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. 2021 Sep 8;6(37):23913–23923. doi: 10.1021/acsomega.1c02925

Effect of Sonochemical Treatment on Thermal Stability, Elemental Mercury (Hg0) Removal, and Regenerable Performance of Magnetic Tea Biochar

Adnan Raza Altaf †,*, Haipeng Teng †,*, Liu Gang ‡, Yusuf G Adewuyi §, Maosheng Zheng †
PMCID: PMC8459432  PMID: 34568670

Abstract

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Elemental mercury (Hg0) removal from a hot gas is still challenging since high temperature influences the Hg0 removal and regenerable performance of the sorbent. In this work, a facile yet innovative sonochemical method was developed to synthesize a thermally stable magnetic tea biochar to capture the Hg0 from syngas. A sonochemically synthesized magnetic sorbent (TUF0.46) exhibited a more prodigious surface area with developed pore structures, ultra-paramagnetic properties, and high dispersion of Fe3O4/γ-Fe2O3 particles than a simply synthesized magnetic sorbent (TF0.46). The results showed that TUF0.46 demonstrated strong thermostability and attained a high Hg0 removal performance (∼98.6%) at 200 °C. After the 10th adsorption/regeneration cycle, the Hg0 removal efficiency of TUF0.46 was 19% higher than that of TF0.46. Besides, at 23.1% Hg0 breakthrough, TUF0.46 achieved an average Hg0 adsorption capacity of 16.58 mg/g within 24 h under complex syngas (20% CO, 20% H2, 5% H2O, and 400 ppm H2S). In addition, XPS results revealed that surface-active components (Fe+, O2–, O*, C=O) were the key factor for high Hg0 removal performance over TUF0.46 from syngas. Hence, sonochemistry is a promising practical tool for improving the surface morphology, thermal resistance, renewability, and Hg0 removal efficiency of a sorbent.

1. Introduction

Mercury (Hg) is a hypertoxic pollutant of global concern that brutally affects the environment and human health.1 The high persistence, extreme toxicity, and bioaccumulation of mercury may cause various severe kidney and nervous system disorders.2 Coal-fired power plants account for the significant source of anthropogenic mercury emission (∼65%) in the atmosphere.3 As the world’s largest coal-consuming country, assume that China emits 25–45% of global mercury annually.4 Thus, the Chinese government has promulgated the latest national air pollutants emission standard (GB13223-2011) for coal-fired power plants,5 enacted to protect the people from mercury hazards.

Mercury exists in three gaseous states, (i) particulate mercury (HgP), (ii) oxidized mercury (Hg2+), and elemental mercury (Hg0), during the coal utilization process.6 Among these, Hg0 is the chemically inert and stubborn form of mercury that is comparatively tricky to capture by the existing gas purification devices.7 In recent years, coal gasification has attracted a great deal of interest worldwide owing to its wide adaptability for raw materials, abundant byproducts, and environmental perspectives relative to conventional coal-fired techniques.8 Nevertheless, coal syngas emits a high fraction of elemental mercury (∼80 μg/m3) than flue gas due to high temperature and a worse reducing environment.9 In addition, higher content of Hg0 is expected to expedite the corrosion of aluminum components that ultimately results in catastrophic industrial accidents.10 Thus, Hg0 removal from syngas faces more challenges than from flue gas.

To date, upstream sorbent injection is the most common and feasible approach to capturing Hg0 from syngas,11−13 for which choosing an effective and economical sorbent is a crucial step. Activated carbon (AC) has been a widely employed sorbent to control Hg0 pollution from thermal power plants.14 However, the high operating costs, low thermal stability, and weak renewability of AC have impeded its commercial application. Therefore, the exploitation of more efficient and economical alternatives for Hg0 removal is greatly appealing. Biomass is an inexpensive and abundant resource for producing a competent carbon material called biochar.15 Biochar has gained significant attention worldwide owing to its excellent textural characteristics (C=O, R–COOH, C(O)–O–C, −OH) and a high affinity for environmental pollutants.16 In addition, in the gas purification and storage field, biochar accounts for an attractive carbon material.17 Tea is a common aromatic beverage all over the world; according to the statistical analysis, China is ranked first among the tea-consuming countries.18 Food and Agriculture Organization (FAO) has reported that about 2.1 million tonnes of tea is consumed yearly by China, which accounts for 1/3 of the global tea consumption.19 This huge consumption of tea generates a massive tea waste problem that needs to be managed appropriately. Hence, recycling this tea waste into biochar synthesis could be an effective approach in terms of waste management and the development of an alternative material to produce activated carbon.

