1. Introduction
With the growth of human activities, heavy metal contamination of soil and groundwater has become a great concern. Heavy metals are highly toxic and carcinogenic due to their persistence, bioaccumulation, and risk of reappearance according to changes of aquatic chemistry. Among various heavy metals, lead and nickel are considered the most dangerous contaminants when they are released into natural waters from various industrial activities including mining, leather processing, and metal plating. Lead can easily accumulate in human organs, causing anemia and severe damage to the nervous system, reproductive system, liver, brain, and kidneys [
1]. Nickel is another toxic metal ion commonly present in steel and battery wastewater, causing vomiting, chest pain, and shortness of breath [
2].
Various methods for heavy metal removal have been developed including reverse osmosis, ion exchange, electrodialysis, electrolysis, and adsorption. Among these technologies, adsorption has been acknowledged as a fast, inexpensive, and widely applicable technique [
3,
4,
5,
6]. In this regard, many studies have been focused on the effectiveness of various types of adsorbents, including biomass, clays, activated carbon, and metal particulates [
7,
8,
9,
10]. Among the listed adsorbents, adsorption onto natural materials and biomass have been the most commonly tested techniques due to their cost-effectiveness when compared to those of other adsorbents such as metal particulates and activated carbon, while specific adsorption capacities have generally been found to be lower than that of metal particulates [
7]. Moreover, effluent concentration for heavy metals during water treatment is very low in most cases, and the adsorption capacity is not the only criterion; the affinity between adsorbent and heavy metals should also be considered [
1].
Nano-sized metal oxide particles have fascinated many researchers due to their high adsorption capacities as well as their extremely fast adsorption of and high affinity for heavy metals [
11,
12]. Among them, manganese oxide nanoparticles (MONPs), with notably high surface area and uniform size, are of great importance due to their novel applications in heavy metal removal [
13,
14]. However, due to the strong tendency of MONPs to form aggregates, synthesis of particles of well-controlled shape and size has been critical for maintaining their high affinity for heavy metals and their surface area. To minimize the aggregation of MONPs, various substrates with large surface area and uniform surface energy have been considered [
15]. Specifically, graphene oxide (GO), a two-dimensional (2D) atomically thick carbon crystal with a honeycomb lattice, huge surface area, mechanical and thermal stability, and durability, has attracted great scientific and technological interest because of its potential applications in various areas including as a substrate for novel nano-particle deposition [
16,
17]. However, it is difficult to obtain homogeneous metal deposition on the hydrophobic surface because graphene sheets are not wetted by water, which leads to the infeasibility of traditional metal loading methods such as wet impregnation. Although GO is suitable and versatile, able to decorate various organic and inorganic chemicals, the subsequent reduction of GO to grow metal nanoparticles has been regarded as a complex, time-consuming, and highly toxic process [
18,
19]. Many efforts have been made to immobilize MONPs on various supporting materials such as sand, zeolite, and crushed brick [
20,
21,
22,
23], but their adsorption performance for heavy metals removal are still facing multiple challenges as discussed above.
Recently, several groups have investigated the use of expanded graphite (EG) because EG could achieve the requirements for nanoparticle support, including high porosity, large surface area, and low price [
24,
25]. Expanded graphite prepared from graphite intercalated compounds is a kind of hydrophobic carbon crystalline material with mainly macro- and mesopores; it has a unique worm-like structure [
26,
27], making it suitable as a support for nanoparticles such as MONPs. In addition, to the best of our knowledge, the combination of MONPs with EG for adsorptive removal of heavy metals has not been reported in literature.
In the present work, we introduced a facile and effective approach to uniformly immobilize MONPs on an expanded graphite surface; the newly formed composite (MONPs-EG) was then applied as an adsorbent for comparative and competitive removal of Pb2+ and Ni2+ in aqueous solution. Batch experiments were performed at various temperatures for a thermodynamic and kinetic study of the processes involved. The potential application of MONPs-EG composite material for the removal of heavy metals in the presence of humic acids (HA) was also investigated.
