Simple Milligram-Scale Extraction of Formaldehyde from Wood

Abstract. Abstract Image. Lignocellulose naturally contains formaldehyde, and generates much more when heated. A simple quantitation of such biogenic ...
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Research Article pubs.acs.org/journal/ascecg

Simple Milligram-Scale Extraction of Formaldehyde from Wood Mohammad Tasooji†,‡ and Charles E. Frazier*,†,‡ †

Sustainable Biomaterials, Virginia Tech, Cheatham Hall, RM 230, 310 West Campus Drive, Blacksburg, Virginia 24061, United States ‡ Macromolecules Innovation Institute, Virginia Tech, 1075 Life Science Circle, Suite 110 (0201), Blacksburg, Virginia 24061, United States ABSTRACT: Lignocellulose naturally contains formaldehyde, and generates much more when heated. A simple quantitation of such biogenic formaldehyde is desirable for the analysis and utilization of lignocellulose. Heretofore, a boiling toluene extraction (the perforator method) was the best known technique to determine biogenic formaldehyde in wood. Described here is a simple milligram-scale water extraction that avoids specimen heating. This method was validated by comparison to a laborious extraction using poly(allylamine), PAA, beads that strongly sorb formaldehyde. The PAA-based extraction and the water-only extraction were found to be effectively equivalent, recovering about 94% of wood formaldehyde. The incomplete formaldehyde recovery is offset by experimental simplicity, and suitability for large sampling. For instance, the new method was applied to the analysis of tree increment cores; formaldehyde levels measured in never-heated Pinus virginiana ranged from 1 to 5 μg/g dry wood, and were comparable to published values using the perforator method. Heating at 200° for 10 min generated about 10−20 times more biogenic formaldehyde. This simple extraction is useful to document biogenic formaldehyde levels in wood, and the formaldehyde generation potential associated with heating, as in the manufacture of woodbased composites. KEYWORDS: Lignocellulose, Wood, Biogenic Formaldehyde, Emissions, Regulations

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INTRODUCTION As biomass utilization advances, it will be useful to have a simple determination of natural, biogenic formaldehyde that is produced and retained within lignocellulose. For instance, it is well established that wood contains biogenic formaldehyde and that heating generates far greater quantities.1−6 Indications are that lignin could be the principal source of lignocellulosic formaldehyde.5 Consequently, a simplified formaldehyde quantitation might offer new insights on lignin transformations that impact biomass utilization. In the meantime, a current practical application concerns regulations of allowable formaldehyde emissions from nonstructural wood-based composites. Global trends have shown steady reductions in allowable formaldehyde emissions from these consumer products. The intended regulatory target has been hydrolytically unstable amino resins such as urea−formaldehyde. However, current regulations restrict allowable emissions to such low levels that biogenic formaldehyde may impact regulation compliance. Our industry interactions indicate that the latest formaldehyde emissions regulations have been met with new amino resin technologies. Nevertheless there are persistent anecdotal reports of complications postulated to be attributable to biogenic formaldehyde. As part of an industry/university cooperation, this effort seeks to achieve a more thorough accounting of all formaldehyde sources in nonstructural woodbased composites, both synthetic and biogenic. Furthermore, we wish to understand more about the chemical transformations that occur during composite hot-pressing, when large quantities of biogenic formaldehyde are produced at temperatures ranging from 150 to 200 °C. After hot-pressing, © XXXX American Chemical Society

total panel emissions decline with time, suggesting that biogenic formaldehyde dissipates, perhaps completely.1,4 In analogy with paper and cellulose,7 we could expect formaldehyde to exhibit great solubility in wood, where the subsequent release is controlled by pH, temperature, moisture content, and reactions producing hemiformals and formals. It was shown that dry wood retains formaldehyde,8 and that increasing relative humidity progressively accelerates release. This same work suggested that formaldehyde may be rapidly and completely removed from wood through simple water extraction.8 This was of interest because of our desire to thoroughly document the occurrence and formation of biogenic formaldehyde in the living tree, and forward through subsequent processing. Because thorough documentation will require a great deal of sampling and analysis, a simplified, milligram-scale, water extraction was preferred so that living tress could be sampled nondestructively using a common tree increment bore. The only comparable formaldehyde extraction is the well-known perforator method that uses 100 g specimens in boiling toluene.9 For our purposes, the perforator method is undesirable because of the large specimen size, and that the specimen is heated to 110 °C in boiling toluene, a treatment that likely generates some formaldehyde. Given the affinity and reactivity between wood and formaldehyde, the intention was to develop a simple water extraction for nearly complete removal of free formaldehyde, Received: June 28, 2016 Revised: July 25, 2016

