As a kind of wastewater produced by papermaking industry, bamboo pulp black liquor (BPBL) discharged into water causes serious environmental problems. In this work, BPBL was successfully converted into porous carbon after activation with potassium hydroxide (KOH) through one-step carbonization, and adsorption properties of porous carbon derived from bamboo pulp black liquor (BLPC) for tetracycline hydrochloride (TCH) and malachite green (MG) were studied. The adsorption capacities of BLPC for TCH and MG are 1047 and 1277 mg/g, respectively, due to its large specific surface area of 1859.08 m2/g. Kinetics and isotherm data are well fitted to the pseudo-second-order rate model and Langmuir model, respectively. Adsorption experiments and characterizations reveal that the adsorption mechanism involved in TCH and MG adsorption on BLPC mainly depends on the synergistic effect of pore filling, H-bonding, π-π interactions and weak electrostatic interactions. In addition, BLPC shows excellent photothermal properties, and the adsorption capacity of TCH and MG on BLPC can reach 584 and 847 mg/g under the irradiation of near infrared lamp for 50 min, respectively. The synthesized BLPC with high adsorption efficiency, good recovery ability, improved adsorption under near-infrared irradiation can be a promising and effective adsorbent for TCH or MG or other pollutes.

  • Bamboo pulp black liquor porous carbon (BLPC) was prepared by one-step carbonization.

  • BLPC was prepared by making full use of bamboo pulp black liquor.

  • The maximum adsorption capacity of tetracycline hydrochloride (TCH) and malachite green (MG) is 1047 and 1277 mg/g, respectively.

  • Suitable for adsorption reaction in various water bodies.

  • The adsorption of TCH and MG can be improved under near infrared light irradiation.

Graphical Abstract

Graphical Abstract
Graphical Abstract

In recent years, water contamination has become an imperative global environmental problem with the fast development of urbanization, industry and healthcare. Especially, various wastewaters containing antibiotic and dyes have been emitted from pharmaceutical factory and textile dyeing (Voigt et al. 2020). Tetracycline hydrochloride (TCH), as a kind of antibiotic, has been used for the treatment and prevention of diseases for human and animals (Zheng et al. 2021). However, most of the TCH ingested by animals is excreted from the body through urine, and then enters rivers and oceans through ecological circulation, which in turn leads to the production of drug-resistant bacteria and pollution for the environment, therefore affecting ecosystems and human health (Mohapatra et al. 2019). Additionally, malachite green (MG), a blueish green cationic dye, has been widely used to impart color in textile and leather industries. MG shows a distinct color at very low concentrations and greatly affects photosynthesis in aquatic plants. Currently, various methods, such as photocatalytic degradation (Zhao et al. 2010; Wang et al. 2021a), electrochemical degradation (Pizan-Aquino et al. 2020; Zhao et al. 2021a), biodegradation (Wang et al. 2021b), advanced oxidation processes (Gao et al. 2021), adsorption (Yang et al. 2021a, 2021b) and membrane separation (Pandele et al. 2020) have been developed to remove TCH and MG in wastewater. Among these technologies, the adsorption method has attracted widespread interest due to no secondary pollution, high efficiency and low cost, and easy operation (Zhang et al. 2020a). Nowadays, a variety of adsorbents have been used for removal of TCH and MG such as graphene oxide (Miao et al. 2019), biochar (Dai et al. 2020), MOFs (Chen et al. 2017), porous carbon (Rivera-Utrilla et al. 2009) and their derivatives. Among these materials, porous carbon has been widely used as an adsorbent for TCH and MG removal due to its large specific surface area, excellent porous structure, low cost, abundant surface functional groups and abundant raw materials (Zhao et al. 2021b; Chen et al. 2022).

Bamboo pulp black liquor (BPBL) is waste from the conventional bamboo pulping process in the paper industry. These black liquors exist in industrial wastewater and can be easily released into the hydrosphere, therefore affecting water resources. Traditionally, BPBL is burnt for energy. It is well known that BPBL is rich in lignin, cellulose and hemicellulose, and it can be converted into useful products. In general, lignin is extracted from BPBL and used to prepare bio-asphalt (Ren et al. 2020), super capacitors (Liu et al. 2019) and activated carbon (He et al. 2016). In recent years, activated carbon has attracted extensive attention due to its rich porous structure surface active groups for excellent adsorption performance. The preparation process of porous carbon was derived from lignin as precursor, and the activation effect of potassium compounds (K2CO3, potassium bicarbonate and KOH) was studied (Zhang et al. 2020b). In addition, porous carbon based on lignin using phosphoric acid as activator was prepared by conventional pyrolysis and microwave pyrolysis, and the effect of process parameters on the adsorption performance of porous carbon was investigated (Brazil et al. 2020). However, these studies only use lignin in BPBL and cannot fully utilize other organic matter.

