Bamboo and coffee, which are abundant and inexpensive, have been used as green adsorbents for the adsorption of industrial dye rhodamine B (RB). Bamboo and coffee are natural sources of cellulose, hemicellulose, and lignin, making them promising green materials for industrial dye removal. The effects of various adsorption conditions, such as contact time, temperature, dose of bamboo powder (BP), coffee ground (CG), initial concentration of RB, and pH values of RB solution, were measured. Consequently, the kinetics of RB adsorption onto bamboo and coffee was in accordance with the pseudo-second-order model, with an activation energy of 29.51 kJ·mol− 1 for bamboo and 27.46 kJ·mol− 1 for coffee. The Langmuir model is well fitted to the whole adsorption period at different temperatures, indicating maximum adsorption capacities of 6.76, 6.96, and 7.64 mg⋅g− 1 (BP) as well as 6.53, 6.80, and 7.51 mg⋅g− 1 (CG) at 30°C, 40°C, and 50°C, respectively. Moreover, the spontaneous nature of the adsorption was based on the negative Gibbs free energy values obtained (i.e., from − 11.09 to − 14.30 kJ⋅mol− 1 [BP] and from − 10.34 to − 13.07 kJ⋅mol− 1 [CG]), revealing that RB adsorption occurred at physical and chemical adsorption states. In addition, the recycling capability of adsorbents was determined in five cycles. Therefore, these materials are promising candidates for low-cost adsorbents.
Article
Natural bamboo powder and coffee ground as low-cost green adsorbents for the removal of rhodamine B and their recycling performance
https://doi.org/10.21203/rs.3.rs-3216705/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 06 Dec, 2023
You are reading this latest preprint version
At present, dye pollution is regarded as a major issue in various industrial applications. Conventional water purification methods, such as physicochemical and biological treatments, are almost ineffective for dye removal because of dye stability 1–3. Consequently, many researchers have focused on developing adsorbent materials to efficiently remove dyes from wastewater 4–6. Previous studies have successfully predicted the adsorption isotherms of dyes on various materials such as activated carbon 7, activated carbon from oil palm wood 8, peach stones 9, phoenix tree leaf powder 10, coir pith carbon 11, sepiolite 12, and pumice powder 13. Among these, activated carbon is a well-known material with a high surface area (~ 3000 m2⋅g− 1), and it is known for its excellent adsorption ability for all kinds of pollutants 14. However, the high cost associated with activated carbon regeneration limits its practical applications. Therefore, the use of low-cost adsorbents made from bio-materials, natural materials, and waste materials from agriculture and industry has become increasingly popular as a cost-effective alternative for water treatment 2,15,16. Recently, a wider range of low-cost adsorbents that can effectively remove dyes, organics, and heavy metals from water in an environmentally-friendly manner have been developed. One promising new resource that has been identified is non-chemically modified bamboo powder (BP) and coffee ground (CG) because of its cost-effectiveness, renewable nature, and environmental friendliness 17,18. In particular, BP (Fig. 1A) is a natural composite material that is abundant in tropical countries, which is also considered as a composite material because of the cellulosic fibers implanted in a lignin matrix 19. For CG (Fig. 1B), it consists of polysaccharides, hemicelluloses, lignin, pectin, cellulose, and a small amount of extractives 20. The surface of CG contains active functional groups (amino, hydroxyl, and carboxyl groups; Figs. 1C–D); thus, CG has great application potential in dye removal because of its cost-effective, renewable, and environmentally friendly features 21. In addition, the negatively charge carboxyl group is considered as the major functional group in the adsorption of cationic dye molecules, but it is not effective in the adsorption of anionic dye molecules 22,23.
Herein, BP and CG are used directly with non-chemical modification for adsorption purposes. Although the adsorption capacity of these non-chemically modified adsorbents is limited, their cost is nearly zero, which are washed only with distilled water, dried at room temperature, and directly applied in the adsorption process. However, the adsorption of dye using these non-chemically modified adsorbents has rarely been investigated, and the adsorption mechanism has not been adequately interpreted. In addition, the adsorption capacity has not been compared between non-chemically modified BP and CG. Therefore, this study aims to examine the adsorption capacity of rhodamine B (RB, a cationic dye that is widely used as a colorant and a tracer fluorescent with toxic and carcinogenic nature) by using non-chemically modified adsorbents. The effects of contact time, temperature, the dose of non-chemically modified adsorbents, initial RB concentration, and pH values of RB solution are also investigated in detail. Moreover, experimental data are analyzed through the adsorption equation of kinetics, isotherms, and thermodynamics; the specific parameters for each model and their recycling ability are also investigated. Therefore, these materials can be considered as potential low-cost adsorbents.
