# Adsorptive Removal of Copper by Using Surfactant Modified Laterite Soil

Authors: Tien Duc Pham; Hoang Hiep Nguyen; Ngoc Viet Nguyen; Thanh Tu Vu; Thi Ngoc Mai Pham; Thi Hai Yen Doan; Manh Ha Nguyen; Thi Mai Viet Ngo
Journal: Journal of Chemistry
 (2017)

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## Abstract

Removal of copper ion (Cu2+) by using surfactant modified laterite (SML) was investigated in the present study. Characterizations of laterite were examined by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), inductively coupled plasma mass spectrometry (ICP-MS), and total carbon analysis. The optimum conditions for removal of Cu2+ by adsorption using SML were systematically studied and found as pH 6, contact time 90 min, adsorbent dosage 5 mg/mL, and ionic strength 10 mM NaCl. The equilibrium concentration of copper ions was measured by flame atomic absorption spectrometry (F-AAS). Surface modification of laterite by anionic surfactant sodium dodecyl sulfate (SDS) induced a significant increase of the removal efficiency of Cu2+. The surface modifications of laterite by preadsorption of SDS and sequential adsorption of Cu2+ were also evaluated by XRD and FT-IR. The adsorption of Cu2+ onto SML increases with increasing NaCl concentration from 1 to 10 mM, but at high salt concentration this trend is reversed because desorption of SDS from laterite surface was enhanced by increasing salt concentration. Experimental results of Cu2+/SML adsorption isotherms at different ionic strengths can be represented well by a two-step adsorption model. Based on adsorption isotherms, surface charge effects, and surface modification, we suggest that the adsorption mechanism of Cu2+ onto SML was induced by electrostatic attraction between Cu2+ and the negatively charged SML surface and nonelectrostatic interactions between Cu2+ and organic substances in the laterite.


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## Body

## 1. Introduction

Removal of heavy metal ion from aqueous solution is important in environmental concern because heavy metal can induce serious problems to human health through the water resources. Therefore, so many projects focus on the removal of heavy metal ion to protect water resources [1–7]. Numerous treatment techniques have been used for heavy metal ion removal from the aquatic environment such as adsorption, ion exchange, coagulation/flocculation, chemical precipitation, photodegradation, and electrochemical oxidation [8–11]. Among these techniques, adsorption is one of the most common technologies for removing heavy metal ions [8, 10, 12–15]. Although activated carbon and activated metal oxides are effective adsorbent for the removal of heavy metal ions [2, 4, 14–17], a big challenge is costly. Thus, cheap adsorbents or/and natural adsorbents or modified cheap adsorbents are preferable in developing countries [5, 8, 12, 18]. To increase the removal efficiency of heavy metal ion by natural minerals, a modification of adsorbent surfaces is needed because the surface of such materials is nonactive. The modified solid adsorbents to remove metal ion was successfully investigated by many researches and,in these cases, not only adsorption capacity but also the adsorptive selectivity was significantly enhanced [19–21].Copper, which is well-known heavy metal, is toxic and nonbiodegradable to aquatic ecosystems and living organisms [22]. Therefore, copper can accumulate in sediments and tissues of living organisms, and separation of copper ions is essential to discharge wastewater into environment. Recently, many studies investigated the removal of copper ion, Cu(II), by adsorption techniques using novel adsorbent [3, 16, 22–25]. Adsorptive removal of Cu(II) by chemically modified adsorbent was investigated in previous studies [3, 26].Laterite is a common soil in tropical countries. Laterite is also easily collected in Vietnam, meaning that laterite is very low cost. Basically, laterite has positive charge in the acid and neutral media so it has been widely used for the removal of many toxic anionic ions including anionic heavy metal [5, 27, 28]. Nevertheless, the removal of cations is rather hard due to the strong electrostatic repulsive force. Thus, in order to increase the removal efficiency of the heavy metal cations by adsorption technique, surface modification of laterite is necessary. Anionic surfactant such as sodium dodecyl sulfate (SDS) is an ecofriendly chemical that can be used in environmental remediation to remove both inorganic and organic pollutants [29]. However, SDS modified laterite to remove Cu2+ has not been studied [5].Adsorption is normally conducted under isothermal condition so that adsorption isotherms fitted by theoretical models are useful to better understand adsorption mechanisms and to explain the interactions between the laterite and copper ion. As for describing adsorption characteristics of heavy metal ions, Langmuir and Freundlich isotherm models are often discussed [30, 31]. However, Langmuir and Freundlich models cannot be applied for adsorption isotherms of surfactants. Thus, adsorption of inorganic pollutants onto surfactant modified laterite could not be fitted by Langmuir and Freundlich models. Fortunately, a two-step model presented by Zhu and Gu [32] with a general adsorption isotherm equation was successfully applied to various types of surfactants, polymers, and dyes adsorption isotherms for numerous systems [32–37]. Adsorptive removal of both organic and inorganic pollutants by using surfactant modified alumina was thoroughly studied by Pal and coworkers [2, 16, 38–41], indicating that surfactant modified solid adsorbent is a novel adsorbent. Nevertheless, the authors have not studied adsorption of Cu(II) on surfactant modified laterite.In this study, we investigate adsorptive removal copper ion using surfactant modified laterite (SML). To the best of our knowledge, this is the first systematic study on the adsorption of copper ion from aqueous solution on SML. Characterizations of laterite and SML were determined by X-ray diffraction (XRD), inductively coupled plasma mass spectrometry (ICP-MS), Fourier transform infrared spectroscopy (FT-IR), and total carbon analysis. The optimum parameters for adsorptive removal copper ion by using SML are systematically studied. The surface modification before and after adsorption is also investigated by XRD and FT-IR. The adsorption mechanism is also proposed on the basis of adsorption isotherms and surface modification.


## 2. Experimental

### 2.1. Materials

Raw laterite was collected from a local place of Thach That, Hanoi, Vietnam. The laterite was treated before measurements as follows: the laterite was washed various times by ultrapure water to reach neutral pH and then dried at 110°C. The treated laterite was cooled in a desiccator at room temperature and stored in a polyethylene container. The dried laterite was sieved in order to collect the particles with the size lower than 0.1 mm. The laterite which was modified with 0.01 M SDS (solid/liquid ratio 200 mg/mL) in 0.01 M NaCl at pH 4 by shaking for 3 h and then washed with ultrapure water was called surfactant modified laterite (SML).Standard solution of copper (1000 ppm in 0.5 M HNO3) was supplied by Merck (Germany). All chemicals were of analytical reagent grade and were used without further purification. Copper nitrate salt, Cu(NO3)2·3H2O, was purchased from Merck (Germany). Anionic surfactant, sodium dodecyl sulfate (SDS, with purity > 95%), from Scharlau (Spain, EU), was used to modify the surface of laterite. The effect of ionic strength was studied by the addition of NaCl (Merck). In order to adjust solution pH, HCl, and NaOH (Merck) were used. Ultrapure water system (Labconco, USA) with resistivity 18.2 MΩ was used to produce ultrapure water in preparing all aqueous solutions.


### 2.2. Adsorption Studies

All adsorption experiments were conducted by batch technique. Initially, the copper stock solution of 2000 ppm was prepared by dissolving precisely calculated amount of copper nitrate salt. Then, the stock solution was appropriately diluted based on experimental requirement.A known amount of adsorbents solution was thoroughly mixed with 50 mL aqueous copper solution (Cu2+) of 10 ppm in 250 mL Erlenmeyer flasks at 25±2°C controlled by an air conditioner. The effect of operating condition (pH, adsorbent dosages, contact time, ionic strength, and initial absorbate concentration) on removal of Cu2+ was studied. The concentration of Cu2+ was determined by flame atomic absorption spectrometry (F-AAS). The removal (R, %) of Cu2+ was calculated by (1)RemovalR,%=Ci-CfCi×100%where Ci and Cf are initial concentration and final concentration of Cu2+, respectively.To study adsorption isotherms, the concentration of Cu2+ was varied from 10 ppm to 2000 ppm and pH was adjusted to the desired value. The adsorption capacity of Cu2+ (Γ Cu2+) onto SDS modified laterite was determined by the different concentrations of Cu2+ solutions before adsorption and after equilibrium process by F-AAS.


