Abstract
A solid adsorbent for SO2 adsorption was prepared from magnesium salt/rice straw via co-precipitation/calcination. The effects of various preparation conditions, including the calcination temperature, the weight ratio of Mg/rice straw, magnesium salts, and amine precipitants, were investigated relative to their effects on the desulfurization performance of adsorbents. Maximum sulfur adsorption capacity (260 mg/g) was obtained with MgO/rice straw biochar adsorbent using tetraethylenepentamine (TEPA) as the precipitants, and the preparation conditions included a calcination temperature of 400 °C, a Mg/rice straw weight ratio of 1.2, and magnesium chloride hexahydrate (MgCl2·6H2O) as the magnesium source. The rice straw biochar-supported MgO sample displayed a high SO2 adsorption capacity due to its excellent textural properties, large specific surface areas, small crystallite size, numerous surface active sites of MgO nanoparticles, and introduced N-H groups. The physical and chemical properties of samples were investigated by field emission scanning electron microscopy (FESEM), Brunauer-Emmett-Teller (BET) analyses, Fourier transform infrared spectrometer (FT-IR), elemental analysis (EA), and X-ray diffraction (XRD).
Download PDF
Full Article
Desulfurization Performance of MgO/ Rice Straw Biochar Adsorbent Prepared by Co-precipitation/ Calcination Route
Jiali Chen, Lianhan Huang, Liming Sun, and Xiaofan Zhu *
A solid adsorbent for SO2 adsorption was prepared from magnesium salt/rice straw via co-precipitation/calcination. The effects of various preparation conditions, including the calcination temperature, the weight ratio of Mg/rice straw, magnesium salts, and amine precipitants, were investigated relative to their effects on the desulfurization performance of adsorbents. Maximum sulfur adsorption capacity (260 mg/g) was obtained with MgO/rice straw biochar adsorbent using tetraethylenepentamine (TEPA) as the precipitants, and the preparation conditions included a calcination temperature of 400 °C, a Mg/rice straw weight ratio of 1.2, and magnesium chloride hexahydrate (MgCl2·6H2O) as the magnesium source. The rice straw biochar-supported MgO sample displayed a high SO2 adsorption capacity due to its excellent textural properties, large specific surface areas, small crystallite size, numerous surface active sites of MgO nanoparticles, and introduced N-H groups. The physical and chemical properties of samples were investigated by field emission scanning electron microscopy (FESEM), Brunauer-Emmett-Teller (BET) analyses, Fourier transform infrared spectrometer (FT-IR), elemental analysis (EA), and X-ray diffraction (XRD).
Keywords: Rice straw; Biochar; MgO; Co-precipitation/calcination; Sulfur dioxide; Adsorption
Contact information: Analytical and Testing Center, Sichuan University, Chengdu 610065, China;
* Corresponding author: zxf62@scu.edu.cn
INTRODUCTION
Sulfur dioxide (SO2) is the main component of acid rain, and its corrosive properties have detrimental effects on both the environment and human health, making it urgent to reduce SO2 emissions. Among the various flue gas desulfurization technologies to remove SO2, the adsorption methods have shown promising characteristics because of its advantages of simple operation procedures, low energy consumption for regeneration, little or no water consumption, lack of secondary pollution, no volatility or corrosion issues, and a wide variety of adsorbents (Mochida et al. 2000; Tailor et al. 2014a). Popular solid adsorbents include activated carbon (Yan et al. 2012, 2013; Guo et al. 2013; Yi et al. 2014), carbon fibers (Zhou et al. 2012; Bai et al. 2016), zeolites (Al-Harahsheh et al. 2014; Vellingiri et al. 2016), polymer-supported metal oxides (Wang et al. 2009), and metal-supported materials (Guo et al. 2012; Li et al. 2012), and metal–organic frameworks (MOFs) (Savage et al. 2016; Tan et al. 2017; Elder et al. 2018; Wang et al. 2018). Magnesium oxide (MgO) nano-materials have attracted significant attention due to their large surface areas, plentiful surface active sites, and excellent adsorption properties (Dhal et al. 2015; Li et al. 2017). MgO combined with activated carbon has been used also to adsorb dyes (Liu et al. 2015; Nassar et al. 2017), CO2 (Vu et al. 2016), fluoride (Jin et al. 2016), phosphates (Zhou et al. 2011), and toxic heavy metal ions (Chowdhury et al. 2016). Furthermore, the alkalinity of MgO, an alkaline earth metal oxide, has strong adsorption ability for acidic SO2 gas.
Currently, large amounts of rice straw residues are discarded or burned in the open air without effective utilization, which causes serious environmental pollution. As a result, effective and environmentally-friendly methods for taking advantage of rice straw should be developed. Modifying rice straw via catalytic oxidation, followed by alkaline nitrogen reagents treatment results in effective SO2 adsorption (Yang et al. 2012; Sun and Zhu 2018). However, the modification methods have some drawbacks; they are not suitable for high temperature, require a complicated process, and have a poor regeneration effect. Hence, biochar derived from waste biomass is widely used as an adsorbent due to its stability and high sorption ability. Biochar has similar pore structure and surface chemical property as activated carbon, and its simple preparation process and low investment used as a potential adsorption substitute for activated carbon (Ahmad et al. 2014). Biochar derived from waste biomass has a highly porous structure and contain various functional groups that are effective in the adsorption of metals (Wu et al. 2012) and CO2 capture (Huang et al. 2015). Meanwhile, biochar is a carbon-rich adsorbent with inorganic constituents, which has enormous potential sorption ability for SO2 (Xu et al. 2016).