Materials
BP and CG were provided from Hanyang Advanced Materials (South Korea) and Trung Nguyen Coffee (Vietnam), respectively (Figs. 1A–B). They were washed several times with distilled (DI) water under magnetic stirring to remove residual organics, dirt, and color. Then, the residues of CG were used in powder form (140–280 µm) after sieving (CHUNG GYE SANG GONG SA, KOREA). RB was obtained from Sigma Aldrich. Filter papers (Φ = 47 mm) were purchased from CHMLAB GROUP. DI water was utilized through a Milli-Q ultrapure water purification system.
RB adsorption experiments
Adsorption experiments were conducted by stirring (200 rpm) non-chemically modified adsorbents (BP and CG) into 50 mL of RB solutions for 230 min to reach equilibrium at various temperatures (30°C, 40°C, and 50°C; Fig. 2). The effect of adsorbent dose (0.05, 0.1, 0.3, 0.5, and 1.0 g) was studied in 50 mL of 20 mg⋅L− 1 RB solutions at various temperatures. Next, initial RB concentrations (5, 10, 20, 30, and 50 mg⋅L− 1) were also utilized to examine their effect at the abovementioned temperatures during adsorption. In addition, the effect of pH values (2, 3, 6, 8, and 9) was investigated in 6 g⋅L− 1 adsorbent dose with 20 mg⋅L− 1 of RB at the abovementioned temperatures. Finally, RB concentrations in aqueous solutions after filtering using a filter paper were determined on the basis of the absorbance value at 550 nm through UV–vis analysis. Moreover, RB solutions of Co = 20 mg⋅L− 1 were utilized in the reuse process to investigate the recycling performance of non-chemically modified adsorbents (Fig. 2). Non-chemically modified adsorbents after adsorbing RB were washed several times with ethanol, and the regenerated sample was used to adsorb RB. The RB removal efficiency (%) at time t was calculated using Eq. (1); Co and Ct (mg⋅L− 1) correspond to RB concentrations at an initial time and time t (the tested period is 0–300 min).
1
Adsorption kinetics
Kinetic studies were performed at various constant temperatures by stirring 0.3 g of non-chemically modified adsorbents with 50 mL of 20 mg⋅L− 1 RB solutions in each glass vial (200 rpm). All aqueous samples of each glass vial were filtered by using filter papers to remove solid materials at various time intervals of 0–300 min. Then, RB concentrations in aqueous solutions were examined on the basis of the absorbance value at 550 nm through UV–vis analysis. The amount of adsorption at time t (qt, mg⋅g− 1) was calculated by using Eq. (2); Co and Ct (mg⋅L− 1) correspond to RB concentrations at an initial time and any time t; V (L) and m (g) refer to the volume of the RB solution and the mass of non-chemically modified adsorbents utilized, respectively.
2
Adsorption isotherms
Adsorption isotherms were conducted using 0.3 g of non-chemically modified adsorbents in 50 mL of different initial RB concentrations (5–200 mg⋅L− 1), and each sample was stirred for 230 min (30°C, 40°C, or 50°C) to obtain an equilibrium mixture (200 rpm). Then, the final RB concentration in the aqueous solution was analyzed at ~ 230 min. The amount of adsorption at equilibrium (qe, mg⋅g− 1) was calculated using Eq. (3); Co and Ce (mg⋅L− 1) correspond to RB concentrations at initial and equilibrium times; V (L) and m (g) refer to the volume of the RB solution and the mass of non-chemically modified adsorbents utilized, respectively.
3
Analysis instruments
The structure of materials was measured by using a FESEM JSM-7600F instrument. FT-IR spectroscopy was determined in a wavenumber range of 4000–400 cm− 1 using an FT-IR spectrophotometer (IFS-66/S, TENSORR27, Bruker Co.) and the KBr method. X-ray diffraction (XRD) was conducted using an X-ray diffractometer (D8-ADVANCE, Bruker Co.) in the 2θ range of 5°–70°. Thermal gravimetric (TG) curves were achieved in a temperature range of 0℃–800°C with a heating rate of 20°C⋅min− 1 using a Seiko Exstar6100 instrument. UV–visible spectroscopy was measured at room temperature using a SpectraMaxM5 instrument.
Characterization of non-chemically modified adsorbents
Figure 3 shows the SEM images of non-chemically modified adsorbents (BP and CG) before adsorption. In particular, BP is observed as fibers with a smooth surface and a size range of 1–30 µm (Figs. 3A–C). By contrast, CG is observed as porous grains, with a rough surface (Figs. 3D–F), a size range of 140–280 µm, and various cavities. These cavities could be characterized as channels onto the surface of CG instead of pores. The enlarged image of non-chemically modified adsorbents is shown in Fig. 3 (C, F), which suggests the hierarchical morphology of BP and CG. As shown in Fig. 3, the non-chemically modified adsorbents are micrometer in size, and they consist of numerous nanomaterials that are hundreds of nanometers in size. Therefore, non-chemically modified adsorbents with different morphologies and sizes affect the dye removal ability from aqueous solution and the collision between them and the adsorbate.