Nonetheless, virgin biochar shows a low Hg0 adsorption performance than AC.20 To achieve an adequate level of Hg0 adsorption, multiple surface activators are used, such as halides, sulfides, alkali, acids, and bases.21,22 Although various surface activators (such as HNO3, H2PO3, H2O2, NaOH) can improve the mercury adsorption efficiency,23,24 this process entails a high loading cost with complicated preparatory steps. Second, sorbent recycling is still a huge challenge because nonseparable mercury sorbents could contaminate the fly ash. In addition, Hg-contaminated fly ash will also cause mercury re-emission hazards on the reuse of fly ash.

Iron oxides (Fe3O4/γ-Fe2O3) are the eminent choice of many scholars because of their high SBET and super-paramagnetic characteristics.25,26 In addition, iron oxides are commonly used for the catalytic adsorption/oxidation of various gas pollutants in an acidic environment.27 George et al. studied that iron oxide-modified sorbents exhibit an excellent removal performance of multiple air pollutants in hot gas.28 Coincidentally, Altaf et al. investigated that Fe(NO3)3-laden biochar reveals an ultramagnetic property with an excellent Hg0 removal performance of about 96% from simulated syngas.11 Likewise, Yang et al. reported that Fe modification promotes the Hg0 removal efficiency at high temperatures with the strongest magnetism in the flue gas.29 In addition, the regenerable property makes the Fe sorbent a recyclable material for mercury removal.30 However, Fe3O4/γ-Fe2O3 mainly shifted and agglomerated on the sorbent surface during the pyrolysis process that may affect the thermal stability, Hg0 removal, and regenerable performance of a sorbent.31 Studies have shown that the migration and agglomeration of metal particles deteriorate the surface texture (pore blockage) that has led to impede the Hg0 removal performance of the sorbent.32 Therefore, it is urgent to improve the stability of iron oxide particles during the pyrolysis process to ensure the high dispersion of metal oxide particles on the sorbent surface.

Sonochemistry is a cogent tool to achieve a high mass transfer at a liquid–solid interface, wherein powerful ultrasound irradiation (0.02–10.0 MHz) induces various physiomechanical effects (e.g., heat, intense shock waves, and microjets) that could resolve the aggregation issue.33 In addition, acoustic cavitation demonstrates high metal particle isolation on the supporting material and reduces the particle size, which eventually improves the surface area and microstructures.34,35 Therefore, the current study aims to contribute to the development of more efficient and economic Hg-sorbent materials with (i) high Fe3O4/γ-Fe2O3 dispersion over the sorbent surface, (ii) excellent thermal stability, and (iii) good regenerable performance, using a generic sonochemical approach.

2. Results and Discussion

2.1. Sample Characterization

2.1.1. Brunauer–Emmett–Teller (BET) Analysis

Table 1 presents the specific surface area and pore volume of synthesized sorbents. It was observed that the pore volume and specific surface area (SBET) were significantly improved in TUF0.46 as compared to TF0.46. This might be due to the higher mass transfer interference at the liquid–solid interface resulting from microjets induced during the ultrasonic treatment. Study has shown the powerful ultrasound irradiation induces various physiomechanical effects that could resolve the aggregation issue.36 In addition, acoustic cavitation demonstrates high metal particle isolation on the supporting material and reduces the particle size, which eventually improves the surface area and microstructures.37 Hence, sonochemistry is a simplistic approach to improve the surface area of a sorbent.

