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Synthesis of ammonia molecularly imprinted adsorbents and ammonia adsorption separation during sludge aerobic composting
Bioresource Technology 300 (2020) 122670
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Case Study
Synthesis of ammonia molecularly imprinted adsorbents and ammonia adsorption separation during sludge aerobic composting Zhangliang Han, Yangjie Xu, Hui Wang, Haozhong Tian, Bin Qiu, Dezhi Sun
T
⁎
Beijing Key Lab for Source Control Technology of Water Pollution, Engineering Research Center for Water Pollution Source Control & Eco-remediation, College of Environmental Science & Engineering, Beijing Forestry University, Beijing 100083, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Ammonia separation Dynamic adsorption Hydrogen energy Molecular imprinted Odor
Ammonia (NH3) is the predominant harmful odor emitted from sludge aerobic composting plants, however, this NH3 could be recycled and used as energy or nitrogen fertilizer. Therefore, the aim of this study was to use molecular imprinting technology to prepare an adsorbent that could separate NH3 from mixed gases. An NH3 molecular imprinted polymer (NH3-MIP) was prepared by precipitation polymerization and optimal synthesis was determined by testing several different ratios of reaction components. NH3 adsorption capacity of the optimal NH3-MIP was 1.62 times that of non-imprinted material. NH3 separation factors increased from 154 (dimethyl sulfides) and 217 (dimethyl disulfides) for non-imprinted material, to 213 (dimethyl sulfides) and 302 (dimethyl disulfides) for the NH3-MIP. The adsorption mechanism was identified as physical adsorption and hydrogen bonding between H–O on the –COOH in NH3-MIP and the nitrogen in NH3. Effective desorption at 150 °C with vacuum maintained over 95% of the NH3 adsorption capacity.
1. Introduction
disulfides (DMDS) emitted during sewage sludge aerobic composting may even cause serious medical and mental harm to MSSACP employees and surrounding residents (Paustenbach and Gaffney, 2006; Han et al., 2019). NH3 contributes the most to odor emissions (Jiang et al., 2016; Zhu et al., 2016), and an NH3 odor intensity of Scale 5 (excessively strong odor) was recently identified in a MSSACP
Construction of new municipal sewage sludge aerobic composting plants (MSSACPs) and operation of existing ones has been limited due to strong odors emitted during the composting process (Cheng et al., 2005, 2019). Ammonia (NH3), dimethyl sulfides (DMS), and dimethyl
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Corresponding author at: College of Environmental Science & Engineering, Beijing Forestry University, 35 Tsinghua East Road, Beijing 100083, China. E-mail address: [email protected] (D. Sun).
https://doi.org/10.1016/j.biortech.2019.122670 Received 31 October 2019; Received in revised form 19 December 2019; Accepted 21 December 2019 Available online 24 December 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.
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by using suitable agents and optimal ratios. Similarly, an NH3-MIP may promote NH3 separation due to cavity matching, and by using a polar functional monomer and crosslinker which could inhibit DMS and DMDS adsorption. Thus, the aims of this study were to: (1) prepare an NH3-MIP and optimize its NH3 adsorption capacity; (2) identify the adsorption mechanism between NH3 and the NH3-MIP; (3) measure the selectivity between NH3, DMS, and DMDS, stability, and regeneration of the synthesized NH3-MIP.
composting workshop (Han et al., 2019). The maximum [NH3] was over 100 mg·m−3 in this composting workshop, which is 3.2 times that of permissible concentration–time weighted averages, and enough to cause serious eye damage and upper respiratory tract irritation (US ACGIH, 2010; Han et al., 2019). Nevertheless, NH3 is an excellent fuel to store hydrogen (Zhao et al., 2010; Miyaoka et al., 2018; Lamb et al., 2019). Gaseous NH3 is easily converted into liquid NH3 in conditions at –33.5 °C to −77.8 °C or 700–800 kpa. Liquid NH3 is safer to store and easier to transport than molecular hydrogen (Nasharuddin et al., 2019). In addition, NH3 fuel has been identified as a method to address ever-increasing energy and environmental concerns (Hui et al., 2019; Yapicioglu and Dincer, 2019). The Haber process is the most commonly used industrial method to produce synthetic NH3, and occurs by the following reaction:
N2 +3H2
High temperature and pressure
⟺
2NH3
2. Materials and methods 2.1. Reagents Analytical reagent grade methacrylic acid (MAA), acrylamide (AM), ethylene glycol dimethacrylate (EGDMA), divinylbenzene (DVB), azodiisobutyronitrile (AIBN), toluene, methanol, trichloromethane, and NH3 solution (25%) were all purchased from Beijing Lanyi Chemical Products Co., Ltd. High purity water was purchased from Beijing Huihan Technology Co., Ltd. All gases used in this study were supplied by Beijing Huatong Jingke Gas Chemical Co., Ltd. Concentrations of NH3, DMS, and DMDS were 5000, 300, and 300 ppm, respectively. Nitrogen and oxygen had purities higher than 99.999%.
(1)
NH3 produced by the Haber process is used as inorganic fertilizer to promote food production, which helped initiate the green revolution of modern agriculture (Smil, 1999; Borlaug, 2002). However, because the Haber process requires high temperatures (> 400 °C) and pressures (150 bar) (Jennings, 1991), industrial NH3 synthesis consumes about 1%-3% of the world’s energy (Zhao et al., 2019) and causes 1% of annual CO2 emissions (Service, 2018). A more efficient method to accumulate NH3 would be to recycle it from sludge aerobic composting emissions. Developing a method to separate NH3 from DMS and DMDS would not only help eliminate odors from MSSACPs, but also eliminate energy consumption and greenhouse gas emissions caused by industrial NH3 synthesis. For example, NH3 emissions were between 4.35 and 10.95 g·dry·kg−1 throughout different seasons in a MSSACP in Beijing, China (Han et al., 2018). Based on this data, approximately 1186–2989 tons of NH3 could be recycled each year from sludge aerobic composting plants in Beijing. However, there have been few studies investigating NH3 separation from DMS or DMDS. Activated carbon is a commonly used material for removing and preventing odors, which adsorbs NH3 by hydrogen bonding or Lewis acid–base interactions (Seredych et al., 2009). To enhance adsorption performance, the surface of activated carbon is modified using chemical, mechanical, thermal, or electrical discharge methods (Park and Kim, 2005). However, the nonpolar surface of activated carbon also facilitates adsorption of DMS and DMDS, which is not beneficial for separation of NH3. For example, the NH3 adsorption capacity of activated carbon is only 0.075–0.45 mmol·g−1 (Rezaeia et al., 2017), but is 0.13 mmol·g−1 for DMS and 1.15 mmol·g−1 for DMDS (Vega et al., 2013). Therefore, using activated carbon is not a suitable strategy for separating NH3 from DMS and DMDS. A different approach that may be useful for NH3 separation would be to use molecularly imprinted adsorbents prepared by self-assembly. During the molecular imprinting process, the target adsorption molecule is used as a template and bound with functional monomers, which together adhere like a “lock and key” (Chen et al., 2011). The template molecule and functional monomers form complexes that get crosslinked. From there, the crosslinker and functional monomers are polymerized to form a molecular imprinted polymer (MIP), and the template molecules are removed in the eluent. A unique cavity is left behind in the MIP that matches the exact size and shape of the template molecule, and also contains functional groups which can specifically bind to the target substance (Khasawneh et al., 2001). A successful MIP will have a high separation factor, which measures the ability of the MIP to distinguish between two different analytes (Zhao et al., 2012; Curk et al., 2015). Oxalic acid has been used as a template molecule for CO2 adsorption and separation by bulk polymerization (Zhao et al., 2012), suspension polymerization (Nabavi et al., 2016), and precipitation polymerization (Liu et al., 2018a). These studies found that the MIPs were highly selective for CO2, and the separation factor (CO2/N2) reached up to 340