The FTIR spectrum of non-chemically modified adsorbents was measured to determine the functional groups of BP and CG. As shown in Fig. 4A, BP and CG spectra show lignocellulosic characteristics based on the major constituents (i.e., fatty acids, lignin, hemicellulose, cellulose, and polysaccharides) 24. In particular, the broad peak at 3423 and 3441 cm− 1 belongs to the O–H stretching vibration in the BP and CG spectra, respectively. The peaks at 2924 cm− 1 (asymmetry) and 2853 cm− 1 (symmetry) belong to the C-H stretching vibration in the CG spectra; however, only one peak of the C-H stretching vibration is found at 2918 cm− 1 (asymmetry) in the BP spectrum. Moreover, the peaks at 1740–1605 cm− 1 (BP) and 1744–1628 cm− 1 (CG) correspond to the carbonyl C = O stretching of hemicellulose and chlorogenic acids 24,25. The peaks at 1461–1375 cm− 1 (BP) and 1465–1383 cm− 1 (CG) are involved in the -CH2 and -CH3 bending modes (hemicellulose, lignin, etc.). The peaks at ∼1242 and 1163 cm− 1 correspond to the C–O–C bonds of chlorogenic acid, lignin in BP, and CG spectra. In addition, the broad peak at 1100–990 cm− 1 belongs to the C–O–H bond of polysaccharides 26 in the BP and CG spectra.
XRD analysis was performed to confirm the structure of non-chemically modified adsorbents (BP and CG, Fig. 4B). The diffraction pattern of such adsorbents exhibited two broad peaks at 15.94° and 22.33° (BP) as well as at 9.43° and 19.24° (CG), indicating that cellulosic materials appear in BP and CG 24. Furthermore, hemicellulose and other components of non-chemically modified adsorbents have an amorphous structure 24. In addition, TG analysis was used to investigate the thermal properties of non-chemically modified adsorbents (Figs. 4C–D). In particular, the first stage of CG and BP occurs at 69.7°C and 72.5°C respectively, because of water evaporation and volatile compounds. The next stage (300°C–400°C) is related to the decomposition of polysaccharides and several fats in BP and CG; moreover, the decomposition temperature of hemicellulose and cellulose is observed at 305.1°C (BP), 316.1°C, and 358.2°C (CG) 24,27. The last stage (> 400°C) could be assigned to the formation of carbonaceous materials and consolidation of carbon structures 24,27.
RB adsorption testing
Adsorption capacity is governed by several operational factors; herein, the effects of contact time, temperature, the dose of non-chemically modified adsorbents, initial RB concentration, and pH value of RB solution were investigated. In addition, the adsorption rate on smaller materials is faster than that on larger ones because of the large external surface area of smaller materials, which increases the collision between the adsorbate and adsorbent (i.e., the RB and non-chemically modified adsorbents). Thus, the movement of small materials in solution is faster than that of large ones, as well as their shear is more on their surface 28,29.
Effect of contact time and temperature
The effect of contact time and temperature is shown in Fig. 5 (A–B). The RB removal percentage increases quickly with contact time, which indicates a possibly strong interaction between the RB and non-chemically modified adsorbents (BP and CG). Here, the equilibrium time is selected at 230 min for 20 mg⋅L− 1 of RB concentration (30°C–50°C), and the RB removal percentage remains almost constant. In particular, the RB removal percentage at 230–300 min is 87.3–87.9% (BP) and 81.6–82.2% (GC); 94.9–95.0% (BP) and 89.2–89.3% (CG); and 97.2–98.9% (BP) and 91.5–93.3% (CG) at 30°C, 40°C, and 50°C.
The contact time occurring between non-chemically modified adsorbents (BP and CG) and the adsorbate (RB) is important to the adsorption process. In particular, the adsorbed adsorbate take a short contact time in the physical adsorption, which is contrary to the chemical adsorption because the chemical bonds between the adsorbent and adsorbate can reach equilibrium with a longer contact time. Furthermore, the uptake of adsorbate is quick at the initial time of the contact period (i.e., 215 min/30°C; 200 min/40°C; 170 min/50°C for BP and CG) because of a large number of available active sites on the surface of non-chemically modified adsorbents (BP and CG) 11. Then, it slows down to obtain near equilibrium 12. In addition, the RB adsorption is considered as an endothermic reaction because of the increasing adsorption capacity with temperature. In promoting the diffusion rate of RB molecules crossing the external boundary layers and being into the internal pores of non-chemically modified adsorbents (BP and CG), the temperature should be increased to decrease the solution viscosity, and different temperatures will change the equilibrium capacity 12.
