chemengineering
Article Application of Sodium Dodecyl Sulfate/Activated Carbon onto the Preconcentration of Cadmium Ions in Solid-Phase Extraction Flow System
Mai Furukawa 1,*, Ikki Tateishi 2, Hideyuki Katsumata 1 and Satoshi Kaneco 1,2
1 Department of Chemistry for Materials, Graduate School of Engineering, Mie University, Mie 514-8507, Japan 2 Global Environment Center for Education & Research, Mie University, Mie 514-8507, Japan * Correspondence: [email protected]; Tel.: +81-59-231-9426
Received: 21 May 2019; Accepted: 26 July 2019; Published: 1 August 2019
Abstract: In the present study, activated carbon (AC) surface modified with sodium dodecyl sulfate (SDS), written as SDS/AC, was applied as an adsorbent for preconcentration and determination of trace amount of cadmium ions in environmental sample waters. The SDS modification on AC was performed at the same time, while cadmium ions were concentrated in the flow system as solid-phase extraction. After the separation and preconcentration steps, cadmium retained on SDS/AC was eluted with HNO3 and was subsequently determined by flame atomic absorption spectrometry (FAAS). The analytical parameters that influence the quantitative determination of trace cadmium, such as SDS concentration, pH and volume of sample solution, eluent conditions, and interference, were optimized. At the optimum conditions, the general matrix elements had little interference on the proposed procedure. The detection limits was 17 ng L 1, and the relative standard deviation (RSD) · − for 12 experiments at 10 µg L 1 cadmium solutions was 2.8%. The developed method was applied · − into the analysis of environmental samples spiked cadmium.
Keywords: cadmium; activated carbon; sodium dodecyl sulfate; solid-phase extraction; flame atomic absorption spectrometry; preconcentration
1. Introduction Now, the environmental pollution with the heavy metals is one of the important problems. Cadmium (Cd) exists in the natural environment and is known to be a toxic environmental pollutant. Although Cd has been applied into the industrial materials, it causes many health problems in organs such as kidney, lung, and liver for the human body [1,2]. World Health Organization (WHO) and Environmental Protection Agency (EPA) have limited Cd concentration in drinking water to 3 µg L 1 · − and 5 µg L 1, respectively [3,4]. In Japan, Ministry of the Environment established 30 µg L 1 for · − · − effluent standard [5] and 3 µg L 1 for environmental quality standards [6] for human health. Thus, it is · − necessary to control the exposure even at trace levels and monitor Cd concentration with sensitive analytical techniques. Currently, the analytical methods for trace determination include flame atomic absorption spectrometry (FAAS), graphite furnace atomic absorption spectrometry (GFAAS) [7], inductively coupled plasma-optical emission spectrometry (ICP-OES) [8] and inductively coupled plasma mass spectrometry (ICP-MS) [9]. Among them, FAAS has been widely employed in the determination of heavy metals because of inexpensive instrument, easy operation, good selectivity, and high sample throughput [10,11]. On the other hand, this method often suffers from worse detection limits [12]. The overcome of this limitation needs the preconcentration and separation of analytes from matrices in analytical procedure [13].
ChemEngineering 2019, 3, 67; doi:10.3390/chemengineering3030067 www.mdpi.com/journal/chemengineering ChemEngineering 2019, 3, 67 2 of 11
Preconcentration techniques have contained liquid–liquid extraction (LLE) [14], coprecipitation [15], cloud point extraction (CPE) [16], and solid phase extract (SPE). In particular, the SPE procedure has been used for the separation and preconcentration techniques of trace metal ions because of high recovery and enrichment factor, short extraction time, variation of adsorbent, and unnecessary of organic solvents [17–19]. In the SPE technique for metal ions in aqueous solution, activated carbon (AC) has been applied as the efficient adsorbent due to high adsorption capacity and low cost [20–22]. Moreover, the modification of functional groups onto AC surface can improve the selectivity and adsorption behavior of heavy metal ions. It has been reported that the metal complexes more effectively adsorbed onto activated carbon. For example, sodium diethyl dithiocarbamate [23], tetrabutylammonium hydroxide [24], ammonium pyrrolidinedithiocarbamate [25], and xylenol orange [26] were modified on activated carbon surfaces and were applied for separation/preconcentration. These techniques required the preparation of the adsorbent prior to preconcentration. Sodium dodecyl sulfate (SDS), an anionic surfactant, is used as cleaning and hygiene products [27]. Recently, the modification that is based on SDS adsorption at the solid–fluid interface has been applied for the determination of various analytes [28–30]. Ahn et al. reported the removal of Cd from aqueous solution by AC impregnated with SDS [31]. Hon et al. investigated the AC modified with SDS as a solid phase sorbent for the separation and preconcentration of trace Cd in water samples [32]. It was necessary, in the study, to fabricate SDS/AC before the preconcentration. Therefore, we have developed the preconcentration technique that Cd was concentrated to AC at the same time while SDS, which was added in a sample solution, was modified to AC by batch method [33]. However, there is little information on the preconcentration flow system that Cd is concentrated to AC in aid of simultaneous modification of SDS. In the present work, the novel preconcentration method was developed for the preconcentration of Cd in environmental water samples based on solid-phase extraction using sodium dodecyl sulfate/activated carbon with the flow system. In the flow system, SDS was adsorbed onto AC during the preconcentration of Cd.
2. Materials and Methods
2.1. Reagents and Materials Cadmium standard stock solution (1000 mg L 1 of Cd2+ in 0.1 mol L 1 nitric acid) was purchased − · − from NACALAI TESQUE, INC. (Tokyo, Japan). Working solutions were carefully diluted from the stock solution with nitric acid solution. Activated carbon (DarcoKB-G) was purchased from SIGMA-ALDRICH® (St. Louis, MO, USA). Sodium dodecyl sulfate (SDS) from Kanto Chemical Co. (Tokyo, Japan) was employed without further purification. Other surfactants—cetyl trimethyl ammonium bromide (CTAB), and octyl phenol ethoxylate (OPE)—were purchased from FUJIFILM Wako Pure Chemical Corporation (Osaka, Japan). Matrix solutions of Al3+, Ca2+, Cu2+, Fe2+,K+, Mg2+, Ni2+, and Zn2+ were prepared by dissolution of nitrates and hydrochlorides. All reagents were in analytical grade. Ultrapure water (18.2 MΩ cm) was obtained from the Advantec ultrapure water system CPW-102 (Tokyo, Japan). The pH of the sample solutions was adjusted with 0.1 mol L 1 HNO · − 3 and/or 0.1 mol L 1 NaOH. · − 2.2. Apparatus The determination of cadmium ions was carried out with Thermo Solaar S2 (Thermo Fisher Scientific K.K., Tokyo, Japan) flame atomic absorption spectrometer (FAAS) equipped with deuterium background corrector. Cadmium hollow cathode lamp (Hamamatsu Photonics K.K., Shizuoka, Japan) was used as the radiation source with wavelength of 228.8 nm. An air/acetylene flame was employed in all measurements, and the slit length of the burner was 5 cm. The instrumental parameters recommended by the manufacturer were used for FAAS measurements. ChemEngineering 2019, 3, 67 3 of 11
A pH/DO METER D-55 (HORIBA, Ltd., Kyoto, Japan) pH meter was used to monitor the pH of the solutions.
