The adsorption behavior and the underlying mechanism of methylene blue (MB) sorption on biochars prepared from different feedstocks at 500 °C were evaluated. The biochar feedstocks included Magnolia grandiflora Linn. leaves biochar (MBC), pomelo (Citrus grandis) peel biochar (PBC) and badam shell biochar (BBC). The results of characterizing and analyzing the samples showed that different biochars had different effects on the adsorption of MB. It could be found that MBC had the best adsorption effect on MB due to its largest average pore diameter of 5.55 nm determined by Brunauer-Emmett-Teller analysis. Under the optimal conditions, the maximum adsorption capacities of BBC, PBC and MBC were 29.7, 85.15 and 99.3 mg/g, respectively. The results showed that the amount of adsorption was affected by the pH value. The maximum adsorption capacity of MBC was 46.99 mg/g when it was at pH of 3, whereas for the same experimental conditions the maximum adsorption capacity of BBC and PBC was 25.29 mg/g at pH of 11 and 36.08 mg/g at pH of 7, respectively. Therefore, MBC was found to be a most efficient low-cost adsorbentl for dye wastewater treatment compared with BBC and PBC, and it had the best removal effect under acidic conditions.
Many industrial processes involving a large amount of dyes cause the formation of dye wastewater, such as textiles, leather nitrification, paper production and food technology (Tan et al. 2007). With the rapid development of the chemical industry in our country, large amounts of dye wastewater have been directly discharged into the environment which has caused negative consequences to the environment and human health. Methylene blue (MB) is a common basic dye which has very large applications for dyeing cotton, wool and clothing besides having a great use in medicine and colored paper. Acute intake of a large amount of MB would cause heart rate increase, vomiting, shock, purpura, jaundice, limb paralysis and tissue gangrene (Lyu et al. 2018). In addition, high dose MB could induce methemoglobinemia, lead to Heinz-body anemia or other symptoms of red cell morphology change and reduce red blood cell life. Therefore, it is an urgent and important global problem to effectively remove dyes.
Dye wastewater treatment methods mainly include coagulation and flocculation, chemical oxidation, membrane separation and adsorption (Rafatullah et al. 2010). Among them, adsorption technology is widely used in the treatment of dye wastewater because of its easy handling, low cost and high efficiency. Adsorption is considered as a valid method for balanced separation and water purification applications (Harikishore Kumar Reddy et al. 2017). Activated carbon can effectively remove active, alkaline and azo dyes in wastewater. However, it is generally applied to dye wastewater treatment or advanced treatment with lower concentration due to its difficulty in regeneration and high cost. In recent years, domestic and foreign scholars have begun to look for new cheap adsorbent materials to try to replace activated carbon. Biomaterials had been found to have the advantages of low price, easy availability, wide application conditions and abundant sources. Waste plant materials also have adsorption potential and can be used as biosorbents. The cell wall of plant materials is rich in lignin, cellulose and hemicellulose. The main reason for the adsorption of dyes by plant materials is that their cellulose and hemicellulose are rich in hydroxyl groups and polyphenol groups. These groups can adsorb dye molecules through different mechanisms such as electrostatic adsorption, complexation and hydrogen bonding.
This paper describes the use of pomelo (Citrus grandis) peel (fruit peel), badan shell (dried fruit shell) and Magnolia grandiflora Linn. leaves (woody plant) as raw materials to study the adsorption behavior of MB. Firstly, the factors affecting the adsorption effect were investigated, and then the adsorption process was optimized by analyzing various parameters of adsorption kinetics. The purpose of this study was to explore the adsorption efficiency and adsorption process of three biochars, and to measure and compare the differences in adsorption capacity of dyes on biochars.
MATERIAL AND METHODS
Preparation of biochars
Magnolia grandiflora Linn. leaves were taken from a local tree (30° 26′ 48″ N, 114° 15′ 25″ E), pomelo (Citrus grandis) peel was obtained from a local fruit market (30° 34′ 56″ N, 114° 25′ 38″ E) and badam shell was purchased from Urumqi (43° 50′ 10″ N, 7° 37′ 24″ E), Xinjiang, China. The raw materials were naturally dried first, soaked in deionized water to remove surface impurities, and placed in an oven to dry at 80 °C. Then these biochars were pyrolyzed in a tubular furnace under anoxic conditions at 500 °C. After the pyrolysis residue was cooled to room temperature, it was ground into powder to pass through a 100-mesh sieve and then rinsed repeatedly with distilled water to neutral. It was put in the oven and dried to constant weight at 80 °C. Finally, it was stored in plastic bottles for later use. The biochars prepared from three different raw materials are named Magnolia grandiflora leaves biochar (MBC), pomelo peel biochar (PBC) and badam shell biochar (BBC), respectively.