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DOI: 10.1021/acssuschemeng.6b01477 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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times sequentially. H2SO4 (15 mL, 0.1M) was added to the beads, and the solution was stirred under dry N2 (25 °C for 60 min for first washing and 30 min for washings 2−4). After each washing, the supernatant (15 mL) was removed by syringe filtration and analyzed by fluorimetric acetylacetone detection. The total desorbed formaldehyde mass was calculated as the sum of formaldehyde determined from all four acid washings. Corresponding blank specimens were treated similarly using HPLC water instead of the formaldehyde standard; measured fluorescence was subtracted from that of the experimental specimen. Biogenic Formaldehyde Analysis in Pine Flakes. Wood flakes were dried by cycling between vacuum (0.15 mmHg) and dry N2 gas, for three times; thereafter, flakes were stored over anhydrous P2O5 and N2 for 48 h prior to use. From 2 to 3 dry flakes, small pieces (∼2.5 × 5 mm) were cut and randomized (earlywood and latewood tissues were not separated); 70 mg was used for control specimens (never-heated), 5 mg for heated specimens (200 °C, 60 min, then cooled to room temperature for 15 min). Step 1: flakes were sealed in serum bottles, which were subjected to vacuum (0.2 mmHg, 1 min) via a needle inserted through the septum, and then (after removing the needle) subjected to treatment (control or heating). Step 2: via syringe and without releasing the serum bottle vacuum, HPLC water (15 mL) was added to solvate formaldehyde (1 h, room temp, 25 °C), and occasional shaking was imposed to wet the total interior surface. Step 3: serum bottle vacuum was released to N2 (1 atm). Step 4: supernatant (10 mL) was removed by syringe filtration; 4 mL was analyzed by fluorimetric acetylacetone detection. Corresponding blanks (control and heated) were prepared similarly with no wood added; measured fluorescence was subtracted from that of the experimental specimens. When PAA beads were used, the following steps occurred after step 3. Step 3-1 (sorption): PAA beads (35 mg, free amine form) and magnetic stir bar were added; serum bottle resealed under dry N2 and stirred (1 h, 25 °C). Step 3-2: flakes were removed by tweezer, taking care to dislodge any attached beads. Step 3-3 (desorption): The formaldehyde/PAA adduct was acid washed four times sequentially: H2SO4 (15 mL, 0.1M) added to the beads; stirred under dry N2 (25 °C, 60 min for first washing; 30 min for washings 2−4). After each washing, supernatant (15 mL) was removed by syringe filtration; 4 mL was analyzed by fluorimetric acetylacetone detection. The total desorbed formaldehyde mass was calculated as the sum of formaldehyde determined from all four acid washings. Corresponding blanks (35 mg beads, 15 mL water) were subjected to the same treatments described; measured fluorescence was subtracted from that of the experimental specimen. Biogenic Formaldehyde Analysis in Pine Increment Cores. Specimen preparation: From an 80 year old Pinus virginiana, six cores were sampled at ∼1.4 m above ground, from an area of approximately 10 cm2, using a 5 mm tree increment bore; the cores were sealed in plastic straws. Juvenile (first 10 rings) and mature (last 40 rings) tissues were separated, razor cut into small disks (∼1 mm thickness, 5 mm diameter), and specimens within tissue types were randomized (earlywood and latewood tissues were not separated). At harvest, the moisture contents of the juvenile and mature tissues were 14% and 40%, respectively. Specimens were dried by cycling between vacuum (0.15 mmHg) and dry N2 gas, for three times, stored over molecular sieves and N2 for 48 h then sealed prior to use. Never-dried specimens were sealed and placed in a second container (purged with N2) and stored at −18 °C. Prior to use, cold-storage sample vials were warmed to room temperature by placing in a water bath. Biogenic formaldehyde determination: Wood specimens (∼50 mg when never heated, ∼10 mg when heated, based on dry mass) were placed in a serum bottle and sealed after purging with N2. All samples were subjected to either heat treatment (200 °C, 10 min) or no heating. After cooling to room temperature, HPLC water (15 mL) was added via syringe, (1 h, 25 °C). Serum bottle pressure was released to N2 (1 atm) and 4 mL was sampled and analyzed by fluorimetric acetylacetone detection.

