Adsorption of Cu(II) and Zn(II) onto Activated Carbon from Oil Palm Empty Fruit Bunch Prepared by Two-Step Acid Treatment and Microwave-Assisted Pyrolysis

Abstract

This study explores the feasibility of biochar-based activated carbon derived from oil palm empty fruit bunch (EFB) as a potential precursor for the preparation of activated carbon via 2-step H₃PO₄ activation under microwave-assisted pyrolysis (2ACEFB). The characterization of EFB and 2ACEFB was observed by FTIR and BET, and chemical composition was determined using proximate and elemental analysis data. The adsorptive removal of Cu(II) and Zn(II) from an aqueous solution was studied, and the effects of metal concentration and solution pH were also investigated. The pseudo-second-order equation was properly described, providing the best fit to the observed experimental data. The adsorption capacities of Cu(II) and Zn(II) onto the EFB were 20.28 and 18.06 mg/g, respectively, and improved by 2.04- and 1.89-fold onto the 2ACEFB. The potential of 2ACEFB was also proved by adsorbent reusability with five consecutive circles of the batch experiment without regeneration or treatment. This study demonstrated that 2ACEFB is an efficient adsorbent for eliminating heavy metals from aqueous solutions.

1. Introduction

The amount of heavy metals, harmful to humans and most animal species, is released from chemical industries such as the production of electric batteries, mining, and glass manufacturing factories. Several techniques, e.g., chemical reduction/precipitation, ion exchange, membrane separation, and adsorption, have been developed to eliminate heavy metal ions from wastewater. Among these techniques, adsorption is a widely used method for the removal of heavy metal ions due to more benefits that have been recognized, including high efficiency, easy handling, economic effectiveness, and the availability of many types of adsorbents [1–3].

In biomass adsorbents, activated carbons are excellent adsorbents because of their high performance with high surface area and a high degree of microporosity. However, the efficiency expected of the activated carbons dramatically depends on their precursors, activators, and treatment/production techniques. So, relatively expensive, nonrenewable precursors, including long-time heating production methods are a significant shortcoming for commercial activated carbon [2, 4]. Numerous researches have focused on different sources of adsorbent and found that the use of biological material is one of the most effective ways to use as precursors. The biological precursors (agricultural waste, biomass, etc.) have been considered an alternative and high-potential material for activated carbons due to their plenty and renewable nature [5]. Either with or without activation treatment, the initial process of activated carbon is required. Carbonization is the most widely used method, being a low-cost technique in which organic matter can be transferred to elemental carbons by either conventional or microwave-assisted pyrolysis.

Microwave-assisted pyrolysis becomes advantageous because it provides a fast and selective heating mechanism [6] which can overcome many restrictions obtained from conventional heating regulation. Furthermore, during the microwave-assisted pyrolysis process, the media molecules hold a high frequency of vibration and frictional collision, resulting in a sudden increase in temperature. Thus, the treatment time can be decreased due to microwave energy. Numerous studies have shown that microwave-assisted pyrolysis can produce activated carbon with a higher specific surface area compared to conventional pyrolysis methods. This is due to the more rapid heating rates, which can lead to more efficient carbonization and activation of the material, resulting in a higher surface area and more uniform pore size distribution which can enhance its adsorption capacity “Microwave-assisted pyrolysis of coconut shell for production of activated carbon: Optimization using response surface methodology” [7], An et al. 2016 “Microwave-assisted pyrolysis of biomass for production of high-quality biooil and biochar with catalyst recycling by using activated carbon [8].

Biochar-based activated carbons have been used in several applications, providing high efficiency for sorption [9–11]. Acid activation is a simple and effective process, generally performed in one-step activation, for enhancing the adsorptive properties, functionalities, specific surface area, and selectivity of different adsorbates. Previously, two-stage activation was a process used in the preparation of activated carbon, providing more suitable activated carbon with a highly porous structure and useful functional groups for the removal of heavy metal ions from aqueous media (a novel route for the preparation of chemically activated carbon from pistachio wood for highly efficient Pb(II) sorption, activated carbon from wood wastes for the removal of uranium and thorium ions through modification with mineral acid). However, it is necessary to prove the efficiency of the produced activated carbon with different adsorbates. Therefore, this study attempted to examine the potential of activated carbons derived from palm oil empty fruit bunch via 2-step H₃PO₄ activation. The effects of different response parameters such as solution pH, metal ion concentration, and contact time were studied. The adsorption capacity of activated carbon as an adsorbent onto Cu(II) and Zn(II) was evaluated by a two- and three-parameter adsorption isotherm.

Heavy metals are toxic to the human body. Whether in the form of pure elements, organic compounds, and inorganic compounds, heavy metal’s toxicity manifests itself when sufficient amounts are accumulated in the body. Copper and zinc are used mainly in the automotive and industrial paint industries. In the study of the removal of Cu(II) and Zn(II), the purpose was to use the synthesized activated carbon in the paint industry effluent treatment before releasing it into the natural water source.

2. Experimental and Methodology

2.1. Material and Chemicals

The oil palm empty fruit bunches utilized as raw materials in this investigation were collected in Surat Thani, South of Thailand. All of the substances used in this study were of analytical grade. Heavy metal salts CuSO₄·5H₂O and Zn(NO₃)₂, as well as H₃PO₄, purchased from Ajax Finechem and Loba Chemie, respectively, were utilized to prepare Cu(II) and Zn(II) stock solutions. Copper(II) sulfate pentahydrate (CuSO₄·5H₂O) for analysis in EMSURE® and zinc standard solution traceable to SRM from NIST Zn(NO₃)₂ in HNO₃ 0.5 mol/l 1000 mg/l Zn Certipur®.