2.2. Preparation of NH3-MIP A series of NH3-MIP adsorbents were synthesized by first dissolving functional monomer (MAA or AM), template (NH3 solution), crosslinker (EGDMA or DVB), and AIBN in solvent (toluene, methanol, or trichloromethane). The mixtures were made airtight using parafilm and degassed with N2 for 5 min to remove oxygen (Liu et al., 2018b). Next, the mixtures were sealed and reacted for 24 h at 60 °C (Zhao et al., 2012). Sedimentary polymers were then washed with D.I water to remove template and solvent, and filtered. The washing procedure was repeated several times until template could not be detected in filtrate (Liu et al., 2018a). Finally, the polymer was dried under vacuum at 60 °C for 12 h. Non-imprinted polymer (NIP) without using template was also prepared in parallel by the same synthetic procedure. In order to obtain optimal NH3 adsorption capacity, functional monomer was first optimized, followed by the crosslinker and solvent (Table 1). Optimal ratios for components used to synthesize NH3-MIPs were also determined using normal multinomial regressive experimental design (Tables 2 and 3). Briefly, varying ratios of initiator (1%, 3%, or 9%), template: functional monomer: crosslinker ratios (1:2:10, 1:4:20, or 1:4:10), and polymeric monomers (2.5%, 5%, or 10%) were compared. Polymeric monomers are the sum of functional monomers and crosslinkers. The ratio of initiator is initiator percent of the polymeric monomers. The ratio of polymeric monomers is polymeric monomer percent of solvent. 2.3. Characterization Sample functional groups were obtained using a FTIR spectrometer (PerkinElmer Spectrum 100, BRUKER, USA). The resolution of determination was 4 cm−1, and the scanning range was between 400 and 4000 cm−1. Pore structure and specific surface area (SBET) were recorded using a specific surface and micropore analyzer (SSA-7000, Beijing BIoode Electronic Technology Co., Ltd., China). Each sample was outgassed under vacuum for 3 h at 120 °C prior to measurement. The pore distribution was measured using the current international BJH method. Thermal stabilities of adsorbents were characterized by thermo-gravimetric analysis (TGA-55, TA Instruments-Waters Co., Ltd., USA). Adsorbents were pre-dried at 120 °C before thermal characterization to remove moisture. 10 mg of adsorbents were heated from 15 to 800 °C with 25 ml·min−1 of N2 flow rate. Particle size and morphological analysis were obtained using a scanning electron microscope 2
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Table 1 Optimization of functional monomer, crosslinker, and solvent. Functional monomer optimization
Reagent Reagent
Functional monomer MAA AM
Dosage (μl) 340.95 214.94
Solvent Alcohol Alcohol
Dosage (ml) 60 60
Crosslinker DVB DVB
Dosage (ml) 2.82 2.82
Initiator AIBN AIBN
Dosage (mg) 60 60
NH3 solution (μl) 74.61 75.91
Dosage (μl) 232.34 340.95 232.34 340.95 232.34 340.95
Solvent Alcohol Alcohol Trichloromethane Trichloromethane Toluene Toluene
Dosage (ml) 60 60 60 60 60 60
Crosslinker EGDMA DVB EGDMA DVB EGDMA DVB
Dosage (ml) 2.57 2.82 2.57 2.82 2.57 2.82
Initiator AIBN AIBN AIBN AIBN AIBN AIBN
Dosage (mg) 60 60 60 60 60 60
NH3 solution (μl) 28.76 41.09 28.76 41.09 28.76 41.09
Crosslinker and Solvent optimization
Reagent Reagent Reagent Reagent Reagent Reagent
Functional monomer MAA MAA MAA MAA MAA MAA
Table 2 Experimental design used for ratio optimization. Group number
Initiator Ratio (%)
Template: functional monomer: crosslinker
Polymeric monomer ratio (%)
1 2 3 4 5 6 7 8 9
1 1 1 3 3 3 9 9 9
1: 1: 1: 1: 1: 1: 1: 1: 1:
2.5 5.0 10.0 5.0 10.0 2.5 10.0 2.5 5.0
2: 4: 4: 2: 4: 4: 2: 4: 4:
10 20 10 10 20 10 10 20 10
Table 3 Experimental design used for optimization of MIP-forming component proportions. Group number
Initiator (AIBN), mg
Template (NH3·H2O), μl
Functional monomer (MAA), μl
Crosslinker (EGDMA), ml
Solvent (Toluene), ml
1 2 3 4 5 6 7 8 9
15 30 60 90 180 45 540 135 270
52.4 52.4 194.6 104.8 104.8 48.7 209.6 26.2 97.3
118.71 237.41 879.36 237.41 464.82 219.84 464.82 118.71 439.58
1.31 2.63 4.86 2.63 5.25 1.22 5.25 1.31 2.43
60 60 60 60 60 60 60 60 60
Fig. 1. Diagram of the device constructed for gas adsorption.
Table 4 NH3 adsorption capacity of NH3-MIP in different conditions.
(SEM) (JSM-6700F, JEOL, Japan). 2.4. NH3 adsorption and desorption 2.4.1. NH3 adsorption NH3 adsorption was conducted on a home-made fixed bed by Beijing Camino Technology Co., Ltd. (Fig. 1). 0.2 g of the prepared adsorbent was loaded into a stainless-steel column. [NH3], [DMS], [DMDS], and [O2] could be adjusted with ratios between flow rates of their standard gases and N2 using a mass flow control meter (S49 32/ MT, Beijing Houchang Huibolong Precision Instrument Co., Ltd., China). During optimization experiments, the total gas flow rate was 200 ml·min−1, [NH3] was 100 ppm (i.e. 76 mg·m−3), and room temperature was 20 °C. Total gas flow rates, [NH3], adsorption temperatures, and oxygen content used for stability experiments are shown in Table 4. Experimental conditions used for selectivity testing were the same as those used in optimization experiments except [DMS] or [DMDS]. [DMS] and [DMDS] were identified by on-situ maximum emission rate ratios among them and NH3 (Han et al., 2019). [DMS] and [DMDS] were 10 ppm (i.e. 29 mg·m−3) and 10 ppm (i.e.
Temperature (°C)
Oxygen content (%)
Flow rate (ml·min−1)
[NH3] (ppm)
Adsorption capacity (mmol·g−1)
0 20 40 20 20 20 20 20 20
0 0 0 10 20 0 0 0 0
200 200 200 200 200 120 280 200 200
100 100 100 100 100 100 100 300 500
0.47 0.42 0.31 0.40 0.38 0.39 0.51 0.45 0.48
42 mg·m−3), respectively. Gas samples from the outlet of the adsorption column were sampled in a Teflon bag (FEP-ptfe-0.5 L, Beijing Haochentiancheng environmental protection and technology company, China), and the concentrations were detected in order to construct corresponding gas breakthrough curves to calculate adsorption capacity. When the detected concentration was equal to the inlet concentration, the adsorption experiment ended.
2.4.2. NH3 desorption Two desorption methods were used in this study: (1) after the NH3 adsorption experiment, adsorbents in the column were heated to approximately 150 °C using a heating furnace (Fig. 1) with 100 ml·min−1 of N2, until NH3 from the outlet of adsorption column was not detected. Thereafter, heating stopped but N2 flow rate was maintained. When the 3
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column temperature decreased to room temperature, NH3 adsorption could start again. Adsorption conditions were the same as those described in section “2.4.1 NH3 adsorption”. (2) Adsorbents were transferred to a vacuum oven (DZF, Beijing Kewei Yongxing Instrument Co., Ltd., China). When the temperature increased to 150 °C, gases in the oven were exhausted by vacuum pump until the pressure in the oven was < 133 pa. Thereafter, the inlet valve of the vacuum pump was opened. The pressure in the oven then recovered to atmospheric pressure, and the cycle was repeated 15 times. 2.5. Analysis methods 2.5.1. Analysis of gases [NH3] was directly measured using an NH3 sensor (JA908-NH3, Dongguan Yong Qi Electronic Equipment Co., Ltd., China). The measurement range of the sensor was 0.1–500 ppm with a 3% relative standard deviation. Before sampling, the NH3 sensor was calibrated with two NH3 standard gases (10 ppm and 100 ppm standard gases) after purging in a nitrogen atmosphere. DMS and DMDS were analyzed by a gas chromatography (GC7890, Agilent Technologies, USA) equipped with a flame photometric detector. The gas chromatography analysis conditions were as follows: A DB-1 capillary column (30 m × 0.32 mm × 5 μm, Agilent Technologies, USA) was used with nitrogen as the carrier gas (1.5 ml·min−1); The temperature of the oven was set at 40 °C initially (holding for 4 min), and then was increased to 120 °C (at 25 °C·min−1) holding for 4 min, after that the temperature was increased to 220 °C (at 25 °C·min−1) holding for 4 min.
Fig. 2. NH3 adsorption capacity of NH3-MIPs synthesized using varying (a) functional monomers, (b) crosslinks and solvents, and (c)/(d) reaction compound ratios. Group 10 corresponding NIP of Group 6.