defined as adsorbed formaldehyde, methylene glycol, and hemiformal. The efficacy of water extraction was determined by comparison to a laborious method using beads prepared from poly(allylamine), PAA, which are known for outstanding efficiency in formaldehyde sorption.10,11 PAA beads contain primary amines that form hemiaminals with equilibrium constants about 2 times greater than that measured for hemiformals.12,13 Described here is the use of PAA beads for validating a milligram-scale, simple extraction and quantitation of free formaldehyde from solid wood.

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MATERIALS AND METHODS

A 70 year old Pinus virginiana tree was harvested locally, and mature tissue was flaked into 85 × 75 × 0.7 mm (RxLxT) specimens. High purity reagents were obtained from commercial sources and used as received. Poly(allylamine hydrochloride) was obtained from Alfa Aesar. Deionized water was generated using a Millipore Milli-Q D3UV; otherwise, HPLC-grade water was used as received from Fisher Scientific. Precleaned vials (40 mL, PTFE septa, polypropylene cap) and serum bottles (50 mL, silicone septa, aluminum seal) were purchased from Sigma-Aldrich. Test tubes (Kimax, rubber-lined screw cap, O.D. × L: 16 × 125 mm), glass shell vials (O.D. × H: 12 mm × 35 mm, plastic stopper), and 0.45 μm PTFE syringe filters were obtained from Fisher scientific. Formaldehyde Analysis. Formaldehyde standard solutions were prepared daily using sodium sulfite standardization.14 Acetylacetone reagent was prepared weekly and stored refrigerated in amber bottles.15 When sampling, all liquid specimens were drawn through 0.45 μm PTFE syringe filters, producing 4 mL specimens that were reacted with the acetylacetone reagent (4 mL) in sealed tubes (60 °C, 10 min). Specimens were cooled to room temperature and analyzed fluorimetrically,16 (PerkinElmer LS55, 30 °C, excitation: 410 nm, emission: 510 nm, scan range: 490−530 nm, scan speed: 200 nm/min, excitation and emission slit width 10 nm; the final fluorimetric intensity was averaged between 505 and 515 nm). During this process all vials/tubes were prerinsed with the respective solutions to improve analytical reproducibility. The fluorimetric calibration curve was established in the low concentration, linear response region,16 with a lower quantitation limit of approximately 2.5 ng/mL. Preparation of Poly(allylamine), PAA, Beads. Poly(allylamine hydrochloride) (20 g, 0.215 mol primary amine) was mixed with sodium hydroxide (9 g, 0.22 mol) in HPLC water (500 mL). Epichlorohydrin (8 g, 86 mmol) and toluene (150 mL) were added to the PAA solution (prechilled by overnight refrigeration), and the mixture was stirred under dry N2 (200 rpm, 3 h, 60 °C). The mixture was cooled to room temperature, toluene decanted, and the crosslinked beads centrifuged (500 rpm, 3 min)/washed with isopropyl alcohol (50 mL × 5) and deionized water (50 mL × 5). The theoretical functionality of the cross-linked beads was 10.75 mmol of total free amine/g, and 2.1 mmol of primary amine/g. To remove all adventitiously sorbed formaldehyde, the beads were added to H2SO4 (750 mL, 2M), and the solution was stirred under dry N2 (700 rpm, 4 h, 70 °C), centrifuged (5000 rpm, 3 min)/washed using deionized water (50 mL, × 5), vacuum-dried (0.15 mmHg) overnight, and stored refrigerated. The final PAA bead yield was ∼54% (free amine form). Prior to use, PAA beads (∼2 g) were converted to the free amine form: mixed with NaOH (100 mL, 3 M), stirred under dry N2 (1 h, 25 °C), centrifuged/washed (500 rpm, 3 min) with deionized water (50 mL × 5), vacuum-dried (0.15 mmHg) overnight, and transferred to a precleaned vial. Beads (5 or 35 mg) were accurately weighed and transferred to sealed glass shell vials and used as described below. PAA Bead Sorption/Desorption of Formaldehyde Standard Solutions. (1) Sorption: PAA beads (5 mg, free amine form) and a magnetic stir bar were added to standard formaldehyde solutions (15 mL, 0.1 μg/mL); primary amine/CH2O mole ratio = 200. Specimens stirred under dry N2 (25 °C, 2 h); 4 mL specimen removed by syringe filtration and analyzed using fluorimetric acetylacetone detection. (2) Desorption: The formaldehyde/PAA adduct was acid washed four B

