMMDTC as a chelating ligand. The DMMDTC ligand and the formation of stable complexes with the metal ions in the specific pH range also increase the selectivity of the proposed method with the spectrophotometric determination of Cu(II), Co(II), and Ni(II). The complexation reagent is chosen based on its rapid and efficient reaction with metal ions, strength over a wide pH range, and water solubility. The advantage of DMMDTC is that it provides exceptional analytical qualities, particularly for the separation of heavy metals with these properties. The AXAD-1180N resin used in this study differs from other resins in its pore structure and volume, particle size, and surface area, it has been an advantage for our method