2. Materials and Methods
2.1. Material Preparation
2.1.1. Chemicals and Materials
Expandable graphite flakes (EXP-527) were supplied by Hyundai Coma Co., Seoul, Korea. Manganese(III) acetylacetonate or Mn(acac)3 (C15H21MnO6, 99.5%), manganese(II, III) oxide (Mn3O4, 97.0%), lead(II) nitrate (N2O6Pb, ≥99.0%), nickel(II) nitrate hexahydrate (H12N2NiO12, 99.9%), methanol (CH4O, 99.8%), and humic acid sodium salt (HA, technical grade) were purchased from Sigma-Aldrich Inc., Saint Louis, Missouri, United States.
2.1.2. Preparation of Manganese Oxide Nanoparticles-Expanded Graphite (MONPs-EG)
The graphite flakes was first expanded in a preheated muffle furnace (DMF-05, HumanLab Instrument Co., Gyeonggi, Korea) at 800 °C for 1 min to obtain EG. The prepared EG was then decorated with manganese oxide nanoparticles through a simple method using manganese acetylacetonate in the presence of methanol. In a typical experiment, 1.0 g EG and 0.5 g Mn(acac)3 were loaded into a 450 mL Parr acid digression bomb (4767, Parr Instrument Co., Moline, Illinois, United States), followed by the addition of an appropriate amount of MeOH. The reaction mixture was carefully sealed and allowed to react at 300 °C for 10 min on an analog hot plate (HS-18, HumanLab Instrument Co., Gyeonggi-do, Korea). After cooling to room temperature, the solid composite was collected by vacuum filtration through a 1.0 μm membrane filter (JAWP, Merck KGaA, Darmstadt, Germany), thoroughly washed with ethanol, then dried overnight at 60 °C. Finally, the product was calcined at 350 °C for 4 h and was denoted MONPs-EG.
2.2. Material Characterization
Surface morphology of the prepared materials was observed using a scanning electron microscope (SEM, Stereoscan 440 FIB, LEO Electron Microscopy Ltd., Cambridge, United Kingdom) and transmission electron microscope (TEM, JEM-2100F, JEOL Ltd., Tokyo, Japan). An X-ray diffraction (XRD) study was carried out using an X-ray diffractometer (D8 advance, Bruker, Billerica, Massachusetts, United States) to examine the crystallographic property of the materials. The Brunauer-Emmett-Teller (BET) surface area and pore volume were measured by nitrogen adsorption/desorption isotherm method using a surface analyzer (MicrotracBEL Corp., Osaka, Japan).
2.3. Experiment
Batch adsorption experiments of Pb2+ and Ni2+ on the prepared MONPs-EG were carried out in single and binary metal systems. Adsorption isotherm experiments were studied with seven different concentrations of heavy metals ranging from 0.01 to 1.0 mM. The initial pH solution was adjusted to pH 5 using NaOH (0.1 M) and/or HCl (0.1 M). The experiments were performed using a fixed amount of MONPs-EG (0.5 g/L) for 100 mL metal solutions contained in series amber glass bottles, which were then shaken in an incubator (WIS-20, Daihan Scientific Co., Gangwon, Korea) for 12 h at a speed of 150 rpm and a temperature of 25 °C. The kinetic adsorption experiments studied 500 mL metal solutions of Pb2+ and/or Ni2+ prepared at initial concentrations of 0.1 and 0.01 mM, respectively. The experiment was performed for 4 h and samples were taken at given time intervals of the reaction, while other conditions were maintained the same as those in the isotherm experiments. In addition, to investigate the influence of natural organic matter on the adsorption behavior of the MONPs-EG, 10.0 mg/L humic acid was used. The concentration of metal ions in the solution was analyzed using inductively coupled plasma optical emission spectroscopy (Optima 7300 DV ICP-OES, Perkin Elmer Inc., Waltham, Massachusetts, United States), with method detection limits (MDL) of 1.7 μg/L and 0.5 μg/L for Pb2+ and Ni2+, respectively.
The amount of metal adsorbed onto the prepared MONPs-EG (q
e) was calculated by subtracting the equilibrium concentrations of metal from the initial concentration, as expressed by the following Equation:
where
C0 and
Ce are the initial and equilibrium concentrations of metals (mg/L), respectively,
V is the volume of the solution (L), and
m is the mass of the MONPs-EG (g).
