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Removal of copper ions using sodium hexadecanoate by ionic flocculation
Separation and Purification Technology 200 (2018) 294–299
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Removal of copper ions using sodium hexadecanoate by ionic flocculation a,⁎
T
b
Giselle Kalline Gomes Carvalho Barros , Ricardo Paulo Fonseca Melo , Eduardo Lins de Barros Netoa a Universidade Federal do Rio Grande do Norte, Centro de Tecnologia – CT, Departamento de Engenharia Química – DEQ – PPGEQ, Campus Universitário, Av. Senador Salgado Filho 3000, Natal, RN 59072-970, Brazil b Universidade Federal Rural do Semi-Árido – UFERSA, Campus Pau dos Ferros, Rodovia BR-226, No Number, Pau dos Ferros, RN 59900-000, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords: Ionic flocculation Sodium hexadecanoate Metal removal Krafft temperature
The mining, electroplating, and electronics industries release increasing amounts of heavy metals, such as copper, into aquatic ecosystems. Industrial effluent often contains metals at concentrations above the maximum limits set by law. In this study, ionic flocculation using the anionic surfactant sodium hexadecanoate is shown to be an alternative method for the removal of copper from industrial effluent. The effects of temperature, metal solution pH, and surfactant concentration on copper removal were analyzed using experimental design techniques. A statistical experimental design of the study showed that the process is directly dependent on the variations in the pH and molar ratio between sodium hexadecanoate and copper ions (Cu2+) but inversely proportional (and less dependent) on the temperature. An individual study of the temperature effect shows that the process is strongly dependent on the Krafft temperature of both surfactants, i.e., that used as the extractive agent and that obtained after the reaction of the surfactant with the metal. Subsequently, the thermodynamic parameters were determined, which showed that the process is spontaneous and either endothermic or exothermic depending on the temperature range.
1. Introduction Wastewater containing heavy metals is often directly or indirectly discharged into the environment as a result of heavy industry, especially in developing countries [1]. Certain heavy metal ions are capable of accumulating in the tissues of living organisms and, even in small amounts, cause disease and disorder [2]. In addition, they may inhibit the operation of biological treatment plants and affect aquatic life, deteriorating their self-purification power [3]. Faced with these problems, there is a need to use treatment methods that reduce or eliminate heavy metals from industrial effluent and watercourses. A variety of metal removal methods have been studied, such as chemical precipitation [4], ion exchange [5], adsorption [6,7], and membrane filtration [8,9]. Ionic flocculation [10] is an alternative removal method. In this process, an anionic surfactant forms a new water-insoluble surfactant when in contact with bivalent and trivalent metal ions. Siska [11] described this as a process of cation exchange between a sodium carboxylate and a metal ion: 2RCOONa + Me2+ → (RCOO)2Me + 2Na+
(1)
This interaction depends on the alkyl chain length of the surfactant ⁎
Corresponding author. E-mail address: [email protected] (G.K.G. Carvalho Barros).
https://doi.org/10.1016/j.seppur.2018.01.062 Received 11 July 2017; Received in revised form 6 January 2018; Accepted 29 January 2018 1383-5866/ © 2018 Elsevier B.V. All rights reserved.
and its concentration. The anionic surfactant formed by the reaction is a metal carboxylate that is insoluble in water but may be soluble in various organic solvents. One of the primary advantages of this process is that the solubilities of the metal compounds formed with carboxylates are lower than those of the corresponding metal hydroxides. In addition, the carboxylates remain insoluble over a wide pH range while simultaneously precipitating a wide variety of different metals at a single pH [12]. Sodium carboxylates are easily prepared by the saponification of fatty acids or triglycerides and can be obtained from natural materials such as coconut oil, sunflower oil, and animal fat. In the chemical industry, these salts, especially those of lithium, sodium, and potassium, have great applicability as foaming agents for concrete, toilet soap bars, and for the degumming of silk [13]. Some studies have been carried out concerning the removal of metals using salts of carboxylic acids, such as sodium octanoate [14], sodium decanoate [15,16], and sodium dodecanoate [17]. These surfactants commonly show Krafft-type behavior [18]. At temperatures above the Krafft temperature, the surfactant solubility increases sharply with increasing temperature; at this point, the solubility is equal to the critical micelle concentration (CMC) [19]. This work presents an innovative approach to metal removal by
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identifying the Krafft point as a limiting factor for the ionic flocculation process. We used sodium hexadecanoate, a surfactant commonly found in natural materials, as the extracting agent. To understand the copper removal process, the thermodynamic parameters and a statistical design of the copper ion removal process were evaluated. In this statistical experimental design, the influence of three factors was considered: temperature, metal solution pH, and surfactant concentration.