DOI: 10.1021/acssuschemeng.6b01477 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Table 1. Formaldehyde Desorption Data for PAA Beads Treated with Formaldehyde Standard Solutionsa acid-wash (desorption)

specimen

fluorimetric intensity

CH2O recovered μg

sum μg

CH2O recovery %

1

blank experimental blank experimental blank experimental blank experimental

41.55 (1.26) 101.03 (1.54) 46.61 (2.20) 81.49 (6.5) 39.31 (0.84) 63.31(3.98) 37.83 (0.72) 49.73 (2.58)

0.59 (0.02)

1.40 (0.04)

94.06 (2.74)

2 3 4

0.34 (0.07) 0.23 (0.04) 0.23 (0.06)b

a

Primary NH2/CH2O mole ratio = 200, determined from four sequential acid-washes; (standard deviation, with n = 3). bbased upon 30 mL volume; steps 1−3 based upon 15 mL volume.

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RESULTS AND DISCUSSION Formaldehyde determination using the acetylacetone fluorimetric detection was selected because it is simple and equally sensitive to the dinitrophenylhydrazine method using liquid chromatography.17 Not only is fluorescence detection very sensitive, but it is also highly formaldehyde-selective because acetylacetone derivatives of higher aldehydes exhibit very weak fluorescence.18 Cross-linked poly(allylamine), PAA, beads were prepared and tested for efficiency in formaldehyde sorption/ desorption by exposure to standardized formaldehyde solutions. The sorption/desorption procedures described in the literature employed elevated temperatures for brief periods (60−70 °C for 10−15 min).11 However, in this work, room temperature conditions over longer sorption/desorption periods resulted in less data variation. It was found that PAA beads sorbed very nearly all formaldehyde from standard solutions; sorption ranged from 98 to 100% when the primary NH2/CH2O mole ratio was varied from 10 to 1000 (data not shown). Likewise, formaldehyde recovery through acid desorption was less efficient because of the strong afinity between PAA and formaldehyde. Table 1 shows an example of formaldehyde desorption via acid washing conducted in four sequential steps, where the cumulative formaldehyde recovery was 94%. As expected PAA beads were so highly active that carefully prepared blank specimens (beads with no added formaldehyde) were essential for accurate quantitation. Notice for instance that the fluorimetric intensity of the blank specimens declined after the third and fourth acid washings. In other words, formaldehyde-free beads were never produced. Consequently, experimental and blank specimens required careful and simultaneous preparation from a common and fresh PAA bead sample. In this case, the low formaldehyde recovery of 94% was probably caused by incomplete desorption; notice in Table 1 that the fluorimetric intensity of the experimental specimens did not level off after 4 desorption cycles. Furthermore, instead of risking potential errors associated with sequential quantitation at each desorption stage, a higher recovery is probable by simply combining separate acid washes for one direct quantitation. In any case, a suitably efficient formaldehyde extraction using PAA beads was established and subsequently employed to evaluate the efficiency of formaldehyde recovery using a simple water extraction without PAA beads. All solid wood extractions were conducted with thinly sliced wood specimens ranging from 5−70 mg, in 15 mL of water, all within 50 mL serum bottles. The serum bottle seals very effectively and provides a simple way to heat specimens and retain volatiles prior to extraction. Figure 1 shows the formaldehyde recovery from wood specimens using PAA