This paper exhibits a new synthetic route to obtain rice straw biochar-loaded magnesium oxide (RSB-MgO) materials by co-precipitating and calcining magnesium salt /rice straw using amine reagents as precipitants. The effects of preparation conditions were investigated, including calcination temperature, the weight ratios of Mg/rice straw, magnesium salts, and amine-containing precipitants. The adsorption temperature and simulated flue gas flow rate were also studied. The prepared RSB-MgO adsorbents were characterized using field emission scanning electron microscopy (FESEM), Brunauer-Emmett-Teller (BET) analyses, Fourier transform infrared spectrometer (FT-IR), element analysis (EA), and X-ray diffraction (XRD).
EXPERIMENTAL
Materials and Reagents
Rice straw specimens were collected from farmland in Chengdu, Sichuan Province, China. Magnesium chloride hexahydrate (MgCl2·6H2O), magnesium nitrate hexahydrate (Mg(NO3)2·6H2O), magnesium acetate tetrahydrate (Mg(CH3COO)2·4H2O), ethylene-diamine (EDA), tetraethylenepentamine (TEPA), diethanolamine (DEA), 30% hydrogen peroxide (H2O2), sodium hydroxide (NaOH), bromocresol green, and methyl red indicators were used as raw materials. All reagents were analytical grade and used without further purification. They were supplied from Ke Long Chemical Reagent Company (Chengdu, China), except for magnesium acetate tetrahydrate (Mg (CH3COO) 2·4H2O), which was obtained from Aladdin Industrial Corporation. The carrier gas (compressed air, Sichuan Peace Gas Co., Ltd., China), simulated flue gas (SO2/N2~0.3%, Chengdu Jin Ke Xing Gas Co., Ltd., China), vacuum tube high-temperature sintering furnace (OTF-1200X, Ke Jing Material Technology Co., Ltd., China).
Preparation of RSB-MgO Adsorbent
Rice straw was crushed into powder and passed through a 20-mesh sieve. MgCl2·6H2O (200 mL, 0.01 M) (MgCl2·6H2O = 20.33 g) and 2.0 g rice straw powder were mixed in a beaker (250 mL) in a 60 °C water bath. EDA was added dropwise to the solution with continuous stirring until the pH reached 11. The reaction was stirred for 1 h and then allowed to stand for 3 h at room temperature. The precipitate was dried at 105 °C for 2 h, then calcined in a high-temperature vacuum sintering furnace to obtain RSB-MgO adsorbent. The calcination conditions were as follows: compressed air gas (180 mL/min) was used as the carrier gas, a heating rate of 15 °C /min, and a calcination time of 1 h at 400 °C. The schematic diagram for the preparation of RSB-MgO is shown in Fig. 1.
Fig. 1. Schematic of the preparation of RSB-MgO adsorbent through precipitation with amine agents and high temperature calcination
Characterization
The morphology of RSB-MgO adsorbent was characterized by FESEM (JSM-7500F, JEOL, Tokyo, Japan). The phase retrieval and grain sizes were obtained by XRD (EMPYREAN, PANalytical B.V, Almelo, Netherlands). Specific surface areas, pore volumes, and average pore diameters were measured using a Brunauer–Emmett–Teller (BET) surface area analyzer (Fully automatic specific surface and micro-pore size analyzer, 3Flex, Micromeritics, Norcross, GA, USA). FT-IR analyses were conducted using an FT-IR spectrometer (Nicolet 6700, Thermo Electron Corporation, Waltham, MA, USA). The elemental compositions of samples were investigated by element analyzer (EURO EA3000, Elementar Analysen-systeme GmbH, Langenselbold, Germany).
Adsorption Experiments
First, 1.0 g of RSB-MgO was loaded onto a glass adsorption column (Φ30 mm × 300 mm) in a 25 °C water bath. The simulated flue gas with an inlet SO2 concentration (initial SO2 concentration, 0.3% volume concentration) was passed through the adsorption column at a flow rate of 400 mL·min-1. The adsorption experiment was considered complete when the SO2 concentration in the exhaust gas exceeded 80% of the inlet SO2 concentration (i.e., when the desulfurization efficiency decreased to 20%). The SO2 in the exhaust gas was absorbed by H2O2 (3%) solution and titrated by NaOH (0.01 M) to neutrality. Sulfur adsorption capacity is used to indicate the SO2 adsorption capacity of a sample, and the desulfurization efficiency (Eq. 1) and sulfur adsorption capacity (Eq. 2) were calculated according to the following formulas,
where η is the desulfurization efficiency (%); C in is the inlet SO2 concentration of the simulated flue gas (ppm); C out is the outlet concentration of SO2 in the exhaust gas (ppm); Cs is the sulfur adsorption capacity (mg·g-1); m is the weight of adsorbent (g); Q is the gas flow rate (L·min-1); and t20% is the contact time when the desulfurization efficiency decreased to 20% (min). The flow chart of SO2 adsorption is shown in Fig. 2.
