Metallic copper removal optimization from real wastewater using pulsed electrodeposition

Journal Pre-proof Metallic copper removal optimization from real wastewater using pulsed electrodeposition Thayane Carpanedo de Morais Nepel, Richard Landers, Melissa ´ ˆ Gurgel Adeodato Vieira, Ambrosio Florencio de Almeida Neto

PII:

S0304-3894(19)31370-6

DOI:

https://doi.org/10.1016/j.jhazmat.2019.121416

Reference:

HAZMAT 121416

To appear in:

Journal of Hazardous Materials

Received Date:

2 February 2019

Revised Date:

21 September 2019

Accepted Date:

6 October 2019

Please cite this article as: de Morais Nepel TC, Landers R, Gurgel Adeodato Vieira M, de Almeida Neto AF, Metallic copper removal optimization from real wastewater using pulsed electrodeposition, Journal of Hazardous Materials (2019), doi: https://doi.org/10.1016/j.jhazmat.2019.121416

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Metallic copper removal optimization from real wastewater using pulsed electrodeposition

Thayane Carpanedo de Morais Nepel*,1, Richard Landers2, Melissa Gurgel Adeodato Vieira1, Ambrósio Florêncio de Almeida Neto1 1

University of Campinas, School of Chemical Engineering, 500, Albert Einstein Avenue. Zip

Code: 13083-852, Campinas-SP, Brazil University of Campinas, “Gleb Wataghin” Institute of Physics, 500, Albert Einstein Avenue.

Zip Code: 13083-852, Campinas-SP, Brazil

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(*) corresponding author: [email protected]

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Graphical abstract3

Highlights 

Optimization for copper removal from real wastewater with pulsed electrodeposition



Optimum copper removal of 33.59% with 84.36% of efficiency in 30 minutes



Operation parameters enables the formation from Cu2O to crystalline metallic Cu



Current and ton: main factors responsible for Cu removal and deposition efficiency

ABSTRACT – The recovery of metals from wastewater is a recurrent problem due to numerous productive activities that produce wastewaters rich in toxic metals. Within this

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context, this research presents the study and optimization of copper recovery of real

wastewater using pulsed electrodeposition. The studied parameters – method, current,

temperature, and rotation– influence both the removal of Cu and the composition of the

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formed deposit, noting that the variation of these parameters enables the removal of copper

with formation from crystalline oxides to crystalline copper in its metallic form. The process

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was optimized, and a 33.59% copper removal from a real wastewater with a deposition

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efficiency of 84.36% in 30 minutes was deemed optimal, using fast galvanic pulse, ton= 1ms, 190 mA, 70 rpm, and 37 °C. For coating in the optimum point, a metallic and crystalline

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copper with 100% purity was obtained.

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Keywords: copper removal; real wastewater; pulsed electrodeposition; experimental design

1. Introduction

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Wastewater from different industrial activities and containing heavy metals consist in

a global issue, since heavy metals tend to accumulate in living organisms, whether toxic or carcinogenic [1]. Among these wastewaters, the rinsing water from the electroplating industry outstand due to its higher copper concentration than that standardized by different countries [2]. For electroplating of veneers, the coating process to provide color and brightness to the gems generates rising water with high concentration of copper, since copper baths are used at

the beginning of the production process to increase thickness and to decrease the irregularities of the raw material [3]. Such wastewaters are conventionally treated with chemical precipitation of salts, creating insoluble hydroxides with formation of a concentrated residual sludge, which should be stored in special landfills and, thus, transferring the contaminant from the liquid to the solid phase [4,5]. As an alternative method to this issue, electrodeposition is noteworthy for providing the recovery of metal ions through selective removal or even for the production of

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new materials [3,6]. Among electrodeposition techniques, the application of direct current between the

substrate and the anode is the most known. Some recent researches pointed out the production

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of different alloys such as CuW, CoW, CoCuW [7], and NiCoW [8] using this galvanostatic method. It was also show in the literature the use of this method for copper removal from

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alkaline synthetic wastewater baths [9,10]. However, studies to obtain metallic coatings using

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electrodeposition have pointed to the benefits of using pulses during chronoamperometry in the properties and characteristics of the obtained metallic coatings, such as: smoothness, uniformity of the coating, and composition [11–13]. Researches on NiCu electrodeposition

homogeneity [14].