Figure 1. Average formaldehyde recovery from Virginia pine flake specimens using water extraction and extraction with PAA beads, for never-heated (control) and heat-treated (200 °C, 60 min) specimens. Error bars = ±1 standard deviation; n = 3.

bead extraction as compared to simple water extraction. Pine specimens heated at 200 °C for 60 min generated formaldehyde at levels about 40 times greater than observed in never-heated control specimens. The formaldehyde recovery was independent of the extraction method for heated specimens. Surprisingly, for never-heated controls, simple water extraction recovered more formaldehyde than PAA extraction. This is explained by the stoichiometries employed; the NH2/CH2O mole ratio during PAA extraction was about 1000 for heated specimens, and about 3200 for never-heated control specimens. In the latter case it was noted that the PAA beads turned yellow, which is a characteristic sign of imine formation.19,20 This characteristic imine color was otherwise never observed when the NH2/CH2O mole ratio was 1000 or less. Imines are more resistant to acid hydrolysis, whereas hemiaminals are readily hydrolyzed during the room temperature acid washing used here. Consequently, formaldehyde recovery from the never-heated control specimens was deemed incomplete due to imine formation, and that the imines were not hydrolyzed during room temperature acid washing. In spite of the complications caused by imine formation, the results indicate that the PAA beads effectively stripped formaldehyde away from the wood specimens, as expected. Given that the two extraction methods were effectively equivalent, we conclude that simple water extraction of small wood specimens is effective for the nearly complete removal of free formaldehyde. This result is consistent with previous findings for wood specimens that were artificially impregnated with formaldehyde. Using the perforator method as an analytical reference, Meyers showed that dry wood retains formaldehyde and that water-saturated wood rapidly releases C

DOI: 10.1021/acssuschemeng.6b01477 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering nearly all formaldehyde.8 Meyers observation suggests that formaldehyde is bound as the hemiformal when wood moisture is low, and that increasing moisture hydrolyzes hemiformals into methylene glycol units that are readily extractable. Results from the PAA method support this hypothesis while demonstrating that 5−70 mg wood specimens are easily analyzed using water extraction. Formaldehyde present as formal, or that might substitute into lignin aromatic rings, is not water extractable under the conditions employed here. Only free formaldehyde, that which is simply adsorbed or loosely bound as hemiformal, is easily water extractable. Considering the latter case, a crude estimate of the hydroxyl equivalent weight in dry wood is about 93 g/mol (assuming inaccesible crystalline cellulose at 50% crystallinity); so the extraction employed here achieves a water/wood-hydroxyl mole ratio that exceeds 1000. No systematic effort was made to study formaldehyde recovery as a function of the water/wood mass ratio, but we find that a range of about 200−300 is the minimum in a water/wood mass ratio that varied from about 200−3000 in this work. This also depends upon the lower quantitation limit (2.5 ng/mL in this case) and the formaldehyde content of the wood specimen. For instance, a 5−10 mg wood specimen is reliably analyzed when heated; whereas the minimum mass for never-heated specimens is 50−70 mg because the corresponding formaldehyde level is so low. In any event, it is clear that a simple, single-stage, water extraction cannot remove all free formaldehyde from wood. Because the PAA extraction resulted in 94% formaldehyde recovery, the simple water extraction must be comparably efficient. We suggest that a recovery of ∼94% formaldehyde is acceptable when the objective is to achieve a simple, nondestructive, milligram-scale method that is amenable to sampling many living trees. Having established the simplified water extraction method, biogenic formaldehyde was measured in sections from 6 tree increment cores sampled from one Virginia pine tree. Figure 2

juvenile wood. The same tissue dependency was also reported by Weigl et al.6 Curiously, Figure 2 indicates that room temperature, vacuum drying resulted in higher levels of formaldehyde, in cores 2−6, as compared to never-dried specimens from core 1. Although Figure 2 presents too little data to comment reliably, this surprising result was also reported by Meyer and Boehme.2 Furthermore, we confirmed this effect in larger data sets that will be reported elsewhere. When wood drying resulted in greater formaldehyde concentrations, Meyer and Boehme2 suggested that formaldehyde generation must have occurred even after specimen drying (at 30 °C). This hypothesis cannot now be refuted, but it seems questionable since elevated temperatures are required to form biogenic formaldehyde over short periods. At present, we offer no explanation for this observation. Among the dried cores 2−6, the formaldehyde content varied slightly between cores and also within cores. Note that earlywood and latewood tissues were not isolated, nor were they randomized and this might explain the variations observed. Figure 3 shows that heating (200 °C, 10 min) increased the formaldehyde content by a factor of about 15−20 in never-