Fig. 2. (a) SO2 adsorption apparatus. Gas supply system: 1. SO2/N2 0.3%, 2. Pressure reducing valve, 3. Rotameter, 4. Three-way valve Adsorption system, 5. Fixed reactor and water bath, 6. H2O2 absorption bottle (Detection system), 7. NaOH absorption bottle (Exhaust gas absorption system); (b) schematic diagram of the adsorption column
RESULTS AND DISCUSSION
Saturated Sulfur Capacity
The adsorption experiment results of native rice straw, rice straw biochar (RSB), and RSB-MgO adsorbent (ethylenediamine (EDA) as a precipitant) are shown in Fig. 3, and an analysis of the physical properties and elemental compositions of RSB and RSB-MgO are presented in Table 1. The sulfur capacity S.D values of native rice straw, RSB, and RSB-MgO under three parallel experiments were 3.0%, 3.9%, and 4.9% (Fig. 3), all of which are less than 5%, indicating that the data were within experimental errors. Sulfur capacity increased from 5.1 to 11.1 mg/g after the rice straw was carbonized into RSB. After loaded MgO on RSB, the saturated sulfur capacity, BET surface area, and N content of adsorbent increased dramatically from 11.1 to 194.6 mg/g, 10.6 to 138.1 m²/g, and 0.3 to 1.8 %, respectively. This indicated that the RSB-MgO had better desulfurization effect compared with rice straw or rice straw biochar, which was attributed to increase in large specific surface area and N content of adsorbent. The contact time in adsorption process of native rice straw, RSB, and RSB-MgO were 16, 30, and 120 min, respectively, which demonstrated that large specific surface area increased the contact time between SO2 in adsorbent. In addition, the high saturated sulfur capacity of RSB-MgO was attributed to the basicity and large number of surface active sites (oxygen defects and magnesium defects active) of MgO. The SO2 adsorption performance of MgO depends on these active sites (R. Li et al. 2017). The SO2 adsorption on MgO is both physical and chemical adsorption. The physical adsorption of SO2 is dependent on morphology and structure, causing gas molecules to be adsorbed on solid surfaces. In addition, chemical adsorption is related to surface chemical activity, and SO2 may react with MgO to generate MgSO3 at a highly active site. Furthermore, the increased N content in RSB-MgO implied that EDA could have introduced N to RSB-MgO, which promoted SO2 adsorption (Tailor et al. 2014b). The results suggested that RSB, MgO, and N content synergistically affect the SO2 adsorption on RSB-MgO. The composite materials of inorganic material and carbon have good desulfurization effect, and the maximum sulfur capacity of MnO2 and Fe2O3 modified activated carbon were 157.8 and 140.6 mg/g (Fan et al. 2013). This proves that MgO enhances SO2 adsorption stronger than MnO2 and Fe2O3.
Fig. 3. Saturated sulfur capacities of native rice straw, rice straw biochar and RSB-MgO
Table 1. Analysis of the Physical Properties and Elemental Composition
Effect of Calcination Temperature
Precipitate prepared from rice straw (RS) (2.0 g) and MgCl2·6H2O (20.33 g, 200 mL) solution was calcined at 300, 400, 500, 600, and 700 °C to obtain RSB-MgO adsorbent. The effect of calcination temperature on the adsorption of SO2 using RSB-MgO was studied using a carrier gas flow rate of 180 mL/min and a heating rate of 15 °C /min. The sulfur adsorption capacities of samples calcined at these five temperatures were 99.0, 194.6, 125.8, 70.6, and 24.2 mg/g, respectively (Fig. 4a), indicating that RSB-MgO adsorbents have good desulfurization performances at various temperatures. The highest sulfur adsorption capacity was observed at 400 °C, and the sulfur capacities were lower on either side of this maximum.
The lower sulfur capacity at 300 °C was due to insufficient carbonization, which resulted in a large amount of residual lignin and cellulose in the rice straw biochar and a portion of the Mg(OH)2 was not calcined to MgO. When the calcination temperature increased from 400 °C to 700 °C, the ash and carbon loss of rice straw biochar increased, which caused the sulfur capacity to gradually decrease due to excessive carbonization. In addition, increased ash content could have contributed to blocking the pores and active adsorption sites, thus reducing the sulphur capacity. It also increased the crystallite size of MgO due to the sintering of MgO grains.