Table 1. Specific Area and Surface Pore Characteristics of Synthesized Sorbents.
    pore volume (cm3/g)
 average pore width (nm)
samples SBET (m2/g) VT   Vmic Vmeso (%) V% (Vmeso/VT)
TF0.46 141.42 0.491 0.253 0.374 77 2.19
TUF0.46 196.28 0.703 0.173 0.648 92 1.42
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2.1.2. Scanning Electron Microscopy (SEM) Analysis

Sonochemistry may affect the surface morphology; hence, sonochemically modified and unmodified sorbents were analyzed by scanning electron microscopy (SEM). The results reveal that TUF0.46 exhibited an excellent dispersion of metal particles with well-developed pore structures (Figure 1). Remarkable improvements in the surface morphology might be the removal of debris from the sorbent surface. In addition, powerful acoustic irradiations certainly induced intense shock waves that allow a high mass transfer.38 In comparison, many immature pore structures and pore obstructions were seen on the TF0.46 surface. The SEM results coincided with the BET analysis results (Table 1).

Figure 1.

Figure 1

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SEM results: (a–c) TF0.46 and (d–f) TUF0.46.

2.1.3. X-ray Diffraction (XRD) Analysis

The XRD peaks of the prepared sorbents are shown in Figure 2. It was observed that major peaks indicative of SiC, Fe3O4 (JCPDS card no. 74-0748), and Fe2O3 (JCPDS card no. 39-1346)39 appeared on TF0.46. The formation of Fe3O4 and Fe2O3 is described below (R1–R6). However, it was seen that low peaks were observed on the TUF0.46 surface. Previously reported, sonochemistry mediates the catalytic activities by changing the surface crystallinity of a sorbent,40 and thus low diffraction peaks were observed.

Figure 2.

Figure 2

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XRD patterns: (a) Fe(NO3)3-loaded sorbent and (b) sonochemically treated sorbent.

Formation of Fe3O4

2.1.3. R1
2.1.3. R2
2.1.3. R3
2.1.3. R4

During the thermal decomposition (Δm) of cellulose and hemicellulose, the CO/H2 gas contents are simultaneously generated, which transforms Fe3+ into Fe2+ (R5, R6).41

Formation of Fe2O3

2.1.3. R5
2.1.3. R6

2.1.4. Magnetic Characteristics

The magnetic property of prepared sorbents is a key feature, which certainly permits their separation from fly ash. It was observed that TUF0.46 exhibited ferromagnetic characteristics with negligible residual magnetization (Figure 3). The saturation magnetization value was increased from 27.66 to 52.48 emu/g by the ultrasonic treatment. This suggested that the sonochemical application favors the ultra-paramagnetic features of the sorbent.

Figure 3.

Figure 3

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Paramagnetic features of synthesized sorbents.

2.2. Effect on Hg0 Removal Performance

A parallel adsorption bench test was performed to investigate the influence of the sonochemical treatment on the Hg0 removal efficiency (η) of synthesized sorbents. Results showed that the Hg0 removal efficiency was significantly improved in sonochemically treated sorbent relative to the directly synthesized sorbent. It can be seen in Figure 4 that the η value in TUF0.46 is about 9% higher than TF0.46. This might occur because of the high dispersion and penetration of active species (O2–, O*, Fe3+) caused by the ultrasonic treatment42 over the TUF0.46 surface, which is beneficial for chemisorption of Hg0. This Hg0 removal performance coincides with SEM (Figure 1) and XRD (Figure 2) analysis. As shown in Figure 2, the high dispersion of Fe3O4/Fe2O3 was detected over the TUF0.46 surface than TF0.46. In addition, the flourished surface area (SBET = 196.28 m2/g) in TUF0.46 reveals that the sonochemical treatment played a significant role in capturing the Hg0 from syngas.

Figure 4.

Figure 4

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Hg0 removal efficiency of prepared sorbents under simulated syngas (20% H2, 20% CO, 5% H2O, 400 ppm H2S gas, Hg0 85 μg m–3) at 120 °C reaction temperature for 120 min; gas hourly space velocity (GHSV) = 48 000 h–1.

