Open Access
Issue
Wuhan Univ. J. Nat. Sci.
Volume 29, Number 2, April 2024
Page(s) 165 - 176
DOI https://doi.org/10.1051/wujns/2024292165
Published online 14 May 2024

© Wuhan University 2024

Licence Creative CommonsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

0 Introduction

Rhodamine B (RhB), widely utilized in the fields such as textiles, leather and paper-making[1] is highly toxic to the surrounding because of its high aromaticity and rigid structure. Numerous strategies have been taken for the removal of these organic dyes in wastewater such as chemical oxidation[2], photo-catalysis[3,4] and adsorption[5,6]. Among these methods, adsorption is widely paid attention due to its high efficiency, low cost and easy operation. In recent years, researchers have focused on exploring new adsorbent materials with wide sources, low cost and well-controllable surface properties.

Biomass-derived biochar (BC) has provoked great interest in the field of pollution control, which not only reduces the environmental burden for realizing the carbon capture and sequestration, but also achieves the effect of "treating waste by waste"[7]. The conventional preparation methods of multi-porous BC are based on the etching of carbon atoms from a carbonaceous source by high-temperature oxidation processes using gases (i.e., CO2 or O2)[8] , solids (e.g., KOH[9,10], NaOH[11,12], KMnO4[13], (NH4)3PO4[14], etc.) or liquids (e.g., H3PO4[15], etc.) as chemical activating agents. Another approach is molten salt-assisted synthesis that involves the carbonization of biomass coupled with metal salts and the generation of nano-pores in the generated carbon with the removal of the salts. The widely-used molten salts are ZnCl2[16], CaCl2[17] and MgCl2[18]-based system in which ZnCl2 (CaCl2 or MgCl2) is a dehydration catalyst and a molecular template. A hierarchical porous biochar was formed by pyrolysis of cellulose (or starch)-magnesium nitrate solution with acid leaching[19,20] and used for supercapacitors. However, there have been few reports of producing porous carbon for pollution control by solid grinding of waste biomass and metal salt followed by pyrolysis.

Herein, we demonstrate an easy and large-scale strategy to fabricate multi-level pore biochar (MN-TRB750) using waste biomass, i.e. tea residue (TR) as the carbon source and magnesium nitrate as the metal salt. The as-prepared MN-TRB750 exhibites a frizzly flake-like morphology and possesses meso-porous and macro-porous structure with a high specific surface area up to 839.54 m2·g-1. MN-TRB750 was used for the removal of RhB, exhibiting a maximum adsorption capacity of 809.0 mg·g-1 with isotherms fitting well to Freundlich and Dubinin-Radushkevic (D-R) models and an equilibrium adsorption capacity of 757.58 mg·g-1 following the pseudo-second-order kinetics.

Overall, this work clarified the effect calcination temperature on microstructure of MN-modified biochar with the help of characteristic methods such as X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), scanning electron microscope (SEM), thermogravimetric analysis and differential scanning calorimetry (TG-DSC) and Fourier transform infrared spectroscopy (FT-IR), evaluated the adsorption abilities of MN-TRB750 for RhB from wastewater and probed the possible adsorption mechanism.

1 Materials and Methods

1.1 Materials

Tea residue (TR) was supplied by a drink shop at a campus dining room. The reagents of magnesium nitrate hexahydrate (Mg(NO3)2·6H2O, MN), magnesium oxide (MO) and RhB are of analytical level and are supplied by Zhiyuan Chemical Reagent Co., Ltd (Tianjin, China). Deionized water is used in all experiments. The stock solution of RhB was prepared by dissolving accurately weighed RhB in distilled water to give a concentration of 1 200 mg·L-1. The required concentration was obtained by successive dilutions in further experiments.

1.2 Preparation

TR, the raw material for the preparation of biochar, was firstly washed with deionized water and dried in an oven at 80 ℃ for 12 h. Then it was evenly mixed with MN at a mass ratio of 1:1 in a solid grinding manner. Then the mixture was transferred into a porcelain crucible with a cover, wrapped by aluminum foil. Then carbonization was performed in a muffle at 200 ℃ for 0.5 h followed with calcination temperature (600-950 ℃) for 2 h. After cooling down, the obtained carbon material was washed with HCl solution (2 mol·L-1) followed by deionized water for several times and dried at 80 ℃ for 12 h. The product was denoted as MN-TRBT which was obtained by calcination of TR and MN with the mass ratio of 1:1. In contrast, TRBT was obtained by direct calcination of TR. Herein T represents pyrolysis temperature (T = 600-950 ℃).