2.2. Adsorbent Preparation

Oil palm empty fruit bunches were gathered in Surat Thani, Southern Thailand, for usage as a raw material in this study. The activated carbon was made in the following processes: the raw materials were crushed and sieved into tiny sizes ranging from 1.0 to 2.0 mm and then pyrolyzed in a fixed bed reactor with flowing N₂ at 450°C for 1.5 hours, yielding biochar (hereafter named EFB) [12, 13]. The biochar was soaked in a phosphoric acid solution with a biochar/H₃PO₄ impregnation ratio of 1 : 1.75 (wt%) and periodic stirring [14]. The activation process was carried out at room temperature overnight. The resultant-activated carbon was obtained and then placed in the microwave-induced reactor for 10 min at 600 W and then washed with DI water until it reached pH 6-7. Overall activation processes were performed as duplicates, hereafter named 2ACEFB, in order to improve the adsorbent surface and adsorption efficiency, providing more efficacious [15].

2.3. Batch Adsorption

A series of batch experiments were conducted to evaluate the performance of 2ACEFB for the removal of Cu(II) and Zn(II) in an aqueous solution. Stock solutions were initially prepared by dissolving 3.929 g of CuSO₄·5H₂O and 2.896 g of Zn(NO₃)₂ in 1 L of distilled water. Standard metal solutions ranged 50-800 mg/L were then prepared appropriately by diluting the stock solution. Eventually, the pH of the metal solutions was adjusted by 0.1 M NaOH or HCl.

In a batch adsorption study, 50 mL of prepared solution was placed in a 250 mL conical flask, and 1 g of adsorbent was added to the flask placing it in an incubator under 150 rpm at a controlled temperature (298 K). Factors affecting the adsorption efficiency such as pH solution (2-6), contact time (0, 10, 20, 40, 60, 90, 120, 180, 240, and 360 min), and initial metal concentration (12.5, 25, 50, 100, 150, 200, 300, 400, 600, and 800 mg/L) were intentionally investigated to determine the optimal condition, qualifying the adsorption and reusability performance of the adsorbent. The amount of adsorption adsorbed can be calculated as follow: $\begin{matrix} (1) & q_{e} = \frac{(C_{o} - C_{e}) V}{m}, \end{matrix}$ where $q_{e}$ is the amount of metal ions adsorbed per mass of adsorbent at equilibrium (mg/g), $V$ is the volume (L) of solution used for batch experiments, $m$ is the mass (g) of adsorbent, and $C_{o}$ and $C_{e}$ are the initial concentration of metal ions and the equilibrium concentration of the solution (mg/g), respectively.

2.4. Instrumentation

The concentrations of Cu(II) and Zn(II) were measured by atomic absorption spectrophotometry (AAS) using AAnalyst 100 Spectrometer, PerkinElmer, Norwalk, CT/USA. EFB and 2ACEFB were characterized with an X-ray fluorescence spectrometer, PW2400, Philips, Netherlands. The FTIR spectra of the biochar EFB and 2ACEFB were recorded from KBr pellets with a Fourier transform infrared spectrometer, Vertex 70, Bruker, Germany. Surface area and porosity were determined using BET-technique, ASAP2460, Micromeritics, USA.

3. Result and Discussion

3.1. Chemical Composition and BET

The proximate analysis, in terms of moisture content, ash content, volatile matter, and fixed carbon, was carried out according to the ASTM standard. The result revealed that the composition of biochar EFB was 4.82 wt% moisture, 28.70 wt% volatile, 60.54 wt% fixed carbon, and 5.95 wt% ash (Table 1). It is clear that EFB contains high fixed carbon and low ash content, making it possible to be used as an activated carbon precursor [16]. After the 2-step H₃PO₄ treatment with a 1.75 impregnation ratio, the proximate composition of the 2ACEFB was 6.15 wt% moisture, 25.50 wt% volatile, 64.49 wt% fixed carbon, and 3.87 wt% ash.

Table 1

Physical and chemical composition of adsorbents.

Item	Sample
Item	EFB	2ACEFB
BET surface area (m²/g)	700.65	816.68
Pore volume (cm³/g)	0.46	0.56
Pore size (Å)	32.16	22.43
Proximate analysis $^{*}$ (wt%)
Moisture	4.82	6.15
Volatile	28.70	25.50
Fixed carbon	60.54	64.49
Ash	5.95	3.87
Ultimate analysis (wt%)
C	71.44	79.25
N	2.23	2.10
O	26.33	14.11
P	—	4.54

$^{*}$ ASTM D7582-15 [17].

The BET result showed that the surface area of 2ACEFB reached 816.68 m²/g, an increase over biochar EFB, as shown in Table 1. This result is similar to those previously reported in the comprehensive literature [18, 19]. The pore volume of the adsorbent increased corresponding with a high surface area which can imply a large number of active sites, and the high mesopore volume enhances the mass transfer rate [19]. This behavior can be described as phosphoric acid acting as a template for creating microporosity during the activation stage [20].

3.2. Surface Functionality and Morphology

Carbon matric is a complex nature that not only consists of the carbon atom but is also formed by others such as oxygen, nitrogen, halogen, sulfur, and phosphorus. These heteroatoms are bonded to the carbon surface layer at the edges [18]. The FTIR spectra of biochar (EFB) and 2-step H₃PO₄-activated carbon (2ACEFB) are shown in Figure 1. The broadband at the peak of 3400 cm^-1 corresponds to the –OH groups, hydroxyl groups from carboxyls, phenols, and adsorbed water [21, 22]. The band at 2922 cm^-1 is due to the C–H stretching of methyl and methylene groups [21]. The region of the spectrum 1300 and 900 cm^-1 consists of different bands being assigned to the C–O stretching in acids, alcohols, phenols, ethers, and esters found in oxidized carbons. However, it is noteworthy to mention that the phosphorous and phosphorous carbonaceous compounds existing in the phosphoric acid-activated carbon are presented in the band region between 900 and 1300 cm^-1 [22].