2.3. Effect on Thermal Stability

Biochar is highly anisotropic, and its thermal stability varies with the degree (°C) of heat flow. Previous works have reported that the high temperature (above 100 °C) influences the thermal stability of biochar,43−45 which is not conducive to capturing the Hg0; thus, thermal stability is a key factor for removing Hg0 from syngas. Therefore, a series of experiments were carried out to investigate the thermal stability and Hg0 removal performance over synthesized sorbents at high temperatures (100–400 °C).

The parallel experiment revealed that both magnetic sorbents exhibited an average Hg0 adsorption performance of about 87% (TUF0.46) and 81% (TF0.46) at 100 °C (Figure 5). Notably, at 200 °C, the sorbents attained their maximum Hg0 removal efficiency, prominently TUF0.46 (η = 98.6%). Despite the extreme temperature (400 °C), TUF0.46 showed incredible thermal stability for capturing Hg0 from syngas, yet the η% was maintained to 46%. Studies have shown that the thermal conductivity of a sorbent is proportional to its surface porosity that ultimately leads to enhance the thermal stability of the sorbent.46,47 SEM analysis demonstrated an excellent dispersion of metal particles with well-developed pore structures in TUF0.46, as shown in Figure 1. In addition, BET analysis (Table 1) witnessed the high porosity and larger surface area in TUF0.46. The high Hg0 removal performance with strong thermal stability in TUF0.46 could be attributed to the above-discussed phenomenon.

Figure 5.

Figure 5

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Hg0 removal efficiency of prepared sorbents under simulated syngas (20% H2, 20% CO, 5% H2O, 400 ppm H2S gas, Hg0 85 μg m–3) for 120 min; gas hourly space velocity (GHSV) = 48 000 h–1.

To verify the above results, thermogravimetric (TG) analysis is conducted and shown in Figure 6. Three mass degradation phases (Δm) were recorded in TF0.46, and the first weight loss of 11% at 130 °C was mainly ascribed to water evaporation.48 An obvious weight loss was observed from 250 to 380 °C, which is attributed to the decomposition of organic compounds of tea leaves.49 The gradual weight loss turned slow above 500 °C. At last, 43% residual weight was achieved at 800 °C.

Figure 6.

Figure 6

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Thermogravimetric analysis of prepared sorbents: (a) TG% and (b) DTG%.

Although the thermogravimetric (TG) and derivative thermogravimetric (DTG) curves in TUF0.46 were different, the overall observed weight loss was comparatively low. However, a systematic weight loss (Δm = 24%) was noticed in the range of 310–435 °C by the fast degradation process of hemicellulose, cellulose, and lignin.46 Finally, 71% residual weight was obtained at 800 °C. The low mass shifting behavior in TUF0.46 indicates that diverse cross-linked oxidized compounds might be formed during the ultrasonic treatment (2 h at 80 °C) that become more stable during pyrolysis and prevent from further oxidation/degradation at higher temperatures.50 Therefore, it is reasonable to conclude that the ultrasonic treatment strengthened the thermal resistance of the sorbent.

2.4. Effect on Sorbent Regeneration Performance

Sorbent recycling is of prime importance for its commercialization and real-world implication. However, continuous thermal regeneration performance may affect the prolonged adsorption efficiency of the sorbent. So, the sorbent regeneration experiment was performed after the Hg0 adsorption test. For each experiment, 50 mg spent sample of TF0.46/or TUF0.46 was heated at 450 °C for 1 h under continuous N2 flow to decompose the adsorbed mercury species. The desorbed mercury was condensed into NaOH solution to avoid secondary mercury pollution. The Hg0 adsorption performances of recycled TF0.46/TUF0.46 over 10 repeated adsorption/regeneration cycles can be seen in Figure 7. The Hg0 removal of TF0.46 was gradually dropped in each cycle and reached 71% after the 10th regeneration cycle. As previously discussed, the migration and/or agglomeration of metal particles stagnates the adsorption efficiency of the sorbent.31 Thus, the inconsistencies in the adsorptive/regenerative capabilities might have resulted from the Fe3O4/γ-Fe2O3 particle instability or/and aggregation over the TF0.46 surface (Figure 1).