1.3 Characterization

The morphology and species phase of samples were monitored via a scanning electron microscope (FlexSEM1000, Hitachi, Japan) and an X-ray diffractometer (DX-2700BH, China) with Cu Ka radiation, respectively. The pyrolysis process of the sample was monitored by a thermogravimetry analysis instrument (Netzsch STA449F3, Germany). The texture structure of MN-TRBT was determined via the N2 adsorption-desorption apparatus (ASAP2460, Micromeritics, America). The functional groups of MN-TRB750 were detected via Fourier-transform infrared spectroscopy (FTIR, Spectrum65, Germany).

1.4 Batch Adsorption Experiments

For each experiment, a given dose (0.010 0-0.010 5 g) of adsorbent and 30 mL of RhB solution with a fixed concentration (150-260 mg·L-1) were added into a 50 mL PVP flask and vibrated at 140 r/min for 2 h. The concentration of RhB was determined with a UV-visible spectrophotometer (UV-8000S) at the wavelength of 551 nm. The removal rate (RD, %) and the equilibrium adsorption amount (qe, mg·g-1) were obtained from Eqs. (1) and (2), respectively.

In some cases, the RhB-loaded MN-TRB750 was separated through centrifugalization and sonicated using ethanol as eluent for 2 h. The regenerated MN-TRB850 was washed with ethanol and ultrapure water successively three times, then dried at 80 ℃ and repeated for the subsequent reuse experiment.

(1)

(2)

where C0 and Ce (mg·L-1) are the initial and equilibrium concentrations of RhB, respectively; V (L) is 0.03 L and w (g) is the weight of the adsorbent.

The adsorption kinetics data was obtained by recording RhB concentration at a fixed time interval as C0 is 260 mg·L-1 at 20 ℃ and pH=7.0. The adsorption capacity at a time interval (qt) was calculated in the same way as Eq. (4), except Ce was replaced with the concentration of a time interval (Ct).

Pseudo-first-order (PFO), pseudo-second-order (PSO), intraparticle diffusion (IPD) and Elovich model (EM) were used to fit the kinetic data[21]. The corresponding equations are as the following:.

(3)

(4)

(5)

(6)

where qt (mg·g-1) and qe (mg·g-1) are the RhB adsorption capacities at various and equilibrium times t (min), respectively; kid (g·mg-1·min-1/2), k1 (min-1) and k2 (g·mg-1·min-1 ) are the rate constants corresponding to IPD, PFO and PSO model, respectively; C (mg·g-1) is a constant that involves the thickness and the boundary layer. α (g·mg-1·min-1) and 1/β (g·mg-1) are the initial sorption rate and desorption constant that involve activation energy and surface cover corresponding to EM, respectively.

To examine the influence of temperature on isothermal adsorption, the adsorption equilibrium experiments were conducted at 293, 303 and 313 K at pH=7.0. Three models of Langmuir (Eq. (7)), Freundlich (Eq. (8)) and D-R (Eq. (9)) were applied to describe the adsorption equilibrium.

(7)

(8)

(9)

where qe (mg·g-1) and qm (mg·g-1) are the equilibrium and maximum adsorption capacity of RhB, respectively; b (L·mg-1) and Kf (mg·g-1 (L·mg-1)1/n) are the constants of the Langmuir and Freundlich models, respectively; ε is the Polanyi factor, β (mol2/J2) denotes the D-R model constant, R is the universal constant of gases, and T (K) is the absolute temperature. The mean free energy of adsorption (E,kJ·mol-1) per molecule of the adsorbate when it is transferred to the surface of the sorbent from infinity in the solution was calculated from the following equation[22].

(10)

2 Results and Discussion

2.1 Characterization Result

2.1.1 XRD and FTIR

The XRD patterns of TRBT and MN-TRBT are shown in Fig. 1. There are two broad and low peaks at 2θ = 23° and 44°for all samples that were calcinated at 450-950 ℃, which are assigned to the (0 0 2) and (1 0 1) planes of graphitic carbon, respectively. It could be found from Fig. 1(a) that the main component of TR is cellulose (CS) because the characteristic diffraction peak of amorphous CS appeared at 2θ value of ~21.6°. The XRD patterns of TRB100-300 are consistent with that of the original CS, indicating that the structure of TR is unchanged at the calcination temperature of 100-300 ℃. The XRD pattern of TRB450 obviously changed, i.e., a broad peak occurs at 2θ = ~23° coupled with the color of the sample changing from brown to black indicating the formation of amorphous carbon. A higher-intensity change of peak at 23° and a sharper diffraction peak at 26.5° occurred as the calcination temperature was increased to 750-850 ℃, indicating that a more orderly carbon layer structure is formed[23]. The ordered graphite structure can facilitate the formation of π-π bond that is favorable for the adsorption of pollutants containing conjugated structure in the adsorption process[24].

thumbnail Fig.1 XRD patterns and color change of TRB (a) and MN-TRBT (b)

Inset of (b): The characteristic peaks of MgO at 42.8° and 62.2° (2θ)

