Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 11493−11499
pubs.acs.org/journal/ascecg
Biomass Bricks with Excellent Indoor Formaldehyde Capture and Transformation Performance Shuai Zhang,† Meng Wang,‡ Zeguang Lu,*,§ Chao Ma,§ and Wanda Jia§ †
College of Chemistry and Material Science, ‡College of Water Conservancy and Civil Engineering, and §College of Forestry, Shandong Agricultural University, No. 61, Daizong Road, Taian, 271018 Shandong, China
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S Supporting Information *
ABSTRACT: Indoor formaldehyde capture and conversion to nontoxic materials, a widespread focal problem, is crucial for human health and suitable residence and also sufficiently challenging. Herein we present a kind of practical construction of biomass/ wooden bricks with excellent indoor formaldehyde capture and transformation performance. These biomass bricks are prepared from Ca(OH)2, waste poplar wood fiber, corn stalk fiber, and nanoTiO2, presenting excellent compressive strength (average 2.94 MPa) and capacity for indoor formaldehyde removal. In the HCHO adsorption test (desiccator testing methods), the concentration of HCHO decreased to 25% of the initial value in 3 days, and the HCHO concentration after the biomass brick absorption experiment is lower than the E2 limiting one (GB 18580-2001 national standard, China), creating a relatively safe indoor environment. On the basis of the results of a series of tests, including scanning electron microscopy, X-ray diffraction, nuclear magnetic resonance, etc., indoor HCHO is not only captured but also transformed to safer products by Cannizzaro disproportionation, hydroxymethylation, and photochemical oxidation. These biomass bricks not only meet construction needs and improve biomass utilization, but they also simultaneously reduce indoor HCHO concentration, providing a environmentally friendly, clean, and healthy residence environment for high-quality human life. KEYWORDS: biomass brick, formaldehyde absorption, biomass, construction materials, corn stalk
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by facilitating the provision of hydroxyl groups.5 The Liu group developed a kind of heterostructured photocatalyst consisting of graphitic carbon nitride (g-C3N4), TiO2, and waste zeolites, which exhibited superior visible-light-responsive activities toward formaldehyde removal. More than 90% of lowconcentration formaldehyde can be oxidized under a commercial LED light.11 Kanjwal prepared titanium-based composite−graphene nanofbers as high-performance photocatalysts, which manifested superior photocatalytic performance. The graphene incorporation helped to harvest more energy from the entire UV−vis spectrum and almost doubled the surface area when the maximum amount of graphene was embedded into the nanofbers.12 Despite great achievements achieved on removal of HCHO, direct integration of building materials (such as biomass bricks) and HCHO-removal performance is also a promising strategy for indoor HCHO removal, which has both practical value and health benefits. It not only upgrades waste biomass resources (especially avoiding the problem of burning crop residue)13,14 but also simultaneously reduces indoor HCHO concentration. Biomass bricks, as green and environmentally
INTRODUCTION Indoor air quality is crucial for environmental/human health and suitable residence, because people generally spend more than 80% of their time in houses and offices.1 Formaldehyde (HCHO), one of the predominant pollutants in houses emitted from household products, building supplies, and decorative materials, has been classified as a human carcinogen.2,3 Long-time exposure to HCHO may cause adverse effects on human health, such as nasal tumors, skin irritation, respiratory tract infection, and cancer.1,4 Many strategies for the removal of HCHO have been explored, including physical/chemical adsorption, photochemical (PC) degradation, and catalytic oxidation.5−7 For indoor HCHO, it is highly desired to develop bifunctionalized materials with both HCHO adsorption and degradation capacities, to prevent the release of the captured HCHO. HCHO, as a type of active chemical, can be consumed under alkaline conditions by several kinds of reactions, facilitating the chemical conversion of HCHO to less-toxic chemicals. Porous materials possessing a highly specific surface with immobilized nanotitanium dioxide (TiO2) present both excellent adsorption and PC degradation activity of HCHO under UV irradiation.8−10 The abundant surface hydroxyl groups and the presence of moisture contribute to the HCHO adsorption performance and promote the photocatalytic oxidation of HCHO to CO2 © 2019 American Chemical Society
Received: March 7, 2019 Revised: May 22, 2019 Published: June 4, 2019 11493
DOI: 10.1021/acssuschemeng.9b01349 ACS Sustainable Chem. Eng. 2019, 7, 11493−11499
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ACS Sustainable Chemistry & Engineering
as CO2 and H2O) by catalytic oxidation of adjacent TiO2 under sunlight.1,21 Therefore, these functionalized biomass bricks present good HCHO capture/conversion ability to safer products, providing an environmentally friendly and health strategy for high-quality human life.