2.3. PreconcentrationChemEngineering 2019 Procedure, 3, x FOR PEER REVIEW 3 of 11
2.3.Fifty Preconcentration mg activated Procedure carbon was added and stirred into 50 mL pure water. For the adsorption, the suspendedFifty solution mg activated was poured carbon intowas added the column and stirred (ϕ40 into100 50 mL mm) pure and water. AC wasFor the put adsorption, on the membrane the × 1 1 filter.suspended After pH solution adjustment, was poured the sample into the solution column ( containingφ40 × 100 mm) 10 µandg L AC− Cdwas andput on 100 the mg membraneL− SDS was 1 · · passedfilter. through After pH AC-loaded adjustment, filter the withsample the solution flow rate containing of 1 mL 10s µg·L. Subsequently,−1 Cd and 100 mg·L the− eluent1 SDS was of 3 mL · − was pouredpassed through with the AC-loaded flow rate filter of with 0.5 the mL flows 1 rateto desorb of 1 mL·s the−1. Subsequently, adsorbed Cd the ion eluent from of AC 3 mL adsorbent. was · − The collectedpoured witheluent the wasflow determinedrate of 0.5 mL·s by flame−1 to desorb atomic the absorption adsorbed spectrometry.Cd ion from AC This adsorbent. preconcentration The procedurecollected is showneluent was in Figure determined1. by flame atomic absorption spectrometry. This preconcentration procedure is shown in Figure 1.
FigureFigure 1. Experimental1. Experimental procedure procedure forfor thethe preconcentrationpreconcentration of ofcadmium cadmium using using sodium sodium dodecyl dodecyl sulfatesulfate/activated/activated carbon carbon (SDS (SDS/AC)./AC).
AnalyticalAnalytical techniques techniques employed employed in the in preconcentration the preconcentration provide provide analyte analyte isolation isolation and enrichment and factor.enrichment Enrichment factor. factor Enrichment is one of importantfactor is one parameters of important to compare parameters the analyteto compare concentration the analyte before and afterconcentration the preconcentration before and after method the preconcentrati and is obtainedon bymethod the following and is obtained Equation by (1).the following Equation (1).
EnrichmentEnrichment factor factor (EF) (EF)= =C Celuenteluent/C/Csamplesample (1) (1) Additionally,Additionally, since since some some analytes analytes may may be lost be duringlost during the preconcentration, the preconcentration, recovery recovery is calculated is calculated by the following Equation (2) in order to evaluate the analyte amount in sample solution by the following Equation (2) in order to evaluate the analyte amount in sample solution before and before and after the preconcentration. after the preconcentration. Recovery = EF × (Veluent/Vsample) × 100 (2)
Here, Celuent, Csample, VeluentRecovery, and Vsample= EF are the(V concentration/V ) in the100 eluent, the concentration in (2) × eluent sample × sample solution, the eluent volume, and the sample volume, respectively. Here, C ,C ,V , and V are the concentration in the eluent, the concentration in Theeluent analyticalsample parameters,eluent such assample surfactant type and concentration, pH of sample, eluent sampleconditions, solution, interfering the eluent elements, volume, sample and the volume, sample and volume, reusability respectively. were evaluated and optimized for theThe recovery analytical of Cd parameters, ion in the preconcentration. such as surfactant type and concentration, pH of sample, eluent conditions, interfering elements, sample volume, and reusability were evaluated and optimized for the recovery3. Results of Cd ion in the preconcentration. The preconcentration cadmium on SDS/AC while modifying SDS to AC surface was attempted 3. Results by passing the sample solution containing relatively low concentration (100 mg·L−1) of SDS through AC-loadedThe preconcentration filter. cadmium on SDS/AC while modifying SDS to AC surface was attempted by passingSigma-Aldrich the sample solutionactivated containing carbon Darco relatively® KB-G low concentrationwas applied (100as the mg Ladsorbent.1) of SDS The through · − AC-loadedcharacterization filter. of DarcoKB-G was reported in the previous work [33]. It was reported from the nitrogen adsorption/desorption isotherm analysis that the curve shape Sigma-Aldrich activated carbon Darco® KB-G was applied as the adsorbent. The characterization of the activated carbon belonged to type IV isotherm with hysteresis loops characteristic of of DarcoKB-Gmesoporous was structure. reported BET in specific the previous surface,work total pore [33]. volume and average pore diameter were 1414 mIt2 was·g−1, reported1.2 cm3·g− from1, and the 3.3nitrogen nm, respectively. adsorption The/desorption peak of pore isotherm diameter analysis distribution that the was curve ~9 nm, shape of the activatedaccording carbon to pore belongeddistribution to calculation. type IV isotherm with hysteresis loops characteristic of mesoporous structure.In BETthe present specific work, surface, normal total sodium pore dodecyl volume su andlfate average(n-SDS) was pore applied diameter for improvement were 1414 mof2 g 1, · − 1.2 cmrecovery.3 g 1, and As the 3.3 result nm, respectively. of previous work, The peakFT-IR of sp poreectra diameterfor AC and distribution AC/SDS showed was ~9it that nm, AC according could to · − porebe distribution modified with calculation. SDS. SEM and TEM images of the samples indicated the surface morphology and conditions of activated carbon could hardly be changed due to the SDS modification. ChemEngineering 2019, 3, 67 4 of 11
In the present work, normal sodium dodecyl sulfate (n-SDS) was applied for improvement of recovery. As the result of previous work, FT-IR spectra for AC and AC/SDS showed it that AC could be modified with SDS. SEM and TEM images of the samples indicated the surface morphology and conditions of activated carbon could hardly be changed due to the SDS modification.