It can be seen from Fig. 1(b) that the XRD pattern of MN-TRB100 shows the characteristic peaks of Mg(NO3)2·6H2O (PDF#14-0101) and the color of the sample is still brown, indicating that no apparent changes occur for the structure of TR and Mg(NO3)2·6H2O. As the calcination temperature increases to 200 ℃, the characteristic peaks of MgO occur at 42.8° and 62.2° (2θ) (PDF#45-0469) indicating that Mg(NO3)2·6H2O has been decomposed into MgO. Simultaneously, the characteristic peak of cellulose disappears at 2θ value of 21.6° coupled with the occurrence of amorphous carbon at 23°. The color of MN-TRB200 also turns into dark black. The conclusion may be obtained that the carbonization temperature of TR cellulose decreased from 450 ℃ to 200 ℃ in the existence of Mg(NO3)2·6H2O by comparing the XRD patterns of TRB200-450 and MN-TRB200-450.

2.1.2 TG-DSC

In order to further elucidate the pyrolysis process of TR and MN-TR, the TG-DSC curves were displayed in Fig. 2. For the TR, about 7.5% of weight loss occurs at pre-72 ℃ with the loss of free and bound water exited in the TR matrix (Fig. 2(a)). The maximum weight loss (52%) happens at 245-369 ℃ corresponding to the rapid decomposition of hemicellulose to volatile matters and tars coupled with an exothermic peak of 412 ℃. The final weight loss (15%) occurs at 370-700 ℃ coupled with an exothermic peak of 642 ℃, which attributes to the carbonization of cellulose and lignin[25]. TG-DSC curves of MN-TR are obviously different from those of TR alone (Fig. 2(b)). The first stage of weight loss (23%) occurs at pre-200 ℃ coupled with two endothermic peaks at 106 and 162.8 ℃, which ascribes to the loss of matrix-water (7.5%) and hemicellulose carbonization (28.5%). The second weight loss (20%) occurs at 185-360 ℃ for the further carbonization of initial biochar, cellulose and lignin. The final weight loss (12%) occurs at 360-500 ℃ for the further decomposition of Mg(OH)2. It can be concluded from the results of Figs. 1 and 2 that MN obviously decreases the TR's initial carbonization temperature.

thumbnail Fig. 2 TG-DSC curves of TR (a) and TR mixed with MN (b)

2.1.3 SEM

The effects of calcination temperature on the surface morphology and pore structure of TRB and MN-TRB are displayed in Fig. 3. TRB200 and TRB300 keep the power state of TR until the pyrolysis temperature of 450 ℃ (Fig. 3(a)-(c)), which is consistent with the results of Fig. 1. Carbon particles occur on the gulf and curl structure as temperature increases to 950 ℃ (Fig. 3(d)-(g)). In contrast, the gulf and curl biochar have been formed in MN-TRB200 (Fig. 3(h)). Then a loose porosity packed structure appears as the pyrolysis temperature is 300-600 ℃ (Fig.3(i)-(k)). A frizzly flake-like structure has been formed at a calcination temperature of 750-950 ℃ (Fig.3(l)-(n)). The results of Fig. 3 illustrated that MN obviously decreases the carbonization temperature of TR and changes the biochar morphology.

thumbnail Fig.3 SEM images of TRB and MN-TRB at different calcination temperature

(a) TRB200, (b) TRB300, (c) TRB450 , (d) TRB600 , (e) TRB750 , (f) TRB850 , (g) TRB950 , (h) MN-TRB200 , (i) MN-TRB300 , (j) MN-TRB450,

(k) MN-TRB600, (l) MN-TRB750, (m) MN-TRB850, (n) MN-TRB950

Moreover, the effect of calcination temperature on the adsorption capacity of TRB and MN-TRB samples toward RhB was also studied. As shown in Fig. 4, the adsorption capacity of MN-TRB is significantly higher than that of TRB at a calcination temperature of 750-950 ℃ due to the obvious difference in morphology and porosity structure between MN-TRB and TRB. The adsorption capacity of MN-TRB750 (466.7 mg·g-1) is obviously higher than that of MN-TRB600 (331.1 mg·g-1) and slightly lower than those of MN-TRB850 (501.9 mg·g-1) andMN-TRB950(511.7 mg·g-1). Meanwhile, the biochar yield obviously decreases from 13.5% to 7.61% and 2.91% with the increment of calcination temperature from 750 to 850 and 950 ℃ (inset of Fig. 4). So, the results of XRD, SEM and Fig. 4 indicate that the structure and adsorption behavior of MN-TRB750 are the optimum. So only MN-TRB750 and its contrast with TRB750 are further characterized.

thumbnail Fig. 4 Effects of calcination temperature on adsorption capacity of TRB and MN-TRB samples toward RhB

Inset: The biochar yield with the calcination temperature; Adsorption conditions: C0 = 180 mg·L-1,T = 313 K, t = 2 h and pH = 7.5

The infrared spectra of MN-TRB750 and TRB750 are illustrated in Fig. 5(a). The C=C stretching vibration of the aromatic ring occurs at 1 567.4 cm-1.The C—OOH vibration appears at 1 414.8-1 349.2 cm-1 group except for the stretching band of CO2 at 2 371.5 cm-1, indicating the existence of aromatic structure and carboxylic acid in biochar. For TRB750, the stretching vibration of C=O derived from RCOOH occurs at 1 727.5 cm-1 and the bending vibration of =C—H occurs at 716.0 and 603.1 cm-1.

thumbnail Fig. 5 (a) The infrared spectra, (b) N2 adsorption-desorption isotherms and (c) dV/dlogD with pore size distribution of MN-TRB750 and TRB750

Inset of (c): dV/dlogD with pore size distribution of MN-TRB750 and TRB750 beyond 40 nm

It can be suggested from Fig. 5(a) that MN improved the extent of aromatic structure and carboxylic acid. The N2 adsorption-desorption isotherms and pore size distribution of MN-TRB750 and TRB750 are shown in Fig. 5(b) and (c), respectively. A porous structure was observed for MN-TRB750 (Fig. 5(b)), which was clearly indicated from the type IV isotherm together with an H4-type hysteresis loop based on the IUPAC classification. The volume of the adsorbed N2 was steeply increased with increasing p/p0in the range of p/p0< 0.1, indicating the presence of rich micropores. Further, the gradual increment of the amount of N2 adsorbed in the medium p/p0 (0.41-0.80), high p/p0(0.8-1.0) and the near parallel part of the hysteresis loop above p/p0= 0.41-0.8 indicated the presence of numerous meso-pores and macro-pores in MN-TRB750.

In contrast, the adsorbed N2 volumes of TRB are markedly lower than those of MN-TRB750 in the low p/p0<0.1, medium p/p0 (0.3-0.8) and high p/p0(0.8-1.0). The differences of pore structure between MN-TRB750 and TRB750 can be further confirmed by the results of Fig. 5(c) and Table 1. As shown in Fig. 5(c), the pore size distribution calculated by the Barrett-Joyner-Halenda (BJH) method from the desorption branch of the isotherm, displays that the pore peaks of MN-TRB750 and TRB750 are at 3-4 nm. However, the value of dV/dlogD of MN-TRB750 is obviously higher than that of TRB750 within 20 nm. No dV/dlogD appears as pore size is beyond 50 nm for TRB750, indicating that TRB750 has no macro-pores (inset of Fig. 5(c)). As shown in Table 1, the BET surface area (SBET) and meso-pore volume MN-TRB750 are obviously higher than those of TRB750. On the whole, the results of Fig. 5(b) and (c) illustrated that MN promoted the generation of meso- and macro-pores in biochar.

On the whole, the results of Figs. 1-5 illustrate that MN obviously decreases the carbonization temperature of TR, improves the extent of aromatic structure and carboxylic acid of MN-TRB750, promotes the formation of frizzly flake-like morphology and obviously increases the volume of meso-pore, macro-pore and adsorption capacity of RhB.

Table 1

The texture information of MN-TRB750 and TRB750

2.2 Effects of Various Factors on RhB Adsorption onto MN-TRB750

2.2.1 Effect of pH

Figure 6 shows the effect of solution pH on RhB adsorption in MN-TRB750 at 20 ℃ as C0 is 240.0 mg·L-1. The qe and RD of RhB are 647.3-666.7 mg·g-1 and 95.76%-93.49% at all adjusted pH values (4.5-13.0) (Fig. 6(a)). As shown in Fig.6(b), the initial pH of pure RhB solution is fixed at about 3.70 due to the existence of the COOH group in the RhB structure. Each adsorption equilibrium pH (pHe) is consistent with each corresponding adjusted pH value, which indicates that solution pH has no obvious changes before and after adsorption. So solution pH has little influence on the present adsorption process. This may be ascribed to the strong buffering effect between MN-TRB750 and RhB because of the existence of —COOH in the structure of MN-TRB750 and RhB (Fig.5(a))[26].

thumbnail Fig. 6 Effect of pH on MN-TRB750 for RhB adsorption (a) and initial pH of pure RhB solution (pH0) and pH value (pHe) of adsorption equilibrium (C0 = 260.0 mg·L-1, T = 20 ℃) (b)

The results of Fig. 6 indicate that the electrostatic force between MN-TRB750 and RhB is not the main factor for the adsorption.