The XRD patterns of RSB-MgO adsorbents calcined at different temperatures are shown in Fig. 4b. All Bragg diffraction peaks in the XRD patterns were assigned to MgO, which is consistent with bulk MgO crystal (ICSD Patterns-89/59522). Five diffraction peaks corresponding to MgO were identified and indexed to the (111), (200), (220), (311), and (222) crystal planes, corresponding to 2θ values of 36.8°, 42.8°, 62.1°, 74.5°, and 78.4°, respectively. Strong and sharp diffraction peaks indicated highly crystalline MgO particles in the sample. However, the diffraction peaks of Mg(OH)2 were observed in the XRD pattern of RSB-MgO calcined at 300 °C, illustrating that Mg(OH)2 was not completely converted to MgO at 300 °C. The crystallite size (D) of the synthesized MgO was calculated from the full width at half maximum (FWHM) of all peaks (111), (200), (220), (311), and (222) using the Debye–Scherrer formula (Makhluf et al. 2005),
where k is the shape factor with a typical value of about 0.9, λ is the X-ray wavelength, β is the breadth of a diffraction peak at half of its maximum intensity, and θ is the Bragg angle. The average crystallite sizes of RSB-MgO samples were 16.7, 14.9, 13.8, 14.5, and 16.8 nm (Table 2). The larger crystallite size of MgO calcined at 300 °C was attributed to the coexistence of MgO with incompletely decomposed Mg(OH)2. When the calcination temperature was between 400 °C to 600 °C, MgO crystallites showed a relatively uniform size distribution due to the complete decomposition of Mg(OH)2 into MgO. In contrast, a larger MgO crystallite size at a calcination temperature of 700 °C was caused by the sintering of MgO grains. Therefore, 400 °C was judged to be the most suitable calcination temperature.
Fig. 4. (a) Sulfur adsorption capacity of RSB-MgO at different calcining temperatures, (b) XRD patterns of RSB-MgO adsorbents calcined at different temperatures, (i) 300 °C, (ii) 400 °C, (iii) 500 °C, (iv) 600 °C, (v) 700 °C
Table 2. Crystallite Sizes of Different Crystal Planes of RSB-MgO Calcined at Different Temperatures
Effect of Mg/Rice Straw Weight Ratio
Different masses of rice straw (RS) (2.0 g, 4.0 g, 8.0 g, and 20.0 g) were added to a solution containing MgCl2·6H2O (20.33 g, 200 mL) to obtain RSB-MgO adsorbent with Mg/RS weight ratios of 1.2, 0.6, 0.3, and 0.1, respectively. The results in Fig. 5 show that increasing the weight ratio of Mg/RS from 0.1 to 1.2 increased the sulfur capacity from 77.9 to 194.6 mg/g. This was attributed to the formation of more vacancies and activation centers on the surface of RSB-MgO due to an increase in the MgO proportion. Therefore, an Mg/RS weight ratio of 1.2 was used in subsequent experiments.
Fig. 5. Sulfur capacities of RSB-MgO with Mg/RS weight ratios of 0.1, 0.3, 0.6, and 1.2
Effect of Magnesium Salts
Chowdhury et al. (2016) reported that the anions of different magnesium salts may affect the morphology of MgO crystals. In this work, the effect of different magnesium salts on the SO2 adsorption using RSB-MgO was investigated. Magnesium chloride hexahydrate (MgCl2·6H2O), magnesium nitrate hexahydrate (Mg(NO3)2·6H2O), and magnesium acetate tetrahydrate (Mg(CH3COO)2·4H2O) were used as magnesium sources to obtain RSB-MgO. Rice straw (RS) (2.0 g) were added to the solution containing MgCl2·6H2O (20.33 g, 200 mL), Mg(NO3)2·6H2O (25.64 g, 200 mL), and Mg(CH3COO)2· 4H2O (21.45 g, 200mL), respectively to obtain RSB-MgO adsorbent. The sulfur capacities of the resulting adsorbents are shown in Fig. 6a. RSB-MgO specimens prepared by three different magnesium salts all displayed good desulfurization performances, with respective sulfur capacities of 194.6, 118.0, and 118.4 mg/g. Amongst them, the sulfur capacity of RSB-MgO prepared by MgCl2·6H2O was much higher than the other two, possibly due to the role of different magnesium salt anions on MgO crystal morphology and crystallinity.
Figure 6b presents the XRD patterns of RSB-MgO samples obtained from different magnesium salts. The same diffraction peaks were observed in all patterns, indicating that all products were highly pure. The high peak intensity and sharp peak shape of RSB-MgO obtained from MgCl2·6H2O demonstrates its high crystallinity, which was conducive to SO2 adsorption. The FESEM images of RSB-MgO adsorbents were prepared using MgCl2·6H2O, Mg(NO3)2·6H2O, and Mg(CH3COO)2·4H2O (Figs. 6c, d, e). Clear morphology differences can be observed among samples, and the FESEM images of RSB-MgO formed from MgCl2·6H2O and Mg(NO3)2·6H2O showed circular lamella particles with sizes of 100 nm, while Mg(CH3COO)2·4H2O displayed a pleated sheet morphology. The growth of nanocrystals is controlled simultaneously by thermodynamics and kinetics (Hu et al. 2010). Different magnesium salt anions could affect the growth rate of crystal and free energy of the crystalline surfaces form different surface morphologies; thus, different surface morphologies of RSB-MgO have different surface properties, which affect the SO2 adsorption on MgO crystalline surfaces. Therefore, MgCl2·6H2O was the most suitable magnesium source.