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also indicate alloy formation with higher purity, low porosity, fine crystalline grains and

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Despite the technical benefits of electrodeposition techniques, especially the pulsed

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ones, copper removal using electrochemistry is little addressed in the literature, and most research are performed on synthetic effluent with basic pH, using electrochemical techniques without pulse [9,11]. Research for synthetic acid effluent showed promise results using combination process of electrolysis and electrodialysis [15]. Hence, there is a need for research over electrochemical pulsed techniques for metal recovery from real wastewater, as well as regarding the influence of the process parameters in

the composition and characteristics of the obtained coating, thus enabling to optimize and to make the use of electrodeposition feasible as a method of recovery and recycling of metals. Within this context, this study adds significant knowledge about the influence of the electrochemical method and its parameters – temperature, rotation, and electrical current – in the removal and deposition efficiency of copper, in addition to the composition of the deposit. Moreover, we highlight the importance of the experimental design methodology used, the Response Surface Model (RSM), known for providing the study on the synergy between

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parameters as well as for the optimization of products and processes [8], with the benefit of use a minimum number of experiments [16,17]. The central composite (CCD) design was

used for building the quadratic model, once it is more effective compared with others, such as

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Box-Behnken (BBD), when analyzing designs with variable and level number higher than 3.

2. Material and Methods

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variables studied in the present study [17].

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The CCD provide a more complete experimental data to analyze the information from the 4

2.1 Experimental Design and Operating Parameter

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The study on removal of copper from electroplating wastewater by electrodeposition was performed through experimental design. Initially, a complete factorial design 24 was

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developed, in which four factors were studied: temperature, electrodeposition method,

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electrical current and cathodic rotation. Since the central point of this planning was the experiment that featured better results, considering electrodeposition efficiency, removal, and composition, we performed the 24 central composite design with the same parameters for process optimization. These factors and their respective levels studied in the central composite design are described in Table 1. All experiments were randomly conducted in order to avoid systematic error, and replicates of the central point (0) were carried to verify the

reproducibility of the process. Table 1 – Levels and coding of the studied factors. Factors Temperature (°C) Method = ton + toff(ms) Current (mA) Rotation (rpm)

Level (-2) 23

Level (-1) 30

Level (0) Level (+1) 37 44

Level (+2) 51

0.1+0.99 10 0

0.5+0.95 100 35

1+9 190 70

2+8 370 140

1.5+8.5 280 105

2.2 Chemical Composition of Electrolyte bath

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The electrolytic bath consisted of a real wastewater generated by the jewelry industry due to successive steps of washing of the jewelry after each electrodeposition process. The

wastewater was previously characterized regarding its chemical composition by inductively

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coupled plasma optical emission spectroscopy, with an equipment model Optima 8300 -

Perkin Elmer, for determining the concentrations of silver, copper, and nickel. The atomic

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absorption spectrometry, AA-7000, Shimadzu, was used to determine possible traces of gold. The analyses indicate that the studied wastewater has in its composition 0.21 mg/L silver,

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0.79 mg/L nickel, 430 mg/L copper and gold concentration below the equipment detection limit (0,015 ppm). In addition to the metals, the bath features high concentrations of cyanide,

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since it is used as a complexing agent for electrodeposition in the jewelry industry. Therefore, it was used as a complexing agent in the process for copper removal during the study. The

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concentration of cyanide was analyzed by potentiometric titration with silver nitrate, and we

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obtained a concentration of 8.3397 ± 0.66 g/L cyanide. This wastewater was directly used in electrodeposition, without adjusting the hydrogen potential (pH = 1.74). 2.2 Substrate and Electrodeposition Electrodepositions were performed using metallic copper foil as substrate, in a square dimension with 0.0008 m² of area. This substrate was previously treated with polishing by using sandpapers no. 400 and no.1200, followed by chemical treatment with 10% wt.% sodium hydroxide solutions, and 1% vol.% sulphuric acid, in this respective order [8].

Electrodeposition experiments were conducted in an electrochemical cell of 50 mL capacity, operating as a batch reactor and using a rotating electrode, Pattern 616A – Princeton Applied Research, for controlling the cathodic rotation of the substrate. A potentiostat/galvanostat, VersaStat3 – Princeton Applied Research in the control of the current density between the cathode and the anode and to control the current application (ton) and the open circuit (toff) times. The anode was a hollow cylindrical platinum mesh and the duration of each electrodeposition was 30 minutes. For the fast-pulsed chronoamperometry, used as

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electrodeposition method, the total time of each cycle was kept in 10 ms, only changing the ratio between ton and toff according to the experimental design, and thus applying 180,000

cycles in each experiment. The electrolytic bath temperature was controlled by a thermostatic

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water bath. Figure 1 illustrates the experimental system.