2.2.2 Effect of contact time and kinetic studies

The effect of contact time and kinetic fit curves of MN-TRB750 for RhB adsorption are shown in Fig. 7. In Fig. 7(a), the removal rate of RhB on MN-TRB750 is sharply increased to 71.2% with an adsorption capacity of 622.5 mg·g-1 at the contact time of 5 min. The adsorption tends to be equilibrium at 40 min. The equilibrium adsorption capacity and removal rate are 731.85 mg·g-1 and 91.3%, respectively. In the initial stage, MN-TRB750 provides sufficient adsorption sites for RhB, and the significant concentration difference between the adsorbent and RhB results in quick RhB adsorption. With the adsorption progress, the adsorption sites reduce and the driving force of the adsorption process weakens, leading to a slower adsorption rate.

thumbnail Fig. 7 Effect of contact time (a) and kinetic fit curves of MN-TRB750 for RhB adsorption

C0 = 260 mg·L-1,T = 20 ℃, pH=7.0

The adsorption process is fitted linearly with four kinetic models (Fig. 7(b)-(e)) and the corresponding kinetic parameters are listed in Table 2. From Fig. 7 and Table 2, PSO is the best model describing the sorption of RhB onto MN-TRB750 with R2=1.0. Moreover, the value of qe (757.6 mg·g-1) obtained from PSO is similar to the experimental qe. So the sorption of RhB onto MN-TRB750 is mainly controlled by chemical adsorption.

The IPD shows two crossed lines due to the difference of mass transfer rate within 30 min, indicating that the adsorption process is divided into two stages (Fig. 7(d)). The linear relationship of the first stage passes through the origin, which means intraparticle diffusion is the rate-controlling step. The rate constant of the first stage (k1d) is nearly 10 times that of the second stage (k2d) due to the difference of the diffusion rate of RhB perpetrating MN-TRB750 at the two stages. RhB molecules fast diffuse into the exterior surface from the meso- and macro-pores of MN-TRB750 until saturation in the first adsorption step. In the second step, RhB molecules tardily perpetrate the interior surface of the meso-, macro-pores and micro-cavities of MN-TRB750, which causes a sharp decrement of k2d.

EM is helpful in energetically heterogeneous adsorbent surfaces. The correlation coefficients of EM are also beyond 0.99, which may support the chemical sorption of RhB molecular on MN-TRB750 heterogeneous surface. The plot between qt and lnt gives α and β values (Fig. 7(e)). As shown in Table 2, the value of α (9 660 g·mg-1·min-1) was much higher than that of β (0.008 174 g·mg-1·min-1), which indicates that RhB sorption onto MN-TRB750 is nonreversible to a higher degree.

On the whole, the results of Fig. 7 and Table 2 illustrate that the present adsorption is an irreversible chemical process with the intraparticle diffusion control.

Table 2

The kinetic model parameters for the adsorption of RhB onto MN-TRB750

2.2.3 Adsorption isotherms and thermodynamics of RhB adsorption onto MN-TRB750

The simulated isotherms of 293-313 K are presented in Fig. 8. The parameters corresponding to Langmuir and Freundlich model are listed in Table 3. Compared with the results of Fig. 8 and Table 3, the Freundlich model is fitted better with the present thermodynamic adsorption process than the Langmuir model due to the higher value of R2 (0.995-0.999), implying the multilayer adsorption of RhB on MN-TRB750 surface.

thumbnail Fig. 8 Isotherm fit curves and various model-fitted curves of MN-TRB750 for RhB adsorption

The results of the D-R model also gave a higher value of R2 (0.984-0.996) at various temperatures (Table 3), indicating pore filling dominates the adsorption process[29]. The free energy of adsorption (E) can predict whether an adsorption process is chemical or simply physical in nature. If 8 kJ·mol-1<E<16 kJ·mol-1, it is considered that the adsorption occurs through chemical adsorption, whereas E<8 kJ·mol-1 demonstrates a physical adsorption process. Herein the E valuedecreases with the increasing of adsorption temperature and is 9.05-12.31 kJ·mol-1 suggesting a chemical process. The maximum monolayer adsorption (qm) derived from the D-R model and the Freundlich constant related to adsorption capacity (KF) significantly decrease with the increase of the temperature from 293 to 313 K (Table 3) due to an exothermic nature.

As shown in Table 4, the internal energy change and interaction mechanism are investigated by the free energy change (ΔG), enthalpy change (ΔH), and entropy change (ΔS). The higher the temperature, the lower the negative ΔG value, implying an unfavorable adsorption energy. ΔH is negative indicating that RhB adsorption onto MN-TRB750 is endothermic and increasing temperature is adverse for the adsorption process. The reason may be that higher temperatures induce rapid inter-molecular movements and increase the mobility of RhB molecules on the MN-TRB750 surface. Further, ΔS is also negative, which means the randomness is of decrement for RhB due to the anchoring role of multi-active center adsorption in the adsorbent/solution interface. On the whole, it is concluded from the results of Figs. 7 and 8 that the adsorption reaction between RhB and MN-TRB750 is spontaneous and exothermic.