3.1. Effect of Surfactants ChemEngineering 2019, 3, x FOR PEER REVIEW 4 of 11 In the preconcentration with the catch system, the addition of SDS was effective for the improvement3.1. Effect of of Surfactants recovery of Cd. Therefore, the effect of surfactant on the preconcentration of Cd by theIn flow thesystem preconcentration was investigated. with the catch system, the addition of SDS was effective for the Asimprovement shown in of Figure recovery2a, of in Cd. the Therefore, case of the cetyl effect trimethyl of surfactant ammonium on the preconcentration bromide(CTAB, of Cd by cationic surfactant),the flow the system recovery was becameinvestigated. worse compared with those obtained with only Cd solution. The reason As shown in Figure 2a, in the case of cetyl trimethyl ammonium bromide (CTAB, cationic may besurfactant), due to electrostatic the recovery repulsionbecame worse between compared positively with those charged obtained hydrophilic with only Cd groups solution. and The cadmium ions. Thereason presence may be ofdue nonionic to electrostatic surfactant repulsion (octyl between phenol positively ethoxylate charged (OPE)) hydrophilic gave the groups slightly and decrease in recovery.cadmium Although ions. The presence the addition of nonionic of surfactant SDS in Cd(octyl solution phenol ethoxylate deteriorated (OPE)) the gave recovery the slightly with high 1 1 concentrationsdecrease in (1000 recovery. mg AlthoughL− ), in thethe Cdaddition sample of SD withS in lowCd solution concentration deteriorated SDS the (100 recovery mg L −with), the factor · −1 · −1 was almosthigh concentrations the same as (1000 that withoutmg·L ), in the the surfactants. Cd sample with low concentration SDS (100 mg·L ), the factor was almost the same as that without the surfactants. Next, in Figure2b, the influence of SDS concentration on the recovery in the presence of interfering Next, in Figure 2b, the influence of SDS concentration on the recovery in the presence of elementinterfering (Ca) was element investigated. (Ca) was investigated. In the condition In the condition without without SDS, calcium SDS, calcium ions severelyions severely affected the recoveryaffected of Cd. the Onrecovery the other of Cd. hand, On the with other the hand, addition with the of SDS,addition the of recovery SDS, the ofrecovery Cd was of improvedCd was in the flow system.improved The in the recovery flow system. of Cd The in the recovery presence of Cd of in SDS the presence was not of so SDS bad was with not the so Cabadmatrix with the(for Ca instance; 16% andmatrix 58% (for in instance; the absence 16% and and 58% presence in the absence of SDS, and respectively);presence of SDS, however, respectively); the however, effect of the matrix Ca effect of matrix Ca element in the flow system was negligible, which is very advantageous. 1 element in the flow system was negligible, which is very advantageous. Therefore, 100 mg L− of SDS Therefore, 100 mg·L−1 of SDS was selected as co-existing surfactant in the Cd solution with the flow· was selected as co-existing surfactant in the Cd solution with the flow system. system.
(a) (b)
Figure 2. Effect of (a) various surfactants (1000 mg·L−1) and1 (b) SDS concentrations on recovery of Figure 2. Effect of (a) various surfactants (1000 mg L− ) and (b) SDS concentrations on recovery of cadmium ions. Experimental conditions: Cd 10 µg·L−1;· AC 501 mg; pH 5; sample volume 200 mL; flow cadmium ions. Experimental conditions: Cd 10 µg L− ; AC 50 mg; pH 5; sample volume 200 mL; rate 1 mL·s−1; eluent 0.1 mol·L−1 HNO3 3 mL. · flow rate 1 mL s 1; eluent 0.1 mol L 1 HNO 3 mL. · − · − 3 3.2. Effect of Initial PH 3.2. Effect of Initial PH The pH of the sample solutions is an important analytical factor in the solid-phase extraction for TheCd element. pH of the Also, sample the chemical solutions species is anfor importantCd in the solution analytical can depend factor on in the solution solid-phase pH. Hence, extraction for Cd element.solution Also,pH can the regulate chemical the release species of forfree CdCd species in the solutionthrough the can forms depend that are on chemically the solution bound pH. Hence, solutionto mineral, pH can organic regulate and the colloidal release components of free Cd in speciesenvironmental through samples. the forms At pH that < 3.5, are all chemicallymetals are bound to mineral,generally organic released, and except colloidal for componentsthe most strongly in environmental bound fractions samples. of the mineral, At pH organic,< 3.5, alland metals are colloidal components [34]. Hot acid digestion is the only reliable technique for charging the total generally released, except for the most strongly bound fractions of the mineral, organic, and colloidal amount of metal to a simple ionic form. Finally, the solution pH can be adjusted to the desired value. componentsTherefore, [34]. Hot the effect acid of digestion sample pH is on the the only recovery reliable of Cd technique was investigated for charging in the range the of total 3 to 10. amount of metalThe to a results simple show ionic it form.that the Finally, recovery the of solutionCd became pH a canmaximum be adjusted at pH around to the 5, desired as illustrated value. in Therefore,Figure 3. the effect of sample pH on the recovery of Cd was investigated in the range of 3 to 10. The resultsThe show tendency it that may the result recovery from of the Cd charges became of AC a maximum surface condition at pH and around chemical 5, as species illustrated for Cd. in Figure3. The distribution of cadmium species at carious pHs has been already repointed by Ahn et al. [35,36]. ChemEngineering 2019, 3, 67 5 of 11
The tendency may result from the charges of AC surface condition and chemical species for Cd. The distribution of cadmium species at carious pHs has been already repointed by Ahn et al. [35,36]. ChemEngineering 2019, 3, x FOR PEER REVIEW 5 of 11 The zero point charge (ZPC) pHZPC of AC Darco KB-G is ~2.2 [37]. The AC surface is positively chargedThe in zero strong point acidic charge media (ZPC) (pH pH