MB supplied by Sinopharm Chemical Reagent Co., Ltd was used as an adsorbate without further purification. A certain amount of MB solid was obtained, and then formulated into a 300 mg/L solution with distilled water as stock solution. Solutions of the corresponding mass concentration were diluted proportionally with deionized water accordingly.
Batch sorption experiment
Effect of solution pH
The effect of pH on adsorption was studied by adsorption studies under different pH conditions. The initial concentration of 100 mL dye solution was set to 50 mg/L, adjusted to different pH values (pH 2, 3, 5, 7, 9, and 11), and 0.1 g of biochar was added. The pH was measured by using a pH meter (ST3100, Changzhou, China). Then, the solution was placed in a constant temperature oscillator and controlled. After shaking for 24 h, the sample was taken out and the amount of adsorption at this time was measured.
For the kinetic experiment, 1.0 g of sorbent was mixed with 100 mL of MB solution (C0: 70 mg/L, 150 mg/L) at pH 5. While the temperature was maintained at 298 K, the agitation speed was set at 150 rpm and 2.0 mL solution was extracted with a disposable needle filter (0.45 µm) at regular intervals. Then the residual concentrations were determined and the samples were analyzed.
All experiments were repeated three times, and the experimental error limit of three experiments is ±5%.
RESULTS AND DISCUSSION
Physical–chemical properties of biochar
Biochar particles were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy and Brunauer–Emmett–Teller (BET) analysis. The BET specific surface area of PBC was very small at 0.026 m2/g and BJH (Barrett-Joyner-Halenda) desorption cumulative volume of pores was 0.00145 cm3/g. The BET surface area, total pore volume and average pore diameter of the BBC were found to be 269.02 m2/g, 0.13532 cm3/g and 3.2554 nm, respectively, and for MBC were 98.89 m2/g, 0.028 cm3/g and 5.55 nm, respectively. It can be seen that the average pore diameter of the MBC is the largest. Table 1 lists these results.
|Adsorbent||BET surface area||Total pore volume||Adsorption average pore diameter|
|Adsorbent||BET surface area||Total pore volume||Adsorption average pore diameter|
The Fourier transform infrared (FTIR) spectra of different biochars are listed in Figure 1. The FTIR spectra obtained showed that the functional groups of biochar samples after adsorption had changed, and some peaks disappeared while new peaks appeared. For BBC, the medium peak near 3,065.41 cm−1 indicated the presence of –OH stretching. Various bands in the spectrum were identified as corresponding to the C=O stretching (at wavenumber of 1,761.98 cm−1), the C–H bending (1,477.28 cm−1) and the symmetric bending of C–O–C (922.61 cm−1). Figure 1(a) shows the FTIR spectra of PBC. The FTIR analysis indicated that there was a distinct absorption peak at 3,662.29 cm−1, representing the stretching vibration absorption of –OH groups. It indicated that a large amount of hydroxyl groups existed in the pomelo peel. A strong peak at 1,740 cm−1 was due to C=O stretching, which indicated that the pomelo peel contains carbonyl. These carbonyls are likely to come from acids or esters since the pomelo peel contains a lot of cellulose and pectin. A peak at 1,511.22 cm−1 was considered to be due to the presence of alkene (C–H) bending. A peak at 912.42 cm−1 could be due to symmetric bending of C–O–C and a medium peak at 643.59 cm−1 signified C–OH twist. For MBC, the band at 1,782.35 cm−1 could be due to C=O stretching and the medium band at 1,507.45 cm−1 to C–H bending. The peak around 1,273.69 cm−1 corresponded to the C–O–C stretching. Moreover, the medium peak at 836.09 cm−1 was caused by C–H bending (out of the plane). The peak at 580.5 cm−1 was the C–C group. The strengths of C–H and C=O decreased significantly before and after adsorption, indicating that these groups are the main reactive groups of these three biochars interacting with MB.