Table 3

Parameters derived from three models for RhB adsorption onto MN-TRB750

Table 4

Adsorption thermodynamic parameters of RhB adsorbed onto MN-TRB750 in aqueous solution

2.2.4 Adsorption mechanism

The results of PSO, EM and D-R models suggested the adsorption of RhB onto MN-TRB750 is controlled by chemical adsorption via pore filling and surface diffusion because of the enormous specific surface area and ideal pore structure of MN-TRB750. Hydroxyl bonds can occur between –OH derived from MN-TRB750 surface and aromatic ring or O and N atoms in RhB dye[27]. The benzene ring of RhB can interact with the benzene ring of MN-TRB750 through π-π interactions[28]. Thus, it can be inferred that high pore filling, hydrogen bond, π-π interaction determined the adsorption capacity of MN-TRB750. The possible adsorption mechanisms of MN-TRB750 are illustrated in Fig. 9.

thumbnail Fig. 9 Schematic diagram of adsorption mechanism for RhB on MN-TRB750

2.2.5 Comparison of the adsorption capacity of MN-TRB750 with other biomass-derived adsorbents

Directly comparing the adsorption capacity of MN-TRB750 with other waste biomass-derived adsorbents is challenging due to the various fabrication conditions and different adsorption parameters. Considering that TR belongs to plants, the adsorption capacity of biomass-based adsorbents for RhB are summarized in Table 5. As shown in Table 5, compared with the data as cited in the literature, it is evident that MN-TRB750 is an excellent adsorbent for RhB due to the facile synthesis method and higher adsorption performance.

Table 5

Comparison of adsorption capacity of various adsorbents for RhB

2.2.6 Regeneration and potential of MN-TRB750

Reuse performance is a critical indicator of the adsorbent for practical applications. Herein ethanol solvent is used to desorb RhB from MN-TRB750. The removal rate declines with the increase in cycle numbers (see Fig. 10). However, the removal efficiency remains at 88.7% after five cycles. The decline in removal rate results from the fact that some adsorption sites are irreversible. It may also be that some meso-pores or micropores adsorbed by RhB are not washed off during the elution process[33]. The reuse results show that MN-TRB750 is a well potential adsorbent for removing RhB because of its good regeneration performance.

thumbnail Fig.10 The reuse of MN-TRB750 for RhB adsorption

3 Conclusion

A novel MN-TRB750 was successfully synthesized by direct pyrolysis of the mixture of TR and MN. MN-TRB750 exhibited a frizzly flake-like morphology with a BET surface area of 839.54 m2·g-1 and an average pore diameter of 3.75 nm. MN decreased the carbonization temperature of biomass, promoted the extent of aromatics and improved the meso- and macro-pores in biochar. The adsorption of RhB onto MN-TRB750 was a spontaneous, exothermic and chemical process. The pore filling, hydrogen bond and π-π interaction dominated the adsorption.