Effect of dose of non-chemically modified adsorbents
The effect of adsorbent dose (1–20 g⋅L− 1) is also investigated using contact time, initial RB concentration, and pH value at different temperatures as constants (Figs. 5C–D). Consequently, the overall trend for different temperatures is the same, and the increasing adsorbent dose (1–6 g⋅L− 1) increases the RB removal percentage because of the increasing adsorbent (BP and CG) surface area and available adsorption sites. However, the RB removal percentage remains almost unchanged with the increase in adsorbent dose (6–20 g⋅L− 1), which can be due to the overlap or aggregation of the surface area of non-chemically modified adsorbents (BP and CG) to RB, thereby increasing the diffusion path length 30. Comparing between BP and CG, the RB removal percentage onto BP is higher than that onto CG at different temperatures (i.e., BP: 24.6–88.9%/30°C, 37.9–96.0%/40°C, 50.1–98.3%/50°C and CG: 22.9–85.1%/30°C, 33.9–91.7%/40°C, 45.3–94.3%/50°C with an increase in the adsorbent dose [1–20 g⋅L− 1]), which is primarily due to the morphology and size of BP. Based on the SEM result, BP is a fiber with a smaller size compared with CG. As previously described, the collision between the RB and non-chemically modified adsorbents with a smaller size and larger external surface area increases; thus, the movement in the solution is faster than that of large ones 28,29.
Effect of initial RB concentrations
Initial RB concentrations (5–50 mg⋅L− 1) are used to examine the effect of RB at different temperatures with the adsorbent dose, contact time, and pH value as constants (Figs. 6A–B). Consequently, the RB removal percentage decreases with the increase of initial RB concentrations (i.e., BP: 94.6–45.3%/30°C, 97.1–58.1%/40°C, 99.0–63.9%/50°C and CG: 93.0–40.0%/30°C, 94.3–54.9%/40°C, 96.1–59.1%/50°C with an initial RB concentration of 5–50 mg⋅L− 1) because of possible chemical and physical interactions between the adsorbate (RB) and non-chemically modified adsorbents (BP and CG). This result is similar to the overall trend for different temperatures. In particular, the RB removal percentage rapidly decreases in RB concentration of 20–50 mg⋅L− 1 because of the overlap of the adsorption sites and the lack of adsorbent 30.
Effect of pH value
The pH value of a corresponding solution is considered as an important condition in the whole adsorption process, which affects the dye chemistry and surface charge of non-chemically modified adsorbents in the aqueous solution 7,12,13,31,32. In the RB adsorption removal, the effect of pH value is presented in Fig. 6 (C–D), which is similar to the overall trend for different temperatures. The results indicate that the RB removal efficiency decreases with the increase of solution pH (i.e., BP: 91.6–58.8%/30°C, 96.9–67.7%/40°C, 98.8–78.9%/50°C and CG: 89.9–47.7%/30°C, 93.9–60.6%/40°C, 98.0–70.4%/50°C with a RB solution pH range of 2–9]. In particular, the RB removal percentage decreases rapidly in the solution pH range of 6–9 (i.e., BP: 28.5%/30°C, 27.3%/40°C, 18.3%/50°C and CG: 33.8/30°C, 28.6%/40°C, 21.1%/50°C).
Here, the RB is dissolved in DI water to release colored dye cations in the solution. The adsorption onto the adsorbent surface from these charged dye groups primarily involves adsorbent surface charges, which is influenced by solution pH 30 Therefore, either high or low pH supports the adsorption of adsorbate (RB) onto non-chemically modified adsorbents (BP and CG). Consequently, the positively charged surface sites on non-chemically modified adsorbents are not advantageous to support the adsorption of cationic dye molecules at low pH, which can be due to the electrostatic repulsion occurring between the RB and non-chemically modified adsorbents (BP and CG, Fig. 7), and the RB still shows high removal ability 11. Hence, other interactions between the RB and non-chemically modified adsorbents (BP and CG) could consist of hydrogen bonds, van der Waals forces, π–π interactions, and hydrophobic–hydrophobic mechanisms 33,34 instead of an electrostatic mechanism. Furthermore, -OH− groups contained on the surface of non-chemically modified adsorbents contribute to the adsorption of cationic dye molecules at high pH 11,35. Otherwise, the adsorption of cationic dye is low, resulting in an inter-molecular interaction. Therefore, such conjunctive pollutant removal will be advantageous for the effluent treatment from various industries because of the well-established chemical nature of pollutants. Concomitantly, the negatively charge carboxyl group is considered as the major functional group in the adsorption of cationic dye molecules, but it is not effective in the adsorption of anionic dye molecules 22,23.
