# Utilization of Macadamia Nutshell Residue for the Synthesis of Magnetic Activated Carbon toward Zinc (II) Ion Removal

Authors: Minh-Trung Dao; Thi-Phuong-Linh Tran; Duc-Thuong Vo; Van-Kieu Nguyen; Le-Thuy-Thuy-Trang Hoang
Journal: Advances in Materials Science and Engineering
 (2021)

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## Abstract

In this study, macadamia nutshell residue, a prevalent leftover and green agricultural waste in Vietnam, was utilized to prepare a magnetic activated carbon adsorbent. The obtained material was characterized by its surface functionalities, elemental composition, crystalline structure, and magnetic properties. The characterization results revealed that the composite comprised Fe3O4 nanoparticles attached to the carbon matrix. The saturation magnetization (Ms) of the composite was found to be 38.2 emu g−1, indicating a convenient separation of the solid adsorbent from aqueous media using an external magnetic field. The feasibility of removing zinc (II) ion from an aqueous solution of the activated carbon/Fe3O4 (AC/Fe3O4) composite was examined. The adsorption kinetics were best explained by the Elovich model and the pseudo-second-order model. The adsorption capacity at equilibrium and the initial rate of Zn2+ adsorption determined by the pseudo-second-order model were 22.73 mg g−1 and 4.18 mg g−1 min−1, respectively. The implications of this study are that a low-cost, green, and magnetically separable material prepared by a large-scale available solid waste can be a promising adsorbent for the elimination of heavy metals.


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## Body

## 1. Introduction

Ceaseless industrialization and population growth across the world have led to a severe deterioration in water quality [1]. The use of chemicals and industrial compounds has continued to increase, posing a significant pollution risk to existing watercourses. Water pollution is a global environmental challenge that destroys the food sources and contaminates drinking water, leading to immediate and long-term effects on human health [2]. Many water pollutants are reported to act as toxic chemicals, including dyes, antibiotics, heavy metals, fertilizers, and pesticides [1, 2]. Heavy metal pollution is a major water pollution hazard and presents a serious risk to human health and aquatic environments [3, 4]. The release wastewater can contain various heavy metals, for instance, nickel, mercury, lead, cadmium, zinc, arsenic, and copper [5]. Zinc is among the metals that have a widespread use in galvanization, electroplating, and the manufacture of alloys, allowing it to accumulate in the environment [6]. This element is essential for living organisms as playing a central role in the immune system and taking part of many metal-enzymes and metal-proteins; however, its excess in water can cause eminent health problems, such as stomach cramps, skin irritations, vomiting, nausea, and anemia [6, 7]. Therefore, for many nations, the search for effective heavy metal treatments, especially for zinc ion, is a priority among their environmental and sustainable development strategies.In order to eliminate heavy metals, various physicochemical techniques have conventionally employed, including chemical precipitation [8], coagulation and flocculation [9], electrochemical method [10], adsorption [11], ion exchange [12], and reverse osmosis [12]. Among them, adsorption has attracted considerable attention from scientists due to its cost-effectiveness, environmental friendliness, and easy operation [13, 14]. Many adsorbents have been developed, including zeolite [15], metal-organic frame materials [16], layered double hydroxides [14], graphene [13], nano- and mesoporous silica [17], and activated carbon (AC) [18]. In particular, AC is popular and widely applied due to its chemical stability, high surface area, and microporous structure [19]. However, the use of commercial AC has the disadvantage of being expensive due to the relatively high cost of its starting materials (wood or coal) [20]. Thus, it is highly desirable to prepare a low-cost, efficient, and locally available activated carbon, derived from renewable biomass materials [21]. Macadamia nuts, a high-value agricultural product, have been grown worldwide since they were first discovered in Australia in 1857. Approximately 70–77% of a macadamia nut comprises its shell. With the rapidly increasing demand for macadamia kernels across the globe, their leftover nutshells have come to represent a large amount of solid residue, creating a serious waste disposal problem. One promising process of recycling macadamia nutshells is to produce biochar, which can be used either for heating purposes or to prepare activated carbon thanks to these shells’ high level of carbon content [22]. However, whereas the role of other agricultural wastes in producing AC has been extensively investigated, macadamia nutshell residue has received limited attention [23]. Moreover, in Vietnam, there remains little study on the potential of macadamia nutshells as AC-based materials even though the production of these nuts has increased rapidly here each year.The use of AC in water treatments has also faced difficulties in the recovery of solid adsorbents from aqueous media [24]. In recent years, a combination of magnetic materials and AC to facilitate the separation via the use of an external magnetic field has emerged as an economic and efficient choice to offset this problem [24, 25]. AC/iron oxide composites are among the magnetically separable adsorbents that serve as effective adsorbents for heavy metals removal [26]. To the best of our knowledge, the study on magnetic AC sorbent for removal of zinc (II) ion is still limited. Therefore, the work reported here sought to prepare an AC/Fe3O4 composite derived from Vietnamese macadamia nutshell residue in order to explore its feasibility for zinc (II) ion removal. The structural, textural, and magnetic properties of the synthesized material were characterized. The effects of initial pH, adsorbent dose, adsorption kinetics, and adsorption mechanism were also determined.