Figure 3. Effect of initial pH in sample solution on Cd recovery. Experimental conditions: Cd 10 µg L 1; Figure 3. Effect of initial pH in sample solution on Cd recovery. Experimental conditions: Cd 10 − 1 1 · AC 50 mg;−1 SDS 100 mg L− ; sample−1 volume 200 mL; flow rate 1 mL s− ; eluent−1 0.1 mol/L HNO3 3 mL. µg·L ; AC 50 mg; SDS· 100 mg·L ; sample volume 200 mL; flow rate· 1 mL·s ; eluent 0.1 mol/L HNO3 3.3. Optimization3 mL. of Elution Conditions The3.3. Optimization effect of eluent of Elution on the Conditions recovery of Cd ions was also studied. The elution (acid) conditions can affect the strippingThe effect of analyteseluent on fromthe recovery SDS/AC, of andCd ions sometimes was also acid studied. elution The mayelution give (acid) impart conditions damage to the atomizercan affect for the AAS. stripping The strippingof analytes power from dependsSDS/AC, and on thesometimes type, concentration acid elution may and give volume impart of acid eluent.damage Therefore, to the the atomizer different for eluents AAS. The including stripping HNO power3, HCl depends and their on the mixed type, acid concentration (HNO3:HCl and= 1:1) were usedvolume in of order acid toeluent. acquire Therefore, the most the suitable different eluent. eluents The including results HNO are shown3, HCl and inTable their 1mixed. acid (HNO3:HCl = 1:1) were used in order to acquire the most suitable eluent. The results are shown in TableTable 1.1. Effect of type, concentration, and volume of eluent on enrichment factor (EF) and recovery. According to the comparison between1 the recovery with various1 acid types, better result were Experimental conditions: Cd 10 µg L− ; AC 50 mg; SDS 100 mg L− ; sample volume 200 mL; flow rate obtained1 with nitric acid, while lower· recovery was observed with· hydrochloric acid and mixed acid 1 mL s− . (HNO· 3:HCl = 1:1). In the case of eluent volume, 2 + 1 mL, It is considered that it takes slightly long time forType the desorptionConcentration process and the (mol movingL 1) actionVol. has (mL) occurred. EF The adsorbent Rec. (%) becomes RSD% wet and · − further desorption process proceeds0.1 after first elution of 6 2 mL HNO 26.53. Furthermore, 79.5 the adsorbent 5.4 is washed away by addition 1 mL0.1 eluent. Considering 5the atomizer 35.3 exhibits 88.3 low tolerance 2.9 to the −1 introductionHNO3 of strong acid, the optimum0.1 elution conditions 3 were 47.30.1 mol·L 77.3HNO3 (2 + 1)4.9 mL for all further experiments. 0.1 2 + 1 58.9 88.4 1.8 1 3 62.5 93.8 1.3 Table 1. Effect of type, concentration, and volume of eluent on enrichment factor (EF) and recovery. 0.1 3 51.6 77.5 3.9 ExperimentalHCl conditions: Cd 10 µg·L−1; AC 50 mg; SDS 100 mg·L−1; sample volume 200 mL; flow rate 1 3 52.1 78.1 3.9 1 mL·s−1. HCl:HNO3 0.1:0.1 3 58.2 87.3 5.5 Type Concentration (mol·L−1) Vol. (mL) EF Rec. (%) RSD% 0.1 6 26.5 79.5 5.4 According to the comparison0.1 between the recovery5 with35.3 various acid88.3 types, better2.9 result were obtained withHNO nitric3 acid, while lower0.1 recovery was observed3 with47.3 hydrochloric77.3 acid4.9 and mixed acid 0.1 2 + 1 58.9 88.4 1.8 (HNO3:HCl = 1:1). In the case of eluent1 volume, 2 + 31 mL, It is62.5 considered 93.8 that it takes1.3 slightly long time for the desorption process and0.1 the moving action3 has occurred.51.6 The adsorbent77.5 becomes3.9 wet and HCl further desorption process proceeds1 after first elution3 of 2 mL52.1 HNO 3. Furthermore,78.1 3.9 the adsorbent ChemEngineering 2019, 3, 67 6 of 11
is washedChemEngineering away by2019 addition, 3, x FOR PEER 1 mL REVIEW eluent. Considering the atomizer exhibits low tolerance 6 of 11 to the introduction of strong acid, the optimum elution conditions were 0.1 mol L 1 HNO (2 + 1) mL for all · − 3 further experiments.HCl:HNO3 0.1:0.1 3 58.2 87.3 5.5
3.4. Interferences3.4. Interferences The oneThe ofone main of main problems problems is is interference interference byby matrixmatrix elements elements for for the the determination determination of heavy of heavy metals in real samples with atomic absorption spectrometry. Therefore, the interference of coexisting metals in real samples with atomic absorption spectrometry. Therefore, the interference of coexisting elements on the preconcentration and determination must be conducted, because it links directly to elements on the preconcentration and determination must be conducted, because it links directly to the the performance of the proposed analytical method. For the interference study on cadmium, the performanceelement ofcontained the proposed in environmental analytical sample method. such For as theAl, Ca, interference Cu, Fe, K, studyMg, and on Zn cadmium, were selected.the element In containedthese inexperiments, environmental the interfering sample element such as was Al, added Ca, Cu, into Fe, 200 K, mL Mg, of andsample Zn solution were selected. (10 µg·L−In1 of these experiments,cadmium the ion). interfering As shown element in Table was 2, because added the into cadmium 200 mL ofrecovery sample was solution almost (10 quantitativeµg L 1 of in cadmium the · − ion).presence As shown of inexcessive Table2, becauseamount of the the cadmium interferin recoveryg cations, was there almost was quantitativeinsignificant invariation the presence of of excessivedifference amount value calculated of the interfering by comparing cations, EF with there and was without insignificant interference variation ions. As ofa consequence, difference value calculatedthe proposed by comparing preconcentration EF with technique and without was independent interference on ions. matrix As interferences. a consequence, the proposed preconcentration technique was independent on matrix interferences. Table 2. Effect of interfering ions on EFin the matrix/EF of Cd2+ ions. Experimental conditions: Cd 10 −1 −1 −1 −1 µg·L ; AC 50 mg; SDS 100 mg·L ; sample volume 200 mL; flow2+ rate 1 mL·s ; eluent 0.1 mol·L Table 2. Effect of interfering ions on EFin the matrix/EF of Cd ions. Experimental conditions: HNO3 2 + 1 mL. 1 1 1 Cd 10 µg L− ; AC 50 mg; SDS 100 mg L− ; sample volume 200 mL; flow rate 1 mL s− ; eluent · 1 Interference Concentration· (mg·L−1) EFin the matrix/EF × 100 (%) RSD (%) · 0.1 mol L− HNO3 2 + 1 mL. · K⁺ 5 94.0 1.4 Mg2⁺ 3 1 81.2 1.4 Interference Concentration (mg L− ) EFin the matrix/EF 100 (%) RSD (%) Ca2⁺ 3 · 91.4 × 0.8 K+ 5 94.0 1.4 Al3⁺ 2 101.9 1.5 Mg2+ 3 81.2 1.4 Fe3⁺ 1 102.7 5.7 Ca2+ 3 91.4 0.8 Ni2⁺ 5 90.0 4.7 Al3+ 2 101.9 1.5 Cu2⁺ 5 95.8 1.5 Fe3+ 1 102.7 5.7 Zn2⁺ 5 98.2 0.4 Ni2+ 5 90.0 4.7 Cu2+ 5 95.8 1.5 3.5. Effect of Sample Volume Zn2+ 5 98.2 0.4 In the case of preconcentration and determination for trace concentration of analytes, the 3.5. Evolumeffect of Sample of sample Volume solution as an analytical parameter has an important role because the enrichment factor becomes large by increasing the sample volume and/or decreasing the eluent volume as Inshown the case in Equation of preconcentration (1). In order andto investigate determination the effect for traceof the concentration volume of sample of analytes, solution, the three volume of sampledifferent solution volumes as of an 200, analytical 500, and parameter100 mL containing has an 10 important µg·L−1 were role examined, because asthe shown enrichment in Figure 4. factor becomes largeWithout by the increasing addition theof an sample interfering volume element and (Ca),/or decreasingthe recoverythe would eluent not be volume affected as by shown the in Equationsample (1). volume. In order On to investigatethe other hand, the ewhenffect ofthe the Ca volume matrix ofelement sample was solution, added, threea slight di ffdecreaseerent volumes in recovery was observed. Taking into account1 that determination of trace Cd requires relative large of 200, 500, and 100 mL containing 10 µg L− were examined, as shown in Figure4. sample volume, 500 mL of sample volume· was chosen for the subsequent experiment.