Adsorption kinetics
For wastewater treatment, the investigation of adsorption kinetics and equilibrium is necessary to provide basal information as well as to design and operate the adsorption procedure. Here, two different kinetic models (i.e., pseudo-first-order [equation (4)] and pseudo-second-order [equation (5)]) were applied to investigate the RB adsorption mechanism. As shown in Table 1 and Fig. 8, the pseudo-second-order equation is a well fitted one to compare the experimental data of the whole adsorption period at different temperatures, and its regression coefficient (R2) is closer to 0.999 for the concentration range used in this study. Hence, the RB adsorption kinetics occurring onto non-chemically modified adsorbents (BP and CG) is in accordance with the pseudo-second-order model, indicating that the rate-limiting step could be regarded to chemical adsorption 36. Therefore, the RB adsorption involves surface exchange reactions until the fully occupied surface functional sites, and then RB molecules will diffuse into the network of the adsorbent (BP and CG) to continue the interaction 32.
In addition, in the pseudo-second-order model, rate constants (k2, Table 1) have been used to determine the activation energy (Ea, J·mol− 1) of RB adsorption on non-chemically modified adsorbents (BP and CG) by using the Arrhenius equation [equation (6)], and its linear form is shown in Eq. (7) 12,36: R (8.314 J·mol− 1·K− 1), ko (g·mg− 1·min− 1), and T (K) indicate the gas constant, temperature-independent factor, and tested temperature, respectively.
$$\frac{1}{{\text{q}}_{\text{t}}}=\frac{{\text{k}}_{1}}{{\text{q}}_{\text{e}}\text{t}}+\frac{1}{{\text{q}}_{\text{e}}}$$
4
$$\frac{\text{t}}{{\text{q}}_{\text{t}}}= \frac{1}{{\text{k}}_{2}{\text{q}}_{\text{e}}^{2}}+ \frac{\text{t}}{{\text{q}}_{\text{e}}}$$
5
$${\text{k}}_{2}= {\text{k}}_{\text{o}}{\text{e}}^{\frac{{-\text{E}}_{\text{a}}}{\text{R}\text{T}}}$$
6
$$\text{ln}{\text{k}}_{2}= \frac{{-\text{E}}_{\text{a}}}{\text{R}\text{T}}+ \text{ln}{\text{k}}_{\text{o}}$$
7
The adsorption system has two major types: physical adsorption and chemical adsorption (activated and non-activated ones). In particular, the activated chemical adsorption indicates that the rate changes with different temperatures in accordance with Ea in the Arrhenius equation. By contrast, Ea in non-activated chemical adsorption is near zero. Based on Eq. (7), Ea has positive values of 29.51 kJ·mol− 1 (BP) and 27.46 kJ·mol− 1 (CG) based on the determined slope from the Arrhenius plot of lnk2 versus 1/T. Therefore, increasing the temperature will promote adsorption, as well as this process, which is considered as an endothermic one (Ea>0). In addition, the major types of adsorption could be based on the calculated activation energy value. In particular, the activation energy range of 5–40 kJ⋅mol− 1 will reveal physical adsorption, or that of 40–800 kJ⋅mol− 1 will indicate chemical adsorption 37,38. Hence, the adsorption activation energy for RB has confirmed the physical adsorption of RB on the surface of non-chemically modified adsorbents (BP and CG). The results indicate that RB adsorption has occurred during physical and chemical adsorptions.