## 2. Materials and Methods

### 2.1. Materials

The chemicals used were ferrous sulfate heptahydrate (FeSO4.7H2O), iron(III) chloride hexahydrate (FeCl3.6H2O), sodium hydroxide (NaOH), hydrochloric acid (HCl), and anhydrous potassium carbonate (K2CO3). They were analytical grade and supplied by Merck, Germany. Macadamia nutshell residue was collected as biomass waste in Bao Lam district, Lam Dong province, Vietnam.


### 2.2. Synthesis of AC from Macadamia Nutshells Using K2CO3

AC derived from macadamia nutshells was synthesized based on the method of Dao et al. [27]. Typically, the cleaned and dried nutshells were mechanically ground into a powder. They were then immersed in K2CO3 solution for 24 h using an impregnation of 1.0. The impregnation ratio was the weight of K2CO3 to the weight of the macadamia nutshells. The obtained solid was washed with deionized water and calcined at 650°C for 1 h. The fine powder was then stored in a vacuum for subsequent experiments. The whole preparation process of magnetic AC is illustrated in Figure 1.Figure 1
Schematic representation of the synthesis of magnetic AC derived from macadamia nutshells.


### 2.3. Synthesis of Magnetic AC

The synthesis of magnetic AC was performed based on the method of Han et al. [28], with some modifications. Typically 100 mL of 0.13 M ferrous sulfate and 100 mL of 0.25 M ferric chloride solutions were mixed to obtain an Fe2+/Fe3+ molar ratio of 1 : 2. Subsequently, AC (5 g) was added, and the mixture was heated to 65°C. Having cooled the mixture to 40°C, 5M sodium hydroxide solution was added until the solution’s pH reached 10.0. After stirring for another 1 h, the mixture was impregnated overnight. The precipitate was then washed with deionized water until a pH of 7 was reached and dried at 80°C.


### 2.4. Material Characterizations

The crystalline phases of the composite were identified via X-ray diffraction (XRD) using an X-ray diffractometer (Equinox 5000, Thermo Scientific, France) equipped with a Cu Kα radiation source (λ = 1.5406 Å). The diffractometer was set at an accelerating voltage of 35 kV, an applied current of 25 mA, and the step size of 0.015. The surface morphology of the materials and elemental mapping were monitored by scanning electron microscopy equipped with an energy dispersive X-ray (SEM-EDX; S4800, Hitachi, Japan, at 1.0–5.0 keV). Transmission electron microscopy (TEM) images were obtained using a JEOL JEM-1010 microscope, Japan, at 80 kV. The Fourier-transform infrared spectroscopy (FTIR) spectra were recorded using an FTIR/NIR spectrometer (Frontier/PerkinElmer, USA). The specific surface areas of the materials were measured via the N2 adsorption-desorption method using a Micromeritics-TriStar II Plus instrument.


### 2.5. Adsorption Experiments

The adsorption of zinc ion on magnetic AC was carried out using batch experiments. Each bottle containing 15 mg adsorbents and 50 mL metal ion solution was stirred at 150 rpm. All the experiments were conducted in triplicate with an initial Zn2+ concentration of 25 ppm. The sample solutions were collected by separating the solid through centrifugation. The effect of the solution pH (adsorbent dose of 0.3 g L−1 and adsorption time of 60 min) was studied in the initial pH range of 2–5. The solution pH was adjusted by adding 0.1 M NaOH or 0.1 M HCl solutions. To study the effect of adsorbent dose, the dose was varied from 0.2 to 1.8 g L−1 at a pH of 4 and an adsorption time of 60 min. The sorption kinetics were evaluated at various sorption times (20–120 min) with an initial zinc concentration of 25 ppm and an adsorbent dose of 1.4 g L−1 at a pH of 4. After being stirred continuously for a certain amount of time, the solutions were collected and the equilibrium concentration of zinc ion was determined using an atomic absorption spectrometer (ContraAA800D, Germany). The adsorption capacity (mg g−1) and the removal percentage (%) of metal ion were calculated by the following equation [11, 29]:(1)qe=C0−CeVm,removal%=C0−CeC0×100,where C0 and Ce (mg L−1) are the initial concentration and the equilibrium concentration of Zn(II), respectively, while mg is the adsorbent mass and V (L) is the solution volume.


## 2.1. Materials

The chemicals used were ferrous sulfate heptahydrate (FeSO4.7H2O), iron(III) chloride hexahydrate (FeCl3.6H2O), sodium hydroxide (NaOH), hydrochloric acid (HCl), and anhydrous potassium carbonate (K2CO3). They were analytical grade and supplied by Merck, Germany. Macadamia nutshell residue was collected as biomass waste in Bao Lam district, Lam Dong province, Vietnam.


## 2.2. Synthesis of AC from Macadamia Nutshells Using K2CO3

AC derived from macadamia nutshells was synthesized based on the method of Dao et al. [27]. Typically, the cleaned and dried nutshells were mechanically ground into a powder. They were then immersed in K2CO3 solution for 24 h using an impregnation of 1.0. The impregnation ratio was the weight of K2CO3 to the weight of the macadamia nutshells. The obtained solid was washed with deionized water and calcined at 650°C for 1 h. The fine powder was then stored in a vacuum for subsequent experiments. The whole preparation process of magnetic AC is illustrated in Figure 1.Figure 1
Schematic representation of the synthesis of magnetic AC derived from macadamia nutshells.