When bivalent metals are in solution and come in contact with the anions of fatty acid soaps, they interact with these anions forming a new surfactant composed of a metal cation and two anions of the original carboxylic acid. Because of the steric hindrance between the carboxyl groups, hydrophobic parts, of this new molecule, turn to opposite sides, while the metal remains in the center. This configuration of the new surfactant gives it a hydrophobic character, which favors the formation of colloids (Fig. 3). The agitation of the system promotes aggregation of the precipitate, forming flocs which can be separated from the system by a filtration process. It is difficult to check the size of these flocs because of their growth kinetics. The ionic flocculation process is based on the attraction between the copper ion and the negative part of the surfactant molecule, forming a salt of low solubility, which results in an effluent with low metal concentration, making its study important as an alternative to existing processes. These flocs were separated by vacuum filtration using quantitative filter paper with a particle retention of 1–2 μm. The obtained filtrate was analyzed using an atomic absorption spectrophotometer (Spectra A-20 plus, Varian). pH control, where necessary, was achieved by adding 0.1 M HCl to the metal solution before the addition of the surfactant solution.
2. Materials and methods 2.1. Materials The synthetic effluent containing the copper ion was prepared using copper sulfate (VETEC). Copper sulfate, sodium hydroxide (NEON), hydrochloric acid (MERCK), and hexadecanoic acid (IMPEX) were analytical grade and used as received without purification. Sodium hexadecanoate was prepared by neutralizing hexadecanoic acid with sodium hydroxide. 2.2. Methods 2.2.1. Ionic flocculation tests Removal tests were performed in a thermostated bath (Koehler Instrument Company, Inc., USA), showed in Fig. 1, where it was possible to control the temperature and the stirring speed of the samples throughout the experiment. The contact of the metal solution and surfactant solution in the equipment was divided into three stages: rapid mixing (stirring speed of 216 rpm for 2 min), slow mixing (stirring speed of 50 rpm for 10 min), and resting period (without stirring for 12 min) [20–22]. To obtain each experimental data point, a copper solution was placed into the tubes of the equipment, reaching a metal concentration of 124.7 ppm, and then subjected to temperature control in the thermostatic bath. Separately, the sodium hexadecanoate solution was heated on a heating plate to the same temperature as the copper solution. After the solutions were at the same temperature they were mixed, allowing the reaction between the surfactant and the metal (Fig. 2).
2.2.2. Temperature evaluation and high-pressure tests One of the factors evaluated in the process was the temperature, because it directly affects the equilibrium. The temperature range evaluated was between 30 and 109.2 °C. To reach temperatures above 100 °C without boiling the water, a high-pressure cell was used. This apparatus has been described in detail in previous papers [23–26]. The surfactant/metal mixture was placed inside the cell, and the pressure was raised gradually to prevent the water from boiling. The temperature was gradually increased to the desired value while the system was stirred constantly. 2.2.3. Experimental design A 23-factorial design with replicas at the central points was used to investigate the influence of three factors on the copper removal process. The selected factors were: the molar ratio between sodium hexadecanoate and Cu2+ ions (P), initial pH of the copper solution (pH), and temperature (T). The response was the percentage of copper removed (%R). Table 1 shows the levels of each factor, used in the design matrix. The minimum level (−1) for the temperature parameter was set at a value above the Krafft temperature of sodium hexadecanoate, 57.5 °C [27]. Therefore, to ensure that the surfactant was fully dissolved, a temperature of 60 °C was used. The maximum level (+1) was set to 80 °C because of the operational limitations of the thermostatic bath used. For the pH parameter, the maximum value was set at the natural pH of the metal salt solution at room temperature, which is 4.5. We did not use values higher than 4.5 because the addition of NaOH to the copper solution promoted the precipitation of metal hydroxides at pH values greater than 6. The minimum pH was set at 2.5 because this approximates the values obtained from copper leaching experiments [28]. The maximum metal/surfactant ratio was defined based on the stoichiometric concentration of the surfactant, taking into account the hypothetical complete reaction of the copper ions. The experiments were performed in random order and were conducted in duplicate. The effect of each factor was analyzed through the generation of response surfaces by the STATISTICA 7.0 program. 2.2.4. Determination of the solubility of the copper surfactant flocs as a function of temperature A more detailed study of the effect of temperature on the solubilization of the copper surfactant flocs was carried out between 30 and 109.2 °C (the value defined as the Krafft point of the copper hexadecanoate) to analyze the temperature influence on the metal removal process. The concentration of surfactant and pH of the metal solution
Fig. 1. Thermostated bath used in the removal tests.
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Fig. 2. Reaction between Sodium hexadecanoate and a bivalent metal (Me2+).
Fig. 3. Floc formation.
Table 1 Levels for each parameter evaluated in the 23-factorial design. Factor
ln K =
(8)
ΔH 0 and
ΔS 0 can
be determined from the linear Here, the values of and angular coefficients, respectively, of the line formed by plotting K × 1/T.
Level
T (°C) P pH
ΔS 0 ΔH 0 − . R RT
−1
0
+1
60 0.350 2.5
70 1.175 3.5
80 2.000 4.5
3. Results and discussion 3.1. Response surfaces analysis
used in the tests were 590 ppm and 4.5, respectively. At this point, a good floc formation was observed, facilitating the analysis of the process.