friendly materials, have attracted wide attention recently and present the potential application in construction of indoor nonbearing walls. Biomass bricks, with good compressive strength, originate from waste wood fiber and crop straw fiber, which not only meet construction needs but also improve biomass utilization. Biomass bricks can balance the relative humidity of indoor air by moisture desorption and absorption, improving the indoor environmental quality. Besides, biomass bricks can also serve as decorations inside a room (Figure 1), facilitating direct contact with indoor air.
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EXPERIMENTAL SECTION
Preparation of Raw Biomass Bricks. Corn stalk fiber, poplar wood fiber, Ca(OH)2, and amorphous nano-TiO2 are commercially available. The ratio of these raw materials in biomass bricks is closely investigated. The optimized mass ratio for these biomass bricks is corn stalk fiber (42 g), poplar fiber (297 g), Ca(OH)2 (1869 g), and amorphous nano-TiO2 (10 g), respectively. Under this optimized ratio, these biomass bricks present the highest compressive strength (average 2.94 MPa). These biomass bricks are molded according to the technology roadmap (Figure S1). Corn stalk fiber, poplar fiber, Ca(OH)2, and TiO2 are mixed completely and put into a squeeze die (235 mm × 110 mm × 150 mm, Figure S2). Then bricks are molded in the cold press machine (10 MPa, 10 min, room temperature, Figure S3, model MY 50B). To dry the wet brick, a hot air drying house (Figure S4, 4.5 m × 2.8 m × 3.0 m, length × width × height) is employed, which is set at 0.20 m/s in velocity, 30.1% in relative humidity and 60 °C in temperature, 48 h. Finally, 30 pieces of biomass bricks (235 mm × 110 mm × 55 mm) are molded and marked as T8694−T8723 and subjected to mechanical property tests, HCHO adsorption experiments, characterization tests, etc. Bricks T8694−T8708 are testsed for moisture content, compressive strength, and density. As shown in Table S2, these tests are repeated five times to obtain the average values. Bricks T8709−T8714 are characterized by SEM, IR, XRD, NMR, DSC, and TG tests. Bricks T8715−T8719 are used for the HCHO adsorption and exposure experiments. Bricks T8720−T8723 are held for some standby characterizations and applications. Tests of Compressive Strength. The compressive strength of the dry bricks is tested using an electrical strength test machine. The loaded direction is in thickness at 1.0 mm/min in movement velocity. Compressive strength is judged by the deformation of 5.0 mm (deformation rate test method is referenced to the “Drying shrinkage property of wood”, GB/T 1932-2009, China). In these biomass bricks, the average volume fraction of plant fiber is 73%. Therefore, these bricks are also named as wooden bricks, and the compressive strength test method is referenced to the “Compressive strength of wood”, GB/T 1935-2009, China, which is different from the destructive strength commonly used for building materials. Indoor HCHO Adsorption Test (desiccator testing methods). HCHO adsorption experiments of bricks T8718 and T8719 are performed in a glass dryer (40 L) and located in a thermostatic chamber (20 ± 1 °C) in the dark, with the concentration of HCHO in distilled water (300 mL) detected by a visible spectrophotometer. Notably, the HCHO concentration in distilled water detected in the desiccator testing methods is a relative value, not the real HCHO concentration in air. Notably, the HCHO concentration in distilled water is employed to estimate that the indoor environment is relatively safe or harmful by comparing with the limiting value (E1 ⩽ 1.0 mg/L, E2 ⩽ 5.0 mg/L, GB 18580-2001, China). HCHO adsorption experiments of these biomass