Until now, the conversion of all the organic compounds of BPBL such as lignin, cellulose and hemicellulose to porous carbons and their application still has been little studied, and particularly, there was no relevant report about the research of BPBL derived BLPC for applications in antibiotic and dye wastewater treatment. BPBL waste was turned into high value-added porous activated carbon in this work. The porous carbon was prepared by fully utilizing all the carbon containing substances in the BPBL, which has a simple process and higher adsorption effect. In addition, the photothermal effect of porous carbon is utilized for enhancing adsorption of the activated carbon. The temperature of porous carbon can be rapidly increased under the irradiation of near-infrared lamp, which can effectively promote the adsorption process of porous carbon.

In this study, BPBL and KOH were used as raw materials as activators, and various organic compounds in black liquor were fully utilized to prepare high-performance black liquid derived porous carbon (BLPC) from bamboo pulp black liquid by one-step method. The structure, morphology and surface functional groups of BLPC were characterized. The effects of mixing ratios of KOH and BPBL and activation temperatures on adsorption properties of BLPC for TCH and MG were investigated. The dynamics, isotherms and reusability of the BLPC were studied. In addition, the photothermal effect of porous carbon is utilized for enhancing adsorption of the activated carbon, and the adsorption effect of BLPC on TCH and MG under the irradiation of near infrared lamp was studied.

Materials

Bamboo pulp black liquor (BPBL) was obtained from Huanlong New Materials Co. (Zheijang, China), tetracycline hydrochloride (TCH), malachite green (MG), hydrochloric acid (HCI) and potassium hydroxide (KOH, AR) were purchased from Chengdu Chron Chemicals Co. Ltd. (Chengdu, China).

Preparation of BLPC

BPBL were used as raw material for porous carbon and KOH was utilized as activating agent. First, 10 mL of deionized water was mixed with 20 g BPBL, and a certain amount of KOH (0–3.6 g) was added and stirred to make the KOH and BPBL mix evenly. After that, the mixture was dried in an oven and then activated in a tube furnace by a sequentially increasing temperature to the target temperature (750–850 °C) at a heating rate of 5 °C min−1 under N2 atmosphere for 1 h to obtain the bamboo pulp black liquid porous carbon (BLPC). The obtained BLPC products were fully ground and thoroughly washed with 1 mol /L HCl solution to remove any inorganic salts and further washed with deionized water until pH was neutral. Finally, the product was dried at 105  °C for 12 h. The samples were named BLPC-T-X, where T (750, 800 and 850 °C) stands for activation temperature and X (0, 12, 15 and 18%) means the ratio of KOH to BPBL.

Characterization

Surface morphologies of the BPBL, BLPC-800-0, LPC-800-12, BLPC-800-15 and BLPC-800-18 were observed at an acceleration voltage of 10 kV and 10 mm of working distance by SEM (JSM-5900LV). Chemical functional groups of the BPBL, BLPC-800-0, LPC-800-12, BLPC-800-15 and BLPC-800-18 were identified by Fourier transform infrared spectroscopy (IRTracer-100). BLPC-800-0 and BLPC-800-15 were tested by XRD (X'Pert Pro MPO) at a scan rate of 5° per minute in an angle range of 5–80°. N2 adsorption-desorption isotherms of the BLPC-800-0, LPC-800-12 and BLPC-800-15 were measured at 77 K (Gemini VII 2390). The surface area was estimated by application of the Brunauer Emmett Teller (BET) equation, the pore size distribution was determined according to the Density Functional Theory (DFT) and Horvath-Kawazoe (HK) method, using the nitrogen adsorption isotherm data.

Adsorption experiments

TCH and MG adsorption experiments were conducted to evaluate the adsorption performance of BLPC. Effects of different pH, temperature and pollutant concentration on TCH and MG adsorption of BLPC were studied. In order to investigate adsorption kinetics, a batch of experiments was conducted by adding 30 mg of BLPC-T-X into the TCH and MG solutions (30 mL) with different initial concentrations of 1000 and 1500 mg/L at 288, 298 and 308 K for different designated interval times, respectively. Different initial concentrations (5, 10, 20, 30, 40, 60, 80, 100 and 120 mg/L) at three different temperatures (288, 298 and 308 K) for 24 h were adopted to study adsorption equilibrium property. The final concentrations of TCH and MG were determined by UV spectrophotometer (UV-2450, Shimadzu, Japan) at 357.0 nm and 617.0 nm respectively. Adsorption amounts of TCH and MG were calculated according to Equations (1) and (2):
formula
(1)
formula
(2)
where Co and Ce (mg/L) are the initial and ultimate concentrations, respectively. Ct (mg/L) is the remaining contaminant concentration at time t (min). V (mL) is the solution volume and M (mg) is the weight of adsorbent.