Figure 1- Representation of the experimental system. The electrodeposition efficiency () was calculated by the applied load and the

equivalent mass acquired by cathode according to Equation (1), 𝜀=(

𝑚∙𝐹 𝑛𝐶𝑢 ∙ 𝑤𝐶𝑢 ) ∙ (∑ ∙ 100) (1) 𝑖 ∙ 𝑡𝑜𝑛 ∙ 𝑛°𝑐𝑦𝑐𝑙𝑒𝑠 𝑀𝐶𝑢

Where: m (g) – the mass of the deposit; i (A) – total current used; 𝒕𝒐𝒏 – the time that the current was applied in each cycle; wCu – the mass fraction of copper in the deposit and determined by EDX; nCu = 2, Mj (g/mol) – the molar mass of Cu; F = 96.485 C/mol; and no. of cycles = 180,000. The percentage of copper removal (%Rem) from wastewater was calculated by Equation 2 using copper concentrations obtained by absorption spectroscopy in the UV-

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Visible at λ = 800 nm, and the 𝐶𝑖 and 𝐶𝑓 concentrations obtained, respectively, before and after each electrodeposition experiment performed.    100 

(2)

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 Ci  C f % Re m    Ci

2.2. Characterization of deposits

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Topographical characterization and chemical composition of deposits were obtained

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by scanning electron microscopy with Energy-dispersive X-ray spectroscopy (EDX) (LEO Electron Microscopy/Oxford 440i). The analysis of copper cyanide formation was performed using Fourier-Transform Infrared Absorption Spectroscopy (FTIR) in the reflectance mode, in

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the range from 4000 to 650 cm-1 and with a resolution of 4 cm-1. The crystallinity of the obtained deposits was determined by X-ray Diffraction (XRD) (Philips X’PERT) with Cu Kα

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radiation, 1.52 Å wavelength, step size of 0.02 2θ, and time per step of 1 second.

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Confirmation of copper oxides and copper cyanide on the surface of some deposits, as well as the determination of the oxidation state of copper on the surface of the coatings, were performed with XPS. The XPS spectra were obtained in a VSW HA-100 spherical analyzer, with Al K non-monochromatic radiation (hv = 1486.6 eV), keeping pressure less than 2x108 mbar. High-resolution spectra were measured with constant energy of the 44 eV analyzer. Binding energy spectra were recorded in the regions of C1s, Cu2p, O1s, N1s.

3. Results and Discussion The results of electrodeposition efficiency and removal percentage for each experiment of the 24 central composite design were calculated according to Equations 1 and 2 and are presented in Table 2. Table 2 – Results of deposition efficiency and removal of the 24 central composite design. (g)

ε (%)

Removal (%)

1

30

90.0

0.100

35

0.0020

67.49

12.79

2

44

90.0

0.100

35

-0.0020

-67.49

-7.35

3

30

270

0.100

35

0.0057

64.11

21.37

4

44

270

0.100

35

0.0051

57.37

16.50

5

30

90.0

0.280

35

0.0043

51.82

20.30

6

44

90.0

0.280

35

7

30

270

0.280

35

8

44

270

0.280

35

9

30

90.0

0.100

10

44

90.0

11

30

270

12

44

270

13

30

90.0

14

44

15

30

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ω (rpm)

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T (°C) ton total (s) I (A)

59.05

17.83

0.0011

4.419

24.30

0.0028

11.25

24.47

105

0.0019

64.13

64.13

0.100

105

-0.0020

-67.49

-3.61

0.100

105

0.0060

67.49

21.19

0.100

105

0.0053

59.62

15.96

0.280

105

0.0079

95.21

27.49

90.0

0.280

105

0.0059

71.10

19.25

270

0.280

105

0.0053

21.29

47.79

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0.0049

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Exp.