MAA and NH3 is likely stronger than between the -N–H of AM and NH3·NH3 adsorption capacities of both the MAA- and AM-NH3-MIPs were higher than that of the corresponding NH3-NIPs, which indicated that the molecular imprinting technology (MIT) was effective, and that pore spatial structures and arrangement of functional monomers were able to adsorb NH3. Therefore, MAA was utilized as the functional monomer for subsequent experiments. In order to identify the optimal crosslinker and solvent for NH3-MIP synthesis, two commonly used crosslinkers (EGDMA and DVB) and three different solvents (toluene, ethanol, and trichloromethane) were tested (Fig. 2(b)). All of the MIP NH3 adsorption capacities were higher than that of the corresponding NIP, indicating that molecular imprinting was effective. Maximum NH3 adsorption capacity (0.34 mmol·g−1) occurred in the NH3-MIP synthesized with a combination of EGDMA and toluene. When EGDMA was used as the crosslinker in combination with ethanol or trichloromethane, NH3 adsorption capacities of the NH3-MIPs were only 0.088 mmol·g−1 and 0.089 mmol·g−1, respectively. Using DVB as the crosslinker resulted in lower NH3 adsorption capacities in combination with all three solvents, however adsorption capacity was still nearly twice as high with toluene. These results indicate that toluene is an effective solvent for synthesizing highly effective NH3-MIPs, especially in combination with EGDMA. Consistent with these findings, toluene has also been shown to be an optimal solvent for CO2 adsorption by CO2-MIPs (Zhao et al., 2012). Based on these results, EGDMA and toluene were used as the crosslinker and solvent for all further experimentation.
2.5.2. Statistical analysis methods Statistical analyses including correlation analysis, mean, and standard deviation of three replicates were calculated using SPSS Statistics 24. NH3/DMS/DMDS adsorption capacities were calculated according to their breakthrough curve by origin 85, as calculated by Eq. (2).
(
m= t×C0 −
∫0
t
)
Ct dt × Q× 22. 4−1 × 10−6 × m 0−1
(2) −1
where: m was NH3/DMS/DMDS adsorption capacity, (mmol·g ); t was adsorption saturation time of NH3/DMS/DMDS, (min); C0 was inlet [NH3]/[DMS]/[DMDS], (ppm); Ct was outlet [NH3]/[DMS]/[DMDS], (ppm); Q was total gases flow rate, (ml·min−1); m0 was the mass of adsorbents in adsorption column, (g). NH3 separation factor (P) was calculated using Eq. (3).
P= m1 × m2- 1 where: m1 was NH3 adsorption capacity, (mmol·g DMS adsorption capacity, (mmol·g−1).
(3) −1
); m2 was DMDS or
3. Results and discussion
3.1.2. Optimization of compound ratios for NH3-MIP synthesis Ratios of polymeric monomers and initiators among template, crosslinker, and functional monomers are key factors that have been shown to effect adsorption capacity by influencing characteristics such as particle size, specific surface area, and morphology (Koeber et al., 2001; Vasapollo et al., 2011; Xia et al., 2017). Therefore, nine different ratio combinations (Group1-9) were used to prepare NH3-MIPs in order to determine the optimal quantities for maximum NH3 adsorption capacity. Preparation of the Group 1 NH3-MIP was unsuccessful, likely because there was not sufficient contact between the minimum ratios of initiator (1%) and polymeric monomer (2.5%). After preparation of the NH3-MIPs, the NH3 adsorption capacity was tested for each group (Fig. 2(c) and (d)). Group 6, which used 3% initiator, 2.5% polymeric monomer, and a template: functional monomer: crosslinking agent ratio of 1:4:10, had the maximum NH3 adsorption capacity of 0.42 mmol·g−1. A NIP control (Group 10), that was prepared using these same ratios, exhibited a substantially lower NH3
3.1. NH3 adsorption capacity and mechanism analysis 3.1.1. Optimization of functional monomer, crosslinker, and solvent for NH3-MIP synthesis MAA and AM are commonly used functional monomers that form hydrogen bonds with imprinted molecules (Saloni et al., 2010; Zhang et al., 2010). Therefore, NH3 adsorption capacity was first screened using MAA or AM as functional monomers for NH3-MIP production (Fig. 2(a)). MAA- and AM-NH3-NIPs, which were synthesized without using NH3 template, were also tested in parallel as controls·NH3 adsorption capacities of MAA-NH3-MIP (0.082 mmol·g−1) and MAA-NH3NIP (0.074 mmol·g−1) were higher than that of corresponding AM-NH3MIP (0.056 mmol·g−1) and AM-NH3-NIP (0.051 mmol·g−1). This indicted that MAA is a more suitable functional monomer for adsorption of NH3, compared with AM. The hydrogen bond between –COOH of 4
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In addition to hydrogen bonding, it is possible that physical adsorption may occur between NH3-MIP and NH3. Temperatures under 100-150 °C with vacuum are typically needed to break hydrogen bonds between O–H in –COOH and nitrogen in NH3 (Kondo et al., 2006). Therefore, physical adsorption was tested by measuring the variation of NH3 adsorption capacity after adsorbing NH3 to saturation then increasing temperature·NH3 adsorption capacity at 20 °C was 0.42 mmol·g−1, but decreased to 0.26 mmol·g−1 when temperatures increased to 60 °C. This confirmed that at least 38.10% of NH3 adsorbed to the NH3-MIPs by physical adsorption. Group 6 NH3 adsorption capacity was 1.62 times of that of the Group 10 control (Fig. 2(d)), while Group 6 and Group 10 had similar pore structure and surface area (Table 5). This indicated that the size and shape of the NH3-MIP target substance contributed to its physical adsorption. In summary, there was hydrogen bonding between O–H in –COOH of NH3-MIP and nitrogen, and a portion of the NH3 was also captured by physical adsorption by using MIT.
Table 5 Pore structure characteristics of NH3-MIPs synthesized using varying compound ratios. Groups
Total pore volume (m3)
Surface area (m2·g−1)
Mean pore size (nm)
2 3 4 5 6 7 8 9 10
0.03 0.03 0.05 0.03 0.07 0.08 0.05 0.03 0.07
6.52 6.02 8.56 9.05 28.94 10.85 17.96 7.45 27.52
2.56 3.26 2.98 2.54 2.52 2.82 3.46 2.56 2.53
Table 6 Correlation analysis between NH3 adsorption capacity and pore structure data for Group 2–9. Total pore volume
Surface area
Mean pore size
Adsorption capacity
1
0.717*
−0.026
0.561
0.717* −0.026 0.561
1 −0.090 0.933**
−0.090 1 −0.147
0.933** −0.147 1
3.2. Adsorption stability Total pore volume Surface area Mean pore size Adsorption capacity
NH3 adsorption capacities of NH3-MIP at different temperatures, flow rates, inlet [NH3], and oxygen content were also tested (Table 4). A decrease in temperature from 40 °C to 0 °C resulted in an increase in NH3 adsorption capacity by 0.16 mmol·g−1. The improvement in NH3 adsorption capacity at lower temperatures was due to enhanced physical adsorption between NH3 and NH3-MIP (Zhao et al., 2014). An increase in NH3 flow rate from 120·ml min−1 to 280 ml·min−1 correlated with an increase in NH3 adsorption capacity from 0.39 to 0.51 mmol·g−1·NH3 adsorption capacity also increased from 0.42 to 0.48 mmol·g−1 when [NH3] increased from 100 ppm to 500 ppm. Oxygen content between 0 and 20%, which would normally be found in anaerobic compositing materials during sludge aerobic compositing, did not have a significant effect on NH3 adsorption capacity. These findings were consistent with CO2 adsorption capacities using CO2-MIPs (Zhao et al., 2014). Increasing temperatures also caused CO2 adsorption capacity to decrease from 0.55 mmol·g−1 at 45 °C to 0.19 mmol·g−1 at 90 °C, and increasing CO2 from 5% to 17% led to a 0.14 mmol·g−1 adsorption increase. Based on this data, adjusting temperature or aeration frequency and volume may be useful methods for improving NH3 adsorption capacity during sludge aerobic composting.