Figure 1

FTIR of untreated EFB and 2ACEFB.

The morphology of empty fruit bunches (EFB) demonstrates a surface that is heterogeneous, containing varying amounts of debris which arise as a consequence of the pyrolysis process. The imaging utilizing energy-dispersive X-ray spectroscopy (EDS) indicated the existence of several organic components, including phosphorus, which has the potential to be incorporated in the formation of a phosphate skin on the surface of EFB after the treatment with phosphate acid (Figure 2).

Figure 2

SEM-EDS of EFB and 2ACEFB.

(b)

3.3. Adsorption Kinetics

To examine the controlling mechanism of the adsorption process, the adsorption data were evaluated in terms of pseudo-first- and second-order-kinetic models. [23] The model expressions in terms of nonlinear and linearized forms are shown in Table 2.

Table 2

Nonlinear and linearized form of the kinetic models.

Kinetic model	Nonlinear form	Linearized form	Model parameters
Pseudo-first-order	$q_{t} = q_{e} - q_{e} e^{- k_{1} t}$	$\log (q_{e} - q_{t}) = \log q_{e} - \frac{k_{1} t}{2.303}$	$q_{e}$ , $k_{1}$
Pseudo-second-order	$q_{t} = \frac{k_{2} q_{e}^{2} t}{1 + k_{2} q_{e} t}$	$\frac{t}{q_{t}} = \frac{1}{k_{2} q_{e}^{2}} + \frac{t}{q_{e}}$	$q_{e}$ , $k_{2}$

The results revealed that models were relevant to the experimental data with a coefficient of determination ( $R^{2}$ ) of data with a coefficient of determination ( $R^{2}$ ) was more than 0.95. However, the kinetics of Cu(II) and Zn(II) onto EFB and 2ACEFB were adequately described by the pseudo-second-order model, in which the kinetic parameters determined are tabulated in Table 3. It can be seen from Figure 3, a nonlinear plot with the kinetic parameters from Table 3, that a relationship between the adsorption uptake ( $q_{e}$ ) from the experiments ( $q_{e, \exp}$ ) and the calculated value ( $q_{e, cal}$ ) was a good fit with $R^{2}$ ranged from 0.998 to 0.999. This means that the pseudo-second-order model can be appropriately used to describe the adsorption kinetics. The amount of uptake for all experiments reached 84% by 60 min and gradually increased to the equilibrium time of 240 min. However, to ensure equilibrium uptake, 360 min equilibration time was used when determining the adsorption isotherms in the next section.

Table 3

Kinetic model parameters of Cu(II) and Zn(II) onto EFB and 2ACEFB.

Kinetic model	Parameters	Adsorbent-adsorbate
Kinetic model	Parameters	EFB-Cu	2ACEFB-Cu	EFB-Zn	2ACEFB-Zn
Pseudo-first-order model	$k_{1}$ (min^-1) $q_{e}$ (mg/g) $R^{2}$	0.05848.630.974	0.05848.570.974	0.04178.130.960	0.05338.450.962
Pseudo-second-order model	$k_{2}$ (min^-1) $q_{e}$ (mg/g) $R^{2}$	0.00949.260.998	0.01389.120.999	0.01098.470.998	0.01029.220.998

Figure 3

Nonlinear fitting of pseudo-first (a) and second-order models (b) of Cu(II) and Zn(II) onto EFB and 2ACEFB.

(b)

3.4. Effect of Concentration

Figure 4 illustrates the relationship of metal ion concentrations for Cu(II) and Zn(II) removals at equilibrium. The overall response shapes are similar across all cases, but 2ACEFB was a better adsorbent than EFB at high concentrations of heavy metals. The maximal improvement in removal by adsorption on comparing 2ACEFB to EFB was 40.80 and 37.04% for Cu(II) and Zn(II), respectively. This is due to the fact that high metal concentration provides a higher driving force for the transfer process to overcome the mass transfer resistance [24–26].

Figure 4

Effect of initial metal ion concentration (solid and dashed line represented removal percentage and metal uptake, respectively).

3.5. Effect of Solution pH

Solution pH is an essential factor that can affect the adsorption process. In fact, chemical precipitation became a dominant process for metal removal at pH 7 [27]. This condition was not solely considered as adsorption because the metal precipitation may lead to a misinterpretation of adsorption capacity. The effect of pH on metal Cu(II) and Zn(II) adsorption onto EFB and 2ACEFB was shown in Figure 5. The result revealed that the maximal adsorption of Cu(II) and Zn(II) onto the adsorbents was at pH 5.0. This result had a similar trend with adsorption study on several metal ions reported by numerous studies [26, 28, 29]. Therefore, the optimal condition used to determine the adsorption capacity of EFB and 2ACEFB was as follows: 1 g of adsorbent dose, pH 5, contact time of 360 min, and room temperature.

Figure 5

Effect of pH solution.

3.6. Adsorption Isotherm

Adsorption data were evaluated by both two-parameter (Langmuir, Freundlich, and Temkin) and three-parameter (Redlich-Peterson, Sips model, and Toth model) isotherm models. Each model explains a specific feature of the adsorption process, and the model expressions and linearized forms are presented in Table 4. To examine the adsorption data, the mean absolute percentage error (MAPE) was used to quantify the experimental results compared with these isotherm models, which can be expressed as follows: $\begin{matrix} (2) & MAPE (%) = \frac{\sum_{i = 1}^{N} |(q_{e, \exp} - q_{e, pre}) / q_{e, \exp}|}{N} \times 100, \end{matrix}$ where $q_{e, \exp}$ and $q_{e, pre}$ are the experimental and predicted uptake capacity (mg/g) and $N$ is the number of data points.

