Background

With the rapid development of industrialization, water pollution has become an ever-increasing global concern up to now [1, 2]. More importantly, water pollution is becoming more and more serious due to the existence of organic dyes stemming from paper-making, leather, rubber and cosmetics [3]. Most of them not only cause serious environmental problems, but also threaten human life [4, 5]. Therefore, it is necessary to explore an environmentally friendly, low-cost and efficient method to remove organic dyes from contaminated water. In order to address this issue, various methods, such as chemical oxidation [6], photocatalysis [Adsorption experiments

The adsorption of MB or MO from aqueous solution on the AC samples was performed in a batch process. Typically, 20.0 mg C-X or PC-X-Y was added into 50.0 ml MB or MO solution with desired initial concentrations under stirring in a flask. The pH was adjusted with 0.1 M HCl or 0.1 M NaOH until it reached 6. After the equilibrium was reached, 1.0 ml of MB or MO solution was separated for subsequent analysis. The amount of MB or MO adsorption at equilibrium qe was calculated by the following equation:

$${q}_{e}=\frac{{(C}_{0}-{C}_{e})V}{W},$$
(1)

where c0 and ce (mg/l) are the liquid-phase concentration of MB before adsorption and at equilibrium, respectively. V (L) is the volume of the aqueous solution and W (g) is the mass of the adsorbent. The concentration of MB or MO was measured by UV–Vis–NIR spectrophotometry (Lambda 900, λmax = 665 ± 1 nm or λmax = 461 ± 1 nm). The experiments were repeated twice for reproducibility purposes.

Results and discussion

Physical structure

The surface structure and morphologies of the carbon samples were characterized by SEM and HRTEM, respectively. Figure 1a shows the SEM image of C-500, a cross-linked and microporous structure with open channel is clearly observed. After KOH activation, the porous carbons still remain with a 3D laminated structure as shown in Fig. 1b, c. The corresponding element map** clearly shows the uniform distribution of carbon and oxygen element, which is shown in Additional file 1: Figure S1. HRETM was applied to give evidence of the morphology and structure of the carbon samples before and after activation in Fig. 1d–f. The carbon materials before and after activation are obviously different. The raw carbon material before activation is composed of mesopores and macropores, whereas the material after activation has more micropores. As shown in Fig. 1e, f, most of the regions consist of dark and light microstructures indicating that disordered graphitic structure and activation process produced more defects in AC samples. There are still graphitic layers with a spacing distance of 0.35 nm [26].

Fig. 1
figure 1

SEM images of a C-500 and b PC-500-6 and c PC-800-6; HRTEM images of d C-500 and e PC-500-6 and f PC-800-6

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Material characterization

To determine the crystalline structure of the carbon materials, XRD patterns are recorded as displayed in Fig. 2a. There are two broad peaks at 2θ = 26 and 43° in C-500, which correspond to the (002) and (100) diffraction planes of graphitic carbon, respectively [27]. After chemical activation, significant changes in XRD files were observed for PC-500-Y. As shown in Fig. 2a, both peaks almost disappear when the mass ratio of KOH/carbon increases from 2 to 6, indicating KOH has destroyed the graphitic structure and more defects have been produced. The above results agree very well with the TEM analyses (Fig. 1e, f).

Fig. 2
figure 2

a XRD curves, b Raman spectra, c full XPS spectra of C-500 and PC-500-Y. d High-resolution C 1s XPS spectra of PC-500-6

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Raman spectroscopy can be employed to characterize the chemical structure of the carbon samples. As shown in Fig. 2b, there are two peaks located at 1350 cm−1 and 1580 cm−1 related to disordered carbon and E2g mode of sp2-hybridized graphitic carbon, respectively [28, 29]. The ratio of the D band to G band (ID/IG) is commonly used to estimate the degree of disordering of the samples [30, 31]. The higher value of ID/IG indicates the presence of more defects in carbon samples. The ID/IG of C-500 is 1.05, which is much higher than that for PC-500-2 (0.84), PC-500-4 (0.82) and PC-500-6 (0.73), indicating that the chemical activation process led to more defects appearing in the samples.