| Sample | Temp. (°C) | qe, exp (mg⋅g− 1) | Pseudo-first-order model | Pseudo-second-order model | ||||
| qe, cal (mg⋅g− 1) | k1 (min− 1) | R2 | qe, cal (mg⋅g− 1) | k2 (×10− 4 g⋅mg− 1⋅min− 1) | R2 | |||
| BP | 30 | 4.30 | 4.10 | 101.93 | 0.995 | 4.04 | 25.31 | 0.999 |
| 40 | 4.59 | 4.27 | 83.75 | 0.993 | 4.18 | 30.47 | 0.999 | |
| 50 | 5.04 | 4.34 | 66.20 | 0.984 | 3.95 | 52.49 | 0.993 | |
| CG | 30 | 4.06 | 3.95 | 113.78 | 0.992 | 3.93 | 22.77 | 0.998 |
| 40 | 4.41 | 4.07 | 89.88 | 0.990 | 4.04 | 28.14 | 0.997 | |
| 50 | 4.83 | 4.11 | 71.52 | 0.994 | 3.84 | 44.85 | 0.995 | |
| Sample | Temp. (°C) | qe, exp (mg⋅g− 1) | Langmuir model | Freundlich model | ||||
| qm, cal (mg⋅g− 1) | KL (L⋅mg− 1) | R2 | n | KF (L⋅mg− 1) | R2 | |||
| BP | 30 | 4.30 | 6.76 | 0.17 | 0.980 | 3.39 | 1.76 | 0.855 |
| 40 | 4.59 | 6.96 | 0.30 | 0.994 | 3.84 | 2.28 | 0.792 | |
| 50 | 5.04 | 7.64 | 0.43 | 0.996 | 4.11 | 2.81 | 0.842 | |
| CG | 30 | 4.06 | 6.53 | 0.13 | 0.964 | 3.24 | 1.53 | 0.871 |
| 40 | 4.41 | 6.80 | 0.23 | 0.994 | 3.31 | 1.83 | 0.846 | |
| 50 | 4.83 | 7.51 | 0.27 | 0.995 | 3.27 | 2.06 | 0.871 | |
| Sample | Temp. (°C) | KL (m3⋅mol− 1) | ∆G° (kJ⋅mol− 1) | ∆H° (kJ⋅mol− 1) | ∆S° (kJ⋅mol− 1⋅K− 1) |
|---|---|---|---|---|---|
| BP | 30 | 81.64 | −11.090 | 37.44 | 0.1604 |
| 40 | 141.71 | −12.891 | |||
| 50 | 205.23 | −14.298 | |||
| CG | 30 | 60.67 | −10.342 | 30.77 | 0.1362 |
| 40 | 108.02 | −12.185 | |||
| 50 | 129.76 | −13.066 |
Adsorption isotherms
The design of the RB adsorption system can constitute the most optimal correlation for the equilibrium curves 11. Here, the Langmuir [equation (8)] and Freundlich [equation (9)] equations, which are common isotherm equations, have been used to determine the equilibrium data of the RB adsorption onto non-chemically modified adsorbents at the abovementioned various temperatures. As shown in Table 1 and Fig. 9, the Langmuir isotherm is well fitted (R2 ∼ 0.99) in the entire adsorption period at different temperatures, thereby indicating the homogeneous nature of the surface of non-chemically modified adsorbents. Furthermore, the increase in the tested temperature has improved the adsorption capacity (i.e., BP: 6.76 mg⋅g− 1/30°C, 6.96 mg⋅g− 1/40°C, 7.64 mg⋅g− 1/50°C and CG: 6.53 mg⋅g− 1/30°C, 6.80 mg⋅g− 1/40°C, 7.51 mg⋅g− 1/50°C).
Therefore, the adsorption capacity increases with temperature. Hence, the Gibbs-free energy (ΔG°, kJ⋅mol− 1), entropy (ΔS°, kJ⋅mol− 1⋅K− 1), and enthalpy (ΔH°, kJ⋅mol− 1) values are determined at the abovementioned temperatures based on the van’t Hoff equation [equation (10)]; R (8.314 J·mol− 1·K− 1), KL (m3⋅mol− 1), and T (K) refer to a gas constant, Langmuir constant, and tested temperature, respectively. In particular, ΔH° and ΔS° values are based on a determined slope of the linear plot of ΔG° versus T.
$$\frac{{\text{C}}_{\text{e}}}{{\text{q}}_{\text{e}}}=\frac{1}{{\text{q}}_{\text{m}}{\text{K}}_{\text{L}}}+\frac{{ \text{C}}_{\text{e}}}{{\text{q}}_{\text{m}}}$$
8
$$\text{log}{\text{q}}_{\text{e}}=\text{log}{\text{K}}_{\text{F}}+\frac{1}{\text{n}}\text{log}{\text{C}}_{\text{e}}$$
9
$${\varDelta \text{G}}^{\text{o}}= -\text{R}\text{T}\text{ln}{\text{K}}_{\text{L}}={\varDelta \text{H}}^{\text{o}}-{\text{T}\varDelta \text{S}}^{\text{o}}$$
10
| Adsorbent | Adsorbate | Contact time (min) | Adsorbent dose (g⋅L− 1) | Temp. (°C) | pH | Isotherms | Kinetics | Thermodynamics | Adsorption capacity (mg⋅g− 1) | Ref. |
|---|---|---|---|---|---|---|---|---|---|---|
| Non-chemically modified BP | RB | 230 | 6 | 30, 40, 50 | 6 | Langmuir | Pseudo-second order | Spontaneous endothermic | 6.7, 6.96, 7.64 | This work |
| Non-chemically modified CG | RB | 230 | 6 | 30, 40, 50 | 6 | Langmuir | Pseudo-second order | Spontaneous endothermic | 6.5, 6.80, 7.51 | This work |
| THMW | RG | 120 | 8 | 30 | Langmuir | Pseudo-second order | -- | 3.40 | 39 | |
| CWOP | RB | 45 | 10 | 29±2 | 5.2 | Langmuir, Freundlich | Pseudo-first order | -- | 3.23 | 40 |
| PO | 45 | 10 | 29±2 | 5.2 | Langmuir, Freundlich | Pseudo-first order | -- | 1.33 | 40 | |
| CA | RB | -- | -- | 30 | 6.2 | Langmuir, Freundlich | -- | -- | 2.86 | 41 |
| MB | -- | -- | 30 | 6.2 | Langmuir, Freundlich | -- | -- | 2.26 | 41 | |
| ANZ | RB | -- | 0.25 | 30, 50 | 6 | Langmuir, Freundlich | Pseudo-second order | Spontaneous endothermic | 2.1, 2.76 | 34 |
| MB | -- | 0.25 | 30, 50 | 6 | Langmuir, Freundlich | Pseudo-second order | Spontaneous endothermic | 6.8, 7.91 | 34 | |
| ZM | RO | 60 | 0.05 | 25–40 | -- | Langmuir | Pseudo-second order | Spontaneous endothermic | 1.06 | 42 |
| IC | 60 | 1 | 25–40 | -- | Langmuir | Pseudo-second order | Spontaneous endothermic | 0.58 | 42 | |
| Na+-MMT | RG | 10 | 5 | 18–34 | 6 | Freundlich, Dubinin-Radushkevich | Pseudo-first order | Exothermic | 0.40 | 43 |
a THMW: Trichoderma harzianum mycelial waste, CWOP: cellulosic waste orange peel, CA: coal ash, ANZ: Australian natural zeolite, ZM: zeolite from fly ash-iron oxide magnetic nanocomposite, Na+-MMT: Na+-montmorillonite, RG: rhodamine 6G, PO: procion orange, MB: methylene blue, RO: reactive orange 16, IC: indigo—carmine.
As summarized in Table 2, the negatively attained ΔG° values show the spontaneous nature of RB adsorption at different temperatures, the affinity between the non-chemically modified adsorbents (BP and CG), and the adsorbate (RB). In addition, the positive values of ΔH° (BP 37.44 kJ⋅mol− 1 and CG 30.77 kJ⋅mol− 1) and ΔS° (BP 0.1604 kJ⋅mol− 1⋅K− 1 and CG 0.1362 kJ⋅mol− 1⋅K− 1) indicate the endothermic properties of adsorption and the increased randomness at the interface between the solid–liquid phases, respectively. Moreover, the attained ΔH° values are less than 40 kJ⋅mol− 1, which indicates physical adsorption 38, but those in the range of 40–800 kJ⋅mol− 1 indicate chemical adsorption 37,38. Thus, these results are similar to the abovementioned RB adsorption that occurs during physical and chemical adsorptions. For various adsorbents, based on the adsorption isotherms, the saturated adsorption capacity for adsorbate could be compared by determining the amount of adsorbate adsorbed on the adsorbents. As listed in Table 3, the adsorption capacity of non-chemically modified adsorbents in this study is greater than that of modified waste materials (trichoderma harzianum mycelial waste [THMW], cellulosic waste orange peel [CWOP], and coal ash [CA]) and modified natural materials (Australian natural zeolite [ANZ], zeolite from fly ash-iron oxide magnetic nanocomposite [ZM], and Na+-montmorillonite [Na+-MMT]) 34,39−43. Therefore, non-chemically modified materials or the reuse of CG are potential candidates that are directly applied as low-cost adsorbents in wastewater treatment.
Characterization of used non-chemically modified adsorbents
In addition, the RB adsorption mechanism can be based on the FTIR spectra of pure RB and RB-loaded adsorbents (Fig. 10A). In particular, in pure RB spectra, a broad peak at 3433 cm− 1 belongs to the -OH/-NH stretching vibration. The peaks at 2974 and 2930 cm− 1 indicate asymmetric and symmetric -CH stretching vibrations, respectively. Two weak peaks at 1645 and 1707 cm− 1 are related to Chet = N+(CH3)2 stretching vibrations; concomitantly, a peak at 1589 cm− 1 indicates C = C vibration in the aromatic ring structure 44. Moreover, the peaks at 1468, 1339, and 1065 cm− 1 are attributed to C-N stretching [-CN(C2H5)2], -CH3 bending, and C–O–C stretching (an aromatic ring) vibrations, respectively 45,46. In particular, a characteristic peak of–C–H blending vibration appearing in the di-substituted benzene ring of the RB molecule was also observed at 818 cm− 1 47. By contrast, the FTIR spectra of RB-loaded adsorbents have few changes in characteristic peaks, which might reveal relatively weak forces during RB adsorption. Notably, the -OH/-NH stretching vibrations are broader, and they shift to low wavenumber regions after RB adsorption (i.e., 3423 to 3414 cm− 1 [BP] and 3441 to 3429 cm− 1 CG]), which is related to the potential hydrogen bonds between non-chemically modified adsorbents (hydroxyl groups) and the RB. Concomitantly, the C = O stretching vibration of non-chemically modified adsorbents (1740–1605 cm− 1 [BP] and 1744–1628 cm− 1 [CG]) and C-N stretching vibrations of RB (1468 cm− 1) are attenuated, which show charge neutralization between the -COO− ions of adsorbents and -N+ ions of RB 48. Therefore, possible interactions are intervened among the functional groups of non-chemically modified absorbents (BP and CG) and the adsorbate (RB), thereby supporting the abovementioned isotherm data.
