Electroanalytical determination of heavy metals in aqueous solutions by using a carbon paste electrode modified with spent coffee grounds

Journal Pre-proof Electroanalytical determination of heavy metals in aqueous solutions by using a carbon paste electrode modified with spent coffee grounds

J. Estrada-Aldrete, J.M. Hernández López, A.M. García León, J.M. Peralta Hernández, F.J. Cerino Córdova PII:

S1572-6657(19)30931-2

DOI:

https://doi.org/10.1016/j.jelechem.2019.113663

Reference:

JEAC 113663

To appear in:

Journal of Electroanalytical Chemistry

Received date:

22 July 2019

Revised date:

29 October 2019

Accepted date:

15 November 2019

Please cite this article as: J. Estrada-Aldrete, J.M.H. López, A.M.G. León, et al., Electroanalytical determination of heavy metals in aqueous solutions by using a carbon paste electrode modified with spent coffee grounds, Journal of Electroanalytical Chemistry(2019), https://doi.org/10.1016/j.jelechem.2019.113663

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© 2019 Published by Elsevier.

Journal Pre-proof

Electroanalytical determination of heavy metals in aqueous solutions by using a carbon paste electrode modified with spent coffee grounds J. Estrada-Aldretea, J. M. Hernández Lópeza, A. M. García Leóna, J.M. Peralta Hernándezcc, F.J. Cerino Córdovab a

Universidad Autónoma de Nuevo León, UANL, Facultad de Ciencias Químicas, Av.

Universidad s/n, Cd. Universitaria, C.P. 66455, San Nicolás de los Garza, Nuevo León,

Universidad Autónoma de Nuevo León, UANL, Facultad de Ingeniería Mecánica y

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b

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México

Eléctrica (FIME). Av. Universidad s/n. Cd. Universitaria, C.P. 66455, San Nicolás de los

Departamento de Química, División de Ciencias Naturales y Exactas, Universidad de

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c

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Garza, Nuevo León, México

Corresponding author:

Felipe J. Cerino-Córdova

Phone number:

52-8183294000, ext. 1503

Fax number:

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a,b*

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Guanajuato, Cerro de la Venada s/n, Pueblito de Rocha, 36040, Guanajuato, México.

52-8183294000, ext. 1503

E-mail: [email protected]; [email protected]

Journal Pre-proof Abstract In this paper, the quantification of Pb2+ and Cd2+ at trace levels was carried out by differential pulse anodic stripping voltammetry technique, using a paste carbon electrode modified with spent coffee grounds (SCG) as working electrode. Different percentages of SCG (40, 50, 60, 70% of SCG) were tested in the experiments; the best electrochemical response was obtained with the electrode modified with 50% of SCG. An experimental design was performed in order to maximize the intensity of the oxidation current, using the

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accumulation time and the reduction potential as factors. The best experimental conditions for the electrodeposition of the cadmium ions were obtained at an accumulation time of

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76s, and the reduction potential of -1.2V; while for the electrodeposition of lead ions, the best performance was obtained at an accumulation time of 120s and a reduction potential of

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-1.2V. The detection limit of the carbon paste electrode modified with 50% of SCG was

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less than 3x10-4 M, showing the potential of these electrodes as a viable and sustainable

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alternative for the determination of heavy metals in aqueous solutions.

Keyword: Carbon paste electrode, Anodic stripping voltammetry, Heavy metal, coffee

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1. Introduction

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waste

The anthropogenic activities have always contributed significantly to the environmental pollution, and the industrial and technological advances have increased considerably the generation of waste. The mining, metallurgical and coatings industries generate a high quantity of waste containing heavy metals, which are dangerous to human beings due to their high toxicity, non-biodegradability and bioaccumulation. Therefore, it is necessary to quantify and dispose them in an adequate way, in order to avoid environmental and health matters [1-3]. Due to strict environmental regulations, analytical methods are required to measure heavy metals concentrations at trace levels. Thus, several analytical techniques to quantify heavy metals have been developed, such as: mass spectrometry (ICP-MS), neutron activation

Journal Pre-proof analysis (NAA), atomic absorption spectrometry (AAS), ion selective electrode (ISE) and emission spectrometry (ES) [4, 5]. However, most of these techniques require long experimental time, sophisticated instrumentation and special training for their use, in addition to these techniques, cannot be used in situ for real-time monitoring [6]. Therefore, it is necessary to develop sensors to overcome these problems, being the electrochemical techniques a feasible solution, due to its fast response and ease of transport. The anodic stripping voltammetry technique (ASV) has been one of the techniques most frequently used for the metal detection [4, 6-9]. One of its variant is the differential pulse anodic