The elemental composition of the carbon samples was estimated by XPS; the full survey spectrum and high-resolution XPS spectra of the samples are shown in Figs. 2c and 6b. There are three characteristic peaks at 285 eV, 400 eV and 531 eV, which correspond to C 1s, N 1s and O 1s, respectively. After KOH activation, nitrogen element almost disappears. As shown in Fig. 2d, the deconvolution of the C1s from PC-500-6 indicated the presence of –C–C–, –C–OH, –C=O and –COOH; the corresponding peaks are located at 284.6 eV, 285.0 eV, 286.8 eV and 288.5 eV, respectively [32, 33]. In Fig. 6b, the O 1s spectrum consists of two peaks located at 533.7 eV and 532.3 eV, which correspond to O–C=O and –C–OH groups. [34] From XPS analyses, it was found that AC samples are mainly composed of carbon as well as a few functional groups on the surface of the samples.

The N2 adsorption/desorption technique was used to investigate the porosity properties of the samples: the corresponding parameters of the AC samples are summarized in Table 1. As shown in Fig. 3a, b, AC samples show a typical type-I isotherm with high adsorption capacity in the low relative pressure range (0–0.05), corresponding to the existence of micropores [35]. Moreover, the broadening of the adsorption knee in the relatively lower pressure range (0.05–0.4) suggested the formation of mesopores during the activation process. The pore size distributions also confirmed the existence of the mesopores in the AC samples [36, 37]. As the mass ratio of KOH/C increases from 2 to 6, the adsorption knee becomes wider, indicating more mesopores are generated in PC-500-6. This conclusion is consistent with details of the porosity parameters of C-X and PC-X-Y in Fig. 3c, d and Table 1. It can be clearly observed that the values of SSA and percentage of V0.8–3.0 in Vtotal go up when the KOH/C mass ratio is enhanced. For example, SBET of PC-500-2, PC-500-4 and PC-500-6 is 2200, 3090 and 3203 m2/g, respectively. In the same time, the value of V(0.8–3.0)/Vtotal increase from 53.2 to 86.9%.

Table 1 Porosity properties of C-X and PC-X-Y samples
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Fig. 3
figure 3

N2 adsorption/desorption isotherms of a C-500 and PC-500-Y, b C-800 and PC-800-Y. Pore size distribution of c C-500 and PC-500-Y, d C-800 and PC-800-Y

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Adsorption isotherms

The adsorption isotherm describes how the interaction happens between AC samples and MB molecules the interface of liquid and solid phase during the adsorption process. In this article, three adsorption isotherms were used to analyze the adsorption data by nonlinear equations, such as Langmuir, Freundlich and Temkin isotherms.

The Langmuir model based on the assumption of monolayer adsorbates on a homogenous surface [10, 38] was tested. It is expressed by:

$${q}_{e}={q}_{m}\frac{{K}_{L}{c}_{e}}{1+{K}_{L}{c}_{e}},$$
(2)

where KL is the Langmuir adsorption constant (L/mg), and qm is Langmuir monolayer adsorbate capacity (mg/g).

After linearization of the Langmuir isotherm Eq. (2), the linear equation is:

$$\frac{{C}_{e}}{{q}_{e}}= \frac{1}{{q}_{m}{K}_{L}}+\frac{1}{{q}_{m}}{c}_{e}.$$
(3)

Multilayer adsorbates are described using the Freundlich model, which assumes an energetically heterogeneous surface. The Freundlich isotherm is represented by the following equation [39, 40]:

$${q}_{e}={K}_{F}{c}_{e}^{1/n}.$$
(4)

The linear form is as following:

$$ln{q}_{e}=ln{K}_{F}+\frac{1}{n}ln{c}_{e},$$
(5)

where qe is the equilibrium concentration of dye on the adsorbent (mg/g), ce is the equilibrium concentration of dye in solution (mg/l), KF and n are the Freundlich constants, which represent the adsorption capacity (mg/g) and the adsorption strength, respectively. The magnitude of 1/n quantifies the favorability of adsorption and the degree of heterogeneity of the adsorbent surface.

The well-known form of the Temkin isotherm indicates the effects of some indirect adsorbate/adsorbate interactions on adsorption isotherms, the isotherm can be expressed as:

$${q}_{e}=\frac{RT}{{b}_{T}}\mathrm{ln}\left({K}_{T}{c}_{e}\right).$$
(6)

The linear form of the Temkin equation is: [41]

$${q}_{e}={B}_{T}ln{K}_{T}+{B}_{T}ln{c}_{e},$$
(7)

where BT = RT/bT, T is the absolute temperature in K, R is the universal gas constant (8.314J/mol/K), KT is the equilibrium binding constant (L/mg), and BT is related to the heat of adsorption and constant value of Temkin isotherm.