### 2.3. Instrumental Analytical Methods

The concentration of Cu2+ was determined by using an atomic absorption spectrometer (AA-6800, Shimadzu, Japan). Hollow cathode lamp (HCL) was used to emit a narrow wavelength of 324.8 nm. The slit width was kept as constant of 0.5 nm for all AAS measurements. The linear relationship between the absorbance and concentrations of Cu2+ had a correlation coefficient of at least 0.999.Concentration of anionic surfactant, SDS used for surface modification of laterite, was determined by spectrophotometry with Ultraviolet Visible spectrophotometer (UV-1650 PC, Shimadzu, Japan) followed our previous paper [36].Potentiometric method was used to determine pH of all solutions. The method was carried out using a HI 2215 Hanna Instruments pH meter by a glass combination electrode. We use three standard buffers (Hanna) to calibrate the electrode before measuring pH of solutions. All measurements were carried out at25±2°C.


### 2.4. Characterization Methods

The chemical compositions of laterite were examined by inductively coupled plasma mass spectrometer (ICP-MS Elan 9000, Perkin Elmer, USA) and total carbon analyzer using a TOCVCPH (Shimadzu, Japan).X-ray diffraction (XRD) was collected on a Bruker D8 Advance X-ray diffractometer, CuKα radiation (λ = 0.1549 nm). Intensity for the diffraction peaks was recorded in the 10°–70° (2θ) range with a step size of 0.03°.To evaluate functional groups of laterite and to confirm surface modification of laterite after Cu2+ adsorption, Fourier transform infrared spectroscopy was performed with an Affinity-1S (Shimadzu, Japan). The FT-IR spectra were obtained under at the same conditions: 25°C, atmospheric pressure, and resolution of 4 cm−1.


## 2.1. Materials

Raw laterite was collected from a local place of Thach That, Hanoi, Vietnam. The laterite was treated before measurements as follows: the laterite was washed various times by ultrapure water to reach neutral pH and then dried at 110°C. The treated laterite was cooled in a desiccator at room temperature and stored in a polyethylene container. The dried laterite was sieved in order to collect the particles with the size lower than 0.1 mm. The laterite which was modified with 0.01 M SDS (solid/liquid ratio 200 mg/mL) in 0.01 M NaCl at pH 4 by shaking for 3 h and then washed with ultrapure water was called surfactant modified laterite (SML).Standard solution of copper (1000 ppm in 0.5 M HNO3) was supplied by Merck (Germany). All chemicals were of analytical reagent grade and were used without further purification. Copper nitrate salt, Cu(NO3)2·3H2O, was purchased from Merck (Germany). Anionic surfactant, sodium dodecyl sulfate (SDS, with purity > 95%), from Scharlau (Spain, EU), was used to modify the surface of laterite. The effect of ionic strength was studied by the addition of NaCl (Merck). In order to adjust solution pH, HCl, and NaOH (Merck) were used. Ultrapure water system (Labconco, USA) with resistivity 18.2 MΩ was used to produce ultrapure water in preparing all aqueous solutions.


## 2.2. Adsorption Studies

All adsorption experiments were conducted by batch technique. Initially, the copper stock solution of 2000 ppm was prepared by dissolving precisely calculated amount of copper nitrate salt. Then, the stock solution was appropriately diluted based on experimental requirement.A known amount of adsorbents solution was thoroughly mixed with 50 mL aqueous copper solution (Cu2+) of 10 ppm in 250 mL Erlenmeyer flasks at 25±2°C controlled by an air conditioner. The effect of operating condition (pH, adsorbent dosages, contact time, ionic strength, and initial absorbate concentration) on removal of Cu2+ was studied. The concentration of Cu2+ was determined by flame atomic absorption spectrometry (F-AAS). The removal (R, %) of Cu2+ was calculated by (1)RemovalR,%=Ci-CfCi×100%where Ci and Cf are initial concentration and final concentration of Cu2+, respectively.To study adsorption isotherms, the concentration of Cu2+ was varied from 10 ppm to 2000 ppm and pH was adjusted to the desired value. The adsorption capacity of Cu2+ (Γ Cu2+) onto SDS modified laterite was determined by the different concentrations of Cu2+ solutions before adsorption and after equilibrium process by F-AAS.


## 2.3. Instrumental Analytical Methods

The concentration of Cu2+ was determined by using an atomic absorption spectrometer (AA-6800, Shimadzu, Japan). Hollow cathode lamp (HCL) was used to emit a narrow wavelength of 324.8 nm. The slit width was kept as constant of 0.5 nm for all AAS measurements. The linear relationship between the absorbance and concentrations of Cu2+ had a correlation coefficient of at least 0.999.Concentration of anionic surfactant, SDS used for surface modification of laterite, was determined by spectrophotometry with Ultraviolet Visible spectrophotometer (UV-1650 PC, Shimadzu, Japan) followed our previous paper [36].Potentiometric method was used to determine pH of all solutions. The method was carried out using a HI 2215 Hanna Instruments pH meter by a glass combination electrode. We use three standard buffers (Hanna) to calibrate the electrode before measuring pH of solutions. All measurements were carried out at25±2°C.


## 2.4. Characterization Methods

The chemical compositions of laterite were examined by inductively coupled plasma mass spectrometer (ICP-MS Elan 9000, Perkin Elmer, USA) and total carbon analyzer using a TOCVCPH (Shimadzu, Japan).X-ray diffraction (XRD) was collected on a Bruker D8 Advance X-ray diffractometer, CuKα radiation (λ = 0.1549 nm). Intensity for the diffraction peaks was recorded in the 10°–70° (2θ) range with a step size of 0.03°.To evaluate functional groups of laterite and to confirm surface modification of laterite after Cu2+ adsorption, Fourier transform infrared spectroscopy was performed with an Affinity-1S (Shimadzu, Japan). The FT-IR spectra were obtained under at the same conditions: 25°C, atmospheric pressure, and resolution of 4 cm−1.


## 3. Modeling by General Isotherm Equation

The obtained isotherms were fitted by a general isotherm equation. The equation was derived by assuming that two adsorption steps can occur at the solid-liquid interface [32].The general isotherm equation is(2)Γ=Γ∞k1C1/n+k2Cn-11+k1C1+k2Cn-1,where Γ is amount of Cu2+ adsorbed; Γ∞ is the maximum adsorption amount; k1 and k2 are equilibrium constants for the first layer adsorption and clusters of n molecules or multilayer adsorption. C denotes the equilibrium concentration of Cu2+ in solution.The selected fitting parameters were described in our previously published papers [35–37].


## 4. Results and Discussion

### 4.1. Characterization of Laterite

#### 4.1.1. Characterization of Raw Laterite

Table1 shows the chemical compositions of raw laterite material. The inorganic components were determined by ICP-MS while carbon content was measured by TC. The results in Table 1 indicate that the main chemical compositions of laterite are metal oxides in which Fe2O3, SiO2, and Al2O3 are dominated. The composition of laterite used in the present study is similar to the published papers [45, 46].Table 1
The chemical composition of raw laterite.
Content (%)
Value
Al2O3
15.0
Fe2O3
40.1
SiO2
33.2
MnO2
0.7
P2O5
0.5
Na2O,
~0.8
K2O,
CaO
C
0.7The XRD pattern of laterite is represented in Figure1. The predominated morphologies were found as quartz (SiO2), hematite (Fe2O3), and goethite (FeO(OH)). The peaks at 2θ = 45, 66 indicated Al2O3 with very low intensity. The mineralogical phases of laterite used in this study is close to laterite used in other researches [5, 45].Figure 1
The XRD pattern of raw laterite.Figure2 indicates the FT-IR spectra of laterite without any modification. As can be seen in Figure 2, the band at 3622.32 and 3406.29 was assigned to –OH group of Si and Al. Another band at 1645.28 was assigned to inner layer water molecules [5, 45]. The bands at 1031.92, 1036.84, 912.33, and 796.60 appeared in the spectra because of the presence of Si–O–Fe, Al–OH, Fe–OH vibrations. The Fe–O bonds stretching at 534.28 and 464.84 were also obtained due to the presence of hematite in structure of laterite. The results of FT-IR are in good agreement with ICP-MS and XRD given above.Figure 2
The FT-IR spectra of raw laterite.


