Abstract
A new perspective on the effect of unconditioned indoor (especially storage areas) and outdoor environments on wood acidity is provided in this work. A comparison between the quantity and types of the organic acids formed in the unconditioned indoor environment and different outdoor environments was made. Moreover, the acidity of some wood samples due to different environmental conditions was determined using a pH meter and high-performance liquid chromatography (HPLC). Fourier transform infrared (FTIR) was used to detect the changes in wood components at the molecular level due to environmental conditions. The results suggest that the unconditioned indoor environment was more aggressive than the outdoor environment with respect to wood deterioration. The polluted atmosphere increased the wood acidity and motivated polysaccharide breakdown.
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Investigating the Impact of Weathering and Indoor Aging on Wood Acidity Using Spectroscopic Analyses
Safa Abdel-Kader Mohamed Hamed,a Mohamed Z. M. Salem,b,* Hayssam M. Ali,c,d and Kareem Mohamed El-Sayed Ahmed e
A new perspective on the effect of unconditioned indoor (especially storage areas) and outdoor environments on wood acidity is provided in this work. A comparison between the quantity and types of the organic acids formed in the unconditioned indoor environment and different outdoor environments was made. Moreover, the acidity of some wood samples due to different environmental conditions was determined using a pH meter and high-performance liquid chromatography (HPLC). Fourier transform infrared (FTIR) was used to detect the changes in wood components at the molecular level due to environmental conditions. The results suggest that the unconditioned indoor environment was more aggressive than the outdoor environment with respect to wood deterioration. The polluted atmosphere increased the wood acidity and motivated polysaccharide breakdown.
Keywords: Wood degradation; Organic acids; pH; HPLC; FTIR
Contact information: a: Conservation Department, Faculty of Archaeology, Cairo University, Giza 12613, Egypt; b: Forestry and Wood Technology Department, Faculty of Agriculture (EL-Shatby), Alexandria University, Alexandria 21545, Egypt; c: Botany and Microbiology Department, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia; d: Timber Trees Research Department, Sabahia Horticulture Research Station, Horticulture Research Institute, Agriculture Research Center, Alexandria 21526, Egypt; e: Conservator at Tell-Basta, Sharqia, Ministry of Antiquities, Egypt;
* Corresponding author: zidan_forest@yahoo.com
INTRODUCTION
Wood has been recognized worldwide as a building material that has special engineering and structural properties. It has a long history of use in Egypt because many architectural elements in historic buildings are made of wood, such as ceilings, doors, floors, domes, and mashrabiyas. In addition, movable architectural elements can be found in the museum environment as objects (Hamed 2014). However, wood is susceptible to the forces of the surrounding environment such as biological agents (Mansour et al. 2020), and climate changes (Sivrikaya and Can 2016; Oberhofnerová et al. 2017). It is exposed outdoors to the deleterious effect of weathering, which can be ascribed to a complex set of reactions induced by solar radiation (UV light), moisture, oxygen, temperature, and sometimes atmospheric gaseous pollutants (Anderson et al. 1991; Ayadi et al. 2003). The combination of oxygen and solar radiation rapidly induces the oxidation of lignin and hemicellulose and depolymerization of cellulose (Lionetto et al. 2012). Furthermore, wood undergoes further degradation in the indoor environment, especially when the wooden objects displaced away from their native setting. Because the degradation mechanisms are active during long-term exhibition and storage due to temperature, moisture, and lighting effects, a stable (controlled) environment should be provided for wooden objects (Harvey and Freedland 1990). That is, the storage conditions determine the chemical processes that may occur in wood. Thus, they have a significant role in the aging process (Fengel 1991).
It is known that almost all types of wood have an acidic behavior that ranges from weak to moderate. The source of wood acidity is the acidic wood components, i.e., the acetyl groups and uronic acid residues attached to the polyoses (xylans in hardwood and mannans in softwood) (Anderson et al. 1991). While many of the organic acids are found bound as esters, some of them exist in the form of salts, and a few of them are found as free acids (Fengel and Wegener 1984; Matteoli et al. 1992; Balaban and Uçar 2001; Yaşar 2018).
Wood has several weak acids or acidic groups, but the most acidic ones control its acidity (Uçar and Uçar 2012). Furthermore, there is no correlation between wood acidity and the content of volatile acids (Balaban and Uçar 2003).
Most of the studies investigating the acidic behavior of wood address volatile organic acids, basically formic and acetic, and they have neglected the others that increase the acidity, notably those resulting from the degradation of wood components due to the changing environment. Thus, the purpose of this investigation is to determine the wood acidity and the differences in the organic acids within wood due to the exposure to different environmental conditions. The conditions considered included indoor vs. outdoor environments and polluted and highly populated areas.
EXPERIMENTAL
Wood Samples
Wooden samples (Table 1) were collected in August 2018 from architectural elements in different places. Four samples were taken from objects stored in the museum storage at the Faculty of Archaeology, Cairo University, Giza, which is the third largest city in Egypt. One sample was taken from Al-Jawhara Palace located in Salah El-Din Citadel in Cairo. Two samples were taken from different buildings on El-Moez street, surrounded by densely populated areas in all directions and considered one of the oldest streets in Cairo. Moreover, two samples were taken from palaces located in Helwan, which is heavy industrial and residential site that lies in south east of Cairo city and is considered the main air pollution source in Cairo according to Alkhdhairi et al. (2018).