Figure 4, Table 2, Additional file 1: Figure S5, Table S1 display the equilibrium adsorption data for C-X and PC-X-Y linearly fitted to Langmuir, Freundlich and Temkin isotherms and corresponding parameters, respectively. As shown in Fig. 4a, the amount of adsorbed MB increases at a low initial concentration and reaches a plateau at a high equilibrium concentration, indicating Langmuir isotherms fitted well with the experiment data [42]. It means that the adsorption took place at the specific homogenous sites within the carbon materials and MB molecules as a single monolayer [43]. According to Fig. 4b–d, the Langmuir isotherms exhibit higher correlation coefficients (R2 > 0.999) than Freundlich or Temkin model, indicating that Langmuir isotherm is a more precise model to describe the adsorption process. Moreover, maximum adsorption capacity is found to increase from 751.88 to 925.93 mg/g with the increase of mass ratio (KOH/carbon) from 2 to 6 (Table 2). Among all AC samples, PC-500-6 exhibits the highest adsorption capacity of MB (925.93 mg/g), more than 35 times higher than that of C-500. The adsorption results indicate that PC-500-6 delivers superior performance in dye adsorption compared to published results (Additional file 1: Table S2). In addition, optical photographs were taken before and after MB adsorption (Additional file 1: Figure S2a). For example, after the adsorption of MB with an initial concentration of 300 mg/L on PC-500-6, the polluted water became clear and colorless, which further revealed the efficient adsorption and distinct discoloration for wastewater using PC-500-6.

Fig. 4
figure 4

a Equilibrium adsorption isotherms, b Langmuir, c Freundlich d and Temkin isotherms of MB on C-500 and PC-500-Y. (20 mg of C-500 or PC-500-Y added to a 50 ml MB solution (10–80 or 100–550 mg/L) at a designated concentration after stirring for 12 h.)

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Table 2 Langmuir, Freundlich and Temkin isotherm parameters for C-500 and PC-500-Y for MB adsorption
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In order to investigate the adsorption kinetics of MB adsorption on AC, the experimental kinetic data are fitted to pseudo-first-order and pseudo-second-order kinetic models. The applied kinetic equations are as follows.

The pseudo-first-order kinetic model can be expressed as [44]:

$$\mathrm{ln}\left({q}_{e}-{q}_{t}\right)=\mathrm{ln}\left({q}_{e}\right)-{k}_{1}t,$$
(8)

where qt is the amount of dye on the adsorbent at t min (mg/g), and k1 is the rate constant of the pseudo-first-order kinetic model for the adsorption (min1). The values of qe and k1 can be determined from the intercept and the slope of the linear plot of ln(qe − qt) versus t, respectively. The pseudo-second-order kinetic model is represented by the following equation [45, 46]:

$$\frac{t}{{q}_{t}}=\frac{1}{{k}_{2}{q}_{e}^{2}}+\frac{t}{{q}_{e}},$$
(9)

where k2 is the rate constant of the pseudo-second-order kinetic model for adsorption (g/mg/min). The slope and intercept of the linear plot of t/qt against t yield the values of qe and k2, respectively.

Figure 5, Additional file 1: Figure S6 and Figure S7 display the kinetic curves and linear fits for the adsorption of MB by PC-X-Y at 20 °C using the pseudo-first-order and pseudo-second-order models. Table 3, Additional file 1: Tables S3, S4 present the coefficients for the pseudo-first-order and pseudo-second-order kinetic models. All the experimental data comply better with the pseudo-second-order kinetic model in terms of higher correlation coefficient values (R2 > 0.99). This demonstrates that the pseudo-second-order kinetic model is more suitable to describe the adsorption behavior of MB by the PC-X-Y.

Fig. 5
figure 5

Kinetic curves of a the pseudo-second-order kinetic model, b PC-500-Y for the adsorption of MB. (Experimental conditions: MB concentration was 250 mg/L for PC-500-2, 300 mg/L for PC-500-4 and PC-500-6 and adsorbent concentration was 20 mg/L.)