#### 4.1.2. Characterization of Laterite after SDS Preadsorption and Copper Adsorption

Anionic surfactant sodium dodecyl sulfate (SDS) with a concentration of 0.01 M (higher than critical micelle concentration, CMC) was used to modify the surface of laterite in 0.01 M NaCl (pH 5). In order to evaluate the change in mineral phase and surface modification and of laterite after preadsorption of SDS (SML) and sequential adsorption of Cu2+ with initial concentration of 100 ppm in 10 mM NaCl (pH 5), the XRD and FT-IR techniques are also used.Figures3(a) and 3(b) show that the morphologies still contain quartz (SiO2), hematite (Fe2O3), and goethite (FeO(OH)). These are similar to raw laterite, demonstrating that the structure of laterite does not change after adsorption SDS and adsorption of Cu2+. However, the intensity of special peaks assigned for each mineral phase was changed. The intensity of specific peak for SiO2 is enhanced while the others of Fe2O3 and FeO(OH) are reduced after preadsorption of SDS (Figure 3(a)) compared with original laterite. SDS is an anionic surfactant that can easily attract positive mineral (Fe2O3 and FeO(OH)) than negative one of SiO2. For the case of laterite after consequential adsorption of cation Cu2+, the peak indicated for SiO2 decreased dramatically while the others for Fe2O3 and FeO(OH) slightly increased [5, 27].Figure 3
The XRD patterns of laterite after preadsorption of SDS (a) and after sequential adsorption of Cu2+ (b).
(a)
(b)Figure4 shows that FT-IR spectra of modified laterite by preadsorption of SDS are similar to the raw one (Figure 4(a)). In additive, the relative intensity of asymmetrical and symmetrical stretching of –CH2– presented at 2926.01 and 2854.65 cm−1 decreases dramatically in the spectra of SML while these peaks appear with very high intensity in spectra of SDS powder (data not shown). This confirms that the hydrophobic interaction can work on the surface of laterite. In addition, the characteristic peaks of SO42- at about 1247 cm−1 and 1218 cm−1 appear very strong in spectra of SDS while all bands disappear in the spectra of SML. It is demonstrated that SDS has sulfate head groups in contact with the surface of laterite via the electrostatic attraction at the current salt concentration (10 mM NaCl). In other words, the modification of laterite was successful due to the presence of bilayer and/or admicelles on the surface of laterite [36].Figure 4
The FT-IR spectra of laterite after preadsorption of SDS (a) and after sequential adsorption of Cu2+ (b).
(a)
(b)As can be seen in Figure4(b), the band at 3442.94 assigned to –OH is very big due to the hydroxo complex of Cu(II). The bands at 1031.92, 1036.84, and 912.33 that appeared in the spectra of raw laterite are shifted to lower wavenumber at 1029.99, 1004.91, and 910.40 because the surface of laterite is changed. In addition, the peaks of –CH2– present at 2926.01 and 2854.65 cm−1 of SML could not be seen in the spectra of SML after Cu2+ adsorption. These results suggest the adsorption of copper molecules onto SML via electrostatic attraction and nonelectrostatic interactions between Cu2+ and organic compounds in laterite.


### 4.2. Surface Modification of Laterite by SDS

Anionic surfactant SDS with a concentration of 0.01 M was used to modify the surface of laterite in 0.01 M NaCl (pH 5). The obtained adsorption capacity of SDS on laterite (with solid-liquid ratio 200 mg/mL) is 0.01 mmol/g. Although the adsorption SDS on laterite is small, the loading implies the presence of bilayer and/or admicelles of SDS [36]. As a result, the surface charge of laterite is negative which can enhance adsorption of cationic heavy metal Cu2+. Figure 5 indicates that the removal efficiency of Cu2+ in 1 mM NaCl (pH 5) with initial concentration of 10 ppm increases from 61.6% to 89.5% after the surface modification of laterite by SDS.Figure 5
The removal of Cu2+ at the initial concentration of 10 ppm in 1 mM NaCl (pH 5) using laterite and surfactant modified laterite (SML).Typical concentration of Cu(II) in wastewater and surface water is quite low. The limited concentration according to national technical regulation on water quality of Vietnam is equal to 1 mg/L for class B (low quality water). It should be noted that when the concentration of Cu(II) in aqueous solution is less than 2.5 mg/L the removal efficiency of copper through adsorption using SML is approximately 99%. Furthermore, the maximum contaminant level (MCL) standard according to USEPA for Cu is 0.25 mg/L. It implies that the SML is a promising material to remove Cu(II) from aqueous solutions.Effective parameters on adsorptive removal of copper ion by using SML are systematically studied, as given below.


### 4.3. Adsorptive Removal of Copper Ion by Using Surfactant Modified Laterite

#### 4.3.1. Effect of pH

Solution pH plays an important role in the adsorption of copper ion onto surfactant modified laterite (SML) because it can affect the surface charge of SML and charging behavior of chemical speciation of copper. At pH > 7, the high precipitation of Cu2+ can take place which influences the concentration of Cu2+ in solution. The effect of initial pH on the adsorption of Cu2+ by SML was investigated in the pH range of 3–7 in 1 mM NaCl (Figure 6). This implies that the negatively charged SML surface can easily attract Cu2+ by electrostatic attraction.Figure 6
The removal of Cu2+ by surfactant modified laterite (SML) as a function of pH (Ci (Cu2+) = 10 ppm; contact time 90 min; adsorbent dosage 5 mg/mL; 1 mM NaCl).As can be seen in Figure6, the removal of Cu2+ using SML increases with increasing solution pH from pH 4 to pH 6 because of the competition between H+ (at low pH) and cation Cu2+ on the surface of SML. However, the desorption of SDS is increased at pH > 7 so that the removal efficiency of Cu2+ decreases. Thus, optimum pH for removal of Cu2+ by SML is pH 6 which agrees well with optimum pH in the case of adsorption heavy metal ions on surfactant modified alumina (SMA) [2, 3]. However, alumina is much more expensive compared with laterite so SML is more suitable than SMA for removing heavy metal ions in developing countries.The point of zero charge of laterite is about 7.4 [45, 46], meaning that at pH < 7.4 laterite has positive charge, since surface modification of laterite with SDS was conducted at pH 4.0 to promote preadsorption of SDS on laterite surface. After preadsorption of SDS, the concomitant of proton occurred so pH of solution increased around 0.6 units (from pH 5.0 to 5.6). This trend is similar to the case of SDS adsorption on Al2O3 [36]. However, net surface charge of SML is negative which enhances adsorption of Cu2+ significantly. After copper adsorption, pH of solutions decreased (from 5.0 to 4.7), demonstrating that adsorption of cation Cu2+ induces the desorption of SDS with proton into solution.


#### 4.3.2. Effect of Contact Time

Contact time affects the completeness of adsorption equilibration. The effect of contact time on the adsorptive removal of Cu2+ by using SML is presented in Figure 7. Figure 7 shows the removal efficiency of Cu2+ from aqueous solution by using SML growing with time from 10 min to 180 min. It is suggested that adsorption reaches equilibrium at 90 min. After 180 min, the removal decreases because high concentration of Na+ in NaCl background can displace Cu2+. The equilibrium adsorption in this study is longer than the case of adsorptive removal of Cu2+ by natural kaolin clay (only 30 min) and the removal of Cu2+ by goethite mineral (60 min) [47, 48]. Nevertheless, 90 min is acceptable and is selected as the optimum contact time for removal of Cu2+ by using SML in 1 mM NaCl (pH 6).Figure 7
The removal of Cu2+ by SML at different contact time (Ci (Cu2+) = 10 ppm; pH 6; adsorbent dosage 5 mg/mL; 1 mM NaCl).