## 2.3. Synthesis of Magnetic AC

The synthesis of magnetic AC was performed based on the method of Han et al. [28], with some modifications. Typically 100 mL of 0.13 M ferrous sulfate and 100 mL of 0.25 M ferric chloride solutions were mixed to obtain an Fe2+/Fe3+ molar ratio of 1 : 2. Subsequently, AC (5 g) was added, and the mixture was heated to 65°C. Having cooled the mixture to 40°C, 5M sodium hydroxide solution was added until the solution’s pH reached 10.0. After stirring for another 1 h, the mixture was impregnated overnight. The precipitate was then washed with deionized water until a pH of 7 was reached and dried at 80°C.


## 2.4. Material Characterizations

The crystalline phases of the composite were identified via X-ray diffraction (XRD) using an X-ray diffractometer (Equinox 5000, Thermo Scientific, France) equipped with a Cu Kα radiation source (λ = 1.5406 Å). The diffractometer was set at an accelerating voltage of 35 kV, an applied current of 25 mA, and the step size of 0.015. The surface morphology of the materials and elemental mapping were monitored by scanning electron microscopy equipped with an energy dispersive X-ray (SEM-EDX; S4800, Hitachi, Japan, at 1.0–5.0 keV). Transmission electron microscopy (TEM) images were obtained using a JEOL JEM-1010 microscope, Japan, at 80 kV. The Fourier-transform infrared spectroscopy (FTIR) spectra were recorded using an FTIR/NIR spectrometer (Frontier/PerkinElmer, USA). The specific surface areas of the materials were measured via the N2 adsorption-desorption method using a Micromeritics-TriStar II Plus instrument.


## 2.5. Adsorption Experiments

The adsorption of zinc ion on magnetic AC was carried out using batch experiments. Each bottle containing 15 mg adsorbents and 50 mL metal ion solution was stirred at 150 rpm. All the experiments were conducted in triplicate with an initial Zn2+ concentration of 25 ppm. The sample solutions were collected by separating the solid through centrifugation. The effect of the solution pH (adsorbent dose of 0.3 g L−1 and adsorption time of 60 min) was studied in the initial pH range of 2–5. The solution pH was adjusted by adding 0.1 M NaOH or 0.1 M HCl solutions. To study the effect of adsorbent dose, the dose was varied from 0.2 to 1.8 g L−1 at a pH of 4 and an adsorption time of 60 min. The sorption kinetics were evaluated at various sorption times (20–120 min) with an initial zinc concentration of 25 ppm and an adsorbent dose of 1.4 g L−1 at a pH of 4. After being stirred continuously for a certain amount of time, the solutions were collected and the equilibrium concentration of zinc ion was determined using an atomic absorption spectrometer (ContraAA800D, Germany). The adsorption capacity (mg g−1) and the removal percentage (%) of metal ion were calculated by the following equation [11, 29]:(1)qe=C0−CeVm,removal%=C0−CeC0×100,where C0 and Ce (mg L−1) are the initial concentration and the equilibrium concentration of Zn(II), respectively, while mg is the adsorbent mass and V (L) is the solution volume.


## 3. Results and Discussion

### 3.1. Characterizations of the Material

The crystal structure of the obtained AC/Fe3O4 composite was determined using the XRD technique. The XRD pattern of the composite (Figure 2) was analyzed using JCPDS, with all the peaks matching the reported magnetite pattern (PDF no. 01-071-6336). The pattern appeared in the form of broad bands, possibly due to the low iron content and/or the small crystalline size of the magnetite formation dispersed over the carbon matrix. The existence of magnetite in the composite was also confirmed by the vibration peak of Fe-O at about 584 cm−1 [30] in the FTIR spectrum (Figure 3). The characteristic vibration peaks corresponding to AC were clearly observable, including the hydroxyl–OH group (3,635 cm−1), carbonyl C=O (1,722 cm−1), C=C (1,573 cm−1), and C-O (1,260 cm−1).Figure 2
XRD pattern of the AC/Fe3O4 composite.Figure 3
FTIR spectrum of the AC/Fe3O4 composite.In addition to the structure and functional groups, surface morphology of materials is known to play an important role in the application processes [31]. TEM and SEM-EDX techniques were then employed to study the morphology and the distribution of each component in the composites. The qualitative analysis conducted using SEM (Figure 4(a)) revealed that the composite possessed a rough and irregular surface. The presence of Fe and O elements by elemental mapping using EDX is presented in Figures 4(b) and 4(c), respectively. These elements appeared throughout the composite, indicating a high dispersion of Fe3O4 over the material. The TEM images (Figures 5(a) and 5(b)) further confirmed the attachment of Fe3O4 to the AC matrix.Figure 4
SEM image (a) and EDX elemental mapping of Fe (b) and O (c) of the AC/Fe3O4 composite.
(a)(b)(c)Figure 5
TEM images of the AC-Fe3O4 composite (a) and (b).
(a)(b)The surface area of the sample was determined using nitrogen adsorption-desorption analysis. The composite possessed a specific surface area of 72.23 m2 g−1 according to the Brunauer–Emmett–Teller (BET) method. This result is similar to the specific surface area of 70.06 m2 g−1 of the AC/iron oxide composite obtained by Wongcharee et al. [32]. In agreement with Wongcharee et al. [32], these low values may owe to the filling of magnetite particles into some pore structures of AC. This may reduce the adsorption capacity of parent AC; however, the dispersion of the magnetic material over the AC matrix could be desirable for a convenient separation.In order to examine the magnetic properties of the composite, the magnetization hysteresis curve was recorded at room temperature (Figure6). The nearly zero values of magnetic remanence and coercivity in the hysteresis exhibited the superparamagnetic property of the composite [11]. Such behavior is to be expected for spherical magnetite nanoparticles with a diameter of approximately 10 nm [33]. The saturation magnetization (Ms) was found to be 38.2 emu g−1. The superparamagnetic nature allowed the convenient adsorbent recovery of the spent composite from treated media using an external magnetic field. These results indicate that the AC/Fe3O4 composite was prepared successfully.Figure 6
Magnetization hysteresis curve of the AC/Fe3O4 composite.