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Table 3 Kinetic parameters of the pseudo-second-order kinetic model for MB on PC-500-Y
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Adsorption thermodynamics of the PC-500-6

During the adsorption process, thermodynamic parameters are important in controlling the adsorption behavior. The adsorption free energy (\(\Delta G\)), the adsorption entropy (\(\Delta S\)), and the adsorption enthalpy (\(\Delta H\)) were calculated using Eqs. (10) and (11) expressed as follows:

$$\mathrm{ln}\left(\frac{{q}_{e}}{{C}_{e}}\right)=\frac{-\Delta H}{RT}+\frac{\Delta S}{R},$$
(10)
$$\Delta G=\Delta H-T\Delta S,$$
(11)

where \({q}_{e}\) (mg/g) is the adsorption capacity, \({C}_{e}\) (mg/L) is the MB concentrations at equilibrium, \(\Delta G\) (kJ/mol) is the standard Gibbs free energy change, \(\Delta H\) (kJ/mol) is the standard enthalpy change, \(\Delta S\) (J/mol K) is the standard entropy change, R (8.314J/mol/K) is the gas constant, and T (K) is the absolute temperature.

The values of \(\Delta G\), \(\Delta H\) and \(\Delta S\) were calculated and shown in Table 4. The negative enthalpy change (\(\Delta H\) = − 0.91 kJ/mol) indicates that the adsorption reaction is an exothermal reaction, which is supported by the decreased adsorption capability with increasing of the temperature. The negative value of \(\Delta G\) reveals that adsorption of MB onto membrane is a spontaneous and feasible process. The negative entropy change (\(\Delta S\) = − 2.56 kJ/mol) indicates that the adsorption decreases randomly at the solid–solute interface during the adsorption. The thermodynamic calculations indicated that methylene blue adsorption was spontaneous and exothermic in nature.

Table 4 Thermodynamic parameters for the adsorption of MB on PC-500-6
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Figure 6a shows clearly the linear relationship between qm and specific surface area (SBET). It can be observed that the adsorption of MB has a strong correlation with SSA, indicating that SSA has a great impact on adsorption ability. For example, among all AC samples, PC-500-6 has the highest adsorption value of 917.43 mg/g (qm) due to the highest SSA of 3203 m2/g. Moreover, the pore volume and pore size distribution also have some influence on the adsorption capacity. In Table 1, AC samples with higher value of V(0.8–3.0 nm)/Vtotal have higher adsorption capacity. For example, PC-500-2, PC-500-4 and PC-500-6 show high adsorption values (751.88, 884.96 and 917.43 mg/g) as well as high values of V(0.8–3.0 nm)/Vtotal (53.2%, 88.1% and 86.9%). To understand the underlying mechanism more deeply, the molecular size of MB and pore size of AC are compared. As shown in Fig. 6c, the size of MB is about 0.7 nm × 1.6 nm, which can easily get into the pores with a diameter in the range of 0.8 nm to 3.0 nm. The dye adsorption capacity is also related to the pore volume of AC samples. As depicted in Table 1, the adsorption capacity increases as the pore volume was enhanced, such as the values of Vtotal for PC-500-2, PC-500-4 and PC-500-6 being 1.0, 1.7 and 2.1 cm3/g, and the corresponding adsorption values are 751.88, 884.96 and 917.43 mg/g, respectively. Based on above analyses, the adsorption capacity of AC is determined by three parameters: SSA, pore volume, and pore size distribution. The larger SSA provides more sites for the adsorption of MB molecules, and the suitable pore size distribution and large pore volume can also increase the adsorption capacity as well. Moreover, according to the high-resolution O 1s XPS spectra of PC-500-6, there were small quantities of oxygen-containing group on the surface of porous carbon. The presence of carboxylic and hydroxyl groups is considered to be good adsorption sites for cationic dyes driven by electrostatic interaction. The adsorption of MB dyes over the PC-500-6 adsorbent are collectively driven by the π–π interaction between the graphitic domains of carbon skeleton and aromatic rings of MB dye molecules, hydrogen linkages between the oxygen functionalities of PC-500-6 and nitrogen/oxygen centers in the dye molecules (Fig. 6c).

Fig. 6
figure 6

a Correlations between the qm and SBET (R2 = 0.95022); b high-resolution C 1s XPS spectra of PC-500-6; c a scheme for MB adsorption in the 3D porous carbon

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Conclusions

In summary, a 3D porous carbon was prepared from jujube at optimized carbonization temperatures and activation agent addition. The resulting 3D porous carbon shows a specific surface area (3203 m2/g) and suitable pore size distribution, leading to an excellent adsorption performance on both cationic dye and anionic dye from an aqueous solution, such as methylene blue (MB) and methylene (MO). PC-500-6 displays the high performance in adsorption MB and MO as high as 917.43 mg/g and 1281.39 mg/g, respectively. The present method paves a way to prepare biomass-derived AC for the removal of organic dyes from polluted water.