* significant correlation; ** extremely significant correlation.
adsorption capacity. In order to explore the internal factors affecting the adsorption capacity of the NH3-MIPs, each adsorbent was analyzed using a specific surface analyzer with statistical analysis to measure pore size, pore volume, and specific surface area (Table 5). Group 6 had the maximum specific surface area (28.94 m2·g−1) and minimum mean pore size (2.52 nm), which likely promoted physical adsorption (Zhao et al., 2012). Correlations between NH3 adsorption capacity, mean pore size, total pore volume, and specific surface area for Groups 2–9 were also analyzed (Table 6). A significantly positive correlation was shown between NH3 adsorption capacity and specific surface area, consistent with the characteristics of Group 6. Because the component ratios used to prepare the Group 6 NH3-MIP were ideal, these conditions were used for synthesis in subsequent experiments. 3.1.3. Analysis of adsorption mechanisms Adsorption forces between adsorbent and adsorbate may be divided into hydrogen bonding, chemisorption, and physical adsorption. There are two possible hydrogen bonds that may form between –COOH of the MAA-NH3-MIP and NH3 (Van Humbeck et al., 2014; Luo et al., 2019). The first possible bond is between the carbon–oxygen on the –COOH and hydrogen of NH3, and the second is between the hydrogen–oxygen on the –COOH and nitrogen of NH3. Stretching vibrations of the O–H in –COOH appeared around 2500–3300 cm−1, and C–O stretching vibrations appeared around 1715–1720 cm−1. Before adsorption, there were two O–H peaks for –COOH (i.e. 2945 and 2985 cm−1) and one C–O peak (i.e. 1719 cm−1) in the NH3-MIP. Stretching vibration of the O–H in –COOH decreased from 2945 cm−1 to 2900 cm−1, and from 2985 cm−1 to 2979 cm−1 after NH3 adsorption, but the C–O peak in –COOH (1719 cm−1) did not change. These results indicate that the O–H in –COOH formed a hydrogen bond with nitrogen in NH3 rather than with the C–O of –COOH. No significant changes were noted in the NH3-MIP FTIR spectra before or after adsorption, confirming that adsorption interactions between NH3 and the NH3-MIP adsorbent were not chemical reactions (Zhao et al., 2012). Additionally, there was no C=C absorption peak at 1660–1600 cm−1, indicating that all C=C bonds in MAA and EGDMA were broken.
3.3. Adsorption selectivity In order to assess the selectivity of the NH3-MIP to adsorb NH3, various combinations of DMS and DMDS with and without NH3 were screened (Fig. 3). The NH3-MIP was most selective for NH3 (0.42 mmol·g−1) compared to DMS (0.0025 mmol·g−1), and DMDS (0.0020 mmol·g−1) (Fig. 3(b)). DMS and DMDS are non-polar substances, therefore they are less likely to adsorb to the polar NH3-MIP prepared by polar EGDMA and MAA. In addition, the pore structures of NH3-MIP has a stronger affinity for NH3. When NH3 was combined with DMS (0.38/0.0023 mmol·g−1), DMDS (0.35/0.0017 mmol·g−1), or both DMS and DMDS (0.32/0.0015/0.00106 mmol·g−1), DMS and DMDS slightly competed with NH3 for adsorption to NH3-MIP, resulting in a minor decrease in NH3 adsorption capacity. Although NH3 separation factors with DMS or DMDS have not been previously investigated, NH3/CO2 separation factors have been shown to be between 3.4 and 52.5 in [Cnmim]2[Co(NCS)4] and [Cnmim] [SCN] (Zeng et al., 2018). The separation factor for NH3/SO2 was 65.88 for maximum NH3 adsorption to IRMOF-3 (Britt et al., 2008). The NH3MIP separation factors for both NH3/DMS (213) and NH3/DMDS (302) were substantially higher compared with the NIP control (154 and 217, respectively). This shows that MIT prompted NH3 separation from DMS and DMDS. 5
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Fig. 3. (a) NH3 breakthrough curve with individual or mixed gases; (b) NH3/DMS/DMDS adsorption capacity with individual or mixed gases.
(room temperature and atmospheric pressure) and desorption conditions (150 °C and vacuum) are low-cost, safe, and can easily be achieved in on-site MSSACPs. Furthermore, this technology may also be applicable for NH3 separation and recovery at other sites with high NH3 emissions (e.g. pig manure facilities and refuse landfill plants). 4. Conclusions An NH3-MIP was successfully prepared in this study, and was demonstrated to be efficient for separating NH3 from other odors in MSSACPs. Physical adsorption and hydrogen bonding, between H–O on the –COOH of methacrylic acid and the nitrogen in NH3, were identified as the main mechanisms of adsorption between the NH3-MIP and NH3. Future studies applying this technology to adsorb and separate NH3 from on-site odors during sludge aerobic composting are warranted. It is expected that this will be a beneficial strategy for elimination of odors and ammonia recovery in MSSACPs. Fig. 4. NH3-MIP regeneration using 150 °C with 100 ml·min−1 of N2 in an adsorption column, or 150 °C and vacuum (< 133 pa) in a vacuum drying oven.
Credit authorship contribution statement Zhangliang Han: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Visualization, Funding acquisition. Yangjie Xu: Validation, Formal analysis, Investigation. Hui Wang: Data curation, Project administration. Haozhong Tian: Validation, Formal analysis, Investigation. Bin Qiu: Writing - review & editing, Supervision. Dezhi Sun: Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition.
3.4. Adsorbent regeneration To determine the optimum desorption method for NH3-MIP regeneration, two different methods were compared (Fig. 4). In the first method, desorption occurred at 150 °C with 100 ml·min−1 of N2 in an adsorption column. Using this method resulted in a 47.67% decrease in NH3 adsorption capacity after the first desorption cycle (Fig. 4). This approximate level of NH3 adsorption capacity (42–52%) was maintained after a subsequent five adsorption/desorption cycles. Desorption in the second method occurred at 150 °C with vacuum (< 133 pa) in a vacuum drying oven. Using this method, NH3 adsorption capacity of the NH3-MIP remained above 95% even after ten cycles of adsorption/ desorption (Fig. 4). The strength of hydrogen bonds is about 5–10 times stronger than that of van der Waals, and can desorb at 100–150 °C with vacuum (Kondo et al., 2006). Therefore, the hydrogen bonds between NH3 and NH3-MIP could be broken at 150 °C in combination with the vacuum. The results presented in this manuscript describe a novel approach for using molecular imprinting technology to adsorb and separate NH3 from DMS and DMDS. This technology will be beneficial for eliminating the key odor, NH3, from MSSACPs (Han et al., 2018, 2019), and the recycled NH3 can be used as energy, nitrogen fertilizer, or liquid NH3 to store hydrogen. In comparison to currently used NH3-production methods, this molecular imprinting technology could also decrease energy consumption and greenhouses emissions. Based on selective adsorption of NH3 to the MIP-NH3, optimal adsorption conditions
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. Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (Grant Number: 2019YC10) and the Beijing Science and Technology Commission Foundation (Grant Number: Z181100005518008). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biortech.2019.122670. 6