Figure 7.

Figure 7

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Hg0 removal efficiency of prepared sorbents in 10 repeated regenerated/adsorption cycles under simulated syngas at 200 °C reaction temperature for 120 min; gas hourly space velocity (GHSV) = 48 000 h–1.

On the other hand, TUF0.46 maintained its Hg0 removal efficiency above 90% after continuous thermal adsorption/regeneration cycles. The prolonged adsorptive/regenerative performance of TUF0.46 is in good agreement with SEM (Figure 1), BET (Table 1), and TG analysis (Figure 6). Hence, the current findings reveal that sonochemistry is a worthwhile approach to enhance the sorbent renewability and Hg0 removal efficiency.

2.5. Effect of CO and H2 on Hg0 Removal Performance and the Role of Active Components over TUF0.46

CO/H2 are the primary components of syngas and extreme reducing agents,51 which may affect the Hg0 removal efficiency of the sorbent. Thus, the Hg0 removal performance of TUF0.46 sorbent was examined in CO and H2 atmosphere. As shown in Figure 8, the Hg0 removal efficiency of TUF0.46 was decreased from 78% to 27% and 42% at 30% CO and 30% H2 concentration with balanced N2 flow, respectively. As previously reported, the CO/H2 gas molecules could compete with gaseous Hg0 for the available active sites on the sorbent surface, reducing the Hg0 removal effectiveness from the syngas.9 The worst inhibition in the Hg0 removal performance of TUF0.46 might have occurred due to a homogeneous reaction between Hg0 and CO/H2.

Figure 8.

Figure 8

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Effect of CO/H2 on the Hg0 removal performance over TUF0.46 at 200 °C reaction temperature for 120 min; gas hourly space velocity (GHSV) = 48 000 h–1.

However, there was no significant difference in Hg0 removal efficiencies between 10 and 20% supplementation of CO and H2 under pure N2 flow, and the average η value was between 46–42 and 73–69%, respectively (Figure 8). Therefore, 20% CO and 20% H2 concentration flow rate was selected for all experimental conditions. The assertive behavior against CO/H2 poisoning in TUF0.46 exists with the rational agreement of the high saturation of active components (Fe+, O2–, O*, C=O), which facilitate the active removal of mercury in a reducing atmosphere.52 Hence, TUF0.46 has proven to be a good CO/H2 tolerant material, which is highly desirable for capturing the Hg0 from coal syngas. To further investigate the role of active components over TUF0.46 in capturing the Hg0, the valance states of Fe, O, and C were studied by XPS before and after the Hg0 adsorption experiment.

2.5.1. Role of Fe+

Figure 9a represents the Fe 2p spectra; three peaks were raised over fresh TUF0.46 at 710.5, 711.4, and 713.8 eV that were assigned to Fe2+ and Fe3+ in octahedral coordination [Fe3+(O)], and Fe3+ in tetrahedral coordination [Fe3+(t)] of Fe3O4, respectively.53 After the Hg0 removal experiment, a prominent change in Fe3+(t) coordination was observed over the spent TUF0.46. However, a significant increase was observed in Fe2+ content (Table 2). This suggests that Fe3+(t) coordination in Fe3O4 could serve as an active site for Hg0 adsorption/oxidation, resulting in Fe amalgamation (R7).11

2.5.1. R7
Figure 9.

Figure 9

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XPS analysis over fresh and spent TUF0.46 sample: (a) Fe 2P; (b) O 1s; and (c) C 1s.