2.4. Data Analysi
2.4.1. Adsorption Isotherms
The experimental equilibrium data of Pb
2+ and Ni
2+ adsorption onto the prepared MONPs-EG in single and binary metals systems were characterized using Langmuir and Freundlich isotherms [
28,
29,
30]; their mathematical expressions are given by the following Equations (2) and (3), respectively.
where
qmax is the maximum adsorption capacity of MONPs-EG for the metal ions (mmol/g),
KL is the Langmuir constant related to the affinity of the adsorption sites (L/mmol),
KF is the Freundlich constant related to the adsorption capacity (mmol/g) (L/mmol)
1/n, and
1/n is the Freundlich constant related to the adsorption intensity (dimensionless).
The comparison between adsorption performances of the prepared MONPs-EG with its precursor or with various manganese oxide-based adsorbents were evaluated. In addition to adsorption capacity, we also investigated the partitioning coefficient (PC) of the adsorbent toward target adsorbate, which represents the ratio of the adsorbate amount in the solid adsorbent phase to its concentration in the liquid phase at equilibrium [
31,
32]. The
PC can be calculated by the following Equation:
where
PC (mmol/g/mM) is the partitioning coefficient,
qe (mmol/g) is the adsorption capacity at equilibrium, and
Ce (mM) is the concentration of metal ions at equilibrium.
2.4.2. Adsorption Kinetics
The kinetics of metal ions adsorption in single and binary system is also studied to determine the type of mechanism and the potential rate-controlling step of the processes. In the present study, two different reaction kinetic models (i.e., pseudo-first order and pseudo-second order) were applied to determine the kinetics of Pb2+ and Ni2+ on to the prepared MONPs-EG.
The pseudo-first order model was first presented by Lagergren in 1898 [
33]; it has been widely used for adsorption kinetics study [
34,
35]. The equation can be expressed in non-linear and linear forms, as shown in Equations (5) and (6), respectively.
The pseudo-second order kinetic model was initially proposed by Blanchard et al. (1984) [
36], and was then developed by Ho and McKay (1999) [
37]. Its non-linear and linear forms can be expressed as shown in Equations (7) and (8), respectively.
where
qe and
qt (mmol/g) are the adsorption capacities at equilibrium and at time
t, respectively,
k1 (1/h) is the pseudo-first order rate constant,
k2 (g/mmol/h) is the pseudo-second order rate constant, and
t (h) is the adsorption time.
2.4.3. Thermodynamic Analysis
The thermodynamic parameters of the adsorption process, such as the Gibbs free energy change (ΔG
0), enthalpy change (ΔH
0), and entropy change (ΔS
0) were analyzed through variations of the thermodynamic equilibrium constant (
K0) at the investigated temperatures (298, 308, and 323 K). In this work,
K0 values of the adsorption of metal ions onto the prepared MONPs-EG were calculated based on the method proposed by Biggar and Cheung (1973) [
38], which can be defined as in the following Equation:
where
as and
γs are respectively the activity and activity coefficient of adsorbed metal ions on the MONPs-EG;
ae and
γe are respectively the activity and activity coefficient of metal ions in the solution at equilibrium;
Cs (mM) is the concentration of metal ions that adsorbed on the prepared adsorbent; and
Ce (mM) is the concentration of metal ions in the solution at equilibrium.
When the concentration of metal ions in the solution approaches zero, resulting in
Cs → 0 and
Ce → 0, and the activity coefficients approach unity [
38]. Equation (9) can be rewritten as:
The thermodynamic parameters were calculated as follows:
Gibbs free energy change of the adsorption process was directly estimated by Equation (11):
where
R is the universal gas constant (8.314 J/mol/K); and
T (K) is the absolute solution temperature.
The Gibbs free energy change is a function of the change in enthalpy and entropy, which is expressed by Equation (12).
Combining Equations (11) and (12) resulted in Equation (13).
ΔH0 and ΔS0 of the adsorption process will be, respectively, obtained from the slope (−ΔH0/R) and the intercept (ΔS0/R) of the plots of ln(K0) as a function of 1/T.