Table 4

Expression of two-parameter and three-parameter isotherm models.

Isotherm model	Nonlinear form	Linearized form	Model parameters
Two-parameter model
Langmuir	$q_{e} = \frac{q_{m} K_{L} C_{e}}{1 + K_{L} C_{e}}$	$\frac{1}{q_{e}} = \frac{1}{q_{m}} + \frac{1}{K_{L} q_{m} C_{e}}$	$K_{L}$ , $q_{m}$
Freundlich	$q_{e} = K_{F} C_{e}^{1 / n}$	$\ln_{q_{e}} = \ln_{K_{F}} + \frac{1}{n} \ln C_{e}$	$K_{F}$ , $n$
Temkin	$q_{e} = \frac{R T}{b_{T}} \ln (K_{T} C_{e})$	$q_{e} = \frac{R T}{b_{T}} \ln (K_{T}) + \frac{R T}{b_{T}} \ln (C_{e})$	$K_{T}$ , $b_{T}$

Three-parameter model
Redlich-Peterson	$q_{e} = \frac{A C_{e}}{1 + B C_{e}^{g}}$	—	$A$ , $B$ , $g$
Sips model	$q_{e} = \frac{q_{m S} K_{S} C_{e}^{m S}}{1 + K_{S} C_{e}^{m S}}$	—	$q_{m S}$ , $K_{S}$ , $m S$
Toth model	$q_{e} = \frac{q_{m T} C_{e}}{{((1 / K_{T}) + C_{e}^{m T})}^{1 / m T}}$	—	$q_{m T}$ , $K_{T}$ , $m T$

The adsorption study results are illustrated in Figure 6, and associated parameters are tabulated in Table 5. Linear regression analysis was used to evaluate two-parameter isotherm models, providing the parameters used for nonlinear fitting. The experimental data fitted well with the Langmuir isotherm better than others, whose correlation coefficients were 0.996-0.997, suggesting that the mechanism was monolayer adsorption. The assumption of the Langmuir model involves an account of the formation of a monolayer of adsorbate molecules on the adsorbent surface. This is consistent with the assumption that the adsorption occurs at specific sites on the adsorbent surface [30].

Figure 6

Nonlinear data fitting of isotherm models for Cu(II) and Zn(II) adsorption onto EFB and 2ACEFB under optimal condition for (a, b) a two-parameter model and for (c, d) three parameters.

(b)

(c)

(d)

Table 5

Isotherm parameters for the adsorption of Cu(II) and Zn(II) onto EFB and 2ACEFB.

Isotherm model	Parameters	Adsorbent-adsorbate
Isotherm model	Parameters	EFB-Cu	2ACEFB-Cu	EFB-Zn	2ACEFB-Zn
Two-parameter model
Langmuir	$K_{L}$ $q_{m}$ $R^{2}$ $MAPE$	0.05220.020.9976.11	0.03844.290.9963.80	0.03617.820.9976.28	0.02936.430.9974.60
Freundlich	$K_{F}$ $n$ $R^{2}$ $MAPE$	2.512.620.92920.58	2.581.670.95515.35	1.9202.550.88518.38	1.931.720.92916.56
Temkin	$K_{T}$ $b_{T}$ $R^{2}$ $MAPE$	315.970.6040.98411.27	350.210.4030.97218.65	697.390.8250.93775.52	757.950.5270.96448.41

Three-parameter model
Redlich-Peterson	$A$ $B$ $g$ $R^{2}$ $RMSE$ $MAPE$	1.0090.0491.0000.9772.9856.05	1.9380.0660.93360.9981.3273.41	0.6210.0341.0000.9712.8246.41	1.1010.0311.0000.9932.6514.82
Sips model	$q_{m S}$ $K_{S}$ $m S$ $R^{2}$ $RMSE$ MAPE	20.010.0401.0880.9782.9187.97	41.660.0460.9550.9981.4843.21	17.800.0301.0530.9722.8057.57	33.580.0221.1500.9952.2947.26
Toth model	$q_{m T}$ $K_{T}$ $m T$ $R^{2}$ $R M S E$ M $A P E$	20.280.0530.9870.9763.0166.02	40.670.0450.9870.9971.5552.41	18.060.0341.0000.9712.826.41	36.000.0311.0000.9932.664.82

The selected three-parameter models were fitted with the same experimental data (see Table 6). These three-parameter models provided good fits which MAPE ranged 2.41-7.97% was better than for the two-parameter isotherms (MAPE ranged 3.80-75.52%) for all experiments. It is also noteworthy that the parameters $g$ , $m_{S}$ , and $m_{T}$ in the Redlich-Peterson, Sips, and Toth models, respectively, were approximately close to unity. This study result suggested that the adsorption of Cu(II) and Zn(II) onto 2ACEFB and EFB can be properly described by the Langmuir model with a homogenous surface.

Table 6

Adsorption capacity of activated carbon with various raw materials toward heavy metals.