Figure 3. Average biogenic formaldehyde levels in heated (200 °C, 10 min) specimens sectioned from Virginia pine increment cores (1−6), as a function of tissue type (juvenile and mature) and drying history (never-dried and dried). Error bars = ±1 standard deviation; n = 3−4.

dried specimens taken from core 1, and by a factor of about 3− 10 in dried specimens taken from cores 2−6. Furthermore, heat-generated formaldehyde levels varied between cores 2−6, and at times susbstantially within cores. Within-core variations in formaldehyde generation might be related to wood anatomical variations that were not randomized during sectioning of the increment cores. For instance, resin canals (epithelial parenchyma) are sufficiently dispersed that they could be excluded or included in the core sections used here (∼1 mm thick in the radial direction, 5 mm dia.). Whereas earlywood/latewood effects (if they occur) would be more likely to undergo randomization with less or perhaps no detectable impact. One could determine earlywood/latewood effects by using more precise sectioning techniques that were not used in this study.

Figure 2. Average biogenic formaldehyde levels in never-heated specimens sectioned from Virginia pine increment cores (1−6), as a function of tissue type (juvenile and mature) and drying history (never-dried and dried). Error bars = ±1 standard deviation; n = 3−4.

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shows that formaldehyde levels in never-heated specimens ranged from about 1−5 μg/g of dry wood. These levels are quite comparable to those found by Meyer and Boehme2 and also by Weigl et al.6 when using the perforator (boiling toluene) extraction on pine specimens. Figure 2 shows that never-heated specimens exhibited a tissue effect where it was seen that mature wood had a higher formaldehyde content than

CONCLUSIONS Because lignocellulose naturally contains formaldehyde, and generates much more when heated, a simple quantitation of this biogenic formaldehyde is considered desirable for the analysis of lignocellulose. Heretofore, the perforator method was the best known technique, but this procedure exposes 100 D

DOI: 10.1021/acssuschemeng.6b01477 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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(5) Schäfer, M.; Roffael, E. On the formaldehyde release of wood. Holz als Roh- und Werkstoff 2000, 58, 259−264. (6) Weigl, M.; Wimmer, R.; Sykacek, E.; Steinwender, M. Woodborne formaldehyde varying with species, wood grade, and cambial age. Forest Prod J. 2009, 59 (1−2), 88−92. (7) Hennebert, P. Solubility and diffusion coefficients of gaseous formaldehyde in polymers. Biomaterials 1988, 9 (2), 162−7. (8) Myers, G. E. Mechanisms of Formaldehyde Release from Bonded Wood Products 1986, 316, 87−106. (9) ISO. Wood-based panels  Determination of formaldehyde release  Part 5: Extraction method (called the perforator method); ISO: Geneva, 2015. (10) Kiba, N.; Sun, L.; Yokose, S.; Kazue, M. T.; Suzuki, T. T. Determination of nano-molar levels of formaldehyde in drinking water using flow-injection system with immobilized formaldehyde dehydrogenase after off-line solid-phase extraction. Anal. Chim. Acta 1999, 378, 169−175. (11) Kiba, N.; Yagi, R.; Sun, L. M.; Tachibana, M.; Tani, K.; Koizumi, H.; Suzuki, T. Poly(allylamine) beads as selective sorbent for preconcentration of formaldehyde and acetaldehyde in high-performance liquid chromatographic analysis. J. Chromatogr A 2000, 886 (1− 2), 83−87. (12) Guthrie, J. P. Carbonyl addition reactions. Factors affecting the hydrate-hemiacetal and hemiacetal-acetal equilibrium constants. Can. J. Chem. 1975, 53 (6), 898−905. (13) Kallen, R. G.; Jencks, W. P. Equilibria for the reaction of amines with formaldehyde and protons in aqueous solution. A re-examination of the formol titration. J. Biol. Chem. 1966, 241 (24), 58645−5878. (14) Walker, J. F. Formaldehyde; Kriger Publishing Co.: Huntington, NY, 1975. (15) Nash, T. The colorimetric estimation of formaldehyde by means of the Hantzsch reaction. Biochem. J. 1953, 55 (3), 416−21. (16) Belman, S. The fluorimetric determination of formaldehyde. Anal. Chim. Acta 1963, 29 (2), 120−126. (17) Salthammer, T.; Mentese, S. Comparison of analytical techniques for the determination of aldehydes in test chambers. Chemosphere 2008, 73 (8), 1351−1356. (18) Vairavamurthy, A.; Roberts, J. M.; Newman, L. Methods for determination of low molecular weight carbonyl compounds in the atmosphere: a review. Atmos. Environ., Part A 1992, 26 (11), 1965− 1993. (19) Bandi, S.; Mehta, S.; Schiraldi, D. A. The mechanism of color generation in poly (ethylene terephthalate)/polyamide blends. Polym. Degrad. Stab. 2005, 88 (2), 341−348. (20) Feng, L.; Musto, C. J.; Suslick, K. S. A simple and highly sensitive colorimetric detection method for gaseous formaldehyde. J. Am. Chem. Soc. 2010, 132 (12), 4046−4047.