44

270

0.280

105

0.0126

50.62

48.41

23

180

0.190

70

0.0045

39.96

32.09

51

180

0.190

70

0.0069

61.27

20.30

19

37

18

0.190

70

-0.0027

-239.76

-8.992

20

37

360

0.190

70

0.0039

17.32

44.32

21

37

180

0.010

70

-0.0013

-219.34

-3.02

22

37

180

0.370

70

0.0013

5.928

43.00

23

37

180

0.190

0

0.0027

23.98

10.64

24

37

180

0.190

140

0.0102

90.58

34.31

16 17

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18

25 (C) 37

180

0.190

70

0.0095

84.36

33.25

26 (C) 37

180

0.190

70

0.0093

82.58

37.33

27 (C) 37

180

0.190

70

0.0102

90.58

37.17

28 (C) 37

180

0.190

70

0.0092

81.67

28.90

29 (C) 37

180

0.190

70

0.0096

85.25

33.58

The data evaluation was done using Statistica® 8.0 software, considering pure error, 95% confidence level, and using the backward elimination method. In this method, a factor

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studied in the experimental design is removed from the statistical empiric model if this procedure promotes an increase of the model R2 adjust parameter and thus a better

mathematical adjust from the model to the experimental data. [18]. Hence, in this study, the

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models for deposition efficiency of removal that best fit to the experimental data were

obtained by eliminating terms with low significance (p>0.05), using ANOVA to analyze the

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significance and fitness of the obtained models, as reported in the literature [19,20,3,21].

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3.1. Influence of electrodeposition parameters in the deposition efficiency Statistical analysis of the deposition efficiency response pointed out that the variables

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temperature, method, current, and the synergy between these factors significantly influence the deposition efficiency, as shown in Pareto chart (Figure 2), where the numbers 1, 2 and 3

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represent, respectively, the temperature, method, and electric current. Moreover, we can perceive that the quadratic (Q) and linear (L) terms for the current and method factors

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influence the deposition efficiency, confirming a mathematical model of second-order. The temperature parameter has no direct influence, but interacts with both the method and the current, highlighting the importance of the synergy between temperature and current and between temperature and method in the result of the deposition efficiency. Synergy between the main factors, current and method, is also statistically significant.

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Figure 2. Pareto chart considering the deposition efficiency response.

Other authors [22, 23] emphasize the complex nature of the parameters of the pulsed

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electrodeposition method and the various phenomena that occur during electrodeposition such

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as kinetics and hydrodynamics. They present calculations of relaxation time of copper using Sand equation, indicating that for deposition of copper due to nickel in NiCu alloys, the

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relaxation time of copper must be higher than the ton. Furthermore, they mention the importance of the synergy between current and ton for the selective recovery of copper, which was obtained by Cherkaoui et al. [24] with ton = 0, 5ms and I = 0.20-0. 25A/cm2 or ton = 2ms

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and I = 0.1 A/cm2 on baths containing copper and nickel with copper concentration of 0.125

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Mol/L. However, it is the first time presented in the literature the statistic importance of this synergy on copper electrodeposition using real wastewater. From the analysis of the results of the influence of studied factors on the response of

electrodeposition efficiency with backward elimination and ANOVA methods, mathematical model presented in Equation 3 was obtained, which is significant, according to Fisher tests conducted from the ANOVA results (Table 3).

𝜀 (%) = 91.88 + 24.02 ∙ 𝑚 − 39.11 ∙ 𝑚2 + 23.75 ∙ 𝑐 − 37.98 ∙ 𝑐 2 + 19.06 ∙ 𝑇 ∙ 𝑚 + 18.78 ∙ 𝑇 ∙ 𝑐 − 27.60 ∙ 𝑚 ∙ 𝑐 (3) Where ε is the deposition efficiency and m, c, and T are, respectively, the encoded values of the levels of method, current, and temperature parameters. Table 3 – ANOVA table of the results of deposition efficiency of the 24 central composite design. Degree of Freedom

Quadratic Mean

Regression

123183.6

7

17597.66

Residual

66830.1

21

Lack of fit

66781.8

17

Pure Error

48.3

4

Total

190013.7

28

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Quadratic Sum

3182.386 3928.341

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12.075

Figure 3. Response surface illustrating the influence of variables method and electrical current on deposition efficiency.

The optimization analysis of the studied factors was carried out using response surface methodology (RSM)[25] obtained through Statistica® 8.0 software. The Figure 3 shows the response surface obtained with temperature and rotation parameters set in the central point (37 °C temperature and 70 rpm rotation). The analysis indicates higher efficiencies, represented in black color, when the current and method parameters are close to the zero encoded level, in which the applied current is 190 mA and ton is equal to 1ms.