Heavy metal	Raw material	Adsorbent dose (g/L)	pH	$Q_{m}$ (mg/g)	Best fit isotherms model	Reference
Cu(II)	Tahiti lemon peels	—	—	28.85	Langmuir	[36]
	Green vegetable waste	0.6	3.5	75.0	Langmuir	[37]
	Water hyacinth	12	6	3.53	Langmuir	[38]
	Pomelo peels	0.25	5	151.35	Langmuir	[39]
	Sugarcane bagasse	2	4.5	225	Langmuir	[40]
	Sewage sludge	0.5	5	26.06	Langmuir-Freundlich	[41]
	Banana leaves	2	5	66.2	Langmuir	[42]
	Calcium-rich biochars	1	5	141.76	Langmuir	[43]
	Raw cow manure	1.25	5	41.83	Freundlich	[44]
	Henna leaves	1.2	7	3.65	Langmuir	[45]
	Agricultural waste	3	9	56.2	Freundlich	[46]
	Oil palm empty fruit bunch	1	5	44.29	Langmuir	This study

Zn(II)	Coconut shell based	1	12	7.87	Langmuir	[47]
	Macadamia nutshell	0.3	4	22.73	Langmuir	[48]
	Water hyacinth	12	6	9.42	Langmuir	[38]
	Chlorella sp.	0.5	6	24.76	Langmuir	[49]
	Palm kernel shell	15.5	5	44.8	Langmuir	[50]
	Oil palm empty fruit bunch	1	5	36.43	Langmuir	This study

3.7. Maximal Adsorption Capacity

For comparison propose of available model types, the three-parameter models obviously fit with the experimental data better than the two-parameter type in the range of initial concentration used and also supported the two-parameter Langmuir isotherm appropriately. It has been clearly shown that the adsorptive potential of Cu(II) was stronger than that of Zn(II). The maximal adsorption capacity of EFB onto Cu(II) and Zn(II) was observed as 20.01-20.28 and 17.80-18.06 mg/g, respectively. When EFB was activated by H₃PO₄, the adsorption capacity improved 2.04- and 1.886-fold to 40.67-41.66 and 33.58-36.00 mg/g over EFB (Table 5). The Cu(II) and Zn(II) uptake capacities obtained from this study are compared to relevant prior studies in the literature presented in Table 6. Therefore, it is concluded that the two-step acid treatment by H₃PO₄ improved both the physical and chemical properties, which can become more wealth capacity in Cu(II) and Zn(II) removal over untreated biochar from EFB.

3.8. Adsorption Mechanism

The surface functional groups of an adsorbent play a vital role in establishing a connection between the adsorbent and the adsorbate [31, 32]. Upon conducting FTIR analysis, it was observed that the process of phosphorylation resulted in an increase in porosity, carbon surface area, and the presence of carboxyl (-COOH) groups. Moreover, the introduction of the phosphoric acid group in 2ACEFB expanded the pore structure during the adsorption of Cu(II) and Zn(II), thus enhancing the exposure of anions on the surface. This leads to an increased ability of 2ACEFB to adsorb Cu(II) and Zn(II) cations. According to Kriaa et al. [33], P-containing surface groups play a critical role in the adsorption of Cu(II) and Zn(II) in solution. This result is in agreement with earlier studies which demonstrated a strong correlation between the quantities of carboxylic phosphate groups, pyrophosphate groups, and the adsorption capacities of Cu(II) and Zn(II) [34, 35]. Figure 7 illustrated the scheme of Cu(II) and Zn(II) adsorption mechanism using phosphorylated sorbents in an aqueous solution, which can be summarized as follows.

Figure 7

A proposed mechanism of the adsorption process of Cu(II) and Zn(II) onto activated carbon. R-OH+CuSO₄⟶R-O-Cu+H₂SO₄. R-COOH+CuSO₄⟶R-COO-Cu+H₂SO₄. R-OH+Zn(NO₃)₂⟶R-O-Zn+HNO₃. R-COOH+Zn(NO₃)₂⟶R-COO-Zn+HNO₃.

3.9. Adsorbent Reusability

Adsorbent reusability is considered to be a significant advantage in determining the adsorbent’s feasibility for practical application [51]. The reusability of the adsorbent in this study was accessed using a series of adsorption reactions under the similar conditions mentioned above (25°C temperature, 6 hrs, 1 g of adsorbent, pH 5, concentration 200 mg/L) by five consecutive reactions. After each reaction was completed, the spent adsorbents were collected from an aqueous solution by filtration and were dried at 105° C.

It is revealed that the removal of Cu(II) and Zn(II) by 2ACEFB was obviously decreased to 79.26 and 72.41%, as shown in Figure 8, respectively, after five consecutive reactions, which indicated inferior reusability of the prepared adsorbents. This result was repeatedly confirmed by the FTIR, as shown in Figure 9. The phosphate groups (band P-OH 919 cm^-1 and amino phosphonic acid functional group 1191 cm^-1) have not been significantly changed in band shedding (1st-5th reuse). There was a slight decrease in the removal of Cu(II) and Zn(II) which may be due to the inferior reusability of the prepared adsorbent [52].

Figure 8

Effect of adsorbent reuse cycle on the removal of Cu(II) and Zn(II).

Figure 9

FTIR spectrum of 2ACEFB with five consecutive reactions. (a) Cu(II) and (b) Zn(II).

(b)

4. Conclusions

In this study, activated carbon was prepared via 2-step H₃PO₄ activation under microwave-assisted pyrolysis in order to examine its feasibility for the adsorption of Cu(II) and Zn(II) in an aqueous solution. A series of batch experiments was successfully done, and the conclusions can be drawn. (1)

The 2-step H₃PO₄-activated carbon (namely, 2ACEFB) derived by oil palm empty fruit bunch (EFB) was successfully performed in this study, totally proved by FTIR, BET, and proximate analysis. The specific surface area and pore volume of the adsorbent increase due to the H₃PO₄ activation

(2)

Adsorption equilibrium reached 84% in approximately 60 min and developed to equilibrium time in 240 min for all cases. The pseudo-second-order model was the best description, with the coefficient of determination ( $R^{2}$ ) ranged 0.998-0.999

(3)

The 2ACEFB was clearly a better adsorbent than EFB, especially at high initial concentration, and improved the removal of Cu(II) and Zn(II) over EFB by 40.80 and 37.04% at pH 7, respectively. The maximal adsorption capacity of 2ACEFB was approximately 2.04- and 1.89-fold related to EFB, respectively, as described adequately by the Langmuir model with monolayer and homogenous surface adsorption

(4)

The 2ACEFB showed good ability in adsorption reusability by five consecutive reactions, with the removal efficiency reduced to 79.26 and 72.41% for Cu(II) and Zn(II), respectively. Therefore, it is implied that the 2ACEFB acts as a promising adsorbent for metal ion adsorption

Footnotes

Data Availability

The data that support the findings of this study are available from the corresponding author, C.S., upon reasonable request.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Authors’ Contributions

Thotsaporn Somsiripan was responsible for the conceptualization, methodology, investigation, writing—original draft, and resources. Chayanoot Sangwichien was in charge of the conceptualization, and the validation, supervision, writing—review and editing, and funding acquisition. Kanogwan Tohdee contributed to the validation and formal analysis. Surat Semmad helped with the formal analysis.