Table 2. Quantitative Analysis of Valance State Elements (%) from XPS Spectra of Fresh and Spent TUF0.46 Sorbents.
sample Fe3+ Fe2+ O2– O* C=O C–O
fresh 87.2 11.6 29.3 48.7 26.5 9.43
spent 74.3 18.4 9.7 61.1 11.2 20.1
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2.5.2. Role of O2– and O*

Figure 9b reveals that three peaks corresponding to O 1s were observed at 530.1 eV (lattice oxygen in metal oxides, O2–), 531.3 eV (chemisorbed oxygen, O*), and 533.1 eV (C–O) on fresh TUF0.46. The appearance of O2– on fresh TUF0.46 might be a result of Fe3O4. Previous studies have reported that metal oxide modification can increase the amount of adsorbed oxygen over the sorbent surface.54 However, a significant increase in O* concentration from 48.70 to 61.10% was observed after the Hg0 adsorption test. This change in O* contents might be attributed to the Fe3+ → Fe2+ transformation that promotes active sites for Hg0 adsorption (Table 2). This can be further explained by the below reactions (R8–R10)

2.5.2. R8
2.5.2. R9
2.5.2. R10

2.5.3. Role of C=O

It was reported that C=O groups could serve as electron acceptors and consequently promote chemisorption and/or oxidation of Hg0.29 To verify this hypothesis, C 1s XPS spectra were studied. As shown in Figure 9c, three peaks were deconvoluted on both fresh and used TUF0.46 at 284.9 eV (C–C group), 286.1 eV (C–O group), and 289.2 eV (C=O group). After the Hg0 adsorption test, an obvious decrease was recorded from 26.5 to 11.2% in the C=O group content. At the same time, an increase in the C–O group content from 9.43 to 20.1% was acheived (Table 2). This prominent decrease suggests that C=O groups acted as electrodes for taking electrons and participated in the electron transfer process to oxidize the Hg0 (R11, R12). Thus, TUF0.46 demonstrated the high Hg0 removal performance in complex syngas.

2.5.3. R11
2.5.3. R12

2.6. Breakthrough Analysis and Hg0 Adsorption Comparison of TUF0.46 with Previously Reported Materials

The breakthrough analysis is a common and widely employed approach to determine the adsorbate removal efficiency of a sorbent under a shorter contact period. Therefore, the Hg0 adsorption capacity of TUF0.46 was testified under simulated syngas at 200 °C to further assess the sorbent performance. As seen in Figure 10a, the sorbent TUF0.46 attained an average adsorption capacity (qt) of about 16.582 mg/g after 24 h of adsorption reaction with a 23.1% Hg0 breakthrough threshold. The high adsorption (physical and/or chemical) efficiency with strong thermal resilience to remove Hg0 from syngas is a key criterion for selecting an adsorbent. However, chemisorption mainly governs the Hg0 removal process. Thus, we employed the most suitable kinetic models (1) Elovich model and (2) pseudo-second-order model to further estimate the Hg0 adsorption ability of TUF0.46 based on 23.1% breakthrough curve.

Figure 10.

Figure 10

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(a) Hg0 breakthrough threshold curve analysis; TUF0.46 dose = 10 mg; Hg0 feed = 150 μg/m3; under N2+ 20% H2 + 20% CO + 400 ppm H2S + 5% H2O atmosphere at 200 °C reaction temperature for 24 h; gas hourly space velocity (GHSV) = 48 000 h–1. (b) Adsorption kinetic models.

2.6.1. Kinetic Simulations

The Elovich and pseudo-second-order models are mainly applied to describe the entire kinetic process of gas chemisorption over the sorbent surface.55 Elovich and pseudo-second-order models were described by the following equations E1 and E2, respectively