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3.2. Influence of electrodeposition parameters in the removal percentage Comparing the percentage results shown in Table 2, experiments no. 15 and 16 have

removal of about 15% higher than the central point, justifying a possible modification of the

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central point in the central composite design for removal optimization. However, when

comparing with the deposition efficiency response, these experiments showed a reduction

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greater than 35% of the deposition efficiency, thus expressing a significant energy

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inefficiency. Therefore, although the central point presents average removal 10% lower than the maximum values found in the factorial planning, it was kept for the optimization with the central composite design.

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The result of the statistical treatment of removal responses indicates that all studied parameters have significant influence on removal response, as shown by Pareto chart (Figure

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rotation.

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4). In this figure, the parameters 1, 2, 3, 4 are respectively: temperature, method, current, and

( 3 ) C u r r e n t( L )

1 4 .0 9 7 1

( 2 ) M e th o d ( L )

1 3 .7 9 7 1

M e th o d ( Q )

- 6 .5 2 8 9

( 4 ) R o ta tio n ( L )

5 .8 8 5 7

C u r r e n t( Q )

- 5 .6 7 3 8

R o ta tio n ( Q )

- 4 .7 5 9 8

( 1 ) T e m p e r tu r e ( L )

- 4 .3 3 7 4

3Lby 4L

4 .3 1 1 8

T e m p e r tu r e ( Q )

- 3 .3 8 8 9

2 .9 6 5 7

1Lby 2L

2 .2 6 2 3

p = 0 .0 5

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2Lby 4L

S ta n d a r d iz e d Ef f e c t Es tim a te ( A b s o lu te V a lu e )

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Figure 4. Pareto chart considering the copper removal.

The result shown in the Pareto chart emphasize that all parameters influence the

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response with quadratic (Q) and linear (L) terms, as presented in the second-order

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mathematical model (Equation 4). Furthermore, the rotation parameter positively interacts with both the current and the method, i.e., the increase in rotation, together with the increase

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in the current or ton total, results in increased removal. The temperature parameter also positively interacts with the method, its effect is lower, but deemed important by the

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backward elimination method. The model obtained was analyzed according to ANOVA (Table 4) and Fisher tests, being significant and predictive, which indicates that it is properly

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fitted to the experimental data and it can be used for prediction of new results within the studied range.

𝑅 (%) = 34.05 − 3.06 ∙ 𝑇 − 2.30 ∙ 𝑇 2 + 9.74 ∙ 𝑚 − 4.43 ∙ 𝑚2 + 9.95 ∙ 𝑐 − 3.85 ∙ 𝑐 2 + 4.16 ∙ 𝑟 − 3.23 ∙ 𝑟 2 + 1.96 ∙ 𝑇 ∙ 𝑚 + 2.56 ∙ 𝑚 ∙ 𝑟 + 3.73 ∙ 𝑐 ∙ 𝑟 (4) Where T, m, c, and r are, respectively, the encoded values of the temperature, method,

current, and rotation parameters.

Table 4 – ANOVA table of the results of removal efficiency of the 24 central composite design. Degree of Freedom

Quadratic Mean

Regression

6553.836

11

595.8033

Residual

473.477

17

27.85159

Lack of fit

425.62

13

32.74

Pure Error

47.857

4

11.96425

Total

7027.313

28

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Quadratic Sum

By using the response surface method, optimization was performed by modifying the variables set in the analysis. The analysis pointed out that the central point provides high

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removal, and higher removal values are achieved by increasing the current and the ton total up

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to the level 1 (280 mA and ton = 1.5 ms). However, it is worth noting that these processes occur with decreasing deposition efficiency. In Figure 5 we present the response surface

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obtained with temperature and rotation parameters set in the central point. As shown in the response surface (Figure 5) and in Table 2, the maximum copper

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removal was 48.4%, reaching levels similar to those reported in the literature [1], however, using real copper wastewater instead of synthetic one. The maximum removal percentage

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using 30 minutes of reaction is also similar with results for Nickel removal using Bofe-type bentonite clay in a bed porous system and synthetic effluent [25]. Additional tests using the

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optimized conditions (ton = 1 ms, 190 mA, 70 rpm, and 37 °C) and processing time of 115 min enabled higher removals, reaching 82.49% of copper removal, in the same range of reported by Vieira et al. [26], for Nickel removal during 250min using Bofe bed porous system absorbent.

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Figure 5. Response surface illustrating the influence of variables method and electrical

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current on copper removal.