bricks are performed by Research Institute of Wood Industry, Chinese Academy of Forestry. The HCHO adsorption test method (desiccator testing methods) refers to “Indoor decorating and refurbishing materials - Limit of formaldehyde emission of wood-based panels and finishing products”, GB 18580-2017, China. Spectrophotometric methods are applied to determine the HCHO capture capacity of bricks. Laminate flooring serves as the source of indoor HCHO. In this desiccator testing method, 40 L dryer volume, 1800 cm2 surface area of laminate flooring, 3 days, and 300 mL of distilled water are used for HCHO adsorption test. The test report of HCHO adsorption capacity is shown in Supporting Information. HCHO Adsorption and Exposure Experiments for Further Characterization. Taking the low concentration in HCHO
Figure 1. Model indoor wall with biomass bricks.
Our group has developed a series of wood/crop waste fiberbased biomass bricks, which possess high compressive strength, are lightweight, and have good water resistance and porous characteristics.15−17 Herein we describe a new kind of functionalized Ca(OH)2/TiO2-based biomass bricks, which are prepared from Ca(OH)2, waste poplar wood fiber and corn stalk fiber with nano-TiO2 as an additive. These biomass bricks present excellent stability and durability. No fissures are observed for more than 1 year, when these bricks are employed to construct housing and indoor walls. Furthermore, these bricks possess excellent capacity for HCHO capture and conversion. As shown in Scheme 1, under the strong basicity of Scheme 1. Possible Chemical Conversion Routes of Absorbed Formaldehyde
Ca(OH)2, the absorbed HCHO converts to less-toxic carboxylates, methanol (Scheme 1a), and hydroxymethyl moieties (Scheme 1b) through intra/intermolecular Cannizzaro disproportionation reaction18 and hydroxymethylation reaction.19,20 Furthermore, these generated intermediates can be subsequently transformed into neutralized products (such 11494
DOI: 10.1021/acssuschemeng.9b01349 ACS Sustainable Chem. Eng. 2019, 7, 11493−11499
Research Article
ACS Sustainable Chemistry & Engineering
standard (desiccator methods, limiting value E1 ≤ 1.0 mg/L, E2 ≤ 5.0 mg/L, GB 18580-2001, China, “Indoor decorating and refurbishing materials - Limit of formaldehyde emission of wood-based panels and finishing products”). This result indicates that the HCHO concentration decreased from a dangerous level to a relatively safe one. In this experimental test, closed indoor environment contaminated by HCHO is simulated by sealing laminate flooring in glass dryers, which is a better approach to practical situations and quite different from HCHO concentration employed in the literature.1−5 In our test, the HCHO concentration after the biomass brick absorption experiment is lower than the E2 limiting value of GB 18580-2001 national standard (China), creating a relatively healthy indoor environment. Due to the differences in test methods and standard, no clear data can be used for comparison between our results and those reported in the literature. Therefore, on the basis of these results, these biomass bricks present excellent HCHO capture capacity, and this strategy is a promising protocol for indoor HCHO removal. It should be noted that the captured HCHO will be converted into relatively safer chemicals with the aid of alkaline Ca(OH)2, preventing the release of captured HCHO. Strong bases, such as Ca(OH)2 and NaOH, can trigger the Cannizzaro disproportionation of HCHO with the formation of carboxylate (calcium formate) and methanol at room temperature.1 Phenol moieties of lignin generated in the hot air drying process (30.1% humidity, 60 °C, 48 h) can react with the absorbed HCHO to produce hydroxymethyl moieties through hydroxymethylation reaction.16,17 Furthermore, neutralized products (such as CO2 and H2O) will be generated by catalytic oxidation of adjacent TiO2 under sunlight.1,15 To obtain clear and exact evidence on the transformation of captured HCHO, a series of characterization tests, including SEM, XRD, IR, SSNMR, TG, and DSC, are performed. SEM Characterization test. Figure 2a clearly shows the rough surface of fibers in brick powder, which is probably