Thirty mg BLPC was added to 30 mL TCH and MG solutions (C0  =  1500 mg/ L), respectively. The pH values of TCH solution and MG solution were adjusted to 5–9 and 3–11 with 0.05 mol/L HCl and 0.05 mol/L NaOH, respectively. The effects of BLPC on the adsorption of TCH and MG at different pH values were investigated after adsorption at 298 K for 24 h.

One hundred mg BLPC-800-15 was added to 100 mL TCH and MG solution (C0 = 1500 mg/ L) and reacted for 24 h. The BLPC-800-15 was filtered, washed, and placed in 100 mL ethanol under ultrasonic irradiation for 30 min for desorption. The regenerative BLPC-800-15 was dried for 24 h and then added to TCH and MG solutions to evaluate regeneration efficiency.

The BLPC-800-15 adsorption experiment and photo-thermal temperature of BLPC-800-15 were performed under 250 W near-infrared light at room temperature (25 °C), and the surface temperature of BLPC-800-15 was recorded by infrared imager. The distance between the BLPC-800-15 surface and infrared light (40, 50 and 60 cm) was adjusted to measure photothermal performance and the effect of near-infrared lamp irradiation on the adsorption properties of porous carbon was evaluated.

SEM analysis

Figure 2 show the surface morphology of BPBL, BLPC-800-0, BLPC-800-12, BLPC-800-15 and BLPC-800-18, respectively. The SEM images revealed noteworthy changes in surface morphology of BPBL after activation. The surface of BPBL shows irregular structure, which is loaded with crystalline salt on organic matter such as lignin (Figure 2(a)). However, it is evident from Figure 1(b) that carbonization at 800 °C causes elimination of volatile matter from biomass resulting in a rough and partially porous surface of BLPC-800-0. Subsequently, a highly porous surface is observed on the BLPC in the presence of KOH after carbonization at 800 °C produced, as shown in Figure 1(c)–1(e). The amount of KOH for activation also influences the surface morphology and structure of porous carbon. Abundant pores are observed on the surface of the BLPC-800-12 as shown in Figure 2(c). indicating that appropriate KOH can promote pores. The pore size is further developed when the ratio of KOH increases from 12 to 15%. However, excessive KOH leads to the corrosion of carbon structure when the ratio of KOH further increases. The pores of BLPC-800-18 collapse and the pore size increases when the KOH addition ratio is 18% as shown in Figure 2(e). The activation mechanism of KOH is shown in Equations (3)–(7) (Gao et al. 2020):
formula
(3)
formula
(4)
formula
(5)
formula
(6)
formula
(7)
Figure 1

Preparation process of BLPC.

Figure 1

Preparation process of BLPC.

Close modal
Figure 2

SEM images of (a) BPBL, (b) BLPC-800-0, (c) BLPC-800-12, (d) BLPC-800-15 and (e) BLPC-800-18.

Figure 2

SEM images of (a) BPBL, (b) BLPC-800-0, (c) BLPC-800-12, (d) BLPC-800-15 and (e) BLPC-800-18.

Close modal

According to the formula, KOH consumes the carbon-containing material in the raw material and pores are formed in the original position at high temperature. Gases are produced during activation, which directly connects the pores with the pores. However, excessive KOH leads to excessive expansion of pores and collapse and the adsorption capacity decreases.

The N2 adsorption–desorption isotherms and pore size distribution are shown in Figure 3 and corresponding specific surface aera and pore volume are presented in Table 1. According to IUPAC classification, isotherms of BLPC can be classified as type I, indicating that a large number of micropores and mesopores exist in BLPC, corresponding to a Langmuir single-layer reversible adsorption process (Al-Ghouti & Da'ana 2020). The specific surface areas of porous carbon prepared without KOH activation and BPBL and BPBL activated with 15% KOH are 928.6 cm²/g and 1859.08 m²/g, respectively. The result indicates that specific surface areas of porous carbon derived from BPBL with KOH activation are higher than that without KOH activation. The pore volumes of porous carbon prepared without KOH activation and BPBL with 15% KOH activation ranges from 1.96 to 2.11 cm3/g, respectively. The pore volume arrives at 2.22 cm3/g when the ratio of KOH is 18%, indicating that the number of intermediary pores and macropores in porous carbon increases because KOH has a pore expanding effect. The specific surface area of BLPC-800-18 is 1494.28 m²/g, indicating that the excessive pore-promoting effect of KOH leads to collapse of micropores and formation of micropores and macropores, resulting in reduced specific surface area. Figure 3(b) shows pore size distribution of the BLPC and the pore sizes of three kinds of porous carbon are mainly below 3 nm, mainly consisting of micropores and mesopores which is conducive to the adsorption of TCH and MG by BLPC.
Table 1