The design matrix, along with the percentage of copper removed, which was obtained experimentally for each combination of levels, is shown in Table 2. From a preliminary analysis of the metal removal results, the point of maximum copper removal (95.7%) is at the maximum pH, maximum molar ratio (+1), and minimum temperature level (−1). The response surfaces for the copper removal process were constructed from the data presented in Table 2, always setting one factor as constant. For the analysis of the simultaneous influence of temperature and surfactant/metal ratio, the pH was set at its maximum level (+1) (Fig. 4a). For the analysis of the simultaneous influence of temperature and pH, the surfactant/metal ratio was fixed at its maximum level (+1) (Fig. 4b). Finally, for the analysis of the simultaneous influence of the surfactant/metal ratio and the pH, the temperature was fixed at its minimum level (−1) (Fig. 4c). Analysis of the response surface shown in Fig. 4a shows that the effect of temperature, compared to the effect caused by the variation of the surfactant/copper ratio, is negligible. Therefore, for a pH value of 4.5 and an initial copper ion concentration of 124.7 ppm, the percentage of metal removed will increase with increasing surfactant concentration, regardless of the temperature. The linearity of the isoresponse curves with respect to temperature corroborates this observation. As shown in Fig. 4b, as the pH increases, more metal is removed. The acidic conditions are not favorable to metal removal because most functional groups present in the molecule of the surfactant are protonated, leaving few available ionized groups. Thus, the competition
2.2.5. Thermodynamic analysis The Gibbs free energy (ΔG0) is the criterion determining the spontaneity of a chemical reaction. It is defined in Eq. (2) [29].
ΔG 0 = −RT ln K
(2)
Here, R is the universal gas constant (8.314 J/(mol K)), and K is the equilibrium constant of the reaction. A negative value of ΔG0 indicates that the process is spontaneous. Eq. (1) describes the reaction between the surfactant and a bivalent metal, showing the formation of the metal salt of the surfactant. The constant, K, for this reaction is given by Eq. (3).
k=
1 2 [RCOO−]2 [Me 2 +] fRCOONa fMe2 +
(3)
Here, [Me 2 +] is the concentration of free metal in solution, and [RCOO-] is the concentration of the free surfactant in the aqueous phase. The parameters fRCOONa and fMe2 +are the activity coefficients of the surfactant and the metal, respectively. The activity coefficients can be found using the Debye–Hückel expression [30].
log fRCOONa =
log fMe2 + =
−AI 0.5 1 + BI 0.5
−4AI 0.5 1 + BI 0.5
(4)
(5)
Table 2 Design matrix.
Here, I is the ionic strength. The parameters A and B in Eqs. (4) and (5) were reported by Klotz and Rosenberg (2008) [30] and were estimated for the temperatures used in this work. Eq. (6) shows the expression used to calculate the ionic strength [31].
I = [RCOONa] + 4 [CuSO4]
(6)
Here, [RCOONa] and [CuSO4] are the concentrations of each species in solution. The relationship between K , the enthalpy (ΔH 0 ), and the entropy (ΔS 0 ) can be expressed by the following equations.
ΔG 0 = ΔH 0−T ΔS 0
(7)
Substituting Eq. (2) into Eq. (7), 296
pH
P
T (°C)
R (%)
1 1 −1 −1 0 0 1 1 −1 −1
−1 1 −1 1 0 0 −1 1 −1 1
−1 −1 −1 −1 0 0 1 1 1 1
23.40 95.70 5.92 51.00 63.00 60.30 24.30 91.60 2.31 36.50
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Fig. 4. Response surfaces for copper removal. (a) Analysis of the temperature and surfactant/copper variables at pH = 4.5; (b) Analysis of the temperature and pH variables, P = 2; (c) Analysis of the surfactant/copper ratio and pH variables, T = 60 °C.
between protons and metal species explains the lower removal efficiency in acidic media [32]. In addition, the response surface of Fig. 4b slopes slightly in the direction of temperature, showing that, once again, the process is affected very little by temperature, but the removal of the metal decreases slightly when the temperature increases. The optimum operating point lies in the coded value of around 0.8, which corresponds to a pH close to 4.0 and a temperature between 60 and 75 °C. The response surface in Fig. 4c shows that the pH and the molar ratio have significant effects on the response, where the effect of the molar ratio is considerable. The optimum operating conditions at a temperature of 60 °C is at pH values greater than 4.0 and a surfactant/ copper ratio at its maximum point, which corresponds to 2.0. 3.2. The copper surfactant floc solubility as a function of temperature On studying the effect of temperature on the solubility of the floc (Fig. 5), it was observed that, until the temperature reaches 45 °C, the removal percentage is, at most, 5%, as shown in Fig. 6. This is because, in this temperature range, the surfactant is practically insoluble in water. As the temperature increases and approaches the Krafft point of sodium hexadecanoate (57.5 °C), the solubility of the surfactant increases and, consequently, the percentage of removed copper increases. The maximum removal percentage occurs between 60 and 70 °C. After 70 °C, the removal percentage decays. The determination of the Krafft point of the surfactant formed from the reaction of sodium hexadecanoate and copper ions was carried out in a high-pressure cell. The Krafft point (the temperature at which the surfactant was totally dissolved in water) was determined visually. For copper hexadecanoate, this point was 109.2 °C. The copper removal decreases because a new copper surfactant is formed, copper hexadecanoate, which, from 70 °C, begins to be solubilized gradually in the aqueous solution. At 109.2 °C, the copper hexadecanoate is completely dissolved. To test this observation, the solubility curve of copper hexadecanoate as a function of temperature was determined (Fig. 7).