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stripping voltammetry (DPASV), which is a sensitive technique that has been most widely employed for trace electroactive species determination and its detection limits of metallic

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ions concentrations, lower than parts per billion reported [4, 10, 11]. The DPASV technique

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was employed for the determination of Cu2+ using a glass carbon electrode, and its detection limit was 5.0 x 10-10 mol/L. Nevertheless, its cost was greater than the cost of

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carbon paste electrode [4]. In this sense, the study of modified carbon electrodes is a research domain that is becoming more important, because it allows obtaining electrodes

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with characteristics superior to commercial electrodes, and also eliminates the use of mercury electrodes, thus, avoiding their toxicity [5, 9, 12-14]. The carbon paste electrode is

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one of the most used materials for electrochemical studies due to its low cost and the ease of dispersion of different solids [15]. Among various materials proposed in the literature for

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the detection of heavy metals, one that may be very promising in this field is coffee

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residues. This waste has been studied for: the removal of heavy metals, the detection of phenolic and methylxanthine components by spectroscopy or the generation of activated charcoal [16-20]. These applications allow the revalorization of this agricultural waste and the reduction of environmental pollution caused by its incineration [21] The aim of this study is to evaluate the performance of a paste carbon electrode modified with the spent coffee grounds (SCG), for the quantification of Cd (II) and Pb (II) in aqueous solution. Different compositions of the electrode were studied, and the experimental conditions, where the carbon paste electrode modified (CPEM) with the spent coffee grounds (SCG) to maximize its analytical response, were determined by factorial designs.

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2. Material and methods 2.1 Spent coffee grounds pretreatment The SCG was obtained locally from coffee-makers after the beverage preparation. The SCG was washed repeatedly with deionized water, in order to remove soluble compounds.

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After that, the SCG was dried in an oven at 60 ºC for 24 h; thereafter, it was milled in a

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vibratory mill and stored until before its use. The chemical modifications of SCG were carried out by the following procedure: samples of 30 g of SCG were dipped in 210 mL of

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0.8 M citric acid solution at 60 ºC for 12 h. Then, the temperature was raised to 100 ºC for

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90 min until the liquid solution was evaporated. Subsequently, the modified SCG was dried in an oven at 60 ºC for 24 h, and thereafter washed with 1 L of deionized water for 1 h in

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order to remove the citric acid unreacted materials. Finally, the modified SCG was dried at 60ºC for 24 h and stored until before its use.

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2.2 Carbon paste electrode preparation

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The preparation of the carbon paste electrodes (CPEs) was carried out by mixing the unmodified SCG (40%, 50%, 60% and 70%), graphite powder (Alfa Aesar), mineral oil,

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and Nafion®. The resulting paste was packed into an insulin syringe (0.1963 cm2 transverse area), and the electric contact was made with a graphite tip. In a similar manner, the carbon paste electrodes, made with the modified SCG (CPEM), were prepared. The electrode surface was regenerated in each experiment, extracting the edge of the electrode paste and homogenizing with SiC papers of No. 600. 2.3 Electrochemical characterization of carbon paste electrode modified In order to evaluate the electrochemical behavior and to determine the electroactivity domain of the CPEM, a support electrolyte of 0.01 M HClO4 was used. The CPEM was employed as working electrode, Ag/AgCl (3 M KCl) as reference electrode and graphite tip as counter electrode. A polarographic analyzer (Basi-Epsilon) was used in all experiments.

Journal Pre-proof The electrochemical characterization of CPEMs (40%, 50%, 70% wt. SCG) was performed by Cyclic voltammetry of 25 mL of 5 mM K4[Fe(CN)6]3H2O in 0.01 M HClO4 at room temperature. Ten successive cycles were carried out at a potential range from -0.6 to 1.2 V vs Ag/AgCl (3 M KCl), using a scan rate of 50 mVs-1 in positive direction. 2.4 Differential pulse anodic stripping voltammetry In order to obtain the optimal experimental condition for the quantification of heavy metal (Cd2+ and Pb2), a two factor three level (32) central composite face-centered were used.

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The electrodeposition potential (Ed) and accumulation time (td) were employed as factors, and their levels are shown on the Table 1. All the experiments were carried out at a

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potential range from -0.6 to 1.2 V vs Ag/AgCl (3 M KCl) with a scan rate of 50 mVs -1

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using 25 ml of heavy metals concentration solution of 30 M and HClO4 (0.01 M) as electrolyte support. A gas inert (N2)) was used to disperse heavy metal solution.