The porosity characteristics of BLPC synthesized at different parameters

ParametersBLPC-800-15BLPC-800-18BLPC-800-0
Specific surface area (m2/g) 1859.08 m²/g 1494.28 m²/g 928.60 m²/g 
Total pore volume (cm3/g) 2.11 cm3/g 2.22 cm3/g 1.96 cm3/g 
ParametersBLPC-800-15BLPC-800-18BLPC-800-0
Specific surface area (m2/g) 1859.08 m²/g 1494.28 m²/g 928.60 m²/g 
Total pore volume (cm3/g) 2.11 cm3/g 2.22 cm3/g 1.96 cm3/g 
Figure 3

N2 adsorption–desorption isotherms (a) and pore size distributions and pore volume of BLPC-T-x (b).

Figure 3

N2 adsorption–desorption isotherms (a) and pore size distributions and pore volume of BLPC-T-x (b).

Close modal

FTIR, Raman and XRD analyses

FTIR spectra of BPBL, porous carbon derived from BPBL without and with KOH activation are illustrated in Figure 4(a). For BPBL, an obvious peak near 3410 cm−1 is attributed to stretching vibration of hydroxyl and carboxyl of BPBL. The strong absorption peak around 2300 cm−1 is assigned to the characteristic peaks of carbon dioxide trapped on the surface of BPBL (Gil et al. 2019; Jawad et al. 2019). The peaks located around 1500 and 1600 cm−1 are attributed to symmetric C = C stretching vibrations groups and stretching of carbonyl (C = O) groups of esters and aldehydes. The sharp peak around 1100 cm−1 is assigned to C-O-C linkages (Wu et al. 2018). The weak absorption peaks at 1597 and 1100 cm−1 imply an existence of C = O and C-O-C, respectively. The results show that intensities of O-H, C = O and C = C peaks decrease after carbonization. These functional groups can produce hydrogen bond interactions and π-π interactions with TCH and MG and promote the adsorption of TCH and MG.
Figure 4

FTIR spectra of BPBL and BLPC (a); Raman spectra of BLPC (b); XRD patterns of BLPC-800-0 and BLPC-800-15 (c).

Figure 4

FTIR spectra of BPBL and BLPC (a); Raman spectra of BLPC (b); XRD patterns of BLPC-800-0 and BLPC-800-15 (c).

Close modal

Raman spectra of porous carbon derived from BPBL without and with KOH activation are shown in Figure 4(b) and two distinct peaks can be obviously observed at 1597 (G-band) and 1349 (D-band, disorder) cm−1, corresponding to graphitic and defected carbon, respectively, indicating that the main component of as-prepared BLPC is carbon. Generally, the G band means that all sp2 atoms in the rings and chains show C-C bond stretching (Wei et al. 2016). D band is related to the disorder and defects in the crystal structure, which is dominated by defects, distortions and displacements. The intensity ratio between D band and G band (R = D/G) represents the disorder of carbon structure. The D/G specific value of BLPC-800-15 and BLPC-800-0 are 1.09 and 1.07, respectively, demonstrating that the graphitization degree of porous carbon derived from BPBL with KOH activation is lower than that without KOH activation. The porous carbon with lower graphitization degrees contains more oxygen-containing functional groups, higher defect degree, higher specific surface area and a larger number of thin layers than porous carbon with a higher graphitization degree, which can effectively improve the adsorption capacity of porous carbon (Jiao et al. 2017). Namely, porous carbon derived from BPBL with KOH activation is better than that without KOH activation.

XRD characteristics were conducted to analyze the crystallinity of porous carbon derived from BPBL without and with KOH activation, as shown in Figure 4(c). There are two broad peaks in XRD patterns of both porous carbon derived from BPBL without and with KOH activation. The peaks located at 23.5 and 43.0° correspond to (002) and (101) planes of graphite (Hou et al. 2017), respectively, which was consistent with amorphous and disordered carbon material.