3.4. Characterization of the obtained deposits

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The results pointed out that the modification of the studied parameters (temperature, method, current, and rotation) allow to obtain different coatings, since metallic copper with

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high adherence up to copper oxide without adhesion. In addition to the copper deposits, for experiments no. 2, 10, 19, and 21, a corrosive process of copper substrate deterioration was

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observed (Table 2). Corrosion can be inferred due to the combined result of negative deposition and removal efficiency, thus indicating mass loss of the substrate with consequent

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increase in the concentration of copper in electrolytic bath from the eroded substrate. By analyzing the considered parameters, these experiments were performed with the highest temperatures (37 and 44 °C), low time of applied electrical current (ton = 90. 18, and 180 s), and low current (10, 100, and 190 mA). In addition, with the low hydrogen potential (1.74) of the wastewater it is possible to indicate that under these circumstances, the corrosion is the dominant phenomena throughout the experiment.

Figure 6 represents the different types of coating obtained during the experiment: corrosion, represented by Exp. 10; metallic copper with high purity, represented in Exp. 25; and copper oxides, as shows in Exp. 23. When evaluating the results of SEM/EDX and XPS, we perceive that the brownish-black color, represented by experiment no. 23, is associated with the formation of copper I oxides (Cu2O) and represents a decrease in the deposit adhesion. Exp. 23

Exp. 10

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Exp. 25

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Figure 6. Image of different types of the obtained metallic coatings.

3.4.1. Deposits composition

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Micrographs and composition were obtained for the experiments no. 8, 9, 10, 13, 15, 16, 23, and 25C for characterizing the different types of obtained deposits. The results are

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presented in Table 5. All micrographs indicated homogeneity in deposits and lack of cracks. The analysis of these results shows the purity of the optimum point (represented by

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experiment no. 25C), which featured 100% copper according to SEM/EDX results. These results indicate a correlation between the appearance and composition of

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coatings, thus indicating, due to the qualitative detection of oxygen with the energy-dispersive X-ray technique, that the dark coloration is associated with oxide formation. By analyzing the parameters used in the experiments, it is highlighting the influence and synergy between all factors in the formation of copper oxide instead of metallic copper, and the high current, low rotation, and high temperature are the main parameters that contribute to oxide production.

Table 5 – Results concerning deposition efficiency, removal percentages, and chemical composition of coatings of each experiment obtained by SEM/EDX.

8

T (°C) 44

ton total (s) 270

I (A) 0.280

ω (rpm) 35

ε (%) 11.25

Removal Composition (%) (%) 24.47 5.36O/ 94.64Cu

9

30

90.0

0.100

105

64.13

64.13

10

44

90.0

0.100

105

-67.49 -3.61

0.56O /99.1Cu

13

30

90.0

0.280

105

95.21

27.49

9.64Cu/ 3.36N

15

30

270

0.280

105

21.29

47.79

1.45 O/ 98.55Cu

16

44

270

0.280

105

50.62

48.41

0.65O/99.35Cu

23

37

180

0.190

0

23.98

10.64

6.16O/ 93.84Cu

25 (C) 37

180

0.190

70

84.36

33.25

100Cu

0.56O/ 96.06Cu/4.94N

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Exp.

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XPS results of the coatings obtained in experiments 23, 13 and 25C were presented in Figure 7. For experiment 23, Oxygen was detected in agreement with the SEM/EDX

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measurements. Comparing the Cu2p spectra obtained with reference spectra for Cu, CuO and Cu2O one observes the absence of the characteristic shake-up satellites for CuO [27, 28], thus

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the presence of copper oxide II can be ruled out. Combining the SEM/EDX visual results with the XPS it was possible to confirm the formation of Cu2O. XPS results also showed the

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presence of Nitrogen as copper cyanide in coating 13 and indicated a small amount of this compound on the surface of the coating mid-way through our series (Exp. 25C).

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The spectra from samples 13 seemed to indicate the existence of regions with two

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different types of charging [27] as indicated in the lower plots in Figure 7. This effect was also observed in the spectra for the other elements from sample. Thus, for the regions rich in CuCN it was necessary to correct for charging shifting the spectra 3.58eV resulting in a Cu peak at 933.2eV consistent with Cu in CuCN. For the other surface regions, the shift was much smaller at 1.11eV leading to a Cu peak at 932.2eV compatible with Cu0 or Cu2O.

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Figure 7- XPS spectra obtained for samples 23, 13 and 25C.