#### 4.3.3. Effect of Adsorbent Dosage

The adsorbent dosage has a significant effect on the adsorption process because it can influence the total surface area of adsorbent and number of binding site. The amount of SML was varied from 0.05 to 6.0 g of adsorbent that corresponded from 1.0 to 12 mg/mL (Figure8).Figure 8
Removal (R) and adsorption capacity (Γ) of Cu2+ by SML as a function of adsorbent dosage (Ci (Cu2+) = 10 ppm; pH 6; contact time 90 min; 1 mM NaCl).Figure8 reveals that the removal of Cu2+ by SML increases with increasing adsorbent dosage from 1 to 12 mg/mL. It may be explained by the increased large number of available binding sites for adsorption or increased net specific surface area with an increase of dosage [49]. However, an increase in adsorbent causes the increase of the adsorption capacity; then the adsorption capacity decreases due to the aggregation of colloidal particles at high adsorbent dosage [50]. Optimum adsorbent dosage is found to be 5 mg/mL and is fixed for the remaining studies.


#### 4.3.4. Effect of Ionic Strength

Ionic strength affects electrostatic attraction between ionic adsorbates and charged surface adsorbent. For adsorption of Cu2+ onto SML, ionic strength also induces a change of existence of SDS molecules on laterite surface. As can be seen in Figure 9, the removal efficiency is lowest in the absence of salt, demonstrating that the electrostatic interaction causes adsorption. Furthermore, adsorption at 50 mM NaCl concentration is lower than that at 10 mM. This result can be explained by desorption of SDS at high salt concentration. Desorption of SDS is enhanced with increasing NaCl concentration from 10 to 50 mM. When increasing salt, the concentration of Cl− is high so the removal is decreased. However, the removal increases with an increase of NaCl concentration from 1 to 10 mM, suggesting that not only electrostatic but also nonelectrostatic between Cu(II) and/or Cu(II) speciation and organic substances in laterite induces adsorption. The effect of salt concentration will be discussed in the adsorption isotherms section.Figure 9
The removal of Cu2+ by SML at different NaCl concentrations (Ci (Cu2+) = 10 ppm; pH 6; contact time 90 min; adsorbent dosage 5 mg/mL).


### 4.4. Characteristics and Mechanisms of Copper Ion Adsorption Isotherms on SDS Modified Laterite

#### 4.4.1. Adsorption Isotherms of Copper Ion on SDS Modified Laterite by a Two-Step Adsorption Model

The effect of ionic strength on adsorption of copper ion on SDS modified laterite (SML) is clearly demonstrated in the isotherms (Figure10). At pH 6, the Cu2+ adsorption capacity increases with increasing NaCl from 1 to 10 mM. At higher salt concentration (from 10 to 50 mM), the adsorption capacity reduces with an increase of salt. The increase in salt concentration increases the number of cations Na+ (counterions) on the negatively charged layer of SML, reducing the electrostatic effect of SML Cu2+ ion. It is quite different from the case of adsorption of copper ion on strawberry leaf powder [51] in which the effect of Na+, K+, Mg2+, and Ca2+ on the copper adsorption is not significant. In our case, the electrostatic attraction between the positive charge of Cu2+ ion and negative charge of SML is effectively screened by increasing NaCl concentrations from 10 to 50 mM. Nonelectrostatic interactions are more important in adsorption at low ionic strength (1 to 10 mM NaCl). These results agree well with the effect of ionic strength on the removal of copper ion using SML shown in previous section.Figure 10
Adsorption isotherms of Cu2+ onto SDS modified laterite (SML) as a function of equilibrium concentration of Cu2+ at different NaCl concentrations. Points are experimental data; solid lines are results of the 2-step adsorption model.Figure10 indicates that, at different salt concentrations, the experimental results can be represented well by the general isotherm equation (2) with the fitting parameters in Table 2.Table 2
The fit parameters for Cu2+ adsorption onto SML which are maximum adsorbed amount Γ∞,Cu2+, the equilibrium constants k1,Cu2+ and k2,Cu2+ for first layer adsorption and multilayer adsorption, respectively, and the number of cluster of Cu2+ ion nCu2+.
C salt(mM NaCl)
Γ
∞
,
C
u
2
+  (mg/g)
k
1
,
C
u
2
+  (g/mg)
k
2
,
C
u
2
+  (g/mg)n-1
n
C
u
2
+
1
150
3.0
×
10
2
8.0
×
10
2
2.4
10
185
5.0
×
10
2
8.0
×
10
2
2.5
50
110
1.5
×
10
2
7.5
×
10
2
2.5As shown in Table2, increasing ionic strength from 1 to 10 mM causes an increase in k1,Cu2+ but a decrease in k1,Cu2+ when increasing salt from 10 to 50 mM. It is noted that the changes in k2,Cu2+ and the number of Cu2+ clusters are not significant (k2,NH4+≈8.0×102 and nCu2+≈2.5). The value of k1,Cu2+ is also related to the slope of isotherm. As a result, at high salt concentration, the slope is lower than that at low salt. It suggests that desorption of SDS from the bilayer of admicelles is enhanced by increasing salt concentration from 10 to 50 mM while at low salt concentration the link between sulfate groups of SDS an Cu2+ is not strong as other interactions.


#### 4.4.2. Adsorption Mechanisms and Advantages of Copper Ion onto SDS Modified Laterite

Adsorptive removal of copper ion (Cu2+) is enhanced by using anionic surfactant, SDS modified laterite (SML). The effective conditions on adsorption of Cu2+ ion by SML were systematically studied in Section 4.3. The two-step model was established to describe the Cu2+ adsorption onto SML, suggesting that adsorption of Cu2+ could replace admicellar bilayers of SDS molecules on the laterite surface at high salt concentration. Adsorption of Cu2+ decreases with an increase of ionic strength at high salt concentration because desorption of SDS is enhanced by increasing salt concentration [35] which is in good agreement with discussion in Section 4.3.1. It is also represented by two-step adsorption model with decreasing k1,Cu2+ when increasing NaCl concentration from 10 to 50 mM. At low salt concentration, the main interactions inducing adsorption may be lateral, hydrogen bonding, surface complexation, and Van der Waals interactions between Cu2+ and organic substances in the laterite.Adsorption mechanisms of Cu2+ ion on SML are also supported by the results of FT-IR spectra (see Section 4.1.2). The bands of CH2– of SML did not occur after adsorption of Cu2+ in 10 mM NaCl, suggesting that the adsorption of Cu2+ onto SML is mainly controlled by the electrostatic attraction between the negatively charged sulfate groups of bilayer admicelles and positive charge of Cu2+. It seems to be similar to the case of Cu2+ on natural kaolinite clays [48]. Nevertheless, at low ionic strength, the attraction between positive Cu2+ and negative SML is not so strong compared with other interactions. In this case, nonelectrostatic interactions between Cu2+ and/or speciation of Cu2+ with organic substances in laterite mainly contribute to adsorption.Some published papers focused on sorption of Cu(II) onto different kinds of sorbents. Nevertheless, sorption of Cu(II) onto SML has not been reported. Furthermore, SML used in the present study has very high adsorption capacity (185 mg/g) compared with other natural sorbents or modified natural materials (Table3). It demonstrates that SML is novel material for removal of Cu(II) from aqueous solution.Table 3
Adsorption capacity of Cu2+ on natural materials or modified natural materials and SML.
Adsorbent
Adsorption capacity (mg/g)
References
Zeolite, clinoptilolite
1.64
[12]
HCl-treated clay
83.3
[42]
Marine alga,Ecklonia maxima
90
[43]
Red paddy soil
0.21
[44]
SDS modified laterite
185
This study


## 4.1. Characterization of Laterite

### 4.1.1. Characterization of Raw Laterite

Table1 shows the chemical compositions of raw laterite material. The inorganic components were determined by ICP-MS while carbon content was measured by TC. The results in Table 1 indicate that the main chemical compositions of laterite are metal oxides in which Fe2O3, SiO2, and Al2O3 are dominated. The composition of laterite used in the present study is similar to the published papers [45, 46].Table 1
The chemical composition of raw laterite.
Content (%)
Value
Al2O3
15.0
Fe2O3
40.1
SiO2
33.2
MnO2
0.7
P2O5
0.5
Na2O,
~0.8
K2O,
CaO
C
0.7The XRD pattern of laterite is represented in Figure1. The predominated morphologies were found as quartz (SiO2), hematite (Fe2O3), and goethite (FeO(OH)). The peaks at 2θ = 45, 66 indicated Al2O3 with very low intensity. The mineralogical phases of laterite used in this study is close to laterite used in other researches [5, 45].Figure 1
The XRD pattern of raw laterite.Figure2 indicates the FT-IR spectra of laterite without any modification. As can be seen in Figure 2, the band at 3622.32 and 3406.29 was assigned to –OH group of Si and Al. Another band at 1645.28 was assigned to inner layer water molecules [5, 45]. The bands at 1031.92, 1036.84, 912.33, and 796.60 appeared in the spectra because of the presence of Si–O–Fe, Al–OH, Fe–OH vibrations. The Fe–O bonds stretching at 534.28 and 464.84 were also obtained due to the presence of hematite in structure of laterite. The results of FT-IR are in good agreement with ICP-MS and XRD given above.Figure 2
The FT-IR spectra of raw laterite.