References

  1. Khan N, Chowdhary P, Gnansounou E, et al. Biochar and environmental sustainability: Emerging trends and techno-economic perspectives[J]. Bioresource Technology, 2021, 332: 125102. [Google Scholar]
  2. Sun P, Zhang K K, Gong J Y, et al. Sunflower stalk-derived biochar enhanced thermal activation of persulfate for high efficient oxidation of p-nitrophenol[J]. Environmental Science and Pollution Research, 2019, 26(26): 27482-27493. [Google Scholar]
  3. Orooji Y, Ghanbari M, Amiri O, et al. Facile fabrication of silver iodide/graphitic carbon nitride nanocomposites by notable photo-catalytic performance through sunlight and antimicrobial activity[J]. Journal of Hazardous Materials, 2020, 389: 122079. [Google Scholar]
  4. Orooji Y, Mohassel R, Amiri O, et al. Gd2ZnMnO6/ZnO nanocomposites: Green sol-gel auto-combustion synthesis, characterization and photocatalytic degradation of different dye pollutants in water[J]. Journal of Alloys and Compounds, 2020, 835: 155240. [Google Scholar]
  5. Wasilewska M, Marczewski A W, Deryło-Marczewska A, et al. Nitrophenols removal from aqueous solutions by activated carbon—Temperature effect of adsorption kinetics and equilibrium[J]. Journal of Environmental Chemical Engineering, 2021, 9(4): 105459. [Google Scholar]
  6. Ma H F, Xu Z G, Wang W Y, et al. Adsorption and regeneration of leaf-based biochar for p-nitrophenol adsorption from aqueous solution[J]. RSC Advances, 2019, 9(67): 39282-39293. [Google Scholar]
  7. Davarazar M, Jahanianfard D, Sheikhnejad Y, et al. Underground carbon dioxide sequestration for climate change mitigation—A scientometric study[J]. Journal of CO2 Utilization, 2019, 33: 179-188. [Google Scholar]
  8. Díez N, Fuertes A B, Sevilla M. Molten salt strategies towards carbon materials for energy storage and conversion[J]. Energy Storage Materials, 2021, 38: 50-69. [Google Scholar]
  9. Mu Y K, Du H X, He W Y, et al. Functionalized mesoporous magnetic biochar for methylene blue removal: Performance assessment and mechanism exploration[J]. Diamond and Related Materials, 2022, 121: 108795. [Google Scholar]
  10. Liu N, Liu Y G, Zeng G M, et al. Adsorption of 17 β-estradiol from aqueous solution by raw and direct/pre/post-KOH treated lotus seedpod biochar[J]. Journal of Environmental Sciences, 2020, 87: 10-23. [Google Scholar]
  11. Mu Y K, Ma H Z. NaOH-modified mesoporous biochar derived from tea residue for methylene blue and orange II removal[J]. Chemical Engineering Research and Design, 2021, 167: 129-140. [Google Scholar]
  12. Wu Y J, Zhong J M, Liu B. Effective removal of methylene blue with zero-valent iron/tea residual biochar composite: Performance and mechanism[J]. Bioresource Technology, 2023, 371: 128592. [Google Scholar]
  13. Zhao N, Deng L B, Luo D W, et al. One-step fabrication of biomass-derived hierarchically porous carbon/MnO nanosheets composites for symmetric hybrid supercapacitor[J]. Applied Surface Science, 2020, 526: 146696. [Google Scholar]
  14. Li J Q, He F F, Shen X Y, et al. Pyrolyzed fabrication of N/P Co-doped biochars from (NH4)3PO4-pretreated coffee shells and appraisement for remedying aqueous Cr(VI) contaminants[J]. Bioresource Technology, 2020, 315: 123840. [Google Scholar]
  15. Zeng H T, Zeng H H, Zhang H, et al. Efficient adsorption of Cr (VI) from aqueous environments by phosphoric acid activated eucalyptus biochar[J]. Journal of Cleaner Production, 2021, 286: 124964. [Google Scholar]
  16. Zou K X, Guan Z X, Deng Y F, et al. Nitrogen-rich porous carbon in ultra-high yield derived from activation of biomass waste by a novel eutectic salt for high performance Li-ion capacitors[J]. Carbon, 2020, 161: 25-35. [Google Scholar]
  17. Pampel J, Mehmood A, Antonietti M, et al. Ionothermal template transformations for preparation of tubular porous nitrogen doped carbons[J]. Materials Horizons, 2017, 4(3): 493-501. [Google Scholar]
  18. Mehmood A, Ali G, Koyutürk B, et al. Nanoporous nitrogen doped carbons with enhanced capacity for sodium ion battery anodes[J]. Energy Storage Materials, 2020, 28: 101-111. [Google Scholar]
  19. Zhang W Z, Wang H L, Liao R X, et al. Salt-assisted in situ formation of N-doped porous carbons for boosting K+ storage capacity and cycling stability[J]. New Carbon Materials, 2021, 36(1): 167-178. [Google Scholar]
  20. Cao J H, Zhu C Y, Aoki Y, et al. Starch-derived hierarchical porous carbon with controlled porosity for high performance supercapacitors[J]. ACS Sustainable Chemistry & Engineering, 2018, 6(6): 7292-7303. [Google Scholar]
  21. Sun Y C, Wang T T, Han C H, et al. Facile synthesis of Fe-modified lignin-based biochar for ultra-fast adsorption of methylene blue: Selective adsorption and mechanism studies[J]. Bioresource Technology, 2022, 344: 126186. [Google Scholar]
  22. Sarkar B, Xi Y F, Megharaj M, et al. Synthesis and characterisation of novel organopalygorskites for removal of p-nitrophenol from aqueous solution: Isothermal studies[J]. Journal of Colloid and Interface Science, 2010, 350(1): 295-304. [Google Scholar]
  23. Trinh V T, Nguyen T M P, Van H T, et al. Phosphate Adsorption by silver nanoparticles-loaded activated carbon derived from tea residue[J]. Scientific Reports, 2020, 10: 3634. [Google Scholar]
  24. Yu W C, Lian F, Cui G N, et al. N-doping effectively enhances the adsorption capacity of biochar for heavy metal ions from aqueous solution[J]. Chemosphere, 2018, 193: 8-16. [Google Scholar]
  25. Shang H S, Lu Y J, Zhao F, et al. Preparing high surface area porous carbon from biomass by carbonization in a molten salt medium[J]. RSC Advances, 2015, 5(92): 75728-75734. [Google Scholar]
  26. Mu Y K, Du H X, He W Y, et al. Functionalized mesoporous magnetic biochar for methylene blue removal: Performance assessment and mechanism exploration[J]. Diamond and Related Materials, 2022, 121: 108795. [Google Scholar]
  27. Jawad A H, Mubarak N S A, Sabar S. Adsorption and mechanism study for reactive red 120 dye removal by cross-linked chitosan-epichlorohydrin biobeads[J]. Desalination and Water Treatment, 2019, 164: 378-387. [Google Scholar]
  28. Gai S, Zhang J, Fan R Q, et al. Highly stable zinc-based metal-organic frameworks and corresponding flexible composites for removal and detection of antibiotics in water[J]. ACS Applied Materials & Interfaces, 2020, 12(7): 8650-8662. [Google Scholar]
  29. Gupta N, Kushwaha A K, Chattopadhyaya M C. Adsorption studies of cationic dyes onto Ashoka (Saraca asoca) leaf powder[J]. Journal of the Taiwan Institute of Chemical Engineers, 2012, 43(4): 604-613. [Google Scholar]
  30. Goswami M, Phukan P. Enhanced adsorption of cationic dyes using sulfonic acid modified activated carbon[J]. Journal of Environmental Chemical Engineering, 2017, 5(4): 3508-3517. [Google Scholar]
  31. Vigneshwaran S, Sirajudheen P, Nikitha M, et al. Facile synthesis of sulfur-doped chitosan/biochar derived from tapioca peel for the removal of organic dyes: Isotherm, kinetics and mechanisms[J]. Journal of Molecular Liquids, 2021, 326: 115303. [Google Scholar]
  32. Lv L J, Huang Y, Cao D P. Nitrogen-doped porous carbons with ultrahigh specific surface area as bifunctional materials for dye removal of wastewater and supercapacitors[J]. Applied Surface Science, 2018, 456: 184-194. [Google Scholar]
  33. Yao X X, Ji L L, Guo J, et al. Magnetic activated biochar nanocomposites derived from wakame and its application in methylene blue adsorption[J]. Bioresource Technology, 2020, 302: 122842. [Google Scholar]