Adsorptive property

Figure 5 shows adsorption capacities of TCH and MG onto the BLPC-T-x at 298 K. The effect of activation temperature and BPBL/KOH ratio on adsorption performance were investigated. Obviously, the improvement of adsorption performance of BLPC is related to the ratio of KOH, the adsorption capacities of the prepared BLPC for TCH are 487, 733, 884 and 738 mg/g, and the adsorption capacities for MG are 678, 933, 1084 and 942 mg/g when the addition ratio of KOH is 0, 12, 15 and 18% at 750 °C, respectively.
Figure 5

The comparisons of TCH (a) and MG (b) adsorption by BLPC-T-x adsorbents, the comparison of specific surface area and adsorption capacity of TCH (c) and MG (d) with different adsorbents.

Figure 5

The comparisons of TCH (a) and MG (b) adsorption by BLPC-T-x adsorbents, the comparison of specific surface area and adsorption capacity of TCH (c) and MG (d) with different adsorbents.

Close modal

The adsorption capacity of BLPC increases at higher activation temperature, but decreases when the activation temperature exceeds 762 °C (the boiling point of K), because KOH is reduced to free metallic K. Free metal K can be embedded in the carbon and widen the space between the carbon layers, increasing the specific surface area of BLPC.

Therefore, the adsorption performance of BLPC for TCH and MG is improved when the activation temperature rises from 750 to 800 °C, the adsorption performance decreases when the activation temperature rises to 850 °C. It can be seen from Figure 5 that the adsorption capacity of BLPC-800-15 for TCH and MG is 1047 and 1277 mg/g, respectively, implying that the BLPC-800-15 exhibits excellent adsorption performance. Figure 5(c) and 5(d) lists the comparison of the adsorption performance of BLPC-800-15 on TCH and MG with other porous carbon adsorbents. It can be observed that the adsorption capacities of BLPC-800-15 for both pollutants are significantly higher than that of adsorbents recently reported (Ahmad & Alrozi 2011; Akar et al. 2013; Singh et al. 2016; Ahmed et al. 2017, 2020; Bhatti et al. 2017; Yu et al. 2017; Zhang et al. 2017; Jang et al. 2018; Rawat & Singh 2018; Cai et al. 2019; Jang & Kan 2019; Haghighat et al. 2020; Zhang et al. 2021), illustrating that the prepared BLPC-800-15 is a promising adsorbent for the removal of antibiotics and dyes.

Desorption experiments

The recycling performance of adsorbents is a critical parameter in practical and commercial applications. The adsorbed BLPC was desorbed by ethanol ultrasonic desorption for four consecutive adsorption-desorption experiments, and the results are shown in Figure 6(a). The adsorption efficiency of TCH and MG on porous carbon still possesses 81.1 and 85.2% after four cycles, respectively, indicating the satisfactory cycle performance of BLPC-800-15. The removal efficiency of BLPC-800-15 in actual waterbodies are required to be studied. Figure 6(b) shows the adsorption efficiency of BLPC for TCH and MG in tap water, river water and seawater. The results show that the adsorption capacity of the two pollutants in tap water is close to deionized water, and the adsorption capacity of BLPC to TCH and MG is 1043 and 1277 mg/g, respectively. Additionally, the removal efficiencies of TCH and MG slightly decrease using river and sea as water resources, and the adsorption capacities of BLPC to TCH and MG in sea water are 936 and 1186 mg/g, respectively, and those in river water are 879 and 1103 mg/g, respectively. The main reason may be the existence of natural organic matter in rivers and seawater (Zhao et al. 2020), which competes with the adsorption of BLPC for TCH and MG.
Figure 6

Regeneration performance of BLPC-800-15 for TCH and MG removal (a), the adsorption of TCH and MG by BLPC-800-15 in different waterbodies (b).

Figure 6

Regeneration performance of BLPC-800-15 for TCH and MG removal (a), the adsorption of TCH and MG by BLPC-800-15 in different waterbodies (b).

Close modal

Adsorption kinetics of TCH and MG onto BLPC

Kinetic studies of adsorption play an important role in understanding the adsorption dynamics and mechanism through order of rate constant. Two different concentrations (1000 and 1500 mg/L) and three different temperature (288, 298 and 308 K) were used to study the adsorption kinetics with data fitted using pseudo-first-order and pseudo-second-order for both TCH and MG (Figure 7). It can be observed that the adsorption capacity of BLPC increases sharply within 100 min due to pore filling, and then increases slightly until equilibrium is reached. Almost all the adsorption processes can reach equilibrium within 200 min, indicating the efficient adsorption capacity of as-prepared adsorbent. As the temperature rises from 288 to 308 K, the adsorption capacity of BLPC for TCH and MG increases continuously because the adsorption of TCH and MG is an endothermic process (Qu et al. 2019; Yang et al. 2021b). The results indicate that the rise of temperature helps in the adsorption of pollutants. The models of pseudo-first-order and pseudo-second-order were used to analyze the adsorption kinetics of BLPC for TCH and MG according to Equations (8) and (9), respectively:
formula
(8)
formula
(9)
Figure 7

Time profiles of adsorption of TCH (a,b) and MG (c,d) onto BLPC-800-15 at the different initial solution concentration and temperature and the corresponding fitting curves of the pseudo-first-order rate model and the pseudo-second-order rate model.