### 4.1.2. Characterization of Laterite after SDS Preadsorption and Copper Adsorption

Anionic surfactant sodium dodecyl sulfate (SDS) with a concentration of 0.01 M (higher than critical micelle concentration, CMC) was used to modify the surface of laterite in 0.01 M NaCl (pH 5). In order to evaluate the change in mineral phase and surface modification and of laterite after preadsorption of SDS (SML) and sequential adsorption of Cu2+ with initial concentration of 100 ppm in 10 mM NaCl (pH 5), the XRD and FT-IR techniques are also used.Figures3(a) and 3(b) show that the morphologies still contain quartz (SiO2), hematite (Fe2O3), and goethite (FeO(OH)). These are similar to raw laterite, demonstrating that the structure of laterite does not change after adsorption SDS and adsorption of Cu2+. However, the intensity of special peaks assigned for each mineral phase was changed. The intensity of specific peak for SiO2 is enhanced while the others of Fe2O3 and FeO(OH) are reduced after preadsorption of SDS (Figure 3(a)) compared with original laterite. SDS is an anionic surfactant that can easily attract positive mineral (Fe2O3 and FeO(OH)) than negative one of SiO2. For the case of laterite after consequential adsorption of cation Cu2+, the peak indicated for SiO2 decreased dramatically while the others for Fe2O3 and FeO(OH) slightly increased [5, 27].Figure 3
The XRD patterns of laterite after preadsorption of SDS (a) and after sequential adsorption of Cu2+ (b).
(a)
(b)Figure4 shows that FT-IR spectra of modified laterite by preadsorption of SDS are similar to the raw one (Figure 4(a)). In additive, the relative intensity of asymmetrical and symmetrical stretching of –CH2– presented at 2926.01 and 2854.65 cm−1 decreases dramatically in the spectra of SML while these peaks appear with very high intensity in spectra of SDS powder (data not shown). This confirms that the hydrophobic interaction can work on the surface of laterite. In addition, the characteristic peaks of SO42- at about 1247 cm−1 and 1218 cm−1 appear very strong in spectra of SDS while all bands disappear in the spectra of SML. It is demonstrated that SDS has sulfate head groups in contact with the surface of laterite via the electrostatic attraction at the current salt concentration (10 mM NaCl). In other words, the modification of laterite was successful due to the presence of bilayer and/or admicelles on the surface of laterite [36].Figure 4
The FT-IR spectra of laterite after preadsorption of SDS (a) and after sequential adsorption of Cu2+ (b).
(a)
(b)As can be seen in Figure4(b), the band at 3442.94 assigned to –OH is very big due to the hydroxo complex of Cu(II). The bands at 1031.92, 1036.84, and 912.33 that appeared in the spectra of raw laterite are shifted to lower wavenumber at 1029.99, 1004.91, and 910.40 because the surface of laterite is changed. In addition, the peaks of –CH2– present at 2926.01 and 2854.65 cm−1 of SML could not be seen in the spectra of SML after Cu2+ adsorption. These results suggest the adsorption of copper molecules onto SML via electrostatic attraction and nonelectrostatic interactions between Cu2+ and organic compounds in laterite.


## 4.1.1. Characterization of Raw Laterite

Table1 shows the chemical compositions of raw laterite material. The inorganic components were determined by ICP-MS while carbon content was measured by TC. The results in Table 1 indicate that the main chemical compositions of laterite are metal oxides in which Fe2O3, SiO2, and Al2O3 are dominated. The composition of laterite used in the present study is similar to the published papers [45, 46].Table 1
The chemical composition of raw laterite.
Content (%)
Value
Al2O3
15.0
Fe2O3
40.1
SiO2
33.2
MnO2
0.7
P2O5
0.5
Na2O,
~0.8
K2O,
CaO
C
0.7The XRD pattern of laterite is represented in Figure1. The predominated morphologies were found as quartz (SiO2), hematite (Fe2O3), and goethite (FeO(OH)). The peaks at 2θ = 45, 66 indicated Al2O3 with very low intensity. The mineralogical phases of laterite used in this study is close to laterite used in other researches [5, 45].Figure 1
The XRD pattern of raw laterite.Figure2 indicates the FT-IR spectra of laterite without any modification. As can be seen in Figure 2, the band at 3622.32 and 3406.29 was assigned to –OH group of Si and Al. Another band at 1645.28 was assigned to inner layer water molecules [5, 45]. The bands at 1031.92, 1036.84, 912.33, and 796.60 appeared in the spectra because of the presence of Si–O–Fe, Al–OH, Fe–OH vibrations. The Fe–O bonds stretching at 534.28 and 464.84 were also obtained due to the presence of hematite in structure of laterite. The results of FT-IR are in good agreement with ICP-MS and XRD given above.Figure 2
The FT-IR spectra of raw laterite.


## 4.1.2. Characterization of Laterite after SDS Preadsorption and Copper Adsorption

Anionic surfactant sodium dodecyl sulfate (SDS) with a concentration of 0.01 M (higher than critical micelle concentration, CMC) was used to modify the surface of laterite in 0.01 M NaCl (pH 5). In order to evaluate the change in mineral phase and surface modification and of laterite after preadsorption of SDS (SML) and sequential adsorption of Cu2+ with initial concentration of 100 ppm in 10 mM NaCl (pH 5), the XRD and FT-IR techniques are also used.Figures3(a) and 3(b) show that the morphologies still contain quartz (SiO2), hematite (Fe2O3), and goethite (FeO(OH)). These are similar to raw laterite, demonstrating that the structure of laterite does not change after adsorption SDS and adsorption of Cu2+. However, the intensity of special peaks assigned for each mineral phase was changed. The intensity of specific peak for SiO2 is enhanced while the others of Fe2O3 and FeO(OH) are reduced after preadsorption of SDS (Figure 3(a)) compared with original laterite. SDS is an anionic surfactant that can easily attract positive mineral (Fe2O3 and FeO(OH)) than negative one of SiO2. For the case of laterite after consequential adsorption of cation Cu2+, the peak indicated for SiO2 decreased dramatically while the others for Fe2O3 and FeO(OH) slightly increased [5, 27].Figure 3
The XRD patterns of laterite after preadsorption of SDS (a) and after sequential adsorption of Cu2+ (b).
(a)
(b)Figure4 shows that FT-IR spectra of modified laterite by preadsorption of SDS are similar to the raw one (Figure 4(a)). In additive, the relative intensity of asymmetrical and symmetrical stretching of –CH2– presented at 2926.01 and 2854.65 cm−1 decreases dramatically in the spectra of SML while these peaks appear with very high intensity in spectra of SDS powder (data not shown). This confirms that the hydrophobic interaction can work on the surface of laterite. In addition, the characteristic peaks of SO42- at about 1247 cm−1 and 1218 cm−1 appear very strong in spectra of SDS while all bands disappear in the spectra of SML. It is demonstrated that SDS has sulfate head groups in contact with the surface of laterite via the electrostatic attraction at the current salt concentration (10 mM NaCl). In other words, the modification of laterite was successful due to the presence of bilayer and/or admicelles on the surface of laterite [36].Figure 4
The FT-IR spectra of laterite after preadsorption of SDS (a) and after sequential adsorption of Cu2+ (b).
(a)
(b)As can be seen in Figure4(b), the band at 3442.94 assigned to –OH is very big due to the hydroxo complex of Cu(II). The bands at 1031.92, 1036.84, and 912.33 that appeared in the spectra of raw laterite are shifted to lower wavenumber at 1029.99, 1004.91, and 910.40 because the surface of laterite is changed. In addition, the peaks of –CH2– present at 2926.01 and 2854.65 cm−1 of SML could not be seen in the spectra of SML after Cu2+ adsorption. These results suggest the adsorption of copper molecules onto SML via electrostatic attraction and nonelectrostatic interactions between Cu2+ and organic compounds in laterite.