Acknowledgments

The authors would like to acknowledge the financial support provided by the Prince of Songkla University Graduate School.

References

Hameed

B. H.

Evaluation of papaya seeds as a novel non-conventional low-cost adsorbent for removal of methylene blue

Journal of Hazardous Materials 2009 162

8122712

2-3 939 944

10.1016/j.jhazmat.2008.05.120

2-s2.0-57649218733

Gupta

V. K.

Carrott

P. J. M.

Carrott

M. M. L.

Suhas

Low-cost adsorbents: growing approach to wastewater treatment—a review

Critical Reviews in Environmental Science and Technology 2009 39

8122712

10 783 842

10.1080/10643380801977610

2-s2.0-70350630302

Atar

Olgun

Wang

Adsorption of cadmium (II) and zinc (II) on boron enrichment process waste in aqueous solutions: batch and fixed-bed system studies

Chemical Engineering Journal 2012 192

8122712

1 7

10.1016/j.cej.2012.03.067

2-s2.0-84862337713

Jagtoyen

Derbyshire

Some considerations of the origins of porosity in carbons from chemically activated wood

Carbon 1993 31

8122712

7 1185 1192

10.1016/0008-6223(93)90071-H

2-s2.0-0027834313

Rao

M. M.

Ramesh

Rao

G. P. C.

Seshaiah

Removal of copper and cadmium from the aqueous solutions by activated carbon derived from Ceiba pentandra hulls

Journal of Hazardous Materials 2006 129

8122712

1-3 123 129

10.1016/j.jhazmat.2005.08.018

2-s2.0-33644523742

16191464

Mahari

W. A.

Zainuddin

N. F.

Wan Nik

W. M. N.

Chong

C. T.

Lam

S. S.

Pyrolysis recovery of waste shipping oil using microwave heating

Energy 2016 9

8122712

10 780

10.3390/en9100780

2-s2.0-84990847957

Ismail

Ishak

M. A.

Ab Ghani

Abdullah

M. F.

Safian

M. T.

Idris

S. S.

Tahiruddin

Yunus

M. F.

Hakimi

N. I.

Microwave-assisted pyrolysis of palm kernel shell: optimization using response surface methodology (RSM)

Renewable Energy 2013 55

8122712

357 365

10.1016/j.renene.2012.12.042

2-s2.0-84873266447

Zhang

Zhong

Xie

Liu

Ruan

Two-step fast microwave-assisted pyrolysis of biomass for bio-oil production using microwave absorbent and HZSM-5 catalyst

Journal of Environmental Sciences 2016 45

8122712

240 247

10.1016/j.jes.2015.12.019

2-s2.0-84962349387

27372139

Gale

Nguyen

Moreno

Gilliard-AbdulAziz

K. L.

Physiochemical properties of biochar and activated carbon from biomass residue: influence of process conditions to adsorbent properties

ACS Omega 2021 6

8122712

15 10224 10233

10.1021/acsomega.1c00530

34056176

10.

Park

Hung

Gan

Rojas

O. J.

Lim

K. H.

Park

Activated carbon from biochar: influence of its physicochemical properties on the sorption characteristics of phenanthrene

Bioresource Technology 2013 149

8122712

383 389

10.1016/j.biortech.2013.09.085

2-s2.0-84885452377

11.

Dissanayake

P. D.

You

Igalavithana

A. D.

Xia

Y. A.

Bhatnagar

Gupta

Y. S. O.

Kua

H. W.

Kim

Kwon

J. H.

Tsang

D. C. W.

Y. S.

Biochar-based adsorbents for carbon dioxide capture: a critical review

Renewable and Sustainable Energy Reviews 2020 119, article 109582

8122712

10.1016/j.rser.2019.109582

12.

Choojit

Sangwichien

Preparation of activated carbon production from oil palm empty fruit bunch and its application

Kasem Bundit Engineering Journal 2018 8

8122712

2 48 67

13.

Mohamed

A. R.

Hamzah

Daud

M. Z.

Optimization of the pyrolysis process of empty fruit bunch (EFB) in a fixed-bed reactor through a central composite design (CCD)

AIP Conference Proceedings 2014 1605

8122712

1 1172 1177

10.1063/1.4887756

2-s2.0-84904686531

14.

Foo

K. Y.

Hameed

B. H.

Dynamic adsorption behavior of methylene blue onto oil palm shell granular activated carbon prepared by microwave heating

Chemical Engineering Journal 2012 203

8122712

81 87

10.1016/j.cej.2012.06.073

2-s2.0-84865647013

15.

Zhou

Niu

Activity of the carbon-based heterogeneous acid catalyst derived from bamboo in esterification of oleic acid with ethanol

Energy Conversion and Management 2016 114

8122712

188 196

10.1016/j.enconman.2016.02.027

2-s2.0-84958985273

16.

Prahas

Kartika

Indraswati

Ismadji

Activated carbon from jackfruit peel waste by H₃PO₄ chemical activation: pore structure and surface chemistry characterization

Chemical Engineering Journal 2008 140

8122712

1-3 32 42

10.1016/j.cej.2007.08.032

2-s2.0-44049094873

17.