Table 1. Sample Numbers with their Locations and Codes Measured in the Present Work
The annual climatic (humidity and temperature) variations to the places where samples were collected were in Cairo with average temperature ranging between max. 37º C (99 °F) and min. 22 °C (71.8 °F) and average relative humidity 46%, in Giza with average temperature ranging between max. 35 °C (96 °F) and min. 24 °C (76 °F) and average relative humidity 46% and, in Helwan with average temperature ranging between max. 37 °C (99 °F) and min. 24 °C (76 °F) and average relative humidity 45%.
For comparison, artificially seasoned new wood blocks (ASNW) and naturally aged wood (NAW) blocks, for 50 years, were cut (approximately 15 x 5 x 5 cm3). The two collected wood blocks from each sample as well as ASNW and NAW blocks were milled and ground to fine powder using a laboratory Wiley mill (A-47054; Weverk, Karlstad, Sweden) (Salem et al. 2020). Then, the wood powders were screened to obtain 20-mesh sized samples, which were used for the chemical analysis.
Optical Microscopy
Optical microscopy (OM) was used to examine the prepared sections of the collected wooden samples to identify their species. Optical microscopy images were taken with a Zeiss Stereo DV 20 microscope equipped with an Axio Cam MRC5 (Zeiss, Oberkochen, Germany).
Scanning Electron Microscopy (SEM)
In order to detect any fungal decay that can affect the results, small pieces (2 × 2 × 5 mm3) were removed from the collected wood samples, then they were mounted on aluminium stubs with double-sided cellophane tape and they were coated with gold (K550X sputter coater; EMITECH, Ashford, England). Finally, they were examined by SEM (JEOL scanning electron microscope JXA-840A; JEOL Ltd.).
Determination of pH Value
For pH measurement, the wood samples were ground. A total of 2.5 g of oven-dried sawdust of each sample were weighed and soaked in 25 mL of distilled water and left at room temperature overnight. Then, they were mixed well via stirring and filtered with filter papers. After that, the pH values of the filtrate (the liquid) were determined using a pH meter (BOECO, 20422, Boeckel + Co (GmbH + Co) KG, Hamburg, Germany) (Mansour et al. 2020).
Preparation of Sample and Organic Acids Extraction
Eleven samples (nine collected samples, ASNW, and NAW) in the form of powered materials (20 mesh) were each weighed out as 0.1 g and used for the extraction and analysis (Clausen et al. 2008). The extraction was completed with H2SO4 (0.1 N) for 1 h at 25 °C using rotary mixing, then centrifuged (21,000 × g, 10 min) and filtered through a 0.45-µm filter (Kenealy et al. 2007).
High-performance Liquid Chromatography (HPLC)
High-performance liquid chromatography was used to separate and quantify the organic acids in the wooden samples under investigation. Chromatographic separation of organic acids was performed using an HPLC Knauer (Wissenschaftliche Geräte GmbH, Berlin, Germany) with a Rezex@ column (Phenomenex, USA catalogue, 2014/2015) equipped with a binary pump. The flow rate was set at 0.6 mL /min, the UV detector was set at 214 nm, and the column oven temperature was kept constant at 65 °C. The column used was a Rezex@ column for organic acids analysis, and the mobile phase was 0.005 M H2SO4. Data were integrated by ClarityChrom@ Version 7.2.0, Chromatography Software (KnauerWissenschaftliche Geräte GmbH, Berlin, Germany). Standard organic acids of analytical grade of oxalic, citric, tartaric, succinic, glutaric, acetic, propionic, and butyric acids were used for the HPLC analysis (Mansour et al. 2020).
Fourier Transform Infrared Spectroscopy (FTIR)
The FTIR spectra of the selected samples were collected using an FTIR spectrophotometer (Nicolet 380; Thermo Fisher Scientific, Waltham, MA, USA) The potassium bromide disk containing finely ground samples was prepared and then analyzed and recorded in the transmission mode within the frequency range of 4000 to 400 cm–1. All spectra were recorded at 4 cm–1 resolution. The peak heights and width of absorption bands were measured by the essential FTIR (eFTIR) software (Operant LLC, version 3.50, Burke, VA, USA).
RESULTS AND DISCUSSION
Identification of Wood Samples
The microscopic examination indicated that the wooden samples were identified as pine (Pinus sp.), where the diagnostic characteristics (Fig. 1) showed that growth ring boundaries were distinct – latewood tracheids thick walled and axial intercellular resin canals present with epithelial cells thin walled in the transverse section (TS). In addition, the tangential longitudinal section (TLS) showed uniseriate rays with a medium average height (5 to 15 cells) and radial intercellular resin canals. In the radial longitudinal section (RLS), the ray tracheid commonly presented with dentate cell walls and ray parenchyma’s end walls were smooth (Crivellaro and Schweingruber 2013).
Fig. 1. The anatomical characteristics of the wooden samples collected for this study that identified as Pinus sp. by optical microscopy in transmitted light: A- Transverse section; B- Tangential section; and C- Radial section
Scanning Electron Microscope Analysis
Scanning electron microscope micrographs (Fig. 2) revealed the presence of nonbiological deterioration in the examined wood samples, mostly resulting from weathering and mechanical deterioration, which are considered the most important factors for architectural wooden elements according to their location and their function. Moreover, the micrographs confirmed the absence of fungal attack. Sustained cracks, fractures, and separations along the cell walls were evident in all samples due to loads and stresses that the wooden elements are exposed to. In addition, the damage results from weathering can be seen in the form of extraneous material presence, destruction of bordered pits, and cell wall checking.