## 4.2. Surface Modification of Laterite by SDS

Anionic surfactant SDS with a concentration of 0.01 M was used to modify the surface of laterite in 0.01 M NaCl (pH 5). The obtained adsorption capacity of SDS on laterite (with solid-liquid ratio 200 mg/mL) is 0.01 mmol/g. Although the adsorption SDS on laterite is small, the loading implies the presence of bilayer and/or admicelles of SDS [36]. As a result, the surface charge of laterite is negative which can enhance adsorption of cationic heavy metal Cu2+. Figure 5 indicates that the removal efficiency of Cu2+ in 1 mM NaCl (pH 5) with initial concentration of 10 ppm increases from 61.6% to 89.5% after the surface modification of laterite by SDS.Figure 5
The removal of Cu2+ at the initial concentration of 10 ppm in 1 mM NaCl (pH 5) using laterite and surfactant modified laterite (SML).Typical concentration of Cu(II) in wastewater and surface water is quite low. The limited concentration according to national technical regulation on water quality of Vietnam is equal to 1 mg/L for class B (low quality water). It should be noted that when the concentration of Cu(II) in aqueous solution is less than 2.5 mg/L the removal efficiency of copper through adsorption using SML is approximately 99%. Furthermore, the maximum contaminant level (MCL) standard according to USEPA for Cu is 0.25 mg/L. It implies that the SML is a promising material to remove Cu(II) from aqueous solutions.Effective parameters on adsorptive removal of copper ion by using SML are systematically studied, as given below.


## 4.3. Adsorptive Removal of Copper Ion by Using Surfactant Modified Laterite

### 4.3.1. Effect of pH

Solution pH plays an important role in the adsorption of copper ion onto surfactant modified laterite (SML) because it can affect the surface charge of SML and charging behavior of chemical speciation of copper. At pH > 7, the high precipitation of Cu2+ can take place which influences the concentration of Cu2+ in solution. The effect of initial pH on the adsorption of Cu2+ by SML was investigated in the pH range of 3–7 in 1 mM NaCl (Figure 6). This implies that the negatively charged SML surface can easily attract Cu2+ by electrostatic attraction.Figure 6
The removal of Cu2+ by surfactant modified laterite (SML) as a function of pH (Ci (Cu2+) = 10 ppm; contact time 90 min; adsorbent dosage 5 mg/mL; 1 mM NaCl).As can be seen in Figure6, the removal of Cu2+ using SML increases with increasing solution pH from pH 4 to pH 6 because of the competition between H+ (at low pH) and cation Cu2+ on the surface of SML. However, the desorption of SDS is increased at pH > 7 so that the removal efficiency of Cu2+ decreases. Thus, optimum pH for removal of Cu2+ by SML is pH 6 which agrees well with optimum pH in the case of adsorption heavy metal ions on surfactant modified alumina (SMA) [2, 3]. However, alumina is much more expensive compared with laterite so SML is more suitable than SMA for removing heavy metal ions in developing countries.The point of zero charge of laterite is about 7.4 [45, 46], meaning that at pH < 7.4 laterite has positive charge, since surface modification of laterite with SDS was conducted at pH 4.0 to promote preadsorption of SDS on laterite surface. After preadsorption of SDS, the concomitant of proton occurred so pH of solution increased around 0.6 units (from pH 5.0 to 5.6). This trend is similar to the case of SDS adsorption on Al2O3 [36]. However, net surface charge of SML is negative which enhances adsorption of Cu2+ significantly. After copper adsorption, pH of solutions decreased (from 5.0 to 4.7), demonstrating that adsorption of cation Cu2+ induces the desorption of SDS with proton into solution.


### 4.3.2. Effect of Contact Time

Contact time affects the completeness of adsorption equilibration. The effect of contact time on the adsorptive removal of Cu2+ by using SML is presented in Figure 7. Figure 7 shows the removal efficiency of Cu2+ from aqueous solution by using SML growing with time from 10 min to 180 min. It is suggested that adsorption reaches equilibrium at 90 min. After 180 min, the removal decreases because high concentration of Na+ in NaCl background can displace Cu2+. The equilibrium adsorption in this study is longer than the case of adsorptive removal of Cu2+ by natural kaolin clay (only 30 min) and the removal of Cu2+ by goethite mineral (60 min) [47, 48]. Nevertheless, 90 min is acceptable and is selected as the optimum contact time for removal of Cu2+ by using SML in 1 mM NaCl (pH 6).Figure 7
The removal of Cu2+ by SML at different contact time (Ci (Cu2+) = 10 ppm; pH 6; adsorbent dosage 5 mg/mL; 1 mM NaCl).


### 4.3.3. Effect of Adsorbent Dosage

The adsorbent dosage has a significant effect on the adsorption process because it can influence the total surface area of adsorbent and number of binding site. The amount of SML was varied from 0.05 to 6.0 g of adsorbent that corresponded from 1.0 to 12 mg/mL (Figure8).Figure 8
Removal (R) and adsorption capacity (Γ) of Cu2+ by SML as a function of adsorbent dosage (Ci (Cu2+) = 10 ppm; pH 6; contact time 90 min; 1 mM NaCl).Figure8 reveals that the removal of Cu2+ by SML increases with increasing adsorbent dosage from 1 to 12 mg/mL. It may be explained by the increased large number of available binding sites for adsorption or increased net specific surface area with an increase of dosage [49]. However, an increase in adsorbent causes the increase of the adsorption capacity; then the adsorption capacity decreases due to the aggregation of colloidal particles at high adsorbent dosage [50]. Optimum adsorbent dosage is found to be 5 mg/mL and is fixed for the remaining studies.


### 4.3.4. Effect of Ionic Strength

Ionic strength affects electrostatic attraction between ionic adsorbates and charged surface adsorbent. For adsorption of Cu2+ onto SML, ionic strength also induces a change of existence of SDS molecules on laterite surface. As can be seen in Figure 9, the removal efficiency is lowest in the absence of salt, demonstrating that the electrostatic interaction causes adsorption. Furthermore, adsorption at 50 mM NaCl concentration is lower than that at 10 mM. This result can be explained by desorption of SDS at high salt concentration. Desorption of SDS is enhanced with increasing NaCl concentration from 10 to 50 mM. When increasing salt, the concentration of Cl− is high so the removal is decreased. However, the removal increases with an increase of NaCl concentration from 1 to 10 mM, suggesting that not only electrostatic but also nonelectrostatic between Cu(II) and/or Cu(II) speciation and organic substances in laterite induces adsorption. The effect of salt concentration will be discussed in the adsorption isotherms section.Figure 9
The removal of Cu2+ by SML at different NaCl concentrations (Ci (Cu2+) = 10 ppm; pH 6; contact time 90 min; adsorbent dosage 5 mg/mL).