ASTM Committee D-5 on Coal and Coke

Standard Test methods for proximate analysis of coal and Coke by macro thermogravimetric analysis (ASTM D7582-15) 2010

ASTM International

18.

Yakout

S. M.

Sharaf El-Deen

Characterization of activated carbon prepared by phosphoric acid activation of olive stones

Arabian Journal of Chemistry 2016 9

8122712

S1155 S1162

10.1016/j.arabjc.2011.12.002

2-s2.0-84855732428

19.

Valero-Romero

M. J.

Calvo-Muñoz

E. M.

Ruiz-Rosas

Rodríguez-Mirasol

Cordero

Phosphorus-containing mesoporous carbon acid catalyst for methanol dehydration to dimethyl ether

Industrial & Engineering Chemistry Research 2019 58

8122712

10 4042 4053

10.1021/acs.iecr.8b05897

2-s2.0-85062903832

20.

Marsh

Rodríguez-Reinoso

Activation processes (chemical)

Activated Carbon 2006 6

8122712

Oxford

Elsevier Science Ltd

322 365

21.

Yang

Qiu

Preparation of activated carbons from walnut shells via vacuum chemical activation and their application for methylene blue removal

Chemical Engineering Journal 2010 165

8122712

1 209 217

10.1016/j.cej.2010.09.019

2-s2.0-78149495828

22.

Puziy

A. M.

Poddubnaya

O. I.

Martínez-Alonso

A. F.

Suárez-García

J. M. D.

Tascón

J. M. D.

Surface chemistry of phosphorus-containing carbons of lignocellulosic origin

Carbon 2005 43

8122712

14 2857 2868

10.1016/j.carbon.2005.06.014

2-s2.0-24344459048

23.

Pathania

Sharma

Effect of surfactants and electrolyte on removal and recovery of basic dye by using Ficus carica cellulosic fibers as biosorbent

Journal Tenside Surfactants Detergents 2012 49

8122712

4 306 314

10.3139/113.110196

24.

Wang

X. S.

Tang

Y. P.

Tao

S. R.

Kinetics, equilibrium and thermodynamic study on removal of Cr (VI) from aqueous solutions using low-cost adsorbent alligator weed

Chemical Engineering Journal 2009 148

8122712

2-3 217 225

10.1016/j.cej.2008.08.020

2-s2.0-61649123296

25.

Mittal

Gupta

V. K.

Malviya

Mittal

Process development for the batch and bulk removal and recovery of a hazardous, water-soluble azo dye (metanil yellow) by adsorption over waste materials (bottom ash and de-oiled soya)

Journal of Hazardous Materials 2008 151

8122712

2-3 821 832

10.1016/j.jhazmat.2007.06.059

2-s2.0-38749112599

26.

Tohdee

Enhancement of Cu(II), Zn(II), Ni(II) and humic acid adsorption in modified bentonite, [Ph.D. thesis] 2018

Prince of Songkhla University

27.

Kanogwan

Kaewsichan

Enhancement of adsorption efficiency of heavy metal Cu(II) and Zn(II) onto cationic surfactant modified bentonite

Journal of Environmental Chemical Engineering 2018 6

8122712

2 2821 2828

10.1016/j.jece.2018.04.030

2-s2.0-85045945133

28.

Abollino

Giacomino

Malandrino

Mentasti

Interaction of metal ions with montmorillonite and vermiculite

Applied Clay Science 2008 38

8122712

3-4 227 236

10.1016/j.clay.2007.04.002

2-s2.0-38549103098

29.

Chen

W. J.

Hsiao

L. C.

Chen

K. K. Y.

Metal desorption from copper(II)/nickel(II)-spiked kaolin as a soil component using plant-derived saponin biosurfactant

Process Biochemistry 2008 43

8122712

5 488 498

10.1016/j.procbio.2007.11.017

2-s2.0-41549162311

30.

Swenson

Stadie

N. P.

Langmuir’s theory of adsorption: a centennial review

Langmuir 2019 35

8122712

16 5409 5426

10.1021/acs.langmuir.9b00154

2-s2.0-85064838843

30912949

31.

Gómora-Hernández

J. C.

Carreño-de-León

M. D.

Flores-Alamo

Hernández-Berriel

M. D.

Fernández-Valverde

S. M.

Kinetic and thermodynamic study of corncob hydrolysis in phosphoric acid with a low yield of bacterial inhibitors

Biomass and Bioenergy 2020 143, article 105830

8122712

10.1016/j.biombioe.2020.105830

32.

Huang

Chen

Zhang

Chen

Adsorption of Pb(II) on mesoporous activated carbons fabricated from water hyacinth using H₃PO₄ activation: adsorption capacity, kinetic and isotherm studies

Applied Surface Science 2014 293

8122712

160 168

10.1016/j.apsusc.2013.12.123

2-s2.0-84895060226

33.

Kriaa

Hamdi

Srasra

Removal of Cu (II) from water pollutant with Tunisian activated lignin prepared by phosphoric acid activation

Desalination 2010 250

8122712

1 179 187

10.1016/j.desal.2008.12.056

2-s2.0-71849101447

34.

Choi

S. S.

Choi

T. R.

Cho

H. J.

Surface modification of phosphoric acid–activated carbon in spent coffee grounds to enhance Cu(II) adsorption from aqueous solutions

Applied Chemistry for Engineering 2021 32

8122712

5 589 598

10.14478/ace.2021.1074

35.

Wan

Shang

Gao

Kobayashi

Luo

Surface modification of phosphoric acid activated carbon by using non-thermal plasma for enhancement of Cu(II) adsorption from aqueous solutions

Separation and Purification Technology 2018 197

8122712

156 169

10.1016/j.seppur.2018.01.007

2-s2.0-85040061069

36.