Figure 7

Time profiles of adsorption of TCH (a,b) and MG (c,d) onto BLPC-800-15 at the different initial solution concentration and temperature and the corresponding fitting curves of the pseudo-first-order rate model and the pseudo-second-order rate model.

Close modal

where Qt (mg/g) is the amount of TCH and MG adsorbed at t (min), and qe (mg/g) represents the amount of TCH and MG adsorption at equilibrium. K1 (min−1) is the pseudo-first-order adsorption rate constant. K2 (min−1) is the pseudo-second-order adsorption rate constant. Qe,cal and Qe,exp represent the theoretical adsorption capacity and actual adsorption capacity, respectively. Fitting results of adsorption kinetics are shown in Figure 7 and the detailed kinetics parameters are presented in Tables 2 and 3. It can be seen from Tables 2 and 3 that R2 value of the pseudo-second-order rate model is higher (>0.95). In addition, the value of Qe,cal obtained by the pseudo-second-order rate model is close to the Qe,exp value. Therefore, the pseudo-second order model is suitable for describing the adsorption kinetics of TCH and MG on BLPC-800-15.

Table 2

Kinetics parameters for the adsorption of TCH onto porous carbon in different conditions

Pseudo-first-order model
Pseudo-second-order model
Co(mg/L)T (K)Qe,exp (mg/g)k1(10−2 min−1)Qe,cal (mg/g)R2k2(10−3 min−1)Qe,cal (mg/g)R2
1,000 288 634 2.26 611.41 0.94 0.044 692.90 0.98 
298 660 2.28 631.05 0.92 0.044 711.38 0.97 
308 682 2.60 642.44 0.86 0.054 713.88 0.95 
1,500 288 980 2.20 929.0 0.93 0.018 1029.15 0.97 
298 1002 1.76 924.4 0.93 0.021 1063.21 0.97 
308 1020 4.21 963.0 0.87 0.027 1088.88 0.97 
Pseudo-first-order model
Pseudo-second-order model
Co(mg/L)T (K)Qe,exp (mg/g)k1(10−2 min−1)Qe,cal (mg/g)R2k2(10−3 min−1)Qe,cal (mg/g)R2
1,000 288 634 2.26 611.41 0.94 0.044 692.90 0.98 
298 660 2.28 631.05 0.92 0.044 711.38 0.97 
308 682 2.60 642.44 0.86 0.054 713.88 0.95 
1,500 288 980 2.20 929.0 0.93 0.018 1029.15 0.97 
298 1002 1.76 924.4 0.93 0.021 1063.21 0.97 
308 1020 4.21 963.0 0.87 0.027 1088.88 0.97 
Table 3

Kinetics parameters for the adsorption of MG onto porous carbon at the different conditions

Pseudo-first-order model
Pseudo-second-order model
Co(mg/L)T (K)Qe,exp (mg/g)k1(10−2 min−1)Qe,cal (mg/g)R2k2(10−3 min−1)Qe,cal (mg/g)R2
1000 288 820 2.60 775.42 0.86 0.033 841.33 0.96 
298 863 2.73 903.21 0.88 0.036 890.12 0.96 
308 942 3.57 854.32 0.90 0.046 957.11 0.95 
1500 288 1032 4.11 939.5 0.89 0.057 1039.98 0.97 
298 1113 3.52 1001.8 0.86 0.069 1102.90 0.97 
308 1232 7.40 1105.9 0.87 0.083 1206.32 0.98 
Pseudo-first-order model
Pseudo-second-order model
Co(mg/L)T (K)Qe,exp (mg/g)k1(10−2 min−1)Qe,cal (mg/g)R2k2(10−3 min−1)Qe,cal (mg/g)R2
1000 288 820 2.60 775.42 0.86 0.033 841.33 0.96 
298 863 2.73 903.21 0.88 0.036 890.12 0.96 
308 942 3.57 854.32 0.90 0.046 957.11 0.95 
1500 288 1032 4.11 939.5 0.89 0.057 1039.98 0.97 
298 1113 3.52 1001.8 0.86 0.069 1102.90 0.97 
308 1232 7.40 1105.9 0.87 0.083 1206.32 0.98 