adsorption desiccator tests into consideration, no significant testing results can be obtained for bricks T8718 and T8719. To obtain clear evidence on the transformation of captured HCHO, bricks T8715 and T8716 are used for subsequent characterization tests instead of bricks T8718 and T8719. Bricks T8715 and T8716 are placed in two sealed dryers (20 mL of 30% HCHO solution are placed beforehand) for 2 weeks in the dark at room temperature, respectively. Then the surface of brick T8715 is used for characterization tests. Subsequently, brick T8716 is exposed to sunlight for 2 weeks (indoor, 9 d sunny, 5 d cloudy/rainy, placed in sealed, transparent plastic pockets), preceding the photocatalytic degradation process. After 2 weeks, the brick (T8716) surface is cut for characterization tests. Test Characterization. Micromorphologies of corn stalk fiber, poplar wood fiber, Ca(OH)2, nano-TiO2, and bricks are observed separately with scanning electron microscopy (SEM). Functional groups are analyzed with the Fourier transform infrared spectroscopy (FTIR). Phase transformations are studied with X-ray diffraction analysis (XRD). Structure changes are examined with solid-state nuclear magnetic resonance (13C SSNMR, model Agilent 600 MHz DD2, Agilent Technologies Co., Palo Alto, CA, no solvent, number of scans: 1024). Thermal stability is investigated with thermogravimetric analysis (TG). The temperature ranges from room temperature to 800 °C in a N2 environment with 20 °C/min of heating rate. Thermodynamics characters are examined with differential scanning calorimetry (DSC). The temperature ranges from room temperature to 400 °C in a N2 environment with 10 °C/min of heating rate. Specific surface area is investigated by a Micromeritics instrument ASAP 2020 automatic specific surface analyzer.
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RESULTS AND DISCUSSION Indoor HCHO Adsorption Capacity. According to the technology roadmap (Figure S1), raw biomass bricks (marked as T8694−T8723, 30 bricks, Figure S4) are molded, their properties, including moisture content, compressive strength, and density, are tested, and the results are listed in Table S2. These biomass bricks present valuable characteristics of being lightweight (average 1.070 g/cm3) and having high strength (average 2.94 MPa), which meet the requirements of nonbearing wall materials. Furthermore, the existence of moisture (average 12.43%) inside the brick provides hydroxyl groups, facilitating the HCHO adsorption and degradation.5,22 Subsequently, the indoor HCHO adsorption test of these biomass bricks was performed in the darkness, in which laminate flooring was chosen as the source of HCHO, simulating the indoor environment HCHO concentration. Inspiringly, as shown in Table 1, the HCHO concentration in Table 1. HCHO Capture Capacity of Biomass Bricks entry 1 2 3 4
sample blank sample laminate flooring laminate flooring + brick T8718 laminate flooring + brick T8719
absorbance
HCHO concentration in distilled water (mg/L)
0.013 1.111 0.281
0 13.4 3.3
0.291
3.4
Figure 2. Characterization by scanning electron micrography. (a) Brick powder; (b) the surface of biomass brick; (c, d) brick surface after HCHO adsorption; (e, f) brick surface after exposure to sunshine.