Pérez

Ulloa

Flórez

Acelas

Ocampo-Pérez

Padilla-Ortega

Forgionny

Valorization of lemon peels wastes into a potential adsorbent for simultaneous removal of copper ion (Cu²⁺) and Congo red from wastewater

Environmental Nanotechnology, Monitoring & Management 2023 20, article 100795

8122712

10.1016/j.enmm.2023.100795

37.

Sabela

M. I.

Kunene

Kanchi

Xhakaza

N. M.

Bathinapatla

Mdluli

Sharma

Bisetty

Removal of copper (II) from wastewater using green vegetable waste derived activated carbon: an approach to equilibrium and kinetic study

Arabian Journal of Chemistry 2019 12

8122712

8 4331 4339

10.1016/j.arabjc.2016.06.001

2-s2.0-84977639268

38.

Nyamunda

B. C.

Chivhanga

Guyo

Chigondo

Removal of Zn (II) and Cu (II) ions from industrial wastewaters using magnetic biochar derived from water hyacinth

Journal of Engineering 2019 2019

8122712

5656983

10.1155/2019/5656983

2-s2.0-85066397574

39.

Liu

Wei

K₂CO₃-activated pomelo peels as a high-performance adsorbent for removal of Cu(II): preparation, characterization, and adsorption studies

Journal of Chemistry 2021 2021

8122712

9940577

10.1155/2021/9940577

40.

Amoo

T. E.

Amoo

K. O.

Adeeyo

O. A.

Ogidi

C. O.

Kinetics and equilibrium studies of the adsorption of copper(II) ions from industrial wastewater using activated carbons derived from sugarcane bagasse

International Journal of Chemical Engineering 2022 2022

8122712

6928568

10.1155/2022/6928568

41.

Fan

Wang

Miao

Liu

Cai

Chen

Cheng

Chen

Luo

Zhang

Na₄P₂O₇-modified biochar derived from sewage sludge: effective Cu(II)-adsorption removal from aqueous solution

Adsorption Science & Technology 2023 2023

8122712

8217910

10.1155/2023/8217910

42.

Darweesh

M. A.

Elgendy

M. Y.

Ayad

M. I.

Ahmed

A. M.

Elsayed

N. M. K.

Hammad

W. A.

Adsorption isotherm, kinetic, and optimization studies for copper (II) removal from aqueous solutions by banana leaves and derived activated carbon

South African Journal of Chemical Engineering 2022 40

8122712

10 20

10.1016/j.sajce.2022.01.002

43.

Liu

Yang

Liu

Cheng

Bao

Modification of calcium-rich biochar by loading Si/Mn binary oxide after NaOH activation and its adsorption mechanisms for removal of Cu(II) from aqueous solution

Colloids and Surfaces A: Physicochemical and Engineering Aspects 2020 601, article 124960

8122712

10.1016/j.colsurfa.2020.124960

44.

Zhang

Yuan

Xie

Han

Characteristics, adsorption behaviors, Cu(II) adsorption mechanisms by cow manure biochar derived at various pyrolysis temperatures

Bioresource Technology 2021 331, article 125013

8122712

10.1016/j.biortech.2021.125013

33773414

45.

Shanthi

Selvarajan

V. M.

Removal of Cr(VI) and Cu(II) ions from aqueous solution by carbon prepared from henna leaves

Journal of Chemistry 2013 2013

8122712

304970

10.1155/2013/304970

2-s2.0-84869034413

46.

Özsin

Kılıç

Apaydın-Varol

Pütün

A. E.

Chemically activated carbon production from agricultural waste of chickpea and its application for heavy metal adsorption: equilibrium, kinetic, and thermodynamic studies

Applied Water Science 2019 9

8122712

3 56

10.1007/s13201-019-0942-8

47.

Yusop

M. F. M.

Azmie

E. M. J.

Ahmad

M. A.

Single-stage microwave assisted coconut shell based activated carbon for removal of Zn(II) ions from aqueous solution - optimization and batch studies

Arabian Journal of Chemistry 2022 15

8122712

8, article 104011

10.1016/j.arabjc.2022.104011

48.

Dao

M. T.

Tran

T. P. L.

D. T.

Nguyen

V. K.

Hoang

L. T. T. T.

Utilization of macadamia nutshell residue for the synthesis of magnetic activated carbon toward zinc (II) ion removal

Advances in Materials Science and Engineering 2021 2021

8122712

2543197

10.1155/2021/2543197

49.

Amin

Chetpattananondh

Biochar from extracted marine Chlorella sp. residue for high efficiency adsorption with ultrasonication to remove Cr(VI), Zn(II) and Ni(II)

Bioresource Technology 2019 289, article 121578

8122712

10.1016/j.biortech.2019.121578

2-s2.0-85066926968

31195297

50.

Karri

R. R.

Sahu

J. N.

Modeling and optimization by particle swarm embedded neural network for adsorption of zinc (II) by palm kernel shell based activated carbon from aqueous environment

Journal of Environmental Management 2018 206

8122712

178 191

10.1016/j.jenvman.2017.10.026

2-s2.0-85032867399

29065359

51.

Ezechi

E. H.

Kutty

S. R. . M.

Malakahmad

Isa

M. H.

Characterization and optimization of effluent dye removal using a new low cost adsorbent: equilibrium, kinetics and thermodynamic study

Process Safety and Environmental Protection 2015 98

8122712

16 32

10.1016/j.psep.2015.06.006

2-s2.0-84941330164

52.

Jiang

Barr

Ren

Gao

Wang

Reusable polyacrylonitrile-sulfur extractor of heavy metal ions from wastewater

Advanced Functional Materials 2021 31

8122712

51, article 2105845

10.1002/adfm.202105845