Fig. 5. Appearance of the floc formed during the extraction process.
As shown in the solubility curve in Fig. 7, higher temperatures favor the dissolution of copper hexadecanoate; in other words, from 72.5 °C, part of the surfactant formed by the reaction between sodium hexadecanoate and copper ions starts to dissolve in the aqueous medium. This is corroborated by the variation of the Krafft point of the fatty acid salt surfactants [33].
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% Removal Efficiency
100
3.3. Thermodynamic parameter analysis
90 80
The thermodynamic parameters, ΔG°, ΔH°, and ΔS°, for the copper removal process were obtained in the temperature ranges of 323.15–333.15, 335.65–343.15, and 345.65–353.15 K using Eqs. (2) and (8). The initial copper concentration was set at 124.7 ppm and the surfactant concentration at 590 ppm. Table 3 presents the residual copper concentrations in solution within different temperature ranges. The concentration of copper ions interacting with the surfactant was obtained based on the stoichiometric ratio of the reaction between sodium hexadecanoate and copper ions based on the reaction model described by Siska [17]. The values of the thermodynamic constants for the three temperature ranges are shown in Table 4. The values for the Gibbs free energy were determined from Eq. (2), while the values of ΔH° and ΔS° were obtained from the resultant equation derived from the ln(K) vs. 1/T plot. The negative values of ΔG° show that the copper removal process occurs spontaneously in all studied temperature ranges. From the results presented in Table 4, it is clear that copper removal using sodium hexadecanoate is a spontaneous process. For the temperature ranges of 323.15–333.15 and 333.15–343.15 K, copper removal is an endothermic process, and, as seen in Fig. 6, higher temperatures favor metal extraction. One possible explanation for this endothermicity is that, as the copper ions move to a solid phase, they lose their hydration layer. This dehydration process consumes energy that exceeds the exothermicity of the extraction process [29]. Analyzing the enthalpy from 343.65 to 353.15 K, we concluded that the process is exothermic. The negative value of the enthalpy accounts for the previously observed reduction in the removal efficiency of the metals with increasing temperature. Because it is an exothermic process, the addition of energy to the system is unfavorable to the formation of the products. At all temperatures, the entropy is positive, reflecting the ionic interaction of the Cu2+ cations with the surfactant anions [34,35].
70 60 50 40 30 20 10 0
30
35
40
45
50
55
60
65
70
75
80
85
90
95 100 105 110
Temperature (°C) Fig. 6. Percentage of copper removed as a function of temperature.
Copper surfactant solubility (ppm)
250 200 150 100 50 0 72,5
75
77,5
80
82,5
85
87,5
90
92,5
Temperature (°C) Fig. 7. Solubility curve of copper hexadecanoate.
Table 3 Residual concentration of copper in solution ([Cu]res) as a function of temperature. Temperature (K)
[Cu]res (ppm)
323.15 325.65 328.15 330.65 333.15 335.65 338.15 340.65 343.15 345.65 348.15 350.65 353.15
18.39 31.25 39.33 52.04 68.93 72.96 72.11 72.36 71.19 63.55 62.63 58.35 56.61
4. Conclusions Sodium hexadecanoate efficiently removes copper from aqueous solution (by around 95%) when the molar ratio of surfactant/metal is 2, the pH value is 4.6, and the temperature is 60 °C. Analysis of the effects of temperature, pH, and surfactant/copper ratio on the copper removal showed that the pH and the surfactant/copper ratio affect the metal removal significantly while the temperature has a less significant effect. However, the temperature has an unfavorable effect at high temperatures. The study at high temperatures showed that the ionic flocculation is governed by the Krafft temperature of the surfactant. We found that, from about 75 °C, the copper hexadecanoate flocs begin to solubilize, and total solubilization occurs at approximately 109 °C, where copper removal was no longer observed. In the determination of the thermodynamic parameters the results showed that the process studied is endothermic for the temperature range of 323.15 K to 343.15 K, exothermic for the temperature range of 343.65 K to 353.15 K, and spontaneous for all the temperature ranges studied.