distilled water decreases significantly from 13.4 mg/L to 3.3 and 3.4 mg/L in 3 days. This indicates that approximately 75% of HCHO released from laminate flooring can be captured or transformed by biomass bricks. Of particular note is that the HCHO concentration in distilled water is a relative value, not the real HCHO concentration in air. This value is employed to estimate that the indoor HCHO concentration is relatively safe or harmful by comparing with the limiting value of the national
ascribed to the generation of calcification. Under the catalysis of Ca(OH)2, some ether bonds are broken and lignin is partially depolymerized during the hot air drying process of molding brick.13 It facilitates the generation of calcification, which adheres to fiber and Ca(OH)2 to provide organic/ inorganic composites. Figure 2b shows a smooth brick surface 11495
DOI: 10.1021/acssuschemeng.9b01349 ACS Sustainable Chem. Eng. 2019, 7, 11493−11499
Research Article
ACS Sustainable Chemistry & Engineering with relatively few fibers, suggesting that most fibers are wrapped inside brick, providing a smooth and compact surface. Figure S11 shows the micrographs of the fracture surface. A lot of fibers are embedded in the Ca(OH)2 matrix as shown in Figure S11. It is difficult for fibers to be pulled out from the matrix. Therefore, the embedded structure and calcification provide a strong fiber−matrix bonding. Fibers perform good connection and support function, significantly improving mechanical strength of these biomass bricks. As shown in Figure S11, the majority of disordered pores inside brick are around 1−5 μm in diameter, and a relatively good surface area (21.56 m2/g) is obtained, thereby facilitating the contact and interaction among Ca(OH)2, fibers, and HCHO. These pores are unhindered, allowing gas vapor (HCHO) to diffuse into the brick. As illustrated in Figures S14 and S15, yellow, green, blue, and red colors represent C, O, Ca, and Ti atoms, respectively. The EDS elemental mapping images clearly show that Ti species (in the form of nano-TiO2), Ca species (in the form of Ca(OH)2 and CaCO3), and O species (with a composition of Ca(OH)2, CaCO3, moisture, and carbohydrate in fiber) spread over both the smooth surface and the fractured one. The C species exist in both fibers and CaCO3. On the smooth surface of brick, the presence of C species indicates that CaCO3 is distributed on the surface, providing better strength and good water resistance.12 Interestingly, after the HCHO adsorption (Figure 2c,d) and exposure to sunlight (Figure 2e, f), the brick surface is roughened. The water content decreases significantly, probably because of volatilization. Compared with the smooth surface of raw bricks, lots of pores are generated on the surface, facilitating gas (such as HCHO) to further diffuse and be consumed inside the brick. XRD Characterization Test. Figure 3 clearly shows the XRD spectra of surface powder of raw bricks and bricks after HCHO adsorption/exposure to sunshine. The diffraction peaks (2θ = 18.0°, 28.7°, 34.1°, 47.1°, 50.9°, 54.4°, 59.4°, 62.6°, 64.3°, 71.8°, 84.8°) with the largest intensity in most of the prepared bricks are assigned to portlandite (JCPDS no. 441481). Peaks at 2θ = 22.8°, 25.4° belong to amorphous TiO2
and anatase, respectively, indicating that some amorphous nano-TiO2 is transformed to anatase during press molding and hot air drying of bricks.23,24 In addition, peaks at 2θ = 23.0°, 29.4°, 39.4°, 43.1°, and 48.6°, corresponding to (012), (104), (113), (202), and (116), indicate the existence of calcite (CaCO3, JCPDS no. 47-1743) in brick. The disappearance of peaks of cellulose (Figure 3 vs Figure S16) proves that the surface of fiber is partially destroyed by alkaline Ca(OH)2 and that calcification appears on the exposed cellulose surfaces. Notably, after HCHO adsorption, although no obvious changes are obtained in the XRD spectrum, the appearance of new small diffraction peaks at 2θ = 14.9°, 16.2° corresponding to calcium formate (JCPDS no. 26-0908) indicates the generation of calcium formate from Ca(OH)2 and the absorbed HCHO through intramolecular Cannizzaro reaction. In other words, absorbed HCHO has partially converted to calcium formate and methanol, suggesting good HCHO capture capacity and transformation ability of these biomass bricks. After further exposure of these biomass bricks to sunshine, peaks belonging to calcium formate are significantly weakened (Figure 3c), suggesting the catalytic oxidation of formate with the aid of nano-TiO2 to generate nontoxic CO2 and H2O. The released CO2 will probably be captured by Ca(OH)2 to generate CaCO3 inside the brick. IR Characterization Test. The IR spectra of raw brick and bricks after HCHO capture/exposure to sunlight are shown in Figure 4. The peaks at 2850, 2927 cm−1 are attributed to the
Figure 4. IR spectra of raw brick (a, black), brick after HCHO adsorption (b, red), and that after exposure in sunshine (c, blue).