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Different theories are described for electrodeposition of copper by electrodeposition in cyanide-containing electrolytic bath [29]. According to Brenner [30], in solutions with high

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concentrations of cyanide, the formation of the tricyanocuprate anion (I) is favored because this is the most stable complex. Hence, considering the direct discharge deposition, the

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proposed mechanism is described in Equation 5. Other authors [31] indicated that in acidic

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solutions, with hydrogen potential similar to the one used in our study, the ion [Cu(CN)2]- was formed. However, the temperature and the ratio between the cyanide anions and copper cations changes the stability of different Cu(CN)x complexes, such as [Cu(CN)2]- and [Cu(CN)3]-2, hindering the stability of species present in each process [15]. − − [𝐶𝑢(𝐶𝑁)3 ]−2 𝑎𝑞 + 𝑒 → 𝐶𝑢𝑠 + 3 𝐶𝑁𝑎𝑞 (5)

In the literature it is also proposed a mechanism based on the formation of the tricyanocuprate anion (I) complex; however, unlike direct discharge, such mechanism involves adsorption of copper cyanide with consequent reduction of copper and cyanide release in the solution, as shown in Equations 6 and 7 [29]. According to the results of the coatings characterization obtained in our study and to this mechanism, the used parameters of electrodeposition are critical for the complete reduction of copper (closer to coating 25C characteristics) instead of the formation and maintenance of CuCN adsorbed in coating, as

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noted in experiments no. 9 and 13. − [𝐶𝑢(𝐶𝑁)3 ]−2 𝑎𝑞 → 𝐶𝑢𝐶𝑁𝑎𝑑 + 2 𝐶𝑁𝑎𝑞 (6)

(7)

-p

− 𝐶𝑢𝐶𝑁𝑎𝑑 + 1 𝑒 − → 𝐶𝑢𝑠 + 𝐶𝑁𝑎𝑞

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Besides the formation of CuCN, the production of oxides detected by SEM/EDX and

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XPS indicates the favoring of parallel reactions of incomplete copper reduction influenced by the parameters used in the process. Noting the presence of dissolved oxygen in the used electrolytic baths and since copper is the cathode of the process, thus receiving the current, we

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state that parallel reaction of oxygen reduction occurs with production of copper I oxide as presented in equation 8. Regarding the oxide, it is important to add that the deposits are not

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adherents; they can be removed easily and used in other applications. The copper I oxide are

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semiconductors type p with low toxicity and therefore have shown applicability in different areas such as photocatalysis, photo voltaic cells and gas sensors [11]. − − 2[𝐶𝑢(𝐶𝑁)3 ]−2 𝑎𝑞 + 2𝑒 + 𝑂2 → 𝐶𝑢2 𝑂 + 6 𝐶𝑁𝑎𝑞 (8)

3.4.2. Morphology

SEM-EDX results add information about corrosion observed in experiments no. 2, 10, 19 and 21. Together with the negative values of deposition efficiency and removal percentage, such results corroborate the corrosion of the substrate when the parameters of the experiment involve high temperature, low time of applied current, and low current. By analyzing the image of experiment no. 10 (performed at 44 °C temperature, applied current time of 90 s, and applied current of 280 mA) presented in Figure 8, the micrograph shows no deposit on the substrate, unlike the observed in other images, but rather a characteristic of

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metallic corrosion image as presented in the literature [32]. Regarding crystalline formations, in the micrographs of experiments no. 8 and 16,

featuring high temperature (44 °C), high current (280 mA), and pulse time (ton = 1.5 ms), it is

-p

verified the dendritic formation. A formation similar to that obtained by the combined

processes of electrodialysis and electrolysis [2]. These dendritic formations are preferred

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when the deposition of atoms at the ends of structures facilitates the transfer of mass, and they

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occur due to the fast electron transfer in the electrode, compared with the diffusion of copper ions in the electrolyte Pletcher and Walsh [33]. Therefore, since the increased current and time of application of the current, the concentration of copper ions in the electrolytic bath

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decrease and these formations are favored in such conditions. Furthermore, it is highlighting the importance of temperature in the reaction kinetics and, therefore, in this process of

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dendritic formation, illustrated by the fact that the deposit no. 15 was obtained with a similar

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current and pulse, but in lower temperature, and showed no such crystalline formations.

Exp. 23

Exp. 8

Exp. 16

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Exp. 10

Exp. 25C

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Exp. 15

Figure 8 – SEM micrographs of metallic coatings obtained with 5000x magnification concerning the experiments no. 10, 23, 8, 16, 15, and 25C.