## 4.3.1. Effect of pH

Solution pH plays an important role in the adsorption of copper ion onto surfactant modified laterite (SML) because it can affect the surface charge of SML and charging behavior of chemical speciation of copper. At pH > 7, the high precipitation of Cu2+ can take place which influences the concentration of Cu2+ in solution. The effect of initial pH on the adsorption of Cu2+ by SML was investigated in the pH range of 3–7 in 1 mM NaCl (Figure 6). This implies that the negatively charged SML surface can easily attract Cu2+ by electrostatic attraction.Figure 6
The removal of Cu2+ by surfactant modified laterite (SML) as a function of pH (Ci (Cu2+) = 10 ppm; contact time 90 min; adsorbent dosage 5 mg/mL; 1 mM NaCl).As can be seen in Figure6, the removal of Cu2+ using SML increases with increasing solution pH from pH 4 to pH 6 because of the competition between H+ (at low pH) and cation Cu2+ on the surface of SML. However, the desorption of SDS is increased at pH > 7 so that the removal efficiency of Cu2+ decreases. Thus, optimum pH for removal of Cu2+ by SML is pH 6 which agrees well with optimum pH in the case of adsorption heavy metal ions on surfactant modified alumina (SMA) [2, 3]. However, alumina is much more expensive compared with laterite so SML is more suitable than SMA for removing heavy metal ions in developing countries.The point of zero charge of laterite is about 7.4 [45, 46], meaning that at pH < 7.4 laterite has positive charge, since surface modification of laterite with SDS was conducted at pH 4.0 to promote preadsorption of SDS on laterite surface. After preadsorption of SDS, the concomitant of proton occurred so pH of solution increased around 0.6 units (from pH 5.0 to 5.6). This trend is similar to the case of SDS adsorption on Al2O3 [36]. However, net surface charge of SML is negative which enhances adsorption of Cu2+ significantly. After copper adsorption, pH of solutions decreased (from 5.0 to 4.7), demonstrating that adsorption of cation Cu2+ induces the desorption of SDS with proton into solution.


## 4.3.2. Effect of Contact Time

Contact time affects the completeness of adsorption equilibration. The effect of contact time on the adsorptive removal of Cu2+ by using SML is presented in Figure 7. Figure 7 shows the removal efficiency of Cu2+ from aqueous solution by using SML growing with time from 10 min to 180 min. It is suggested that adsorption reaches equilibrium at 90 min. After 180 min, the removal decreases because high concentration of Na+ in NaCl background can displace Cu2+. The equilibrium adsorption in this study is longer than the case of adsorptive removal of Cu2+ by natural kaolin clay (only 30 min) and the removal of Cu2+ by goethite mineral (60 min) [47, 48]. Nevertheless, 90 min is acceptable and is selected as the optimum contact time for removal of Cu2+ by using SML in 1 mM NaCl (pH 6).Figure 7
The removal of Cu2+ by SML at different contact time (Ci (Cu2+) = 10 ppm; pH 6; adsorbent dosage 5 mg/mL; 1 mM NaCl).


## 4.3.3. Effect of Adsorbent Dosage

The adsorbent dosage has a significant effect on the adsorption process because it can influence the total surface area of adsorbent and number of binding site. The amount of SML was varied from 0.05 to 6.0 g of adsorbent that corresponded from 1.0 to 12 mg/mL (Figure8).Figure 8
Removal (R) and adsorption capacity (Γ) of Cu2+ by SML as a function of adsorbent dosage (Ci (Cu2+) = 10 ppm; pH 6; contact time 90 min; 1 mM NaCl).Figure8 reveals that the removal of Cu2+ by SML increases with increasing adsorbent dosage from 1 to 12 mg/mL. It may be explained by the increased large number of available binding sites for adsorption or increased net specific surface area with an increase of dosage [49]. However, an increase in adsorbent causes the increase of the adsorption capacity; then the adsorption capacity decreases due to the aggregation of colloidal particles at high adsorbent dosage [50]. Optimum adsorbent dosage is found to be 5 mg/mL and is fixed for the remaining studies.


## 4.3.4. Effect of Ionic Strength

Ionic strength affects electrostatic attraction between ionic adsorbates and charged surface adsorbent. For adsorption of Cu2+ onto SML, ionic strength also induces a change of existence of SDS molecules on laterite surface. As can be seen in Figure 9, the removal efficiency is lowest in the absence of salt, demonstrating that the electrostatic interaction causes adsorption. Furthermore, adsorption at 50 mM NaCl concentration is lower than that at 10 mM. This result can be explained by desorption of SDS at high salt concentration. Desorption of SDS is enhanced with increasing NaCl concentration from 10 to 50 mM. When increasing salt, the concentration of Cl− is high so the removal is decreased. However, the removal increases with an increase of NaCl concentration from 1 to 10 mM, suggesting that not only electrostatic but also nonelectrostatic between Cu(II) and/or Cu(II) speciation and organic substances in laterite induces adsorption. The effect of salt concentration will be discussed in the adsorption isotherms section.Figure 9
The removal of Cu2+ by SML at different NaCl concentrations (Ci (Cu2+) = 10 ppm; pH 6; contact time 90 min; adsorbent dosage 5 mg/mL).


## 4.4. Characteristics and Mechanisms of Copper Ion Adsorption Isotherms on SDS Modified Laterite

### 4.4.1. Adsorption Isotherms of Copper Ion on SDS Modified Laterite by a Two-Step Adsorption Model

The effect of ionic strength on adsorption of copper ion on SDS modified laterite (SML) is clearly demonstrated in the isotherms (Figure10). At pH 6, the Cu2+ adsorption capacity increases with increasing NaCl from 1 to 10 mM. At higher salt concentration (from 10 to 50 mM), the adsorption capacity reduces with an increase of salt. The increase in salt concentration increases the number of cations Na+ (counterions) on the negatively charged layer of SML, reducing the electrostatic effect of SML Cu2+ ion. It is quite different from the case of adsorption of copper ion on strawberry leaf powder [51] in which the effect of Na+, K+, Mg2+, and Ca2+ on the copper adsorption is not significant. In our case, the electrostatic attraction between the positive charge of Cu2+ ion and negative charge of SML is effectively screened by increasing NaCl concentrations from 10 to 50 mM. Nonelectrostatic interactions are more important in adsorption at low ionic strength (1 to 10 mM NaCl). These results agree well with the effect of ionic strength on the removal of copper ion using SML shown in previous section.Figure 10
Adsorption isotherms of Cu2+ onto SDS modified laterite (SML) as a function of equilibrium concentration of Cu2+ at different NaCl concentrations. Points are experimental data; solid lines are results of the 2-step adsorption model.Figure10 indicates that, at different salt concentrations, the experimental results can be represented well by the general isotherm equation (2) with the fitting parameters in Table 2.Table 2
The fit parameters for Cu2+ adsorption onto SML which are maximum adsorbed amount Γ∞,Cu2+, the equilibrium constants k1,Cu2+ and k2,Cu2+ for first layer adsorption and multilayer adsorption, respectively, and the number of cluster of Cu2+ ion nCu2+.
C salt(mM NaCl)
Γ
∞
,
C
u
2
+  (mg/g)
k
1
,
C
u
2
+  (g/mg)
k
2
,
C
u
2
+  (g/mg)n-1
n
C
u
2
+
1
150
3.0
×
10
2
8.0
×
10
2
2.4
10
185
5.0
×
10
2
8.0
×
10
2
2.5
50
110
1.5
×
10
2
7.5
×
10
2
2.5As shown in Table2, increasing ionic strength from 1 to 10 mM causes an increase in k1,Cu2+ but a decrease in k1,Cu2+ when increasing salt from 10 to 50 mM. It is noted that the changes in k2,Cu2+ and the number of Cu2+ clusters are not significant (k2,NH4+≈8.0×102 and nCu2+≈2.5). The value of k1,Cu2+ is also related to the slope of isotherm. As a result, at high salt concentration, the slope is lower than that at low salt. It suggests that desorption of SDS from the bilayer of admicelles is enhanced by increasing salt concentration from 10 to 50 mM while at low salt concentration the link between sulfate groups of SDS an Cu2+ is not strong as other interactions.