Table 4 Thermodynamic parameters for temperature ranges from 323.15 to 333.15, 333.15 to 343.15, and 343.15 to 353.15 K. Temperature (K)
ΔG0 (kJ/mol)
ΔH° (kJ/mol)
ΔS° (kJ/(mol K))
323.15 325.65 328.15 330.65 333.15 335.65 338.15 340.65 343.15 345.65 348.15 350.65 353.15
−35.61 −36.26 −36.82 −37.58 −38.64 −39.18 −39.47 −39.82 −40.10 −40.06 −40.35 −40.50 −40.77
59.51
0.29
8.96
0.14
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Removal of copper ions using sodium hexadecanoate by ionic flocculation a,⁎
T
b
Giselle Kalline Gomes Carvalho Barros , Ricardo Paulo Fonseca Melo , Eduardo Lins de Barros Netoa a Universidade Federal do Rio Grande do Norte, Centro de Tecnologia – CT, Departamento de Engenharia Química – DEQ – PPGEQ, Campus Universitário, Av. Senador Salgado Filho 3000, Natal, RN 59072-970, Brazil b Universidade Federal Rural do Semi-Árido – UFERSA, Campus Pau dos Ferros, Rodovia BR-226, No Number, Pau dos Ferros, RN 59900-000, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords: Ionic flocculation Sodium hexadecanoate Metal removal Krafft temperature
The mining, electroplating, and electronics industries release increasing amounts of heavy metals, such as copper, into aquatic ecosystems. Industrial effluent often contains metals at concentrations above the maximum limits set by law. In this study, ionic flocculation using the anionic surfactant sodium hexadecanoate is shown to be an alternative method for the removal of copper from industrial effluent. The effects of temperature, metal solution pH, and surfactant concentration on copper removal were analyzed using experimental design techniques. A statistical experimental design of the study showed that the process is directly dependent on the variations in the pH and molar ratio between sodium hexadecanoate and copper ions (Cu2+) but inversely proportional (and less dependent) on the temperature. An individual study of the temperature effect shows that the process is strongly dependent on the Krafft temperature of both surfactants, i.e., that used as the extractive agent and that obtained after the reaction of the surfactant with the metal. Subsequently, the thermodynamic parameters were determined, which showed that the process is spontaneous and either endothermic or exothermic depending on the temperature range.
1. Introduction Wastewater containing heavy metals is often directly or indirectly discharged into the environment as a result of heavy industry, especially in developing countries [1]. Certain heavy metal ions are capable of accumulating in the tissues of living organisms and, even in small amounts, cause disease and disorder [2]. In addition, they may inhibit the operation of biological treatment plants and affect aquatic life, deteriorating their self-purification power [3]. Faced with these problems, there is a need to use treatment methods that reduce or eliminate heavy metals from industrial effluent and watercourses. A variety of metal removal methods have been studied, such as chemical precipitation [4], ion exchange [5], adsorption [6,7], and membrane filtration [8,9]. Ionic flocculation [10] is an alternative removal method. In this process, an anionic surfactant forms a new water-insoluble surfactant when in contact with bivalent and trivalent metal ions. Siska [11] described this as a process of cation exchange between a sodium carboxylate and a metal ion: 2RCOONa + Me2+ → (RCOO)2Me + 2Na+
(1)
This interaction depends on the alkyl chain length of the surfactant ⁎
Corresponding author. E-mail address: [email protected] (G.K.G. Carvalho Barros).
https://doi.org/10.1016/j.seppur.2018.01.062 Received 11 July 2017; Received in revised form 6 January 2018; Accepted 29 January 2018 1383-5866/ © 2018 Elsevier B.V. All rights reserved.
and its concentration. The anionic surfactant formed by the reaction is a metal carboxylate that is insoluble in water but may be soluble in various organic solvents. One of the primary advantages of this process is that the solubilities of the metal compounds formed with carboxylates are lower than those of the corresponding metal hydroxides. In addition, the carboxylates remain insoluble over a wide pH range while simultaneously precipitating a wide variety of different metals at a single pH [12]. Sodium carboxylates are easily prepared by the saponification of fatty acids or triglycerides and can be obtained from natural materials such as coconut oil, sunflower oil, and animal fat. In the chemical industry, these salts, especially those of lithium, sodium, and potassium, have great applicability as foaming agents for concrete, toilet soap bars, and for the degumming of silk [13]. Some studies have been carried out concerning the removal of metals using salts of carboxylic acids, such as sodium octanoate [14], sodium decanoate [15,16], and sodium dodecanoate [17]. These surfactants commonly show Krafft-type behavior [18]. At temperatures above the Krafft temperature, the surfactant solubility increases sharply with increasing temperature; at this point, the solubility is equal to the critical micelle concentration (CMC) [19]. This work presents an innovative approach to metal removal by
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identifying the Krafft point as a limiting factor for the ionic flocculation process. We used sodium hexadecanoate, a surfactant commonly found in natural materials, as the extracting agent. To understand the copper removal process, the thermodynamic parameters and a statistical design of the copper ion removal process were evaluated. In this statistical experimental design, the influence of three factors was considered: temperature, metal solution pH, and surfactant concentration.