### 3.2. Sorption Study

The adsorption of zinc (II) ion on the AC/Fe3O4 composite was examined using batch experiments. The adsorption properties of the sorbent including the effects of initial pH, adsorbent dose, adsorption kinetics, and adsorption mechanism were investigated.


#### 3.2.1. Effect of pH

pH is an important variable influencing the binding of heavy metal ions onto the surfaces of adsorbents [34]. The effect of pH on the adsorption of zinc (II) ion was therefore studied in the initial pH range of 2.0–5.0. The increase in pH from 2.0 to 3.5 was seen to lead to a rapid enhancement in the extent of metal ion removal (Figure 7). The further increase in pH from 3.5 to 5.0 caused a slow enhancement in the removal percentage, reaching a maximum value at a pH of 4.0. These behaviors can be explained based on the interaction between the adsorbent and the adsorbate that the oxygenic functional groups of AC/Fe3O4 composites can interact with Zn2+ to form metal complexes [35]. At low pH, the H+ ion may compete with Zn2+ and occupy the surface of the adsorbent, thereby causing the metal adsorption to become more unfavorable. Surface complexation is expected to be predominant mechanism in the adsorption process in this study. The mechanism of cadmium adsorption will be discussed hereafter.Figure 7
The effect of initial pH.


#### 3.2.2. Effect of Adsorbent Dose

Studying the effect of adsorbent dose on adsorption plays a crucial role in large-scale applications. It is valuable not only for minimizing adsorption costs but also for avoiding the excessive use of adsorbents during wastewater treatment [36]. The effect of adsorbent dose on the removal of zinc ion is demonstrated in Figure 8. The removal percentage capacity enhanced as the dose increased from 0.2 g L−1 to 1.4 g L−1. This behavior can be attributed to the increase in the number of accessible active sites for zinc (II) ion. When the adsorbent dose further increased from 1.4 g L−1 to 1.8 g L−1, there was no statistically significant difference in removal efficiencies (one-way analysis of variance, P = 0.99) at a 95% confidence interval (α = 0.05). This could be explained by the agglomeration of sorbent caused from high dose of sorbent in solution, resulting in a saturation of active sites and a surface equilibrium state of the adsorption [37].Figure 8
The effect of adsorbent dose.


#### 3.2.3. Adsorption Kinetics

The field of adsorption kinetics studies changes in the particular adsorbent/adsorbate system over time that can determine the reaction rate, the factors that affect the rate, and the probable mechanisms involved [38]. The adsorption of Zn2+ ion by the AC/Fe3O4 composite was observed at different contact times, varying from 20 min to 120 min (Figure 9). In the first 40 min, the adsorption process occurred rapidly, reaching equilibrium after 60 min at 298 K.Figure 9
Zinc adsorption kinetics on the AC/Fe3O4 composite.The obtained experimental kinetics data were analyzed using pseudo-first-order kinetics equation (2), pseudo-second-order kinetics equation (3), the Elovich model equation (4), and the intraparticle diffusion kinetics model equation (5) [11, 39, 40].(2)logqe−qt=logqe−k12.303t,(3)tqt=1k2qe2+tqe,(4)q1=1βlnαβ+1βlnt,(5)qt=kit1/2+C,where qe (mg g−1) and qt (mg g−1) are the sorption capacities of a target at equilibrium and at a time t, respectively, and k1 (min−1), k2 (g mg−1 min−1), α (mg g−1 min−1), and ki (mg g min−0.5) are the rate constants of the pseudo-first-order, the pseudo-second-order, the Elovich kinetics model equation, and intraparticle diffusion, respectively. β (mg g−1) is the desorption constant in the Elovich equation, and C is related to the boundary layer thickness.The linear plotting of the four models is presented in Figure10, and the obtained kinetics model parameters are listed in Table 1. The Elovich model was found to be the best fit of the experimental data, followed closely by the pseudo-second-order model. The pseudo-first-order and intraparticle diffusion kinetics models did not fit well with the kinetics data. The close values of adsorption capacity at equilibrium calculated using the pseudo-second-order model (qe,cal = 22.73 mg g−1) and the experimental value (qe,exp = 24.02 mg g−1) indicated the good fit of the model to the experimental data. The initial rate (h = k2qe2) of Zn2+ adsorption on ACP/Fe3O4 was calculated to be 4.18 mg g−1 min−1. The good fit of the kinetics data with the Elovich model is suitable for a system with a heterogeneous adsorbing surface, like the AC/Fe3O4 composite in this study. Moreover, this model is related to chemisorption, indicating that the surface complexation can be a predominant mechanism of the adsorption process.Figure 10
Pseudo-first-order model (a), pseudo-second-order model (b), Elovich kinetics model (c), and intraparticle diffusion kinetics model (d) for the adsorption of Zn2+ ion by the AC/Fe3O4 composite.
(a)(b)(c)(d)Table 1
Kinetics model parameters for the adsorption of Zn2+ ion by the ACP/Fe3O4 composite.
ModelsKinetics model parameters and regression coefficientsPseudo-first-orderk1 (min−1)qe (mg g−1)R20.03019.500.970Pseudo-second-orderk2 (g mg−1 min−1)qe (mg g−1)R20.008122.730.993Elovichα (mg g−1 min−1)β (mg g−1)R22.760.0550.998Intraparticle diffusionki (mg g min−0.5)CR21.0711.320.824The optimum adsorption conditions and removal efficiency of AC/Fe3O4 composite in Zn2+ ion adsorption are compared with previously reported studies (Table 2). It can be observed that the results obtained in this study are comparable with the literature under similar conditions. Moreover, the adsorbent in this study has the advantage of possessing higher pseudo-second-order rate constant compared with many other materials. These results make AC/Fe3O4 composite an interesting material with respect to its environmentally friendliness, cost-effectiveness, convenient separation, and relatively high adsorption efficiency.Table 2
Comparison of adsorption conditions and adsorption efficiency of various materials for Zn2+ removal.
MaterialspHAdsorbent dosage (g L−1)Contact time (min)Initial concentration of Zn2+ (ppm)Adsorption capacity (mg g−1)Re (%)Pseudo-second-order parametersRef.qe,cal (mg g−1)k2 (g mg−1 min−1)Zeolites—518020036.76—34.720.002[41]Fish bone5.01872020—981.930.00023∗[42]Palm oil mill effluent-based AC5.54∗5035059.88—29.500.0020[43]Tire-derived AC—4∗360∗50071.9—50.250.0057[35]Commercial AC—50∗1440∗50014.0—5.570.0019[35]AC/Fe3O44.06∗18010—70.5——[44]AC/Fe3O44.0–5.01.46025—9022.730.0081This study∗calculated from the published papers.