### 4.4.2. Adsorption Mechanisms and Advantages of Copper Ion onto SDS Modified Laterite

Adsorptive removal of copper ion (Cu2+) is enhanced by using anionic surfactant, SDS modified laterite (SML). The effective conditions on adsorption of Cu2+ ion by SML were systematically studied in Section 4.3. The two-step model was established to describe the Cu2+ adsorption onto SML, suggesting that adsorption of Cu2+ could replace admicellar bilayers of SDS molecules on the laterite surface at high salt concentration. Adsorption of Cu2+ decreases with an increase of ionic strength at high salt concentration because desorption of SDS is enhanced by increasing salt concentration [35] which is in good agreement with discussion in Section 4.3.1. It is also represented by two-step adsorption model with decreasing k1,Cu2+ when increasing NaCl concentration from 10 to 50 mM. At low salt concentration, the main interactions inducing adsorption may be lateral, hydrogen bonding, surface complexation, and Van der Waals interactions between Cu2+ and organic substances in the laterite.Adsorption mechanisms of Cu2+ ion on SML are also supported by the results of FT-IR spectra (see Section 4.1.2). The bands of CH2– of SML did not occur after adsorption of Cu2+ in 10 mM NaCl, suggesting that the adsorption of Cu2+ onto SML is mainly controlled by the electrostatic attraction between the negatively charged sulfate groups of bilayer admicelles and positive charge of Cu2+. It seems to be similar to the case of Cu2+ on natural kaolinite clays [48]. Nevertheless, at low ionic strength, the attraction between positive Cu2+ and negative SML is not so strong compared with other interactions. In this case, nonelectrostatic interactions between Cu2+ and/or speciation of Cu2+ with organic substances in laterite mainly contribute to adsorption.Some published papers focused on sorption of Cu(II) onto different kinds of sorbents. Nevertheless, sorption of Cu(II) onto SML has not been reported. Furthermore, SML used in the present study has very high adsorption capacity (185 mg/g) compared with other natural sorbents or modified natural materials (Table3). It demonstrates that SML is novel material for removal of Cu(II) from aqueous solution.Table 3
Adsorption capacity of Cu2+ on natural materials or modified natural materials and SML.
Adsorbent
Adsorption capacity (mg/g)
References
Zeolite, clinoptilolite
1.64
[12]
HCl-treated clay
83.3
[42]
Marine alga,Ecklonia maxima
90
[43]
Red paddy soil
0.21
[44]
SDS modified laterite
185
This study


## 4.4.1. Adsorption Isotherms of Copper Ion on SDS Modified Laterite by a Two-Step Adsorption Model

The effect of ionic strength on adsorption of copper ion on SDS modified laterite (SML) is clearly demonstrated in the isotherms (Figure10). At pH 6, the Cu2+ adsorption capacity increases with increasing NaCl from 1 to 10 mM. At higher salt concentration (from 10 to 50 mM), the adsorption capacity reduces with an increase of salt. The increase in salt concentration increases the number of cations Na+ (counterions) on the negatively charged layer of SML, reducing the electrostatic effect of SML Cu2+ ion. It is quite different from the case of adsorption of copper ion on strawberry leaf powder [51] in which the effect of Na+, K+, Mg2+, and Ca2+ on the copper adsorption is not significant. In our case, the electrostatic attraction between the positive charge of Cu2+ ion and negative charge of SML is effectively screened by increasing NaCl concentrations from 10 to 50 mM. Nonelectrostatic interactions are more important in adsorption at low ionic strength (1 to 10 mM NaCl). These results agree well with the effect of ionic strength on the removal of copper ion using SML shown in previous section.Figure 10
Adsorption isotherms of Cu2+ onto SDS modified laterite (SML) as a function of equilibrium concentration of Cu2+ at different NaCl concentrations. Points are experimental data; solid lines are results of the 2-step adsorption model.Figure10 indicates that, at different salt concentrations, the experimental results can be represented well by the general isotherm equation (2) with the fitting parameters in Table 2.Table 2
The fit parameters for Cu2+ adsorption onto SML which are maximum adsorbed amount Γ∞,Cu2+, the equilibrium constants k1,Cu2+ and k2,Cu2+ for first layer adsorption and multilayer adsorption, respectively, and the number of cluster of Cu2+ ion nCu2+.
C salt(mM NaCl)
Γ
∞
,
C
u
2
+  (mg/g)
k
1
,
C
u
2
+  (g/mg)
k
2
,
C
u
2
+  (g/mg)n-1
n
C
u
2
+
1
150
3.0
×
10
2
8.0
×
10
2
2.4
10
185
5.0
×
10
2
8.0
×
10
2
2.5
50
110
1.5
×
10
2
7.5
×
10
2
2.5As shown in Table2, increasing ionic strength from 1 to 10 mM causes an increase in k1,Cu2+ but a decrease in k1,Cu2+ when increasing salt from 10 to 50 mM. It is noted that the changes in k2,Cu2+ and the number of Cu2+ clusters are not significant (k2,NH4+≈8.0×102 and nCu2+≈2.5). The value of k1,Cu2+ is also related to the slope of isotherm. As a result, at high salt concentration, the slope is lower than that at low salt. It suggests that desorption of SDS from the bilayer of admicelles is enhanced by increasing salt concentration from 10 to 50 mM while at low salt concentration the link between sulfate groups of SDS an Cu2+ is not strong as other interactions.


## 4.4.2. Adsorption Mechanisms and Advantages of Copper Ion onto SDS Modified Laterite

Adsorptive removal of copper ion (Cu2+) is enhanced by using anionic surfactant, SDS modified laterite (SML). The effective conditions on adsorption of Cu2+ ion by SML were systematically studied in Section 4.3. The two-step model was established to describe the Cu2+ adsorption onto SML, suggesting that adsorption of Cu2+ could replace admicellar bilayers of SDS molecules on the laterite surface at high salt concentration. Adsorption of Cu2+ decreases with an increase of ionic strength at high salt concentration because desorption of SDS is enhanced by increasing salt concentration [35] which is in good agreement with discussion in Section 4.3.1. It is also represented by two-step adsorption model with decreasing k1,Cu2+ when increasing NaCl concentration from 10 to 50 mM. At low salt concentration, the main interactions inducing adsorption may be lateral, hydrogen bonding, surface complexation, and Van der Waals interactions between Cu2+ and organic substances in the laterite.Adsorption mechanisms of Cu2+ ion on SML are also supported by the results of FT-IR spectra (see Section 4.1.2). The bands of CH2– of SML did not occur after adsorption of Cu2+ in 10 mM NaCl, suggesting that the adsorption of Cu2+ onto SML is mainly controlled by the electrostatic attraction between the negatively charged sulfate groups of bilayer admicelles and positive charge of Cu2+. It seems to be similar to the case of Cu2+ on natural kaolinite clays [48]. Nevertheless, at low ionic strength, the attraction between positive Cu2+ and negative SML is not so strong compared with other interactions. In this case, nonelectrostatic interactions between Cu2+ and/or speciation of Cu2+ with organic substances in laterite mainly contribute to adsorption.Some published papers focused on sorption of Cu(II) onto different kinds of sorbents. Nevertheless, sorption of Cu(II) onto SML has not been reported. Furthermore, SML used in the present study has very high adsorption capacity (185 mg/g) compared with other natural sorbents or modified natural materials (Table3). It demonstrates that SML is novel material for removal of Cu(II) from aqueous solution.Table 3
Adsorption capacity of Cu2+ on natural materials or modified natural materials and SML.
Adsorbent
Adsorption capacity (mg/g)
References
Zeolite, clinoptilolite
1.64
[12]
HCl-treated clay
83.3
[42]
Marine alga,Ecklonia maxima
90
[43]
Red paddy soil
0.21
[44]
SDS modified laterite
185
This study


## 5. Conclusions

For the first time, we investigated adsorption characteristics of copper ion (Cu2+) on sodium dodecyl sulfate (SDS) surfactant modified laterite (SML) in aqueous solution. The adsorption experiments were quantified by flame atomic absorption spectrometry (F-AAS). The raw laterite, laterite after preadsorption of SDS, and sequential adsorption of Cu2+ were characterized by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). The optimum conditions for adsorptive removal of Cu2+ using SML were found as pH 6, contact time 90 min, adsorbent dosage 5 mg/mL, and ionic strength 10 mM NaCl. Adsorption isotherms of Cu2+ on SML at different NaCl concentrations were fitted well by a two-step adsorption model. The adsorption of Cu2+ on SML decreased with an increase of NaCl concentration due to the enhancement of SDS desorption with increasing salt concentration but at low NaCl concentrations (1 to 10 mM), adsorption Cu2+ on SML increased with an increase of salt. Adsorption mechanisms of Cu2+ onto SML were mainly controlled by electrostatic attraction between the positive charge of Cu2+ and the negatively charged layer SML at high ionic strength while at low ionic strength nonelectrostatic interactions between Cu2+ and organic compounds in laterite induced adsorption. The SML was demonstrated as a novel adsorbent to remove Cu2+ from aqueous solutions.



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