When bivalent metals are in solution and come in contact with the anions of fatty acid soaps, they interact with these anions forming a new surfactant composed of a metal cation and two anions of the original carboxylic acid. Because of the steric hindrance between the carboxyl groups, hydrophobic parts, of this new molecule, turn to opposite sides, while the metal remains in the center. This configuration of the new surfactant gives it a hydrophobic character, which favors the formation of colloids (Fig. 3). The agitation of the system promotes aggregation of the precipitate, forming flocs which can be separated from the system by a filtration process. It is difficult to check the size of these flocs because of their growth kinetics. The ionic flocculation process is based on the attraction between the copper ion and the negative part of the surfactant molecule, forming a salt of low solubility, which results in an effluent with low metal concentration, making its study important as an alternative to existing processes. These flocs were separated by vacuum filtration using quantitative filter paper with a particle retention of 1–2 μm. The obtained filtrate was analyzed using an atomic absorption spectrophotometer (Spectra A-20 plus, Varian). pH control, where necessary, was achieved by adding 0.1 M HCl to the metal solution before the addition of the surfactant solution.
2. Materials and methods 2.1. Materials The synthetic effluent containing the copper ion was prepared using copper sulfate (VETEC). Copper sulfate, sodium hydroxide (NEON), hydrochloric acid (MERCK), and hexadecanoic acid (IMPEX) were analytical grade and used as received without purification. Sodium hexadecanoate was prepared by neutralizing hexadecanoic acid with sodium hydroxide. 2.2. Methods 2.2.1. Ionic flocculation tests Removal tests were performed in a thermostated bath (Koehler Instrument Company, Inc., USA), showed in Fig. 1, where it was possible to control the temperature and the stirring speed of the samples throughout the experiment. The contact of the metal solution and surfactant solution in the equipment was divided into three stages: rapid mixing (stirring speed of 216 rpm for 2 min), slow mixing (stirring speed of 50 rpm for 10 min), and resting period (without stirring for 12 min) [20–22]. To obtain each experimental data point, a copper solution was placed into the tubes of the equipment, reaching a metal concentration of 124.7 ppm, and then subjected to temperature control in the thermostatic bath. Separately, the sodium hexadecanoate solution was heated on a heating plate to the same temperature as the copper solution. After the solutions were at the same temperature they were mixed, allowing the reaction between the surfactant and the metal (Fig. 2).
2.2.2. Temperature evaluation and high-pressure tests One of the factors evaluated in the process was the temperature, because it directly affects the equilibrium. The temperature range evaluated was between 30 and 109.2 °C. To reach temperatures above 100 °C without boiling the water, a high-pressure cell was used. This apparatus has been described in detail in previous papers [23–26]. The surfactant/metal mixture was placed inside the cell, and the pressure was raised gradually to prevent the water from boiling. The temperature was gradually increased to the desired value while the system was stirred constantly. 2.2.3. Experimental design A 23-factorial design with replicas at the central points was used to investigate the influence of three factors on the copper removal process. The selected factors were: the molar ratio between sodium hexadecanoate and Cu2+ ions (P), initial pH of the copper solution (pH), and temperature (T). The response was the percentage of copper removed (%R). Table 1 shows the levels of each factor, used in the design matrix. The minimum level (−1) for the temperature parameter was set at a value above the Krafft temperature of sodium hexadecanoate, 57.5 °C [27]. Therefore, to ensure that the surfactant was fully dissolved, a temperature of 60 °C was used. The maximum level (+1) was set to 80 °C because of the operational limitations of the thermostatic bath used. For the pH parameter, the maximum value was set at the natural pH of the metal salt solution at room temperature, which is 4.5. We did not use values higher than 4.5 because the addition of NaOH to the copper solution promoted the precipitation of metal hydroxides at pH values greater than 6. The minimum pH was set at 2.5 because this approximates the values obtained from copper leaching experiments [28]. The maximum metal/surfactant ratio was defined based on the stoichiometric concentration of the surfactant, taking into account the hypothetical complete reaction of the copper ions. The experiments were performed in random order and were conducted in duplicate. The effect of each factor was analyzed through the generation of response surfaces by the STATISTICA 7.0 program. 2.2.4. Determination of the solubility of the copper surfactant flocs as a function of temperature A more detailed study of the effect of temperature on the solubilization of the copper surfactant flocs was carried out between 30 and 109.2 °C (the value defined as the Krafft point of the copper hexadecanoate) to analyze the temperature influence on the metal removal process. The concentration of surfactant and pH of the metal solution
Fig. 1. Thermostated bath used in the removal tests.
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Fig. 2. Reaction between Sodium hexadecanoate and a bivalent metal (Me2+).
Fig. 3. Floc formation.
Table 1 Levels for each parameter evaluated in the 23-factorial design. Factor
ln K =
(8)
ΔH 0 and
ΔS 0 can
be determined from the linear Here, the values of and angular coefficients, respectively, of the line formed by plotting K × 1/T.
Level
T (°C) P pH
ΔS 0 ΔH 0 − . R RT
−1
0
+1
60 0.350 2.5
70 1.175 3.5
80 2.000 4.5
3. Results and discussion 3.1. Response surfaces analysis
used in the tests were 590 ppm and 4.5, respectively. At this point, a good floc formation was observed, facilitating the analysis of the process.