2.6.1. E1
2.6.1. E2

where qt and qe are the adsorption capacities (mg/g) of TUF0.46 at time t and the equilibrium time (E1 and E2). α can be defined as the initial rate of adsorption (E1) and β is related to the activation energy for the chemical adsorption of Hg0 over TUF0.46.56k2 is the rate constant of the pseudo-second-order kinetic model (E2). The fitting curves of kinetic models for Hg0 adsorption on TUF0.46 are displayed in Figure 10b. It was determined that a consistent agreement is present between experimental and simulated data of both kinetic models. In addition, the correlation coefficient (R2) values of fitted models were closer to 1, which suggests that chemisorption is the main route of the Hg0 removal process from syngas. The Hg 4f XPS spectra further justified the chemisorption phenomenon, as can be seen in Figure 11; two strong peaks at 101.4 and 105.7 eV appeared on the spent TUF0.46 surface, which mainly attributed to Hg 4f7/2 and Hg 4f5/2 chemisorbed mercury (Hg2+).57 Notably, the obtained R2 value of the pseudo-second-order model was higher than the Elovich model (Figure 10b), indicating the Hg0 adsorption process over TUF0.46 corresponds to the pseudo-second order.58 Thus, the pseudo-second-order model can comprehensively describe the Hg0 adsorption process of TUF0.46. A detailed Hg0 adsorption comparison of different sorbents reported in previous literature with TUF0.46 is summarized in Table 3. However, the calculated and simulated equilibrium adsorption capacity (qe) of TUF0.46 was up to 66.36 mg/g, which is much higher than that of previously reported sorbents (Table 3). This implies that TUF0.46 is a competent sorbent material to capture mercury. Hence, TUF0.46 can be considered a versatile sorbent material to control the mercury emission from coal-thermal power plants.

Figure 11.

Figure 11

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XPS Hg 4f analysis over spent TUF0.46 sample.

Table 3. Hg0 Adsorption Potential Comparison of Different Sorbents Reported in Previous Literature with TUF0.46.
sorbents stable Hg0 feed (μg/m3) R/T (°C) simulated time (h) qe (mg/g) reference
AC 55 140 - 2.2 (62)
[MoS4]2–/CoFe-LDH 350 75 50 16.4 (63)
T7F5 90 140 2.5 1.24 (64)
CoMn2O4 150 160 36 39.05 (9)
Fe3O4–xSey 65 100 50 8.8 (65)
CoMoS/γ-Al2O3 30 50 2250 18.9 (66)
TUF0.46 150 200 96 66.36 this study
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2.7. Economic Implications for the Real-World Application of TUF0.46

The affordable cost of the sorbent for capturing mercury is the key for its practical application in coal-thermal power plants. Therefore, in this study, the production cost analysis of TUF0.46 has been thoroughly investigated. Tea waste collection and sample preparation are well described in our previous work.11 A stepwise cost analysis (in USD) is given below:

  • (1)

    Raw material = 0.0 USD (waste tea was collected from nearby tea stalls).

  • (2)

    Sample processing = 0.055 USD (tap water was used for initial wash. Distilled water was only used for rinsing [0.5 l × 0.11 USD per liter cost])

  • (3)

    Sonochemical modification for 10 g of tea = 0.204 USD [5.5 g of Fe(NO3)3 was dissolved in 50 mL of distilled water for 10 g tea feedstock + added 0.3 M citric acid solution + 2 h ultrasound treatment (5.5 g × 0.027 USD per gram cost + 0.041 USD + 0.23 units × 0.066 USD)]

  • (4)

    Pyrolysis and steam activation = 0.451 USD [6 units × 0.066 USD + 0.055 USD]

  • (5)

    Total cost per gram of TUF0.46 = 0.0779 USD [0.779 ÷ 10]

  • (6)

    Total cost per kilogram of TUF0.46 = 77.9 USD

According to the U.S. Environmental Protection Agency, the activated carbon injection takes a 500 USD cost per hour, achieving about 90% mercury removal level in coal-fired power plants.59,60,3 However, the above estimated cost of production in TUF0.46 indicates that sorbent preparation using tea waste is a comparatively more inexpensive and environmental approach. In addition, the prolonged regenerable property of TUF0.46 will reduce more mercury removal costs. Thus, excellent Hg0 removal efficiency coupled with facile and robust synthesis procedure, incredible resistance in complex syngas, high thermal stability, and outstanding renewability make the TUF0.46 a propitious sorbent for practical application of Hg0 elimination from coal syngas.

