Abstract
A new bio-sorbent derived from green algae biomass, Arthrospira platensis (Spirulina), was found to be economically practical for water decontamination. This biosorbent comprised of microalgae cellulose, poly(lactic acid) (PLA), Dabai activated carbon (AC), and montmorillonite (MMT), each plays a distinctive role in removing methylene blue (MB) dye. The presence of hydroxyl and carbonyl functional groups in algae cellulose, confirmed by the FTIR analysis, offered binding sites for dye removal. Scanning electron microscopy demonstrated the morphological structure of the biosorbent, highlighting the combined effect of microalgae cellulose, PLA, Dabai AC, and MMT mixtures. The inclusion of Dabai AC and MMT improved micropores and mesopores, enhancing adsorption reactions. The Brunauer-Emmett-Teller (BET) analysis confirmed that the sample containing microalgae cellulose, Dabai AC, and MMT clay in PLA had a specific surface area of 0.784 m2/g, three times higher than the PLA + cellulose sample. Additionally, adding 1% MMT to the sample improved the particle dispersion on the surface of the hydrophobic PLA, thereby improving its thermal properties. Remarkably, the biosorbent effectively eliminated 86.8% of MB dye from an initial concentration of 50 mg/L after 60 min of Vis-light irradiation using Ultraviolet-visible spectroscopy.
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Extraction and Characterization of Modified Algae Derivative Cellulose and its Mixtures for Dye Removal
Md. Rezaur Rahman,a,* Anthonette Anak James,a Al-Khalid bin Othman,a Muhammad Khusairy Bin Bakri,a Jamal Uddin,g Ain Zaienah Sueraya,a Mohammed Mahbubul Matin,b Sulaiman Y. Alfaifi,c O. Madkhali,d Mahmood D. Aljabri,e and Mohammed M. Rahman f
A new bio-sorbent derived from green algae biomass, Arthrospira platensis (Spirulina), was found to be economically practical for water decontamination. This biosorbent comprised of microalgae cellulose, poly(lactic acid) (PLA), Dabai activated carbon (AC), and montmorillonite (MMT), each plays a distinctive role in removing methylene blue (MB) dye. The presence of hydroxyl and carbonyl functional groups in algae cellulose, confirmed by the FTIR analysis, offered binding sites for dye removal. Scanning electron microscopy demonstrated the morphological structure of the biosorbent, highlighting the combined effect of microalgae cellulose, PLA, Dabai AC, and MMT mixtures. The inclusion of Dabai AC and MMT improved micropores and mesopores, enhancing adsorption reactions. The Brunauer-Emmett-Teller (BET) analysis confirmed that the sample containing microalgae cellulose, Dabai AC, and MMT clay in PLA had a specific surface area of 0.784 m2/g, three times higher than the PLA + cellulose sample. Additionally, adding 1% MMT to the sample improved the particle dispersion on the surface of the hydrophobic PLA, thereby improving its thermal properties. Remarkably, the biosorbent effectively eliminated 86.8% of MB dye from an initial concentration of 50 mg/L after 60 min of Vis-light irradiation using Ultraviolet-visible spectroscopy.
DOI: 10.15376/biores.18.3.5967-5992
Keywords: Algae; Cellulose; Mixtures; Synthesis; Clay
Contact information: a: Faculty of Engineering, Universiti Malaysia Sarawak, Jalan Datuk Mohammad Musa, Kota Samarahan, Sarawak, Malaysia; b: Bioorganic and Medicinal Chemistry Laboratory, Department of Chemistry, Faculty of Science, University of Chittagong, Chittagong, Bangladesh; c: Department of Chemistry, King Abdulaziz University, Jeddah 21589, Saudi Arabia; d: Department of Physics, College of Science, Jazan University, P.O. Box 114, Jazan 45142, Kingdom of Saudi Arabia; e: Department of Chemistry, University College in Al-Jamoum, Umm Al-Qura University, Makkah 21955, Saudi Arabia; f: Center of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University, Jeddah 21589, Saudi Arabia; g: Department of Natural Science, Coppin State University, Science and Technology Center, Baltimore, Maryland, 21216, U.S.A; *Corresponding author: rmrezaur@unimas.my
INTRODUCTION
Approximately 700,000 tons of various colorants are produced annually, with 100,000 commercially available dyes used in industrial applications (Samsami et al. 2020). These dyes are frequently discarded into water bodies without further treatment. The textile industry generates 54% of dye effluent from using 75% of global dye for coloration and printing on fabrics, followed by the dyeing industry, which accounts for 21% (Slama et al. 2021). The failure of the dye mixture to adhere entirely to the fabric is the primary cause of the high amount of dye effluent generation. Methylene blue (MB) dye is one of the most dangerous dye effluents released by the textile and tanning industries (Moghazy et al. 2020). The accumulation of dyes poses a significant threat to the environment as it reduces light penetration, preventing photosynthesis and lowering oxygen levels in the water. As a result, viruses and bacteria multiply, making the water unsafe to consume and use.