The design matrix, along with the percentage of copper removed, which was obtained experimentally for each combination of levels, is shown in Table 2. From a preliminary analysis of the metal removal results, the point of maximum copper removal (95.7%) is at the maximum pH, maximum molar ratio (+1), and minimum temperature level (−1). The response surfaces for the copper removal process were constructed from the data presented in Table 2, always setting one factor as constant. For the analysis of the simultaneous influence of temperature and surfactant/metal ratio, the pH was set at its maximum level (+1) (Fig. 4a). For the analysis of the simultaneous influence of temperature and pH, the surfactant/metal ratio was fixed at its maximum level (+1) (Fig. 4b). Finally, for the analysis of the simultaneous influence of the surfactant/metal ratio and the pH, the temperature was fixed at its minimum level (−1) (Fig. 4c). Analysis of the response surface shown in Fig. 4a shows that the effect of temperature, compared to the effect caused by the variation of the surfactant/copper ratio, is negligible. Therefore, for a pH value of 4.5 and an initial copper ion concentration of 124.7 ppm, the percentage of metal removed will increase with increasing surfactant concentration, regardless of the temperature. The linearity of the isoresponse curves with respect to temperature corroborates this observation. As shown in Fig. 4b, as the pH increases, more metal is removed. The acidic conditions are not favorable to metal removal because most functional groups present in the molecule of the surfactant are protonated, leaving few available ionized groups. Thus, the competition
2.2.5. Thermodynamic analysis The Gibbs free energy (ΔG0) is the criterion determining the spontaneity of a chemical reaction. It is defined in Eq. (2) [29].
ΔG 0 = −RT ln K
(2)
Here, R is the universal gas constant (8.314 J/(mol K)), and K is the equilibrium constant of the reaction. A negative value of ΔG0 indicates that the process is spontaneous. Eq. (1) describes the reaction between the surfactant and a bivalent metal, showing the formation of the metal salt of the surfactant. The constant, K, for this reaction is given by Eq. (3).
k=
1 2 [RCOO−]2 [Me 2 +] fRCOONa fMe2 +
(3)
Here, [Me 2 +] is the concentration of free metal in solution, and [RCOO-] is the concentration of the free surfactant in the aqueous phase. The parameters fRCOONa and fMe2 +are the activity coefficients of the surfactant and the metal, respectively. The activity coefficients can be found using the Debye–Hückel expression [30].
log fRCOONa =
log fMe2 + =
−AI 0.5 1 + BI 0.5
−4AI 0.5 1 + BI 0.5
(4)
(5)
Table 2 Design matrix.
Here, I is the ionic strength. The parameters A and B in Eqs. (4) and (5) were reported by Klotz and Rosenberg (2008) [30] and were estimated for the temperatures used in this work. Eq. (6) shows the expression used to calculate the ionic strength [31].
I = [RCOONa] + 4 [CuSO4]
(6)
Here, [RCOONa] and [CuSO4] are the concentrations of each species in solution. The relationship between K , the enthalpy (ΔH 0 ), and the entropy (ΔS 0 ) can be expressed by the following equations.
ΔG 0 = ΔH 0−T ΔS 0
(7)
Substituting Eq. (2) into Eq. (7), 296
pH
P
T (°C)
R (%)
1 1 −1 −1 0 0 1 1 −1 −1
−1 1 −1 1 0 0 −1 1 −1 1
−1 −1 −1 −1 0 0 1 1 1 1
23.40 95.70 5.92 51.00 63.00 60.30 24.30 91.60 2.31 36.50
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Fig. 4. Response surfaces for copper removal. (a) Analysis of the temperature and surfactant/copper variables at pH = 4.5; (b) Analysis of the temperature and pH variables, P = 2; (c) Analysis of the surfactant/copper ratio and pH variables, T = 60 °C.
between protons and metal species explains the lower removal efficiency in acidic media [32]. In addition, the response surface of Fig. 4b slopes slightly in the direction of temperature, showing that, once again, the process is affected very little by temperature, but the removal of the metal decreases slightly when the temperature increases. The optimum operating point lies in the coded value of around 0.8, which corresponds to a pH close to 4.0 and a temperature between 60 and 75 °C. The response surface in Fig. 4c shows that the pH and the molar ratio have significant effects on the response, where the effect of the molar ratio is considerable. The optimum operating conditions at a temperature of 60 °C is at pH values greater than 4.0 and a surfactant/ copper ratio at its maximum point, which corresponds to 2.0. 3.2. The copper surfactant floc solubility as a function of temperature On studying the effect of temperature on the solubility of the floc (Fig. 5), it was observed that, until the temperature reaches 45 °C, the removal percentage is, at most, 5%, as shown in Fig. 6. This is because, in this temperature range, the surfactant is practically insoluble in water. As the temperature increases and approaches the Krafft point of sodium hexadecanoate (57.5 °C), the solubility of the surfactant increases and, consequently, the percentage of removed copper increases. The maximum removal percentage occurs between 60 and 70 °C. After 70 °C, the removal percentage decays. The determination of the Krafft point of the surfactant formed from the reaction of sodium hexadecanoate and copper ions was carried out in a high-pressure cell. The Krafft point (the temperature at which the surfactant was totally dissolved in water) was determined visually. For copper hexadecanoate, this point was 109.2 °C. The copper removal decreases because a new copper surfactant is formed, copper hexadecanoate, which, from 70 °C, begins to be solubilized gradually in the aqueous solution. At 109.2 °C, the copper hexadecanoate is completely dissolved. To test this observation, the solubility curve of copper hexadecanoate as a function of temperature was determined (Fig. 7).
Fig. 5. Appearance of the floc formed during the extraction process.
As shown in the solubility curve in Fig. 7, higher temperatures favor the dissolution of copper hexadecanoate; in other words, from 72.5 °C, part of the surfactant formed by the reaction between sodium hexadecanoate and copper ions starts to dissolve in the aqueous medium. This is corroborated by the variation of the Krafft point of the fatty acid salt surfactants [33].
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% Removal Efficiency
100
3.3. Thermodynamic parameter analysis
90 80
The thermodynamic parameters, ΔG°, ΔH°, and ΔS°, for the copper removal process were obtained in the temperature ranges of 323.15–333.15, 335.65–343.15, and 345.65–353.15 K using Eqs. (2) and (8). The initial copper concentration was set at 124.7 ppm and the surfactant concentration at 590 ppm. Table 3 presents the residual copper concentrations in solution within different temperature ranges. The concentration of copper ions interacting with the surfactant was obtained based on the stoichiometric ratio of the reaction between sodium hexadecanoate and copper ions based on the reaction model described by Siska [17]. The values of the thermodynamic constants for the three temperature ranges are shown in Table 4. The values for the Gibbs free energy were determined from Eq. (2), while the values of ΔH° and ΔS° were obtained from the resultant equation derived from the ln(K) vs. 1/T plot. The negative values of ΔG° show that the copper removal process occurs spontaneously in all studied temperature ranges. From the results presented in Table 4, it is clear that copper removal using sodium hexadecanoate is a spontaneous process. For the temperature ranges of 323.15–333.15 and 333.15–343.15 K, copper removal is an endothermic process, and, as seen in Fig. 6, higher temperatures favor metal extraction. One possible explanation for this endothermicity is that, as the copper ions move to a solid phase, they lose their hydration layer. This dehydration process consumes energy that exceeds the exothermicity of the extraction process [29]. Analyzing the enthalpy from 343.65 to 353.15 K, we concluded that the process is exothermic. The negative value of the enthalpy accounts for the previously observed reduction in the removal efficiency of the metals with increasing temperature. Because it is an exothermic process, the addition of energy to the system is unfavorable to the formation of the products. At all temperatures, the entropy is positive, reflecting the ionic interaction of the Cu2+ cations with the surfactant anions [34,35].
70 60 50 40 30 20 10 0
30
35
40
45
50
55
60
65
70
75
80
85
90
95 100 105 110
Temperature (°C) Fig. 6. Percentage of copper removed as a function of temperature.
Copper surfactant solubility (ppm)
250 200 150 100 50 0 72,5
75
77,5
80
82,5
85
87,5
90
92,5
Temperature (°C) Fig. 7. Solubility curve of copper hexadecanoate.
Table 3 Residual concentration of copper in solution ([Cu]res) as a function of temperature. Temperature (K)
[Cu]res (ppm)
323.15 325.65 328.15 330.65 333.15 335.65 338.15 340.65 343.15 345.65 348.15 350.65 353.15
18.39 31.25 39.33 52.04 68.93 72.96 72.11 72.36 71.19 63.55 62.63 58.35 56.61
4. Conclusions Sodium hexadecanoate efficiently removes copper from aqueous solution (by around 95%) when the molar ratio of surfactant/metal is 2, the pH value is 4.6, and the temperature is 60 °C. Analysis of the effects of temperature, pH, and surfactant/copper ratio on the copper removal showed that the pH and the surfactant/copper ratio affect the metal removal significantly while the temperature has a less significant effect. However, the temperature has an unfavorable effect at high temperatures. The study at high temperatures showed that the ionic flocculation is governed by the Krafft temperature of the surfactant. We found that, from about 75 °C, the copper hexadecanoate flocs begin to solubilize, and total solubilization occurs at approximately 109 °C, where copper removal was no longer observed. In the determination of the thermodynamic parameters the results showed that the process studied is endothermic for the temperature range of 323.15 K to 343.15 K, exothermic for the temperature range of 343.65 K to 353.15 K, and spontaneous for all the temperature ranges studied.
Table 4 Thermodynamic parameters for temperature ranges from 323.15 to 333.15, 333.15 to 343.15, and 343.15 to 353.15 K. Temperature (K)
ΔG0 (kJ/mol)
ΔH° (kJ/mol)
ΔS° (kJ/(mol K))
323.15 325.65 328.15 330.65 333.15 335.65 338.15 340.65 343.15 345.65 348.15 350.65 353.15
−35.61 −36.26 −36.82 −37.58 −38.64 −39.18 −39.47 −39.82 −40.10 −40.06 −40.35 −40.50 −40.77
59.51
0.29
8.96
0.14
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