All Tables

Table 1

The texture information of MN-TRB750 and TRB750

Table 2

The kinetic model parameters for the adsorption of RhB onto MN-TRB750

Table 3

Parameters derived from three models for RhB adsorption onto MN-TRB750

Table 4

Adsorption thermodynamic parameters of RhB adsorbed onto MN-TRB750 in aqueous solution

Table 5

Comparison of adsorption capacity of various adsorbents for RhB

All Figures

thumbnail Fig.1 XRD patterns and color change of TRB (a) and MN-TRBT (b)

Inset of (b): The characteristic peaks of MgO at 42.8° and 62.2° (2θ)

In the text
thumbnail Fig. 2 TG-DSC curves of TR (a) and TR mixed with MN (b)
In the text
thumbnail Fig.3 SEM images of TRB and MN-TRB at different calcination temperature

(a) TRB200, (b) TRB300, (c) TRB450 , (d) TRB600 , (e) TRB750 , (f) TRB850 , (g) TRB950 , (h) MN-TRB200 , (i) MN-TRB300 , (j) MN-TRB450,

(k) MN-TRB600, (l) MN-TRB750, (m) MN-TRB850, (n) MN-TRB950

In the text
thumbnail Fig. 4 Effects of calcination temperature on adsorption capacity of TRB and MN-TRB samples toward RhB

Inset: The biochar yield with the calcination temperature; Adsorption conditions: C0 = 180 mg·L-1,T = 313 K, t = 2 h and pH = 7.5

In the text
thumbnail Fig. 5 (a) The infrared spectra, (b) N2 adsorption-desorption isotherms and (c) dV/dlogD with pore size distribution of MN-TRB750 and TRB750

Inset of (c): dV/dlogD with pore size distribution of MN-TRB750 and TRB750 beyond 40 nm

In the text
thumbnail Fig. 6 Effect of pH on MN-TRB750 for RhB adsorption (a) and initial pH of pure RhB solution (pH0) and pH value (pHe) of adsorption equilibrium (C0 = 260.0 mg·L-1, T = 20 ℃) (b)
In the text
thumbnail Fig. 7 Effect of contact time (a) and kinetic fit curves of MN-TRB750 for RhB adsorption

C0 = 260 mg·L-1,T = 20 ℃, pH=7.0

In the text
thumbnail Fig. 8 Isotherm fit curves and various model-fitted curves of MN-TRB750 for RhB adsorption
In the text
thumbnail Fig. 9 Schematic diagram of adsorption mechanism for RhB on MN-TRB750
In the text
thumbnail Fig.10 The reuse of MN-TRB750 for RhB adsorption
In the text

Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.

Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.

Initial download of the metrics may take a while.