The environmental legislature has added a new law regarding the discharge of toxic colored effluent into the water bodies, which requires dye-utilizing industries to adhere to The International Dye Industry Wastewater Discharge Quality Standards. There are a few ways to remove dyes from water which include oxidation (Nidheesh et al. 2018; Alshahrani et al. 2022), adsorption (Harja et al. 2022; Sultana et al. 2022), flocculation-precipitation (Berber-Villamar et al. 2018), and membrane separation (Moradihamedani 2021). According to Royanudin et al. (2021), adsorption is one of the most widely used methods due to its simplicity, low cost, and high success rate. However, using bio-sorbent for adsorption has emerged as an appealing approach among researchers to treat domestic and industrial wastewater due to the need for more abundant, environmentally acceptable, cost-effective, and biodegradable materials (Moghazy and Abdo 2018; Aka et al. 2019; Qi et al. 2021; Biswas and Pal 2021). The unique characteristics of each bio-sorbent, including their adsorbing surface nature, pore structure, and porosity, have contributed to their diverse roles in the removal of dyes from industrial effluent. Arthrospira platensis (Spirulina), a promising natural inexhaustible source of raw materials, is suitable for developing algae-based biosorbents. The Arthrospira platensis (Spirulina) algae biomass is commonly used for heavy metal removal. However, there is limited information on its use for dye removal from wastewater. Algal biomass contains a variety of functional groups due to the presence of alginate gel in their cell walls, which may promote excellent binding sites (Moghazy et al. 2020). To date, no research has been reported on developing Spirulina algae biomass mixtures with the addition of activated carbon (AC), clay, and polymer to develop bio-sorbent.
This study aimed to synthesize cellulose from Spirulina platensis algae to develop environmentally friendly biosorbent mixtures with varying percentages of Dabai AC, montmorillonite (MMT) clay, and polylactic acid (PLA). Polylactic acid, MMT, and Dabai AC were chosen for their bio-derived properties. Dabai is a native fruit exclusively found in Borneo Island, and its nutshell was later on converted into an activated biocarbon. MMT, on the other hand, was used as a clay mineral, which contributes to the increased surface area and porous structure of the sample. In this work, the properties of the developed bio-sorbents were investigated via several characterization tests such as Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) with energy dispersive X-Ray analysis (EDX), Brunauer-Emmet-Teller (BET) analysis, and thermogravimetric analysis (TGA). To validate the ability of the developed bio-sorbent in treating dye effluent, a comparison study with commercial adsorbents was conducted.
EXPERIMENTAL
Materials
Dried green algae biomass provided by the Sarawak Biodiversity Center (SBC) was used as the main material for cellulose synthesis. Sodium hydroxide (NaOH) pellet (CAS Number: 1310-73-2), sodium chlorite (NaClO2) (CAS Number: 80 7758-19-2), glacial acetic acid (CAS Number: 64-19-7), montmorillonite (MMT) (CAS Number: 1318-93-0), polylactic acid (PLA) (CAS Number: 26100-51-6) and methylene blue (MB) dye (CAS Number: 61-73-4) were purchased from Merck. Dabai-activated carbon (AC) was provided by Universiti Malaysia Sarawak (UNIMAS).