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Borlaug, N.E., 2002. The green revolution revisited and the road ahead. Spec. Anniversary Lect. Britt, D., Tranchemontagne, D., Yaghi, O.M., 2008. Metal-organic frameworks with high capacity and selectivity for harmful gases. Proc. Natl. Acad. Sci. U.S.A. 105 (33), 11623–11627. Chen, L., Xu, S., Li, J., 2011. Recent advances in molecular imprinting technology: current status, challenges and highlighted applications. Chem. Soc. Rev. 40 (5), 2922–2942. Cheng, X., Peterkin, E., Burlingame, G.A., 2005. A study on volatile organic sulfide causes of odors at Philadelphia's Northeast Water Pollution Control Plant. Water Res. 39 (16), 3781–3790. Cheng, Z., Sun, Z., Zhu, S., Lou, Z., Zhu, N., Feng, L., 2019. The identification and health risk assessment of odor emissions from waste landfilling and composting. Sci. Total Environ. 649, 1038–1044. Curk, T., Dobnikar, J., Frenkel, D., 2015. Rational design of molecularly imprinted polymers. Soft Matter 12 (1), 35–44. Han, Z., Qi, F., Wang, H., Li, R., Sun, D., 2019. Odor assessment of NH3 and volatile sulfide compounds in a full-scale municipal sludge aerobic composting plant. Bioresour. Technol. 282, 447–455. Han, Z., Sun, D., Wang, H., Li, R., Bao, Z., Qi, F., 2018. Effects of ambient temperature and aeration frequency on emissions of ammonia and greenhouse gases from a sewage sludge aerobic composting plant. Bioresour. Technol. 270, 457–466. Hui, L., Xue, Y., Yu, H., Liu, Y., Fang, Y., Xing, C., Huang, B., Li, Y., 2019. Highly efficient and selective generation of ammonia and hydrogen on a graphdiyne-based catalyst. J. Am. Chem. Soc. 141 (27), 10677–10683. Jennings, J.R., 1991. Catalytic Ammonia Synthesis: Fundamentals and Practice. Plenum Press, New York. London. Jiang, T., Ma, X., Tang, Q., Yang, J., Li, G., Schuchardt, F., 2016. Combined use of nitrification inhibitor and struvite crystallization to reduce the NH3 and N2O emissions during composting. Bioresour. Technol. 217, 210–218. Khasawneh, M.A., Vallano, P.T., Remcho, V.T., 2001. Affinity screening by packed capillary high performance liquid chromatography using molecular imprinted sorbentsII. Covalent imprinted polymers. J. Chromatogr. A. 922 (1), 87–97. Koeber, R., Fleischer, C., Lanza, F., Boos, K.S., Sellergren, B., Damià, B., 2001. Evaluation of a multidimensional solid-phase extraction platform for highly selective on-line cleanup and high-throughput LC-MS analysis of triazines in river water samples using molecularly imprinted polymers. Anal. Chem. 73 (11), 2437–2444. Kondo, S., Ishikawa, Y., Abe, L., 2006. Adsorption Science. Chemical Industry Press, Beijing, pp. 26. Lamb, K.E., Dolan, M.D., Kennedy, D.F., 2019. Ammonia for hydrogen storage: a review of catalytic ammonia decomposition and hydrogen separation and purification. Int. J. Hydrogen Energy 44 (7), 3580–3593. Liu, F., Kuang, Y., Wang, S., Chen, S., Fu, W., 2018a. Preparation and characterization of molecularly imprinted solid amine adsorbent for CO2 adsorption. New J. Chem. 42 (12), 10016–10023. Liu, C., Shang, L., Yoshioka, H.T., Chen, B., Hayashi, K., 2018b. Preparation of molecularly imprinted polymer nanobeads for selective sensing of carboxylic acid vapors. Anal. Chim. Acta 1010, 1–10. Luo, X., Qiu, R., Chen, X., Pei, B., Lin, J., Wang, C., 2019. Reversible construction of ionic networks through cooperative hydrogen bonds for efficient ammonia absorption. ACS Sustain. Chem. Eng. 7 (11), 9888–9895. Miyaoka, H., Ichikawa, T., Ichikawa, T., Kojima, Y., Hiroki, M., Hikaru, M., Tomoyuki, I., Takayuki, I., Yoshitsugu, K., 2018. Highly purified hydrogen production from ammonia for PEM fuel cell. Int. J. Hydrogen Energy 43 (31), 14486–14492. Nabavi, S.A., Vladisavljević, G.T., Eguagie, E.M., Li, B., Georgiadou, S., Manovic, V., 2016. Production of spherical mesoporous molecularly imprinted polymer particles
7
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Case Study
Synthesis of ammonia molecularly imprinted adsorbents and ammonia adsorption separation during sludge aerobic composting Zhangliang Han, Yangjie Xu, Hui Wang, Haozhong Tian, Bin Qiu, Dezhi Sun
T
⁎
Beijing Key Lab for Source Control Technology of Water Pollution, Engineering Research Center for Water Pollution Source Control & Eco-remediation, College of Environmental Science & Engineering, Beijing Forestry University, Beijing 100083, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Ammonia separation Dynamic adsorption Hydrogen energy Molecular imprinted Odor
Ammonia (NH3) is the predominant harmful odor emitted from sludge aerobic composting plants, however, this NH3 could be recycled and used as energy or nitrogen fertilizer. Therefore, the aim of this study was to use molecular imprinting technology to prepare an adsorbent that could separate NH3 from mixed gases. An NH3 molecular imprinted polymer (NH3-MIP) was prepared by precipitation polymerization and optimal synthesis was determined by testing several different ratios of reaction components. NH3 adsorption capacity of the optimal NH3-MIP was 1.62 times that of non-imprinted material. NH3 separation factors increased from 154 (dimethyl sulfides) and 217 (dimethyl disulfides) for non-imprinted material, to 213 (dimethyl sulfides) and 302 (dimethyl disulfides) for the NH3-MIP. The adsorption mechanism was identified as physical adsorption and hydrogen bonding between H–O on the –COOH in NH3-MIP and the nitrogen in NH3. Effective desorption at 150 °C with vacuum maintained over 95% of the NH3 adsorption capacity.
1. Introduction
disulfides (DMDS) emitted during sewage sludge aerobic composting may even cause serious medical and mental harm to MSSACP employees and surrounding residents (Paustenbach and Gaffney, 2006; Han et al., 2019). NH3 contributes the most to odor emissions (Jiang et al., 2016; Zhu et al., 2016), and an NH3 odor intensity of Scale 5 (excessively strong odor) was recently identified in a MSSACP
Construction of new municipal sewage sludge aerobic composting plants (MSSACPs) and operation of existing ones has been limited due to strong odors emitted during the composting process (Cheng et al., 2005, 2019). Ammonia (NH3), dimethyl sulfides (DMS), and dimethyl
⁎
Corresponding author at: College of Environmental Science & Engineering, Beijing Forestry University, 35 Tsinghua East Road, Beijing 100083, China. E-mail address: [email protected] (D. Sun).
https://doi.org/10.1016/j.biortech.2019.122670 Received 31 October 2019; Received in revised form 19 December 2019; Accepted 21 December 2019 Available online 24 December 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.
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by using suitable agents and optimal ratios. Similarly, an NH3-MIP may promote NH3 separation due to cavity matching, and by using a polar functional monomer and crosslinker which could inhibit DMS and DMDS adsorption. Thus, the aims of this study were to: (1) prepare an NH3-MIP and optimize its NH3 adsorption capacity; (2) identify the adsorption mechanism between NH3 and the NH3-MIP; (3) measure the selectivity between NH3, DMS, and DMDS, stability, and regeneration of the synthesized NH3-MIP.
composting workshop (Han et al., 2019). The maximum [NH3] was over 100 mg·m−3 in this composting workshop, which is 3.2 times that of permissible concentration–time weighted averages, and enough to cause serious eye damage and upper respiratory tract irritation (US ACGIH, 2010; Han et al., 2019). Nevertheless, NH3 is an excellent fuel to store hydrogen (Zhao et al., 2010; Miyaoka et al., 2018; Lamb et al., 2019). Gaseous NH3 is easily converted into liquid NH3 in conditions at –33.5 °C to −77.8 °C or 700–800 kpa. Liquid NH3 is safer to store and easier to transport than molecular hydrogen (Nasharuddin et al., 2019). In addition, NH3 fuel has been identified as a method to address ever-increasing energy and environmental concerns (Hui et al., 2019; Yapicioglu and Dincer, 2019). The Haber process is the most commonly used industrial method to produce synthetic NH3, and occurs by the following reaction:
N2 +3H2
High temperature and pressure
⟺
2NH3
2. Materials and methods 2.1. Reagents Analytical reagent grade methacrylic acid (MAA), acrylamide (AM), ethylene glycol dimethacrylate (EGDMA), divinylbenzene (DVB), azodiisobutyronitrile (AIBN), toluene, methanol, trichloromethane, and NH3 solution (25%) were all purchased from Beijing Lanyi Chemical Products Co., Ltd. High purity water was purchased from Beijing Huihan Technology Co., Ltd. All gases used in this study were supplied by Beijing Huatong Jingke Gas Chemical Co., Ltd. Concentrations of NH3, DMS, and DMDS were 5000, 300, and 300 ppm, respectively. Nitrogen and oxygen had purities higher than 99.999%.