stretching vibration of C−H in the fibers. Peaks at 3643, 875 cm−1 are the featured ones of Ca(OH)2. Wide peaks appear in the regions of 3400−3450 and 1550−1660 cm−1, which are assigned to the stretching and bending vibrations of OH and H−O−H in the matrix (hydroxyl groups in fibers, H2O, and Ca(OH)2).25,26 Besides the porous structure and relatively high specific surface area (21.56 m2/g), the substantial surface hydroxyls also contribute to the superb HCHO adsorption performance, especially at low concentration of HCHO, leading to enhanced adsorption of HCHO through the hydrogen bonding between HCHO and hydroxyl groups. Under the strong basicity of Ca(OH)2, the absorbed HCHO will be converted to carboxylates and methanol in the presence of H2O via Cannizzaro reaction. Furthermore, moisture
Figure 3. XRD spectra of surface powder of raw brick (a, black), brick after HCHO adsorption (b, red), and that after exposure to sunshine (c, blue). 11496
DOI: 10.1021/acssuschemeng.9b01349 ACS Sustainable Chem. Eng. 2019, 7, 11493−11499
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Ca(OH)2, some ether bonds of lignin are cleaved, and lignin is partially depolymerized to be dissolved in Ca(OH)2, resulting in the generation of organic/inorganic calcification adhered to the fiber surface. A new peak at δ = 55.5 ppm, mainly ascribed to hydroxymethyl moieties, increases significantly, suggesting that HCHO is captured and subsequently reacts with the exposed phenol moieties through hydroxymethylation reaction. Notably, new peaks at δ = 168.4, 182.3 ppm, corresponding to the generated carboxylates, increase obviously. Furthermore, a slight reduction of peak at δ = 168.4 ppm is observed when the brick is exposed to sunlight, suggesting that part of the generated carboxylates, especially formate calcium, are photocatalytic degraded to neutral CO2 and H2O by neighboring nano-TiO2 under sunshine. TG and DSC Tests. TG and DSC characterizations are illustrated in Figure 6. Among the three TG curves (Figure 6I), an obvious difference (at the range of 450−550 °C) is obtained. For raw bricks, an interesting weight increment phenomenon appeared, which is ascribed to the reaction between the thermally generated CaO and CO2 (CO2 is released during the process of thermal degradation of fiber, and CaO is generated from the thermal decomposition of Ca(OH)2).13 However, after the HCHO adsorption test, carboxylates are formed and partial depolymerization of lignin occurs, which results in pyrolysis taking place in advance to release CO2 before the thermal generation of CaO. Therefore, no similar weight-increment results are obtained for bricks of the HCHO adsorption test (Figure 6b) and those exposed to sunshine (Figure 6c). For raw bricks, the strong endothermic peak at 75.0 °C corresponds to the volatilization of free H2O in bricks. After the HCHO adsorption test, no obvious endothermic peak below 100 °C is found, indicating that free water is significantly reduced, which is agreement with that of the IR spectrum. At 156.0 °C, the second obvious endothermic peak appears, which corresponds to the pyrolysis process of fiber.13,30 Compared to those of raw fibers (Figure S20) and raw bricks (Figure 6a), this endothermic peak (Figure 6b) occurs in advance.13,34 This phenomenon is ascribed to the partial depolymerization of lignin assisted by Ca(OH)2. Furthermore, a sharp peak at 312.5 °C appears. This peak can probably be attributed to the thermal decomposition of calcium formate in the presence of Ca(OH)2, giving further evidence of the Cannizzaro reaction of HCHO in bricks. After further exposure to sunlight, the second endothermic peak takes place in advance (from 156.0 to 126.5 °C), indicating that lignins in fiber are further degraded under photocatalytic oxidation of TiO2.35,36 The sharp peak at 312.5 °C disappears, indicating that calcium formate is photocatalytic oxidized by neighboring nano-TiO2. These results are consistent with those obtained from XRD, IR, and NMR experiments. On the basis of these valuable results, it can be concluded that the HCHO is captured and converted by biomass brick, providing a safer indoor environment. Therefore, based on these results and previous reports, a possible mechanism about the capture and conversion of HCHO by biomass bricks is proposed (Scheme 2). As shown in Scheme 2, Ca(OH)2, waste poplar wood fiber, corn stalk fiber, and nano-TiO2 are molded together with an optimized ratio to provide these biomass bricks. During the hot air drying process of molding brick, calcifications are generated with the aid of Ca(OH)2 and adhered to fiber and Ca(OH)2 closely to provide a strong fiber−matrix bonding. Fibers perform good connection and support function, and disordered pores are
facilitates the generation of active OH-free radicals, improving the photocatalytic activity of nano-TiO2 for further conversion to safe materials.22,27−29 After HCHO adsorption, peaks at 3400−3450 cm−1 and 1550−1660 cm−1 are weakened, which is ascribed to the volatilization of H2O. Although calcium formate presents a relatively strong absorption at 1589, 1630 cm−1, no obvious new peaks in this region are observed in Figure 4, probably due to the low concentration of the generated calcium formate. The peak at 1408 cm−1, belonging to the C−H bending vibration, agrees well with those of fibers. Peaks at 950−1250 cm−1, belonging to vibration of the C−O bond,25,26 weakened and broadened. This suggests that some C−O bonds (such as the ether bond of lignin) are broken and new C−O bonds (such as in hydroxymethyl moieties from hydroxymethylation reaction) are generated. Interestingly, peaks in the region of 1450−1500 cm−1, corresponding to the C−H bending vibration and skeletal vibration of an aromatic ring, increase significantly. With aid from HCHO30 and alkaline Ca(OH)2,31−33 lignin can be partially depolymerized. More phenol moieties will be exposed and adjacent to the absorbed HCHO, facilitating the occurrence of hydroxymethylation reaction between phenol moieties of lignin and HCHO.30 The generated hydroxymethyl moieties can form complexes with Ca(OH)2 as adhesive materials, improving the strength of bricks. No peaks at about 1700 cm−1 are observed, suggesting that carboxylates rather other carboxylic acids are generated from Cannizzaro reaction. After subsequent exposure to sunlight, compared with that after HCHO adsorption, no obvious change is obtained, which is probably ascribed to the low concentration of generated carboxylates after HCHO adsorption. 13 C SSNMR Characterization Test. The 13C SSNMR spectra of raw brick, brick of HCHO adsorption test, and that exposed to sunshine are depicted in Figure 5. After the HCHO adsorption process, peaks at δ = 23.9, 32.2, 46.0 ppm, mainly indexed to the carbon atoms of side alkyl chains in lignin, enhance significantly. New peaks at δ = 152.0, 158.8, 120−150 ppm appear obviously, which correspond to aryl carbon and the double bond in the lignin structure. The increase of these peaks indicates that under assistance from HCHO and
Figure 5. 13C SSNMR spectra of raw brick (a, black), brick after HCHO adsorption (b, red), and that after exposure to sunshine (c, blue). 11497
DOI: 10.1021/acssuschemeng.9b01349 ACS Sustainable Chem. Eng. 2019, 7, 11493−11499
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Figure 6. TG (I) and DSC (II) characterizations of raw brick (a, black), brick of HCHO adsorption test (b, red), and that exposed to sunshine (c, blue).