Conclusions

Mercury is a common neurological toxic pollutant after lead (Pb) in the environment;61 therefore, its control is inevitable. In this study, efficient magnetic tea biochar was synthesized using a facile sonochemical technique accompanied by the one-step pyrolysis method. The analytical findings revealed that the sonochemical application improves the sorbents’ morphology, textural characteristics (SBET), and paramagnetic features. Furthermore, the adsorption bench test showed that the sonochemically treated sample (TUF0.46) exhibited unprecedented Hg0 removal efficiency (≥98%) with strong thermal stability at high temperatures. At 23.1% Hg0 breakthrough, the qt value was 16.58 mg/g, and qe value up to 66.36 mg/g was achieved (Table 3). In addition, after 10 continuous adsorption/regeneration periods, the Hg0 removal efficiency of TUF0.46 was still above 90%, representing its competency for real-world application. The current study suggests that sorbent selection criteria could not merely depend upon high Hg0 adsorption aspects; it should also be cheap and easily recyclable. In that case, TUF0.46 possessed all desirable characteristics; moreover, the unique paramagnetic property would prevent the high risk of fly ash deterioration.

3. Materials and Methods

3.1. Sorbent Preparation and Characterization

The sorbent was synthesized by the sonochemically assisted one-step pyrolysis method. Typically, 10 g of processed waste tea feedstock was dispersed in 0.46 mol L–1 solution of Fe(NO3)3 under continuous thermostatic agitation (setup: 300 rpm at 80 °C) for 6 h. Afterward, 0.3 M citric acid solution was applied and ultrasonically treated for 2 h at 80 °C. Subsequently, the impregnated sample was amassed and oven-dried for 5 h at 105 °C. Finally, the sample was pyrolyzed in a vertical furnace (500 ± 20 °C min–1) for 2 h under pure N2 atmosphere. After pyrolysis, the Fe(NO3)3-loaded sample attained a magnetic property. Then, the synthesized magnetic sorbent was further post-pyrolyzed under a mixed gas atmosphere (H2O steam 5 mL min–1 + N2 60 mL min–1) at 700 °C for 45 min to activate and flourish the pore structures. Finally, the prepared magnetic sorbent was preserved in airtight glass jars and marked as TUF0.46, here ‘T’ represents tea char, ‘U’ stands for the ultrasonic treatment, and ‘F’ is Fe(NO3)3 application. The magnetic sorbent without the ultrasonic treatment was synthesized with the consistent preparatory method and labeled as TF0.46. The sorbent characterization methods are described in the Supporting Information S2.

3.2. Elemental Mercury Removal Experiment

The bench-scale Hg0 removal experiment over prepared sorbents was investigated in a fixed-bed reactor system shown in Figure 12. The total syngas flow rate was adjusted at 1 L/min, composed of N2, 20% H2, 20% CO, 5% H2O, and 400 ppm H2S gas. Mercury vapors were continuously generated by the Hg0 permeation device at 85 μg m–3: 1 ± 60 °C. For each Hg0 adsorption test, normally 50 mg of sorbent mixed with 0.2 g of quartz sand was used. The quartz sand was employed to elevate the height of the sorbent bed about 20 mm, thus increasing the residence time of syngas. A continuous mercury analyzer (MAX-L Hg analyzer) was used to measure the Hg0 concentration. The Hg0 removal efficiency (η) and adsorption capacity (qt) of sorbent (μg/g) were calculated by the following equations E3 and E4

3.2. E3
3.2. E4

where Co and Ce represent the Hg0 concentration (μg/m3) at the inlet and outlet of the reactor in eqs E3 and E4. While F is the gas flow rate (L/min), W is the weight of the sorbent (g), and t is the reaction time (min).

Figure 12.

Figure 12

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Fixed-bed reactor for Hg0 adsorption test.

Acknowledgments

The project was supported by the Natural Science Basic Research Plan in Shaanxi Province of China (Program No. 2019JLM-13).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c02925.

  • Sorbent characteristics including BET, scanning electron microscopy (SEM), X-ray diffraction (XRD), vibrating sample magnetometer (VSM), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao1c02925_si_001.pdf (90.3KB, pdf)

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Supplementary Materials

ao1c02925_si_001.pdf (90.3KB, pdf)

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