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The experimental conditions, obtained with the experimental design, were used in the

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quantification of heavy metal experiments by standard addition method. The Cd2+ and Pb2 concentrations were varied from 30 to 269.35 M. The detection (L.D.) and quantification (L.C.) limits were evaluated using Eqs. (1) and (2) [22].

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L.D. = 3 /m

(2),

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L.C. = 10 /m

(1)

relationship.

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where is the standard deviation of the data points, and m is the slope of the linear

3. Results and discussion 3.1 Electrochemical characterization of CPEM The electrochemical response of the CPEM electrode at different percentages of SCG (40, 50, 60, 70% wt.) in 5 mM K4[Fe(CN)6]3H2O and 0.01 M HClO4 was evaluated using cyclic voltammetry. The results are shown at the Figure 1. The ferrocyanide oxidation (Epa) and the reduction (Epcc) peaks using CPEM with 40%, 50% and 60% of SCG are shown on

Journal Pre-proof the Table 2, which are characteristic of the peaks of oxidation and reduction of ferrocyanides [23]. The peak-to-peak separation (ΔEp), which is related to the electron transfer kinetics, showed values higher than 500 mV for all CPEMs; this can be attributed to various factors, such as a slow electron transfer rate on the surface of the modified electrodes as a result of uncompensated resistance along the electrochemical system, or a low concentration of the species present in the electrolyte, as reported by other authors [23-25].

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However, oxidation and reduction bands do not appear in the cyclic voltammogram of the CPEM with 70% of SCG (Fig. 1d). Transition regions at both ends of the potential sweeps

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are observed, referred to as Faradaic feature, suggesting that the ions not only interact with

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the electrode surface, but also penetrate through the intrinsic porosity due to the high content of SCG in the CPEM, what inhibits the oxidation and reduction reactions of the

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Fe(CN)63-/4- [26, 27].

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On the other hand, the CPEM with 50% of SCG (CPEM50) improves both: the shape of redox peaks and the magnitude of the cathodic and anodic peak currents, indicating a major sensitivity. The anodic (ipa) and cathodic (ipc) currents increase while increasing

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successive cycle, which indicates that Fe was deposited over the surface of the CPEM, since it is the only specie in the solution [28].

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This behavior becomes more evident if we compare it with the glassy carbon electrode

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(Fig. 1f). In both voltammograms, it was observed the presence of oxidation and reduction peaks related to ferrocyanides. However, the CPEM50 peaks were more intense and defined in comparison with the glassy carbon electrode. This is an indication of a better performance of the CPEM50 over the glassy carbon electrode. According to these results, the CPEM50 was chosen to determine the optimal conditions to perform the differential pulse voltammetry anodic studies.

3.2 Effect of the accumulation time and the reduction potential To determine the conditions of electrodeposition potential (Ed) and accumulation time (td), where the anodic peak current response is maximized, on the surface of the modified

Journal Pre-proof electrodes (CPEM50), an experimental design was carried out (Table 1). The analysis of variances (ANOVA) for Pb and Cd are shown on the Table 3 and Table 4, respectively. The lead ANOVA (Table 3) shows that both linear variables studied and the interaction td, Ed and td*Ed, are significant with 95% confidence. Whereas, the Cd ANOVA (Table 4) showed that with 95% confidence, the linear and quadratic variable Ed is significant, followed by the interaction td2*Ed. The hierarchical quadratic modeling equation of the Pb anodic peak current at 95%

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confidence, in actual factors terms (Equation 3): Ipa (Pb) = 1.13035E-004 – 3.21483E-006 * td + 1.32919E-007 * Ed – 4.926E-005 * td * Ed

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(3)

The model variables explain 92.94% of the variation in the Pb anodic peak current (Adj R-

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Squared = 0.9294). The results do not approach significance because of the lack of fit (pValue = 0.4298), then, the statistic model is adequate to plot the region studied of operation

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conditions.

In the same way, the hierarchical quadratic modeling equation of the Cd anodic peak

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current at 95% confidence, in actual factors terms, will be shown in the Equation 4:

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Ipa (Cd) = 2.15684E-004 – 4.07108E-006 * td + 3.59538E-007 * Ed – 4.49374E-009 * td * (4)

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Ed + 2.70824E-008 * td2 + 1.33804E-010 * Ed2 + 2.98090E-011 * td2 * Ed.

The model variables explain 77.88% of the variation in the Cd anodic peak current (Adj RSquared = 0.7788). Nevertheless, the results do not approach significance because of the lack of fit (p-Value < 0.0001); there is only a 0.01% chance that the lack of fit could occur due to noise. The response surface plots described by the statistical models are shown at Figure 2, where the behavior of the anodic current is shown as a function of the variables, proposed in the experimental design.

Journal Pre-proof The Pb response (Fig. 2a) shows that there is an increase in the intensity of the current, as the electrodeposition potential and the accumulation time were increased; the maximum anodic peak current (2.4 x 10-4 A) was obtained at -1200 mV of potential and 120 seconds of accumulation time. This optimal experimental condition was employed to carry out the Pb quantitative analysis (Section 3.3). It is important to mention that a decreasing tendency of anodic current was observed with decreasing the deposit time, by maintaining the potential at its maximum value (-1200 mV); its minimum anodic current value reached was

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approximately 1 x 10-7 A at 30 seconds. In the case of the anodic current intensity associated with the Cd electrodeposition (Fig.

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2b), it shows that there is no variation in the anodic current at potentials below -1100 mV,

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regardless of the electrodeposit time. However, a maximum response of 2.716 x 10- 5 A was reached at -1200 mV and 76 seconds; if the time is increased maintaining the maximum

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potential, the anodic current is decreased. This optimal experimental condition was used in

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the Cd quantification by differential pulse anodic stripping voltammetry (Section 3.3).

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3.3 Differential pulse anodic stripping voltammetry The electroanalytical response of the CPEM50 was evaluated in different heavy metals

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concentrations in the range from 30 to 263.8 M, using the best experimental condition (Ed

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and td), obtained in the previous section. In Figure 3, the anodic peak current increases proportionally to the heavy metal concentration increase. These experimental data were adjusted to a linear equation, with the correlation coefficients higher than 0.9, which means that the relationship between anodic peak current and the heavy metal concentration are linear on the range of heavy metal concentration employed (Table 2). Then, the CPEM50 could be used to quantify heavy metals in aqueous solution, in the range of heavy metal concentration studied. The detection and quantification limits were calculated using Equations 1 and 2, and their values are shown on the Table 5. The results showed that the limit of detection and quantification of Pb(II) and Cd(II) are similar in the order of magnitude (10-4) and the standard deviations were 1.9245 x 10-5 and 1.8752 x 10-4 for Pb2+ and Cd2+, respectively .

Journal Pre-proof 4. Conclusions In this paper, the feasibility to use carbon paste electrode modified with spent coffee grounds as a sustainable alternative to quantify lead and cadmium by differential pulse anodic stripping voltammetry has been demonstrated. The carbon paste electrode using 50% of spent coffee grounds as modifying agent showed the best performance in the experiments of electrochemical characterization in K4[Fe (CN)6]3H2O-HClO4 solution by cyclic voltammetry. This modified electrode showed a

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performance better than glassy carbon electrode, and it was determined that its magnitude of signal is highly dependant of percentage of spent coffee grounds. The increase of % SCG

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in carbon paste electrode higher than 60% had a negative effect in the electrochemical

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response; the oxidation and reduction peaks did not appear in these experiments. On the other hand, the experimental design was a useful tool for obtaining the best

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experimental conditions to heavy metal quantification by differential pulse anodic stripping voltammetry (75 s and 1.2 V for Cd and 120 s, and 1.2 V for Pb). The limit of detection

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and quantification of Pb and Cd were not greatly different (in the order of 10-4). This paper demonstrates that coffee grounds could be an environmental and cost-effective

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Acknowledgements

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aqueous solution.

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alternative to modify carbon paste electrode for the quantification of heavy metals in an

Estrada-Aldrete is very grateful for the Master’s grant awarded by the Mexican Council of Science and Technology (CONACyT).

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Journal Pre-proof Table List.

Table 1. The factors and levels used in the experimental design. Table 2. Parameters of the electrochemical characterization of the CPEM. Table 3. ANOVA for Pb anodic peak current. Table 4. ANOVA for Cd anodic peak current. Table 5. Detection and quantification limits of Pb(II) and Cd(II) using CPEM50.

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Table 6. Comparison of detection limits of Pb(II) and Cd(II) of different electrodes

Journal Pre-proof Table 1.

Coded levels Independent variable Coded factor

-1

0

1

Actual levels Accumulation time td (s) Electrodeposition

30

75

120

B

-1200

-950

-700

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potential, Ed (mV)

A

Journal Pre-proof Table 2. Electrode

Epa(mV)

Epcc(mV)

Ep (mV)

CPEM40 (40% SCG)

950

-150

1100

CPEM50 (50% SCG)

650

130

520

CPEM60 (60% SCG)

800

-40

840

CPEM70 (70% SCG)

ND

ND

ND

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ND: Not detected

Journal Pre-proof Table 3.