Recycling performance
In upholding the adsorption capacity during their repeated utilization in wastewater treatment, the reuse performance of non-chemically modified absorbents (BP and CG) was investigated at 30°C during the reuse process, which is conducted by immersing and washing the RB-loaded BP and CG with ethanol. Then, these samples were used to re-adsorb the RB (20 mg⋅L− 1) at 230 min. In several recent correlative works, CWOP, which is a modified waste material 40, has shown good adsorption ability in an acidic medium (acetic acid) and good desorption ability in an alkaline medium (pH 12.0) for anionic dyes (CR and procion orange). Thus, these anionic dyes were held by the CWOP (i.e., ion exchange). However, for cationic dye (RB), the desorption performance did not remarkably change (17.0–27.0%) with the increase of pH from 3.0 to 11.0, as well as it is similar to the adsorption ability, indicating that ion exchange could not be considered as a major part in the adsorption process. In addition, THMW, which is a modified waste material 39 obtained from 0.1–0.5 N NaOH, successfully desorbed rhodamine 6G (i.e., 26.9–57.9%). Therefore, THMW can be regenerated and recycled. In particular, the non-chemically modified BP and CG as low-cost adsorbents have not been identified for recycling performance. Here, the RB removal percentage of non-chemically modified adsorbents almost did not decrease in each RB solution from the 1st cycle to the 5th cycle; however, their RB removal percentage rapidly and slightly decreases from the 4th cycle to the 5th cycle (i.e., 83.8–78.3% [BP] and 80.0–75.0% [CG], Fig. 10B). Therefore, these low-cost adsorbents are considered as reusable adsorbents in five cycles. BP and CG are used as low-cost materials (particularly as zero-cost adsorbent) and suggested as potential adsorbents because of the following characteristics: (i) high recycling ability, (ii) easy recycling stage, and (iii) nearly zero cost of their preparation (non-chemically modified form) 49. Hence, the abovementioned points could provide a cost potential for their applications in a real industrial-scale system.
The natural abundance materials such as BP and CG are used directly with non-chemical modification for RB adsorption. The adsorption capacity of RB using low-cost adsorbents (non-chemically modified BP and CG) is evaluated during adsorption. Several effects of adsorption conditions were investigated, such as the contact time, temperature, dose of non-chemically modified adsorbents, initial concentration of RB, and pH values of RB solution. In addition, the experimental data well fitted the pseudo-second-order model, with Ea of 29.51 kJ·mol− 1 (BP) and 27.46 kJ·mol− 1 (CG). The positive values of Ea indicate that the increase in temperature supports adsorption, and the process involves endothermic properties of adsorption. Furthermore, the Langmuir model well fitted the experimental data; the maximum adsorption capacities are 6.76, 6.96, and 7.64 mgg− 1 for BP, as well as 6.53, 6.80, and 7.51 mgg− 1 for CG at 30°C, 40°C, and 50°C, respectively. The attained ΔG° with negative values (i.e., from − 11.09 to − 14.30 kJ⋅mol− 1 [BP] and from − 10.34 to − 13.07 kJ⋅mol− 1 [CG]) shows the spontaneous nature of the adsorption; concomitantly, the values of ΔH° and ΔS° are calculated to be 37.44 kJ⋅mol− 1/0.1604 kJ⋅mol− 1⋅K− 1 (BP) and 30.77 kJ⋅mol− 1/0.1362 kJ⋅mol− 1⋅K− 1 (CG). Therefore, the RB adsorption is an endothermic reaction, which is at the state of physical and chemical adsorptions, and its recycling ability is determined in five cycles. Furthermore, these materials are considered as potential low-cost adsorbents.
Acknowledgments
This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean Government (MSIT) (NRF-2023R1A2C2005617).
Author contributions statement
Thi Sinh Vo: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Visualization, Writing—original draft and editing. Muhammad Mohsin Hossain: Writing—review and editing. Kyunghoon Kim: Conceptualization, Writing—review and editing, Supervision, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Data availability statement
The data obtained and/or analyzed during the current study are available from the corresponding author on reasonable request.
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No competing interests reported.