Figure 4. Effect of sample volume on recovery of cadmium ions. Experimental conditions: Cd 10 µg L 1; · − AC 50 mg; SDS 100 mg L 1; sample volume 200 mL; flow rate 1 mL s 1; eluent 0.1 mol L 1 HNO 3 mL. · − · − · − 3 ChemEngineering 2019, 3, 67 7 of 11 ChemEngineering 2019, 3, x FOR PEER REVIEW 7 of 11
WithoutFigure 4. the Effect addition of sample of an volume interfering on recovery element of (Ca),cadmium the recoveryions. Experimental would not conditions: be affected Cd by10 the sampleµg·L volume.−1; AC 50 On mg; the SDS other 100 hand,mg·L−1; when sample the volume Ca matrix 200 mL; element flow rate was 1 added,mL·s−1; eluent a slight 0.1 decrease mol·L−1 in recoveryHNO was3 3 mL. observed. Taking into account that determination of trace Cd requires relative large sample volume, 500 mL of sample volume was chosen for the subsequent experiment. 3.6. Analytical Performance 3.6. AnalyticalUnder the Performance optimized conditions, the enrichment factor (EF), recovery, dynamic range (liner range),Under detection the optimized limit, and conditions, reproducibility the enrichment were investigated. factor (EF), When recovery, the dynamic sample solution range (liner and range), eluent detectionvolumes limit,were and500 and reproducibility 3 mL, respectively, were investigated. the calibration When curves the sample was linear solution in the and dynamic eluent volumes range of were0.1–15 500 µg·L and− 31 mL,and respectively,the regression the equation calibration was curves A (absorbance) was linear in = the 0.0762 dynamicC (concentration range of 0.1–15 of µCdg L2+ in1 · − andµg·L the−1) regression+ 0.058 (R2 = equation 0.9982). wasThe Aenrichment(absorbance) factor= 0.0762 was 150C (concentration with the recovery of Cd of2 90.2%.+ in µg TheL 1 )detection+ 0.058 · − (Rlimit2 = 0.9982).(3S/N) Thewas enrichment17 ng·L−1, factorwhich was was 150 calculated with the recoveryby dividing of 90.2%. the Cd The concentration, detection limit (3Swhich/N) waswas 17obtained ng L 1, whichfrom 3 was times calculated the standard by dividing deviation the Cdof th concentration,e blank by the which enrichment was obtained factor. from The 3 value times was the · − standardbetter than deviation the detection of the blanklimit (8 by µg·L the enrichment−1) obtained factor.with graphite The value atomizer was better [38]. than Since the FAAS detection is in much limit (8moreµg L general1) obtained use throughout with graphite the atomizer world, the [38 ].prop Sinceosed FAAS preconcentration is in much more is very general advantageous. use throughout The · − thereproducibility world, the proposed (variability preconcentration of the preconcentration is very advantageous. effect with The each reproducibility system) was (variabilitystudied by ofFAAS the preconcentrationand preconcentration effect by with adsorption. each system) The was rela studiedtive standard by FAAS deviation and preconcentration (RSD) for 10 µg·L by− adsorption.1 of Cd was The2.8% relative for 12 standardmeasurements. deviation (RSD) for 10 µg L 1 of Cd was 2.8% for 12 measurements. · − 3.7.3.7. ReusabilityReusability TheThe reusabilityreusability ofof adsorbentadsorbent is one of of the the significant significant factors factors in in assessing assessing the the performance performance of ofadsorbent adsorbent materials. materials. In Inorder order to to examine examine the the reus reusabilityability of of activated activated carbon carbon on thethe filter,filter, thethe preconcentrationpreconcentration experiments,experiments, thatthat is,is, adsorption–desorption adsorption–desorption cycles,cycles, werewere repeatedlyrepeatedly performedperformed byby 1 usingusing the the same same AC AC adsorbent. adsorbent. After After each each preconcentration preconcentration operation, operation, 5 5 mL mL of of 0.1 0.1 mol mol·LL −1HNO HNO3and and · − 3 purepure water water were were added added into into the the AC AC filter filter for for the the perfect perfect removal removal of of cadmium cadmium ions ions onto onto AC. AC. TheThe recoveries recoveries of of Cd Cd until until 3 cycles 3 cycles became became almost almost the same, the andsame, after and 4 cycles, after the4 cycles, values decreasedthe values slightlydecreased because slightly of the because remaining of the and remaining accumulation and accumulation of SDS into AC, of asSDS shown into inAC, Figure as shown5. Consequently, in Figure 5. ACConsequently, could be reused AC could for 3 cyclesbe reused without for 3 losing cycles the without significant losing adsorption. the significant adsorption.