(1)
NH3 produced by the Haber process is used as inorganic fertilizer to promote food production, which helped initiate the green revolution of modern agriculture (Smil, 1999; Borlaug, 2002). However, because the Haber process requires high temperatures (> 400 °C) and pressures (150 bar) (Jennings, 1991), industrial NH3 synthesis consumes about 1%-3% of the world’s energy (Zhao et al., 2019) and causes 1% of annual CO2 emissions (Service, 2018). A more efficient method to accumulate NH3 would be to recycle it from sludge aerobic composting emissions. Developing a method to separate NH3 from DMS and DMDS would not only help eliminate odors from MSSACPs, but also eliminate energy consumption and greenhouse gas emissions caused by industrial NH3 synthesis. For example, NH3 emissions were between 4.35 and 10.95 g·dry·kg−1 throughout different seasons in a MSSACP in Beijing, China (Han et al., 2018). Based on this data, approximately 1186–2989 tons of NH3 could be recycled each year from sludge aerobic composting plants in Beijing. However, there have been few studies investigating NH3 separation from DMS or DMDS. Activated carbon is a commonly used material for removing and preventing odors, which adsorbs NH3 by hydrogen bonding or Lewis acid–base interactions (Seredych et al., 2009). To enhance adsorption performance, the surface of activated carbon is modified using chemical, mechanical, thermal, or electrical discharge methods (Park and Kim, 2005). However, the nonpolar surface of activated carbon also facilitates adsorption of DMS and DMDS, which is not beneficial for separation of NH3. For example, the NH3 adsorption capacity of activated carbon is only 0.075–0.45 mmol·g−1 (Rezaeia et al., 2017), but is 0.13 mmol·g−1 for DMS and 1.15 mmol·g−1 for DMDS (Vega et al., 2013). Therefore, using activated carbon is not a suitable strategy for separating NH3 from DMS and DMDS. A different approach that may be useful for NH3 separation would be to use molecularly imprinted adsorbents prepared by self-assembly. During the molecular imprinting process, the target adsorption molecule is used as a template and bound with functional monomers, which together adhere like a “lock and key” (Chen et al., 2011). The template molecule and functional monomers form complexes that get crosslinked. From there, the crosslinker and functional monomers are polymerized to form a molecular imprinted polymer (MIP), and the template molecules are removed in the eluent. A unique cavity is left behind in the MIP that matches the exact size and shape of the template molecule, and also contains functional groups which can specifically bind to the target substance (Khasawneh et al., 2001). A successful MIP will have a high separation factor, which measures the ability of the MIP to distinguish between two different analytes (Zhao et al., 2012; Curk et al., 2015). Oxalic acid has been used as a template molecule for CO2 adsorption and separation by bulk polymerization (Zhao et al., 2012), suspension polymerization (Nabavi et al., 2016), and precipitation polymerization (Liu et al., 2018a). These studies found that the MIPs were highly selective for CO2, and the separation factor (CO2/N2) reached up to 340
2.2. Preparation of NH3-MIP A series of NH3-MIP adsorbents were synthesized by first dissolving functional monomer (MAA or AM), template (NH3 solution), crosslinker (EGDMA or DVB), and AIBN in solvent (toluene, methanol, or trichloromethane). The mixtures were made airtight using parafilm and degassed with N2 for 5 min to remove oxygen (Liu et al., 2018b). Next, the mixtures were sealed and reacted for 24 h at 60 °C (Zhao et al., 2012). Sedimentary polymers were then washed with D.I water to remove template and solvent, and filtered. The washing procedure was repeated several times until template could not be detected in filtrate (Liu et al., 2018a). Finally, the polymer was dried under vacuum at 60 °C for 12 h. Non-imprinted polymer (NIP) without using template was also prepared in parallel by the same synthetic procedure. In order to obtain optimal NH3 adsorption capacity, functional monomer was first optimized, followed by the crosslinker and solvent (Table 1). Optimal ratios for components used to synthesize NH3-MIPs were also determined using normal multinomial regressive experimental design (Tables 2 and 3). Briefly, varying ratios of initiator (1%, 3%, or 9%), template: functional monomer: crosslinker ratios (1:2:10, 1:4:20, or 1:4:10), and polymeric monomers (2.5%, 5%, or 10%) were compared. Polymeric monomers are the sum of functional monomers and crosslinkers. The ratio of initiator is initiator percent of the polymeric monomers. The ratio of polymeric monomers is polymeric monomer percent of solvent. 2.3. Characterization Sample functional groups were obtained using a FTIR spectrometer (PerkinElmer Spectrum 100, BRUKER, USA). The resolution of determination was 4 cm−1, and the scanning range was between 400 and 4000 cm−1. Pore structure and specific surface area (SBET) were recorded using a specific surface and micropore analyzer (SSA-7000, Beijing BIoode Electronic Technology Co., Ltd., China). Each sample was outgassed under vacuum for 3 h at 120 °C prior to measurement. The pore distribution was measured using the current international BJH method. Thermal stabilities of adsorbents were characterized by thermo-gravimetric analysis (TGA-55, TA Instruments-Waters Co., Ltd., USA). Adsorbents were pre-dried at 120 °C before thermal characterization to remove moisture. 10 mg of adsorbents were heated from 15 to 800 °C with 25 ml·min−1 of N2 flow rate. Particle size and morphological analysis were obtained using a scanning electron microscope 2
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Table 1 Optimization of functional monomer, crosslinker, and solvent. Functional monomer optimization
Reagent Reagent
Functional monomer MAA AM
Dosage (μl) 340.95 214.94
Solvent Alcohol Alcohol
Dosage (ml) 60 60
Crosslinker DVB DVB
Dosage (ml) 2.82 2.82
Initiator AIBN AIBN
Dosage (mg) 60 60
NH3 solution (μl) 74.61 75.91
Dosage (μl) 232.34 340.95 232.34 340.95 232.34 340.95
Solvent Alcohol Alcohol Trichloromethane Trichloromethane Toluene Toluene
Dosage (ml) 60 60 60 60 60 60
Crosslinker EGDMA DVB EGDMA DVB EGDMA DVB
Dosage (ml) 2.57 2.82 2.57 2.82 2.57 2.82
Initiator AIBN AIBN AIBN AIBN AIBN AIBN
Dosage (mg) 60 60 60 60 60 60
NH3 solution (μl) 28.76 41.09 28.76 41.09 28.76 41.09
Crosslinker and Solvent optimization
Reagent Reagent Reagent Reagent Reagent Reagent
Functional monomer MAA MAA MAA MAA MAA MAA
Table 2 Experimental design used for ratio optimization. Group number
Initiator Ratio (%)
Template: functional monomer: crosslinker
Polymeric monomer ratio (%)
1 2 3 4 5 6 7 8 9
1 1 1 3 3 3 9 9 9
1: 1: 1: 1: 1: 1: 1: 1: 1:
2.5 5.0 10.0 5.0 10.0 2.5 10.0 2.5 5.0
2: 4: 4: 2: 4: 4: 2: 4: 4:
10 20 10 10 20 10 10 20 10
Table 3 Experimental design used for optimization of MIP-forming component proportions. Group number
Initiator (AIBN), mg
Template (NH3·H2O), μl
Functional monomer (MAA), μl
Crosslinker (EGDMA), ml
Solvent (Toluene), ml
1 2 3 4 5 6 7 8 9
15 30 60 90 180 45 540 135 270
52.4 52.4 194.6 104.8 104.8 48.7 209.6 26.2 97.3
118.71 237.41 879.36 237.41 464.82 219.84 464.82 118.71 439.58
1.31 2.63 4.86 2.63 5.25 1.22 5.25 1.31 2.43
60 60 60 60 60 60 60 60 60
Fig. 1. Diagram of the device constructed for gas adsorption.
Table 4 NH3 adsorption capacity of NH3-MIP in different conditions.
(SEM) (JSM-6700F, JEOL, Japan). 2.4. NH3 adsorption and desorption 2.4.1. NH3 adsorption NH3 adsorption was conducted on a home-made fixed bed by Beijing Camino Technology Co., Ltd. (Fig. 1). 0.2 g of the prepared adsorbent was loaded into a stainless-steel column. [NH3], [DMS], [DMDS], and [O2] could be adjusted with ratios between flow rates of their standard gases and N2 using a mass flow control meter (S49 32/ MT, Beijing Houchang Huibolong Precision Instrument Co., Ltd., China). During optimization experiments, the total gas flow rate was 200 ml·min−1, [NH3] was 100 ppm (i.e. 76 mg·m−3), and room temperature was 20 °C. Total gas flow rates, [NH3], adsorption temperatures, and oxygen content used for stability experiments are shown in Table 4. Experimental conditions used for selectivity testing were the same as those used in optimization experiments except [DMS] or [DMDS]. [DMS] and [DMDS] were identified by on-situ maximum emission rate ratios among them and NH3 (Han et al., 2019). [DMS] and [DMDS] were 10 ppm (i.e. 29 mg·m−3) and 10 ppm (i.e.
Temperature (°C)
Oxygen content (%)
Flow rate (ml·min−1)
[NH3] (ppm)
Adsorption capacity (mmol·g−1)
0 20 40 20 20 20 20 20 20
0 0 0 10 20 0 0 0 0
200 200 200 200 200 120 280 200 200
100 100 100 100 100 100 100 300 500
0.47 0.42 0.31 0.40 0.38 0.39 0.51 0.45 0.48
42 mg·m−3), respectively. Gas samples from the outlet of the adsorption column were sampled in a Teflon bag (FEP-ptfe-0.5 L, Beijing Haochentiancheng environmental protection and technology company, China), and the concentrations were detected in order to construct corresponding gas breakthrough curves to calculate adsorption capacity. When the detected concentration was equal to the inlet concentration, the adsorption experiment ended.