To further explore the surface structure of the samples, the BC samples before and after adsorption were observed with scanning electronic microscopy (SEM) and energy dispersive spectroscopy (EDS). The results are shown in Figure 2. It could be seen from Figure 2(a), 2(c) and 2(e) that the prepared adsorbent has a rich pore network structure, and the pore distribution is uniform and dense, which ensured that the sample has a high specific surface area and pore volume (Esquena 2012). The surface of dye-loaded adsorbent (Figure 2(b), 2(d) and 2(f)), indicates that the surface of biochars was covered with material. The structure of PBC was similar to that of MBC because of their fibre structure. The pore in BBC was smaller than that in PBC and MBC. Their elemental compositions were further investigated by EDS spectrum. The elements of different biochars are basically unchanged after adsorption, indicating that these elements do not participate in reactions. The types of elements of different biochars are also inconsistent.
Effects of contact time and initial dye concentration
The effects of contact time and initial dye concentration on the adsorption of MB by biochar were studied. It could be seen from Figure 3 that the adsorption amount of MB on three biochars increased with time. The adsorption results of the three biochars showed that the amount of biochars adsorbed to MB increased rapidly with time at the beginning of the reaction, and then the amount of adsorption changed slowly and gradually stabilized. It was because there were a large number of empty positions on the surface of the biochar for adsorption in the initial stage, and when the reaction was in process, the remaining empty surface positions become more and more difficult to occupy due to the repulsive force between the solute molecules (Jia et al. 2018). When the initial concentrations of MB were 70 mg/L and 150 mg/L, the adsorption amount of MB on adsorbent increased rapidly in the first 60 min. The results showed that the rate of dye adsorption was fast. It could be due to the electrostatic attraction between biochar and MB. When the initial MB concentration was changed from 20 to 200 mg/L under certain external adsorption conditions, the removal rate of MB decreased as the initial concentration of the solution increased. This phenomenon was related to the mass transfer driving force (Sadaf & Bhatti 2014). The adsorption load increases as the concentration of the MB solution increases, which was not conducive to promoting the diffusion of the adsorbate from the liquid to the surface and inside of the adsorbent, causing a decrease in adsorption rate (Dhodapkar et al. 2007). The results showed the removal efficiency of BBC had the largest change, which decreased from 92.06% to 9.63%.
|Qe (mg/g)||k1||R2||Qe (mg/g)||k2||R2|
|Qe (mg/g)||k1||R2||Qe (mg/g)||k2||R2|
The correlation coefficient associated with the linear fit and the average chi-square error () analysis were used to evaluate the best model describing the MB adsorption data (Elwakeel et al. 2017). is determined according to Equation (3). The kinetic parameters are listed in Table 2. It could be seen from Figure 4(a) and 4(b) that the pseudo-first-order kinetic adsorption data slightly deviated from the fitting curve, and the correlation coefficient R2 was 0.7250–0.9624. The fitting effect of the pseudo-second-order kinetic equation was relatively good, and the adsorption data were basically consistent with the fitting curve. The calculation shows that the average chi-square error (χ2) of the pseudo-second-order kinetic equation was smaller than that of the pseudo-first-order kinetic equation. R2 was 0.9906–0.9997. This phenomenon indicated that the adsorption mechanism of biochar for the MB solution was diverse but it was still based on chemistry (Smith et al. 2016). The adsorption mechanism of biochar for organic pollutants mainly includes distribution and surface adsorption, and also includes other microscopic adsorption mechanisms. The pseudo-second-order kinetic model provides a better fit than the pseudo-first-order model, suggesting that MB adsorption to these biochars might involve a multi-nucleus rather than single-nucleus adsorption process. Similar phenomena were reported for malachite green adsorption onto sugarcane bagasse biochar (Vyavahare et al. 2018) and removal of levofloxacin using rice-husk and wood-chip biochars (Yi et al. 2016).
The values and the constants of two isotherm models for MB adsorption on BBC, PBC and MBC at 298 K are listed in Table 3. The results showed that the linear correlation coefficient of the Langmuir isotherm equation was 0.9626–0.9986, which was higher than the linear correlation coefficient fitted by the Freundlich isotherm equation. It could be seen from Figure 4(c) and 4(d) that the Langmuir model could better describe the MB adsorption behavior of these biochars. The Langmuir adsorption model proved that these three kinds of biochar as adsorbents had an abundant-pore structure and uniform adsorption point distribution, and mainly monolayer adsorption occurred between MB molecules and biochar.
|Sorbent||Freundlich equation||Langmuir equation|
|Sorbent||Freundlich equation||Langmuir equation|
Similar results were reported for MB adsorption on activated carbon made of dehydrated peanut hull (Özer et al. 2007). The result that the equilibrium parameter values were between 0 and 1 showed that these biochars were beneficial for the adsorption of MB under the experimental condition. The maximum adsorption capacities of BBC, PBC and MBC were 25.52, 32.71 and 67.05 mg/g, respectively. The adsorption capacities of BBC and PBC were close, and the value of MBC was highest.