Fig. 6. (a) Sulfur capacities of RSB-MgO prepared from different magnesium salts, (b) XRD patterns of RSB-MgO samples prepared from different magnesium salts: (i) MgCl2·6H2O, (ii) Mg(NO3)2·6H2O, (iii) Mg(CH3COO)2·4H2O, FESEM images of RSB-MgO prepared from: (c) MgCl2·6H2O, (d) Mg(NO3)2·6H2O, (e) Mg(CH3COO)2·4H2O
Effect of Amine Reagents on Sulfur Capacity
Four amine reagents (ethylenediamine (EDA), tetraethylenepentamine (TEPA), cyclohexylamine (CA), and diethanolamine (DEA)) were used as precipitants and added dropwise respectively to the solution (200 mL) containing rice straw (RS) (2.0 g) and MgCl2·6H2O (20.33 g) to obtain RSB-MgO-EDA, RSB-MgO-TEPA, RSB-MgO-CA, and RSB-MgO-DEA adsorbents. The resulting sulfur capacities were respectively 194.6 mg/g, 263.6 mg/g, 253.5 mg/g, and 113.5 mg/g (Fig. 7a), showing that all RSB-MgO adsorbents prepared using different amine precipitants have excellent desulfurization effects, but they showed notably different sulfur capacities.
Fig. 7. (a) The sulfur capacities of RSB-MgO prepared using different amines, (b) XRD patterns of RSB-MgO prepared using different amine agents, (i) RSB-MgO-EDA, (ii) RSB-MgO-TEPA, (iii) RSB-MgO-CA, and (iv) RSB-MgO-DEA
Table 3 presents the BET parameters (specific surface area, pore volume, and average pore diameter) of RSB-MgO-EDA, RSB-MgO-TEPA, RSB-MgO-CA, and RSB-MgO-DEA. The specific surface areas and pore volumes of these samples considerably increased compared with RSB, which resulted in their higher sulfur capacities. Moreover, the XRD patterns of RSB-MgO-EDA, RSB-MgO-TEPA, RSB-MgO-CA, and RSB-MgO-DEA are presented in Fig. 7 (b). Only diffraction peaks of MgO were detected, indicating that these products were highly pure. The average crystallite sizes were 15.0, 13.8, 14.8, and 12.3 nm, respectively (Table 3). It can be seen that the MgO crystallite size prepared by the amines is DEA (hydroxylamine) <TEPA (long-chain amine) <CA (cyclic amine) <EDA (short-chain amine).
The BET results (Table 3) show that RSB-MgO-TEPA and RSB-MgO-EDA have nearly equal BET surface area values, but RSB-MgO-TEPA exhibited a higher sulfur capacity than RSB-MgO-EDA, which may be related to the smaller crystallite size of RSB-MgO-TEPA. However, the reason why RSB-MgO-CA has a higher sulfur capacity than RSB-MgO-EDA is that RSB-MgO-CA has a smaller crystallite size than RSB-MgO-EDA even though the BET surface area of RSB-MgO-CA was slightly smaller than RSB-MgO-EDA. Therefore, it can be concluded that the smaller crystallite size of adsorbent produced a better desulfurization effect when the BET surface area is not significant difference. It is worth noted that the RSB-MgO-DEA displayed the smaller specific surface area than other three products, which may resulted in a lower number of vacancies and activation sites on its surface, and the main reason why a lower sulfur capacity even though the RSB-MgO-DEA have the smallest crystallite size.
The elemental analysis results in Table 3 show that the nitrogen content of RSB-MgO-EDA, RSB-MgO-TEPA, RSB-MgO-CA, and RSB-MgO-DEA were 1.82%, 1.75%, 1.80%, and 1.85%, respectively, which were all about 6 times higher than that of biochar (0.28%). This indicates that amines were introduced into RSB-MgO, whose alkalinity assisted in the adsorption of acidic SO2. However, the nitrogen content was not significantly different between the samples, indicating that it was not the main influencing factor for the different SO2 adsorptions of RSB-MgO samples prepared using different amines. Moreover, The FT-IR spectra of RSB-MgO-EDA, RSB-MgO-TEPA, RSB-MgO-CA, and RSB-MgO-DEA are presented in Fig 8. The two peaks at 1450 and 1650 cm-1 were assigned to N-H deformation and bending vibrations, which demonstrated the presence of N-H groups in adsorbents. The N-H group may form hydrogen bond with SO2 in the absence of water. It can be concluded that introducing amine groups in RSB-MgO prepared by TEPA long chain amines with multiple amine groups has better SO2 adsorption effect.
Table 3. Physical Properties, Elemental Content, Average Crystallite Sizes, and Crystallinity of RSB-MgO Prepared Using Different Amines
Fig. 8. FT-IR spectra of RSB-MgO samples prepared by different amine agents, (i) RSB-MgO-EDA, (ii) RSB-MgO-TEPA, (iii) RSB-MgO-CA, (iv) RSB-MgO-DEA
Effect of Adsorption Conditions on Sulfur Capacity
Effect of flow rate of flue gas
Under the above adsorption experimental conditions (calcination temperature of 400 °C, Mg/rice straw weight ratio of 1.2, MgCl2·6H2O as magnesium source, tetraethylenepentamine (TEPA) as the precipitant, and adsorption temperatures of 25 °C), the sulfur capacity of the RSB-MgO-TEPA adsorbent was measured with flue gas flow rates of 250, 300, 350, 400, and 450 mL/min (Fig. 9).
Figure 9 shows that RSB-MgO-TEPA had a higher sulfur capacity at a flue gas flow rate of 400 mL/min, while higher or lower flow rates showed similar sulfur capacities. This may be because the lower gas flow rate causes less SO2 molecules to diffuse on the solid adsorbent surface. As the adsorption time increases, the SO2 molecules physically adsorbed on the solid adsorbent surface will desorb, which results the lower sulfur capacity. When increasing the gas flow rate, the gas driving force increased, and the gas driving force could be assisted in the diffusion and adsorption of SO2 molecules on the solid adsorbent surface. However, when the flue gas velocity was too high, SO2 molecules were ejected without being adsorbed because of short contact time, resulting in a lower desulfurization effect. Therefore, suitable gas flow rate should be selected, 400 mL/min was applied in this experiment.
Fig. 9. Sulfur capacities of RSB-MgO-TEPA under different flue gas flow rates
Effect of adsorption temperature
Under the above adsorption experiment conditions (calcination temperature of 400 °C, Mg/rice straw weight ratio of 1.2, MgCl2·6H2O as the magnesium source, tetraethylenepentamine (TEPA) as the precipitant, and flue gas flow rates of 400 mL/min), the sulfur capacity of RSB-MgO-TEPA was analyzed at adsorption temperatures of 15, 25, 35, 45, and 55 °C (Fig. 10).
Fig. 10. Sulfur capacities of RSB-MgO-TEPA at different adsorption temperatures
Figure 10 shows that when the temperature was raised from 15 °C to 45 °C, the sulfur capacity of RSB-MgO-TEPA gradually increased. When the temperature was further raised to 55 °C, the sulfur capacity decreased, possibly due to the activity of gas molecules. SO2 gas molecules has a low molecular activity at lower temperatures, which caused a high kinetic resistance during diffusion to RSB-MgO-TEPA. As the temperature increased, the activity of SO2 and the amine groups in the adsorbent were enhanced, which was beneficial for the interactions of SO2 with the amine group and diffusion into the adsorbent. However, if the temperature was too high, desorption may occur, resulting in a lower sulfur capacity.
CONCLUSIONS
- RSB-MgO adsorbents prepared from the co-precipitation/calcination route displayed excellent desulfurization effects, and sulfur capacities of RSB-MgO adsorbents were much higher than that of native rice straw and rice straw biochar.
- At a calcination temperature of 400 °C, a Mg/rice straw weight ratio of 1.2, and using MgCl2·6H2O as the magnesium source and tetraethylenepentamine (TEPA) as the precipitant, the sulfur capacity of the adsorbent reached 260 mg/g at a flue gas flow rate of 400 mL/min and adsorption temperature of 25 °C.
- The SO2 adsorption was affected by the synergistic action of MgO, rice straw biochar and N-H groups, and MgO played a dominant role in SO2 adsorption.
- The RSB-MgO-TEPA has a good desulfurization effect because of its circular lamella morphology, large BET surface area, small crystallite size, high crystallinity, and N-H groups introduced.
- The RSB-MgO adsorbent may be a potential alternative material for SO2 removal in the future, and the use of MgCl2∙6H2O and biomass residues to synthesize SO2 adsorbents provides remarkable economic and environmental advantages.
ACKNOWLEDGMENTS
The authors gratefully appreciate the use of FESEM, XRD, FT-IR, and EA in the Analytical Testing Center of Sichuan University.
REFERENCES CITED
Ahmad, M., Rajapaksha, A. U., Lim, J. E., Zhang, M., Bolan, N., Mohan, D., Vithanage, M., Lee, S. S., and Ok, Y. S. (2014). “Biochar as a sorbent for contaminant management in soil and water: A review,” Chemosphere 99, 19-33. DOI: 10.1016/j.chemosphere.2013.10.071
Al-Harahsheh, M., Shawabkeh, R., Batiha, M., Al-Harahsheh, A., and Al-Zboon, K. (2014). “Sulfur dioxide removal using natural zeolitic tuff,” Fuel Processing Technology 126, 249-258. DOI: 10.1016/j.fuproc.2014.04.025
Bai, B. C., Lee, C. W., Lee, Y.-S., and Im, J. S. (2016). “Metal impregnate on activated carbon fiber for SO2 gas removal: Assessment of pore structure, Cu supporter, breakthrough, and bed utilization,” Colloids and Surfaces A: Physicochemical and Engineering Aspects 509, 73-79. DOI: 10.1016/j.colsurfa.2016.08.038