2.4.2. NH3 desorption Two desorption methods were used in this study: (1) after the NH3 adsorption experiment, adsorbents in the column were heated to approximately 150 °C using a heating furnace (Fig. 1) with 100 ml·min−1 of N2, until NH3 from the outlet of adsorption column was not detected. Thereafter, heating stopped but N2 flow rate was maintained. When the 3
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column temperature decreased to room temperature, NH3 adsorption could start again. Adsorption conditions were the same as those described in section “2.4.1 NH3 adsorption”. (2) Adsorbents were transferred to a vacuum oven (DZF, Beijing Kewei Yongxing Instrument Co., Ltd., China). When the temperature increased to 150 °C, gases in the oven were exhausted by vacuum pump until the pressure in the oven was < 133 pa. Thereafter, the inlet valve of the vacuum pump was opened. The pressure in the oven then recovered to atmospheric pressure, and the cycle was repeated 15 times. 2.5. Analysis methods 2.5.1. Analysis of gases [NH3] was directly measured using an NH3 sensor (JA908-NH3, Dongguan Yong Qi Electronic Equipment Co., Ltd., China). The measurement range of the sensor was 0.1–500 ppm with a 3% relative standard deviation. Before sampling, the NH3 sensor was calibrated with two NH3 standard gases (10 ppm and 100 ppm standard gases) after purging in a nitrogen atmosphere. DMS and DMDS were analyzed by a gas chromatography (GC7890, Agilent Technologies, USA) equipped with a flame photometric detector. The gas chromatography analysis conditions were as follows: A DB-1 capillary column (30 m × 0.32 mm × 5 μm, Agilent Technologies, USA) was used with nitrogen as the carrier gas (1.5 ml·min−1); The temperature of the oven was set at 40 °C initially (holding for 4 min), and then was increased to 120 °C (at 25 °C·min−1) holding for 4 min, after that the temperature was increased to 220 °C (at 25 °C·min−1) holding for 4 min.
Fig. 2. NH3 adsorption capacity of NH3-MIPs synthesized using varying (a) functional monomers, (b) crosslinks and solvents, and (c)/(d) reaction compound ratios. Group 10 corresponding NIP of Group 6.
MAA and NH3 is likely stronger than between the -N–H of AM and NH3·NH3 adsorption capacities of both the MAA- and AM-NH3-MIPs were higher than that of the corresponding NH3-NIPs, which indicated that the molecular imprinting technology (MIT) was effective, and that pore spatial structures and arrangement of functional monomers were able to adsorb NH3. Therefore, MAA was utilized as the functional monomer for subsequent experiments. In order to identify the optimal crosslinker and solvent for NH3-MIP synthesis, two commonly used crosslinkers (EGDMA and DVB) and three different solvents (toluene, ethanol, and trichloromethane) were tested (Fig. 2(b)). All of the MIP NH3 adsorption capacities were higher than that of the corresponding NIP, indicating that molecular imprinting was effective. Maximum NH3 adsorption capacity (0.34 mmol·g−1) occurred in the NH3-MIP synthesized with a combination of EGDMA and toluene. When EGDMA was used as the crosslinker in combination with ethanol or trichloromethane, NH3 adsorption capacities of the NH3-MIPs were only 0.088 mmol·g−1 and 0.089 mmol·g−1, respectively. Using DVB as the crosslinker resulted in lower NH3 adsorption capacities in combination with all three solvents, however adsorption capacity was still nearly twice as high with toluene. These results indicate that toluene is an effective solvent for synthesizing highly effective NH3-MIPs, especially in combination with EGDMA. Consistent with these findings, toluene has also been shown to be an optimal solvent for CO2 adsorption by CO2-MIPs (Zhao et al., 2012). Based on these results, EGDMA and toluene were used as the crosslinker and solvent for all further experimentation.
2.5.2. Statistical analysis methods Statistical analyses including correlation analysis, mean, and standard deviation of three replicates were calculated using SPSS Statistics 24. NH3/DMS/DMDS adsorption capacities were calculated according to their breakthrough curve by origin 85, as calculated by Eq. (2).
(
m= t×C0 −
∫0
t
)
Ct dt × Q× 22. 4−1 × 10−6 × m 0−1
(2) −1
where: m was NH3/DMS/DMDS adsorption capacity, (mmol·g ); t was adsorption saturation time of NH3/DMS/DMDS, (min); C0 was inlet [NH3]/[DMS]/[DMDS], (ppm); Ct was outlet [NH3]/[DMS]/[DMDS], (ppm); Q was total gases flow rate, (ml·min−1); m0 was the mass of adsorbents in adsorption column, (g). NH3 separation factor (P) was calculated using Eq. (3).
P= m1 × m2- 1 where: m1 was NH3 adsorption capacity, (mmol·g DMS adsorption capacity, (mmol·g−1).
(3) −1
); m2 was DMDS or
3. Results and discussion
3.1.2. Optimization of compound ratios for NH3-MIP synthesis Ratios of polymeric monomers and initiators among template, crosslinker, and functional monomers are key factors that have been shown to effect adsorption capacity by influencing characteristics such as particle size, specific surface area, and morphology (Koeber et al., 2001; Vasapollo et al., 2011; Xia et al., 2017). Therefore, nine different ratio combinations (Group1-9) were used to prepare NH3-MIPs in order to determine the optimal quantities for maximum NH3 adsorption capacity. Preparation of the Group 1 NH3-MIP was unsuccessful, likely because there was not sufficient contact between the minimum ratios of initiator (1%) and polymeric monomer (2.5%). After preparation of the NH3-MIPs, the NH3 adsorption capacity was tested for each group (Fig. 2(c) and (d)). Group 6, which used 3% initiator, 2.5% polymeric monomer, and a template: functional monomer: crosslinking agent ratio of 1:4:10, had the maximum NH3 adsorption capacity of 0.42 mmol·g−1. A NIP control (Group 10), that was prepared using these same ratios, exhibited a substantially lower NH3
3.1. NH3 adsorption capacity and mechanism analysis 3.1.1. Optimization of functional monomer, crosslinker, and solvent for NH3-MIP synthesis MAA and AM are commonly used functional monomers that form hydrogen bonds with imprinted molecules (Saloni et al., 2010; Zhang et al., 2010). Therefore, NH3 adsorption capacity was first screened using MAA or AM as functional monomers for NH3-MIP production (Fig. 2(a)). MAA- and AM-NH3-NIPs, which were synthesized without using NH3 template, were also tested in parallel as controls·NH3 adsorption capacities of MAA-NH3-MIP (0.082 mmol·g−1) and MAA-NH3NIP (0.074 mmol·g−1) were higher than that of corresponding AM-NH3MIP (0.056 mmol·g−1) and AM-NH3-NIP (0.051 mmol·g−1). This indicted that MAA is a more suitable functional monomer for adsorption of NH3, compared with AM. The hydrogen bond between –COOH of 4
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In addition to hydrogen bonding, it is possible that physical adsorption may occur between NH3-MIP and NH3. Temperatures under 100-150 °C with vacuum are typically needed to break hydrogen bonds between O–H in –COOH and nitrogen in NH3 (Kondo et al., 2006). Therefore, physical adsorption was tested by measuring the variation of NH3 adsorption capacity after adsorbing NH3 to saturation then increasing temperature·NH3 adsorption capacity at 20 °C was 0.42 mmol·g−1, but decreased to 0.26 mmol·g−1 when temperatures increased to 60 °C. This confirmed that at least 38.10% of NH3 adsorbed to the NH3-MIPs by physical adsorption. Group 6 NH3 adsorption capacity was 1.62 times of that of the Group 10 control (Fig. 2(d)), while Group 6 and Group 10 had similar pore structure and surface area (Table 5). This indicated that the size and shape of the NH3-MIP target substance contributed to its physical adsorption. In summary, there was hydrogen bonding between O–H in –COOH of NH3-MIP and nitrogen, and a portion of the NH3 was also captured by physical adsorption by using MIT.
Table 5 Pore structure characteristics of NH3-MIPs synthesized using varying compound ratios. Groups
Total pore volume (m3)
Surface area (m2·g−1)
Mean pore size (nm)
2 3 4 5 6 7 8 9 10
0.03 0.03 0.05 0.03 0.07 0.08 0.05 0.03 0.07
6.52 6.02 8.56 9.05 28.94 10.85 17.96 7.45 27.52
2.56 3.26 2.98 2.54 2.52 2.82 3.46 2.56 2.53
Table 6 Correlation analysis between NH3 adsorption capacity and pore structure data for Group 2–9. Total pore volume
Surface area
Mean pore size
Adsorption capacity
1
0.717*
−0.026
0.561
0.717* −0.026 0.561
1 −0.090 0.933**
−0.090 1 −0.147
0.933** −0.147 1
3.2. Adsorption stability Total pore volume Surface area Mean pore size Adsorption capacity
NH3 adsorption capacities of NH3-MIP at different temperatures, flow rates, inlet [NH3], and oxygen content were also tested (Table 4). A decrease in temperature from 40 °C to 0 °C resulted in an increase in NH3 adsorption capacity by 0.16 mmol·g−1. The improvement in NH3 adsorption capacity at lower temperatures was due to enhanced physical adsorption between NH3 and NH3-MIP (Zhao et al., 2014). An increase in NH3 flow rate from 120·ml min−1 to 280 ml·min−1 correlated with an increase in NH3 adsorption capacity from 0.39 to 0.51 mmol·g−1·NH3 adsorption capacity also increased from 0.42 to 0.48 mmol·g−1 when [NH3] increased from 100 ppm to 500 ppm. Oxygen content between 0 and 20%, which would normally be found in anaerobic compositing materials during sludge aerobic compositing, did not have a significant effect on NH3 adsorption capacity. These findings were consistent with CO2 adsorption capacities using CO2-MIPs (Zhao et al., 2014). Increasing temperatures also caused CO2 adsorption capacity to decrease from 0.55 mmol·g−1 at 45 °C to 0.19 mmol·g−1 at 90 °C, and increasing CO2 from 5% to 17% led to a 0.14 mmol·g−1 adsorption increase. Based on this data, adjusting temperature or aeration frequency and volume may be useful methods for improving NH3 adsorption capacity during sludge aerobic composting.
* significant correlation; ** extremely significant correlation.
adsorption capacity. In order to explore the internal factors affecting the adsorption capacity of the NH3-MIPs, each adsorbent was analyzed using a specific surface analyzer with statistical analysis to measure pore size, pore volume, and specific surface area (Table 5). Group 6 had the maximum specific surface area (28.94 m2·g−1) and minimum mean pore size (2.52 nm), which likely promoted physical adsorption (Zhao et al., 2012). Correlations between NH3 adsorption capacity, mean pore size, total pore volume, and specific surface area for Groups 2–9 were also analyzed (Table 6). A significantly positive correlation was shown between NH3 adsorption capacity and specific surface area, consistent with the characteristics of Group 6. Because the component ratios used to prepare the Group 6 NH3-MIP were ideal, these conditions were used for synthesis in subsequent experiments. 3.1.3. Analysis of adsorption mechanisms Adsorption forces between adsorbent and adsorbate may be divided into hydrogen bonding, chemisorption, and physical adsorption. There are two possible hydrogen bonds that may form between –COOH of the MAA-NH3-MIP and NH3 (Van Humbeck et al., 2014; Luo et al., 2019). The first possible bond is between the carbon–oxygen on the –COOH and hydrogen of NH3, and the second is between the hydrogen–oxygen on the –COOH and nitrogen of NH3. Stretching vibrations of the O–H in –COOH appeared around 2500–3300 cm−1, and C–O stretching vibrations appeared around 1715–1720 cm−1. Before adsorption, there were two O–H peaks for –COOH (i.e. 2945 and 2985 cm−1) and one C–O peak (i.e. 1719 cm−1) in the NH3-MIP. Stretching vibration of the O–H in –COOH decreased from 2945 cm−1 to 2900 cm−1, and from 2985 cm−1 to 2979 cm−1 after NH3 adsorption, but the C–O peak in –COOH (1719 cm−1) did not change. These results indicate that the O–H in –COOH formed a hydrogen bond with nitrogen in NH3 rather than with the C–O of –COOH. No significant changes were noted in the NH3-MIP FTIR spectra before or after adsorption, confirming that adsorption interactions between NH3 and the NH3-MIP adsorbent were not chemical reactions (Zhao et al., 2012). Additionally, there was no C=C absorption peak at 1660–1600 cm−1, indicating that all C=C bonds in MAA and EGDMA were broken.
3.3. Adsorption selectivity In order to assess the selectivity of the NH3-MIP to adsorb NH3, various combinations of DMS and DMDS with and without NH3 were screened (Fig. 3). The NH3-MIP was most selective for NH3 (0.42 mmol·g−1) compared to DMS (0.0025 mmol·g−1), and DMDS (0.0020 mmol·g−1) (Fig. 3(b)). DMS and DMDS are non-polar substances, therefore they are less likely to adsorb to the polar NH3-MIP prepared by polar EGDMA and MAA. In addition, the pore structures of NH3-MIP has a stronger affinity for NH3. When NH3 was combined with DMS (0.38/0.0023 mmol·g−1), DMDS (0.35/0.0017 mmol·g−1), or both DMS and DMDS (0.32/0.0015/0.00106 mmol·g−1), DMS and DMDS slightly competed with NH3 for adsorption to NH3-MIP, resulting in a minor decrease in NH3 adsorption capacity. Although NH3 separation factors with DMS or DMDS have not been previously investigated, NH3/CO2 separation factors have been shown to be between 3.4 and 52.5 in [Cnmim]2[Co(NCS)4] and [Cnmim] [SCN] (Zeng et al., 2018). The separation factor for NH3/SO2 was 65.88 for maximum NH3 adsorption to IRMOF-3 (Britt et al., 2008). The NH3MIP separation factors for both NH3/DMS (213) and NH3/DMDS (302) were substantially higher compared with the NIP control (154 and 217, respectively). This shows that MIT prompted NH3 separation from DMS and DMDS. 5
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Fig. 3. (a) NH3 breakthrough curve with individual or mixed gases; (b) NH3/DMS/DMDS adsorption capacity with individual or mixed gases.
(room temperature and atmospheric pressure) and desorption conditions (150 °C and vacuum) are low-cost, safe, and can easily be achieved in on-site MSSACPs. Furthermore, this technology may also be applicable for NH3 separation and recovery at other sites with high NH3 emissions (e.g. pig manure facilities and refuse landfill plants). 4. Conclusions An NH3-MIP was successfully prepared in this study, and was demonstrated to be efficient for separating NH3 from other odors in MSSACPs. Physical adsorption and hydrogen bonding, between H–O on the –COOH of methacrylic acid and the nitrogen in NH3, were identified as the main mechanisms of adsorption between the NH3-MIP and NH3. Future studies applying this technology to adsorb and separate NH3 from on-site odors during sludge aerobic composting are warranted. It is expected that this will be a beneficial strategy for elimination of odors and ammonia recovery in MSSACPs. Fig. 4. NH3-MIP regeneration using 150 °C with 100 ml·min−1 of N2 in an adsorption column, or 150 °C and vacuum (< 133 pa) in a vacuum drying oven.
Credit authorship contribution statement Zhangliang Han: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Visualization, Funding acquisition. Yangjie Xu: Validation, Formal analysis, Investigation. Hui Wang: Data curation, Project administration. Haozhong Tian: Validation, Formal analysis, Investigation. Bin Qiu: Writing - review & editing, Supervision. Dezhi Sun: Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition.
3.4. Adsorbent regeneration To determine the optimum desorption method for NH3-MIP regeneration, two different methods were compared (Fig. 4). In the first method, desorption occurred at 150 °C with 100 ml·min−1 of N2 in an adsorption column. Using this method resulted in a 47.67% decrease in NH3 adsorption capacity after the first desorption cycle (Fig. 4). This approximate level of NH3 adsorption capacity (42–52%) was maintained after a subsequent five adsorption/desorption cycles. Desorption in the second method occurred at 150 °C with vacuum (< 133 pa) in a vacuum drying oven. Using this method, NH3 adsorption capacity of the NH3-MIP remained above 95% even after ten cycles of adsorption/ desorption (Fig. 4). The strength of hydrogen bonds is about 5–10 times stronger than that of van der Waals, and can desorb at 100–150 °C with vacuum (Kondo et al., 2006). Therefore, the hydrogen bonds between NH3 and NH3-MIP could be broken at 150 °C in combination with the vacuum. The results presented in this manuscript describe a novel approach for using molecular imprinting technology to adsorb and separate NH3 from DMS and DMDS. This technology will be beneficial for eliminating the key odor, NH3, from MSSACPs (Han et al., 2018, 2019), and the recycled NH3 can be used as energy, nitrogen fertilizer, or liquid NH3 to store hydrogen. In comparison to currently used NH3-production methods, this molecular imprinting technology could also decrease energy consumption and greenhouses emissions. Based on selective adsorption of NH3 to the MIP-NH3, optimal adsorption conditions
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. Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (Grant Number: 2019YC10) and the Beijing Science and Technology Commission Foundation (Grant Number: Z181100005518008). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biortech.2019.122670. 6
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