Effect of temperature on dye adsorption
The temperature has an effect on the adsorption amount of adsorbent. The diffusion rate of the adsorbed molecules through the outer boundary layer and the internal pores of the adsorbed particles increased with increasing temperature (Al-Qodah 2000). As the temperature increased, the adsorption behavior of BBC, PBC and MBC was also different. For BBC, the adsorption capacity decreased with increasing temperature. It indicates that the process of BBC adsorbing MB is exothermic. This was because the physical bond between the MB and the active site of the adsorbent decreased as the temperature increased. It showed that MB was adsorbed onto the BBC, which caused the adsorbate molecules to lose some degrees of freedom. Similar trends were reported for adsorption of MB on activated carbon prepared from coconut husk (Tan et al. 2008).
Completely different behaviors were obtained for PBC and MBC; the amount of adsorption became larger as the temperature increased, which meant that the adsorption of MB by both biochars was an endothermic process. This may be caused by the increase of the migration rate of dye molecules and the number of active centers as the temperature increased (Salleh et al. 2011). Thus more and more molecules got enough energy to interact with the active center on the surface.
Effect of solution pH on dye adsorption
The pH value of the solution is one of the important factors affecting the adsorption performance of the adsorbent since the pH in the solution not only affects the existent morphology of MB cations in solution but also affects the active sites on the surface of the adsorbent (Khattri & Singh 2009). The effect of pH on biochar adsorption was investigated at an initial dye concentration of 50 mg/L, an oscillation time of 24 h, and BC concentration and temperature fixed at 0.10 g and 298 K, respectively. The effect of pH on MB adsorption under equilibrium (qe) is shown in Figure 5(a). For PBC and BBC, a low amount of dye was found to be adsorbed when the solution was in low pH. MB is a cationic dye: a large amount of H+ in the solution will compete and occupy the adsorption site when in low pH, resulting in a low adsorption rate. At near-neutral pH conditions, the surface of the adsorbent would accumulate more negative charges to promote adsorption between the positive dye ions and the adsorbent through electrostatic interaction. As the pH rises, the adsorption effect increases slowly. At pH of 7 the adsorption capacity of PBC was at maximum (36.08 mg/g) whereas the maximum adsorption capacity of BBC was 25.29 mg/g at pH 11. Nevertheless, MBC at high pH value showed a slight decrease in dye adsorption. The maximum adsorption capacity of MBC was 46.99 mg/g at pH of 3. There might be other adsorption mechanisms such as ion exchange or chelation (Hamdaoui 2006). Similar results were observed for the adsorption of MB on cedar sawdust and broken bricks (Hamdaoui 2006). As shown in Figure 5(b), the pH of the equilibrium solution was increased compared to the initial pH because the addition of biochars, which usually had an alkaline pH, neutralized the acidity of the solution.
The research shows that the BBC, MBC and PBC all could be used to remove MB dye as a cheap and available adsorbent. The adsorption of MB onto MBC was found to be more effective than that of BBC and PBC. The amount of MB uptake on BBC, MBC and PBC increased with increasing of initial dye concentration and contact time. It was observed that the pH dependency of adsorption by BC was large. The above phenomena of the three adsorbents suggested adsorption of MB on MBC and PBC was endothermic while exothermic for BBC. The adsorption isotherms of BBC, PBC and MBC for MB were in accordance with the Langmuir equation and belong to monolayer adsorption. Their maximum adsorption capacities were 29.7, 85.15 and 99.3 mg/g, respectively. The adsorption kinetics were consistent with the pseudo-second-order kinetics models. The maximum adsorption capacities of PBC and MBC were close to each other and both higher than BBC. It was found that MBC has the best adsorption effect on MB, indicating that MBC could be used as the best biological material for dye wastewater treatment compared with BBC and PBC.
This research was financially supported by the Project (2017zy011) of Hubei Key Laboratory for Efficient Utilization and Agglomeration of Metallurgic Mineral Resources, the Natural Science Foundation of Hubei Province of China (2017CFB350). Also the authors would like to thank teacher Zhou for support with the BET and FTIR analysis.