Source Model

1.108E-008 4.321E-009 1.913E-008 9.705E-009 8.474E-010 1.978E-010 1.573E-010 2.383E-010

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td Ed td * Ed td *Ed2 Residual Lack of Fit Pure Error Total

Mean Square

F-Value

p-Value

56.03 21.84 96.68 49.06 4.28

< 0.0001 0.0016 < 0.0001 0.0001 0.0723

0.66

0.6514

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Degree of Freedom 4.434E-008 4 4.321E-009 1 1.913E-008 1 9.705E-009 1 8.474E-010 1 1.583E-009 8 6.294E-010 4 9.534E-010 4 4.592E-008 12 Sum of Squares

Journal Pre-proof Table 4.

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td Ed Ed2 td2 * Ed Ed3 Residual Lack of Fit Pure Error Total

3.010E-010 2.211E-012 2.478E-010 1.880E-012 2.506E-010 2.477E-010 1.086E-013 1.360E-013 8.811E-014

F-Value

p-Value

3325.05 20.35 2281.69 17.31 2307.06 2252.89

< 0.0001 0.0028 < 0.0001 0.0042 < 0.0001 < 0.0001

1.54

0.3339

of

Model

Mean Square

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Degree of Freedom 1.806E-009 6 2.211E-012 1 2.478E-010 1 1.880E-012 1 2.506E-010 1 2.447E-010 1 7.603E-013 7 4.079E-013 3 3.524E-013 4 1.807E-009 12 Sum of Squares

Source

Journal Pre-proof Table 5.

Range of Metal Concentration

Linear equation

R2



(M)

LOD

LOQ

(M)

(M)

Pb2+

30-189

Ipa=2 x 10-7 [Pb2+]+1x10-5

0.926

6.01x10-6

90

300

Cd2+

30-269

Ipa=2 x10-6 [Cd2+]-1x10-5

0.945

5.96 x 10-5

89

297

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Ipa: A [Pb2+], [Cd2+]: M

Journal Pre-proof

Technique

Reference

5 -50

Detection Limit (LOD) g L-1 Pb2+ 0.2

SWASV

10.0–60.0

0.65

SWASV

PE-CPE

2- 8

10

DPASV

Tesarova et al., 2009 [33] Ashrafi et al., 2014 [34]

BiF-ZDCPE

1-20

0.10

of

BiNP/N-MPGE

10-150

31.07

SCG-CPEM

30-189*

re

Table 6.

89* Cd2+ 0.8

SWASV

0.77

SWASV

0.08

DPSV

SbF-CPE

DPSV ASV

DPASV

5 -50

SbNP-MWCNT

10–60

BiF-ZDCPE

1-20

BiNP/N-MPGE

10-150

7.31

ASV

Sb/NaMM- CPE

4–150

0.25

SWASV

SnF-CPE

1.13

SWASV

89*

DPASV

*mol

L-1

na

2–90

Jo

SCG-CPEM

lP

SbF-CPE

ur

SbNP-MWCNT

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Range of Concentration g L-1

-p

Electrode

30-269*

Ouangpipat et al, 2003 [35] Cao et al., 2008 [36] Palisoc et al., 2019 [37] This Work Tesarova et al., 2009 [33] Ashrafi et al., 2014[34]

Cao et al., 2008 [36] Palisoc et al., 2019[37] Chen et al., 2016 [38] Li et. al., 2012 [39] This work

Journal Pre-proof

Figures List

Figure 1. Carbon paste electrode modified with spent coffee grounds

Figure 2 Cyclic voltammograms of 5 mM K4[Fe(CN)6]·3H20 and 0.01 M HClO4 solution with different working electrodes: a) CPEM40 (40% SCG), b) CPEM50 (50% SCG), c)

Comparison of CPEM50 and glassy carbon electrode.

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CPEM60 (60% SCG), d) CPEM70 (70% SCG), e) glassy carbon electrode and f)

-p

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Figure 3 Response surface of the current oxidation intensity of Pb (a) and Cd (b) Figure 4 Quantification of (a) Pb2+ and (b) Cd2+ using CPEM50 by differential pulse

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anodic stripping voltametry.

Journal Pre-proof Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Journal Pre-proof Highlights

Spent coffee grounds could be used as a modifying agent of a carbon paste electrode modified (CPEM)

CPEM is an environmentally friendly for the electroanalytical quantification of heavy

of

metals

The sensitivity of the CPEM is related to the amount of coffee waste inside the

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electrode

-p

CPEM with 50% of SCG improves the shape and magnitude of redox peaks of

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ferrocyanide reaction

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The limits of detection of CPEM are 79 and 80 M for Pb(II) and Cd(II), respectively

Figure 1

Figure 2

Figure 3

Figure 4