Methods
Preparation of algae biomass, alkali, and bleach solution
The algal biomass was weighed and ground until forming a powder. The ground algae were then placed in a petri dish for further use. An amount of 0.4 g NaOH pellets were poured into a beaker filled with 250 mL distilled water and stirred until fully dissolved. The solution was transferred into a 1000 mL volumetric flask. The beaker was rinsed repeatedly with distilled water and poured into the volumetric flask until the liquid level was about 2 cm below the graduated mark. Next, distilled water was added using a dropper to the flask until the meniscus showed the same level as the graduated line. The volumetric flask was closed with a stopper and shaken. The flask was labeled as ‘0.01 M NaOH’.
A bleach solution was prepared with an equal volume of acetate buffer and aqueous sodium chlorite. Approximately 40.5 g of NaOH (MW: 39.997 gmol-1) and 112.5 mL of glacial acetic acid were diluted to 1000 mL of distilled water and stirred until the mixture dissolved to produce acetate buffer with a pH of 4.74. The acetate buffer was kept in a 2000 mL screw cap bottle and labeled as ‘acetate buffer.’ Next, roughly 102 mL of 0.25 M NaClO2 was dissolved in 1398 mL distilled water to produce a 0.017 M NaClO2 solution. The NaClO2 solution was then kept in a 2000 mL screw cap bottle labeled ‘0.017 M NaClO2’. Finally, 500 mL of each acetate buffer and NaClO2 were poured into a beaker for bleaching treatment.
Separation of cellulose from algae
The experiment was conducted with a ratio of 1:20 of algae biomass to the solution during the treatment. First, 50 g of ground algae was weighed and placed into a beaker with a weighing scale. Then, 1000 mL of distilled water was heated to 60 °C using a hotplate stirrer. The raw algae were then poured into the heated distilled water and stirred continuously for 1 h. After 1 h, the mixture was left to cool at room temperature until a separated layer was formed. The top layer was poured as waste, leaving the bottom layer filled with wet algae. The cellulose extraction was conducted as described by Sebeia et al. (2019). A volume of 1000 mL of 0.01 M NaOH was heated to 80 °C to prepare for alkali treatment. Wet algae were then poured into the heated NaOH and stirred continuously for 1 h. Following the treatment, the mixture was allowed to cool to room temperature, and distilled water was gradually added until a supernatant layer formed in the solution.
The top layer was poured out as waste, leaving the bottom layer of alkali-treated algae. The bleach solution was made with an equal amount of acetate buffer and aqueous NaClO2 and then heated to 80 °C. The alkali-treated algae were poured into the bleach solution and treated three times again for 40 min under continuous stirring until pure white-colored cellulose was obtained. The mixture was then left to cool before pouring into the centrifuge tube and centrifuged for 15 min at 15000 RPM using a centrifuge machine. After centrifugation, the top layer was poured out, leaving the bleached algae cellulose at the bottom. Next, distilled water was added to the bleached algae-containing tube, mixed, and put for centrifuged again to obtain pH 7. The cellulose was poured into a petri dish once pH 7 was obtained and placed in a drying oven at 60 °C for 6 h. The dried cellulose was left to cool to room temperature, ground with a mortar and pestle, and kept for further use.
Preparation of sample
The mixtures were prepared by blending the materials in accordance with the compositions shown in Tables 1, 2, 3, and 4. The purpose of blending was to ensure the materials were thoroughly mixed and resulted in a homogeneous mixture. The mixture ratios were estimated using the maximum and minimum values found in the literature.
Table 2. PLA + Cellulose
Table 3. PLA + Cellulose + AC
Table 4. PLA + Cellulose + AC + MMT
Dye adsorption test
First, 0.1 g of MB dye powder was dissolved in distilled water in a 250 mL beaker, and the solution was transferred into a 500 mL volumetric flask. The beaker was then rinsed repeatedly with distilled water and poured into the volumetric flask until the liquid level was 2 cm below the graduated mark. The volumetric flask was closed with a stopper and shaken. The flask was labeled as ‘200 ppm MB’. Then, 250 mL of 200 ppm MB was taken from the flask and placed into a beaker. The solution in the beaker was poured into a 1000 mL volumetric flask.
Similarly, the beaker was then rinsed repeatedly with distilled water and poured into the volumetric flask until the liquid level was about 2 cm below the graduated mark. Distilled water was added to the flask until the meniscus showed the same level as the graduated line using a dropper. Again, the volumetric flask was closed with a stopper and shaken. Finally, the flask was labeled as ‘50 ppm MB’ and kept aside for the application of dye adsorption.
Dye adsorption was done using the samples of microalgae cellulose, 100% PLA, PLA + 2% cellulose, PLA + 2% cellulose + 2% AC, and PLA + 2% cellulose + 2% AC + 1% MMT. Due to the fast adsorption of the samples, the time taken for the adsorption of MB dye was set to 60 min with 10 min interval for sample taking. Mixing the adsorbent and adsorbate was kept constant at 150 RPM. 5 beakers were prepared, and 1000 mL of 50 ppm MB was poured into each beaker. 1.0 g of each sample was weighed. Then, the adsorbents were poured into the beaker and stirred continuously for 60 min using a hotplate stirrer and a magnetic stirrer. At every 10 min interval, 10 to 15 mL of samples were extracted using a syringe and filtered with filter paper into a centrifuge tube. The samples were then labeled accordingly and measured using UV-vis.
Characterizations and Tests
FTIR analysis of the neat PLA, algae cellulose, and developed mixtures
An FTIR spectrophotometer (Shimadzu, Kyoto, Japan) was used in a spectral range between 4000 to 400 cm-1 to examine the functional groups of the neat PLA, algae cellulose, and the mixture samples. Approximately 0.1 to 1.0% of the samples were mixed with 200 mg of dry potassium bromide (KBr) on a small pestle to create a small pellet. The analysis was started once the FTIR software was programmed following ASTM E1252-98 (2021) and ASTM E168-16 (2016) standards.
SEM-EDX analysis of the neat PLA, algae cellulose, and developed mixtures
The neat PLA, algae cellulose surface morphology, and the developed mixtures were observed using a Hitachi analytical tabletop SEM (benchtop) (TM-3030, Hitachi High Technologies, Mannheim, Germany) under an accelerating voltage of 15.0 kV. The characterization procedure was based on the ASTM E2015-04 (2014) standard. The elemental compositions of the samples were analyzed using EDX. Dried powdered samples were pasted on the SEM disc and placed in the chamber. The chamber was then let to EVAC before obtaining the results from the SEM and EDX software.
TGA analysis of the mixtures
The thermal stability of the algae cellulose biosorbent and its mixtures were analyzed using the TG machine. The dried samples were then heated from 25 to 700 °C at 10 °C/min, kept in a zip and lock bag, and handed to the laboratory assistants for analysis. The test was done according to ASTM E1131-20 (2020) standard.
BET analysis of the mixtures
The surface area of the mixtures labeled PLA + 2% cellulose, PLA + 2% cellulose + 2% AC, and PLA + 2% cellulose + 2% AC + 1% MMT were analyzed using the BET surface analyzer. The condition of the BET analysis was set at the degassing condition of 10 °C/min at 25 to 1000 °C for 6 h. The sample was dried by placing it in an oven at 60 °C. Upon drying, the sample was kept in a zip and lock bag and handed to the laboratory assistants for analysis. The test was based on the ASTM D6556-14 (2014) standard.
UV-vis analysis
The adsorption capacity of the cellulose, PLA, PLA + 2% cellulose, PLA + 2% cellulose + 2% AC, and PLA + 2% cellulose + 2% AC + 1% MMT were measured using a UV-vis spectrophotometer with wavelength ranging between 500 and 700 nm. Then 3 to 4 mL of samples were dropped into the UV-vis cuvette and placed into the UV-vis spectrophotometer to obtain the analysis results. The test was done according to ASTM E169-16 (2016). The biosorption capacity of the samples was calculated using the equation adopted from Paska et al. (2014) for evaluating the adsorption capacity (Eq. 1) and the methylene blue dye removal percentage (Eq. 2).
(1)
(2)
In these equations, qt is the dye absorbed per unit of biosorbent (mg/g); Co is the concentration of dye at initial state(mg/L); Ct is the concentration of dye at different periods of time (mg/L); W is the weight of biosorbent (g); V is the volume of solution (L); and Ce is the concentration of dye at equilibrium (mg/L)
RESULTS AND DISCUSSION
FTIR analysis
The appearance of new peaks and the differences in intensities were considered based on the FTIR results in Fig. 1.The absorption bands of cellulose, Dabai AC, and MMT in Figs. 1 (a), (c) and (d) were observed between the regions of 400 to 4000 cm-1. In contrast, the absorption bands of PLA, PLA/C, PLA/C/AC, and PLA/C/AC/MMT in Figs. 1 (b), (e), (f), and (g) were identified in the wavenumber range of 400 to 2000 cm-1. Based on the peaks in Fig. 1, the peak at 3277 cm-1 revealed an O-H stretching vibration of the hydroxyl group in cellulose. The appearance of the O-H hydroxyl group was similar to the result of algae cellulose reported by Sebeia et al. (2019) through the detection of an absorbance band at 3292 cm-1. The peaks between 1630 to 900 cm-1 were the typical absorbance bands observed in cellulose (Hospodarova et al. 2018). A sharp peak observed at 1638 cm-1 was attributed to the O-H bending variation of water molecules. This peak indicated the presence of water absorption within the void spaces of cellulose after the removal of lignin and hemicellulose (Liew et al. 2015). This finding further supported the hygroscopic behavior of cellulose in absorbing and retaining moisture from its surroundings. Other studies conducted by Samia et al. (2016) and Aragaw and Bogale (2021) discussed that the peak within the range of 1510 to 1650 cm-1 was responsible for C=O stretching variations. This possibly indicated the presence of the carbonyl group found in various compounds containing carboxylic acids, ketones, amides, aldehydes, and esters. Based on Mallamace et al. (2014), the band of amide I, which represented the stretching vibration of the C=O commonly observed within the range of 1600 to 1700 cm-1, whereas the band of amide II, which showed the combination of N-H bending and C-N stretching vibration, is commonly observed in the range of 1500 to 1570 cm-1. In Fig. 1(a), the appearance of the peak at 1650 cm-1 could also be due to the presence of amide I, whereas the peak at 1526 cm-1 was due to amide II. The presence of several polysaccharide components and proteins allowed the formation of acid-binding sites on the surface of microalgae which results in biosorbents with a high affinity toward dye (Singh et al. 2018; Siddiqui et al. 2019). Both Figs. 1 (a) and (c) showed the appearance of the C=O stretching variation. This makes it an appealing material for dye removal. In addition, the presence of a strong band at 1749 cm-1 in Fig. 1 (b), referred to as the C=O stretch, resulted in a more pronounced occurrence of peaks at 1749, 1748, and 1748 cm-1 in Figs. 1 (e), (f), and (g) of the mixture samples. Besides, the presence of the inorganic compounds in the lignocellulosic samples of cellulose and AC also caused the appearance of peaks corresponding to the C=O carbonyl stretch that indicate the presence of carboxylic acid functional groups. However, comparative studies revealed that the intensities of the peaks in the mixture samples were lower than that of pure PLA in Fig. 1 (b).
Fig. 1. FTIR spectra (a) cellulose; (b) pure PLA; (c) Dabai AC; (d) MMT; (e) PLA + cellulose; (f) PLA + cellulose + AC; (g) PLA + cellulose + AC + MMT
Besides, the peak at 1452 cm-1 shown by the PLA spectrum corresponds to the CH3 asymmetric bending vibration, which also appeared at 1452 cm-1 (Chieng et al. 2013). Meanwhile, the peak at 1182 cm-1 demonstrates the characteristic of C-O-C stretching in PLA, which has also been observed in the spectra of PLA/C, PLA/C/AC, and PLA/C/AC/MMT mixtures. The C-O stretching frequency showed the characteristic of an ester group at wavenumber 1084 cm-1 in Figs. 1 (b), (e), (f), and (g), similar to the findings reported by Khui et al. (2021), James et al. 2022 and Choksi and Desai (2017). However, the peak intensities were slightly lower in Figs. 1 (e), (f), and (g) due to the addition of cellulose, AC, and MMT in the respective mixtures, which could influence the thermal properties displayed by the TGA results, as indicated by Deng et al. (2020).
Meanwhile, in Fig. 1 (d), the appearance of peaks at 1032 cm-1 and 529 cm-1 represented the Si-O stretching and bending vibrations. This observation aligned with the findings of Patel et al. (2006), who assigned the peak at 1035 cm-1 to Si-O stretching (in-plane), whereas the peak within the range of 1100 to 1000 cm-1 was assigned to the Si-O stretch (out-of-plane) for MMT. The appearance of the aforementioned peaks as well as the peak at 1084 cm-1 in Fig. 1 (g) indicated that the MMT had a good adhesion interaction with the composite (James et al. 2023). The FTIR results in Figs. 1 (f) and (g) showed a negligible difference in peak intensity. This provided evidence that the inclusion of 1% MMT clay had little effect on the FTIR results. Upon the addition of MMT to the mixtures, the intensity of the C-O-C asymmetric stretching peak at 1182 cm-1 in Fig. 1 (f) showed a slight reduction to 1180 cm-1, as displayed in Fig. 1 (e), similar to the observation of Arjmandi et al. (2016). According to the authors, the reduction was initiated by the formation of hydrogen bonds between the MMT and PLA, which led to a good compatibility outcome. Hong et al. (2021) also indicated that the absorbance band of 1180 cm-1 was attributed to the glycosidic bonds which hold the cellulose units.
Two peaks at the wavenumber regions 868 and 754 cm-1 were attributed to the amorphous and crystalline phase of C-C stretching vibrations. These peaks were only visible in Fig.1 (b) of the pure PLA, since there were no obvious peaks in the mixture samples. The finding was similar to Rocca-Smith et al. (2017), who noticed peaks at 863 and 758 cm-1 in the FTIR result of a PLA film. The peaks of PLA/C in Fig. 1 (e) were more pronounced than the spectra of PLA/C/AC and PLA/C/AC/MMT mixtures in Figs. 1 (f) and (g). This indicates that the addition of AC and MMT to PLA/C does not affect the FTIR patterns of the mixtures, only their intensities due to the absence of a new functional group.
SEM/EDX analysis
The SEM results shown in Figs. 2, 3, 4, 5, and 6 are the morphological images of cellulose, pure PLA, PLA/C, PLA/C/AC, and PLA/C/AC/MMT at 800x magnification. Based on the observation, the cellulose structure derived from algae, as depicted in Fig. 2, exhibits a coarse surface due to several chemical treatments during the extraction. Similar to the morphological characteristic of Aegagropila linnaei macroalgae (Sebeia et al. 2019), cellulose’s rough surface can increase the likelihood of trapping numerous dye molecules. The presence of mesopores and micropores on the structures of microalgae increases the surface area of the material. This expanded surface area offers more available binding sites for the adsorption of dye molecules. In addition, the dissolution of algae fiber in an aqueous alkali solution caused a splitting effect and the formation of both intra- and intermolecular H-bonds, which changed the cellulose structure. Kannam et al. (2017) reported that the degradation of microfibrils resulted in numerous small particles combining to form large microfibrils due to Van der Waals interaction and the effect of the hydrogen bond.
Figure 3 illustrates the morphological image of a pure PLA sample. PLA displays a smooth structural image compared to the samples of PLA/C, PLA/C/AC, and PLA/C/AC/MMT presented in Figs. 4, 5, and 6. A similar surface condition to Fig. 3 was noticed in Fig. 4 (a), although it contained 0.5% cellulose. As the percentage of cellulose increased, the sample structure exhibited small shallow cracks, causing slightly ductile behavior (Khui et al. 2021). More voids and deep crevasses (red circles) were created as AC and MMT were introduced to the sample, as observed in Figs. 5 and 6. This problem correlated with the one raised by Ramesh et al. (2019), who highlighted the importance of considering the concentration of MMT in a sample as it could affect its mechanical properties, causing the formation of micro-voids, accumulation, and fiber bending. However, there were no obvious agglomeration issues in any of the samples except for samples Figs. 5 (b) and (c), although there were poor dispersion and lack of interfacial interaction between the cellulosic particles and the PLA matrix due to the hydrophilic nature of cellulose (Sousa et al. 2019).
Fig. 2. SEM with EDX spectrum of cellulose at 800x magnification
Fig. 3. SEM with EDX spectrum of pure PLA at 800x magnification
Fig. 4. SEM with EDX spectrum of (a) PLA + 0.5% cellulose; (b) PLA + 1% cellulose; (c) PLA + 2% cellulose at 800x magnification
Fig. 5. SEM with EDX spectrum of (a) PLA + 2% cellulose + 0.5% AC; (b) PLA + 2% cellulose + 1% AC; (c) PLA + 2% cellulose + 2% AC at 800x magnification Non-uniform dispersion
Fig. 6. SEM with EDX spectrum of (a) PLA + 1% cellulose + 1% AC + 1% MMT; (b) PLA + 2% cellulose + 1% AC + 1% MMT; (c) PLA + 2% cellulose + 2% AC +1% MMT at 800x magnification
Table 5. Elemental Composition Analysis Based on the EDX Spectra
Table 5 indicates the elements in the samples, which correlates with the EDX spectra shown in Figs. 2, 3, 4, 5, and 6. Carbon (C) and oxygen (O) were the most frequently detected elements in all samples, with C accounting for the highest percentage of elemental composition, followed by O. In addition, the table also shows the presence of nitrogen (N) in relatively low quantities in samples with high cellulose content. As an example, Fig. 2 exhibits 1.71% N, whereas Fig. 4 (c) and Fig. 6 (b) show 1.48% N and 1.31% N respectively.
This occurrence was attributed to the ability of cellulosic materials to permanently retain nitrogen compounds, albeit usually in a low percentage range due to the amino functions of the cellulose (Goto et al. 2021). Despite undergoing mild acidic washing which typically removes nitrogen compounds, certain nitrogen components remained resistant and tightly bound, leaving a small residue of nitrogen in the samples (Goto et al. 2021).
Additionally, the presence of MMT in samples depicted in Fig. 6 was associated with the presence of Silicon (Si) compounds. However, the amount was relatively low which accounted for 1.04%, 0.56%, and 0.45%. This can be attributed to the chemical structure of MMT which is a phyllosilicate mineral consisting of stacked layers where each layer is made up of two O-Si-O tetrahedral sheets sandwiching one O-Al(Mg)-O octahedral sheet (Zhou et al. 2019). The presence of Si due to MMT could also be seen in the FTIR spectra as explained previously for Fig. 1 (d) where Si-O stretching and bending were present.
BET analysis
Analysis of the nitrogen adsorption/desorption isotherms at 77 K, as in Figs. 7 (a), 8 (a), and 9(a), were conducted to estimate the specific surface areas of the prepared samples and their porosity, which correlate with their capabilities in removing dye molecules. Based on the displayed BET graphs, it was found that all of the adsorption/desorption curves were close to BET-type IV based on the International Union of Pure and Applied Chemistry (IUPAC) classification (Ambroz et al. 2018), which indicates the presence of both micro and mesopores on the surface of the samples with H2-type hysteresis loop.
The mesoporous structure has a relatively large surface area, including a high pore volume making the sample suitable for dye removal (Blachnio et al. 2021). The generated curves represent the characteristics of the pores and their sizes in which the SEM could not carry out the identification (Phothong et al. 2021). Based on the literature, the hysteresis loop for nitrogen at 77K occurs when the pores exceed 4 nm (Schlumberger and Thommes 2021). The appearance of the H2 hysteresis ring in Figs. 7(a), 8(a), and 9(a) signify the occurrence of pore blocking (Yuan et al. 2021).
The multi-point BET curves shown in Figs. 7 (b), 8(b), and 9(b) were used to determine the surface area of the mixtures, as tabulated in Table 6. The results show the addition of AC to PLA/cellulose mixtures increased the specific surface area of the samples from 0.297 m2/g to 0.448 m2/g. Further increment of specific surface area was obtained with the addition of 1% MMT, which reported 0.784 m2/g.
Fig. 7. Graph of (a) linear isotherm; (b) multi-point BET plot of PLA + 2% cellulose