#### 3.2.4. Proposed Mechanism of the Adsorption Process

Depending on the properties of metal ions, surface charges, pore sizes, and functional groups of adsorbents, there may be different interactions governing adsorption of heavy metals on AC-based materials [45]. In this study, the results obtained supported the hypothesis that surface complexation should be the predominant mechanism of the adsorption process. A proposed mechanism is presented in Figure 11. AC/Fe3O4 composite contains carboxylic and hydroxyl groups that can serve as ligand-like surface functional groups. These oxygenic functional groups can interact with divalent zinc (II) ion to form specific metal complexes. This facilitates the stabilization of Zn2+ on the surface of the adsorbent and can then be recovered by an external magnet.Figure 11
A proposed mechanism of the adsorption process.


## 3.1. Characterizations of the Material

The crystal structure of the obtained AC/Fe3O4 composite was determined using the XRD technique. The XRD pattern of the composite (Figure 2) was analyzed using JCPDS, with all the peaks matching the reported magnetite pattern (PDF no. 01-071-6336). The pattern appeared in the form of broad bands, possibly due to the low iron content and/or the small crystalline size of the magnetite formation dispersed over the carbon matrix. The existence of magnetite in the composite was also confirmed by the vibration peak of Fe-O at about 584 cm−1 [30] in the FTIR spectrum (Figure 3). The characteristic vibration peaks corresponding to AC were clearly observable, including the hydroxyl–OH group (3,635 cm−1), carbonyl C=O (1,722 cm−1), C=C (1,573 cm−1), and C-O (1,260 cm−1).Figure 2
XRD pattern of the AC/Fe3O4 composite.Figure 3
FTIR spectrum of the AC/Fe3O4 composite.In addition to the structure and functional groups, surface morphology of materials is known to play an important role in the application processes [31]. TEM and SEM-EDX techniques were then employed to study the morphology and the distribution of each component in the composites. The qualitative analysis conducted using SEM (Figure 4(a)) revealed that the composite possessed a rough and irregular surface. The presence of Fe and O elements by elemental mapping using EDX is presented in Figures 4(b) and 4(c), respectively. These elements appeared throughout the composite, indicating a high dispersion of Fe3O4 over the material. The TEM images (Figures 5(a) and 5(b)) further confirmed the attachment of Fe3O4 to the AC matrix.Figure 4
SEM image (a) and EDX elemental mapping of Fe (b) and O (c) of the AC/Fe3O4 composite.
(a)(b)(c)Figure 5
TEM images of the AC-Fe3O4 composite (a) and (b).
(a)(b)The surface area of the sample was determined using nitrogen adsorption-desorption analysis. The composite possessed a specific surface area of 72.23 m2 g−1 according to the Brunauer–Emmett–Teller (BET) method. This result is similar to the specific surface area of 70.06 m2 g−1 of the AC/iron oxide composite obtained by Wongcharee et al. [32]. In agreement with Wongcharee et al. [32], these low values may owe to the filling of magnetite particles into some pore structures of AC. This may reduce the adsorption capacity of parent AC; however, the dispersion of the magnetic material over the AC matrix could be desirable for a convenient separation.In order to examine the magnetic properties of the composite, the magnetization hysteresis curve was recorded at room temperature (Figure6). The nearly zero values of magnetic remanence and coercivity in the hysteresis exhibited the superparamagnetic property of the composite [11]. Such behavior is to be expected for spherical magnetite nanoparticles with a diameter of approximately 10 nm [33]. The saturation magnetization (Ms) was found to be 38.2 emu g−1. The superparamagnetic nature allowed the convenient adsorbent recovery of the spent composite from treated media using an external magnetic field. These results indicate that the AC/Fe3O4 composite was prepared successfully.Figure 6
Magnetization hysteresis curve of the AC/Fe3O4 composite.