Adsorption isotherm

Two adsorption isotherms, namely the Langmuir and Freundlich isotherms were used to analyze the mechanism for the adsorption of TCH and MG onto BLPC-800-15. The Langmuir isotherm assumes a monolayer coverage of adsorbate over a homogenous adsorbent surface. The Langmuir equation represents the reference (Equation 10)):
formula
(10)
Qe (mg/g) is the adsorption capacity of adsorbent per unit mass under equilibrium state. Qo (mg/g) is the maximum adsorption capacity. Kt (L/mg) represents the Langmuir adsorption constant and Ce (mg/L) stands for equilibrium concentration of TCH and MG. The Freundlich isotherm assumes multilayer coverage of adsorbate over adsorbent surface. The Freundlich equation represents the reference (Equation (11)):
formula
(11)
where Qe (mg/g) is the adsorption amount of adsorbent per unit mass under the equilibrium state. Ce (mg/L) is the equilibrium concentration. Kf and n are Freundlich constants, which represent adsorption capacity and adsorption intensity of the adsorbent, respectively. The adsorption isotherm fitted with Langmuir and Freundlich models are illustrated in Figure 8 and the calculated parameters are listed in Tables 4 and 5. The adsorption capacity of porous carbon increases with the rise of TCH and MG initial concentration and temperature. It can be seen from Tables 4 and 5 that R2 values calculated from the Langmuir model are higher than those from the Freundlich model for both TCH and MG at 288, 398 and 308 K, indicating that monomolecular layer adsorption plays a major role in adsorption of TCH and MG on BLPC. Qm values of Langmuir isotherm for TCH are 1050, 1171 and 1215 mg/g at 288, 298 and 308 K, respectively. Additionally, Qm values of Langmuir isotherm for MG are 1218, 1284 and 1435 mg/g, respectively. An increase of Qm values at higher temperature indicates that higher temperature is beneficial for adsorption. Generally, the Kf parameter is used to evaluate the adsorption capacity and a large KF value suggests high adsorption capacity of the adsorbent. The KF value is directly proportional to temperature, indicating that high temperature is favorable for adsorption.
Table 4

Isotherm parameters for the adsorption of TCH onto BLPC-800-15

ModelLangmuir
Freundlich
T (K)Qm (mg/g)KL (L/mg)R2KF (L/g)R2
288 1050.15 0.53 0.99 431.27 0.97 
298 1171.10 0.51 0.99 479.06 0.97 
308 1215.15 0.517 0.99 495.64 0.97 
ModelLangmuir
Freundlich
T (K)Qm (mg/g)KL (L/mg)R2KF (L/g)R2
288 1050.15 0.53 0.99 431.27 0.97 
298 1171.10 0.51 0.99 479.06 0.97 
308 1215.15 0.517 0.99 495.64 0.97 
Table 5

Isotherm parameters for the adsorption of TCH onto BLPC-800-15

Langmuir
Freundlich
T (K)Qm (mg/g)KL (L/mg)R2KF (L/g)R2
288 1218.81 0.51 0.99 495.04 0.97 
298 1284.20 0.56 0.99 541.72 0.97 
308 1435.78 0.63 0.99 632.32 0.97 
Langmuir
Freundlich
T (K)Qm (mg/g)KL (L/mg)R2KF (L/g)R2
288 1218.81 0.51 0.99 495.04 0.97 
298 1284.20 0.56 0.99 541.72 0.97 
308 1435.78 0.63 0.99 632.32 0.97 
Figure 8

Adsorption isotherms of TCH (a) and MG (b) onto BLPC-800-15 at 288, 298 and 308 K and their corresponding curves fitted by Langmuir and Freundlich models.

Figure 8

Adsorption isotherms of TCH (a) and MG (b) onto BLPC-800-15 at 288, 298 and 308 K and their corresponding curves fitted by Langmuir and Freundlich models.

Close modal

Effect of pH value on adsorption of the porous carbon

In order to further investigate the effect of pH values on adsorption of TCH and MG, the adsorption capacity of BLPC to TCH and MG at different pH values was determined. The adsorption capacity of BLPC to TCH reaches the maximum at pH = 7 (Figure 9(b)). TCH mainly exists in the form of TCH3+ when pH is less than 7. However, TCH is mainly converted into the state of TCH2, TCH and TC2− under neutral and alkaline conditions. Therefore, electrostatic repulsion inhibits the TCH adsorption on the BLPC because porous carbon has a positive charge when pH is less than 4.1 as shown in Figure 9(a) and 9(b). On the contrary, the electrostatic interaction between TCH and BLPC increases at high pH values, leading to an increase in TCH adsorption capacity. Surfaces of BLPC are negatively charged at pH above 4.1. The electrostatic repulsion between TCH and negatively charged BLPC results in a reduction of the adsorption capacity of BLPC to TCH. It can be seen from Figure 9(c) that the removal capacity for MG increases when pH rises from 3 to 9. The adsorption rate remains stable when the pH value ranges from 9 to 11. As MG is a cationic dye, the adsorption capacity decreases with the rise of the positive charge on the surface of BLPC, both the surface of BLPC and MG are positively charged and the adsorption performance is relatively low when the pH is less than 4.1. The negative charge on the surface of BLPC increases at higher pH, and the electrostatic adsorption is improved, leading to the enhancement of the adsorption performance, and the adsorption effect of BLPC for dyes is mainly affected by the electrostatic interaction between pollutants and the surface of BLPC. The increase in the number of H+ ions in aqueous solution further weakens the adsorption capacity, and H+ ions occupy the active site on the surface of BLPC, limiting its interaction with cationic dye molecules.
Figure 9