g specimens to boiling toluene. We developed a simple water extraction that avoids specimen heating while using very small specimens that facilitate nondestructive sampling. The method described here was validated by comparison to a laborious extraction using poly(allylamine), PAA, beads that strongly sorb formaldehyde. PAA-based extraction recovered about 94% of the formaldehyde from wood specimens, and was effectively equivalent to a much simplified water-only extraction. For our purposes, the incomplete formaldehyde recovery is offset by the experimental simplicity, and its suitability for large sampling. For instance, the water-only extraction was applied to the analysis of increment cores sampled from a single Pinus virginiana tree. Formaldehyde levels measured in never-heated 50 mg specimens ranged from 1 to 5 μg/g dry wood and were comparable to published values using the perforator method. Heating at 200° for 10 min generated about 10−20 times more biogenic formaldehyde. The formaldehyde generation potential associated with wood heating is easily measured using 5−10 mg specimens that are conveniently heated and subsequently extracted in the same small glass container. This simple, milligram-scale extraction is useful to document biogenic formaldehyde in wood, and the potential to form heatgenerated formaldehyde, as in the manufacture of woodbased composites. The precise relationship between biogenic formaldehyde levels and actual composite emissions is currently unknown. However, a direct relationship seems reasonable, as demonstrated by Birkeland et al., who measured significant biogenic emissions from panels bonded with no-added formaldehyde adhesives.1 Hopefully the documentation of formaldehyde generation potential in different woods will help us determine if biogenic formaldehyde impacts compliance with regulations of allowable formaldehyde emissions from nonstructural wood-based composites.

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AUTHOR INFORMATION

Corresponding Author

*C. E. Frazier. E-mail: [email protected]. Phone: +1 540 231 8318. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the Wood-Based Composites center, a National Science Foundation Industry/University Cooperative Research Center (Award #1035009). Partial funding was also supplied by the Virginia Agricultural Experiment Station and the McIntire Stennis Program of the National Institute of Food and Agriculture, U.S. Department of Agriculture.

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REFERENCES

(1) Birkeland, M. J.; Lorenz, L.; Wescott, J. M.; Frihart, C. R. Determination of native (wood derived) formaldehyde by the desiccator method in particleboards generated during panel production. Holzforschung 2010, 64 (4), 429−433. (2) Meyer, B.; Boehme, C. Formaldehyde emission from solid wood. Forest Prod J. 1997, 47 (5), 45−48. (3) Roffael, E. Volatile organic compounds and formaldehyde in nature, wood and wood based panels. Holz als Roh- und Werkstoff 2006, 64 (2), 144−149. (4) Salem, M. Z. M.; Bohm, M. Understanding of formaldehyde emissions from solid wood: an overview. BioResources 2013, 8 (3), 4775−4790. E

DOI: 10.1021/acssuschemeng.6b01477 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX