Chemical properties of waste mangrove bark and its potential uses

Chemicals and raw materials

The bark of five mangrove species, namely Rhizophora apiculata, Rhizophora mucronata, Bruguiera gymnorhiza, Bruguiera parviflora and Ceriops tagal, were harvested along Bintuni Bay, West Papua, Indonesia. Samples were collected from three different locations and were harvested from an adult tree ready for harvesting, approximately 30–40 years of age. Bark of the same species was combined, dried for 5 days and powdered using a hand-operated blender. Folin-Ciocalteau phenol reagent, benzoic acid, gallic acid, methanol, anhydrous sodium carbonate (Na₂CO₃), polyvinylpolypyrrolidone (PVPP) were purchased from Sigma-Aldrich (St. Louis, MO,U.S.A.).

Proximate analysis

In the evaluation of biomass for its potential in fuel application, proximate analysis is usually done to measure the gross composition of the biomass in terms of moisture, ash, volatile matter and fixed carbon content (Basu and Basu 2013). The determination of proximate analysis is standardized through the American Society for Testing and Materials D3172-13 (ASTM D3172-13 2002). In this standard test, the order of determination is as follows: moisture, followed by volatile matter and finally ash and this is shown in Figure 1. The fixed carbon is computed by difference.

OPEN IN VIEWER

Figure 1

Sequence of steps for the determination of proximate composition.

In determining the %moisture of mangrove bark powders, about 2 g of sample was oven-dried at 105°C to constant weight. The %moisture of the sample was calculated based on the loss in weight of the powders. The dried sample obtained after this analysis was subjected to volatile matter content determination. For this analysis, the furnace was initially heated to 950°C. Then, the dried sample was covered and placed in the outer ledge of the furnace for 2 min with the furnace open. Afterwards, the sample was transferred to the edge of the furnace and heated for 3 min with the furnace open. Finally, the sample was moved to the rear of the furnace for 6 min with the furnace closed. The samples were then cooled and weighed. The %volatile matter of the sample was also calculated based on the loss in weight of the powders.

The sample obtained after analysis of volatile matter content was then subjected to ash content determination. The sample was heated to 750°C in the furnace for 6 h to constant weight. Similar to the prior analyses, the %ash of the samples was computed based on the loss of weight of the sample. Finally, the %fixed carbon of the sample was computed using Equation (1). (1) where M = % Moisture content, VM = % Volatile matter content, A = % Ash content.

Fourier transform infrared spectroscopy

FT-IR spectroscopy is employed to examine the chemical structure of the wood bark for its reliability (Özgenç et al. 2017; Pandey 2004). It has been used to identify the presence of certain functional groups in a molecule, and the collection of absorption bands can confirm the identity of compounds and specific impurities in the sample. FT-IR spectroscopy was used to determine the presence of cellulose, hemicellulose and lignin on the surface of bark samples. The FT-IR spectra were generated using Cary 600 FT-IR spectrometer (Agilent Technologies) in attenuated total reflectance (ATR) mode. A total of 128 scans were obtained under ambient conditions within the frequency bands ranging from 380 to 4000 cm⁻¹ for each mangrove bark sample. The differences in the functional groups and fingerprint bands for each sample were noted using Origin software.

Calorific value content

The gross calorific values of the mangrove bark samples were determined based on American Society for Testing Materials (ASTM D5865-13 2013) Standard Test Method for Gross Calorific Value of Coal and Coke Designation: D5865 – 10a using Parr 1108 oxygen bomb calorimeter. During the analysis, the pressure inside the calorimeter was maintained at 25 atm and less than 1 gram of sample was used. The computation of calorific value was based on the residual fuse wire after the analysis. Benzoic acid was used as a standard. Data correlation were noted using SPSS software.

Total polyphenol content

About 1 g of the sample was mixed with 50 mL methanol and was shaken at 40°C for 18 h until fully dissolved. An aliquot of the solution (about 1 mL) was diluted with water to 7 mL and was added with 0.5 mL of Folin-Ciocalteau phenol reagent. Exactly 5 min later, 1.0 mL of saturated sodium carbonate solution was added. The mixture was then diluted to 10 mL with water. The sample was incubated for 2 h at room temperature and the absorbance was determined at 765 nm using a spectrophotometer (UV-1700, Shimadzu Corp.). A standard calibration curve using gallic acid was also prepared to obtain the total polyphenol concentration as gallic acid equivalence (GAE) per amount of powder(Swain and Hillis 1959; Vieira et al. 2011).

Total tannin content

In determining the total tannin content of the sample, PVPP was used to precipitate the phenolic groups in tannins from solutions. The amount of PVPP used for this analysis was based on a mass ratio equal to 100:1 (PVPP: total polyphenol content). About 1 g of the sample was mixed with 50 mL methanol and was shaken at 40°C for 18 h until fully dissolved. The extract was diluted with water and the total phenolic content was determined. PVPP was then added to the solution, mixed and then the pH was adjusted to 3. The total phenolic content of the resulting mixture was again determined. The total tannin content (expressed as mg GAE per gram sample) was calculated as the difference between the initial and final total polyphenol content.

Proximate composition of mangrove bark

It is presented in Figure 2 is the proximate composition of the mangrove bark. The proximate composition of each variety of mangrove bark has notable differences. The mangrove bark is largely composed of volatile matter across varieties, which is around 60% of the proximate composition of the samples. Volatile matter can comprise carbon oxides, hydrogen and traces of nitrogen compounds and it dictates ignition, flame stability and the mass of unburned carbon in the fly ash. The amount of volatile matter impacts fuel application of specific biomass, particularly its combustion characteristics (Miller 2013). A comparison of volatile matter content of some biomass shows that mangrove bark has relatively higher value, and is similar to lithotype coals (Brockway and Higgins 2013). This implies further that mangrove bark can be used as fuel substitute since coal is commonly used with volatile matter equal to 40%. In most practice, wood is usually used as fuel having a volatile matter ranging from 50% to 80% (White 1987).

OPEN IN VIEWER

Figure 2

Proximate composition of the bark of five mangrove species (values represent average for three trials; error bars represent standard deviation).

In terms of fixed carbon content, Bruguiera gymnorhiza has the highest fixed carbon content (18%), followed by Bruguiera parviflora (17%) among the mangrove bark species. The carbon content of the material is expressed in its fixed carbon amount. Fixed carbon content of mangrove bark is important as it signifies char yield after devolatilization (Zhang et al. 2009) and is dependent on mangrove habitat and species (Sudirman et al. 2014). Char yield refers to the remaining mass after complete degradation of the sample including residual carbon. This could mean that Bruguiera gymnorhiza and Bruguiera parviflora mangrove barks can retain high amounts of char when used as fuel or fuel substitute.

On the other hand, ash represents the total amount of mineral nutrients in the form of inorganic residue that is left after water and organic matter has been removed by heating. The ash content is an indicator for a raw material to be considered in fertilizer and composts applications (Petrovský et al. 2018). The ash content of Rhizophora apiculata (8.91%) and Rhizophora mucronata (8.41%) are much higher among the mangrove bark species.

FTIR spectra of mangrove bark

The infrared spectra of the mangrove bark samples of various species are characterized by the absorption peaks between 1030 and 3332 cm⁻¹ regions as shown in Figure 3. The occurrence of peaks in the spectra of the mangrove bark are all similar but with differences in intensities. Each peak represents functional groups that are attributed to the presence of cellulose, hemicellulose and lignin in the bark samples. This is summarized in Table 1.

OPEN IN VIEWER

Figure 3

FR-IR spectrum of the bark of five mangrove species.

OPEN IN VIEWER

Table 1

Characteristic FT-IR bands of Mangrove Bark Components (Özgenç et al. 2017).

Wavenumber (cm−1)	Assignment	Attributed to
1030	C –O stretching	Cellulose and hemicellulose
1232	syringyl ring and C –O stretch	Lignin and xylan
1315	CH2 rocking vibration	Cellulose
1373	C –H deformation vibration	Cellulose
1448	C = C and C –H bond; O –H in plane deformation	Lignin and hemicellulose
1513	C = C stretching of aromatic skeletal vibration	Lignin
1606	Aromatic skeletal and C = O stretch vibration	Lignin
2917	C –H stretching	Cellulose
3332	O –H stretching (hydrogen bonding)	Cellulose

The peaks at 1232 cm⁻¹ (C–O stretching) and the region between 1448 and 1606 cm⁻¹ (C = C stretching, C – H bond, O –H in plane deformation and C = O stretch vibration) are attributed to the lignin content of the bark samples. On the other hand, the absorption peak at 1030 cm⁻¹ corresponds to C–O stretching and this is attributed to the presence of hemicellulose. The bands at 2917 cm⁻¹ (C–H stretching) and 3332 cm⁻¹ (O–H stretching) were attributed to the cellulose content of the sample.

In all the observed peaks, Ceriops tagal species imparts the strongest intensities. This is followed by Bruguiera gymnorhiza, Rhizophora mucronata, Rhizophora apiculata and finally Bruguiera parviflora. These results would indicate that Ceriops tagal contains the highest cellulose, hemicellulose and lignin contents among the mangrove species. In a study by Demirbaş (2001), a direct relationship was established between the lignin content and higher heating value (HHV) of biomass fuels. A material with high lignin content would also have higher HHV.

Calorific values of mangrove bark

Basically, the heating value is the amount of heat produced by complete combustion of a material and it is measured as a unit of energy per unit mass or volume of a sample. The heat of combustion of a material is expressed by the higher and lower heating values. HHV is the gross calorific value, and it is defined as the amount of heat released when a material is combusted and the products have returned to a temperature of 25^oC (ÖzyuǧUran and Yaman 2017).

The calorific values (kJ g sample⁻¹) of mangrove bark are shown in Figure 4. Bruguiera gymnorhiza has the highest calorific value (65 kJ g⁻¹) followed by Rhizophora mucronata (47 kJ  g⁻¹) and Ceriops tagal (37 kJ g⁻¹). Both Rhizophora apiculata and Bruguiera parviflora have the lowest gross calorific value among the samples tested. These results indicate that Bruguiera gymnorhiza can be the best alternative for solid fuel among the mangrove species studied as it has the highest heating value.

OPEN IN VIEWER

Figure 4

Calorific values (kJ/g sample) of the bark of five mangrove species (values represent average for three trials; error bars represent standard deviation).

A comparison of Figures 2 and 4 reveals that Bruguiera gymnorhiza has the highest fixed carbon (18%) and has the highest calorific value among the mangrove species. Materials with high fixed carbon were found to have high HHV (Pérez-Arévalo et al. 2015). Bruguiera parviflora has the highest volatile matter (63%) and Rhizophora apiculata has the lowest (57%). The role of volatile matter in combustion is high, accounting from 70% to 80% of the energy created (White 1987). However, a comparison of Figures 3 and 4 shows that Ceriops tagal has a relatively low HHV even if it contains the highest lignin. In a study by White (1987), it was found that extractives (non-chemically bound components like sucrose, protein, ash, chlorophyll and waxes) in biomass samples could also affect the HHV of a material even at high lignin content.

Total phenolic and tannin content of mangrove bark

The total polyphenol contents of the mangrove bark expressed as GAE are shown in Figure 5. Ceriops tagal had the highest phenolic content at 140.78 GAE, followed by Bruguiera gymnorhiza (119.14) and Bruguiera parviflora (87.47). The difference in total polyphenol contents between the variety of both Bruguiera spp. and Rhizophora spp. is significant. Between the same species, Rhizophora apiculata has 12 times the amount of polyphenol compared to Rhizophora mucronata while Bruguiera gymnorhiza has 38% more polyphenol compared to Bruguiera parviflora.

OPEN IN VIEWER

Figure 5

Total Phenolic Content of Mangrove Bark (values represent average for three trials; error bars represent standard deviation).

Total tannin contents of mangrove bark are shown in Figure 5. The total tannin contents of the mangrove bark samples are also expressed in mg GAE per gram sample. Among the barks, Ceriops tagal had the highest tannin content of 125.64 GAE, followed by Bruguiera gymnorhiza (124.13) and Bruguiera parviflora (85.4). The difference in tannin contents between the variety of both Bruguiera spp. and Rhizophora spp. is significant. Between the same species, Rhizophora apiculata has twice the amount of polyphenol compared to Rhizophora mucronata while Bruguiera gymnorhiza has 45% more tannin content compared to Bruguiera parviflora. Figure 5 also shows that phenolic content and tannin content of the different mangrove bark are correlated as there is negligible difference per variety.

Ceriops tagal bark may be processed further to get valuable chemical compounds like tannin that will serve as another product from mangroves. A study by Tan and Kassim (2011) has shown that the choice of extraction solvents greatly affects the phenolic and tannin contents extracted from mangrove bark. Using alcohol, the total phenolic content of the Rhizophora apiculata bark is at 8.57 GAE but using 70% acetone and 30% water, its total phenolic content is at 191.82 GAE while its tannin content remains the same. As the study used methanol as the main extraction solvent, total phenolic content may differ if it were to use other solvents; in this way, other phenolic compounds may be extracted.