Zeta potential of BLPC-800-15 at different pH value conditions (a), adsorption performance of TCH at different pH value conditions (b), adsorption performance of MG at different pH value conditions (c).

Figure 9

Zeta potential of BLPC-800-15 at different pH value conditions (a), adsorption performance of TCH at different pH value conditions (b), adsorption performance of MG at different pH value conditions (c).

Close modal

Adsorption mechanism

The adsorption mechanism of BLPC for TCH and Mg is not only the pore filling caused by high specific surface area, but also other adsorption mechanisms. In addition, the BLPC surface is negatively charged when pH is higher than pHpzc as shown in Figure 10. TCH and MG with positive charge are easily attracted by static electricity and adsorbed on the surface of BLPC. The carboxyl group and hydroxyl group in BLPC act as hydrogen donors and are bonded with the H-bond receptor between TCH and MG to produce hydrogen bond interaction. In addition, π-π interaction between the benzene ring on BLPC and TCH and MG contributes to adsorption of TCH and MG on BLPC (Sharma et al. 2019). TCH and MG molecules have a large flat ring structure and may form van der Waals force on the surface of BLPC, leading to further adsorption.
Figure 10

TCH and MG adsorption mechanism.

Figure 10

TCH and MG adsorption mechanism.

Close modal

Photo-thermal adsorption property

For highly efficient solar energy promoted adsorption, it is very important to develop photothermal materials that efficiently absorb solar light and convert to thermal energy. BLPC has a higher content of graphite, thus it has a higher light absorption rate in the infrared region, and can be heated up rapidly in a short time under the irradiation of a near infrared lamp. As shown in Figure 11(a), BLPC has a fast heating response and good photothermal performance. Figure 11(c)–11(e) shows infrared images of saturation temperature on the BLPC surface at distances of 60, 50, and 40 cm, the surface temperature of BLPC is significantly higher than the ambient temperature. Figure 11(b) shows the comparison of the adsorption performance of BLPC on TCH and MG without or with light. It can be seen that the adsorption capacity of BLPC on TCH and MG at 50 min is 552 and 790 mg/g, respectively, and the adsorption capacity of BLPC on TCH and MG at near infrared light is 584 and 847 mg/g, respectively. The excellent photothermal effect of BLPC surface can be applied to promote the adsorption efficiency of outdoor solar energy.
Figure 11

(a) The temperature of BLPC under different illumination time and illumination distance, (b) adsorption rate of porous carbon with or without light contrast, (c-e) infrared images of porous carbon saturation temperature were obtained at 40, 50 and 60 cm, respectively.

Figure 11

(a) The temperature of BLPC under different illumination time and illumination distance, (b) adsorption rate of porous carbon with or without light contrast, (c-e) infrared images of porous carbon saturation temperature were obtained at 40, 50 and 60 cm, respectively.

Close modal

The BPBL waste residue was fully utilized to convert BPBL into porous carbon by one step activation at high temperature by KOH. The specific surface area of the prepared BLPC is 1859.08 m2/g, and the adsorption capacity of the BLPC to TCH and MG is 1047 and 1277 mg/g, respectively. In addition, BLPC can improve the adsorption properties of TCH and MG under near-infrared light due to the photothermal effect of BLPC and can be applied to promote the adsorption effect under outdoor solar light. Compared with other adsorbents, the preparation process of BLPC is simple, low cost, offers environmental protection, and can make full use of all the carbon-containing substances in BPBL. BLPC also has excellent adsorption performance and can absorb a large amount of TCH and MG in a short time, which can be used for the adsorption and purification of TCH and MG wastewater. The preparation of BLPC provides a way for the treatment of BPBL, and the photothermal adsorption process has certain reference significance.

This work was financially supported by the Science and Technology Planning Project of Sichuan Province (No. 2020YFN0150).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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