Tables S1 and S2, Figures S1−S20, and test report of HCHO adsorption provided by Research Institute of Wood Industry, Chinese Academy of Forestry (PDF)
Scheme 2. Mechanism of HCHO Capture and Conversion by Biomass Bricks
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Shuai Zhang: 0000-0001-7320-9081 Zeguang Lu: 0000-0002-8543-2317 Author Contributions
S. Zhang performed the relative tests, analyzed the data, and wrote the paper; M. Wang discussed the idea and modified the paper, Z. Lu put forward the idea, designed the research, and wrote the paper, and C. Ma and W. Jia manufactured the bricks and performed the relative mechanical tests. All authors have given approval to the final version of the manuscript. Funding
generated, allowing indoor HCHO to diffuse and be captured. In the presence of Ca(OH)2, the captured HCHO converts to less-toxic carboxylates, methanol, and hydroxymethyl moieties through disproportionation reaction and hydroxymethylation reaction. Subsequently, these generated intermediates can be transformed into CO2 and H2O and so on by catalytic oxidation of adjacent TiO2 under sunlight, providing a safer indoor environment.
We thank the Project of Special Fund for Forest Scientific Research in the Public Welfare, no. 201504506, China. Engineer Eriyang in Shanghai Chenmai Technology Co., Ltd., is thanked for his help with the relative tests. Thanks to Yuejin Fu and Xianwu Zou in Research Institute of Wood Industry, Chinese Academy of Forestry, for their assistance on the HCHO adsorption experiments.
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Notes
The authors declare no competing financial interest.
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CONCLUSIONS We have thus provided a new kind of biomass brick as a promising material for indoor HCHO removal. These functionalized Ca(OH)2/TiO2-based biomass bricks are proved to possess both excellent HCHO capture capacity and conversion efficiency and transform the captured HCHO to relatively safe materials, controlling the contamination from indoor HCHO. These biomass bricks not only meet the construction needs but also simultaneously provide an environmentally friendly and healthy residence environment for high-quality human life.
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REFERENCES
(1) Yu, J.; Li, X.; Xu, Z.; Xiao, W. NaOH-modified ceramic honeycomb with enhanced formaldehyde adsorption and removal performance. Environ. Sci. Technol. 2013, 47, 9928−9933. (2) Luo, B.; Tang, H.; Cheng, Z.; Ji, Y.; Cui, X.; Shi, Y.; Wang, B. Detecting the photoactivity of anatase TiO2(001)-(1×4) Surface by Formaldehyde. J. Phys. Chem. C 2017, 121, 17289−17296. (3) Xia, Y.; Zhu, K.; Kaspar, T. C.; Du, Y.; Birmingham, B.; Park, K. T.; Zhang, Z. J. Atomic structure of the anatase TiO2(001) surface. J. Phys. Chem. Lett. 2013, 4, 2958−2963. (4) Kim, M.; Park, E.; Jurng, J. Oxidation of gaseous formaldehyde with ozone over MnOx/TiO2 catalysts at room temperature (25 °C). Powder Technol. 2018, 325, 368−372. (5) Sun, X.; Lin, J.; Guan, H.; Li, L.; Sun, L.; Wang, Y.; Miao, S.; Su, Y.; Wang, X. Complete oxidation of formaldehyde over TiO2 supported subnanometer Rh catalyst at ambient temperature. Appl. Catal., B 2018, 226, 575−584.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b01349. 11498
DOI: 10.1021/acssuschemeng.9b01349 ACS Sustainable Chem. Eng. 2019, 7, 11493−11499
Research Article
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DOI: 10.1021/acssuschemeng.9b01349 ACS Sustainable Chem. Eng. 2019, 7, 11493−11499