Micrographs also pointed out the cyanide on the coating surface. Figure 9 shows the images for the coating obtained in experiment 13 and 9 where it is identified a denser grained deposits formation, similar with reported by Burcu and coworkers [34] for CuCN in aluminum substrate. For the central point (25C) it is also verified that this formation (highlighted with white circles in Figure 8) is over an initial layer of copper. These results emphasize and explain the XPS results, once it was observed a differential charging due to the different coating layers, and confirm copper cyanide formation and the adsorption step during

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the copper metallic production. Exp. 13

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Exp. 9

Figure 9 - SEM micrographs of metallic coatings obtained with 5000x magnification

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3.4.3. FTIR

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concerning the experiments no. 13 and 25C.

FTIR results (Figure 10) for the coatings 9, 13 and 25C also confirmed the

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presence of copper cyanide with a negative-going band in the 2000-2200 cm-1 range, characteristic of νC ≡ N bond stretch. Similar results were obtained by Souto and coworkers [35] for copper cyanide adsorbed in platinum, and as in this paper, small contribution of atmospheric CO2 from the gas phase present in the FTIR spectrometer was verified at 2350 cm-1.

Exp. 25C

Transmitance (%)

Exp. 13

4000

3500

3000

2500

2000

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Exp. 9

1500 -1

wavenumbers (cm )

1000

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Figure 10 – FTIR for the coatings obtained in the experiments 9, 13 and 25C.

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3.4.4. Crystallinity

The obtained diffractograms indicated that all deposits are crystalline and mostly consist of copper, as observed by the SEM-EDX technique. Diffractograms showed three high

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intensity peaks in 2θ = 44, 50, and 74° referring, respectively, to the crystallographic points (111), (200), and (220) of the face-centered cubic structure with the unit cell of the S.G. space

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group. Fm3m [35,7]. Figure 11 present the x-ray diffraction analysis carried out for the

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deposit obtained in optimum point and for the experiment no. 23, corroborating the presence of oxide in the copper coating composition obtained in such experiment. The difficulty in identification of species in low concentration is known in the literature

as a limitation of this technique, which requires an amount of microconstituents and precipitated phases being present in a concentration greater than 5% in volume in the microstructure in such a way that their peaks are not mistaken with the background. Hence,

since sample no. 23 features oxide concentration of approximately 6% (SEM-EDX data), we can observe a broad peak and close to the noise, about 2θ = 36.4 that is related to the crystallographic face (111) of copper I oxide (JCPDS 05-0667). The other coatings containing smaller oxide amounts were also analyzed by X-ray diffraction (XRD); however, the

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diffractogram was similar to that of pure copper due to the low oxide concentration.

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Figure 11 – Cu diffractograms obtained in experiment no. 23 and in the central point,

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with expansion in the region 2θ = 35-45.

4. Conclusion

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This paper presents the electrodeposition parameters optimization for copper removal from real wastewater in the electroplating industry. According to the results, the parameters method, current, temperature, and rotation influenced both the removal of Cu and the composition of the formed deposit, highlighting that the variation of these parameters enables the removal of copper with formation from oxides to copper in its metallic form, with different crystalline formations. The results indicate the effectiveness of the fast-pulsed

chronoamperometry technique and the importance of synergy between variables to maximize the deposition efficiency, and that the electrical current and the ton time are the main factors responsible for the increase in the removal percentage. Using statistical methods of backward elimination and ANOVA, we obtained models with 95% significance for modeling the experiments and removal prediction for future processes. The process was optimized, and a 33.59% copper removal from a real wastewater of jewelry industry was deemed optimal, with a deposition efficiency of 84.36% in 30 minutes. Optimization was achieved by using fast-

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pulsed chronoamperometry, ton = 1 ms, 190 mA, 70 rpm, and 37 °C. For coating in the optimum point, we obtained metallic and crystalline copper, with higher level of purity. In

115 minutes of processing using the optimized conditions, we achieved higher removal values

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of 82.49%, as well as using other studied parameters, although with formation of copper I

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oxide in small amount.

Acknowledgements

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This research was funded by FAPESP (Project no. 2017/24887-5) and CNPq (Process no. 140776/2016-8). The authors thank Espaço da Escrita – Pró Reitoria de Pesquisa –

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UNICAMP – for the language services provided.

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