FigureFigure 5.5. EffectEffect of thethe adsorbentadsorbent reusereuse onon preconcentrationpreconcentration andand determinationdetermination ofof cadmium.cadmium. 1 1 Experimental conditions: Cd 10 µg−L1 ; AC 50 mg; SDS 100− mg1 L ; sample volume 200 mL; Experimental conditions: Cd 10 µg·L· ;− AC 50 mg; SDS 100 mg·L ; sample· − volume 200 mL; flow rate flow rate 1 mL s 1; eluent 0.1 mol L 1 HNO 2 + 1 mL. 1 mL·s−1; eluent· − 0.1 mol·L−1 HNO·3 2− + 1 mL.3 3.8. Reaction Mechanism 3.8. Reaction Mechanism A mechanism for the adsorption of cadmium on the SDS/AC is proposed in Figure6 as well. A mechanism for the adsorption of cadmium on the SDS/AC is proposed in Figure 6 as well. Because the zero point charge (ZPC) pHZPC is 2.2 [37], the AC surface may become negatively in the Because the zero point charge (ZPC) pHZPC is 2.2 [37], the AC surface may become negatively in the pH range of 3 to 10. Hence, the hydrophobic tail as alkyl group (-C12H25) can be linked to the AC pH range of 3 to 10. Hence, the hydrophobic tail as alkyl group (-C12H25) can be linked to the AC surface, and a negatively charged hydrophilic head, such as a sulfonate group, is oriented to bulk surface, and a negatively charged hydrophilic head, such as a sulfonate group, is oriented to bulk solution. The cadmium ion with positively charge can be related to the adsorption site of AC surface or solution. The cadmium ion with positively charge can be related to the adsorption site of AC surface or hydrophilic head. Furthermore, it can be considered that the adsorption of SDS associated with ChemEngineering 2019, 3, 67 8 of 11
ChemEngineering 2019, 3, x FOR PEER REVIEW 8 of 11 hydrophilic head. Furthermore, it can be considered that the adsorption of SDS associated with Cd ion mightCd ion also might occur also as occur the preconcentration as the preconcentration mechanism. mechanism. According According to the effect to ofthe SDS, effect the of interaction SDS, the processinteraction between process the hydrophilicbetween the head hydrophilic and Cd2+ headwill becomeand Cd predominant2+ will become for thepredominant preconcentration. for the preconcentration.
Figure 6. ReactionReaction mechanism that cadmium ions are attracted/absorbed attracted/absorbed by SDS and AC.
3.9. Analysis of Environmental Samples In orderorder toto confirmconfirm the the validity validity of of the the proposed proposed method, method, the the present present procedure procedure was was applied applied into theinto preconcentration the preconcentration and determinationand determination of Cd of inCd di infferent different environmental environmental samples samples such such as rain as rain and mineraland mineral waters. waters. Since thethe concentrationconcentration ofof Cd Cd in in the the environmental environmental samples samples was was less less than than the the detection detection limit, limit, the spikedthe spiked recoveries recoveries were were evaluated evaluated for spikedfor spiked samples. samples. The The equation equation of spike of spike recovery recovery is given is given as as
SpikeSpike recovery recovery= (C = (Cfound C− Coriginal)/)/CCspiked × 100100 (3) (3) found − original spiked × where Cfound, Coriginal, and Cspiked are the concentration obtained by determination using the established C C C wherepreconcentrationfound, original procedure,, and spiked theare concentration the concentration in initial obtained sample by solution, determination and spiked using theconcentration, established preconcentrationrespectively. procedure, the concentration in initial sample solution, and spiked concentration, respectively.The recovery for the spiked Cd was in the range of 92 to 116% as seen in Table 2. The relative standardThe recoverydeviations for for the 3 experiments spiked Cd was for insamples the range spiked of 92with to 116%Cd were as seenbetter in than Table 1.9%.3. The Hence, relative the standardresults obtained deviations that for the 3 experiments present flow for system samples could spiked be with employed Cd were for better the than preconcentration 1.9%. Hence, theof resultscadmium obtained in environmental that the present samples. flow system could be employed for the preconcentration of cadmium in environmental samples. Table 2. Determination of cadmium ions in different real samples using the proposed method (n = 3). Table 3. Determination of cadmium ions in different real samples using the proposed method (n = 3). Sample Added (μg·L−1) Found (μg·L−1) Spike/Recovery (%) Sample Added (µg L 1) Found (µg L 1) Spike/Recovery (%) Rain water 1 - · − N.D.· − (Tsu,Rain Mie) water 1 5.0- N.D. 5.80 ± 0.02 116 (Tsu, Mie) 5.0 5.80 0.02 116 Rain water 2 - ± N.D. (Tsu,Rain Mie) water 2 5.0- N.D. 5.32 ± 0.10 106 (Tsu, Mie) 5.0- 5.32 0.10N.D. 106 ± Mineral water 5.0- N.D. 5.21 ± 0.09 104 Mineral water 10.05.0 5.21 9.170.09 ± 0.01 104 92 ± 10.0 9.17 0.01 92 N.D.: Not detected.± N.D.: Not detected. 3.10. Comparative Analysis 3.10. Comparative Analysis The comparison of this proposed procedure with other SPE procedures is shown in Table 3 [39– 42]. The comparisonproposed preconcentration of this proposed procedure has good with or othercomparable SPE procedures enrichment is shown factor, in Tablelinear4[ 39range,–42]. Thedetection proposed limit preconcentration(LOD) in comparison has good with or other comparable preconcentration enrichment techniqu factor,es linear combined range, with detection SPE, limitand atomic (LOD) absorption in comparison spectrometry. with other preconcentration techniques combined with SPE, and atomic absorption spectrometry. Table 3. Comparison of analytical performances of the present work with those reported in the literatures for SPE methods combined with atomic absorption spectrometry for cadmium determination.
Adsorbent EF Linear Range (μg·L−1) LOD (ng·L−1) Detection Ref. Fe3O4@FePO4 10 0.05–0.5 21 ETAAS [39] Tween 80 coated alumina 9.5 1.0–400.0 200 FAAS [40] ChemEngineering 2019, 3, 67 9 of 11
Table4. Comparison of analytical performances of the present work with those reported in the literatures for SPE methods combined with atomic absorption spectrometry for cadmium determination.
Adsorbent EF Linear Range (µg L 1) LOD (ng L 1) Detection Ref. · − · − Fe3O4@FePO4 10 0.05–0.5 21 ETAAS [39] Tween 80 coated alumina 9.5 1.0–400.0 200 FAAS [40] Dowex Marathon C 250 1–10 130 FAAS [41] Cd(II)-II-MMS 50 0.01–0.2 6.1 GFAAS [42] SDS/AC 150 0.1–15 17 FAAS This work
4. Conclusions Sodium dodecyl sulfate-modified activated carbon (SDS/AC) was used as a sorbent for efficient preconcentration and determination of trace cadmium ions in the flow system. As a result of optimizing the conditions, the proposed procedure gave quantitative and reproducible results. This method is simple and rapid, and serves high enrichment factor. Furthermore, it is very advantageous that the organic solvent is unnecessary for the present method.
Author Contributions: M.F. and S.K. conceived and designed the experiments. M.F. performed the experiments and wrote the paper. I.T. and H.K. analyzed the results and advised the project. Funding: The present research was partly supported from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. Notes: All experiments were conducted at Mie University. Any opinions, findings, conclusions, or recommendations expressed in this paper are those of the authors and do not necessarily reflect the view of the supporting organizations. Conflicts of Interest: The authors declare no conflicts of interest.
References
1. Satarug, S.; Ujjin, P.; Vanavanitkun, Y.; Nishijo, M.; Baker, J.R.; Moore, M.R. Effects of cigarette smoking and exposure to cadmium and lead on phenotypic variability of hepatic CYP2A6 and renal function biomarkers in men. Toxicology 2004, 204, 161–173. [CrossRef] 2. Sasaki, Y.; Yamamoto, N.; Suzuki, Y.; Ueta, I.; Kawakubo, S. Kinetic spectrophotometric determination of trace cadmium in environmental fresh water with 5,10,15,20-tetraphenyl-21H,23H-porphinetetrasulfonic acid. Anal. Sci. 2015, 31, 551–555. [CrossRef] 3. World Health Organization (WHO). Guidelines for Drinking-Water Quality, 4th ed.; Incorporating the 1st Addendum. Available online: https://www.who.int/water_sanitation_health/publications/drinking-water- quality-guidelines-4-including-1st-addendum/en/ (accessed on 15 April 2019). 4. Environmental Protection Agency (EPA). 2018 Drinking Water Standards and Health Advisories Tables. Available online: https://www.epa.gov/dwstandardsregulations/2018-drinking-water-standards- and-advisory-tables (accessed on 15 April 2019). 5. Ministry of the Environment, Government of Japan. National Effluent Standards. Available online: https://www.env.go.jp/en/water/wq/nes.html (accessed on 15 April 2019). 6. Ministry of the Environment, Government of Japan. Environmental Quality Standards for Groundwater Pollution. Available online: https://www.env.go.jp/en/water/gw/gwp.html (accessed on 15 April 2019). 7. Thongsaw, A.; Chaiyasith, W.C.; Sananmuang, R.; Ross, G.M.; Ampiah-Bonney, R.J. Determination of cadmium in herbs by SFODME with ETAAS detection. Food Chem. 2017, 219, 453–458. [CrossRef] 8. Feist, B.; Mikula, B. Preconcentration of heavy metals on activated carbon and their determination in fruits by inductively coupled plasma optical emission spectrometry. Food Chem. 2014, 147, 302–306. [CrossRef] 9. Cervantes, A.; Rodríguez, R.; Ferrer, L.; Cerdá, V.; Leal, L.O. Automatic solid phase extraction of cadmium exploiting a multicommutated flow system previous ICP-MS detection: Application to tobacco samples. Microchem. J. 2017, 132, 107–111. [CrossRef] 10. Gonzáles, A.P.S.; Firmino, M.A.; Nomura, C.S.; Rocha, F.R.P.; Oliveira, P.V.; Gaubeur, I. Peat as a natural solid-phase for copper preconcentration and determination in a multicommuted flow system coupled to flame atomic absorption spectrometry. Anal. Chim. Acta 2009, 636, 198–204. [CrossRef] ChemEngineering 2019, 3, 67 10 of 11
11. Duran, A.; Tuzen, M.; Soylak, M. Preconcentration of some trace elements via using multiwalled carbon nanotubes as solid phase extraction adsorbent. J. Hazard. Mater. 2009, 169, 466–471. [CrossRef] 12. Marahel, F.; Ghaedi, M.; Montazerozohori, M.; Biyareh, M.N.; Kokhdan, S.N.; Soylak, M. Solid-phase extraction and determination of trace amount of some metal ions on Duolite XAD 761 modified with a new Schiff base as chelating agent in some food samples. Food Chem. Toxicol. 2011, 49, 208–214. [CrossRef] 13. Ngeontae, W.; Aeungmaitrepirom, W.; Tuntulani, T.; Imyim, A. Highly selective preconcentration of Cu (II) from seawater and water samples using amidoamidoxime silica. Talanta 2009, 78, 1004–1010. [CrossRef] 14. Swain, S.S.; Nayak, B.; Devi, N.; Das, S.; Swain, N. Liquid–liquid extraction of cadmium(II) from sulfate medium using phosphonium and ammonium based ionic liquids diluted in kerosene. Hydrometallurgy 2016, 162, 63–70. [CrossRef] 15. Mohammadi, S.Z.; Shamspur, T.; Baghelani, Y.M. Determination of copper, nickel, manganese and cadmium ions in aqueous samples by flame atomic absorption spectrometry after simultaneous coprecipitation with
Co(OH)2. Arab. J. Chem. 2014.[CrossRef] 16. Rezende, H.C.; Nascentes, C.C.; Coelho, M.M. Cloud point extraction for determination of cadmium in soft drinks by thermospray flame furnace atomic absorption spectrometry. Microchem. J. 2011, 97, 118–121. [CrossRef] 17. Parham, H.; Pourreza, N.; Rahbar, N. Solid phase extraction of lead and cadmium using solid sulfur as a new metal extractor prior to determination by flame atomic absorption spectrometry. J. Hazard. Mater. 2009, 163, 588–592. [CrossRef] 18. Ngeontae, W.; Aeungmaitrepirom, W.; Tuntulani, T. Chemically modified silica gel with aminothioamidoanthraquinone for solid phase extraction and preconcentration of Pb (II), Cu (II), Ni (II), Co (II) and Cd (II). Talanta 2007, 71, 1075–1082. [CrossRef] 19. Omidi, F.; Behbahani, M.; Bojdi, M.K.; Shahtaheri, S.J. Solid phase extraction and trace monitoring of cadmium ions in environmental water and food samples based on modified magnetic nanoporous silica. J. Magn. Magn. Mater. 2015, 395, 213–220. [CrossRef] 20. Sanchez-polo, M.; Rivera-utrilla, J. Adsorbent-Adsorbate Interactions in the adsorption of Cd (II) and Hg (II) on ozonized activated carbons. Environ. Sci. Technol. 