## 3.2. Sorption Study

The adsorption of zinc (II) ion on the AC/Fe3O4 composite was examined using batch experiments. The adsorption properties of the sorbent including the effects of initial pH, adsorbent dose, adsorption kinetics, and adsorption mechanism were investigated.


### 3.2.1. Effect of pH

pH is an important variable influencing the binding of heavy metal ions onto the surfaces of adsorbents [34]. The effect of pH on the adsorption of zinc (II) ion was therefore studied in the initial pH range of 2.0–5.0. The increase in pH from 2.0 to 3.5 was seen to lead to a rapid enhancement in the extent of metal ion removal (Figure 7). The further increase in pH from 3.5 to 5.0 caused a slow enhancement in the removal percentage, reaching a maximum value at a pH of 4.0. These behaviors can be explained based on the interaction between the adsorbent and the adsorbate that the oxygenic functional groups of AC/Fe3O4 composites can interact with Zn2+ to form metal complexes [35]. At low pH, the H+ ion may compete with Zn2+ and occupy the surface of the adsorbent, thereby causing the metal adsorption to become more unfavorable. Surface complexation is expected to be predominant mechanism in the adsorption process in this study. The mechanism of cadmium adsorption will be discussed hereafter.Figure 7
The effect of initial pH.


### 3.2.2. Effect of Adsorbent Dose

Studying the effect of adsorbent dose on adsorption plays a crucial role in large-scale applications. It is valuable not only for minimizing adsorption costs but also for avoiding the excessive use of adsorbents during wastewater treatment [36]. The effect of adsorbent dose on the removal of zinc ion is demonstrated in Figure 8. The removal percentage capacity enhanced as the dose increased from 0.2 g L−1 to 1.4 g L−1. This behavior can be attributed to the increase in the number of accessible active sites for zinc (II) ion. When the adsorbent dose further increased from 1.4 g L−1 to 1.8 g L−1, there was no statistically significant difference in removal efficiencies (one-way analysis of variance, P = 0.99) at a 95% confidence interval (α = 0.05). This could be explained by the agglomeration of sorbent caused from high dose of sorbent in solution, resulting in a saturation of active sites and a surface equilibrium state of the adsorption [37].Figure 8
The effect of adsorbent dose.


### 3.2.3. Adsorption Kinetics

The field of adsorption kinetics studies changes in the particular adsorbent/adsorbate system over time that can determine the reaction rate, the factors that affect the rate, and the probable mechanisms involved [38]. The adsorption of Zn2+ ion by the AC/Fe3O4 composite was observed at different contact times, varying from 20 min to 120 min (Figure 9). In the first 40 min, the adsorption process occurred rapidly, reaching equilibrium after 60 min at 298 K.Figure 9
Zinc adsorption kinetics on the AC/Fe3O4 composite.The obtained experimental kinetics data were analyzed using pseudo-first-order kinetics equation (2), pseudo-second-order kinetics equation (3), the Elovich model equation (4), and the intraparticle diffusion kinetics model equation (5) [11, 39, 40].(2)logqe−qt=logqe−k12.303t,(3)tqt=1k2qe2+tqe,(4)q1=1βlnαβ+1βlnt,(5)qt=kit1/2+C,where qe (mg g−1) and qt (mg g−1) are the sorption capacities of a target at equilibrium and at a time t, respectively, and k1 (min−1), k2 (g mg−1 min−1), α (mg g−1 min−1), and ki (mg g min−0.5) are the rate constants of the pseudo-first-order, the pseudo-second-order, the Elovich kinetics model equation, and intraparticle diffusion, respectively. β (mg g−1) is the desorption constant in the Elovich equation, and C is related to the boundary layer thickness.The linear plotting of the four models is presented in Figure10, and the obtained kinetics model parameters are listed in Table 1. The Elovich model was found to be the best fit of the experimental data, followed closely by the pseudo-second-order model. The pseudo-first-order and intraparticle diffusion kinetics models did not fit well with the kinetics data. The close values of adsorption capacity at equilibrium calculated using the pseudo-second-order model (qe,cal = 22.73 mg g−1) and the experimental value (qe,exp = 24.02 mg g−1) indicated the good fit of the model to the experimental data. The initial rate (h = k2qe2) of Zn2+ adsorption on ACP/Fe3O4 was calculated to be 4.18 mg g−1 min−1. The good fit of the kinetics data with the Elovich model is suitable for a system with a heterogeneous adsorbing surface, like the AC/Fe3O4 composite in this study. Moreover, this model is related to chemisorption, indicating that the surface complexation can be a predominant mechanism of the adsorption process.Figure 10
Pseudo-first-order model (a), pseudo-second-order model (b), Elovich kinetics model (c), and intraparticle diffusion kinetics model (d) for the adsorption of Zn2+ ion by the AC/Fe3O4 composite.
(a)(b)(c)(d)Table 1
Kinetics model parameters for the adsorption of Zn2+ ion by the ACP/Fe3O4 composite.
ModelsKinetics model parameters and regression coefficientsPseudo-first-orderk1 (min−1)qe (mg g−1)R20.03019.500.970Pseudo-second-orderk2 (g mg−1 min−1)qe (mg g−1)R20.008122.730.993Elovichα (mg g−1 min−1)β (mg g−1)R22.760.0550.998Intraparticle diffusionki (mg g min−0.5)CR21.0711.320.824The optimum adsorption conditions and removal efficiency of AC/Fe3O4 composite in Zn2+ ion adsorption are compared with previously reported studies (Table 2). It can be observed that the results obtained in this study are comparable with the literature under similar conditions. Moreover, the adsorbent in this study has the advantage of possessing higher pseudo-second-order rate constant compared with many other materials. These results make AC/Fe3O4 composite an interesting material with respect to its environmentally friendliness, cost-effectiveness, convenient separation, and relatively high adsorption efficiency.Table 2
Comparison of adsorption conditions and adsorption efficiency of various materials for Zn2+ removal.
MaterialspHAdsorbent dosage (g L−1)Contact time (min)Initial concentration of Zn2+ (ppm)Adsorption capacity (mg g−1)Re (%)Pseudo-second-order parametersRef.qe,cal (mg g−1)k2 (g mg−1 min−1)Zeolites—518020036.76—34.720.002[41]Fish bone5.01872020—981.930.00023∗[42]Palm oil mill effluent-based AC5.54∗5035059.88—29.500.0020[43]Tire-derived AC—4∗360∗50071.9—50.250.0057[35]Commercial AC—50∗1440∗50014.0—5.570.0019[35]AC/Fe3O44.06∗18010—70.5——[44]AC/Fe3O44.0–5.01.46025—9022.730.0081This study∗calculated from the published papers.


### 3.2.4. Proposed Mechanism of the Adsorption Process

Depending on the properties of metal ions, surface charges, pore sizes, and functional groups of adsorbents, there may be different interactions governing adsorption of heavy metals on AC-based materials [45]. In this study, the results obtained supported the hypothesis that surface complexation should be the predominant mechanism of the adsorption process. A proposed mechanism is presented in Figure 11. AC/Fe3O4 composite contains carboxylic and hydroxyl groups that can serve as ligand-like surface functional groups. These oxygenic functional groups can interact with divalent zinc (II) ion to form specific metal complexes. This facilitates the stabilization of Zn2+ on the surface of the adsorbent and can then be recovered by an external magnet.Figure 11
A proposed mechanism of the adsorption process.


## 3.2.1. Effect of pH

pH is an important variable influencing the binding of heavy metal ions onto the surfaces of adsorbents [34]. The effect of pH on the adsorption of zinc (II) ion was therefore studied in the initial pH range of 2.0–5.0. The increase in pH from 2.0 to 3.5 was seen to lead to a rapid enhancement in the extent of metal ion removal (Figure 7). The further increase in pH from 3.5 to 5.0 caused a slow enhancement in the removal percentage, reaching a maximum value at a pH of 4.0. These behaviors can be explained based on the interaction between the adsorbent and the adsorbate that the oxygenic functional groups of AC/Fe3O4 composites can interact with Zn2+ to form metal complexes [35]. At low pH, the H+ ion may compete with Zn2+ and occupy the surface of the adsorbent, thereby causing the metal adsorption to become more unfavorable. Surface complexation is expected to be predominant mechanism in the adsorption process in this study. The mechanism of cadmium adsorption will be discussed hereafter.Figure 7
The effect of initial pH.


## 3.2.2. Effect of Adsorbent Dose

Studying the effect of adsorbent dose on adsorption plays a crucial role in large-scale applications. It is valuable not only for minimizing adsorption costs but also for avoiding the excessive use of adsorbents during wastewater treatment [36]. The effect of adsorbent dose on the removal of zinc ion is demonstrated in Figure 8. The removal percentage capacity enhanced as the dose increased from 0.2 g L−1 to 1.4 g L−1. This behavior can be attributed to the increase in the number of accessible active sites for zinc (II) ion. When the adsorbent dose further increased from 1.4 g L−1 to 1.8 g L−1, there was no statistically significant difference in removal efficiencies (one-way analysis of variance, P = 0.99) at a 95% confidence interval (α = 0.05). This could be explained by the agglomeration of sorbent caused from high dose of sorbent in solution, resulting in a saturation of active sites and a surface equilibrium state of the adsorption [37].Figure 8
The effect of adsorbent dose.


## 3.2.3. Adsorption Kinetics

The field of adsorption kinetics studies changes in the particular adsorbent/adsorbate system over time that can determine the reaction rate, the factors that affect the rate, and the probable mechanisms involved [38]. The adsorption of Zn2+ ion by the AC/Fe3O4 composite was observed at different contact times, varying from 20 min to 120 min (Figure 9). In the first 40 min, the adsorption process occurred rapidly, reaching equilibrium after 60 min at 298 K.Figure 9
Zinc adsorption kinetics on the AC/Fe3O4 composite.The obtained experimental kinetics data were analyzed using pseudo-first-order kinetics equation (2), pseudo-second-order kinetics equation (3), the Elovich model equation (4), and the intraparticle diffusion kinetics model equation (5) [11, 39, 40].(2)logqe−qt=logqe−k12.303t,(3)tqt=1k2qe2+tqe,(4)q1=1βlnαβ+1βlnt,(5)qt=kit1/2+C,where qe (mg g−1) and qt (mg g−1) are the sorption capacities of a target at equilibrium and at a time t, respectively, and k1 (min−1), k2 (g mg−1 min−1), α (mg g−1 min−1), and ki (mg g min−0.5) are the rate constants of the pseudo-first-order, the pseudo-second-order, the Elovich kinetics model equation, and intraparticle diffusion, respectively. β (mg g−1) is the desorption constant in the Elovich equation, and C is related to the boundary layer thickness.The linear plotting of the four models is presented in Figure10, and the obtained kinetics model parameters are listed in Table 1. The Elovich model was found to be the best fit of the experimental data, followed closely by the pseudo-second-order model. The pseudo-first-order and intraparticle diffusion kinetics models did not fit well with the kinetics data. The close values of adsorption capacity at equilibrium calculated using the pseudo-second-order model (qe,cal = 22.73 mg g−1) and the experimental value (qe,exp = 24.02 mg g−1) indicated the good fit of the model to the experimental data. The initial rate (h = k2qe2) of Zn2+ adsorption on ACP/Fe3O4 was calculated to be 4.18 mg g−1 min−1. The good fit of the kinetics data with the Elovich model is suitable for a system with a heterogeneous adsorbing surface, like the AC/Fe3O4 composite in this study. Moreover, this model is related to chemisorption, indicating that the surface complexation can be a predominant mechanism of the adsorption process.Figure 10
Pseudo-first-order model (a), pseudo-second-order model (b), Elovich kinetics model (c), and intraparticle diffusion kinetics model (d) for the adsorption of Zn2+ ion by the AC/Fe3O4 composite.
(a)(b)(c)(d)Table 1
Kinetics model parameters for the adsorption of Zn2+ ion by the ACP/Fe3O4 composite.
ModelsKinetics model parameters and regression coefficientsPseudo-first-orderk1 (min−1)qe (mg g−1)R20.03019.500.970Pseudo-second-orderk2 (g mg−1 min−1)qe (mg g−1)R20.008122.730.993Elovichα (mg g−1 min−1)β (mg g−1)R22.760.0550.998Intraparticle diffusionki (mg g min−0.5)CR21.0711.320.824The optimum adsorption conditions and removal efficiency of AC/Fe3O4 composite in Zn2+ ion adsorption are compared with previously reported studies (Table 2). It can be observed that the results obtained in this study are comparable with the literature under similar conditions. Moreover, the adsorbent in this study has the advantage of possessing higher pseudo-second-order rate constant compared with many other materials. These results make AC/Fe3O4 composite an interesting material with respect to its environmentally friendliness, cost-effectiveness, convenient separation, and relatively high adsorption efficiency.Table 2
Comparison of adsorption conditions and adsorption efficiency of various materials for Zn2+ removal.
MaterialspHAdsorbent dosage (g L−1)Contact time (min)Initial concentration of Zn2+ (ppm)Adsorption capacity (mg g−1)Re (%)Pseudo-second-order parametersRef.qe,cal (mg g−1)k2 (g mg−1 min−1)Zeolites—518020036.76—34.720.002[41]Fish bone5.01872020—981.930.00023∗[42]Palm oil mill effluent-based AC5.54∗5035059.88—29.500.0020[43]Tire-derived AC—4∗360∗50071.9—50.250.0057[35]Commercial AC—50∗1440∗50014.0—5.570.0019[35]AC/Fe3O44.06∗18010—70.5——[44]AC/Fe3O44.0–5.01.46025—9022.730.0081This study∗calculated from the published papers.


## 3.2.4. Proposed Mechanism of the Adsorption Process

Depending on the properties of metal ions, surface charges, pore sizes, and functional groups of adsorbents, there may be different interactions governing adsorption of heavy metals on AC-based materials [45]. In this study, the results obtained supported the hypothesis that surface complexation should be the predominant mechanism of the adsorption process. A proposed mechanism is presented in Figure 11. AC/Fe3O4 composite contains carboxylic and hydroxyl groups that can serve as ligand-like surface functional groups. These oxygenic functional groups can interact with divalent zinc (II) ion to form specific metal complexes. This facilitates the stabilization of Zn2+ on the surface of the adsorbent and can then be recovered by an external magnet.Figure 11
A proposed mechanism of the adsorption process.


## 4. Conclusions

The magnetic AC derived from macadamia nutshell residue was successfully prepared and utilized for the removal of zinc (II) ion in an aqueous solution. Fe3O4 was found to be attached to the carbon matrix, forming a magnetically separable adsorbent. The adsorption capacity was affected by the initial pH, the adsorbent dose, and the sorption time. It reached a maximum value at pH of 4, an adsorbent dose of 1.4 g L−1, and a sorption time of 1 h. The kinetics data followed the Elovich model and the pseudo-second-order model, indicating that chemisorption could occur on the heterogeneous surface of the solid adsorbent. Surface complexation mechanism was then proposed to be responsible for the adsorption. This study has provided information about the properties and the feasibility of zinc (II) ion removal of magnetic AC derived from macadamia nutshell residue. This composite is a promising adsorbent for being low-cost, environmentally friendly, and magnetically separable. Its utilization in wastewater treatments also provides opportunity for converting the leftover agricultural waste to valuable materials.



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*Source: 2543197-2021-09-21.xml*