Chowdhury, I. H., Chowdhury, A. H., Bose, P., Mandal, S., and Naskar, M. K. (2016). “Effect of anion type on the synthesis of mesoporous nanostructured MgO, and its excellent adsorption capacity for the removal of toxic heavy metal ions from water,” RSC Advances 6(8), 6038-6047. DOI: 10.1039/c5ra16837f
Dhal, J., Sethi, M., Mishra, B., and Hota, G. (2015). “MgO nanomaterials with different morphologies and their sorption capacity for removal of toxic dyes,” Materials Letters 141, 267-271. DOI: 10.1016/j.matlet.2014.10.055
Elder, A. C., Bhattacharyya, S., Nair, S., and Orlando, T. M. (2018). “Reactive adsorption of humid SO2 on metal–organic framework nanosheets,” The Journal of Physical Chemistry C 122(19), 10413-10422. DOI: 10.1021/acs.jpcc.8b00999
Fan, L., Chen, J., Guo, J., Jiang, X., and Jiang, W. (2013). “Influence of manganese, iron and pyrolusite blending on the physiochemical properties and desulfurization activities of activated carbons from walnut shell,” Journal of Analytical and Applied Pyrolysis 104, 353-360. DOI:10.1016/j.jaap.2013.06.014
Guo, J.-x., Liang, J., Chu, Y.-H., Sun, M.-C., Yin, H.-Q., and Li, J.-J. (2012). “Desulfurization activity of nickel supported on acid-treated activated carbons,” Applied Catalysis A: General 421, 142-147. DOI: 10.1016/j.apcata.2012.02.010
Guo, Y., Li, Y., Zhu, T., Ye, M., and Wang, X. (2013). “Adsorption of SO2 and chlorobenzene on activated carbon,” Adsorption 19(6), 1109-1116. DOI: 10.1007/s10450-013-9539-y
Huang, Y.-F., Chiueh, P.-T., Shih, C.-H., Lo, S.-L., Sun, L., Zhong, Y., and Qiu, C. S. (2015). “Microwave pyrolysis of rice straw to produce biochar as an adsorbent for CO2 capture,” Energy 84, 75-82. DOI: 10.1016/j.energy.2015.02.026
Hu, J., Song, Z., Chen, L., Yang, H., Li, J., and Richards, R. (2010). “Adsorption properties of MgO(111) nanoplates for the dye pollutants from wastewater,” Journal of Chemical and Engineering Data 55, 3742-3748. DOI: 10.1021/je100274e
Jin, Z., Jia, Y., Zhang, K.-S., Kong, L.-T., Sun, B., Shen, W., Meng, F. L., and Liu, J. H. (2016). “Effective removal of fluoride by porous MgO nanoplates and its adsorption mechanism,” Journal of Alloys and Compounds 675, 292-300. DOI: 10.1016/j.jallcom.2016.03.118
Li, P., Liu, C., Zhang, L., Zheng, S., and Zhang, Y. (2017). “Enhanced boron adsorption onto synthesized MgO nanosheets by ultrasonic method,” Ultrasonics Sonochemistry 34, 938-946. DOI: 10.1016/j.ultsonch.2016.07.029
Li, R., Wang, J. J., Zhou, B., Zhang, Z., Liu, S., Lei, S., and Xiao, R. (2017). “Simultaneous capture removal of phosphate, ammonium and organic substances by MgO impregnated biochar and its potential use in swine wastewater treatment,” Journal of Cleaner Production 147, 96-107. DOI: 10.1016/j.jclepro.2017.01.069
Li, Y., Qi, H., and Wang, J. (2012). “SO2 capture and attrition characteristics of a CaO/bio-based sorbent,” Fuel 93, 258-263. DOI: 10.1016/j.fuel.2011.08.043
Liu, X., Niu, C., Zhen, X., Wang, J., and Su, X. (2015). “Novel approach for the synthesis of Mg (OH) 2 nanosheets and lamellar MgO nanostructures and their ultra-high adsorption capacity for Congo red,” Journal of Materials Research 30(10), 1639-1647. DOI: 10.1557/jmr.2015.113
Makhluf, S., Dror, R., Nitzan, Y., Abramovich, Y., Jelinek, R., and Gedanken, A. (2005). Microwave‐assisted synthesis of nanocrystalline MgO and its use as a bacteriocide,” Advanced Functional Materials 15(10), 1708-1715. DOI: 10.1002/adfm.200500029
Mochida, I., Korai, Y., Shirahama, M., Kawano, S., Hada, T., Seo, Y., and Yoshikawa, M., Yasutake, A. (2000). “Removal of SOx and NOx over activated carbon fibers,” Carbon 38(2), 227-239. DOI: 10.1016/s0008-6223(99)00179-7
Nassar, M. Y., Mohamed, T. Y., Ahmed, I. S., and Samir, I. (2017). “MgO nanostructure via a sol-gel combustion synthesis method using different fuels: An efficient nano-adsorbent for the removal of some anionic textile dyes,” Journal of Molecular Liquids 225, 730-740. DOI: 10.1016/j.molliq.2016.10.135
Savage, M., Cheng, Y., Easun, T. L., Eyley, J. E., Argent, S. P., Warren, M. R., Lewis, W., Murray, C., Tang, C. C., Frogley, M. D., et al. (2016). “Selective adsorption of sulfur dioxide in a robust metal–organic framework material,” Advanced Materials 28(39), 8705-8711. DOI: 10.1002/adma.201602338
Sun, L., and Zhu, X. (2018). “Practical and theoretical study of the adsorption performances of straw-based tertiary amine-supported material toward sulfur dioxide in flue gas,” BioResources 13(1), 1132-1142.
Tailor, R., Abboud, M., and Sayari, A. (2014a). “Supported polytertiary amines: highly efficient and selective SO2 adsorbents,” Environmental science & technology 48(3), 2025-2034. DOI: 10.1021/es404135j
Tailor, R., Ahmadalinezhad, A., and Sayari, A. (2014b). “Selective removal of SO2 over tertiary amine-containing materials,” Chemical Engineering Journal 240, 462-468. DOI: 10.1016/j.cej.2013.11.002
Tan, K., Zuluaga, S., Wang, H., Canepa, P., Soliman, K., Cure, J., Li, J., Thonhauser, T., and Chabal, Y. J. (2017). “Interaction of acid gases SO2 and NO2 with coordinatively unsaturated metal organic frameworks: M-MOF-74 (M= Zn, Mg, Ni, Co),” Chemistry of Materials 29(10), 4227-4235. DOI: 10.1021/acs.chemmater.7b00005
Vellingiri, K., Kim, K.-H., Kwon, E. E., Deep, A., Jo, S.-H., and Szulejko, J. E. (2016). “Insights into the adsorption capacity and breakthrough properties of a synthetic zeolite against a mixture of various sulfur species at low ppb levels,” Journal of Environmental Management 166, 484-492. DOI: 10.1016/j.jenvman.2015.10.053
Vu, A.-T., Ho, K., Jin, S., and Lee, C.-H. (2016). “Double sodium salt-promoted mesoporous MgO sorbent with high CO2 sorption capacity at intermediate temperatures under dry and wet conditions,” Chemical Engineering Journal 291, 161-173. DOI: 10.1016/j.cej.2016.01.080
Wang, A., Fan, R., Pi, X., Zhou, Y., Chen, G., Chen, W., and Yang, Y. L. (2018). “Nitrogen-doped microporous carbons derived from pyridine ligand-based metal-organic complexes as high-performance SO2 adsorption sorbents,” ACS Applied Materials & Interfaces 10(43), 37407-37416. DOI: 10.1021/acsami.8b12739
Wang, X., Ma, X., Zhao, S., Wang, B., and Song, C. (2009). “Nanoporous molecular basket sorbent for NO2 and SO2 capture based on a polyethylene glycol-loaded mesoporous molecular sieve,” Energy and Environmental Science 2(8), 878-882. DOI: 10.1039/b906476a
Wu, W., Yang, M., Feng, Q., McGrouther, K., Wang, H., Lu, H., et al. (2012). “Chemical characterization of rice straw-derived biochar for soil amendment,” Biomass and Bioenergy 47, 268-276. DOI: 10.1016/j.biombioe.2012.09.034
Xu, X., Huang, D., Zhao, L., Kan, Y., and Cao, X. (2016). “Role of inherent inorganic constituents in SO2 sorption ability of biochars derived from three biomass wastes,” Environ. Sci. Technol. 50(23), 12957-12965. DOI: 10.1021/acs.est.6b03077
Yan, Z., Liu, L. L., Zhang, Y. L., Liang, J. P., Wang, J. P., Zhang, Z. T., and Wang, X. (2013). “Activated semi-coke in SO2 removal from flue gas: Selection of activation methodology and desulfurization mechanism study,” Energy and Fuels 27(6), 3080-3089. DOI: 10.1021/ef400351a
Yan, Z., Wang, J., Zou, R., Liu, L., Zhang, Z., and Wang, X. (2012). “Hydrothermal synthesis of CeO2 nanoparticles on activated carbon with enhanced desulfurization activity,” Energy and Fuels 26(9), 5879-5886. DOI: 10.1021/ef301085w
Yang, C., Tan, T., and Zhu, X. (2012). “Adsorptive capacity of ethylenediamine treated oxidised rice straw for sulfur dioxide,” Carbohydrate Polymers 87(2). DOI: 10.1016/j.carbpol.2011.10.013
Yi, H., Wang, Z., Liu, H., Tang, X., Ma, D., Zhao, S., Zhang, B., Gao, F., and Zuo, Y. (2014). “Adsorption of SO2, NO, and CO2 on activated carbons: Equilibrium and thermodynamics,” Journal of Chemical and Engineering Data 59(5), 1556-1563. DOI: 10.1021/je4011135
Zhou, J., Yang, S., and Yu, J. (2011). “Facile fabrication of mesoporous MgO microspheres and their enhanced adsorption performance for phosphate from aqueous solutions,” Colloids and Surfaces A: Physicochemical and Engineering Aspects 379(1-3), 102-108. DOI: 10.1016/j.colsurfa.2010.11.050
Zhou, X., Yi, H., Tang, X., Deng, H., and Liu, H. (2012). “Thermodynamics for the adsorption of SO2, NO and CO2 from flue gas on activated carbon fiber,” Chemical Engineering Journal 200, 399-404. DOI: 10.1016/j.cej.2012.06.013
Article submitted: January 8, 2020; Peer review completed: April 3, 2020; Revised version received and accepted: April 19, 2020; Published: May 5, 2020.
DOI: 10.15376/biores.15.2.4738-4752