2002, 36, 3850–3854. [CrossRef] 21. Channarong, B.; Lee, S.H.; Bade, R.; Shipin, O.V. Simultaneous removal of nickel and zinc from aqueous solution by micellar-enhanced ultrafiltration and activated carbon fiber hybrid process. Desalination 2010, 262, 221–227. [CrossRef] 22. Moreno-Pirajan, J.C.; Giraldo, L.; Liliana, G. Heavy metal ions adsorption from wastewater using activated carbon from orange peel. J. Chem. 2012, 9, 926–937. [CrossRef] 23. Tavakkoli, N.; Habibollahi, S.; Tehrani, S.A. Separation and preconcentration of Arsenic (III) ions from aqueous media by adsorption on MWCNTs. Arab. J. Chem. 2014.[CrossRef] 24. Tsoi, Y.K.; Leung, K.S.Y. Toward the use of surface modified activated carbon in speciation: Selective preconcentration of selenite and selenate in environmental waters. J. Chromatogr. A 2011, 1218, 2160–2164. [CrossRef] 25. Jiang, Y.Y.; Wu, Y.W.; Liu, J.F.; Xia, X.Q.; Wang, D.L. Ammonium pyrrolidinedithiocarbamate-modified activated carbon micro-column extraction for the determination of as (III) in water by graphite furnace atomic absorption spectrometry. Microchim. Acta 2008, 161, 137–142. [CrossRef] 26. Ensafi, A.A.; Shiraz, A.Z. On-line separation and preconcentration of lead (II) by solid-phase extraction using activated carbon loaded with xylenol orange and its determination by flame atomic absorption spectrometry. J. Hazard. Mater. 2008, 150, 554–559. [CrossRef] 27. Fu, W.; Qu, F.; Yu, G.; You, J. High selectivity for sodium dodecyl sulphate by polymer nanoparticles and detection of proteins based on the polymer nanoparticles-sodium dodecyl sulphate system. Sens. Actuators B Chem. 2017, 245, 774–779. [CrossRef] 28. Shawabkeh, R.A.; Abu-Nameh, E.S.M. Absorption of phenol and methylene blue by activated carbon from pecan shells. Colloid J. 2007, 69, 355–359. [CrossRef] 29. Manbohi, A.; Ahmadi, S.H. In-tube magnetic solid phase microextraction of some fluoroquinolones based
on the use of sodium dodecyl sulfate coated Fe3O4 nanoparticles packed tube. Anal. Chim. Acta 2015, 885, 114–121. [CrossRef] ChemEngineering 2019, 3, 67 11 of 11
30. Marahel, F.; Ghaedi, M.; Shokrollahi, A.; Montazerozohori, M.; Davoodi, S. Sodium dodecyl sulfate coated poly (vinyl) chloride: An alternative support for solid phase extraction of some transition and heavy metals. Chemosphere 2009, 74, 583–589. [CrossRef] 31. Ahn, C.K.; Kim, Y.M.; Woo, S.H.; Park, J.M. Removal of cadmium using acid-treated activated carbon in the presence of nonionic and/or anionic surfactants. Hydrometallurgy 2009, 99, 209–213. [CrossRef] 32. Hou, G.; Li, Y.; Cheng, Y. Activated carbon modified with sodium dodecyl sulfonate as a solid phase sorbent for the separation and preconcentration of trace cadmium in water samples with detection by electrothermal atomic absorption spectrometry. Anal. Lett. 2013, 46, 1978–1990. [CrossRef] 33. Furukawa, M.; Katsumata, H.; Suzuki, T.; Kaneco, S. Determination of cadmium in environmental samples by flame atomic absorption spectrometry with preconcentration using sodium dodecyl sulfate/activated carbon. Bunseki Kagaku 2016, 65, 419–424. [CrossRef] 34. Nurul, A.; Satoshi, K.; Yukitake, N.; Hideyuki, K.; Tohru, S.; Kiyohisa, O. Preconcentration of trace lead by adsorption onto a tantalum wire for electrothermal atomization atomic absorption spectrometry with a tungsten tube atomizer. Microchem. J. 2007, 86, 89–93. [CrossRef] 35. Ahn, C.K.; Park, D.; Woo, S.H.; Park, J.M. Removal of cationic heavy metal from aqueous solution by activated carbon impregnated with anionic surfactants. J. Hazard. Mater. 2009, 164, 1130–1136. [CrossRef] 36. Choi, J.; Ide, A.; Truong, Y.B.; Kyratzis, I.L.; Caruso, R.A. High surface area mesoporous titanium–zirconium oxide nanofibrous web: A heavy metal ion adsorbent. J. Mater. Chem. A 2013, 1, 5847–5853. [CrossRef] 37. Martín-González, M.A.; González-Díaz, O.; Susial, P.; Araña, J.; Herrera-Melián, J.A.; Doña-Rodríguez, J.M.; Pérez-Peña, J. Reuse of Phoenix canariensis palm frond mulch as biosorbent and as precursor of activated carbons for the adsorption of Imazalil in aqueous phase. Chem. Eng. J. 2014, 245, 348–358. [CrossRef] 38. Hiroki, H. Multielement profiling analyses of biological, geochemical, and environmental samples as studied by analytical atomic spectrometry. Bull. Chem. Soc. Jpn. 1999, 72, 1163–1186. [CrossRef] 39. Xiaoxing, Z.; Changle, S.; Li, Z.; Hui, L.; Binxia, C.; Libo, L.; Weimin, G. Adsorption studies of cadmium
onto magnetic Fe3O4@FePO4 and its preconcentration with detection by electrothermal atomic absorption spectrometry. Talanta 2018, 181, 352–358. [CrossRef] 40. Mohammadi, S.Z.; Hamidian, H.; Karimzadeh, L.; Moeinadini, Z. Tween 80 coated alumina: An alternative support for solid phase extraction of copper, nickel, cobalt and cadmium prior to flame atomic absorption spectrometric determination. Arab. J. Chem. 2016, 9, S1290–S1296. [CrossRef] 41. Teslima, D.; ¸Serife,S.; Ahmet, Ü.; ¸Senol,K. A solid phase extraction procedure for the determination of Cd (II) and Pb (II) ions in food and water samples by flame atomic absorption spectrometry. Food Chem. 2015, 174, 591–596. [CrossRef] 42. Bingshan, Z.; Man, H.; Beibei, C.; Bin, H. Novel ion imprinted magnetic mesoporous silica for selective magnetic solid phase extraction of trace Cd followed by graphite furnace atomic absorption spectrometry detection. Spectrochim. Acta B 2015, 107, 115–124. [CrossRef]
© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).